Evaluation of dislocation position in patients with recurrent anterior shoulder dislocation

The usual mechanism of anterior shoulder dislocation is widely believed to be a combination of glenohumeral joint abduction, extension, and external rotation forces, even though no published reports to date have investigated the arm position of anterior shoulder dislocation in detail. Understanding the exact position of anterior shoulder dislocations is important for the management of anterior shoulder instability.

The study included 40 shoulders of 38 patients (32 males, 6 females), aged 28.0 (range, 13-73) years with symptomatic post-traumatic recurrent anterior shoulder instability. While patients were under general anesthesia, but before shoulder-stabilizing surgery, we evaluated the angle of external rotation with 90° elevation in the scapular plane at which the humeral head showed anterior translations over the glenoid rim.

The center of anterior instability at 90° elevation in the scapular plane was at 25.9° of external rotation. Anterior translations were detected in the range of 3.4° of internal rotation to 55.1° of external rotation, and no shoulders (except one) showed anterior translation at maximal external rotation.

Gross anterior translation was seen in the middle range of rotation at approximately 25° of external rotation, and anterior translation decreased close to the end of external and internal rotation. Shoulders with grade III translation showed anterior translation in a wider range of rotation, especially in external rotation. These data will help to further our understanding of the management and the prevention of anterior shoulder dislocations.