Improving the Sustainability of Metal On Metal Hip Implants Via Better Machining

This is a report on ?work still in progress? in which metal on metal (MoM) hip implants are used as an example of manufacturing for better product sustainability. Debris generated during normal use (wear and fatigue) of the hip implants is the main cause of prosthetic replacement. This debris (some less than 30 nm in size) elicits an inflammatory reaction which can lead to pain and failure of the implant that requires revision surgery. Hence, the sustainability issue is to improve the quality of life by manufacturing prostheses that generate less debris and last longer with reduced pain..

Fortunately, the recent literature contains a detailed description Co-Cr-Mo alloy ball and head prostheses that were removed from patients along with a study of their properties. The metallurgical microstructure of the material adjacent to the articulating surface adjusts with use. This region has several layers of differing characteristics. In the used implants, a thin layer near the interface seems to be composed of nanograin sized material, which was not the case when they were implanted. Such small grained materials are hard but brittle. Hence, the sustainable solution is to induce larger compressive residual stresses in the surface during the manufacturing (machining) process.

In order to study the implant debris generation, a purpose-built biaxial pin on disk (PoD) testing system was used to study wear of Co-Cr-Mo cylindrical specimens (0.3125 inch diameter by 1 inch long) while immersed in a simulated biofluid environment. The discs were prepared from 2 inch bar stock of the same alloy. The average sliding speed between the pin and disk was 50 mm/s. Three pins were used in each experiment for statistical purposes. Mass loss, debris generation and change in surface roughness were measured at 100,000 cycle increments up to 500,000 cycles. Preliminary data shows that the cutting tool nose radius and feed rate were the major factors affecting pin wear rate. Micro-hardness and residual stresses are also measured. Pin wear rate was found to be proportional to bearing surface microhardness. These results demonstrate that machining

conditions can be selected which will induce compressive residual stresses in C0-Cr-Mo alloy MoM hip implants.