The lumbrical tendons arise uniquely from the flexor tendons themselves (FDP) rather than from bone, and insert into the extensor expansions. This unique origin-insertion arrangement means lumbricals can coordinate flexor and extensor forces: when the FDP flexes (bringing the lumbrical origin closer to its insertion), the lumbricals relax rather than shorten, preventing paradoxical IP joint flexion during MCP flexion — the basis of the lumbrical-plus phenomenon.
MCP joint flexion and IP joint extension simultaneously; the intrinsic plus position
The lumbrical-plus finger is a paradoxical clinical finding where FDP tendon division or excessive FDP tendon graft length causes the lumbrical to be stretched rather than relaxed during FDP action, producing IP joint extension when the patient attempts finger flexion. This is recognised by the patient complaining that their finger extends when they try to make a fist. Treatment requires lumbrical release or reduction of FDP graft tension.
Paradoxical IP extension during attempted flexion from excessive lumbrical tension after FDP tendon repair or grafting, managed with lumbrical release or graft tension adjustment.
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