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Supinator muscle

The supinator is a deep muscle of the posterior compartment of the forearm, primarily responsible for supination of the forearm—turning the palm upward. It wraps around the proximal third of the radius in a spiral manner, forming a key anatomical structure in the lateral elbow and proximal forearm region.

It consists of two layers — superficial and deep — that enclose the upper part of the radius. The deep branch of the radial nerve (posterior interosseous nerve) passes between these layers, making this region a common site for nerve entrapment (supinator or arcade of Frohse syndrome).

Synonyms

  • Posterior supinator

  • Short supinator of the forearm

Origin, Course, and Insertion

  • Origin:

    • Lateral epicondyle of humerus (via the common extensor tendon)

    • Radial collateral ligament of the elbow joint

    • Annular ligament of the radius

    • Supinator crest and fossa of the ulna

  • Course:

    • Fibers spiral obliquely around the proximal radius.

    • The deep and superficial layers form a tunnel through which the posterior interosseous nerve passes.

    • The fibers wind laterally around the neck of the radius to reach its anterior and lateral surfaces.

  • Insertion:

    • Lateral surface of the upper third of the radius, including its neck and adjacent areas of the shaft.

Relations

  • Anteriorly: Brachioradialis, extensor carpi radialis longus, and extensor carpi radialis brevis

  • Posteriorly: Supinator fat pad and proximal portion of extensor muscles

  • Medially: Anconeus and ulna

  • Laterally: Radius and radial recurrent vessels

  • Deep to: Biceps tendon and radial head of flexor muscles

  • Pierced by: Deep branch of the radial nerve (posterior interosseous nerve)

Nerve Supply

  • Deep branch of the radial nerve (C6–C7) — also known as the posterior interosseous nerve after it passes through the muscle

Arterial Supply

  • Recurrent interosseous artery (from posterior interosseous branch of ulnar artery)

  • Radial recurrent artery (branch of radial artery)

  • Posterior interosseous artery contributes minor branches

Venous Drainage

  • Radial veins and posterior interosseous veins, draining into the brachial venous system

Function

  • Primary action: Supinates the forearm when the elbow is extended (biceps brachii assists when flexed)

  • Stabilization: Maintains the position of the radius during pronation-supination movements

  • Dynamic balance: Coordinates with pronator teres and pronator quadratus for rotational control of the forearm

Clinical Significance

  • Nerve entrapment: The deep branch of the radial nerve may be compressed as it passes through the supinator (Arcade of Frohse), leading to posterior interosseous nerve syndrome with weakness of finger and wrist extension.

  • Tendinous inflammation: Overuse or repetitive rotational strain (e.g., in racket sports, manual work) can cause lateral elbow pain.

  • Surgical relevance: Important landmark for approaches to the proximal radius and nerve decompression procedures.

  • Imaging significance: MRI and CT are essential for assessing muscle tears, nerve compression, or mass lesions in the supinator canal.

MRI Appearance

  • T1-weighted images:

    • Muscle belly: intermediate signal intensity (typical for skeletal muscle)

    • Surrounding fat: bright, enhancing contrast between layers

    • Nerve (posterior interosseous): low-to-intermediate signal passing through muscle

    • Atrophy or fatty infiltration: increased intramuscular brightness

  • T2-weighted images:

    • Normal muscle: intermediate-to-low signal, darker than on T1

    • Edema, strain, or inflammation: bright hyperintense signal within fibers

    • Nerve compression: perineural hyperintensity or signal alteration in adjacent muscle due to denervation edema

  • STIR:

    • Normal muscle: intermediate-to-dark signal

    • Pathology: bright hyperintense signal in edema, myositis, or early denervation changes

  • Proton Density Fat-Saturated (PD FS):

    • Normal: intermediate-to-dark homogeneous muscle signal

    • Pathologic: bright foci or linear hyperintensity in muscle indicating strain or edema

    • Enhances visualization of perineural fluid or inflammatory changes around the radial nerve

  • T1 Fat-Sat Post-Contrast:

    • Normal muscle: mild, uniform enhancement

    • Inflammatory myopathy or infection: diffuse enhancement with indistinct margins

    • Denervation: minimal enhancement early, followed by atrophy and fatty infiltration on chronic stages

CT Appearance

Non-Contrast CT:

  • Muscle: homogeneous soft-tissue density lateral to proximal radius

  • Clear distinction between superficial and deep layers

  • Detects muscle hypertrophy, atrophy, or intramuscular calcification

  • Useful for evaluating osseous landmarks, radial head fractures, and periarticular pathology

Post-Contrast CT (standard):

  • Muscle enhances uniformly

  • Inflamed or edematous areas: show increased enhancement

  • Mass lesions or nerve entrapment zones: seen as asymmetric soft-tissue thickening or perineural enhancement

MRI images

Supinator muscle axial cross sectional anatomy 3T MRI AI enhanced radiology image -img-00000-00000

MRI images

Supinator muscle axial cross sectional anatomy 3T MRI AI enhanced radiology image -img-00000-00000_00001

MRI images

Supinator muscle axial cross sectional anatomy 3T MRI AI enhanced radiology image -img-00000-00000_00002

MRI images

Supinator muscle axial cross sectional anatomy 3T MRI AI enhanced radiology image -img-00000-00000_00003

MRI images

Supinator muscle coronal cross sectional anatomy 3T MRI AI enhanced radiology image -img-00000-00000_00004