Retinal Straylight before and after Implantation of the
Bag in the Lens IOL

Abstract

Purpose.: To quantify the changes in retinal straylight that occur after implantation of the Bag in the Lens (BIL) IOL, which by design avoids the formation of posterior capsule opacification or any influence of the posterior lens capsule.

Methods.: This prospective study included 81 eyes of 53 cataract patients planned for surgery with the BIL. Preoperatively and 6 months postoperatively their straylight level was determined using the compensation comparison method.

Results.: After implantation of the BIL straylight significantly improved from 1.59 ± 0.26 preoperatively to 1.19 ± 0.21 log units postoperatively (paired t-test, P < 0.001). Postoperative straylight was within age-normal levels in 56.1% of the eyes and went below age-normal values in 40.2% of the eyes. Average postoperative straylight remained 0.27 log units (or 1.87×) above the age-independent base straylight value of 0.931 + 0.200. Postoperatively straylight was not significantly correlated with age (r ² = 0.001), but it was significantly correlated with a regression model that combines both age and axial length (r ² = 0.199).

Conclusions.: Retinal straylight after implantation of the BIL is comparable to what has been published for the early follow-up of other IOL types, suggesting that a clear posterior lens capsule does not seem to add to straylight. However, for all IOL types average postoperative straylight remains above the expected base level, possible due to nonlenticular age-related parameters or to the physiological response of the ocular tissues to the surgical act. This should be examined in further detail in future studies.

Introduction

Intraocular straylight is present to some degree in all eyes. In healthy eyes it is known to remain constant until approximately 45 years of age, after which it gradually increases as the patient gets older, ^1–3 mostly due to an increase in the light scatter of the crystalline lens. Straylight is often more pronounced in individuals with a light iris color or low skin pigmentation. ^2,4 It is known to increase in myopic eyes ⁵ and to decrease after refractive correction by either laser surgery ^6–8 or using phakic intraocular lenses. ⁹ The origin of this effect is not yet understood. 

Cataract significantly increases straylight in the eye, depending on cataract type and density. ^10,11 Since it is well known that straylight gradually increases with age, cataract can therefore be considered as a rapid aging of the crystalline lens as far as light scatter is concerned. ² From this viewpoint cataract surgery would be a return of the straylight to a level determined only by the nonlenticular structures of the eye and the intraocular lens (IOL). Pseudophakic eyes could therefore be used to study light scatter by these nonlenticular structures. However, in practice straylight measurements in pseudophakic eyes are influenced by the lens capsule and posterior capsule opacification (PCO), ^3,12–14 a transformation and proliferation of lens epithelial cells left behind after cataract surgery. This may cause the postoperative straylight to be higher than expected, ^15–17 severely hindering any attempt to study the nonlenticular origins of straylight. 

PCO or visual axis reproliferation was solved with the introduction of the Morcher 89A Bag in the Lens IOL (BIL), which has been shown to eliminate the risk of visual axis reproliferation due to its special design and implantation technique using both an anterior and posterior capsulorhexis. ¹⁸ Moreover, this lens type is not subjected to the effects of capsular changes over time, ¹⁹ resulting in a good stability in terms of postoperative shifts ²⁰ or rotations. ²¹ The BIL is therefore ideal to study straylight changes after cataract surgery and verify whether nonlenticular parameters may influence straylight as a function of age, which is the aim of this study. 

Patients and Methods

Patients

All patients were recruited at the Ophthalmology Department of the Antwerp University Hospital. Exclusion criteria were a history of previous ocular surgery, profound amblyopia, corneal scars or haze, vitreous floaters, systemic diseases (e.g., diabetes and systemic macular diseases), and any postoperative complications of the cataract surgery. Patients were tested preoperatively and 6 months postoperatively. 

The study adhered to the tenets of the Declaration of Helsinki and received approval of the ethical committee of the Antwerp University Hospital (ref. no. 9/53/297). Informed consent was obtained from participating subjects prior to testing. 

Methods

The retinal straylight measurements in this work were performed with the C-Quant (Oculus Optikgeräte, Wetzlar, Germany), a commercial version of the compensation comparison technique proposed by the research group of van den Berg. ^22,23 Only subjects with preoperative and postoperative straylight measurements of acceptable quality were included (i.e., a repeated measures standard deviation parameter, Esd, below 0.08 and a measurement quality parameter, Q, above 0.5). ²⁴  

In healthy eyes retinal straylight has been shown^1,2 to increase with the fourth power of age after the age of 45 and can be modeled as follows³:  

 

which can be considered as the age-normal curve, supplemented by a range of ±0.2 log units to account for the normal biological variation in straylight. By subtracting Equation 1 from the measured straylight it is possible to define the “base and age-corrected” (BAC) straylight, which compensates for the age related straylight increase.⁵ Moreover, since axial length L also influences retinal straylight, a “base, age, and axial length-corrected” (BALC) straylight can be defined as well as the difference between the following equation and the measured straylight value⁵:  

 

However, as the age-related increase in straylight is mostly due to changes in the crystalline lens, Equations 1 and 2 can no longer be used after cataract extraction. We therefore modeled this using the reference regression recently proposed by van Bree et al.¹⁴ for pseudophakic eye without posterior capsule opacification:  

 

which has a 95% confidence interval of ±0.35 log units. Subtracting Equation 3 from the pseudophakic straylight measurement, we can define a “pseudophakic base and age corrected” straylight BAC_IOL.  

