Oxygen Tension Beneath Scleral Lenses of Different Clearances

Abstract

Purpose 

To evaluate the relative partial pressure in oxygen (pO₂) at the corneal surface under Boston XO₂ scleral lenses (SL) fitted with targeted clearances of 200 and 400 μm (SL200 and SL400).

Methods 

During this prospective study, the right eyes of eight normal subjects were fitted with SL200 and SL400. Clearance, validated after 5 minutes of wear with an optical coherence tomograph, was used with lens thicknesses to calculate transmissibility and estimate pO₂. Corneal pO₂s were measured with an oxygen electrode after 5 minutes of (1) corneal exposure to calibrating gases with various pO₂ or of (2) SL wear. Decays in pO₂ were modeled to an exponential. Linear regression between exponent k of these decays and calibrating gas pO₂s allowed for the calculation of corneal pO₂ under SL. Differences between pO₂s beneath SL200 and SL400 were tested with a mixed ANOVA.

Results 

The estimated transmissibility based on thicknesses and clearances (239.7 ± 34.7; 434.5 ± 33.2 μm) predicted a corneal pO₂ of 8.52 ± 0.51 and 6.37 ± 0.28% for SL200 and SL400. These values were close to measured pO₂: 9.07 ± 0.86 and 6.19 ± 0.87% (mean ± SEM) (P < .05) for SL200 and SL400, respectively. Both pO₂ measurements fall short of the theoretical values needed to prevent hypoxia during lens wear (9.9% and above).

Conclusions 

As shown in vivo for the first time, an 18-mm scleral lens fitted with a 400-μm clearance reduces the oxygen tension available to the cornea by 30% compared to a similar lens fitted with a 200-μm clearance after 5 minutes of wear.