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Posterior shoulder dislocation

Radswiki et al.

Posterior shoulder dislocation are far less common than an anterior shoulder dislocation and can be tricky to identify if only AP projections are obtained. 

Epidemiology

Posterior shoulder dislocations account for only 2-4% of all shoulder dislocations (the vast majority are anterior) 1,3

Mechanism

Typically the humeral head is forced posteriorly in internal rotation while the arm is abducted 1,3. In adults, convulsive disorder is the most common cause. Electrocution is a classic but uncommon cause of posterior shoulder dislocation. In both situations bilateral dislocations are not infrequent 1-3.

Occasionally, they can be the result of strength imbalance within the rotator cuff muscles. Posterior dislocations may even go unnoticed, especially in elderly patients 1

Radiographic features

Plain films usually suffice in making the diagnosis, although cross-sectional imaging (CT  / MRI)  is often used to assess for the presence and extent of articular surface injury (reverse Hill-Sachs lesion), glenoid injury (reverse Bankart lesion) or ligamentous injury.

Plain films

Posterior dislocation may be missed initially on frontal radiographs in 50% of cases, as the humeral head appears to be almost normally aligned with the glenoid 1-2. The absence of external rotation on images in a standard shoulder series is a clue to posterior dislocation, the internally rotated humeral head takes on a rounded appearance known as the lightbulb sign 2.  Acute angle of the scapulohumeral arch (Moloney's arch) is also present and can be used to distinguish from anterior dislocation.

Signs on frontal radiographs are subtle, including the trough line sign and the loss of normal half-moon overlap sign.

Axillary, scapular Y, or posterior oblique projections are needed for confirmation. 

A number of associated injuries are recognised including 2:

Reporting checklist

In addition to stating that a posterior dislocation is present, any evidence of proximal humeral fractures or glenoid fractures should be sought and commented upon. 

Treatment and prognosis

In most cases, acute posterior dislocations have spontaneously reduced prior to imaging 3

When a posterior dislocation presents to the emergency department, unlike anterior shoulder dislocations which are relatively easily reduced, posterior dislocations are more problematic and attempts at closed reduction should only be performed in consultation with a treating orthopaedic surgeon 2. Additionally if the shoulder has been dislocated for 3 or more weeks (particularly common in elderly debilitated patients) or if the anterior humeral articular injury (reverse Hill-Sachs lesion) involves more than 20% of the articular surface, then closed reduction is contraindicated 2

Fortunately neurovascular compromise is uncommon, but associated gleno-labral and capsular injuries can lead to posterior shoulder instability 2-3

See also

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