A quantitative analysis of astigmatism induced by pterygium

Abstract

An analytical model has been developed for the localized corneal deformation produced in the region of the head of a pterygia. Astigmatism results when this localized deformation enters the central region or optical zone of the cornea. The amount and direction of the pterygia-induced astigmatism may be predicted from the values of the corneal curvature within the optical zone. The analytical solution of Lur'e based on the Papkovich-Neuber theory was applied to the anatomical and mechanical conditions affecting the cornea and conjunctiva. The force exerted by the head of a pterygia was measured experimentally for the first time. This force is of the order of that produced by the extraocular muscles in primary gaze. Using this model it is possible to predict that 2.35 diopters of the pterygia-induced astigmatism would result from a pterygia exerting 5 g of force along a meridian passing through the center of the cornea, and whose head is located 2.38 millimeters from the optical center of the cornea of an eye having a 4 mm pupillary diameter.