AL-adjusted Km residual groups and anterior chamber depth heterogeneity in patients with age-related cataract

To characterize the nonlinear relationship between axial length (AL) and mean keratometry (Km) and evaluate whether Km residual groups identify anterior chamber depth (ACD) differences beyond conventional AL-based stratification. This retrospective cross-sectional study included 15,082 eyes with age-related cataract undergoing swept-source optical coherence tomography (SS-OCT) biometry; one eye was randomly selected per participant. Segmented regression identified the AL–Km breakpoint, and a generalized additive model (GAM) modeled the AL–Km trajectory. Associations between Km residual groups and ACD were assessed using multivariable linear regression adjusted for AL, lens thickness (LT), central corneal thickness (CCT), age, sex, and group-by-AL interactions. Model comparisons included raw Km, residual groups, and nonlinear models. Robustness was assessed by bootstrap resampling and AL-conditional residual standardization. The AL–Km relationship was nonlinear, with a breakpoint at 24.20 mm (95% CI, 24.15–24.25 mm). GAM residuals classified 2,247 eyes (14.9%) as low-Km group (LKG), 10,579 (70.1%) as coupled-Km group (CKG), and 2,256 (15.0%) as high-Km group (HKG). Compared with CKG, LKG was associated with shallower ACD (β = −0.125 mm), whereas HKG was associated with deeper ACD (β = 0.115 mm; both P < 0.001). In 10-fold cross-validation, the model including the categorical Km residual group had a lower CV RMSE than the model including raw Km (0.2454 vs. 0.2515 mm), but a higher CV RMSE than the nonlinear models (0.2331–0.2360 mm). The categorical association between Km residual group and ACD was strongest at 22 ≤ AL < 24 mm (Cramer’s V = 0.146). At AL ≥ 26 mm, unadjusted ACD differences among the three groups were not significant ( P = 0.404), whereas adjusted analysis showed a LKG–CKG difference (− 0.115 mm) but no HKG–CKG difference (+ 0.005 mm). AL-conditional residual standardization yielded 95.4% agreement with the primary classification. The AL–Km relationship is nonlinear, with a cohort-specific change point around 24–24.5 mm. Km residual groups were associated with ACD differences among eyes with similar AL that were not fully captured by AL stratification alone, and provide an interpretable representation of individual deviation from the population AL–Km relationship.