Ulnar Nerve Anatomy

The ulnar nerve is the nerve of fine manipulation. It powers the intrinsic muscles behind grip strength, pinch and complex finger movements, and power grip relies heavily on ulnar-innervated intrinsic function to stabilise the MCP joints. The profound impact of this innervation is seen in the intrinsic-minus hand.

Where it is caught. The cubital tunnel at the elbow is the commonest site of compression, and Guyon's canal at the wrist is the second.

Motor supply. In the forearm the nerve supplies flexor carpi ulnaris and the ring and little finger portions of FDP, one and a half muscles between them, which flex the wrist and digits 4 and 5. In the hand it supplies most of the intrinsics:

• Hypothenar muscles - abductor, flexor and opponens digiti minimi
• Interossei - the palmar interossei (3), which adduct, and the dorsal interossei (4), which abduct
• Lumbricals 3 and 4, the medial two
• Adductor pollicis, the muscle Froment's sign tests
• Deep head of flexor pollicis brevis

Sensory supply comes through three branches:

• Palmar cutaneous branch - arises proximal to the wrist and supplies the hypothenar eminence
• Dorsal cutaneous branch - arises about 5 cm proximal to the wrist, passes dorsal to FCU and supplies the dorsal medial hand
• Superficial (terminal) branch - supplies the volar little finger and the ulnar half of the ring finger

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Axilla and arm. The ulnar nerve is a terminal branch of the medial cord (C8, T1) and often receives C7 fibres from the lateral cord. It lies medial to the axillary and brachial arteries and runs at first in the anterior compartment. At mid-arm it pierces the medial intermuscular septum into the posterior compartment and runs distally towards the groove behind the medial epicondyle. This proximal course is relatively safe from compression.

The elbow. The nerve passes posterior to the medial epicondyle in the retrocondylar groove and enters the cubital tunnel, a fibro-osseous tunnel, which it leaves to enter the forearm between the humeral and ulnar heads of flexor carpi ulnaris.

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The boundaries of the cubital tunnel:

• Roof - the cubital tunnel retinaculum (Osborne's, or arcuate, ligament), taut in flexion
• Floor - the MCL (ulnar collateral ligament), its posterior bundle, and the capsule
• Anterior - medial epicondyle
• Posterior - medial head of triceps
• Lateral - olecranon

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Why flexion matters. Elbow flexion increases intraneural pressure, elongates the nerve by 4-7 mm and reduces the volume of the tunnel by 50%. Hence splinting in extension is effective, and this dynamic compression explains why symptoms are worse at night.

Five sites of compression. From proximal to distal, these are the layers a complete decompression releases:

1. Arcade of Struthers - a band of fascia about 8 cm proximal to the medial epicondyle; a potential compression site, especially after transposition
2. Medial intermuscular septum, where the nerve passes through it
3. Cubital tunnel retinaculum (Osborne's ligament)
4. Aponeurosis of FCU, as the nerve enters between the two heads
5. Deep flexor-pronator aponeurosis, as the nerve exits

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The forearm. Beyond FCU the nerve lies on FDP and is joined by the ulnar artery in the distal two-thirds, then gives off the dorsal cutaneous branch.

Guyon's canal. A fibro-osseous tunnel at the wrist containing the ulnar nerve and ulnar artery, the nerve lying on the ulnar (medial) side of the artery:

• Roof - volar carpal ligament
• Floor - transverse carpal ligament (flexor retinaculum) and pisohamate ligament
• Medial wall - pisiform
• Lateral wall - hook of hamate

Within the canal the nerve divides into a superficial sensory branch and a deep motor branch. The deep branch turns around the hook of hamate, which explains its vulnerability to a hook fracture, a ganglion and repetitive handlebar pressure. The level of a lesion relative to this bifurcation defines the three zones of the Akahori classification.

