Today nonconforming glenohumeral implants are a common choice for total shoulder arthroplasty (TSA). Use of the conforming implant has decreased because of the “rocking-horse” effect [1], which leads to high stress and moments at the glenoid rim when the humeral head subluxes during range of motion. Retrieval studies have provided evidence that the rocking-horse effect is the major cause of implant loosening and fracture, delamination and deformation of the glenoid rim. Nonconforming glenoid implants, owing to a larger radius of the glenoid articular surface, can reduce the rocking-horse effect by improving the rim-head contact during subluxation. However, the nonconforming shape increases the contact stress and instability when the humeral head is in the central region, where motion frequently occurs [2].

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