Effect of intraocular lens asphericity on posterior capsule opacification between two intraocular lenses with same acrylic material: a fellow-eye study

Authors


Mayank A. Nanavaty, DO, MRCOphth, MRCSEd
Department of Ophthalmology
St Thomas’ Hospital
London SE1 7EH
UK
Tel: + 020 7188 4331
Fax: + 020 7188 4318
Email: mayank_nanavaty@hotmail.com

Abstract.

Purpose:  To evaluate intra-individual differences in posterior capsule opacification (PCO) and visual performance between spherical AcrySof SN60AT and an aspheric AcrySof SN60WF intraocular lens (IOL) with a posterior aspheric surface, both of which are made of same hydrophobic acrylic material.

Setting:  Ophthalmology Department, St Thomas’ Hospital, London, UK.

Methods:  In this prospective randomized, fellow-eye comparison, an aspheric IOL, which is 9% thinner in comparison with the spherical IOL, was randomized to the first eye of 47 patients and fellow-eye surgery was performed within 3 weeks. Follow-up was at 1, 3, 6, 12 and 24 months. Corrected logMAR visual acuity (CDVA) was measured at 100% and 9% contrast. After pupil dilation, digital retroillumination photographs were taken and the mean PCO percentage was calculated using poco software at each follow-up visit.

Results:  At 1, 3, 6, 12 and 24 months, 47 (94 eyes), 44 (88 eyes), 42 (84 eyes) and 41 (82 eyes) patients were followed-up respectively. Hundred per cent and 9% of LogMAR CDVA was not significantly different between the two IOLs (p = NS at all time-points). Percentage area PCO scores (mean ± SD) at 1, 3, 6, 12 and 24 months with the spherical IOL was 5.82 ± 9.89, 7.76 ± 16.83, 7.21 ± 12.46, 9.29 ± 18.25 and 14.39 ± 25.42, respectively, and with an aspheric IOL was 8.91 ± 12.79, 5.97 ± 10.32, 5.15 ± 7.92, 7.68 ± 11.18 and 12.18 ± 20.10, respectively (p = NS at all time-points).

Conclusions:  Posterior capsule opacification was not significantly different between the spheric and aspheric IOLs in this fellow-eye, randomized comparison. Additional asphericity on the existing model of IOL does not influence PCO performance.

Introduction

Cataract surgery has evolved from a sight saving procedure to a refractive procedure in which quality of vision and optical outcomes are of crucial importance. Posterior capsule opacification (PCO) still remains the main complication of cataract surgery. The development is multifactorial, involving patient factors, surgical technique (Apple et al. 2000; Davidson et al. 2000; Ram et al. 2001) and intraocular lens (IOL) design and biomaterial (Ursell et al. 1998; Hollick et al. 1999; Georgopoulos et al. 2003; Wejde et al. 2003; Heatley et al. 2005). Clinical studies show that IOLs with a square-edged optic profile are associated with less PCO than those with a round-edged profile (Nagamoto & Eguchi 1997; Nishi & Nishi 1999; Nishi et al. 2000; Peng et al. 2000; Hayashi & Hayashi 2005; Nanavaty et al. 2008).

IOL technology improves at a rapid pace. Newer aspheric IOL designs reduce spherical aberration and improve contrast sensitivity, and various aspheric IOL designs are available which correct corneal spherical aberration completely, partially or are neutral to the pre-existing corneal spherical aberration. These newer aspheric IOLs are different in design to their spheric equivalent with different optical surfaces and thicknesses of the IOL. There are numerous published studies on the visual and optical performances of these aspheric IOLs (Packer et al. 2002, 2004; Kershner 2003; Mester et al. 2003; Rocha et al. 2006; Denoyer et al. 2007; Pandita et al. 2007; Tzelikis et al. 2007, 2008; Nanavaty et al. 2009; Lee et al. 2011), but there is a paucity of published literature on influence of the IOL asphericity on PCO.

