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Innovations | Mar 2011

Hydrophobic Acrylic IOLs: A Primer

The material properties and designs of different manufacturers influence surgical results.

The 2003 American Society of Cataract and Refractive Surgery (ASCRS) survey of practice styles and preferences indicated that, since 1998, hydrophobic acrylic has been the preferred IOL optic material.1 Because surgeons appreciate the controlled unfolding characteristics associated with this class of materials, the popularity of hydrophobic acrylic lenses has increased worldwide since their introduction.

In the August 2010 issue of CRST Europe, Khiun F. Tjia, MD, of the Netherlands, summarized the characteristics of hydrophobic acrylic IOLs that were new to the European market.2 As a follow-up, this article provides an overview of biocompatibility issues related to hydrophobic acrylic lenses and a discussion of new material and design trends in IOL manufacturing (Table 1) and the possible influences of these trends on surgical outcomes. This information was obtained through the corresponding manufacturers and peer-reviewed literature.3 The list of IOLs is not exhaustive; some were included in Dr. Tjia’s previous publication.2

BIOCOMPATIBILITY

Uveal biocompatibility. This is defined as the reaction of the uvea to an IOL. In the body’s response to the presence of a foreign body—the IOL—monocytes (lymphocytes) and macrophages migrate through blood vessel walls to the aqueous humor and eventually to the IOL surface. As this occurs, the monocytes and macrophages transform—the monocytes into small round cells and macrophages into epithelioid and foreign-body giant cells. These cells, which are responsible for the phagocytosis of debris, bacteria, and foreign material, reflect a natural immunologic process in a foreign-body reaction.

Abela-Formanek et al4 conducted a comparison of the uveal biocompatibility of hydrophilic acrylic (MemoryLens; Mentor Ophthalmics, Inc., Santa Barbara, California, and Hydroview; Bausch + Lomb, Rochester, New York), hydrophobic acrylic (AcrySof; Alcon Laboratories, Inc., Fort Worth, Texas, and Sensar AR40; Abbott Medical Optics Inc., Santa Ana, California), and hydrophobic silicone IOLs (CeeOn 920; formerly Pharmacia Corp., Peapack, New Jersey; technology since purchased by Abbott Medical Optics Inc.). The researchers found that the highest incidence of late foreign-body cell reaction was with hydrophobic acrylic IOLs (AcrySof, 30%; Sensar AR40, 17%), followed by the hydrophilic acrylic (MemoryLens, 8%; Hydroview, 4%) and silicone (CeeOn, 4%; CeeOn 911A, 0%) IOLs. A low-grade cellular reaction was clinically insignificant in all cases. Giant cells generally degenerate and detach from IOL surfaces, and only an acellular proteinaceous membrane is left around the IOL, isolating it from the surrounding ocular tissues.

Capsular biocompatibility. Capsular biocompatibility has two components, the reaction of lens epithelial cells (LECs) and the reaction of the capsule to IOL material and design. Abela-Formanek et al4 also evaluated LEC outgrowth, anterior capsular opacification (ACO), and posterior capsular opacification (PCO) outcomes in the same lenses enumerated above. The highest incidence of LEC outgrowth on the anterior IOL surface (originating from the anterior capsular rim) was in the hydrophilic acrylic group (Hydroview, 85%; MemoryLens, 27%), followed by the hydrophobic acrylic (AcrySof, 4%; Sensar AR40, 3%). No LECs were found on the anterior surface of the silicone IOLs. The difference among IOL groups was significant (P=.0001). ACO was more predominant in the hydrophobic IOL groups.

Other studies have shown more fibrosis of the anterior capsule and ACO in association with silicone IOLs, especially plate-haptic designs.5 Additionally, some studies4,6 have indicated that PCO was lower with square-edge–design IOLs, regardless of IOL material.

DESIGN FEATURES AFFECT PCO

The preventive PCO effect associated with the square edge may be due to the mechanical barrier effect,7 the contact inhibition of migrating LECs at the capsular bend created by the edge,8 the higher pressures exerted by IOLs with a square-edged optic profile on the posterior capsule,9 or combinations of these factors.

