Ring corneal clouding in keratoconus.

Rings of corneal edema were noted in one eye of each of three keratoconic patients, all wearing McGuire design rigid contact lenses of limited oxygen transmissibility (Dk/La). This finding appears to be analogous to the previously reported "central circular clouding" (CCC) with this specific appearance associated with the particulars of the keratoconic cornea-contact lens relation. Estimation of local oxygen transmissibilities suggests that a Dk/L < 3 x 10(-9) may produce this gross clinical sign underneath the lens midperiphery, whereas central Dk/L > or = 6 x 10(-9) may be sufficient to prevent its appearance.