Background: Reverse total shoulder arthroplasty offers pain relief and functional improvement for patients with rotator cuff-deficient shoulders. The purpose of this study was to determine the optimal amount of humeral retroversion for this prosthesis. Materials and methods: Six cadaveric shoulders underwent computed tomography (CT) imaging and were then dissected of soft tissues, except for their tendinous attachments. A reverse total shoulder arthroplasty was implanted in 0°, 20°, 30°, and 40° of retroversion, and the shoulders were mounted on a simulator to determine the muscle forces required to achieve 30° and 60° of scaption. CT images were converted into 3-dimensional models, and the amount of internal and external rotation was determined with computer modeling at various scaption angles. Results: No differences were found in the forces required for 30° or 60° of scaption for any muscle, at any retroversion. With increasing retroversion, more impingement-free external rotation was obtained, with a concomitant decrease in the amount of internal rotation. Above 60°, the humerus was allowed to rotate around the glenosphere unencumbered. Conclusions: Increasing retroversion did not affect the muscle force requirements for scaption across the shoulder. Placing the humeral component in 0° to 20° of retroversion allows maximum internal rotation with the arm at the side, a movement that is required for daily activities. This limits external rotation with the arm at the side, but has no effect on external rotation with the arm elevated.
Bibliographical noteFunding Information:
Funding for this study was obtained through departmental funds under the auspices of the Sports Medicine and Shoulder Service and the Biomechanics Laboratory at the Hospital for Special Surgery.
- Biomechanical study
- Computer modeling
- Reverse total shoulder arthroplasty
- Shoulder simulator