PCO performance of spheric AcrySof SN60AT IOL has already been published (Hancox et al. 2008). We designed this prospective, randomized, fellow-eye controlled study to compare the spherical AcrySof SN60AT and an aspheric AcrySof SN60WF (Alcon Labs, Fort Worth, TX, USA), which are both single-piece IOLs made of same acrylic material, to evaluate the impact of asphericity on visual acuity and PCO outcomes.

Material and Methods

This is a prospective, randomized, fellow-eye controlled study performed on patients undergoing phacoemulsification for bilateral senile cataracts between November 2006 and July 2007 at the Department of Ophthalmology, St Thomas’ Hospital, London. The study was approved by the Hospital’s Ethics Committee and followed the tenets of the Declaration of Helsinki. This study is registered on http://www.clinicaltrials.gov (Registration number: NCT00762021).

The inclusion criterion was uncomplicated age-related bilateral cataract with the potential to see 20/40 or better in each eye. Exclusion criteria were any concurrent medication apart from ocular lubricants, any coexisting ocular pathology, unilateral amblyopia, previous intraocular surgery or laser treatment, retinal complications, pupil dilatation <7 mm, any surgical complications or inability to co-operate or maintain follow-up. A complete preoperative ophthalmic examination was performed including slit lamp and fundus evaluation after mydriasis. Axial length was measured by partial coherence interferometry (IOLMaster; Carl Zeiss Meditec, Oberkochen, Germany); if this was not possible because of the density of the cataract, it was measured by ultrasonography.

All eyes had a standardized phacoemulsification procedure using a ‘stop and chop’ technique performed by the same surgeon (DJS). During the surgery, particular care was taken to make sure that the size of the capsulorhexis was fashioned to overlap the IOL optic edge for 360° in all eyes. The randomization plan was generated by computer prior to the study, and either the spherical IOL (AcrySof SN60AT) or an aspheric IOL (AcrySof SN60WF, both from Alcon Laboratories) was injected through a 2.75 mm temporal clear corneal incision in the first eye and the other IOL to the second eye within 3 weeks. All patients were treated with G Maxitrol (Alcon Laboratories) eye drops four times a day for 3 weeks after surgery.

Both the spherical AcrySof SN60AT and the aspheric AcrySof SN60WF are single-piece hydrophobic acrylic IOLs with yellow chromophobe with 6 mm optic diameter and 13 mm overall length. The design of aspheric AcrySof SN60WF is similar to AcrySof SN60AT except that it is 9% thinner and it has a posterior aspheric surface (Trueb et al. 2009) that corrects 0.17 μm of spherical aberration.

All patients were examined postoperatively at 1, 3, 6, 12 and 24 months. At all follow-up visits, the patients had a manifest subjective refraction and best corrected distance visual acuity was assessed for 100% and 9% contrast sensitivity by ETDRS LogMAR visual acuity chart at 4 m.

Pupils were then dilated with G Tropicamide 1% and G Phenylephrine hydrochloride 10% eye drops. A digital retroillumination image of each eye was taken at each visit with a dedicated retroillumination camera system. The images were analysed with poco software (Barman et al. 2000) to measure the percentage area of PCO in the capsulorhexis area. Capsulorhexis size was also quantified by measuring the area within the capsulorhexis (pixels) in each image. This area was then converted to square millimetres (1 mm2 = 40572 pixels).

Statistical analysis

Sample size was calculated for comparison of two independent groups. For this calculation, the mean PCO of group one (AcrySof SN60AT in this study) was presumed to be nine (Hancox et al. 2008) and mean PCO for group two (AcrySof SN60WF in this study) was speculated to be 18. A standard deviation of 15 was presumed for both groups. For achieving 80% power with the type 1 error of 5%, the sample size was calculated to be 44 in each group. Considering 18–20% dropout at follow-up visits, we decided to enrol 52 patients bilaterally.

All data were entered on to Excel spreadsheets (Microsoft Office 2003). All further calculation and statistical analysis was carried out using standard software [Excel (Microsoft Office 2003)]. Normal distribution of the data was checked using Kolmogorov–Smirnov Test. A paired t-test (two tailed) was employed to analyse the data; p < 0.05 was considered to be significant.

