The Impact of Hydrogel Lens Settling on the Thickness of the Tears and Contact Lens

Abstract

purpose. To investigate the effect of contact lens insertion on the thickness on the prelens tear film (PLTF), the contact lens, and the postlens tear film (PoLTF).

methods. Twelve contact lens wearers (mean age, 32.7 years; four males) inserted etafilcon A hydrogel lenses (power: −2.00 D, base curve: 8.3 mm) in both eyes immediately before testing. Previously described interference techniques, based on oscillations in reflectance spectra, were used to measure the thickness of the PLTF, contact lens, and PoLTF. The thickness of the layers is derived from the frequency of the oscillations. Spectra were captured 1 minute after lens insertion and every minute thereafter for 30 minutes. Least squares regression fits were used to determine the relation between thickness of each layer and time.

results. The combined data from all subjects for PLTF thickness were fit with an exponential decay plus a constant thickness; the initial thickness was 4.5 micrometers, the time constant was 7.1 minutes (P < 0.001), and final thickness was 2.5 micrometers. The apparent thickness of the contact lens declined linearly at an average rate of 0.051 micrometers/minute (P < 0.001). The PoLTF thickness remained constant at 2.5 micrometers (P = 0.46).

conclusions. For most subjects, the PLTF thinned significantly over the course of the first 30 minutes of lens wear. The apparent thinning of the contact lens may be caused by a real thinning of the lens, but also may have a contribution from improved centration over the 30-minute period. The PoLTF remained relatively stable during this period.

The normal, daily process of lens settling on the ocular surface for experienced lens wearers remains somewhat elusive. The term ‘lens settling’ is somewhat broad, but probably should be thought to encompass tear film disruption, contact lens hydration changes, and resultant dimensional and lens movement alterations after a contact lens is applied to the ocular surface. 

There has been considerable research in the evaluation of contact lens-related dehydration during the initial wearing period. Understanding the hydration properties of hydrogel materials is important, as dehydration has several clinical implications, including potential alterations in fitting characteristics (i.e., lens steepening, tightening, and discomfort), ^{1 2 3 4 5 6} decreased oxygen transmissibility, ^{7 8 9} and increased corneal desiccation. ^{10 11 12} Andrasko ¹³ showed that for the first 30 minutes of lens wear, a hydrogel lens can dehydrate by 7% to 11%, with thicker lenses dehydrating less than thin ones made of the same material. He also showed that the water content of high-water-content lenses decreases more substantially than lower-water-content lenses of the same thickness. The vast majority of dehydration was found to be within the first 5 minutes of wear, especially for high-water-content materials. Efron and coworkers ¹⁴ confirmed the finding that most dehydration occurs within the first period of wear, and that high-water-content lenses seem to continue to dehydrate for longer periods of time than low-water-content lenses. In yet another study, Brennan and coworkers ¹⁵ showed that lenses made of etafilcon A (Acuvue II; Vistakon, Johnson and Johnson, Jacksonville, FL) dehydrate approximately 6.2% after only 20 minutes of lens wear. Pritchard and Fonn ¹⁶ showed this dehydration was related to an increase in dryness symptoms at the end of the wearing time. One could infer from these demonstrations of hydrogel dehydration that the lens may reduce in thickness while on the eye, although this has not been studied. 

Several studies have evaluated lens movement during the lens settling period. Martin and Holden ¹⁷ showed that hydrogels decrease in movement by approximately 20% during the first 10 minutes of wear. Brennan and coworkers ¹⁸ showed that there is a significant decrease during the first 25 minutes after application of a lens. Golding and coworkers ¹⁹ examined the impact of blinking on lens movement during lens settling, with the assumption that a more frequent blink rate is associated with reduced postlens tear film (PoLTF) thickness. They found that lens movement was significantly lower when the blink rate was high, and suggested that lens settling is determined by PoLTF expulsion due to the eyelid forces on the PoLTF. Little and Bruce ²⁰ showed that hypotonicity of the tear film is related to a reduction in lens movement; this is consistent with the osmotic theory proposed by Harris and Mandell. ²¹  

