Young's modulus measurements of gas permeable contact lens materials

The Young's modulus of elasticity of a range of commercially available gas permeable contact lens materials was measured using specially prepared samples. Significant differences were found in materials such that groupings could be made on the basis of oxygen permeability. The highest value of Young's modulus was found with the lowest Dk polymethyl methacrylate (PMMA) and the lowest measured value with the highest Dk (Fluoroperm 90). The value for the 3M (Advent) material correlated with its high Dk (100). The results confirm that the greater the oxygen permeability of a material the greater will be the flexibility. When fitting and designing lenses for a toric cornea practitioners must balance the increase in center thickness to offset flexing with the corresponding decrease in oxygen transmissibility. Conversely it may be that lenses can be designed to flex where this would be an advantage. It is suggested that material manufacturers provide Young's modulus data to help practitioners understand more fully the materials they prescribe.