Silicone Hydrogels: Is There a Comfort Advantage?
By Eric Papas, PhD, MCOptom, DipCL, FAAO
Silicone hydrogels were introduced just over a decade ago and at their current rate of uptake will all but obliterate traditional materials from the market before 10 more years go by. This phenomenal growth is testimony to the clinical benefits these lenses bring. Chief among them, of course, is their excellent oxygen transmissibility. For most clinicians, the evidence that lens wearers no longer have corneal and limbal signs of hypoxia is unequivocal and relatively easy to spot. The nature and extent of any additional benefits may present a more difficult clinical challenge, especially when it comes to the important area of comfort. How can we really determine if silicone hydrogels offer better performance than traditional hydrogels do in this regard?
In theory, the experiments necessary to provide the answer to this conundrum would seem simple to set up and carry out. The reality is that, while several attempts have been made, the study designs employed have varied broadly. Thus, interpreting the various results that emerge may be trickier than a first glance would suggest. For example, some groups have taken the route of simply assessing the comfort of several existing hydrogel wearers, refitting them with silicone hydrogels, and then reevaluating the comfort response with the new modality. All of the studies that have done this have been consistent in reporting better comfort after the refit than previously (Brennan et al, 2002; more references available at clspectrum.com/references.asp). Given that this refit strategy mimics what clinicians would often do in practice, there will certainly be a temptation to view these papers as providing compelling evidence for the practical superiority of silicone hydrogels in terms of comfort. If we take a step back for a moment, however, and think about this particular method, it becomes obvious that caution is needed before accepting that conclusion.
One issue is that the wearers (and investigators) were typically well aware of the lens types being worn, i.e., they were not masked. Furthermore, because subjects were given new lenses that were different from their habitual pair, we cannot be sure that reports of better comfort would not have happened in any case. Any freshly replaced lens might well have given the same result irrespective of its material, i.e., the order of wear was not randomized. There was no real opportunity for this “new lens” effect to wear off, as the follow-up periods in these studies were generally quite short.
Exceptionally, there was one study in which long-term follow-up was included, and after 3 years of silicone hydrogel wear, the frequency of reported symptoms remained low compared to those reported with the original lenses (Schafer et al, 2007). Although this adds a degree of confidence to the overall view, the lack of masking remains a concern because human beings have many usually subconscious ways of altering their behavior based on what they know or think they know. Thus, treatment masking and order randomization are important elements to look for when interpreting the data from clinical trials in which subjects are switched from one modality to another.
Perhaps the ultimate means of head-to-head comparison is the crossover study. In the example relevant to our question, subjects were randomized to wear either silicone hydrogel or traditional hydrogel lenses for a period and then switched to the alternative lens type after a suitable break, during which they wore neither (Guillon and Maissa, 2007). This design exposes subjects to both lens types with a wearing experience that resembles reality in the sense that the same lens type is applied to both eyes, care systems are used normally, and wearers go through regular wearing and occupational procedures. Patients in this study rated the silicone hydrogel lens more comfortable, but, once again, there is a caveat. Because the recommended replacement frequencies differed between the two lens types—the hydrogels were worn for twice as long as the silicone hydrogels before being discarded—the ages of the lenses at the time of the comfort assessment were different. There is nothing intrinsically wrong with this study design; comparing lenses used according to manufacturers' recommendations is a perfectly reasonable goal. However, it is important for us to understand details like this if we choose to use these results for our own purposes, especially if they differ from the original intentions of the study designer.
A practical issue often raised about crossover trials is that they are quite slow. Basically, researchers must wait until all subjects have finished wearing both lens types before obtaining an answer. To circumvent this, we might conduct a direct comparison in which subjects simultaneously wear a traditional hydrogel lens on one eye and a silicone hydrogel on the other, with the lens assignments masked and randomized. In a study in which this was done, subjects found no significant differences between the two lens types in terms of comfort (Cheung et al, 2007). Allowing wearers to simultaneously experience both lens types in this way is appealing because it increases the likelihood that the same subjective criteria will be used to make the individual comfort judgments in each case. Additional confidence can be placed in this particular result because both lens types were made by the same manufacturer, and this goes some way toward eliminating lens design differences as a factor in the comfort response.
Two Groups, Two Lenses
Another potential solution to the speed problem with crossover studies is to assess the comfort of two groups of wearers at the same point in time, one group already well adapted to hydrogels and the other similarly accustomed to silicone hydrogels (Santodomingo-Rubido et al, 2010). Again, no significant comfort differences emerged between the groups with this method. Critics will point out, however, that because different individuals were in each group and the sample size was relatively small (~20), judging criteria may have varied sufficiently to confound the outcome. So if we conduct “snapshot” studies like this one, large samples are desirable to reduce the potential for bias. By collecting a sufficiently large data set, we would hope to see overall trends emerging, despite underlying variations in lens type, wearing schedule, care system, etc. among individuals.
Two recent studies have taken this approach. Each included more than 900 subjects, and both found a comfort advantage for silicone hydrogel lenses. In one study, the proportion of hydrogel wearers reporting discomfort toward the end of the day was not only greater than for silicone hydrogel wearers, but also increased slightly with the patient's age, at least up to the mid-30s (Chalmers et al, 2009). In the other study (Young et al, 2011), better overall and end-of-day comfort ratings were reported for silicone hydro-gel materials. Whether these trends identified within the larger population are large enough to be appreciated by a typical wearer remains to be seen, but this may be the reason for the general lack of similar findings in direct comparisons in which samples have been small.
A Critical Eye
How should we evaluate all of these various outcomes to answer the question posed earlier? Hopefully, it will be clear that having a greater number of studies pointing in one direction is not necessarily a reliable indicator on its own. We must also consider their individual quality in terms of design and conduct, as well as relevance to the specific question.
This being said, it seems the body of evidence currently available suggests a comfort advantage for silicone hydrogel materials over their lower-transmissibility, traditional counterparts. More work needs to be done to confirm this view and the size of the effect, however. Ideally, these studies will use appropriate, well-considered designs, but if not, we can at least hope that they will be critically scrutinized and accurately interpreted. CLS
For references, please visit www.clspectrum.com/references.asp and click on document #192.
Associate Professor Papas is executive director of Research & Development and director of Post Graduate Studies, Brien Holden Vision Institute and Vision Cooperative Research Centre, and senior visiting fellow, School of Optometry & Vision Science, University of New South Wales, Sydney, Australia. He has received research funds from Ciba Vision, Alcon, AMO, and Allergan. You can reach him at firstname.lastname@example.org.