Author: Nathan Haas, MDUniversity of Michigan
Originally Published: Modern Resident, June/July 2014
Posterior shoulder dislocations are relatively uncommon, comprising only 2-4% of all shoulder dislocations. Thus, posterior dislocations often go undiagnosed, and can lead to severe consequences for both the patient and emergency physician (EP). A high index of suspicion and a firm grasp of associated radiologic findings are key to making the diagnosis.
Posterior shoulder dislocations are classically associated with seizures, electrocution and severe trauma. As a group, the internal rotators of the humerus (teres major, pectoralis major and latissimus dorsi) are more powerful than the external rotators (infraspinatus, posterior deltoid and teres minor), leading to internal rotation during global muscle contraction from electrical activity (seizure, electrocution, electroconvulsive therapy, etc.). This internal rotation is what allows the humeral head to dislocate posteriorly from the glenoid fossa, and also produces the characteristic "light bulb sign" of the humeral head seen in posterior shoulder dislocations.
The AP view of the normal shoulder demonstrates the normal asymmetry of the humeral head in anatomic position. The larger portion is on the medial side, seated in the glenoid fossa. With internal rotation in the setting of a posterior dislocation, this larger portion rotates out of view producing the more round and symmetric "light bulb sign" of the humeral head in the second image. It is important to note that this pertains only to the AP view, and not the axillary or lateral view of the shoulder.
*Image 1: Normal AP view of shoulder
Source: Dr. M Daya; ebmedicine.net
Reprinted with permission from EB Medicine, publisher of Emergency Medicine Practice, from: Daya M, Nakamura Y. Shoulder girdle fractures and dislocations. Emergency Medicine Practice. 2007; 9(10):4, www.ebmedicine.net
*Image 2: Posterior dislocation, "light bulb sign"
Source: Dr. Alexandra Stanislavsky; radiopaedia.org
While the axillary or scapular Y views often help demonstrate posterior shoulder dislocations, the "light bulb sign" of the humeral head is often present on the AP view. Other signs include the rim sign (>6mm gap between the medial humeral head and anterior glenoid rim), the trough sign/reverse Hill-Sachs lesion (compression fracture of anteromedial humeral head), or fracture of the lesser tuberosity.
- Shoulder Girdle Fractures And Dislocations. EB Medicine. Web. 20 May 2014. http://www.ebmedicine.net/topics.php?paction=showTopicSeg&topic_id=120&seg_id=2471
- Stanislavsky A. Posterior Shoulder Dislocation. Radiopaedia. Web. 20 May 2014. http://radiopaedia.org/cases/posterior-shoulder-dislocation
- Tosif, S. Posterior Shoulder Dislocation. Life in the Fast Lane. Web. 20 May 2014. http://lifeinthefastlane.com/posterior-shoulder-dislocation/