GP Problem Solving: A Material Matter
BY LORETTA B. SZCZOTKA-FLYNN, OD, MS, FAAO
Don't forget about GP material properties when problem solving non-ideal contact lens fits. Specifically, you can vary GP lens material properties to enhance or limit flexure, wetting, comfort and oxygen transmissibility.
The following case demonstrates how I used material properties to my advantage when fitting a unilateral keratoconus patient.
Patient AB has keratoconus in the right eye only and classic with-the-rule astigmatism in the left eye. I originally fit him in spherical GP lenses made in Boston ES (Bausch & Lomb) in each eye, which easily corrected the OD keratoconic eye to 20/20.
|Figure 1. Bar values represent manufacturer reported Dk values (ISO/Fatt) followed by the GP material name of various Boston products.|
Test your GP fitting skills to diagnose the following visual problem OS:
Spectacle refraction: –3.00 –1.50 x 178
Keratometry: 44.00 @ 180/ 46.62 x 090
VA with spherical GP: 20/25
Over-refraction: +0.50 –1.00 x 084 20/20
Give yourself a pat on the back if you correctly determined that the residual astigmatism resulted from a non-flexing GP lens that induced the patient's internal, physiologic astigmatism to reduce his lens-corrected visual acuity.
You can easily solve this problem by taking advantage of material properties to induce planned flexure. I changed the lens material OS to Boston XO, and with that change the patient's visual acuity improved to 20/20 with no appreciable over-refraction. Effectively, the higher Dk and lower modulus Boston XO material conformed somewhat to the corneal toricity, correcting only the degree of astigmatism that matched the spectacle refraction.
Inherent rigidity of the material can determine flexure. For any lens material, there's a critical thickness below which flexure can prove clinically significant. PMMA has a critical thickness of 0.12. GP lenses typically have higher critical thicknesses that increase as the Dk increases.
The modulus of the plastic relates to its stiffness and its ability to mask corneal astigmatism. As Figure 1 shows, modulus goes down as Dk goes up. In my case the Boston XO lens flexed in a planned manner to reduce AB's residual astigmatism whereas the Boston ES material did not.
If changing to a different material doesn't help, other fitting properties that manage flexure include lens thickness, corneal topography, the base curve-to-cornea fitting relationship and lens positioning. For example, higher corneal toricity as well as thinner and steeper lenses will result in greater amounts of flexure.
Dr. Szczotka-Flynn is an associate professor at Case Western Reserve University Dept. of Ophthalmology and is director of the Contact Lens Service at University Hospitals of Cleveland.