Current hydrophobic acrylic IOLs generally have a square optic edge. However, not all square edges are the same. In an experimental study including 19 hydrophobic acrylic lenses, the area above the posterior-lateral edge of each IOL was measured to determine the deviation from a perfect square.10 Variation was found not only among the IOL designs but also among different powers of the same IOL. This type of evaluation can help manufacturers optimize their IOLs’ optic edges. For example, Hoya Corp. (Tokyo) modified the edge profile of several AF-1 models after results of a single-surgeon prospective felloweye comparison showed that the PCO rate was significantly higher with the AF-1 YA-60BB IOL than with the AcrySof SN60AT.11 The study was based on results in 36 patients who were followed for 24 months. The result is not surprising, considering the differences in edge design between these lenses in the amount of area deviating from a perfect square (AF-1, 329.7 *µm2; SN60AT [20.00 D], 97.2 µm2).10 Hoya made several manufacturing changes, including variations in polishing process, and different prototypes were sent for evaluation.12 The area deviating from a perfect square in the current Hoya AF-1 family now measures 39.1 µm2. Such quantification of the area deviating from a perfect square—and therefore assessment of the sharpness of the posterior optic edge—is not available for many new models of hydrophobic acrylic lenses.

Another design trend observed in hydrophobic acrylic IOL manufacturing is the availability of new IOLs in a one-piece platform. Most prospective, randomized studies have shown no statistically significant differences between one- and three-piece hydrophobic acrylic IOLs in terms of PCO rate;13 however, it has been shown that, whenever PCO occurs with one-piece designs, it has the tendency to start at the level of the optic-haptic junction.7

Several manufacturers have modified the optic-haptic junctions of their IOLs to obtain a continuous 360° square edge. In one study, patients were implanted with a one-piece hydrophobic acrylic IOL with an interrupted optic edge (AcrySof) in one eye and a newer one-piece hydrophobic acrylic IOL with a continuous optic edge (Tecnis 1-Piece; Abbott Medical Optics Inc.) in the other.14 Two years after surgery, the extent and density of PCO were significantly greater in eyes with the AcrySof IOL that those with the Tecnis IOL.

BLUE-LIGHT FILTERING CHROMOPHORES

Some hydrophobic acrylic lenses incorporate blue-light filtering chromophores, as it has been suggested that blue-light–filtering IOLs protect lipofuscin-containing retinal pigment epithelial cells from blue-light damage.15 There is indirect evidence suggesting that blue-light filters may result in a reduced risk of development or progression of macular degeneration, but the value of these yellowchromophore– containing lenses is not fully understood. Potential side effects of constantly filtering blue light may include negative impacts on color vision, night vision, and sleep and circadian rhythms.16

One photochromic lens (Matrix Aurium; Medennium, Irvine, California) turns yellow when exposed to ultraviolet (UV) light because the blue light is absorbed. The lens behaves as a standard UV-absorbing IOL in an indoor environment; the optic remains colorless, and there is no filtering of blue light. This innovative concept theoretically eliminates the potential side effects of yellow IOLs and provides patients with the potential for improved night vision after cataract surgery, additional protection of the macula against blue light in daylight conditions, and improved contrast sensitivity (as demonstrated in clinical studies with yellow lenses).17

In a rabbit study conducted in our laboratory, the long-term biocompatibility and photochromic stability of the photochromic IOL were evaluated under extended UV light exposure (simulating 20 years of exposure).17 The photochromic change was found to be reversible (ie, the optic returned to its colorless state after discontinuation of UV light projection) and reproducible (ie, the photochromic change was observed at different time points in the clinical follow-up). Additionally, the photochromic property of the optic material remained stable and was present and unchanged 12 months after implantation. The rabbit eyes exhibited clinical and histopathologic changes that are normally expected in this model, and there was no difference between study and control eyes in terms of biocompatibility.

