Articles  |   October 2013
The TFOS International Workshop on Contact Lens Discomfort: Report of the Contact Lens Interactions With the Tear Film Subcommittee
Author Affiliations & Notes
  • Jennifer P. Craig
    Department of Ophthalmology, New Zealand National Eye Centre, University of Auckland, Auckland, New Zealand
  • Mark D. P. Willcox
    School of Optometry and Vision Science, University of New South Wales, Sydney, New South Wales, Australia
  • Pablo Argüeso
    Schepens Eye Research Institute and Massachusetts Eye and Ear, Harvard Medical School, Boston, Massachusetts
  • Cecile Maissa
    Optometric Technology Group Research & Consultancy, London, United Kingdom
  • Ulrike Stahl
    Centre for Contact Lens Research, School of Optometry and Vision Sciences, University of Waterloo, Waterloo, Ontario, Canada
  • Alan Tomlinson
    Glasgow Caledonian University, Glasgow, United Kingdom
  • Jianhua Wang
    University of Miami, Miller School of Medicine Department of Ophthalmology, Bascom Palmer Eye Institute, Miami, Florida
  • Norihiko Yokoi
    Department of Ophthalmology, Kyoto Prefectural University of Medicine, Kyoto, Japan
  • Fiona Stapleton
    School of Optometry and Vision Science, University of New South Wales, Sydney, New South Wales, Australia
  • Correspondence: Fiona Stapleton, School of Optometry and Vision Science, University of New South Wales, Rupert Myers Building, Barker St, Sydney, 2052, New South Wales, Australia;
Investigative Ophthalmology & Visual Science October 2013, Vol.54, TFOS123-TFOS156. doi:
  • Views
  • PDF
  • Share
  • Tools
    • Alerts
      This feature is available to authenticated users only.
      Sign In or Create an Account ×
    • Get Citation

      Jennifer P. Craig, Mark D. P. Willcox, Pablo Argüeso, Cecile Maissa, Ulrike Stahl, Alan Tomlinson, Jianhua Wang, Norihiko Yokoi, Fiona Stapleton; The TFOS International Workshop on Contact Lens Discomfort: Report of the Contact Lens Interactions With the Tear Film Subcommittee. Invest. Ophthalmol. Vis. Sci. 2013;54(11):TFOS123-TFOS156. doi:

      Download citation file:

      © ARVO (1962-2015); The Authors (2016-present)

  • Supplements
The aim of the subcommittee report was to review published evidence describing tear film changes secondary to contact lens wear and to examine the evidence for associations between tear film changes and contact lens–related discomfort (CLD) in order to identify potential etiologies for CLD and strategies for the optimization of comfort. 
The report comprises two main sections; the first describes biophysical interactions between the contact lens and the tear film, and the second deals with biochemical changes to the tear film associated with contact lens wear. 
In first considering the tear film structure, recent tomographic, interferometric, and reflectance spectral techniques indicate central corneal tear film thickness values around 3 μm, 13 aligning closely with earlier measurements. 4,5  
The thin outermost lipid layer of the tear film, on the order of 50 to 100 nm in thickness, 6,7 forms the barrier between the environment and the eye. The lipid secretions arise mainly from the meibomian glands through orifices located at the mucocutaneous lid margin junctions, and are combined with a lesser lipid contribution from the eyelid glands of Moll and Zeiss. 8 Spread over the tear film surface by blinking, the lipid layer comprises a thin inner polar layer, overlaid by a thicker outer nonpolar layer. 9,10 In addition to preventing overspill of tear fluid onto the eyelids and contamination of the tear film by skin lipids, 9,11 the most significant role of the lipid envelope is considered to be in retarding evaporation from the ocular surface. 12,13  
The aqueous phase of the tear film forms the bulk of the tear film thickness. 8 It arises primarily from the main lacrimal gland and accessory lacrimal glands of Krause and Wolfring, 14 with additional fluid and electrolytes secreted by the ocular surface epithelial cells. The tear flow rate varies according to the level of sensory stimulation, in response to the demands of the external environment. The overnight tear production rate is significantly lower than that during the day. 15  
The role of the aqueous phase is to nurture and protect the epithelia by providing a medium for the transfer of oxygen and nutrients to the avascular corneal tissue; conveying signals between the structures bathed in aqueous; and flushing away epithelial debris, toxins, and foreign bodies. 16 The electrolytes within the aqueous phase dictate the osmolarity of the tear fluid, as well as playing a role in regulating pH and maintaining epithelial integrity. 16 Hyperosmolarity, which reflects an increased electrolyte concentration, is recognized to damage the ocular surface. 17 Aqueous layer proteins contribute to ocular surface defense 18 and maintenance of tear film stability. 9 The proportions of plasma-derived and conjunctiva-derived proteins relative to lacrimal gland proteins within the tear film are dependent upon the tear flow rate and the level of ocular surface stimulation. As well as electrolytes and proteins, the tear film contains antioxidants to scavenge free radicals and growth factors important in epithelial regeneration and wound healing. 19 Inflammation causes changes in the tear film constituents with release of inflammatory markers precipitating an escalating cycle of inflammation with ocular surface irritation, tear film instability, epithelial cell dysfunction, and apoptosis, which ultimately affects corneal epithelial barrier function. 17  
The aqueous phase of the tear film contains soluble, gel-forming mucins, produced by the conjunctival goblet cells. 20,21 These mucins are important for removing pathogens and debris from the ocular surface, smoothing the ocular surface, and for protecting the surface, through lubrication, from the blink and from environmental insult. 16 Anchored to the apical plasma membrane of the corneal and conjunctival epithelial cells are the transmembrane mucins, which contribute to forming the glycocalyx. These molecules can interact with multiple proteins through both the extracellular and intracellular domains. Carbohydrate structures present on the highly glycosylated extracellular region allow interaction with carbohydrate-binding proteins, such as galectin-3, to promote barrier function on the most apical epithelial cell layer. 20  
In situ, contact lenses (CLs) divide the tear film into pre- and postlens films (Fig. 1). This compartmentalization impacts the tear film in a number of ways, affecting both the biophysical and biochemical properties of the tear film. Contact lens wearers are recognized to exhibit significantly more ocular symptoms than nonwearers. 2224 In an attempt to determine the relevance of tear film changes to CLD, each of the tear film parameters is described for the non–CL-wearing eye and CL-wearing eye. Wherever possible, associations between the tear film changes and reported discomfort in CL wearers are discussed. 
Figure 1
Tear film structure with a contact lens in situ (P. Argüeso).
Figure 1
Tear film structure with a contact lens in situ (P. Argüeso).
Changes in the Biophysical Properties of the Tears With Contact Lens Wear and Their Effect on Comfort
Blink Impact on Precorneal and Pre-Lens Tear Film Spread and Volume
The integrity of the tear film and of the superficial layer of ocular surface epithelium is codependent. Therefore, if abnormalities are present in either one, a cycle of damage may be triggered at the ocular surface. For example, abnormalities in the tear film (presenting as an unstable tear film) can induce abnormalities in the ocular surface epithelium (presenting as decreased wettability and decreased barrier function) and vice versa, such that a suboptimal interaction between the tear film and the ocular surface epithelium ensues and is maintained in a cyclical fashion. Such possible interrelated vicious cycle mechanisms are presented here in the context of the tear film–CL interactions. 
To help maintain clear vision and ocular surface health, eye blinks occur to distribute natural tears over the ocular surface, especially the corneal surface. Two major types of blink can be distinguished, complete and incomplete (partial), in which the eyelid covers more or less than 67% of the cornea, respectively. 25  
It is reported that in healthy subjects, the proportion of incomplete blinking, for an unspecified vision task, can reach up to 20% of the total blinks. 26 The insertion of a CL onto the ocular surface might not modify the overall blinking frequency immediately; however, the following findings are repeatedly reported: 
    There is a higher percentage of incomplete blinks in rigid CL wearers. 27,28
    Although no clear difference is observed in the frequency of incomplete blinks between soft CL wearers and control subjects, the correlation between the percentage of incomplete blinks and the grade of corneal fluorescein staining is much stronger in the eyes of subjects who wear soft CLs. 25,29,30 Moreover, subjects with incomplete blinks reportedly suffer more from discomfort and dryness, and more lens deposits. 25,31
The ratio between the tear film breakup time (TBUT) and the interblink interval (IBI) defines the Ocular Protection Index (OPI). The ocular surface is considered to be protected when the TBUT matches or exceeds the IBI (OPI ≥ 1). In the case of a blink rate of 12 per minute (mean IBI, 5 seconds) and a TBUT of 4 seconds, an incomplete blink creates an approximate 10-second IBI, thus resulting in exposure of the corneal, conjunctival, and/or CL surface areas due to the lack of tear film integrity. 
When the blink rate is reduced to 8 per minute during reading, 32 the mean IBI is 7.5 seconds, and an incomplete blink increases the IBI of the exposed cornea, conjunctiva, or CL surface to approximately 15 seconds. However, when the blink rate is only 4 per minute, as is commonly observed during computer use, 33 the mean IBI is 15 seconds, and an incomplete blink creates an IBI of approximately 30 seconds for the exposed cornea, conjunctiva, or CL surface. Thus, incomplete blinks lead to a prolonged IBI, which in turn might result in increased evaporation and impaired tear film lipid layer (TFLL) spread over the ocular surface. 
Studies have shown that in patients with dry eye and in subjects with CL (soft or rigid)-related dry eye symptoms, the blinking frequency is increased from 15.5 blinks/minute to more than 20.3 blinks/minute in order to compensate for the tear film instability, 34,35 In the case of rigid CLs, the increased blinking frequency might also be the result of continuous increased frictional wear between the CL, palpebral conjunctiva, and the cornea. 
Thus, serious attention must be paid to the reports 36,37 that a CL care solution containing wetting agents might restore the normal blinking frequency (an indication of restored ocular comfort and tear film stability), while rewetting drops 38,39 have just a temporary (up to 10 minutes) effect. The difference in the time scale of the beneficial effect of CL care solutions compared to the rewetting formulations indicates the possibility that the polymeric agents adsorbed to the CL surface gain much longer residence time at the ocular surface in comparison to compositions instilled as eye drops. 
Lipid Layer Interferometry
The multilayer TFLL located adjacent to the mucoaqueous tear layer comprises nonpolar meibomian lipids (primarily wax and sterol esters) spread on top of a polar lipid surface. 9 Two major functions of the lipid layer are to lower the tear surface tension, thus allowing the tear film to maintain its high area-to-volume ratio, and to inhibit the aqueous tear evaporation. 10 Various instruments (and associated techniques) have been developed to visualize the tear film compartments and lipid layer, including the Doane interferometer, 40 King-Smith interferometer, 2 the Keeler Tearscope 41 (Keeler Ltd., London, UK), and the DR-1 specular microscope. 42 On the basis of these microscopy observations, six- and five-grade scales for visually assessing the TFLL have been proposed, which consider the thickness and uniformity of the lipid layer. 43,44  
Over a CL surface, the grade of the lipid layer frequently deteriorates, indicating a thinning lipid layer, 43 due to lack of a sufficiently thick aqueous layer (a necessary prerequisite for TFLL spread), and/or forms patches with poor wettability. 44 Although soft CLs with high water content maintain a thicker aqueous layer immediately after insertion of the lens (and thus a high-grade lipid layer), they do not provide a long-term resolution. 45 Experiments evaluating the effect of low air temperature and low relative humidity on the tear film on the surface of soft CLs with different water content revealed that CLs with higher water content were more vulnerable to drying and after prolonged wear, these lenses resulted in thinner tear films with shorter noninvasive breakup time (NIBUT). 46 If the aqueous tear layer becomes too thin, direct interaction between tear film lipids and the CL surface becomes possible, and the formation of lipid deposits takes place. 47 The lipid deposition is especially problematic for silicone hydrogel CLs where, after continuous wear, hydrophobic lipid-attractive patches readily appear over the CL surface. Once formed, lipid deposits result in impaired optical quality and nonwettability of the lens surface (with the latter resulting in instantaneous breakup of the film). 48  
The quality of the TFLL and of the tear film in general can be evaluated by measurement of lipid layer spread with its subsequent fitting to exponential kinetics, for example, the Voigt model of viscoelasticity. 44 It has been found that a thicker lipid layer in eyes with sufficient aqueous tear shows significant elastic contribution in spreading, while in aqueous tear-deficient dry eye induced due to CL wear or other reasons, the extent of lipid layer spread decreases and the viscous contribution to its spread becomes dominant. Indeed, lipid layer spread becomes much slower after 8 hours of soft CL wear (measured as a >4-fold increase of the exponential time constant describing the spread kinetics). 49 The impaired lipid spread is thought to correlate with the thinner aqueous layer formed over the soft CL surface, particularly over the surface of silicone hydrogel CLs, in which the nonwettable hydrophobic silicone moieties might reorient themselves toward the lens surface after an initial breakup event. 47 This dramatic delay of lipid layer spread 49 is somewhat indicative of the deterioration of the CL surface experienced in the course of daily use, and reflects the decreased aqueous tear volume observed during daily CL wear. 50  
Tear Film Stability
Qualitative changes in the lipid layer appearance manifest clinically as alterations in the tear film integrity, described as the stability of the preocular tear film, a clinical index of which is the tear breakup time. In the non–CL-wearing eye, thinner lipid layers have been associated with shorter tear breakup time measurements, and thicker layers with increased breakup times. 12,5155  
Tear film stability does not remain constant throughout the day. Decreases in TBUT have been observed in non-CL wearers immediately after awakening 56 and also toward the end of the day, 57 with the latter observation proposed to contribute toward increased end-of-day discomfort reported by office workers and CL wearers. Over the longer term, TBUT has been shown by a number of investigators to reduce with age 5860 while others have observed no difference with age. 61 Sex also appears to have an effect on tear film stability, with females reported to exhibit reduced TBUTs relative to age-matched males. 24,60,61 Conflicting effects of low ambient relative humidity on noninvasive tear film breakup in the non–lens-wearing eye have been reported. 46,62  
Contact lenses disrupt the TFLL 53,63 and reduce tear film thickness. 3,64 The disruption is most marked with rigid lens wear, where typically no pre-lens lipid layer is visible clinically, 65 and tear breakup occurs within 2 to 3 seconds in contrast to values around 5 to 6 seconds over a soft CL. 41,66 Larger, less mobile soft CLs have greater potential to support a pre-lens TFLL, but this lipid layer tends to be thinner and consequently susceptible to more rapid breakup than the non-CL tear film, 6769 irrespective of lens material. 70,71 Overall tear film thinning has been shown to be significantly faster on the surface of a CL than on the corneal surface, 72 This instability may be related to a thinner pre-lens film, but it has been proposed that even where the pre-lens and preocular tear films are similar in thickness, the pre-lens tear film is still considerably less stable. 2 The location of tear film breakup is also influenced by the presence of a CL, with the locus of tear film breakup of the pre-lens film most often being central, while that of the non–lens-wearing eye is more frequently parameniscal. 73 Although CL dehydration has been implicated as a major factor in the development of CL-related dry eye in high water content soft lens wearers, 74 more recent evidence suggests that dehydration plays a less significant role 75 ; and the mechanism involved in the thinned lipid layer and reduced stability is related more to alterations in the lipid layer structure, possibly due to the affinity of the polar components of the lipid layer to the CL surface, resulting in increased tear evaporation and lens surface dewetting. 55  
Comparison of the preocular tear film before and after CL wear highlights significant decreases in breakup time initially 70,76,77 ; but, over the longer term, preocular tear film breakup (without a CL in situ) appears to be largely unaffected by CL wear, irrespective of the material or wear regimen, with similar effects observed for continuous and daily wear. 53,77,78  
In CL wearers, reduced pre-lens breakup times have been associated with increased symptoms of discomfort, both in hydrogel and silicone hydrogel lens wearers. 70,75,7886 Based on the differences in pre-lens breakup time between symptomatic and asymptomatic individuals, Hom and Bruce 87 suggested a cutoff TBUT value of 3 seconds as a suitable criterion for identifying tear film dysfunction likely to cause dryness symptoms in CL wearers. 
With respect to tear film stability, consistent comfort differences relating to lens material have not been established, but environmental conditions have been described as further affecting symptoms of dryness and stability of the pre-lens tear film. Maruyama and colleagues 46 found reduced tear film thickness and breakup times in conditions of low relative humidity (20%), suggesting that increased evaporation plays a role in this process. In the presence of a soft CL, this was associated with increased symptoms of discomfort. Higher lens water content was found to correlate with increasing dryness symptoms, but not to tear film breakup. 46  
Significant differences in NIBUT have been reported between CL wearers described as either tolerant or intolerant on the basis of their ability to tolerate lens wear for a period of at least 6 hours. 76 Tolerant wearers averaged a NIBUT of around 20 seconds in comparison to 13 seconds for intolerant wearers. Interestingly, the pattern of pre-lens tear film drying 88 on the CL surface was shown to vary with tolerance to CL wear, with all intolerant CL wearers exhibiting a streak pattern of breakup in comparison to tolerant wearers, in whom more spot breakup patterns were observed. Stepwise discriminant function analysis, used to predict tolerance or otherwise to ≥6 hours of lens wear, indicated that, of the broad range of tests performed, tear film stability indices (NIBUT and drying pattern) were the most highly predictive measures of tolerance. 76 Others, too, have conceded from a range of tests that NIBUT, combined with lid parallel conjunctival folds (LIPCOF) and Ocular Surface Disease Index (OSDI) score, provided the best predictive power (positive predictive value 87%, accuracy 91%) for symptom development in new CL wearers. 89  
In a subsequent study by the same group, the effect of 6 hours of soft CL wear on a similarly large range of tear parameters was compared in tolerant and intolerant wearers. 90 At baseline, NIBUT was confirmed to be significantly shorter in the intolerant group than in the tolerant group. However, it was found, with CLs in situ, that NIBUT significantly declined over the 6-hour wear period in the tolerant group only, such that their mean NIBUT postwear was reduced to a level not significantly different from that of the intolerant wearers, with or without CL wear. It was postulated that reduced tear flow rates and phenol red thread test results in the intolerant group compared to the tolerant group after CL wear could be attributed perhaps to tear film instability. 90 The predictive nature of NIBUT in this study is consistent with that in the non–lens-wearing eye, where stepwise multiple regression analysis has shown that, along with ocular surface sensitivity, NIBUT is a significant predictor of symptoms as measured by the OSDI. 91 Chui and colleagues, 77 conversely, were unable to confirm NIBUT as a predictor of CL success (defined as ≥12 hours of wear without signs or symptoms), either alone or in combination with the phenol red thread test, as a measure of tear secretion. 
As the tear film breaks up on the CL surface, a further consequence is degradation in visual performance. 9294 Changes in the CL hydration or changes in the quality of the tear film may also result in variations in visual performance. 93,95 Given a shorter NIBUT associated with CLD and intolerance to CLs, there may be greater visual compromise associated with discomfort, and possibly blurring of the vision may provide one of the stimuli to blink in those patients in whom the pre-lens NIBUT is shorter than the interblink period. 
Tear Film Evaporation
The normal tear film is lost from the ocular surface by evaporation, absorption, and drainage. Tear film evaporation is believed to be the main determinant of tear film thinning, 96,97 and is acknowledged as a key component in tear dynamics and the development of dry eye. 98 Excessive evaporation of the tear film is recognized to cause tear hyperosmolarity triggering a cycle of ocular surface inflammation. 17  
With few exceptions, 99 the published literature reports that tear evaporation rates increase in dry eye, 100105 typically in association with a loss of integrity of the tear lipid layer. 12 The lipid layer has long been recognized to play an important role in inhibiting tear film evaporation 13,106 on the basis of animal studies, and this has been confirmed in human studies where a 4-fold increase in tear evaporation is observed in the absence of a clinically visible or continuous lipid layer. 12 Reduced lipid layer integrity in the non–CL-wearing eye most commonly results from posterior blepharitis (most commonly meibomian gland dysfunction [MGD]) and anterior blepharitis 17,107,108 ; and associations of these conditions with symptoms of dry eye, tear film instability, age, and sex have been reported. 102,109,110 Environmental conditions can also affect the rate of tear film evaporation, with increased rates apparent in healthy eyes, under conditions of low relative humidity. 62  
It is generally accepted that tear evaporation increases with age, particularly in females. 111113 Although other studies also reported higher evaporation rates in females than in males, the relationship with age was not confirmed. 61,114  
Paradoxically, the application of artificial aqueous supplements has been shown to cause an immediate increase in the rate of tear film evaporation, 115 a phenomenon attributed to tear film disruption upon instillation, most specifically disruption of the superficial lipid layer. Supporting this hypothesis are the more recent counter observations that the rate of tear evaporation can be decreased following supplementation with natural or artificial lipids. 116,117  
Inconsistencies in evaporation rates described in the literature have been attributed to subject selection, measurement techniques, and instrumentation. 118 Published studies reporting evaporation rates in the non–CL-wearing eye are summarized in Table 1
Table 1. 
Evaporation Rates Reported for the Non–Contact Lens–Wearing Eye
Table 1. 
Evaporation Rates Reported for the Non–Contact Lens–Wearing Eye
Authors TER ± SD, ×10−7 g/cm 2 /s TER Measurement Technique
Hamano and colleagues (1980) 119 26.9 Pressure gradient, open chamber (in contact with tears)
Tomlinson and Cedarstaff (1982) 120 109.2 ± 49.3, at 70% Resistance hygrometry
Cedarstaff and Tomlinson (1983) 121 119.8 ± 39.9, at 50% RH Resistance hygrometry
Rolando and Refojo (1983) 122 4.1 ± 0.4, at 29.5% RH Change in relative humidity measured within closed chamber filled with dry air
Herold (1987) 123 68.9 ± 18.9 Vapor pressure
Tomlinson and colleagues (1991) 124 12.5 ± 6.9 Vapor pressure
Tomlinson and Cedarstaff (1992) 125 166.7 ± 5.0, at 70% Resistance hygrometry
Tsubota and Yamada (1992) 99 15.6 ± 3.8, at 40% RH Vapor pressure
Tomlinson and Giesbrecht (1994) 114 10.6 ± 6.6 Vapor pressure
Mathers and colleagues (1993) 103 14.7 ± 6.4, at 30% RH Change in relative humidity measured within closed chamber filled with dry air
12.1 ± 5.5, at 40% RH
Craig and Tomlinson (1997) 12 0.39 ± 0.37, at 48% RH Vapor pressure
Goto and colleagues (2003) 109 4.1 ± 1.4 Change in relative humidity measured in ventilated chamber system with air flow
Thai and colleagues (2004) 126 10.8 ± 5.3 Vapor pressure
Guillon and Maissa (2010) 111 16.6, median 15.9, at 30% RH Change in relative humidity measured within closed chamber filled with dry air
13.7, median 11.4, at 40% RH
Khanal and colleagues (2009) 101 5.8 ± 2.8 Vapor pressure
Dogru and colleagues (2011) 82 4.1 ± 0.3, at 30%–50% RH Quartz crystal humidity sensor
Arciniega and colleagues (2011) 127 5.5 ± 2.0, at 30% RH Change in relative humidity measured within closed chamber filled with dry air
3.8 ± 1.3, at 40% RH
Kimball and colleagues (2010) 96 53.9 ± 71.5 Extrapolated from spectral interferometry*
Petznick and colleagues (2013) 128 26.8 ± 2.4 Extrapolated from infrared thermography
The rate of tear film evaporation has been demonstrated to increase with a CL in situ. 98,120,126,129 It is generally accepted that the physical presence of a CL disrupts the normal tear film structure, and in particular the lipid layer, facilitating a more rapid loss of tear fluid by evaporation. This is supported by research describing decreased tear film stability in the presence of a CL. 70,71 Under constant environmental conditions, researchers have failed to demonstrate consistent differences in the tear evaporation rate with different lens materials, even between rigid and soft lenses. 120 As with non–CL-wearing eyes, there is significant variation in tear evaporation rate values between research groups for CL wearers; however, overall, the literature shows that CLs typically result in a 1.2× to 2.6× increase in the rate of tear evaporation relative to the non–lens-wearing eye, with no clear pattern relating to either lens form or water content (Table 2). 120,126,129 It does, however, appear possible to differentiate lens types under adverse environmental conditions. Kojima and colleagues 130 found significant increases in evaporation rate in wearers of hydrogel lenses but not silicone hydrogel lens wearers 130 following exposure to a controlled adverse environment chamber that was significantly drier at 18% relative humidity (RH) than the ambient environment (30%–40% RH). 
Table 2. 
Summary of Studies Reporting Tear Evaporation Rates in Contact Lens Wearers
Table 2. 
