The biomechanics of keratoconus and rigid contact lenses

To examine aspects of the genesis and progression of keratoconus through an analysis of the biomechanical forces associated with this condition, including those generated by rigid contact lenses.

The biomechanics of applanation tonometry and rigid contact lenses serve as a basis for examining contact lens adherence and the potential for contact lens induced mid peripheral corneal applanation and apical molding with apical clearance fittings.

Some physiologic and pathologic mechanisms for increases in intraocular pressure are reviewed. The possibility is raised that hard squeeze blinks, for example, could significantly increase the distending forces that bear on the corneal apex in keratoconus and that some apical clearance contact lens fittings could increase the risk of progressive ectasia. Support for these hypotheses is found among healthy eyes and eyes with keratoconus that show increased curvature when apical clearance fittings are worn.

The known risk of scarring responses to excessively flat fitting rigid contact lenses must be balanced against the possible risk of molding and ectasia advancement responses to tight definite apical clearance lenses. Fittings within the range of minimal apical clearance and minimal apical contact (divided support) may be the most appropriate. The appearance and performance of these fittings may be difficult to distinguish clinically. Because some patients may develop pathologic levels of intraocular pressure with vigorous eye rubbing, strong squeeze blinks, inverted body positions, and strenuous muscular effort, patients with, or at risk for, keratoconus, glaucoma, or progressive myopia should be advised of the possible adverse significance of these activities.