Posterior and Anterior Interosseous Nerve: Function and Clinical Examination
Posterior Interosseous Nerve (PIN)
Function
The posterior interosseous nerve innervates all finger and thumb extensors, providing motor function to the extensor digitorum communis, extensor indicis proprius, extensor digiti minimi, extensor pollicis longus and brevis, abductor pollicis longus, and extensor carpi ulnaris 1. Additionally, the PIN contains deep sensory fibers directed to the wrist ligaments and joints, contributing to proprioception 2. Terminal branches can extend as far as the metacarpophalangeal joints in 15% of cases, potentially contributing to sensation on the dorsal hand 3.
Clinical Examination
Key clinical findings in PIN palsy include:
- Weak wrist extension with radial drift (due to preserved extensor carpi radialis longus/brevis function from the radial nerve proper)
- Inability to extend the fingers at the metacarpophalangeal joints
- Paralysis of thumb extension and weak thumb abduction 1
For PIN compression without motor paralysis (radial tunnel syndrome), examine for:
- Pain in the proximal dorsal forearm (present in all cases)
- Sensory collapse test over the radial tunnel (positive in 57% of cases)
- Pain with forearm pronation combined with compression over the radial tunnel (positive in 71% of cases)
- Tinel sign over the radial tunnel (positive in 36% of cases) 4
The PIN typically originates from the radial nerve approximately 5.4 cm distal to the medial epicondyle 5. Common compression sites include the arcade of Frohse (the fibrous arch at the proximal edge of the supinator muscle), which accounts for nearly half of entrapment cases 1.
Common pitfall: Unlike radial nerve palsy, PIN palsy spares wrist extension strength (though with radial deviation) because the extensor carpi radialis longus and brevis are innervated proximal to the PIN branching point. This distinguishes PIN lesions from more proximal radial nerve injuries.
Anterior Interosseous Nerve (AIN)
Function
The anterior interosseous nerve provides motor innervation to the flexor pollicis longus, the lateral half of the flexor digitorum profundus (index and middle fingers), and the pronator quadratus 6. It is a pure motor branch with no cutaneous sensory distribution.
Clinical Examination
Complete AIN palsy presents with:
- Inability to flex the distal phalanx of the thumb (interphalangeal joint)
- Inability to flex the distal phalanx of the index finger (distal interphalangeal joint)
- Classic "OK sign" test failure: Ask the patient to make an "OK" sign by touching the tips of the thumb and index finger together. With AIN palsy, the patient cannot flex the distal phalanges and instead forms a pinch using the interphalangeal joints in extension, creating a triangular rather than circular shape 6.
Incomplete AIN palsy demonstrates less axonal damage, affecting either the thumb or index finger selectively rather than both 6.
The AIN originates from the median nerve approximately 5.4 cm distal to the medial epicondyle and 21 cm proximal to the ulnar styloid 5. The branch to the flexor pollicis longus typically arises about 4 cm distal to the AIN origin 5.
Compression sites to consider:
- Fibrous arch between the humeral and ulnar heads of the pronator teres (most common, occurring in 82% of cases)
- Fibrous arch of the flexor digitorum superficialis
- Hypertrophied Gantzer muscle (accessory head of flexor pollicis longus) when positioned anterior to the AIN 6
Common pitfall: AIN palsy has no sensory loss because it is a pure motor nerve. If sensory deficits are present in the median nerve distribution, consider a more proximal median nerve lesion rather than isolated AIN involvement. Additionally, weakness of the flexor digitorum superficialis or thenar muscles indicates median nerve pathology proximal to the AIN takeoff.
Electrodiagnostic Testing
Both motor and sensory nerve conduction studies can be performed along the PIN, with the sensory component measuring deep sensory fibers to wrist structures 2. For AIN evaluation, electromyography of the flexor pollicis longus, pronator quadratus, and lateral flexor digitorum profundus can localize the lesion and assess severity 1.