contact lens materials
Oxygen Permeability and Transmissibility, Part 1
BY WILLIAM J. BENJAMIN, OD, MS, PHD
Oxygen permeability (Dk) of a traditional hydrogel material is limited due to the water content of the material. The highest practical water content for a hydrogel lens is about 80 percent, so a hydrogel's hydrated permeability (Dk) could reach a maximum of 40 to 50 Fatt Dk units (Figure 1).
However, the Dk of balafilcon A (PureVision, Bausch & Lomb) is reportedly 91 and that of lotrafilcon A (Night & Day, CIBA Vision) reportedly 140. Other silicone hydrogels have come onto the market with reported Dk values of 60 to 128 Fatt Dk units.
Figure 1. For hydrogel materials, Dk is directly related to water content.
Figure 2. Most hydrogel lenses fall into the medium or low Dk/t category shown here.
Dk Versus Dk/t
The increased permeability translates into impressive oxygen transmissibility (Dk/t) values for silicone hydrogel lenses. Because these lenses in minus powers are centrally less than 0.1mm thick, the central Dk/t of the balafilcon A material is at least 95 Fatt Dk/t units, and the central Dk/t of the lotrafilcon A material is at least 150 Fatt Dk/t units, depending on the actual central thickness.
Of course, the central thickness doesn't describe the Dk/t in the periphery of a minus lens. Still, even in the periphery, the amount of oxygenation is significantly greater than that of soft hydrogel lenses. Most silicone hydrogel materials are near or above the level achieving hyperpermeability, for which thickness variations across the lens surfaces have subdued impacts on the concentrations of oxygen that reach the cornea.
With a central Dk of 40 to 50 (for an 80-percent-water material in minus lenses) — representing the upper limit of hydrogel materials — and with a typical center thickness, we'd get a maximum Dk/t of 30 to 40 and 12 percent to 14 percent oxygen at the central cornea. Almost all popular hydrogel lenses result in much less oxygen than this, falling into the medium or even low category (Figure 2).
When we prescribe silicone hydrogels, except for those intended only for daily wear, our patients are getting 18 percent to 19 percent oxygen in the openeye state — a significant amount of additional oxygenation. Silicone hydrogel materials allow much more oxygen to reach the central cornea.
So Why Silicone Hydrogels?
There's no doubt in my mind that greatly reducing hypoxia during contact lens wear is a significant benefit for daily and especially for extended wear. However, in Part 2 of this series of columns, we'll begin to explore why many practitioners still prescribe hydrogel lenses in spite of the oxygen advantages of silicone hydrogel lenses. CLS
For references, please visit www.clspectrum.com/references.asp and click on document #147.
Dr. Benjamin is a professor of Optometry and Vision Science, a senior scientist at the Vision Science Research Center, and a clinician Contact Lens Practice and Primary Eye Care the University of Alabama at Birmingham.