Approximately half of the magnitude of ocular residual astigmatism comes from posterior corneal astigmatism in myopic patients aged 18-40 years

To determine the main sources of ocular residual astigmatism (ORA) and the contribution of posterior corneal astigmatism (PCA) to ORA. Anterior corneal astigmatism (ACA) and PCA was obtained from Pentacam. ORA was calculated by Thibos vector analysis. Pearson correlation coefficient was used to assess relationships between variables. The contribution of PCA to ORA was evaluated by a physical method. Mean magnitude of PCA was 0.40 ± 0.15 D. Mean amount of ORA was 0.69 ± 0.33 D. There was a significant correlation between the magnitudes of PCA and ORA ( r  = 0.38, P  < 0.001). The PCA in 171 eyes (96.61%) supplemented ORA. The mean superimposed values (SV) was 0.37 ± 0.16 D (rang 0.01 to 0.70 D). The against-the-rule PCA can not only superimpose but also counteract with-the-rule and oblique ORA. The against-the-rule ORA can be superimposed by with-the-rule, against-the-rule and oblique PCA. Of the 171 eyes, the SV of 85 eyes(49.71%) were larger than half of the magnitude of ORA. In a minority of eyes, PCA can have a negative effect on ORA and may be greater than it. The SV/ORA ratio may be used to personalize the surgical method proposed by Alpins, resulting in better outcomes.