Mechanical testing of shoulder prostheses and recommendations for glenoid design

Glenoid component loosening generates the greatest concern among surgeons performing total shoulder arthroplasty. Laboratory testing of glenoid prostheses may lead to improved design, subsequently leading to a reduction in the incidence of clinical loosening. The goals of this study were to develop a laboratory test method to address glenoid loosening and to apply this method to a variety of prosthesis designs. With use of a biaxial apparatus, glenoid components were cyclically subjected to superoinferior edge loading, mimicking the off-center rocking-horse phenomenon thought to contribute to glenoid loosening clinically. Before and after the rocking test was performed, compression and distraction of the superior and inferior edges were measured with the humeral head displaced to each edge. Rocking performance could not be predicted from initial measurements, indicating the necessity for dynamic loading to evaluate the likelihood of loosening. A roughened fixation surface for outperformed a smooth fixation surface, a curved backing showed almost half the distraction of a flat backing, and a nonconstrained prosthesis distracted less than a more constrained prosthesis.