Rethinking the Link Between Dk and RGP Lens Performance

BY JOHN L. SCHACHET, OD; LEE E. RIGEL, OD; KEVIN M. REEDER, OD; THOMAS M. HIXSON, OD; DAVID W. HANSEN, OD; AND SUSAN CONNELY, FCLSA

This study challenges the traditional belief that high oxygen permeability and good lens performance are mutually exclusive in a workhorse RGP material.

Increasing the Dk of RGP contact lenses is often accompanied by compromises in other lens properties vital to the clinical performance of the material. High oxygen permeability has frequently been associated with poor surface wetting, increased lens deposits, greater lens flexure and dimensional instability. However, recent advances in polymer chemistry have resulted in a new generation of lens materials that challenge the belief that low Dk lenses provide superior on-eye performance.

Study Design

To address the clinical questions related to the relationship between Dk and material performance, we conducted a double-masked study at six independent clinical sites comparing three RGP materials: Paragon HDS (Dk=58), Boston ES (Dk=31) and Boston IV (Dk=26.3). Criteria for patient inclusion was a history of over one year of successful RGP lens wear for at least eight hours a day. Thirty-four subjects enrolled and 33 patients (66 eyes) completed the three-month study. One patient discontinued due to loss of interest. Table 1 and Table 2 display baseline patient characteristics.

Three sets of lenses (one in each material) were manufactured by an independent laboratory in lens parameters identical to the patients' current RGP specifications. Each investigation site inspected the finished lenses to verify their accuracy and quality.

All lenses were unknown to the subjects and to the investigators. Investigators collected each worn pair of lenses upon dispensing the new pair. Patients wore each of the three pairs of lenses for 30 days with a minimum wearing time of eight hours and a maximum of 16 hours per day. They were instructed to use their habitual lens care regimen throughout the course of the study.

Follow-up protocol consisted of: evaluating lens position, lens movement and apical lens-to-cornea relationship; performing spherocylindrical overrefraction (SCOR), logMAR high and low contrast visual acuity and biomicroscopy (Table 3); and evaluating four aspects of in vivo wetting -- pre lens tear breakup time, percent tear lens coverage after the blink, tear thickness and surface deposition (see sidebar on p. 46). Patients completed a questionnaire rating the lens performance of their habitual lenses and each pair of study lenses at the two-week and four-week follow-up visit.

Dispensing Visit Results

At dispensing, lenses made from Paragon HDS demonstrated a higher mean and median for initial comfort than those made from Boston ES. One patient reported blurred vision with Paragon HDS
lenses compared to five patients with Boston IV. Four patients reported a need for cleaning at dispensing with Boston IV lenses compared to two with
Paragon HDS lenses.

Lenses made of Paragon HDS demonstrated the longest mean pre lens tear break up time (TBUT) and the highest mean percent tear lens coverage when compared to Boston IV and ES. These are apparent trends, but a larger sample size would be required to determine statistical significance.

Four-Week Visit Results

Biomicroscopy findings after four weeks of wearing lenses made of the Paragon HDS material demonstrated less staining when compared to four weeks of wearing Boston IV and ES. An increase in staining occurred in five eyes with HDS, 13 eyes with Boston IV and 12 eyes with ES.

The mean and median for overall image sharpness with the Paragon HDS lenses were greater than that with both Boston IV and Boston ES lenses. The mean for the night vision response as well as mean consistency of vision was greater with Paragon HDS lenses than with Boston IV lenses for both variables. All three materials demonstrated equivalent median responses for night vision and consistency of vision responses.

Subjects' mean perception of their near vision was higher with Paragon HDS than with Boston IV or ES. Subjects also reported the highest overall satisfaction with Paragon HDS lenses compared to Boston IV and ES lenses. The median values for these responses were equivalent. Table 4 and Table 5 show how patients rated each material in various categories, however a larger sample size would be required to determine statistical significance for these apparent trends.

The mean pre lens tear break up time was greater for lenses manufactured in Paragon HDS material than for those manufactured in Boston IV. Paragon HDS demonstrated the greatest median tear lens break up time of all three materials. While the median percent tear lens coverage values for all three materials were equivalent, the mean value was greater for the Paragon HDS material than for the Boston ES material (Table 6).

We also noted an apparent trend of a lower mean deposition with lenses manufactured in Paragon HDS than with those manufactured in Boston IV and ES, but a larger sample size would be required to establish statistical significance for this measure as well.

Of the three materials, there was an apparent trend for subjects to report fewest symptoms after wearing the lenses made of Paragon HDS material and to report the least need for cleaning with the Paragon HDS lenses (Table 7). However, these trends did not prove to be statistically significant.

We analyzed the comfort reported at the four-week visit by assessing the number of responses at each level by material (Table 8). We compared the overall satisfaction patients reported on the questionnaire for the four-week visit to the overall satisfaction with their habitual lenses reported on the baseline questionnaire. Thirty-four eyes were reported as having equal or improved overall satisfaction with Paragon HDS, 28 eyes with Boston IV and 26 eyes with Boston ES.

Clinical Measures of Lens Fit

We assessed lens centration, movement on the blink and lens-to-cornea relationship to compare the fit of the lenses. There was no difference in the measures of lens fit for the three materials. Upon inspection, we found all contact lenses to be well within the tolerance of the ANSI standards.

In Vivo Dimensional Stability

The measures of cylindrical overrefraction and logMAR low contrast visual acuity with the spherocylindrical overrefraction in place reflect in vivo lens flexure, which may result in a higher mean and standard deviation of the cylindrical overrefraction. There was no difference between the cylindrical overrefraction of the three materials at dispensing, two-week follow-up or at four-week follow-up. The standard deviation of the cylindrical overrefraction reflects similar variation in the findings for the three materials.

Supporting the Equivalence Hypothesis

This clinical trial demonstrates the equivalence of Paragon HDS material to currently marketed materials of lower Dk values.

Paragon HDS provides contact lens practitioners with an alternative workhorse material having twice the Dk value of others widely used. The equivalent clinical performance provides practitioners and patients alike with advantages from higher Dk coupled with very good functional wetting, low soiling, minimized staining, very good vision, few symptoms, comfort, stability and great overall satisfaction.

References are available upon request to the editors at Contact Lens Spectrum. To receive them via fax, call (800) 239-4684 and request document #40.

This study was sponsored by Paragon Vision Sciences.

Drs. Schachet, Rigel, Reeder, Hixson and Hansen and Ms. Connelly are all in practices that fit and manage large populations of RGP lens wearers. All were clinical investigators for this study.