Abstract
BACKGROUND: Rotational alignment of the femoral component is pivotal in total knee arthroplasty (TKA). However, fixed rules such as posterior condylar axis (PCA) + 3° external rotation may neglect patient-specific anatomy and cartilage loss in knee osteoarthritis (KOA). MRI can visualize cartilage and may refine rotational planning. This study aimed to determine, on MRI, how posterior condylar cartilage defects affect femoral rotational alignment (cartilage-aware surgical epicondylar axis (SEA)-PCA), to assess variation by coronal alignment and KOA severity, and to evaluate the utility of tangential anterior line (TAL) -SEA as a complementary descriptor. METHODS: In this cross-sectional study, 548 knees (≥ 60 years) underwent standardized MRI measurements of SEA-PCA and TAL-SEA. Coronal alignment was classified as neutral/varus/valgus; KOA severity as none/early/advanced (Kellgren-Lawrence classification). We calculated a PCA + 3° femoral rotation and computed resultant malrotation relative to SEA. RESULTS: Both angles declined with increasing KOA severity (SEA-PCA: 3.0° vs. 2.6° vs. 2.2°; TAL-SEA: 4.7° vs. 4.4° vs. 2.9°; all p < 0.001). Among KOA knees, valgus alignment exhibited larger rotational angles than neutral/varus (e.g., advanced KOA SEA-PCA 4.4° valgus vs. 2.0°/1.8°; TAL-SEA 5.7° valgus vs. 2.8°/2.7°; all p < 0.001). The PCA + 3° rule produced frequent malrotation: internal in most valgus KOA knees and external in most varus KOA knees; notably, this pattern reversed in non-KOA knees. Alignment-angle correlations were positive but modest. CONCLUSION: Rotational axes vary with KOA severity and coronal alignment on MRI, and these phenotype-dependent differences explain why a uniform PCA + 3° approach produced a high malrotation rate (internal in many valgus KOA knees and excessive external in many varus KOA knees). Therefore, femoral rotational alignment planning should be individualized using anatomy-based references-prioritizing the SEA, cross-checking with TAL-SEA, and applying directional tolerances that avoid internal rotation relative to the SEA-rather than relying on a fixed-angle rule.