The Use of Finite Element Methods to Predict Tibiofemoral Joint Mechanics of Prosthetic Knee Implants

This paper on "The Use of Finite Element Methods to Predict Tibiofemoral Joint Mechanics of Prosthetic Knee Implants" was presented at the NAFEMS World Congress on The Evolution of Product Simulation From Established Methods to Virtual Testing & Prototyping - 24-28 April 2001, The Grand Hotel, Lake Como, Italy.

Summary

Different methods are available to measure the contact area in articulating joint prostheses. A common method is to use pressure sensitive film. This method requires many design iterations in order to achieve the desired contact area and contact stress. The ability to predict the contact mechanics of a particular knee design prior to the creation of prototypes would allow the engineer to optimise the design. With finite element analysis (FEA) software, a virtual test bed for optimising the implant's tibiofemoral joint mechanics is now possible. But how accurate is the finite element analysis? This study compared tibiofemoral contact areas and contact stresses for a knee implant, measured with pressure sensitive film and analysed with FEA. The set-up for both techniques was the same.
The results showed a 17% to 35% lower contact area for the finite element method for different load levels, even though non-linear material properties for UHMWPE were used. There are some reasons for this difference. For the investigation with FEA a CAD model (computer aided design) was used which was modelled with nominal dimensions. If a component is manufactured, the dimensions can fall anywhere within a tolerance field. That has the consequence that the articulating surfaces are not uniformly round. A further reason may be the load history of the polyethylene component. All Fuji film measurements were made sequentially with one component. The material data originate from compression tests, but the influence of creep was not considered. For the FEA, the type of element also has an influence since the tetrahedral elements are not appropriate for this kind of contact analysis.
The following steps to solve the problem are planned. It is necessary to obtain the same articulating surfaces both for measuring and for analysing.