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A 32-year-old carpenter presents with difficulty in performing screwing in movements of the examination, the clinician notices weakness of the supinator muscle. There is associated we deep extensors of forearm but actions of triceps is unaffected.

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posterior interosseous nerve anatomy radial nerve forearm branches

Clinical intraoperative photographs demonstrating a surgical approach to the forearm. Figure A shows an open incision with the brachioradialis (BCRL) and extensor carpi radialis longus (ECRL) muscles identified. Self-retaining or handheld retractors are used to develop the plane between these two muscle bellies using blunt dissection. Figure B illustrates deeper exposure achieved through further lateral retraction of the brachioradialis. This maneuver reveals the superficial radial nerve, which appears as a pale, cord-like longitudinal structure situated in the deep plane. The surgical field highlights the relevant anatomy for decompression of the radial nerve or its branches, such as in cases of posterior interosseous nerve (PIN) entrapment. The images serve as an educational guide for surgical planes in the proximal forearm, emphasizing the relationship between mobile wad musculature and the radial nerve's sensory branch.

Clinical intraoperative photographs demonstrating a surgical approach to the forearm. Figure A shows an open incision with the brachioradialis (BCRL) and extensor carpi radialis longus (ECRL) muscles identified. Self-retaining or handheld retractors are used to develop the plane between these two muscle bellies using blunt dissection. Figure B illustrates deeper exposure achieved through further lateral retraction of the brachioradialis. This maneuver reveals the superficial radial nerve, which appears as a pale, cord-like longitudinal structure situated in the deep plane. The surgical field highlights the relevant anatomy for decompression of the radial nerve or its branches, such as in cases of posterior interosseous nerve (PIN) entrapment. The images serve as an educational guide for surgical planes in the proximal forearm, emphasizing the relationship between mobile wad musculature and the radial nerve's sensory branch.

This clinical photograph displays a surgical dissection of the right proximal forearm and elbow region, utilizing an extended Henry approach to visualize the radial nerve and its distal branches. The surgical field is retracted, exposing the deep muscular and neurovascular anatomy. A white arrow identifies the main trunk of the radial nerve as it descends between the brachialis and brachioradialis muscles. The branching pattern is clearly demarcated: a black arrow points to the posterior interosseous nerve (PIN), which dives toward the supinator muscle; a blue arrow indicates the superficial radial nerve, which continues distally under the brachioradialis; and a yellow arrow points to the motor branch supplying the extensor carpi radialis brevis (ECRB). The nerves appear as glistening, off-white cord-like structures set against the deep red, striated muscle tissue and yellow lobulated subcutaneous fat. This image serves as a high-fidelity anatomical reference for peripheral nerve surgery, demonstrating the critical spatial relationships between the radial nerve bifurcation and the surrounding musculature.

This clinical photograph displays a surgical dissection of the right proximal forearm and elbow region, utilizing an extended Henry approach to visualize the radial nerve and its distal branches. The surgical field is retracted, exposing the deep muscular and neurovascular anatomy. A white arrow identifies the main trunk of the radial nerve as it descends between the brachialis and brachioradialis muscles. The branching pattern is clearly demarcated: a black arrow points to the posterior interosseous nerve (PIN), which dives toward the supinator muscle; a blue arrow indicates the superficial radial nerve, which continues distally under the brachioradialis; and a yellow arrow points to the motor branch supplying the extensor carpi radialis brevis (ECRB). The nerves appear as glistening, off-white cord-like structures set against the deep red, striated muscle tissue and yellow lobulated subcutaneous fat. This image serves as a high-fidelity anatomical reference for peripheral nerve surgery, demonstrating the critical spatial relationships between the radial nerve bifurcation and the surrounding musculature.

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Diagnosis: Posterior Interosseous Nerve (PIN) Palsy

This presentation is a textbook case of PIN compression/injury, also called deep branch of radial nerve palsy or low radial nerve palsy.

Anatomical Basis - Why These Specific Muscles?

