Introduction

Shoulder dislocations occur when the humeral head dislodges from the glenoid cavity of the scapula. The shoulder is the most common joint in the body to dislocate, accounting for approximately 50% of all major joint dislocations. This is a common condition, particularly in males, with an incidence of 40 per 100,000 person-years, observed most frequently in males aged 16-20 years old.

Classification

Shoulder dislocations are most commonly classified into three categories, depending on the direction in which the dislocation has occurred. The direction refers to the location of the humeral head in relation to the glenoid cavity. The majority of shoulder dislocations are anterior (>95%), followed by posterior (4%) and inferior dislocations (1%).

Anterior
  • In anterior dislocations, the humeral head lies anterior to the glenoid fossa
  • These dislocations are divided into three categories: subcoracoid, subglenoid and subclavicular
  • Subcoracoid
    • These account for the majority of anterior dislocations
    • Lie anterior to the glenoid fossa but inferior to the coracoid process
  • Subglenoid
    • Account for 1/3 of anterior dislocations
    • The head of the humerus lies anterior and below the glenoid fossa
  • Subclavicular
    • The rarest form of anterior dislocation
    • Humeral head lies medial to the coracoid process, and inferior to the lower clavicle border

Posterior
  • Account for only 2-4% of dislocations
  • Humeral head will lie posterior to the glenoid fossa

Inferior
  • Account for less than 1% of dislocations
  • In inferior dislocations, the humeral head lies directly inferior and slightly medial to the glenoid fossa
  • This can appear quite similar to a subglenoid anterior dislocation, as they both lie inferior and medial. However, in an inferior dislocation, the humeral shaft's position will be parallel to the scapular spine, and this is the key differentiating factor

Epidemiology

  • Incidence: 24.00 cases per 100,000 person-years
  • Peak incidence: 20-30 years
  • Sex ratio: more common in males 2.5:1
<1 1-5 6+ 16+ 30+ 40+ 50+ 60+ 70+ 80+

Aetiology

The shoulder joint is the most likely joint in the body to dislocate due to an anatomical predisposition, where the head of the humerus is larger than the shallow glenoid fossa on the scapula. Although this allows the shoulder to be incredibly mobile and manoeuvrable in multiple positions, it certainly compromises the stability of the joint.

Certain people are more prone to shoulder dislocations than others, including:
  • Prior rotator cuff tears
  • Those with an anatomically loose joint capsule
  • Previous damage to the glenohumeral ligament

Anterior dislocation
  • Mechanism of injury is normally a force to an abducted, externally rotated and extended arm
  • More rarely, these can be caused by a force to the posterior side of the humerus, or a fall on an outstretched arm
  • Common causes include high-energy sporting collisions and falls

Posterior dislocation
  • The mechanism of injury is usually a force to the anterior surface of the shoulder, or axial loading of an adducted and internally rotated arm
  • The most common causes can be remembered using the '3 E's':
    • Epilepsy - seizures are the most common cause. The dislocation occurs either from a fall itself or from strong muscular contractions that may occur during the clonic phase
    • Electrocution
    • Ethanol - typically following a fall

Inferior dislocation
  • The mechanism of injury is usually a very high energy injury, commonly a due to a hyperabduction impact which forces the humeral head onto the acromion process, pushing the humeral head into the infraglenoid region

Clinical features

The clinical features tend to vary depending on the type of shoulder dislocation which has been sustained, however there are a few features which are common across all classifications. These include:
  • Significant shoulder pain
  • Inability and hesitancy to move shoulder and arm
  • History of fall or traumatic event
  • In very slim individuals, it may be possible to palpate a dent in the glenoid fossa where the humeral head normally lies
  • If axillary nerve damage occurs as a result of dislocation (in approximately 40% of cases), clinical symptoms might include:
    • Loss of normal sensation in the lateral surface of the shoulder region, known as the 'regimental badge' area
    • Weakness of the deltoid muscle (weak arm abduction)

Anterior dislocation
  • Arm position:
    • Slightly abducted
    • Externally rotated
  • Acromion process may appear prominent, particularly in slim individuals
  • Loss of normal rounded appearance of shoulder
  • It may be possible to palpate the humeral head below the position of the coracoid process

Posterior dislocations
  • Arm position:
    • Adducted
    • Internally rotated
  • The posterior shoulder will appear much more prominent than usual
  • The anterior shoulder will appear more flattened than usual
  • Prominent coracoid process
  • Inability to externally rotate, both mechanically and due to pain

Inferior dislocations
  • Arm position:
    • Arm held above the head
    • Fixed, abducted position
  • Inability to actively adduct the arm

Investigations

In suspected shoulder dislocations, usually the only investigation required is plain radiography. However, there are certain circumstances where computed tomography scans may also be necessary.