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Martin-Gruber anastomosis. Crossover fibres run from the median to the ulnar nerve in the forearm in 15% of the population, so the ulnar intrinsics are innervated by median fibres. In a high ulnar lesion hand function may be surprisingly preserved (pseudo-preservation), and in median lesions ulnar signs appear. The anomaly can confuse clinical and EMG findings.

Riche-Cannieu anastomosis. Its hand-level counterpart is a communication in the palm between the recurrent (motor) branch of the median nerve and the deep (motor) branch of the ulnar nerve. It is common in anatomical studies and, when extensive, can produce an all-ulnar hand, in which the ulnar nerve effectively supplies all the intrinsic muscles, including the thenar group.

Inspection. Look for wasting of the hypothenar eminence and of the first dorsal interosseous on the dorsum of the first web space. Then look at the ring and little fingers for clawing, which is hyperextension of the MCP joints with flexion of the IP joints.

The ulnar paradox. A high lesion claws less than a low one. In a low lesion (wrist) FDP is intact, and its unopposed flexion of the IP joints produces severe clawing. In a high lesion (elbow) the FDP to the ring and little fingers is paralysed as well, so the IP joints do not flex and the claw deformity is masked.

Wartenberg's sign. The little finger sits abducted, from weakness of the palmar interossei (adduction) and the unopposed action of EDM (radial nerve) and abductor digiti minimi, which is ulnar-supplied but often spared or less affected relative to mechanics.

Froment's and Jeanne's signs. Froment's sign tests adductor pollicis. The patient grips a sheet of paper between the thumbs; if adductor pollicis is weak, the patient flexes the thumb IP joint with FPL (median nerve) to compensate, and that flexion is the positive sign. Jeanne's sign is hyperextension of the thumb MCP joint during pinch (instability).

Provocation tests.

• Tinel's sign - tap over the cubital tunnel and Guyon's canal
• Elbow flexion test - the patient flexes the elbow fully with the wrist extended and holds it for 60 seconds; reproduction of symptoms is positive
• Scratch collapse test - its sensitivity is controversial

Localising the level. The dorsal cutaneous branch leaves before Guyon's canal, so dorsal ulnar sensation is lost in cubital tunnel and other high lesions and often spared in Guyon's canal and wrist lesions.

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Differential diagnosis. A lower trunk plexopathy can mimic an ulnar nerve lesion.

Other causes of clawing. Ulnar nerve palsy is the classic cause of a claw hand, but the differential also includes leprosy (a thickened nerve), neuritis (viral or autoimmune), amyotrophy (diabetic or neuralgic) and C8/T1 radiculopathy.

Radiographs. Look for bone spurs, a cubitus valgus deformity, a supracondylar spur (rare) or, at the wrist, a hook of hamate fracture.

Ultrasound. A cross-sectional area over 10 mm² suggests compression, and the scan can show nerve instability (subluxation) in real time. Its sensitivity is only modest (Pompe and Beekman), so a normal scan cannot exclude disease.

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MRI. Shows space-occupying lesions, such as a ganglion cyst in Guyon's canal.

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Nerve conduction studies and EMG. Neurophysiology is the gold standard for grading severity.

• Conduction velocity - slowing across the elbow to under 50 m/s, or an absolute drop of over 10 m/s compared with the forearm
• EMG of FCU - denervation suggests the lesion is at or above the elbow; a normal FCU suggests a distal lesion, or a very mild one
• Guyon's canal - prolonged distal latency

Conservative care is for mild symptoms (McGowan I) and intermittent paraesthesia:

• Night splinting that prevents elbow flexion over 45 degrees
• Activity modification - avoid resting the elbows on tables, and headset use
• NSAIDs as an adjunct

It is about 50% effective in mild cases, and patient compliance is key. The Cochrane review puts the splint in its place: in mild disease, advice on avoiding provocative positions improved symptoms, and adding splinting or nerve gliding gave no further benefit. The advice is therefore the core of conservative care.

