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Ten Reasons to Fit Silicone Hydrogel Contact Lenses
Find out why this latest contact lens material could open up more opportunities in your practice.
BY MILTON HOM, O.D., F.A.A.O.

Silicone hydrogels are the most innovative advance in soft contact lenses in recent years. Looking back, it seems only natural that the industry developed a safe alternative for continuous wear. Patients simply want safe contact lenses that they can sleep in, but resistance still exists among eyecare practitioners to use them. Now that they're readily available, why fit them? This article will present 10 reasons.

REASON 1: THE PRESENT AND THE FUTURE

Silicone hydrogels came to the United States in the late 1990s, yet Kyoichi Tanaka and four others began their development more than 25 years ago when they described copolymerizing silicone with hydrophilic monomer to make a soft contact lens. Their challenge was as difficult as combining oil and water to form an optically clear product. Researchers and scientists invested considerable intellectual input and financial resources to achieve today's high-Dk soft lenses.

The investment has been great because companies are committed to silicone hydrogels as their future. At a product launch last year, the speaker stated emphatically that it is a matter of when, not if silicone hydrogels will take over the marketplace. Presently, four silicone hydrogels are available (or soon will be) on the U.S. market, with more on the way. They are CIBA Vision's Night & Day (lotrafilcon A) and O₂Optix (lotrafilcon B), Bausch & Lomb's PureVision (balafilcon A) and Vistakon's Acuvue Advance (galyfilcon A).

REASON 2: HIGHER DK VALUES

Present HEMA-based materials may share the fate of PMMA contact lenses and glass spectacle lenses. While still available, their use is limited. There's a good chance that the standard mid-water contact lenses of today may no longer be around in the future, or their use may be similarly limited.

Conventional materials rely on water content to increase Dk levels (higher water means higher Dk). The Dk of water is only 80, which places a ceiling on how high a conventional material's Dk can be. The relationship between Dk and water content in silicone hydrogels is quite the opposite, as higher water content means lower Dk. Figure 1 shows the Dk/t values and water content for several lenses, both conventional and silicone hydrogels.

Increasing the water content in conventional or mid-water contact lenses increases the resultant Dk/t. With silicone hydrogel lenses, Dk/t climbs significantly as the silicone content -- not water content -- of the lens increases. The overall Dk/t values of silicone hydrogel contact lenses are much higher than those of older soft lens materials.

Currently, the highest Dk/t material available is lotrafilcon A (175 Barrers), but even higher Dk silicone hydrogel materials are on the horizon.

REASON 3: UNIQUE CORNEAS

All corneas need oxygen. What makes one cornea different from another is the amount of oxygen that it needs. Figure 2 represents individual variation in swelling response. Some corneas surely do fine with Dks in the 20s or 30s (represented by the green boxes). Corneas with greater needs, however, don't fare as well. It seems reasonable that higher-Dk lenses would better serve them.

REASON 4: DK/t CONTROVERSY

Recent controversy has surrounded silicone hydrogels and Dk/t values. Noel Brennan of Australia feels that higher-Dk lenses don't make much difference for the cornea. For lenses that have Dk/t of 50 or more, a law of diminishing returns applies. Mr. Brennan states, ". . . there is not much difference in terms of oxygenation between the two silicone hydrogel lenses [Night & Day and PureVision]." Although there is a 60% difference in Dk/t between lotrafilcon A and balafilcon (175 vs. 110), it only results in a 1% advantage in terms of corneal oxygen consumption.

As an analogy, if someone scuba dives for 20 minutes, would they want a tank with a 20-minute supply or a 30-minute supply of oxygen? The thinking behind the analogy is that lenses that have higher Dk/t values give more oxygen to the cornea. The tank with the 20-minute supply represents the balafilcon lens; the 30-minute tank represents the lotrafilcon lens.

Mr. Brennan believes the scuba tank analogy is inaccurate. Applying the law of diminishing returns, a truer analogy might be that the balafilcon tank supplies 30 minutes of oxygen and the lotrafilcon tank supplies 30 minutes and 15 seconds of oxygen. Greater oxygen is supplied by a lotrafilcon lens, but the advantage is a mere 15 seconds.

Mr. Brennan bases the law of diminishing returns on a reworked mathematical oxygen model. For daily wear, he feels that no differences exist in the amount of oxygen that the cornea consumes between any of the silicone hydrogel contact lenses on the market.

