The effect of optical zone decentration on lower- and higher-order aberrations after photorefractive keratectomy in a cat model

Abstract

Purpose: To simulate the effects of decentration on lower- and higher-order aberrations (LOAs and HOAs) and optical quality, by using measured wavefront error (WFE) data from a cat photorefractive keratectomy (PRK) model.

Methods: WFE differences were obtained from five cats' eyes 19 +/-7 weeks after spherical myopic PRK for -6 D (three eyes) and -10 D (two eyes). Ablation-centered WFEs were computed for a 9.0 mm pupil. A computer model was used to simulate decentration of a 6-mm subaperture in 100-microm steps over a circular area of 3000 microm diameter, relative to the measured WFE difference. Changes in LOA, HOA, and image quality (visual Strehl ratio based on the optical transfer function; VSOTF) were computed for simulated decentrations over 3.5 and 6.0 mm.

Results: Decentration resulted in undercorrection of sphere and induction of astigmatism; among the HOAs, decentration mainly induced coma. Decentration effects were distributed asymmetrically. Decentrations >1000 microm led to an undercorrection of sphere and cylinder of >0.5 D. Computational simulation of LOA/HOA interaction did not alter threshold values. For image quality (decrease of best-corrected VSOTF by >0.2 log units), the corresponding thresholds were lower. The amount of spherical aberration induced by the centered treatment significantly influenced the decentration tolerance of LOAs and log best corrected VSOTF.

Conclusions: Modeling decentration with real WFE changes showed irregularities of decentration effects for rotationally symmetric treatments. The main aberrations induced by decentration were defocus, astigmatism, and coma. Treatments that induced more spherical aberration were less tolerant of decentration.

Figure 1

Second-order refraction change decentration maps for the right eye of cat 5-005. (A) Change in sphere, 3.5-mm PD. (B) Change in cylinder magnitude, 3.5-mm PD. (C) Change in sphere, 6-mm PD. (D) Change in cylinder magnitude, 6-mm PD. The center (crosshair) is set to zero; dotted lines: 0.25-D steps.

Figure 2

Mean effects of decentration on 2nd-order refraction change (averaged data from 0°, 90°, 180°, 270° meridians for the five eyes). (A) Change in sphere magnitude. (B) Change in cylinder magnitude. The center is set to zero. Dotted line: −0.5-D threshold.

Figure 3

Decentration map for the change of coma RMS in the right eye of cat 5-005. (A) 3.5-mm PD; (B) 6-mm PD. The center (crosshair) is set to zero.

Figure 4

Mean effects of decentration on the change of coma RMS (averaged data from 0°, 90°, 180°, and 270° meridians for the five eyes). The center is set to zero.

Figure 5

Decentration map for the change of image quality with best correction (Δ log BCVSOTF) for the right eye of cat 5-005. (A) 3.5-mm PD; (B) 6-mm PD. The center (cross-hair) is set to zero; the dots mark the outmost coordinates with a Δ log BCVSOTF > −0.2; the circle shows the mean tolerance r̄.

Figure 6

Mean effects of decentration on the change of log BCVSOTF (averaged data from 0°, 90°, 180°, and 270° meridians for the five eyes). The center is set to zero; dotted line: −0.2-log VSOTF threshold.

Figure 7

Scatterplot showing the influence of spherical aberrations induced by the centered treatment on the decentration tolerance r̄ of image quality (Δ log BCVSOTF). Error bars: SD of r̄ for each eye indicating irregularity of the decentration pattern.