GLISTENINGS

Glistenings are fluid-filled microvacuoles that form within an IOL optic when the lens is in an aqueous environment. 3 They can be observed with any type of IOL, although they have mainly been described in association with hydrophobic acrylic lenses. Experimental and clinical studies suggest that different hydrophobic acrylic lenses exhibit different tendencies for the occurrence of glistenings. Factors that may influence their formation include IOL material composition, IOL manufacturing technique, IOL packaging, patient-associated conditions such as glaucoma or those leading to breakdown of the blood-aqueous barrier, and concurrent use by the patient of ocular medications.

The exact impact of glistenings on postoperative visual function and an understanding of their evolution in the late postoperative period remains a matter of controversy, partly because IOL explantation due to glistenings is rarely reported. Although it is likely that each hydrophobic acrylic IOL exhibits different tendencies toward glistening formation, no peer-reviewed articles are currently available for the IOLs listed in Table 1. Most of the peerreviewed literature describes glistening formation, incidence, and severity in the AcrySof material;3 a relatively small number of articles and presentations evaluate glistening formation in the Acryfold VA60CA/CB (Hoya Corp.), Acrylmex (Ophthalmic Innovations International, Inc.; now Aaren Scientific, Ontario, California), Sensar AR40(e), and XACT (Advanced Vision Science, Inc., Goleta, California) IOLs.3

In one Japanese clinical study of glistenings,18 74 eyes were implanted with the AcrySof SA30AL (n=17), the Acryfold VA60CB (n=37), or the Sensar AR40e (n=20). Follow-up ranged from 6 to 18 months. Slit-lamp examination showed the presence of grade 1C glistenings (Miyata classification) in 35.3% of the AcrySof IOLs and 62.1% of the Acryfold IOLs but in 0% of the Sensar IOLs. Hoya investigated the reports of glistenings (a total of 29) in its early IOL models VA60CA and VA60CB, which were never commercially released outside of Japan and were subsequently discontinued. Improvements were implemented in the polymerization and cleaning processes of the company’s lenses, and continued monitoring of the manufacturing processes and a low level of customer complaints indicate that these changes have been effective.

STORAGE MEDIA CAN CAUSE GLISTENINGS

According to information contained in the US Food and Drug Administration (FDA) Summary of Safety and Effectiveness Data and on IOL labeling (obtained through the manufacturer, Advanced Vision Science, Inc.), the XACT IOL was originally packaged in 10.0% saline. The presence of glistenings was reported in 40 eyes (40 patients) between 1 week and 1 month after surgery, with decreased density at each visit thereafter.3 No differences in contrast sensitivity were observed between the study and control eyes, except for a clinically insignificant difference in contrast sensitivity detected in the highest spatial frequencies at 1 month. It was postulated that, after implantation, the osmotic differential between the IOL and aqueous humor caused more water to be absorbed into the IOL optic, resulting in the appearance of glistenings. However, a change of hydration/storage media from 10.0% to 0.9% saline was implemented, and laboratory testing demonstrated elimination of the glistenings. This was confirmed in an additional clinical trial conducted in the Dominican Republic and Germany, the data from which were included in the FDA marketing application for the IOL.19 In this study, 172 eyes (142 patients) were examined at least once between 1 and 6 months, and 123 eyes (101 patients) were examined at least once between 6 months and 2 years. No glistenings were observed at any time.

The XACT IOL was approved for use in the United States in February 2009 (subsequently licensed to Bausch + Lomb, Rochester, New York) and in Japan in 2008 (trade name, Santen Eternity). This hydrophobic acrylic IOL, packaged in 0.9% saline, is also gamma sterilized.

NO REPORTED GLISTENINGS

Other IOLs with no reported glistenings include the Aurolab lens (Aurolab, New Delhi, India), the Acrylmex IOL (three- and one-piece models EC-3 and EC-1Y, respectively), Hanita hydrophobic acrylic lenses (Hanita Lenses, Kibbutz Hanita, Israel), and the Artis (Cristalens, France).

