Definition

Due to the significant wide range of movement (close to 360º) and complex anatomy, the shoulder is subject to a number of pathologies. A shoulder dislocation occurs when the humerus head is forced out of the shoulder socket - the glenoid.

Pathology

The humeral head is stabilised within the shoulder joint by the glenoid labrum, a fibro-cartilaginous rim surrounding the glenoid cuff that is attached to the tendon of the long head of the biceps muscle. A tear or a violent stretch of these structures causes the exit of the humeral head from the glenoid, making the shoulder highly unstable. A shoulder dislocation can be complete or partial. In complete dislocation the head of the humerus moves entirely out of the socket, requiring medical intervention, whereas in partial dislocation, also named subluxation, the head of the humerus slips out of the socket only temporarily to often returning into place spontaneously.

Classification

Anterior dislocation of the shoulder is by far the most common type of dislocation (>90%) causing inability to abduct the arm (raise the arm laterally). This occurs when the humeral head has moved to a position in front of the joint. Such dislocation is easily detected by clinical inspection due to the evident loss in the round contour of the shoulder. The most severe cases of anterior dislocation of the shoulder are associated with injury to the axillary artery and the axillary nerve.

Posterior dislocation is less common (<3%) and occasionally left unnoticed during medical examination. It occurs when the top of the humerus is displaced toward the back of the body out of the glenoid. This type of shoulder dislocation can be the consequence of a high-energy trauma and a fall due to seizures.

Superior dislocation is the least frequent type of dislocation (1%). It occurs when the humeral head is driven upward through the rotator cuff. It can be associated with fracture of the humerus, clavicle and acromion.

Causes

A shoulder dislocation is most often related to a strong force pulling the shoulder upwards, outwards or from an extreme external rotation of the humerus. The pathology is common in young men between 20 and 30 years and elderly women above 60 years of age. It can be caused by a fall, a force applied on the outstretched arm or by a direct impact on the shoulder area as it may happen in road traffic accidents. In sport, a shoulder dislocation may arise by a forceful throwing, lifting or hitting. A shoulder dislocation can also occur without trauma while performing simple movements such as outstretching the arm or turning over in bed when lying on the shoulder. This type of dislocation mostly involves both shoulders, does not cause major pain and is linked to excessive elasticity of the ligaments in some individuals resulting in joint laxity. It can also depend on the abnormality in the sequence in which the muscles of the shoulder are activated with movement. Some people can even dislocate the shoulder voluntarily.

Risk factors

A previous shoulder dislocation or subluxation predisposes to a second episode of the pathology particularly in young men (incidence of 80-90%). A particularly high risk is found in athletes involved in sports such as football, rugby, hockey and skiing due to the frequent contact impacts, throwing activities and falls. Congenital conditions causing loosening of the joints, such as Ehlers-Danlos Syndrome confer an intrinsic poor stability of the shoulder joint facilitating the exit of the humeral head. Weakness of the muscles around the shoulder and core muscles due to lack of training can predispose to a dislocation. Incorrect posture and inadequate sporting technique are all contributing factors to a shoulder dislocation.

Symptoms

Sudden, intense pain, deformity, instability and weakness of the shoulder are the characteristic symptoms indicative of a shoulder dislocation. These signs are associated with the inability to move the shoulder as well as possible local changes such as swelling and bruising. The external shape of the shoulder may appear altered with loss of normal anatomical contour. As a consequence of a dislocation the patient may feel numbness and tingling around the shoulder area, possibly extending to the arms and fingers.

Treatment

Nonoperative treatment

Upon a diagnosis of shoulder dislocation, a closed reduction is performed usually under anaesthesia in the Emergency Department. It consists of manual reposition the humeral head in the glenoid using different methods. This is followed by the immobilisation of the shoulder for approximately four weeks, aided by local treatment with ice and/or heat and non-steroidal antiinflammatory drugs (NSAIDs). Physiotherapy is recommended at a later stage.

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Surgical treatment

Surgery is performed if a closed reduction is not successful or when a traumatic dislocation is associated with injuries to the labrum (e.g. Superior Labral Tear from Anterior to Posterior also named SLAP tear) or glenoid (Bankart lesion), damage to the humeral head (Hill-Sachs lesion) or to the ligaments of the rotator cuff. These secondary pathologies produce significant shoulder instability and require surgical repair to prevent further dislocations. Various approaches are available including arthroscopic surgery and open surgery. After surgery the shoulder is immobilised for 3-4 weeks prior to commence physical therapy.

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