Introduction: Although clinical results for the Metal-on-Metal (MoM) devices have been excellent, recently some concerns have been raised regarding the occurrence of periprosthetic soft tissue lesions (PSTL) in some patients with MoM devices. Clinical studies and retrieval analyses have shown that devices revised due to groin pain and PSTL generally have significantly higher wear that has been attributed to edge loading of the implants.
Aim: The retrieval study was to investigate the cause of edge-loading of MoM devices in vivo.
Materials and Methods: In this study 13 retrieved Birmingham Hip Resurfacing (BHR) devices were examined. All devices were supplied with radiographs showing the in vivo position of the implant. Linear wear was assessed using a Taylor-Hobson Talyrond 290 roundness machine. Multiple roundness profiles were obtained to locate the area of maximum wear on each component. Edge loaded devices were identified when the maximum linear wear occurred at the edge of the cup. Non-edge loaded pairs showed wear area within the articulating surface of the cup.
The in vivo abduction angle and version angle of the cup were determined by superimposing the BHR models onto the radiographs (ProEngineer Wildfire 4 with ISDX II extension software) using anatomical references and specific features of the BHR.
Results: Linear wear: Among the 13 devices investigated, 11 were edge loaded with the maximum linear wear occurred at the edge of the cup. The remaining 2 pairs were non-edge loaded. The average joint linear wear rate of the edge loaded devices was 49.9 μm per year, and that for the two non-edge loaded devices was 2.4 μm per year. Edge loaded pairs had far greater linear wear than non-edge loaded components.
Cup orientation: The abduction angles of the two non-edge loaded cups were 31° and 39°, and their version angles were 12 and 16° respectively. These angles were within recommended orientation for the BHR. In contrast, the adduction angles and/or version angles of all edge loaded devices were outside the recommended orientation. Their abduction angle varied from 40° to 66° and version angle from 5° to 46°.
The edge loaded devices with higher inclination angles and/or higher version angels generally had higher linear wear. There is strong correlation between the cup orientation and the linear wear of the implant.
Conclusion: Mal-orientated devices in this study showed clear signs of edge loading which in turn resulted in significant increase in wear compared to the well orientated/non-edge loaded devices.
