Pathomechanics in Atraumatic Shoulder Instability: Scapular Positioning Correlates with Humeral Head Centering.von Eisenhart-Rothe, R MD*; Matsen, F A III MD†; Eckstein, F MD‡; Vogl, T MD§; Graichen, H MD*Author Information From the *Research Group for Kinematics and Biomechanics, Department of Orthopedic Surgery, University of Frankfurt, Frankfurt, Germany; the †Department of Orthopaedics and Sports Medicine, University of Washington, Seattle, WA; the ‡Musculoskeletal Research Group, Anatomische Anstalt, Ludwig-Maximilians-Universität, Munich, Germany; and the §Department of Clinical and Interventional Radiology, University of Frankfurt, Frankfurt, Germany. Received: February 6, 2004 Revised: August 26, 2004 Accepted: October 20, 2004 The authors have received funding from the Deutsche Forschungsgemeinschaft (DFG; Grant 1638/5-1, 5-2) and the Klein and Riese Foundation. Each author certifies that his or her institution has approved the human protocol for this investigation and that all investigations were conducted in conformity with ethical principles of research, and that informed consent was obtained. Correspondence to: R. von Eisenhart-Rothe, MD, Research Group for Kinematics and Biomechanics, Orthopedic Department, University of Frankfurt, Marienburgstr. 2, 60528 Frankfurt, Germany. Phone: 49 69 6705 0; Fax: 49 69 6705 375; E-mail: [email protected] Clinical Orthopaedics and Related Research (1976-2007): April 2005 - Volume 433 - Issue - pp 82-89 doi: 10.1097/01.blo.0000150338.27113.14 Buy Metrics Abstract The objective was to analyze three-dimensional scapular positioning and glenohumeral centering of normal and atraumatic unstable shoulders. We hypothesized that changes of humeral head position correlate with alterations of scapular positioning. The shoulders of 28 healthy volunteers and 14 patients with atraumatic instability were examined in various arm positions using open magnetic resonance imaging. After segmentation and three-dimensional reconstruction, three-dimensional analyses of scapular positioning and humeral head position relative to the glenoid were done. The coefficient of correlation (r) between both parameters was determined using the correlation z test. The glenohumeral to scapulothoracic ratio in the scapular plane was increased in nine of 14 patients and decreased in three patients, whereas the scapular internal rotation in the transverse plane was increased in all unstable shoulders. The unstable shoulders also had malcentering (greater than two times the standard deviation in the healthy volunteers) of the humeral head in the direction of instability during various arm positions. In healthy and unstable shoulders, the correlation between scapular position and glenohumeral positioning was high during passive elevation (r = 0.60-0.87). The high correlation suggests that scapular positioning is relevant for humeral head decentering. Therefore, physiotherapeutic strategy should consider the malpositioning of the scapula and be adapted to the direction of instability. © 2005 Lippincott Williams & Wilkins, Inc.