Knee Joint, Nonlinear
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Given an alloy femoral knee joint component (one of two "condyles") and a UHWMPe insert representing the tibial component (bone/cartilage), what is the stress/strain in the insert as a function of force loading and lower leg extension (sitting to standing articulation)?

3D CAD model setup with elastic material model for femoral component, "plastic" material model (as in hydro-carbon polymer) for tibial insert, and contact surfaces with friction between knee joint components.

Nodal strains, stresses, and displacements.

This was a particularly difficult problem to solve for two reasons: firstly, the hard-drive on the primary computer assigned to solve this problem was in the process of "going south" and ultimately failed (MTBF "was" 50,000 hours) and secondly, the UHMWPe is a "plastic" material (hyper-elastic, incompressible, high Poisson's Ratio), so some user intervention was required to facilitate convergence. The first problem masked the second problem. The HD was replaced and the results turned out quite nicely. The UHMWPe is not smooth because the machining of the interfacial surface tears away material leaving micro-cracks that can open up, leading to failure of the surface, and the body below. This compromises the artificial knee and perhaps will require the knee replacement to be "replaced." Ouch! The longevity of the joint can be increased if the interfacial loading can be minimized by spreading the local load across a larger bearing surface, i.e., decrease the pressure. The FEA done here was to baseline the load effect for a mature product line for future design changes if required.