A Performance Comparison of Two Hydrogel Lenses

KENNETH A. LEBOW, OD, BRUCE BRIDGEWATER, OD, & WALTER WEST, OD
APRIL 1998

This contralateral study compares Proclear, a high water content lens, to CSI, a low water content lens often renowned for good comfort, deposit resistance and on-eye hydration.

Since its introduction in the early 1980s, CSI (crofilcon A) has been widely regarded as a benchmark soft contact lens with good all-around performance. This lathe-cut design offers superior and consistent visual acuity, a relatively stable, non-coating surface and good handleability. It has been suggested that these clinical advantages are due to the polymer's low water content (38%), non-ionic surface characteristic and relative stiffness compared to other hydrogel polymers. Because of these clinical attributes, the CSI lens, manufactured by Wesley Jessen, has been described as a superior choice for maximizing visual performance, masking astigmatism, problem-solving for improved lens comfort and resolving giant papillary conjunctivitis.

Proclear, a lathe-cut, 59 percent water content lens manufactured by Biocompatibles in omafilcon A material, offers low on-eye dehydration, good oxygen transmissibility and superior deposit resistance to proteins and lipids. Conventional wisdom holds that lower water content materials are less prone to dehydration and deposit formation than higher water content materials. Reports have suggested that the dehydration characteristics of Proclear are more similar to low water content lenses than to those with equivalent water contents. This study was designed to test the clinical performance of Proclear lenses in comparison to CSI lenses, which are often described as a standard of excellence in hydrogels.

Methods

In this three-month, double-masked, daily wear study, nine centers across the United States agreed to fit a minimum of five patients with the Proclear lens in one randomly selected eye and the CSI lens in the contralateral eye. A total of 54 patients participated (Table 1). Base curve and diameter selection was limited to the median 8.5mm, 14.2mm design for Proclear and 8.3mm, 13.8mm for CSI. Patients cared for both lenses with Bausch & Lomb's ReNu multipurpose solution. CSI lenses were enzymatically cleaned weekly, while Proclear lenses were not.

At the initial visit, patients' general ocular and contact lens histories as well as their baseline keratometric, refractive and slit lamp measurements were recorded. Each patient was examined one week, one month, two months and three months after beginning lens wear. Table 2 details the range of measurements taken at the follow-up visits. Refractive and keratometric measurements were repeated at the exit visit for comparison with baseline values.

Subjective comfort was assessed at each visit using a 10cm visual analogue scale (VAS). Patients marked the scale according to their perceived lens comfort, with 10 indicating very comfortable (unable to feel the lens) and zero indicating extremely uncomfortable. Patients also completed a comparison comfort log during the first 30 days of lens wear.

Lens water content was assessed at the exit visit using a hand-held Atago refractometer. The error of measurement for the refractometer is one percent.

Results

Subjective Ratings (Comfort, Symptoms and Lens Preference) -- The mean comfort score was higher for Proclear at each visit, but this was statistically significant only at the one-month visit (Fig. 1). On average, patients preferred Proclear lenses to CSI lenses throughout the study (Fig 2). The patient log of preference showed an identical proportion of preferences for Proclear at 30 days as there was at 90 days (2:1). During the first 30 days of wear, patients reported an increasing preference for Proclear and a decreasing preference for CSI (Fig. 3). Patients rated their subjective symptoms of dryness, itchiness or grittiness as none, mild, moderate or severe. A significantly greater level of symptoms (p<0.05) were reported for the CSI lenses at the one-month visit; otherwise there were no significant differences in the number or level of symptoms (Fig. 4).

At the conclusion of the study, the lens preference data for previous contact lens wearers was grouped to show those who had previous CSI wearing experience and those with no previous experience with CSI lenses (Table 3). Twelve out of 24 subjects who had previously worn CSI lenses preferred the Proclear lens, while eight preferred CSI and four had no lens preference. Of the 10 patients with no previous experience with CSI, four preferred Proclear, two preferred CSI and the remaining four had no lens preference.

Objective Findings (Lens Fit, Ocular Physiology, Lens Dehydration) -- There were no statistically significant changes in keratometry, overrefraction, ocular physiology, lens fitting assessment and best corrected visual acuity for each lens type or between Proclear and CSI at the completion of the study. At both the one-month and two-month visits, the CSI lenses showed significantly less post-blink movement than the Proclear lenses. By the three-month visit, however, there were no significant differences between the Proclear and CSI lenses. There were no significant differences in ocular physiology between the Proclear and CSI lenses at any visit. The relative percentage dehydration for Proclear was similar to that of the CSI lenses (4.1% vs 4.9% respectively). The lens hydration results are shown in Table 4.

