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

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