The Biomechanics of Keratorefractive Surgery

Corneal biomechanics is the study of the mechanical properties and responses of the cornea. The cornea’s remarkable transparency and strength allow it to contain the intraocular pressure, serve as a protective layer and act as the major refracting surface of the eye. The shape of the cornea, and hence its refractive properties, is directly determined by its ultrastructural and biomechanical properties. In keratorefractive surgery the aim is to alter the cornea’s refractive power by changing its shape. In the early days of laser vision correction it was assumed that the postoperative change in corneal shape was determined directly by the pattern of tissue ablation.1 Now it is clear that this is an oversimplification because biomechanical and wound healing changes also influence final corneal shape.2 This is illustrated by the hyperopic shift that typically occurs following phototherapeutic keratectomy, a procedure that a simple shape subtraction theory would predict to be refractively neutral. Like all technical subjects biomechanics has its own language and this complicates understanding for the non-specialist. The purpose of this article is not to review biomechanical theory in general or even to describe all that is known about corneal biomechanics but rather to provide the reader with an understanding of the most clinically relevant concepts and principles so enabling an appreciation of how this important subject can impact on clinical practice.