Effects of glenosphere positioning on impingement-free internal and external rotation after reverse total shoulder arthroplasty

Introduction: Patients may experience a loss of internal rotation (IR) and external rotation (ER) after reverse total shoulder arthroplasty (RTSA). We hypothesized that alterations in the glenosphere position will affect the amount of impingement-free IR and ER. Materials and methods: Computed tomography (CT) scans of the scapula and humerus were obtained from 7 cadaveric specimens, and 3-dimensional reconstructions were created. RTSA models were virtually implanted into each specimen. The glenosphere position was determined in relation to the neutral position in 7 settings: medialization (5 mm), lateralization (10 mm), superior translation (6 mm), inferior translation (6 mm), superior tilt (20 degrees), and inferior tilt (15 degrees and 30 degrees). The humerus in each virtual model was allowed to freely rotate at a fixed scaption angle (0 degrees, 20 degrees, 40 degrees, and 60 degrees) until encountering bone-to-bone or bone-to-implant impingement (180 degrees of limitation). Measurements were recorded for each scaption angulation. Results: At 0 degrees scaption, only inferior translation, lateralization, and inferior tilt (30 degrees) allowed any impingement-free motion in IR and ER. At the midranges of scaption (20 degrees and 40 degrees), increased lateralization and inferior translation resulted in improved rotation. Supraphysiologic motion (>90 degrees rotation) was seen consistently at 60 degrees of scaption in IR. Superior translation (6 mm) resulted in no rotation at 0 degrees and 20 degrees of scaption for IR and ER. Conclusions: Glenosphere position significantly affected humeral IR and ER after RTSA. Superior translation resulted in significant restrictions on IR and ER. Optimal glenosphere positioning was achieved with inferior translation, inferior tilt, and lateralization in all degrees of scaption. Level of evidence: Basic Science Study, Computer Modeling. (C) 2013 Journal of Shoulder and Elbow Surgery Board of Trustees.