Shoulder Instability
The shoulder joint is made up of a ball (humeral head) and a socket (glenoid). The ball is much larger than the shallow, flat socket which makes it inherently unstable. The labrum is a thin piece of tissue that lines the rim of the glenoid. It improves stability of the shoulder by serving as a bumper to help keep the ball in the socket through a large range of motion. As a result of its incredible arc of motion, the shoulder carries a higher risk of dislocation compared to other joints, such as the hip, where the socket is much deeper. Dislocations of the shoulder occur most frequently in the setting of traumatic injury such as a fall, or participation in contact sports, including football or hockey.
The shoulder can dislocate in any direction, but it most commonly comes out the front and bottom of the joint. Your doctor would describe this as an anteroinferior dislocation. Less commonly the shoulder will dislocate out the back, posterior dislocation, or get stuck with your arm raised above your head, luxatio erecta. The act of dislocation can damage both the ball (humerus) and socket (glenoid) components of the joint. Injuries can include small fractures or dents in the bone, known as a Hill-Sachs lesion, or injuries to the surrounding soft tissue, such as the labrum or rotator cuff. Most commonly, dislocations result in damage to the labrum related to the direction the shoulder dislocated, typically the anteroinferior labrum. This type of injury is referred to as a Bankart tear. In addition to tearing tissue, dislocations can stretch out and damage the joint capsule. All of these injuries together make it more likely that the shoulder will dislocate again.
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