Summary of Studies Reporting Tear Evaporation Rates in Contact Lens Wearers
Authors % Increase in TER During Lens Wear Lens Type Lens WC, % RH, %
Tomlinson and Cedarstaff (1982)120 216 PMMA
216 Paragon-18 (PMMA)
191 Silicone elastomer: Silsoft (Bausch and Lomb, Rochester, NY) 70
187 Hydrogel Sauflon (Sauflon Pharmaceuticals Ltd., Twickenham, UK) 70
258 Hydrogel Cibasoft (Alcon, Fort Worth, TX) 38
Cedarstaff and Tomlinson (1983)121 135 Hydrogel 70% 70
138 Hydrogel 55% 55 70
155 Hydrogel 38% 38
Thai and colleagues (2004)126 127 Hydrogel: polymacon 38
123 Hydrogel: omafilcon A 62
142 Hydrogel: phemfilcon A 38
138 Silicone hydrogel: balafilcon A 36 NS
140 Hydrogel: etafilcon A 58
Guillon and Maissa (2008)98 156 Hydrogel NS 30
167 Hydrogel NS 40
Increased tear evaporation rates lead to dryness and discomfort symptoms in CL wearers. Kojima and colleagues 130 noted a relationship between increased tear evaporation and ocular discomfort in nonadapted CL wearers fitted with hydrogel lenses (etafilcon A) and exposed to a controlled adverse chamber environment with RH of 18%. Interestingly, however, no differences in either symptoms or evaporation rate were observed for neophytes fitted with silicone hydrogel lenses (narafilcon A) under the same conditions. 130  
Tear Film Temperature
The temperature of the normal ocular surface, and thus the adjacent tear film, is lower than core body temperature on account of its exposed location, somewhere on the order of 32°C to 36°C. 131 Mean ocular surface temperature in dry eyes is reported to be similar to 132,133 or slightly higher than that in normal eyes. 134,135 Increased ocular surface temperatures measured in uveitis 136 suggest that the increased temperature in dry eye is likely attributable to ocular surface inflammation, a key element in dry eye. 17,134  
Technological advances, resulting in the advent of noncontact infrared thermography, have enabled measurement of ocular temperature with significantly improved sensitivity as well as spatial and temporal resolution. 133,134,137,138 This has led to the observation by most researchers that the ocular surface temperature varies across the exposed surface, with the normal cornea being warmest at the limbus and coolest centrally. 134,139 With the exception of one study, 133 this temperature differential between the limbus and corneal center has been shown to be greater in dry eye, such that the central cornea is significantly cooler, relative to the limbus, in dry eyes than in normal eyes. 100,134,135,140 The faster rate of cooling observed postblink in dry eyes has been attributed to a more rapid rate of tear film evaporation. 135  
The relationship recognized to exist between tear film stability and ocular surface temperature 46,100,141 suggests that ocular surface thermography is an indirect measure of tear film stability. 142 More recently, thermography has also become recognized as a surrogate for evaporation rate measurement. 113,128  
Variations in participant characteristics, study design, and methodology between published studies evaluating the effect of CLs on tear film temperature preclude direct comparison of their results. Participant age and health status, as well as the environmental conditions and the precise measurement site on the eye, can affect the results. The invasive nature of some earlier techniques, in contrast to the noncontact techniques reported more recently, further compounds this issue. 
In one of the earliest reports, with thermistors embedded in scleral lenses, Hill and Leighton 143 observed insignificant temperature differences in the presence of the CLs, although temperatures increased significantly during eye closure. Hamano, 144 too, observed only small differences (<0.5°C) between eyes with and without rigid CLs using a noncontact radiometer. Conversely, Fatt and Chaston, 145 with a noncontact bolometer, recorded a more significant difference in ocular surface temperature, of 0.5 to 1.5°C, between hard CL wearers and non–lens-wearing eyes. They found the ocular surface temperature in soft CL wearers to differ no more than 0.5°C from the naked eye temperature. Also evaluating soft CLs, Martin and Fatt 146 detected insignificant differences in temperature beneath hydrogel lenses, using a technique with thermistors sandwiched between thin hydrogel CLs, although again they were able to observe significant temperature increases during eye closure. Montoro and colleagues, 147 as one of the first groups to use the current standard technique of infrared thermography with customized analysis software, identified irregular thermal patterns in a group of 19 CL wearers. 
More sensitive current techniques indicate that the temperature of the pre-lens tear film in soft lens wearers is cooler than that of the non–CL-wearing eye, 148 while the temperature of the postlens tear film beneath the CL is higher. 131 Lens materials with high water content and a correspondingly rapid rate of water loss show lower lens surface temperatures in situ than those with low water content. 148 This difference between lens materials is reflected in ocular surface temperatures (postlens tear film), which increase beneath all CL materials, 146 but more so with silicone hydrogel lenses than with hydrogel lenses. 131 This is attributed to the higher bound-to-free water ratio, which results in a lower rate of water loss from silicone hydrogel lenses. 131  
Few studies have directly evaluated comfort and ocular surface temperature in CL wearers. Hill and Leighton 149 experienced some success in correlating temperature-related sensations described by subjects to corneal temperatures assessed by thermistor units embedded in scleral CLs. Some predictive value could be assigned to specific descriptors, and it was concluded that tear film temperature could reflect the neural contributions that influence subjective experience. 149 However, such a study has not been performed with the high-resolution thermographic technology available now. Lowering the ocular surface temperature with cooled (4°C) artificial tears reduces ocular surface sensitivity and improves comfort. 150 This might suggest that, if ocular surface temperature is raised in CL wear, the concept of reducing ocular surface temperature could be advantageous. However, this should be viewed in conjunction with the hypothesis put forward that cold neuro-receptors in the corneal and lid margin may be partly responsible for CLD (see Report by the Subcommittee on Neurobiology). 
Tear Film Thickness
The precorneal tear film (PCTF) is regarded as an important layer in keeping ocular surface wet and smooth so that epithelial integrity and sharp vision can be maintained. The thickness of the PCTF is a key parameter that relates to tear secretion, spreading, evaporation, and drainage. Previously no consensus could be reached on tear film thickness, mainly on account of the difficulty in measuring a fluid layer that is highly dynamic in nature. 151 However, it is generally accepted that the total tear film thickness is around 3 μm. 14  
Each blink alters the tear distribution, resulting in variation of the tear film thickness. 151,152 During each blinking cycle, the tear film thickness varies within a couple of microns. 151 If the blinking is delayed (by the subject's consciously refraining from blinking), 151 the tear film thickness increases to approximately 7 μm due to reflex tearing. Recently, ultrahigh-resolution optical coherence tomography (OCT) has been used to corroborate these measurements. 153155 The tear film begins to thin during the open-eye period due to redistribution and evaporation. 1,72,151  
Nichols and King-Smith 64 found that the pre-lens tear film (PLTF) was approximately 2 μm with interferometry, which was about the same as measured at 3 minutes after lens insertion using ultrahigh-resolution OCT. 153 When the measurement was taken at the time of lens insertion, the PLTF was higher, at around 6 μm, due to reflex tearing or surplus lens wetting solution. 153 The PLTF thickness can be altered by adding drops onto the lens; however, the increase of the PLTF is transient (approximately 10 minutes) (Fig. 2). 153 Interferometry measurements reveal that PLTF thickness might be approximately 1 μm thinner compared to the 3.5-μm-thick PCTF and that the PLTF thinning rate is higher compared to that of PCTF (which in turn leads to a shorter TBUT of the PLTF). 2,64,72,156  
Figure 2
Changes in the PLTF and PoLTF after lens insertion followed by one drop of artificial tears on the convex surface of the lens. (A) A soft CL was inserted onto the cornea (CO) without the aid of artificial tears. Immediately afterward, the PLTF (green) and PoLTF (red) were visualized by spectral-domain ocular coherence tomography. (B) Three minutes later, the PLTF and PoLTF were no longer visible, as they became too thin. (C) One drop (35 μL) of artificial tears instilled onto the lens did not increase the PoLTF, whereas the PLTF was clearly increased immediately afterward. (D, E) In the following minutes, the PoLTF never increased sufficiently to become visible, using ultrahigh-resolution (UHR)-OCT in this study, and the PLTF decreased gradually with blinking. Reproduced from Chen et al. Ultrahigh-resolution measurement by optical coherence tomography of dynamic tear film changes on contact lens. IOVS. 2010;51:1988–1993. 153
Figure 2
Changes in the PLTF and PoLTF after lens insertion followed by one drop of artificial tears on the convex surface of the lens. (A) A soft CL was inserted onto the cornea (CO) without the aid of artificial tears. Immediately afterward, the PLTF (green) and PoLTF (red) were visualized by spectral-domain ocular coherence tomography. (B) Three minutes later, the PLTF and PoLTF were no longer visible, as they became too thin. (C) One drop (35 μL) of artificial tears instilled onto the lens did not increase the PoLTF, whereas the PLTF was clearly increased immediately afterward. (D, E) In the following minutes, the PoLTF never increased sufficiently to become visible, using ultrahigh-resolution (UHR)-OCT in this study, and the PLTF decreased gradually with blinking. Reproduced from Chen et al. Ultrahigh-resolution measurement by optical coherence tomography of dynamic tear film changes on contact lens. IOVS. 2010;51:1988–1993. 153
The postlens tear film (PoLTF) may play an important role in the interactions with the ocular surface and may impact lens movement and ocular comfort. 157159 Depletion of the PoLTF may also cause lens adherence 160 and surface staining, 31,159 which have to do with CL-related complications 154,161 and discontinuation. 23,162 The thickness of the PoLTF at the center location of the cornea is approximately 1 to 3 μm (Fig. 3), 3,36,64,153,156 in agreement with other studies. In contrast, Lin and colleagues 163 found the central PoLTF to be approximately 11.5 μm using optical pachymetry. It may be slightly thicker at the time of lens insertion, and rapid thinning is evident. 64,153 The PoLTF remains thin irrespective of the instillation of artificial tears. 153  
Figure 3
Changes in the PLTF and postlens tear film PoLTF after lens insertion. (A) One drop (35 μL) of artificial tears was placed on the concave surface of the lens before insertion. The PLTF and PoLTF were immediately increased compared with the precorneal tear film (PCTF, post hoc tests, P < 0.05). The PoLTF decreased continuously for the next 8 minutes (post hoc test, P < 0.05), and the PLTF decreased in a similar fashion (P < 0.05). (B) After lens insertion without prior application of a drop of artificial tears to the concave surface, the PLTF was immediately thicker than the PCTF at baseline (P < 0.05). After 3 minutes of lens wear, both the PLTF and PoLTF decreased significantly compared with the moment of lens insertion (P < 0.05). When 35 μL artificial tears was instilled on the lens, the PLTF increased significantly and then decreased gradually in the following 8 minutes (P < 0.05). However, the PoLTF did not increase immediately after drop instillation and also did not change in the following 10 minutes (Repeated measures ANOVA, P > 0.05). Reproduced from Chen et al. Ultrahigh-resolution measurement by optical coherence tomography of dynamic tear film changes on contact lens. IOVS. 2010:51:1988–1993. 153
Figure 3
Changes in the PLTF and postlens tear film PoLTF after lens insertion. (A) One drop (35 μL) of artificial tears was placed on the concave surface of the lens before insertion. The PLTF and PoLTF were immediately increased compared with the precorneal tear film (PCTF, post hoc tests, P < 0.05). The PoLTF decreased continuously for the next 8 minutes (post hoc test, P < 0.05), and the PLTF decreased in a similar fashion (P < 0.05). (B) After lens insertion without prior application of a drop of artificial tears to the concave surface, the PLTF was immediately thicker than the PCTF at baseline (P < 0.05). After 3 minutes of lens wear, both the PLTF and PoLTF decreased significantly compared with the moment of lens insertion (P < 0.05). When 35 μL artificial tears was instilled on the lens, the PLTF increased significantly and then decreased gradually in the following 8 minutes (P < 0.05). However, the PoLTF did not increase immediately after drop instillation and also did not change in the following 10 minutes (Repeated measures ANOVA, P > 0.05). Reproduced from Chen et al. Ultrahigh-resolution measurement by optical coherence tomography of dynamic tear film changes on contact lens. IOVS. 2010:51:1988–1993. 153
Tear Production/Turnover
Quantification of tear production during CL wear has received limited attention, in part because of the technical difficulty of measurement. The earliest attempts by Hamano and colleagues (1983) 119 employed a wetting measure, the phenol red thread test, to overcome the problems of repeatability and consistency with the Schirmer test. Thread wetting was not found to increase with CL wear. Sørensen and colleagues (1980) 164 were the first to use tear clearance as a measure of tear flow in 14 individuals before and after 1 month of adaptation to a Soflens CL (Bausch & Lomb, Rochester, NY). They used a gamma camera to assess the rate of new tear production through observation of the elimination of a radioactive tracer, technetium, from the conjunctival sac. The fractional turnover rate with a soft CL presoaked in the technetium solution was similar to the rate with a solution instilled directly into the eye. 164  
Other investigators have observed the elimination of a fluorescein dye from the eye to measure tear production during CL wear. 165168 Puffer and colleagues (1980) 165 studied 51 normal subjects with a simple method that permitted measurement of the rate of fluorescein loss from the central PTCF. No statistically significant correlations were found between tear elimination coefficient and sex, eye color, or CL wear. 165 Occhipinti and colleagues (1988) 166 were the first to employ the automated scanning fluorophotometer (Fluorotron; OcuMetrics, Mountain View, CA) and found no significant difference in tear turnover rate (TTR) in CL wearers compared with nonwearers. 
The use of either an automated scanning fluorophotometer or slit lamp–mounted fluorophotometer offers potentially the most accurate measure of TTR in CL wear without the cost and inherent restrictions of the gamma camera. The use of small molecular weight fluorescent tracers, however, confounds measurement due to dye penetration into soft CLs. 168,169,170 Notwithstanding this difficulty, the TTR appears to decrease significantly in CL wear compared with the non–CL-wearing eye. 107 An average TTR of 15.5%/minute is typical of normal young subjects without lenses. 167,171175 Further experiments with a more likely nonpenetrating tracer, 70-kDa fluorescein-isothiocyanate (FITC) dextran, were carried out on a group of 20 habitual wearers. 168 The measures with a conventional hydrogel lens, etafilcon A (Acuvue 2; Johnson & Johnson Vision Care, Inc., Jacksonville, FL), and silicone hydrogel lens, balafilcon A (PureVision; Bausch & Lomb), and with no lens showed TTRs of 12.4%/minute, 13.2%/minute, and 16.4%/minute, respectively. Therefore fluorophotometric measurements of TTR with this tracer showed no statistically significant difference in the presence of a CL, consistent with the consensus from previous studies. 
In an attempt to relate tear production in CL wear to the discomfort experienced by some CL wearers, Tomlinson and colleagues 176 compared tear physiology in symptomatic and asymptomatic wearers. Subjects with symptoms of CL dry eye (CLDE) had a significantly lower basal TTR (in the absence of a lens) than asymptomatic subjects (Fig. 4); TTR in this study was measured with the Fluorotron (OcuMetrics) immediately after CL removal. This finding is in accord with the speculation of Glasson and colleagues 90 of reduced tear flow in intolerant wearers. Thai 177 had previously shown that values on CL removal were consistent with the normal basal tear flow rate. No significant differences between the groups were found for tear evaporation, osmolarity, or tear breakup time. The greater basal tear production facility in asymptomatic patients may offset the loss of tear fluid due to the increased tear evaporation rate induced by CL wear. 104,120,178  
Figure 4
Tear turnover and evaporation rates in symptomatic (of contact lens dry eye) and asymptomatic contact lens wearers. Adapted with permission from Tomlinson et al. Why do some contact lens wearers avoid contact lens dry eye symptoms? Poster presented at: 91st Annual Meeting of the American Academy of Optometry; October 24–27, 2012; Phoenix, AZ. 176 Copyright [date if known] [name of copyright holder].
Figure 4
Tear turnover and evaporation rates in symptomatic (of contact lens dry eye) and asymptomatic contact lens wearers. Adapted with permission from Tomlinson et al. Why do some contact lens wearers avoid contact lens dry eye symptoms? Poster presented at: 91st Annual Meeting of the American Academy of Optometry; October 24–27, 2012; Phoenix, AZ. 176 Copyright [date if known] [name of copyright holder].
Tear Volume
Tears are secreted by the lacrimal gland and approximately 4.5 μL is distributed into the cul-de-sac, approximately 2.9 μL into the tear menisci, and approximately 1.1 μL into the preocular tear film. 179 During the blinking cycle, some of the tears drain into the drainage system (via the puncta) or evaporate into the air. The volume remains as a dynamic balance so that a certain amount of tears is always available to form the tear film by lid spreading. 151 In the normal tear system, as little as 2 to 4 μL of the tear volume on the ocular surface is required for the maintenance of a wet surface. 151,179 With each blink, the tears are mixed and redistributed. 180 The movement of the eyelids acts as a pump by compressing the canaliculi and lacrimal sac and promoting drainage of tears. The tear volume must maintain a relatively steady state. 152,179,181184 Under normal circumstances, the drainage system itself is thought to contain a negligible volume of tears. 185 It appears that only a small variation in the tear menisci occurs during blinking and the small amount of tears constantly available is sufficient to keep the ocular surface wet. While blinking and eye opening have little effect on normal tear volume, spreading during blinking and evaporation during the open-eye period may cause minor variations in tear distribution (Fig. 5). 151 In contrast, the tear volume increases during restricted blinking, 151 instillation of artificial tears, 186 and punctal occlusion. 153 Through overloading of the tear volume with repeated instillation of saline solution, increased blink output into the drainage system is evident. 151,187,188 The phenomenon indicates that the capacity of the ocular surface to hold excessive tears is limited. It may also indicate that the system may be regulated, presumably by the lower tear meniscus through the drainage system. 151,152  
Figure 5
Total tear volume during normal and delayed blinks in 21 subjects. The upper tear meniscus volume (UTMV), tear film volume (TFV), and lower tear meniscus volume (LTMV) were estimated during normal (A) and delayed (B) blinks. The tear volume was greater during delayed blinking than during normal blinking (P < 0.01). Most of the change was due to increases in the LTMV (B). Both UTMV and LTMV were higher (P < 0.001) during delayed blinking (B) compared with normal blinking (A). The UTMV and LTMV increased significantly at the end of the eye-opening period compared with the beginning during delayed blinking (P < 0.05). Reproduced from Palakuru et al. Effect of blinking on tear dynamics. IOVS. 2007;48:3032–3037. 151
Figure 5
Total tear volume during normal and delayed blinks in 21 subjects. The upper tear meniscus volume (UTMV), tear film volume (TFV), and lower tear meniscus volume (LTMV) were estimated during normal (A) and delayed (B) blinks. The tear volume was greater during delayed blinking than during normal blinking (P < 0.01). Most of the change was due to increases in the LTMV (B). Both UTMV and LTMV were higher (P < 0.001) during delayed blinking (B) compared with normal blinking (A). The UTMV and LTMV increased significantly at the end of the eye-opening period compared with the beginning during delayed blinking (P < 0.05). Reproduced from Palakuru et al. Effect of blinking on tear dynamics. IOVS. 2007;48:3032–3037. 151
Tear volumes with CLs in situ have been estimated by imaging the upper and lower tear menisci using OCT. 153 Compared to the tear meniscus volume (approximately 1.5 μL) on the ocular surface, 151 a reduced tear meniscus volume (approximately 1 μL) on the CL surface is evident. 153 It is important to note that the tear volume decreases over time during CL wear. 189 Chen and colleagues 189 studied tear meniscus volumes in symptomatic lens wearers, asymptomatic lens wearers, and asymptomatic non-lens wearers. New lenses were worn by these groups for 10 hours, and tear menisci were imaged using OCT. The results showed significant decreases in the tear meniscus volume over the study period (Fig. 6). 
Figure 6
Ocular surface comfort ratings (A) and UTMV (B), LTMV (C), and TTMV (D) during 10 hours of contact lens wear. Group 1, experienced contact lens wearers with dry eye complaints; group 2, experienced contact lens wearers without dry eye complaints; group 3, inexperienced contact lens wearers without dry eye complaints. Reproduced from Chen et al. Tear menisci and ocular discomfort in symptomatic wearers. IOVS. 2011;52:2175–2180. 50
Figure 6
Ocular surface comfort ratings (A) and UTMV (B), LTMV (C), and TTMV (D) during 10 hours of contact lens wear. Group 1, experienced contact lens wearers with dry eye complaints; group 2, experienced contact lens wearers without dry eye complaints; group 3, inexperienced contact lens wearers without dry eye complaints. Reproduced from Chen et al. Tear menisci and ocular discomfort in symptomatic wearers. IOVS. 2011;52:2175–2180. 50
In a follow-up study, 50 a significant relationship between ocular comfort and upper, lower, and total tear meniscus volumes was established following 10 hours of lens wear in symptomatic and asymptomatic lens wearers (Fig. 7). The weak but significant relationships demonstrated a negative impact of the tear meniscus volume on ocular discomfort, although this would suggest, perhaps, that the tear meniscus volume may not be solely responsible for the decreased ocular discomfort. 
Figure 7
Ocular comfort ratings and tear meniscus volumes. Group 1, experienced contact lens wearers with dry eye complaints; group 2, experienced contact lens wearers without dry eye complaints; group 3, inexperienced contact lens wearers without dry eye complaints. For the individuals of each group, the comfort level at each time point during the 10 hours of lens wear was plotted against the simultaneous UTMV (A), LTMV (B), and TTMV (C). During the 10 hours of contact lens wear, there were significant positive correlations between the ocular comfort ratings and each of the calculated volumes (P < 0.05). Reproduced from Chen et al. Tear menisci and ocular discomfort in symptomatic wearers. IOVS. 2011;52:2175–2180. 50
Figure 7
Ocular comfort ratings and tear meniscus volumes. Group 1, experienced contact lens wearers with dry eye complaints; group 2, experienced contact lens wearers without dry eye complaints; group 3, inexperienced contact lens wearers without dry eye complaints. For the individuals of each group, the comfort level at each time point during the 10 hours of lens wear was plotted against the simultaneous UTMV (A), LTMV (B), and TTMV (C). During the 10 hours of contact lens wear, there were significant positive correlations between the ocular comfort ratings and each of the calculated volumes (P < 0.05). Reproduced from Chen et al. Tear menisci and ocular discomfort in symptomatic wearers. IOVS. 2011;52:2175–2180. 50
In summary, there is no direct evidence based on published studies showing the relationship between tear film thickness (pre- or posttear film) and ocular discomfort in CL wearers. However, decreased tear meniscus volumes appear to be related to ocular discomfort at the end of the day. 50 Similar findings with intolerant CL wearers have been reported by Glasson and colleagues. 76  
Tear Film Profile at the Edge of a Soft CL
Soft CLs cover a portion of conjunctiva, which is soft tissue. 50 The conjunctiva appears to distort at the lens edge. The interaction between the lens edge and conjunctiva may occur because of different pressure profiles 190 that are produced across the ocular surface underneath each lens. 191,192 It may be possible to have a tear meniscus around the lens edge at the point of lens insertion or on instillation of artificial tears. 154 However, the tear meniscus around the soft CL edge appears much smaller than that around the hard lens edge. 154 With excessive tears from tearing or instillation of artificial tears, the tear film can be augmented around the periphery of the lens, with the thickest layers at the inferior portion of the lens due to gravity. 
