Ulnar Nerve
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Direct answer
Failure to spread the fingers, a clawed little finger and a wasted first web space all name the ulnar nerve (C8, T1), the continuation of the medial cord and the chief nerve of the hand. It runs down the medial arm, passes behind the medial epicondyle in the groove every student has bumped, enters the forearm between the heads of flexor carpi ulnaris, and crosses into the hand superficial to the flexor retinaculum through Guyon's canal, lateral to the pisiform. Injury claws the fourth and fifth fingers and numbs the medial one and a half digits.
What you must remember
- Arm: medial to the brachial artery above mid-arm, then pierces the medial intermuscular septum to pass behind the medial epicondyle — no branches in the arm.
- Forearm: enters between the humeral and ulnar heads of flexor carpi ulnaris; supplies flexor carpi ulnaris and the medial half of flexor digitorum profundus; joined laterally by the ulnar artery in the lower two-thirds.
- Forearm cutaneous branches: dorsal branch (medial one and a half digits and dorsal hand) and palmar cutaneous branch (medial palm) — both arise above the wrist, escaping a Guyon's canal lesion.
- Hand entry: superficial to the flexor retinaculum in Guyon's canal, lateral to the pisiform, dividing into superficial sensory and deep motor branches — it does not pass through the carpal tunnel.
- Motor supply in the hand: all interossei, medial two lumbricals, adductor pollicis, deep head of flexor pollicis brevis, the three hypothenar muscles and palmaris brevis.
- Ulnar paradox: a wrist lesion claws the hand more than an elbow lesion, because the medial flexor digitorum profundus — paralysed only at the elbow — no longer flexes the interphalangeal joints of the clawed fingers.
- Bedside signs: clawing of ring and little fingers, positive Froment's sign, wasting of the first dorsal interosseous, and tingling on tapping behind the medial epicondyle.
Two lesions, compared properly
Compare a man who cut his wrist at the pisiform with a woman who fractured her medial epicondyle years ago. The wrist lesion spares the forearm — flexor carpi ulnaris, the medial profundus and both cutaneous branches still work — and only the intrinsic muscles die. The result is a marked claw: the lumbricals and interossei can no longer extend the interphalangeal joints of the fourth and fifth fingers, while the intact profundus flexes them and the long extensors hyperextend the knuckles. Adductor pollicis is paralysed, so pinching paper flexes the thumb's interphalangeal joint — Froment's sign, flexor pollicis longus substituting for the dead adductor.
The elbow lesion adds forearm weakness and subtracts clawing — the ulnar paradox. With the medial profundus paralysed, the distal interphalangeal joints lose their flexor, so the claw is paradoxically milder; flexor carpi ulnaris weakness shows as radial-deviated wrist flexion, and sensory loss extends up the dorsal medial forearm through the dorsal branch. Stating the paradox with its mechanism — the claw needs the profundus working — is the sentence that distinguishes a prepared candidate.
Where students slip
The ulnar nerve is sent through the carpal tunnel; it crosses in Guyon's canal, which is why carpal tunnel syndrome never numbs the little finger — a deduction examiners love. The paradox is recited backwards under pressure; anchor it by mechanism (low lesion, intact profundus, worse claw). The forearm branches are forgotten, so candidates expect dorsal numbness in a Guyon's canal lesion and are confused when it is absent. And the "one and a half" formula is over-applied: the nerve supplies one and a half forearm muscles and one and a half digits of sensation, but most of the hand's intrinsic muscles — the discrepancy between its small sensory and large motor territory is its whole clinical character.
Frequently asked questions
What is the root value of the ulnar nerve?
C8 and T1, from the medial cord — the same cord as the medial root of the median nerve.
How does the ulnar nerve enter the hand?
Superficial to the flexor retinaculum through Guyon's canal, lateral to the pisiform, dividing into superficial sensory and deep motor branches — not through the carpal tunnel.
Which muscles does the ulnar nerve supply?
Flexor carpi ulnaris and the medial half of flexor digitorum profundus; in the hand, all interossei, the medial two lumbricals, adductor pollicis, deep head of flexor pollicis brevis, the hypothenar muscles and palmaris brevis.
What is the ulnar paradox?
Clawing is more severe in wrist lesions than elbow lesions, because the medial flexor digitorum profundus — paralysed only in the elbow lesion — is needed to flex the interphalangeal joints of a fully developed claw.
What is Froment's sign?
While pinching a paper card, the affected thumb flexes at its interphalangeal joint: adductor pollicis paralysis forces flexor pollicis longus, a median-supplied muscle, to substitute for pinch.
Why is the little finger spared in carpal tunnel syndrome?
Because the medial one and a half digits travel through Guyon's canal, not the carpal tunnel; little-finger numbness points away from median nerve compression.