Third Flexor Digitorum Profundus Tendon: Anatomy, Function, Imaging

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Third flexor digitorum profundus tendon

The third flexor digitorum profundus (FDP) tendon, corresponding to the ring finger, is one of the four terminal tendons of the flexor digitorum profundus muscle, located in the deep anterior compartment of the forearm. This tendon passes through the carpal tunnel and along the flexor sheath of the finger to insert on the base of the distal phalanx of the ring finger.

It provides powerful flexion of the distal interphalangeal (DIP) joint, contributing to composite finger flexion during gripping and fine motor tasks. The ring finger FDP tendon often shares a muscle belly with the middle finger tendon, making it anatomically and functionally interconnected—an important consideration in tendon injury, repair, and rehabilitation.

Synonyms

FDP tendon to the fourth digit
Profundus tendon of the ring finger
Deep flexor tendon of the ring finger

Origin, Course, and Insertion

Origin (muscle): Arises from the upper three-fourths of the anterior and medial surfaces of the ulna and adjacent interosseous membrane.
Course:
- The tendon for the ring finger separates distally within the forearm and passes deep to the flexor digitorum superficialis (FDS).
- It enters the carpal tunnel beneath the flexor retinaculum, enclosed in a common synovial sheath with other FDP tendons.
- Within the finger, it runs deep to the corresponding FDS tendon and emerges through the FDS split at the level of the proximal phalanx.
- It continues distally within the fibrous flexor sheath, surrounded by synovial fluid for smooth gliding.
Insertion: Base of the distal phalanx of the ring finger (fourth digit), on its palmar surface.

Tendon Attachments and Sheaths

Enclosed within a synovial sheath continuous with the digital flexor sheath of the ring finger.
Surrounded by the fibrous flexor pulley system (A1–A5 pulleys), which maintains tendon alignment during flexion.
The vincula tendinum (vinculum longum and breve) connect the tendon to the phalanges, providing vascular and structural support.
Works in coordination with the FDS tendon and lumbrical muscle for smooth, coordinated flexion.

Relations

Proximally: Lies deep to the FDS in the forearm and palm.
Distally: Deep to the flexor sheath and between A4 and A5 pulleys.
Anteriorly: Covered by fibrous pulley and flexor sheath.
Posteriorly: Lies directly over the phalanges and volar plate of the interphalangeal joints.
Laterally: Flanked by synovial sheath and adjacent digital nerves and vessels.

Nerve Supply

Anterior interosseous nerve (branch of median nerve, C8–T1).
Occasionally dual innervation with ulnar nerve slips supplying ulnar-side tendons (ring and little fingers).

Function

Flexion of distal interphalangeal joint: Primary flexor of the distal phalanx of the ring finger.
Assists in flexion of proximal and metacarpophalangeal joints during full grip.
Power grip and precision tasks: Critical in hand closure and force transmission.
Synergy: Works with FDS and lumbricals to provide smooth graded flexion across joints.

Clinical Significance

Tendon laceration: Common at distal phalanx or mid-finger levels (Zone I and II injuries).
Avulsion injury: Known as “jersey finger,” where FDP tendon avulses from the distal phalanx—frequent in ring finger due to shared muscle belly and high tension.
Adhesions and tenosynovitis: Restrict tendon gliding within sheath, common post-trauma or in rheumatoid arthritis.
Trigger finger (stenosing tenosynovitis): May affect the tendon near the A1 pulley, causing catching or locking.
Surgical importance: Precise repair essential for restoring DIP flexion and overall hand grip strength.

MRI Appearance

T1-weighted images:
- Tendon: Low signal (dark) linear structure within the flexor sheath.
- Surrounding fat: Bright, highlighting tendon margins.
- Muscle belly: Intermediate signal intensity proximally.
- Pathology: Partial tears appear as localized discontinuity or focal thickening with intermediate signal; complete tears show tendon gap.
T2-weighted images:
- Normal tendon: Low signal, darker than muscle.
- Muscle: Intermediate signal, darker than on T1.
- Pathology:
  - Tendinitis or partial tear: Bright hyperintense areas within or around tendon.
  - Tenosynovitis: Bright fluid signal within the sheath.
  - Complete rupture: Discontinuity with surrounding hyperintense fluid gap.
STIR:
- Normal tendon: Dark low signal.
- Pathology: Bright hyperintense sheath fluid or surrounding edema in inflammation, tear, or tenosynovitis.
- Excellent for early detection of soft-tissue edema and tendon sheath inflammation.
Proton Density Fat-Saturated (PD FS):
- Normal tendon: Dark homogeneous signal with sharp margins.
- Pathologic tendon: Bright hyperintensity indicating edema, strain, or fluid around tendon.
- Highlights subtle peritendinous inflammation and pulley system pathology.
T1 Fat-Sat Post-Contrast:
- Normal tendon: Minimal to no enhancement.
- Inflamed tendon sheath: Diffuse enhancement along sheath margins.
- Chronic tenosynovitis or adhesions: Irregular peripheral enhancement.
- Postoperative repair: Enhancing granulation tissue may appear around tendon repair site.

CT Appearance

Non-Contrast CT:

Tendon visualized as soft-tissue density structure anterior to the phalanges.
Flexor sheath appears as a subtle tubular outline along the palmar aspect of the ring finger.
Indirect findings include swelling, calcifications, or bony avulsion at the distal phalanx base in jersey finger.

Post-Contrast CT (standard):

Enhanced soft-tissue definition highlights tendon margins.
Inflamed tendon sheath or postoperative scarring shows increased enhancement.
Helpful in evaluating chronic thickening, tendon retraction, and ossified scars when MRI is contraindicated.

MRI image

CT image

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