Shoulder dislocation

Associate Editors-In-Chief: Jumana Nagarwala, M.D., Senior Staff Physician, Department of Emergency Medicine, Henry Ford Hospital and

Overview
In posterior dislocation, the humeral head is forced posteriorly in internal rotation. Posterior dislocations account for 2%–4% of all shoulder dislocations. In adults, convulsive disorder is the most common cause. Electrocution is a classic but uncommon cause of posterior shoulder dislocation. Bilateral dislocations are not infrequent.

Anterior dislocation is usually the result of direct or indirect trauma, with the arm forced into abduction and external rotation. It is the most frequent type of shoulder dislocation (represents more than 90% of injuries).
 * Associated lesions:
 * Hill-Sach lesions (describes a characteristic defect of the posterolateral surface of the humeral head, and represents a compression fracture)
 * Labral lesions (i.e., Bankart lesion)
 * Bony glenoid lesions - Osseous anterior glenoid rim fractures (44%), bony Bankart lesions, fracture of the greater tuberosity
 * Intraarticular loose body
 * Rotator cuff lesions - Supraspinatus tears or subscapularis tears

Diagnostic Findings

 * Posterior dislocation may be missed initially on frontal radiographs in 50% of cases.
 * Signs on frontal radiographs are subtle, including the trough line sign and the loss of normal "half-moon overlap".
 * Axillary, scapular Y, or posterior oblique projections are needed for confirmation.
 * The absence of external rotation on images in a standard shoulder series is a clue to posterior dislocation.
 * Loss of half-moon overlap: On a normal true anteroposterior image, there is a half-moon overlap between the humeral head and the glenoid. In a posteriorly dislocated shoulder, there is lateral displacement of the humeral head with respect to the glenoid (losing the half-moon overlap)
 * Trough line sign: In posterior dislocation, the anterior aspect of the humeral head becomes impacted against the posterior glenoid rim. With sufficient force, this causes a compression fracture on the anterior aspect of the humeral head. This compression fracture is analogous to the Hill-Sachs compression fracture seen with anterior dislocation of the glenohumeral joint. Frontal radiographs reveal two nearly parallel lines in the superomedial aspect of the humeral head.

(Images courtesy of RadsWiki)