Wear in the prosthetic shoulder: association with design parameters

Total replacement of the glenohumeral joint provides an effective means for treating a variety of pathologies of the shoulder. However, several studies indicate that the procedure has not yet been entirely optimized. Loosening of the glenoid component remains the most likely cause of implant failure, and generally this is believed to stem from either mechanical failure of the fixation in response to high tensile stresses, or through osteolysis of the surrounding bone stock in response to particulate wear debris. Many computational studies have considered the potential for the former, although only few have attempted to tackle the latter. Using finite-element analysis an investigation, taking into account contact pressures as well as glenohumeral kinematics, has thus been conducted, to assess the potential for polyethylene wear within the artificial shoulder. The relationships between three different aspects of glenohumeral design and the potential for wear have been considered, these being conformity, polyethylene thickness, and fixation type. The results of the current study indicate that the use of conforming designs are likely to produce slightly elevated amounts of wear debris particles when compared with less conforming joints, but that the latter would be more likely to cause material failure of the polyethylene. The volume of wear debris predicted was highly influenced by the rate of loading, however qualitatively it was found that wear predictions were not influenced by the use of different polyethylene thicknesses nor fixation type while the depth of wearing was. With the thinnest polyethylene designs (2 mm) the maximum depth of the wear scar was seen to be upwards of 20% higher with a metal-backed fixation as opposed to a cemented design. In all-polyethylene designs peak polymethyl methacrylate tensile stresses were seen to reduce with increasing polyethylene thickness. Irrespective of the rate of loading of the shoulder joint, the current study indicates that it is possible to optimize glenoid component design against abrasive wear through the use of high conformity designs, possessing a polyethylene thickness of at least 6 mm.