The flexor pollicis brevis has the unusual distinction of dual innervation, with the superficial head supplied by the median nerve and the deep head by the ulnar nerve, making it the muscle that straddles the thenar innervation boundary. It flexes the thumb MCP joint and assists in opposition, working alongside the abductor pollicis brevis and opponens pollicis to produce the full opposition movement. The radial sesamoid within its insertion is the sesamoid that most commonly fractures from a fall on the thumb.
| Origin | Flexor retinaculum and trapezium; Trapezoid and capitate |
|---|---|
| Insertion | Radial side of the base of the thumb proximal phalanx and the radial sesamoid |
| Nerve Supply | Superficial head: recurrent branch of the median nerve (C8, T1); Deep head: deep branch of the ulnar nerve (C8, T1) |
| Blood Supply | Superficial palmar arch |
| Actions | Flexion of the thumb MCP joint; Assists in thumb opposition |
|---|
Its MCP flexion positions the thumb for gripping, and alongside the opponens and APB it contributes to the combination of movements that bring the thumb pad to face the fingertips during opposition.
The dual innervation of the FPB superficial and deep heads means that after isolated median nerve palsy, the deep head remains functional and can provide some residual MCP flexion. This must be taken into account when testing for thenar muscle function in nerve injury. The radial sesamoid, within the FPB tendon, is the most commonly fractured sesamoid in the hand from a hyperextension injury.
The FPB is palpable as the central thenar muscle between the more radial APB and the more ulnar adductor pollicis, becoming firm during resisted thumb MCP flexion with the IP joint held extended.
Fracture of the radial sesamoid within the FPB insertion from a fall on the thumb or direct impact, producing MCP joint pain and tenderness on the radial plantar thumb, managed with immobilisation.
