The effect of varus deformity on rotational geometry of the distal femur in osteoarthritic knees

Background: Accurate rotational positioning of the femoral component is one of the most important factors for successful total knee arthroplasty (TKA). The effect of varus deformity severity on distal femoral condylar geometry in the axial plane remains unclear. The purpose was to investigate the relationship between varus deformity severity and change in distal femoral posterior condyle phenotype. Methods: A total 400 patients who underwent TKA for medial osteoarthritis were enrolled between August 2015 and May 2021. Preoperative computed tomography (CT) scan was used. The angle formed by surgical and clinical transepicondylar axes (TEA) and posterior condylar line (PCL) were measured: posterior condylar angle (PCA) and condylar twist angle. The anteromedial condyle, anterolateral condyle, posteromedial condyle (PMC) posterolateral condyle lengths were also measured on CT. The distal femur geometry parameters were compared between severe (hip-knee-ankle [HKA] angle ≥10°) and mild (HKA varus <10°) varus groups. Results: The PCA angle was 0.79° smaller in the severe varus group (n = 228) than in the mild varus group (n = 172). The PMC distance was 0.86 mm lower in the severe varus group. Multiple linear regression identified the HKA angle, medial proximal tibia angle (MPTA), and PMC dimensions as predictors of the PCA. ROC analysis showed that the optimum cutoff value for a PCA <3° was an HKA angle of 8.9° (P = 0.010) and an MPTA of 85.5° (P < 0.001). Conclusion: The PCA and PMC were smaller in severe varus than in mild varus osteoarthritic knees. Study design: Case series; Level of Evidence IV.