Shoulder dislocation and instability

• What is shoulder dislocation and instability?
• Statistics on shoulder dislocation and instability
• Risk factors for shoulder dislocation and instability
• Progression of shoulder dislocation and instability
• Symptoms of shoulder dislocation and instability
• Clinical examination of shoulder dislocation and instability
• How is shoulder dislocation and instability diagnosed?
• Prognosis of shoulder dislocation and instability
• Treatment of shoulder dislocation and instability
• Shoulder dislocation and instability prevention
• References

What is shoulder dislocation and instability?

Shoulder anatomy and stability

The shoulder joint (also known as the glenohumeral joint) is the most mobile joint in the body, owing mainly to its unique anatomical structure which includes a very shallow socket. Unfortunately, it is also the most frequently dislocated joint in the human body. The joint is kept in place or stable by a combination of bone, ligaments and muscles, which all have an important part to play in shoulder stability.  The bony part of the joint socket is very shallow, so it is important that all these structures are working well to prevent the joint from dislocating.

For more information, see Anatomy of the Shoulder.

Anterior shoulder dislocation

Shoulder dislocation occurs in the anterior direction in 95% of cases. This means that the ball (head of the humerus or, in other words, the top part of the long bone in the upper arm) ‘pops out‘ in the forward direction. The first episode of an anterior dislocation usually occurs when an individual has their arm positioned above their head, and an unexpected small further injury forces the arm that little bit further, pushing the shoulder into an extreme position which overcomes the structures that stabilise the shoulder joint, causing the ball to pop out of its socket.

Bankhart and Hill–Sachs lesions

The force of the head of the humerus (ball) popping out of the socket usually causes part of the gristle of cartilage (labrum) running around the rim of the socket to be torn off. This is called a Bankhart lesion, or labral tear. Sometimes a small piece of bone can be torn off with the labrum.
When the ball has popped out of position, the back of the ball is jammed up against the socket. This causes a dent in the ball known as a Hill–Sachs lesion. In people under 40 years with anterior shoulder dislocation, around half experience Hill–Sachs lesions.

Risk of further shoulder dislocations

After somebody has had a shoulder dislocation, the damage to their shoulder means that they have a higher chance of further shoulder dislocations. The chance of further instability can be estimated at approximately 60%, but does depend on age and activity level. Physiotherapy does help the shoulder recover from an initial shoulder dislocation, but does not reduce the chance of further shoulder dislocations. A 60% risk of further instability is not acceptable to some of the population, who choose surgery early to lower their chance of further instability to 5%.
There are some significant disadvantages to sustaining repeat shoulder dislocations – for example, 15% of drownings are thought to be associated with a dislocated shoulder. There is a greater risk of osteoarthritis (wear and tear and loss of cartilage in the joint) with increasing numbers of dislocations. In addition, each dislocation usually results in more bone loss from the humeral head and glenoid, which lowers the chance of success of minimally invasive (arthroscopic) surgery, and may mean that an open bone grafting procedure (Laterjet procedure) will be required.

Injury to nerves and blood vessels

Damage to the axillary artery (which runs through the armpit and supplies the arm) occurs in 1–2% of shoulder dislocations, more commonly in the older age group as their blood vessels are less elastic.
Nerve injuries are more common, particularly of the axillary nerve, which occurs in around 10–40% of shoulder dislocations. The axillary nerve wraps around the neck of the humerus, and supplies the deltoid muscle, one of the most important muscles around the shoulder joint, and the skin overlying the side of the shoulder (where a shoulder badge would be worn on the sleeve of a jacket). Most people with axillary nerve damage recover without treatment, as the symptoms go away when the shoulder is put back in position. Less commonly there may be damage to the brachial plexus, which is a network of nerves that run from the neck and supply the arm. This is more likely when dislocation occurs with the arm out to the side, with the elbow bent at 90 degrees and the hand hanging down. Again, the recovery from damage to the brachial plexus associated with shoulder dislocation is usually good.