Evaluation of the PoLTF peripherally may reveal more information on lens fitting tightness and matching between lens design and ocular surface. Using ultrahigh-resolution OCT, two gaps beneath the lens, filled with PoLTF, can be visualized. 50,154 One type of PoLTF can be found at the peripheral cornea, and the other one can be found at the limbal junction area. The thickness of the PoLTF beneath the lens edge ranges from several micrometers up to approximately 60 μm. 154 It appears that the PoLTF may vary depending on lens design and material. 189  
Tear Exchange
From mathematical formulas, Weissman 193 inferred that flexure of a −3.0-diopter (D) lens should exchange approximately 0.01 μL fluid per diopter. Lubrication theory predicted 10% to 20% tear exchange at each blink for a normal blink with the usual tear film thickness. 194 Using fluorophotometry and a nonpenetrating tracer, the measured T95 (time to deplete 95% of a fluorescent dye from beneath a CL) was 27.3 minutes, and the tear exchange turnover rate was calculated to be 9.0%/minute. 195 In another study conducted by McNamara and colleagues, 196 the mean tear mixing rate was 1.82%/minute with a 12-mm-diameter CL, 1.61%/minute with a 12.5-mm-diameter CL, 1.34%/minute with a 13-mm-diameter CL, and 1.24%/minute with a 13.5-mm-diameter CL. Ocular surface OCT has been used to track tear mixing beneath the CL edge (Wang J, et al. IOVS 2011;52:ARVO E-Abstract 3628). In a small sample of five eyes, tear mixing was evident. Preliminary data showed that the 95% decay time was approximately 10 to 20 minutes (Wang J, et al. IOVS 2011;52:ARVO E-Abstract 3628). The tear exchange or mixing during lens wear may be regulated by the interrelationships between four variables: lens diameter and movement, the blink, and tear replenishment rate. 196  
Tear osmolarity or the saltiness of tears can be regarded as an indicator of the balance between the production of tears and their elimination via evaporation, drainage, and absorption. 105 The main contributors to tear film osmolarity are the electrolytes of the aqueous phase, principally the cations sodium and potassium and the anions chloride and bicarbonate; proteins and sugars play a minor role only. 197199 Mean tear film osmolarity measurements in the normal eye range between 283 and 318 mmol/kg, with an average value of approximately 302 mmol/kg. 197,200 It must be noted that osmolarity is commonly determined for tears from the lower meniscus, and it has been speculated that the osmolarity across the ocular surface might be significantly higher due to the variable effects of evaporation. 201,202 Generally, the sex of an individual and the hormonal cycle do not affect tear osmolarity, but increased osmolarity can be observed throughout the day and with increasing age. 112,172,203209 While there seems to be agreement that reflex tearing results in decreased tear film osmolarity, measurements on patients with epiphora remain equivocal. 210215 Tear osmolarity measurements find their most frequent application in the diagnosis of dry eye. The measurement of tear osmolarity has been suggested as a gold standard in the diagnosis of dry eye, as the often observed elevated levels (hyperosmolarity) are considered a core mechanism in symptoms and ocular surface damage in this condition. 17,63,214 A comprehensive summary of causes and effects of tear film hyperosmolarity in dry eye is provided in the 2007 TFOS Report of the Dry Eye WorkShop. 17  
During CL wear, tear film osmolarity undergoes a series of changes. Initially, the insertion of a CL results in a reduction of tear film osmolarity, potentially caused by some reflex tearing during the early adaption to the lens. 215217 This initial reduction has been considered as a cause for PoLTF depletion with subsequent lens adherence and a contributor to corneal swelling, although all osmolarity measurements at the current time are limited to the tear meniscus. A subsequent increase in osmolarity is often observed. 216218 However, there remains some debate as to the level of tear osmolarity over time, the effect of lens type and wear modality, and the effect on ocular comfort. Some authors have reported that tear film osmolarity will return to or remain at its pre-CL insertion level, 217219 while others report an increased level, postremoval, compared to baseline. 216,220,221 A summary of tear osmolarity values during CL wear is given in Table 3. Farris 222 demonstrated that wear of soft CLs on an extended-wear basis and hard lenses on a daily-wear basis significantly increased tear osmolarity, but such effect was not observed with soft lenses worn on a daily-wear basis. In contrast, other authors have shown significantly increased tear film osmolarity with soft daily-wear lenses. 221 So far, no differences in tear osmolarity have been demonstrated between hydrogel and silicone hydrogel CLs. 219221,223,224  
Table 3. 
Summary of Tear Film Osmolality Findings During CL Wear
Table 3. 
Summary of Tear Film Osmolality Findings During CL Wear
Authors Subject Group Tear Osmolarity, mmol/kg Type of Osmometer
Sarac and colleagues 219 Hydrogel 295.0 ± 1.4 In situ tear osmolarity system
Silicone hydrogel 298.8 ± 7.2
Stahl and colleagues 221 Baseline 314.4 ± 13.9 Vapor pressure osmometer
Hydrogel 323.1 ± 13.3
Silicone hydrogel 321.5 ± 17.6
Glasson and colleagues 226 Baseline 322.4 ± 16.7 Vapor pressure osmometer
Hydrogel 318.1 ± 12.8
Nichols and Sinnott 55 Subjects with CL-induced dry eye 307.7 ± 32.4 Freezing point depression osmometer
Subjects without CL-induced dry eye 297.1 ± 31.4
Miller and colleagues 223 Control, non-CL wear 305 ± 21 Vapor pressure osmometer
Hydrogel daily wear 319 ± 30
Silicone hydrogel continuous wear 319 ± 32
RGP 324 ± 25
Iskeleli and colleagues 227 Hydrogel daily wear, 55% H2O 312.2 ± 16.0 Freezing point depression osmometer
Hydrogel daily wear, 38.6% H2O 316.5 ± 12.1
RGP, 90 Dk 313.1 ± 9.7
RGP, 52 Dk 316.4 ± 11.6
Dabney and colleagues 224 Control, non-CL wear 309.0 ± 17.0 Vapor pressure osmometer
Hydrogel 313.7 ± 28.5
Silicone hydrogel 324.3 ± 41.7
RGP 317.0 ± 13.0
Martin 216 Baseline 316 Freezing point depression osmometer
Hydrogel lens eye 331
Contralateral eye 320–326
Farris 222 Aphakic nonwear control 321 ± 9 Freezing point depression osmometer
Aphakic extended wear 318 ± 7
Phakic RGP daily wear 316 ± 6
Phakic hydrogel daily wear 309 ± 8
Phakic hydrogel extended wear 318 ± 7
Until recently, most studies assessing tear film osmolarity required a large tear volume and consequently the collection of large amounts or dilution of the sample with subsequent recalculation. It must be considered that these requirements may have hindered the observation of subtle differences between lens types. Increased tear film osmolarity during CL wear has been attributed to two main factors: reduced tear production due to reduced corneal sensitivity, and excessive evaporation due to a disrupted tear film and reduced tear film stability. 214,225 Considering that these mechanisms are similar to those in dry eye, there has been some interest in the impact of tear osmolarity in CL wear on ocular comfort. Nichols and Sinnott 55 demonstrated significantly higher osmolarity values in participants with CL-induced dryness. Glasson and colleagues 76 found that symptomatic CL wearers tended to display a high tear film osmolarity even without CLs. However, in a study by Stahl and colleagues, 221 an association between tear osmolarity and ocular comfort during CL wear could not be shown. 
Tear ferning refers to the distinct crystallization pattern that appears when tears are allowed to air dry on a glass slide. This image of fern-like crystals is most commonly assessed, under white light microscopy, on a simple qualitative grading scale from I (complete, uninterrupted ferning pattern with no spaces between ferns) to IV (total absence of ferning), 229238 although other quantitative methods, such as area assessment through counting the number of micrometer lattice squares 239,240 or digital image analysis have been applied. 241,242 As outlined by Golding and Brennan, 243 as well as Pearce and Tomlinson, 244 there remains some discussion as to which components of the tear film are responsible for the successful development of tear ferns. However, there seems to be agreement that it is not the level of a single component but rather the ratio between the organic salts and macromolecules that will determine the quality of the ferning pattern. 243245 The majority of individuals display a tear ferning pattern of grade I or II. 231,246 An increase in the tear ferning grade, reflecting abnormal tear functionality, can be seen with age, 234 CL wear, 229 in the morning, 233 and in conditions such as keratoconjunctivitis sicca, 231,232,246 Sjögren's syndrome, 247 Down syndrome, 248 and cystic fibrosis. 249  
The potential benefits of tear ferning in predicting CL tolerance were first described by Kogbe and Liotet. 250 Besides being able to predict discomfort, it was also possible to identify individuals with excessive protein deposition, as they would display a grade I ferning pattern but with subtle differences such as big and very closely branched ferns, with the branches being significantly more curved. Defining CL intolerance as cessation of CL wear due to ocular symptoms, deposition, or ocular health issues, Ravazzoni and colleagues 251 found that grades I and II before the first lens fit can be used as predictors for CL tolerance with a sensitivity of 57.9% and a specificity of 88.5%. With a more strict approach using only grade I as predictive of tolerance, a sensitivity of 78.95% and specificity of 78.35% were achieved. The sensitivity and specificity of the prediction could even be improved further if tear ferning was performed after 1 month of CL wear. Using tear ferning in a group of established CL wearers and nonwearers, Evans and colleagues 229 demonstrated a significantly higher tear ferning grade in lens wearers. However, the authors were not able to show a significant correlation to ocular symptoms, assessed via the Ocular Comfort Index questionnaire, or to demonstrate a difference in tear ferning patterns between symptomatic and asymptomatic lens wearers. The authors concluded that tear ferning provided good accuracy for discriminating between lens wearers and non-lens wearers but that the prediction of dry eye symptoms was rather poor. However, a negative predictive value of 86% indicated that normal tear ferning grades could be considered a good predictor for good ocular comfort during CL wear. 
Tear ferning is an indication of tear functionality, and only limited information is available about tear ferning in CL wearers or the relationship to ocular comfort. To draw valuable conclusions about tear ferning in CL wear and the association to ocular comfort, more controlled studies are needed, including studies that assess the correlation between tear ferning in neophytes and ocular comfort during lens wear assessed via questionnaires, the impact of lens type, or the impact of length of lens wear on tear ferning. Currently, Rolando's grading scale is the most commonly used method to assess tear ferning. 232 Although Pensyl and Dillehay 252 showed good intra- and interobserver repeatabilities when assessing proportions of tear ferning samples, Rolando's method is based on a subjective grading, and Norn 239,240 demonstrated poor repeatability using this latter system. 
pH Measurement
Different methodological approaches have estimated the pH of the normal tear film to be within the 6.5 to 7.8 range. 253258 The tear film pH varies throughout the day, shifting from acid to alkaline, but such variations are contained within fairly narrow limits, usually a range of approximately 0.6 of a pH unit. 34,256,259 Stimulation of tear secretion and blinking lead to acidification, whereas eyelid opening leads to alkalization by equilibration with the partial pressure of the CO2 in the surrounding air. 257  
The tear film has been shown to be more acidic in CL wearers, decreasing between 0.27 and 0.53 pH units. 255,258 This decrease has been observed in the tear fluid behind the CL, both in gas-permeable and impermeable lenses, and has been attributed to the lens preventing CO2 loss from the eye. 255  
There is limited evidence to support the notion that alteration in tear film pH affects CLD. It has been suggested that acidic changes in pH during CL wear could contribute to tight lens syndrome, based on data demonstrating decreased hydration of soft CLs induced by acidification. 256,260  
Natural tears display non-Newtonian behavior, dependent upon the shear rate. 261 This shear-thinning behavior with higher-viscosity fluids at low shear rates, as occurs during the open-eye state, is necessary for the tears to contribute to the lubrication of the ocular surface without damage during a high shear rate situation such as blinking. Viscosity values of normal human tears have been reported in the range of 1 mPa·s at high shear rate (∼120 s−1) to up to 10 mPa·s at lowest shear rate (∼0 s−1). 261264 The underlying mechanism is not well understood, and the components involved are still questioned. It was originally thought that soluble mucins were the main components contributing to the viscosity of tears, 261,262,265 but more recently the involvement of tear proteins and lipids has been suggested. 262264 Loss of shear-thinning behavior has been reported if the tear lipids are removed, but artificial mixtures of proteins containing lysozyme or lactoferrin can also exhibit shear-thinning behavior 263,264 ; subsequently, Gouveia and Tiffany 264 have proposed an interactive role between the proteins and lipids with protein–protein and protein–lipid interactions responsible for the viscous properties of tears. 263 Slight differences in viscosity have been reported with dry eye disease 263 ; however, the effect of CL wear on tear film viscosity is currently unknown, and it is not known whether there is a change in viscosity with CLD. 
Surface Tension
The stability and spreading of the tear film is governed by the balance of the interfacial forces acting at the air–tear film, tear film–cornea, and cornea–air interfaces. A negative correlation has been shown between surface tension measures of the tear film and the rate of tear film breakup; that is, the higher the surface tension, the quicker the tear film breakup. 266 The tear film surface tension is approximately two-thirds that of water 267 or saline. 268 Therefore, evaluation of the equilibrium surface tension (pressure) at the air–tear film interface is important for understanding the stability of tear film and its ability to spread. 269  
At one time it was believed that mucin in tears was a major contributor to surface tension, 270 but more recent evidence suggests that the concentration of mucin needed (0.5% or 5 mg/mL) 268,270 exceeds that present in normal tears (estimated as 32 ng/mL Mucin 5AC a protein encoded by the gene MUC5AC). 271 Another possible confounder of previous results was the use of nonocular mucin (often bovine submaxillary mucin or porcine gastric mucin) in place of ocular mucin. Tests have shown that purified bovine ocular mucin has no surface activity even at concentrations 100 times higher than those normally occurring in tears 272 and that purified rabbit ocular mucin has only weak surface activity. 273 Furthermore, the initial assays demonstrating that removal of tears and mucin from the cornea left a hydrophobic surface 274,275 have been questioned, as the treatments were harsh. Using more gentle removal of mucin has been shown to leave behind a wettable hydrophilic epithelium. 276281 The tear film lipids are likely to be the most important contributors to the surface tension of tears, as delipidating tears increases their surface tension, and adding back the lipids restores this to its previous value. 273 The polar components of the tear film with their amphipathic nature are likely to be key contributors to the spread of the lipid layer upon the aqueous component of tears. 
There are several methods available to measure surface tension. These include a Wilhelmy plate used together with a Langmuir trough for tension measurements at a planar air–water interface 272,282,283 ; an axisymmetric drop/bubble shape analysis for tension determination at curved surfaces of pendant drops 284,285 or sessile bubbles. 286288 Using a capillary tube and determining the pressure needed to flatten a meniscus of tears, the surface tension of reflex or basal tears collected by capillary tubes is 42 to 46 mN/m 273,289 and is 46.6 ± 3.8 mN/m using a Wilhelmy balance. 268  
Data using an artificial TFLL 290 have shown that during increases in surface pressures (as would be seen during blinking), 291 the area/molecule may be too small to accommodate all lipids (polar and nonpolar) at the air–aqueous interface, and it is most likely that the nonpolar lipids deposit upon the polar lipids. These in vitro data also demonstrated that at all pressures the lipid layer was most likely inhomogeneous, with condensed domains of nonpolar lipids above a layer of polar lipids. 290 Results of other in vitro experiments, performed by adding saturated or unsaturated cholesterol or wax esters to lipid films made of human meibum, have been taken to indicate that the bulk nonpolar layer of the tear film contains liquid crystals of cholesterol esters interacting with wax esters. To date, work on the polar tear film lipids has focused on the role of phospholipids. Phosphatidylcholine or sphingomyelin can restore the surface tension of delipidated tears 273 in a capillary meniscus model, and dipalmitoylphosphatidylcholine can interact strongly with meibum lipids. 292  
Although the role of mucin in the surface tension of tears has been largely discredited (see above), there may be a role for tear film proteins. Several investigations using monolayers of meibomian gland lipids have shown that proteins or mucins can penetrate them, and some proteins change the associated surface pressure. 272,283,285,293,294 Lipocalins in tears have been shown to have some surface activity, 285 and adding back a mimic of tear film lipocalin (bovine beta-lactoglobulin) with lipids can improve the surface tension of saline. 273 Millar and colleagues 295 concluded that the effect of lipocalin on the TFLL was complex and depended on the types of lipids present in the lipid film and adsorbed to lipocalin. Proteins that can be isolated by high-performance liquid chromatography (HPLC) in the 23-minute fraction (likely to be lipophilins) from rabbit tears are surface active and can decrease tear surface tension. They are associated with increases in tear breakup time in vitro. 296 The presence of divalent cations in rabbit tears may also influence tear film stability, as chelators of these can decrease the tear film surface pressure and decrease the breakup time of rabbit tears on eye, but these ions do not appear to play the same role in human tears. 297  
Most of the studies discussed above have used monolayers of lipids, but lipids derived from the meibomian glands form multilayers, not monolayers, 292 and have a thickness of approximately 20 molecules. 2,10,298301 Svitova and Lin 269 used lipids extracted from the surface of lotrafilcon A silicone hydrogel CLs (using toluene/isopropyl alcohol) and demonstrated that thick multilayers of these extracted lipids exhibited a low surface tension (using sessile bubble apparatus) of 32 to 22.5 mN/m (depending on the thickness of the film) compared to monolayers (40 mN/m). 272,285,294 Addition of lysozyme to these thick lipid layers did not alter their surface tension, but there was evidence that lysozyme could adsorb irreversibly to the lipid layer and increase the relaxation time of the layer. 269  
There is a paucity of information on the effect of CL wear on the surface tension of tears, or indeed on the components that may influence the surface tension of tears. One study has examined the role of different lipids on the ability of an artificial tear fluid (ATF) to wet the surface of a tefilcon A Hydroxyethyl methacrylate (HEMA)-based soft CL. Addition of various phospholipids, in particular phosphatidylinositol, was able to improve the ability of the ATF wet the lens surface, 301 although surface tension of the ATF with or without phospholipids was not measured. 
Dry eye tears have increased surface tension, reported as 44 to 53 mN/m compared to 42 to 46 mN/m 266,273 using the capillary tube method or 52.9 ± 7.4 mN/m compared to 46.6 ± 3.8 mN/m 302 using a Wilhelmy balance. As yet there appear to be no studies examining whether any form of CL wear changes the surface tension of tears, or evidence of a link with discomfort associated with CL wear. 
Since the CL divides the tear film and creates new interfaces, it is critical that the PLTF components be able to spread over the anterior CL surface. Most hydrophobic tear lipids are unable to spread over the aqueous, and polar lipids are required to attain more favorable spreading conditions. In contrast to the non-CL situation, where tear aqueous components are able to spread over the cornea, in the presence of a CL, tear film aqueous spreading over a surface that is potentially already coated by tear lipids is compromised. 303  
Summary of Biophysical Changes to the Tear Film With CL Wear and Their Influence on Comfort
The physical presence of a CL in situ divides the tear film into a pre- and a postlens tear film and creates new interfaces with and within the ocular environment. This partition and new interaction has been shown to lead to biophysical changes of the tear film properties, including a decrease in tear film stability, pre-lens lipid layer thickness, and tear volume as well as an increase in evaporation rate, as summarized in Table 4. To date, the effect on comfort of many of these biophysical properties is unknown or inconclusive. However, evidence points toward a link between decreased stability, increased evaporation, reduced tear turnover, and ferning and CLD. Further evidence is required to establish the associations of tear volume, surface tension, osmolarity, pH, and ocular temperature with CLD. 
Table 4. 
Summary of Major Effects of Contact Lenses on the Tear Film With Evidence, Where Possible, of Links to Contact Lens Discomfort
Table 4. 
Summary of Major Effects of Contact Lenses on the Tear Film With Evidence, Where Possible, of Links to Contact Lens Discomfort
Parameter Range in Precorneal Tear Film Range in CL Wear, Pre-Lens Tear Film Evidence of Relation to Contact Lens Discomfort
 Blink Interblink interval in healthy eye is 5–15 s. Most of the blinks are complete blinks. In cases of people working on display or exposed to other dry eye factors, interblink time and percentage of incomplete blink increase.32,33,45 Prolonged CL wear results in increased percentage of incomplete blinks (for rigid CL) and stronger association between tear film instability and percentage of incomplete blinks (for soft CL). CL wear frequently reduces blink frequency. CL wetting solutions can maintain normal blink frequency.25,27–31,34,35 Reduced blink frequency or increased percentage of incomplete blink resulted in CL discomfort.2,27–30,47,64,72,156
 Lipid layer integrity TFLL spreading and integrity can be analyzed to evaluate the viscoelasticity of the lipid layer. Elasticity prevails in healthy eyes, while the contribution of viscosity increases in dry eyes.9,11,42,44,304 TFLL spread depends on the CL material. Prolonged wear of CL frequently delays the kinetics of TFLL spread and worsens the TFLL integrity. 2,9,46,49,53,305,306 Impaired TFLL integrity and spread correlates with CL discomfort. 9,44,45,49,50,53,307
 Tear film stability NIBUT 4.6–>60 43,51,53,54,56,57, 59,70,308311
TBUT 4.2–14.4 s 53,54,57,309,311,312
Soft: 5–10.1 53,6670
PLTT: 8.23–11.03 2,55,72
RGP: 2–3 s 41,66
Decreased tear film stability associated with CL discomfort 70,75,7886
 Evaporation Range 0.4–167 g/m 2 /h
12,81,96,99,101,103,109,111,114, 119128
1.2–2.6× ↑ in evaporation 98,120,121,126 Discomfort associated with increased tear evaporation in neophytes fitted with hydrogel CL (but not SiHy) at 18% relative humidity. 130
 Ocular surface  temperature profile 32°C–36°C 131 Pre-lens tear film: cooler than without CL 148 No clear relationship demonstrated between tear film temperature and discomfort in CL wear, although artificially lowering the ocular surface temperature, with cooled (4°C) artificial tears, reduces ocular surface sensitivity and improves comfort. 150
Postlens tear film: warmer than without CL 131
 Tear film thickness 1–7 μm 1,3,72,151,154,185,306 Pre-lens: 1–7 μm 3,72,153,154,158,163,306
Postlens: <3 μm 3,72,153,154,158,163,306
No evidence showing a link between tear film thickness and discomfort.
 Tear turnover rate 16.9 ± 6.8 167
16.2 ± 5.1
Range = 10.9–22.2 101
13.2 ± 4.5
Range = 5–32 168 *
15.6 ± 5.9 167

16.3 ± 7.2
Range = 5–29 168 *
Symptomatic wearers
20.6 ± 6.0
Range = 16–36
Asymptomatic wearers
33.8 ± 8.8
Range = 27–42 175
 Tear volume 2–4 μL 50,151,152,185,188 1–2 μL 185,188 Lower tear volume has a weak but significant relation to discomfort in CL wear. 50,76
 Tear exchange 10%–20% per blink 193 9.0%/min 194 No link between tear exchange and ocular discomfort.
1.82%/min with a 12-mm-diameter CL, 1.61%/min with a 12.5-mm-diameter CL, 1.34%/min with a 13-mm-diameter CL, and 1.24%/min with a 13.5-mm-diameter CL 195
 Osmolality/  electrolytes 280–318 197,200 297−331 55,216,219,221224,226,227 No association between tear film osmolarity and ocular comfort has been established, 221 although tendency toward higher tear film osmolarity in patients with CL discomfort. 55,76
 Ferning Grades I to IV 231,232,234,235,239,252 Grades I–IV 251
Mean 2.02 ± 0.60 229
0.87 mm 2 /μl 240
No correlation to comfort assessed via Ocular Comfort Index, but Grades I and II are good predictors for good ocular comfort. 229
Grades I and II are predictors for CL tolerance. 251
 pH 6.5–7.8 254259 ↓ in CL wear 255,258 Limited evidence to support link between pH and discomfort. 256,260
 Viscosity High shear rate 1 mPa·s No data No evidence linking tear viscosity with contact lens discomfort.
Low shear rate 10 mPa·s 262265
Changes in Tear Composition With Contact Lens Wear and Their Effect on Comfort
Tear Types.
Challenges facing analysis of the tear film proteome include the volume and type of tears that can be collected. Tears have been classified into four types: basal, reflex, emotional, and closed-eye tears. Basal 313 (sometimes also referred to as open-eye) tears bathe the mucous membranes of the eye during the day and have a turnover rate between 3.4 μL/min 314 and approximately 1 μL/min, 183,315 and a volume of approximately 7 μL. 183,314 Reflex tears are produced upon stimulation of the lacrimal reflex by irritant substances or foreign particles. Emotional tears are produced as a result of various emotions, such as sadness. Closed-eye tears are those tears that bathe the eye during sleep. The protein component of these tear types is known to be different; for example, the levels of secretory immunoglobulin-A (sIgA) decreases in concentration from closed-eye to basal to reflex tears. 313,316320 Other tear proteins such as lactoferrin, lipocalin-1, and lysozyme do not appreciably change their concentration in closed-eye, basal, and reflex tears. 313,319 These findings led to the classification of different proteins in the tear fluid into constitutive (i.e., those that have a constant level of production and so their concentration decreases during increases in tear fluid production, e.g., sIgA), 313 regulated (i.e., those that have changes in production during changes in amount of tears, e.g., lysozyme, lactoferrin, and lipocalin-1), 313 and serum derived (which also decrease during increases in tear fluid production, such as albumin). 317 Emotional tears may differ from reflex tears by containing chemosignals (pheromones) that affect behavior 321 and having a slightly higher total protein concentration of 6 mg/mL compared to 4 mg/mL. 322 Table 5 lists the major tear proteins and their changes during CL wear. 
Table 5. 
Concentration of Some of the Major Proteins in Tears and the Effect of CL Wear
Table 5. 
