First Flexor Digitorum Superficialis Tendon: Anatomy & Imaging

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First flexor digitorum superficialis tendon

The first flexor digitorum superficialis (FDS) tendon is the specific tendon of the index finger arising from the FDS muscle in the forearm. It is one of four tendons that pass through the carpal tunnel, lying superficial to the tendons of the flexor digitorum profundus (FDP). It provides powerful flexion at the proximal interphalangeal (PIP) joint and contributes to flexion at the metacarpophalangeal (MCP) joint.

This tendon is essential for independent finger flexion and precision grip. It forms a bifurcated insertion (Camper’s chiasm) that allows the deeper FDP tendon to pass through, creating a strong, interdependent flexor system in the digits.

Synonyms

FDS tendon to index finger
Superficial digital flexor tendon of the index
Sublimis tendon (index)

Origin and Course

Origin: Continuation of the FDS muscle belly arising from the medial epicondyle of the humerus, coronoid process of ulna, and anterior border of the radius.
Course:
- Passes through the carpal tunnel, lying superficial to the FDP tendons.
- In the palm, the FDS tendon to the index lies superficial and radial to the tendons of the middle, ring, and little fingers.
- Enters the fibrous digital sheath of the index finger along with the FDP tendon.
- At the level of the proximal phalanx, the FDS tendon splits into two slips (Camper’s chiasm) to allow the FDP tendon to pass through.

Insertion

Each slip of the split tendon reunites and inserts on the sides of the shaft of the middle phalanx of the index finger.
The arrangement allows FDP to continue distally to the base of the distal phalanx, flexing the DIP joint while the FDS acts primarily at the PIP joint.

Relations

Proximal (forearm): Lies deep to palmaris longus and flexor carpi radialis tendons.
At wrist: Within the carpal tunnel, superficial to FDP tendons and deep to flexor retinaculum.
In palm: Covered by palmar aponeurosis and lumbrical origin.
At finger level:
- Anteriorly: Fibrous digital sheath.
- Posteriorly: Flexor digitorum profundus tendon (passing through the split).
- Laterally: Digital neurovascular bundles (proper digital arteries and nerves).

Tendon Attachments and Sheath

Enclosed within a synovial sheath inside a fibrous digital tunnel along the finger.
The A1 to A5 annular pulleys and C1 to C3 cruciform pulleys maintain tendon position and prevent bowstringing during flexion.
At the PIP joint, the FDS tendon splits, forming the Camper’s chiasm, through which the FDP tendon passes.

Nerve Supply

Median nerve (C7–T1) via muscular branches to FDS in the forearm.
Though not a direct tendon nerve, proprioceptive fibers within the tendon sheath contribute to fine motion coordination.

Function

Flexion of PIP joint: Primary action of the FDS tendon in the index finger.
Flexion of MCP joint: Assists in bending the finger at the metacarpophalangeal level.
Grip strength: Enhances precision grip and fine manipulation.
Dynamic stability: Works synergistically with FDP and intrinsic hand muscles to maintain finger posture and control.

Clinical Significance

Tendon lacerations: Common in palmar or digital injuries; isolated FDS damage limits PIP joint flexion while preserving DIP movement.
Tenosynovitis: Inflammation of the tendon sheath (e.g., trigger finger) causes pain and restricted movement.
Adhesions: Post-surgical or traumatic scarring restricts tendon glide within the sheath.
Tendon rupture: May result from forceful extension or flexion injuries.
Carpal tunnel syndrome: Compression may indirectly affect FDS function.
Imaging role: MRI and CT assist in evaluating tears, inflammation, and pulley integrity.

MRI Appearance

T1-weighted images:
- Tendon: Uniformly low signal (dark band) due to dense collagen.
- Surrounding fat and muscle: Bright on T1, providing contrast.
- Marrow in phalanges: Bright signal (fatty marrow).
- Tear or degeneration: Focal or linear intermediate signal within tendon substance.
T2-weighted images:
- Normal tendon: Low signal (dark) compared to muscle.
- Tendon sheath: Thin low-signal rim surrounding tendon.
- Pathology: Partial tear or tendinopathy shows bright hyperintense signal within or around tendon.
- Fluid in sheath (tenosynovitis): Bright hyperintense signal encircling tendon.
STIR:
- Normal tendon: Dark flow void-like structure.
- Pathology: Bright signal in surrounding soft tissue or within tendon indicates edema, inflammation, or rupture.
- Sensitive for early tendonitis or pulley strain.
Proton Density Fat-Saturated (PD FS):
- Normal tendon: Dark linear band with smooth margins.
- Inflamed tendon or tenosynovitis: Bright peritendinous signal.
- Partial tear: Focal bright intratendinous area with surrounding edema.
T1 Fat-Sat Post-Contrast:
- Normal tendon: Minimal or no enhancement.
- Active inflammation: Peritendinous and synovial enhancement.
- Chronic tendinosis: Mild peripheral enhancement with low-signal fibrotic core.
- Post-surgical scarring: Irregular enhancement along sheath.

CT Appearance

Non-Contrast CT:

Tendon: Linear soft-tissue density extending along the palmar aspect of the index finger.
Bone landmarks: Middle phalanx insertion visible as subtle groove or cortical irregularity.
Pathology: Detects calcific tendinitis, bony avulsions, or sheath thickening.
Useful in cases of trauma or foreign body assessment.

Post-Contrast CT (standard):

Normal tendon: Mild enhancement of peritendinous sheath only.
Inflamed tendon sheath or tenosynovitis: Diffuse linear enhancement.
Partial tear: Contrast may extend into peritendinous gap or sheath.
Valuable for postoperative tendon evaluation and inflammatory disorders when MRI is unavailable.

MRI image

CT image

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