Flexor Digitorum Profundus

Clinical Notes

• The dual innervation is the single most important clinical fact about FDP. The index and middle finger portions are innervated by the anterior interosseous nerve (AIN, a branch of the median nerve) — loss of DIP flexion in these fingers suggests AIN palsy (Kiloh-Nevin syndrome). The ring and little finger portions are innervated by the ulnar nerve — loss of DIP flexion in these fingers localizes to the ulnar nerve proximal to the wrist (since the ulnar nerve branch to FDP arises in the forearm).
• The "pinch test" for AIN palsy: ask the client to make an "OK" sign (thumb tip to index fingertip). In AIN palsy, the DIP of the index finger and the IP of the thumb cannot flex, producing a triangle or flat pinch instead of a round circle.

• FDP tendons pass through the carpal tunnel — like FDS, hypertonic FDP contributes to carpal tunnel pressure. Treatment of conditions/carpal-tunnel-syndrome should include forearm flexor release.
• Jersey finger — rupture of the FDP tendon from its distal phalanx insertion, caused by forced extension of a flexed DIP (e.g., grabbing a jersey during a tackle). Inability to flex the DIP of the affected finger is diagnostic. This is a surgical condition — refer immediately.
• Anterior interosseous syndrome (Kiloh-Nevin syndrome) — compression of the AIN produces weakness of FDP to the index finger, FPL, and pronator quadratus without sensory loss (AIN is pure motor). The pinch test is the quick clinical screen.

• In clients with heavy gripping occupations (manual labor, climbing, musical instrument players), FDP is chronically loaded and hypertonic. Because it is deep, it is rarely treated directly — students work the superficial flexors and assume the deep layer will follow.
• Tenderness on deep palpation against the ulna in the proximal-medial forearm is common and often produces a "deep ache" that the client finds difficult to localize.

• Requires deep, slow pressure accessed from the medial forearm against the ulna. The muscle does not respond well to rapid or superficial techniques because it is deep and thick.
• Post-treatment, grip strength and finger dexterity often improve. In musicians (especially guitarists and pianists), FDP release can significantly improve finger independence and reduce forearm fatigue.

• The ulnar artery and ulnar nerve run along the medial forearm superficial to FDP — when pressing deeply against the ulna, monitor for arterial pulsation and avoid sustained compression of the neurovascular bundle.
• The anterior interosseous nerve runs on the anterior surface of the interosseous membrane between FDP and FPL — deep pressure in the central forearm can irritate this motor nerve.

• When assessing hand function, always test DIP flexion independently from PIP flexion. Many students only test "finger flexion" as a single movement and miss isolated FDP weakness. The DIP flexion test (stabilize PIP, flex DIP) is the fastest way to check FDP and, by extension, to screen for AIN palsy (index and middle) or ulnar nerve lesions (ring and little).