Using metrology to bridge the gap in understanding between engineering and biological failure: the case of metal-on-metal hip replacements

A number of metal-on-metal (MoM) hip replacement devices have been recently withdrawn from the market due to concerns at reported failure rates. Higher failure rates have been reported for modular large head metal-on-metal (LHMoM) hip replacements when compared to the same bearing used as a resurfacing. The 2010 NJR reports a failure rate of LHMoM hips at five years of 7.8% compared to 6.3% for hip resurfacings and 2-3% for conventional cemented THRs, one centre reporting a 48.8% 6 year revision rate for LHMoM. This concern has led to the recent decision by the British Hip Society to stop using these implants until more evidence on their performance becomes available.
The higher failure rate has largely been attributed to material loss from the head-stem taper junction, although this contribution to total material loss remains poorly understood. Furthermore it is not clear whether this material is lost as a result of mechanical wear or corrosion, a result with significant clinical implications. Indeed clinically, wear and corrosion products have been identified and linked to incidents of severe soft tissue reaction resulting in patient pain and the need for revision.
Clinically, blood metal ion levels are commonly used as a test of MoM implant performance. This had previously been correlated to bearing wear, however, the possibility of material loss from the taper junction and other interfaces mean that this relationship is more complex. Debate is ongoing as to what can be considered a ‘safe’ or ‘critical’ level of blood metal ions, but due to the complex relationship of factors how this relates to implant performance and material loss is not well understood. Clinical studies comparing the levels of metal ions in the blood of patients with well functioning LHMoM and resurfacing hips show elevated levels for the LHMoM suggesting that the taper junction may have a significant influence on ion levels.
These concerns have highlighted the need for a standardised approach to wear analysis of such components at retrieval.
This study has been setup in collaboration with the London Implant Retrieval Centre, Imperial College and UCL to address the need for a traceable metrological approach to the quantification and characterisation of in vivo wear. The purpose of this study is to quantify the material loss – either through wear or corrosion - of both bearing surfaces and the taper junction, and investigate any relationships with blood metal ion levels and commonly reported clinical variables.

Significance Statement
This is an urgent area of research given the lack of standardisation in this area of analysis, consequently there are a number of approaches that have been adopted in literature and industry with little reference paid to good and reliable measurement practice.
The overall aim of this research is to produce an overarching measurement methodology that is established from first principles and can be universally applied to the measurement of retrieved hip replacements. This is the first study to join biological and engineering analysis through use of best practice techniques to understand the performance of a biomechanical joint replacement by building a correlation between blood ion levels and material loss. The furthering of understanding in this area has great implications for society through the ability to close the loop of understanding and develop more effective, longer lasting joint replacements.