What is this condition?
This occurs when the ball of the shoulder (the humeral head) moves abnormally in relation to the shoulder socket (glenoid), producing either a sensation of the ball quickly popping in and out of the socket (termed “subluxation”), or a full dislocation – when the head comes fully out of the socket.
What causes it?
An unstable shoulder is usually caused by a traumatic event, typically during sports such as rugby or in a fall, that damages internal structures within the shoulder that are responsible for the stability of the joint. Shoulder instability is more commonly seen in younger patients, since they are more likely to be involved in contact sport and typically have looser joints. The most common type of dislocation is an “anterior” dislocation, where the ball moves to the front of the socket. The classic internal structure to be damaged following an anterior dislocation is the anterior labrum, producing the so-called “Bankart tear” or lesion. The labrum is a ring of cartilage that encircles the glenoid and deepens it. Once it has torn it can either fail to heal or alternatively heal in the wrong position, leading to instability. Another common pattern is the “ALPSA lesion”, where both the capsule (lining of the joint) and labrum are stripped from the front of the socket and heal in an abnormal position causing the shoulder to become loose and recurrently unstable. Damage to the bone can also occur during a dislocation and contribute towards ongoing symptoms of instability. The so-called “Hill-Sachs lesion” is a common finding on MRI scans and x-rays and arises when the relatively soft bone of the back of the humeral head impacts onto the harder bone of the edge of the glenoid/socket when the shoulder is fully dislocated, causing a dent. After multiple dislocations bone loss both on the glenoid and humeral head side can lead to the shoulder becoming increasingly loose and can increase the failure rate of soft tissue reconstructive surgeries.
MRI Arthrogram of shoulder showing Bankart lesion of the labrum
MRI Arthrogram of shoulder showing Hill-Sachs lesion in the humeral head
What is the long term outlook?
Some people who have a shoulder dislocation may never have further trouble with their shoulder in the future. Alternatively, others may go on to develop laxity in the shoulder, feelings of insecurity and perhaps even further dislocations. Whether this happens or not depends on a number of factors, including your age at the time of first dislocation (the younger you are the more likely it is to come out again in the future), the degree of damage to internal structures, and your desire to return to contact sport. Someone who is under 18 at the time of their first dislocation and who intends to play high level competitive rugby is very likely to develop problems with recurrent instability or dislocations if they do not have surgery. However, if you are in your mid 30’s and do not intend to get back to sport, a more conservative approach may be all that is needed. An individualised approach that is based on your circumstances, examination findings and MRI scans is typically required.