Clinincal performance and fitting principles of “rigid” super permeable contact lenses

Abstract

It is not surprising that the structural changes created in rigid polymers to substantially increase theiroxygen permeability alter other physical properties that influence contact lens performance. These include polymer-tear interface compatibility, dimensional stability, hardness, and fracture and flexural resistance. Clinical experience with the newly available superpermeable rigid lenses have confirmed these expectations. However, of special interest to contact lens practitioners is the finding that the increased flexure of these lenses can have a profound effecton their wearing comfort and requires a major modification of the fitting objectives in order to fully exploit their performance potential. The increased flexibility of superpermeable lenses reduces their capability of resisting the compression forcescreated during the closing phase of each blink. Depending on the lens design, blink induced compression may create a temporary seal between the periphery of the lens and the underlying cornea. As the lens decompresses during the opening phase of each blink cycle, the resulting hydrodynamic forces can create a transient negative pressure at the corneal surface that will cause increasing lens awareness as well as unstable contact lens optics. Thus, the choice of base curve radius for a specific lens diameter has been identified as one of the most critical lens design considerations in that the lens must have a sufficiently flat fit so as to avoid creating a suction effect during the blink cycle.