Shoulder joint instability is unfortunately a common sequela following particular injuries to the shoulder. Instability is particularly common due to the anatomical shape of the joint surfaces which are incongruent and built to allow a large range of motion rather than stability.
Rather than the bony congruence of the joint, the shoulder joint relies on other supporting structures to provide its stability. These include:
- Rotator cuff muscles (group of four muscles which form a supportive sling around the shoulder joint)
- glenoid labrum (fibrous deepening to the shoulder socket which deepens the socket and increases contact area, thereby, contributing to stability)
- proximal biceps tendon (one portion of the biceps runs across the centre of the shoulder joint and attaches to the rim of the glenoid labrum)
- shoulder joint capsule (a thick band of soft tissue reinforced by several strong ligaments which hold the upper arm bone in the socket at rest and during movements)
When one or several of these soft tissue structures are injured this may lead to traumatic instability of the shoulder. Several types of trauma may lead to injury to these structures:
Anterior shoulder dislocation
As the name suggests this injury is where the upper arm bone is forced out of the front of the shoulder socket (glenoid). It is the most common cause of ongoing traumatic instability of the shoulder as the shoulder is vulnerable when it is elevated and externally rotated which may occur with shoulder tackling in sports and falling. An anterior dislocation may lead to tearing and laxity to the anterior joint capsular as well as tearing to the glenoid labrum and sometimes fractures to the rim of the glenoid.
Posterior & inferior shoulder dislocation
Dislocation of the shoulder joint backwards (posterior) and downwards (inferior) are relatively uncommon causes of ongoing traumatic shoulder instability. These injuries are normally due to a a violent fall on to an outstretched hand. Fits (epileptic or otherwise) and electricution are other causes.
SLAP and bankhart tear to the glenoid labrum
A SLAP tear and a bankhart tear are both tears to the glenoid labrum which lies as a fibrous rim attached to the edge of the shoulder socket. It aids shoulder stability by deepening the socket. Injury to the labrum is normally due to subluxation or full dislocation of the shoulder or due throwing injuries. Once torn, the glenoid labrum does not repair and as such can contribute to ongoing traumatic instability of the shoulder.
Rotator cuff tear
The rotator cuff muscles and tendons provide one of the most powerful contributory factors to stabilising the shoulder joint. Although a rotator cuff tear in isolation will not cause a definite instability of the shoulder it is a common associated injury with labral and joint capsule tears and significantly impairs the ability of the shoulder to compensate for these injuries.
Due to the inherent structural nature of traumatic shoulder instability the treatment approach either involves rehabilitating the muscles which support the shoulder blade and glenohumeral joint of the shoulder or surgery to correct the particular structural fault. In some cases shoulder braces may be used as part of a rehabilitation or post-surgical strategy to stabilise the shoulder during certain activities such as sports.
Rehabilitation exercises will generally involve three components. The first is a program of exercises known as proprioceptive exercises which develop the shoulders ability to recognise it’s current location and joint angles. This is extremely important as when this is optimised muscle co-ordination and timing improve when the shoulder is placed in positions of vulnerability making it less likely to dislocate.
The second exercise component is to optimise the strength of the muscles which support the shoulder blade. The shoulder socket (glenoid) is part of the bony anatomy of the shoulder blade and if these muscles are able to position and stabilise the position of the shoulder socket optimally then this allows the rotator cuff muscles to work to stabilise the shoulder joint more efficiently.
Finally, rehabilitation of the rotator cuff muscles themselves allows the shoulder joint to be able to control the centring of the upper arm bone in the shoulder socket and also resist any rotational forces in an optimal way to help to prevent dislocation.
Depending on patient choice, age (the younger the patient the more likely the need for surgery to adequately stabilise the shoulder), activity level and the degree of structural damage present surgery may be considered as the first line of treatment. Many variations of surgery will be considered but in the vast majority of cases the surgery will be completed using a keyhole technique (arthroscopy).
Depending on the structural problems present surgery will stitch any tears to the rotator cuff tendons and labrum and then pull the remaining part of the shoulder joint capsule across the region of instability, securing it to keep it tight. If required various forms of bony block grafts may be used to replace glenoid fractures.