Scalene muscles and the brachial plexus: anatomical variations and their clinical significance.

Anatomical variations may be clinically significant, but many are inadequately described or quantified. Variations in neck anatomy are important to surgeons performing surgical procedures in this region. Thirty-two female and 19 male adult cadavers were studied. The commonly described anatomical relationship of the brachial plexus (BP) lying between the anterior scalene (AS) and middle scalene (MS) muscles was found in only 60% of instances. Scalenus minimus was present in 46% of instances (bilateral in 14 cadavers). The most common variation was the penetration of the AS by the C5 and/or C6 ventral rami. The C5 and C6 roots may fuse before piercing AS (15% cases, bilateral in 4 cadavers), or the C5 root alone pierce the belly of AS (13% cases, bilateral in 3 cadavers). The roots also may pierce AS independently (6% cases, bilateral in 1 cadaver). In 3%, the C5 root was found to be completely anterior to AS.