Ulnar Nerve

Anatomy

Terminal branch of medial cord of the brachial plexus.

C8 and T1, with occasional contributions from C7.

Upper arm

Runs medial to brachial art. beneath pectoralis major, medial to the coracobrachialis and anterior to the long head of the triceps. At the level of the distal attachment of the coracobrachialis, the ulnar nerve pierces the medial intermuscular septum to enter the posterior compartment lying on the anterior border of the medial head of the triceps. A thick fascial band that connects the medial head of the triceps to the intermuscular septum crosses the ulnar nerve at approximately 8 cm proximal to the medial epicondyle (the arcade of Struthers). Found in approximately 70% of the population, the arcade of Struthers is more common than the ligament of Struthers (median nerve entrapment) which is found in 1% of the population.

Elbow

The ulnar nerve wraps around the posterior aspect of the medial epicondyle at the level of the elbow, encased within a fibrous sheath (Osborne’s ligament) laterally, and the head of the flexor carpi ulnaris posteromedially. Together, these two structures form the cubital tunnel.

First branch is sensory to elbow capsule. It exits the cubital tunnel passing between the two heads of the flexor carpi ulnaris and enters the anterior compartment of the forearm.

Forearm

After exiting the cubital tunnel, the ulnar nerve gives off motor branches to the flexor carpi ulnaris. It then lies on the anterior surface of the flexor digitorum profundus. +- 5 cm distal to the medial epicondyle, the ulnar nerve gives off branches to the ulnar half of the long flexors, ie. the ring and small fingers. In the middle of the forearm the ulnar nerve becomes superficial and meets with the ulnar artery. Before the flexor carpi ulnaris becomes tendinous, the ulnar nerve divides. The more superficial of the two branches courses dorsally toward the distal ulna and dorsum of the hand and becomes the dorsal sensory branch of the ulnar nerve.

Wrist

Near the wrist the ulnar nerve rises superficial to the flexor retinaculum and lies under the tendon of the flexor carpi ulnaris before its attachment to the pisiform. The ulnar nerve then turns radial to the pisiform to lie in Guyon’s Canal. Within the canal, the ulnar nerve divides into motor and sensory branches. The superficial branch first gives off motor fibers to palmaris brevis and divides to innervate the ulnar side of the palm and ring finger, and the entire small finger. The deep branch provides innervation to the adductor pollicis, opponens digiti minimi, abductor digiti minimi, flexor digiti minimi brevis, flexor pollicis brevis (deep head), the interossei and the lumbricals to the ring and small fingers
Several variations in the position of the ulnar nerve at the wrist have been described.

Connections between the median and ulnar nerves

Martin-Gruber or proximal anastomosis - Occurs in 10% to 25%, connection between the median and ulnar nerve in the forearm.
Riche-Cannieu anastomosis - distally, communication between the palmar cutaneous branches of the median and ulnar nerves.
These crossovers may lead to an underestimation of an injury to the ulnar nerve clinically and electrodiagnostically.

Clinical

Signs and symptoms of ulnar neuropathy

Medial forearm pain progressing to numbness and dysesthesia along the ulnar aspect of the wrist and ring and small fingers
Wasting of flexor carpi ulnaris if the lesion is proximal
Wasting of the hypothenar musculature and wasting of the first web spaced as adductor pollicis atrophies.
Froment's sign - grasp a piece of paper between thumb and index metacarpal. The patient compensates for weakness in Adductor policis by using FPL and flexing the IP joint

Possible areas of entrapment

Medial intermuscular septum
Arcade of Struthers
Cubital tunnel - most common
Fascia of the flexor carpi ulnaris
Guyon’s Canal

Ulnar neuropathy after elbow trauma

Scarring in and around the cubital tunnel can lead to compression and tethering of the ulnar nerve at any of the sites described.

Immobilization of the elbow may potentiate scarring and fibrosis and exacerbate the problem, which is magnified after elbow release of contracture where increased joint motion is not matched by ulnar nerve glide.
Decompression with or without anterior transposition has been described as a treatment for ulnar neuropathy.

There is no consensus as to the optimal location of the transposition although it has been suggested that submuscular positioning is better for patients undergoing revision surgery. Routine in situ release during the initial surgery is thought to decrease the need for future nerve surgery. Good function can be achieved after neurolysis and anterior transposition during late reconstruction of the posttraumatic elbow. Of course if ulnar neuritis is present at initial treatment, decompression and/or transposition must be performed.

Decompression plus or minus anterior transposition of the ulnar nerve has been recommended after acute trauma to prevent tardy ulnar neuritis in patients with or without pre-injury symptoms. Early motion may help prevent the development of scarring with subsequent neuropathy.

References

Mazurek, Michael T. MD. Shin, Alexander Y. MD. Upper Extremity Peripheral Nerve Anatomy: Current Concepts and Applications. Clinical Orthopaedics & Related Research. 1(383):7-20, February 2001.

Ristic, Sasha MD. Strauch, Robert J. MD. Rosenwasser, Melvin P. MD. The Assessment and Treatment of Nerve Dysfunction After Trauma Around the Elbow. Clinical Orthopaedics & Related Research. (370):138-153, January 2000.