Imaging description

When the humeral head is dislocated posteriorly, it is also displaced laterally by the posterior glenoid rim; therefore the shoulder joint may appear widened on frontal projection (Figure 7.1). In many patients with posterior shoulder dislocation, two parallel lines of cortical bone may be identified on the superomedial aspect of the humeral head (Figure 7.1). One line represents the articular cortex of the humeral head and the other denotes the margin of a trough-like impaction fracture. Reverse Hill–Sachs fracture is an impaction fracture of the anteromedial humeral head after a posterior humeral dislocation (Figures 7.1 and 7.2). Reverse osseous Bankart fracture is a fracture of the posteroinferior rim of the glenoid that may occur after posterior glenohumeral dislocation.

Importance

The primary problem with posterior dislocation of the shoulder is in making the diagnosis. More than 50% of cases are missed at the initial examination.

Typical clinical scenario

Seizures are the most common cause of posterior shoulder dislocation. Injury to the dynamic posterior shoulder stabilizers after posterior dislocation is often seen in young patients presenting with shoulder pain or posterior glenohumeral instability.

Differential diagnosis

Hemarthrosis or lymphedema of the extremity may lead to some confusion with posterior dislocation by the widened joint space on the frontal radiographs.

Teaching point

The trough sign may be the only indication of posterior shoulder dislocation on frontal radiographs. Although anterior instability is the most common type of glenohumeral instability, a recent increase in the diagnosis of posterior instability after athletic injuries can be attributed to growing awareness.