Abstract

SA.12.01

Are micro-decentrations a reason for residual refractive error after corneal refractive laser surgery for myopia?

Bühren J.¹, Yoon G.², Kenner S.^1,2, MacRae S.², Huxlin K.^1,2
1Department of Ophthalmology, ²Center for Visual Science, ³Institute of Optics, Univerisity of Rochester, Rochester, USA

Objective: Micro-decentrations (decentrations of the optical zone [OZ] <500 µm) are ubiquitous after corneal laser refractive surgery (CLRS). The purpose of the study was to check the hypothesis that micro-decentrations are responsible for residual refractive error after CLRS using a cat photorefractive keratectomy (PRK) model.
Methods: Five cat eyes were treated with a conventional spherical PRK (217, Bausch and Lomb) for -6 D (3 eyes) and -10 D (2 eyes). Wavefront errors (WFEs) were measured in the awake fixating 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 in lower and higher order aberrations (LOA and HOA) was computed over simulated decentrations for 3.5 and 6.0 mm sub-pupil diameters. The combination of lower-order terms that yielded the best image quality (expressed as the Visual Strehl ratio based on the optical transfer function-BCVSOTF) was considered as the simulated endpoint for the subjective manifest refraction.
Results: In all eyes, decentration resulted in under-correction of sphere and induction of astigmatism. An under-correction of sphere of more than 0.5 D was observed for decentrations >1255±160 µm; the threshold value for cylinder was 1304±130 µm. If the interaction with HOA was taken into account the critical values increased to 1157±213 µm and 1323±141 µm, respectively. For pupil diameters of 6 mm, similar values could be obtained.
Conclusions: Decentration of the optical caused 2nd order under-correction and induction of higher order aberrations that were significant only for decentrations >1000µm. The interaction of lower- with higher- order aberrations increased tolerance of decentration.

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