Radial Nerve Anatomy

Motor Innervation Summary

• Arm: Triceps (Long, Lateral, Medial heads), Anconeus.
• Lateral Epicondyle: Brachioradialis, ECRL, ECRB (variable).
• Forearm (PIN): Supinator, EDC, EDM, ECU, APL, EPB, EPL, EIP.

These muscle groups represent the ordered motor recovery.

Sensory Distribution

• Posterior Cutaneous Nerve of Arm: Posterior arm.
• Lower Lateral Cutaneous Nerve of Arm: Lateral aspect of lower arm.
• Posterior Cutaneous Nerve of Forearm: Strip down middle of posterior forearm.
• Superficial Radial Nerve: Dorsal aspect of thumb, index, middle, and radial half of ring finger (proximal to PIP joints). Autonomous zone: First dorsal webspace.

This sensory map is crucial for localizing lesion levels.

Origin and Proximal Course

Origin:

• Continuation of the Posterior Cord of the Brachial Plexus.
• Roots: C5, C6, C7, C8, (T1).
• Lies posterior to the axillary artery.

The Triangular Interval:

• The nerve exits the axilla entering the posterior compartment through the Triangular Interval.
• Boundaries: Teres Major (superior), Long Head Triceps (medial), Humerus (lateral).
• Contents: Radial Nerve + Profunda Brachii Artery.

Spiral Groove:

• Winds around the humerus in the spiral groove (musculospiral groove) between lateral and medial heads of triceps.
• Passes medial to lateral.
• Innervates: Triceps (Lateral and Medial heads), Anconeus.

The spiral groove is the classic site for "Saturday Night Palsy".

Distal Arm and Elbow

Transition to Anterior:

• Pierces the Lateral Intermuscular Septum approx. 10cm proximal to the lateral epicondyle.
• Moves from Posterior → Anterior Compartment.
• Lies between Brachialis (medial) and Brachioradialis (lateral).

Innervation Order:

1. Brachioradialis
2. ECRL
3. ECRB (Variable - can be PIN)

Bifurcation:

• Anterior to the lateral epicondyle (level of radiocapitellar joint).
• Splits into:
  1. Superficial Radial Nerve (SRN): Purely sensory.
  2. Posterior Interosseous Nerve (PIN): Purely motor.

Forearm Course

Posterior Interosseous Nerve (PIN):

• Passes beneath the Arcade of Frohse (proximal edge of Supinator). Or supinator arch.
• This is the most common site of compression.
• Winds around radial neck within substance of Supinator.
• Emerges in posterior compartment to innervate extensor muscles.
• Terminates as a sensory twig to the wrist capsule.

Superficial Radial Nerve (SRN):

• Runs deep to Brachioradialis in the forearm.
• In distal third, it emerges posteriorly between BR and ECRL tendons ("Wartenberg's Point").
• Becomes subcutaneous and divides into dorsal digital nerves.

Injury here leads to Wartenberg's Syndrome (purely sensory).

Injury Levels & Patterns

Radiological Assessment

• Plain X-rays: Humeral shaft fracture (Holstein-Lewis), Radial head fracture/dislocation.
• Ultrasound: Can visualize nerve continuity in Holstein-Lewis fractures (nerve entrapped in callus?).
• MRI: For soft tissue masses (lipoma, ganglion) compressing PIN.

Reliable imaging confirms the diagnosis.

EMG / NCS

Indications:

• Assessing continuity in closed injuries (wait 3-4 weeks for Wallerian degeneration signs).
• Monitoring recovery (looking for re-innervation potentials).
• Differentiating PIN syndrome from Radial Tunnel Syndrome.

Timing: Baseline study immediately is of limited value. 3-4 weeks is optimal for first assessment.

Treatment Algorithm

Posterior Approach to Humerus

Indication: Distal 1/3 fractures, Exploration of Radial Nerve.

technique:

• Midline posterior incision.
• Identify interval between Long and Lateral heads of Triceps (proximal).
• Or split Triceps midline (distal).
• Identify Nerve: In spiral groove with Profunda Brachii artery.
• Trace distally through lateral intermuscular septum.

Careful dissection preserves the nerve.

Anterolateral Approach

Indication: Mid/Proximal shaft, PIN decompression.

Technique:

• Interval: Brachioradialis (Radial N) and Brachialis (Musculocutaneous N).
• Nerve identified between these muscles.
• CAUTION: Nerve pierces septum 10cm proximal to elbow. Don't strip too vigorously proximally on the lateral side.

Protection of the nerve is paramount.

Henry Approach (Volar)

Indication: PIN Decompression / Radius Fixation.

Technique:

• Interval: BR (Radial) and FCR (Median).
• Identify Superficial Radial Nerve under BR.
• Follow proximally to bifurcation.
• Ligate Leash of Henry.
• Identify PIN entering Supinator.
• Supinate forearm to move PIN away from incision site.

This approach gives excellent exposure of the radius.

Wartenberg's Syndrome

Compression of Superficial Radial Nerve (SRN).

• Site: Between Brachioradialis and ECRL tendons during pronation (scissoring effect).
• Symptoms: Pain/Paresthesia in dorsal thumb/webspace. +ve Tinel's.
• DDx: De Quervain's (Finkelstein test distinguishes).

Surgical release is rarely needed but effective.

Radial Tunnel Syndrome

Pain vs Palsy.

• PIN Syndrome: Motor palsy (finger drop, radial deviation). Painless.
• Radial Tunnel Syndrome: Pain in proximal forearm. No motor weakness. controversial diagnosis. Resembles Tennis Elbow but pain is 4cm distal to epicondyle.

Conservative management is the mainstay.