The Effect of Pupil Decentration on Lower- ond Higher-Order Aberrations After Myopic Photorefractive Keratectomy (PRK) in a Cat Model

Abstract

Purpose:: To investigate the effect of pupil decentration relative to the optical zone (OZ) on lower and higher order aberrations (LOA and HOA) in a cat photorefractive keratectomy (PRK) model.

Methods:: Four cat eyes were treated with a conventional spherical PRK (217, Bausch and Lomb) for -6D (3 eyes) and -10D (one eye). Wavefront errors (WFEs) were measured in the awake behaving state before and 22±4 weeks after PRK and were computed for a pupil diameter (PD) of 9.0 mm. A computer model was used to simulate decentration of a sub-pupil relative to the measured WFE difference in 100 µm steps. The change of image quality expressed as the logarithm of the Visual Strehl ratio based on the optical transfer function for best correction (log BCVSOTF) and the change in LOA and HOA were computed for simulated decentrations for 3.5 and 6.0 mm sub-pupil diameters. The combination of lower-order terms that yielded the best image quality (BCVSOTF) was considered as the simulated endpoint for the subjective manifest refraction.

Results:: The mean decrease in BCVSOTF for a 6 mm PD was: centered treatment: -0.57 ±0.16 (decrease by 25% from preop centered value); 200 µm decentration: -0.61±0.23 (30% decrease); 500 µm decentration:-0.75±0.24 (45% decrease); 1000 µm decentration:-0.95±0.3 (57% decrease). For the 3.5 mm PD the slope of the precentual VSOTF decrease was flatter. There was increasing undercorrection of 2nd order terms with decentration. For 200 µm of supero-temporal decentration, the spherical equivalent for a 6 mm pupil was -0.03 ±0.01 D lower than after the measurement (3.5 mm: -0.02 ±0.07 D), at 500 µm there was an undercorrection of -0.28 ±0.09 D (3.5 mm: -0.17 ±0.19 D). For a decentration of 1000 µm the undercorrection increased to -0.74 ±0.33 D (-0.81 ±0.61 D). For both 3.5 and 6 mm pupils, the BCVSOTF-based simulated subjective SE was more myopic than the refraction based on 2nd order Zernike terms. There was also an increase HOA induction with decentration, particularly for primary coma terms which showed a linear increase with decentration along their axis.

Conclusions:: Decentration of the optical zone causes 2nd order undercorrection and induction of HOA. Micro-decentrations of <500 µm appear to be of limited clinical relevance since they had an only minor effect on the undercorrection and the decrease of the BCVSOTF.

Keywords: refractive surgery: optical quality • refractive surgery: complications • refractive surgery: PRK