The deep branch of the ulnar nerve arises at the wrist as the ulnar nerve divides in Guyon canal. It curves around the hook of the hamate with the deep branch of the ulnar artery, passes between the abductor and flexor digiti minimi, enters the palm, and sweeps across the deep palmar space to supply all the hypothenar muscles, the medial two lumbricals, all the palmar and dorsal interossei, the adductor pollicis, and the deep head of flexor pollicis brevis.
| Origin | Ulnar nerve in Guyon's canal at the level of the pisiform (C8, T1) |
|---|---|
| Course | Passes between abductor digiti minimi and flexor digiti minimi brevis, pierces opponens digiti minimi, curves around the hook of the hamate, and runs laterally across the palm in the concavity of the deep palmar arch |
| Motor |
|
The deep branch of the ulnar nerve is the primary motor nerve to the intrinsic muscles of the hand and is critical for fine motor function including power grip, precision pinch, and key pinch. Isolated deep branch palsy produces a hand that can flex and extend the fingers but cannot abduct or adduct them (interossei loss) and cannot pinch between the thumb and index finger (adductor pollicis loss), producing a positive Froment sign. Compression at the hook of the hamate (handlebar palsy, hypothenar hammer syndrome) and within the palm produces purely motor deficits without sensory loss.
Compression of the deep branch at the hook of the hamate from prolonged cycling or vibrating tools weakens the intrinsic hand muscles without sensory loss, with a positive Froment sign; padding and activity change usually help.
A blow from a bat, club or racquet handle can fracture the hook of the hamate and compress the deep branch, causing pure motor ulnar weakness; plain films often miss it, CT confirms it, and excision of the fragment relieves the nerve.
Repeated use of the hand as a hammer damages the ulnar artery near the hook of the hamate, causing finger ischaemia and sometimes irritation of the adjacent deep branch.
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