The critical Dk/L to avoid oedema for daily wear RGP contact lenses

Abstract

Background: A study was conducted to investigate the critical Dk/L required to avoid open‐eye oedema with rigid gas‐permeable (RGP) contact lenses.Methods: RGP contact lenses in matched designs were fabricated from three materials of nominal Dk of 12, 30 and 45 times 10−11 (cm2.mlO2)/(s.ml.mmHg), with a centre thickness of 0.15 mm. A randomised, double‐masked study design was used. Over two lens‐wearing sessions one week apart, nine young adult subjects wore one lens made of each of the three materials for four hours. In one session the non‐lens‐wearing eye served as a control. Corneal thickness was measured using an ultrasonic pachometer before and after lens wear and the extent of corneal oedema calculated.Results: Lenses with a Dk/L of 8 times 10−9 (cm.mlO2)/(s.ml.mmHg) induced statistically significant levels of corneal oedema (1.8 ± 1.5 per cent) after four hours of open‐eye wear. No corneal oedema was detected with lenses above this critical Dk/L value. No evidence could be found to support the concept of ‘osmotic’ corneal oedema induced by reflex lacrimation in unadapted subjects.Conclusion: RGP lenses with a Dk/L of 20 times 10−9 (cm.mlO2)/(s.ml.mmHg) or greater worn for daily wear are unlikely to induce corneal oedema in the majority of patients.