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Shoulder instability/trauma

Unstable shoulder

The shoulder is a ball and socket joint, it is made up of 3 bones that move together as one; the scapula (socket of the shoulder blade) and the head of the arm bone (ball section) and the collar bone.

The joint is designed to give a large amount of movement. This also means that it has the potential to be too loose. There are various structures that help to keep the joint in position. The most important ones are:

  • Ligaments, which hold the bones together
  • Labrum (A rim of cartilage, which deepens the socket)
  • Muscles, which keep the shoulder blade and ball in the correct position when moving or using the arm
  • Joint capsule (lining that covers the joint).

Shoulder instability is when the ball is not suitably controlled within the socket. Shoulder instability can be classified into 3 types.


Polar type 1

Polar type 1 is when trauma to the shoulder results in a dislocation. This is the most common type of shoulder instability and occurs after a high force injury or event.

90% of shoulder dislocations are anterior, with the remaining 10% being posterior.

During a dislocation, the ligaments and other structures supporting the humeral head into the shoulder cup are stretched, which increases the chances of a further dislocation.

It is important to get these checked over by a medical professional, as there can be associated injuries such as rotator cuff tear, fracture of the greater tuberosity (on the upper arm) and neurological injury.

As it is a traumatic event, other injuries may have occurred such as further fractures, dislocations or muscle/tendon tears.

After a dislocation, certain movements and activities should be avoided to prevent further dislocations and an individualised exercise programme should be given to regain the strength and stability in the joint.


Polar type 2 and 3

Polar type 2 and Polar type 3 instability is when there has not been any trauma involved, but the shoulder feels unstable. This is associated with apprehension and pain and may result in subluxations or dislocations.

These can be due to various factors but can involve a lack of stability from the surrounding muscles and ligaments leading to the shoulder ‘falling out’, or abnormal muscle patterning that results in the shoulder being ‘pulled out’.

Most of the time, surgery is not required for shoulder instability and it can be managed through rehabilitation, aimed at correcting muscle patterns and increasing the strength and stability of the shoulder and surrounding structures.

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