With respect to overnight wear, a study by Mueller, et al. evaluated overnight corneal swelling responses using lenses of reported Dk/t of 110 (balafilcon) and 175 (lotrafilcon) to determine whether swelling with either or both of these lenses was greater than that found with no lens wear. Central and peripheral overnight corneal swelling with the lotrafilcon lens was not significantly different from no lens wear (p>0.05). The balafilcon lenses caused significantly higher central corneal swelling values upon awakening and at a one-hour, measurement compared to no lens wear (p<0.05). The authors concluded that the minimum Dk/t to prevent corneal swelling appears to be greater than 110, but less than 175. Lower amounts of corneal edema with overnight wear were seen with lotrafilcon than balafilcon.

There is also the complication of limbal hyperemia. Papas looked at the relationship between induced limbal hyperemia and lens oxygen transmissibility. Subjects wore a range of soft contact lenses for eight hours each and Papas observed the degree of induced limbal hyperemia. His findings support the theory that hypoxia induces limbal hyperemia. Based on this study, a Dk/t of at least 125 was needed to avoid limbal hyperemia.

Finally, lens thickness plays an important role in oxygen transmissibility. Oxygen delivery to the cornea depends on both the Dk of the material and the lens thickness (t), with thinner lenses providing the cornea with more oxygen, because there is less of a barrier for the oxygen to diffuse through. Dk/t refers to how much oxygen passes through the lens at a specific point, dependent on the thickness at that point. Therefore, Dk/t varies across the profile of any given lens. For industry standards, Dk/t is generally specified at the center, or thinnest portion, of a ­3.00 spherical lens. The center of a plus-powered lens and the periphery of a high-minus lens will have different (lower) Dk/t values.

Older materials, with Dk values in the 20s and 30s, caused corneal swelling -- we just debated how much was acceptable. Preventing limbal hyperemia was a pipe dream. Silicone hydrogels, however, provide options to entirely eliminate them.

REASON 5: SLEEPING IN LENSES

According to CIBA Vision market data, 84% of soft contact lens wearers nap in their lenses, and one-third admit to doing so frequently or all the time. Twenty-five percent of patients wearing one- to two-week disposable-type lenses with low oxygen transmissibility occasionally or routinely sleep in their lenses overnight. Seventy-two percent of soft contact lens wearers said they prefer a schedule that includes overnight wear at least occasionally if their eyecare practitioner assured them it was safe (see Figure 3).

The numbers are overwhelming and the demand is great. Nearly three-quarters of our patients want the capability of sleeping in their lenses. The FDA has approved three of the four silicone hydrogels (Night & Day and O₂Optix and PureVision) for overnight wear.

REASON 6: WEARABILITY

Silicone, by nature, is intensely hydrophobic. A lens made with pure silicone would have tremendous oxygen transmission, but would be unwearable. The lens wouldn't wet at all. To make the surfaces of silicone hydrogel lens materials hydrophilic, techniques incorporating plasma into the surface processing of the lens have been developed. These surface treatments mask the hydrophobic silicone from the tear film, increasing the surface wettability of the materials and reducing lipid deposition.

The surfaces of Night & Day and O₂Optix lenses are permanently modified in a gas plasma reactive chamber to create a permanent, ultrathin (25 nm), high-refractive index, continuous hydrophilic surface. PureVision lenses are surface treated in a gas plasma reactive chamber that transforms the silicone components on the surface of the lenses into hydrophilic silicate compounds. Glassy, discontinuous silicate "islands" result, and the hydrophilicity of the transformed surface areas "bridges" over the underlying hydrophobic balafilcon A material. These surface modifications don't impede flow of oxygen and fluids through the lenses. Both surface treatments become an integral part of the lens; they aren't surface coatings that patients can easily "strip" away from the base material during daily handling and cleaning.

The Acuvue Advance lens material is the first non surface-treated silicone hydrogel and uses an internal wetting agent based on poly vinyl pyrollidone (PVP). It's designed to provide a hydrophilic layer at the surface of the material to "shield" the silicone at the material interface, thereby reducing the degree of hydrophobicity typically seen at the surface of silicone hydrogels.

One disadvantage of PVP may be a tendency to attract lipid deposits. Lipids preferentially deposit onto hydrophobic surfaces because of hydrophobic-to-hydrophobic interactions. Materials that have higher percentages of PVP have shown greater amounts of lipid deposits.

The surface and other wetting treatments make a silicone lens possible to wear. Without it, silicone hydrogels wouldn't be successful.

REASON 7: LESS DEPOSITION

Historically, protein deposition has been a problem with contact lenses. Three major variables affect the development of deposits:

1. length of wear

2. individual tear chemistry

3. lens material.

Length of wear. Frequent replacement schedules have greatly reduced length of wearing time and subsequent risk factors for deposits.

Individual tear chemistry. It's not easy to determine this variable in a clinical setting. Care system management can somewhat compensate for a tear chemistry that's prone to depositing. Most multi-purpose solution (MPS) care systems are labeled as "no rub," but modifying the regimen to include rubbing and rinsing can help patients who have a history of deposits.

