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Articularis cubiti muscle

The articularis cubiti, also known as the subanconeus muscle, is a small, variable muscle located on the posterior aspect of the distal humerus. It is considered a deep derivative of the triceps brachii muscle, specifically its medial head. The muscle’s primary role is to elevate the synovial membrane of the elbow joint during extension, preventing entrapment between the olecranon and the humerus.

Although small, this muscle plays an important functional role in maintaining smooth elbow joint motion and is of interest in imaging and surgery due to its proximity to the olecranon fossa and posterior capsule.

Synonyms

  • Subanconeus muscle

  • Small capsular muscle of elbow

  • Deep part of medial head of triceps

Origin, Course, and Insertion

  • Origin: From the distal part of the posterior surface of the humerus, just proximal to the olecranon fossa and deep to the medial head of the triceps brachii.

  • Course: Fibers descend vertically, thin and flat, closely applied to the posterior capsule of the elbow joint.

  • Insertion: Into the posterior aspect of the elbow joint capsule, blending with its fibrous tissue near the olecranon fossa.

Relations

  • Anteriorly: Posterior capsule of the elbow joint

  • Posteriorly: Medial head of triceps brachii

  • Medially: Ulnar nerve and posterior branch of the inferior ulnar collateral artery

  • Laterally: Olecranon fossa and lateral head of triceps

  • Inferiorly: Olecranon process and posterior surface of ulna

Arterial Supply

  • Deep brachial artery (profunda brachii) – via muscular branches supplying triceps and adjacent capsule

  • Posterior branch of inferior ulnar collateral artery – small capsular branches

Venous Drainage

  • Companion veins of the profunda brachii artery (venae comitantes)

  • Drains into brachial veins accompanying the brachial artery

Nerve Supply

  • Radial nerve (C7–C8) – via a branch to the medial head of the triceps brachii

Function

  • Elevation of elbow joint capsule: Pulls the synovial membrane superiorly during elbow extension to prevent entrapment or pinching.

  • Capsular stabilization: Keeps the posterior capsule taut during extension.

  • Assists triceps: Acts synergistically with triceps brachii in the terminal phase of elbow extension.

  • Protective role: Prevents impingement of soft tissue between the olecranon and the humerus.

Clinical Significance

  • Anatomical variation: Often blends with or indistinguishable from the medial head of triceps; may be absent in some individuals.

  • Surgical landmark: Relevant during posterior approaches to the elbow joint and triceps tendon repairs.

  • Post-traumatic fibrosis: May adhere to the posterior capsule after trauma or surgery, causing extension limitation.

  • Imaging importance: Recognized on MRI as a thin muscular layer separating the posterior capsule from triceps fibers; helps differentiate normal anatomy from pathology.

MRI Appearance

  • T1-weighted images:

    • Muscle belly: intermediate signal intensity similar to adjacent triceps.

    • Capsule: low signal (dark line) deep to the muscle.

    • Surrounding fat: bright, highlighting the muscle boundaries.

    • Chronic fibrosis: may appear as low-signal fibrotic bands between muscle and capsule.

  • T2-weighted images:

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

    • Capsule: remains dark.

    • Pathology (strain, edema): bright hyperintense areas within or deep to the muscle belly.

    • Joint effusion: appears as bright signal deep to the capsule near the olecranon fossa.

  • STIR:

    • Normal muscle: intermediate-to-dark signal.

    • Pathologic muscle (strain, inflammation): hyperintense signal adjacent to the capsule.

    • Sensitive to subtle edema or capsular inflammation.

  • Proton Density Fat-Saturated (PD FS):

    • Normal: intermediate-to-dark signal homogeneous throughout the muscle.

    • Abnormal: focal bright hyperintense signal with indistinct margins in strain or small tears.

    • Highlights thin fascial plane between articularis cubiti and triceps.

  • T1 Fat-Sat Post-Contrast:

    • Normal: uniform mild enhancement.

    • Inflamed capsule or postoperative scar: shows enhancement along posterior capsule.

    • Distinguishes between synovial thickening (enhancing) and simple effusion (non-enhancing).

CT Appearance

Non-Contrast CT:

  • Muscle appears as a thin soft-tissue density layer over the posterior humerus near the olecranon fossa.

  • Capsule visualized as a thin low-density line separating muscle and joint.

  • Difficult to distinguish from triceps without contrast; best appreciated with high-resolution thin slices.

Post-Contrast CT (standard):

  • Muscle enhances mildly and homogeneously.

  • Capsular enhancement indicates inflammation, fibrosis, or postoperative changes.

  • Useful for identifying posterior elbow joint effusion, calcifications, or periarticular fibrosis.

MRI image

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