guest editorial

A Transitional Period For Dk Values

BY WILLIAM "JOE" BENJAMIN, OD, MS, PHD
OCTOBER 1998

There is some controversy brewing about Dk values of new materials that have been disclosed lately, partly because of the recently published contact lens standard from the American National Standards Institute (ANSI). The new standard, which went through the consensus-based ANSI process for over six years, became available in May 1998. The measurement of oxygen permeability (Dk) of contact lens materials is covered by ANSI Z80.20, which is also called "Contact Lenses -- Standard Terminology, Tolerances, Measurements, and Physiochemical Properties." ANSI Z80.20 specifies the correction for the edge effect, resulting in a substantially smaller Dk value.

You can imagine the difficulty for contact lens companies in disclosing smaller edge-corrected Dk values for their materials. Some merely forgot that the edge effect existed, and many others were forced to disclose only non-edge corrected values in order to avoid a competitive marketing disadvantage. The overall result is that many or most Dk values that clinicians have been seeing in trade journals and company-sponsored advertisements are higher. There are several other ways to heighten Dk values, but ANSI Z80.20 further specifies that Dk measurements be calibrated according to a set of standard materials, such as those contained in the Permeability Reference Material Repository in my lab at the University of Alabama at Birmingham School of Optometry. Regardless of the manner in which the Dk is measured, it should remain within 10 percent of its actual value if a proper calibration is performed. In my view, we should support compliance with ANSI Z80.20 in the area of Dk measurement because a truer, more consistent analysis of the Dk values will enhance our ability to select contact lens materials.

We should not expect perfect compliance with ANSI Z80.20 immediately. I believe most companies will eventually specify Dk values that are edge-corrected and calibrated. But in the interim, the Dk values at every company will be reassessed, and this will take a couple of years to play out. My guess is that the companies will not go back and reissue Dk values for the materials already on the market, but will focus on appropriately stating the Dk values of new materials.

An excellent interim method of specification is to advertise two designated Dk values for each new material -- the lower, edge-corrected one, and the higher, non-edge-corrected one. This way, the practitioner is able to receive the more appropriate factual information and can also gauge the higher Dk value against those values already cited by other manufacturers. Menicon Company Ltd., for instance, specifies the Dk of its new Menicon Z material as 163 Dk units (edge-corrected and calibrated) and 189 Dk units (not edge-corrected). Polymer Technology Corporation (PTC) recently specified the Dk of the new Boston EO material as 58 Dk units (polarographic technique) and 82 Dk units (gas-to-gas technique). Thus, PTC has likely given an appropriate Dk for the Boston EO, but has also given a higher value for comparison to those already advertised by other companies.

We're going through a transition period in which contact lens companies and clinicians are adapting to Dk values derived from a calibrated method that corrects for the edge effect. We will now likely see two Dk values for new materials: a lower value calibrated and corrected for edge effect and a higher one used in comparison with Dk values stated before ANSI Z80.20 originated. I'm hopeful that in a few years, the utility of relative uncorrected Dk values will diminish and that the calibrated, edge-corrected Dk values will remain.

Dr. Benjamin is a professor of optometry and physiological optics and director of clinical research at the University of Alabama at Birmingham, School of Optometry