contact lens materials
Oxygen Permeability and Transmissibility, Part 2
BY WILLIAM J. BENJAMIN, OD, MS, PHD
In my February column, I noted that silicone hydrogel materials broke the dependence of oxygen permeability (Dk) on water content. Soft contact lenses were then able to move out of the categories of low and medium transmissibility (Dk/t) as GP lenses had done 20 years before.
As contact lenses obtained successively greater Dk/t, they paradoxically delivered lesser incremental amounts of oxygen to the cornea for each improvement. For instance, in the early days of soft lenses we went from PMMA (Dk of 0) to polymacon (38 percent water), which measured 10 Fatt Dk/t units at a 0.1mm center thickness. The oxygen difference with that increment of change was on the order of 5 percent.
But as we went from 10 Dk/t units to 20 Dk/t units, we didn't get another 5 percent additional increase. We got about 4 percent. As we went to lenses of even higher Dk/t, the numerical units may have indicated a large increase but the additional amounts of oxygen delivered to the cornea progressively lessened (Figure 1).
Figure 1. As Dk/t increases in steps, the amount of additional oxygen delivered to the cornea becomes progressively less.
With the exception of silicone hydrogel lenses intended only for daily wear, central portions of most silicone hydrogels fall on the upper part of the curve. Relatively large changes in Dk/t with these materials resulted in smaller changes of the amount of oxygen supplied to the cornea. At the top of the curve where it's almost flat — in the hyper-Dk/t range — a big jump in Dk/t would get only a bit more oxygen to the cornea.
How Much Oxygen is Necessary for Success?
The original oxygen value proposed by Polse and Mandell (1970) to prevent daily wear corneal swelling was 2.5 percent, which is achievable with thin polymacon. As finer techniques for measuring corneal swelling were developed, that value rose to 3.5 percent to 5 percent to 8 percent. Holden and Mertz (1984) suggested a Dk/t of at least 24 for daily wear. More recently, Harvitt and Bonanno (1999) thought at least 35 Fatt Dk/t units were necessary.
For extended wear, Holden and Mertz suggested that at least 13.5 percent oxygen was needed, which was then thought equivalent to 87 Dk/t units. Hamano (1983) posed that the value should be 15 percent oxygen. Harvitt and Bonanno raised the criterion to 125 Dk/t units.
Long ago, I could tell the oxygen demand of an unstressed cornea from the same cornea made hypoxic by as little as 2 percent — down to 19 percent oxygen. This roughly agrees with Harvitt and Bonanno's criterion, which is in the hyper Dk/t zone.
There are few clinical signs that distinguish between the higher oxygen criteria. In this range the cornea is not starved for oxygen, unlike at lower Dk levels that generate obvious sequelae. It comes down to how fine you want to cut it as to where you set your limit.
My next column will demonstrate that clinical success with today's soft lenses is not closely associated with central transmissibility in most cases. CLS
For references, please visit www.clspectrum.com/references.asp and click on document #149.
Dr. Benjamin is a professor of Optometry and Vision Science, a senior scientist at the Vision Science Research Center, and a clinician in Contact Lens Practice and Primary Eye Care at the University of Alabama at Birmingham.