The Anterior Interosseous Nerve (AIN)

The Anterior Interosseous Nerve (AIN) is a branch of the median nerve that innervates specific muscles in the forearm, including the flexor pollicis longus, the lateral part of the flexor digitorum profundus, and the pronator quadratus. Isolated AIN injury without involvement of the rest of the median nerve can occur in specific traumatic or compressive situations. Some common scenarios include:

  1. Radial Head Fracture:
  • AIN compression or injury can occur in the setting of a radial head fracture. The nerve can be compressed as it courses through the supinator muscle, leading to isolated AIN dysfunction.
  1. Supinator Syndrome:
  • Supinator syndrome is a condition where the AIN gets compressed as it passes through the supinator muscle. This can occur due to trauma, overuse, or repetitive pronation and supination of the forearm.
  1. Entrapment at Arcade of Frohse:
  • The Arcade of Frohse is a fibrous arch formed by the superficial edge of the supinator muscle. The AIN may be compressed at this point, leading to isolated AIN palsy.
  1. Deep Forearm Laceration:
  • A deep laceration in the deep forearm, especially in the region of the supinator or proximal forearm, can result in isolated AIN injury without affecting the rest of the median nerve.

It’s important to note that AIN palsy without involvement of the rest of the median nerve is relatively uncommon. Most median nerve injuries affect the entire nerve, resulting in a combination of motor and sensory deficits in the hand. Isolated AIN injuries are usually associated with specific anatomical or traumatic factors that selectively affect the AIN. Clinical evaluation, imaging studies, and, in some cases, electrodiagnostic tests can help diagnose and localize the injury.