Risk Stratification for Varus Stem Positioning in Cementless THA: A Dual-Parameter Diagnostic Tool Based on Proximal Femoral Morphology

Abstract Purpose This study aimed to investigate the influence of proximal femoral anatomical parameters on coronal varus alignment of cementless femoral stems in total hip arthroplasty (THA) and to determine predictive thresholds. Methods A retrospective analysis of 315 primary THA cases (2022–2024) receiving a cementless stem (Polarstem) via an anterolateral approach was conducted. Preoperative pelvic radiographs were used to measure the Neck-Shaft Angle (NSA), Femoral Cortical Index (FCI), Canal Flare Index (CFI), and Trochanteric-Diaphyseal Offset Ratio (TDOR). Postoperative stem alignment was categorized as varus ( < − 3°) or non-varus ( ≥ − 3°) based on the stem-canal axis angle. Results Among 159 included patients, 35 (22.01%) were in the varus group and 124 (77.99%) in the non-varus group. The varus group had a significantly smaller NSA (127.10 ± 1.48 vs. 128.07 ± 1.17, P < 0.001) and higher TDOR (0.53 ± 0.02 vs. 0.51 ± 0.03, P < 0.001). ROC analysis identified NSA ≤ 127.85° (AUC = 0.700) and TDOR > 0.53 (AUC = 0.699) as predictive thresholds. Their combination achieved a higher AUC of 0.771. Multivariate logistic regression confirmed NSA ≤ 127.85° (OR = 4.725) and TDOR > 0.53 (OR = 4.468) as independent risk factors for varus alignment (both P < 0.001). No significant differences were found in FCI or CFI ( P > 0.05). Conclusion NSA (≤ 127.85°) and TDOR (> 0.53) are independent risk factors for coronal varus alignment in cementless THA. Their combined use provides superior predictive performance compared to either parameter alone.