Posterior capsule opacification (PCO) is a well-documented complication after cataract surgery, with a 1-year incidence of 2.0% to 15.0%1–3 and a 3-year incidence of 10.0% to 40%.4–6 Advancements in surgical techniques,7,8 intraocular lens (IOL) design,9,10 and pharmacologic treatments11,12 have been applied in an attempt to prevent this complication. Preliminary studies found the effectiveness of an IOL surface modification with ultraviolet–ozone (UV–O3) or argon plasma treatment in preventing PCO formation.13,14 These 2 techniques alter the adhesive property of the IOL and impede lens epithelial cell (LEC) migration and proliferation by increasing the adhesion between the IOL and the capsular bag. However, argon plasma was found to be inferior to UV–O3 treatment in preventing PCO and was associated with surface deterioration secondary to an etching effect.14
The aim of this study was to evaluate a new single-piece hydrophobic acrylic IOL with a development name of SP2 (marketed as Hoya iSert Vivinex, Hoya Surgical Optics, Inc.) made of a new hydrophobic acrylic polymer composition. This IOL is intended to exhibit negligible glistenings and is formed into final shape in a cast-molding process. The IOL underwent UV–O3 treatment on the posterior surface only and is intended to significantly reduce the incidence and severity of PCO, likely by improving the adhesion between the posterior IOL surface and the posterior capsule according to results of preliminary in vitro studies.13,14 We also evaluated uveal biocompatibility as well as the performance of neodymium:YAG (Nd:YAG) laser posterior capsulotomy with this surface-modified IOL.
Materials and methods
Ten New Zealand white rabbits of the same sex and weighing between 2.4 kg and 3.2 kg were acquired from approved vendors and treated in accordance with guidelines set forth by the Animal Welfare Act, the Association for Research in Vision and Ophthalmology, as well as the “Guide for the Care and Use of Laboratory Animals.” All eyes were grossly checked for the presence of anomalies before the surgical procedures and were found to be unremarkable. The study IOL was implanted in the right eyes of the rabbits; the control IOL was implanted in the left eyes. The SP2 IOL (Figure 1) comes sterile in a preloaded inserter. This IOL was compared with an identically prepared IOL except without the posterior surface treatment. The study IOL and control IOL had the same dioptric power (+20.0). The surgical procedures were performed by the same surgeon (N.M.) and recorded on a video system.
Anesthesia, surgical preparation, and bilateral phacoemulsification with IOL implantation were performed as described in previous studies.15,16 Briefly, a fornix-based conjunctival flap was fashioned. A corneoscleral incision was then made using a crescent blade, and the anterior chamber was entered with a 3.0 mm keratome. A capsulorhexis forceps was used to create a well-centered continuous curvilinear capsulorhexis (CCC) with a diameter of approximately 5.0 mm. After hydrodissection, the phacoemulsification handpiece (Alcon Infiniti System) was inserted into the posterior chamber for removal of the lens nucleus and cortical material. One milliliter of epinephrine 1:1000 and 0.5 mL of heparin (10 000 USP units/mL) were added to each 500 mL of irrigation solution to facilitate pupil dilation and control inflammation. The residual cortex was then removed with the irrigation/aspiration handpiece. An ophthalmic viscosurgical device (OVD) (sodium hyaluronate 1.6% [Amvisc Plus]) was used to expand the capsular bag. The IOLs were then implanted in the capsular bag using the corresponding recommended injection system. The IOLs were rotated manually in the capsule to qualitatively compare in vivo IOL material adhesion to the capsular bag of the surface-treated IOLs versus the untreated IOLs, and a description of the differences was taken. The wound was closed with a 10-0 monofilament nylon suture after removal of the OVD using the phacoemulsification handpiece. Appropriate in-the-bag placement of the IOLs, IOL centration, and coverage of the IOL optic by the capsulorhexis were verified at the end of the procedure.
Neomycin–polymyxin B sulfates–dexamethasone ointment was administered after the surgery and during the first postoperative week only. Prednisolone acetate drops were administered during the second postoperative week and then discontinued.
The eyes were dilated and evaluated by slitlamp examination for ocular inflammatory response 1, 2, 3, 4, 5, and 6 weeks postoperatively. Clinical color photographs of each eye at each time point were obtained with a digital camera attached to the slitlamp. A standard scoring method in 11 categories was used at each examination, including assessment of corneal edema and the presence of cells and flare in the anterior chamber. Retroillumination images with the pupil fully dilated were obtained for photographic documentation of CCC size, anterior capsule opacification (ACO), PCO, and capsule fibrosis.
After the 4-week postoperative slitlamp examination, 5 of the rabbits had bilateral Nd:YAG laser posterior capsulotomy (round shape configuration and a diameter of 3.0 to 4.0 mm) to assess for significant differences in the capsulotomy performance between study eyes and control eyes. The capsulotomy was carefully centered behind the IOL optic in each eye.
