[Topographical evaluation of centration of excimer laser photorefractive keratectomy]

To evaluate the situation of centrality of excimer laser photorefractive keratectomy (PRK).

Eyesys Corneal Analysis System was used for the computer-assisted analysis of corneal differential topographical maps gained preoperatively and 10 days postoperatively in 158 eyes of 98 patients with at least six months of follow-up to determine the direction and the distance of the center of ablation zone relative to the center of the entrance pupil, and a comparison in decentration difference was made between one-zone ablated eyes and multi-zone ablated eyes.

Concerning the centration of ablated zone relative to the center of entrance pupil: 100 eyes (63.3%) were centered within 0.50 mm, 55 eyes (34.8%) were decentered more than 0.50 mm with 3 eyes (1.9%) being decentered more than 1 mm, and the mean decentration amount was 0.45 mm. 38 eyes were performed by multi-zone ablation and 120 eyes by one-zone ablation, their mean amount of decentration was 0.55 mm and 0.44 mm respectively, the difference being significant (P < 0.05). 4 eyes with decentration greater than 0.50 mm lost one to two lines of their best corrected visual acuity postoperatively due to the decentered ablation.

Many factors contribute to decentration of the ablated zone. Great decentration will affect the recovery of the postoperative visual acuity. The Passive Eye Tracking System (PET) will contribute to precise centration and reduction of displacement.