Concentration of Some of the Major Proteins in Tears and the Effect of CL Wear
Protein Type Reflex Basal* Closed Eye Lens Wear†
Total protein, mg/mL 6.0 319 9.0 319 18.0 319 5.4 ± 0.4 372
3.9–5.0 317 7.3 317 15.5 ± 8.4 373 § 11.9 ± 2.0 370
4.6 ± 0.2 372 3.4 ± 1.5 373 § 5.6–6.6 340
9.4 ± 3.0 370 7.2 ± 2.3 367
5.9 ± 1.5 340 3.2 ± 1.5 90
6.9 ± 1.4 367
3.5 ± 1.3 76
9.5 ± 1.7 371
7.0 ± 1.7 369
Lysozyme, mg/mL 1.6 319 2.0 319 1.8 319 4.0 ± 0.6 372
1.3–1.6 317 2.1 ± 0.2 317 3.0 375 2.9, closed eye, OK RGP 375
1.5 ± 1.1 222 0.7 ± 0.6 222 3.3 ± 0.4 376
2.7 ± 0.3 372 2.5 375 1.9 377
3.0 ± 0.3 376 1.4–1.9 374
2.2 377 1.1 ± 0.5, RGP‡; 1.2 ±
1.3 ± 0.7 374 0.8 ± 0.4 426 0.4, high water soft‡; 0.8 ± 0.2, low water soft 426
1.6 ± 0.2 76 1.4 ± 0.3 90
Decrease, SiHy, RGP 425
Lactoferrin, mg/mL 1.8 319 2.6 319 1.8 319 1.5–1.7 378
1.3–1.5 317 1.6 ± 0.1 317 3.5 375 3.3, closed eye, OK RGP 375
4.0 ± 3.1 222 2.9 ± 2.8 222 1.8 ± 0.1 379 1.1 ± 0.2 367
1.8 ± 0.4 379 2.5 ± 0.9 379 1.6 ± 0.9 368
1.5–1.8 382 3.2 375 1.6 ± 0.3 376
1.1 ± 0.3 367 0.3–0.8 374
1.7 ± 0.5 368 2.6 ± 1.0 90
1.7 ± 0.1 376
0.7 ± 0.8 374
2.7 ± 1.1 76
Lipocalin-1, mg/mL 1.9 319 1.3 319 1.7 319 3.2, closed eye, OK RGP 375
1.1–1.3 317 1.6 ± 0.1 317 3.3 375
3.5 375
0.5 ± 0.9 359
1.5 ± 0.2 371
sIgA, mg/mL 0.1 319 2.6 319 10.0 319 0.1 ± 0.1 380
0.1–0.2 318 0.2–0.9 318 2.3–8.4 318 5.0, closed eye, OK RGP 375
0.1–0.4 317 0.9 ± 0.1 317 4.6 375 1.1 ± 0.5 370
0.1 ± 0.1 380 2.8 375 0.8 ± 0.3 90 1.6 ± 0.5 367
0.8 ± 0.3 370 3 ± 2 383 1.5 ± 0.6 382
1.7 ± 0.7 367 2.6 ± 1.8 382 1.2. ± 0.2, DW RGP; 0.7 ± 0.1, DW soft; 0.6 ± 0.1, EW soft 381
0.9 ± 0.1 381 1.1 ± 1.6 427
1.3 ± 1.0 427 0.7 377
2.4 377 1 ± 1, closed eye 383
0.9 ± 0.1 76
Specific sIgA, units 2.1 ± 2.7 ( S. epidermidis ) 380 82 ± 15 ( P. aeruginosa ) 381 100% ( P. aeruginosa ) 383 1.3 ± 2.0 ( S. epidermidis ; reflex tears) 380 3.7 ± 3.4 ( E. coli; reflex tears) 380
9.0 ± 12.2 ( E. coli ) 380 2.7 ± 5.8 ( H. influenzae; reflex tears) 380
6.0 ± 12.3 ( H. influenzae ) 380 67 ± 11, DW RGP; 52 ± 9, DW soft; 38 ± 6, EW soft ( P. aeruginosa ) 381
20% ( P. aeruginosa ; DW and EW) 383
Albumin, μg/mL 20 319 100 319 1100 319 540, closed eye, OK RGP 375
3.0–8.4 317 42.0 ± 4.7 317 760–1100 318 10.3–24.1 374
1830 ± 2440 219 380 ± 640 219 200 375 49.8 ± 57.1 380
10–20 318 20–60 318 10.2 ± 3.0 367
72.3 ± 71.2 380 20 375
14.6 ± 8.6 367
12.5 ± 5.1 374
14.6 ± 8.6 367
Tear Collection Methods.
There are essentially three methods for collecting tears: using a microcapillary tube to draw tears into the lumen of the glass tube (it is believed that this causes minimal change to the ocular surface and minimal reflex tearing), using a Schirmer strip placed into the lower fornix to adsorb tears, and using a sponge placed within the fornix to adsorb tears. It is well established that tears collected by Schirmer strip contain higher concentration of serum-derived proteins such as albumin, transferrin, and IgG 323 compared with those collected by capillary tube. Due to the low volume of tears, even with normal eyes (approximately 7 μL per eye), some researchers have used a flush method to help collect tears. This involves instilling a volume of buffered saline onto the ocular surface, allowing that to interact, and collecting it usually using capillary tubes. 324 While this method does dilute the tear sample, it may have advantages where the tear volume is low or it is difficult to collect sufficient tears for biochemical analysis. 
The tear lipids are primarily secreted from the meibomian glands and form the outermost bilayer of the tear film. 274,300 The TFLL has an outer layer of nonpolar lipids at the air interface to retard water evaporation from the tear film and to protect from external contaminants and an inner layer of polar lipids that creates an interface with the aqueous layer to help the spreading of the outer layer and increase its stability. 9,12,13,106 The major lipid components of the meibomian gland secretion are nonpolar wax esters, cholesterol esters, diesters, and triacylglycerol, with smaller concentrations of cholesterol, fatty acids, and other polar lipids. Polar lipids account for 5% to 15% of the total lipids and are suggested to include phospholipids (phosphatidylcholine, phosphatidylethanolamine, sphingomyelin), ceramides, and cerebrosides 298,325,326 and more recently long chain (O-acyl)-omega-hydroxy fatty acid (OAHFA). 327,328 The presence of phospholipids in meibum remains controversial, 329331 and OAHFAs have been suggested as being responsible for creating the interface between the aqueous and the nonpolar lipid layer instead of phospholipids. 326,327 Limited information is available on the compositional analyses of tear film lipids. Recent research has shown that the lipid composition of tears is possibly more complex than that of meibum. 332 Nonpolar lipids in tears differ from those found in meibomian secretions. 332335 Also, in contrast to meibomian secretion lipids, several researchers have confirmed the presence of phospholipids in tears. 335340  
The clinical appearance, thickness, and stability of the pre-lens TFLL have been shown to be disrupted by the presence of a CL (refer to section Lipid Layer Interferometry), and lipids are known to deposit on CLs (see TFOS Report from the Contact Lens Materials, Design and Care Subcommittee). Changes in the tear film lipid composition associated with CL wear could be expected; however, compositional analyses of tear film lipids during CL wear are very limited. Young and Hill 341,342 measured cholesterol levels in normal subjects and subjects with CL problems; the cholesterol levels in the subjects with CL problems ranged from 389 to 435 mg/100 mL whereas the levels measured for normal subjects ranged from 190 to 203 mg/100 mL. In normal subjects, tear cholesterol levels were initially decreased after rigid CL fitting but returned to prefitted levels once adaptation was complete. 342 More recently, another study in soft CL wearers reported a negative association between the level of cholesterol esters in tears and the thickness of the lipid layer as well as a positive association between the level of cholesterol esters and dryness symptoms, with higher levels associated with increased dryness symptoms. 343 Yamada and colleagues 340 reported concentrations of phospholipids of 186 ± 39 and 162 ± 33 μg/mL in tears of subjects wearing polymacon (group I) and etafilcon A (group IV) CLs, respectively, the latter being significantly lower than for the same subjects when not wearing CLs (220 ± 35 μg/mL, P = 0.0023). These findings are in agreement with another study showing an increase in the ratio of nonpolar to polar lipids in the tears of CL wearers. 344 Further, a similar finding was reported in non–CL-wearing subjects with dry eye symptoms. 345 A low level of phospholipids/polar lipids in tears could conceivably be a contributing factor to the dryness symptoms and discomfort reported during soft CL wear. 
When the lipid layer is compromised and no longer able to supply full coverage over the aqueous layer, the tear film stability is significantly decreased and evaporation increased. 12,98,106,346 Degradation of lipid components by autoxidation (or photooxydation), enzymatic oxidation, or enzymatic lysis is expected to have a deleterious effect on the layer. 303  
The presence of one or more double bonds (unsaturation) in the structure of certain lipids makes them susceptible to oxidation due to the availability of the allylic hydrogen. 347 Hence, mono- or di-unsaturated fatty acids or esters present in the tears and, in particular, polyunsaturated fatty acids possibly originating from vascular leakage into the tear layer, are susceptible to autoxidation under the effect of light, atmospheric oxygen, and so on. 303 Primary oxidation products, hydroperoxides, can then be further converted into secondary peroxidation products such as hydrocarbons, aldehydes, hydroxyaldehydes, and epoxides. 347 Two aldehydes commonly used as markers of oxidative stress are 4-hydroxy-2(E) nonenal (4-HNE), a peroxidation product of linoleic acid (18:2 n-6) or arachidonic acid (20:4 n-6), and malondialdehyde, an end product of the degradation of linolenic acid (18:3). 
Phospholipases are lipolytic enzymes found in tears, contributing to their antibacterial properties, 348 that are able to degrade phospholipids into diacylglycerols and lysophospholipids; group II phospholipase A2 (sPLA2 GII) is the most abundant in tears and is secreted by both the acinar and ductal cells of the lacrimal gland. 349351 There have been several reports of PLA2 in tears of CL wearers as well as deposited on hydrogel CL materials 352354 but without consensus on the effect of CL wear, with both no change and a reduction in concentration being reported. Group II phospholipase A2 hydrolyses the ester bond at the sn-2 position of phospholipids, producing a lysophospholipid and a free fatty acid, often arachidonic acid, a precursor in the production of eicosanoids such as prostaglandins and leukotrienes and known to be involved in ocular surface inflammation. 355,356 The suspected presence of diacylglycerols and the lack of phospholipids in tear samples reported by Campbell and colleagues, 357 in conjunction with phospholipase C activity, further highlights the possible role of lipolytic enzymes in modulating tear film lipid composition. In blepharitis patients, PLA2 activity has been shown to be enhanced and is hypothesized to cause the disruption of tear film phospholipids, compromising the function of the polar lipid layer and contributing to a breakdown of tear film structure. 358 Intolerant wearers unable to wear their CLs for longer than 6 hours during the day were found to have an increased level of secretory phospholipase A2 (sPLA2) in the tears (1.86 ± 0.05 ng/μL, P = 0.047) compared to tolerant subjects (1.80 ± 0.08 ng/μL), as well as increased levels of degradation products, malondialdehyde and 4-HNE (0.85 ± 1.0 vs. 0.15 ± 0.15 μM, P = 0.004). 359 The enzymatic activity of sPLA2 was double (13.5 ± 51 vs. 7.3 ± 2.4 × 103 cpm/protein unit, P = 0.004) that of tolerant wearers, with a 10-fold increase in the concentration of lipid degradation products. Both findings suggest greater lipid deterioration in intolerant wearers. The accumulation of sPLA2 on CL surfaces may also promote further hydrolysis of tear film phospholipids. 
Finally, transfer of skin lipids and lipase not normally present in the tear film onto CLs and subsequent release in the tear film can create compositional changes detrimental to tear film stability. 360  
Tear lipid chemistry is likely to be affected by CL wear. This effect will depend upon the characteristics of the CL but also on the individual patient tear composition. Studies of the tear lipid chemistry investigating nonpooled, individual samples are required to further understand the potential role of lipids in CL discomfort. 
The Tear Film Proteome.
The tear film proteome (defined as all the proteins and peptides that can be identified from tears) has not yet been definitively described, although researchers have been examining the proteins of tears for many decades. Various techniques have been used to investigate the tear film proteome. These include one-dimensional polyacrylamide gel electrophoresis (PAGE) and Western blotting to identify and quantify the separated proteins; chromatography and mass-spectrometry (MS) techniques with additions such as isobaric tag for relative and absolute quantitation to enable quantification of proteins in the original sample; and enzyme-linked immunosorbent assays using specific antibodies to quantify proteins in a sample without prior separation of the proteins. The numbers of proteins in the tear proteome have been reported to vary widely. Using sodium dodecyl sulfate (SDS)-PAGE-MS to separate proteins and reverse-phase (RP) 361 capillary HPLC and matrix-assisted laser desorption/ionization (MALDI)-MS to identify peptides, Funke and colleagues 362 identified 267 proteins in tears collected from experienced soft CL wearers. Using nano-HPLC-MS/MS, de Souza and colleagues 363 identified 491 proteins in the tear film of one individual. Zhou and colleagues, 364 using various fractions of tears and nano-RP HPLC-MS/MS, identified 1543 proteins in tears collected from four healthy non-CL wearers, with 714 proteins being present in all samples. It has been estimated that the tear film proteome contains approximately 35% of proteins in common with the proteome of plasma, 364 25% of proteins in common with those found in saliva, 365 and 24% of proteins in common with those found in urine. 366 These findings indicate that tears contain many unique proteins as well as a smaller fraction of proteins in common with other human body fluids. 
Table 5 gives details of the major proteins found in the tear film. The basal tear film contains 3.5 to 9.5 mg/mL total protein 76,317,319,340,367371 (as with all protein analyses, differences are partly due to collection methods, techniques to quantify proteins [i.e., Lowry, Bradford, bicinchoninic acid or fluorescence-based protein assays], and use of different standards [usually albumin but occasionally, e.g., IgG or soybean trypsin inhibitor]). The total protein concentration does not change significantly in reflex tears (Table 5), 317,319,372 but does increase during sleep in closed-eye tears to approximately 16 to 18 mg/mL (Table 1). 319,373 The level of the regulated major tear proteins lysozyme (0.7–3.0 mg/mL), 76,219,317,319,372,374377 lactoferrin (0.7–4.0 mg/mL), 76,222,317,319,367,368,374376,378,379 and lipocalin-1 (0.5–3.5 mg/mL) 317,319,359,371,375 does not change in reflex, basal, or closed-eye tears. The constitutive protein sIgA changes from a low concentration in reflex tears, of 0.06 to 0.38 mg/mL, 317,319,380 to 0.84 to 2.8 mg/mL in basal tears 76,317,319,367,370,375,377,381,382 and to 3 to 10 mg/mL in closed-eye tears. 319,375,383 Sullivan and Allansmith 384 have shown that all IgA in tears in rats is sIgA. Similarly, the serum-derived proteins albumin, complement C3, complement C4, and complement factor B also increase in concentration from reflex to basal to closed-eye tears. 317,319,367,374,375,379,380  
Inflammatory Mediators in Tears.
Tears have been shown to contain a variety of inflammatory mediators, including complement (Table 6), 379 arachidonic acid metabolites (e.g., leukotriene B4 and prostaglandin E2), 385386 and a range of different cytokines (Table 6). The range of cytokines differs depending on the study methodology, but the most commonly observed cytokines in tears are interferon (IFN)-γ; interleukin (IL)-1α, IL-1β, IL-4, IL-6, Il-8, IL-10, and IL-12(p70); and tumor necrosis factor (TNF)-α. 387398 One of the reasons for differences in the descriptions of the cytokines present in tears may be that not all tear samples contain all cytokines. For example, Enriquez-de-Salamanca and colleagues 399 found that epidermal growth factor, CX3CL1, interleukin-1 receptor agonist and CXCL10 were detected in 100% of basal tear samples; IL-8/CXCL8 and vascular endothelial growth factor were detected in >93% of samples; IL-6 in 65%, IL-10 in 48%, IFN-γ in 30%, IL-1β in 30%, IL-17 in 13%, and IL-13 in 9%; GM-granulocyte-monocyte colony stimulating factor in 7%; and TNF-α in 2% of samples. These inflammatory mediators appear to be tightly regulated, as tears also contain inhibitors of the complement cascade including lactoferrin, 379,400 decay accelerating factor, 379,401 and CD59 402 ; soluble receptors of cytokines and growth factors, for example, EGFR, 394,395 IL-2R, 395 IL-6R, 394,395,403 and TNFR2 394 ; and the IL-1 antagonist IL-1Ra 387,399,404 as well as certain cytokines (such as IL-10) that are themselves anti-inflammatory. 
Table 6.
Concentration of Certain of the Inflammatory Proteins, Proteases, and Protease Inhibitors in Tears and Effect of Contact Lens Wear
Table 6.
Concentration of Certain of the Inflammatory Proteins, Proteases, and Protease Inhibitors in Tears and Effect of Contact Lens Wear
Protein Type Reflex Basal* Closed Eye Lens Wear†
Fibronectin, ng/mL 21 ± 25429 120 ± 160 429
Complement C3, μg/mL 4.0 ± 5.6 379 27.4 ± 47.5 379 106.5 ± 84.3 379 5.3 ± 1.7 370
4.4 ± 2.1 370
Complement C4, μg/mL 0.2 ± 0.4 379 1.7 ± 3.1 379 5.6 ± 5.1 379 0.1 ± 0.1 370
0.1 ± 0.1 370
Complement factor B, μg/mL 0.1 ± 0.1 379 4.0 ± 5.0 379 20.8 ± 8.1 379
sPLA2, ng/μL 1.8 ± 0.1 359 5.0–5.1 371
5.0 ± 0.7 340
IL-6, pg/mL ∼45 405 ∼70 405 147 ± 112 435 3.6, RGP lenses 431
2.2 431 4.7, OK RGP 430
2.2 430 33.1 ± 15.0 433
7.1 ± 6.4 433 33.2–40.4, keratoconic Px,
5.8–10.0, keratoconic Px 82,432  RGP or soft piggyback 432
1516 ± 497 82 2505 ± 951 82
0 434 43.8 ± 5.3 434
26.5 ± 21.8 393 13.9 ± 1.1 389 60 ± 32, neophyte‡; 218 ± 45, adapted
3.6 ± 3.4 398
110 ± 142 390
29.3 406
31.7 397
2.2 431
∼10 391
IL-8, pg/mL ∼400 405 ∼1200 405 148 ± 103 439 935.3, OK RGP 430
601.5 430 412.6 ± 104.1 433
107.4 ± 56.8 433 372.6–438.2, keratoconic,
110.5–156.2, keratoconic Px 432 176 ± 72 393 RGP or soft piggyback lenswear 432
37.4 ± 8.7 389 ∼500 391 70 ± 32, neophyte‡; 229 ± 175, adapted 44
572 ± 637 390
1084.3 406
283.0 397
TNF-α, pg/mL 1.8 431 4.2, RGP lenses 431
EGF, pg/mL 698.1 430 2348.7, OK RGP lens wear 430
1277 ± 619 393
MMP-9, ng/mL 9.8 ± 14.3 373 2000.7 ± 1950.7 373 12.9, RGP lenses 431
6.1 431 74.3, OK RGP lens wear 430
39.2 430
8.4 ± 4.7 408
10.5 ± 0.2 416
TIMP-1, ng/mL 74.5 ± 39.7 373 § 277.8 ± 282.2 373
NGAL, ng/mL 680.8 ± 523.3 373 § 3620.3 ± 1832.1 373
Table 6 details the changes that occur to some of these inflammatory mediators in different tear types and in CL wear. The concentration of IL-6 and IL-8 decreases in reflex tears compared to basal tears, whereas the concentration of VEGF, Fas ligand (FasL), and monocyte chemotactic protein (MCP1) does not. 405 In closed-eye tears, IL-1β, IL-6, IL-12(p70), and TNF-α increased in concentration, 406 but there are conflicting reports on whether the concentration of IL-8 and IL-10 increases during sleep, 385,397,406 partly because, for IL-8, its level is already high in basal tears. 
Proteases in Tears.
The major proteinase activities of tears are gelatinolytic and collagenolytic. 388 Tears contain high levels of cathepsin-C activity, but also cathepsin-B, trypsin-like, and urokinase activity. 407 The proteomic study by de Souza and colleagues 363 showed large numbers of proteases (such as matrix metalloproteinase [MMP]-8, MMP-9, leukocyte elastase, plasminogen, cathepsins, and aminopeptidases) as well as antiproteases (such as α-2-macroglobulin, α-1-microglobulin, cystatins, α-1-antitrypsin, α-1-antichymotrypsin, leukocyte elastase inhibitor, plasminogen activator inhibitor-2, thrombospondin-1, secretory leukocyte protease inhibitor, and tissue inhibitor of matrix metalloproteinase [TIMP]-1). 318  
Research has tended to focus on the presence of MMPs and their inhibitors (such as TIMPs) in tears. Tears have been shown to contain MMP-1 (also known as interstitial collagenase), MMP-2 (gelatinase A), MMP-3 (stromelysin-1), MMP-8 (neutrophil collagenase), MMP-9 (gelatinase B), MMP-10 (stromelysin-2), MMP-13 (collagenase 3), TIMP-1, TIMP-2, and TIMP-4 (Table 6). 387,388,395,408411 However, often the active forms of the MMPs have not been demonstrated, and often tears contain only inactive pro-forms 387,412414 or low levels of active forms. 415,416  
Plasmin activity in tears increases during sleep, 320,417 as does the level of pro-MMP-9. 373 However, the level of protease inhibitors such as α-1-antiprotease, α-1-antichymotrypsin, α-2-macroglobulin, secretory leukocyte protease inhibitor, and cystatin C also increases in closed-eye tears to levels between 3 and 23 times those in reflex tears. 318,373 In the case of MMP-9 and TIMP-1, there is the potential for activation of MMP-9 during sleep as its level increases by approximately 200-fold whereas the level of its inhibitor TIMP-1 increases only 3-fold. 373  
Other Types of Inflammatory Mediators.
Tears contain histamine (N-methyl histamine), and its concentration increases from reflex (80 ± 110 pg/mL) to basal (200 ± 140 pg/mL) to closed-eye (840 ± 1150 pg/mL) tears. Its concentration in tears during asymptomatic CL wear (370 ± 80 pg/mL) is not different from that in basal tears of non-lens wearers. 418 Tears also contain the neuromediators: substance P, calcitonin gene-related peptide (CGRP), neuropeptide Y (NPY), vasoactive intestinal peptide, and nerve growth factor (see report from the Subcommittee on Neurobiology for more details). 386,419421  
Effect of CL Types and Wear Schedules on the Tear Film Proteome.
It is possible that different CLs or the disinfecting/cleaning solutions used with them, as well as the wear schedule on which lenses are worn, can affect the tear film proteome, although there is a general lack of information on the effects of CL wear on the tear film proteome. Tables 5 and 6 outline the publications that have examined the effect of CL wear on the tear film proteome. 
Initial studies on total protein in tears by Hill and Uniacke 422 and Callender and Morrison 423 showed that, during adaptation to hard CLs, protein concentration decreased, but returned to normal after the first 7 days of lens wear. 423 However, overall there does not appear to be a change in the total protein content or the concentration of lysozyme, lactoferrin, or lysozyme during lens wear (Table 1) 90,323,340,361,367,368,370,372,374376,378,422,424 (with the exceptions that one study showed a significant decrease in antibody intensity of lysozyme in PAGE from tears of silicone hydrogel and rigid gas-permeable [RGP] lens wearers compared to nonwearers, 425 and one other study showed that the level of lysozyme increased in tears from RGP [1.1 ± 0.5 mg/mL] or high water soft lenses [1.2 ± 0.4 mg/mL] compared to no lens wear [0.8 ± 0.4 mg/mL] or wear of low water content soft lenses [0.8 ± 0.2 mg/mL]). 426  
In contrast, the effect of CL wear on the concentration of sIgA in tears is more controversial. Six studies have reported no effect of lens wear. 367,370,375,380,381,427 However, Kijlstra and colleagues 428 reported that there was a decrease in the concentration of sIgA in tears during the first 3 months of RGP daily lens wear compared to non-lens wear, by approximately 27%, but the concentration returned to normal 1 year after lens fitting. Others have shown a similar effect of a decrease in sIgA concentration in tears of CL wearers (type of lens, wear schedule, or length of wear was not given) 361 in a mixed group of CL wearers (lens type not given; mixed DW and EW; average 8 years of wear), 377 in a group of EW soft lens wearers (at least 6 months lens wear), 382 and in the closed-eye tears collected from a group of either daily wear or extended wear soft lens wearers (average length of wear 4 years). 383 In addition, there appears to be a reduction in the concentration of sIgA specific for Pseudomonas aeruginosa or Escherichia coli but not for Staphylococcus epidermidis or Haemophilus influenzae in tears. 380,381,383  
The concentration of albumin in basal tears does not change during relatively long-term wear of RGP 367 or soft lenses 374 but is increased in closed-eye tears during wear of RGP lenses for orthokeratology. 375 The concentration of complement proteins C3 or C4 does not change with DW or EW of soft lenses. 370 The concentration of fibronectin in basal tears is increased during EW of soft lenses (average wearing time of 33 months). 429 Most studies (with the exception of one measuring the concentration of IL-6 in tears of RGP lens wearers, another measuring the level of IL-6, IL-8, or MMP-9 in the tears of long-term lotrafilcon A silicone hydrogel lens wearers, and one measuring IL-6 in tears after 2 weeks of silicone hydrogel lens wear) 82,430,431 have found that CL wear in general increases the concentration of IL-6, IL-8, TNF-α, EGF, and MMP-9 in tears, 430435 and moreover there appears to be an effect of length of lens wear. 435  
After collecting tears using the flush method and two-dimensional differential gel electrophoresis (2D-DIGE), Markoulli and colleagues 436 found a significant decrease in the level of Zn-alpha2-glycoprotein in the tears of people who had worn silicone hydrogel lenses (lotrafilcon B) on a DW basis compared to their tear film collected prior to lens wear. Kramann and colleagues, 425 using wearers of RGP or silicone hydrogel lenses and a semiquantitative analysis, found that there was a significant increase in the concentration of protein S100 A8 in the tears of both lens-wearing groups, and a significant decrease in concentration of secretoglobin but increase in cystatin in the RGP lens wearers compared to silicone hydrogel or non-lens wearers. There is an increase in plasmin activity in tears during CL wear (soft lens wearers) acknowledged by most 437439 but not all studies. 417  
Association of CL Discomfort With the Tear Proteome.
