Taper Wear in Large Head Metal-on-Metal Hips

Introduction: Large Head Metal on Metal hips were introduced with the promise of lower wear rates, reduced chance of dislocation and an increased range of motion compared to conventional MoM hips. However, the 2010 National Joint Registry reports a 5 year failure rate of LHMoM hips of 7.8% compared to 6.3% for hip resurfacings and comparisons of well functioning hips have shown the LHMoM have significantly increased metal ion levels compared to resurfacings. These differences are despite LHMoM hips and resurfacings having identical bearings surfaces; thus the difference in in-vivo performance must be related to the modular head / neck taper junction. This study aims to examine the taper junctions of explanted LHMoM to try and understand the increased failure and metal ion levels.

Method: The internal taper inside the modular head and external taper on the femoral neck were examined using a Coordinate Measuring Machine (CMM), Roundness Machine and Optical Profilometer. The taper angle, cylindricity and wear were measured using the CMM and roundness machine, while the surface topography was measured using the Roundness machine (with diamond stylus) and optical profilometer. The surface topography was correlated with the wear measurements.

Results and Discussion: There was a range of wear rates measured for the tapers ranging from the un-measureable to a 2 - 3 mm3, which is a similar magnitude to the volumetric wear from the bearing surface for typical hip joints. Examination of the surface topography revealed that in some cases the ridged surface texture of the neck had been worn into the surface of the inside taper of the head. Examination of the taper angles, revealed that the head was a more acute angle than the stem, suggesting that the contact will occur near the end of the neck, in the centre of the head.

Conclusion: Taper wear of LHMoM hips is a significant problem in some cases and may contribute to the elevated failure rate. The examination of explanted tapers provides a valuable insight into the in vivo behaviour of taper junctions and the wear mechanisms.