Numerical analysis of variations in posterior cruciate ligament properties and balancing techniques on total knee arthroplasty loading.

Total knee arthroplasty (TKA) is a widely used and successful orthopaedic procedure. During TKA, the posterior cruciate ligament (PCL) can either be retained or substituted by a post-cam mechanism. One of the main functions of the PCL is to facilitate femoral rollback during knee flexion. For adequate PCL functioning, the PCL should be balanced correctly after TKA. A tight PCL leads to more femoral rollback at the expense of a higher joint compression and potential polyethylene wear. Frequently used surgical techniques to balance a tight PCL are PCL release and increasing the posterior tibial slope. The objective of this study was to evaluate the effects of variations in PCL properties and balancing techniques on the mechanical outcome of a total knee replacement during a weight-bearing squatting movement (flexion range=45-150 degrees). For this purpose, a prosthetic finite element knee model was developed including a PCL having adjustable properties. Varying the PCL stiffness and PCL steepness (elevation angle) with respect to the tibial plateau considerably affected the TKA loading characteristics. Both a relatively high PCL stiffness and a low elevation angle at the start of the flexion cycle led to a high PCL force (1400-1500 N) and a high peak polyethylene contact stress of roughly 52 MPa during deeper knee flexion (120 degrees). Releasing the PCL with roughly 4 mm or increasing the posterior tibial slope to 7 degrees reduced the PCL force to 300-400 N and the polyethylene peak contact stress to 35-42 MPa at 120 degrees of flexion. The femoral rollback patterns during deep knee flexion were only marginally affected when extra posterior tibial slope was added, whereas additional PCL release resulted in paradoxical anterior movement of the femur.