There has been very little research on whether the tear film proteome changes with CL discomfort (by any definition). No significant difference was found in the concentration of total protein, lysozyme, lactoferrin, or sIgA between tears of tolerant or intolerant CL wearers in the absence of lens wear compared to soft lens wear during 1 day. 76,90 However, there is an apparent association between the levels of lipocalin-1 or sPLA2 in tears and intolerance to lens wear, with intolerant individuals in the absence of CL wear having increases in both these proteins (2.40 ± 1.5 vs. 0.45 ± 0.85, P < 0.001; 1.86 ± 0.05 vs. 1.80 ± 0.08, P = 0.047, respectively) compared to tolerant lens wearers. 359  
Nichols and Green-Church 369 analyzed the tears of normal CL wearers and CL wearers classified, using the Contact Lens Dry Eye Questionnaire, as having CL-related dry eye symptoms during lens wear (all wore galyfilcon A silicone hydrogel CLs). Using combinations of SDS-PAGE, 2D-DIGE, and nano-LC-MS/MS, they found that the total protein of tears was significantly reduced in the CL-related dry eye group (P = 0.02). Furthermore, the concentrations in tears of β-2-microglobulin, proline-rich protein-4, lacritin, and secretoglobin 1D1 were found to be decreased, whereas the concentrations of secretoglobin 2A2, albumin, deleted in malignant brain tumor (DMBT)-1, and prolactin-inducible protein were increased in the tears of the CLDE group compared to the normal CL group. 
Mucins and Glycocalyx
Mucins are a family of high molecular weight, heavily glycosylated proteins that form the protective biofilm on the surface of epithelial cells. They are characterized by the presence of multiple tandem repeats of amino acids, rich in serine and threonine, in the central domain of the mucin core peptide; these tandem repeats provide sites for O-glycosylation. 440,441 Epithelial mucins can be divided in two different classes, transmembrane (or cell surface associated) and secreted. The glycosylated regions of these molecules are hydrophilic and contribute to the prevention of ocular surface desiccation by binding water. On the apical glycocalyx, transmembrane mucins and their O-glycans prevent adhesion and maintain epithelial barrier function through interactions with galectins. 20 Other O-glycan-containing glycoproteins, such as lubricin, also promote boundary lubrication between the cornea, conjunctiva, and CL-like materials. 442  
The normal human tear film contains MUC5AC, a secreted mucin produced by goblet cells within the conjunctival epithelium. The stratified corneal and conjunctival epithelia produce three transmembrane mucins: MUC1, MUC4, and MUC16. Transmembrane mucins are concentrated on the tips of the apical cells' microplicae, forming a dense glycocalyx at the epithelial–tear film interface, but they can also be shed from the cell surface and consequently are found in the tear film. 443  
Several studies have demonstrated a decrease in the amount of secreted mucin at the ocular surface of CL wearers. MUC5AC messenger ribonucleic acid (mRNA) in the conjunctiva and MUC5AC protein in tears are significantly reduced in subjects wearing both soft and rigid CL. 82,444446 Also, levels of sialic acid, a terminal carbohydrate in glycoproteins, is reduced in the tears of CL wearers. 447 Studies evaluating transmembrane mucins during CL wear have generated more variable results. Binding of the CA 19-9 antibody to a sialic acid on MUC1 in tear samples decreased significantly during CL wear. 448 Conversely, exposing tear film from CL wearers to immortalized human corneal epithelial cells has resulted in MUC1 upregulation. 449 This variability could be ascribed to the use of different experimental approaches, methods, or CL types in these studies. For instance, use of CLs with different water contents has been shown to differentially influence the levels of MUC1 mRNA. 450  
Contact lens wear is commonly associated with damage to the ocular surface glycocalyx, including physical changes in the form of thinning or compression and signs of biochemical changes reflected as an increase in the number of carbohydrate receptors. 451 Multipurpose CL solutions further contribute to disruption of the integrity of the glycocalyx, affecting the shedding of MUC16 from the cell surface and reducing MUC1 and MUC16 mucin gene expression. 452,453  
Mechanical interaction of the CL with the epithelial surface and the blinking forces of the lid are also involved in formation of so-called mucin balls. 454 This is a common but innocuous phenomenon that appears to cause spherical indentations in the corneal epithelium after lens removal. 455458 Histology shows that mucin balls are negative for lipids and bacteria, but are periodic acid Schiff positive, indicating that glycoproteins constitute a major component of their content. 459 The development of mucin balls does not depend on the CL type worn, but lens type does influence the degree of mucin ball formation. 454 There does not appear to be a link between CLD and mucin ball formation. 
A limited number of studies have attempted to correlate mucin expression during CL wear with comfort. Protein analyses have shown that CL wearers with symptoms of discomfort, as measured using the Contact Lens Dry Eye Questionnaire, have decreased levels of MUC5AC in the tear film. 444 Additional analyses in asymptomatic CL wearers, on the other hand, have produced conflicting results. MUC5AC content in conjunctival goblet cells is low in CL wearers with no subjective symptoms or clinical signs of intolerance compared to healthy controls. 445 However, data from additional studies have shown no significant changes in the levels of transmembrane or secreted mucins, or in the content of glycosidic residues in non-goblet epithelial cell vesicles in tolerant CL wearers. 460,461 These discrepancies in mucin expression in asymptomatic wearers could be attributed to long-term differential inflammatory responses, known to affect mucin biosynthesis. 460 More recently, it has been proposed that the pattern of mucin degradation during CL wear could also affect comfort, since mucin fragmentation in response to a new material has been observed in asymptomatic, but not symptomatic, CL wearers. 462  
Other Tear Film Components
Tears have antioxidant activity 463,464 and contain several antioxidant components, including gamma-glutamyl transpeptidase that protects against oxidative stress via glutathione recapture, 421 cysteine, ascorbic acid/ascorbate, glutathione, uric acid/urate and tyrosine, 465,466 and superoxide dismutase. 19 Ascorbate and lactate dehydrogenase, but not urate, increase in concentration in tears from basal to closed eye. 375  
While CL wear increases the level of the antioxidant tyrosine in tears, 466 it does not increase the concentration of ascorbic acid or the total antioxidant activity. 467 Wearing a RGP orthokeratology lens for one night significantly increases the concentration of ascorbate and lactate dehydrogenase in tears, 375 and lactate dehydrogenase increases with extended wear of highly oxygen-permeable soft or RGP lenses. 224,468 The magnitude of lactate dehydrogenase increase is dependent on the type of CL and especially on the oxygen permeability of the lens. 469,470 Tears contain nucleotides and dinucleotides that have a function in controlling tearing and ocular surface wound healing, 471 but the effect of CL wear on the concentration of these in tears is not known. 
There is no published information on the relationship between antioxidants or nucleotides on the comfort response during CL wear. 
Cellular Content of Tears (PMNs)
The earliest demonstration of white blood cells in tears was by Norn, 472 who observed a relative leucocytosis, first thing in the morning, in tears collected from the conjunctival sac. Subsequently, others have shown that during sleep the tear film and ocular surface are infiltrated by large numbers of polymorphonuclear leukocytes/neutrophils (PMN). 318,435,473475 This recruitment is likely to be mediated by the increased concentrations of chemokines, such as IL-8 and leukotriene B4, 385,476 that are found in closed-eye tears. 
Using neophytes to CL wear and placing a CL in one eye only, Wilson and colleagues 475 demonstrated that there were >6000 leukocytes that could be washed from the ocular surface after sleep and that lens wear did not affect this number. On the other hand, in a study of three separate groups of subjects (non-lens wearers, neophytes to lens wear, and adapted CL wearers), numbers of PMNs were significantly higher from tears/ocular surface wash of neophyte compared to non-lens wearers, but adapted lens wearers had fewer PMNs recovered. 435 The number of PMNs recovered from the two CL-wearing groups was also significantly different. These changes were at least partly the result of changes to chemokine levels in tears of the three groups. 435 Similarly, Stapleton and colleagues 474 demonstrated that there was a significant reduction in the numbers of PMNs washed from the corneal surfaces of experienced (adapted) daily-wear soft lens wearers following sleeping in their lenses. 
There have been no studies relating the role of PMN recruitment onto the ocular surface during sleep to CLD. 
External Components
Multipurpose disinfecting solutions (MPDS) used to clean and disinfect soft CLs overnight often contain surface-active ingredients (e.g., Tetronic, Pluronic), 477 added to improve cleaning efficiency and CL wettability and to maximize comfort. Surface-active agents have the capacity to emulsify the lipid layer and destabilize the tear film. 274 These surfactants are introduced in the tear film upon CL insertion, after overnight soaking in MPDS, and can further destabilize the tear film. 36,269,274 Svitova and Lin 269 have reported some effect of surfactant-containing lens care solutions on the rheological properties of mixed lipids–lysozyme films in vitro. Further, any uptake into the CL material during overnight storage will create a slow release of the surface-active substance during wear. 269,478 No information is currently available on the effect in vivo of MPDS on the tear film. 
Eye cosmetics, even though applied externally, have been shown to migrate onto the ocular surface and through the tear film 479 and deposit onto CLs; cosmetic products include a variety of ingredients (oils, waxes, pigments, powder, stearates, surfactants, diluents, preservatives) that can have a potential destabilizing effect on the tear film. 480 One ingredient commonly found in eye cosmetics, to prevent bacterial growth during storage, is the preservative benzalkonium chloride, which has been associated with a decreased TBUT and dry eye symptoms. 304,481,482 While deposition of cosmetics on the CL surface is recognized to affect CL comfort, 122 no information on cosmetics within the tear film, specifically, has been linked to CL-induced discomfort. 
Future Directions
It is clear from the preceding report that there remain significant gaps in our understanding of the extent to which tear film changes in CL wear might be responsible for inducing symptoms of discomfort in CL wearers. A number of areas in which further research is indicated and should be prioritized to help address the identified shortfalls are described below. 
To understand the relationship between CLD and tear film dynamics and composition, possible major directions of research are as follows: 
    Examining associations between biochemical parameters in the tear film with CLD using a consistent definition of comfort, particularly in establishing parameters that may be predictive in neophyte wearers and understanding changes over time.
    Refining the selection of wetting agents that can be included in CL care solutions to help maintain long-term wettability of the CL surface (e.g., in addition to poloxamer and Tetronic molecules incorporated in current formulations, many other hydrophilic polymers and block copolymer wetting agents require further exploration)
    Development of novel CL materials that can resist evaporation of water content or can maintain a highly wettable surface after a prolonged wearing time
In terms of lipid layer integrity, major priorities for future research include the following: 
    Elucidating the mechanism of lipid/CL and protein/CL interactions responsible for deposit formation to explore the effect of CL surface charge, roughness, and effect of lens surface modification with phospholipid or polymer coatings, and so on
    Designing nonadhesive CL surfaces with long-term resistance when worn or to develop lens care formulations improving the CL wettability
    Gaining an understanding of how wetting agents can modify the spread and the quality of the TFLL over the CL surface
With regard to PLTF stability, future research should be directed toward development of lens materials, designs, and surfaces, with or without the aid of care products that promote biocompatibility, to a level where the tear film can remain stable over the surface. Current evidence leads us to believe that more biocompatible CL surfaces could promote more physiological tear film structure in at least those deemed tolerant of CL wear. 90  
Whether the ocular surface temperature in CL wearers directly impacts comfort has not been established. However, cooled artificial tears have been found, subjectively, to improve comfort in normal non–lens-wearing eyes, suggesting that this area is deserving of further exploration. The close relationship between ocular surface temperature, tear film stability, and tear evaporation would suggest that interventions that modify one aspect will have influence on all. 
Contemporary high-resolution technologies such as OCT, allowing detailed observation of the tear film profile during lens wear, have confirmed the significant physical impact of CLs, and particularly rigid CLs, on the tear film. This approach has benefit in optimizing the fitting relationship and edge characteristics of the lens as related to CLD. 
Osmolarity is recognized as a key property of the tear film, but its assessment in CL wear has been limited to date, in part by the need for large tear volumes for analysis. The design of osmometers that require only minute amounts of tears may help in more accurately defining location-specific tear film osmolarity changes in the pre- and post-CL tear film, particularly in those suffering from CL-induced dry eye. Although a variety of studies have investigated the effect of CLs on tear film osmolarity, there is limited information on the impact of the osmotic level on ocular comfort, unlike the situation with dry eye disease. Of particular relevance would be studies that not only compare the osmolarity of symptomatic and asymptomatic lens wearers but also assess its correlation to ocular comfort indices in order to improve our understanding of the impact of tear film osmolarity on CL-induced dryness. Also, if osmolarity of tears does affect comfort, then investigations on the biochemical/chemical changes that occur may provide insight into methods of alleviating the discomfort. 
Tests such as tear ferning have shown some potential to discriminate between lens wearers and non-lens wearers and perhaps even predict ocular comfort during CL wear. Availability of digital image analysis may allow for more accurate and objective tear ferning quantification in the future and lend support to the investigation of the relationship between tear ferning and ocular comfort during CL wear. 
Clinical investigations have yielded some limited data indicating that CL wear induces a modest decrease in tear film pH. Additional evidence-based data are required to support any mechanistic link between reduced pH in the tear film and CL discomfort. 
With regard to the surface tension of tears, there is a lack of data specifically addressing the questions of whether CL wear alters the surface tension of tears, whether this can be related to discomfort during lens wear, and what might be the underlying biochemical and physical changes to the tears that manifest as changes to surface tension. Furthermore, experiments specifically addressing the issue of whether changes to the surface tension of tears are related to the wettability of CLs during wear are lacking. 
With the advent of newer proteomic, 369,425,436 glycomic, 483,484 and lipidomic 326,485 techniques, reexamination of the role of the proteins, glycoproteins, mucins, and lipids, as well as nonbiological components of the tears in tear film surface tension and effects of CLs, should be undertaken. In recent years the polar lipids OAHFAs and their esters in meibum have been discovered, 486 and their concentration is correlated more strongly than that of phospholipids to dry eye severity 487 ; therefore an examination of the effect of these lipids and other polar lipids on the spreading behavior of the TFLL, surface tension of tears, changes during CL wear, and discomfort is warranted, as well as further evaluation of the effect of tear lipid degradation. 
In relation to effect of CLs on tear composition, it is clear that there is probably no effect of CLs on the concentration of total protein, lysozyme, and lactoferrin (at least with soft lens wear). Research needs to be conducted on the effect of lens wear on the concentration of lipocalin-1 in tears, especially given its potential change in tears in intolerant compared to tolerant lens wearers. 359 The effect of CL wear on the concentration of total or specific sIgA requires further investigation. It should be noted that while substantial progress has been made in defining the tear proteome, this has not taken into account the changes that are known to occur between the different tear types; this should be addressed in future work. 
Concerning the inflammatory mediators in tears, further research on the potential for CL wear to affect arachidonic acid metabolites, neuropeptides, histamine, and other inflammatory mediators would be beneficial. While lens wear does appear to generally increase the level of several cytokines in tears, which if any of these relate to CL discomfort is unknown at present. An effort to relate changes of these mediators, proteases, and all the potential inhibitory factors for inflammatory mediators and proteases in tears with comfort during CL wear is urgently needed. Aside from these major directions, the very large differences in the amount of cytokines reported in tears (see Table 6, IL-6 and IL-8 as examples) also merit investigation. It seems unlikely that these large differences are physiological but are perhaps related to methodological differences, 488 and further research is required for clarification. 
Clinical investigations have yielded some supportive data indicating that symptomatic CL wear might be associated with decreased levels of secreted mucin in tears. However, there is not complete agreement on whether reduced mucin content contributes to ocular surface discomfort, due to some degree of variability in methods across studies. Contact lens wear is clearly associated with physical and biochemical changes to the epithelial glycocalyx. Future investigations on the integrity of this carbohydrate-rich zone on the cell surface could offer potential new information on the mechanisms leading to discomfort during CL wear. At present, there are insufficient molecular data to demonstrate accurately the presence of either transmembrane or secreted mucins within “mucin balls.” 
Numerous opportunities exist for further research to be conducted in this area. Answers to these research questions will foster a better understanding of the impact of tear film changes secondary to CL wear on ocular comfort, in order that we might strive to reduce the effects and optimize CL comfort. Evidence suggests that the biophysical properties of the tear film are interrelated, and thus it is likely that no single component can be isolated as responsible for CL discomfort. This theory is supported by the demonstration that the feature with the strongest link to ocular comfort during CL wear is tear film stability, a property recognized to reflect myriad tear film components and interactions. 
Disclosure: Each workshop participant's disclosure data can be found in the Appendix of the Introduction. 
King-Smith PE Fink BA Fogt N Nichols KK Hill RM Wilson GS. The thickness of the human precorneal tear film: evidence from reflection spectra. Invest Ophthalmol Vis Sci . 2000; 41: 3348–3359. [PubMed]
King-Smith PE Fink BA Hill RM Koelling KW Tiffany JM. The thickness of the tear film. Curr Eye Res . 2004; 29: 357–368. [CrossRef] [PubMed]
Wang J Fonn D Simpson TL Jones L. Precorneal and pre- and postlens tear film thickness measured indirectly with optical coherence tomography. Invest Ophthalmol Vis Sci . 2003; 44: 2524–2528. [CrossRef] [PubMed]
Ehlers N. The thickness of the precorneal tear film. Acta Ophthalmol . 1965; 81: 92–100.
Benedetto DA Shah DO Kaufman HE. The instilled fluid dynamics and surface chemistry of polymers in the preocular tear film. Invest Ophthalmol Vis Sci . 1975; 14: 887–902.
Norn MS. Semiquantitative interference study of fatty layer of precorneal film. Acta Ophthalmol (Copenh) . 1979; 57: 766–774. [CrossRef] [PubMed]
Olsen T. Reflectometry of the precorneal film. Acta Ophthalmol (Copenh) . 1985; 63: 432–438. [CrossRef] [PubMed]
Wolff E. The muco-cutaneous junction of the lid-margin and the distribution of the tear fluid. Trans Ophthalmol Soc U K . 1946; 66: 291–308.
Bron AJ Tiffany JM Gouveia SM Yokoi N Voon LW. Functional aspects of the tear film lipid layer. Exp Eye Res . 2004; 78: 347–360. [CrossRef] [PubMed]
McCulley JP Shine W. A compositional based model for the tear film lipid layer. Trans Am Ophthalmol Soc . 1997; 95: 79–88, discussion 88–93. [PubMed]
Foulks GN. The correlation between the tear film lipid layer and dry eye disease. Surv Ophthalmol . 2007; 52: 369–374. [CrossRef] [PubMed]
Craig JP Tomlinson A. Importance of the lipid layer in human tear film stability and evaporation. Optom Vis Sci . 1997; 74: 8–13. [CrossRef] [PubMed]
Mishima S Maurice DM. The oily layer of the tear film and evaporation from the corneal surface. Exp Eye Res . 1961; 1: 39–45. [CrossRef] [PubMed]
Dartt DA. Dysfunctional neural regulation of lacrimal gland secretion and its role in the pathogenesis of dry eye syndromes. Ocul Surf . 2004; 2: 76–91. [CrossRef] [PubMed]
Sack RA Tan KO Tan A. Diurnal tear cycle: evidence for a nocturnal inflammatory constitutive tear fluid. Invest Ophthalmol Vis Sci . 1992; 33: 626–640. [PubMed]
Rolando M Zierhut M. The ocular surface and tear film and their dysfunction in dry eye disease. Surv Ophthalmol . 2001; 45 (suppl 2): S203–S210. [CrossRef] [PubMed]
The definition and classification of dry eye disease: report of the Definition and Classification Subcommittee of the International Dry Eye WorkShop (2007). Ocul Surf . 2007; 5: 75–92. [CrossRef] [PubMed]
Smolin G. The defence mechanism of the outer eye. Trans Ophthalmol Soc U K . 1985; 104 (pt 4): 363–366. [PubMed]
Chen Y Mehta G Vasiliou V. Antioxidant defenses in the ocular surface. Ocul Surf . 2009; 7: 176–185. [CrossRef] [PubMed]
Argueso P. Glycobiology of the ocular surface: mucins and lectins. Jpn J Ophthalmol . 2013; 57: 150–155. [CrossRef] [PubMed]
Dartt DA. Regulation of mucin and fluid secretion by conjunctival epithelial cells. Prog Retin Eye Res . 2002; 21: 555–576. [CrossRef] [PubMed]
Vajdic C Holden BA Sweeney DF Cornish RM. The frequency of ocular symptoms during spectacle and daily soft and rigid contact lens wear. Optom Vis Sci . 1999; 76: 705–711. [CrossRef] [PubMed]
Doughty MJ Fonn D Richter D Simpson T Caffery B Gordon K. A patient questionnaire approach to estimating the prevalence of dry eye symptoms in patients presenting to optometric practices across Canada. Optom Vis Sci . 1997; 74: 624–631. [CrossRef] [PubMed]
du Toit R Situ P Simpson T Fonn D. The effects of six months of contact lens wear on the tear film, ocular surfaces, and symptoms of presbyopes. Optom Vis Sci . 2001; 78: 455–462. [CrossRef] [PubMed]
Abelson MB Holly FJ. A tentative mechanism for inferior punctate keratopathy. Am J Ophthalmol . 1977; 83: 866–869. [CrossRef] [PubMed]
McMonnies CW. Incomplete blinking: exposure keratopathy, lid wiper epitheliopathy, dry eye, refractive surgery, and dry contact lenses. Cont Lens Anterior Eye . 2007; 30: 37–51. [CrossRef] [PubMed]
Fink BA Hill RM Carney LG. Corneal oxygenation: blink frequency as a variable in rigid contact lens wear. Br J Ophthalmol . 1990; 74: 168–171. [CrossRef] [PubMed]
Van Der Worp E De Brabander J Swarbrick H Hendrikse F. Eyeblink frequency and type in relation to 3- and 9-o'clock staining and gas permeable contact lens variables. Optom Vis Sci . 2008; 85: E857–E866. [CrossRef] [PubMed]
Collins MJ Iskander DR Saunders A Hook S Anthony E Gillon R. Blinking patterns and corneal staining. Eye Contact Lens . 2006; 32: 287–293. [CrossRef] [PubMed]
Wolkoff P Nojgaard JK Troiano P Piccoli B. Eye complaints in the office environment: precorneal tear film integrity influenced by eye blinking efficiency. Occup Environ Med . 2005; 62: 4–12. [CrossRef] [PubMed]
Collins MJ Stahmer D Pearson G. Clinical findings associated with incomplete blinking in soft lens wearers. Clin Exp Optom . 1989; 72: 55–56. [CrossRef]
Doughty MJ. Consideration of three types of spontaneous eyeblink activity in normal humans: during reading and video display terminal use, in primary gaze, and while in conversation. Optom Vis Sci . 2001; 78: 712–725. [CrossRef] [PubMed]
Patel S Henderson R Bradley L Galloway B Hunter L. Effect of visual display unit use on blink rate and tear stability. Optom Vis Sci . 1991; 68: 888–892. [CrossRef] [PubMed]
Hill RM Carney LG. The effects of hard lens wear on blinking behaviour. Int Contact Lens Clin . 1984; 11: 242–246.