Fractures (broken bones) and rotator cuff tears

Significant fractures occur in around one quarter of shoulder dislocations, and are more common when there is a traumatic mechanism of injury, first-time dislocation, or the person is aged over forty.If there is a fracture and a piece of bone has moved from where it should be (displaced), then it usually needs surgery.
Rotator cuff muscle tears are reported to occur in 14–63% of anterior dislocations, more commonly in older people. Any older person who has had a shoulder dislocation 2 weeks ago and now cannot lift their arm above shoulder level most likely has a massive rotator cuff tear, and should have surgery as soon as possible.

Posterior dislocation

Posterior shoulder dislocation means the head of the humerus ‘pops out’ in a backwards direction, which is the case in 2–4% of dislocations. This tends to occur when there is a blow to the front of the shoulder, violent muscle contractions due to a seizure (fit) or electrocution, or the arm is bent across the body and pushed backwards. Injuries that are commonly associated with posterior dislocation include fractures, rotator cuff tears, and Hill–Sachs lesions.

Inferior dislocation

Inferior shoulder dislocation means the head of the humerus sits below the socket once it has ‘popped out’. Only 0.5% of dislocations occur inferiorly. It is caused by forceful moving of the arm over the head toward the other side of the body, or by pulling on the arm when it is fully extended over the head as may occur when grasping an object above the head to break a fall. Injuries associated with inferior dislocation include damage to nerves (60%), rotator cuff tears (80%), and injury to blood vessels (3%).

Multidirectional instability  (MDI)

MDI of the shoulder is a condition that sometimes occurs in people who are “double-jointed“, which is also known as having ligamentous laxity. MDI occurs in younger patients, and can cause both pain and symptoms of instability. The direction of instability can be anterior, posterior and/or inferior, hence the name.
Patients with MDI usually do not have any tear or definable anatomical problem in the shoulder. They are best treated with an extensive physiotherapy program lasting at least 6 months, which involves developing a conscious understanding of the position of the ball in the socket and how to keep it there by contracting the correct muscles through the various range of motions of the shoulder.

Statistics on shoulder dislocation and instability

Around 2% of the population have some instability of the shoulder joint. Overall, 1.7% of people dislocate their shoulder, though this statistic may almost double in people with high physical demands. Almost half of dislocations occur in people aged between 15 and 29.

Risk factors for shoulder dislocation and instability

Risk factors for shoulder dislocation include:

• Gender: Around 70% of shoulder dislocations occur in males;
• Age: Almost half of dislocations occur between the ages of 15 and 29;
• Mechanism: 95% of anterior shoulder dislocations are due to traumatic injury, most commonly resulting from a fall (60%). Almost half of dislocations occur during sports or recreation. Those who play certain sports, or who undertake activities with high physical demands (such as individuals in the armed forces) are at increased risk;
• Location: 47.7% of dislocations occur at home, while 34.5% occur at sites of sports or recreation;
• Anatomy and function: It is likely that factors such as shallow joint sockets, weak shoulder muscles, and loose ligaments increase the risk of shoulder dislocation, although these factors have not been proven in studies; and
• Previous dislocation: The strongest risk factor for shoulder dislocation is previous dislocation.

Progression of shoulder dislocation and instability

The average age of individuals who dislocate their shoulder for the first time occurs in two peaks. The first group tends to be young adult men, with injury occurring due to high-impact mechanisms. The smaller second group are older individuals with lower-impact mechanisms of injury and lower rates of re-dislocation.
Dislocation may result in further instability of the shoulder joint, which may present as subtle joint looseness, or recurrent dislocation. Up to a third of people who experience shoulder dislocation go on to develop long-term shoulder arthritis.

