The geometry of the glenohumeral joint is osseous, naturally nonconforming and minimally constrained, thus the essential requirement of a glenohumeral prosthesis in total shoulder arthroplasty (TSA) is prevention of joint degeneration and glenoid loosening. A variety of glenoid prostheses have been developed. Nonconforming glenohumeral implants are common for TSA. However, the nonconforming shape increases the instability when the humeral head is in the central region, where motion frequently occurs. Conforming implants can increase joint stability, but the “rocking-horse” effect [1] caused by the conforming shape is thought to lead to high stresses and moments at the glenoid rim when the humeral head approaches the periphery during its range of motion. The hybrid design, with a conforming center and a nonconforming periphery, combines the advantages of both nonconforming and conforming implant geometries. It has been shown [2] that the peak stress generated in glenoid components during activities of daily living can be as high as 25 MPa, which exceeds the polyethylene yield strength of the glenoid component and can lead to wear and cold flow of the component. Polyethylene has also been shown to be viscoelastic [3]. Therefore, both elastic-plastic and viscoelastic-plastic models of the glenoid implant were used to determine how viscoelasticity affected stress in the implant. The effects of implant shape on the stresses in the center, transition, and superior zones for the three different glenoid implant shapes, as well as on the stress in the underlying cement and bone, were determined in this study.

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