The FDS is the superficial long finger flexor producing PIP joint flexion. It can be tested in isolation by holding the other three fingers in full extension (blocking FDP by its shared muscle belly) and asking the patient to flex the tested finger. Zone II flexor tendon injuries in the digital sheath involve both FDS and FDP.
| Origin | Medial epicondyle and medial coronoid process; Anterior radial shaft below the radial tuberosity |
|---|---|
| Insertion | Middle phalanx bases of fingers 2-5, splitting into two slips that allow the FDP to pass through (Camper chiasm) |
| Nerve Supply | Median nerve (C7, C8, T1) |
| Blood Supply | Ulnar artery |
| Actions | PIP joint flexion of fingers 2-5; MCP joint flexion assistance; Wrist flexion when finger joints are fully flexed |
|---|
The FDS tendon splits at each finger's A1 pulley into two slips that pass around the FDP tendon before reinserting on the middle phalanx — the Camper chiasm. This arrangement allows FDP to reach the distal phalanx while FDS inserts more proximally.
The FDS tendon is the most commonly transferred tendon for opposition reconstruction (Bunnell transfer using the ring finger FDS routed to the abductor pollicis brevis for median nerve palsy thumb opposition restoration).
Individual FDS tendons are palpable at the wrist flexor crease during isolated PIP flexion of each finger with the others held extended.
Zone II flexor tendon division requiring primary repair in the critical no man's land region.
