Application of Retrieval Analysis, Wear Measurement, and Finite Element Analysis to the Identification of Potential Failure Mechanisms in Metal-on-Metal Total Hip Replacements

Doctors have been performing reconstructive surgery on the hip for over almost 100 years. Over the last 40 years, with the development of new materials and better designs, great strides have been made in the technologies of total joint replacement. The continued success of total hip replacement has led to operations on younger patients, who are placing higher and higher demands on their total hip replacement. Thus, the implants being put in today are expected to last longer than ever before, potentially upwards of 30 years. In the pursuit of these long-lasting implants, numerous alternative bearing materials have been considered, one of which consists of a metal-on-metal (MoM) bearing couple using CoCrMo alloy.;The first generation of MoM implants were implanted in the mid 20 th century, before being eclipsed by the success of polyethylene bearings. While many of these implants saw early failure attributed to variability in manufacturing and surgical practices, other 1st generation MoM implants performed well for 25 years or more. As demand grew for a very low wearing bearing couple, the concept of MoM bearings was revisited. After simulator testing showed extremely low wear rates, a second generation of MoM implants was introduced in the early 2000s, with MoM bearing use reaching as high as 30% in 2010.;Clinical performance of these new MoM bearings did not live up to pre-clinical testing. Reports of higher-than-expected revision rates and bearing wear leading to metal toxicity led to the recall of one design, and widespread controversy regarding the performance of all MoM devices. This study aims to identify mechanisms for MoM failure by analyzing a set of 109 MoM retrievals obtained by the Dartmouth Biomedical Engineering Center. The bearing surfaces will be examined for evidence of in vivo damage and measured using a coordinate measuring machine. Finally, potential mechanisms for observed damage and wear will be explored with finite element analysis.;The retrieval analysis showed that MoM devices were not performing in vivo as expected from in vitro testing. In the 109 MoM devices in the set of retrievals, all of the devices showed unexpected damage. The mean linear wear rate was greater than predicted from in vitro testing. The damage and wear on the retrieved bearings indicates that the hydrodynamic lubrication layer was failing in vivo. Damage and wear do not appear to vary between different device designs, indicating that the issue may lie with MoM articulation. Modeling of edge loading scenarios identified roll-out edge loading, a new mechanism of micro-separation and edge loading, as a potential mechanism for damage and wear. Finite element analysis showed that linear damage features commonly observed on retrieved femoral devices could be due to edge loading due to micro-separation. While these edge loading events were not correlated to increased wear within our set of retrievals, it is clear that edge loading due to micro-separation has the potential to damage both bearing surfaces, possibly leading to inferior performance of these devices. The mechanisms of edge loading in all total hip replacement devices need to be fully understood and accounted for during device design and testing.