Intraoperative variability between the anteroposterior and posterior condylar axes in primary TKA supports a 20° proximal trochlear entrance design for unrestricted kinematic alignment.

Abstract

Unrestricted kinematic alignment (uKA) in total knee arthroplasty (TKA) creates a challenge for trochlear design. Femoral implants designed for mechanical alignment (MA) use asymmetrical posterior bone cuts to adjust axial rotation and optimize patellofemoral tracking. In uKA, posterior bone cuts are calipered resections. There is no rotational adjustment. The prosthetic trochlea is therefore fixed relative to the posterior condylar axis (PCA). The rotational variability between the native anteroposterior axis (APA) and the PCA determines the proximal trochlear entrance width required to capture the patella. This study directly measured this rotational relationship intraoperatively and determined whether a dedicated trochlear design is required for the uKA technique. A prospective cohort study was performed in consecutive patients undergoing primary TKA for osteoarthritis or inflammatory arthritis. A single implant system was used in all cases. All measurements were performed by a single senior surgeon. After distal femoral resection, the APA line was marked on the bone using a dedicated marking jig. A posterior condylar referencing jig with a 1° haptic rotational dial was then used to record the rotational difference of the APA line relative to the PCA line (PCAL). A total of 935 knees in 739 patients were analysed. Three hundred fifty-seven knees (38%) were male. Mean age was 67 years (range: 27-96). Relative to the PCAL, the mean APA line rotation was 1.3° of external rotation (ER; standard deviation 2.7°). The range was wide, from 8° of internal rotation to 10° of ER. The distribution was unimodal and broadly bell-shaped. The intraoperative rotation between the APA line and the PCAL, measured during primary TKA, spans an 18° range, which may exceed the capture of MA-designed proximal trochlear entrances. A 20° trochlear entrance design is proposed (a 1° margin at each end) for implants used for uKA. Prospective biomechanical and clinical validation is needed. Level III, prospective observational cohort study.