Shoulder prosthesis subluxation: theory and experiment

The articular shapes of the humeral and glenoid components in total shoulder arthroplasty affect the loading, translation, and contact stresses in the joint, thereby affecting stability, glenoid loosening, and wear. Experiments were conducted to determine the subluxation load and corresponding translation for 6 types of glenoid components. The effects of shape, size, testing direction, compressive load, testing speed, testing medium, bone substitute properties, and repeated subluxations were investigated and compared with theoretical, rigid-body predictions. The subluxation load, varying from 45% to 98% of the axial load for the prostheses tested, is affected by glenoid constraint (i.e., the maximum slope at the glenoid articular rim), the compressive load, the coefficient of friction, and the deformability of the articular edge. Rigid-body theory overestimated the experimental load, which was not surprising given the visible deformations, but provides a framework to highlight the relevant design parameters. The subluxation translation, ranging from 1 to 13 mm for the prostheses tested, is determined by the glenoid length and by the conformity between the humeral and glenoid radii. Experimental translations were greater than rigid-body predictions for the most conforming prostheses and roughly equal for less conforming prostheses. The goals of this study were to characterize the subluxation load and translation of a variety of types of prostheses, to develop the rigid-body basis for these results, to compare the rigid-body and experimental results, and to locate experimentally the glenoid articular rim for further testing.