Femoral component internal rotation in functionally aligned robotic‐assisted total knee arthroplasty does not impair functional outcomes

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PURPOSE: Femoral component internal rotation tightens the medial flexion gap and medialises femoral trochlea position, thus influencing patellofemoral tracking and function. Therefore, one of the tenets of mechanically aligned total knee arthroplasty (TKA) has been to avoid femoral component internal rotation relative to the transepicondylar axis (TEA). Our aim was to describe femoral component rotation achieved in functionally aligned TKA and assess the impact of femoral component rotation on functional outcomes. METHODS: A retrospective review of 2760 consecutive, functionally aligned (FA), robotic-assisted TKAs, undertaken by two senior arthroplasty surgeons was performed. Femoral component rotation was assessed relative to the surgical TEA (sTEA) at the time of implantation using an image-based TKA system. Primary outcomes were Forgotten Joint Score (FJS) and knee flexion range of motion (ROM) at 1 year follow-up. Secondary outcomes were knee extension, EuroQol 5-Dimension Score, Knee Injury and Osteoarthritis Outcome Score for Joint Replacement, Oxford Knee Score and Visual Analogue Scale pain. RESULTS: Mean femoral component rotation was 0.3° (standard deviation 2.1, range 6.9 internally rotated to 6.5 externally rotated) external to the sTEA. The femoral component was >2° internally rotated to the sTEA in 332 (12%) cases and >4° in 66 (2%) cases. It was rare for the femoral component to be more than 2° internally rotated to the preoperative native posterior condylar axis (0.5%, n = 13). When comparing patients with femoral component internally rotated more than 2° to the sTEA to all other patients, there was no statistically nor clinically relevant difference in any outcome measure at 1 year follow-up. Multivariate analysis found femoral component rotation to have no significant correlation with postoperative FJS or flexion ROM. CONCLUSION: Internally rotating the femoral component relative to the surgical TEA, in a functionally aligned TKA, was not associated with any detrimental impact on clinical outcomes. The ongoing study of alternative alignment strategies that result in relative internal rotation of the femoral component is warranted. LEVEL OF EVIDENCE: Level III, retrospective comparative study.

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