Symptoms of shoulder dislocation and instability

Dislocation is associated with symptoms of pain, deformity and an inability to move the shoulder joint. There may be numbness, tingling, or coldness of the arm if nerves or blood vessels are involved. The person may also be aware of the feeling of the humerus ‘popping out’ of the socket as the injury occurs.
It is important for the doctor to be able to establish the cause of instability if one is apparent. This may involve a traumatic dislocation event, repetitive micro-trauma (for example due to overhead activity), or inherited hyper-mobile joints. Pain or instability may be associated with specific positions. Symptoms of instability or dislocation at night may suggest severe instability.
Following shoulder dislocation, there may be some remaining instability of the shoulder once it is back in place. Anterior (forward) instability following shoulder dislocation usually is evident due to pain, fear or discomfort when the arm is in a ‘stop sign’ position. Posterior (backward) instability results in symptoms when the arm is crossing the body and turned inward. Inferior (downward) instability may present as pain or tingling while carrying heavy objects or with pulling downwards on the arm. In some instances, instability is multidirectional (in several directions).

Clinical examination of shoulder dislocation and instability

People with anterior (forward) shoulder dislocation tend to hold their arm by their side, supported by their other arm. They tend to resist attempts at movement. On inspection, there may be some deformity at the shoulder joint, due to the head of the humerus sitting in the wrong place.
When the shoulder is dislocated posteriorly (backwards), there is a flattening of the shoulder at the front, and a fullness at the back of the shoulder where the dislocated humeral head is sitting. The arm is held across the body and turned in, and cannot be turned out. With inferior (downward) shoulder dislocation, the arm is unable to be moved out to the side, and tends to be held above the person’s head with the forearm resting on the head.
The doctor will assess for damage to bone, nerves and blood vessels before they put the shoulder joint back in place (known as ‘reducing‘ the shoulder).
In the more chronic setting, several tests may be performed to assess for shoulder instability. Pain and instability in certain positions may indicate the direction and degree of instability. The apprehension test involves moving the shoulder into a ‘stop sign’ position. If the patient shows fear (apprehension), discomfort or guarding, the test is considered positive and indicates anterior instability is present.

How is shoulder dislocation and instability diagnosed?

Following shoulder dislocation, the doctor will arrange a shoulder x-ray, usually before they attempt to put the joint back in place (reduce it). This is to confirm the dislocation, and to look for any fractures which would make reducing the joint unsafe. Once the joint has been put back, another x-ray will be done to confirm the joint is in the correct position. When there are more complicated injuries, CT or MRI scans may be necessary to check for injury to muscles, cartilage, other ‘soft tissues’ and subtle bone damage.

Prognosis of shoulder dislocation and instability

The consequences of traumatic anterior shoulder dislocation depend on the severity of injury and the age of the person when they first dislocate their shoulder. The younger the person at first dislocation, the more likely they are to dislocate again. In athletic people aged younger than 20 years, rates of recurrent dislocation are as high as 90%,decreasing to 50–75% in people aged 20–25 years. In people older than 40 years, anterior shoulder dislocation is associated with lower rates of instability, but higher rates of rotator cuff muscle tears (15%). The risk of rotator cuff tears increases to 40% in people older than 60 years. The recurrence rate of dislocation is also related to the degree of injury of structures that support the shoulder joint at the time of dislocation.

Treatment of shoulder dislocation and instability

Dislocated shoulders should be put back in place (reduced) as soon as x-rays have been taken. It is essential that this procedure is carried out by trained medical staff, as there is a risk of damage to blood vessels, nerves and the possibility of fracture if it is not done properly.

Pain relief options

Options for pain relief while shoulders are put back in place include:

• No sedation: Anaesthesia may not be required for people who have dislocated their shoulder before, and have done it again with minimal trauma. This works best with techniques that do not require significant force, such as external rotation, scapular manipulation, or Milch technique (see below);
• Nitrous oxide (laughing gas);
• Sedation with injection of anaesthetic into the veins may be required to reduce muscle spasm and relieve pain. The patient is briefly asleep while the shoulder is put back in place. Common options include the drugs known as midazolam, fentanyl, propofol and ketamine. Procedural sedation requires appropriate monitoring and supervision, and the ability to provide airway support if it is required.