When to operate. There is no agreed cut-off for how long to persist with conservative care, and no precise electrodiagnostic threshold that mandates surgery. The pragmatic consensus is to operate for:

• Failure of 3-6 months of conservative nerve-protection measures
• Motor weakness (McGowan II/III), progressive motor loss or wasting
• Persistent constant numbness

The operations.

1. In situ decompression - simple release of Osborne's ligament; success 80-90%
2. Anterior subcutaneous transposition - the nerve is moved anterior to the epicondyle and secured with a fascial sling
3. Anterior submuscular transposition - the nerve is placed under the flexor mass; good for revision or very thin patients
4. Medial epicondylectomy - removes the bony prominence, with a risk of MCL injury and instability

Choosing. For primary compression, multiple meta-analyses show no difference in outcome between decompression and transposition, and for idiopathic disease the RCTs and the Cochrane review agree, with fewer complications after decompression. Decompression is also faster, less invasive and preserves vascularity, so it is standard for primary simple cases. Transposition is indicated for a subluxating nerve, valgus deformity, stricture or scarring, and revision cases, and is often indicated when the bed is irregular (arthritic osteophytes). Beyond that, the choice depends on surgeon preference and pathology, and whether subluxation alone justifies transposition is disputed (see Controversies).

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Set-up. Mark the incision and confirm the symptomatic side before starting, to avoid wrong-site surgery.

• Position - supine on an arm board, tourniquet high on the arm
• Anaesthesia - general or regional (block)
• Equipment - loupes or microscope (optional but recommended) and a nerve stimulator
• Instruments - basic plastic set, tenotomy scissors, vessel loops

In situ decompression.

1. Incision - a short curvilinear posteromedial incision over the cubital tunnel, centred on the medial epicondyle. Identify and protect the MABCN (medial antebrachial cutaneous nerve) branches.
2. Release - expose the nerve without circumferential devascularisation and release every constricting layer proximally and distally: all five sites, from the FCU aponeurosis between the two heads and the deep flexor-pronator aponeurosis distally, through Osborne's ligament over the tunnel, to the arcade of Struthers and medial intermuscular septum proximally.
3. Stability check - flex the elbow after release. If the nerve subluxes, transpose; if not, close.

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Anterior submuscular transposition. The technique is robust for recurrent cases.

1. Extended exposure - extend the incision proximally and distally.
2. Mobilise the nerve - release proximally to the arcade of Struthers and distally deep into the forearm, and excise the medial intermuscular septum completely.
3. Prepare the bed - elevate the flexor-pronator mass from the medial epicondyle.
4. Transpose - place the nerve anterior to the epicondyle, deep to the muscle mass.
5. Reattach - repair the flexor origin.

After decompression, move early in a soft dressing to prevent stiffness, and avoid direct pressure. After transposition, immobilise for 1-2 weeks to allow the nerve's new position to stabilise, then begin range of motion.

The order of recovery. Sensory recovery typically precedes motor: paraesthesia resolves first, then strength returns, and sensation may take months.

Severity decides. Severity at presentation is the single most important determinant of recovery. Motor recovery is unpredictable in severe cases (McGowan III): "time is muscle". Established (pre-operative) intrinsic atrophy is a poor prognostic sign and rarely reverses fully. Other factors:

• Age - patients over 50 have poorer outcomes
• Duration - symptoms for over 1 year correlate with incomplete recovery, and intrinsic function may not fully return in elderly or long-standing cases
• Site - distal (wrist) lesions reinnervate faster than proximal (elbow) ones, but intrinsic demand is high

Global Epidemiology

• Cubital tunnel syndrome is the second most common compressive (entrapment) neuropathy of the upper limb, after carpal tunnel syndrome.
• Reported population incidence is approximately 20-30 per 100,000 person-years, with a male predominance and a peak in the 5th-6th decades.
• Strong associations: prolonged or repetitive elbow flexion, prolonged direct pressure on the elbow, diabetes, smoking, manual/vibrating-tool occupations and prior elbow trauma (tardy ulnar palsy after cubitus valgus).
• Guyon's canal (ulnar tunnel) syndrome is far less common; ganglion cyst is the single most frequent space-occupying cause, followed by hook-of-hamate pathology and ulnar artery aneurysm/thrombosis (hypothenar hammer syndrome).