Aurolab. In a study of 120 patients implanted with the Aurolab lens at the Aravind Eye Hospital in Madurai, India, BCVA was 6/12 or better, and no eye had IOLinduced intraocular pressure increase (information obtained from the manufacturer). Decentration was not significant, and IOL tilt, discoloration, or opacity was not reported. However, adverse events included one case of iritis and one case of secondary surgical intervention due to haptic damage.

Acrylmex. Two European clinical studies presented at the 2009 ASCRS meeting20,21 showed the absence of glistenings at 1 year after implantation of the Acrylmex IOL. The first study included at least 10 cases with a minimum follow-up of 18 months.

Hanita hydrophobic acrylic lenses. These IOLs are manufactured with a hydrophobic acrylic material provided by Benz Research. The material supplier assesses a glistenings severity index for each batch of material for quality assurance. IOLs immersed in saline were observed for 6 months; the severity index was significantly lower than that of the AcrySof lenses (personal communication with Benz Research & Development).

Artis. Results are available for 20 consecutive eyes implanted with the Artis (follow-up, 60 days). These IOLs were implanted in five centers by five surgeons through a sub-2–mm incision (1.7 mm). The refractive results were within ±0.50 D of intended correction and remained stable. No glistenings or any form of IOL opacification was observed. Due to the overall design—four haptic components with 5° angulation and a square optic edge, including at the optic-haptic junctions—the IOL did not induce ovalization of the capsular bag. Posterior capsular contact with the optic was noted between 8 and 15 days postoperatively; thus far, no lens epithelial cell migration has been detected behind the IOL optic, and the totally transparent anterior capsule is not attached to the IOL’s anterior optic surface.

CALCIFICATION

Postoperative surface or intraoptic deposition of calcium phosphate has been described in association with hydrophilic acrylic lenses.22 The four major designs involved in the problem are the Hydroview (Bausch + Lomb), the MemoryLens, the SC60B-OUV (Medical Developmental Research, Inc., Clearwater, Florida), and the Aqua-Sense (Aaren Scientific).

Calcium phosphate deposits have also been reported to occur on the posterior optic surface of silicone lenses implanted in eyes with asteroid hyalosis.22 To the best of our knowledge, no confirmed case of calcification of hydrophobic acrylic lenses is described in the literature; therefore, postoperative calcification does not appear to be an issue with hydrophobic acrylic materials.

CONCLUSION

Various hydrophobic acrylic IOLs have been launched in the international market, with manufacturing trends that include one-piece platforms, a 360° square posterior optic edge with attention to the barrier effect at the level of the optic-haptic junctions, and incorporation of blue-light–filtering chromophores. A photochromic IOL, filtering blue light only under photopic conditions, is also available.

All hydrophobic acrylic lenses are not manufactured from the same materials or using the same processes. Therefore, each lens exhibits different characteristics, including refractive index, water content, and tendency for glistenings formation. In terms of PCO formation, it appears that the square posterior optic edge is the most important IOL design factor for prevention of this complication. However, all square edges in the market are not equally square. Only long-term clinical studies will indiindicate if there is superiority of a particular hydrophobic acrylic IOL over others.

Nick Mamalis, MD, is a Professor and the Director of the Intermountain Ocular Research Center, John A. Moran Eye Center, University of Utah, Salt Lake City. Dr. Mamalis states that he is a paid consultant to Abbott Medical Optics Inc. He may be reached at e-mail: nick.mamalis@hsc.utah.edu.

Steele McIntyre, MD, is an ophthalmic research and pathology fellow at the Intermountain Ocular Research Center, John A. Moran Eye Center, University of Utah, Salt Lake City. Dr. McIntyre states that he has no financial interest in the products or companies mentioned. He may be reached at e-mail: steelemcintyre@hotmail.com.

Liliana Werner, MD, PhD, is an Associate Professor and the Co-Director of the Intermountain Ocular Research Center, John A. Moran Eye Center, University of Utah, Salt Lake City. Dr. Werner states that she is a paid consultant to Powervision and Abbott Medical Optics Inc. She may be reached at e-mail: liliana.werner@hsc.utah.edu.

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