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We present a case of quadrilateral space syndrome (QSS) in a patient with left arm pain. The patient sustained a trauma to his left arm, and QSS was successfully diagnosed by physical examination, magnetic resonance image, electromyographic evaluation, and nerve conduction studies. Surgery was performed to decompress the axillary nerve and the patient recovered fully with minimal residual symptoms.

Background: Variations in the axillary nerve branching patterns have been reported. The aim of the study is to investigate the extra- and intra-muscular course of the axillary nerve and quantify the regional innervation of the deltoid.

Methods: In fresh frozen specimens, the origin of the axillary nerve from the posterior cord of the brachial plexus and its extra- and intra-muscular course were identified. Muscle dimensions, branching patterns and the distance from the axillary nerve origin to major branches were measured. The weights of muscle segments supplied by major branches of the axillary nerve were recorded.

Results: Twenty-three cadaveric dissections were completed. The axillary nerve bifurcated within the quadrangular space in all cases. The mean distance from the origin to bifurcation of the axillary nerve was 39 ± 13 mm; from axillary nerve bifurcation to the teres minor branch was 13 ± 6 mm; and from axillary nerve bifurcation to the middle branch of anterior division was 26 ± 11 mm. The nerve to teres minor and superior lateral brachial cutaneous nerve originated from the posterior division or common trunk in all cases. No fibrous raphe were identified separating anterior, middle and posterior deltoid segments. The anterior division of axillary nerve supplied 85 ± 4% of the deltoid muscle (by weight). The posterior division supplied 15 ± 4% of the deltoid muscle (by weight). The posterior deltoid was supplied by both anterior and posterior divisions in 91.3% of cases.

Conclusions: This study demonstrates a consistent branching pattern of the axillary nerve. The anterior division of the axillary nerve innervates all three deltoid segments in most instances (85% of the deltoid by weight). This study supports the concept of re-innervation of the anterior division alone in isolated axillary nerve injuries.