York M Ong J Robbins JC. Variation in blink rate associated with contact lens wear and task difficulty. Am J Optom Arch Am Acad Optom . 1971; 48: 461–467. [CrossRef] [PubMed]
Nichols JJ Mitchell GL King-Smith PE. The impact of contact lens care solutions on the thickness of the tear film and contact lens. Cornea . 2005; 24: 825–832. [CrossRef] [PubMed]
Yang SN Tai YC Sheedy JE Kinoshita B Lampa M Kern JR. Comparative effect of lens care solutions on blink rate, ocular discomfort and visual performance. Ophthalmic Physiol Opt . 2012; 32: 412–420. [CrossRef] [PubMed]
Luchs JI Nelinson DS Macy JI. Efficacy of hydroxypropyl cellulose ophthalmic inserts (LACRISERT) in subsets of patients with dry eye syndrome: findings from a patient registry. Cornea . 2010; 29: 1417–1427. [CrossRef] [PubMed]
Wander AH. Long-term use of hydroxypropyl cellulose ophthalmic insert to relieve symptoms of dry eye in a contact lens wearer: case-based experience. Eye Contact Lens . 2011; 37: 39–44. [CrossRef] [PubMed]
Doane MG. An instrument for in vivo tear film interferometry. Optom Vis Sci . 1989; 66: 383–388. [CrossRef] [PubMed]
Guillon JP Guillon M. Tear film examination of the contact lens patient. Contax . 1988; May: 14–20.
Yokoi N Takehisa Y Kinoshita S. Correlation of tear lipid layer interference patterns with the diagnosis and severity of dry eye. Am J Ophthalmol . 1996; 122: 818–824. [CrossRef] [PubMed]
Guillon JP Guillon M. Tear film examination of the contact lens patient. Optician . 1993; 206: 21–29.
Yokoi N Yamada H Mizukusa Y Rheology of tear film lipid layer spread in normal and aqueous tear-deficient dry eyes. Invest Ophthalmol Vis Sci . 2008; 49: 5319–5324. [CrossRef] [PubMed]
Efron N. Tear film. In: Efron N ed. Contact Lens Complications . London: Butterworth-Heinemann; 2012: 76–102.
Maruyama K Yokoi N Takamata A Kinoshita S. Effect of environmental conditions on tear dynamics in soft contact lens wearers. Invest Ophthalmol Vis Sci . 2004; 45: 2563–2568. [CrossRef] [PubMed]
Lorentz H Jones L. Lipid deposition on hydrogel contact lenses: how history can help us today. Optom Vis Sci . 2007; 84: 286–295. [CrossRef] [PubMed]
Gellatly KW Brennan NA Efron N. Visual decrement with deposit accumulation of HEMA contact lenses. Am J Optom Physiol Opt . 1988; 65: 937–941. [CrossRef] [PubMed]
Varikooty J Keir N Simpson T. Estimating tear film spread and stability through tear hydrodynamics. Optom Vis Sci . 2012; 89: E1119–E1124. [CrossRef] [PubMed]
Chen Q Wang J Shen M Tear menisci and ocular discomfort during daily contact lens wear in symptomatic wearers. Invest Ophthalmol Vis Sci . 2011; 52: 2175–2180. [CrossRef] [PubMed]
Craig JP Blades K Patel S. Tear lipid layer structure and stability following expression of the meibomian glands. Ophthalmic Physiol Opt . 1995; 15: 569–574. [CrossRef] [PubMed]
Isreb MA Greiner JV Korb DR Correlation of lipid layer thickness measurements with fluorescein tear film break-up time and Schirmer's test. Eye (Lond) . 2003; 17: 79–83. [CrossRef] [PubMed]
Guillon M Styles E Guillon JP Maissa C. Preocular tear film characteristics of nonwearers and soft contact lens wearers. Optom Vis Sci . 1997; 74: 273–279. [CrossRef] [PubMed]
Nichols JJ Nichols KK Puent B Saracino M Mitchell GL. Evaluation of tear film interference patterns and measures of tear break-up time. Optom Vis Sci . 2002; 79: 363–369. [CrossRef] [PubMed]
Nichols JJ Sinnott LT. Tear film, contact lens, and patient-related factors associated with contact lens-related dry eye. Invest Ophthalmol Vis Sci . 2006; 47: 1319–1328. [CrossRef] [PubMed]
Patel S Bevan R Farrell JC. Diurnal variation in precorneal tear film stability. Am J Optom Physiol Opt . 1988; 65: 151–154. [CrossRef] [PubMed]
Lira M Oliveira ME Franco S. Comparison of the tear film clinical parameters at two different times of the day. Clin Exp Optom . 2011; 94: 557–562. [CrossRef] [PubMed]
Patel S Farrell JC. Age-related changes in precorneal tear film stability. Optom Vis Sci . 1989; 66: 175–178. [CrossRef] [PubMed]
Patel S Boyd KE Burns J. Age, stability of the precorneal tear film and the refractive index of tears. Cont Lens Anterior Eye . 2000; 23: 44–47. [CrossRef] [PubMed]
Cho P Yap M. Age, gender, and tear break-up time. Optom Vis Sci . 1993; 70: 828–831. [CrossRef] [PubMed]
Craig JP Tomlinson A. Age and gender effects on the normal tear film. Adv Exp Med Biol . 1998; 438: 411–415. [PubMed]
Abusharha AA Pearce EI. The effect of low humidity on the human tear film. Cornea . 2013; 32: 429–434. [CrossRef] [PubMed]
Lemp MA. Report of the National Eye Institute/Industry workshop on Clinical Trials in Dry Eyes. CLAO J . 1995; 21: 221–232. [PubMed]
Nichols JJ King-Smith PE. The effect of eye closure on the post-lens tear film thickness during silicone hydrogel contact lens wear. Cornea . 2003; 22: 539–544. [CrossRef] [PubMed]
Guillon JP. Tear film structure and contact lenses. In: Holly FJ ed. The Preocular Tear Film in Health, Disease and Contact Lens Wear . Lubbock, TX: Dry Eye Institute; 1986; 914–939.
Morris CA Holden BA Papas E The ocular surface, the tear film, and the wettability of contact lenses. Adv Exp Med Biol . 1998; 438: 717–722. [PubMed]
Ruben M Guillon M. Contact Lens Practice . London: Chapman & Hall; 1994: 453–480.
Young G Efron N. Characteristics of the pre-lens tear film during hydrogel contact lens wear. Ophthalmic Physiol Opt . 1991; 11: 53–58. [CrossRef] [PubMed]
Guillon M Guillon JP. Hydrogel lens wettability during overnight wear. Ophthalmic Physiol Opt . 1989; 9: 355–359. [CrossRef] [PubMed]
Faber E Golding TR Lowe R Brennan NA. Effect of hydrogel lens wear on tear film stability. Optom Vis Sci . 1991; 68: 380–384. [CrossRef] [PubMed]
Alonso-Caneiro D Iskander DR Collins MJ. Tear film surface quality with soft contact lenses using dynamic-area high-speed videokeratoscopy. Eye Contact Lens . 2009; 35: 227–231. [CrossRef] [PubMed]
Nichols JJ Mitchell GL King-Smith PE. Thinning rate of the precorneal and prelens tear films. Invest Ophthalmol Vis Sci . 2005; 46: 2353–2361. [CrossRef] [PubMed]
Bruce AS Mainstone JC Golding TR. Analysis of tear film breakup on Etafilcon A hydrogel lenses. Biomaterials . 2001; 22: 3249–3256. [CrossRef] [PubMed]
Efron N Brennan NA. A survey of wearers of low water content hydrogel contact lenses. Clin Exp Optom . 1988; 71: 86–90. [CrossRef]
Fonn D Situ P Simpson T. Hydrogel lens dehydration and subjective comfort and dryness ratings in symptomatic and asymptomatic contact lens wearers. Optom Vis Sci . 1999; 76: 700–704. [CrossRef] [PubMed]
Glasson MJ Stapleton F Keay L Sweeney D Willcox MD. Differences in clinical parameters and tear film of tolerant and intolerant contact lens wearers. Invest Ophthalmol Vis Sci . 2003; 44: 5116–5124. [CrossRef] [PubMed]
Chui WS Cho P Brown B. Soft contact lens wear in Hong Kong-Chinese: predicting success. Ophthalmic Physiol Opt . 2000; 20: 480–486. [CrossRef] [PubMed]
Santodomingo-Rubido J Wolffsohn JS Gilmartin B. Changes in ocular physiology, tear film characteristics, and symptomatology with 18 months silicone hydrogel contact lens wear. Optom Vis Sci . 2006; 83: 73–81. [CrossRef] [PubMed]
Wolffsohn JS Hunt OA Chowdhury A. Objective clinical performance of “comfort-enhanced” daily disposable soft contact lenses. Cont Lens Anterior Eye . 2010; 33: 88–92. [CrossRef] [PubMed]
Sengor T Aydin Kurna S, Ozbay N, Ertek S, Aki S, Altun A. Contact lens-related dry eye and ocular surface changes with mapping technique in long-term soft silicone hydrogel contact lens wearers. Eur J Ophthalmol . 2012; 22 (suppl 7): S17–S23. [CrossRef] [PubMed]
Fonn D Dumbleton K. Dryness and discomfort with silicone hydrogel contact lenses. Eye Contact Lens . 2003; 29: S101–S104; discussion S115–S118, S192–S194. [CrossRef] [PubMed]
Dogru M Ward SK Wakamatsu T The effects of 2 week senofilcon-A silicone hydrogel contact lens daily wear on tear functions and ocular surface health status. Cont Lens Anterior Eye . 2011; 34: 77–82. [CrossRef] [PubMed]
Glasson MJ Hseuh S Willcox MD. Preliminary tear film measurements of tolerant and non-tolerant contact lens wearers. Clin Exp Optom . 1999; 82: 177–181. [CrossRef] [PubMed]
Bitton E Jones L Simpson T Woods C. Influence of the blink interval on tear meniscus height in soft contact lens and nonlens wearers. Eye Contact Lens . 2010; 36: 156–163. [CrossRef] [PubMed]
Nilsson SE Andersson L. Contact lens wear in dry environments. Acta Ophthalmol (Copenh) . 1986; 64: 221–225. [CrossRef] [PubMed]
Pritchard N Fonn D Weed K. Ocular and subjective responses to frequent replacement of daily wear soft contact lenses. CLAO J . 1996; 22: 53–59. [PubMed]
Hom MM Bruce AS. Prelens tear stability: relationship to symptoms of dryness. Optometry . 2009; 80: 181–184. [CrossRef] [PubMed]
Bitton E Lovasik JV. Longitudinal analysis of precorneal tear film rupture patterns. Adv Exp Med Biol . 1998; 438: 381–389. [PubMed]
Pult H Murphy PJ Purslow C. A novel method to predict the dry eye symptoms in new contact lens wearers. Optom Vis Sci . 2009; 86: E1042–E1050. [CrossRef] [PubMed]
Glasson MJ Stapleton F Keay L Willcox MD. The effect of short term contact lens wear on the tear film and ocular surface characteristics of tolerant and intolerant wearers. Cont Lens Anterior Eye . 2006; 29: 41–47, quiz 49. [CrossRef] [PubMed]
Situ P Simpson TL Fonn D Jones LW. Conjunctival and corneal pneumatic sensitivity is associated with signs and symptoms of ocular dryness. Invest Ophthalmol Vis Sci . 2008; 49: 2971–2976. [CrossRef] [PubMed]
Thai LC Tomlinson A Ridder WH. Contact lens drying and visual performance: the vision cycle with contact lenses. Optom Vis Sci . 2002; 79: 381–388. [CrossRef] [PubMed]
Timberlake GT Doane MG Bertera JH. Short-term, low-contrast visual acuity reduction associated with in vivo contact lens drying. Optom Vis Sci . 1992; 69: 755–760. [CrossRef] [PubMed]
Tutt R Bradley A Begley C Thibos LN. Optical and visual impact of tear break-up in human eyes. Invest Ophthalmol Vis Sci . 2000; 41: 4117–4123. [PubMed]
Lohmann CP Fitzke F O'Brart D Muir MK Timberlake G Marshall J. Corneal light scattering and visual performance in myopic individuals with spectacles, contact lenses, or excimer laser photorefractive keratectomy. Am J Ophthalmol . 1993; 115: 444–453. [CrossRef] [PubMed]
Kimball SH King-Smith PE Nichols JJ. Evidence for the major contribution of evaporation to tear film thinning between blinks. Invest Ophthalmol Vis Sci . 2010; 51: 6294–6297. [CrossRef] [PubMed]
King-Smith PE Nichols JJ Nichols KK Fink BA Braun RJ. Contributions of evaporation and other mechanisms to tear film thinning and break-up. Optom Vis Sci . 2008; 85: 623–630. [CrossRef] [PubMed]
Guillon M Maissa C. Contact lens wear affects tear film evaporation. Eye Contact Lens . 2008; 34: 326–330. [CrossRef] [PubMed]
Tsubota K Yamada M. Tear evaporation from the ocular surface. Invest Ophthalmol Vis Sci . 1992; 33: 2942–2950. [PubMed]
Kamao T Yamaguchi M Kawasaki S Mizoue S Shiraishi A Ohashi Y. Screening for dry eye with newly developed ocular surface thermographer. Am J Ophthalmol . 2011; 151: 782–791, e781. [CrossRef] [PubMed]
Khanal S Tomlinson A Diaper CJ. Tear physiology of aqueous deficiency and evaporative dry eye. Optom Vis Sci . 2009; 86: 1235–1240. [CrossRef] [PubMed]
Mathers WD. Ocular evaporation in meibomian gland dysfunction and dry eye. Ophthalmology . 1993; 100: 347–351. [CrossRef] [PubMed]
Mathers WD Binarao G Petroll M. Ocular water evaporation and the dry eye. A new measuring device. Cornea . 1993; 12: 335–340. [CrossRef] [PubMed]
Nichols JJ. Evaporative Tear Film and Contact Lens Factors Associated with Dry Eye Symptoms in Contact Lens Wearers . Columbus, OH: Ohio State University; 2004.
Tomlinson A Khanal S. Assessment of tear film dynamics: quantification approach. Ocul Surf . 2005; 3: 81–94. [CrossRef] [PubMed]
Iwata S Lemp MA Holly FJ Dohlman CH. Evaporation rate of water from the precorneal tear film and cornea in the rabbit. Invest Ophthalmol Vis Sci . 1969; 8: 613–619.
Thai LC. Profiling the Changes in Tear Physiology with Contact Lens Wear . Glasgow, UK: Department of Vision Sciences, Glasgow Caledonian University; 2007: 60.
Lemp MA Nichols KK. Blepharitis in the United States 2009: a survey-based perspective on prevalence and treatment. Ocul Surf . 2009; 7: S1–S14. [CrossRef] [PubMed]
Goto E Endo K Suzuki A Fujikura Y Matsumoto Y Tsubota K. Tear evaporation dynamics in normal subjects and subjects with obstructive meibomian gland dysfunction. Invest Ophthalmol Vis Sci . 2003; 44: 533–539. [CrossRef] [PubMed]
Shimazaki J Sakata M Tsubota K. Ocular surface changes and discomfort in patients with meibomian gland dysfunction. Arch Ophthalmol . 1995; 113: 1266–1270. [CrossRef] [PubMed]
Guillon M Maissa C. Tear film evaporation--effect of age and gender. Cont Lens Anterior Eye . 2010; 33: 171–175. [CrossRef] [PubMed]
Mathers WD Lane JA Zimmermann MB. Tear film changes associated with normal aging. Cornea . 1996; 15: 229–334. [CrossRef] [PubMed]
Tan JH Ng EY Acharya UR. Evaluation of tear evaporation from ocular surface by functional infrared thermography. Med Phys . 2010; 37: 6022–6034. [CrossRef] [PubMed]
Tomlinson A Giesbrecht C. Effect of age on human tear film evaporation in normals. Adv Exp Med Biol . 1994; 350: 271–274. [PubMed]
Trees GR Tomlinson A. Effect of artificial tear solutions and saline on tear film evaporation. Optom Vis Sci . 1990; 67: 886–890. [CrossRef] [PubMed]
Goto E Endo K Suzuki A Fujikura Y Tsubota K. Improvement of tear stability following warm compression in patients with meibomian gland dysfunction. Adv Exp Med Biol . 2002; 506: 1149–1152. [PubMed]
Khanal S Tomlinson A Pearce EI Simmons PA. Effect of an oil-in-water emulsion on the tear physiology of patients with mild to moderate dry eye. Cornea . 2007; 26: 175–181. [CrossRef] [PubMed]
Mathers W. Evaporation from the ocular surface. Exp Eye Res . 2004; 78: 389–394. [CrossRef] [PubMed]
Hamano H Hori M Hamano T A new method for measuring tears. CLAO J . 1983; 9: 281–289. [PubMed]
Tomlinson A Cedarstaff T. Tear evaporation from the human eye: the effects of contact lens wear. J Br Contact Lens Assoc . 1982; 5: 141–150. [CrossRef]
Cedarstaff TH Tomlinson A. Human tear volume, quality and evaporation: a comparison of Schirmer, tear break-up time and resistance hygrometry techniques. Ophthalmic Physiol Opt . 1983; 3: 239–245. [CrossRef] [PubMed]
Rolando M Refojo MF. Tear evaporimeter for measuring water evaporation rate from the tear film under controlled conditions in humans. Exp Eye Res . 1983; 36: 25–33. [CrossRef] [PubMed]
Herold W. Rate of evaporation of tear fluid in the human compared with a physical model [in German]. Klin Monbl Augenheilkd . 1987; 190: 176–179. [CrossRef] [PubMed]
Tomlinson A Trees GR Occhipinti JR. Tear production and evaporation in the normal eye. Ophthalmic Physiol Opt . 1991; 11: 44–47. [CrossRef] [PubMed]
Tomlinson A Cedarstaff T. Diurnal variation in human tear evaporation. J Br Contact Lens Assoc . 1992; 15: 77–79. [CrossRef]
Thai LC Tomlinson A Doane MG. Effect of contact lens materials on tear physiology. Optom Vis Sci . 2004; 81: 194–204. [CrossRef] [PubMed]
Arciniega JC Wojtowicz JC Mohamed EM McCulley JP. Changes in the evaporation rate of tear film after digital expression of meibomian glands in patients with and without dry eye. Cornea . 2011; 30: 843–847. [CrossRef] [PubMed]
Petznick A Tan JH Boo SK Lee SY Acharya UR Tong L. Repeatability of a new method for measuring tear evaporation rates. Optom Vis Sci . 2013; 90: 366–371. [CrossRef] [PubMed]
Cedarstaff TH Tomlinson A. A comparative study of tear evaporation rates and water content of soft contact lenses. Am J Optom Physiol Opt . 1983; 60: 167–174. [CrossRef] [PubMed]
Kojima T Matsumoto Y Ibrahim OM Effect of controlled adverse chamber environment exposure on tear functions in silicon hydrogel and hydrogel soft contact lens wearers. Invest Ophthalmol Vis Sci . 2011; 52: 8811–8817. [CrossRef] [PubMed]
Purslow C Wolffsohn JS Santodomingo-Rubido J. The effect of contact lens wear on dynamic ocular surface temperature. Cont Lens Anterior Eye . 2005; 28: 29–36. [CrossRef] [PubMed]
Fujishima H Toda I Yamada M Sato N Tsubota K. Corneal temperature in patients with dry eye evaluated by infrared radiation thermometry. Br J Ophthalmol . 1996; 80: 29–32. [CrossRef] [PubMed]
Mori A Oguchi Y Okusawa Y Ono M Fujishima H Tsubota K. Use of high-speed, high-resolution thermography to evaluate the tear film layer. Am J Ophthalmol . 1997; 124: 729–735. [CrossRef] [PubMed]
Morgan PB Tullo AB Efron N. Infrared thermography of the tear film in dry eye. Eye (Lond) . 1995; 9 (pt 5): 615–618. [CrossRef] [PubMed]
Craig JP Singh I Tomlinson A Morgan PB Efron N. The role of tear physiology in ocular surface temperature. Eye (Lond) . 2000; 14 (pt 4): 635–641. [CrossRef] [PubMed]
Mapstone R. Corneal thermal patterns in anterior uveitis. Br J Ophthalmol . 1968; 52: 917–921. [CrossRef] [PubMed]
Purslow C Wolffsohn JS. Ocular surface temperature: a review. Eye Contact Lens . 2005; 31: 117–123. [CrossRef] [PubMed]
Morgan PB Soh MP Efron N Tullo AB. Potential applications of ocular thermography. Optom Vis Sci . 1993; 70: 568–576. [CrossRef] [PubMed]
Efron N Young G Brennan NA. Ocular surface temperature. Curr Eye Res . 1989; 8: 901–906. [PubMed]
Su TY Hwa CK Liu PH Noncontact detection of dry eye using a custom designed infrared thermal image system. J Biomed Opt . 2011; 16: 046009. [CrossRef] [PubMed]
Purslow C Wolffsohn J. The relation between physical properties of the anterior eye and ocular surface temperature. Optom Vis Sci . 2007; 84: 197–201. [CrossRef] [PubMed]
Kottaiyan R Yoon G Wang Q Yadav R Zavislan JM Aquavella JV. Integrated multimodal metrology for objective and noninvasive tear evaluation. Ocul Surf . 2012; 10: 43–50. [CrossRef] [PubMed]
Hill RM Leighton AJ. Temperature changes of human cornea and tears under a contact lens. I. The relaxed open eye, and the natural and forced closed eye conditions. Am J Optom Arch Am Acad Optom . 1965; 42: 9–16. [CrossRef] [PubMed]
Hamano H Minami S Sugimori Y. Experiments in thermometry of the anterior portion of the eye wearing a contact lens by means of infra-red thermometer. Contacto . 1969; 13: 12–22.
Fatt I Chaston J. Temperature of a contact lens on the eye. Int Contact Lens Clin . 1980; 7: 195–198.
Martin DK Fatt I. The presence of a contact lens induces a very small increase in the anterior corneal surface temperature. Acta Ophthalmol (Copenh) . 1986; 64: 512–518. [CrossRef] [PubMed]
Montoro JC Haverly RF D'Arcy SJ Use of digital infrared imaging to objectively assess thermal abnormalities in the human eye. Thermology . 1991; 3: 242–248.
Ooi EH Ng EY Purslow C Acharya R. Variations in the corneal surface temperature with contact lens wear. Proc Inst Mech Eng H . 2007; 221: 337–349. [CrossRef] [PubMed]
Hill RM Leighton AJ. Temperature changes of human cornea and tears under contact lenses. 3. Ocular sensation. Am J Optom Arch Am Acad Optom . 1965; 42: 584–588. [CrossRef] [PubMed]
Fujishima H Yagi Y Shimazaki J Tsubota K. Effects of artificial tear temperature on corneal sensation and subjective comfort. Cornea . 1997; 16: 630–634. [CrossRef] [PubMed]
Palakuru JR Wang J Aquavella JV. Effect of blinking on tear dynamics. Invest Ophthalmol Vis Sci . 2007; 48: 3032–3037. [CrossRef] [PubMed]
Palakuru JR Wang J Aquavella JV. Effect of blinking on tear volume after instillation of midviscosity artificial tears. Am J Ophthalmol . 2008; 146: 920–924. [CrossRef] [PubMed]
Chen Q Wang J Tao A Shen M Jiao S Lu F. Ultrahigh-resolution measurement by optical coherence tomography of dynamic tear film changes on contact lenses. Invest Ophthalmol Vis Sci . 2010; 51: 1988–1993. [CrossRef] [PubMed]
Wang J Jiao S Ruggeri M Shousha MA Chen Q. In situ visualization of tears on contact lens using ultra high resolution optical coherence tomography. Eye Contact Lens . 2009; 35: 44–49. [CrossRef] [PubMed]
Wang J Shen M Cui L Wang MR. The watery eye. Curr Allergy Asthma Rep . 2011; 11: 192–196. [CrossRef] [PubMed]
Nichols JJ King-Smith PE. The impact of hydrogel lens settling on the thickness of the tears and contact lens. Invest Ophthalmol Vis Sci . 2004; 45: 2549–2554. [CrossRef] [PubMed]
Bruce AS Mainstone JC. Lens adherence and postlens tear film changes in closed-eye wear of hydrogel lenses. Optom Vis Sci . 1996; 73: 28–34. [CrossRef] [PubMed]
Little SA Bruce AS. Postlens tear film morphology, lens movement and symptoms in hydrogel lens wearers. Ophthalmic Physiol Opt . 1994; 14: 65–69. [CrossRef] [PubMed]
Little SA Bruce AS. Role of the post-lens tear film in the mechanism of inferior arcuate staining with ultrathin hydrogel lenses. CLAO J . 1995; 21: 175–181. [PubMed]
Little SA Bruce AS. Hydrogel (Acuvue) lens movement is influenced by the postlens tear film. Optom Vis Sci . 1994; 71: 364–370. [CrossRef] [PubMed]
Brennan NA Efron N. Symptomatology of HEMA contact lens wear. Optom Vis Sci . 1989; 66: 834–838. [CrossRef] [PubMed]
Fonn D. Discontinuation of contact lens wear and its effect on the growth of the business. Contact Lens Spectrum . 1996; 11: 4–5.
