SURFACE RESIDUAL STRESSES AND MICROMECHANISMS OF WEAR ASSESSED ON RETRIEVED CERAMIC FEMORAL HEADS

Abstract

The experimental determination of residual stress fields on the surface of retrieved femoral heads represents a fundamental step in understanding their wear degradation behavior and the tribological mechanisms, which are operative on the femoral joint during its working life time. In this work, the surface of retrieved alumina and zirconia (Al2O3 and ZrO2) femoral heads were investigated by piezo-spectroscopic tecniques based both on photoluminescence and Raman effects. The high spatial resolution of the laser, impinging on the investigated surface (typically about 1 micron of lateral resolution), enabled us estimating patterns and magnitude of residual stress in extremely narrow zones, comparable with the grain size of the material.

Four retrieved ceramic femoral heads were analyzed. Two balls were made of alumina with a typical grain size of from 4 to 10 microns. Both alumina balls were retrieved after only few years from implantation, due to septic and aspetic loosening. The remaining two femoral heads were made of zirconia with a typical grain size of 1 micron. These latter balls were retrieved after 2 and 13 years, respectively (both for loosening problems). With a systematic collection of a large number of data on a microscopic level it was possible to assess the retrieved femoral heads in to to, thus extending the microscopic analysis to the entire joint.

In allumina balls retrieved after short time implantation, a macroscopic stress field was found, which arose from manufacturing, loading history, and the displacements acting on the femoral head during its lifetime. This stress field was completely overcome by a microscopic residual stress field due to local contacts (e.g., local shocks owing to microseparation, impinging and wear contacts). On the other hand, in zirconia femural heads, the major amount of surface deterioration after long-term exposure arose from tetragonal-to-monoclinic transformation in biological environment. These data allowed us to draw interesting considerations about the role of the material microstructure and the peculiar kinematic mechanisms involved with the use of femoral heads made of different materials.

Spectroscopic techniques, which are complementary to in vitro testing procedures and stress analyses based on finite-element methods, can be very useful for improving the design of the femoral head and for optimizing the microstructural characteristics of the ceramic materials employed. Based on this and previous fluorescence and Raman spectroscopic studies, we also propose that a systematic screening of the ceramic implants before implantation can strongly reduce the probability of failure of the implant.