The flexor pollicis brevis has two heads: the superficial head (median nerve innervated) arises from the flexor retinaculum and inserts on the radial sesamoid and proximal phalanx base via the radial tendon, while the deep head (ulnar nerve innervated) arises from the deep carpal bones and inserts via the ulnar tendon, often blending with adductor pollicis. The dual innervation of this muscle is unique among thenar muscles and explains its variable presentation in isolated nerve injuries.
The dual innervation of FPB explains why patients with high median nerve injuries may still retain some thumb flexion through the ulnar-innervated deep head, and why ulnar nerve palsy incompletely abolishes MCP flexion. In thenar reconstruction after nerve injury, the FPB tendon contributions are assessed electrophysiologically. FPB overactivity in spasticity or cerebral palsy produces a thumb-in-palm deformity managed with botulinum toxin injection or surgical lengthening of the FPB tendon.
Spastic overactivity of the flexor pollicis brevis and adductor pollicis in cerebral palsy or stroke produces a fixed flexion deformity at the thumb MCP joint with the thumb held across the palm, impairing grip and grasp, managed with botulinum toxin, splinting, or surgical tendon release and lengthening.
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