Introduction: Recently several retrieval reports of PS TKA have demonstrated that the wear and deformation can occur on the anterior tibial post and the hyperextension of femorotibial components was a pivotal factor in the mechanism of anterior tibial post impingement. The objective of this study was to investigate the in vivo mechanism of anterior tibial post impingement during gait in PS TKAs.

Methods: Twenty knees with PS TKAs implanted by single surgeon were assessed in this study. The Review Board Committee of the author’s institution approved this study and informed consent was obtained from all patients. In this study ten knees implanted with Scorpio NRG PS (Stryker Orthopedics, Mahwah, NJ) and ten knees implanted with NexGen Legacy-flex fixed (Zimmer, Warsaw, IN) were examined. Each patient was asked to perform treadmill gait under fluoroscopic surveillance in the sagittal plane. Treadmill gait speed was 0.5–1.0 m/s at Patients selected comfortable speed. Patients were explicitly encouraged to fully extend their knees at heel-strike and to avoid a shuffling gait pattern. Patients used light-touch hand-support to maintain the foot position on the treadmill. In vivo 3D poses of the knee prostheses were computed using a two- to three-dimensional (2D/3D) registration technique, which uses CAD models to reproduce spatial postures of the femoral and tibial components from calibrated single-view fluoroscopic images. We evaluated range of motion and the anterior-posterior (AP) translations between femoral and tibial inserts. The anterior tibial post impingement was determined when the proximity between tibial post and femoral cam is within the 0.5mm threshold.

Results: The maximum flexion during gait was 41.9° (25°–56°) in NRG and 42.3° (23°–59°) in Legacy-flex. The minimum flexion during gait was 1.5° (−9.2° − 8.4°) in NRG and 1.8° (−13.0° − 17.0°) in Legacy-flex. The AP translations of the medial and lateral contact points in stance phase were significantly larger in Legacy-flex compared with NRG (medial ; p=0.02,lateral ; p=0.007, Mann-Whitney’s U test). Anterior tibial post impingement was recognized in four knees implanted with Legacy-flex, and in two of three knees, the knee was not hyper extended. On the other hand, no impingement was recognized in knee implanted with NRG.

Discussion: In this study, the anterior tibial post impingement occurred not only in hyper extended knee but also in slightly flexed knee in Legacy-flex. One of the reasons why the anterior tibial post impingement was recognized in Legacy-flex was the large amount of the AP translation in stance phase. Posterior translation in stance phase may be one pivotal factor in the mechanism of the anterior tibial post impingement.