Specific Gravity and RGP Lens Performance

BY ALAN P. LEVITT, O.D.
OCT. 1996

Enhance overall RGP lens performance by linking appropriate designs and powers to materials that have complementary specific gravities. Among the factors that influence rigid gas permeable lens performance and ocular tolerance are: diameter, optic zone, power, base curve, peripheral curves, center thickness, edge design, material wettability and oxygen permeability. As part of the process of prescribing RGPs, practitioners select and combine these variables to provide the best possible fit and vision.

These variables directly affect lens mass, an important parameter that cannot be specified when ordering an RGP lens without the assistance of a specific computer program. Lens mass is especially important for positioning moderate to high lens prescriptions and thicker-than-standard designs such as bitorics and prism ballasted lenses. In addition to diameter, power, thickness and lenticulation, specific gravity is another significant, but often overlooked factor that affects mass.

OPTIMIZING RGP LENS PERFORMANCE THROUGH SPECIFIC GRAVITY

Specific gravity is the ratio of the mass of a solid or liquid to the mass of an equal volume of distilled water at 4º C. High specific gravity materials have greater mass than equal volumes of lower specific gravity materials. Thus, the mass of an RGP lens can be altered significantly just by selecting a material with a high or low specific gravity, even when all other parameters remain equal.

Choosing materials based on specific gravity, while keeping all other parameters the same, can create up to a 20 percent change in lens mass and significantly affect lens position. Compare this to lenticulation alone, which reduces lens mass only 10 percent to 15 percent.

The greatest changes in lens mass can be accomplished by manufacturing RGP lenses in lenticular or prism ballasted designs from materials selectively chosen for their specific gravity. RGP materials range in specific gravity from 1.050 to 1.270 (Table 1).

The following methods link lens prescription, design and material selection to optimize RGP lens performance.

MODERATE TO HIGH PLUS LENSES

Fitting hyperopic patients in a with-the-lid or Korb design is a special challenge because plus lenses have anteriorly located centers of gravity compared to minus lenses, and these patients often have relatively flat corneas. Together, these factors may cause plus lenses to drop off the upper lid and seek an inferior position on the cornea.

By combining minus lenticulation with a low specific gravity material, the mass of a plus lens can be maximally reduced, which minimizes its tendency to ride inferiorly and lose its attachment to the upper lid. This is especially evident as plus power increases. In contrast, ordering a lenticular design in a high specific gravity material will be counterproductive.

MODERATE TO HIGH MINUS LENSES

Consider using low specific gravity materials when lenses ride too low. Combine a plus lenticular design with a low specific gravity material to create a lighter, thinner-edged lens that may now be captured by the upper lid. Recent studies have demonstrated that high specific gravity materials have little lowering effect on high riding minus lenses.

PRISM BALLASTED BIFOCAL OR TORIC LENSES

High specific gravity materials may help a lens position inferiorly by increasing its mass, thus improving performance. Consider ordering a high specific gravity material as an alternative to increasing prism ballast when these designs position higher than desired. By choosing materials with high Dk and high specific gravity, lens position may be improved without creating a thicker lens that further reduces oxygen flow to the cornea.

CONSIDER ALL THE VARIABLES

Specific gravity should be recognized as another significant quality of RGP materials, in addition to Dk and wetting angle, that influences rigid lens performance on the human eye. No single material quality or design variable assures successful fitting of all patients with contact lenses. Total lens design can be optimized for each patient by considering all design variables and material qualities that affect on-eye performance. CLS

Dr. Levitt is in private practice in North Miami Beach, Fla. He is a preceptor for Pennsylvania College of Optometry.