A Comparative Study

of RGP Materials in Thin Lens Designs

FRANK R. TOSCANO, O.D., F.A.A.O., & BRUCE BRIDGEWATER, O.D.
OCT. 1997

This study compares the clinical performance of thin lens designs manufacturered in Boston ES and FluoroPerm 30, with emphasis on masking astigmatism.

In a study six years ago, we described how we successfully refit 20 adapted, thin-design RGP lens wearers with FluoroPerm 30 and FluoroPerm 60 materials in thin, Polycon-type designs. At the time, practitioners were concerned about flexure and instability which were idiosyncratic to many of these high-Dk fluorosilicone acrylate materials. Our study demonstrated that these factors were not problematic, and we continued to prescribe FluoroPerm 30 in the thin designs for most of our RGP patients.

Since then, RGP lenses of about 30 Dk have become the standard of care, and the variety of 30 Dk materials has increased. Among the more recent introductions is Boston ES manufactured by Polymer Technology. According to the manufacturer, the basic chemistry of this material improves its rigidity, making it superior to other fluorinated materials. The company cites in vitro flexural resistance testing to support its claims of enhanced optical quality and masking of astigmatism, particularly in thin designs. We conducted this study to see if these qualities carried over into clinical contact lens practice.

PATIENT SELECTION & STUDY DESIGN

We selected 23 current spherical RGP lens wearers for this four-month, double-blind study. All patients exhibited at least 0.75D of corneal astigmatism measured with the keratometer, and more than half (12 patients) had corneal astigmatism of 1.50D or greater, the threshold at which lens flexure can be especially troublesome (Table 1).

All patients received two pairs of lenses over the course of two months (5 visits). Each pair consisted of one lens made from FluoroPerm 30 and one lens from Boston ES. Conforma Laboratories of Norfolk, Va., manufactured the study lenses and maintained records to assure that lenses were dispensed according to protocol. If a patient's first pair of lenses was FluoroPerm 30 OD and Boston ES OS, then the second pair was Boston ES OD and FluoroPerm 30 OS. Neither practitioner nor patient knew which material was worn on either eye.

The patient visit schedule was as follows:

At the initial visit, we performed refraction, keratometry, slit lamp biomicroscopy, and when necessary, diagnostic lens fitting. At each follow-up visit, we measured Snellen acuity with lenses, and performed overrefraction, overkeratometry and slit lamp biomicroscopy. We also charted lens position, movement and surface quality.

At the one-month visit when lens pairs were exchanged, and at the concluding two-month visit, patients completed a questionnaire to rate right-eye and left-eye lenses on vision, lens comfort upon insertion, lens comfort just prior to removal and overall comfort. The simple rating scale ranged from 1 (right much better) to 5 (left much better), with the intervening scores of 2 (right somewhat better), 3 (equal) and 4 (left somewhat better).

RESULTS AND OBSERVATIONS

In evaluating post-fit residual cylinder, it appeared that both materials performed well in masking astigmatism. Figure 1 illustrates post-fit residual cylinder on overrefraction as a percentage of refractive cylinder. FluoroPerm 30 averaged 36.3 percent of pre-fit cylinder on dispensing versus 47.6 percent for Boston ES. By the one-week visit, that gap had closed somewhat (35.6% for FluoroPerm 30 and 38.6% for Boston ES) and all but disappeared by the one-month visit (31.8% for FluoroPerm 30 and 32.9% for Boston ES).

Spherical overrefraction yielded somewhat even results. Twenty-nine of the 46 eyes accepted additional correction. Figure 2 shows the average overrefraction for these eyes in absolute diopters. On the dispensing visit, both materials performed equally, with overrefraction averaging slightly less than a quarter-diopter (0.2112D) for each material. At the one-week visit, Boston ES eyes accepted slightly more correction (0.2715D) than FluoroPerm 30 eyes (0.2198D). At the one-month visit, the situation reversed, with FluoroPerm 30 eyes accepting slightly more correction (0.2025D) than Boston ES eyes (0.1681D). Claims that the laboratory-measured stiffness of Boston ES provides better optical clarity did not seem to translate to enhanced clinical performance.

In terms of Snellen acuity, 34 eyes experienced a slight loss (far less than one line on average) from best corrected visual acuity for both lens materials. There were no significant differences in the other observable clinical performance areas. Lenses of both materials positioned well and moved appropriately.

Biomicroscopy revealed no edema or neovascularization on any visit. There were a few incidents of superficial and transitory (Grade 1) fluorescein staining. In each case, staining was bilateral and, therefore, not related to lens material. We observed a single case of bilateral Grade 2 staining on one visit. We referred the patient to a staff member for blink training and the patient's corneas were clear on the next visit.

PATIENT PREFERENCE

As indicated by the two one-month questionnaires, patients stating a preference favored FluoroPerm 30 over Boston ES (Fig. 3). Fourteen patients preferred the vision provided by the FluoroPerm 30 lenses and four patients preferred the Boston ES lenses. Patient assessment of lens comfort upon insertion was equal; four preferred FluoroPerm 30 and four preferred Boston ES.

Patients having a lens preference for comfort upon removal gave a slight edge to FluoroPerm 30, eight to five, across the two questionnaires. In terms of overall preference, FluoroPerm 30 was preferred in 10 cases compared with eight for Boston ES. Among patients having a preference, the FluoroPerm 30 lens was most often preferred in both lens pairs (Fig. 4).

WORKHORSE MATERIALS RUN NECK-AND-NECK

These results demonstrate that both materials are clinically useful and will perform well in thin, Polycon-type designs. However, our data does not indicate that either material is superior in terms of masking more astigmatism or providing enhanced optical quality. In fact, patient preferences tended to favor FluoroPerm 30.

We believe that competent practitioners can achieve quality of vision, comfort and ease of handling with either material. As Dr. Robert Koetting says, "The way a contact lens is fitted is more important than the material from which it was made." CLS

Dr. Toscano is in an M.D./O.D. partnership practice in Virginia Beach, Va., where he specializes in contact lenses and primary care.

Dr. Bridgewater is in private practice in Phoenix, specializing in unique contact lens applications.

This study was sponsored by Paragon Vision Sciences. The authors have no financial interest in the company.