Caring for Silicone Hydrogel Lenses

Optometric Management, Jul 2006 by Sindt, Christine

The right contact lens care system can make or break your patients' ability to wear their lenses comfortably.

In the last few years, the silicone hydrogel contact lens options aimed at the daily- or flexible-wear patient have exploded to include spherical, toric and bifocal wearers. Consumers rank a "healthier lens" as an important need (CIBA Vision data) and since the price point is now reasonable for even the most conservative budget, many patients are asking to be refit. Optometrists who originally weren't comfortable with the idea of continuous wear have seen the physiological benefits of increased oxygen transmission in their daily wear patients and now have the confidence to recommend these lenses. With both patients and doctors driving this market, it is the fastest growing lens category and some studies have projected that silicone hydrogel sales will overtake HEMA-based lens sales in 2007-2008. Daily lens wear, however, requires a daily lens care regime and daily wear silicone hydrogel lenses have specific considerations. Here, we'll review all the things you need to consider when prescribing this modality for your patients.

CARING FOR LENSES

Silicone hydrogels absorb less protein, deposit more lipids and have decreased surface wettability compared with conventional HEMA-based materials. Numerous reports have indicated corneal staining with certain silicone hydrogel lens/solution combinations. So, it's imponant to prescribe specific lens care regimes based on the patient's symptoms, signs and lens brand. Dire consequences may occur if the patient continues using her habitual solution or randomly changes solutions without regard for the lens material. If a patient experiences discomfort, such as dryness, irritation or red eyes with a new silicone hydrogel material, the patient and doctor could wrongly assume that the lens is at fault and not the lens/solution combination.

The recent outbreak of Pusarium keratitis associated with one lens solution is a strong reminder of the complexity of lens care systems and the opportunity for disaster given the right mix of circumstances. Fortunately, this has given us the opportunity to reconnect with patients about lens care issues such as deposits, wettability, solution compatibility and care habits.

PROTEIN AND LIPID DEPOSITION

Proteins, lipids and mucins deposit rapidly after lens insertion. Deposits may come from the tears, the environment or lens handling. Tear proteins include lysozymes, lactoferrin and albumin, with lysozyme accounting for 90% of the total lens protein deposits. In its natural state, lysozyme is a bacteriolytic enzyme, which plays an important role in the eye's defense against pathogens. Once they deposit on a contact lens, however, these proteins are subject to denaturation, which may incite an immunological response, such as giant papillary conjunctivitis (GPC).

Lens surface charge, protein size and charge, lens pore size (water content) and the age of the material are all factors that effect deposition. Lysozyme is a very small, positively charged protein that is easily adsorbed by negatively charged materials with relatively large pore size. High levels of lysozyme deposition have been measured on conventional HEMA-based materials, particularly Group FV materials. Silicone hydrogel materials, on the other hand, have shown reduced protein deposition. Contact lens solutions facilitate protein removal with negatively charged molecules that pull the positively charged proteins from the lens surface. Ironically, silicone hydrogel materials show a greater percentage of denatured lysozyme, albeit in very small amounts. The relatively hydrophobic nature of silicone hydrogel materials may account for this.

The hydrophobic nature of silicone is also responsible for the increased deposition of lipids.

Patients often report fogging or hazing secondary to lipid deposits. If lipid deposition occurs, a digital massage is necessary. Adding a separate surfactant cleaner will adequately remove lipids. Patients with meibomian gland disease (MGD) may see more lipid deposition. Initiate lid hygiene procedures and treatment if a patient suffers from repeated lipid films.

WEnABILITY

Surface wettability (wetting angle or contact angle) describes how a liquid spreads across a surface. It is the relationship of the liquid surface tension with the contact lens material. It can be determined by placing a liquid drop on the material surface and measuring the angle the drop makes with the horizontal surface. A high wetting angle (>90 degrees) means the surface does not wet well and there are many dry areas. A lower wetting angle (

Silicone hydrogel lenses are notorious for their high wetting angles. Wetting is improved by modifying the surface of the lens. Balafilcon (PureVision, Bausch ocLomb) lenses are treated in a gas plasma reactive chamber, which transforms the silicone components on the surface of the lens into hydrophilic silicate compounds. This process results in glassy, discontinuous silicate "islands," the hydrophilicity of which "bridges" over the underlying hydrophobic material.

Lotrafilcon A (Night & Day, CIBA Vision) and lotrafilcon B (02Optix, CIBA Vision) materials are surface-modified in a gas plasma reactive chamber to create a permanent, ultra thin (25nm), high-refractive index, continuous hydrophilic surface. Galyfilcon A (Acuvue Advance, Vistakon) and Senofilcon A (Acuvue Oasys, Vistakon) are not manufactured with surface treatments. These materials permanently incorporate an internal wetting agent, which shields the silicone at the material interface, functioning both as humectant (moisture-loving agent) and lubricant.