"Boot Sign" of Anterior Femoral Condylar Resectional Shape during Total Knee Arthroplasty Is More Frequent in Asian Patients

Purpose: There is lack of intraoperative consensus on the distal femur anterior resected surface shape that allows reliable rotational alignment assessment during total knee arthroplasty (TKA). We aimed to evaluate the ratio and prevalence of anterior femoral resection surface intraoperatively. Materials and Methods: The study included 234 osteoarthritis patients with varus knees and not valgus knees or deformities. After conventional medial parapatellar approach, measured resection technique based on the mechanical axis of the femur and preoperative TEA-PCA angle on CT with anterior reference was used among all the patients. The anteroposterior (AP) lengths after distal femoral resection were measured as the femoral lateral AP (FLAP) and femoral medial AP (FMAP) lengths. Based on the medial (MD) and lateral condyle (LD) vertical distance ratios of the femur anterior resected surface, the groups were classified into "boot sign", "grand-piano", and "butterfly sign" groups. For comparison of the mean values, the data were assessed for normality with the Shapiro-Wilk test. One-way ANOVA with post hoc analysis using Tukey's honestly significant difference (HSD) test was used to compare the mean values among the groups. The correlations between the MD/LD and variables were analyzed using the Pearson correlation coefficient. Linear regression analyses were used to find the associated factors to the anterior femoral resection surface shape. Results: Mean intraoperative femoral rotation and distal femoral cutting angles were 4.9 degrees +/- 1.2 and valgus 5.0 degrees +/- 0.7, respectively. Mean FLAP was 52.9 +/- 4.2 mm. Mean MD/LD (0.61 +/- 0.13) was lower than that of typical "grand-piano sign". The morphological shape incidence of the "boot sign" was 62.4%. In the "boot sign" group, the FLAP was found to be smaller than that in the other groups (52.4 +/- 4.2 vs. 53.7 +/- 4.2 vs. 54.9 +/- 2.7; p = 0.02), while the intraoperative femoral rotation angle was found to be larger than in the other groups (5.0 +/- 1.2 vs. 4.6 +/- 1.1 vs. 4.7 +/- 1.2; p = 0.039). The MD/LD-associated factors were FLAP, intraoperative femoral rotation, and distal femoral cutting angles (R2 = 0.268). Conclusion: The femur anterior resection surface shape in TKA was found in the "boot sign" rather than the "grand-piano sign" in Korean ethnics owing to an asymmetric morphology of femoral condyles. Ethnic differences, including distal femoral morphology, should be considered for assessment of the femoral rotation angle using the femur anterior resection surface shape.