Refractive changes after phototherapeutic keratectomy

Purpose: To evaluate refractive error changes after phototherapeutic keratectomy (PTK). Setting: University Eye Hospital, Kiel, and University Eye Hospital, Hulle, Germany. Methods: The MEL 60 excimer laser (Aesculap Meditec) was used in all cases. To even out the peaks and valleys of irregular surfaces, modulating agents were applied. The study included 45 patients with various preoperative corneal diseases: central scars, recurrent erosions, corneal dystrophies, and surface irregularities. Subjective and objective refraction, keratometry, slitlamp photography, and corneal topography were performed preoperatively and postoperatively. The follow-up was up to 24 months. Results: Twenty-six patients had stable postoperative refractions. Thirteen patients developed a hyperopic shift; the highest observed amount was +4.0 diopters. In seven patients, the astigmatic error increased, although no significant change in axis was measured. Three patients had a myopic shift. Conclusion: After PTK, all types of refractive change can occur. The greatest risk is that of a hyperopic shift. We saw a correlation between the degree of hyperopia and the ablation depth. Methods for preventing such changes include (1) a large treatment zone, (2) use of a polishing program involving a low viscosity fluid at the end of the laser procedure, (3) a two-step treatment in selected cases to avoid ablations that are too deep.