Second Flexor Digitorum Profundus Tendon: Anatomy, Function, Imaging

Topic

Second flexor digitorum profundus tendon

The second flexor digitorum profundus tendon, also known as the middle finger FDP tendon, is one of four distal tendinous slips of the flexor digitorum profundus muscle, located in the deep anterior compartment of the forearm. It provides the primary flexion force to the distal interphalangeal (DIP) joint of the middle finger, enabling precision grip and fine digital control.

This tendon travels deep within the carpal tunnel, then passes along the flexor sheath of the middle finger, deep to the flexor digitorum superficialis (FDS) tendon, to insert onto the base of the distal phalanx. The second FDP tendon is essential in coordinated finger movement, working in concert with the lumbrical muscle arising from its radial side.

Synonyms

Middle finger FDP tendon
Deep flexor tendon to the second digit
Profundus tendon of the middle finger

Origin and Course

Origin: From the anterior and medial surfaces of the ulna and interosseous membrane (as part of the common FDP muscle belly).
Course:
- Descends in the deep flexor plane of the forearm beneath the flexor digitorum superficialis.
- Passes through the carpal tunnel, surrounded by the common synovial sheath of the flexor tendons.
- In the palm, the muscle divides into four tendons for the index, middle, ring, and little fingers.
- The second tendon proceeds toward the middle finger, entering its fibro-osseous flexor sheath.
- It passes deep to the FDS tendon, which splits at the proximal phalanx (Camper’s chiasm), allowing the FDP tendon to pass through and continue distally.
- Terminates by inserting into the palmar surface of the base of the distal phalanx of the middle finger.

Insertion

Distal attachment: Base of the distal phalanx of the middle finger.
The insertion allows flexion of the distal interphalangeal joint (DIP) and contributes to overall digital flexion synergy.

Relations

Proximally: Deep to flexor digitorum superficialis; adjacent to median nerve and ulnar artery in forearm.
Distally: Surrounded by the digital synovial sheath within the flexor tunnel of the finger.
Superficial to: Flexor digitorum profundus muscle belly in the forearm.
Deep to: Flexor digitorum superficialis split at proximal phalanx (chiasm of Camper).
Laterally: Paired neurovascular bundles (digital arteries and nerves, though excluded here by request).

Tendon Sheath and Pulley System

Enclosed within a fibro-osseous digital sheath, reinforced by the annular (A1–A5) and cruciate (C1–C3) pulleys.
These structures maintain the tendon’s proximity to the phalanges, preventing bowstringing and ensuring efficient flexion.
Synovial sheath provides lubrication, allowing smooth tendon gliding during movement.

Nerve Supply

Median nerve (C8–T1) via the anterior interosseous branch — controlling flexion and proprioception of the middle finger DIP joint.

Function

Primary flexor of the distal interphalangeal joint (DIP) of the middle finger.
Assists in flexion of the proximal interphalangeal (PIP) and metacarpophalangeal (MCP) joints through synergistic contraction.
Provides power grip and precision grasp control during fine motor tasks.
Works with the lumbrical muscle (arising from its lateral side) to aid coordinated flexion of MCP with extension of interphalangeal joints.

Clinical Significance

Tendon rupture or avulsion: May occur from forceful hyperextension of DIP joint (“jersey finger”).
Tenosynovitis: Inflammation within the flexor sheath causes pain and restricted motion.
Adhesions: Post-traumatic or postsurgical scarring impairs smooth tendon gliding.
Trigger finger: Thickening of tendon or pulley system can cause locking during flexion.
Surgical relevance: Commonly evaluated during tendon repair, grafting, or carpal tunnel procedures.
Imaging importance: MRI is preferred for detecting partial or complete tears, tenosynovitis, or pulley injuries.

MRI Appearance

T1-weighted images:
- Tendon: Low signal (dark), homogeneous and sharply marginated.
- Muscle belly: Intermediate signal.
- Marrow (phalanges): Bright fatty signal.
- Tendon sheath: Thin low-signal line; fluid minimal in normal state.
- Pathology: Partial tear or thickening shows intermediate-to-bright intratendinous signal.
T2-weighted images:
- Tendon: Dark, low signal; normal fiber continuity preserved.
- Muscle belly: Intermediate-to-dark, slightly darker than on T1.
- Sheath fluid: Bright (hyperintense); small amount normal, excess indicates tenosynovitis.
- Tears or degeneration: Focal or linear bright intratendinous signal extending to surface.
STIR:
- Normal tendon: Intermediate-to-dark signal with clear margins.
- Inflammation or edema: Bright hyperintense signal surrounding or within the tendon.
- Useful for: Detecting early tenosynovitis, partial tears, or pulley inflammation.
Proton Density Fat-Saturated (PD FS):
- Normal tendon: Intermediate-to-dark homogeneous signal.
- Abnormal tendon: Bright hyperintense focal area indicating tendinopathy or tear.
- Sheath effusion: Bright rim of fluid encasing tendon, common in overuse or inflammatory conditions.
T1 Fat-Sat Post-Contrast:
- Normal tendon: No significant enhancement.
- Tenosynovitis: Enhancing synovial sheath surrounding non-enhancing tendon.
- Partial tear: Irregular peripheral enhancement; complete rupture shows enhancement in retracted ends or granulation tissue.

CT Appearance

Non-Contrast CT:

Tendon: Soft-tissue density, linear and low attenuation relative to bone.
Bony insertions: Smooth cortical attachment at distal phalanx base.
Pathology: Calcific tendinitis may appear as small hyperdense foci along tendon course.
Fracture-avulsion: Bony fragment visible at distal insertion in “jersey finger.”

Post-Contrast CT (standard):

Normal tendon: Minimal homogeneous enhancement.
Inflammatory or post-traumatic changes: Enhancing peritendinous soft tissue or synovial sheath.
Useful for: Evaluating tendon integrity and bone avulsion in patients unable to undergo MRI.

MRI image

CT image

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