Trochlea of Humerus: Anatomy, Structure, Function, Imaging

Topic

Trochlea of humerus

The trochlea of the humerus is a prominent pulley-shaped articular surface located on the distal end of the humerus, forming part of the elbow joint. It articulates with the trochlear notch of the ulna, creating the humeroulnar articulation, which allows flexion and extension of the forearm. The term “trochlea” means “pulley,” reflecting its grooved, spool-like shape that guides the ulna during elbow motion.

The trochlea is medially positioned on the distal humerus and separated laterally from the capitulum by a shallow groove. It is larger and extends more distally than the capitulum, contributing to the carrying angle of the elbow. The articular surface is covered with hyaline cartilage, continuous with the capitulum superiorly and wrapping anteriorly and posteriorly to allow a wide range of motion.

Synonyms

Humeral trochlea
Pulley of the humerus
Medial condyle articular surface (functional term)

Location and Structure

Position: Distal medial articular surface of the humerus.
Shape: Pulley-like, with a deep central groove and two ridges—medial ridge larger and more prominent than the lateral.
Articular surface: Covered with smooth hyaline cartilage, extending more distally and posteriorly on the medial side.
Axis: Oblique downward and medially, contributing to the physiological carrying angle of the elbow.
Separated from the capitulum: By a shallow groove known as the trochlear groove or sulcus.

Articulation

Articulates with the trochlear notch of the ulna, particularly with the coronoid process anteriorly and the olecranon process posteriorly.
Forms the hinge-type (ginglymus) humeroulnar joint, permitting flexion and extension in a single sagittal plane.
Stability is maintained by the tight congruence of the trochlear notch with the trochlea and reinforcement by collateral ligaments.

Relations

Anteriorly: Coronoid fossa above, accommodating the coronoid process during flexion.
Posteriorly: Olecranon fossa above, receiving the olecranon during extension.
Laterally: Capitulum of humerus (articulating with the head of radius).
Medially: Medial epicondyle and origin of common flexor muscles.
Inferiorly: Articular cartilage continuous with that of the ulna during joint motion.

Attachments

The joint capsule of the elbow attaches just proximal to the margins of the trochlear articular surface.
Collateral ligaments:
- Ulnar collateral ligament: Medial side, strongly attached to the trochlear ridge and medial epicondyle.
- Annular ligament of radius: Indirectly stabilizes the humeroulnar articulation during rotation.
Provides attachment to the synovial membrane and periosteum proximally.

Function

Hinge motion: Allows flexion and extension of the forearm at the elbow.
Load transmission: Distributes compressive forces between humerus and ulna during weight bearing or lifting.
Stability: Deep trochlear groove maintains alignment of the ulna throughout movement.
Carrying angle: The oblique orientation of the trochlea contributes to the valgus angle between the arm and forearm.
Joint congruence: Ensures smooth articulation and even distribution of synovial fluid during motion.

Clinical Significance

Fractures: Involvement seen in distal humeral fractures (e.g., intercondylar or coronal shear fractures).
Osteochondral defects: May result from trauma or repetitive microstress, causing pain and limited motion.
Arthritis: Degenerative or post-traumatic arthritis leads to cartilage thinning and subchondral sclerosis.
Dislocations: Posterior elbow dislocations may impact or shear the trochlea.
Osteonecrosis: Rare but may occur secondary to trauma or systemic disease.
Imaging role: MRI and CT are vital for evaluating cartilage defects, fractures, and joint congruity.

MRI Appearance

T1-weighted images:

Cortical bone: Low signal (dark).
Subchondral bone marrow: Bright signal due to fatty marrow.
Articular cartilage: Smooth intermediate-to-low signal covering the trochlear surface.
Joint capsule: Thin, low-signal margin surrounding the articulation.
Fractures: Appear as linear low-signal defects crossing cortex or subchondral region.

T2-weighted images:

Cortex: Very dark (low signal).
Bone marrow: Bright signal, slightly less than on T1.
Cartilage: Intermediate-to-bright signal—increased brightness may indicate cartilage degeneration or edema.
Joint fluid: Hyperintense, outlining articular surfaces.
Pathology: Bone contusions or osteochondral lesions appear as focal hyperintense areas in subchondral bone.

STIR:

Normal bone marrow: Intermediate-to-dark signal.
Pathologic areas: Bright hyperintensity in marrow edema, fracture, or inflammation.
Sensitive for early detection of stress injury or osteitis.

Proton Density Fat-Saturated (PD FS):

Normal cartilage: Smooth intermediate-to-dark signal surface.
Abnormal: Focal bright hyperintense areas indicating cartilage softening, fissures, or chondromalacia.
Ideal for assessing cartilage thickness and joint effusion.

T1 Fat-Sat Post-Contrast:

Normal bone: Homogeneous mild enhancement.
Cartilage defects or osteochondritis dissecans: Show marginal or focal enhancement around subchondral bone.
Synovitis or inflammation: Enhancing synovium and pericapsular tissue.

CT Appearance

Non-Contrast CT:

Cortex: High attenuation, sharply defined articular margins.
Subchondral bone: Fine trabecular pattern with smooth cortical outline.
Articular cartilage: Appears as a low-density line overlying bone surface.
Pathology: Excellent visualization of fractures, erosions, subchondral sclerosis, osteophytes, and cortical irregularities.
3D reconstructions: Useful for preoperative assessment of distal humeral fractures and joint reconstruction planning.

Post-Contrast CT (standard):

Cartilage and capsule: Show mild enhancement of pericapsular tissues.
Inflammatory or infectious changes: Demonstrated as irregular enhancement or soft-tissue thickening.
Arthritic changes: Marginal osteophytes and cortical sclerosis appear more prominent.

CT VRT 3D image