FDP is the only flexor of the DIP joint and is uniquely innervated by two different nerves — the AIN for the radial two slips (index and middle) and the ulnar nerve for the ulnar two slips (ring and little). This dual innervation makes it a key nerve injury localiser.
| Origin | Upper three-quarters of the anterior and medial ulna and interosseous membrane |
|---|---|
| Insertion | Bases of the distal phalanges of fingers 2-5 |
| Nerve Supply | Anterior interosseous nerve (AIN) for index and middle (C7, C8, T1); Ulnar nerve for ring and little (C8, T1) |
| Blood Supply | Anterior interosseous artery |
| Actions | Flexes the DIP joints of all four fingers — the only DIP flexor; Assists PIP and MCP flexion; Assists wrist flexion |
|---|
The "profundus advancement" or "FDP to FDS" repair is used in zone II flexor tendon injuries. FDP avulsion (jersey finger) at the distal phalanx insertion requires urgent repair to restore DIP flexion. Inability to flex the DIP joint with the PIP held extended (FDP test) is specific for FDP discontinuity.
Tested by DIP flexion with the PIP held in extension by the examiner.
FDP avulsion from the distal phalanx base in a jersey-grabbing injury producing inability to flex the DIP joint, requiring urgent surgical reattachment to prevent permanent DIP extension deformity.