Reduction techniques

Again, it must be emphasised that putting a dislocated shoulder back in place must not be attempted by untrained individuals.
There is no one technique that is better than the others for reducing a dislocated shoulder. The choice is usually made based on the condition of the individual and the preference of the clinician. The individual will be informed of the risks of reduction, which include risks of anaesthetic, fractures, rotator cuff injuries (usually present before reduction), and damage to arteries and nerves, which are rare when good technique is used.
Certain people may be referred to orthopaedic surgeons (specialist bone doctors) before an attempt is made to put the joint back in place. People who may be considered for this course of action include elderly patients (due to their increased risk of fracture or nerve and blood vessel injury during relocation attempts), and anyone younger than 10 years of age (due to risk of fractures that may affect the growth of the bone).
Options for reduction of anterior shoulder dislocation include:

• Traction/counter-traction: This is the least traumatic and most advisable method to use. A sheet or towel is wrapped under the armpit to apply counter-traction from the opposite side of the patient to the clinician who is providing gentle continuous traction at the wrist or elbow, in a direction that is slightly abducted (30 degrees) and forwardly flexed (30 degrees);^. 
• Scapular manipulation: With the individual lying on their stomach, or sitting upright, the shoulder blade is rotated by pushing the tip toward the spine and the socket downwards, while an assistant provides gentle forward or downward traction on the arm. This disengages the humeral head from the socket, and allows it to reduce into anatomical position. This technique is quick (1–5 mins) and well tolerated, and is successful in over 80% of attempts;
• External rotation: This manoeuvre takes 5–10 minutes. The individual is lying on their back, with the elbow bent to 90 degress. The clinician holds the individual’s elbow and wrist, and very slowly the individual allows their arm to rotate outwards as the clinician guides the movement. Whenever pain or spasm is felt, the movement is stopped and the muscles are allowed to relax. The “clunk” associated with reduction using other techniques may not be evident. Reduction usually occurs once the arm is externally rotated between 70 and 110 degrees;
• Milch technique: If reduction using external rotation is unsuccessful, the Milch technique can be added. The fully outwardly rotated arm is taken out to the side into an overhead position, maintaining external rotation throughout. Gentle traction is applied in line with the arm bone, and there is direct pressure placed over the humeral head via the clinician’s thumb in the armpit. This technique is successful in over 86% of cases.
• Spaso technique: Gentle vertical traction and outward rotation is applied to the dislocated arm while the individual is lying on their back.

Other techniques have been described, but are considered complex, or associated with complications. Reduction is achieved when there is a “clunk” as the humeral head returns to the socket, and the individual is able to place the affected hand on the opposite shoulder. Reduction will be confirmed with an x-ray.

Immobilisation

The period of immobilisation following reduction is variable, though immobilisation in a standard sling has not been shown to effect the chance of recurrence. Immobilisation is usually achieved using a sling to maintain a position of internal rotation.
Recent reports have concluded that immobilisation in a position of external rotation of the humerus may result in lower rates of recurrent dislocation. The problem with this technique is the significant inconvenience the individual experiences by having their arm positioned in an externally rotated position for 24 hours per day. This concept requires further studying before it is adopted for routine use.
The period of immobilisation is followed by rehabilitation with physiotherapy targeted towards restoring the range of motion and improving the strength of the shoulder joint to maximise its stability.

Surgical management

Some people who dislocate their shoulder will require surgery in order to stabilise the joint. This is particularly the case for people whose joint is still unstable following conservative management as outlined above, and who have damage to the joint (such as a Hills–Sachs or Bankhart lesion). Surgery is associated with a 95% reduction in re-dislocation. The procedure may be performed as an open or a keyhole procedure, and there is no significant difference in terms of shoulder stability, re-injury, requirement for further surgery, or shoulder function between the two methods.

Shoulder dislocation and instability prevention

Nothing has been proven to reduce the chances of dislocating a shoulder for the first time. Once a shoulder has been dislocated, surgery to stabilise the joint has been shown to reduce the chances of it dislocating again. Physiotherapy assists in rehabilitating the shoulder after a dislocation, though it will not reduce the risk of it recurring.
 

Kindly reviewed by:
Dr John Trantalis
MBBS FRACS
Specialist Surgeon of Shoulder & Elbow Surgery at the Orthopaedic and Joint Replacement Centre, NSW; and Editorial Advisory Board Member of the Virtual Bone and Rheumatology Centres.

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