Fig. 1

Guyon Tunnel, regional anatomy. Cadaveric specimens (a, c) demonstrate the ulnar nerve (arrow) as it engages the Guyon tunnel running on the radial side of the pisiform (Pis). Distal to the pisiform the deep branch (outlined arrowhead) crosses the pisohamate hiatus (dashed line), which is a tendinous arch formed by the proximal insertion of the abductor digiti minimi (ADM) and flexor digiti minimi brevis (FDM), whereas the superficial branch (arrowhead) keeps to its path superficial to the hypothenar muscles before branching in its terminal division. FR, flexor retinaculum; FCU, flexor carpi ulnaris; FT, flexor tendons; UA, ulnar artery; TM, thenar muscles. b Schematic drawing demonstrates the terminal divisions of the superficial (outlined arrowhead) and deep branch (arrowhead) distal to the Guyon tunnel. Thin arrow, proper ulnar palmar digital nerve for the V finger; dashed thin arrow, common palmar digital nerve for the IV webspace; #1 nerve for the hypothenar muscles, fourth lumbrical, fifth carpometacarpal and metacarpophalangeal joints; #2 nerve for the interossei of the third space; #3 nerve for the oblique head of the adductor pollicis; #4 nerve for the interossei of the fourth space and the fourth metacarpophalangeal joint; #5 nerve for the third lumbrical and the fourth carpometacarpal joint; #6 nerve for the interossei of the first and second spaces, the first and second intermetacarpal and metacarpophalangeal joints, the two heads of the adductor pollicis, and the deep head of the flexor pollicis brevis