What is Shoulder Instability?
The shoulder joint is the most mobile joint in the body. However it is also the most potentially unstable joint. Although it is a so called “ball and socket” joint the socket is actually very shallow. The ball is balanced on the socket rather than in it. This is quite different for instance to the hip joint where the ball component of the joint is physically within the socket. Thus a dislocation of the hip generally requires severe trauma.
Stability of the shoulder joint depends not so much on the bony structure of the joint, but rather the soft tissues around the joint including ligaments, the glenoid labrum, and surrounding muscles. The ligaments which collectively form the capsule of the shoulder joint are the most important of these structures. Undue laxity of these ligaments due to injury or inherent ligament laxity will contribute to shoulder instability. The instability can range from minor slippage of the ball on the socket when the shoulder is in a vulnerable position, through to Subluxation, and complete Dislocation.
- Subluxation refers to abnormal translation of the head of the humerus on to the rim of the glenoid but just short of complete dislocation. The ball may be temporarily caught on the edge of the socket and then relocate either spontaneously or with a little assistance. It might do so with a painful clunk.
- Dislocation is a step beyond subluxation.The head of the humerus either falls or is forced over the edge of the glenoid. The ball will often remained locked out of position and require assistance to relocate the ball in the socket. The degree of assistance required varies. Some patients may with some difficulty manoeuvre the shoulder back in to place. More often medical assistance is required. This degree of instability is typically much more painful than a subluxation.
Classification of Shoulder Dislocation
Shoulder subluxation and dislocation may be classified in a number of ways. This can include the Direction of the instability and the Mechanism or cause of the instability.
By far the most common direction of shoulder instability is Anterior. This is where the head of the humerus translates in a forward direction relative to the glenoid. It is usually a result of a significant injury, particularly on the sporting field.
Posterior shoulder instability is where the ball slips in a backwards direction. It is much less common than anterior instability. It may result from inherent ligament laxity. Trauma in the form of a direct blow to the front of the shoulder may also force the ball backwards. Posterior dislocation may also result from the violent muscular contractions caused by an epileptic seizure or an electrical shock.
Shoulder instability and dislocation may occur without a major trauma. The shoulder may slip when taking the arm suddenly in to an extreme overhead positon. Examples of this phenomenon might include a forceful tennis serve or a throwing action. Sometimes a seemingly trivial injury might trigger the instability. This type of instability mechanism is often referred to as “Atraumatic Instability”. It is often a reflection of inherent laxity of the ligaments that normally stabilise the joint. It is quite common in teenagers and young adults, particularly women. This type of instability will often respond well to a muscle strengthening program, without needing to progress to a surgery.
Dislocations that occur due to a much more significant trauma are more likely to result in tearing of the important ligaments at the front of the shoulder. This can set the scene for repeated episodes of instability but without the same violence of the initial injury. This type of instability is often referred to as “Post-Traumatic Instability”. Common injuries that trigger this type instability include the arm being wrenched in to an extreme overhead position such as with football or rugby. Falls on to the outstretched arm, bicycle and motorcycle accidents are also common causes.
Diagnosis of Shoulder Dislocation
Diagnosis of shoulder instability or complete dislocation is made by a step wise assessment of:
- Mechanism of the Injury: The degree of force applied to your shoulder and the position your arm was forced in to at the time of the injury may raise a strong suspicion of instability.
- Symptoms: The pain is usually severe. You may notice that your shoulder clearly looks and feels out of place. It might feel like the shoulder is stuck in a certain position. Patients often notice numbness and tingling and a sense of weakness in the arm.
- Clinical Assessment: There may be a visible shoulder deformity. Structures in the region of the shoulder apart from the joint can also be injured at the time of the dislocation. In particular nerves can be stretched and rotator cuff tendons can be torn. They need to be assessed.
- X-rays: will confirm the diagnosis. They will also reveal associated shoulder fractures. Fractures of the greater tuberosity of the humerus or the edge of the glenoid might be revealed by this test. X-rays are also advised after the dislocated joint has been relocated to confirm that the ball is in the correct position relative to the socket.
- MRI Scan: This test will be helpful in assessing the extent of tearing of the ligaments at the font of the shoulder. Significant tearing away of the ligaments away from the edge of the socket may be an indicator that the shoulder may have ongoing instability issues.
Anterior Shoulder Dislocation
& Humerus Fracture
Treatments for Shoulder Dislocation
Following diagnosis of a shoulder dislocation the joint needs to be relocated. The technique used is called a closed reduction. After the administration of pain relieving medication and sedation, a gentle manipulation of the joint may achieve reduction. Patients with a strong muscular build may be best suited to having a brief general anaesthetic. This relaxes the muscles around the shoulder to then reduce the joint.
The severe pain related to the dislocation usually reduces dramatically after the joint has been relocated. However there can be residual soreness that takes weeks and sometimes several months to fully subside. A sling is advised in the first week or so after the injury. The shoulder can be gradually mobilised as the soreness settles but taking care initially to avoid taking the arm I to more extreme positions such as a throwing type of action.
There may be a role for a physiotherapy program aimed at improving muscle strength. This can be especially helpful for patients with so call “atraumatic” instability. It may also be of assistance for patient who have had a more traumatic dislocation, but will not necessarily eliminate the risk of further dislocations.
Traumatic dislocation of the shoulder, especially in patients with an active lifestyle, carries a significant risk of ongoing instability symptoms. These can include a sense of apprehension and fear when the arm is taken in to vulnerable positions. Some patient notices subtle slippage of their shoulder or more obvious subluxation with certain activities. At the extreme end of the symptom scale some may experience unexpected dislocation of their shoulder with an innocent movement.
It is often only during the rehabilitation phase or sometimes long afterwards that it becomes clear as to whether there will be ongoing instability symptoms. The severity of any ongoing instability symptoms will help in deciding whether a Shoulder Stabilisation surgery will be a consideration.