Analysis of contact mechanics for composite cushion knee joint replacements

Recent research in the area of cushion form bearings for total joint replacements has primarily used thin, soft, elastomeric layers with similar elastic modulus to articular cartilage, bonded to rigid substrates. These are designed to promote the body's natural lubricants to separate the articulating surfaces and prevent wear. Applications to joint replacements have revealed that the abrupt change in stiffness between the soft layer and the rigid substrate and the relatively low strength of the interface resulted in high shear stresses and debonding of the soft layer from the substrate. The approach adopted in this study is to use components with a graded modulus or composite construction. The composite construction consists of a soft compliant layer of polyurethane and a second stiffer polyurethane layer thought to be rigid enough to mechanically interlock to a metallic tibial tray. In this composite structure the deformation of the more rigid polyurethane underlay may generally influence the stress distribution and deformation in the softer upper layer and at the interface between the two materials. A simple analysis technique is presented in the present study where the composite double layer was approximated as an equivalent modulus single layer. Single layer theory, which is readily available in the literature, can then be used to determine the contact parameters, including the maximum contact pressure and the contact radius, for the composite structure. By varying the layer thicknesses and material properties of both the soft surface layer and the stiffer structural support layer, the magnitudes and locations of stresses can be controlled. Results of a parametric stress analysis are presented to assist in the selection of the most appropriate composite layers for cushion knee designs. A 4 mm thick surface layer with an elastic modulus of 20 MPa and a 4 mm thick structural support layer with an elastic modulus of 1000 MPa were considered suitable for this application.