Biomechanics of posterior-substituting total knee arthroplasty: an in vitro study.

The cam-spine system in posterior-substituting total knee arthroplasty was designed to improve posterior stability and to increase posterior femoral translation (rollback). Little is known on its effectiveness in the restoration of femoral rollback under functional loads. In the current study, the effect of cam-spine engagement on knee motion under simulated muscle loads was investigated using knees from cadavers. The translations of the lateral and medial femoral condyles of the knee before and after total knee arthroplasty were compared from 0 degrees to 120 degrees flexion. Cam-spine contact forces were measured under the same muscle loads. The posterior translations of both femoral condyles in the total knee arthroplasty were significantly lower than that of the native knee beyond full extension. Cam-spine engagement occurred between 60 degrees and 90 degrees flexion followed by an increase in posterior translation of both femoral condyles. However, the resultant femoral translation of the total knee arthroplasty was still lower than that of the native knee from 90 degrees to 120 degrees flexion. Knee motion after cam-spine engagement was independent of muscle loads, indicating the importance of the cam-spine mechanism at high flexion angles. Decreased posterior translation of both femoral condyles after total knee arthroplasty may be a limiting factor at high flexion.