Flexor Digitorum Superficialis Tendons: Anatomy, Function, Imaging

Topic

Flexor digitorum superficialis tendons

The flexor digitorum superficialis (FDS) is a key muscle in the anterior compartment of the forearm whose tendons provide powerful flexion of the proximal interphalangeal (PIP) joints of the fingers. Each tendon also contributes to flexion of the metacarpophalangeal (MCP) joints and assists wrist flexion.

The FDS lies deep to the flexor carpi radialis and palmaris longus but superficial to the flexor digitorum profundus (FDP). Its tendons travel through the carpal tunnel and separate within the hand to reach the index, middle, ring, and little fingers. Each tendon splits into two slips near the proximal phalanx — known as Camper’s chiasm — allowing the deeper FDP tendon to pass through en route to the distal phalanx.

Synonyms

FDS tendons
Sublimis tendons
Superficial finger flexor tendons

Origin, Course, and Insertion

Origin (muscle):
- Humeroulnar head from the medial epicondyle of humerus, ulnar collateral ligament, and coronoid process of ulna
- Radial head from the anterior border of the radius (superior half)
Course (tendons):
- Four tendons arise in the distal forearm, pass through the carpal tunnel within a common synovial sheath deep to the flexor retinaculum.
- In the palm, the tendons fan out to the four medial digits, each enclosed within a fibrous digital sheath.
- At the level of the proximal phalanx, each tendon splits into two slips (Camper’s chiasm), through which the FDP tendon passes.
Insertion:
- Two slips reunite on the sides of the shaft of the middle phalanx, forming a broad insertion into the base of the middle phalanx of digits II–V.

Relations

Proximal forearm: Deep to palmaris longus and flexor carpi radialis
At wrist: Deep to flexor retinaculum, within the carpal tunnel
In palm: Superficial to flexor digitorum profundus tendons
At fingers: Surrounded by digital synovial sheaths and fibrous pulleys
Medially: Flexor carpi ulnaris
Laterally: Flexor pollicis longus tendon (thumb side)

Tendon Attachments

Enclosed in fibrous digital sheaths with annular (A1–A5) and cruciform (C1–C3) pulleys for stabilization
Each tendon divides at Camper’s chiasm, allowing FDP to pass through and continue distally
Tendon slips insert on the sides of the middle phalanx, transmitting force for PIP flexion

Nerve Supply

Median nerve (C7–T1), specifically the anterior interosseous branch for deep fibers and direct median branches for superficial heads

Function

Primary: Flexes the proximal interphalangeal (PIP) joints of digits II–V
Secondary: Aids in flexion of metacarpophalangeal (MCP) joints and wrist flexion
Coordination: Works synergistically with FDP and lumbricals for fine digital control
Clinical role: Isolated testing of FDS helps differentiate between FDP and median nerve dysfunction

Clinical Significance

Tendon injury: Common in lacerations or penetrating trauma to the volar hand or fingers
Tendinopathy: Repetitive strain from gripping or climbing causes focal thickening or tenosynovitis
Trigger finger: Thickening of FDS tendon or pulley constriction causing painful catching
Carpal tunnel syndrome: Compression of the median nerve beneath FDS tendons
Surgical relevance: Critical in tendon repair, graft harvesting, and tendon transfer procedures

MRI Appearance

T1-weighted images:
- Normal tendon: low signal (dark, fibrous structure)
- Muscle belly: intermediate signal with fascicular pattern
- Surrounding fat: bright, providing excellent contrast
- Partial tear: focal intermediate signal within the tendon
- Complete tear: discontinuity with retraction and surrounding intermediate signal hematoma
T2-weighted images:
- Normal tendon: low signal (dark); fibrillar architecture visible on high-resolution imaging
- Pathology: bright hyperintense signal within tendon substance indicating tendinosis or tear
- Tenosynovitis: bright fluid signal within the tendon sheath surrounding the tendon
STIR:
- Normal tendon: dark (flow void appearance)
- Edema or inflammation: bright hyperintense signal in peritendinous tissues or within the tendon sheath
- Excellent for identifying early tendinitis and partial tears
Proton Density Fat-Saturated (PD FS):
- Normal tendon: dark uniform signal
- Pathologic tendon: focal or diffuse bright signal representing fiber disruption or synovial fluid
- Ideal for detecting tenosynovitis, partial thickness tears, and pulley injuries
T1 Fat-Sat Post-Contrast:
- Normal tendon: mild to no enhancement
- Inflamed sheath or granulation tissue: peritendinous enhancement
- Chronic tendinopathy: peripheral rim enhancement with central low-signal scar tissue

CT Appearance

Non-Contrast CT:

Tendons appear as soft-tissue density strands anterior to flexor profundus tendons
Useful for detecting calcific tendinitis, tendon thickening, or traumatic displacement
Digital sheath pulleys may be visualized as subtle low-density bands

Post-Contrast CT (standard):

Enhanced differentiation of synovial sheath and tendon margins
Tenosynovitis: shows enhancement of the sheath and mild thickening of tendon
Post-traumatic hematoma: localized soft-tissue hyperdensity along tendon course

MRI image