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Trochlear notch of ulna

The trochlear notch of the ulna, also known as the semilunar notch, is a large, curved articular depression on the anterior aspect of the proximal ulna. It forms the main articulating surface with the trochlea of the humerus, creating the hinge-type (ginglymus) ulnohumeral joint of the elbow.

This deep, crescent-shaped concavity extends between the olecranon process superiorly and the coronoid process inferiorly, forming a continuous articular surface that allows flexion and extension of the elbow. Its congruency and the tight fit with the trochlea ensure joint stability, while the surrounding capsule and ligaments maintain alignment and strength.

Synonyms

  • Semilunar notch of ulna

  • Greater sigmoid cavity

  • Trochlear surface of ulna

Location and Structure

  • The notch lies on the anterior proximal end of the ulna, facing anteriorly and slightly superiorly.

  • It is formed by two bony projections: the olecranon process above and the coronoid process below.

  • Its articular surface is smooth, concave, and covered with hyaline cartilage, forming a deep groove that perfectly accommodates the trochlea of the humerus.

  • The curvature of the notch corresponds to the trochlear pulley, allowing hinge-like movement of the elbow.

  • The longitudinal ridge in the center of the notch divides it into medial and lateral facets that articulate with the matching grooves of the humeral trochlea.

Relations

  • Superiorly: Olecranon process, forming the posterior boundary of the elbow joint

  • Inferiorly: Coronoid process and radial notch (lateral to it)

  • Anteriorly: Joint capsule and brachialis tendon insertion

  • Posteriorly: Olecranon fossa of humerus during extension

  • Laterally: Radial notch and radial head articulation (proximal radioulnar joint)

  • Medially: Ulnar collateral ligament and medial capsule of elbow

Articular Connections

  • Superior articulation: With the trochlea of the humerus, forming the ulnohumeral joint

  • Lateral association: Near the radial notch, adjacent to the proximal radioulnar articulation

  • Inferior continuity: With the anterior surface of the coronoid process, providing insertion for the brachialis tendon

Function

  • Hinge movement: Enables flexion and extension at the elbow joint

  • Joint stability: Provides the primary bony constraint against anteroposterior displacement

  • Load transmission: Transfers forces from the humerus to the ulna during weight-bearing and lifting

  • Attachment support: Serves as a bony framework for the capsule, ligaments, and muscle insertions that stabilize the elbow

Clinical Significance

  • Fractures: Commonly involve the olecranon or coronoid process, affecting the notch’s integrity and joint congruence

  • Dislocation: Ulnohumeral dislocations often include disruption of the trochlear notch articulation

  • Arthritis: Degenerative changes may cause narrowing, subchondral sclerosis, and osteophyte formation

  • Coronoid fractures: Small avulsions at the inferior border of the notch destabilize the elbow (“terrible triad” injury)

  • Osteochondral defects: Result from trauma or repetitive stress, leading to restricted motion and pain

  • Imaging relevance: Crucial in assessing elbow stability, fracture alignment, and degenerative or inflammatory joint changes

MRI Appearance

  • T1-weighted images:

    • Cortex: Low signal (dark), sharply defined contour

    • Marrow: Bright, representing fatty content in the proximal ulna

    • Articular cartilage: Smooth, thin intermediate-to-low signal layer over the notch surface

    • Joint capsule: Low signal band enclosing anteriorly

    • Fractures: Appear as linear low-signal defects with surrounding bright marrow changes in acute stages

  • T2-weighted images:

    • Cortex: Low signal

    • Marrow: Bright, less intense than on T1

    • Cartilage: Intermediate-to-bright with uniform thickness; irregularity indicates chondral damage

    • Joint fluid: Hyperintense, outlining the trochlear articulation

    • Pathology: Effusions, cartilage loss, or subchondral cysts appear hyperintense

  • STIR:

    • Normal bone: Intermediate-to-dark signal

    • Pathologic: Bright marrow signal indicating edema, contusion, or osteitis

    • Highlights acute injury, occult fracture, or early osteomyelitis

  • Proton Density Fat-Saturated (PD FS):

    • Normal bone and cartilage: Intermediate-to-dark signal intensity

    • Abnormal: Bright hyperintense areas at articular surface in chondral fissures or osteochondral injury

    • Excellent for evaluating cartilage defects, joint effusions, and ligament attachments

  • T1 Fat-Sat Post-Contrast:

    • Normal: Homogeneous mild enhancement of marrow and synovium

    • Inflammatory arthritis: Enhancing synovium with effusion

    • Fracture or osteomyelitis: Patchy marrow enhancement, cortical irregularity, and soft-tissue enhancement

CT Appearance

Non-Contrast CT:

  • Cortex: High attenuation, sharply defined curved contour of the notch

  • Articular surface: Smooth concavity between olecranon and coronoid

  • Trabecular bone: Fine, uniform pattern with dense subchondral bone

  • Pathology:

    • Fractures: visualized as lucent lines through olecranon or coronoid region

    • Arthritis: shows joint space narrowing, osteophytes, or subchondral sclerosis

    • Post-traumatic deformities easily detected

Post-Contrast CT (standard):

  • Bone: Mild homogeneous enhancement

  • Soft tissues: Pericapsular enhancement in inflammation or synovitis

  • Utility: Useful in assessing complex fractures, nonunion, or chronic post-surgical changes

CT VRT 3D image

Trochlear notch of ulna 3D

MRI image

trochlear notch of ulna axial cross sectional anatomy 3T MRI AI enhanced radiology image -img-00000-00000

MRI image

trochlear notch of ulna sag cross sectional anatomy 3T MRI AI enhanced radiology image -img-00000-00000

CT image

trochlear notch of ulna ct sag image