Comparing Hydrogel and Silicone Hydrogels

See how a particular hydrogel lens fared going head-to-head with two silicone hydrogels.

Comparing Hydrogels and Silicone Hydrogels
See how a particular hydrogel lens fared going head-to-head with two silicone hydrogels.
By Brian Chou, OD, FAAO

Silicone hydrogel contact lenses are well recognized for their high oxygen transmissibility. High-Dk/t lenses minimize hypoxic corneal changes that include microcysts, limbal hyperemia and neovascularization. Meanwhile, the contact lens industry is correcting some of the early surprises associated with silicone hydrogels such as solution incompatibility. Some solution manufacturers have introduced lens care systems, such as Opti-Free Replenish (Alcon), that are designed to be compatible with all soft contact lenses including silicone hydrogels.

Furthermore, the market entry of lower-modulus silicone hydrogels should reduce modulus-related surprises such as mucin-balls, superior epithelial arcuate lesions (SEALs), contact lens papillary conjunctivitis (CLPC) and conjunctival epithelial flaps.

Finally, the contact lens market is experiencing a notable shift in silicone hydrogel prescribing away from continuous wear toward daily wear. This shift toward daily wear should further enhance clinical safety for patients wearing silicone hydrogel lenses.

Figure 1. Conjunctival staining with Acuvue Advance.

Does More Oxygen = More Comfort?

While the evolution of silicone hydrogel lenses continues, some unanswered questions still exist. Among them, does the high oxygen transmissibility of silicone hydrogels convey optimal wearing comfort? Some of the commercially available silicone hydrogel lenses are marketed in a way that implies a direct connection between higher oxygen transmission and greater comfort. As a result, many practitioners appear to believe that silicone hydrogels offer better wearing comfort than do hydrogel materials.

This article reviews two recent studies that compared the subjective comfort of omafilcon A (Proclear Compatibles, CooperVision), a biomimetic hydrogel lens material, against two leading silicone hydrogel lenses. Several papers in the peer-reviewed literature have demonstrated that contact lens wearers of omafilcon A typically experience less symptoms and signs of lens-related discomfort vs. wearers of other hydrogel materials, while others have not. The two studies I review here were designed to compare the comfort and patient preference of silicone hydrogels against the Proclear lens.

Proclear vs. Acuvue Advance

The first study (Iravani N et al, 2005) was presented at the 2005 BCLA conference and won first place in the poster contest over more than 60 entries. In this bilateral, two-part crossover study, 60 subjects wore omafilcon A and galyfilcon A (Acuvue Advance, Vistakon) contact lenses in random succession for four weeks, each with daily lens removal. Study participants were asked to use Opti-Free Express (Alcon) to care for their test lenses. Although galyfilcon A is labeled for two-week replacement, a four-week replacement schedule was selected to allow for comparison against omafilcon A under similar time intervals. The researchers performed clinical assessments and patients completed questionnaires at two weeks and four weeks after beginning wear of each test lens.

Figure 2. Fluorescence from lipid and protein deposition from a representative sample of lenses.

Approximately half (55 percent) of the subjects enrolled reported a maximum comfortable wearing time (WT) of 10 hours or less with their habitual lenses. A significant number of subjects reported that both omafilcon A and galyfilcon A lenses improved comfortable WT compared with their habitual lenses. There were no significant differences between the two test lenses with respect to comfort or comfortable WT.

The slit lamp findings were similar between lens types except for conjunctival staining, which was graded significantly higher for Acuvue Advance (Figure 1); at four weeks, conjunctival staining was noted in 70 percent of eyes wearing Acuvue Advance compared with 33 percent of eyes wearing Proclear Compatibles. The mean scores for limbal hyperemia were similar for both lenses. Similar proportions of subjects stated a preference for each lens type; for overall comfort, 46 percent of subjects expressed a preference for Proclear Compatibles compared to 35 percent for Acuvue Advance, and 19 percent indicated no preference. 

In a post-study analysis of lens surface deposits, Acuvue Advance showed greater deposition as graded by slit lamp than did Proclear Compatibles (Figure 2). During the study, two of the Acuvue Advance lenses required replacement because of deposits (Figure 3) compared to none for the Proclear lenses.

With the exception of the conjunctival staining and surface deposits with Acuvue Advance, the overall clinical performance of Proclear lenses was similar to that of Acuvue Advance. Both lenses showed improved comfort and WT compared to conventional hydrogel contact lenses. Other researchers have since reported a higher-than-baseline level of conjunctival staining with Acuvue Advance as well (Brennan, Coles, 2006).

Figure 3. Deposits on Acuvue Advance after two weeks of daily wear for one subject.

