The lumbrical tendons arise uniquely from the FDP tendons in the palm rather than from bone, pass on the radial side of the MCP joints, and insert into the radial lateral bands of the extensor expansions. This arrangement links the flexor and extensor systems: when a lumbrical contracts it flexes the MCP joint, pulls the FDP tendon distally to slacken it, and tightens the extensor apparatus, making the lumbricals effective IP joint extensors. Together with the interossei they produce the intrinsic plus position of MCP flexion with IP extension.
| Muscle |
|
|---|---|
| Attachment | Arise from the FDP tendons in the palm and insert on the radial lateral bands of the extensor expansions of fingers 2 through 5 |
MCP joint flexion and IP joint extension simultaneously; the intrinsic plus position
The lumbrical-plus finger is a paradoxical finding after FDP division distal to the lumbrical origin, or after an FDP graft that is too long. FDP pull is then transmitted through the lumbrical to the extensor apparatus, producing IP joint extension when the patient attempts to make a fist. Treatment requires lumbrical release or correction of FDP graft tension. The intrinsic plus position is also the safe position for splinting the injured hand, and intrinsic tightness from spasticity, burns or fibrosis is assessed with the Bunnell test.
Paradoxical IP extension during attempted flexion after FDP division distal to the lumbrical origin or an overly long FDP graft, managed with lumbrical release or graft tension adjustment.
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