These authors performed in vitro testing of 6 nonretentive and 6 retentive reverse total shoulder (rTSA) non-cross-linked polyethylene liners, subjecting them to 4.5 million cycles of alternating cycles of abduction-adduction and flexion-extension motion. The rTSA liners were assessed for gravimetric wear loss, 3-dimensional volumetric loss, and particulate wear debris analysis. Wear took place for both retentive and non-retentive liners, increasing essentially linearly with the number of cycles. Retentive liners underwent significantly greater volume loss and greater surface deviation than non retentive liners, especially after high numbers of cycles. The authors suggest that the additional stability afforded by retentive liners should be balanced against the potential for increased wear and potential for subsequent polyethylene wear-induced aseptic loosening.
Comment: The real story here is that the polyethylene of a reverse total shoulder wears increasingly with increasing use as shown in the figure below. However by their method volumetric wear was not detectable with fewer that 250,000 cycles under loading conditions approximating body weight. It seems likely that wear rates in patients would depend on the number of cycles, the excursion of each cycle, the compressive load across the joint, and the presence of abrasive particles within the articulation.
It is recognized that polyethylene debris can cause inflammation, accelerated wear, and bone resorption.
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