In functionally aligned total knee arthroplasty, femoral component rotation follows the transepicondylar axis to achieve flexion balance

Abstract

PURPOSE: In functionally aligned (FA) total knee arthroplasty (TKA), femoral component rotation (FCR) is personalised to optimise flexion gap balance. As axial malalignment has been attributed to patellofemoral complications, this study assessed FA FCR in relation to the surgical transepicondylar axis (TEA) and early implant survivorship. METHODS: We analysed 446 robotic-assisted primary TKAs in 393 patients using FA with preresection gap balancing. Femoral and tibial coronal resection boundaries were 6° valgus to 3° varus and 6° varus to 3° valgus, respectively. The TEA and posterior condylar axis (PCA) were identified on preoperative computed tomographic scans. Intraoperatively, FCR was initially set parallel to the PCA, then rotated to achieve flexion balance within 6° boundaries to the TEA. The primary outcome was the mean angular difference between the TEA and final FCR and the proportion of patients with FCR within 2°, 4° and 6° from the TEA. Secondary outcomes included all-cause revisions for the robotic-assisted TKAs in this series from our in-house database and a national joint replacement registry at a mean of 3.3 years postoperatively (range 2.2-5.5). RESULTS: Mean FCR was 0.0° (SD 2.1°) relative to the TEA and 1.8° (SD 1.4°) externally rotated to the PCA. Final FCR was within 2° in 74.9%, within 4° in 97.3% and within 6° in 99.6% of patients relative to the TEA. There were seven postoperative procedures (1.6%) within the study period: two reoperations (0.5%) for early arthrotomy failure requiring repair (both exhibiting FCR within 1° to the TEA) and three revisions (0.7%) for deep surgical site infection (one after early traumatic arthrotomy failure, one revision for instability and one manipulation for stiffness). CONCLUSION: In FA TKA, femoral components more closely align to the TEA than the PCA to achieve a balanced flexion gap. In conjunction with a low incidence of revision surgeries, this suggests the FA technique does not result in significant patellofemoral complications. LEVEL OF EVIDENCE: Level III.