Humeral component retroversion in reverse total shoulder arthroplasty: a biomechanical study

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 degrees, 20 degrees, 30 degrees, and 40 degrees of retroversion, and the shoulders were mounted on a simulator to determine the muscle forces required to achieve 30 degrees and 60 degrees 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 degrees or 60 degrees 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 degrees, 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 degrees to 20 degrees 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. Level of evidence: Basic Science Study, Biomechanical Study. (C) 2012 Journal of Shoulder and Elbow Surgery Board of Trustees.