The shoulder is a ball and socket joint. It is the most commonly dislocated joint in the body - this happens because the design of the joint makes movement easy in all directions. This design however also leaves the shoulder vulnerable to becoming unstable and to dislocation.

95% of all shoulder dislocation episodes occur after trauma and most of the remainder happen in joints that are already unstable.

95% of all shoulder dislocations dislocate to the front of the joint - called anterior dislocation. Posterior and inferior (into the armpit region) are also possible depending on the nature of the injury

The incidence of shoulder dislocation in the whole community is about 17 cases per 100,000 people in the population. It is more common than that in athletes or active people.

More than seven out of every ten cases of anterior shoulder dislocation occur in athletes. The most common age groups to be injured are in the 20's and in the 60's - with men more often affected than women. Frozen shoulder can develop after subluxation or dislocation in the older female population.

 The following factors have been clearly identified as risk factors for shoulder dislocation:

• previous injury - especially previous dislocation or subluxation of the joint
• an increase in joint laxity due to genetic factors or to previous injury
• violent sports or sports that force the shoulder into extreme positions of movement
• unusual position of the humerus in the joint - the ball in the socket
• problems with muscle control such as cerebral palsy or similar medical ailments

 Shoulder joints become unstable and dislocate for a variety of reasons. These include both factors to do with the joints themselves and also factors to do with the way they move. These are sometimes called "static" and "dynamic" factors respectively.

Joint related static factors include things like the shape of the individual persons shoulder joint, the nature of the soft gristle tissue that surrounds the bones and the ability of that joint to create a vaccuum pressure effect by using it's capsule to hold things in place.

Dynamic stability is created by the strength and the coordinated tone of the muscles around the shoulder. These muscles include the so called rotator cuff, the deltoid muscle and the biceps and triceps muscles.

A deep ring of gristle - shaped a bit like a donut - sits in a ring shape round the socket part of the joint. This is called the labrum and provides a vital role in keeping the joint stable. The labrum is often injured in shoulder joint trauma and it is this injury which can leave the joint unstable and vulnerable to dislocation.

Shoulder dislocation can also be related to injury to the rotator cuff group of muscles - particularly in the elderly population.

There is nearly always a history of trauma or injury.

When the shoulder dislocates anteriorly the trauma usually involves an indirect force to the arm in extension and rotation at the same time.

Severe immediate sickening pain is the main symptom and is often the most pressing reason that the patient seeks urgent shoulder pain treatment.

Nerves around the shoulder can be damaged during a dislocation and it's always important for the doctor to check for this.

X-rays from two directions are needed to confirm the diagnosis - although it is usually pretty obvious what has happened. Sometimes a fracture can accompany the dislocation so it's important to check on this.

CT and MRI scanning are also often used for a clearer picture of what is going on inside the joint. MRI scanning in particular is good at showing tears in the rotator cuff or in the labrum tissue. Die can be injected to create what is known as an MRI Arthrogram

 All acute dislocated shoulders need reduction performed as soon as possible. There are a variety of ways to do this but the old "foot in the armpit method" - called the Hippocratic method - is falling from favour because of a risk of causing a fracture or nerve damage.

Most athletes who sustain a shoulder dislocation will need surgical treatment to stabilise the joint - otherwise the problem is likely to keep recurring. The rate of second or more dislocation in young athletes is higher than 90% if a stabilising surgery is not performed.

It can take as long as six months for an athlete to return to their sport after shoulder stabilisation surgery

This Hub page was created by Doctor Gordon Cameron - shoulder pain specialist from Edinburgh, Scotland