Shoulder anatomy and stability

The shoulder joint (also known as the glenohumeral joint) is the most mobile joint in the body, owing mainly to its unique anatomical structure which includes a very shallow socket. Unfortunately, it is also the most frequently dislocated joint in the human body. The joint is kept in place or stable by a combination of bone, ligaments and muscles, which all have an important part to play in shoulder stability. The bony part of the joint socket is very shallow, so it is important that all these structures are working well to prevent the joint from dislocating.

Anterior shoulder dislocation

Shoulder dislocation occurs in the anterior direction in 95% of cases. This means that the ball (head of the humerus or, in other words, the top part of the long bone in the upper arm) ‘pops out‘ in the forward direction. The first episode of an anterior dislocation usually occurs when an individual has their arm positioned above their head, and an unexpected small further injury forces the arm that little bit further, pushing the shoulder into an extreme position which overcomes the structures that stabilise the shoulder joint, causing the ball to pop out of its socket.

Bankhart and HillSachs lesionsThe force of the head of the humerus (ball) popping out of the socket usually causes part of the gristle of cartilage (labrum) running around the rim of the socket to be torn off. This is called a Bankhart lesion, or labral tear. Sometimes a small piece of bone can be torn off with the labrum.

When the ball has popped out of position, the back of the ball is jammed up against the socket. This causes a dent in the ball known as a HillSachs lesion. In people under 40 years with anterior shoulder dislocation, around half experience HillSachs lesions.

Risk of further shoulder dislocations

After somebody has had a shoulder dislocation, the damage to their shoulder means that they have a higher chance of further shoulder dislocations. The chance of further instability can be estimated at approximately 60%, but does depend on age and activity level. Physiotherapy does help the shoulder recover from an initial shoulder dislocation, but does not reduce the chance of further shoulder dislocations. A 60% risk of further instability is not acceptable to some of the population, who choose surgery early to lower their chance of further instability to 5%.

There are some significant disadvantages to sustaining repeat shoulder dislocations for example, 15% of drownings are thought to be associated with a dislocated shoulder. There is a greater risk of osteoarthritis (wear and tear and loss of cartilage in the joint) with increasing numbers of dislocations. In addition, each dislocation usually results in more bone loss from the humeral head and glenoid, which lowers the chance of success of minimally invasive (arthroscopic) surgery, and may mean that an open bone grafting procedure (Laterjet procedure) will be required.

Posterior dislocation

Posterior shoulder dislocation means the head of the humerus ‘pops out’ in a backwards direction, which is the case in 24% of dislocations. This tends to occur when there is a blow to the front of the shoulder, violent muscle contractions due to a seizure (fit) or electrocution, or the arm is bent across the body and pushed backwards. Injuries that are commonly associated with posterior dislocation include fractures, rotator cuff tears, and HillSachs lesions.

Inferior dislocation

Inferior shoulder dislocation means the head of the humerus sits below the socket once it has ‘popped out’. Only 0.5% of dislocations occur inferiorly. It is caused by forceful moving of the arm over the head toward the other side of the body, or by pulling on the arm when it is fully extended over the head as may occur when grasping an object above the head to break a fall. Injuries associated with inferior dislocation include damage to nerves (60%), rotator cuff tears (80%), and injury to blood vessels (3%).

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