Ulnar Nerve Entrapment

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Clinically Relevant Anatomy

• Clinically Relevant Anatomy 1,2,3
      o Ulnar nerve entrapment typically occurs at two main sites: the cubital tunnel and guyon’s canal. Guyon’s canal                syndrome, also known as, ulnar tunnel syndrome is much less common than cubital tunnel syndrome.
      o The ulnar nerve can also be disrupted at other sites if a fracture of the humerus, radius, or ulna is sustained. Rarely,          vascular pathology can lead to compression at atypical sites as well.
      o Function of the nerve can be compromised distal to the site of the lesion.
      o Course of the Ulnar nerve:
            - Originates from the C8-T1 nerve roots.
                  • There is a possible minor involvement of the C7 nerve root as well.

                                        File:Brachial Plexus.jpg
            - The ulnar nerve branches off the medial cord of the brachial plexus.
            - The ulnar nerve runs just medial to the axillary artery, and continues down the humerus.
            - About halfway down the humerus, the ulnar nerve pierces the medial intermuscular septum, and passes back                   over the medial head of the triceps.
            - The nerve continues around the posterior aspect of the medial epicondyle and goes through the ulnar groove.
            - It then courses underneath the tendonous arch also known as the cubital tunnel (humeral-ulnar aponeurosis),                   which is formed by the two heads of the flexor carpi ulnaris, and enters into the anterior compartment of the                     forearm.
            - In the forearm the nerve branches off into:
                  • Muscular branches of the ulnar nerve
                        o Innervates
                              - Flexor carpi ulnaris
                                    • flexion and adduction of wrist
                              - Ulnar portion of the Flexor digitorum profundus
                                    • Flexes distal phalanges at distal interphalangeal joints of digits 2-5 and assists with flexion of                                         hand
                  • Dorsal cutaneous sensory branch
                        o Supplies sensation to the dorsal aspect of the fifth finger and the dorsal medial portion of the 4th finger.
                  • Palmar cutaneous branch of the ulnar nerve
                        o Supplies sensation to the palmar aspect of the fifth finger and the palmar medial portion of the 4th                                    finger.
            - The ulnar nerve continues down the forearm medially and enters into the hand though the Guyon’s canal.
                  • Guyon’s canal
                        o Proximal wall
                              - Pisiform bone
                        o Distal wall
                              - Hook of the hamate
                        o Floor:
                              - Combination of the thick transverse carpal ligament, the hamate and triquetrum bones
                        o Roof
                              - Narrowed by a ligament running from pisiform to hamate.
            - In the hand the ulnar nerve branches off into:
                  • Superficial branch of ulnar nerve
                        o Supplies the Palmaris brevis
                              - steadies and corrugates skin of palm to help with grip
                  • Deep branch of ulnar nerve
                        o Supplies the interossei, third and fourth lumbricals, the adductor pollicis, and part of the flexor pollicis                              brevis.

Mechanism of Injury / Pathological Process

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Clinical Presentation

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