Anteroposterior radiograph of a 29-year-old man with left shoulder pain and immobility following a motor vehicle accident
Outlet radiograph of the patient
Axial computed tomography scan is performed shortly after radiographic examination of the patient
A 29-year-old man presents to the emergency department with left shoulder pain following a motor vehicle accident. He lost control of his vehicle, and the car rolled over. He believes something hit the front of his left shoulder during the accident, and he has been unable to move the shoulder since. Results of the patient’s radiographs and computed tomography scan show a left shoulder posterior dislocation with glenoid engagement.
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A positive light bulb sign can be seen on anteroposterior radiograph of the left shoulder. When a posterior dislocation occurs, the humerus rotates internally and the humeral head contour appears projected like a light bulb on anteroposterior radiograph.
Radiographic findings in cases of posterior shoulder dislocation are subtle and are often missed. In fact, up to 50% of cases of posterior shoulder dislocation seen in the emergency department are undiagnosed. Additionally, anterior shoulder dislocation occurs 15 to 20 times more commonly than posterior shoulder dislocation, compounding the difficulty with diagnosis. Axillary radiograph is the best view to diagnose posterior shoulder dislocation; however, most people cannot abduct their arm sufficiently to comply for this study. A Velpeau view, performed with the arm adducted, can be used in these patients as an alternative to the axillary view.1
The most frequent cause of posterior dislocation is direct high-energy trauma with the shoulder in adduction, flexion, and internal rotation. Other causes include seizures and electrocution. These causes are remembered as the 3 Es: epilepsy, electric shock, and extreme trauma.
Anyone who presents to the emergency department with these types of injuries and complains of shoulder pain and lack of motion but in whom radiographs are normal (by report) should undergo further workup for posterior shoulder dislocation. Examination should begin with axillary or Velpeau view radiographs, and if the diagnosis is still unclear, computed tomography can be performed in the emergency department.1,2
Dagan Cloutier, MPAS, PA-C, practices in a multispecialty orthopedic group in the southern New Hampshire region and is editor-in-chief of the Journal of Orthopedics for Physician Assistants (JOPA).
- Rouleau DM, Hebert-Davies J, Robinson CM. Acute traumatic posterior shoulder dislocation. J Am Acad Orthop Surg. 2014;22:145-152.
- Hatch D. Posterior Instability and Posterior Dislocation. http://www.orthobullets.com/sports/3051/posterior-instability-and-posterior-dislocation. Accessed January 31, 2017.