After the final clinical examination at 6 weeks, the animals were anesthetized and then humanely killed with a 1 mL intravenous injection of pentobarbital sodium–phenytoin sodium (Euthasol). Their globes were enucleated and placed in 10% neutral buffered formalin. The globes were then bisected coronally just anterior to the equator. Gross examination and photographs from the posterior aspect (Miyake-Apple view) were performed to assess ACO and PCO development as well as IOL fixation and other parameters. The extent and severity of ACO and PCO were scored according to methods established at our laboratory.17,18
After gross examination and photographs, all globes were sectioned and the anterior segments including the capsular bags were processed for standard light microscopy and stained with hematoxylin–eosin.
Overall, the injection procedures were unremarkable in all rabbit eyes. No significant differences were noted in surface tackiness or ease of rotation of the IOL inside the capsular bag during surgery between the 2 IOLs. In all eyes (study and control), 100% coverage of the IOL optic periphery by the CCC was observed at the end of the surgical procedure.
The slitlamp examination at 1 week showed a mild inflammatory reaction with mild fibrin formation in front of the IOL in practically all eyes with the study IOL and eyes with the control IOL. This fibrin formation resolved by the 2-week slitlamp examination except in the case of 1 study IOL, in which the fibrin persisted until the 4-week examination. This eye also had blood in front of the IOL as well as a mild hyphema, observed since the 1-week examination. The 2-week examination showed mild honeycomb PCO generally starting at the optic–haptic junctions in both IOL groups. There was also protrusion of cortex and pearl material in the front of the IOL in some eyes in both groups, leading to synechia formation in some eyes by the 4-week examination. The surface of some IOLs in both groups had mild amounts of giant cell deposits. A partial pupil capture was observed in 2 eyes, 1 containing a study IOL and the other a control IOL, at the end of the study. This was associated with a relatively large capsulorhexis.
Starting with the slitlamp examination at 1 week, there were no observable differences in the distance between the posterior surface of the IOL and the posterior capsule between study IOLs and control IOLs. Study IOLs and control IOLs had the exact same design, and the capsule appeared to be in contact with the posterior optic surface. Later postoperatively, however, there was progressively more proliferative material between the posterior surface of the control IOL and the posterior capsule. At the 4-week examination, the mean PCO score was 0.88 ± 0.33 (SD) in right eyes and 2.55 ± 1.13 in left eyes (P=.003, 2-tailed paired t test) (Figure 2). This score reflects the PCO in 9 rabbit eyes. One rabbit was excluded from this analysis as a result of poor visualization of the posterior chamber secondary to synechia formation in the right eye (same eye with blood and fibrin in front of the IOL at the 1-week examination) and capsulorhexis phimosis in the left eye.
No difficulties or differences were noted in performing the Nd:YAG procedure in the study eyes relative to the control eyes in the 5 rabbits used in this subset of the study.
Postmortem gross examination under the Miyake-Apple posterior view showed that all IOLs were fixated in the capsular bag. Very mild decentration was noted in some eyes in both IOL groups. In the eyes treated with the Nd:YAG laser, peripheral proliferative material behind the optic was scored (mean value) as 2.6 ± 0.54 in the right eye and 3.4 ± 0.54 in the left eye (P=.01, 2-tailed paired t test). In the untreated Nd:YAG eyes, peripheral PCO was scored as 1.8 ± 0.8 in the right eyes and 2.8 ±1.09 in the left eyes (P=.03, 2-tailed paired t test) (Figure 3).
Analysis of multiple histopathologic sections from each eye confirmed clinical and gross findings relative to capsular bag opacification and Soemmerring ring formation. There was no difference in the findings between the study eyes and the control eyes, and there were no signs of untoward inflammation or toxicity in any eye evaluated (Figure 4).
Posterior capsule opacification, or secondary cataract, remains the most frequent complication of cataract surgery.1–6 Numerous advancements have been made to surgical techniques and IOLs in an attempt to reduce secondary cataracts. These changes include polishing the capsule to remove residual LECs7 as well as IOL-specific changes that mostly target their design.9,10 Despite these attempts, PCO continues to be a significant postoperative complication.1–6
We tested 1 IOL-related factor for PCO prevention in this study, specifically the effect of IOL posterior optic surface modification. Both study IOLs and control IOLs had the same design and were manufactured from the same hydrophobic acrylic material.
