Functional alignment improves femoral joint line obliquity preservation in comparison with the classical measured resection technique

Abstract Purpose (1) To analyse the femoral anatomical–mechanical axis (AA–MA) and posterior condylar–transepicondylar axis (PCA–TEA) relationships using computed tomography (CT)‐based measurements. (2) To quantify the coronal and axial errors that would occur when positioning the femoral component using mechanically aligned TKA modelled as 5° of valgus from the AA and 3° of external rotation from the PCA. (3) To compare coronal and rotational femoral positioning between this systematic approach and robotic‐assisted functionally aligned TKA. Methods It is a multicentric cross‐sectional observational study. Preoperative CT scans of 318 patients were analysed to determine AA–MA and PCA–TEA relationships. Then, femoral positioning was simulated, intending a measured‐resection mechanical aligned TKA defined as 5° of valgus from the AA and 3° of external rotation from the PCA. It was compared with the real femoral component placement performed in each patient using robotic‐assisted functionally aligned TKA. Results Mean AA–MA relationship was 6.0 ± 0.9° and mean PCA–TEA was 3.1 ± 1.8°. In measured‐resection, mechanically aligned TKA, 74.4% and 45.8% of patients would present a femoral component implanted in varus from the MA and internal rotation from the TEA, respectively. Functional alignment was significantly closer to the native LDFA than mechanical alignment (2.0° vs 3.5°). With mechanical alignment, 45.8% of femoral components would be implanted in internal rotation from the TEA, compared to only 8.8% with functional alignment. Conclusions Measured resection based on average population data (5° of valgus from the AA and 3° of external rotation from the PCA) may result in a considerable proportion of femoral components implanted in varus and internal rotation. In contrast, robotic‐assisted functionally aligned TKA provided significantly improved restoration of the native femoral joint line obliquity and effectively prevented excessive internal rotation. Level of Evidence Level IV.