Posterior Interosseous Nerve (Full)

nervus interosseus posterior

The posterior interosseous nerve (PIN) is the motor branch of the radial nerve that enters the posterior forearm through the supinator at the Arcade of Frohse. Its entrapment at the Arcade produces PIN syndrome — finger and thumb drop without wrist drop (ECRL is preserved above the entrapment, maintaining wrist extension). PIN compression from radial head fracture, synovitis, or muscle hypertrophy at the Arcade requires surgical decompression through the Frohse arcade.

Region: Forearm

Anatomical Data

Origin, Insertion & Supply

Origin	Deep branch of the radial nerve — enters the posterior compartment through the supinator (Arcade of Frohse)

Clinical Relevance

Clinical Notes

PIN syndrome is distinguished from complete radial nerve palsy by preserved wrist extension (ECRL is above the entrapment), preserved brachioradialis, and preserved sensation. Radial tunnel syndrome — pain rather than weakness at the radial tunnel — may represent a sensory variant of PIN compression. The PIN is the nerve at risk during radial head arthroplasty and proximal radius ORIF.

Pathology

Common Injuries & Conditions

PIN Syndrome

Posterior interosseous nerve entrapment at the Arcade of Frohse producing finger and thumb drop with preserved wrist extension managed with surgical decompression.