Mechanical testing of shoulder prostheses and recommendations for glenoid design

Glenoid component loosening generates the greatest concern among surgeons p erforming total shoulder arthroplasty laboratory testing of glenoid prosthe ses 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 meth od to a variety of prosthesis designs. With use of a biaxial apparatus, gle noid components were cyclically subjected to superoinferior edge loading, m imicking the off-center rocking-horse phenomenon thought to contribute to g lenoid loosening clinically. Before and after the rocking test was performe d, compression and distraction of the superior and inferior edges were meas ured with the humeral head displaced to each edge. Rocking performance coul d not be predicted from initial measurements, indicating the necessity For dynamic loading to evaluate the likelihood of loosening. A roughened fixati on surface far outperformed a smooth Fixation surface, a curved backing sho wed almost half the distraction of a flat backing, and a nonconstrained pro sthesis distracted less than a more constrained prosthesis.