Less research has been conducted on physical tear film disruption during the lens settling period. Clinically, one might predict that on application of a hydrogel lens to the ocular surface, there might be a thick tear film surrounding the contact lens, including both the prelens tear film (PLTF) and PoLTF. This increased tear thickness might be followed by reductions to normal pre- and postlens tear thicknesses over the initial period of wear. However, many questions remain to be answered about this process. Little and Bruce ²² qualitatively examined the PoLTF thickness during the first 6 hours of etafilcon A lens wear. They showed via specular reflections that the PoLTF was thick (i.e., amorphous) immediately after lens insertion. However, during the next 30 minutes of lens wear, the PoLTF showed colored reflections indicating significant thinning, with concurrent reduced lens movement. The PoLTF returned to thicker values for the remainder of the lens wearing period. Chauhan and Radke ²³ also studied the impact of lens settling on the tear film thickness using a theoretical model. The model predicts that the postlens tear film does not come to a steady state, but rather, continues to thin toward the corneal surface, especially in the periphery. They suggest that this might be the mechanism of superior epithelial arcuate lesions (SEALs) and adherent lenses, which occur in a small number of patients. These studies suggest that there may be changes in the tear film during the first hour or so of lens settling. However, no quantitative data have been reported on the thickness of the tear film or contact lens during the lens settling period. The purpose of the present report was to examine the thickness of the PLTF, contact lens, and PoLTF during the initial lens settling period. 

Methods

Interferometry

The details of our interferometric methods, including the optical system, and the validity and reliability of the technique, were previously described for contact lens wearers. ²⁴ However, some general principles of our method of measuring the PoLTF are presented here. Interference occurs when light waves are reflected from two surfaces at the front of the eye causing spectral oscillations (i.e., a series of maxima and minima). In this method, reflectance spectra from the front of an eye wearing a contact lens are measured at normal incidence. Interference between reflections from four surfaces—the front of the tear film, the front and back of the contact lens, and the front of the cornea—can give rise to as many as six oscillations in the reflectance spectra, three from simple layers, (Layer A, prelens tear film; B, postlens tear film; C, contact lens) and three from composite layers, (Layer D = A + C; E = C + B; F = A + C + B). The thickness of any layer is given by the ‘frequency’ of the corresponding oscillations on a plot of reflectance as a function of 2n/λ where n is the refractive index of tears and λ is wavelength in a vacuum. ²⁵ Because thickness of any layer is given by its frequency, the Fourier analysis of the reflectance spectrum shows corresponding peaks due to Layers A, C, D, and F. These four layers tend to give detectable oscillations, whereas oscillations from Layers B and E are weak and hard to detect. ²⁴ Therefore, indirect estimates (Layer F − D) are used to derive the thickness of the PoLTF; when direct estimates of PoLTF thickness (Layer B) can be made, they correlate well with these indirect estimates. The reflection spectrum (562 to 1030 nm) is recorded on a CCD camera that samples the spectral image from a spectrograph. For these measures, the exposure duration was 0.5 second made approximately 2 seconds after a blink, the mean temperature was 24°C, the mean humidity was 26%, and the measurement area was nominally 12 × 33 micrometers (in practice probably larger due to aberrations, defocus, and eye movements). Refractive index of the tears was assumed to be 1.337 at 589 nm and a correction was made for dispersion. ^{24 26} The refractive index of the hydrogel lens (etafilcon A) was assumed to be 1.405, as nominally reported by the manufacturer. The current method for measuring the thickness of these layers is limited to values of greater than 1 μm. ²⁷  

Clinical Study

All patients recruited for the study were required to review and sign informed consent documents, which were approved by the Biomedical Institutional Review Board of The Ohio State University according to the Tenets of the Declaration of Helsinki. Twelve experienced contact lens-wearing subjects (mean age, 32.7 ± 10.0 years; four males) were recruited to participate in this study. Each subject was free of ocular disease (including dry eye), and each was a current Acuvue II (Vistakon) lens wearer. Each subject was fitted in etafilcon A contact lenses, which were worn in both eyes during the experimental procedure (power, −2.00 D; base curve, 8.3 mm). After completion of the experimental trial described below, the fit of the lenses was verified using standard clinical techniques, including the assessment of movement, centration, and coverage. 

