EFFECTS OF ABLATION DIAMETER, DEPTH, AND EDGE CONTOUR ON THE OUTCOME OF PHOTOREFRACTIVE KERATECTOMY

PURPOSE: To investigate the effects of the ablation diameter, depth, a nd edge contour on the outcome of excimer laser photorefractive kerate ctomy (PRK). METHODS: A prospective study was conducted in which 60 pa tients (60 eyes) were randomly allocated to 5.00-mm, 6.00-mm, or 5.00 to 6.00-mm multizone treatment groups. All eyes underwent a -6.00 diop ter (D) correction using a Summit Omnimed excimer laser. RESULTS: In e yes treated with 6.00-mm diameter zones, the initial hyperopic shift w as reduced, with significant differences at 1 and 4 weeks (p<0.01). At 6 and 12 months, the refractive changes were closer to the intended c orrection with 6.00-mm diameters. The predictability of PRK was improv ed with 6.00-mm zones, with a significant reduction in variance of the refractive changes, at all stages postoperatively (p<0.05 to p<0.001) . Objective measurements of haze were significantly less at 1, 3, and 6 months with 6.00-mm ablations (p<0.05). There were no differences be tween the 5.00-mm and the 5.00- to 6.00-mm multizone groups. Computeri zed measurements of ''night'' halo were significantly smaller in the 6 .00-mm treatment group at 1 week and 1 month (p<0.05). At 12 months, t wo patients treated with 5.00-mm zones and three with the 5.00- to 6.0 0-mm multizone complained of severe night vision disturbances. No 6.00 -mm eyes were similarly affected. CONCLUSIONS: Treatment with a 6.00-m m spherical ablation diameter produced less initial overcorrection, im proved predictability, and was associated with a reduction in postoper ative halos and night vision disturbances. Creating a superficial blen d zone with a 5.00- to 6.00-mm multizone treatment had no beneficial e ffect on the outcome.