The effect of stem length on reverse total shoulder humeral fixation

Background: The purpose of this study was to evaluate the relative contribution of the reverse shoulder arthroplasty (RSA) humeral stem length on initial implant fixation, which may contribute to the long-term success of uncemented shoulder implants. Methods: Three RSA humeral components were randomly divided into the following testing groups: (1) 100% Stem length (n = 7); (2) 50% Stem length (n = 7); (3) 0% Stem length (n = 7). Each humeral stem was press-fit into twenty-one 4th Generation Osteoporotic Sawbones humeri. Torque and compressive axial load were applied to the humerus for 10,000 cycles at ±2.5 Nm and −392 N, respectively. Rotational micromotion of the implant was measured. Results: Micromotion for all 3 testing groups was below the 150 µm micromotion threshold throughout testing. The overall micromotion in Group 2 was significantly larger than Group 1 (P < .001) and Group 3 (P < .001). Overall micromotion between Group 1 and Group 3 were not significantly different (P = .686). Conclusion: All stem lengths studied maintained micromotion below the 150 µm threshold, suggesting that any of these treatment groups would provide adequate initial fixation to allow bony on-growth. Clinically, stemless or short-stemmed prosthesis design offer theoretical benefits such as the degree of freedom for humeral stem placement independent of diaphysis. This study concluded that the zero-length stem will allow for this and provide adequate fixation comparable to the full-length stem. Level of Evidence: Basic Science Study.