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Capitulum of humerus

The capitulum of the humerus is a smooth, rounded eminence forming the lateral part of the distal articular surface of the humerus. It articulates with the head of the radius, forming the humeroradial (radiocapitellar) joint, a key component of the elbow complex. The capitulum facilitates flexion, extension, and rotation of the forearm, contributing to forearm pronation and supination.

Structurally, the capitulum is covered with hyaline cartilage and forms the anterior and inferior aspects of the lateral condyle. Posteriorly, it is non-articular and merges with the lateral epicondyle. Its rounded contour allows smooth articulation with the concave surface of the radial head during elbow motion.

Synonyms

  • Capitellum of the humerus

  • Lateral condyle articular surface

  • Humeroradial condyle

Location and Structure

  • Position: Lateral portion of the distal humerus, immediately lateral to the trochlea.

  • Articular surface: Smooth, convex, hemispherical; covered by hyaline cartilage for articulation with the radial head.

  • Orientation: Projects anteriorly and inferiorly, allowing contact with the radius during flexion.

  • Boundaries:

    • Medially continuous with the trochlea

    • Laterally adjacent to the lateral epicondyle

    • Superiorly continuous with the humeral shaft

    • Inferiorly forming the radiocapitellar joint with the radius

  • Non-articular regions: Posterior aspect and part of lateral surface devoid of cartilage.

Relations

  • Superiorly: Distal humeral shaft and lateral supracondylar ridge

  • Inferiorly: Head of radius (radiocapitellar joint)

  • Anteriorly: Articular capsule of elbow and origins of extensor muscles

  • Posteriorly: Non-articular bone continuous with lateral epicondyle

  • Medially: Trochlea and ulnar articulation

Attachments

  • Capsular attachment: Joint capsule of the elbow attaches around the capitulum’s anterior and lateral margins.

  • Ligamentous relation: The annular ligament of the radius indirectly stabilizes the radiocapitellar articulation.

  • Muscular relation: Common extensor origin arises from the lateral epicondyle just posterior to the capitulum.

Function

  • Articulation: Forms the humeroradial component of the elbow joint, articulating with the radial head.

  • Motion: Permits flexion and extension of the forearm and allows rotation of the radius during pronation and supination.

  • Force transmission: Transfers axial and rotational loads from the radius to the humerus.

  • Joint stability: Contributes to lateral stability of the elbow joint.

Clinical Significance

  • Fractures: Capitellar fractures (Hahn–Steinthal and Kocher–Lorenz types) are intra-articular injuries commonly seen after falls on the outstretched hand.

  • Osteochondritis dissecans: Common in adolescent athletes, involving the capitulum and radial head from repetitive microtrauma.

  • Arthritis and chondromalacia: Degenerative or post-traumatic cartilage wear causing pain and limited motion.

  • Panner’s disease: A self-limiting osteochondrosis of the capitulum seen in children, leading to temporary collapse of the articular surface.

  • Imaging relevance: Radiographs, CT, and MRI help assess fractures, osteochondral defects, and degenerative changes.

MRI Appearance

  • T1-weighted images:

    • Cortex: Low signal (dark) with sharply defined margins.

    • Marrow: Bright signal representing fatty marrow in adults.

    • Cartilage: Smooth intermediate-to-low signal cap covering articular surface.

    • Fractures: Linear low-signal lines through cortex or subchondral bone.

    • Osteochondral lesions: Focal low-signal defects or subchondral collapse.

  • T2-weighted images:

    • Cortex: Low signal.

    • Marrow: Bright, slightly lower than on T1 due to fat and trabecular structure.

    • Cartilage: Intermediate-to-bright; thinning or irregularity indicates chondral loss.

    • Joint fluid: Hyperintense, outlining the capitellar contour.

    • Pathology: Bone marrow edema appears hyperintense; subchondral cysts appear bright and rounded.

  • STIR:

    • Normal bone: Intermediate-to-dark signal in marrow; cortex remains dark.

    • Edema or inflammation: Bright hyperintense areas within subchondral bone or adjacent soft tissue.

    • Excellent for detecting bone bruise, Panner’s disease, and early osteochondritis dissecans.

  • Proton Density Fat-Saturated (PD FS):

    • Normal bone marrow: Intermediate-to-dark signal.

    • Abnormal: Focal bright signal representing marrow edema, cyst, or osteochondral injury.

    • Clearly delineates cartilage defects and joint effusions.

  • T1 Fat-Sat Post-Contrast:

    • Normal capitulum: Mild uniform enhancement of marrow and synovium.

    • Inflammation or arthritis: Synovial enhancement and subchondral hyperemia.

    • Osteochondritis dissecans: Peripheral rim enhancement around unstable fragment.

    • Osteomyelitis or infection: Patchy, irregular enhancement of bone and surrounding soft tissue.

CT Appearance

Non-Contrast CT:

  • Cortex: High attenuation, smooth contour outlining the capitular dome.

  • Trabecular bone: Fine, uniform spongy pattern.

  • Cartilage: Appears as thin, low-attenuation layer on articular surface.

  • Fractures: Linear or displaced lucencies through the articular surface; 3D reformats improve evaluation.

  • Osteochondral lesions: Focal flattening or irregularity with subchondral sclerosis.

  • Degenerative change: Osteophyte formation, subchondral cysts, and joint space narrowing.

Post-Contrast CT (standard):

  • Normal: Mild, homogeneous enhancement of adjacent soft tissues and periarticular capsule.

  • Inflammation or arthritis: Enhancing synovium or pericapsular tissue.

  • Osteochondritis or infection: Enhancement around subchondral lucencies or sequestrum.

CT VRT 3D image

Capitulum of humerus CT 3D VRT IMAGE

MRI image

Capitulum of humerus sag cross sectional anatomy 3T MRI AI enhanced radiology image -img-00000-00000

MRI image

Capitulum of humerus coronal cross sectional anatomy 3T MRI AI enhanced radiology image -img-00000-00000

CT image

Capitulum of humerus ct coronal image