Topics

Topic

design image
Flexor digitorum superficialis muscle (humeroulnar head)

The humeroulnar head of the flexor digitorum superficialis (FDS) is one of the two proximal origins of this important forearm muscle, the other being the radial head. It arises from the common flexor origin at the medial epicondyle of the humerus and from the medial margin of the coronoid process of the ulna.

The humeroulnar head forms the larger and more superficial portion of the muscle and contributes to flexion at the proximal interphalangeal (PIP) joints, metacarpophalangeal joints, and the wrist. It plays a vital role in gripping and coordinated hand movement.

Synonyms

  • Humeroulnar head of FDS

  • Superficial head of flexor digitorum sublimis

Origin, Course, and Insertion

  • Origin:

    • Humeral part: Medial epicondyle of the humerus via the common flexor tendon

    • Ulnar part: Medial margin of the coronoid process of the ulna

  • Course:

    • Fibers run distally and medially, forming a thick muscular belly in the proximal forearm.

    • They pass deep to the pronator teres and palmaris longus, joining the radial head fibers to form a common belly.

    • The muscle divides into four tendons that pass through the carpal tunnel, superficial to the tendons of the flexor digitorum profundus.

  • Insertion:

    • Each tendon splits into two slips (forming a chiasma) that insert into the sides of the middle phalanges of the second to fifth digits.

Relations

  • Superficial: Pronator teres, palmaris longus, flexor carpi radialis, and flexor carpi ulnaris

  • Deep: Flexor digitorum profundus and median nerve (near its entry point)

  • Medial: Ulnar artery and vein (proximal forearm)

  • Lateral: Radial head of FDS and median nerve (between the two heads at origin)

  • Posterior: Ulna and interosseous membrane proximally

Nerve Supply

  • Median nerve (C7, C8, T1) — passes between the humeroulnar and radial heads as it enters the forearm

Arterial Supply

  • Ulnar artery — primary supply

  • Radial artery — minor contribution proximally

  • Anterior ulnar recurrent artery and muscular branches of the radial artery reinforce the proximal region

Venous Drainage

  • Deep veins of the forearm, accompanying the ulnar and radial arteries, drain into the brachial veins

  • Superficial veins drain toward the median cubital and basilic veins

Function

  • Flexes the middle phalanges at the proximal interphalangeal joints (primary action)

  • Flexes proximal phalanges at the metacarpophalangeal joints

  • Assists wrist flexion when fingers are flexed

  • Stabilizes the wrist and finger joints during gripping and precision activities

Clinical Significance

  • Median nerve entrapment: The nerve passes between the humeroulnar and radial heads, making this site a common location for pronator syndrome or median nerve compression

  • Muscle strain or tendinopathy: Overuse in gripping or throwing activities

  • Surgical relevance: The humeroulnar head forms part of the superficial flexor-pronator mass, important in medial epicondylar surgeries

  • Vascular considerations: The proximity of ulnar artery branches makes this area significant in flap and graft harvesting procedures

MRI Appearance

  • T1-weighted images:

    • Muscle belly: Intermediate signal intensity with well-defined fascicular pattern

    • Tendons: Low signal (dark) linear structures

    • Fat planes: Bright surrounding perimuscular fat delineating borders

    • Pathology: Tendinopathy or strain appears as focal intermediate-to-bright signal in the origin or muscle belly

  • T2-weighted images:

    • Normal muscle: Intermediate-to-low signal (slightly darker than on T1)

    • Tendon: Dark, low-signal linear band

    • Pathologic changes: Bright hyperintense signal in cases of edema, partial tear, or inflammation at the common flexor origin

    • Surrounding bursae or fluid collections: Hyperintense

  • STIR:

    • Normal muscle: Intermediate-to-dark signal

    • Pathologic areas: Bright hyperintense regions indicating edema, strain, or tendinitis

  • Proton Density Fat-Saturated (PD FS):

    • Normal muscle: Intermediate-to-dark, homogeneous appearance

    • Tendinopathy or strain: Bright hyperintense foci within proximal fibers or tendon attachment

    • Ideal for detecting subtle inflammation at the medial epicondyle or humeroulnar junction

  • T1 Fat-Sat Post-Contrast:

    • Normal muscle: Mild uniform enhancement

    • Inflamed tendon or myotendinous junction: Focal or linear enhancement

    • Chronic tendinopathy or fibrosis: Low signal with peripheral rim enhancement

CT Appearance

Non-Contrast CT:

  • Muscle: Soft-tissue density, distinct from adjacent fat

  • Tendon origin: Visible at the medial epicondyle as a dense linear band

  • Chronic enthesopathy: Calcification or thickening at the origin

  • Acute injury: Subtle stranding or low-density edema around medial elbow

Post-Contrast CT (standard):

  • Normal muscle: Homogeneous enhancement

  • Inflammation or strain: Increased enhancement at the common flexor origin and myotendinous junction

  • Calcific tendinitis: Focal dense calcified deposits adjacent to medial epicondyle

MRI images

Flexor digitorum superficialis (humeroulnar head) axial cross sectional anatomy 3T MRI AI enhanced radiology image -img-00000-00000_00001

MRI images

Flexor digitorum superficialis (humeroulnar head) axial cross sectional anatomy 3T MRI AI enhanced radiology image -img-00000-00000_00002

MRI images

Flexor digitorum superficialis (humeroulnar head) axial cross sectional anatomy 3T MRI AI enhanced radiology image -img-00000-00000

CT images

Flexor digitorum superficialis (humeroulnar head) ct axial