Ulnar Nerve Anatomy

Motor Innervation Summary

• Forearm: Flexor Carpi Ulnaris (FCU), FDP (Ring, Little).
• Hypothenar: Abductor Digiti Minimi, Flexor Digiti Minimi, Opponens Digiti Minimi.
• Hand: Palmar Interossei (3), Dorsal Interossei (4), Lumbricals (3, 4), Adductor Pollicis, Deep head FPB.

Total: 1.5 Forearm muscles + Most Hand Intrinsics.

The profound impact of this innervation is seen in the 'Intrinisic Minus' hand.

Sensory Distribution

• Palmar Cutaneous Branch: Arises proximal to wrist. Supplies hypothenar eminence.
• Dorsal Cutaneous Branch: Arises 5cm proximal to wrist. Supplies dorsal medial hand.
• Superficial Branch (Terminal): Volar aspect of little finger and ulnar half of ring finger.

Important: In high lesions, dorsal sensation is lost. In low lesions (Guyon's), dorsal sensation (Dorsal Cutaneous Branch) is often SPARED because it branches off before the canal.

Axilla & Arm Course

Origin:

• Terminal branch of Medial Cord (C8, T1).
• Often receives C7 fibers (from Lateral Cord).
• Lies medial to axillary and brachial arteries.

Arm:

• Runs in anterior compartment initially.
• At mid-arm, it pierces the Medial Intermuscular Septum to enter the posterior compartment.
• Arcade of Struthers: A band of fascia approx. 8cm proximal to medial epicondyle. A potential compression site (esp after transposition).
• Runs distally towards the groove behind the medial epicondyle.

This proximal course is relatively safe from compression.

The Cubital Tunnel

Anatomy:

• Passes posterior to the Medial Epicondyle in the retrocondylar groove.
• Enters the Cubital Tunnel.

Cubital Tunnel Boundaries:

• Roof: Cubital Tunnel Retinaculum (Osborne's Ligament) - taut in flexion.
• Floor: MCL (posterior bundle) and capsule.
• Anterior: Medial Epicondyle.
• Posterior: Triceps medial head.
• Lateral: Olecranon.

Biomechanics:

• Elbow Flexion → Increases intraneural pressure.
• Nerve elongates 4-7mm during flexion.
• Volume of tunnel decreases by 50% in flexion.
• Hence, splinting in extension is effective.

This dynamic compression explains why symptoms are worse at night.

Forearm & Guyon's Canal

Forearm:

• Enters forearm by passing between two heads of FCU (Aponeurosis of FCU - another compression site).
• Lies on top of FDP.
• Joined by Ulnar Artery in distal 2/3.
• Gives off Dorsal Cutaneous Branch ~5cm proximal to wrist (passes dorsal to FCU).

Guyon's Canal (Ulnar Tunnel):

• Fibro-osseous tunnel at the wrist.
• Roof: Volar Carpal Ligament.
• Floor: Transverse Carpal Ligament (Flexor Retinaculum), Pisohamate ligament.
• Medial Wall: Pisiform.
• Lateral Wall: Hook of Hamate.
• Contents: Ulnar Nerve + Ulnar Artery (Nerve is Ulnar/Medial to Artery).

Zones of Guyon's Canal:

1. Zone 1 (Proximal): Motor + Sensory. Before bifurcation.
2. Zone 2 (Deep): Purely Motor. Around Hook of Hamate.
3. Zone 3 (Superficial): Purely Sensory. Distal.

McGowan Classification (Cubital Tunnel)

The original three-grade clinical staging system for ulnar nerve compression at the elbow, used to guide management and predict outcome.

Dellon Modification

Dellon expanded McGowan grades into a more granular system that incorporates sensory thresholds, motor power and two-point discrimination, subdividing each grade into mild/moderate/severe. The practical clinical message is unchanged:

• Mild: sensory symptoms only, normal two-point discrimination, no weakness.
• Moderate: measurable weakness and abnormal two-point discrimination, early wasting.
• Severe: fixed motor loss, abnormal two-point discrimination, established atrophy and clawing.

Severity at presentation is the single most important determinant of recovery — established intrinsic atrophy rarely reverses fully.

Radiology

• X-rays: Check for bone spurs, cubitus valgus deformity, supracondylar spur (rare), or hook of hamate fracture (Guyon).
• Ultrasound: Cross-sectional area over 10mm² suggests compression. Can see nerve instability (subluxation) in real-time.
• MRI: Space-occupying lesions (ganglion cystic in Guyon's).

Advanced imaging is reserved for atypical cases or recurrences.

Neurophysiology

• Conduction Velocity: Slowing across elbow (under 50 m/s) or absolute drop of over 10 m/s compared to forearm.
• EMG: Denervation in FCU suggests lesion is AT or ABOVE elbow. Normal FCU suggests lesion distal (or very mild).
• Guyon's: Distal latency prolonged.

Neurophysiology is the gold standard for grading severity.

Non-Operative Management

• Indication: Mild symptoms (McGowan I), intermittent paresthesia.
• Night Splinting: Prevents elbow flexion over 45 degrees. Keep arm straight-ish.
• Activity Modification: Avoid resting elbows on tables, headset use.
• NSAIDs: Adjunct.
• Success: ~50% effective in mild cases.

Patient compliance is key to conservative success.

Surgical Management

• Indications: Failure of conservative (3-6m), Motor weakness (McGowan II/III), Persistent constant numbness.
• Options:
  1. In Situ Decompression: Simple release of Osborne's ligament. Success 80-90%.
  2. Anterior Transposition (Subcutaneous): Moving nerve anterior to epicondyle. secure with fascial sling.
  3. Anterior Transposition (Submuscular): Placing nerve under flexor mass. Good for revision or very thin patients.
  4. Medial Epicondylectomy: Removes the bony prominence. Risk of MCL injury / instability.

Choice depends on surgeon preference and pathology.

Decompression vs Transposition

• Evidence: Multiple meta-analyses show NO difference in outcome for primary compression.
• Decompression: Faster, less invasive, preserves vascularity.
• Transposition: Indicated for: subluxating nerve, valgus deformity, stricture/scarring, or revision cases.

Decompression is standard for primary simple cases.

In Situ Decompression

Room Setup & Logistics

Clinical Handoff

Anterior Transposition (Submuscular)

1. Extended Exposure: Extend incision proximally and distally.
2. Mobilize Nerve: Release proximally to Arcade of Struthers and distally deep into forearm. Excised medial intermuscular septum completely.
3. Prepare Bed: Elevate flexor-pronator mass (CFC) from medial epicondyle.
4. Transpose: Place nerve anterior to epicondyle, deep to muscle mass.
5. Reattach: Repair flexor origin.

This technique is robust for recurrent cases.

Ulnar Tunnel Syndrome

• Causes: Ganglion cyst (most common - 50%), Hook breakdown (Golfer), Ulnar Artery Aneurysm (Hammer syndrome), Cyclist Palsy.

Akahori Classification

• Treatment: Decompression of tunnel, address pathology (remove cyst).

Anatomical knowledge of these zones aids in localizing the lesion.

The Bouvier Test

Martin-Gruber Anastomosis

• Anatomy: Cross-over fibers from Median to Ulnar nerve in the FOREARM.
• Prevalence: 15% of population.
• Effect: Ulnar intrinsics are innervated by Median fibers. In high ulnar lesions, hand function may be surprisingly preserved ("pseudo-preservation"). In median lesions, ulnar signs appear.

This anomaly can confuse clinical and EMG findings.