Two patients have complete ulnar nerve palsies: one at the level of the elbow and one at Guyon's canal. On inspection, the patient with the ELBOW lesion shows LESS obvious clawing of the ring and little fingers than the one with the wrist lesion. What explains this apparent paradox?
- A At the elbow the interossei are spared because they receive collateral innervation
- B At the elbow the lumbricals are intact, actively correcting the deformity
- C At the elbow the flexor digitorum profundus is denervated, removing the deforming flexion force on the distal phalanges ✓
- D Clawing depends only on the site of sensory loss, which is more extensive in low lesions
Correct answer: C. At the elbow the flexor digitorum profundus is denervated, removing the deforming flexion force on the distal phalanges
Explanation
Clawing arises from unopposed long flexors acting on denervated interossei and lumbricals. In a high ulnar lesion, the medial half of flexor digitorum profundus is also paralysed, so the distal interphalangeal joints of ring and little fingers cannot be actively flexed and the classic deformity is attenuated. In a low lesion at the wrist, FDP remains intact and drives the distal joints into flexion, making the claw more marked. This is 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.