Proximal Ulnar Nerve

Within the proximal forearm, the ulnar nerve courses along the length of the medial forearm, lying on the ulnar collateral ligament immediately below the elbow; it subsequently passes between the two heads of the flexor carpi ulnaris (C7, 8, T1), while innervating this muscle, and the medial flexor digitorum profundus (C8,T1), providing flexion of the fourth and fifth distal interphalangeal joints. The palmar and dorsal cutaneous sensory branches leave the ulnar nerve within the lower forearm. As the flexor carpi ulnaris narrows into its tendon, the nerve and artery emerge from under its lateral edge, where it is covered just by skin and fascia.

Entering the hand through the Guyon canal, the ulnar nerve passes anterior, that is, superficial to the flexor retinaculum, almost immediately splitting into its superficial and deep terminal branches. This contrasts with the median nerve, which passes deep to the flexor retinaculum and thereby provides a potential entrapment site. At the wrist, the ulnar nerve divides into the superficial and deep branches. The superficial branch innervates the palmaris brevis muscle and then splits into its terminal sensory nerves, providing sensation to the entire fifth digit and medial ring finger. The deep terminal motor branch (C8, T1) has a purely muscular function, supplying all hypothenar muscles, including the abductor digiti minimi, opponens digiti minimi, and flexor digiti minimi. Subsequently, it curves along the palm, providing motor branches to the third and fourth lumbricals, the four dorsal and three palmar interossei, the adductor pollicis, and the deep head of the flexor pollicis brevis.

The ulnar palmar cutaneous branch arises 7 cm above the wrist, descends near the ulnar artery, pierces the deep fascia, supplying the hypothenar eminence; it communicates with the medial cutaneous nerve of the forearm and the palmar cutaneous branch of the median nerve. The dorsal ulnar branch arises 5 to 10 cm above the wrist, passes posteriorly, deep to the tendon of flexor carpi ulnaris, pierces the deep fascia, and continues distally along the posteromedial side of the wrist. Here it divides into branches that supply the palmaris brevis muscle, and as the superficial terminal branch, the skin on the medial side of the back of the hand and fingers. There are usually two or three dorsal digital nerves: one, the proper palmar digital nerve, supplying the medial side of the little finger, the other splitting into a common palmar digital nerve, communicating with the adjoining common palmar digital branch of the median nerve before dividing into the two proper palmar digital nerves for the adjacent sides of the little and ring fingers. Very rarely, the ulnar nerve supplies 2.5 rather than 1.5 digits, in which case the areas supplied by the median and radial nerves are reciprocally reduced.

There are a variety of interconnections between the ulnar and median nerves, allowing interchanges of fibers from different nerve roots. Their clinical implications are important, allowing explanations for seemingly “nonanatomic” findings.

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