Which deformity results from loss of the extension-controlling forces of the third and fourth lumbricals, leading to hyperextension at certain MCP joints?

• A. Flexion of the ring and small finger MCP joints
• B. Hyperextension of the index and middle finger MCP joints
• C. Uncheck abduction of the ring and small finger MCP joints
• D. Hyperextension of the ring and small finger MCP joints

Answer: (D)

Lumbricals control finger posture by flexing the MCP joints while extending the PIP and DIP joints. The 3rd and 4th lumbricals, which go to the ring and little fingers, are particularly important for opposing the extensor digitorum at the MCP joints of those fingers. When their extension-controlling forces are lost, the extensor digitorum acts unopposed at the MCP joints, causing hyperextension of the ring and little finger MCP joints. This loss of intrinsic balance is a classic pattern seen with ulnar-nerve–related lumbrical dysfunction, often accompanied by IP flexion (clawing) compounding the deformity.