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.
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 of the ulnar nerve at the hook of the hamate from prolonged cycling or vibrating tool use produces weakness of intrinsic hand muscles without sensory loss, presenting as difficulty with fine motor tasks and a positive Froment sign, managed with padding, activity modification, and occasionally surgical decompression.
Repetitive trauma to the hypothenar eminence from using the hand as a hammer damages the ulnar artery and deep branch at the hook of the hamate, producing digital ischaemia and motor weakness requiring vascular and nerve decompression surgery.
