Analysis of Corneal Topographic Optical Zone and Decentration after Myopic Refractive Surgery

ABSTRACT

PURPOSE: To identify factors influencing the ablation center and to evaluate the size of the ablation zone after PRK and LASIK using Technolas 217 C. METHODS: This study examined the results of refractive surgery using Technolas 217 C Excimer laser with eye tracking system in 69 eyes, of which 31 underwent PRK and 38 LASIK. The ablation zone and degree of decentration were determined through corneal topography. We divided the eyes into two groups according to the degree of decentration: less than 0.5 mm as the first group, and from 0.5 to 1.0 mm as the second. The ablation zone diameter in topography was compared with the programmed preoperative optical zone. RESULTS: The only factor influencing the ablation center in the PRK group was preoperative spherical equivalent(p=0.016), and no factor influenced the ablation zone in the LASIK group. In both groups, the long axis of the ablation zone was longer than that of the programmed optical zone(p=0.003), while the short axis was shorter (p=0.001). CONCLUSIONS: After refractive surgery using Technolas 217 C Excimer laser with eye tracking system, there was no clinically significant decentration. With greater preoperative spherical equivalent, more decentered ablation was noted in the PRK group. The topographical long axis of the ablation zone was longer than that of the programmed ablation zone, while the short axis of the ablation zone was shorter. During refractive surgery, we should be aware of the possibility that the short axis of the postoperative ablation zone may be shorter than that of the programmed preoperative optical zone.