Head of Radius: Anatomy, Structure, Relations, Function, Imaging

Topic

Head of radius

The head of the radius forms the expanded proximal end of the radius and plays a crucial role in the elbow and forearm joints. It is circular, flattened, and slightly concave on its superior surface, articulating with the capitulum of the humerus to form the humeroradial joint, which allows flexion, extension, pronation, and supination.

The peripheral circumference of the head articulates with the radial notch of the ulna, forming the proximal radioulnar joint. This dual articulation allows the radius to rotate freely during forearm movements while maintaining stability through the annular ligament, which encircles and secures the head in place.

Synonyms

Proximal head of radius
Radial articular head
Capitellar surface of radius

Location and Structure

Position: Proximal end of the radius, immediately distal to the lateral epicondyle of the humerus.
Shape: Cylindrical with a shallow concavity on the superior surface (fovea of head).
Articular surfaces:
- Superior (fovea): Concave, articulates with the capitulum of the humerus.
- Circumference: Smooth, articulates medially with the radial notch of the ulna.
Neck: Slightly constricted region below the head, connecting to the radial tuberosity.
Surface features: Covered by hyaline cartilage on articular regions; smooth posterior and lateral aspects for ligamentous attachment.

Relations

Superiorly: Capitulum of the humerus (articular surface)
Inferiorly: Neck of the radius and annular ligament
Medially: Radial notch of the ulna and annular ligament
Laterally: Joint capsule of the elbow
Anteriorly: Supinator and radial collateral ligament
Posteriorly: Extensor muscles of the forearm origin and joint capsule

Attachments

Annular ligament: Encircles the radial head and attaches to the anterior and posterior margins of the radial notch of the ulna, stabilizing the proximal radioulnar joint.
Joint capsule: Attaches around the margin of the articular surface except where covered by cartilage.
Synovial membrane: Lines the inner aspect of the capsule and extends around the head beneath the annular ligament.

Function

Elbow articulation: Forms the humeroradial joint with the capitulum, allowing flexion and extension.
Forearm rotation: Rotates within the annular ligament to enable pronation and supination.
Load transmission: Transfers axial loads from the hand and wrist to the humerus through the capitulum.
Joint stability: Contributes to stability of both the elbow and proximal radioulnar joints.

Clinical Significance

Fractures: Radial head fractures are common in falls on an outstretched hand; classified by Mason’s system.
Dislocation: Occurs in children as nursemaid’s elbow (radial head subluxation) from annular ligament displacement.
Degeneration: Osteoarthritis may develop following trauma or chronic instability.
Post-traumatic stiffness: Common after intra-articular fractures involving capitellar surface.
Surgical relevance: Radial head excision or replacement used in complex elbow injuries.
Imaging importance: MRI and CT are crucial for assessing fracture pattern, cartilage damage, and ligamentous injury.

MRI Appearance

T1-weighted images:
- Cortex: Low signal (dark), sharply defined.
- Marrow: Bright due to fatty content in adults.
- Articular cartilage: Thin, smooth, intermediate-to-low signal over the superior fovea.
- Annular ligament: Low-signal band encircling the head.
- Fracture lines: Appear as linear low-signal defects traversing cortex or subchondral bone.
T2-weighted images:
- Cortex: Low signal.
- Marrow: Bright, though slightly less than T1 due to fat and vascular components.
- Articular cartilage: Intermediate to bright; defects or thinning indicate degeneration.
- Joint fluid: Bright hyperintense signal outlining articular surfaces.
- Pathology: Bone contusions, edema, or cartilage injuries show localized hyperintensity.
STIR:
- Normal marrow: Intermediate-to-dark signal.
- Pathologic marrow: Bright hyperintensity in bone contusion, fracture, or osteochondral lesion.
- Highlights associated soft-tissue edema or joint effusion.
Proton Density Fat-Saturated (PD FS):
- Normal marrow: Intermediate-to-dark homogeneous signal.
- Pathologic areas: Bright hyperintense regions indicating marrow edema or early fracture.
- Excellent for evaluating cartilage, annular ligament tears, and synovial inflammation.
T1 Fat-Sat Post-Contrast:
- Normal bone: Mild homogeneous enhancement.
- Inflamed synovium or capsule: Enhancing soft tissue around head.
- Osteochondral defects or infection: Patchy marrow enhancement or irregular cortical enhancement.

CT Appearance

Non-Contrast CT:

Cortex: High-attenuation, sharply marginated dense rim.
Marrow: Low attenuation with fine trabecular pattern.
Articular surface: Smooth and rounded; cartilage seen as thin low-density interface.
Fractures: Clearly visible as cortical discontinuities, depression, or comminution.
Joint evaluation: Excellent for detecting subtle intra-articular fractures, loose bodies, and step deformities.