When posterior shoulder dislocation occurs, the anteromedial aspect of humeral head hits the posterior glenoid resulting in compression fracture of anteromedial humeral head (Reverse Hill Sachs lesion and trough line sign) as well as posterior labral tear, glenoid avulsion fracture and reverse Bankart lesion. 

In this patient the avulsed posterior labrum remains attached to a sleeve of periosteum known as Posterior labroligamentous periosteal sleeve avulsion (POLPSA), a recognized reverse Bankart variant. This lesion is associated with posterior shoulder instability and requires surgical intervention.