In addition to straylight measurements, the axial length was determined with the IOL Master (Version 2; Carl Zeiss, Jena, Germany), the refraction with the AR-700 autorefractometer (Nidek, Gamagori, Japan), and the corrected distance visual acuity (CDVA) in decimal notation with a retro-illuminated Bailey-Lovie high contrast acuity chart (ETDRS) at a distance of 4 m from the patient. The IOL volume as a function of power was calculated using the thickness, diameter, and radii of curvature of the lenses, which were provided by the manufacturer (Morcher GmbH, Stuttgart, Germany). 

Statistical Methodology

All data were processed using MS Excel 2003 (Microsoft, Redmond, WA) and SPSS 12.0 (IBM, Armonk, NY). The significance levels for all statistical tests were chosen at 0.05. There were three eyes of two young patients with ages far below the average age of 70.9 ± 13.8 years. Being outliers from the main group, these eyes would have a disproportionally large impact on age-based analyses. We therefore decided to exclude these three eyes from statistical analyses, but we included them in the figures for illustrative purposes. 

Results

Patients

This prospective study includes data from 81 eyes of 53 cataract patients that were planned for implantation with the BIL IOL. The preoperative subject and biometry data of this group are given in Table 1. 

Table 1. 

 

Patient Data

Parameter	Value
No. of subjects	53
Male/female	23/30
Age, y, mean ± SD (range)	70.9 ± 13.8 (14.3, 86.7)
No. of eyes	81
Right/left eyes	45/36

Changes in Visual Acuity and Spherical Equivalent Refraction

All of the included eyes underwent an uneventful cataract extraction and IOL implantation. Six months postoperatively CDVA had improved significantly from 0.25 ± 0.18 preoperatively to 0.03 ± 0.08 (paired t-test; P < 0.001) postoperatively (Table 2). The spherical equivalent (SE) refraction improved markedly from a range of −14.00 to +5.50 D preoperatively to a range of −2.50 to +1.00 D postoperatively. 

Table 2. 

 

Pre- and Postoperative Data for 81 Eyes

	Preop	Postop	Paired t-test
SE refraction, D*	−0.59 ± 3.64 (−14.00, +5.50)	−0.57 ± 0.67 (−2.50, +1.00)	P = 0.992
CDVA, logMAR*	0.25 ± 0.18 (−0.10, 0.69)	0.03 ± 0.08 (−0.10, 0.30)	P < 0.001†
Straylight log(s)*	1.59 ± 0.26 (0.93, 2.15)	1.19 ± 0.21 (0.73, 1.68)	P < 0.001†
Straylight above age normal‡	58.0%	3.7%
Straylight within age normal‡	35.8%	55.6%
Straylight below age normal‡	6.2%	40.7%
Straylight between age normal‡ and base + 0.200	3.6%	7.2%
Straylight below base + 0.200	4.9%	43.2%

*  Mean ± SD (range).

†  Significant change: P < 0.05.

‡  Age normal: Equation 1 ± 0.200.

Comparison of Straylight before and after BIL Implantation

Cataract extraction and BIL implantation caused the retinal straylight to decrease significantly by −0.40 ± 0.29 log units from 1.59 ± 0.26 to 1.19 ± 0.21 log units (paired t-test; P < 0.001; Table 2; Fig. 1). Preoperative straylight was above age normal (i.e., Equation 1 + 0.2) for 58.0% of the eyes (Table 2). Postoperatively this reduced to 3.7% of the eyes, all of which had an axial length longer than 27.50 mm. The number of eyes in the age-normal range (Equation 1 ± 0.2) increased from 35.8% to 55.6%, and the percentage of eyes with a below age-normal straylight (i.e., Equation 1 − 0.2) increased from 6.2% to 40.7%. 

Figure 1. 

 

Straylight values before and after BIL implantation for 81 eyes in comparison with the reference of Equation 1 and the base value (i.e., 0.931 ± 0.200).

