Anterior Interosseous Nerve | AIN

Gray’s Anatomy cross-section of the forearm
Cross-section through the middle of the forearm. The anterior interosseous nerve and vessels lie on the interosseous membrane between flexor pollicis longus and flexor digitorum profundus. Image by Henry Vandyke Carter, Gray’s Anatomy (1918), Wikimedia Commons, public domain.

Anatomy

  • Motor branch of the median nerve arising about 5 cm distal to the elbow joint
  • Runs on the interosseous membrane between FPL and FDP
  • Ends in pronator quadratus and sends sensory fibres to the wrist joint
  • No cutaneous supply

Motor Supply

  • Flexor pollicis longus
  • FDP to the index and middle fingers
  • Pronator quadratus

Clinical Features

  • Unable to make an OK sign because thumb IP and index DIP flexion are lost
  • Pinch becomes pulp to pulp with straight joints
  • No sensory loss
  • The nerve most often injured in extension type supracondylar humerus fractures with posterolateral displacement
  • Other causes are forearm fractures, fibrous bands, an accessory head of FPL (Gantzer muscle) and neuralgic amyotrophy
  • Test pronator quadratus with resisted pronation in full elbow flexion

Investigations

  • EMG to confirm the lesion and exclude a partial median nerve injury
  • MRI if neuritis or a mass is suspected

Differential Diagnosis

Management

  • Most fracture related and neuritic palsies recover, so observe for 3 to 6 months
  • Exploration when there is no recovery
  • Tendon transfers for permanent loss, such as brachioradialis to FPL and side to side FDP tenodesis

Structures at Risk

  • Anterior approach to the proximal radius
  • Reduction and pinning of supracondylar humerus fractures

Reviewed by Professor Phong Tran, Head of Orthopaedic Surgery, Western Health. Last updated 10 October 2026.