Tibial post failure has been reported in several retrieval studies for knee surgeries using posterior-stabilized (PS) prostheses. Fractures of the tibial post are considered to be caused by high stress-induced destruction (bending and tensile forces). The design of polyethylene inserts requires a certain level of plastic deformation to take place without the risk of deformation. The design of the tibial post varies markedly among manufacturers. In the present study, Simpleware ScanIP and LS-DYNA software were used to examine the mechanical forces in operation in the tibial post in three different commercially available knee prostheses. The purpose of the study was to use FEA to find the safest tibial post design (i.e., the optimal shape, length, width, and height) using the same load conditions.
Three commercially available knee prostheses with different types of tibial post designs were digitized using a Siemens SOMATOM Definition Flash CT scanner. The DICOM files were loaded in Simpleware ScanIP where the metallic femoral component and the polyethylene insert were reconstructed. 3D finite element meshes were created in Simpleware FE using four-node tetrahedral elements, and exported to LS-DYNA for simulation.
In LS-DYNA (ver. 971, LSTC, USA), stress and strain levels were analyzed. The femoral component was simulated as a rigid model, and the tibial insert as a multi-linear approximate elastoplasticity model. Analyses were carried out to simulate two types of tibial post impingement during different kinds of knee motion: 1. Anterior tibial post impingement (flexion angle: -10°; applied load to the femoral component: 500N); 2. Posterior tibial post impingement (flexion angle: 120°; applied load: 1000N; internal rotation of the tibial insert: 10°). The distributed values of von Mises stress and plastic strain on the tibial post were shown as the results of the analysis.