Analysis of reverse total shoulder joint forces and glenoid fixation.

Reverse total shoulder arthroplasty (rTSA) implants are intended to restore stability and function to shoulders with rotator cuff deficiency. The implant consists of a glenosphere projecting from a glenoid baseplate and articulating in a socket at the proximal end of a humeral component. Despite the demonstrated clinical efficacy, little information is available regarding the joint forces about this construct and the stability of the glenoid component against these forces. Our hypotheses were that the joint forces about the rTSA were comparable to that about a normal shoulder joint, and that the micromotion between the baseplate and the scapula against these loads would be sufficiently low to induce bone ingrowth. To investigate this, a custom testing rig was constructed to simulate active shoulder elevation in fresh-frozen shoulder specimens. The forces about the rTSA were calculated and found to include compressive and shear forces up to 0.7 and 0.4 BW, respectively. In contrast to a normal shoulder, where the joint forces peak at 90° of abduction, forces about the rTSA were highest at about 60° of abduction. These forces were then applied in cyclic loading conditions to the glenoid baseplate, and the micromotion of the implant relative to the bone was measured in the four quadrants of the component. For two different rTSA designs (DePuy Delta III® and Encore RSP®) and in the entire range of the fixation testing, the cyclical micromotions were always less than 62 µm. Thus, under loading conditions similar to physiological shoulder elevation, micromotion of the glenoid component was sufficiently low and within previously published limits to induce bone ingrowth.