Contribution of optical zone decentration and pupil dilation on the change of optical quality after myopic photorefractive keratectomy in a cat model

Abstract

Purpose: To simulate the simultaneous contribution of optical zone decentration and pupil dilation on retinal image quality using wavefront error data from a myopic photorefractive keratectomy (PRK) cat model.

Methods: Wavefront error differences were obtained from five cat eyes 19+/-7 weeks (range: 12 to 24 weeks) after spherical myopic PRK for -6.00 diopters (D) (three eyes) and -10.00 D (two eyes). A computer model was used to simulate decentration of a 6-mm sub-aperture relative to the measured wavefront error difference. Changes in image quality (visual Strehl ratio based on the optical transfer function [VSOTF]) were computed for simulated decentrations from 0 to 1500 mum over pupil diameters of 3.5 to 6.0 mm in 0.5-mm steps. For each eye, a bivariate regression model was applied to calculate the simultaneous contribution of pupil dilation and decentration on the pre- to postoperative change of the log VSOTF.

Results: Pupil diameter and decentration explained up to 95% of the variance of VSOTF change (adjusted R(2)=0.95). Pupil diameter had a higher impact on VSOTF (median beta=-0.88, P<.001) than decentration (median beta=-0.45, P<.001). If decentration-induced lower order aberrations were corrected, the impact of decentration further decreased (beta=-0.26) compared to the influence of pupil dilation (beta=-0.95).

Conclusions: Both pupil dilation and decentration of the optical zone affected the change of retinal image quality (VSOTF) after myopic PRK with decentration exerting a lower impact on VSOTF change. Thus, under physiological conditions pupil dilation is likely to have more effect on VSOTF change after PRK than optical zone decentration.

Figure 1

Color maps showing the simultaneous effect of decentration and pupil diameter (PD) on the change of retinal image quality (visual Strehl ratio based on the optical transfer function [Δ log VSOTF]) for eye #5-005_OD. A) Simulation 1: optimum lower order aberration (LOA) correction obtained at a 3-mm pupil diameter; no correction of LOA for any decentered position or dilated pupil. B) Simulation 2: optimum LOA correction obtained at a 3-mm pupil diameter for any decentered position; no adjustment for any dilated pupil. C) Simulation 3: optimum LOA correction at all decentered positions and adjustment for all pupil dilations. Dotted line = 0.2 log VSOTF decrease threshold

Figure 2

Image simulation of postoperative image quality by convolution for eye #5-005_OD. The convolution images are not corrected for phase shift. A) Simulation 1: optimum lower order aberration (LOA) correction obtained at a 3-mm pupil diameter (PD); no correction of LOA for any decentered position or dilated pupil. B) Simulation 2: optimum LOA correction obtained at a 3-mm pupil diameter for any decentered position; no adjustment for any dilated pupil.

Figure 3

Comparison of change of visual Strehl ratio based on the optical transfer function (Δ VSOTF) as a function of pupil diameter (PD) for three grades of decentration (0, 500, and 1500 μm); average values from all five eyes. The centered value obtained for a 3-mm pupil diameter is set to zero. A) Simulation 1: optimum lower order aberration (LOA) correction obtained at a 3-mm pupil diameter; no correction of LOA for any decentered position or dilated pupil. B) Simulation 2: optimum LOA correction obtained at a 3-mm pupil diameter for any decentered position; no adjustment for any dilated pupil. C) Simulation 3: optimum LOA correction at all decentered positions and adjustment for all pupil dilations.

Figure 4

Comparison of decentration tolerance (maximum permissible decentration to maintain a decrease of visual Strehl ratio based on the optical transfer function [VSOTF] of <0.2 log units). The asterisks denote significance of difference between correction modes (*P<.05, **P<.01 after Bonferroni correction). Asterisks, top row: difference between uncorrected and best spectacle-corrected mode; bottom row: difference between uncorrected and best spectacle-corrected at 3-mm mode. Differences between best-corrected and best-corrected at 3-mm mode did not reach statistical significance.