Findings: AP view of the left shoulder demonstrate marked internal rotation and impaction fracture of the humeral head, i.e., through line. Axial view demonstrates posterior subluxation of the humeral head in the glenoid fossa and the impaction fracture of the humeral head.

Diagnosis: Posterior dislocation with impaction fracture of the humeral head.

Discussion: Posterior dislocation of the shoulder is uncommon and constitutes less than 5% of shoulder dislocations. Posterior dislocation is usually the result of anterior to posterior directed trauma, but can be seen as the result of violent muscle contraction causing the humerus to rotate severely internally and adduct as is seen in convulsive seizures and electric shock.

Radiographic signs of posterior dislocation include:

1. Distortion of the elliptical area created by the overlapping humeral
   head and glenoid fossa on the AP view.
2. A vacant glenoid cavity.
3. >6 mm space separating humeral head and anterior rim of the glenoid.
4. Fixed internal rotation of the humerus
5. Impaction fracture of the humeral head, i.e., through line.

Harris J, Harris W, and Novelline R. The Radiology of
Emergency Medicine, 3rd Edition. Williams and Wilkins,
Baltimore; 1993:283-285.

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