In a preliminary study, hydrophobic acrylic IOLs were prepared and irradiated with UV–O3 or argon plasma to the surface of the IOLs.14 Elemental analysis of the IOL surfaces was performed to confirm surface modification. Changes produced by UV–O3 or argon plasma treatment were examined for fibronectin and LEC adhesion in cell culture. To evaluate the PCO prevention by the treated IOLs, 8-week-old albino rabbits had implantation of 3 different IOLs: the UV–O3-treated IOLs, argon plasma–treated IOLs, and control IOLs. After 2 weeks, the rabbits were killed humanely and their globes prepared for histopathologic evaluation. Surface composition showed an increase in nitrogen content and hydroxyl substitute and carboxyl substitute groups on the surfaces of treated IOLs. The fibronectin adhesion and the LEC adhesion on the UV–O3-treated and argon plasma–treated samples were increased. There was statistically significant inhibition of PCO formation in the UV–O3 group and argon plasma–treated group. Of the 2 surface-treatment methods evaluated, argon plasma was associated with an etching effect that can promote surface deterioration, whereas UV–O3 treatment produced little damage to the IOL surface. Also, UV–O3 treatment was more effective than argon plasma treatment in preventing PCO.14
In the current study, only the UV–O3 treatment was evaluated. The hydrophobic acrylic IOLs had the treatment on the posterior surface only, 10 rabbits were used with direct comparison between the test IOL and the control (untreated) IOL, the follow-up was longer than in the above-mentioned study, and the performance of Nd:YAG laser posterior capsulotomy was also assessed. Limiting the surface treatment to the posterior surface decreases the overall tackiness of the IOL while keeping the potential for adhesion between the posterior optic surface and the posterior capsule. Also, increased adhesion properties with early complete sealing of the anterior capsule remnant against the anterior IOL optic surface may increase the risk for complications such as capsular bag distension syndrome.19
For PCO comparisons, we focused on the scores at the 4-week clinical examination because the rabbit is an accelerated model for PCO. Previous studies20–23 have shown that in terms of proliferation of LECs and PCO, 6 to 8 weeks in the rabbit eye correspond to approximately 2 years in the human eye. The overwhelming proliferative capacity of the New Zealand rabbit LECs usually renders PCO comparisons after 4 weeks inconclusive. At 4 weeks, PCO was significantly less in the group of eyes with the surface-modified IOL. At the postmortem examination performed after a clinical follow-up of 6 weeks, peripheral PCO was still statistically less with the surface-modified IOLs. Uveal biocompatibility was similar between eyes with modified IOLs and unmodified IOLs. Our results also suggest that, if needed, performance of an Nd:YAG laser posterior capsulotomy is not rendered more difficult by eventual adhesion between the posterior optic surface and the posterior capsule. Whether the results obtained in the rabbit model can be directly transferred to humans is unknown. Previous studies24,25 have found differences at the molecular level between human LECs and rabbit LECs in terms of their interaction with modified surfaces.
It is suggested that changing IOL surface properties (eg, adhesion and wettability) does not compromise uveal biocompatibility or structural integrity of the IOL.14,26 Previous studies have also suggested that by increasing the adhesiveness of the posterior IOL surface to anterior chamber cell proteins, adhesion between the IOL and posterior capsule is improved and PCO formation is minimized. The sandwich theory postulated by Linnola et al.27–31 states that an IOL made of a bioadhesive material allows a single layer of LECs to bond to the IOL and the posterior capsule at the same time, producing a sandwich pattern that includes the IOL, a cell monolayer, and the posterior capsule. The sealed sandwich structure would prevent further LEC proliferation. In 2 follow-up studies using pseudophakic human eyes obtained postmortem, Linnola et al.30,31 found that fibronectin was the major extracellular protein involved in the adhesion between a bioadhesive IOL and the capsular bag. Our results support these facts and theory, with similar uveal biocompatibility between the 2 groups of IOLs and enhanced PCO prevention with the surface-modified IOLs. The objective of the current study was to show that the study IOL was associated with less PCO than the control IOL in the rabbit model. Previous in vitro studies13,14 have already shown that the surface treatment of the study IOL enhances adhesion to protein and LECs. Because the results in our current study indicated less PCO with the study IOL, we hypothesized that the adhesion between the IOL and the posterior capsule was the responsible mechanism. Starting with the slitlamp examination at 1 week, there were no observable differences in the distance between the posterior surface of the IOL and the posterior capsule between study IOLs and control IOLs. The study IOLs and control IOLs had the exact same design, and the capsule appeared to be in contact with the posterior optic surface. Whether there was firm attachment of the posterior surface of the study IOL and the posterior capsule is a parameter that is beyond the scope of our study. Later postoperatively, however, there was progressively more proliferative material between the posterior surface of the control IOL and the posterior capsule.
In conclusion, in this rabbit study designed to evaluate a new hydrophobic acrylic IOL with UV–O3 treatment of the posterior surface, the study IOL significantly prevented PCO compared with the control IOL, an identical IOL without the posterior surface treatment. Uveal biocompatibility and the performance of Nd:YAG laser posterior capsulotomy were also similar between the 2 IOL groups.
What Was Known
- Among the IOL-related factors to prevent PCO, the IOL design (eg, square optic edge) has been considered to have a preponderant role in the prevention of this complication.
What This Paper Adds
- A new UV–O3 treatment of the posterior surface of single-piece hydrophobic acrylic IOLs enhanced the PCO preventative effect, likely by increasing the adhesion between the posterior capsule and the posterior IOL surface.
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