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Topic

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Sternal angle

The sternal angle, also known as the angle of Louis, is the palpable anterior angulation between the manubrium and body of the sternum at their articulation. It marks a major anatomical transition point in the thorax and is one of the most important clinical and radiological surface landmarks.

The sternal angle corresponds to several key internal structures, including the 2nd costal cartilage, the level of the T4–T5 intervertebral disc, and the plane that divides the superior and inferior mediastinum. It serves as a crucial reference point during physical examination, procedural planning, and thoracic imaging interpretation.

Synonyms

  • Angle of Louis

  • Manubriosternal angle

  • Manubriosternal joint

Location and Structure

  • Position: At the junction of the manubrium and the body of the sternum on the anterior midline of the thorax.

  • Shape: Forms a slight transverse ridge or prominence palpable beneath the skin.

  • Articulation: Represents the manubriosternal joint, typically a secondary cartilaginous (symphysis) joint that may ossify later in life.

  • Associated structures: Anchors the 2nd rib bilaterally and lies at the level where the aortic arch begins and ends.

Relations

  • Superiorly: Manubrium and attachments of sternocleidomastoid and infrahyoid musculature

  • Inferiorly: Body of sternum and costal cartilages

  • Laterally: 2nd costal cartilages and ribs

  • Posteriorly: T4–T5 vertebral level and mediastinal structures

  • Anteriorly: Subcutaneous tissue, skin, and superficial veins

Attachments

  • Costal cartilage: Articulation with the 2nd costal cartilage, forming the 2nd sternocostal joint

  • Ligamentous: Sternal ligaments reinforcing anterior chest wall stability

  • Muscular (indirect): Related to origins of pectoralis major and attachments of sternocleidomastoid to manubrium

Function

  • Serves as a key anatomical landmark for counting ribs and locating intercostal spaces

  • Marks the division between the superior and inferior mediastinum

  • Indicates the level of:

    • Aortic arch

    • Bifurcation of the trachea

    • Entry of the azygos vein into the superior vena cava

  • Provides structural stability between manubrium and sternal body

  • Assists clinicians in proper placement for cardiac auscultation and thoracic procedures

Clinical Significance

  • Primary reference point for rib counting, thoracic imaging correlation, and emergency procedures

  • Identifies the transverse thoracic plane used in cross-sectional anatomy

  • Important in evaluating sternum positioning in trauma, surgery, and congenital conditions

  • Acts as a palpable guide for surface anatomy in medical education and examination

MRI Appearance

T1-weighted images:

  • Cortical bone: Low signal (dark)

  • Marrow of sternum: Bright signal due to fatty marrow

  • Manubriosternal joint: Low-signal cortical margins with intermediate joint space signal

  • Adjacent soft tissues: Fat appears bright; muscles show intermediate signal

T2-weighted images:

  • Cortex: Dark signal

  • Marrow: Bright, slightly less intense than on T1

  • Cartilage and joint space: Intermediate-to-bright appearance

  • Muscles: Intermediate T2 signal

STIR:

  • Cortex: Very low signal

  • Marrow: Intermediate-to-dark normal signal

  • Soft tissues: Suppressed fat helps outline musculature and soft-tissue planes clearly

T1 Fat-Saturated Post-Contrast:

  • Cortical bone: No enhancement

  • Marrow: Mild homogeneous enhancement

  • Soft tissue planes: Enhanced visualization of surrounding fascia and sternocostal junctions

CT Appearance

Non-Contrast CT:

  • Cortical bone: High attenuation with sharp margins

  • Marrow: Lower attenuation compared to cortex, representing fatty marrow

  • Manubriosternal joint: Visible as a smooth articulation with variable ossification

  • Costal cartilage: Intermediate attenuation, connecting 2nd rib to sternal angle

MRI images

Sternal angle coronal MRI image-img-00000-00000

MRI images

Sternal angle sag image