The FPB flexes the thumb MCP joint and is one of the three thenar muscles innervated predominantly by the median nerve. Its dual innervation makes it resilient to either median or ulnar nerve palsy alone. With the APB and opponens pollicis it produces thumb opposition.
| Origin | Flexor retinaculum and trapezium crest — median nerve supply; Trapezoid and capitate — ulnar nerve supply |
|---|---|
| Insertion | Radial side of the thumb proximal phalanx base and medial sesamoid |
| Nerve Supply | Superficial head: recurrent median nerve (C8, T1); deep head: deep ulnar nerve (C8, T1) — dual innervated |
| Blood Supply | Superficial palmar arch |
| Actions | MCP joint flexion of the thumb; Assists in thumb opposition; Contains the radial sesamoid in its tendon |
|---|
FPB MCP flexion positions the thumb pad to face the finger pads during pinch, complementing APB's palmar abduction and opponens' CMC rotation.
In combined median and ulnar nerve palsy the FPB is completely denervated, producing complete loss of thumb opposition requiring tendon transfer for functional restoration. Isolated FPB paralysis from median nerve lesions is partially compensated by the ulnar-innervated deep head.
The FPB is palpable as the central thenar muscle between the APB (radial) and adductor pollicis (ulnar), becoming firm during resisted thumb MCP flexion.
FPB denervation in median nerve palsy contributing to loss of thumb opposition managed with opponens transfer surgery.