Results

There were 104 eyes of 52 patients recruited in this study. The mean age was 71.7 ± 11.9 years. All the eyes had in-the-bag implantation of IOL. No intraoperative complications were noted in eyes, which were included in this study. Five patients were excluded at 1-month follow-up because one patient developed prolonged postoperative inflammation following first eye surgery that had AcrySof SN60WF IOL. However, we do not believe that this was caused by the IOL as he had similar inflammation in the second eye, which was performed later with AcrySof SA60AT IOL, once excluded from the study. One patient moved out of the city, two withdrew from follow-up because of change in personal circumstances after surgery, and one was noted to have residual posterior capsule plaque with required Nd:YAG laser capsulotomy at 1 month. Therefore, forty-seven patients (94 eyes) attended follow-up at 1 month postoperative visit. Forty-four patients (88 eyes) attended the 3-month and 6-month follow-ups because two patients were unable to attend after cerebrovascular accidents, and one patient declined follow-up. Forty-two patients (84 eyes) attended follow-up at 1 year because another patient died 11 months after surgery and one declined follow-up at 1 year. None of the eyes required Nd:YAG capsulotomy until 2 years. At 2 years, one patient required Nd: YAG laser capsulotomy in the eye with the spheric IOL and one patient required Nd:YAG laser capsulotomy in both eyes. For the purpose of analysis, the data before performing Nd:YAG capsulotomy, for these eyes, were included. This gives a Nd:YAG laser capsulotomy rate of 4.26% (two eyes out of 47) with spheric IOL and 2.13% (one eye out of 47) with the aspheric IOL at 2 years.

The data were found to be normally distributed using Kolmogorov–Smirnov Test. There was no significant difference in keratometry, axial length, IOL power and postoperative spherical equivalent between the two IOL groups (Nanavaty et al. 2009). There was no significant difference in 100% and 9% LogMAR CDVA between the IOL groups at any follow-up visit (Table 1). There was no significant difference in mean capsulorhexis area and percentage area PCO at any follow-up visits between the two IOL groups (Table 2).

Table 1.   100% and 9% LogMAR corrected distance visual acuity (CDVA) comparison between the groups with spheric AcrySof SN60AT and aspheric AcrySof SN60WF at follow-up visits.
Follow-up visitsAcrySof SN60AT
[mean ± SD (95% CI)]
AcrySof SN60WF
[mean ± SD (95% CI)]
p-value*
100% LogMAR CDVA9% LogMAR CDVA100% LogMAR CDVA9% LogMAR CDVA100% LogMAR CDVA9% LogMAR CDVA
  1. SD= standard deviation; CI= confidence interval.

  2. * Paired t-test.

1 month0.02 ± 0.1 (−0.02, 0.06)0.38 ± 0.2 (0.32, 0.44)0.00 ± 0.1 (−0.03, 0.03)0.32 ± 0.2 (0.26, 0.38)0.210.05
3 months−0.02 ± 0.1 (−0.05, 0.01)0.39 ± 0.2 (0.33, 0.45)−0.01 ± 0.1 (−0.04, 0.02)0.34 ± 0.2 (0.28, 0.40)0.080.08
6 months−0.02 ± 0.1 (−0.04, 0.00)0.34 ± 0.1 (0.30, 0.38)0.01 ± 0.1 (−0.02, 0.04)0.33 ± 0.1 (0.29, 0.37)0.070.56
12 months−0.04 ± 0.1 (−0.07, −0.01)0.35 ± 0.2 (0.30, 0.40)−0.02 ± 0.1 (−0.02, 0.04)0.32 ± 0.2 (0.29, 0.37)0.160.18
24 months0.04 ± 0.1 (0.00, 0.08)0.42 ± 0.2 (0.36, 0.48)0.07 ± 0.2 (−0.02, 0.04)0.45 ± 0.2 (0.29, 0.37)0.160.39
Table 2.   Mean percentage posterior capsule opacification (PCO) scores and capsulorhexis area (mm2) comparison between the groups with spheric AcrySof SN60AT and aspheric AcrySof SN60WF at follow-up visits.
Follow-up visitsAcrySof SN60AT [mean ± SD (95% CI)]AcrySof SN60WF [mean ± SD (95% CI)]p-value*
Percentage PCO score (%)Capsulorhexis area (mm2)Percentage PCO score (%)Capsulorhexis area (mm2)Percentage PCO score (%)Capsulorhexis area (mm2)
  1. SD= standard deviation; CI= confidence interval.

