The lumbrical muscles arise from the FDP tendons in the palm and insert via their tendons into the radial lateral band of the extensor mechanism at the level of the proximal phalanx. Their unique origin from the FDP and insertion into the extensor hood creates a mechanism whereby MCP flexion (through FDP contraction) also increases lumbrical tension on the extensor hood, producing simultaneous IP joint extension.
The lumbrical contribution explains the intrinsic-plus position (MCP flexion with IP extension), the position of safe immobilisation for the hand. It also explains the paradox of lumbrical-plus finger: when the FDP is divided distal to the lumbrical origin, grip activates the lumbrical to extend the IP joints rather than flex the finger. In intrinsic tightness (from spasticity, burn contracture, or fibrosis), the lumbricals tighten across the lateral bands producing DIP flexion with attempted MCP extension, tested by the Bunnell intrinsic tightness test.
FDP tendon division distal to the lumbrical origin redirects FDP pull proximally through the lumbrical and extensor hood, producing paradoxical IP joint extension when the patient tries to flex, corrected by lumbrical tenotomy or proximal FDP anastomosis.
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