Subjects were first asked to apply the left contact lens, followed immediately by the right contact lens. One minute after application of the right contact lens, spectral oscillations were recorded each minute for a 30-minute period. Although it would have been desirable to capture earlier spectra (i.e., immediately after lens application), this was not possible due to necessity of aligning the eye in the optical system. No formal assessment of lens comfort was made per se during the trial, although subjects were free to comment on their contact lens-related comfort and perception of excessive tearing if warranted. If a subject were to make such a complaint during the course of the 30-minute trial, the experiment would have been stopped, and restarted at a later time. 

Analysis and Statistical Procedures

Data were considered valid if they satisfied the ‘lax’ criteria of Nichols and King-Smith. ²⁴ It should be noted that mean thickness estimates for PLTF and PoLTF using these lax criteria agree to within 0.05 micrometers with the means using strict criteria, while providing many more valid estimates. ²⁴  

The time variation of thickness of any layer (PLTF, contact lens, or PoLTF) was modeled either as a linear function of time or, if significantly better, as an exponential decay plus a constant. Least squares fits were obtained using SigmaPlot (SPSS, Chicago, IL) for both individual subjects and for combined data from all 12 subjects. For an individual subject, i, fits were obtained by minimizing  

\[{{\sum}_{t}}\ {[}h_{i}(t)\ {-}\ h{^\prime}_{i}(t){]}^{2}\]

where h _i (t) is a valid measurement of thickness at time t, and h′ _i (t) is the function fit to the data, given by either  

\[h{^\prime}_{i}(t)\ {=}\ h{^\prime}_{i}(0)\ {+}\ kt\]

for a linear function of time, or  

\[h{^\prime}_{i}(t)\ {=}\ h{^\prime}_{i}({\infty})\ {+}\ Ce^{{-}t/t_{0}}\]

for an exponential decay plus a constant. In equation 2 , h′ _i (0) and k were parameters adjusted to give the least squares fit, whereas in equation 3 , h′ _i (∞), C and t ₀ were adjusted. For fitting combined data for all subjects, equation 1 was replaced by  

\[{{\sum}_{i}}{{\sum}_{t}}\ {[}h_{i}(t)\ {-}\ h{^\prime}_{i}(t){]}^{2}\]

In this case, all 12 values of h′ _i (0) or h′ _i (∞) were adjusted together with a single parameter, k, for a linear fit (equation 2) , or the two parameters C and t ₀ for the exponential decay plus constant (equation 3) . Significance and confidence intervals for parameters were determined by F-ratio tests (Minitab, State College, PA) derived from SigmaPlot output, as was the significance of the difference between linear and exponential plus constant fits (equations 2 and 3) . Statistical results were confirmed by mixed models regression techniques in SAS Version 8.2 (SAS Institute, Carey, NC), which are not presented. For all models, P values < 0.05 were considered significant. 

Results

Out of 360 spectra, 351 satisfied the criteria for PLTF thickness estimates. However, two of these were eliminated as outliers because they were over 6 standard deviations greater than the mean (over 13 micrometers). In one case, this was the first measurement after lens insertion, perhaps related to fluid introduced with the lens; in the other case, there was an isolated increase in thickness 9 minutes after lens insertion, perhaps due to irritation of the eye. 

Combining data from all subjects, the time variation of PLTF thickness was significantly better fit by an exponential decay plus constant (equation 3) , than by a linear function (F-ratio test, P < 0.001). This fit is illustrated in Figure 1 , where the ordinates of the plotted points are given by  

\[{\bar{h}}(t)\ {+}\ {[}{\bar{H}}{^\prime}({\infty})\ {-}\ {\bar{h}}{^\prime}({\infty}){]}\]

where h̄(t) is the average for all subjects with valid measurements at time t, h̄′(∞) is the average of h′ _i (∞) (see equation 3 ) for those same subjects, and H̄′(∞) is the average of h′ _i (∞) for all subjects. The term [H̄′(∞)-h̄′(∞)] corrects for differences between the subjects with valid measurements compared to all subjects. Compared to the simple average, h̄′(∞), this method of plotting causes an unbiased reduction in the scatter of the data, particularly when there are many missing data points and when there is considerable variance in the individual estimates of the parameters h′ _i (∞). The fitted curve was determined from equations 3 and 4 , averaged for all subjects; it is given by  

\[{\bar{H}}{^\prime}({\infty})\ {+}\ Ce^{{-}t/t_{0}}\]

