Two patients have ulnar nerve injuries, one at the elbow and one at the wrist. The patient with the ELBOW lesion shows LESS obvious clawing of the ring and little fingers than the patient with the wrist lesion. What is the correct explanation for this apparent paradox?
- A At the elbow, paralysis of the medial half of flexor digitorum profundus reduces the deforming force on the ring and little fingers ✓
- B At the elbow, the deep branch of the ulnar nerve escapes injury, preserving interossei
- C At the elbow, denervation oedema splints the metacarpophalangeal joints in extension
- D At the wrist, the lumbricals to the little finger are spared because they receive dual innervation
Correct answer: A. At the elbow, paralysis of the medial half of flexor digitorum profundus reduces the deforming force on the ring and little fingers
Explanation
Clawing results from unopposed long extensors extending the MCP joints while the interossei and ulnar lumbricals, normally MCP flexors, are paralysed. In a high lesion at the elbow, the medial half of flexor digitorum profundus is also denervated, so the IP joints of the ring and little fingers lose their flexion force and the claw looks flatter. B low lesion at the wrist spares FDP, leaving full IP flexion and a more pronounced claw, hence the ulnar paradox.
Reference: Snell's Clinical Anatomy by Regions, 10th ed.
High-yield for: NEET PGINI-CETNExTFMGEUSMLEPLABMRCP
Written and medically reviewed by the StethoPrep medical team.