Coronal stability at mid-stance is preserved in mechanically aligned medial pivot total knee arthroplasty despite intraoperative lateral laxity

Background Medial pivot total knee arthroplasty (MP-TKA) prioritizes medial stability while allowing a degree of lateral laxity. Traditional mechanical alignment may enlarge the lateral gap, but whether such laxity compromises dynamic coronal stability during weight-bearing gait remains unclear. Methods Consecutive patients who underwent mechanically aligned MP-TKA were prospectively enrolled, and fluoroscopic gait analysis using 3D–2D registration was performed to evaluate coronal alignment and joint stability during the mid-stance phase of gait. Intraoperative soft-tissue balance was quantified using standardized gap measurements in both extension and flexion. Dynamic coronal stability was assessed using femoral–tibial implant varus–valgus angle (VVA), mediolateral displacement (MLD) between the implants, and the presence of femoral condylar lift-off (FCLO). Forty knees were analyzed. Results Although lateral laxity exceeded medial laxity intraoperatively in both extension and flexion, coronal alignment during the mid-stance phase of gait remained neutral (mean hip–knee–ankle angle 180.8 ± 2.7°, VVA 0.0 ± 0.3°) with minimal mediolateral translation (MLD 0.2 ± 0.5 mm). No knee demonstrated FCLO. Soft-tissue balance parameters showed no significant correlation with VVA, MLD, or tibial joint line obliquity. Conclusion These findings indicate that for mechanical alignment this medial pivot design may accommodate a certain degree of lateral laxity without compromising coronal stability during the mid-stance phase of gait when medial stability is preserved.