The shoulder is a ball and socket joint. It has the greatest tendency to dislocate (come out of joint) of any joint in the body. This is due to the fact that the socket (glenoid) is very shallow compared with the diameter of the ball. The stability of the shoulder is improved by the presence of a series of ligaments, a thick rim of tissue that deepens the socket called the labrum and a strong but flexible sack, the capsule, the surrounds and contains the whole shoulder joint.
If the ligaments or the capsule stretch or the labrum is torn off the socket then the joint may partly dislocate or sublux. If the ball completely loses contact with the socket this is known as a dislocation. Shoulder instability is a term that is used to describe the shoulder either subluxing or dislocating during movement or exercise
In which direction does the shoulder dislocate?
By far the most common is anterior dislocation (97%) where the ball moves forward in relation to the socket. The shoulder may also dislocate backwards (posteriorly) or downwards (inferiorly) but this is much more rare.
What are the causes of shoulder dislocation?
There are three main categories of dislocation:
The shoulder is injured with sufficient force such as with a heavy fall or road traffic accident to cause the shoulder to come out of joint. The shoulder may stay dislocated and may need reducing (put back into joint) at hospital. Following this injury the arm is normally immobilised in a sling for a period of time and then a course of physiotherapy is undertaken.
Such an injury often causes the labrum to be torn from the front of the socket which can allow the shoulder to become unstable and dislocate more easily with lesser trauma in the future. If this occurs then surgery can be considered to repair the torn part of the labrum. This operation can be carried out either by arthroscopic (keyhole) or open surgery.
A traumatic dislocation
This occurs when the shoulder subluxes or dislocates with minimal force such as when reaching up on to a high shelf. It is usually painful but often the shoulder usually goes back into joint itself. A traumatic dislocation of the shoulder occurs in people who have lax joints (being double jointed) and the first line of treatment is directed towards strengthening and balancing the control of shoulder movement through physiotherapy. If this fails then surgery may be necessary.
Positional non-traumatic dislocations
This occurs due to abnormal pull of the muscles surrounding the shoulder and may, for instance begin as a party trick. It is normally painless and often occurs in both shoulders. Treatment is by physiotherapy to correct the abnormal muscle patterning.
Treatment of Shoulder dislocations
The emergency treatment is to relocate the shoulder into joint. Following a period of rest in a sling, physiotherapy is often helpful in helping to regain mobility and strength of the shoulder.
Surgery may be considered after the initial episode of dislocation or if the shoulder repeatedly dislocates (recurrent dislocation). This may be carried out ether using arthroscopically or by an open technique.