Biomechanics of Subtractive Surgery: From ALK to LASIK

Abstract

Myopic and hyperopic excimer laser in situ keratomileusis (LASIK) have become widely accepted procedures. Although LASIK does not rely on the mechanical response of the cornea to obtain the optical correction, the creation of a flap and ablation of the exposed stromal bed must disturb the state of stress in the tissue below the ablation zone. Little attention appears to have been paid to the mechanical response of the cornea to LASIK [1–3]. For low to moderate correction, the stress change induced by the surgical procedure is probably relatively small, although, to the best of our knowledge, it has not been quantified to date. As interest moves to deeper ablation depths, it is increasingly important to understand the mechanical response of the tissue to its new geometric configuration. Clearly, as ablation depths vary from shallow to very deep, the cornea can be expected to exhibit a corresponding range of deformational responses. The deformational response of the cornea to LASIK can be identified with an instantaneous component associated with the intrinsic elasticity of the tissue and a delayed postoperative component that may be associated with possible regression and that derives from complex and poorly understood mechanisms.