INSTABILITY means that the shoulder dislocates completely (dislocation) or partially (subluxation).
The shoulder consists of a ball (humeral head) and socket (glenoid). The ball is stabilized in the socket by a cartilage rim (“labrum”, which means “lip”) and the ligaments, which attach to the labrum. In other words these ligaments attach to the socket on the one side and to the ball on the other side.
|Arrow shows a normal labrum (left) and a torn labrum (right)|
Shoulder dislocations occur when the ball (humeral head) of the shoulder is forced out of the socket (glenoid). Separation or dislocation can also occur of the smaller joint on top of the shoulder known as the AC joint (acromio-clavicular) – this is the junction between the collar bone and the shoulder bone and this type of separation is not usually included in the term “shoulder dislocation”.
The shoulder dislocates when the ball is forced out of the socket in various situations and there are different types of shoulder dislocation. The shoulder can go out the front (anteriorly) or to the back (posteriorly). The anterior type of shoulder dislocation is by far the most common.
There are various classes of shoulder dislocation:
- Post traumatic dislocation of the shoulder joint – this happens after an injury. The ball is forced out of the socket usually with the arm outstretched and forced to the back–the shoulder is forced out the front– this is called an “indirect” force. A direct blow against the shoulder can also cause a dislocation (a “direct” force). During such a shoulder dislocation, the tissues holding the shoulder in place are usually torn. This may either be the separation of the ligaments and labrum (the labrum is the cartilage rim around the socket) referred to as a “Bankart lesion” or even bone being broken off the edge of the socket.After such a first shoulder dislocation the incidence of repeated shoulder dislocations with minimal force is quite high. In younger patients the recurrence rate is higher than in older people. The reason is that in a young person more force is required because of the laxity and strength of the ligaments. That usually leads to the ligaments being stripped off the edge of the socket and this seldom heals again. In older people the ligament is often simply torn and will heal again but it has to be noted that other associated injuries are not uncommon in older persons – this includes fractures, tears of the rotator cuff tendons and injuries to the blood vessels and nervesThere is a case to be made to repair the tissue after the first shoulder dislocation in a young person, if he or she wants to continue a sporting career. When a recurring dislocation is present the repair of tissues may either be done through a scope (keyhole surgery) by simply repairing the ligaments back to the edge of the socket, or in cases where there has been bone broken off the socket a procedure may have to be done to transfer bone to the damaged area (referred to as a “Latarjet procedure”).
- Dislocations due to lax ligaments: In certain individuals the ligaments are lax to a degree that the shoulder dislocates quite easily. When such a dislocation occurs the damage is not nearly as much as in the type mentioned above where ligaments tear. The persons with laxity can usually first be treated conservatively with exercise to try and strengthen the muscles to contain the shoulder. Should this not be successful a tightening of the ligaments (referred to as a “capsular shift”) can usually be done arthroscopically.
- Voluntary shoulder dislocation: This occurs in individuals where they can “pop out” the shoulder under their own control. This should not be treated with surgery at all.