Determinants of Posterior Corneal Astigmatism Orientation With Emphasis on High Against-The-Rule Anterior Astigmatism: Insights From 17,860 Eyes.

PURPOSE: To identify determinants of posterior corneal astigmatism (PCA) orientation, including anterior corneal astigmatism (ACA) orientation and magnitude. METHODS: In this retropective cross-sectional observational study, anonymized swept-source optical coherence tomography biometry measurements (IOLMaster 700) were analyzed. Eyes with previous refractive surgery, corneal ectasia, or ocular trauma were excluded. ACA and PCA were classified as with-the-rule, against-the-rule (ATR), or oblique according to the steepest meridian, accounting for the optical reversal of the posterior cornea. Associations with ATR PCA were evaluated using chi-square tests and regression models. RESULTS: A total of 17,860 eyes from 17,860 patients were included. Among 17,483 eyes with PCA > 0 D and classifiable orientation, 14,335 (81.99%) had ATR PCA. In bivariate analysis, each additional year of age was associated with lower odds of ATR PCA (odds ratio: 0.955; P < 0.001). ATR PCA was present in 9096 eyes (92.85%) with with-the-rule or oblique ACA versus 4960 eyes (67.19%) with ATR ACA (P < 0.001). Among eyes with ATR ACA ≥ 3.00 D, only 14 of 213 (6.57%) exhibited ATR PCA (unadjusted odds ratio: 0.007; P < 0.001). Male sex remained independently associated with lower odds of ATR PCA. CONCLUSIONS: ACA orientation and magnitude were strongly associated with PCA orientation. Age showed a marked bivariate association, although its independent contribution was attenuated after multivariable adjustment. High-magnitude ATR ACA identified eyes in which PCA often departed from the predominant ATR pattern.