Ablation zone centration after photorefractive keratectomy and its effect on visual outcome

To determine the relation between ablation zone decentration, measured by corneal topography, and visual and refractive outcome, contrast sensitivity, glare, and subjective reports of halos after photorefractive keratectomy (PRK).

Eye Foundation, University of Missouri-Kansas City School of Medicine.

This study comprised 70 eyes of 70 patients enrolled into Phase III of the clinical investigation of the VISX 20/20 excimer laser for the correction by PRK of 1.00 to 6.00 diopters of pre-existing sphere. All patients were treated with 5.0 mm ablation zones. Preoperative and postoperative corneal topography in 67 eyes was done using the EyeSys system. Ablation zone decentration was measured relative to the pupillary center at all postoperative visits as determined from the difference or change map.

Mean distance between the ablation zone center and the pupillary center was 0.62 mm +/- 0.34 (SD) (range 0.07 to 1.67 mm). Twenty seven of 66 patients with corneal topography (40.9%) had decentrations less than 0.5 mm; decentrations in 6 eyes (9.1%) were 1.0 mm or more from the pupillary center. No correlation was observed between the magnitude of decentration and 1 year postoperative best spectacle-corrected acuity (r = -.04, P = .76), uncorrected acuity (r = -.03, P = .84), or spherical equivalent (r = -.07, P = .59); preoperative uncorrected acuity (r = .04, P = .73) or spherical equivalent (r = -.02, P = .90); or reduction in spherical equivalent (r = .02, P = .89). The magnitude of decentration was correlated with preoperative spectacle-corrected acuity (r = -.37, P = .002). There was no difference in contrast sensitivity, glare, or halos between the cases with less than 0.5 mm of decentration and those with 0.5 mm or more of decentration.

The degree of ablation zone decentration did not affect postoperative Snellen visual acuity or contrast sensitivity.