A high-level nerve injury may result in which characteristic deformity?

• A. Flexion of the ring and small finger MCP joints because of loss of the extension-controlling forces of the third and fourth lumbricals
• B. Hyperextension of the index and middle finger MCP joints because of loss of the extension-controlling forces of the first and second lumbricals
• C. Unchecked abduction of the ring and small finger MCP joints because of lack of motor innervation the third and fourth lumbricals
• D. Hyperextension of the ring and small finger MCP joints because of loss of the extension-controlling forces of the third and fourth lumbricals

Answer: (D)

When the intrinsic hand muscles that balance the extensor mechanism are lost, the extensor digitorum can extend the metacarpophalangeal joints without opposition. The lumbricals normally flex the MCP joints and simultaneously extend the IP joints; the third and fourth lumbricals, which are innervated by the ulnar nerve, provide this flexion and extension balance for the ring and little fingers. If those lumbricals are weakened or paralyzed due to a high-level ulnar nerve injury, that flexion force at the MCP joints disappears. The extensor digitorum then dominates at the MCP joints, causing hyperextension of the ring and small finger MCPs. The index and middle fingers are less affected because their MCP flexion is supported by the first and second lumbricals (median nerve) and the overall balance of the hand's extensor mechanism.