Effect of Cataract on Straylight

Following the assumption that cataract is a condition with an effect on straylight that goes beyond what is normal for the patient's age and biometry, the influence of cataract on straylight can be estimated by subtracting Equation 2 from the preoperative straylight to correct for the age-related and axial length components (Fig. 2). Excluding the three eyes younger than 50 years from the analysis, the 78 remaining eyes older than 50 years showed a slight but significant decrease in BALC straylight with age (r ² = 0.052; P < 0.045). The BALC straylight in the three excluded young eyes was very high, which suggested that this decrease in influence of cataract with age may start before the age of 50. 

Figure 2. 

 

Preoperative BALC straylight to estimate the influence of cataract on straylight for 78 eyes older than 50 years (solid markers). The BALC straylight for three eyes younger than 50 were excluded from the regression, but shown for illustrative purposes (open markers).

Correlation between Straylight and Visual Acuity

Before cataract surgery no significant correlation between straylight and CDVA was found (r ² = 0.028, P = 0.133). However, after BIL implantation this correlation became statistically significant [r ² = 0.156, P < 0.001, log(s) = 0.9905·CDVA + 1.1532]. This is also seen in Figure 3. 

Figure 3. 

 

Straylight values before and after BIL implantation for 81 eyes as a function of CDVA. Horizontal lines indicate normal age-related straylight increase in phakic subjects.

Influence of Age on Straylight in Pseudophakic Eyes

Excluding the three eyes younger than 50 years, neither a preoperative nor a postoperative correlation of straylight with age were found to be significant (Fig. 4; preop: r ² < 0.001, P = 0.971; postop: r ² < 0.001, P = 0.787). However, most of the data, including the three young eyes, remained within the 95% confidence interval of the pseudophakic reference Equation 3 by van Bree et al. ¹⁴  

Figure 4. 

 

Straylight in 81 eyes implanted with the BIL as a function of age (solid markers) in comparison with the regression by van Bree et al. ¹⁴ Dashed lines indicate the 95% confidence interval of the van Bree data. The postoperative straylight for three eyes younger than 50 are shown for illustrative purposes (open markers).

Influence of Axial Length on Straylight in Pseudophakes

In pseudophakes the second term of Equation 1 no longer correctly describes the age-dependent increase in retinal straylight since the crystalline lens, the largest contributor to this term, is no longer present in these eyes. Instead the BAC_IOL straylight (Equation 3) was used to investigate whether the axial length dependency of straylight (Equation 2) is still valid after IOL implantation. As is seen in Figure 5 this quadratic dependency is still significant (with P < 0.01 for the quadratic component), and almost coincides with the regression found in phakic eyes. By combining Equation 3 with the axial length regression from Figure 5, we can create a pseudophakic model that depends on both age and axial length:  

 

This improved the fit with our subjects older than 50 years from r ² < 0.001 (P = 0.787) for Equation 3 to r ² = 0.199 (P < 0.001) with the model given in Equation 4.  

Figure 5. 

 

Changes in BAC_IOL straylight in 81 eyes implanted with the BIL as a function of axial length in comparison with the BAC straylight fit in healthy phakic subjects (Equation 2).

Discussion

The influence of cataract on straylight was found to be very different for different age groups. In young cataract patients the straylight increase due to cataract was very large (from 0.93 to 2.00 log units), while in older cataract patients straylight values partially overlapped the age-normal range that gradually increases in healthy eyes (Fig. 1). In older cataract patients straylight values were found to be much more varied within a range from 0.93 to 2.15 log units, suggesting that there may be an upper limit for straylight around 2.15 log units in cataractous eyes without any additional pathology. 

Depending on the definition of cataract, these observations may be considered in two different ways. First, cataract could be interpreted as an age-related lenticular condition that increases straylight levels beyond what is best for the patient's age (i.e., age-normal). This definition would lead to the rather counterintuitive conclusion that the effect of cataract on straylight would significantly decrease with age (Fig. 2). Alternatively one could consider that the normal increase in lens opacities with age, the most likely cause for the age-normal straylight increase, may in fact be considered as a form of early cataract that could have an excessive effect on straylight. ¹⁰ Following this definition one would find that many people in the general population may already have a degree of cataract that increases straylight to levels that cause functional problems (e.g., difficulties with driving at night), even if their visual acuity is still normal. In extreme cases such straylight-related complaints in subjects with a good visual acuity could even justify cataract surgery. ²⁵  

Given that the crystalline lens is the main source of light scatter in the aging eye and that its influence increases even further in case of cataract, it is no surprise that the retinal straylight decreases considerably after the removal of a cataractous lens. In 56.1% of the eyes this resulted in postoperative straylight values that were within the age-normal limits, and in 40.2% of the eyes the straylight was below age-normal. Especially in this second group the postoperative result can be considered as a rejuvenation of the eye as far as straylight is concerned. 

