Oxygen Diffusion and Edema With Modern Scleral Rigid Gas Permeable Contact Lenses

Abstract

PURPOSE. We defined the theoretical oxygen tension behind modern scleral contact lenses (CLs) made of different rigid gas permeable (RGP) materials, assuming different thickness of the tear layer behind the lens. A second goal was to show clinically the effect of the postlens tear film on corneal swelling. METHODS.We simulated the partial pressure of oxygen across the cornea behind scleral CLs made of different lens materials (oxygen permeability Dk, 75-200 barrer) and different thickness (T av , 100-300 lm). Postlens tear film thicknesses (T post-tear ) ranging from 150 to 350 lm were considered. Eight healthy subjects were fitted randomly with a scleral lens with a thin and a thick postlens tear layer in two different sessions for a period of 3 hours under open-eye conditions.RESULTS. The CLs with less than 125 barrer of Dk and a thickness over 200 lm depleted the oxygen availability at the lens-cornea interface below 55 mm Hg for a postlens tear film of 150 lm. For a postlens tear film thickness of 350 lm, no combination of material or lens thickness will meet the criteria of 55 mm Hg. Our clinical measures of corneal edema showed that this was significantly higher (P < 0.001, Wilcoxon signed ranks test) with the thicker compared to the thinner T post-tear (mean 6 SD, 1.66 6 1.12 vs. 4.27 6 1.19%).CONCLUSIONS. Scleral RGP CLs must be comprised of at least 125 barrer of oxygen permeability and up to 200 lm thick to avoid hypoxic effects even under open eye conditions. Postlens tear film layer should be below 150 lm to avoid clinically significant edema.

Scleral lens influence on corneal curvature and pachymetry in keratoconus patients

“…Pullum and Stapleton [30] described a less than 3% corneal swelling in four healthy subjects with scleral lenses. Depending on the thickness of the fluid layer between lens and cornea, a corneal swelling of 1.5-4% in eight healthy subjects was reported recently by Compañ et al [32] When comparing scleral lens Dk, it seems that Pullum and Stapleton used lower scleral lens Dk's, and the group of Compañ et al used comparable Dk's compared to our study. The limited temporary increase in pachymetry in our study group was not expected to cause adverse physiological corneal responses; however, individual hypoxia responses may differ.…”

“…The limited temporary increase in pachymetry in our study group was not expected to cause adverse physiological corneal responses; however, individual hypoxia responses may differ. In general, it is recommended to restrict hypoxia-induced swelling by application of highest Dk available materials and minimizing both lens thickness and the fluid layer between lens and cornea [31,32]. The thickness of the fluid layer between lens and cornea was not investigated in this study.…”

Scleral contact lenses and hypoxia

“…In another study, part clinical and part theoretical, Compañ et al [5] calculated that to avoid corneal edema during open eye wear a scleral contact lens of 200 m thickness and made from a material of at least 125 Dk must not be fitted with a corneal vault greater than 150 m. Subjects fitted with 15.5 mm, 100 Dk material scleral contact lenses and a post-lens tear film thickness along visual axis of 150 m or 350 m had a corneal swelling of 1.6% for the shallower and 3.9% for the deeper reservoir after 3 h wearing time. This magnitude of swelling is within the range of the physiological swelling experienced during closed eye conditions, such as sleep [6,7].…”

“…The above mentioned studies [5,8] reported 1.6-3.9% corneal swelling after scleral lens wear, which is a level of edema not detectable during a normal biomicroscopical corneal evaluation. In a retrospective study on 517 eyes it was reported that 7.4% experienced episodes of edema [12].…”

“…The three most recently published theoretical studies [3][4][5] did not consider the tear exchange that may occur between the post-lens reservoir and tears peripheral to the lens. Such an exchange may not be insignificant but has yet to be assessed.…”