Posterosuperior shoulder dislocation due to the rupture of deltoid posterior fibers: a case report

BACKGROUND: Superior shoulder dislocation is a rare type of shoulder dislocation. Its occurrence is thought to be associated with rupture of the deltoid; however, few reports are available on the mechanism of onset and the treatment of a superior shoulder dislocation. Here we describe a case of dislocation in the direction of the posterior acromion, referred to as posterosuperior shoulder dislocation, caused by the traumatic rupture of deltoid posterior fibers. CASE PRESENTATION: An 83-year-old woman who fell on her right elbow in the hospital presented to our department with complaints of right shoulder pain. She had been undergoing conservative treatment for a massive rotator cuff tear and a rupture of the long head of biceps tendon 5 years previously. X-ray radiography images at the time of the trauma revealed that the right humeral head was dislocated upwards and in the direction of the posterior acromion. Magnetic resonance imaging (MRI) revealed newly ruptured deltoid posterior fibers, in addition to the massive rotator cuff tear. Closed reduction was performed and the shoulder joint was held in external rotation at 30 degrees for 6 weeks. However, re-dislocation was observed at an early stage after the removal of the orthosis and marked instability remained. CONCLUSIONS: This is the first case of posterosuperior shoulder dislocation. It suggests that rupture of the deltoid posterior fibers contributes to the onset of posterosuperior shoulder dislocation in patients with a massive rotator cuff tear. Moreover, in superior shoulder dislocation, conservative treatment may result in continuing instability which requires surgical treatment.