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Article Date: 12/1/2011

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Point: Silicone Hydrogels: A Vast Improvement
Topic: Silicone Hydrogels: Were They Really Worth It?

Point: Silicone Hydrogels: A Vast Improvement

By David A. Berntsen, OD, PhD, FAAO

It has been a decade since silicone hydrogel (SiHy) contact lenses became available. Today, more than ever, I believe SiHy should be our material of choice.

More Oxygen, Whiter Eyes

The most obvious benefit of SiHy lenses is the reduction of hypoxia-related complications associated with hydrogel materials, and the resulting whiter eyes (Maldonado-Codina et al, 2004; Dumbleton et al, 2006). Although SiHy lenses have not eliminated comfort issues, they can reduce subjective dryness and allow patients to comfortably wear lenses longer each day than hydrogel lenses (Young et al, 2007; Dumbleton et al, 2008). Reducing hypoxic corneal stress also helps reduce the "myopic creep" noted with hydrogel materials (Fonn et al, 2002; Jalbert et al, 2004).

Although SiHy lenses have not reduced the risk of microbial keratitis with daily wear or extended wear as initially hoped (Stapleton et al, 2008), they do reduce neovascularization and other hypoxic complications that necessitate reducing or stopping contact lens wear (Covey et al, 2001; Dumbleton et al, 2001).

Many Options

In addition to spherical designs, SiHy lenses are now available in bifocal and toric designs, and replacement schedules include monthly, two-week, and now daily disposable options. With the recent introduction of a latheable SiHy material, even patients who need custom designs are no longer limited to hydrogel lenses.

Complications, Deposits and Compatibility

The higher modulus of first-generation SiHy materials was associated with an increase in noninflammatory events, such as superior epithelial arcuate lesions (Holden et al, 2001; Dumbleton, 2003). The lower modulus of newer-generation SiHy materials and design improvements have greatly decreased these issues, and these complications are easily managed when they do occur.

Some SiHy materials can be more prone to lipid deposition, and we have evidence that certain solution and material combinations are more effective for removing deposits (Carney et al, 2008; Zhao et al, 2009). In addition, rubbing lenses is a beneficial step when trying to remove deposits from SiHy lenses (Nichols, 2006). The greater need to rub SiHy lenses to remove lipid deposits might be viewed as a disadvantage compared to hydrogel lenses, but mounting evidence tells us that all patients should rub their lenses to maximize ocular health. Szczotka-Flynn et al (2010) reported that bacterial contamination of lenses is a significant risk factor for corneal infiltrative events in extended wear, while the presence of corneal staining is not (Szczotka-Flynn et al, 2010). Given that a rub-and-rinse step has been shown to be the most effective way of decreasing bacteria on contact lenses (Zhu et al, 2011), I believe we should be instructing all patients to rub their lenses, regardless of lens material.

Specific SiHy material and preservative combinations have been associated with solution-induced corneal staining. While the etiology of this staining and its clinical relevance to corneal integrity are topics of debate, these reports should not deter us from prescribing SiHy lenses. No one solution or lens is best for every patient, and we should be mindfully using the clinical information available to prescribe the lens/solution combination we feel is best for each patient. For those who wish to avoid preservative issues, peroxide-based systems eliminate the concern, as do daily disposable SiHy lenses for patients who can wear a spherical lens.

Healthy, Comfortable Lens Wear

Because of SiHy advances, we can confidently prescribe lenses that offer improved ocular health over hydrogel lenses by reducing hypoxic stress, which translates into healthy, more comfortable contact lens wearers. CLS

For references, please visit www.clspectrum.com/references.asp and click on document #193.


Dr. Berntsen is an assistant professor at the University of Houston College of Optometry.
Counterpoint

A Fascinating But Disappointing Experiment

By Graeme Young, MPhil, PhD, FCOptom, DCLP, FAAO

In many respects, silicone hydrogel lenses have been a disappointment. They were heralded as "the rebirth of continuous wear" but, instead, have generated the rebirth of superior epithelial arcuate lesions and giant papillary conjunctivitis and the advent of a number of complications not seen before (conjunctival flaps, mucus balls and corneal molding). The longed-for eradication of—or at least reduction in—microbial keratitis did not happen.

In truth, we have all participated in a fascinating experiment on the effect of contact lens physical properties on ocular physiology. Encouraged by the early findings of reduced hyperemia, many pundits predicted safer extended wear, fewer complications, better comfort and a brighter future. Some even expected better wettability, owing to the lower water content of silicone hydrogels. As often happens in clinical science, however, the results have been surprising and forced us to rethink many of our dogmas.

Increased Complications

The most startling finding has been that serious complications have increased with silicone hydrogel lenses. A meta-analysis by Szczotka-Flynn and Diaz (2007) found that corneal infiltrative events are twice as frequent in silicone hydrogel extended wear than with conventional hydrogel lenses. The reason for this is unclear, but modulus is an obvious suspect. Even in daily wear, the risk of inflammatory events with silicone hydrogel lenses is nearly three times that of hydrogels (Efron et al, 2005). Thus, the risk of having to interrupt contact lens wear because of adverse events is greater with silicone hydrogels (Wagner et al, 2011).

Unconvincing Comfort Data

The evidence to suggest that the oxygen boost from silicone hydrogels has improved comfort is unconvincing. Studies from the 1980s found that severe oxygen deprivation resulted in discomfort, but there is no evidence that the relatively modest increase in oxygen with silicone hydrogels versus hydrogels has, in itself, improved end-of-day comfort. Brennan (2009) evaluated the comfort scores from a large sample of studies and found wide differences but was unable to correlate comfort with material Dk/t. Several clinical trials have noted improvements in comfort with silicone hydrogels but, invariably, these have been comparisons with previous habitual lenses rather than controlled randomized crossover studies. It is, perhaps, inevitable that patients and practitioners would ascribe these improvements to a new material rather than the placebo effect or other confounding factors.

Silicone-containing polymers are inherently hydrophobic, and various remedies have been used to render them wettable enough for use in the eye. It seems perverse, therefore, that silicone hydrogels have been proposed as a remedy for lens-related dryness. The reasoning for this usually derives from the belief that low-water lenses dehydrate less and are better suited for patients susceptible to dryness. However, this overlooks the fact that the only lens to receive FDA approval for a dry eye claim is a mid-water rather than a low-water (hydrogel) lens. It is clear that the performance of silicone hydrogels varies between lens types, and it is likely that improvements in dryness symptoms derive from edge design, wetting agents and surface lubricity rather than the properties of silicone.

Not the Last Word

Were silicone hydrogels worth it? They certainly provide useful clinical options, and only the most foolish practitioner would fail to use them. But they did not deserve their hyperbolical fanfare and are certainly not the last word in lens development. I am sure there are other exciting, new contact lens materials around the corner, but let's learn a lesson in caution from the silicone hydrogel experiment. CLS

For references, please visit www.clspectrum.com/references.asp and click on document #193.


Dr. Young is managing director of Visioncare Research Ltd, a UK-based contract research organization specializing in eyecare clinical research.

Contact Lens Spectrum, Issue: December 2011

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