Biomechanical analysis of the post-cam mechanism in a TKA: comparison between conventional and semi-constrained insert designs

Posterior-stabilized (PS) total knee arthroplasty (TKA) designs were introduced to compensate for the resected or deficient posterior cruciate ligament and to avoid paradoxical anterior sliding of the femur. Knee joint stability may differ among patients and, therefore, orthopaedic companies developed several solutions to compensate for these differences. In particular, conventional PS designs are used for conventional TKA and semi-constrained PS designs are mainly applied in revision surgery or in conventional surgery when there is a compromised soft tissue envelope. However, despite good functional long-term results, a better understanding of forces acting on the post of the polyethylene insert is needed to find an eventual correlation with the risk of post-cam failure or loosening of the tibial baseplate for the two PS solutions. To the authors’ best knowledge, no literature data is currently available to compare the two solutions. In this paper, a validated numerical model was developed to analyse and compare the posterior of the post-cam mechanism of the same TKA design with conventional and semi-constrained inserts during several motor tasks. For each solution, different motor tasks show different values of maximum post-cam force. Comparing the two solutions, a general slightly higher force (range between 3 and 5%) is observed in the semi-constrained solution. The results concerning the conventional solution are in agreement with the literature. After this analysis, we can report that a semi-constrained design shows only slightly higher posterior post-cam contact force, and thus would not be expected to significantly increase the risk of failure of the post-cam mechanism, as compared to a conventional PS design.