Abstract
Background
Previous studies have suggested that the modular junction of metal on metal (MoM) total hip replacements (THR) is an important source of metallic debris.
Methods
We carried out a prospective study using custom techniques to analyse one of the largest collections of failed contemporary MoM devices in the world. All explants from patients who had suffered adverse reactions to metal debris (ARMD) were included in this study. These explants included: 82 36mm THRs, and 147 resurfacing head THRs and 140 resurfacing arthroplasties from several manufactures. Volumetric wear analysis of the bearing surfaces and taper junctions was carried out using a coordinate measuring machine. The relationships between total metallic loss and metal ion concentrations and the macroscopic and histological tissue appearance of THR patients were compared to those in resurfacing patients. Mann Whitney test for non-parametric data was used to assess significant differences between groups.
Results
Resurfacing explants retrieved from patients who had suffered ARMD were found to have significantly higher median rates of volumetric wear than the THRs (10.16 versus 2.25mm3/yr (p < 0.001)). Total volumetric material loss from taper junctions ranged from 0.01 to 21.55mm3. When volumetric taper wear was combined with bearing surface wear in the THR patients this total rate of material loss was still significantly less than in the resurfacing patients 2.52 versus 10.16mm3/yr (p < 0.001)). Despite this, macroscopic tissue destruction and extent of ALVAL infiltration was found to be significantly greater in the THR patients.
Conclusion
Taper debris appears to more readily stimulate a destructive immune cascade than debris from primary bearing surfaces. This cascade can culminate in catastrophic tissue necrosis when blood metal ion concentrations appear normal. MHRA guidance should distinguish clearly between MOM THRs and resurfacings.
