Connections between nerves where fibers traditionally carried by one nerve travel within a different nerve and then return to join the original nerve more distally are termed nerve anastomoses.

These can explain unexpected findings in the setting of a specific known nerve injury.


Martin-Gruber Anastomosis:

  • Forearm level connection between the median and ulnar nerve.
  • Can take many forms:
    • One to two branches of fibers.
    • Arise(s) from the median nerve, the median nerve branch to superficial flexor muscles, or the anterior interosseous nerve.
    • Travels to the ulnar nerve in the forearm following an oblique or transverse course.
  • Clinical relevance:
    • In an ulnar nerve injury at a level proximal to the anastomosis, intrinsic ‘ulnar innervated’ hand function will remain preserved because those fibers were traveling with the median nerve and were therefore uninjured.
    • In a median nerve injury at a level proximal to the anastomosis, all intrinsic muscle function will be absent because the median nerve fibers that innervate the thenar musculature and the fibers that go the remaining ‘ulnar innervated’ intrinsic muscles will all be denervated.


  • Hand level connections between the ulnar and median nerve.
  • Can take many forms:
    • Ulnar nerve can contribute motor fibers to the various thenar muscles through branches at the hand.
    • Can innervate the flexor pollicis brevis, abductor pollicis brevis and/or opponens pollicis.
  • Clinical relevance:
    • Despite severe carpal tunnel syndrome, the motor musculature to the thenar eminence might be relatively spared.
    • Despite median nerve injury proximal to the hand, the thenar musculature remains quite functional.