An examination of the 'edge effect' in the measurement of contact lens oxygen transmissibility

The difficulty in interpreting previously published estimates of the oxygen permeability of rigid gas permeable contact lens materials has led to the postulation that the 'edge effect' contributes to an overestimation of individual oxygen transmissibility measurements. The edge is the term used to describe the lateral diffusion of oxygen from the anterior lens surface outside the cathode area during oxygen transmissibility measurement; this serves to reduce the effective resistance to oxygen flow from the atmosphere to the cathode surface. By comparing the oxygen transmissibility of specially prepared composite lenses (in the form of a central button of RGP material surrounded by an impermeable PMMA carrier) to matched lenses of standard form, we empirically demonstrate the existence of a significant edge effect which contributes to an overestimation of Dk/L by approximately 27 % for a 0.4mm thick lens. The edge effect must be taken into account in calculating the intrinsic oxygen permeability of hard and soft contact lens materials; this will allow practitioners to predict more accurately the ocular response to lens wear.