  2. * Paired t-test.

1 month5.82 ± 9.89 4.82, 6.83)17.80 ± 3.54 17.44, 18.16)8.91 ± 12.79 7.60, 10.21)17.80 ± 2.63 17.53, 18.06)0.050.74
3 months7.76 ± 16.83 6.05, 9.47)17.83 ± 3.45 17.48, 18.19)5.97 ± 10.32 4.92, 7.02)18.05 ± 2.95 17.75, 18.35)0.490.68
6 months7.21 ± 12.46 5.94, 8.48)18.09 ± 3.62 17.72, 18.46)5.15 ± 7.92 4.34, 5.95)18.19 ± 3.02 17.89, 18.50)0.270.80
12 months9.29 ± 18.25 7.43, 11.14)18.66 ± 3.36 18.32, 19.01)7.68 ± 11.18 6.54, 8.81)18.79 ± 3.04 18.49, 19.10)0.420.79
24 months14.39 ± 25.42 11.80, 16.97)18.23 ± 3.48 17.87, 18.58)12.18 ± 20.10 10.13, 14.22)18.77 ± 2.70 18.50, 19.05)0.670.60

Figure 1 shows the cumulative frequency of percentage of eyes with PCO scores between 0–10%, 10–20% and >20% at 2 years. The majority of eyes in both groups had <10% PCO score. Figure 2 shows the difference in percentage PCO score between the spheric and aspheric IOLs. The difference in percentage PCO score between the spheric and aspheric IOLs was within −1% and 1% in 40% of eyes.

Figure 1.

 Cumulative frequency of percentage eyes with different posterior capsule opacification scores at 2-year follow-up visit.

Figure 2.

 Cumulative frequency of percentage of eyes versus difference in posterior capsule opacification scores at 2-year follow-up visit.

Discussion

While the struggle to eliminate PCO goes on, technological advancements in the field of cataract and refractive surgery have led to the development of newer aspheric IOLs that are well recognized to improve mesopic contrast sensitivity. Aspheric IOLs have modified aspheric IOL optic surfaces, either anteriorly or posteriorly, depending on the manufacturer, which inevitably changes the design of the IOL. In a randomized, fellow-eye comparison of two multifocal IOLs of same diffractive design but of different materials, Hütz et al. (2010) found no difference in visual performance. There is a paucity of information on whether the aspheric design of the IOL could cause clinically significant change in PCO performance. We designed this prospective, randomized, fellow-eye controlled study to analyse the effect of IOL asphericity on PCO by comparing two IOLs, which are made of same hydrophobic acrylic. They have similar design except that the aspheric IOL is 9% thinner (Trueb et al. 2009) and has a posterior aspheric surface (Fig. 3).

Figure 3.

 Environmental scanning electron microscopy pictures of the optic edge thickness of spherical AcrySof SN60AT and an aspheric Acrysof SN60WF (reproduction of images from a previous project –Nanavaty et al. 2008).

