Review Question - QID 217846

A "ligh-bulb" sign is associated with a posterior shoulder dislocation which is often caused by electrocution or seizures.

Posterior shoulder instability is less common than anterior instability and is often missed. Patients will present with shoulder pain with often equivocal AP and lateral radiographs. However, axillary views and CT imaging will clearly define the asymmetry of the glenohumeral joint with potential reverse Hill-Sachs lesions and reverse Bankart lesions. Electrocution and seizures are common inciting events, which lead to tetanic contraction of the surrounding muscles, pulling the humeral head out of alignment. Anterior instability is still more common with these conditions, but it is important to think about a posterior dislocation in this patient population.

McKean et al. reported a case of a patient that presented with a unilateral posterior shoulder dislocation after experiencing a seizure. Initial imaging of bilateral shoulders did not reveal acute fractures or obvious dislocations, but CT imaging revealed a posterior shoulder dislocation with a large reverse Hill-Sachs lesion and posteriorly displaced reverse Bankart lesion. They concluded a high index of suspicion should be maintained when patients present with shoulder pain following a seizure episode.

Guehring et al. prospectively studied a series of 17 patients with posterior shoulder dislocations with reverse Hill-Sachs lesions and evaluated treatment options and 5-year functional outcomes. They reported that 70% of patients presented with humeral articular defects that measured between 25-40% of the surface and were treated with retrograde chondral elevation or antegrade cylindrical graft with fair results. They concluded an open approach to treat posterior shoulder dislocations with associated reverse Hill-Sachs lesions is not disadvantageous and treatment should be based on the size and depth of the defect and the time since injury.

Illustration A is an AP radiograph of the right shoulder depicting a "light bulb" sign indicating a posterior shoulder dislocation.

Incorrect answers
Answer 1: Glenohumeral internal rotation deficit is associated with posterior impingement, posterior capsular contractures, and anterior subluxation of the shoulder.
Answer 3: Inflammatory arthritis is associated with central erosion of the glenohumeral joint.
Answer 4: Excessive glenoid anteversion is associated with anterior shoulder instability.
Answer 5: A previous anterior band of the inferior glenohumeral ligament injury suggests a prior episode of anterior shoulder instability.