The shoulder joint is a physical complex part of the body. As such, there can be many causes of pain, discomfort or movement issues. A previous article looked at impingement. This feature discusses pain as a result of instability in the shoulder.
What is joint instability?
You may have an unstable joint when you find you have less control of movement in that area. This, in turn, results in an increase of internal movement within the joint, which ultimately becomes a point of ‘weakness’. In this way, the shoulder joint will have unstable movement when you want to move it forwards, backwards or upwards. When two or more directions are affected, this is called “multi-directional instability”.
Happily, instability can be treated conservatively by a physiotherapist, depending on the severity and the structures (surrounding tissues or muscles) involved.
The skeletal ‘ball and socket’
Firstly, the shoulder joint is made up of the head of the humerus bone which fits into the shallow depression or socket in the scapula or wing bone, forming this joint.
To illustrate the ratio between these two bones, think about a golf ball and a golf tee. Achieving stability using muscular control is like balancing the golf ball on a tee. Your shoulder will be at its most stable when it is steady in a resting position (static stability of the labrum, ligaments/capsule) and when it is moving (dynamic stability using muscles: rotator cuff, scapular stabilisers etc.). Most often, instability is a result of congenital abnormalities or a fracture due to trauma, for example, a dislocation of the shoulder.
The labrum is a type of cartilage in the shape of a ridged rim that deepens the socket of the joint to increase the stability of the skeletal structures (mentioned above). Negative pressure occurs between the surfaces, causing a suction force holding the head of the humerus in the socket. It also serves as the point where ligaments attach to the socket from the humeral head.
The labrum does not contract or stretch beyond its original length. However, when it is damaged or torn, it loses the vacuum force created between the two surfaces (negative pressure), thus increasing that unwanted movement within the joint.
Ligaments form a cocoon-like safety around the ball and socket, further increasing the stability of the shoulder. These ligaments resist excessive movement in various directions and add to the ‘static stability’ mentioned earlier. However, the ligaments can be damaged beyond their normal range when a partial or full tear creates a weak spot, causing the humeral head to move too freely within the capsule, increasing unwanted friction between the joint surfaces.
The rotator cuff
There are four muscles that assist with ‘dynamic stability’ – supraspinatus, infraspinatus, teres minor & subscapularis. They work together to keep the balance within the shoulder joint.
The tendons of these four muscles blend with the ligamentous capsule and together form a ‘cuff” at the ‘neck’ of the humeral ‘head’. This is known as the rotator cuff, which plays an important role in the dynamic stability of the shoulder. It does this by controlling and steadying the rotation of the ball and socket within the shoulder when all kinds of movements are made. The rotator cuff’s main function is to ensure that the head of the humerus bone remains in the centre of the glenoid socket. When the rotator cuff muscles or tendons are damaged, this can lead to a decrease in control of movement in the shoulder joint, causing pain.
The bicep tendon
Have a look again at the picture of the labrum. You will see the long head of the biceps bracci (one of the tendons of the muscle) which are attached to the top part of the labrum. The bicep tendon is included in this list of causes of shoulder pain from instability, as it can be involved with labrum injuries and is called a SLAP (superior labrum, anterior to posterior). The bicep muscle also helps to keep the ball and socket stable when you lift your arm.
The scapular stabilisers
As mentioned earlier, it is important that the humeral head is centred properly within the glenoid cavity. Referring again to our illustration of balancing a golf ball on a tee, if the tee is not placed perpendicular to the horizon (ie at 90 degrees to the ground), the ball will be more difficult to balance. In this way, there is a whole group of muscles connected to the scapula (where the glenoid cavity is found) which play an important role in positioning the shoulder joint for optimal movement.
Other reasons for shoulder instability.
- Hypermobility, where joints can move beyond the normal range of motion. This can be measured on a scale out of 9 (the Beighton score). When classified as hypermobile (score >4), it means that the tissue type is more flexible than when classified hypomobile (score <4). In the case of hypermobility, shoulder structures need to be strengthened.
- Weakness or injury to the shoulder area. This can cause other issues, such as subacromial pain syndrome.
The good news is that when you think there may be a problem with stability in your shoulder, a physiotherapist can be your first point of contact. They can help you investigate and determine the next course of action. In fact, physiotherapy is ideal for helping you to strengthen ‘dynamic stability’ before and after an injury or surgery, for example. Treatment also includes pain management and helping you to heal properly. Not all cases of shoulder instability need surgery.
This article was submitted by the Douglasdale and Fourways branches of Lamberti Physiotherapy. You can contact one of the Physios practising at either branch by using our handy appointment form.
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