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Relevance of Arm Position and Muscle Activity on Three-Dimensional Glenohumeral Translation in Patients with Traumatic and Atraumatic Shoulder Instability

Ruediger M. O. von Eisenhart-Rothe, MD^,, Alwin Jäger, MD, Karl-Hans Englmeier, PhD, Thomas J. Vogl, MD^|| and Heiko Graichen, MD

Research Group for Kinematics and Biomechanics, Department of Orthopedic Surgery, University of Frankfurt, Frankfurt
^|| Department of Clinical and Interventional Radiology, University of Frankfurt, Frankfurt
Institute for Medical Informatics, Gesellschaft für Umwelt und Gesundheit GmbH, Neuherberg, Oberschleissheim, Germany

Parts of this study have been presented at the 9th Conference of the European Orthopaedic Research Society, Brussels, June 1999; the 63rd Conference of the Deutschen Gesellschaft für Unfallchirurgie, Berlin, November 1999; the Symposium SECEC Travelling fellows, Munich, June 1999; the 7th Conference of the Deutschen Vereinigung für Schulter- und Ellbogenchirurgie, Bad Homburg, May 2000, and Deutscher Orthopädenkongreß, Wiesbaden, October 2000.

Address correspondence and reprint requests to Ruediger M. O. von Eisenhart-Rothe, MD, Department of Orthopedic Surgery, University of Frankfurt, Marienburgstrasse 2, 60528 Frankfurt, Germany

Background: No quantitative data on glenohumeral translation exist allowing one to distinguish insufficiency of the active or passive stabilizers in different forms of shoulder instability.

Hypothesis: To determine whether 1) in traumatic or atraumatic shoulder instability an increase of glenohumeral translation can be observed in specific relevant arm positions, 2) muscle activity leads to recentering of the humeral head, and 3) there exist differences between traumatic and atraumatic instability.

Study Design: Prospective clinical trial.

Methods: In 12 patients with traumatic and 10 patients with atraumatic instability, both shoulders were examined in different arm positions—with and without muscle activity—by using open magnetic resonance imaging and a three-dimensional postprocessing technique.

Results: At 90° of abduction and external rotation, translation (anterior-inferior) was significantly higher in patients with traumatic unstable shoulders compared with their contralateral side (3.6 ± 1.5 versus 0.7 ± 1.6 mm). In patients with atraumatic instability, significantly increased translation (4.7 ± 2.0 mm) was observed, with the direction being nonuniform. Muscle activity led to significant recentering in traumatic but not in atraumatic instability.

Conclusions: In traumatic instability, increased translation was observed only in functionally important arm positions, whereas intact active stabilizers demonstrate sufficient recentering. In atraumatic instability, a decentralized head position was recorded also during muscle activity, suggesting alterations of the active stabilizers.

Clinical Relevance: These data are relevant for optimizing diagnostics and therapeutic strategies.

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