Wear predictions for reverse total shoulder replacements

Reverse total shoulder arthroplasty (RTSA) has become the gold standard to treat rotator cuff tear arthropathy. RTSA is performed by substituting the humeral head and the glenoid cavity by a plastic cup in UHMWPE and a metallic head, respectively, in a geometrical reversed configuration with respect to the anatomical one. Major complications affect 27% of cases and mainly regard scapular notching due to cup-bone impingement and wear debris. Unfortunately, wear in shoulder prosthesis has not been largely studied as for hip and knee implants. Indeed, no wear test standards or even shoulder simulators exist, also because of a limited knowledge on shoulder/RTSA dynamics. Additionally, only a few numerical wear models for RTSA can be found in the literature, mainly focused on the comparison between anatomical and reverse solutions, and which often simulates simplified conditions, such as planar unloaded motions even neglecting fundamental aspects of wear process, i.e. cross-shearing (CS). The aim of the present study is to numerically investigate wear in RTSAs analysing the effect of: a) wear factor and wear law; a) implant geometry; b) inversion of bearing materials, i.e. plastic head + metallic cup, which should reduce the risks associated to scapular notching.