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Humeroulnar joint

The humeroulnar joint is the primary hinge articulation of the elbow, formed between the trochlea of the humerus and the trochlear notch of the ulna. It allows flexion and extension of the forearm, providing the main axis of elbow motion. Structurally, it is a synovial hinge (ginglymus) joint, reinforced by a strong fibrous capsule and the ulnar collateral ligament.

This joint plays a key role in upper limb biomechanics, transferring forces from the forearm to the humerus and stabilizing the elbow during motion and weight-bearing. Its congruent bony surfaces and tight ligamentous support give it exceptional stability compared to other upper-limb joints.

Synonyms

  • Elbow hinge joint

  • Trochlear articulation of the elbow

  • Ulnohumeral joint

Articular Surfaces

  • Humeral surface: The trochlea of the humerus, spool-shaped and covered with hyaline cartilage, articulates with the ulna.

  • Ulnar surface: The trochlear notch of the ulna, formed by the olecranon and coronoid processes, concave and covered with cartilage.

  • These surfaces are congruent, providing firm stability and a smooth hinge mechanism.

Capsule and Ligaments

  • Joint capsule: Encloses the humeroulnar, humeroradial, and proximal radioulnar joints; thin anteriorly and posteriorly but thickened medially and laterally.

  • Ulnar collateral ligament (UCL):

    • Anterior band: Strongest and primary stabilizer during valgus stress.

    • Posterior band: Weak, limits flexion beyond 120°.

    • Oblique band: Forms the floor of the cubital tunnel and blends with capsule.

  • Annular ligament: Though primarily for the proximal radioulnar joint, it provides secondary stability to the humeroulnar joint by securing the radial head.

Relations

  • Anteriorly: Brachialis muscle, joint capsule, and median nerve (at the medial side).

  • Posteriorly: Olecranon process and triceps tendon.

  • Medially: Ulnar collateral ligament and ulnar nerve (posterior to the medial epicondyle).

  • Laterally: Radial collateral ligament and common extensor tendon origin.

Movements

  • Type: Hinge joint permitting flexion and extension in the sagittal plane.

  • Range of motion:

    • Flexion: ~145°

    • Extension: 0° (sometimes slight hyperextension in females).

  • Axis of motion: Passes through the center of the trochlea and capitulum.

  • Stabilizing factors:

    • Bony congruence between trochlea and trochlear notch.

    • Ulnar collateral ligament.

    • Surrounding musculature (brachialis, triceps, and anconeus).

Function

  • Provides the primary mechanical hinge for forearm flexion and extension.

  • Transmits axial and shear forces from hand to humerus during load-bearing.

  • Maintains elbow stability during pronation and supination movements through the radioulnar articulation.

  • Facilitates precision and power movements of the upper limb.

Clinical Significance

  • Dislocation: Posterior dislocations are common, often involving capsular and ligamentous rupture.

  • Fracture: Involves coronoid process or trochlear notch; may lead to instability.

  • Arthritis: Osteoarthritis or rheumatoid changes cause joint narrowing and osteophyte formation.

  • Ligament injuries: Ulnar collateral ligament sprain common in throwing athletes (“pitcher’s elbow”).

  • Bursitis: Olecranon bursa inflammation may indirectly affect humeroulnar movement.

  • Imaging importance: MRI and CT evaluate structural congruity, cartilage integrity, and post-traumatic deformity.

MRI Appearance

  • T1-weighted images:

    • Cortical bone: low signal (dark)

    • Bone marrow: bright signal (fatty marrow)

    • Articular cartilage: intermediate-to-low signal, forming a smooth cap over the trochlea and trochlear notch

    • Joint capsule and ligaments: low signal linear structures

    • Pathology: fractures as low-signal lines, marrow edema as intermediate-to-bright signal

  • T2-weighted images:

    • Cortical bone: dark

    • Bone marrow: bright, slightly less intense than on T1

    • Cartilage: intermediate-to-bright with smooth contour

    • Joint fluid: bright hyperintense, outlining articular surfaces

    • Pathology: effusion, synovitis, or ligament tear appears as bright or irregular signal areas

  • STIR:

    • Normal marrow: intermediate-to-dark signal

    • Pathologic changes: bright hyperintense signal from edema, contusion, or infection

    • Highlights soft-tissue edema, capsulitis, and bone stress

  • Proton Density Fat-Saturated (PD FS):

    • Normal bone and cartilage: intermediate-to-dark signal

    • Abnormal: bright signal for marrow edema or synovial inflammation

    • Excellent for detecting small effusions, ligament injury, and subtle bone contusions

  • T1 Fat-Sat Post-Contrast:

    • Normal joint: mild synovial enhancement

    • Inflammation or arthritis: diffuse enhancement of synovium and capsule

    • Osteomyelitis: patchy marrow enhancement with cortical irregularity

    • Post-surgical or traumatic scarring: peripheral rim enhancement

CT Appearance

Non-Contrast CT:

  • Excellent bony detail showing the congruency between trochlea and trochlear notch.

  • Cortical bone: high attenuation, sharply defined margins.

  • Cartilage: visualized as a thin low-density interface between subchondral bones.

  • Detects fractures, osteophytes, subchondral sclerosis, and early degenerative changes.

Post-Contrast CT (standard):

  • Demonstrates synovial enhancement in arthritis or inflammation.

  • Useful for assessing joint effusion, capsular thickening, or post-traumatic remodeling.

  • Provides high-resolution bone detail in elbow instability or malunion.

CT VRT 3D image

Humeroulnar joint ct 3d vrt image

MRI image

Humeroulnar joint coronal cross sectional anatomy 3T MRI AI enhanced radiology image -img-00000-00000

MRI image

Humeroulnar joint sag cross sectional anatomy 3T MRI AI enhanced radiology image -img-00000-00000

CT images

Humeroulnar joint ct sagittal