First Flexor Digitorum Profundus Tendon (Index): Anatomy & Imaging

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First flexor digitorum profundus tendon

The first flexor digitorum profundus (FDP) tendon is the portion of the FDP muscle that inserts into the distal phalanx of the index finger. It is one of four terminal tendons of the FDP, lying deep to the flexor digitorum superficialis (FDS) in the forearm and palm.

Unlike the tendons to the middle, ring, and little fingers—which often share a common muscle belly—the index FDP tendon arises from a more distinct muscular portion, allowing independent flexion of the index distal interphalangeal (DIP) joint. This independence provides precision grip and fine motor control essential for tasks like pinching or writing.

Synonyms

FDP tendon to index finger
Radial slip of flexor digitorum profundus
Flexor profundus indicis tendon

Origin, Course, and Insertion

Origin: Deep anterior and medial surfaces of the upper three-quarters of the ulna and interosseous membrane (via the FDP muscle belly).
Course:
- Descends deep to the FDS within the flexor compartment of the forearm.
- Passes through the carpal tunnel deep to the flexor retinaculum.
- In the palm, the tendon runs within its own fibrous digital sheath to the index finger.
- Lies deep to the FDS tendon of the same finger, which splits at the level of the proximal phalanx to allow passage of the FDP.
Insertion:
- Attaches to the palmar base of the distal phalanx of the index finger.
- Provides flexion at the DIP joint specifically.

Tendon Attachments

Enclosed within a synovial tendon sheath that extends from the carpal tunnel to the distal phalanx.
Surrounded by fibrous pulleys (A1–A5), which maintain close apposition to bone and prevent bowstringing.
The FDS tendon bifurcates around the FDP tendon at the level of the proximal phalanx, forming the characteristic “Camper’s chiasm.”

Relations

Proximally: Flexor digitorum superficialis (FDS) and median nerve in the carpal tunnel.
Distally: Distal phalanx and periosteum of the index finger.
Medially: Adjacent FDP tendons to middle and ring fingers.
Laterally: Flexor pollicis longus tendon (in the forearm).
Dorsally: Middle phalanx and volar plate of the distal interphalangeal joint.

Nerve Supply

Anterior interosseous nerve (branch of the median nerve, C8–T1).

Function

Primary: Flexes the distal interphalangeal (DIP) joint of the index finger.
Secondary: Assists in flexion at the proximal interphalangeal (PIP), metacarpophalangeal (MCP), and wrist joints.
Independent movement: The index tendon operates semi-independently due to its distinct muscle belly, enabling isolated finger flexion.
Functional importance: Essential for precision grip, typing, writing, and fine object manipulation.

Clinical Significance

Tendon injury: Lacerations or avulsions can result in loss of DIP flexion (commonly known as “jersey finger”).
Tendinopathy: Repetitive strain or overuse may lead to inflammation or microtears within the tendon sheath.
Adhesions or stenosing tenosynovitis: Scar tissue or sheath thickening may restrict tendon glide, producing stiffness or triggering.
Nerve lesions: Anterior interosseous nerve palsy causes weakness or inability to flex the distal phalanx of the index finger.
Surgical relevance: Critical structure during flexor tendon repair, carpal tunnel release, and tendon grafting procedures.

MRI Appearance

T1-weighted images:
- Normal tendon: Uniform low signal (dark), surrounded by bright peritendinous fat.
- Muscle belly: Intermediate signal intensity, with clear fascicular pattern.
- Tendon sheath: Thin low-signal line enclosing the tendon.
- Pathology: Partial tears or degeneration appear as focal intermediate-to-bright signal within tendon fibers.
T2-weighted images:
- Normal tendon: Low signal (dark linear structure).
- Muscle belly: Intermediate-to-dark signal, slightly darker than T1.
- Tendinopathy: Bright hyperintense signal areas indicating fiber disruption or fluid accumulation.
- Tenosynovitis: Bright signal outlining tendon sheath due to synovial fluid or inflammation.
STIR:
- Normal tendon: Dark to intermediate signal with sharp margins.
- Inflammation or edema: Bright hyperintense signal around or within tendon sheath.
- Partial tear: Focal bright regions within otherwise dark tendon fibers.
Proton Density Fat-Saturated (PD FS):
- Normal tendon: Intermediate-to-dark homogeneous signal.
- Pathology: Bright signal along tendon substance or peritendinous region due to fluid, synovitis, or strain.
- Ideal for visualizing tendinopathy, tenosynovitis, or early microtears.
T1 Fat-Sat Post-Contrast:
- Normal tendon: Minimal enhancement.
- Inflamed tendon sheath: Circumferential enhancement due to synovitis.
- Partial tears: Peripheral enhancement outlining disrupted fibers.
- Chronic scar tissue: Mild, heterogeneous enhancement pattern.

CT Appearance

Non-Contrast CT:

Tendon: Appears as a linear soft-tissue density along the volar aspect of the phalanges.
Muscle belly: Homogeneous soft-tissue attenuation in the deep anterior compartment.
Indirect signs of pathology: Soft-tissue swelling, effusion, or subtle calcification within the tendon sheath (chronic tendinopathy).
Bony interface: Visualizes enthesophytes or cortical irregularity at the distal phalanx insertion.

Post-Contrast CT (standard):

Normal tendon: Minimal enhancement.
Inflamed or thickened sheath: Enhances circumferentially, distinguishing tenosynovitis from pure mechanical injury.
Tendon rupture or avulsion: Demonstrated as discontinuity or proximal tendon retraction.
Useful in: Evaluating postsurgical tendon repair, chronic scarring, or calcific deposits along the flexor sheath.

MRI image

CT image

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