Sternal Angle (Angle of Louis)

The sternal angle, also known as the angle of Louis, is a palpable transverse ridge on the anterior thoracic wall formed at the junction between the manubrium and body of the sternum. This junction is called the manubriosternal joint.

The sternal angle is one of the most important landmarks in thoracic surface anatomy because the second costal cartilage articulates with the sternum at this level. This relationship allows the second rib to be identified and provides the standard starting point for counting ribs and intercostal spaces.

A transverse plane passing through the sternal angle extends posteriorly approximately to the T4-T5 intervertebral disc. This plane is used to describe several important internal thoracic relationships and divides the superior mediastinum from the inferior mediastinum.

Location

The sternal angle lies on the anterior midline of the thorax at the junction of the manubrium and body of the sternum.

It can usually be felt as a slight horizontal ridge or step on the anterior surface of the sternum.

Manubriosternal Joint

The manubriosternal joint connects the inferior border of the manubrium with the superior border of the body of the sternum.

The slight angulation between these two parts of the sternum produces the palpable sternal angle.

Type of Joint

The manubriosternal joint is typically a secondary cartilaginous joint, or symphysis.

It may undergo partial or complete ossification with increasing age.

Angle of Louis

The term angle of Louis is a traditional clinical name for the sternal angle.

Both terms refer to the same palpable landmark at the manubriosternal junction.

Palpation

The sternal angle can be located by beginning at the superior end of the sternum and moving inferiorly along the anterior midline.

The examiner first identifies the jugular notch and then follows the manubrium downward until the transverse ridge of the manubriosternal joint is encountered.

Palpation Technique

1. Identify the jugular notch at the superior border of the manubrium.
2. Move the fingertips inferiorly along the anterior surface of the manubrium.
3. Feel for a slight transverse ridge or change in angulation.
4. Identify this ridge as the sternal angle.
5. Move laterally to locate the second costal cartilage and second rib.

Second Costal Cartilage

The second costal cartilage articulates with the sternum at the level of the sternal angle.

This relationship is the principal reason the landmark is so useful during surface examination.

Second Rib

Moving laterally from the sternal angle leads to the second costal cartilage and then the second rib.

The second rib can then be used as the starting point for counting the remaining ribs inferiorly.

Why Rib Counting Begins Here

The first rib lies largely beneath the clavicle and is difficult to palpate directly.

The second rib, by contrast, can be identified reliably from its relationship with the sternal angle.

Counting Intercostal Spaces

Once the second rib has been identified, the examiner can move inferiorly and count successive ribs and intercostal spaces.

An intercostal space is named for the rib forming its superior border, so the space immediately below the second rib is the second intercostal space.

Transverse Thoracic Plane

An imaginary horizontal plane passing through the sternal angle is known as the transverse thoracic plane.

Posteriorly, this plane passes approximately through the intervertebral disc between the fourth and fifth thoracic vertebrae.

T4-T5 Relationship

The sternal angle is therefore commonly associated with the T4-T5 intervertebral disc level.

This relationship is approximate because posture, respiration, age, and individual anatomy can alter the precise correspondence between surface and internal structures.

Mediastinal Division

The transverse thoracic plane is used to divide the mediastinum into superior and inferior portions.

The superior mediastinum lies above this plane, while the inferior mediastinum lies below it.

Superior Mediastinum

The superior mediastinum extends from the superior thoracic aperture to the transverse thoracic plane.

It contains major vessels, nerves, the trachea, esophagus, thymic tissue, and other structures.

Inferior Mediastinum

The inferior mediastinum lies below the transverse thoracic plane and extends toward the diaphragm.

It is subdivided into anterior, middle, and posterior mediastinal compartments.

Major Relationships at the Sternal Angle Plane

Structure or Event	Relationship
Second costal cartilage	Articulates with sternum at this level
Manubriosternal joint	Forms the sternal angle
Vertebral level	Approximately T4-T5 intervertebral disc
Mediastinum	Plane separates superior and inferior mediastina
Tracheal bifurcation	Typically near this plane, with respiratory variation
Aortic arch	Beginning and end classically related to this plane

Tracheal Bifurcation

The tracheal bifurcation, or carina, is commonly located near the transverse thoracic plane in the adult.

