The 'intrinsic minus' hand (claw hand) deformity is characterized by hyperextension of the MCP joints and flexion of the IP joints. In a low ulnar nerve palsy, which fingers are MOST severely affected and why?

• A Index and middle fingers, because their lumbricals are median-innervated and compensate
• B All four fingers equally, since all interossei are ulnar-innervated
• C Middle and ring fingers only, due to their shared interosseous innervation
• D Ring and little fingers, because their lumbricals (for 4th and 5th digits) are ulnar-innervated and denervated ✓

Explanation

The lumbricals of the index and middle fingers are innervated by the median nerve, while lumbricals of the ring and little fingers are ulnar-innervated. In low ulnar nerve palsy, the ring and little finger lumbricals are denervated, so MCP extension (lumbrical action) is lost and interosseous weakness allows MCP hyperextension via unopposed long extensors, while IP joints flex — producing claw deformity. The index and middle fingers show less clawing because their median-innervated lumbricals partially maintain MCP flexion. This explains the 'ulnar paradox': high ulnar nerve injuries produce less claw deformity because denervated FDP reduces the IP flexion force.

Reference: Maheshwari Essential Orthopaedics, 6th ed.

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Written and medically reviewed by the StethoPrep medical team.