Damage patterns at the head-stem taper junction helps understand the mechanisms of material loss

Background: Material loss at the taper junction of metal-on-metal total hip replacements (MOM THRs) has been implicated in their early failure. The mechanisms of material loss are not fully understood; analysis of the patterns of damage at the taper can help us better understand why material loss occurs at this junction. Methods: We mapped the patterns of material loss in a series of 155 MOM-THRs received at our centre by scanning the taper surface using a roundness-measuring machine. We examined these material loss maps to develop a five-tier classification system based on visual differences between different patterns. We correlated these patterns to surgical, implant and patient factors known to be important for head-stem taper damage. Results: We found that 63 implants had ‘minimal damage’ at the taper (material loss <1mm3 ) and the remaining 92 implants could be categorised by four distinct patterns of taper material loss. We found that (1) head diameter and (2) time to revision were key significant variables separating the groups. Conclusion: These material loss maps allow us to suggest different mechanisms that dominate the cause of the material loss in each pattern: (a) corrosion, (b) mechanically assisted corrosion or (c) intra-operative damage or poor size tolerances leading to toggling of trunnion in taper