Initial glenoid component fixation in "reverse" total shoulder arthroplasty: a biomechanical evaluation.

In patients with rotator cuff arthropathy, a "reverse" shoulder prosthesis resists glenohumeral subluxation and offers the potential for improved function. However, premature mechanical failure due to loosening is a concern with these devices. This in vitro study evaluates initial glenoid component fixation of 2 uncemented "reverse" prostheses during physiologic loading and determines the relationship among lateral offset of the glenosphere, fixation method, and motion. To simulate an excellent glenoid bone stock, a polyurethane foam bone with similar material properties to that of the glenoid cancellous bone was used. Both lateral offset and peripheral screw type affected the magnitude of baseplate motion. Baseplate motion for Delta III components and Reverse Shoulder Prosthesis (RSP) components fixed with 5.0-mm captured screws were below the 150 mum of motion generally accepted as the threshold for bone ingrowth. Stable fixation was achieved for the RSP-neutral components despite a substantially (69%) greater moment at the baseplate-foam interface compared with the Delta III. Obtaining similar results in vivo is partially dependent on surgical placement of the peripheral screws and the patient's glenoid bone stock.