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Anterior fat pad of elbow joint

The anterior fat pad of the elbow joint is a thin, triangular pad of adipose tissue located within the anterior aspect of the distal humerus, interposed between the fibrous capsule and the synovial membrane. It occupies the coronoid and radial fossae of the humerus and moves dynamically during elbow flexion and extension.

In a normal state, it is visible on imaging as a subtle radiolucent or low-signal band along the anterior humeral cortex. It plays an important role as a cushion and space-filler, allowing smooth movement of the elbow capsule and synovium without impingement. Its displacement on imaging is a key diagnostic sign of joint effusion or intra-articular pathology, classically referred to as the “sail sign” on lateral radiographs.

Synonyms

  • Coronoid and radial fossa fat pad

  • Anterior humeral fat pad

  • Elbow anterior fat cushion

Location and Structure

  • The anterior fat pad lies within the anterior compartment of the elbow joint capsule, overlying the coronoid fossa (medially) and radial fossa (laterally) of the distal humerus.

  • Composition: Comprised of lobulated adipose tissue enclosed by synovial membrane and fibrous capsule.

  • Shape: Triangular, with its base directed inferiorly toward the joint space and apex upward along the anterior humeral cortex.

  • Mobility: Shifts position with elbow movement—displaced posteriorly during flexion and anteriorly during extension.

Relations

  • Anteriorly: Fibrous capsule of the elbow joint and overlying brachialis muscle

  • Posteriorly: Synovial membrane and anterior aspects of the humerus, coronoid fossa, and radial fossa

  • Laterally: Radial fossa and capitellum

  • Medially: Coronoid fossa and trochlea of humerus

  • Inferiorly: Communicates with joint space over the coronoid process of the ulna

Function

  • Cushioning: Acts as a protective buffer between the fibrous capsule and synovial membrane.

  • Movement accommodation: Adapts its shape during flexion and extension, preventing capsule entrapment.

  • Pressure regulation: Adjusts intra-articular volume changes during motion, minimizing stress on synovium.

  • Diagnostic indicator: Its displacement is an indirect sign of intra-articular effusion, hemorrhage, or fracture.

Clinical Significance

  • Elbow effusion: The anterior fat pad becomes elevated and triangular (“sail sign”) on lateral radiographs, indicating joint fluid or hemarthrosis.

  • Fracture detection: Especially useful in identifying occult supracondylar or radial head fractures in children and adults.

  • Synovial pathology: May be displaced or thickened by synovitis, lipoma arborescens, or intra-articular masses.

  • Post-traumatic changes: Edema, hemorrhage, or inflammation may alter its contour or signal intensity.

  • Surgical relevance: Serves as an anatomic landmark in anterior elbow arthrotomy and arthroscopy.

MRI Appearance

  • T1-weighted images:

    • Fat pad appears bright (high signal intensity), similar to subcutaneous fat.

    • Thin, low-signal fibrous capsule outlines its surface.

    • Adjacent bone and synovium appear intermediate-to-dark.

    • Displacement: Effusion pushes the fat pad anteriorly, increasing its visibility.

  • T2-weighted images:

    • Normal fat pad: bright signal intensity, slightly less than subcutaneous fat.

    • Joint fluid: very bright hyperintense signal contrasting with the fat pad.

    • Pathology: effusion, hemorrhage, or synovial thickening appears as high-signal material surrounding or lifting the pad.

  • STIR:

    • Normal fat: intermediate-to-dark signal due to fat suppression.

    • Effusion or edema: appears bright hyperintense adjacent to or replacing the suppressed fat signal.

    • Helpful in detecting early inflammation or intra-articular bleeding.

  • Proton Density Fat-Saturated (PD FS):

    • Fat pad: intermediate-to-dark signal after fat suppression.

    • Effusion or hemorrhage: bright hyperintense signal outlining displaced fat pad.

    • Sensitive for small joint effusions, synovitis, and post-traumatic edema.

  • T1 Fat-Sat Post-Contrast:

    • Normal fat pad: minimal enhancement.

    • Pathologic: pericapsular or synovial enhancement suggests inflammation or synovitis.

    • Hemarthrosis or post-traumatic changes may show irregular enhancement surrounding the capsule.

CT Appearance

Non-Contrast CT:

  • Fat pad appears as a low-attenuation (hypodense) triangular structure anterior to the distal humerus.

  • Normal position: closely applied to anterior humeral cortex.

  • Displacement: Elevation or bulging indicates elbow joint effusion.

  • Surrounding changes: Hyperdense joint fluid or blood may outline the displaced fat pad.

  • Best evaluated on lateral reformatted views for the classic “fat pad sign.”

Post-Contrast CT (standard):

  • Fat pad itself does not enhance.

  • Surrounding synovium or capsule may enhance in inflammation or post-traumatic states.

  • Detects small effusions, intra-articular air, or hematoma around the elbow capsule.

MRI image

anterior fat pad of elbow joint axial cross sectional anatomy 3T MRI AI enhanced radiology image -img-00000-00000

MRI image

anterior fat pad of elbow joint sagl cross sectional anatomy 3T MRI AI enhanced radiology image -img-00000-00000