Posterior femoral condylar morphology following mechanical vs. kinematic alignment using a medial-pivot implant designed for mechanical alignment: A CT-based comparative study.

Purpose: The kinematic alignment (KA) technique aims to restore native joint anatomy; however, the extent to which it restores posterior femoral condylar morphology after total knee arthroplasty (TKA) remains unclear. The posterior longitudinal overhang in the femoral condyle (PLOF) has been reported to affect clinical outcomes. This study aimed to compare the PLOF after medial pivot TKA performed using KA and mechanical alignment (MA) techniques.

Methods: This retrospective computed tomography-based study included 50 knees (from 25 men and 25 women) with medial knee osteoarthritis who underwent unilateral TKA. Femoral component-computer-aided design (FC-CAD) models of a medial-pivot prosthesis originally engineered for MA were positioned on preoperative computed tomography images according to KA and MA protocols. The primary outcomes were medial and lateral PLOF, defined as the sagittal overhang of the FC-CAD model beyond the native bone contour. The extent of distal and posterior femoral resections and correlations among femoral morphological parameters, including femoral valgus, lateral distal femoral, and condylar twist angles, and the PLOF were analysed.

Results: The mean medial and lateral PLOF associated with KA was significantly greater (3.8 ± 1.4 vs. 3.2 ± 2.0 mm; p = 0.002) and smaller (3.4 ± 1.6 vs. 4.1 ± 1.9 mm; p < 0.001), respectively, than those associated with MA. Compared with KA, MA decreased valgus alignment and added external rotation. The PLOF associated with KA and femoral morphological parameters was not significantly correlated.

Conclusions: The KA resulted in significantly different ( > 3 mm) medial and lateral PLOF compared with the MA. The lack of correlations between the KA-associated PLOF and femoral morphological parameters suggests a native-prosthetic morphological mismatch. Surgeons should consider strategic downsizing when an MA-oriented medial-pivot FC is implanted using KA or employ KA-specific implants to optimise clinical outcomes.

Level of evidence: Level III, retrospective comparative study.