What muscles are supplied by the Anterior Interosseous Nerve (AIN)?

Muscles Innervated by the Anterior Interosseous Nerve (AIN)

The Anterior Interosseous Nerve (AIN) supplies three muscles in the forearm: the flexor pollicis longus, the radial half of the flexor digitorum profundus (to the index and middle fingers), and the pronator quadratus.

Anatomy and Function of the AIN

• The AIN is a pure motor nerve that branches from the median nerve in the proximal forearm 1, 2
• It runs deep in the forearm along with the anterior interosseous artery 3
• The AIN provides motor innervation exclusively to:
  • Flexor pollicis longus (FPL) - responsible for flexion of the thumb's interphalangeal joint 1, 3
  • Flexor digitorum profundus (FDP) to the index and middle fingers - responsible for flexion of the distal interphalangeal joints 1, 3
  • Pronator quadratus (PQ) - responsible for pronation of the forearm 1, 2
• The AIN contains no sensory fibers, making it a pure motor nerve 2

Clinical Significance of AIN Innervation

• AIN syndrome (Kiloh-Nevin syndrome) presents with weakness or paralysis of the three muscles supplied by this nerve 4
• The classic clinical presentation includes:
  • Inability to form an "OK" sign due to weakness of FPL and FDP to the index finger 3
  • Weakness or inability to flex the distal phalanx of the thumb 3
  • Weakness or inability to flex the distal interphalangeal joint of the index finger 3
  • Weakness of pronation, especially against resistance 2
• No sensory deficits are present in isolated AIN syndrome, which helps distinguish it from more proximal median nerve lesions 2

Common Causes of AIN Compression

• Fibrous bands originating from the deep head of the pronator teres muscle 1
• Fibrous bands from the superficial head of the pronator teres 1
• Compression by the flexor digitorum superficialis arch 3
• Anatomical variations where the nerve runs deep to both heads of the pronator teres 1
• Compression by a thickened lacertus fibrosus (bicipital aponeurosis) 1
• Iatrogenic causes such as venipuncture or intravenous injections in the cubital region 2

Diagnostic Considerations

• MRI can demonstrate denervation changes in the affected muscles 3
• Electrodiagnostic studies are essential for confirming the diagnosis and determining the severity of nerve compression 2, 3
• Differential diagnosis must include flexor tendon rupture, flexor tendon adhesions, and stenosing tenosynovitis 1

Understanding the specific muscles innervated by the AIN is crucial for accurate diagnosis and appropriate management of patients with suspected AIN syndrome or forearm motor deficits.