The superior (upper) trunk forms where the C5 and C6 roots join near the lateral border of the anterior scalene, close to a surface landmark called Erb point. It crosses the posterior triangle of the neck and gives off the suprascapular nerve and the nerve to subclavius before splitting into anterior and posterior divisions behind the clavicle.
| Origin | Union of the C5 and C6 roots |
|---|---|
| Course | Runs downward and laterally across the posterior triangle of the neck above the middle trunk and divides behind the clavicle into anterior and posterior divisions |
| Motor |
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| Sensory |
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The superior trunk is the part of the plexus most often injured when the head and shoulder are forced apart, as in shoulder dystocia at birth or a fall onto the shoulder. The result is Erb palsy: the arm hangs adducted and internally rotated with the elbow extended and the forearm pronated (the waiter tip posture). Brief traction or compression of the upper trunk causes the burning arm pain of a stinger or burner in contact sports.
Traction injury of C5-C6 or the superior trunk paralyses the deltoid, rotator cuff, biceps and brachialis, producing the waiter tip posture. Many birth injuries recover spontaneously, but those without biceps recovery by about 3-6 months are considered for surgery.
A sudden blow that depresses the shoulder and bends the neck to the opposite side stretches the upper trunk, causing seconds to minutes of burning pain and weakness down the arm. Recurrent or prolonged episodes need assessment before return to play.
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