Rigid contact lens design equivalencies of overall diameter and axial edge lift: individual variations

Abstract

We compared the effects on rigid contact lens tear pump efficiency of changes in lens overall and optic zone diameter and of changes in lens axial edge lift by measuring corneal oxygen uptake on the right eyes of six human subjects after 5 min of both static and dynamic contact lens wear. Two sets of five lenses were worn by each subject. In set 1, overall diameter varied from 8.2 to 9.4 mm in 0.3-mm steps, with the optic zone diameter being 1.4 mm smaller and a 0.09 mm axial edge lift. In set 2, axial edge lift varied from 0.05 to 0.13 mm in 0.02-mm steps; overall diameter was 8.8 mm, and optic zone diameter was 7.4 mm. For the pooled data, a 0.3-mm decrease in overall diameter was approximately equivalent to a 0.03-mm increase in axial edge lift as a means of enhancing tear pump efficiency. However, considerable intersubject differences were exhibited, with the following increases in axial edge lift being equivalent to the 0.3-mm reduction in overall diameter: 0.01, 0.01, 0.04, 0.06, 0.09, and > 0.20 mm. This illustrates that subject factors (i.e., palpebral aperture size, lid-to-cornea geometry, corneal toricity, peripheral corneal topography, lid tension) influence lens positioning, movement, and tear pump efficiency.