contact
lens materials
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.
Material Concerns
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.
Managing Flexure
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.
Other Options
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.
Contact Lens Spectrum, Issue: October 2005