contact lens materials
Defining Hypertransmissibility of a Contact Lens Series
BY WILLIAM J. BENJAMIN, OD, MS, PHD
In my August column "Defining Hypertransmissibility of a Contact Lens," we defined oxygen transmissibility (Dk/t) of a contact lens using the mean harmonic thickness over the optic zone. This allowed an estimate of the Dk/t for each lens that was more representative than the values obtained using the central thickness or average thickness. We could then better compare one lens to another in terms of transmissibility. When the Dk/t calculated using the mean harmonic thickness was at least 80 Fatt Dk/t units, we concluded that the single lens was hypertransmissible.
Single Lens Versus Lens Series
But what about a lens series? Is the Night & Day (CIBA Vision) brand hypertransmissible? What about PureVision (Bausch & Lomb) or any other brand of contact lens made of a silicone hydrogel material?
To complete the definition of hypertransmissibility for a lens series, rather than for a single lens, I further propose that hypertransmissibility be required for each spherical power needed to cover at least 99 percent of potential wearers based on the approximate distribution of ametropia in the population (for example, from –8.00D to +6.00D). The transmissibility of optic zones is minimized at the power extremes of this range because the mean harmonic thickness is greatest there (Table 1).
You may recognize that lens brands consisting of materials that have an oxygen permeability (Dk) of less than approximately 100 Fatt Dk units won't produce at least 80 Fatt Dk/t units in the extreme powers. These materials may be capable of hypertransmissibility in lenses of low or moderate power, but the entire series could not be so classified. Hence, there are four brands of silicone hydrogel spherical soft lenses now available that are hypertransmissible for at least 99 percent of the population, as calculated when mean harmonic thickness (MHT) is derived for the optic zones of lenses at the extremes of the refractive error distribution.
In my next column in this series about oxygen permeability and transmissibility, we'll explore how the updated definition of hypertransmissibility complements those Dk/t criteria of Holden and Mertz as well as of Harvitt and Bonanno for extended wear and further develop the meaning of material hyperpermeability. CLS
For references, please visit www.clspectrum.com/references.asp and click on document #155.
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.