Lin MC Graham AD Polse KA Mandell RB McNamara NA. Measurement of post-lens tear thickness. Invest Ophthalmol Vis Sci . 1999; 40: 2833–2839. [PubMed]
Sørensen T Taagehoj F Christensen U. Tear flow and soft contact lenses. Acta Ophthalmol (Copenh) . 1980; 58: 182–187. [CrossRef] [PubMed]
Puffer MJ Neault RW Brubaker RF. Basal precorneal tear turnover in the human eye. Am J Ophthalmol . 1980; 89: 369–376. [CrossRef] [PubMed]
Occhipinti JR Mosier MA LaMotte J Monji GT. Fluorophotometric measurement of human tear turnover rate. Curr Eye Res . 1988; 7: 995–1000. [CrossRef] [PubMed]
Kok JH Boets EP van Best JA Kijlstra A. Fluorophotometric assessment of tear turnover under rigid contact lenses. Cornea . 1992; 11: 515–517. [CrossRef] [PubMed]
Thai LC. Profiling the Changes in Tear Physiology with Contact Lens Wear . Glasgow, UK: Department of Vision Sciences, Glasgow Caledonian University; 2007: 84.
Creech JL Do LT Fatt I Radke CJ. In vivo tear-film thickness determination and implications for tear-film stability. Curr Eye Res . 1998; 17: 1058–1066. [CrossRef] [PubMed]
Miller KL Polse KA Radke CJ. Fenestrations enhance tear mixing under silicone-hydrogel contact lenses. Invest Ophthalmol Vis Sci . 2003; 44: 60–67. [CrossRef] [PubMed]
Keijser S van Best JA Van der Lelij A Jager MJ. Reflex and steady state tears in patients with latent stromal herpetic keratitis. Invest Ophthalmol Vis Sci . 2002; 43: 87–91. [PubMed]
Tomlinson A Pearce EI Simmons PA Blades K. Effect of oral contraceptives on tear physiology. Ophthalmic Physiol Opt . 2001; 21: 9–16. [CrossRef] [PubMed]
van Best JA Benitez del Castillo JM Coulangeon LM. Measurement of basal tear turnover using a standardized protocol. European concerted action on ocular fluorometry. Graefes Arch Clin Exp Ophthalmol . 1995; 233: 1–7. [CrossRef] [PubMed]
Webber WR Jones DP Wright P. Fluorophotometric measurements of tear turnover rate in normal healthy persons: evidence for a circadian rhythm. Eye (Lond) . 1987; 1 (pt 5): 615–620. [CrossRef] [PubMed]
Xu KP Tsubota K. Correlation of tear clearance rate and fluorophotometric assessment of tear turnover. Br J Ophthalmol . 1995; 79: 1042–1045. [CrossRef] [PubMed]
Tomlinson A Fagehi R Manahilov V. Why do some contact lens wearers avoid contact lens dry eye symptoms? Poster presented at: 91st Annual Meeting of the American Academy of Optometry; October 24–27, 2012; Phoenix, AZ.
Thai LC. Profiling the Changes in Tear Physiology with Contact Lens Wear . Glasgow, UK: Department of Vision Sciences, Glasgow Caledonian University; 2007: 61.
Hamano H Hori M Mitsunaga S. Measurement of evaporation rate of water from the pre-corneal film and contact lenses. Contacto . 1981; 25: 7–14.
Mishima S. Some physiological aspects of the precorneal tear film. Arch Ophthalmol . 1965; 73: 233–241. [CrossRef] [PubMed]
Zhu H Chauhan A. A mathematical model for ocular tear and solute balance. Curr Eye Res . 2005; 30: 841–854. [CrossRef] [PubMed]
Mishima S Gasset A Klyce SD Jr Baum JL. Determination of tear volume and tear flow. Invest Ophthalmol . 1966; 5: 264–276. [PubMed]
Acosta MC Gallar J Belmonte C. The influence of eye solutions on blinking and ocular comfort at rest and during work at video display terminals. Exp Eye Res . 1999; 68: 663–669. [CrossRef] [PubMed]
Paulsen FP Schaudig U Thale AB. Drainage of tears: impact on the ocular surface and lacrimal system. Ocul Surf . 2003; 1: 180–191. [CrossRef] [PubMed]
Mishima S Gasset A Klyce SD Jr Baum JL. Determination of tear volume and tear flow. Invest Ophthalmol . 1966; 5: 264–276. [PubMed]
Doane MG. Blinking and the mechanics of the lacrimal drainage system. Ophthalmology . 1981; 88: 844–851. [CrossRef] [PubMed]
Wang J Aquavella J Palakuru J Chung S. Repeated measurements of dynamic tear distribution on the ocular surface after instillation of artificial tears. Invest Ophthalmol Vis Sci . 2006; 47: 3325–3329. [CrossRef] [PubMed]
Sahlin S Gravity Chen E. blink rate, and lacrimal drainage capacity. Am J Ophthalmol . 1997; 124: 758–764. [CrossRef] [PubMed]
Sahlin S Laurell CG Chen E Philipson B. Lacrimal drainage capacity, age and blink rate. Orbit . 1998; 17: 155–159. [CrossRef] [PubMed]
Chen Q Wang J Shen M Lower volumes of tear menisci in contact lens wearers with dry eye symptoms. Invest Ophthalmol Vis Sci . 2009; 50: 3159–3163. [CrossRef] [PubMed]
Allaire PE Flack RD. Squeeze forces in contact lenses with a steep base curve radius. Am J Optom Physiol Opt . 1980; 57: 219–227. [CrossRef] [PubMed]
Carney LG Mainstone JC Carkeet A Quinn TG Hill RM. Rigid lens dynamics: lid effects. CLAO J . 1997; 23: 69–77. [PubMed]
Lieberman DM Grierson JW. The lids influence on corneal shape. Cornea . 2000; 19: 336–342. [CrossRef] [PubMed]
Weissman BA. Power and tear volume exchanges with flexible contact lenses. Am J Optom Physiol Opt . 1980; 57: 711–713. [CrossRef] [PubMed]
Hayashi T Fatt I. A lubrication theory model of tear exchange under a soft contact lens. Am J Optom Physiol Opt . 1976; 53: 101–103. [CrossRef] [PubMed]
Paugh JR Stapleton F Keay L Ho A. Tear exchange under hydrogel contact lenses: methodological considerations. Invest Ophthalmol Vis Sci . 2001; 42: 2813–2820. [PubMed]
McNamara NA Polse KA Brand RJ Graham AD Chan JS McKenney CD. Tear mixing under a soft contact lens: effects of lens diameter. Am J Ophthalmol . 1999; 127: 659–665. [CrossRef] [PubMed]
Murube J. Tear osmolarity. Ocul Surf . 2006; 4: 62–73. [CrossRef] [PubMed]
van Haeringen NJ. Tränenflüssigkeit. In: Hockwin O ed. Biochemie des Auges . Berlin, Germany: Karger, 1985: 1–11.
Yoshimura H Hosokawa K. Studies on the mechanism of salt and water secretion from the lacrimal gland. Jpn J Physiol . 1963; 15: 303–318. [CrossRef]
Tomlinson A Khanal S Ramaesh K Diaper C McFadyen A. Tear film osmolarity: determination of a referent for dry eye diagnosis. Invest Ophthalmol Vis Sci . 2006; 47: 4309–4315. [CrossRef] [PubMed]
Bron AJ Tiffany JM Yokoi N Gouveia SM. Using osmolarity to diagnose dry eye: a compartmental hypothesis and review of our assumptions. Adv Exp Med Biol . 2002; 506: 1087–1095. [PubMed]
Gaffney EA Tiffany JM Yokoi N Bron AJ. A mass and solute balance model for tear volume and osmolarity in the normal and the dry eye. Prog Retin Eye Res . 2010; 29: 59–78. [CrossRef] [PubMed]
Terry JE Hill RM. Human tear osmotic pressure. Diurnal variations and the closed eye. Arch Ophthalmol . 1978; 96: 120–122. [CrossRef] [PubMed]
Craig JP Tomlinson A. Effect of age on tear osmolality. Optom Vis Sci . 1995; 72: 713–717. [CrossRef] [PubMed]
Chen SP Massaro-Giordano G Pistilli M Schreiber CA Bunya VY. Tear osmolarity and dry eye symptoms in women using oral contraception and contact lenses. Cornea . 2013; 32: 423–428. [CrossRef] [PubMed]
Farris RL Stuchell RN Mandel ID. Tear osmolarity variation in the dry eye. Trans Am Ophthalmol Soc . 1986; 84: 250–268. [PubMed]
Benjamin WJ Hill RM. Human tears: osmotic characteristics. Invest Ophthalmol Vis Sci . 1983; 24: 1624–1626. [PubMed]
Gilbard JP Cohen GR Baum J. Decreased tear osmolarity and absence of the inferior marginal tear strip after sleep. Cornea . 1992; 11: 231–233. [CrossRef] [PubMed]
Uniacke NP Hill RM. Osmotic pressure of the tears during adaptation to contact lenses. J Am Optom Assoc . 1970; 41: 932–936. [PubMed]
Nelson JD Wright JC. Tear film osmolality determination: an evaluation of potential errors in measurement. Curr Eye Res . 1986; 5: 677–681. [CrossRef] [PubMed]
White KM Benjamin WJ Hill RM. Human basic tear fluid osmolality. I. Importance of sample collection strategy. Acta Ophthalmol (Copenh) . 1993; 71: 524–529. [CrossRef] [PubMed]
Saleh GM Hussain B Woodruff SA Sharma A Litwin AS. Tear film osmolarity in epiphora. Ophthal Plast Reconstr Surg . 2012; 28: 338–340. [CrossRef] [PubMed]
Stahl U Francis IC Stapleton F. Prospective controlled study of vapor pressure tear osmolality and tear meniscus height in nasolacrimal duct obstruction. Am J Ophthalmol . 2006; 141: 1051–1056. [CrossRef] [PubMed]
Farris RL. Tear osmolarity--a new gold standard? Adv Exp Med Biol . 1994; 350: 495–503. [PubMed]
Harris MG Mandell RB. Contact lens adaption: osmotic theory. Am J Optom Arch Am Acad Optom . 1969; 46: 196–202. [CrossRef] [PubMed]
Martin DK. Osmolality of the tear fluid in the contralateral eye during monocular contact lens wear. Acta Ophthalmol . 1987; 65: 551–555. [CrossRef]
Martin DK Holden BA. Variations in tear fluid osmolality, chord diameter and movement during wear of high water content hydrogel contact lenses. Int Contact Lens Clin . 1983; 10: 332–342.
Benjamin WJ Armitage BS Woloschak MJ Hill RM. Nanoliter tracking of the tears. J Am Optom Assoc . 1983; 3: 243–244.
Sarac O Gurdal C Bostanci-Ceran B Can I. Comparison of tear osmolarity and ocular comfort between daily disposable contact lenses: hilafilcon B hydrogel versus narafilcon A silicone hydrogel. Int Ophthalmol . 2012; 32: 229–233. [CrossRef] [PubMed]
Iskeleli G Karakoc Y Aydin Ö Yetik H Uslu H Kizilkaya M. Comparison of tear-film osmolarity in different types of contact lenses. CLAO J . 2002; 28: 174–176. [PubMed]
Stahl U Willcox M Naduvilath T Stapleton F. Influence of tearfilm and contact lens osmolality on comfort in CL wear. Optom Vis Sci . 2009; 86: 857–867. [CrossRef] [PubMed]
Farris RL. Tear analysis in contact lens wearers. CLAO J . 1986; 12: 106–111. [PubMed]
Miller WL Doughty MJ Narayanan S A comparison of tear volume (by tear meniscus height and phenol thread test) and tear fluid osmolality measurements in non-lens wearers and in contact lens wearers. Eye Contact Lens . 2004; 30: 132–137. [CrossRef] [PubMed]
Dabney BW Robertson DM Tran A Leach N Bergmanson JPG. Tear analysis in contact lens wearers assessing osmolality and volume. Optom Vis Sci . 2000; 77: 265. [CrossRef]
Gilbard JP Gray KL Rossi SR. A proposed mechanism for increased tear-film osmolarity in contact lens wear. Am J Ophthalmol . 1986; 102: 505–507. [CrossRef] [PubMed]
Glasson MJ Stapleton F Keay L Ball M Willcox MD. Effect of contact lens wear on the pre-corneal tear film. In: Zierhut M Stern ME and Sullivan DA eds. Immunology of the Eye . Oxford: Taylor and Francis; 2005: 239–254.
Iskeleli G Karakoc Y Akdeniz-Kayhan B Kayhan U Gurler B Ozkan S. Comparison of tear lactate dehydrogenase activities of different types of contact lens wearers and normal control group. CLAO J . 1999; 25: 101–104. [PubMed]
Stahl U Willcox M Stapleton F. Osmolality and tear film dynamics. Clin Exp Optom . 2012; 95: 3–11. [CrossRef] [PubMed]
Evans KS North RV Purslow C. Tear ferning in contact lens wearers. Ophthalmic Physiol Opt . 2009; 29: 199–204. [CrossRef] [PubMed]
Srinivasan S Joyce E Jones LW. Tear osmolality and ferning patterns in postmenopausal women. Optom Vis Sci . 2007; 84: 588–592. [CrossRef] [PubMed]
Rolando M. Tear mucus ferning test in normal and keratoconjunctivitis sicca eyes. Chibret Int J Ophthalmol . 1984; 2: 32–41.
Versura P Profazio V Campos EC. Performance of tear osmolarity compared to previous diagnostic tests for dry eye diseases. Curr Eye Res . 2010; 35: 553–564. [CrossRef] [PubMed]
Bitton E Keech A Jones L Simpson T. Subjective and objective variation of the tear film pre- and post-sleep. Optom Vis Sci . 2008; 85: 740–749. [CrossRef] [PubMed]
Puderbach S Stolze HH. Tear ferning and other lacrimal tests in normal persons of different ages. Int Ophthalmol . 1991; 15: 391–395. [CrossRef] [PubMed]
Versura P Fresina M Campos EC. Ocular surface changes over the menstrual cycle in women with and without dry eye. Gynecol Endocrinol . 2007; 23: 385–390. [CrossRef] [PubMed]
Iester M Orsoni GJ Gamba G Improvement of the ocular surface using hypotonic 0.4% hyaluronic acid drops in keratoconjunctivitis sicca. Eye (Lond) . 2000; 14: 892–898. [CrossRef] [PubMed]
Horwath J Ettinger K Bachernegg M Bodner E Schmut O. Ocular Ferning test - effect of temperature and humidity on tear Ferning patterns. Ophthalmologica . 2001; 215: 102–107. [CrossRef] [PubMed]
Kogbe O Liotet S Tiffany JM. Factors responsible for tear ferning. Cornea . 1991; 10: 433–444. [CrossRef] [PubMed]
Norn M. Ferning in conjunctival-cytologic preparations. Crystallisation in stained semiquantitative pipette samples of conjunctival fluid. Acta Ophthalmol (Copenh) . 1987; 65: 118–122. [CrossRef] [PubMed]
Norn M. Quantitative tear ferning. Methodologic and experimental investigations. Acta Ophthalmol (Copenh) . 1988; 66: 201–205. [CrossRef] [PubMed]
Julio G Lluch S Cardona G Fornieles A Merindano D. Item by item analysis strategy of the relationship between symptoms and signs in early dry eye. Curr Eye Res . 2012; 37: 357–364. [CrossRef] [PubMed]
Julio G Lluch S Pujol P Merindano MD. Effects of tear hyperosmolarity on conjunctival cells in mild to moderate dry eye. Ophthalmic Physiol Opt . 2012; 32: 317–323. [CrossRef] [PubMed]
Golding TR Brennan NA. The basis of tear ferning. Clin Exp Optom . 1989; 72: 102–112. [CrossRef]
Pearce EI Tomlinson A. Spatial location studies on the chemical composition of human tear ferns. Ophthalmic Physiol Opt . 2000; 20: 306–313. [CrossRef] [PubMed]
Golding TR Baker AT Rechberger J Brennan NA. X-ray and scanning electron microscopic analysis of the structural composition of tear ferns. Cornea . 1994; 13: 58–66. [CrossRef] [PubMed]
Tabbara KF Okumoto M. Ocular ferning test. A qualitative test for mucus deficiency. Ophthalmology . 1982; 89: 712–714. [CrossRef] [PubMed]
Vaikoussis E Georgiou P Nomicarios D. Tear mucus ferning in patients with Sjogren's syndrome. Doc Ophthalmol . 1994; 87: 145–151. [CrossRef] [PubMed]
Filipello M Scimone G Cascone G Zagami A Pantaleoni G. Ferning test in Down's syndrome. Acta Ophthalmol (Copenh) . 1992; 70: 274–277. [CrossRef] [PubMed]
Rolando M Baldi F Calabria G. Tear mucus crystallization in children with cystic fibrosis. Ophthalmologica . 1988; 197: 202–206. [CrossRef] [PubMed]
Kogbe O Liotet S. An interesting use of the study of tear ferning patterns in contactology. Ophthalmologica . 1987; 194: 150–153. [CrossRef] [PubMed]
Ravazzoni L Ghini C Macri A Rolando M. Forecasting of hydrophilic contact lens tolerance by means of tear ferning test. Graefes Arch Clin Exp Ophthalmol . 1998; 236: 354–358. [CrossRef] [PubMed]
Pensyl CD Dillehay SM. The repeatability of tear mucus ferning grading. Optom Vis Sci . 1998; 75: 600–604. [CrossRef] [PubMed]
Abelson MB Udell IJ Weston JH. Normal human tear pH by direct measurement. Arch Ophthalmol . 1981; 99: 301. [CrossRef] [PubMed]
Andres S Garcia ML Espina M Valero J Valls O. Tear pH, air pollution, and contact lenses. Am J Optom Physiol Opt . 1988; 65: 627–631. [CrossRef] [PubMed]
Chen FS Maurice DM. The pH in the precorneal tear film and under a contact lens measured with a fluorescent probe. Exp Eye Res . 1990; 50: 251–259. [CrossRef] [PubMed]
Coles WH Jaros PA. Dynamics of ocular surface pH. Br J Ophthalmol . 1984; 68: 549–552. [CrossRef] [PubMed]
Fischer FH Wiederholt M. Human precorneal tear film pH measured by microelectrodes. Graefes Arch Clin Exp Ophthalmol . 1982; 218: 168–170. [CrossRef] [PubMed]
Norn MS. Tear fluid pH in normals, contact lens wearers, and pathological cases. Acta Ophthalmol . 1988; 66: 485–489. [CrossRef]
Hill RM Carney LG. Tear pH: how predictable? J Am Optom Assoc . 1978; 49: 269–270. [PubMed]
McCarey BE Wilson LA. pH, osmolarity and temperature effects on the water content of hydrogel contact lenses. Contact Intraocul Lens Med J . 1982; 8: 158–167. [PubMed]
Tiffany JM. The viscosity of human tears. Int Ophthalmol . 1991; 15: 371–376. [CrossRef] [PubMed]
Pandit JC Nagyova B Bron AJ Tiffany JM. Physical properties of stimulated and unstimulated tears. Exp Eye Res . 1999; 68: 247–253. [CrossRef] [PubMed]
Tiffany JM Nagyova B. The role of lipocalin in determining the physical properties of tears. Adv Exp Med Biol . 2002; 506: 581–585. [PubMed]
Gouveia SM Tiffany JM. Human tear viscosity: an interactive role for proteins and lipids. Biochim Biophys Acta . 2005; 1753: 155–163. [CrossRef] [PubMed]
Tiffany JM. Viscoelastic properties of human tears and polymer solutions. Adv Exp Med Biol . 1994; 350: 267–270. [PubMed]
Tiffany JM Winter N Bliss G. Tear film stability and tear surface-tension. Curr Eye Res . 1989; 8: 507–515. [CrossRef] [PubMed]
Miller D. Measurement of the surface tension of tears. Arch Ophthalmol . 1969; 82: 368–371. [CrossRef] [PubMed]
Zhao J Wollmer P. Surface activity of tear fluid in normal subjects. Acta Ophthalmol . 1998; 76: 438–441. [CrossRef]
Svitova TF Lin MC. Tear lipids interfacial rheology: effect of lysozyme and lens care solutions. Optom Vis Sci . 2010; 87: 10–20. [CrossRef] [PubMed]
Lemp MA Holly FJ Iwata S Dohlman CH. The precorneal tear film. I. Factors in spreading and maintaining a continuous tear film over the corneal surface. Arch Ophthalmol . 1970; 83: 89–94. [CrossRef] [PubMed]
Liu W Li HL Lu DY The tear fluid mucin 5AC change of primary angle-closure glaucoma patients after short-term medications and phacotrabeculectomy. Mol Vis . 2010; 16: 2342–2346. [PubMed]
Millar TJ Tragoulias ST Anderton PJ The surface activity of purified ocular mucin at the air-liquid interface and interactions with meibomian lipids. Cornea . 2006; 25: 91–100. [CrossRef] [PubMed]
Nagyova B Tiffany JM. Components responsible for the surface tension of human tears. Curr Eye Res . 1999; 19: 4–11. [CrossRef] [PubMed]
Holly FJ. Formation and rupture of the tear film. Exp Eye Res . 1973; 15: 515–525. [CrossRef] [PubMed]
Holly FJ Lemp MA. Wettability and wetting of corneal epithelium. Exp Eye Res . 1971; 11: 239–250. [CrossRef] [PubMed]
Cope C Dilly PN Kaura R Tiffany JM. Wettability of the corneal surface: a reappraisal. Curr Eye Res . 1986; 5: 777–785. [CrossRef] [PubMed]
Dilly PN. Contribution of the epithelium to the stability of the tear film. Trans Ophthalmol Soc U K . 1985; 104 (pt 4): 381–389. [PubMed]
Gipson IK Yankauckas M Spurr-Michaud SJ Tisdale AS Rinehart W. Characteristics of a glycoprotein in the ocular surface glycocalyx. Invest Ophthalmol Vis Sci . 1992; 33: 218–227. [PubMed]
Tiffany JM. Measurement of wettability of the corneal epithelium. 1. Particle attachment method. Acta Ophthalmol . 1990; 68: 175–181. [CrossRef]
Tiffany JM. Measurement of wettability of the corneal epithelium. 2. Contact-angle method. Acta Ophthalmol . 1990; 68: 182–187. [CrossRef]
Watanabe H Fabricant M Tisdale AS Spurrmichaud SJ Lindberg K Gipson IK. Human corneal and conjunctival epithelia produce a mucin-Like glycoprotein for the apical surface. Invest Ophthalmol Vis Sci . 1995; 36: 337–344. [PubMed]
Holly FJ. Surface chemistry of tear film component analogs. J Colloid Interface Sci . 1974; 49: 221–231. [CrossRef]
Mudgil P Torres M Millar TJ. Adsorption of lysozyme to phospholipid and meibomian lipid monolayer films. Colloids Surf B Biointerfaces . 2006; 48: 128–137. [CrossRef] [PubMed]
He Q Zhang Y Lu G Miller R Mohwald H Li J. Dynamic adsorption and characterization of phospholipid and mixed phospholipid/protein layers at liquid/liquid interfaces. Adv Colloid Interface Sci . 2008; 140: 67–76. [CrossRef] [PubMed]
Miano F Calcara M Millar TJ Enea V. Insertion of tear proteins into a meibomian lipids film. Colloids Surf B Biointerfaces . 2005; 44: 49–55. [CrossRef] [PubMed]
Schurch S Bachofen H Possmayer F. Surface activity in situ, in vivo, and in the captive bubble surfactometer. Comp Biochem Physiol A Mol Integr Physiol . 2001; 129: 195–207. [CrossRef] [PubMed]
Svitova TF Radke CJ. AOT and Pluronic F68 coadsorption at fluid/fluid interfaces: a continuous-flow tensiometry study. Indust Eng Chem Res . 2005; 44: 1129–1138. [CrossRef]
Svitova TF Wetherbee MJ Radke CJ. Dynamics of surfactant sorption at the air/water interface: continuous-flow tensiometry. J Colloid Interface Sci . 2003; 261: 170–179. [CrossRef] [PubMed]
Pandit JC Nagyova B Bron AJ Tiffany JM. Physical properties of stimulated and unstimulated tears. Experimental Eye Research . 1999; 68: 247–253. [CrossRef] [PubMed]
Kulovesi P Telenius J Koivuniemi A Molecular organization of the tear fluid lipid layer. Biophys J . 2010; 99: 2559–2567. [CrossRef] [PubMed]
