Total Knee Replacement (TKR) surgery is used to treat sufferers of osteoarthrosis and/or rheumatoid arthritis. Many different designs are available to replace the afflicted knee. The TKR is typically composed of four or more discrete components:​ the patellar component, the femoral component, the tibial component, and the tibial insert. The tibial insert is made of Ultra High Molecular Weight Polyethylene (UHMWPE), is affixed to the top of the tibial tray (usually made of a cobalt-chrome, or titanium alloy), and provides a low friction articulation on its upper surface for the femoral component (cobalt-chrome, or titanium alloy).

Most TKR failures are due to component loosening due to the generation of UHMWPE wear particles. The exact etiology of the wear mechanisms present in TKR is not well understood. Most wear particle production is generally assumed to occur on the upper surface of the UHMWPE tibial insert, because it is subject to high contact stresses and wear associated with the articulation of the femoral component. Depending upon the design of' the tibial tray, the underside of the UHMWPE insert can also be subject to high stresses. The tibial tray may require fixation additional to that provided by the component’s intrinsic design (metal stems with or without various coatings, with or without bone cement). This is accomplished by using bone screws which clamp the tibial tray to the underlying bone. The use of bone screws requires holes in the tibial tray, holes on which the UHMWPE insert rests. Depending on the location and design of the screw holes, deflection of the tibial insert into the screw holes can occur. This can be detrimental to the longevity of the TKR, because the deflected UHMWPE produces wear particles, and removes UHMWPE from major load bearing areas of the tibial insert.

A finite element method (FEM) model was used to determine the stresses in the UHMWPE tibial insert, and the amount of UHMWPE displacement into the screw hole. UHMWPE was modeled using nonlinear material models for 23°C and 37°C. Contact elements were used whenever possible to model metal to UHMWPE contact. A Hertzian analysis was used to compare results with a fully supported UHMWPE layer FEM model. An experimental study was used to determine the amount of UHMWPE displacement into screw holes under static loading, at 21 °C and at 37°C;​ the effect of UHMWPE thickness on displacement;​ the effect o f adding a radius to the screw hole edge;​ and the amount of permanent UHMWPE displacement.

Results of the FEM and experimental models are compared. Recommendations as to placement and geometry of the screw holes, and thickness of UHMWPE, are made.