What causes shoulder dislocation?

Updated: Nov 29, 2018
  • Author: Sharon R Wilson, MD; Chief Editor: Trevor John Mills, MD, MPH  more...
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Answer

Anterior shoulder dislocations usually result from abduction, extension, and external rotation, such as when preparing for a volleyball spike. Falls on an outstretched hand are a common cause in older adults. The humeral head is forced out of the glenohumeral joint, rupturing or detaching the anterior capsule from its attachment to the head of the humerus or from its insertion to the edge of the glenoid fossa. This occurs with or without lateral detachment.

Posterior dislocations are caused by severe internal rotation and adduction. This type of dislocation usually occurs during a seizure, a fall on an outstretched arm, or electrocution. Occasionally, a severe direct blow may cause a posterior dislocation. Bilateral posterior dislocation is rare and almost always results from seizure activity. Misinterpretation of the radiograph appearance of a posterior dislocation may result in misdiagnosis as a soft tissue injury in up to 79% of cases.

Rare, but serious, inferior dislocations (luxatio erecta) may be due to axial force applied to an arm raised overhead, such as when a motorcycle collision victim tumbles to the ground. More commonly, the shoulder is dislocated inferiorly by indirect forces hyperabducting the arm. The neck of the humerus is levered against the acromion and the inferior capsule tears as the humeral head is forced out inferiorly. Luxatio erecta almost always has an associated fracture or soft-tissue injury. One series found 80% of patients to have fracture of the greater tuberosity or tear of the rotator cuff. Neurologic compromise was found in 60% of patients, with the axillary nerve the most commonly injured nerve. Inferior dislocations have the highest incidence (3.3%) of vascular compromise.


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