The deep branch of the ulnar nerve in the forearm supplies flexor carpi ulnaris and the medial two slips of flexor digitorum profundus before the nerve passes deep to the flexor retinaculum to enter Guyon's canal. This forearm-level deep branch is distinct from the deep motor branch at the hand level.
The FCU and FDP4/5 branches arise in the forearm and are spared in Guyon's canal (wrist-level) ulnar nerve entrapment — an important distinction for injury localisation. High ulnar nerve injuries (above elbow) produce clawing of all four fingers alongside FCU and FDP4/5 weakness, while low injuries spare these muscles. EMG of FCU and FDP4/5 localises ulnar nerve injury to high vs low.
High ulnar nerve injury above the elbow denervates FCU and FDP4/5 (forearm branch) in addition to intrinsic hand muscles, paradoxically producing less severe clawing than low lesions (ulnar paradox) due to combined FDP paralysis.
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