Anterior Interosseous Nerve

nervus interosseus anterior

The anterior interosseous nerve is the deep motor branch of the median nerve, supplying only three muscles and having no cutaneous sensory territory. Its paralysis produces the characteristic pinch deformity where the patient cannot form an OK sign because the thumb interphalangeal joint and the index DIP joint both fail to flex, causing the finger and thumb to extend rather than flex at their distal joints.

Region: Forearm

Anatomical Data

Origin, Insertion & Supply

Origin	Median nerve approximately 5 centimetres below the medial epicondyle

Clinical Relevance

Clinical Notes

Anterior interosseous nerve syndrome produces weakness of thumb IP flexion and index DIP flexion without any sensory loss, creating the pathognomonic inability to make an OK sign. Spontaneous onset without trauma is increasingly attributed to neuralgic amyotrophy (Parsonage-Turner syndrome) involving the AIN selectively. The diagnosis is confirmed by EMG showing denervation of FPL, index FDP, and pronator quadratus with normal median sensory conductions.

Pathology

Common Injuries & Conditions

Anterior Interosseous Nerve Syndrome

Motor branch paralysis producing inability to form an OK sign from weakness of thumb IP and index DIP flexion without sensory loss, often from neuralgic amyotrophy with spontaneous recovery over 12 to 18 months.