Background

Shoulder dislocations account for almost 50% of all joint dislocations. Most commonly, these dislocations are anterior (90-98%) and occur because of trauma. Most anterior dislocations are subcoracoid in location. Subglenoid, subclavicular, and, very rarely, intrathoracic or retroperitoneal dislocations may occur.

For patient education resources, see the Breaks, Fractures, and Dislocations Center, as well as Shoulder Dislocation.

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Epidemiology

Frequency

Shoulder dislocations account for almost 50% of all joint dislocations.

Etiology

The usual mechanism of injury is extreme abduction, external rotation, extension, and a posterior directed force against the humerus. Forceful abduction or external rotation alone can also lead to dislocation (about 30% of cases), as can a direct blow to the posterior humerus (29%), forced elevation and external rotation (24%), and a fall onto an outstretched hand (17%).^{[1, 2, 3]}

Posterior dislocations are less common (2-10%) and are the result of an axial load applied to the adducted and internally rotated arm. Classic posterior dislocations also occur as a result of electrocution or seizures because of the strength imbalance between the internal rotators (subscapularis, latissimus dorsi, pectoralis major muscles), which overpower the external rotators (teres minor and infraspinatus muscles).

Inferior dislocations are rare and result from a hyperabduction force that causes the humeral neck to lever against the acromion. Diagnosing inferior dislocations is critical because of the high incidence of complications. Neurologic injuries (particularly axillary nerve lesions) are associated with inferior dislocations in as many as 60% of cases, vascular injuries occur in about 3.3% of cases, rotator cuff tears in occur in 80-100% of cases,^[4] and greater tuberosity fractures and pectoralis major avulsions are also associated with inferior dislocations.

Superior dislocations are extremely rare and result from an extreme force in a cephalic direction to the adducted arm. Acromioclavicular injuries and fractures of the acromion, clavicle, and tuberosities may occur with superior dislocations.

Atraumatic instability is usually multidirectional and commonly occurs in individuals with generalized hyperlaxity due to connective tissue disorders, such as Ehlers-Danlos syndrome and Marfan syndrome. A small or flat glenoid fossa, excessive anteversion or retroversion of the glenoid, weak rotator cuff muscles, neuromuscular disorders, or a redundant capsule may also jeopardize the concavity-compression, adhesion-cohesion, or the glenoid suction-cup phenomena that aid in stability of the shoulder.

Multidirectional instability most commonly occurs in younger populations, usually in patients younger than 30 years, and is often familial and bilateral. The first dislocation often occurs after a minor injury or after a period of disuse. Patients may experience subluxations that progress over time to actual dislocations, which spontaneously reduce. These dislocations may be voluntary or involuntary. Voluntary dislocations have been associated with psychiatric illnesses and may be used in attention seeking behavior. Surgery should be avoided in this population because the instability is likely to recur.

Presentation

Patients with anterior dislocations usually present with the arm in slight abduction and externally rotated. The humeral head can often be palpated in the front of the shoulder. Internal rotation and adduction are limited. Movement is usually very painful as a result of muscle spasms.

Patients with posterior dislocations present with the arm internally rotated and adducted. External rotation is severely limited. A posterior prominence is usually palpable, the anterior shoulder is flattened, and the coracoid process is more prominent. Historically, these dislocations have been missed or misdiagnosed as a frozen shoulder.

Inferior dislocations lead to a condition known as luxatio erecta, which describes a classic presentation of the arm abducted 110-160° with the forearm resting on or behind the patient's head.^{[5, 6]}

Indications

Surgery may be indicated if patients are unable or unwilling to change their occupation or avoid participating in high-risk sports and if they have recurrent dislocations or subluxations.

Relevant Anatomy

See Surgical therapy.

Contraindications

Surgery should be avoided in patients with voluntary shoulder dislocations associated with psychiatric illnesses because the instability is likely to recur.

Workup