The final average thickness, H̄′(∞), was 2.47 ± 0.92 micrometers; the time constant of the exponential decay, t ₀ (equation 3) , was 7.1 minutes, and its amplitude, C, was 2.0 micrometers; thus the initial thickness, H̄′(∞)+C, was approximately 4.5 micrometers. Nine of 12 subjects showed a significantly better fit (F-ratio test, P < 0.05) with an exponential decay plus constant compared to simply a constant thickness. 

The averaged data of Figure 1 conceal considerable individual variability. For the subject of Figure 2A , a rapid decay was observed with a time constant of 1.5 minutes and a 95% confidence interval of 0.9 to 2.4 minutes. For the subject of Figure 2B , the best fitting time constant was 26 minutes with a 95% confidence interval from 9.4 minutes to infinity (implying that the fit was not significantly better than a linear fit). There is clearly no overlap between the confidence intervals of the time constants for these two subjects. For the subject of Figure 2C , the fit for an exponential decay plus constant was not significantly better than for a constant thickness (P = 0.98). 

For contact lens thickness data, 186 valid thickness measurements were obtained; a combined fit for all subjects, using exponential decay plus constant, was not significantly better than a linear fit (F-ratio test, P = 0.48). The solid line in Figure 3 shows a linear fit to the combined data (equations 2 and 4) , where the ordinates of the plotted points are given by  

\[{\bar{h}}(t)\ {+}\ {[}{\bar{H}}{^\prime}(0)\ {-}\ {\bar{h}}{^\prime}(0){]}\]

analogous to equation 5 . Initial average fitted thickness, H̄′(0), was 88.7 ± 2.1 micrometers, with a thinning rate, −k, (see equation 2 ) of 0.051 micrometers/minute; this slope was significant (F-ratio test, P < 0.001; r ² = 0.382). By fitting individual data, all 12 subjects showed thinning (k < 0), but three slopes were small and not significant (P > 0.05). Although, as noted above, the fit using an exponential decay plus constant is not significantly better than the simpler linear fit, it is worth reporting, partly because it is more logical (it does not imply that contact lens thickness will eventually become negative). The best fitting time constant, t ₀, was 60 minutes with an initial amplitude C of 3.9 micrometers. The ninety-five percent confidence interval for t ₀ was 16 minutes to ∞ with corresponding initial amplitudes C of 1.9 micrometers to ∞; the dashed line in Figure 3 shows the fit for the shortest time constant of the confidence interval, t ₀ = 16 minutes. 

For PoLTF data, 176 valid thickness measurements were obtained; a combined fit using exponential plus constant was not significantly better than a linear fit (F-ratio test, P = 0.47). Figure 4 shows a linear fit to the combined data, as in Figure 3 . Initial average fitted thickness, H̄′(0), was 2.50 ± 0.45 micrometers, with a slope k of 0.003 micrometers/minute, indicating a slow but insignificant (P = 0.47; r ² = 0.0034) thickening of the PoLTF. As for the PLTF, the averaged data of Figure 4 conceals considerable individual variability in slope k, which is illustrated in Figure 5 . For the subject of Figure 5A , the tear film thickened at a rate of k = 0.081 micrometers/minute, whereas for the subject of Figure 5B , the tear film thinned at a rate of −k = 0.054 micrometers/minute. Both of these slopes were significant (F-ratio test, P < 0.0001) and remain significant even after correction for multiple (twelve) tests (P < 0.0012). Three out of the 12 subjects gave significant slopes (all P < 0.05), including the two shown in Figure 5 and a third subject who showed a significant thinning rate, as in Figure 5B . 

