Flexor Digitorum Profundus Tendons: Anatomy, Function, Imaging

Topic

Flexor digitorum profundus tendons

The flexor digitorum profundus (FDP) tendons are the distal extensions of the flexor digitorum profundus muscle, located in the deep layer of the anterior forearm. These tendons play a vital role in flexing the distal interphalangeal (DIP) joints of the fingers and contribute to overall grip strength and dexterity of the hand.

The FDP muscle arises from the ulnar shaft and interosseous membrane and divides distally into four separate tendons, each passing through the carpal tunnel to the medial four digits (index to little finger). Each tendon continues distally to insert on the base of the distal phalanx, deep to the flexor digitorum superficialis (FDS) tendons.

Synonyms

Deep flexor tendons of the fingers
Tendons of the flexor digitorum profundus muscle

Parts

Muscular belly: Located in the proximal forearm; gives rise to four tendinous slips.
Tendinous portion: Four tendons that pass under the flexor retinaculum within the carpal tunnel.
Digital segments: Tendons continue into the fingers, each passing through the fibro-osseous digital sheath and beneath the flexor digitorum superficialis split (Camper’s chiasm) before inserting on the distal phalanx.

Origin, Course, and Insertion

Origin (muscular part): Proximal three-quarters of the anterior and medial surfaces of the ulna and adjacent interosseous membrane.
Course:
- Muscle transitions into four tendons near the wrist.
- The tendons pass deep to the flexor retinaculum within the carpal tunnel along with FDS and flexor pollicis longus.
- Each tendon enters a fibro-osseous tunnel of its respective finger, where it passes through the split in the FDS tendon (the chiasm of Camper).
Insertion:
- Each tendon inserts into the base of the distal phalanx of the 2nd to 5th fingers.

Relations

Proximally: Deep to flexor digitorum superficialis muscle.
At the wrist: Lies deep within the carpal tunnel beneath the FDS tendons.
In the hand: Each tendon enclosed in a common synovial sheath (ulnar bursa) before entering individual digital tendon sheaths.
In the digits:
- Superficial: Flexor digitorum superficialis tendon
- Deep: Distal phalanx bone
- Lateral: Digital arteries and nerves
- Dorsal: Middle phalanx and interosseous structures

Tendon Attachments

Each tendon inserts into the palmar base of the distal phalanx.
The tendons are anchored by vincula longum and brevis — small connective tissue bands that transmit blood vessels to the tendon.
Surrounded by the fibro-osseous sheath formed by annular and cruciform pulleys, maintaining tendon alignment and preventing bowstringing during finger flexion.

Nerve Supply

Medial (ulnar) part: Supplied by the ulnar nerve (ring and little fingers).
Lateral (radial) part: Supplied by the anterior interosseous branch of the median nerve (index and middle fingers).

Function

Primary function: Flexion of the distal interphalangeal (DIP) joints of the index, middle, ring, and little fingers.
Secondary actions: Assists in flexion of proximal interphalangeal (PIP) and metacarpophalangeal (MCP) joints and contributes to wrist flexion.
Gripping mechanism: Provides fine control and sustained force during grasping and pinching.
Coordination: Works synergistically with the flexor digitorum superficialis for smooth multi-joint flexion.

Clinical Significance

Tendon rupture: Often due to avulsion (jersey finger), particularly affecting the ring finger; results in loss of DIP flexion.
Tendinopathy: Chronic overuse causes inflammation and thickening within tendon sheath.
Tenosynovitis: Seen in rheumatoid arthritis or infection; causes pain and restricted motion.
Laceration injuries: Common in hand trauma; surgical repair crucial for functional recovery.
Trigger finger: Stenosing tenosynovitis affecting FDP or FDS tendon passage under the pulleys.
Imaging relevance: MRI and CT are key for evaluating integrity, continuity, and postoperative repair of FDP tendons.

MRI Appearance

T1-weighted images:
- Tendons: Low signal (dark linear structures).
- Muscle belly: Intermediate signal intensity.
- Marrow fat in phalanges: Bright.
- Normal sheath fat: Bright, outlining tendons.
- Tear or laceration: Discontinuity or gap with intermediate-to-bright signal from fluid or hemorrhage.
T2-weighted images:
- Tendons: Low signal intensity (dark).
- Muscle: Intermediate signal, slightly darker than on T1.
- Pathology:
  - Tendon tear: bright hyperintense fluid signal filling the gap.
  - Tenosynovitis: bright fluid signal surrounding tendon.
  - Tendon degeneration: focal intermediate signal within normally dark tendon.
STIR:
- Normal tendon: Dark (low signal).
- Inflamed or injured tendon: Bright hyperintense peritendinous or intratendinous signal.
- Postoperative scar: Low signal intensity.
Proton Density Fat-Saturated (PD FS):
- Normal: Dark, smooth tendons surrounded by intermediate-to-dark muscle signal.
- Partial tear or tendinitis: Focal bright signal within tendon.
- Tenosynovitis: Bright signal outlining tendon within its sheath.
T1 Fat-Sat Post-Contrast:
- Normal tendons: No significant enhancement.
- Inflammation or synovitis: Peritendinous and sheath enhancement.
- Post-surgical changes: Peripheral enhancement of scar tissue; non-enhancing gap indicates tendon rupture.

CT Appearance

Non-Contrast CT:

Tendons: Appear as fine soft-tissue density bands within the flexor compartments of fingers and hand.
Bone: High attenuation; cortical margins easily distinguish tendon path.
Calcifications: Chronic tendinopathy may show focal calcified densities along tendon path.
Avulsion fracture: May reveal small bony fragment at distal phalanx insertion.

Post-Contrast CT (standard):

Normal tendon: Mild uniform enhancement of surrounding soft tissues.
Inflamed or infected tendon sheath: Contrast enhancement of sheath walls.
Post-repair imaging: Highlights scar tissue, adhesions, or partial healing defects.

MRI image