Improving the Reliability of the Ceramic-On-Ceramic Wear Couple in THR

The alumina-on-alumina wear couple which was introduced about 20 years ago has received new attention because of its superior wear behavior. Improved material properties and new design concepts provide solutions which overcome the restrictions of the older cup design. The new design makes use of the beneficial properties of both ceramic and metal materials in a modular cup assembly. In this paper the material aspects are briefly reviewed first: the mechanical strength of medical grade alumina has been improved by about 40 % between 1978 and 1996. Results of finite element analysis of the stresses found in the ceramic cup insert are presented: it was found that the metal back cone angle has to be larger than the insert cone angle in order to minimize tensile stresses in the cup insert. In addition, stresses in the cup insert are minimized if the radial gap between cup insert and ball head is around 40 μm. The finite element results are confirmed by experimental findings on the resistance to static load and on fracture patterns. Also reported are results of cyclic fatigue experiments at up to 10⁷ cycles which result in a fatigue limit of at least 19 kN. Preliminary results on the stability of the cup assembly under directionally oscillating loads show that there is no detectable motion between the cup insert and the metal back after a few load cycles.