The shoulder is the most moveable joint in your body. It helps you to lift your arm, to rotate it, and to reach up over your head. It is able to turn in many directions. This greater range of motion, however, can cause instability.
Shoulder instability occurs when the head of the upper arm bone is forced out of the shoulder socket. This typically happens as a result of a sudden injury, such as a fall or accident.
Once a shoulder has dislocated, it is vulnerable to repeat episodes. When the shoulder is loose and slips out of place repeatedly, it is called chronic shoulder instability.
The shoulder is made up of three bones:
Normal shoulder anatomy.
The head, or ball, of the upper arm bone fits into a shallow socket in your shoulder blade. This socket is called the glenoid. Strong connective tissue, called the shoulder capsule, is the ligament system of the shoulder and keeps the head of the upper arm bone centered in the glenoid socket. This tissue covers the shoulder joint and attaches the upper end of the arm bone to the shoulder blade.
The shoulder also relies on strong tendons and muscles to keep it stable.
Once the ligaments, tendons, and muscles around the shoulder become loose or torn, dislocations can occur repeatedly. Chronic shoulder instability is the persistent inability of these tissues to keep the arm centered in the shoulder socket.
(Left) Normal shoulder stability. (Right) Head of the humerus dislocated to the front of the shoulder.
There are three common ways that a shoulder can become unstable:
Severe injury, or trauma, is often the cause of an initial shoulder dislocation. When the head of the humerus dislocates, the socket bone (glenoid) and the ligaments in the front of the shoulder are often injured. The labrum — the cartilage rim around the edge of the glenoid — may also tear. This is commonly called a Bankart lesion. A first dislocation can lead to continued dislocations, giving out, or a feeling of instability.
A Bankart lesion is a tear in the labrum — the cartilage rim around the edge of the shoulder socket.
Some people with shoulder instability have never had a dislocation. Most of these patients have looser ligaments in their shoulders. When this increased looseness is just your normal anatomy, it is called hyperlaxity.
Sometimes, the looseness is the result of repetitive overhead motion. Swimming, tennis, and volleyball are among the sports requiring repetitive overhead motion that can stretch out the shoulder ligaments. Many jobs also require repetitive overhead work.
Looser ligaments can make it hard to maintain shoulder stability. Repetitive or stressful activities can challenge a weakened shoulder. This can result in a painful, unstable shoulder.
In a small minority of patients, the shoulder can become unstable without a history of injury or repetitive strain. In such patients, the shoulder may feel loose or dislocate in multiple directions, meaning the ball may dislocate out the front, out the back, or out the bottom of the shoulder. This is called multidirectional instability. These patients have naturally loose ligaments throughout the body and may be double-jointed.
Common symptoms of chronic shoulder instability include:
