Outcomes of Functional Versus Mechanical Alignment in Robotic Arm-Assisted Total Knee Arthroplasty: A Randomized Controlled Trial

Introduction Mechanically aligned (MA) total knee arthroplasty (TKA) is the gold standard; however, some patients continue to report post- surgical dissatisfaction. An alternative is Functional alignment (FA), which integrates kinematic alignment (KA) principles with pre- resection bone balancing within defined boundaries. FA aims to improve outcomes by restoring native joint lines and optimising soft-tissue balance. There is limited comparative evidence between the two procedures. Methods A prospective, randomised controlled trial compared MA (n=121) and FA (n=123) using robotic-assisted TKA. MA components were positioned perpendicular to the limb mechanical axis, and balance achieved with soft-tissue releases. For FA, initial virtual component positioning was matched with native knee anatomy, with soft-tissue adjustments prior to bone cuts. A mixed-model analysis of variance was performed to compare outcomes. The primary outcome was the Forgotten Joint Score (FJS). Results At two-years, mean FJS for both procedures were similar (MA: 64.4±30.1 vs. FA: 70.1±25.6, p=0.10). More soft-tissue releases were performed in MA than FA (65% vs. 16%, p<0.001). FA patients had higher Knee Injury and Osteoarthritis Outcome (KOOS) Symptoms (86.6±12.9 vs. 82.5±14.0, p=0.01) and Quality of Life (QOL) scores (76.1±20.3 vs. 70.7±22.7, p=0.03), and more ‘would recommend’ the procedure (94% vs. 82%, p<0.01) compared with MA. For constitutionally varus knees (Coronal Plane Alignment of the Knee Type I), FA patients reported higher FJS (71.3±24.8 vs. 56.8±31.6, p=0.02) and KOOS-QOL (76.4±21.7 vs. 64.2±19.2, p=0.02) than MA. Both procedures had similar patient-reported outcomes (Oxford Knee Score, KOOS, EuroQol-5 Dimensions, Pain Visual Analog Scale), clinical outcomes (length of stay, functional physio tests), reoperations, and implant survivorship (FA: 1 vs. MA: 0 revisions). Conclusions FA required fewer soft-tissue releases compared with MA, however outcomes at two years were similar. FA may provide improved outcomes for a particular subgroup of patients based on preoperative alignments.