In this study, we found no difference in 100% and 9% LogMAR CDVA, capsulorhexis area and mean PCO scores between each IOL up to 2 years postoperatively. Biber et al. (2009), in their recent published study, compared PCO rates and its visual significance between three IOL models (multifocal spherical AcrySof SN60D3, monofocal spherical AcrySof SN60AT and monofocal aspherical AcrySof SN60WF) in 225 eyes (75 in each group), all of which were single-piece hydrophobic acrylic IOLs. Patients had a mean follow-up of 15.9 ± 6.5 months. Posterior capsule opacification was diagnosed clinically by retrospective chart review, and the length of time that this occurred after surgery was recorded. The PCO rate was found to be 42.7% at 13.1 ±7.1 months postoperatively in the multifocal spherical group, 28.0% at 14.9 ± 9.5 months in the monofocal spherical group, and 14.7% at 10.3 ± 5.3 months in the monofocal aspheric group. The YAG capsulotomy rate with the multifocal spherical group (25.3%) and monofocal spherical group (17.3%) was higher than the monofocal aspheric group (4%). Although they do not report a significant difference in mean postoperative duration of YAG laser capsulotomy, laser capsulotomy was performed earlier in the monofocal aspheric IOL groups (approximately 9 months postoperatively) compared to multifocal spherical and monofocal spherical IOL (approximately 13 months postoperatively). In our current study, we had two eyes out of 47 (4.26%) with spheric IOL and one eye out of 47 (2.13%) with aspheric IOL, which required laser capsulotomy for visually significant PCO at 2 years. It is not justifiable to compare our capsulotomy rates statistically as the number of eyes requiring capsulotomy was very small. YAG capsulotomy rates with other aspheric IOLs such as the Akreos AO IOL (Bausch and Lomb, Rochester, NY, USA) are reported to be 35% at 1 year by Alió et al. (2009) and 1.4% with Tecnis Z9000 IOL (Abbott Laboratories, Abbott Park, North Chicago, IL, USA) at approximately 40 months by Ram et al. (2009) whereas in a study on the Restor (AcrySof SN60D3; Alcon Laboratories) multifocal IOL Shah et al. (2010) found a capsulotomy rate of 15.49% at a mean follow-up of 22 months. None of these studies can be compared because of the huge variation in the IOL design and material, lack of universal guidelines for performing laser capsulotomy, individual patient requirements, surgeon perception, availability of equipment and financial factors.

Biber et al. (2009) postulated that the reduction in PCO with aspheric IOLs in their study was because of the difference in design of the IOLs. They state that the posterior surface of the aspheric IOL is more convex centrally than that of the relatively flat spherical IOLs and postulate that a decrease in the convexity of the posterior surface of the spherical; IOL reduces the potential for contact between the IOL and the posterior capsule thereby creating a potential space for lens epithelial cells to migrate and proliferate (Biber et al. 2009). This is, in fact, erroneous as the AcrySof SN60WF has an aspheric posterior surface which is less convex that the anterior surface of the same IOL or the posterior surface of the spheric AcrySof SN60AT IOL, and we feel that their results probably reflect the retrospective study design.

In the current prospective, randomized, fellow-eye comparative study, we did not find any difference in PCO score at all follow-up visits up to 2 years postoperatively. This clearly shows that the small difference in design of the aspheric IOL does not produce clinically significant difference in PCO performance. Although glistenings are associated with hydrophobic acrylic material (Mönestam & Behndig 2010), none of the eyes in this study showed significant glistening to affect visual function until 2 years. Like any surgical study, it is hard to implement a double mask design to this prospective, randomized study and this could be a potential limitation.

Although the aspheric IOL is thinner than the spherical IOL, the edge profile and edge thickness are very similar (Fig. 3). We believe that a square edge profile produces its effect by compression of the edge against the posterior capsule as the bag fibroses and collapses after surgery thereby creating a mechanical pressure barrier to the lens epithelial cell migration (Boyce et al. 2002). In theory, reduced IOL thickness could cause less compression of the IOL against posterior capsule as there is less IOL volume to push against the capsule. If this is the case, a reduction of 9% does not appear to affect PCO performance.

In summary, we did not find that the addition of asphericity to this IOL design influences the PCO performance for the first 2 years after surgery.

Acknowledgements

Supported from a grant from Alcon Laboratories, Fort Worth, Texas, USA. David J. Spalton and James Boyce are consultant to Alcon Laboratories, Fort Worth, Texas, USA. Presented at European Society of Cataract & Refractive Surgery Conference, Barcelona, 2009.

Ancillary