The present study confirmed the hypothesis that both pupil dilation and decentration contribute to the decrease of retinal image quality after myopic PRK compared to the image quality of a perfectly centered treatment at a 3-mm pupil diameter. This could be expected as the wavefront error increases as a function of pupil diameter7
However, as shown graphically in (0.2 log VSOTF decrease, black dotted line), the influence of pupil dilation prevailed over decentration-induced decreases in image quality. This was confirmed by statistical analysis () as standardized regression coefficients β for pupil dilation were at least twice as high as those for optical zone decentration. Moreover, if decentration-induced LOAs were corrected (simulation 2), the influence of decentration further decreased (, β = −0.26). This suggests that LOAs are primarily responsible for decreases in image quality in the case of decentration. The induction of LOAs by pupil dilation is limited, as reflected by marginal differences between simulation models 2 and 3 (see ). In the latter model, dilation-induced LOAs were corrected. In contrast to similuations 1 and 2, simulation 3 has only theoretical relevance because in clinical practice it is almost impossible to adapt the LOA correction to the actual pupil diameter. The convolution series in shows the effect of LOA correction in case of pupil dilation and decentration. and also illustrate that in case of micro-decentrations (≤500 μm), the effect on optical quality is rather limited. In particular, we could not support our second hypothesis that micro-decentrations, which are asymptomatic under photopic conditions (in our simulations represented by a 3-mm pupil diameter), become symptomatic under mesopic conditions (6-mm pupil diameter). Only for severe decentrations (eg, 1500 μm) did we observe a somewhat steeper decrease in retinal image quality with pupil dilation (see –) relative to the centered treatment.
Analysis of decentration tolerance as a function of pupil diameter confirmed results from our previous study19
and again underlined the role of decentration-induced LOAs (see ) in lowering retinal image quality. If the strict criterion of a VSOTF decrease of 0.2 log units is applied, the tolerance to decentration at a given pupil diameter was lowest in simulation 1 (LOA no correction, open squares in ). If decentration-induced LOAs (see , gray diamonds) or decentration- and dilation-induced LOAs (see , black triangles) were corrected, decentration tolerance increased. This effect was statistically significant, at least for lower pupil diameters.
With regard to clinical practice and as suggested by our previous study,19
the present simulations showed that ubiquitous optical zone micro-decentrations10
only have a limited effect on optical quality after laser refractive surgery. However, one should consider that the physiological range of pupil diameters is by far higher than that of our model.23
In addition, there is more probability that an eye will reach large pupil diameters than high decentration values.10
This discrepancy of probabilities further decreases the likely influence of decentration on image quality following laser refractive surgery. Even if a relatively strict criterion is applied (as in our decentration tolerance construct), decentrations ~500 μm did not lead to a VSOTF decrease beyond the tolerance threshold (see ).
Although the present results are in line with clinical experience that micro-decentrations do not inevitably induce optical symptoms, they should be interpreted with some caution. The VSOTF is a theoretical construct, as is the simulation of best-corrected refraction based on the VSOTF. Psychometric tests have shown that subjective quality of vision may differ from the VSOTF.24
Some patients may be particularly sensitive to decentration-induced coma blur whereas others may exhibit no impairments. Moreover, aberrations induced by optical zone decentration may become more complex in cases of astigmatic or wavefront-guided ablations. Procedures that induce higher amounts of spherical aberration, such as treatments for high myopia2,25
are also likely to exhibit lower decentration tolerance compared to the treatments investigated in this study.19
Finally, even this small sample was heterogenous regarding its treatment effects and induced HOA (). This heterogeneity was also reflected by a relatively large range of decentration tolerance values and regression coefficients. However, a larger sample size is needed to establish any correlation between the amount of attempted correction and the relative contribution of pupil dilation and decentration of optical quality. Regarding comparability of cat with human PRK, analysis of treatment effects showed that the aberration pattern in the cat model is similar to that observed in humans.14,21
The under-correction on the cat cornea has been observed before and could be explained by a lower ablation rate for the cat cornea.14,19
Although this heterogeneity of the data somewhat decreased comparability with human data, it does not affect the results from the model itself. Moreover, the cat model is unique as wavefront sensing at a large physiological pupil diameter of 9 mm without pharmacological dilation is possible.21
The present model study showed that pupil dilation has a higher impact on the decrease in retinal image quality experienced after myopic PRK than decentration of the ablation optical zone. In particular, for eyes that do not show visual symptoms such as halos and ghosting under photopic conditions but become symptomatic with pupil dilatation, micro-decentrations (≤500 μm) did not induce significant amounts of additional lower or higher order optical aberrations.