research review

Silicone Hydrogel Deposition

BY LYNDON JONES, PHD, FCOPTOM, FAAO

The rapid growth of silicone hydrogel (SiHy) lenses has once more sparked interest in lens deposition, as clinical reports have surfaced of patients exhibiting increased deposition and reduced wettability when using SiHy materials (Cheung et al, 2007). To date, approximately 35 peer-reviewed manuscripts have addressed SiHy materials and deposition over the past eight years.

Protein Deposition on SiHy

The majority of work has focused on protein deposition. The first of these by Jones et al (2003) showed that lysozyme deposition on a traditional FDA group IV material (etafilcon A, Acuvue, Vistakon) was significantly greater than that measured on first-generation SiHy lenses (985μg/lens versus <10μg/lens). They also suggested that the degree to which lysozyme denatured was influenced by the underlying lens material, with the lowest degree of denaturation (22 percent) occurring on the traditional material, as compared with 50 percent for balafilcon A (PureVision, Bausch & Lomb) and 80 percent for lotrafilcon A (Night & Day, CIBA Vision). Further work from the same laboratory confirmed that care regimens had minimal impact on controlling the amount of deposition.

However, two of these studies (Senchyna et al, 2004; Subbaraman et al, 2006) did suggest that the degree of lysozyme denaturing could be impacted by both the composition of the care regimen and the use of in-eye rewetting drops, with solutions containing Tetronic 1304 resulting in a reduced percentage of denatured lysozyme on the materials.

More recent in vitro work investigated the kinetics of both lysozyme (Subbaraman et al, 2006) and lactoferrin (Chow et al, 2009) buildup on SiHy materials over a four-week period. Lysozyme accumulation on FDA group IV traditional lenses reached a maximum on the seventh day and then plateaued at around 1400μg/lens, whereas the SiHy deposition continued to increase during the four weeks but at much lower levels (4-to-20μg/lens after four weeks). In comparison, lactoferrin deposition on both traditional and SiHy lenses increased over the month. At the end of four weeks, etafilcon A and balafilcon A deposited the most lactoferrin (12μg/lens), and lotrafilcon A and B (O₂Optix, CIBA) deposited the least (3μg/lens).

Other studies by Luensmann et al (2007, 2009) have shown that both albumin and lysozyme are blocked from entering the lens matrix by homogenous surface treatments on SiHy materials such as lotrafilcon A and B. This is in stark contrast to other SiHy materials and traditional hydrogels, in which protein deposits both on the lens surface and throughout the lens matrix.

A new technique that shows much promise for analyzing deposits is mass spectrometry (MS). Green-Church (2008) used MS to report on the proteome associated with two SiHy materials and two care regimens and observed 7-to-10μg/lens from each lens/solution combination. In all four combinations they identified six proteins including lipocalin, lysozyme, lacritin, lactoferrin, proline rich 4, and IgA. Zhao et al (2008) also used MS to show a wide array of proteins, most commonly lysozyme and lipocalin, in extracts from worn SiHy lenses.

Further, Emch (2009) used MS to examine the ability of four care regimens to remove protein deposits from two SiHy materials worn for eight hours. He showed that Opti-Free Express (Alcon), which contains Tetronic 1304, was consistently associated with more efficient removal of proteins from the study materials.

Pucker (2009) examined the role of a rinsing step in aiding protein removal from lotrafilcon B lenses when used with "no-rub" Aquify (CIBA). He showed that rinsing the SiHy lens after removal and prior to overnight disinfection removed approximately 50 percent of the protein. Finally, Zhao et al (2009) examined protein extracts from four SiHy materials when used with four care regimens. They reported that balafilcon A lenses exhibited the greatest total protein deposition (5-to-23μg/lens), with Aquify being the most effective regimen at reducing protein deposition on this material. Lotrafilcon B lenses deposited the most protein when used with Opti-Free Express (3.6μg/lens). As Zhao pointed out, within a lens polymer type, lens care solutions exhibit varying effectiveness in reducing protein and lipid accumulation.

