Shoulder instability

The shoulder joint is a marvelous unique structure, being the most mobile joint we have. No other joint can match it in the degrees of freedom it has. However, this mobility comes at a price, the risk of becoming unstable.

Anatomy of the shoulder


Note how the ball (head) of the humerus fits into a shallow socket on the scapula called the glenoid. You can see that this ball does not fit into the glenoid cup exactly, thus allowing for the wide range of movement provided by the shoulder, but at the cost of skeletal stability. Joint stability is provided instead by the rotator cuff muscles , related bony processes and glenohumeral ligaments.

There are 2 main types of shoulder instability:

  1. Trauma related
  2. Excessive laxity of the shoulder ligaments and poor muscle control.


  • The most common cause of instability
  • Usually caused by a fall or direct impact to the shoulder
  • The large force causes structural damage to the ligaments and socket rim (labrum).
  • This tends to lead to repetitive dislocations in some people.
  • Rehabilitation can reduce the chances of this.

Laxity of ligaments and poor muscle control

  • Can be caused through weakness in the rotator cuff muscles
  • Weakness of the muscles that control the scapula and rib cage can also cause instability.
  • It is difficult to keep the ball in a socket that is not controlled through strong ligaments and muscles.
  • Requires specialist rehabilitation to achieve the complex muscle control.
  • Some cases surgery can assist the muscle retraining process through tightening up the capsule and ligaments.


When a shoulder is unstable, it can dislocate. This means that the head of the humerus separates fully from the socket.





Significant trauma is usually required to cause a shoulder to dislocate. The usual direction is anterior. It can go out at the bottom or inferior, or a combination of anterior and inferior. Very rarely does it dislocate posteriorly, but can happen.


An unstable shoulder can also sublux, which is a temporary slip of the head of the humerus. It usually relocates itself on its own or when the patient wiggles their arm or changes the position of the arm.

Additionally an unstable shoulder can cause sensations of instability without any physical separation of the joint. We call this apprehension.


  • Once subluxed or dislocated recurrence is very likely.
  • In people under 25yrs old there is approximately an 80% chance of recurrence.
  • The recurrence rate reduces as age advances.
  • Once a shoulder dislocates a second time, it will almost always continue to re-dislocate with the arm in certain positions and often with less and less trauma.

 Because of the high recurrence rate, the goal of any treatment is to reduce the possibility of recurrence. In spite of this treatment, the recurrence range is still fairly high.

Laxity of ligaments and poor muscle control

When trauma occurs it can cause damage to ligaments, causing them to be stretched, torn or detached from the bone. These can then heal in a loose position of which may increase the risk of future instability.

Alternatively, some people are born with somewhat loose shoulder ligaments. Instability can occur without trauma or injury. The textbooks don’t tell you this do they, but we aren’t all built the same, in fact no two or us are, variation in anatomy is just human!

Muscle control is vital in insuring stability at the shoulder joint and to maintain its correct biomechanics.

Researchers have discovered the importance of your rotator cuff muscles to dynamically stabilise your shoulder joint. These muscles must be strong in order to protect and prevent the shoulder from re-dislocating or subluxing.



As physiotherapist’s at Black Pear we can assess your muscle recruitment pattern and prescribe the best exercises for you specific to your needs and get you back to full function.

Contact us here at Black Pear Physiotherapy on 01905 611 010…check out the website for more tips or why not book an appointment to let us help you get back to full function.

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