Predictive Assessment of Maximum Contact Pressures in Metal-on-Polymer Hip Joint Implants by Analytical Method and FEM

Abstract

In more than 90% of hip replacements, the acetabular cup is made of ultra-high molecular weight polyethylene (UHMWPE) and the femoral head is made of stainless steel, cobalt-chromium-molybdenum alloys, titanium or ceramic. The primary factor in evaluating soft-on-hard endoprostheses is the maximum contact pressure as the contact strength of UHMWPE. The level of these pressures is used as a criterion for the correct selection of the femoral head size and radial clearance in the connection during arthroplasty for a specific patient. According to the previously developed new analytical method is proposed for estimating maximum contact pressures in thermo-diffusion nitrided (TDN) titanium GRADE 2 – UHMWPE hip joint implant. The effects of implant femoral head diameter and radial clearance on maximum contact pressures are determined for the average load. Relationships between maximum contact pressures and the above-mentioned parameters are established. A comparison is made between maximum contact pressures estimated by the proposed analytical method and numerical analysis with ANSYS (2D and 3D). The maximum contact pressures calculated using the 2D model have been found to be the lower limit, and the pressures calculated using the 3D model in ANSYS are the upper limit for analytical calculations. Therefore, it should be expected that at thickness of cup ˜ 7…8 mm the contact stresses will not be significantly different than those obtained from the solution by the above method. The developed analytical method can be effectively used for research not only soft-on-hard endoprostheses (metal-on-polymer, ceramics-on-polymer), but also hard-on-hard endoprostheses (metal-on-metal, ceramics-on-ceramics). The results of analytical calculations were also related to the results of FEM calculations known in the literature and their convergence was established.