Side-by-Side Guideline Comparison

Registry & High-Volume Evidence

• There is no large dedicated international registry for peripheral nerve decompression equivalent to the joint-replacement registries (NJR, AJRR, AOANJRR), so practice rests on RCTs and systematic reviews rather than registry survivorship data.
• Large administrative and insurance datasets consistently show cubital tunnel surgery is among the most frequently performed elective hand operations worldwide, with low but non-trivial revision rates (incomplete release and persistent symptoms being the leading reasons for reoperation).

High- vs Limited-Resource Practice Variation

• High-resource settings: ready access to nerve conduction studies/EMG and high-resolution ultrasound for diagnosis and lesion localisation; endoscopic in-situ release available in selected centres; day-case surgery under regional or local anaesthesia.
• Limited-resource settings: diagnosis is predominantly clinical (Tinel's, elbow flexion test, Froment's, pattern of wasting) with selective or no electrodiagnostics; open simple decompression under local anaesthesia is the pragmatic default — low-cost, effective, and avoids dependence on imaging or specialised endoscopic equipment.
• Globally, the threshold to operate is driven by motor involvement and failure of conservative measures, not by access to advanced imaging.

Ulnar tunnel syndrome. Compression in Guyon's canal is localised by Akahori's zones (under Classification). The causes:

• Ganglion cyst - the most common, at 50%
• Hook of hamate breakdown (the golfer)
• Ulnar artery aneurysm (hammer syndrome)
• Cyclist's palsy

Treatment is decompression of the tunnel and treatment of the pathology, removing a cyst where one is found.

The Bouvier test. It determines whether a claw hand is simple or complex, and so whether an intrinsic transfer is needed. The examiner blocks MCP hyperextension, mimicking the intrinsics.

• Positive (simple claw) - the patient can extend the IP joints using EDC, so EDC and the extensor mechanism are intact. Treatment prevents MCP hyperextension (Zancolli, capsulodesis).
• Negative (complex claw) - the patient cannot extend the IP joints despite the MCP block, indicating a fixed flexion contracture or an incompetent extensor mechanism. It needs more than a simple static block, for example a dynamic transfer.

The cause. Tardy ulnar nerve palsy is a delayed ulnar neuropathy at the elbow that develops months to years after an old elbow injury. The classic cause is a childhood lateral condyle fracture that goes to nonunion or malunion and produces a progressive cubitus valgus deformity; the increased carrying angle stretches and tethers the ulnar nerve over the medial epicondyle, causing a slow traction and friction neuropathy. Supracondylar malunion, old distal humeral fractures and chronic elbow osteoarthritis can do the same.

The presentation is a gradual ulnar neuropathy, with paraesthesia in the ulnar digits, intrinsic weakness and wasting, and clawing, appearing long after an injury the patient may have forgotten. The key examination clue is a valgus elbow with a positive Tinel's sign over a nerve that is often palpably subluxing or thickened.

Treatment. Because the nerve sits in a deformed and often scarred bed, anterior transposition (subcutaneous or submuscular) is generally preferred over simple in situ decompression, sometimes alongside correction of the underlying deformity.

The subluxating nerve. This is where the decompression-versus-transposition debate is still genuine. Bartels found decompression effective even with subluxation, yet many surgeons still transpose an unstable nerve to avoid a mobile, vulnerable target.

Endoscopic or open release. Systematic-review data favour endoscopic release on outcomes and complications, but the evidence is non-randomised and learning-curve sensitive. Open in situ release remains the global default, and endoscopy is centre- and surgeon-dependent.

Routine imaging. Whether ultrasound or MRI should be routine rather than selective is unsettled. Many advocate reserving imaging for atypical, recurrent or mass-suspicious cases.