New physiological paradigms to assess the effect of lens oxygen transmissibility on corneal health.
OBJECTIVE
New paradigms are needed to allow an assessment of the physiological performance of a contact lens. Oxygen transmissibility (Dk/L) is not adequate in several respects. The oxygen supply to the cornea cannot be calculated from Dk/L because the oxygen tension of the cornea-contact lens interface is not known. Also, oxygen delivery to the cornea is not a linear function of Dk/L. Doubling the Dk/L of high Dk/L lenses does not double the oxygen supply to the cornea.


METHODS
We propose a new term, Biological Oxygen Apparent Transmissibility (BOAT). Oxygen supply to the cornea through a lens of known BOAT can be calculated by multiplying BOAT by the oxygen tension in the environment at the anterior surface of the lens. Oxygen supply to the cornea through the lens is linearly related to BOAT. The numerical values for BOAT of any lens of known Dk/L can be read from a graph or calculated from a simple formula. A second paradigm introduced here concerns point-to-point oxygen transmissibility of an optically powered contact lens. We propose that the health of epithelial cells is governed by the oxygen transmissibility of that portion of the contact lens immediately anterior to any small group of epithelial cells. Finally, we propose that, whereas epithelial cell health is governed by the oxygen transmissibility at a point on the lens, the swelling of the cornea is governed by the total oxygen supply to the epithelium. This total oxygen supply is controlled by the harmonic average oxygen transmissibility of the lens.


CONCLUSIONS
These new paradigms suggest that the clinician who is concerned about the patient's epithelium should choose a lens in which the portion with the lowest oxygen transmissibility (i.e., the thickest portion) supplies an adequate supply of oxygen to the epithelial cells. If corneal swelling is the clinical concern, then a lens with the highest harmonic average oxygen transmissibility should be chosen.
Fatt
I
I
School of Optometry, University of California, Berkeley 94720, USA.
D003261:Contact Lenses; D003315:Cornea / Q000502:physiology*; D015715:Corneal Edema / Q000209:etiology; D004058:Diffusion; D004867:Equipment Design; D006801:Humans; D008962:Models, Theoretical; D055095:Optics and Photonics; D010100:Oxygen / Q000378:metabolism*
D010100:Oxygen