INTRODUCTION: Mechanically aligned (MA) total knee arthroplasty (TKA) is regarded as a gold standard
however, some patients report dissatisfaction post-surgery. Functional alignment (FA) is a potential alternative, integrating kinematic alignment (KA) principles with pre-resection bone balancing within defined boundaries. The use of FA aims to improve outcomes by restoring native joint lines and optimizing soft-tissue balance. However, comparative evidence is limited. METHODS: This prospective, randomized controlled trial compared MA (n = 121) and FA (n = 123) in robotic-assisted TKA. For MA, components were positioned perpendicular to the limb mechanical axis, with soft-tissue releases to achieve balance. For FA, initial virtual component positioning was used to match native knee anatomy (KA), with adjustments for soft-tissue balance before bone cuts. The primary outcome was the forgotten joint score (FJS). Outcomes were compared with a mixed-model analysis of variance. RESULTS: At the two-year follow-up, the mean FJS was comparable (MA: 64.4 ± 30.1 versus FA: 70.1 ± 25.6, P = 0.10). The MA cases had more soft-tissue releases than FA (65 versus 16%, P <
0.002). Compared to MA patients, FA patients had higher Knee Injury and Osteoarthritis Outcome (KOOS) Symptoms (86.6 ± 12.9 versus 82.5 ± 14.0, P = 0.01) and KOOS-Quality of Life (QOL) scores (76.1 ± 20.3 versus 70.7 ± 22.7, P = 0.03). More FA patients 'would recommend' the procedure (94 versus 82%, P <
0.01). For patients who had preoperative coronal plane alignment of the knee (CPAK) Type I, FA had higher FJS (71.3 ± 24.8 versus 56.8 ± 31.6, P = 0.02) and KOOS-QOL (76.4 ± 21.7 versus 64.2 ± 19.2, P = 0.02) than MA. No other differences were seen in patient-reported outcomes (Oxford Knee Score, KOOS, EuroQol-5 Dimensions, Pain Visual Analog Scale), clinical outcomes (length of stay, functional physio tests), reoperations, or implant survivorship (FA: 1 versus MA: 0 revisions). CONCLUSIONS: While functional alignment required fewer soft-tissue releases compared to mechanical alignment, at two years patient-reported and clinical outcomes were similar. The use of FA may provide improved outcomes for a specific subgroup of patients, based on their preoperative alignment.