The effect of temperature on soft contact lens modulus and diameter

To examine the relative changes in diameter and modulus of soft contact lenses when the temperature is raised from room temperature (RT) to eye temperature (ET).

Thirteen lens types including 9 silicone hydrogel lenses were measured for diameter and elastic modulus at RT (20 ± 1°C) and ET (34 ± 1°C). Lens diameter measurements were undertaken after equilibration in ISO saline in a temperature-controlled lens analyzer (Optimec, Ltd, Malvern, United Kingdom). Measurements of flexural modulus of elasticity were made using an Instron 3343 tensiometer (Instron, Norwood, MA) with the samples suspended in a temperature-controlled saline bath.

All lens types reduced in diameter when raised to ET. The largest mean changes with silicone hydrogel and conventional hydrogel lenses were with Biofinity (Δ0.35 mm) and Acuvue 2 (Δ0.28 mm), respectively. All the silicone hydrogels showed a statistically significant reduction in modulus when raised to ET ranging from Δ0.06 MPa with comfilcon A to Δ0.78 MPa with balafilcon A. All the conventional hydrogels showed relatively small changes (<0.05 MPa) in modulus. Two of the four conventional hydrogels showed a statistically significant change in modulus (etafilcon A and ocufilcon A), but these were small and believed to be clinically insignificant.

This study has highlighted some clinically relevant changes in soft contact lens modulus and diameter when raised from RT to ET. It has also shown the importance of standardizing modulus measurement technique.