The most common nerve injured during an anterior shoulder dislocation is the axillary nerve. The axillary nerve branches off the posterior cord of the brachial plexus and C5 and C6 nerve roots. The nerve then courses anteriorly to the subscapularis and exits out the axilla posteriorly through the quadrangular space. The nerve then divides into the anterior, posterior, and articular branches. The anterior branch wraps around the surgical neck of the humerus beneath the deltoid muscle. The posterior branch innervates the teres minor and posterior deltoid muscle. The smaller articular branch enters the shoulder behind the subscapularis muscle and innervates the shoulder joint itself.1, 2
The mechanism of injury during a low velocity shoulder dislocation is traction on the axillary nerve wrapped around the humerus as the nerve is pulled forward beyond physiologic limits. Symptomatic axillary nerve injury occurs in about 10% of patients after an anterior shoulder dislocation. However, electromyography studies have reported much higher rates of nerve injury, with some studies approaching 55%. Symptoms of an axillary nerve injury include numbness and a dull achy pain over the lateral and posterior deltoid. A nerve injury should be suspected with any numbness to the lateral arm after a shoulder dislocation. In patients older than age 40 years who sustain a dislocation, it is important to know that weak arm abduction is more likely related to a torn rotator cuff than a nerve injury. Axillary nerve injuries have a very good prognosis with full recovery expected between 12 and 45 weeks. Electromyography studies are generally not warranted as the prognosis is straightforward. Physical therapy should be started early after the injury to help regain motion and strength of the shoulder.1
Dagan Cloutier, MPAS, PA-C, practices in a multispecialty orthopedic group in the southern New Hampshire region and is editor-in-chief of the Journal of Orthopedics for Physician Assistants (JOPA).
- Visser CP, Coene LNJEM, Brand R, Tavy DL. The incidence of nerve injury in anterior dislocation of the shoulder and its influence on functional recovery. J Bone Joint Surg Br. 1999;81:679-685.
- Cetik O, Uslu M, Acar HI, Comert A, Tekdemir I, Cift H. Is there a safe area for the axillary nerve in the deltoid muscle? A cadaveric study. J Bone Joint Surg Am. 2006;88;2395-2399.