Posterior-stabilized (P/S) total knee replacements are designed o replace the function of the knee joint in the absence of the posterior cruciate ligament. The most prevalent P/S tibial components are modular, consisting of a plastic tibial insert which is secured by the locking mechanism to a metallic tibial tray. While these tibial components offer the advantages of modularity including increased flexibility and decreased inventory, they must withstand loading that will be transferred to the tibial insert post, without excessive deformation of the post or disassociation of the tibial insert from the tibial tray. The objective of this study is to develop a test method to determine the fatigue integrity of a modular P/S tibial component under physiologic loading.
Modular P/S tibial components are subjected to a combined compressive and nteroposterior shear load transferred from the femoral component to the tibial insert post and the condylar surfaces. A test method was developed to reproduce this loading by placing a P/S implant at 30° of flexion and transferring load to the tibial component as would occur in vivo. By applying a 5 Hz, sinusoidal load in this manner for ten million cycles, the fatigue integrity of the modular component can be evaluated. Additional information may be gained by measuring the angular deformation of the tibial insert post and determining the static locking strength of the tibial construct after fatigue loading. Failure criteria for the modular component would be either disassociation of the tibial insert or gross deformation of the tibial insert post.