Discussion

The average value of PLTF thickness immediately after lens insertion was 4.5 micrometers, while the average value after the initial exponential decay was H̄′(∞) = 2.47 ± 0.92 micrometers. This finding is not significantly different from the value 2.31 ± 0.82 micrometers, which was measured in 12 subjects approximately 20 minutes after lens insertion in a previous study. ²⁴  

Considerable individual variability is indicated by comparison of the three subjects in Figure 2 . Perhaps different mechanisms may underlie the difference in time constants; for example, the rapid decay in Figure 2A may correspond to a time constant for drainage of excess fluid through the puncta, whereas the slow decay in Figure 2B might correspond to a slow reduction in neural stimulation leading to a corresponding reduction in tear production. For the subject in Figure 2C , the excess fluid from lens insertion may have already been drained through the puncta by the time of the first measurement. Further studies may elucidate this intersubject variability in the PLTF. It seems as though most attention should be related to this particular layer, as it obviously changes the most during the initial phase of lens adaptation. Thus, this layer may be most associated with lens comfort during wear. 

The data of Figure 3 , showing apparent thinning of the contact lenses at 0.051 micrometers/minute, could have two possible explanations. First, an obvious explanation is that the contact lenses really do thin on the eye. It may be that the tear film becomes somewhat hyperosmotic during the initial wear period, as proposed by Harris and Mandell, ²¹ which could lead to some initial dehydration of the lens and resultant reduction in thickness. Similar studies associated with silicone hydrogel type materials may show different results, as these lenses are thought to resist dehydration. ²⁸  

A second possible explanation of apparent contact lens thinning is that lenses were initially decentered, and then became better centered over the course of 30 minutes; this would cause a reduction in lens thickness at the measurement spot, which is at the center of the cornea. Through thick lens calculations, decentration of 1-mm would correspond to an increase in lens thickness of 2.0 micrometers for the −2.00 D lenses used here; this is greater than the observed thinning over 30 minutes. Our analysis of the data of Tranoudis and Efron (unpublished data, 2004), averaged over eight types of soft contact lens, indicates that centration might cause an apparent thinning for a −2.00 D lens of approximately 1.2 micrometers in the first 20 minutes, after which little more thinning may occur. This prediction is fairly consistent with the dashed curve in Figure 3 , which is the exponential decay plus constant fit for the shortest time constant in the 95% confidence interval, t ₀ = 16 minutes. Currently we are uncertain to what extent the two mechanisms, real thinning and centration, contribute to the observed thinning. Reports that lens dehydration occurs during the initial period of lens wear indicate that real thinning probably does occur. ^{14 15 29 30} Real lens thinning would have been better measured with lenses of −0.50 D power as they have nearly uniform thickness. However these lenses have a greater central thickness than the −2.00 D lenses used here; this would have made it more difficult to study the PoLTF with our instrumentation because the spectral oscillations from Layers D and F would have a higher frequency, and would no longer be resolved at the short wavelength end of the spectrum. It remains unclear at this point just how clinically relevant any apparent contact lens thinning might be during these first 30 minutes postinsertion, especially as these subjects were all normal. If there is an abnormal PLTF such as might be the case in contact lens-related dry eye, this lens thinning may be exacerbated, potentially altering the fit and comfort of the lens. 

To our surprise, using combined data for all subjects, no significant time variation in PoLTF thickness was observed (Fig. 4) . The PoLTF seemed to reach its final, steady state value within the first minute of wear. The current method is certainly able to detect changes in PoLTF thickness, such as thinning caused by eye closure and thickening after subsequent eye opening. ²⁷ We were not able to confirm the thinning of the PoLTF over the first 30 minutes after lens insertion suggested by Little and Bruce ³¹ from the presence of colored interference effects from the PoLTF. Also, the question of why lens movement may decrease in the initial period of lens wear could not be explained. ^{17 18} In this regard, it may be noted that our method measured PoLTF thickness at the center of the contact lens, whereas lens movement presumably depends on PoLTF thickness throughout the area of the contact lens. Additionally, lens movement may depend not only on the PoLTF thickness, but also on other factors such as PoLTF viscosity and the thickness and viscosity of the tear fluid between the contact lens and the lids. 

