How well does intra-operative contact path predict post-operative contact path during activities of daily living after medial unicompartmental knee arthroplasty?

Robot-assisted surgical systems can predict post-operative kinematics based upon intra-operative passive kinematics, planned implant position and alignment, and soft-tissue tension. It is currently unknown how well the intra-operative passive kinematics replicate the post-surgical weight bearing active kinematics. This study compared intra-operative and post-operative tibiofemoral implant contact paths after medial unicompartmental knee arthroplasty (mUKA). Passive intraoperative and active postoperative tibiofemoral contact path data was collected from eight patients who underwent mUKA. Intraoperative contact path data was measured using a navigation system. Postoperative contact path data was measured during walking, chair rise, stair ascent, and stair descent using a biplane radiography system and a validated tracking process. A total of 86 movement trials were included in the analysis. The contact point on the femur implant was up to 9.8 mm more medial and up to 8.3 mm less anterior at low flexion angles during activities of daily living than during passive extension intra-operatively, and the contact point on the tibia implant was up to 13.8 mm less lateral and up to 5.8 mm less posterior at low flexion angles during activities of daily living than during intra-operative passive extension. Femoral contact paths primarily differed between 3° and 42° of flexion; and tibial contact paths differed between 3° and 50° of flexion. This pilot study is the first to compare intra-operative and post-operative weight bearing contact paths. The primary conclusions from this study are that contact points on the femur implant are more medial and less anterior at low flexion angles during activities of daily living than during passive extension intra-operatively, and that the contact points on the tibia implant are less lateral at low flexion angles during activities of daily living than during intra-operative passive extension.