Oxygen transmissibility at various locations in hydrogel toric prism-ballasted contact lenses

Corneal metabolism is known to be adversely affected by anterior surface hypoxia. It has been demonstrated that contact lens (CL) oxygen transmissibility (Dk/L in cmmL O2/s mL mm Hg) is a direct function of the oxygen permeability (Dk in cm2 ml O2/mm Hg mL mm Hg) of the plastic divided by the thickness of the lens (L in cm). It has also been shown recently that the cornea should be sensitive to the Dk/L of that portion of a CL immediately overlying the tissue, in the absence of tear exchange and mixing; such is the case for all hydrogel CLs. Because hydrogel toric CLs are known to have anisomorphically variable thickness profiles, we have measured lens thickness along the vertical meridian and calculated local oxygen transmissibilities (Dk/L) for a series of hydrogel CLs.

Thickness was measured at 5 locations along the vertical meridian of 16 different prescription toric hydrogel CLs of each of 6 brands (5 prism-ballasted design and 1 double-slab off design) and 12 different prescription spherical CLs of 2 brands (controls). Dk (at 38 degrees C) was calculated for each material from the nominal water content by a known relation.

We found that the average Dk/L for all lenses was greatest from the center (mean +/- SD of 8 +/- 4 x 10(-9)) to the superior portion (12 +/- 2 x 10(-9) at 6 mm from the CL center and 13 +/- 4 x 10(-9) at 3 mm from the CL center) of prism-ballast design toric CLs, and least in the lower portions (4 +/- 2 x 10(-9) at 6 mm from the lens center and 6 +/- 1 x 10(-9) at 3 mm from the CL center) of these CLs. Analysis of covariance showed that the effect of location for the prism-ballasted CLs is statistically significant (F = 203.11, p < 0.00005). One nonprism ballast toric CL design and the spherical designs showed minimal differences across the vertical meridian. We therefore conclude that clinicians should carefully monitor the physiological hypoxic response of the cornea, in particular the inferior area, during wear of prism-ballasted hydrogel lenses.