Characteristics of Posterior Corneal Astigmatism in 28,551 Chinese Eyes with Cataracts

Background/Objectives: Posterior corneal astigmatism (PCA) varies among individuals, potentially limiting the applicability of population-based estimates. This study aimed to characterize PCA and evaluate high-magnitude PCA and horizontal posterior steep-meridian orientation. Methods: Cataract patients undergoing preoperative biometry with the IOLMaster 700 were assessed for PCA magnitude and orientation. Factors associated with high-magnitude PCA (≥0.50 D) and horizontal orientation were evaluated. Fellow-eye concordance and vector discrepancies between Abulafia–Koch (AK) estimates and measured corneal astigmatism were analyzed. Results: The study included 28,551 patients (28,551 eyes). PCA magnitude was between 0.10 and <0.40 D in 76.6% of eyes, and 91.7% had a vertical posterior steep meridian. High-magnitude PCA and horizontal orientation occurred in 5.1% and 2.5% of eyes. Across PCA magnitude groups, the largest differences involved anterior corneal astigmatism (ACA) magnitude (ε2 = 0.090), total corneal astigmatism (TCA) magnitude (ε2 = 0.047), and age (ε2 = 0.036). Across orientation groups, ACA and TCA magnitude showed the largest differences (ε2 = 0.044 and 0.049). High-magnitude PCA was independently associated with younger age, the female sex, a larger ACA magnitude, and with-the-rule (WTR) ACA. Horizontal orientation was associated with older age, a larger ACA magnitude, against-the-rule (ATR) or oblique ACA, and smaller white-to-white distance (WTW). Discrepancies between AK estimates and measured PCA and TCA were greater in both subgroups, with TCA discrepancies exceeding 0.50 D in 40.6% of high-magnitude and 40.4% of horizontal-orientation eyes. Conclusions: Most eyes followed the conventional PCA pattern, whereas high-magnitude PCA and horizontal posterior steep-meridian orientation showed distinct profiles and greater discrepancies from AK estimates. Population-based estimates may therefore be less representative in these eyes.