A shoulder dislocation is when the humeral head (the ball of the upper arm) comes completely out of its socket at the shoulder joint. The humeral head can also come partially out of the socket (subluxation). In either case, shoulder pain is present and use of the shoulder is restricted.
A shoulder dislocation can damage the joint capsule, the joint surface, the tendons, and/or the muscles. In some cases there may also be injury to blood vessels and nerves.
Usually the humeral head comes out of the front of the shoulder socket, but it can dislocate out of the back of the shoulder; rarely does it dislocate upwards or downwards.
A shoulder dislocation is usually caused by falling on the shoulder or the outstretched arm, or it can be caused by a powerful movement such as throwing or blocking a throwing motion. Certain sports such as football, skiing, hockey, rugby, volleyball, and basketball are more likely to cause an injury that results in a dislocated shoulder.
Sometimes a shoulder dislocation/subluxation is associated with diminished muscle strength or excessive elasticity (hyperlaxity) of the ligaments and/or tendons. In children or adults with extremely elastic ligaments (hyperlaxity) the humeral head pops out more easily (shoulder instability).
A variety of symptoms result from a shoulder dislocation:
- acute pain
- shoulder stiffness
- reluctance to move the shoulder
- swelling or altered shape of the shoulder
Diagnosing a shoulder dislocation is often based on a description of the injury, the symptoms, and the physical examination, and is confirmed by X-ray. Additional physical examination determines whether there is also damage to the cartilage, bone, or nerves.
When the shoulder has previously been dislocated, the chance that it will happen again increases considerably, with younger individuals being particularly prone to re-dislocation. A stiffer shoulder may be present in older adults, making it less likely to come out of the socket again.
Besides being painful and annoying, a recurring shoulder dislocation may damage cartilage, causing wear-related complaints in later life. The shape of the humeral head may change, making it easier to dislocate.