contact lens materials
Defining Hypertransmissibility of a Contact Lens
BY WILLIAM J. BENJAMIN, OD, MS, PHD
In the overwhelming majority of publications, oxygen transmissibility (Dk/t) of contact lenses is specified at the center of a low-minus spherical refractive power, often –3.00D. This simple practice seemingly consolidates a complicated concept down to a single number. However, it over-estimates the transmissibility at other points on the lens that are thicker — which is, indeed, every other point out to the edge.
This method also overstates the Dk/t at the center of other refractive powers for lenses made of the same material. For instance, the Dk/t of a +1.00D lens will invariably be less at its center compared to a –3.00D lens made of the same material.
Figure 1. Hypertransmissibility requires at least 80 Fatt Dk/t units.
Figure 2. Comparison of mean harmonic thickness and average thickness.
The practice also confuses the definition of hypertransmissibility for a contact lens (Figure 1). Hypertransmissibility requires at least 80 Fatt Dk/t units. This value is consistent with the criterion of Holden and Mertz (1984) of 87 Dk/t units over the central lens area, a value that would have been lessened by correction of the edge effect.
The definition of thickness (t) needs to be standardized to allow a more consistent comparison of Dk/t from lens to lens. I propose that the mean harmonic thickness over the optic zone be used to define that standard thickness.
The Devil is in the Details
Mean harmonic thickness (t) is a method of averaging the radial thicknesses at the center of a lens and the peripheries of several annular zones of equal area as specified in ANSI Z80.20:2004 and ISO 18369-1, extending out to the edge of the optic zone. Typically, at least five annular zones are visualized (Figure 2). For a minus lens, the mean harmonic thickness is slightly greater than the average radial thickness of the optic zone, which is an average of the radial thicknesses at the center and peripheries of several annular zones of equal width. It logically follows that the mean harmonic thickness of a plus lens is slightly less than the average thickness.
Oxygen travels through the area of a lens in a radial fashion and, thus, the mean harmonic thickness is conceptually more applicable than the average thickness or central thickness. In addition, using the mean harmonic thickness over the optic zone balances the contributions of the thinner and thicker areas of a lens in calculating Dk/t. Thus, if we must describe Dk/t using a single number, we can produce a Dk/t value that is generally representative of a contact lens. If that value is at least 80 Fatt Dk/t units, then the lens would be considered hypertransmissible.
My next column will expand on the definition of oxygen hypertransmissibility so that it will characterize an entire series (type or brand) of contact lenses. CLS
For references, please visit www.clspectrum.com/references.asp and click on document #153.
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.