Posterior Interosseous Nerve (PIN) –
Study Notes
Overview
The posterior interosseous nerve (PIN) is a purely motor branch of the radial nerve. It
controls the muscles responsible for extending the wrist, fingers, and thumb. Because of
its deep course through the forearm, it is prone to compression or injury, which can
cause weakness in hand movements without affecting sensation.
Anatomical Course
● The radial nerve divides near the elbow into:
○ Superficial branch: sensory
○ Deep branch: motor
● The deep branch passes through the supinator muscle via the arcade of Fröhse
and becomes the PIN.
● PIN supplies:
○ Extensor digitorum communis
○ Extensor carpi ulnaris
○ Extensor pollicis longus & brevis
○ Extensor indicis proprius
○ Abductor pollicis longus
● The extensor carpi radialis brevis is innervated before the PIN, which helps
distinguish PIN palsy from high radial nerve injury.
Common Sites & Causes of Compression
● Arcade of Fröhse: most common site
● Repetitive forearm movements (pronation/supination)
● Trauma or fractures near the elbow
● Ganglion cysts, tumors, or fibrotic bands
● Surgical or iatrogenic injury
Clinical Presentation
● Weakness/paralysis of finger and thumb extension
● Partial wrist extension may remain (extensor carpi radialis longus spared)
● Sensation over the back of the hand remains normal
● Pain or tenderness at the lateral elbow or supinator region
Diagnosis
● Clinical examination: test finger and thumb extension; palpate arcade of Fröhse
● Electrodiagnostic studies: EMG and nerve conduction confirm motor deficit and
localize compression
● Imaging: MRI or ultrasound may detect masses, fibrosis, or nerve enlargement
Management
Conservative Treatment
Surgical Treatment
● Indicated if symptoms persist >3–6 months or if a compressive lesion is found
● Decompression of the PIN by releasing the arcade of Fröhse and surrounding
fibrous tissue
● Minimally invasive techniques (endoscopic or arthroscopic) are emerging
● Severe chronic cases may require tendon transfer to restore extension
Prognosis
● Most patients recover fully with timely treatment
● Delay or severe injury may cause persistent weakness
● Early recognition is essential for optimal functional recovery
Summary
The PIN is a motor nerve critical for finger and thumb extension. Compression at the
arcade of Fröhse is the most common cause of PIN syndrome. Accurate diagnosis,
early conservative care, and surgical decompression when necessary allow restoration
of hand function.
