RGP insights

Choosing Your RGP Designs Without Fear

BY LORETTA B. SZCZOTKA, OD, MS
April 1999

It has taken years of research and development in the oxygen arena, but manufacturers are finally providing proven rigid gas permeable materials and promising new soft contact lens products. These advancements have which has raised our awareness of the benefits of super and hyper Dk. Although high Dk products. These advancements have raised our awareness of the benefits of super and hyper Dk. Although high Dk soft contact lenses will be launched soon, they will undoubtedly be initially released in limited parameters and limited markets. But what product has been available in every parameter to all markets and can be used for any patient indication? High, super and hyper Dk RGP contact lenses.

Dk Classifications

In 1993, Dr. William "Joe" Benjamin published his classifications of oxygen permeability and transmissibility standardized by ranges of equivalent oxygen percentage (EOP) (Fig. 1). Until the release of high Dk soft lenses, only RGP contact lenses could provide EOP values in Benjamin's super- and hyper-transmissible classifications. The EOP value of a material, developed by Richard Hill O.D., Ph.D., is the oxygen percentage (on a scale of 0 to 21, where 21 is the percentage of oxygen in the air) that produces a corneal "oxygen thirst" equal to that which is produced after wearing the test contact lens.

EOPs are lens thickness-dependent. During my graduate years, we reported on lens thicknesses and its impact on corneal hypoxic stress for a variety of RGP contact lens polymers. The findings from this research can be transferred to clinical applications in everyday practice as you decide which RGP material you should use for each of your patients.

Real World Application

The corneal oxygen supply of RGPs in Benjamin's low, medium and high Dk/L classifications is significantly decreased when lens thickness is increased. These materials, if manufactured in high myopic, hyperopic or aphakic corrections, prism ballasted bifocals or front surface RGP designs, will have significantly diminished transmissibility compared to a standard 0.12mm center thickness lens of the same material.

The thickness of RGP contact lenses in the super Dk/L range has reduced impact on corneal oxygen supply. The contact lens practitioner is able to manipulate lens thickness for other purposes, such as controlling lens flexure, centration, drop rate, lid interactions and physical fit, without sacrificing much oxygen transmissibility.

Have No Fear

Hyper Dk/L RGPs are nearly insensitive to lens design in their oxygen performance, and the cornea may not be able to distinguish EOP differences between different designs of the same material.

Use these contact lens materials without fear of diminishing the oxygen supply, even in the most complex and hearty designs. Concurrent advancements in lens stability allow some hyper Dk/L products to be produced in super thin designs, which may provide the ultimate combination for patient comfort.

Courtesy of Menicon USA and adopted from Benjamin, W.J. JAOA 1993; 64 (3).Dr. Szczotka is an assistant professor at Case Western Reserve University Dept. of Ophthalmology and Director of the Contact Lens Service at University Hospitals of Cleveland.