Coronal plane femoral bowing in Far East Asians: implications for the strategy of distal femoral resection in total knee arthroplasty

Background The accuracy of distal femoral resection in intramedullary (IM) guided total knee arthroplasty (TKA) depends on femoral morphology and varies according to individual anatomy. This study aimed to characterise coronal plane femoral bowing in Far East Asians according to age, sex, and severity of varus deformity to identify optimal strategies for distal femoral resection in TKA. Method Femoral anatomical parameters in 656 patients (M/F = 232:424) were assessed using standing long-leg anteroposterior radiography which was fulfilling strict standard. The femur was divided into three longitudinal segments to measure the segmental anatomical axial deviation from the mechanical axis and intersegmental bowing. Coronal plane femoral bowing pattern was categorised based on combined gross bowing and distal bowing. Results Mean hip-knee-ankle angle; neck-shaft angle; proximal, middle, and distal segmental axial differences; mechanical lateral distal femoral angle; and femur length were 6.7 +/- 6.8 degrees, 125.0 +/- 5.5 degrees, 5.9 +/- 1.7 degrees, 6.1 +/- 1.1 degrees, 5.3 +/- 1.6 degrees, 88.4 +/- 2.6 degrees, and 432.3 +/- 23.9 mm in male and 8.4 +/- 5.5 degrees, 126.4 +/- 5.6 degrees, 5.4 +/- 1.5 degrees, 6.6 +/- 0.9 degrees, 5.6 +/- 1.6 degrees, 89.3 +/- 2.6 degrees, and 410.6 +/- 23.3 mm in female, respectively. Mean proximal, distal, and gross femoral bowing was 0.3 +/- 1.8 degrees, - 0.8 +/- 1.8 degrees, and - 0.5 +/- 2.9 degrees in male and 1.2 +/- 1.6 degrees, - 1.0 +/- 1.6 degrees, and 0.2 +/- 2.7 degrees in female, respectively. Conclusions Grossly straight femur with a straight distal part was the most common femoral bowing pattern in Far East Asians. Distal bowing was proved to be a key factor to choose method for distal femoral resection in TKA. Using IM-guide to achieve accurate distal femoral resection in the femora with distal segmental axial deviation between 4-8 degrees and distal bowing less than +/- 1 degrees is considered feasible.