The most important finding of this study was that the mean MD/LD ratio was 0.61 in Korean patients, and it was lower than that of the typical “grand-piano sign” definition. Furthermore, the incidence of the morphological shape of the “boot sign” was found in 62.4% of cases. Furthermore, the MD/LD ratio was associated, especially with FLAP, indicating that the morphology of the distal femur might correlate with the MD/LD ratio.
Several authors have proposed a correlation between the shape of the anterior resection surface and femoral rotation angle in TKA.16–19) The asymmetrical shape of the anterior resection surface of the femur, the so-called “grand-piano sign,” was considered a reference for the correct rotational angle of the femoral condylar resection when aligned parallel with TEA.16–19) However, the shape of the anterior resection surface is known to change with the femoral rotation angle,17–19) distal femoral resection parameters,18,19) and even kinematic aligned TKA.18) Cui et al17) have reported that the MD/LD ratio of the anterior resection surface was 0.66 and 0.69 when using the surgical TEA and 3° external rotation relative to the posterior condylar axis, respectively, by measuring the vertical distance without considering the distal femoral resection. Thus, there might be substantial differences in the vertical length and the ratio of the anterior resection surface according to the distal femoral resection, which would be more frequent in clinical practice. In a study by Ohmori et al19), the MD/LD ratio after distal femoral resection was 0.62 to 0.67, but the ratio increased with the increase in flexion angle of the distal femoral resection. Kim et al18) have reported similar results, but the MD/LD ratio in kinematically aligned TKA (range = 0.73–0.76) was found to be larger (close to the butterfly) compared with that of the mechanically aligned TKA (range = 0.57–0.63), which was lower than the definition of the “grand-piano sign”. In this study, the shape of the anterior resection surface was frequently found as a “boot sign” rather than the “grand-piano sign”. Since the parameters for the anterior resection surface were measured after all femoral bone resection were aligned mechanically, the MD/LD ratio was possibly lower than that of the “grand-piano sign” by definition, similar to previous studies.18,19) Furthermore, the anatomical morphology of the distal femur would be different based on ethnicity or sex, which might affect the femoral rotational alignments.14,27−29) Although there were many studies on the shape of the anterior resection surface during TKA, no study considered the distal femoral morphologies simultaneously.
Distal femoral morphology varies according to sex, ethnicity, or individual.27–30) Previous studies have reported that the femoral posterior condyles are asymmetric in width, with the lateral side smaller than the medial,30) narrower among women than that among men,27,28,30) and longer anteroposteriorly among the Black population than that among the Asian population.29) Furthermore, the femoral condyles in the Korean population were known to be asymmetric, as shown by larger FMAP than FLAP, which is in agreement with the Chinese population. Thus, we assumed that the shape of the anterior resection surface might be correlated with the asymmetric morphologic features of the distal femur, especially the FMAP or FLAP, and not only the femoral rotational angle. According to the results of this study, the shape of the anterior resection surface was correlated with the FLAP and femoral rotation angle, indicating that smaller FLAP correlated with the smaller MD/LD ratio, close to the “boot sign” feature. In brief, the asymmetric distal femoral morphology, which is smaller in the lateral femoral condyle, could affect the shape of the anterior resection surface of the femur, including the femoral rotation and distal femoral cutting angles. Thus, while considering the shape of the anterior resection surface of the femur as a reference for the correct femoral rotation position, the asymmetric distal femoral morphology should be reviewed simultaneously, especially in East Asian ethnics. The intraoperative morphology of the anterior resection surface might be more frequently close to a “boot sign” owing to asymmetry.
This study has several limitations. First, patients with valgus knees were excluded from this study; hence, the results might vary in patients with valgus knees. Second, the results of this study were obtained using an East Asian population; there might be differences among other ethnicities.29) The anatomical morphology of East Asians was found to be smaller in absolute size but wider in ratio with more asymmetric features than the Black or White population29); hence there might be differences in the shape of the anterior femoral resection surface. Third, there might be differences in the ratios according to surgical techniques, such as posterior or anterior referenced TKA, and the conventional mechanical alignment or kinematic alignment during TKA.18) Fourth, considerable individual variations based on the cartilage thickness of the femur could be observed as well as differences owing to the use of different reference axes, such as the surgical or clinical TEA and white side line.22)
The shape of the anterior femoral resection surface was frequently found as a “boot sign” in Korean patients, which might be correlated with the asymmetric distal femoral morphology of East Asian people. Furthermore, while using the shape of the anterior resection surface of the femur as a reference for femoral rotation angle, the distal femoral anatomical morphology, especially the FLAP and distal femoral cutting angle should be considered simultaneously.