Finite Element Analysis of the Tibial Post of a Knee Prosthesis

Overview

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.

Characteristics:

  • 3 knee prostheses digitized using a Siemens CT scanner
  • Image processing and segmentation in Simpleware ScanIP
  • Creation of four-node tetrahedral elements mesh using Simpleware FE
  • FE analysis in LS-DYNA using same load conditions
  • Comparison of results suggest optimal tibial post design

Thanks to

Kitasato University (Graduate School of Medical Sciences): K. Tanaka •
R. Sakai • K. Mabuchi

Image Processing and Meshing

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.

FE 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.

Results

The simulations indicated that larger tibial posts might be better at dispersing stress, even if the contact area is relatively small. Results suggested that the size of the tibial post is generally more important than its shape, and that the design of Product B in this study was almost ideal for reducing the risk of tibial post failure.

3D FE modelling was effective at analyzing tibial post designs under a range of different conditions. The results obtained are useful for understanding the design of the tibial post of knee prostheses.