Shoulder dislocation

Dr Jeremy Jones et al.

In a shoulder dislocation, there is separation of the humerus from the glenoid of the scapula at the glenohumeral joint.

The shoulder is exceptionally manoeuvrable and sacrifices stability to enable an increase in function. Therefore, it is vulnerable to dislocation, and is the most commonly dislocated large joint. In fact, approximately half of major joint dislocations seen in emergency departments are of the shoulder 1.

The shoulder almost always dislocates anteriorly (95-98% of the time) and in uncomplicated cases, analgesia, prompt reduction and immobilisation is all that is required. 

This article contains a general discussion on shoulder dislocation. For specific dislocation types please refer to the following articles:

Epidemiology

Sex distribution is bimodal and relative incidence is dependant on patient age. Younger patients tend to be male and are often related to sporting injury. Older patients tend to be female:

  • younger: 20-30 (male to female ratio of 9:1)
  • older: 60-80 (female to male ratio of 3:1)
Aetiology

Shoulder dislocation almost exclusively occurs following trauma. The shoulder is in its weakest position when it is abducted and externally rotated. Sporting injuries and motor vehicle collisions are common causes.

There is an increased incidence in patients who have had previous shoulder injury, and particularly in those who have dislocated previously.

Clinical presentation

Patients present with severe pain and restriction of movement of the shoulder. The majority of people who present with a shoulder dislocation do so after trauma, e.g. sporting trauma, assault, seizure, falls.

It is useful to determine whether the dislocation is acute, chronic or recurrent.

Mechanisms

A number of biomechanical forces can produce shoulder dislocation, including 1:

  • forced extension, abduction and external rotation
    • most common
    • leads to anterior dislocation
  • direct blow from behind
    • leads to anterior dislocation
  • forceful contraction of shoulder girdle muscles
    • uncommon
    • leads to posterior dislocation
  • forced hyperabduction 4
    • leads to inferior dislocation

Pathology

Shoulder dislocation occurs when the humeral head is traumatically displaced from the glenoid fossa of the scapula. The process of dislocation is massively disruptive to the labrum, joint capsule, supporting ligaments and muscles. This is particularly true in anterior dislocations where there can be injury to the anterior capsule, anterior labrum or biceps tendon.

Relevant anatomy

The glenoid is a saucer-shaped extension of the scapula. Its shape means that it offers limited bony support to the joint. The glenoid is augmented by the cartilaginous labrum with additional support from the joint capsule, surrounding ligaments and the muscles of the rotator cuff. The labrum, capsule and ligaments tend to be stronger in younger patients.

Relevant physiology

The glenohumeral joint is has a wide range of motion in multiple directions. While this is helpful for arm use, this flexibility means that the joint is relatively unstable. 

Type of dislocation

Shoulder dislocations are usually divided according to the direction in which the humerus exits the joint:

Radiographic features

X-rays (AP and lateral +/- axillary view) are sufficient in almost all cases to make the diagnosis, although CT and MR are often required to assess for the presence of subtle fractures of the glenoid rim or ligamentous / tendinous injuries respectively.

Plain film

Anterior and inferior dislocations are usually simple diagnoses, with the humeral head and outline of the glenoid being incongruent.

Where the humeral head is displaced medially and overlies the glenoid, the dislocation is anterior.

Posterior dislocations can be difficult to identify on an AP view only (as may be obtained in the setting of secondary survey of a trauma) as the humeral head moves directly posteriorly and congruency may appear to be maintained (at least at first glance).

All dislocations should be easily identified on trans-scapular Y views. When the humeral head is normally aligned, it will project centred over the centre of the Y formed by the coracoid, blade of the scapula and spine of the scapula (acromion).

Report checklist

In addition to reporting the presence of a dislocation a number of features and associated findings should be sought and commented upon:

It is also important to remember to scrutinise the ribs and portion of the lungs and mediastinum included in the film for unexpected findings. Think about the soft tissue structures that might be injured, particularly the neurovascular bundle with inferior dislocations.

Treatment and prognosis

The only treatment option for a dislocated shoulder is prompt reduction. This is usually performed in the Emergency Department following sedation and appropriate analgesia. A number of techniques can be used to reduce the shoulder.

The ease of reduction is dependant on the age and build of the patient (younger, heavily built guys will be more difficult to reduce) and the time that the joint has been dislocated (the longer it's out, the more difficult it is to get back in).

Rest is required following dislocation, so immobilisation is required: 3 weeks for younger patients (< 30 years old, who have a very high rate of recurrence) and 7-10 days in older patients. During this time gentle active motion should be carried out to preserve range of motion 4.

As a general rule, the shorter the duration of dislocation the fewer complications (size of Hill-Sachs lesion; neurovascular compromise, etc). 

Early arthroscopy, labral repair and debridement may be of use, especially in young patients with anterior dislocation in which there is a high (up to 85%) rate of recurrence 3.

Differential diagnosis

  • shoulder pseudo subluxation: apparent inferior displacement of the humeral head from capsular distension secondary to a haemarthrosis or large effusion

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