Schuett BS Millar TJ. Lipid component contributions to the surface activity of meibomian lipids. Invest Ophthalmol Vis Sci . 2012; 53: 7208–7219. [CrossRef] [PubMed]
Georgiev GA Kutsarova E Jordanova A Krastev R Lalchev Z. Interactions of Meibomian gland secretion with polar lipids in Langmuir monolayers. Colloids Surf B Biointerfaces . 2010; 78: 317–327. [CrossRef] [PubMed]
Miano F Zhao XB Lu JR Penfold J. Coadsorption of human milk lactoferrin into the dipalmitoylglycerolphosphatidylcholine phospholipid monolayer spread at the air/water interface. Biophys J . 2007; 92: 1254–1262. [CrossRef] [PubMed]
Tragoulias ST Anderton PJ Dennis GR Miano F Millar TJ. Surface pressure measurements of human tears and individual tear film components indicate that proteins are major contributors to the surface pressure. Cornea . 2005; 24: 189–200. [CrossRef] [PubMed]
Millar TJ Mudgil P Butovich IA Palaniappan CK. Adsorption of human tear lipocalin to human meibomian lipid films. Invest Ophthalmol Vis Sci . 2009; 50: 140–151. [CrossRef] [PubMed]
Schoenwald RD Vidvauns S Wurster DE Barfknecht CF. The influence of tear proteins on the film stability of rabbit tear extracts. J Ocul Pharmacol Ther . 1998; 14: 15–29. [CrossRef] [PubMed]
Wei XE Markoulli M Millar TJ Willcox MDP Zhao ZJ. Divalent cations in tears, and their influence on tear film stability in humans and rabbits. Invest Ophthalmol Vis Sci . 2012; 53: 3280–3285. [CrossRef] [PubMed]
Butovich IA Millar TJ Ham BM. Understanding and analyzing meibomian lipids--a review. Curr Eye Res . 2008; 33: 405–420. [CrossRef] [PubMed]
King-Smith PE Fink BA Hill RM Koelling KW Tiffany JM. The thickness of the tear film. Curr Eye Res . 2004; 29: 357–368. [CrossRef] [PubMed]
McCulley JP Shine WE. The lipid layer: the outer surface of the ocular surface tear film. Biosci Rep . 2001; 21: 407–418. [CrossRef] [PubMed]
Peters K Millar TJ. The role of different phospholipids on tear break-up time using a model eye. Curr Eye Res . 2002; 25: 55–60. [CrossRef] [PubMed]
Zhao J Manthorpe R Wollmer P. Surface activity of tear fluid in patients with primary Sjogren's syndrome. Clin Physiol Funct Imaging . 2002; 22: 24–27. [CrossRef] [PubMed]
Panaser A Tighe BJ. Function of lipids - their fate in contact lens wear: an interpretive review. Cont Lens Anterior Eye . 2012; 35: 100–111. [CrossRef] [PubMed]
Ishibashi T Yokoi N Kinoshita S. Comparison of the short-term effects on the human corneal surface of topical timolol maleate with and without benzalkonium chloride. J Glaucoma . 2003; 12: 486–490. [CrossRef] [PubMed]
Yokoi N Komuro A Maruyama K Kinoshita S. New instruments for dry eye diagnosis. Semin Ophthalmol . 2005; 20: 63–70. [CrossRef] [PubMed]
Nichols JJ King-Smith PE. Thickness of the pre- and post-contact lens tear film measured in vivo by interferometry. Invest Ophthalmol Vis Sci . 2003; 44: 68–77. [CrossRef] [PubMed]
Bontempo AR Rapp J. Protein-lipid interaction on the surface of a rigid gas-permeable contact lens in vitro. Curr Eye Res . 1997; 16: 1258–1262. [CrossRef] [PubMed]
Mengher LS Bron AJ Tonge SR Gilbert DJ. A non-invasive instrument for clinical assessment of the pre-corneal tear film stability. Curr Eye Res . 1985; 4: 1–7. [CrossRef] [PubMed]
Patel S Murray D McKenzie A Shearer DS McGrath BD. Effects of fluorescein on tear breakup time and on tear thinning time. Am J Optom Physiol Opt . 1985; 62: 188–190. [CrossRef] [PubMed]
Iskander DR Collins MJ Davis B. Evaluating tear film stability in the human eye with high-speed videokeratoscopy. IEEE Trans Biomed Eng . 2005; 52: 1939–1949. [CrossRef] [PubMed]
Cho P Douthwaite W. The relation between invasive and noninvasive tear break-up time. Optom Vis Sci . 1995; 72: 17–22. [CrossRef] [PubMed]
Cho P Brown B. Review of the tear break-up time and a closer look at the tear break-up time of Hong Kong Chinese. Optom Vis Sci . 1993; 70: 30–38. [CrossRef] [PubMed]
Fullard RJ Snyder C. Protein levels in nonstimulated and stimulated tears of normal human subjects. Invest Ophthalmol Vis Sci . 1990; 31: 1119–1126. [PubMed]
Eter N Gobbels M. A new technique for tear film fluorophotometry. Br J Ophthalmol . 2002; 86: 616–619. [CrossRef] [PubMed]
Kuppens EV Stolwijk TR de Keizer RJ van Best JA. Basal tear turnover and topical timolol in glaucoma patients and healthy controls by fluorophotometry. Invest Ophthalmol Vis Sci . 1992; 33: 3442–3448. [PubMed]
Fullard RJ Tucker D. Tear protein composition and the effects of stimulus. Adv Exp Med Biol . 1994; 350: 309–314. [PubMed]
Fullard RJ Tucker DL. Changes in human tear protein levels with progressively increasing stimulus. Invest Ophthalmol Vis Sci . 1991; 32: 2290–2301. [PubMed]
Sack RA Beaton A Sathe S Morris C Willcox M Bogart B. Towards a closed eye model of the pre-ocular tear layer. Prog Retin Eye Res . 2000; 19: 649–668. [CrossRef] [PubMed]
Sack RA Sathe S Beaton A. Tear turnover and immune and inflammatory processes in the open-eye and closed-eye environments: relationship to extended wear contact lens use. Eye Contact Lens . 2003; 29: S80–S82; discussion S83–S84, S192–S194. [CrossRef] [PubMed]
Tan KO Sack RA Holden BA Swarbrick HA. Temporal sequence of changes in tear film composition during sleep. Curr Eye Res . 1993; 12: 1001–1007. [CrossRef] [PubMed]
Gelstein S Yeshurun Y Rozenkrantz L Human tears contain a chemosignal. Science . 2011; 331: 226–230. [CrossRef] [PubMed]
Farris RL. Tear analysis in contact lens wearers. Trans Am Ophthalmol Soc . 1985; 83: 501–545. [PubMed]
Stuchell RN Feldman JJ Farris RL Mandel ID. The effect of collection technique on tear composition. Invest Ophthalmol Vis Sci . 1984; 25: 374–377. [PubMed]
Markoulli M Papas E Petznick A Holden B. Validation of the flush method as an alternative to basal or reflex tear collection. Curr Eye Res . 2011; 36: 198–207. [CrossRef] [PubMed]
Green-Church KB Butovich I Willcox M The international workshop on meibomian gland dysfunction: report of the subcommittee on tear film lipids and lipid-protein interactions in health and disease. Invest Ophthalmol Vis Sci . 2011; 52: 1979–1993. [CrossRef] [PubMed]
Butovich IA. Lipidomics of human Meibomian gland secretions: chemistry, biophysics, and physiological role of Meibomian lipids. Prog Lipid Res . 2011; 50: 278–301. [CrossRef] [PubMed]
Butovich IA Wojtowicz JC Molai M. Human tear film and meibum. Very long chain wax esters and (O-acyl)-omega-hydroxy fatty acids of meibum. J Lipid Res . 2009; 50: 2471–2485. [CrossRef] [PubMed]
Chen J Green-Church KB Nichols KK. Shotgun lipidomic analysis of human meibomian gland secretions with electrospray ionization tandem mass spectrometry. Invest Ophthalmol Vis Sci . 2010; 51: 6220–6231. [CrossRef] [PubMed]
Butovich IA Uchiyama E Di Pascuale MA McCulley JP. Liquid chromatography-mass spectrometric analysis of lipids present in human meibomian gland secretions. Lipids . 2007; 42: 765–776. [CrossRef] [PubMed]
Butovich IA Uchiyama E McCulley JP. Lipids of human meibum: mass-spectrometric analysis and structural elucidation. J Lipid Res . 2007; 48: 2220–2235. [CrossRef] [PubMed]
Shine WE McCulley JP. Polar lipids in human meibomian gland secretions. Curr Eye Res . 2003; 26: 89–94. [CrossRef] [PubMed]
Butovich IA. On the lipid composition of human meibum and tears: comparative analysis of nonpolar lipids. Invest Ophthalmol Vis Sci . 2008; 49: 3779–3789. [CrossRef] [PubMed]
Borchman D Foulks GN Yappert MC Ho DV. Temperature-induced conformational changes in human tear lipids hydrocarbon chains. Biopolymers . 2007; 87: 124–133. [CrossRef] [PubMed]
Borchman D Foulks GN Yappert MC Tang D Ho DV. Spectroscopic evaluation of human tear lipids. Chem Phys Lipids . 2007; 147: 87–102. [CrossRef] [PubMed]
Ham BM Jacob JT Keese MM Cole RB. Identification, quantification and comparison of major non-polar lipids in normal and dry eye tear lipidomes by electrospray tandem mass spectrometry. J Mass Spectrom . 2004; 39: 1321–1336. [CrossRef] [PubMed]
Dean AW Glasgow BJ. Mass spectrometric identification of phospholipids in human tears and tear lipocalin. Invest Ophthalmol Vis Sci . 2012; 53: 1773–1782. [CrossRef] [PubMed]
Ham BM Cole RB Jacob JT. Identification and comparison of the polar phospholipids in normal and dry eye rabbit tears by MALDI-TOF mass spectrometry. Invest Ophthalmol Vis Sci . 2006; 47: 3330–3338. [CrossRef] [PubMed]
Rantamaki AH Seppanen-Laakso T Oresic M Jauhiainen M Holopainen JM. Human tear fluid lipidome: from composition to function. PLoS One . 2011; 6: e19553. [CrossRef] [PubMed]
Saville JT Zhao Z Willcox MD Blanksby SJ Mitchell TW. Detection and quantification of tear phospholipids and cholesterol in contact lens deposits: the effect of contact lens material and lens care solution. Invest Ophthalmol Vis Sci . 2010; 51: 2843–2851. [CrossRef] [PubMed]
Yamada M Mochizuki H Kawashima M Hata S. Phospholipids and their degrading enzyme in the tears of soft contact lens wearers. Cornea . 2006; 25: S68–S72. [CrossRef] [PubMed]
Young WH Hill RM. Cholesterol levels in human tears: case reports. J Am Optom Assoc . 1973; 45: 425–428.
Young WH Hill RM. Tear cholesterol levels and contact lens adaptation. Am J Optom Arch Am Acad Optom . 1973; 50: 12–16. [CrossRef] [PubMed]
Guillon M Maissa C Girard-Claudon K Cooper P. Influence of the tear film composition on tear film structure and symptomatology of soft contact lens wearers. Adv Exp Med Biol . 2002; 506: 895–899. [PubMed]
Maissa C Guillon M Girard-Claudon K Cooper P. Tear lipid composition of hydrogel contact lens wearers. Adv Exp Med Biol . 2002; 506: 935–938. [PubMed]
Kawashima M Yamada M Arita R Kawai M Mashima Y. Measurement of phospholipids in human tears. Folia Ophthalmol Jpn . 2003; 54: 870–873.
Rantamaki AH Javanainen M Vattulainen I Holopainen JM. Do lipids retard the evaporation of the tear fluid? Invest Ophthalmol Vis Sci . 2012; 53: 6442–6447. [CrossRef] [PubMed]
Cyperlipid Center. Lipid peroxidation. Available at: Accessed April 20, 2013.
Qu XD Lehrer RI. Secretory phospholipase A2 is the principal bactericide for staphylococci and other gram-positive bacteria in human tears. Infect Immun . 1998; 66: 2791–2797. [PubMed]
Aho HJ Saari KM Kallajoki M Nevalainen TJ. Synthesis of group II phospholipase A2 and lysozyme in lacrimal glands. Invest Ophthalmol Vis Sci . 1996; 37: 1826–1832. [PubMed]
Nevalainen TJ Aho HJ Peuravuori H. Secretion of group 2 phospholipase A2 by lacrimal glands. Invest Ophthalmol Vis Sci . 1994; 35: 417–421. [PubMed]
Saari KM Aho V Paavilainen V Nevalainen TJ. Group II PLA(2) content of tears in normal subjects. Invest Ophthalmol Vis Sci . 2001; 42: 318–320. [PubMed]
Aho VV Paavilainen V Nevalainen TJ Peuravuori H Saari KM. Diurnal variation in group IIa phospholipase A2 content in tears of contact lens wearers and normal controls. Graefes Arch Clin Exp Ophthalmol . 2003; 241: 85–88. [CrossRef] [PubMed]
Hume EB Cole N Parmar A Secretory phospholipase A2 deposition on contact lenses and its effect on bacterial adhesion. Invest Ophthalmol Vis Sci . 2004; 45: 3161–3164. [CrossRef] [PubMed]
Mochizuki H Yamada M Hatou S Kawashima M Hata S. Deposition of lipid, protein, and secretory phospholipase A2 on hydrophilic contact lenses. Eye Contact Lens . 2008; 34: 46–49. [CrossRef] [PubMed]
Thakur A Willcox MD. Cytokine and lipid inflammatory mediator profile of human tears during contact lens associated inflammatory diseases. Exp Eye Res . 1998; 67: 9–19. [CrossRef] [PubMed]
Granstrom E. The arachidonic acid cascade. The prostaglandins, thromboxanes and leukotrienes. Inflammation . 1984; 8 (suppl): S15–S25. [CrossRef] [PubMed]
Campbell D Griffiths G Tighe BJ. Tear analysis and lens-tear interactions: part II. Ocular lipids-nature and fate of meibomian gland phospholipids. Cornea . 2011; 30: 325–332. [CrossRef] [PubMed]
Song CH Choi JS Kim DK Kim JC. Enhanced secretory group II PLA2 activity in the tears of chronic blepharitis patients. Invest Ophthalmol Vis Sci . 1999; 40: 2744–2748. [PubMed]
Glasson MJ Stapleton F Willcox MD. Lipid, lipase and lipocalin differences between tolerant and intolerant contact lens wearers. Curr Eye Res . 2002; 25: 227–235. [CrossRef] [PubMed]
Campbell D Mann A Hunt O Santos LJ. The significance of hand wash compliance on the transfer of dermal lipids in contact lens wear. Cont Lens Anterior Eye . 2012; 35: 71–76. [CrossRef] [PubMed]
Suttorp-Schulten MS Luyendijk L Kok JH Kijlstra A. HPLC analysis of tear proteins in giant papillary conjunctivitis. Doc Ophthalmol . 1989; 72: 235–240. [CrossRef] [PubMed]
Funke S Azimi D Wolters D Grus FH Pfeiffer N. Longitudinal analysis of taurine induced effects on the tear proteome of contact lens wearers and dry eye patients using a RP-RP-capillary-HPLC-MALDI TOF/TOF MS approach. J Proteomics . 2012; 75: 3177–3190. [CrossRef] [PubMed]
de Souza GA Godoy LM Mann M. Identification of 491 proteins in the tear fluid proteome reveals a large number of proteases and protease inhibitors. Genome Biol . 2006; 7: R72. [CrossRef] [PubMed]
Zhou L Zhao SZ Koh SK In-depth analysis of the human tear proteome. J Proteomics . 2012; 75: 3877–3885. [CrossRef] [PubMed]
Denny P Hagen FK Hardt M The proteomes of human parotid and submandibular/sublingual gland salivas collected as the ductal secretions. J Proteome Res . 2008; 7: 1994–2006. [CrossRef] [PubMed]
Schmidt A Aebersold R. High-accuracy proteome maps of human body fluids. Genome Biol . 2006; 7: 242. [CrossRef] [PubMed]
Balasubramanian SA Pye DC Willcox MD. Levels of lactoferrin, secretory IgA and serum albumin in the tear film of people with keratoconus. Exp Eye Res . 2012; 96: 132–137. [CrossRef] [PubMed]
Ballow M Donshik PC Rapacz P Samartino L. Tear lactoferrin levels in patients with external inflammatory ocular disease. Invest Ophthalmol Vis Sci . 1987; 28: 543–545. [PubMed]
Nichols JJ Green-Church KB. Mass spectrometry-based proteomic analyses in contact lens-related dry eye. Cornea . 2009; 28: 1109–1117. [CrossRef] [PubMed]
Stapleton F Willcox MD Morris CA Sweeney DF. Tear changes in contact lens wearers following overnight eye closure. Curr Eye Res . 1998; 17: 183–188. [CrossRef] [PubMed]
Yamada M Mochizuki H Kawai M Tsubota K Bryce TJ. Decreased tear lipocalin concentration in patients with meibomian gland dysfunction. Br J Ophthalmol . 2005; 89: 803–805. [CrossRef] [PubMed]
Sariri R Khamedi A. Variations in electrophoretic tear protein pattern due to contact lens wear. J Chromatogr A . 2007; 1161: 64–66. [CrossRef] [PubMed]
Markoulli M Papas E Cole N Holden BA. The diurnal variation of matrix metalloproteinase-9 and its associated factors in human tears. Invest Ophthalmol Vis Sci . 2012; 53: 1479–1484. [CrossRef] [PubMed]
Carney FP Morris CA Willcox MD. Effect of hydrogel lens wear on the major tear proteins during extended wear. Aust N Z J Ophthalmol . 1997; 25 (suppl 1): S36–S38. [CrossRef] [PubMed]
Choy CK Cho P Benzie IF Ng V. Effect of one overnight wear of orthokeratology lenses on tear composition. Optom Vis Sci . 2004; 81: 414–420. [CrossRef] [PubMed]
Rapacz P Tedesco J Donshik PC Ballow M. Tear lysozyme and lactoferrin levels in giant papillary conjunctivitis and vernal conjunctivitis. CLAO J . 1988; 14: 207–209. [PubMed]
Vinding T Eriksen JS Nielsen NV. The concentration of lysozyme and secretory IgA in tears from healthy persons with and without contact lens use. Acta Ophthalmol (Copenh) . 1987; 65: 23–26. [CrossRef] [PubMed]
Velasco Cabrera MJ, Garcia Sanchez J, Bermudez Rodriguez FJ. Lactoferrin in tears in contact lens wearers. CLAO J . 1997; 23: 127–129. [PubMed]
Willcox MD Morris CA Thakur A Sack RA Wickson J Boey W. Complement and complement regulatory proteins in human tears. Invest Ophthalmol Vis Sci . 1997; 38: 1–8. [PubMed]
McClellan KA Cripps AW Clancy RL Billson FA. The effect of successful contact lens wear on mucosal immunity of the eye. Ophthalmology . 1998; 105: 1471–1477. [CrossRef] [PubMed]
Cheng KH Spanjaard L Rutten H Dankert J Polak BC Kijlstra A. Immunoglobulin A antibodies against Pseudomonas aeruginosa in the tear fluid of contact lens wearers. Invest Ophthalmol Vis Sci . 1996; 37: 2081–2088. [PubMed]
Pearce DJ Demirci G Willcox MD. Secretory IgA epitopes in basal tears of extended-wear soft contact lens wearers and in non-lens wearers. Aust N Z J Ophthalmol . 1999; 27: 221–223. [CrossRef] [PubMed]
Willcox MD Lan J. Secretory immunoglobulin A in tears: functions and changes during contact lens wear. Clin Exp Optom . 1999; 82: 1–3. [CrossRef] [PubMed]
Sullivan DA Allansmith MR. Source of IgA in tears of rats. Immunology . 1984; 53: 791–799. [PubMed]
Thakur A Willcox MD Stapleton F. The proinflammatory cytokines and arachidonic acid metabolites in human overnight tears: homeostatic mechanisms. J Clin Immunol . 1998; 18: 61–70. [CrossRef] [PubMed]
Yamada M Ogata M Kawai M Mashima Y Nishida T. Substance P in human tears. Cornea . 2003; 22: S48–S54. [CrossRef] [PubMed]
Acera A Rocha G Vecino E Lema I Duran JA. Inflammatory markers in the tears of patients with ocular surface disease. Ophthalmic Res . 2008; 40: 315–321. [CrossRef] [PubMed]
Balasubramanian SA Mohan S Pye DC Willcox MD. Proteases, proteolysis and inflammatory molecules in the tears of people with keratoconus. Acta Ophthalmol . 2012; 90: e303–e309. [CrossRef] [PubMed]
Erdogan-Poyraz C Mocan MC Bozkurt B Gariboglu S Irkec M Orhan M. Elevated tear interleukin-6 and interleukin-8 levels in patients with conjunctivochalasis. Cornea . 2009; 28: 189–193. [CrossRef] [PubMed]
Fodor M Facsko A Rajnavolgyi E Enhanced release of IL-6 and IL-8 into tears in various anterior segment eye diseases. Ophthalmic Res . 2006; 38: 182–188. [CrossRef] [PubMed]
Huang D Xu N Song Y Wang P Yang H. Inflammatory cytokine profiles in the tears of thyroid-associated ophthalmopathy. Graefes Arch Clin Exp Ophthalmol . 2012; 250: 619–625. [CrossRef] [PubMed]
Jun AS Cope L Speck C Subnormal cytokine profile in the tear fluid of keratoconus patients. PLoS One . 2011; 6: e16437. [CrossRef] [PubMed]
Lam H Bleiden L de Paiva CS Farley W Stern ME Pflugfelder SC. Tear cytokine profiles in dysfunctional tear syndrome. Am J Ophthalmol . 2009; 147: 198–205, e191. [CrossRef] [PubMed]
Na KS Mok JW Kim JY Rho CR Joo CK. Correlations between tear cytokines, chemokines, and soluble receptors and clinical severity of dry eye disease. Invest Ophthalmol Vis Sci . 2012; 53: 5443–5450. [CrossRef] [PubMed]
Pannebaker C Chandler HL Nichols JJ. Tear proteomics in keratoconus. Mol Vis . 2010; 16: 1949–1957. [PubMed]
Solomon A Dursun D Liu Z Xie Y Macri A Pflugfelder SC. Pro- and anti-inflammatory forms of interleukin-1 in the tear fluid and conjunctiva of patients with dry-eye disease. Invest Ophthalmol Vis Sci . 2001; 42: 2283–2292. [PubMed]
Uchino E Sonoda S Kinukawa N Sakamoto T. Alteration pattern of tear cytokines during the course of a day: diurnal rhythm analyzed by multicytokine assay. Cytokine . 2006; 33: 36–40. [CrossRef] [PubMed]
Yoon KC Jeong IY Park YG Yang SY. Interleukin-6 and tumor necrosis factor-alpha levels in tears of patients with dry eye syndrome. Cornea . 2007; 26: 431–437. [CrossRef] [PubMed]
Enriquez-de-Salamanca A Castellanos E Stern ME Tear cytokine and chemokine analysis and clinical correlations in evaporative-type dry eye disease. Mol Vis . 2010; 16: 862–873. [PubMed]
Veerhuis R Kijlstra A. Inhibition of hemolytic complement activity by lactoferrin in tears. Exp Eye Res . 1982; 34: 257–265. [CrossRef] [PubMed]
Szczotka LB Cocuzzi E Medof ME. Decay-accelerating factor in tears of contact lens wearers and patients with contact lens-associated complications. Optom Vis Sci . 2000; 77: 586–591. [CrossRef] [PubMed]
Cocuzzi E Szczotka LB Brodbeck WG Bardenstein DS Wei T Medof ME. Tears contain the complement regulator CD59 as well as decay-accelerating factor (DAF). Clin Exp Immunol . 2001; 123: 188–195. [CrossRef] [PubMed]
Shoji J Kawaguchi A Gotoh A Inada N Sawa M. Concentration of soluble interleukin-6 receptors in tears of allergic conjunctival disease patients. Jpn J Ophthalmol . 2007; 51: 332–337. [CrossRef] [PubMed]
Carreno E Enriquez-de-Salamanca A Teson M Cytokine and chemokine levels in tears from healthy subjects. Acta Ophthalmol . 2010; 88: e250–e258. [CrossRef] [PubMed]