Anterior Interosseous Nerve

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Anterior Interosseous Nerve
Nerve Type	Motor nerve
Origin	Major branch of the median nerve and receives fibers from the C8 and T1 spinal nerve roots.
Course	Descends on the anterior interosseous membrane with the anterior interosseous artery between flexor pollicis longus and the radial FDP, passes deep to pronator quadratus, and terminates with articular twigs to the wrist and distal radioulnar joint.
Major Branches	Muscular branches to flexor pollicis longus, the radial half of flexor digitorum profundus (index ± middle), and pronator quadratus, plus articular branches to the carpus and distal radioulnar joint.
Sensory innervation	No cutaneous field, with sensory supply limited to articular afferents to the wrist and distal radioulnar joint.
Motor innervation	Flexor pollicis longus, pronator quadratus, and the radial FDP slip(s) to the index (± middle) finger enabling thumb IP flexion, forearm pronation at end range, and DIP flexion of the index.
Conditions	Kiloh-Nevin syndrome (Anterior Interosseous Nerve Syndrome)

Motor Supply

Note for FDP the AIN only supplies the index and middle finger. The ulnar nerve supplies the ring and little finger.

Muscles supplied by the AIN	Origin	Insertion	Action
Flexor Digitorum Profundus	Anterior and medial surfaces of the proximal three-quarters of the ulna	Four tendons insert into the base of each distal phalanx of digits 2-5	DIP flexion. Also MCP and PIP flexion of fingers 2-5, assists with flexion of the wrist.
Flexor Pollicis Longus	Anterior surface of the middle half of the shaft of the radius and coronoid process of the ulna	Base of the distal phalanx of the thumb	Flexes the interphalangeal joint of the thumb, assists in flexion of MCPJs and CMCJs and wrist.
Pronator Quadratus	Anterior surface and distal quarter of the ulna	Anterior surface and distal quarter of the radius	Pronation

Conditions

Anterior interosseous nerve syndrome (Kiloh–Nevin) presents with pure motor weakness causing an impaired “OK” sign from FPL/FDP paresis and PQ weakness, typically due to compression at pronator teres, the FDS arch, or Gantzer’s muscle, inflammatory neuritis, or iatrogenic/traumatic injury.