The Ulnar Nerve and Why It Matters for Your Hand

The ulnar nerve runs from the cervical spine, around the medial side of the elbow ('the funny bone'), through the wrist via Guyon's canal, and into the ring and small fingers and most of the small muscles inside the hand. Because the ulnar nerve innervates almost all the intrinsic hand muscles that produce grip and pinch strength, compression at any point along its course produces weakness, atrophy, and characteristic deformities far more dramatically than median nerve compression does. The three main compression sites are the cubital tunnel at the elbow (most common), Guyon's canal at the wrist, and rare digital nerve compressions in the ring and small fingers. Early treatment prevents permanent muscle loss; delay does not.

The Course of the Ulnar Nerve

The ulnar nerve carries fibers from the C8 and T1 nerve roots, joining the medial cord of the brachial plexus. From there it runs:

• Down the medial side of the upper arm, behind the medial intermuscular septum, deep to the triceps muscle.
• Behind the medial epicondyle (the bony bump on the inside of the elbow). This is where the nerve runs through the cubital tunnel, beneath Osborne's ligament. The "funny bone" sensation when you bump your elbow is the ulnar nerve being briefly compressed at this spot.
• Between the two heads of the flexor carpi ulnaris (FCU) in the proximal forearm, then deep to the FCU as it travels distally.
• Down the medial forearm, generally without incident.
• Through Guyon's canal at the wrist, a narrow space bordered by the pisiform bone, the hook of the hamate, and the volar carpal ligament. The nerve divides here into a superficial sensory branch and a deep motor branch.
• Into the hand, where the deep motor branch curves around the hook of the hamate to power the intrinsic hand muscles, and the sensory branch supplies the ring and small finger.

What the Ulnar Nerve Innervates

Motor in the forearm:

• Flexor carpi ulnaris (wrist flexion and ulnar deviation)
• Flexor digitorum profundus to ring and small fingers (fingertip flexion of those two fingers)

Motor in the hand (the intrinsic muscles):

• Hypothenar muscles (abductor digiti minimi, flexor digiti minimi, opponens digiti minimi) - the muscles at the base of the small finger
• Adductor pollicis (thumb adduction toward the palm)
• Deep head of flexor pollicis brevis
• All 7 interosseous muscles (finger spreading and crossing)
• Lumbricals 3 and 4 (MCP flexion of ring and small fingers)

This is most of the small muscle mass inside the hand. Loss of these muscles produces dramatic functional impairment.

Sensory:

• Palmar surface of the small finger and the ulnar (small-finger side) half of the ring finger
• Dorsal surface of the small finger and ulnar half of the ring finger via the dorsal cutaneous branch (which arises proximal to Guyon's canal; this is why dorsal hand sensation is preserved in Guyon's canal compression but lost in cubital tunnel compression)
• Ulnar side of the palm via the palmar cutaneous branch

The Three Main Compression Sites

1. Cubital tunnel (at the elbow). The most common ulnar nerve compression and the second most common upper extremity nerve compression overall (after carpal tunnel). The ulnar nerve runs through a tight bony groove behind the medial epicondyle, beneath Osborne's ligament. Activities that involve prolonged elbow flexion (sleeping with the elbow bent, holding a phone, leaning on the elbow) compress the nerve in this tight space. Symptoms: tingling and numbness in the ring and small fingers, hand weakness, and (in advanced cases) intrinsic atrophy. See full cubital tunnel article for details on diagnosis and treatment.

2. Guyon's canal (at the wrist). Less common than cubital tunnel. Compression is often caused by a ganglion cyst arising from the carpal bones, by a hook-of-hamate fracture, by repetitive pressure (cyclists who lean on the handlebars for hours, hammer use), or by anatomic variants. Symptoms: numbness and tingling in the ring and small fingers, intrinsic muscle weakness, but with preservation of dorsal hand sensation (because the dorsal cutaneous branch arose proximal to the canal). See full Guyon's canal article.

3. Digital nerve compressions (in the ring and small fingers). Rare but real. Compression of the ulnar digital nerve to the small finger from a tight ring or anatomic variant. Sometimes referred to as "bowler's thumb of the ulnar side" or "occupational neuritis." Treatment is usually conservative; surgical decompression is rarely needed.

Examination Signs of Ulnar Nerve Dysfunction

Froment's sign. The patient holds a piece of paper between the thumb and the radial side of the index finger. The examiner pulls the paper. Normal patients use the adductor pollicis (ulnar nerve) to grip. Patients with ulnar nerve weakness compensate by flexing the thumb tip with the flexor pollicis longus (median nerve), and the visible thumb-tip flexion gives the sign away.

Wartenberg's sign. The small finger drifts laterally (ulnar) compared to the ring finger when the patient extends the fingers. The drift is caused by unopposed action of the extensor digiti minimi (an ulnar abductor) when the third palmar interosseous (an ulnar adductor, also ulnar nerve innervated) is weakened. The sign is paradoxical: it looks like an ulnar deviation but is actually caused by ulnar nerve weakness.

Claw hand (ulnar claw). The ring and small fingers rest in extension at the MCP joints and flexion at the PIP and DIP joints, producing a clawed appearance. The deformity is caused by paralysis of the lumbrical muscles to those fingers, leading to unopposed extensor tendon action at the MCP and unopposed FDP action at the PIP and DIP. Paradoxically, low ulnar nerve injuries (at the wrist) produce more dramatic clawing than high injuries (at the elbow), because in high injuries the FDP to ring and small is also paralyzed, reducing the over-flexion of the distal joints.

Crossing fingers test. Ask the patient to cross the middle finger over the index finger ("good luck" position). This requires the interossei muscles, all ulnar nerve innervated. Inability to perform this maneuver suggests ulnar nerve weakness.

Intrinsic muscle atrophy. Visible flattening of the muscles between the metacarpals on the back of the hand (especially between the thumb and index finger) and flattening of the hypothenar eminence (the muscle bulk on the ulnar side of the palm). Atrophy is a late and ominous sign.

Why Early Treatment Prevents Permanent Atrophy

When a nerve is compressed long enough, the muscles it innervates begin to atrophy. After enough time, the muscle fibers become permanently non-functional even if the nerve is later decompressed and regenerates. The window for full recovery depends on:

• How long the nerve has been compressed. Symptoms present for less than 6 to 12 months almost always recover fully with appropriate treatment. Symptoms present for years often do not.
• How severe the compression is. Intermittent symptoms have a better prognosis than constant symptoms with weakness and atrophy.
• Where the compression is. The ulnar nerve's long path means that proximal compressions (at the elbow) take longer to recover after surgery than distal compressions (at the wrist). The nerve regenerates at about 1 mm per day, so a 30 cm regeneration takes about 10 months; the muscles need to still be capable of accepting reinnervation when the nerve fibers arrive.
• The patient's age and health. Younger patients regenerate nerves more reliably than older patients. Diabetes and other neuropathies impair recovery.

The take-home message for patients: hand weakness, intrinsic atrophy, or constant ring and small finger numbness are not "wait and see" symptoms. They warrant prompt evaluation by a hand and upper extremity surgeon. The cost of delay is permanent muscle loss that no amount of subsequent surgery can recover.

Frequently Asked Questions

Related Reading

• Cubital Tunnel Syndrome: the most common ulnar nerve compression.
• Guyon's Canal Syndrome: ulnar nerve compression at the wrist.
• Is It Cubital Tunnel or a Pinched Neck Nerve?: distinguishing nerve compression sites.
• The Median Nerve Explained: the companion anatomy article on the other major hand nerve.