Unlike less convenient enzyme cleaners in the past, modern MPS systems have built-in protein removers that work while the lenses soak. Some of these components are hydranate, poloxamine, poloxamer and tetronic. They're so effective that scientists have proposed some of them as in-the-eye cleaners. The protein removers are gentle and effective at the same time.

Lens materials. Silicone hydrogels offer advantages for protein or lysozyme deposition. Jones, et al. investigated differences in lysozyme between mid-water and silicone hydrogel lenses. Lysozyme deposition on mid-water contact lenses was significantly greater than silicone hydrogel lenses (P<0.001). The lens material significantly influences the quantity of lysozyme and lipid deposition. They found that silicone hydrogel materials deposit low levels of lysozyme when compared with mid-water materials.

Despite the advances in care systems, mid-water contact lenses are much more prone to deposits than are silicone hydrogels. Combined with frequent replacement schedules and proper care systems, wearers of silicone hydrogels are relatively immune to protein deposition.

REASON 8: LIFESTYLES

Silicone hydrogels offer benefits that conventional lenses don't. Continuous wear contact lenses can prove especially useful for certain vocations or professions. Members of the emergency work force (medical personnel, fire fighters and the police) often work unpredictable hours and schedules. They may benefit from immediate clear vision on waking. Other shift workers may appreciate similar advantages.

Many parents also wish to see clearly to tend to young children during the night. Additionally, increasing numbers of patients enjoy active outdoor lifestyles. There may be activities such as camping or hiking where patients are unable to disinfect or handle their contact lenses in a hygienic manner. This can present a problem for patients in a number of occupations, including members of the armed forces.

While patients who wear mid-water lenses can also enjoy the same advantages of overnight wear, it comes with a much lesser degree of safety than overnight wear with silicone hydrogel lenses.

REASON 9: DRYNESS PROBLEMS

End-of-day symptoms of discomfort and dryness often reported by mid-water lens wearers are significantly reduced for silicone hydrogels. In a recent study, Chalmers found that more than 30% of daily wear soft lens patients reported moderate to severe end-of-day dryness with their low-Dk lenses. After one week in silicone hydrogel lenses, only 13% reported that degree of end-of-day dryness.

REASON 10: IMPROVEMENTS

More and more companies plan to jump into the silicone hydrogel fray, making increasing options available. Contact lens companies are looking at developing innovative silicone-based hydrogels, and the foreseeable future for this segment of lens materials looks promising.

In all probability, every major contact lens manufacturer will release silicone-based hydrogels over the next 10 years. These materials will address the concerns seen in the current offerings of lenses. More likely, these new materials will have stiffness levels closer to conventional hydrogels and have better surface treatments. The lens technology of tomorrow will truly make the surfaces hydrophilic.

Ideally, the new generation of materials would support a tear film for longer than the typical seven to eight seconds seen with currently available materials. They will consist of polymers that resist contamination with pathogenic organisms, increasing safety beyond the safe levels we already experience today. Such materials should result in increased comfort and reduced inflammatory complications compared with currently available materials. There will be a significant impact on the contact lens market that will hopefully help grow the entire eyecare market.

THE WAVE OF THE FUTURE

Little doubt exists that silicone hydrogel materials will displace conventional contact lens materials over time for both daily and overnight wear. Silicone hydrogel lenses have improved physiological performance, excellent handling characteristics and improved comfort, making them as close to the ideal contact lens material as we have today. We shouldn't reserve lenses for overnight use only or use them as "trouble-shooting" lenses; rather they should become the standard lens type that we fit for all patients. Researchers Lyndon Jones and Kathy Dumbleton state, "We predict that, regardless of the success of continuous wear, highly oxygen permeable soft lens polymers will dominate the contact lens market in 10 years time."

Some practitioners may not have fit many silicone hydrogels yet, but as these 10 reasons have illustrated, those practitioners soon will.

References available on request.

Dr. Hom practices in Azusa, Calif. He is a Diplomate in Cornea and Contact Lenses and is a Distinguished Practitioner in the National Academies of Practice in Optometry. Dr. Hom has written more than 100 publications and has given more than 50 invited lectures.

Figure 1: Dk/t values and water content for several lenses are shown. The overall Dk/t values of silicone hydrogels are much higher than older soft lens materials.

Figure 2: The individual variation in swelling response. The average line (in red) shows the average response of all patients. The green boxes represent the patient who has the least amount of observed edema. The yellow boxes represent the patient who showed the highest edematous response.

Figure 3: Seventy-two percent of the patients desire some form of overnight wear.