Reverse shoulder arthroplasty (RSA) is increasingly utilized to restore shoulder function in patients with osteoarthritis and rotator cuff deficiency. There is currently little known about shoulder function after RSA or if differences in surgical technique or implant design affect shoulder performance. The purpose of this study was to quantify scapulohumeral rhythm in patients with RSA during loaded and unloaded shoulder abduction. Eleven patients with RSA performed shoulder abduction (elevation and lowering) with and without a handheld 3kg weight during fluoroscopic imaging. Three RSA designs were included. We used model-image registration techniques to determine the 3D position and orientation of the implants. Cubic curves were fit to the humeral elevation as a function of the scapular elevation over the entire motion. The slope of this curve was used to determine the scapulohumeral rhythm (SHR). For abduction above 40°, shoulders with RSA exhibited an average SHR of 1.5:1. There was no significant difference in SHR between shoulder abduction with and without 3kg handheld weights (1.6±0.2 unweighted vs. 1.4±0.1 weighted), nor was there a significant difference between elevation and lowering. SHR was highly variable for abduction less than 40°, with SHR ranging from a low of 1 to greater than 10. For these very small groups, there was no apparent pattern of differences between implant designs having differing degrees of lateral offset. At arm elevation angles less than 40°, SHR in RSA shoulders is highly variable and the mean SHR (2–5) with RSA appears higher than SHR in normal shoulders (2–3). At higher elevation angles, SHR in shoulders with RSA (1.5–1.8) is much more consistent and appears lower than SHR in normal shoulders (2–4). With the small subject cohort, it was not possible to demonstrate differences between subjects with different implant designs. Ongoing analysis of reverse shoulder function with larger cohort sizes will allow us to refine our observations and determine if there are differences in shoulder function due to implant design, preoperative condition and rehabilitation protocols.
