Silicone Hydrogels and Choice of Care Regimen

Silicone hydrogel lens materials are unique in the way that they behave. Their extraordinary oxygen transmission [ 1, 2 ] and the physiological benefits that such performance affords these materials is well documented [ 3-7 ]. However, studies have also revealed that there are several other unique characteristics of these materials, particularly in respect to their interaction with the tear film. Compared with conventional hydrogels they dehydrate to a lesser extent and at a slower rate [ 8, 9 ], they adsorb considerably less protein [ 10-13 ], exhibit increased lipid deposition [ 11 ] and display reduced wettability [ 14-16 ] due to the exposure of silicon groups at the material surface [ 17 ]. In view of their unique behaviour, there are growing concerns that their performance warrants them being considered in an FDA grouping that is new and exclusive to such materials [ 18 ].

The recent release of two silicone hydrogel lenses aimed primarily at the daily wear market (Acuvue Advance and O 2 OPTIX) and the reticence of some practitioners to recommend lenses of any type for overnight wear will inevitably result in an increase in the percentage of silicone hydrogels used solely for daily wear. Current estimates indicate that the use of silicone hydrogels for daily wear varies between 10 and 80%, dependent upon the country in question, with an average value of approximately 40% [ 19 ]. Once used in this way, then the issue of which care regimen performs optimally with these lenses becomes a matter of some concern to both practitioner and patient alike. This question becomes particularly relevant, given their unique properties described above.

The major point to consider with regards to solution choice relates to optimizing lens comfort. Up to 50% of contact lens wearers complain of symptoms of dryness and reduced comfort with their lenses [ 20 ], particularly towards the end of the day [ 21 ]. Several studies indicate that patients report fewer symptoms of dryness with silicone hydrogel lenses compared with conventional lenses [ 10, 22-26 ], but it still remains a major reason for dissatisfaction, even with these new materials [ 26, 27 ].

In order to maximize lens comfort and minimise dryness, attempts should be made to enhance the wettability of the lens surfaces - by ensuring that the lens surfaces remain as free of tear deposits as possible - and minimizing any compatibility issues with the solutions used.

Modern generation care regimens are typically used in a “no-rub” format, where lenses are merely removed from the eye and placed in the regimen overnight, thus optimizing convenience for the patient. However, the majority of current regimens were developed for conventional materials and have been formulated to optimize protein removal, frequently including components such as citric acid and Hydranate® (hydroxyalkylphosphonate) that optimally remove or sequester protein from the lens material or solution. As described above, silicone hydrogels deposit little protein but increased amounts of lipid compared with conventional hydrogel materials. Not infrequently, patients refitted from a group IV hydrogel material such as etafilcon A (Acuvue 2) - which adsorbs very little lipid [ 28, 29 ] - into a silicone hydrogel material will display increased surface “coating” of their lens (Figure 1) and occasionally some evidence of lens calculi or “jelly-bumps” (Figure 2), which are primarily composed of lipid [ 30, 31 ]. The lipid deposition is occurring due to the increased hydrophobicity that exists at the surface of these new lens materials and this lipid contamination will often decrease the wettability of the lens surface and result in symptoms of dryness and visual dissatisfaction, particularly at the end of the day.

These patients often display signs of mild blepharitis or meibomian gland dysfunction (MGD) and management of these conditions by using conventional lid hygiene methods will often minimise the lipid deposit on the silicone hydrogel material. In situations where there is no obvious sign of MGD then increased attention to removing the lipid is required. This can be achieved by either using a surfactant cleaner containing alcohol (such as MiraFlow® or AOFlow®) in conjunction with their multipurpose regimen, or using a “no-rub” multipurpose care regimen in a “rubbing” format, where the lens is removed, physically rubbed with the care regimen to aid lipid removal and then soaked as usual overnight. In patients who are primarily using their lenses on an overnight basis, periodic removals and rubbing and rinsing in this manner will often improve lens comfort and overall patient acceptance of their silicone hydrogels.

Compatibility of care regimens with silicone hydrogel materials has recently come under some scrutiny [ 32 ]. The vast majority of modern care regimens used to disinfect contact lenses consist of products that are based upon hydrogen peroxide or two high molecular weight antibacterial agents, namely polyhexamethylene biguanide (PHMB) and polyquaternium-1 (polyquad). The most recent addition to the market-place (Bausch & Lomb ReNu MoistureLoc) uses a novel preservative called Alexidine.

A number of recent studies have suggested that the use of certain care regimens with silicone hydrogels may result in abnormally high levels of relatively asymptomatic corneal staining [ 27, 33-35 ], though other studies have refuted this claim [ 36, 37 ]. While clinical problems associated with the use of “old-type” preservatives and disinfectants are well accepted, complications with newer, high molecular weight preservatives are relatively rare, with millions of patients worldwide successfully using modern, one-step, no-rub, multipurpose products. Studies with silicone hydrogels indicate that the corneal staining seen is frequently annular in appearance (Figure 3), typically increases in severity over a 4-week wearing period [ 27, 35 ], is remarkably asymptomatic [ 27, 35, 38, 39 ] and appears to be at its greatest approximately two hours after insertion, with reduced levels observed six hours after insertion [ 40 ]. This pattern may help to explain why some clinicians and published reports have been relatively unaware of this phenomenon [ 36, 37 ], as practitioners may often see a patient for a follow-up appointment some 6-8 hours post-insertion and may only insert fluorescein if the patient reports any discomfort.

These observations are of clinical note, as practitioners switching patients from conventional lens materials into daily wear silicone hydrogels will typically allow the patient to continue with their previous care regimen. If a patient were to exhibit a solution-type sensitivity or toxicity of the type described above, it would be logical for the practitioner to blame the lens material, as that is the only factor to have changed. However, it is not the lens material that is at fault but a specific interaction between the material and care regimen that is causing the problem; the same patient may not exhibit this staining pattern if they were to use a different care system that does not react with the silicone hydrogel material.

If a practitioner switches a daily wear patient from a conventional hydrogel material to a silicone hydrogel then the potential for increased corneal staining with some solutions should be considered. A careful fluorescein evaluation, using a yellow barrier filter, must be undertaken at all after-care visits to ensure no undesirable interactions between the lens material and care regimen are occurring and it would appear that examination of patients approximately two hours post lens insertion is most likely to reveal any potential incompatibilities. Subjects exhibiting such reactions should be switched to another regimen and a careful examination of the cornea undertaken some 7-10 days later to ensure that the interaction has been overcome with the new combination.

In conclusion, patients being fitted with daily wear silicone hydrogel lens materials should be carefully followed-up to ensure that an appropriate care regimen has been dispensed. It is likely that manufacturers will, in future, develop care regimens optimised towards silicone hydrogels and that increasingly practitioners will be provided with solution systems specifically formulated for this growing category of lens materials.

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