The radial nerve divides at the level of the lateral epicondyle into:
  1. Superficial branch (purely sensory - dorsum of hand/lateral 3.5 fingers)
  2. Deep branch / Posterior Interosseous Nerve (PIN) (purely motor to forearm extensors)
The PIN passes through the supinator muscle via the Arcade of Frohse - the fibrous arch at the proximal edge of the superficial layer of supinator. This is the most common site of entrapment.
Before entering the arcade of Frohse, the PIN gives off branches to:
  • Extensor carpi radialis brevis (ECRB)
After exiting the supinator, it supplies the deep extensors of the forearm:
  • Extensor digitorum communis (EDC)
  • Extensor digiti minimi (EDM)
  • Extensor carpi ulnaris (ECU)
  • Extensor indicis proprius (EIP)
  • Abductor pollicis longus (APL)
  • Extensor pollicis brevis (EPB)
  • Extensor pollicis longus (EPL)

Why Triceps is SPARED

The branches to triceps arise from the radial nerve in the axilla and spiral groove - far proximal to where the PIN begins. By the time the radial nerve reaches the lateral epicondyle/radial head area, the triceps branches have long since left. So PIN injury always spares triceps. - S. Das Manual on Clinical Surgery, 13th Ed.

Why Supinator is Affected

The supinator is innervated by the PIN itself as the nerve passes through it. Compression at or just proximal to the arcade of Frohse can therefore affect the supinator as well. In this case, with supinator and deep extensor weakness, the lesion is at or just proximal to the proximal supinator border.
"PIN innervates the ECRB, supinator, EIP, ECU, EDC, EDM, APL, EPB, and EPL" - Miller's Review of Orthopaedics, 9th Ed.

Why This Patient (Carpenter)?

Carpenters perform repetitive pronation-supination movements (screwing, drilling). This is a well-recognized occupational cause of PIN compression at the arcade of Frohse.
"Repetitive pronation and supination of the forearm can predispose to compression of the deep/posterior interosseous nerve branch of the radial nerve at the fibrous arcade of Frohse." - Imaging Anatomy: Bones, Joints, Vessels and Nerves

Key Clinical Features - PIN Palsy

FeatureFindings
Wrist dropPresent but with radial deviation (ECRL spared, as it is innervated by radial nerve proper before bifurcation)
Finger extensionLost at MCP joints
Thumb extension/abductionLost
Wrist extensionPartially preserved with radial deviation
Triceps powerNormal
BrachioradialisNormal (innervated before bifurcation)
SensationNormal - PIN is purely motor
The radial deviation on wrist extension is a hallmark: ECRL (supplied by the radial nerve proper, above the PIN origin) is intact, but ECU (PIN-supplied) is weak, so the wrist pulls radially when extended.

Sites of Compression (Mnemonic: LEAFS)

  1. L - Recurrent Leash of Henry (radial recurrent artery fan)
  2. E - Proximal edge of Extensor carpi radialis brevis
  3. A - Arcade of Frohse (most common - proximal edge of supinator)
  4. F - Fascial band at the radial head
  5. S - Distal edge of Supinator
  • Miller's Review of Orthopaedics, 9th Ed., p. 694

Differentiating PIN Palsy from Higher Radial Nerve Injuries

Level of InjuryTricepsBrachioradialisECRLSupinatorDeep extensorsSensation
AxillaWeakWeakWeakWeakWeakLost
Spiral groove (Saturday night palsy)SparedWeakWeakWeakWeakLost
PIN (this case)SparedSparedSparedWeakWeakNormal

Management

  • Conservative (first-line): Activity modification (rest from repetitive pro/supination), splinting, NSAIDs - for 3 months
  • Surgical decompression: If no recovery by 3 months - release at the arcade of Frohse; good to excellent results in ~85% of patients
  • Tendon transfer: For irreversible paralysis - FCR to EDC + palmaris longus to EPL
  • Campbell's Operative Orthopaedics, 15th Ed. (2026); Miller's Review of Orthopaedics, 9th Ed.

Surgical Anatomy Image

PIN and radial nerve branches at the proximal forearm
Intraoperative image showing the radial nerve (white arrow), PIN (black arrow) diving toward the supinator, superficial radial nerve (blue arrow), and ECRB branch (yellow arrow).
Summary: This 32-year-old carpenter has PIN (posterior interosseous nerve) compression at the arcade of Frohse due to repetitive supination/pronation. Triceps is spared because its innervation arises from the radial nerve well above the PIN level. Sensation is intact because the PIN is purely motor. The primary occupational activity (screwing) directly stresses the site of compression.
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