Measurements of Elastic Modulus and Hardness of Biocompatible Polyurethanes and the effects on Contact Area within the Hip Prosthesis

Abstract

Synopsis The use of soft layers on the bearing surfaces of artificial hip joints has shown large advantages over current devices. However, the Hardness and Elastic Modulus of the layers are critical for good tribological properties. Thus, these properties must be monitored during optimisation of materials for this application. This paper reveals the interaction between the two mechanical properties and indicates how Elastic Modulus can be calculated from a simple Hardness test. The advantage of the Hardness test is that it may be performed on any size or thickness of sample. This removes processing differences between components and test pieces. The results from Hardness measurements were found to be highly dependent on the backing material and the extent of bonding to it as well as the thickness of the material. In addition, it was noted that the 93 and 100A grades from the Tecoflex range of biocompatible polyurethanes exhibited substantial visco-elastic properties. The work on Hardness / Elastic Modulus interactions has allowed the effective modulus of an acetabular component to be estimated, which can then be applied to the lubrication equations. These theoretical studies support current experimental findings.