Shoulder instability is classified into two categories: subluxations and dislocations. To understand each, we must first understand the shoulder anatomy. The shoulder is a ball-and-socket joint comprised of the humerus (ball) and glenoid or glenoid fossa (socket). The glenoid is a shallow groove which comes off the scapula (shoulder blade) that articulates with the head of the humerus. Due to the glenoid’s shallow groove, the head of the humerus can become dislocated due to a direct force or if the shoulder is severely unstable, the shoulder can dislocate on its own.
A subluxation is: When an unstable shoulder can slip in and out of the joint. When subluxation occurs the shoulder is partially or not completely dislocated. If the shoulder is partially out of the shoulder socket, it may completely dislocate. Subluxations typically reduce on their own.
A dislocation is: When the head of the humerus comes completely out of the glenoid. One occurrence can lead to the condition known as chronic instability, where the shoulder is at a higher risk of subluxation and/or dislocation. Typically with shoulder dislocations, the labrum— the cartilage lining the glenoid— is often torn.