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.