A 25-year-old cricketer sustains a posterior shoulder dislocation during a seizure. On examination, the arm is held in internal rotation and adduction. X-ray shows a 'light bulb' sign. Which additional radiological finding confirms posterior dislocation on a standard AP view?

• A Subcoracoid position of the humeral head
• B Hill-Sachs lesion on the anteromedial humeral head
• C Widening of the glenohumeral joint space >6 mm ('positive rim sign') ✓
• D Fracture of the greater tuberosity with superoinferior displacement

Explanation

Posterior shoulder dislocation is notoriously missed on AP X-ray. The 'light bulb' sign (symmetric appearance of the internally rotated humeral head) suggests posterior dislocation. The positive rim sign — a widened glenohumeral joint space of >6 mm on the AP view — occurs because the posterior dislocation displaces the head away from the glenoid rim anteriorly. An axillary lateral or Y-view clinches the diagnosis. Hill-Sachs lesions (impression fracture) in posterior dislocation occur on the anteromedial head (reverse Hill-Sachs/McLaughlin lesion), not posterolateral.

Reference: Maheshwari Essential Orthopaedics, 6th ed.

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Written and medically reviewed by the StethoPrep medical team.