Third Flexor Digitorum Superficialis Tendon (Ring Finger): Anatomy & Imaging

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Third flexor digitorum superficialis tendon

The third flexor digitorum superficialis (FDS) tendon belongs to the group of four tendons originating from the FDS muscle in the forearm. It specifically serves the ring finger, functioning as the primary flexor of the proximal interphalangeal (PIP) joint and assisting in metacarpophalangeal (MCP) flexion. Among the FDS tendons, the third is often the most developed, contributing significantly to grip and precision tasks.

Origin, Course, and Insertion

Origin: From the humeroulnar and radial heads of the FDS muscle (medial epicondyle of humerus, ulnar coronoid process, and anterior border of radius).
Course: The third tendon travels through the carpal tunnel deep to the flexor retinaculum within the common FDS sheath. In the palm, it lies superficial to the corresponding flexor digitorum profundus (FDP) tendon.
Insertion: At the level of the proximal phalanx of the ring finger, it divides into two slips (Camper’s chiasm) that embrace the FDP tendon and reunite on the sides of the middle phalanx, inserting onto its shaft.

Relations

Superficial: Palmar aponeurosis and flexor retinaculum
Deep: Flexor digitorum profundus tendon (to ring finger)
Lateral/Medial: Adjacent FDS tendons to middle and little fingers
Distal: Flexor sheath and pulley system (A1–A5 pulleys)

Tendon Attachments

Enclosed within a fibro-osseous digital sheath reinforced by annular and cruciate pulleys.
The split around the FDP tendon allows it to pass distally to the distal phalanx.
The tendon merges into the volar plate at the PIP joint for stability.

Nerve Supply

Median nerve (C7–T1) via branches supplying the FDS muscle in the forearm.

Function

Flexion of PIP joint of the ring finger.
Assists MCP flexion and stabilizes finger during grip.
Isolated function testing: Flexion of ring finger PIP joint with other fingers held extended.
Coordination: Works synergistically with FDP and lumbricals for controlled digital motion.

Clinical Significance

Tendon injury or rupture: May occur due to laceration or avulsion at Camper’s chiasm; results in loss of isolated PIP flexion.
Adhesions: Common after flexor tendon repair, causing stiffness or restricted glide.
Trigger finger: Inflammation or thickening at A1 pulley may affect the FDS tendon of the ring finger.
Avulsion fracture: Rare at the middle phalanx insertion.
Imaging role: MRI identifies tears, synovitis, or tenosynovitis; CT detects calcification or partial laceration.

MRI Appearance

T1-weighted images:
- Tendon: low signal (dark) homogeneous band.
- Muscle belly proximally: intermediate signal intensity.
- Surrounding fat: bright, outlining the tendon.
- Partial tears: focal thickening with intermediate intratendinous signal.
T2-weighted images:
- Normal tendon: low signal, slightly darker than muscle.
- Pathology: bright hyperintense areas indicate edema or partial tear.
- Fluid in sheath: high signal intensity outlining tendon.
STIR:
- Normal: intermediate-to-dark signal.
- Pathology: hyperintense sheath fluid or tendon edema in tenosynovitis.
Proton Density Fat-Saturated (PD FS):
- Normal: uniform low to intermediate signal.
- Partial tear: focal bright hyperintensity within tendon fibers.
- Complete tear: tendon discontinuity with fluid-filled gap.
T1 Fat-Sat Post-Contrast:
- Normal: minimal enhancement.
- Tenosynovitis: enhancing tendon sheath.
- Chronic scar tissue: peripheral enhancement with central hypointensity.

CT Appearance

Non-Contrast CT:

FDS tendon: linear soft-tissue density anterior to the middle phalanx.
Detects calcification, thickening, or disruption.
Adjacent bony avulsion or pulley damage seen in high resolution.
Post-Contrast CT (standard):
Normal tendon: minimal enhancement.
Inflamed sheath or scar: focal enhancement outlining tendon path.
Useful for postoperative or traumatic assessment

MRI image

CT image

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