Will Keratectasia be a Major Complication for Lasik in the Long Term?

Abstract

Whereas the biomechanical factors of the cornea with respect to elasticity and posterior corneal asphericity have been partly elucidated clinically by Roberts et al and experimentally by Muller et al, the effect of the lasik flap on the eventual outcome has not been entirely evaluated. While Barraquer described the importance of total corneal thickness (TCT) and residual stromal bed thickness (RSB) in the prevention of post MKM keratectasia, and Ruiz and Casebeer, the significance of leaving at least 30% TCT beneath an ALK incision for hyperopia, it has been shown by Reinstein using Artemis 3D pacymetry in cases of progressive post lasik keratectasia (PPLK), that an inadvertently deep lasik flap can compromise the RSB resulting in frank keratectasia post operatively. The significance of PPLK in eyes with forme frust keratoconus (KC) has recently been questioned by Binder et al in a recent editorial. Bearing in mind the incidence of ordinary KC in the population which varies from 50–230 per 100000 (Yang et al) and the reported incidences of 0.12% and 0.66% PPLK by Reinstein and Pallikaris, one would expect to find a greater number of PPLK reported in the world literature which consists of a total 87 eyes (Binder). In a recent review of 137 eyes of highly myopic patients (Ave −14.75D) ooperated by the author over a five year period between 1994–1999, where intra operative US pachymetry measuring TCT and RSB were carried out, in only one retreatment eye in which this was not done and the RSB compromised, was PPLK produced. All the other eyes showed a completely stable topgraphy and refraction over the 9 year follow up period. In the future, evaluation of these cases, it will be important that other characteristics of the cornea such as viscoelasticity, shear modulus, and plasticity be analysed with techniques such as dynamic corneal imaging, ocular hysteresis and the use of electronic speckle pattern intrerferometry (ESP). From a practical point of view, the use of non touch online optical coherence pachymetry should help to avoid the possibility of excessively deep flaps especially with the use of some mechanical microkeratomes.