Patient function after a posterior stabilizing total knee arthroplasty: cam–post engagement and knee kinematics

Even though posterior substituting total knee arthroplasty has been widely used in surgery, how the cam–post mechanism (posterior substituting mechanism) affects knee joint kinematics and function in patients is not known. The objective of the present study was to investigate posterior femoral translation, internal tibial rotation, tibiofemoral contact, and cam–post engagement of total knee arthroplasty patients during in vivo weight-bearing flexion. Twenty-four knees with a PS TKA were investigated while performing a single leg weight-bearing lunge from full extension to maximum flexion as images were recorded using a dual fluoroscopic system. The in vivo knee position at each targeted flexion angle was reproduced using 3D TKA models and the fluoroscopic images. The kinematics of the knee was measured from the series of the total knee arthroplasty models. The cam–post engagement was determined when the surface model of the femoral cam overlapped with that of the tibial post. The mean maximum flexion angle for all the subjects was 112.5 ± 13.1°. The mean flexion angle where cam–post engagement was observed was 91.1 ± 10.9°. The femur moved anteriorly from 0° to 30° and posteriorly through the rest of the flexion range. The internal tibial rotation increased approximately 6° from full extension to 90° of flexion and decreased slightly with further flexion. Both the medial and lateral contact point moved posteriorly from 0° to 30°, remained relatively constant from 30° to 90°, and then moved further posterior from 90° to maximum flexion. The in vivo cam–post engagement corresponded to increased posterior translation and reduced internal tibial rotation at high flexion of the posterior substituting total knee arthroplasty. The initial cam–post engagement was also mildly correlated with the maximum flexion angle of the knee (R = 0.51, p = 0.019). A later cam–post engagement might indicate an environment conducive to greater flexion. If the factors that affect cam–post engagement timing can be established, proper manipulation of those factors may improve the function of the knee after posterior substituting total knee arthroplasty.