Shoulder Instability

What is Shoulder Instability?

The stability of the shoulder joint is provided by no single structure in the joint. Instead, the bony structure of the joint surfaces, the ligaments and muscles are all key components in maintaining a stable shoulder joint yet permitting a large range of movement in several directions.

Instability is often associated with subluxation (partial dislocation of the shoulder joint), which may be associated with pain and / or dead arm sensation. Indeed this is often what prompts the athlete to seek medical attention. In some people, this is not actually painful but can be quite annoying and prevent them from taking part in daily activities or sports.

The instability of the shoulder joint can be in one direction for example, anterior instability (out the front), posterior instability (out the back) or in more than one direction (known as multidirectional instability). Anterior is the most common form of instability and is probably because the joint capsule is at its weakest at the front of the joint.

What are the causes of shoulder instability?

This condition can be caused by a number of reasons. If the joint surfaces are shaped slightly differently – for example if the glenoid fossa is slightly flatter than usual, or the head of humerus is more of an oval shape – the joint may not be as stable compared with other people who have “normal” joint anatomy.

Other structures support the bony anatomy to help provide stability to the shoulder. These include:
Glenoid Labrum –this is a ring of cartilage which deepens the glenoid fossa, making the “cup” of the socket deeper and hence improving stability
Joint Capsule – this is a membrane which encompasses the entire joint, providing stability but also maintaining the joint complex and holding the lubricating (synovial) fluid in the correct place
Ligaments – these hold the bones together and provide stability by preventing them from moving when they shouldn't.
Muscles – these work alongside the ligaments in preventing unwanted movement, but also initiate and create movement of the joint.

Instability may be caused by:
Trauma (traumatic instability)
This is usually due to a particular accident or injury which damages the structures that provide stability
This is sometimes due to an old injury – for example an injury which weakens the capsule may cause instability – known as “post traumatic instability”
Joint Laxity (a traumatic instability)
Could possibly be due to:
anatomical abnormalities
generalized laxity (known as hypermobility)
muscle weakness
certain conditions, including pregnancy
“Acute on chronic” instability – a traumatic injury to an already lax joint.

Please see the Virtual Sports Injury Clinic for more information on how to treat Shoulder Instability and other shoulder injuries.