What is the action of the Flexor Digitorum Profundus (FDP) tendon?

Action of the Flexor Digitorum Profundus (FDP) Tendon

The primary action of the Flexor Digitorum Profundus (FDP) tendon is to flex the distal interphalangeal (DIP) joints of digits 2-5, allowing for powerful grip and fine motor control of the fingers.

Anatomical Considerations

• The FDP is a deep muscle originating from the anterior surface of the ulna and interosseous membrane
• Its four tendons pass through the carpal tunnel and continue distally through the hand
• Each tendon passes through the corresponding flexor digitorum superficialis (FDS) tendon at Camper's chiasm in zone II
• The FDP tendons insert on the palmar base of the distal phalanges of digits 2-5

Functional Actions

Primary Action

• Flexion of the distal interphalangeal (DIP) joint
  • This is the defining action of the FDP and can be isolated by holding the proximal and middle phalanges stable while asking the patient to flex the distal phalanx

Secondary Actions

• Flexion of the proximal interphalangeal (PIP) joint
• Flexion of the metacarpophalangeal (MCP) joint
• Weak flexion of the wrist joint (when all digits flex simultaneously)

Clinical Significance

Testing FDP Function

• To isolate and test FDP function:
  1. Stabilize the middle phalanx (blocking PIP joint flexion)
  2. Ask the patient to flex the distal phalanx
  3. Inability to flex the DIP joint indicates FDP dysfunction

Pathological Conditions

• FDP avulsion ("Jersey finger") - common in contact sports when the finger is forcibly extended while actively flexed 1
• FDP tendon lacerations in zone II ("no man's land") may occur with superficialis tendon intact 2
• Closed ruptures of FDP are rare but can occur 3

Surgical Considerations

• Digital flexor tenotomy of the FDP is used to treat plantar or apex ulcers on flexible claw or hammer toe deformities in digits 2-5 4
• In cases of isolated FDP injury, reconstruction techniques include FDP-FDS-FDP dual tendon transfer 5
• Dynamic tenodesis procedures can maintain approximately 34° of DIP flexion after FDP injury 6

Biomechanical Importance

• The FDP is essential for:
  • Power grip (holding objects firmly)
  • Precision grip (fine manipulation)
  • Hook grip (carrying bags)
  • Pinch grip (holding small objects)

Understanding the action and function of the FDP tendon is crucial for diagnosing hand injuries and planning appropriate surgical interventions when tendon damage occurs.