Matching the Patient with the Gas Permeable Material

The material you choose is as important as the lens design and fitting relationship.

By EDWARD S. BENNETT, OD, MSEd
St. Louis, Mo.

The No. 1 question laboratory consultants hear from contact lens fitters is “Which gas permeable lens material should I use?” With more than 30 lens materials to choose from, this is no easy question to answer. With assistance from Keith Parker (Accu Lens), Kevin Hing (Danker) and Larry Platt (Platt Contact Lens), here’s which lens materials would be optimum for a given patient.

GP Material’s Critical Functions • Provide sufficient oxygen permeability to meet the cornea’s oxygen needs. • Provide good on-eye surface wettability. • Provide crisp visual acuity.

• Young myopes. Young myopes are excellent candidates for gas permeable lenses due to the ability of these lenses to slow down the progression of their myopia. For these patients, a sturdy, stable material is important to minimize warpage and wettability problems. Our experts recommend a low Dk F-S/A material. As young people are quite active, a larger-than-average diameter (9.6 – 10.5mm, for example) will provide a stable fit, and minimize lens dislodgement and loss. Reinforce at each follow-up visit the need for patients to handle their lenses carefully when cleaning them in the palm of the hand upon removal every night.

• Presbyopes. The most underutilized lens modality for presbyopic patients? Gas permeable multifocal and bifocal lens designs. The benefits of field of view and quality of vision are quite apparent. In addition, the lenses are not complicated to fit. The most common lens design for presbyopia is the aspheric multifocal design.
As these are thin designs that are fit steeper than “K” with the goal of good centration and limited movement with the blink, a low Dk material is often used, notably for myopic patients. The fabricating laboratory typically recommends a specific material for a given multifocal design, therefore eliminating this as a variable for the practitioner. These designs do tend to shift slightly on downward gaze to assist in providing acceptable near vision. Most designs are available in higher add powers, providing as much as +1.75D of add power.

Patients with a low lower lid, current gas permeable lens wearers, athletes and computer users benefit from these designs. For patients who would benefit from an aspheric design but require a high add power, consider a “modified bifocal” approach in which one aspheric lens is slightly overplussed to provide optimum near power. For patients who have the proper lid anatomy and critical vision needs, fit a segmented translating bifocal design. As these lenses are prism ballasted and therefore much thicker than aspheric designs, they are manufactured in a high Dk lens material.

RGP Is Out, GP Is In Rigid gas permeable or RGP lenses have officially undergone a name change to GP (oxygen permeability). The Contact Lens Manufacturers Association (CLMA) will use this terminology on all literature nationwide as well as all RGP Lens Institute resources and programs.

• Astigmats. Gas permeable lenses are almost always indicated for highly astigmatic patients, including those requiring bitoric designs, post-surgical and keratoconus. The ability to mold the cornea to reduce astigmatism while providing a smoother optical surface is a great benefit of gas permeable lenses. Again it is quite important that the material selected is fairly rigid to resist flexure and optimally correct the cylinder. Therefore, the low Dk lens material group is often recommended for myopia patients. This is likewise true for bitoric lenses due to the ease of fabricating the lenses from sturdier lens materials. ©

Dr. Bennett (ebennett@umsl.edu) is director of student affairs at the University of Missouri-St. Louis School of Optometry.