A 28-year-old man sustains a motorcycle injury with a traction injury to the right upper limb. Examination reveals loss of shoulder abduction, loss of sensation over the lateral shoulder, and inability to externally rotate the shoulder. All of the following nerves are likely damaged in this injury EXCEPT:
Loss of shoulder abduction → Supraspinatus (suprascapular nerve) + Deltoid (axillary nerve) paralysis
2.
Loss of sensation over lateral shoulder → Axillary nerve sensory loss (lateral brachial cutaneous nerve)
3.
Loss of external rotation of shoulder → Infraspinatus (suprascapular nerve) + Teres minor (axillary nerve) paralysis
This pattern is classic for an upper trunk (C5–C6) traction injury — i.e., Erb's palsy — which damages nerves arising from or passing through the upper trunk.
Nerve-by-Nerve Analysis
Axillary Nerve (Posterior Cord, C5–C6) — DAMAGED:
Innervates deltoid (shoulder abduction) and teres minor (external rotation)
Provides sensory supply to lateral shoulder via the lateral brachial cutaneous nerve
Directly explains the abduction loss and lateral shoulder sensory loss
Innervates biceps brachii and brachialis (elbow flexion) and coracobrachialis
Provides sensory supply to lateral forearm (lateral antebrachial cutaneous nerve)
In classic Erb's palsy (C5–C6 upper trunk injury), the musculocutaneous nerve is characteristically involved, producing loss of elbow flexion — the "waiter's tip" posture
The stem does not explicitly state elbow flexion is intact; in a true upper trunk traction injury, musculocutaneous nerve damage is expected
Radial Nerve (Posterior Cord, C5–C8, T1) — LEAST LIKELY DAMAGED:
Innervates wrist extensors, finger extensors, triceps, and brachioradialis
Provides sensory supply to dorsal hand and posterior forearm
The radial nerve carries significant C7–C8 contributions; an isolated upper trunk (C5–C6) traction injury does not typically damage the radial nerve
Wrist drop and loss of finger extension are not features of Erb's palsy
The radial nerve is the nerve least likely to be damaged in this clinical scenario
Musculocutaneous nerve → damaged (C5–C6 contribution via lateral cord)
Radial nerve → NOT primarily damaged (predominantly C7–C8; posterior cord involvement requires a much more extensive injury than described)
Key Point
The radial nerve is the EXCEPT answer because it is not a C5–C6 nerve and is not characteristically injured in the upper trunk traction injury pattern described. (Gray's Anatomy; Aids to the Examination of the Peripheral Nervous System, Guarantors of Brain)
Clinical Pearl
Erb's palsy (upper trunk C5–C6 injury) classically presents with loss of shoulder abduction, external rotation, and elbow flexion — the "waiter's tip" posture. The radial nerve (C5–C8, T1) is spared unless the injury extends to involve the entire posterior cord or lower trunk.
