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.