Lipid Deposition on SiHy

Three clinical reports suggest that deposition with what appear to be lipids may be an issue with SiHy materials. Nichols (2006) examined two-week replacement galyfilcon A (Acuvue Advance, Vistakon) lenses and reported that <10 percent of subjects exhibited clinically significant levels of deposition when they disinfected their lenses with a no-rub multipurpose solution, and that adding a rub-and-rinse step significantly reduced this deposition rate. Ghormley (2006) also proposed that rubbing and rinsing with multipurpose solutions, or even using dedicated surfactant cleaners in some instances, would reduce visible deposition. Finally, Cheung et al (2007) examined subjects who wore galyfilcon A lenses in one eye and etafilcon A lenses in the other that were replaced every two weeks. They found that the SiHy material had more visible lipid deposits than the hydrogel material did.

Interestingly, some work has suggested that a small amount of lipid deposition may benefit the wettability of certain SiHy materials. Lorentz (2007) demonstrated that wetting angle can be reduced in surface-treated SiHy materials, which may help to explain why certain SiHy materials appear to improve in comfort for some patients during the first few hours or days of wear. Lira et al (2009) confirmed this for worn lenses. However, eventually any such benefits are overcome and vision and comfort may decrease, confirming the need for appropriate replacement schedules.

Two in vitro studies have quantified lipid deposits on SiHy materials. Carney et al (2008) used a fluorescence assay to quantify the amount of cholesterol (a nonpolar lipid) and phosphatidyl-ethanolamine (PE, a polar lipid) that built up on several SiHy materials over a 20-day period. They showed that cholesterol deposited on all lenses to a greater extent than PE did, and that the surface-treated lotrafilcon materials deposited the least PE (<2μg/lens), with galyfilcon A and senofilcon A (Acuvue Oasys, Vistakon) depositing approximately 5μg/lens. Senofilcon A and balafilcon A deposited approximately 23μg/lens of cholesterol, with lotrafilcon B depositing only 3μg/lens. Iwata et al (2008) confirmed that lotrafilcon A deposited very low levels of lipid and that balafilcon A deposited the highest amount of the six materials tested, with the range being 0.4-to-7.6μg/lens.

The first ex vivo study to quantify lipid deposition on SiHy materials was that of Jones et al (2003) who used HPLC to suggest that lipid deposition on balafilcon A could be as high as 600μg/lens, as compared with 20μg/lens on etafilcon A. These high values were refuted by Maziarz et al (2006), who suggested that cholesterol was the most likely lipid to be detected on SiHy materials at levels of <40μg/lens. More recent work has suggested that these lower values appear more accurate.

Zhao (2009) showed that of four SiHy materials examined, balafilcon A lenses exhibited the greatest amount of cholesterol deposition (4-to-8μg/lens) while lotrafilcon B exhibited the least (<0.5μg/lens). Their work showed that Aquify and Opti-Free Replenish (Alcon) solutions were most effective in reducing cholesterol deposition from senofilcon A and galyfilcon A lenses, respectively. Heynen (2009) quantified lipid deposition using HPLC on senofilcon A lenses when disinfected with a no-rub one-step hydrogen peroxide system (CIBA ClearCare) or a Polyquad and Aldox- preserved system (Opti-Free Replenish) using a five-second rinse. They showed that the predominant lipids detected were cholesterol oleate and cholesterol and that use of Opti-Free Replenish resulted in less total lipid deposition (34 versus 22μg/lens).

Reducing SiHy Deposition

Studies have clearly shown that the deposition profiles of SiHy and traditional materials are dramatically different, and that as subjects transition from one material to another they may exhibit some deposition issues. If so, they may benefit from a rub/rinse step, using a surfactant-containing multipurpose system, or replacing their lenses more frequently. CLS

For references, please visit www.clspectrum.com/references.asp and click on document #168.

Dr. Jones is the associate director of the Centre for Contact Lens Research and a professor at the School of Optometry at the University of Waterloo, Ontario, Canada. He has received research funding from Alcon, AMO, B&L, CIBA Vision, CooperVision, Johnson & Johnson, and Menicon.