A 23-year-old woman presented to the emergency department with a 1-year history of atraumatic right shoulder pain. Over the preceding few months, the pain had worsened and her range of movement had reduced considerably. She was otherwise fit and healthy.

On examination, her shoulder appeared normal and there was no obvious neurovascular deficit, including the axillary nerve. She had no movement in any direction actively, but passively could bring her shoulder to 60° of abduction and forward flexion.

A plain radiograph of the …