In Vivo Determination of the Cam-Post Engagement in Fixed and Mobile Bearing TKA

INTRODUCTION In-vivo data pertaining to the actual cam-post engagement mechanism in PS and Bi-Cruciate Stabilized (BCS) knees is still very limited. Therefore, the objective of this study was to determine the cam-post mechanism interaction under in-vivo, weight-bearing conditions for subjects implanted with either a Rotating Platform (RP) PS TKA, a Fixed Bearing (FB) PS TKA or a FB BCS TKA. METHODS In-vivo, weight-bearing, 3D knee kinematics were determined for eight subjects (9 knees) having a RP-PS TKA (DePuy Inc.), four subjects (4 knees) with FB-PS TKA (Zimmer Inc.), and eight subjects (10 knees) having BCS TKA (Smith&Nephew Inc.), while performing a deep knee bend. 3D-kinematics was recreated from fluoroscopic images using a previously published 3D-to-2D registration technique (Figure 1). Images from full extension to maximum flexion were analyzed at 10° intervals. Once the 3D-kinematics of implant components was recreated, the cam-post mechanism was scrutinized. The distance between the interacting surfaces was monitored throughout flexion and the predicted contact map was calculated. RESULTS Anterior-Contact (BCS TKA): 7/10 knees analyzed had the femoral component engaged with the anterior aspect of the tibial post at full extension (Figure 1). However, the contact between them was lost in very early flexion (average:4.9°; maximum:9.9°). The contact was always located centrally on the anterior aspect of the tibial post. Posterior-Contact: The cam-post engaged at 34° for the BCS, 91° for the FB-PS and at 97° for RP-PS TKA. In the BCS and FB-PS knees, the contact initially occurred on the medial aspect of the tibial post and then gradually moved centrally and superiorly with increasing flexion (Figure 2), while for the RP-PS TKA it was located centrally on the post at all times (Figure 3). One subject each in the RP-PS and FB-PS groups did not experience cam/post engagement. DISCUSSION The anterior cam-post interaction in the BCS group was found to be present in a majority of subjects. However, there were three subjects who did not have any contact with the anterior aspect of the tibial post. This can be explained from the fact that the tibio-femoral contact points (at full extension) for these three patients was more anterior than any of the other seven subjects analyzed in this study. On the posterior side, for the BCS and FB-PS groups, the initial contact with the tibial post was achieved on the medial aspect, before the contact area tended to move centrally and superiorly with increasing flexion. Interestingly, in the RP-PS group, the contact between the cam and post was located centrally on the post at all times when engaged. This is probably due to the mobility of the polyethylene, characteristic for the analyzed TKA design. The polyethylene insert rotated axially in accord with the rotating femur. Therefore the posterior surface of the mobile bearing post was able to remain parallel to the surface of the femoral cam. This phenomenon was not observed in the Fixed Bearing TKAs and may increase the chances of edge loading on the polyethylene, resulting in wear patterns on the post.