Wear-induced loss of mass in reversed total shoulder arthroplasty with conventional and inverted bearing materials.

The notching phenomenon is one of the major concerns with reversed total shoulder arthroplasty. Repetitive contact between the humeral implant and the scapula (mechanical notching) produces progressive abrasion of the implant if the moving part is made of polyethylene. Its debris may then lead to active osteolysis (biological notching). Inversion of bearing materials, i.e. Glenosphere made of polyethylene and humeral Inlay made of metal, aims at the reduction of this phenomenon. However, the question arises if the tribological behavior would then be different. On an experimental setup, the gravimetric wear of both material configurations was measured after loading and moving over 500,000 cycles. The abrasion of the polyethylene Inlay due to mechanical notching was calculated by means of 3D CAD models with different notching stages. The loss of mass due to gravimetric wear was compared to the loss of mass caused by mechanical notching. After 500,000 cycles the measured amount of wear of the polyethylene components was between 8 and 10 mg for both tribological pairings. The calculated loss of mass of the polyethylene Inlay caused by mechanical notching ranged from 73 to 3881 mg. The results of this study indicate that the gravimetric polyethylene wear in the estimated life-time is very low and not significantly different between both material configurations. However, the polyethylene abrasion due to mechanical notching in the configuration with polyethylene Inlay is by far more important than any gravimetric wear. These results support the continued use of inverted bearings in reversed total shoulder arthroplasty.