Stress analysis of hemispherical ceramic hip prosthesis bearings

We present design criteria and test data for ceramic hemispherical ‘resurfacing’ prosthesis bearings that are attached by placement on the conical trunnion of the prosthesis stem. These large diameter bearings will be subjected to higher torques than conventional 28 mm bearings. Prototypes were fabricated and tested. The pattern of failure and subsequent analysis suggests that fracture initiated on the surface of the bore and that this was preceded by the development of large circumferential hoop stresses. Widening the bore will improve stability under torque loads but this may be at the expense of bearing strength. A finite element analysis has shown that the optimum taper bore to bearing diameter ratio is 0.43 and that designs with large taper bore to bearing diameter ratios (over 0.6) should be avoided due to the development of excessively high hoop stresses on the inner bore surface. High meridional tensile stresses can appear where the internal roof surface joins the conical bore surface as the load is removed. A smooth radius between the conical bore and the roof will help to overcome this. Further improvements include a thinner roof that is smoothly arched internally. Thus, the design of the component can be optimised, making it less susceptible to fracture without compromising the stability of the ceramic head on its trunnion.