Initial PoLTF thickness was H̄′(0) = 2.50 ± 0.45 micrometers with an insignificant slope. This value is not significantly different from the value 2.37 ± 0.46 micrometers from a previous study. ²⁴ It should be noted that the PoLTF showed less subject-to-subject variability than the PLTF; causes of these differences of variability in PoLTF and PLTF deserve further study.Figure 5 illustrates that even though the combined data demonstrated no significant time variation (Fig. 4) , individual subjects may show either a significant thickening (Fig. 5A) or thinning (Fig. 5B) over time. Mechanisms determining PoLTF thickness have been discussed by Chauhan and Radke ²³ and involve alternate inflow of fluid between blinks (due to elasticity of the lens) and expulsion by lid pressure during the blink. It seems possible that factors such as lid pressure and contact lens fit (relative to corneal curvature) may contribute to the thickening or thinning seen in Figure 5 . For 9 of the 12 subjects, there was no significant change in PoLTF thickness over 30 minutes; the mechanisms which typically maintain the PoLTF thickness at a rather constant value of approximately 2.5 micrometers need further investigation. This layer may not be much related to initial lens comfort, as it is not as dynamic as the PLTF during this period. However, this layer is probably more important in terms of the development of other contact lens-related complications, such as mechanical and inflammatory events, especially during eye closure. 

In summary, the PLTF thickness was significantly reduced over the course of the first 30 minutes of lens wear; the initial thinning was considerably more rapid in some subjects than in others. The apparent thickness of the contact lens was also reduced, but part of this decrease may have been due to improved lens centration and subsequent measurement through a thinner part of the lens. The PoLTF thickness remained relatively unchanged, although individual subjects showed significant thickening or thinning. It is important to note that these measures relate to the central area of the contact lens, rather than the more peripheral regions. Tear dynamics in those areas remain elusive as methods have not been developed to study those regions, particularly for the PoLTF. It is unclear how these estimates of thickness in the central region relate to the peripheral areas. Additional studies are needed to address the impact of wearing time (e.g., a full day of lens wear) on the thickness of each of these layers, particularly with different types of hydrogel materials (i.e., silicone hydrogel vs. traditional hydrogel). It also should be noted that lenses in this study were removed directly from the blister pack and placed on the eye, rather than after soaking in a care solution. It could be that wearing hydrogel lenses after they are soaked in such a care or wetting solution may be associated with more stable thickness values, particularly for the contact lens. Further studies will need to be conducted in the future, especially to determine the relation between wearing comfort and the thickness of these layers. 

 Supported by grants from Advanced Medical Optics, Inc. (JJN, PEK-S), NIH/NEI K23 EY13766–02 (JJN), and Ohio Lions Eye Research Foundation (PEK-S).

 Submitted for publication February 12, 2004; revised March 3, 2004; accepted March 7, 2004.

 Disclosure: J.J. Nichols, Advanced Medical Optics (F); P.E. King-Smith, Advanced Medical Optics (F)

 The publication costs of this article were defrayed in part by page charge payment. This article must therefore be marked “advertisement” in accordance with 18 U.S.C. §1734 solely to indicate this fact.

 

Figure 1.

 

Prelens tear thickness as a function of time after contact lens insertion, derived from combined data for all 12 subjects. The fitted curve is an exponential decay plus a constant.

Figure 2.

 

(A–C) Prelens tear film thickness as a function of time for three subjects.

Figure 3.

 

Contact lens thickness as a function of time after contact lens insertion, derived from combined data for all subjects. The solid line is a linear fit to the data. The dashed curve is an exponential decay plus a constant, corresponding to the shortest time constant in the 95% confidence interval.

Figure 4.

 

Postlens tear film thickness as a function of time after contact lens insertion, derived from combined data for all subjects. The solid line is a linear fit to the data.

Figure 5.

 

(A–B) Postlens tear film thickness as a function of time for two subjects.