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The most common mechanism of injury to the axillary nerve is closed trauma involving a traction injury to the shoulder,such as stretching of the nerve over the humeral head during an anterior shoulder dislocation. The incidence of axillary nerve injury has been reported to be between 19% and 55% following an anterior shoulder dislocation and up to 58% following proximal humeral fractures.

Several of these injuries may go unnoticed, because the pain of a shoulder fracture or dislocation may mask symptoms. Risk factors associated with axillary nerve injury after shoulder dislocation include advancing patient age at the time of dislocation (especially >40 years of age), length of time the shoulder is left unreduced (>12 hours), and amount of trauma required to cause the dislocation.

Otherwise, blunt trauma to the anterolateral aspect of the shoulder has been reported to be a possible cause of isolated axillary nerve injury (192). Traumatic causes that affect the nerve at the quadrilateral space include direct trauma to the posterior shoulder, traction injury to the upper extremity, and a deep posterior shoulder intramuscular injection.

In quadrilateral space syndrome, compression of posterior circumflex humeral artery and axillary nerve results from nontraumatic causes within confines of the quadrilateral space. Fibrous bands frequently have been seen during surgery within the quadrilateral space of patients with this problem. Unfortunately, the origin of these bands is unknown.

Other suggested causes for compression of these structures have been shearing forces between the teres major and teres minor, friction or irritation of the nerve as it passes around posterior glenoid, compression by a hypertrophied portion of subscapularis muscle that inserts onto the humerus just inferior to lesser tuberosity, dilated veins within the quadrilateral space, or a paralabral cyst. Overuse conditions of the shoulder causing repetitive microtrauma or abnormal biomechanics may lead to this pathologic syndrome.

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