A bullet has lodged in the axilla of a man following a gun battle with the police. The bullet struck a bone, shattered, and completely severed the posterior cord of the brachial plexus. Which muscle(s) would not be affected by this injury?
a. latissimus dorsi
b. subscapularis
c. triceps brachii
d. extensor carpi radialis brevis
e. serratus anterior
f. all of the above would be affected
The answer is: e. serratus anterior
The key to this problem is knowing which nerves branch from the posterior cord. The pneumonic STARS may be useful here: The two S's refer to the upper and lower subscapular nerves which innervate subscapularis; the T refers to the thoracodorsal nerve which innervates latissimus dorsi; the A refers to the axillary nerve which innnervates the deltoid and teres minor; and the R refers to the radial nerve. In this case the posterior cord was completely severed by the bullet and thus all branches would be dysfunctional and the associated muscles paralyzed. Serratus anterior innervated by the long thoracic nerve would remain intact. The long thoracic nerve derives from the 5th, 6th and 7th cervical roots proximal to the formation of the trunks and as such, could be considered a pre-plexus structure and therefore would not be injured with posterior cord involvement. Clinically one would expect this patient to be able to protract the shoulder girdle with little or no difficulty.