Abstract
Purpose: :
As material permeability (Dk) of gas permeable (GP) contact lenses enters the high range of 159 to 250 x 10exp–11: (1) does post–lens tear exchange provide detectable hypoxic relief to the covered cornea; and (2) if so, what are the contributory relationships between lens transmissibility (Dk/t) and post–lens tear exchange to hypoxic relief as Dk/t continues to rise?
Methods: :
Corneal oxygen uptake rates were measured polarographically for each of ten right eye corneas. Each cornea–lens combination was observed twice, with values being averaged with the others to obtain mean responses to both static (non–blinking) and dynamic (blinking) reference periods of 300 s of wear. Oxygen permeability values of the two materials compared here were 159 (SF–P95) and 163–250 (Menicon Z). Using contact lenses of 0.12, 0.16, and 0.20 mm lens center thicknesses, this provided Dk/t values equal to 132, 99, 80 and 136–208, 102–156, and 82–125, respectively. Repeated measures analysis of variance was used to test for statistically significant differences among mean rates for each condition, with Tukey’s test for post–hoc comparisons at the p=0.01 threshold.
Results: :
Corneal hypoxic stress was observed to vary directly with lens center thickness and inversely with lens transmissibility across the Dk/t ranges of both GP materials studied here. In the transmissibility range above 80 x 10exp–9, contributions of the tear exchange pathway were minimal, and in no case was hypoxic stress found to be reduced statistically significantly by blinking (p=0.581).
Conclusions: :
For both of the high Dk GP materials studied here, blinking was found to detectably reduce hypoxic stress, but not at statistically significant levels. The proportional contribution of blink–driven tear exchange, then, was minor across the Dk/t ranges studied, with hypoxic relief being due almost entirely to increase in lens transmissibility. Although of diminishing consequence as a secondary oxygen source with increasing transmissibility, blink–driven post–lens tear exchange does remain essential for waste clearance and prevention of lens binding to the corneal surface.
Keywords: contact lens • cornea: basic science • cornea: epithelium