Development of morphological filtering method for specification and characterisation of hip replacement taper junctions

The introduction of modular large head metal-on-metal (LHMoM) hip replacements promised lower wear rates and reduced chance of dislocation coupled with an increased range of motion compared to conventional metal-on-metal 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. This has led to a widely held hypothesis that the modular head/neck taper junction could be a site of secondary wear debris and indeed wear has been indentified in this area on a number of retrieved bearing tapers. It is clear that the correct specification of this junction is vital to the optimum in-vivo performance of these bearings and that this could reduce the number of observed failures. This study will investigate the example of a number of newly manufactured total hip replacement femoral stems and will be the first to develop a method for specifying and measuring the underlying form of the textured surface of the critical neck taper which is characterised by a micro-thread. This was achieved through developing a filtering routine that allows for the extraction and isolation of the profile points that would contact the femoral head female taper counterface when in operation. In this study a series of measurements were taken on each component, axially along the neck taper. The measurements were performed using a Talyrond 365 roundness machine with a 5 m diamond stylus. The subsequently extracted profiles were firstly levelled using a least squares method in a post-process routine and were then processed using a newly developed morphological filtering routine to extract the envelope profile of contact points from the primary texture inherent in each profile.