contact lens materials
Oxygen Permeability and Transmissibility, Part 3
BY WILLIAM J. BENJAMIN, OD, MS, PHD
In my April column I mentioned that the oxygen difference could be rather small for hypertransmissible contact lenses in spite of seemingly large numerical differences in central oxygen transmissibility. Furthermore, we realized that corneas no longer become starved for oxygen as once was the case when contact lenses had limited oxygen capacity. Thus, we began to question the clinical relevance of theoretical criteria intended to demonstrate at what point enough oxygen is enough.
Another point is that numerical criteria for Dk/t are based on the central area of the cornea and its response to decreasing amounts of oxygenation. The upper part of the cornea gets roughly 1 percent less oxygen than the central cornea does when the eyes are closed, likely because of the distribution of vasculature on the underside of the upper eyelid. For eye closure, as with overnight wear, this is a small difference. But during waking hours the upper eyelid covers the superior area of the cornea in most individuals (Figure 1), and when not wearing lenses the upper cornea gets only a single digit percentage of oxygen while the central cornea gets the full 21 percent.
The most critical part of the cornea for hypoxia — the superior cornea — is not considered in the Dk/t criteria. In minus lenses, this is where the thickest portion of the optic zone resides. There is a disconnect between what the contact lens research community studies and what is necessary for daily or extended wear.
Clinicians study the superior cornea, especially when prescribing contact lenses. They know that they have to look at the superior cornea to make sure that nothing adverse is occurring. And they know through experience that the superior cornea is the most critical area because of the complications they've seen.
Figure 1. The upper eyelid covers the superior cornea in most individuals.
How Do Practitioners React?
In daily wear, practitioners concede to silicone hydrogels in cases of moderate-to-high hyperopia and high myopia for which lenses are noticeably thick and the clinical signs of hypoxia are recognizable. There is also potential for silicone hydrogels in the toric market where soft lenses are thicker by necessity of design.
But practitioners don't notice much benefit from using silicone-hydrogel materials in daily wear for the large majority of their patients who are low-to-moderate myopes and low hyperopes. If some tangible oxygen benefits exist for this majority, they're over-shadowed by variability among patients and other aspects of the lenses such as elasticity, surface biocompatibility, availability, cost, fit, edge design and comfort.
In extended wear, most practitioners concede to silicone hydrogels on the belief that the added oxygenation is beneficial for their patients. However, they don't see the magnitude of clinical benefit that was expected. The variables noted in the previous paragraph often become more significant in extended wear and, in numerous instances, overwhelm the positive effects of lesser hypoxia.
Hence, great reliance remains on daily wear. With less concern over hypoxia, many practitioners prefer daily disposables for safe and comfortable daily wear. CLS
For references, please visit www.clspectrum.com/references.asp and click on document #151.
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.