Proclear vs. O2Optix

This second study (Morgan D et al, 2005) shared similarities in design to the previous evaluation. It was a subject-masked, bilateral crossover investigation. Forty subjects wore omafilcon A and lotrafilcon B (O2Optix, CIBA Vision) lenses in random succession and were evaluated after two weeks and four weeks of wear. The researchers asked study participants to use CIBA Vision AOSept Plus (in the United Kingdom) or Clear Care (in the United States) with their test lenses, wearing the lenses with daily removal and four-week replacement. As in the earlier study, lotrafilcon B is labeled for two-week replacement, but a four-week replacement schedule was selected to allow comparison against omafilcon A under similar time intervals. The researchers performed clinical assessments and patients completed questionnaires at two weeks and four weeks after beginning wear of each test lens.

Both O2Optix and Proclear offered excellent overall clinical performance. The Dk/t of O2Optix (125) and of Proclear (52) both exceed the minimum Dk/t of 35 reported by Harvitt and Bonnano to avoid induced corneal swelling for daily wear. The current study showed similar slit lamp findings except that the O2Optix lens was associated with less limbal redness than was Proclear at the four-week point, which is consistent with previous reports of reduced limbal hyperemia with high-Dk/t silicone hydrogels (Sweeney, 2003).

Mean comfort score on a 0 to 100 visual analogue scale was 79.0 for Proclear, and 68.8 for O2Optix (two-tailed P = 0.0046). Mean comfortable WT for the Proclear lenses was 11.83 hours and for the O2Optix 10.75 hours (two-tailed P = 0.0563). There were no significant differences between lens types for dynamic and static lens fit, visual acuity or subjective ratings of visual quality. "Burning/stinging" and "dryness" symptoms demonstrated a lower frequency with Proclear, but the difference wasn't statistically significant. Sixty-eight percent of subjects preferred Proclear overall compared to 32 percent for O2Optix, and 0 percent reported no preference. For end-of-day comfort, 59 percent preferred Proclear as compared to 24 percent for O2Optix, and 17 percent indicated no preference. Figure 4 summarizes patient questionnaire results.

Factors that Influence Lens Comfort

Figure 4. Results of subject responses to a forced-choice questionnaire at the conclusion of the study. P-values are from Fisher's Exact test.

The clinical evaluations of Proclear against Acuvue Advance and O2Optix suggest that for most patients, Proclear offers equivalent if not better comfort. The logical conclusion is that high Dk/t alone doesn't guarantee optimal lens-wearing comfort. Further support for this conclusion comes from a study (Fonn, Dumbleton, 2003) in which investigators found that symptoms of dryness and discomfort were no better with the silicone hydrogel material lotrafilcon A (Night & Day, CIBA), than they were with three hydrogel materials including omafilcon A.

Taken in sum, it's likely that multiple factors besides Dk/t � including lens modulus, wetting and dehydration characteristics, edge profile and deposit resistance � collectively influence lens-wearing comfort. The empirical "proof" is that many happy and comfortable PMMA lens wearers exist. As we all know, PMMA is completely impermeable to oxygen. If high Dk/t was the sole requisite for comfortable lens wear, no patient would have tolerated PMMA lens wear.

Looked at from a different angle, if high Dk/t really is critical to wearing comfort, why aren't all wearers of hyper-Dk GPs successful? Alas, some patients cannot tolerate the GP lens edge awareness — a factor that's completely independent of Dk/t. So does hypoxia cause dryness-related discomfort? Perhaps that question is best addressed by Noel Brennan, MScOptom, PhD, FAAO, and Philip Morgan, MCOptom, FAAO, who in their December 2005 Readers' Forum "Another View of Oxygen Transmission" wrote, "There's absolutely no evidence in the Medline-abstracted literature to back (such a) claim."

Neither the studies nor any anecdotal clinical experience indicates that comfort and silicone hydrogels are mutually exclusive. Riley and Chalmers at the 2005 American Academy of Optometry meeting demonstrated that one new-generation silicone hydrogel for daily wear can alleviate some common lens-related symptoms. With other daily wear silicone hydrogels in the industry pipeline, we can expect to see additional improvements in wearing comfort. Research and development efforts are shifting away from Dk/t toward other desirable lens characteristics. 

Clearly the key to contact lens comfort is not as simple as high Dk/t alone. Comfort is a more complex equation which deserves further scientific clarification. Meanwhile, practitioners should realize that despite the buzz surrounding silicone hydrogel materials, omafilcon A remains a proven benchmark for daily wear lens comfort, possibly for many years to come.

Dr. Chou is in private group practice in San Diego. He is also a consultant for the California Board of Optometry and a clinical investigator and consultant to CooperVision, SynergEyes and Ophthonix.

For references, please visit and click on document #124.