Functional alignment maintains constant medial stability and a more distinct medial pivot pattern throughout flexion compared to mechanical alignment in medial pivot total knee arthroplasty

Purpose To investigate whether functional alignment (FA) leads to differences in in vivo kinematics and early clinical outcomes compared to mechanical alignment (MA) in patients undergoing total knee arthroplasty (TKA) using a medial pivot (MP) prosthesis. Methods Forty patients (20 MA and 20 FA) who underwent TKA with a MP implant were investigated under fluoroscopy during squatting using a two‐to‐three‐dimensional registration technique 1 year postoperatively. FA started with anatomical implant positioning plan, followed by navigation‐guided adjustments of femoral and tibial cutting plan based on intraoperative varus–valgus stress testing for soft tissue balance. In MA, resections are aimed for neutral coronal alignment. External rotation, varus–valgus angle, anteroposterior translation of medial and lateral contact points and kinematic pathways were assessed from 0° to 110° of flexion. Clinical outcomes were evaluated 1 year postoperatively using the Knee injury and Osteoarthritis Outcome Score (KOOS) and 2011 Knee Society Score (KSS). The range of motion and radiographic alignment were also evaluated. Results A significant group × angle interaction was observed for lateral anteroposterior translation, external rotation and valgus angulation ( p < 0.001), indicating different kinematic pathways between groups. In the MA group, a significant main effect of the flexion angle on valgus angulation was detected ( p < 0.001), whereas in the FA group, the varus–valgus angles remained consistent throughout flexion. The FA group demonstrated significantly greater postoperative flexion; better KOOS scores for pain, ADL and sports; and superior KSS symptom scores ( p < 0.05). Radiographic analysis showed significant differences in MPTA and LDFA in the FA group (both p < 0.001). Conclusion FA and MA techniques resulted in distinct in vivo kinematics. The FA approach demonstrated MP knee motion with consistent varus–valgus stability throughout flexion, leading to improved early clinical outcomes compared to MA. These findings support the kinematic benefits of FA in MP TKA. Level of Evidence Level III.