Plain X-ray
  • X-ray view required include:
    • Antero-posterior
    • Scapular 'Y' view
    • Axillary view
  • Anterior dislocation
    • The humeral head will lie out of the glenoid cavity, usually in a subcoracoid position
    • In a scapular 'Y' view, the humeral head will appear medial to the 'Y'(the 'Y' being formed by the body, spine and coracoid process)
  • Posterior dislocation
    • Not as obvious as anterior dislocations, may be subtle and therefore missed in approximately 50% of cases
    • Absence of external rotation of the arm
    • 'Lightbulb sign' - circular appearance of humeral head due to fixed internal rotation of humerus
    • 'Rim sign' - widened glenohumeral joint space >6mm between medial aspect of the humeral head to anterior glenoid rim
    • 'Trough line sign' - two vertical or arch-like parallel lines of cortical bone seen on medial humeral head
    • Axillary views are the best for posterior dislocations, as 'Y' views are unreliable for diagnosis
  • It is important to also obtain plain X-ray after reduction of the dislocated joint to ensure a successful reduction has been attained, as well as exclude any concurrent fractures caused by the injury (particularly surgical neck of humeral head fractures)

Computed tomography
  • CT scans are not routinely indicated for suspected anterior dislocations
  • May be required in suspected posterior dislocations, particularly if the presence of a dislocation is in question on X-ray, or to determine the size of a concurrent reverse Hills-Sachs lesion
  • UptoDate also suggest that CT scans should be done when the humeral head location is in-determinant on X-ray, or when a CT angiogram is required due to signs of a potential axillary artery injury

Management

The main management for any type of shoulder dislocation is reduction of the joint in a timely manner. The only contraindication to immediate reduction of the joint is if there is concern about causing vascular injury, this is particularly pertinent in elderly patients who present sub-acutely (7-10 days post injury). In these scenarios, it is important to obtain orthopaedic consultation as open reduction may be performed instead.

Analgesia and sedation
  • UptoDate recommend that analgesia and/or sedation should not be used unless necessary, as reduction can be generally achieved if the dislocation is recent (<24 hours), recurrent, or somewhat non-traumatic in nature.
  • If required, common medications used include fentanyl, midazolam, propofol and ketamine
  • UptoDate recommend using intra-articular lidocaine if there is a risk of complications from procedural sedation, such as in the elderly with multiple co-morbidities

Shoulder joint reduction
  • Anterior dislocation
    • There is no evidence to suggest that one technique is better than the other, it is dependant on physician preference
    • UptoDate suggest that the 'upright' technique is preferred, where the patient sits upright and gentle downward traction is applied to the arm, whilst another person gently rotates the scapula from behind
    • Another popular technique is the 'prone' technique, where the arm hangs off the side of the bed and 5-10kg of weight are hung off the arm to provide traction
  • Posterior dislocation
    • Reduction involves applying axial traction to an adducted arm with the elbow flexed
    • As the axial traction is applied, the arm should be internally rotated and adducted, to relocate the shoulder joint
    • If unsuccessful, open reduction procedures may be required
  • Inferior dislocation
    • Reduction involves applying traction-countertraction in line with the humerus, whilst in an adducted position
    • The gradual adduction of the humerus tends to reduce the joint dislocation

Post-reduction X-ray
  • It is important to obtain an anteroposterior and lateral x-ray after reduction techniques have been performed
  • This will both confirm that the humeral head has reduced back into the glenoid fossa, as well as to ensure there are no fractures present

Surgical referral
  • Usually, surgical referral for open reduction is not required unless reduction procedures are unsuccessful
  • BMJ best practice recommend that surgical referral should be provided for patients under 25-years-old, because these groups are likely to have ongoing shoulder instability and therefore recurrent dislocations

Complications

Unfortunately, the most common complication of a shoulder dislocation is subsequent and recurrent shoulder dislocations. This tends to be as a result from ongoing shoulder joint instability. Other complications such as nerve damage are rare.

Shoulder instability
  • With each subsequent shoulder dislocation, there is an ongoing predisposition to develop further dislocations
  • This is caused by stretching, tearing or detachment of shoulder joint structures (including ligaments, the glenoid labrum or capsule)
  • May result in recurrent subluxation (partial dislocation) or complete dislocation

Axillary nerve damage
  • A rare complication of shoulder dislocations themselves, as well as shoulder dislocation reduction techniques
  • Most commonly injured in anterior dislocations
  • According to UptoDate, a degree of axillary nerve injury is present in approximately 40% of patients, but usually will recover without any intervention or surgery
  • More common in patients with inferior shoulder dislocations, and in the elderly
  • Clinical features
    • Loss of normal sensation in the lateral surface of the shoulder region, known as the 'regimental badge' area
    • Weakness of the deltoid muscle (weak arm abduction)
  • Management
    • Conservative initially with rest and physiotherapy
    • Subsequent nerve conduction studies ~1 month after the injury
    • Surgery may be necessary, and UptoDate suggest this is best done within 3-6 months after the injury

Hills-Sachs lesion
  • A Hills-Sachs lesion, or Hills-Sachs fracture, is a cortical depression in the posterolateral humeral head
  • This lesions is caused by the forceful impaction of the head of the humerus against the glenoid cavity rim when the shoulder dislocates
  • Much more common in anterior shoulder dislocations (35-40%)

Bankart lesion
  • A Bankart lesion is a tear of the anterior and inferior glenoid labrum
  • Similar to Hills-Sachs lesions, these are also more common in anterior shoulder dislocations
  • Management is usually conservative
    • Rest
    • Immobilisation
    • Physiotherapy
    • Very rarely, surgery may be required to re-attach the torn portion of the glenoid labrum