In order to estimate the lowest possible pseudophakic straylight values, one has to know the amount of intrinsic straylight caused by a young and clear crystalline lens in vivo. Assuming a clear lens generates some straylight, which is reasonable given the cellular structure of the lens, part of the base term of 0.931 log units would be due to this scatter. Even though no straylight measurements of only the crystalline lens in vivo have been published to date, one would expect that after crystalline lens removal the postoperative straylight decreases to values clearly below the base term of 0.931. 

However, the average postoperative straylight was found to be 0.27 log units (or 1.87×) higher than the base term, which indicates that there are parameters that significantly influence postoperative straylight. Examples of such parameters could be nonlenticular age-related parameters or physiological changes due to the lens implantation procedure (e.g., changes in corneal or vitreal clarity). The exact influence of each of these factors on straylight should be examined in detail in future studies. 

Retinal factors, such as the gradual age-related degradation of retinal nerves, are unlikely to influence straylight measurements ²⁶ because these parameters do not cause straylight and process scattered and nonscattered light in the same manner. One possible exception is the optical density of the macular pigment, which absorbs incident light that might otherwise be backscattered by the sclera. But since macular pigmentation has been shown to remain (mostly) constant with age, ²⁷ and it filters both the retinal image and the scattered light in the same manner, it is unlikely that this has a significant effect on age-related increase in straylight. 

Another possible parameter that may influence straylight is the posterior lens capsule. This can easily be investigated by comparing the average postoperative straylight of the BIL with that of common “lens in the bag” IOLs as the implantation of the BIL requires a capsulorhexis in both the anterior and posterior lens capsule. Comparing our current results with the recent literature on straylight short after implantation of other IOL types (i.e., ≤6 months postoperatively to remain before the onset of PCO) shows that the results of the BIL are comparable with those of other lenses (Table 3). Since the main difference between the BIL and other IOLs is the presence of the posterior capsule, one can conclude that, in absence of PCO, the influence of the posterior capsule on retinal straylight is likely to be small in comparison to the measurement error of the C-Quant (i.e., 0.07 log units). ²⁴  

Table 3. 

 

Comparison with Literature (C-Quant Measurements Only)

Source, y	IOL Type	No. of Eyes	Follow-Up	Postop SL*†
Van Den Berg,3 2007	Various types	220	Various	1.24 (0.61, 1.95)
Cerviño, 292 2008	ThinOptX/Acri.Smart 48 S	32	6 mo	1.24 ± 0.24 (0.80, 1.68)
	AcrySof ReSTOR	35	6 mo	1.24 ± 0.30 (0.93, 1.97)
de Vries,17 2008	AcrySof SA60AT	44	6 mo	1.10 ± 0.19 (0.78, 1.60)
	AcrySof ReSTOR	60	6 mo	1.20 ± 0.16 (0.86, 1.61)
van Gaalen, 30 2010	Tecnis ZA9003	29		1.38 ± 0.26
	Tecnis ZA9003	29		1.38 ± 0.25
de Vries, 31 2010	AcrySof ReSTOR +3	68	6 mo	1.27 ± 0.16
	AcrySof ReSTOR +4	46	6 mo	1.19 ± 0.19
de Vries, 32 2010	AcrySof ReSTOR (S)	45	6 mo	1.16 ± 0.16
	AcrySof ReSTOR (A)	47	6 mo	1.19 ± 0.19
Ehmer,16 2011	AMO ReZoom	10	>3 mo	1.12 (0.95, 1.35)
	AMO Tecnis	10	>3 mo	1.28 (1.04, 1.76)
	Oculentis Mplus	10	>3 mo	1.13 (0.87, 1.51)
van Bree,14 2012	Various types	99		1.12 (0.58, 1.59)
Rozema, Present	Morcher 89A	81	>3 mo	1.19 ± 0.21 (0.73, 1.68)

 SL, straylight.

*  Log units.

†  Mean ± SD (range).

The correlation between straylight and CDVA was found to be nonsignificant in cataract patients prior to lens implantation and significant after lens implantation. This weak postoperative correlation is roughly comparable to the correlation found in phakic eyes, ³ confirming that also in pseudophakes visual acuity and straylight are to be considered as different aspects of the visual experience. 

The quadratic increase of the BAC straylight as a function of axial length found in phakic subjects ⁵ was also found for the BAC_IOL straylight in pseudophakes at almost the exact same rate. However, as for phakic eyes, the origin of this axial length dependency remains unclear for the moment. One possible explanation may be the thinning of the choroid in myopes, ²⁸ which causes the choroidal pigment to be more spread out. However, the connection between retinal straylight and choroidal thinning remains to be confirmed. 

Footnotes

 Disclosure: J.J. Rozema, None; T. Coeckelbergh, None; M. Caals, None; M. Bila, None; M.-J. Tassignon, None