contact lens materials
The Meaning of Hyperpermeability
BY WILLIAM J. BENJAMIN, OD, MS, PHD
In my last two columns, we derived an updated definition of hypertransmissibility for a series of contact lenses. Numerically speaking, at least 80 Fatt Dk/t units were necessary for every contact lens in a type or brand of spherical lenses required to correct at least 99 percent of the population (from –8.00D to +6.00D). Hence, we discovered that four types of currently available silicone hydrogel soft lenses are hypertransmissible as calculated when we determined the mean harmonic thickness of the optic zone for lenses at the extremes of the refractive error distribution.
Further, we noted that the lens material would need oxygen permeability (Dk) greater than approximately 100 Fatt Dk units for a spherical lens series or brand to be considered hypertransmissible. Thus, we determined that the four available silicone hydrogel materials whose Dk values fell at or above 100 Fatt Dk units were hyperpermeable.
Past and Present Definitions
Incredibly, the updated numerical requirement for hyperpermeability is the same as that originally proposed when the hyper-oxygen terms were coined 15 years ago (≥100 Dk units). Because the updated value was determined using a more descriptive concept of thickness, these previously determined values are somewhat validated.
You may be concerned about the disparity between the well-known extended-wear criterion of Holden and Mertz (87 Dk/t units), the corresponding value more recently recommended by Harvitt and Bonanno (125 Dk/t units) and how they compare to these definitions of hypertransmissibility and hyperpermeability. The Holden and Mertz criterion published in 1984 reduces to approximately 70 Fatt Dk/t units when corrected for the edge effect, as later became expected. Hence, the definitions of hypertransmissibility and hyperpermeability require a slightly higher oxygen capability than that predicted by Holden and Mertz.
As noted previously, Dk/t is most often quoted at the center of a low-minus lens. Were we to use this convention instead of the mean harmonic thickness, the definition of hypertransmissibility would demand a central transmissibility of greater than 140 Fatt Dk/t units. Therefore, the definitions of hypertransmissibility and hyperpermeability also imply a slightly higher central oxygen capability compared to the recommendation of Harvitt and Bonanno as typically expressed in the field.
Factoring in Altitude
The added oxygen capacity assumed by the updated definitions may help compensate for the diminishing influence of altitude on oxygen availability. Although 33.5 percent of the world's population live within 100m of sea level, another 65.5 percent live at altitudes up to 2,500m. Mobility in today's society ensures that many contact lens wearers will subject their corneas to various elevations at least temporarily and during air travel. Ultimately, then, the updated definitions are compatible with and complementary to their original definitions — the Dk/t criterion of Holden and Mertz, and the recommendation of Harvitt and Bonanno — with a hedge against the adverse oxygen impact of altitude.
The Final Word
Please turn to my seventh and concluding column in this series about oxygen permeability and transmissibility titled, "Clinical Pearls and Greater Hyperpermeability." CLS
To obtain references for this article, please visit http://www.clspectrum.com/references.asp and click on document #157.
Dr. Benjamin is 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.