Discontinuations -- Ten patients failed to complete the study. Three of these incompletions were lens-related (two related to the CSI lens and one related to both lenses), six were lost to follow-up and one was solution-related.

Discussion

We chose the CSI lens as the benchmark for this clinical performance comparison based upon its reputation for deposit resistance and high quality optics. In addition, it's logical to expect the CSI lens to dehydrate significantly less than polymers of higher water contents, making it a consistent product that would yield a challenging comparison.

Although the higher comfort rating of Proclear compared to CSI was statistically significant at the one-month visit only, a clear trend within the data substantiates a strong patient preference for Proclear. This is confirmed by the subjective patient evaluations of the study which demonstrated that the number of days patients preferred Proclear increased during the first 30 days. Forced patient choice results at the conclusion of the study demonstrate a similar two-to-one preference for Proclear. Moreover, patients reported significantly fewer symptoms of dryness, itchiness or grittiness with Proclear than with CSI at the one-month visit.

Weekly enzymatic cleaning is recommended for the CSI lens to improve long-term patient comfort by reducing surface spoilage. Overall patient preferences for and improved subjective comfort with the Proclear lenses are even more significant in the light of the absence of enzymatic cleaning. This suggests that the CSI lenses, even with weekly enzymatic cleaning, did not provide the long-term physiological advantages that appeared to exist with the Proclear lenses.

Although the results of this study show no significant difference in the relative percentage of dehydration, the Proclear lenses dehydrated less than the CSI lenses. This is impressive, considering the Proclear lens has a higher water content than the CSI lens and would be expected to show a greater amount of relative on-eye dehydration. The oxygen transmission of a contact lens is determined by the water content and center thickness of the lens, so any on-eye lens dehydration will adversely affect the level of oxygen available to the corneal tissue. Holden and Mertz have reported that oxygen transmission (Dk/L) of 24 x 10^-9 through a contact lens is necessary for no change in corneal physiology during daily wear. The CSI lens has an estimated Dk/L of 21.7 x 10^-9, slightly below that recommended by Holden and Mertz, whereas the oxygen transmissibility of Proclear is high (Dk/L of 35.7x 10^-9, Dk=25) due to the higher water content and relatively thin center thickness (center thickness=0.07mm). Proclear's dehydration resistance, which provides sustained oxygen flux, may be partly responsible for its overall preference and improved comfort relative to the CSI lenses.

Conclusion

In this study, the Proclear lens performed as well as the CSI lens in terms of lens fit, ocular physiology and on-eye hydration. The Proclear lenses tended to become more comfortable as the study progressed while the CSI lenses tended to become less comfortable. This comfort data was also consistent with the patient preferences at the completion of the study. Although it is generally presumed that high water content materials exhibit greater on-eye dehydration, Proclear's clinical performance in this study demonstrates that a high water, non-ionic material can possess dehydration characteristics equivalent to low water, non-ionic materials.

We conclude that the Proclear lens provides improved lens comfort through limited on-eye dehydration and sustained high oxygen transport, offering the potential for improved ocular surface biocompatibility. This enhanced biocompatibility provides a potential solution for the rather significant rate of contact lens dropouts resulting from lens discomfort, variable or reduced visual acuity throughout the wearing schedule, physiological compromise or general lens intolerance. Further, Proclear promotes lens care compliance and reduced costs through simplified care.

This study was sponsored by Biocompatibles Eyecare, Inc.

References are available upon request to the editors at Contact Lens Spectrum. To receive references via fax, call (800) 239-4684 and request document #35. (Be sure to have a fax number ready).

Dr. Lebow, a past chair of the AOA Contact Lens Section, is in private practice in Virginia Beach, Va., specializing in contact lenses and clinical research. He is also a consultant for Biocompatibles Eyecare, Inc.

Dr. Bridgewater is a member of the Contact Lens Section of the AOA, the International Academy of Sports Vision, the Contact Lens Association of Ophthalmologists and the International Society of Refractive Keratoplasty. He is in private practice in Phoenix, Ariz.

Dr. West lectures nationally and internationally and is in private practice in Brentwood, Tenn.