The flexor digitorum profundus is the deep finger flexor that alone controls distal phalanx flexion, passing through the flexor tendon sheath alongside the FDS which splits to allow the FDP to reach the distal phalanx. Its unique dual innervation, median nerve for index and middle fingers, ulnar nerve for ring and little, means that no single nerve injury paralyses all four digits, a clinically important anatomical feature used in nerve injury localisation. FDP is the tendon used in Jersey finger avulsion injuries.
| Origin | Proximal three-quarters of the anterior and medial ulnar surface and adjacent interosseous membrane |
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| Insertion | Palmar surface of the bases of the distal phalanges of all four fingers |
| Nerve Supply | Lateral two digits: anterior interosseous nerve (median), C8, T1; Medial two digits: ulnar nerve, C8, T1 |
| Blood Supply | Anterior interosseous artery; Ulnar artery |
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The profundus test isolates FDP function by holding the MCP and PIP joints extended and asking the patient to flex only the DIP joint, possible only through FDP action, as FDS does not reach the distal phalanx.
Zone II flexor tendon repairs in the digital flexor tendon sheath (between the A1 and A3 pulleys) are the most technically challenging tendon repairs because both FDP and FDS must be repaired in the tight sheath. The lumbrical muscles arise from the FDP tendons and can produce lumbrical plus deformity if the FDP is cut, paradoxical IP extension when the patient tries to flex the finger. FDP avulsion (Jersey finger) requires urgent surgical repair within 10 to 14 days before the retracted tendon loses vascularity.
FDP function is assessed by isolating DIP joint flexion with the PIP held extended. The muscle belly is palpable in the mid-forearm medial to the FDS.
FDP avulsion from the distal phalanx during gripping producing inability to flex the DIP joint, requiring urgent surgical repair within 2 weeks.
Overuse degeneration within the flexor tendon sheath producing triggering or locking in avid climbers from sustained gripping, managed with sheath injection and load modification.
