contact lens design and materials
Silicone Hydrogel Lenses: Where Do We Stand? Part 2
BY NEIL PENCE, OD, FAAO
In Part 1, we reviewed how the significantly higher oxygen transmission levels of silicone hydrogel (SiHy) materials have lessened complications resulting from corneal hypoxia. We also discussed that SiHy lenses have not reduced microbial keratitis (MK) risk. Overnight wear, patient hygiene, and lens care habits continue to be important factors for lens-related infection. Now, let's continue the review.
Inflammation, and in particular infiltrative keratitis (IK) or subepithelial corneal infiltrates, is another area in which SiHy lenses have not led to improvements. In fact, an excellent review (Szczotka-Flynn & Diaz, 2006) found that SiHy wearers may be twice as likely to suffer an IK event. Repeat inflammatory events in extended wear patients identify them as poor candidates for overnight wear. The higher inflammation risk may be due to mechanical issues, to poor flushing of debris from behind the lens, to combined effects that create more insult to the corneal surface, or to yet unknown reasons.
The stiffer modulus of SiHy lenses is believed to result in a slightly higher incidence of several complications. Superior epithelial arcuate lesions (SEALs) are likely a mainly mechanical effect. Early FDA trials of SiHy lenses showed a higher incidence of SEALs and mucin balls. With the introduction of steeper base curves for these lenses, and perhaps improved surface wetting, we see both complications less often clinically and anecdotally. Occurrences of SEALs warrant refitting the lens, while mucin balls are benign and asymptomatic.
Likewise, comfort problems were anticipated as a result of stiffer modulus lenses. Most practitioners cautioned patients of an adaptation time when switching from hydrogels. This is stressed much less today, if at all. Perhaps SiHy lenses got better, but more likely we were part of the comfort "problem." In our experience, once we stopped talking about comfort so much, it no longer was an issue with most patients.
While a number of factors likely play a role in GPC (we might have listed this under inflammation), a mechanical component is thought to be mainly responsible for the localized form that occurs with SiHy lenses (Skotnitsky et al, 2006). Seen more in continuous wear, treat localized GPC similarly to the generalized form seen with hydrogels. Changes in lens fitting or modulus may also have some effect on localized GPC.
Conjunctival epithelial flaps have also been reported as a SiHy complication (Løfstrøm and Kruse, 2005) with a possible mechanical component in their etiology. Lens edge shape, thickness, and wetting characteristics may play roles as might stiffer modulus. These seem asymptomatic and non-threatening.
So Where Do We Stand?
SiHy lenses have become the predominant material in the market for most new lenses introduced by major manufacturers. Their higher oxygen transmissions are essential for the safest overnight wear, and they have nearly eliminated corneal hypoxia problems. While mechanical factors from the stiffer modulus may show up rarely, in most patients these are not an issue. Unfortunately, SiHy lenses have not solved MK issues or corneal inflammation secondary to lens wear. These are still areas of concern, especially for overnight wear, and require continued close monitoring. CLS
For references, please visit www.clspectrum.com/references.asp and click on document #174.
Dr. Pence is director of the Contact Lens Research Clinic, Indiana University School of Optometry in Bloomington, Indiana. He is a consultant or advisor to B&L, Ciba Vision, and Vistakon, and has received research funding from AMO.