Quantifying posterior corneal astigmatism's contribution to ocular residual astigmatism: implications for personalized refractive surgery.

Purpose: To determine the main sources of ocular residual astigmatism (ORA) and to quantify the contribution of posterior corneal astigmatism (PCA) to ORA.

Methods: This cross-sectional study was conducted on 177 right eyes of 177 patients. Anterior corneal astigmatism (ACA) and PCA were obtained from Pentacam. ORA was calculated by the vector difference between refractive astigmatism (RA) and ACA. The Pearson correlation coefficient was employed to assess relationships between variables, and the contribution of PCA to ORA was assessed using a physical method.

Results: The mean magnitude of PCA was 0.40 ± 0.15 D (range 0.10-0.70 D), while the mean ORA was 0.69 ± 0.33 D (range 0.05-1.63 D). There was a statistically significant correlation between the magnitudes of PCA and ORA (r = 0.38, P < 0.001). PCA had a positive effect on ORA in 171 eyes (96.61%), with a mean contribution value (CV) of 0.37 ± 0.16 D (range 0.01-0.70 D). The against-the-rule PCA exhibited both positive and negative effects on with-the-rule and oblique ORA, whereas with-the-rule, against-the-rule, and oblique PCA all positively influenced against-the-rule ORA. In 85 eyes (49.71%), the CV exceeded half of the ORA magnitude. In a small subset of eyes, PCA had a negative effect on ORA, potentially exceeding its magnitude.

Conclusions: Approximately half of the magnitude of ORA comes from PCA. The CV/ORA ratio may be used to personalize the surgical method proposed by Alpins, resulting in better outcomes.