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Describe the course of the ulnar nerve and the features of its injury. Explain the ulnar paradox.

Anatomy · 1st Professional — Pre-Clinical

The ulnar nerve (C8, T1) is the largest branch of the medial cord of the brachial plexus.

Forearm — flexor carpi ulnaris and the medial half of flexor digitorum profundus (ring and little fingers) — the "one and a half muscles"
Hand — all the small muscles except the thenar group and the lateral two lumbricals: hypothenar muscles, all interossei, medial two lumbricals, adductor pollicis, and the deep head of flexor pollicis brevis
It is the chief nerve of the fine movements of the hand

The index and middle fingers are spared, because their lumbricals are median-supplied
Wasting of the hypothenar eminence and the dorsal interossei, with guttering between the metacarpals

Feature	Lesion at the elbow (high)	Lesion at the wrist (low)
FDP (medial half)	Paralysed	Intact
IP joint flexion	Weak	Strong
Clawing	Less marked	More marked
Sensory loss	Palmar and dorsal medial 1½ digits	Palmar only — the dorsal branch arises above the wrist

Sign	Basis
Froment sign	Adductor pollicis paralysed; the patient flexes the thumb IP joint (using flexor pollicis longus, median) to grip a sheet of paper
Card test	Inability to adduct the fingers (palmar interossei)
Egawa sign	Inability to abduct the middle finger side to side
Book test	Weak grip when holding a book between the fingers
Guttering	Wasting of the dorsal interossei, especially the first

The nerve is most often injured behind the medial epicondyle, where it is subcutaneous — hence the "funny bone" sensation on striking it
Cubitus valgus after a malunited supracondylar or lateral condyle fracture stretches the nerve, causing a tardy ulnar nerve palsy years later
Fracture of the medial epicondyle and dislocation of the elbow are common acute causes
At the wrist, injury occurs from a laceration or a ganglion in Guyon canal, or from prolonged cycling ("handlebar palsy")
Ulnar nerve injury is functionally serious, since the fine grip depends on the interossei; it is the nerve of skilled hand movement

All questions in Upper Limb