The fourth lumbrical is the most ulnar and is ulnar-nerve innervated. Its origin from two FDP tendons (ring and little) makes it a two-tendon lumbrical — the only one of the four with this anatomy.
| Origin | Adjacent sides of the FDP tendons to the ring and little fingers |
|---|---|
| Insertion | Radial side of the extensor expansion of the little finger |
| Nerve Supply | Deep branch of ulnar nerve (C8, T1) |
| Blood Supply | Digital arteries |
| Actions | Flexes the little finger MCP joint; Extends the little finger IP joints |
|---|
Fourth lumbrical denervation in ulnar nerve palsy contributes to clawing of the little finger. It is the lumbrical that occupies the hypothenar eminence region. Accessory fourth lumbrical (present in some individuals) can compress Guyon's canal contents.
Tested by resisted little finger MCP flexion with IP extended.
Loss of fourth lumbrical in ulnar palsy contributing to little finger clawing — hyperextension of the MCP and flexion of the IP joints from absent intrinsic MCP flexion force.
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