Kinematic Analyses of Patients with Reverse Shoulder Arthroplasty

Abstract

The classic scapulohumeral rhythm (glenohumeral/scapulothoracic motion) first described by Inman et al. (J Bone Joint Surg Am 26:1–30, 1944) has a ratio of 2:1. However, it was demonstrated that two-dimensional methods fail to account for “out-of-plane” motions; therefore a 3-dimensional system, especially an electromagnetic system, seems to be the most suitable method to register the three-dimensional movement of the shoulder. By using the “Flock of Birds” (FoB) system, it was demonstrated that primary reverse shoulder arthroplasty (RSA) patients do not actively use the complete glenohumeral (GH) motion provided by the prosthetic design. However, primary total shoulder arthroplasty (TSA) patients have more active range of motion (ROM) for the complete thoracohumeral (TH) shoulder motion compared to primary RSA patients, because TSA patients completely use their possible GH motion, without differences in active and passive motion. In revision surgery, the GH motion of RSA patients is far inferior, compared to primary RSA’s. Furthermore, external loads applied to the arm in patients with an RSA are enlarging the scapular contribution to the total motion of the arm. With this information, the assumption could be made that optimizing the GH motion of RSA patients by lateralizing the center of rotation, either bony (BIO-RSA) or metallic, could provide more GH elevation, possibly due to the delayed subacromial impingement and more GH axial rotation because of the delayed anterior and posterior notching. However, this needs to be demonstrated in future research, obtaining GH kinematical data, comparing standard RSA patients with RSA patients with a more lateralized center of rotation.