Its exact position varies with respiration, posture, age, and individual anatomy.

Carina

The carina is the internal ridge at the point where the trachea divides into the right and left main bronchi.

Although it cannot be palpated externally, the sternal angle provides a useful surface reference for its approximate level.

Aortic Arch

The arch of the aorta is classically described as beginning and ending near the transverse thoracic plane.

This relationship makes the sternal angle an important landmark for orienting major vessels within the superior mediastinum.

Ascending and Descending Aorta

The ascending aorta transitions into the aortic arch, and the arch subsequently continues as the descending thoracic aorta.

The sternal angle plane provides an approximate anatomical reference for these transitions.

Pulmonary Trunk Relationship

The pulmonary trunk and its major branches occupy the central thoracic region near the heart and great vessels.

The transverse thoracic plane provides a useful reference when studying their relationships to the aortic arch and tracheal bifurcation.

Thoracic Duct Relationship

The thoracic duct ascends through the posterior mediastinum and crosses from the right toward the left side at approximately the mid-thoracic level, commonly described near the T4-T6 region.

The sternal angle plane therefore provides a useful general orientation level when considering its thoracic course.

Azygos Vein Relationship

The azygos vein arches over the right lung root before entering the superior vena cava.

This occurs in the superior mediastinal region near the level represented externally by the upper thoracic landmarks.

Surface Anatomy of the Sternum

The sternum provides several important palpable landmarks along the anterior thoracic midline.

Sternal Landmark	Surface Importance
Jugular notch	Superior border of manubrium
Sternal angle	Manubriosternal junction and second rib landmark
Body of sternum	Central anterior thoracic landmark
Xiphisternal joint	Junction of body and xiphoid process
Xiphoid process	Inferior sternal landmark

Relationship to the Jugular Notch

The jugular notch lies superior to the sternal angle at the upper border of the manubrium.

It serves as the initial palpable reference when locating the sternal angle from above.

Relationship to the Xiphoid Process

The xiphoid process lies substantially inferior to the sternal angle.

Together, the jugular notch, sternal angle, and xiphoid region provide a sequence of midline landmarks along the sternum.

Cardiac Auscultation

The sternal angle is essential for locating specific intercostal spaces used during cardiac auscultation.

After identifying the second rib, the examiner can locate the traditional valve listening areas.

Aortic Auscultation Area

The traditional aortic listening area is located at the right second intercostal space near the sternal border.

Identification begins by locating the sternal angle and second rib.

Pulmonary Auscultation Area

The traditional pulmonary listening area is located at the left second intercostal space near the sternal border.

The sternal angle again provides the starting reference.

Respiratory Examination

Accurate identification of ribs and intercostal spaces is important during inspection, palpation, percussion, and auscultation of the lungs.

The sternal angle allows thoracic findings to be described according to reproducible surface levels.

Chest Surface Mapping

The sternal angle is used together with vertical surface lines such as the midsternal, midclavicular, and axillary lines.

This creates a practical coordinate system for describing findings on the thoracic wall.

Relationship to the Pleura and Lungs

The sternal angle does not directly mark a lung or pleural boundary, but it allows ribs and intercostal spaces to be counted accurately.

This indirectly assists in estimating the surface projections of lung lobes, fissures, and pleural reflections.

Clinical Significance

Rib Counting

The most important clinical use of the sternal angle is identification of the second rib.

From this point, ribs and intercostal spaces can be counted systematically across the chest wall.

Cardiovascular Examination

The landmark allows accurate localization of the second intercostal spaces used for aortic and pulmonary auscultation.

It also provides orientation to the central thoracic structures associated with the transverse thoracic plane.

Pulmonary Examination

Rib counting from the sternal angle helps localize breath sounds, percussion findings, chest wall abnormalities, and other examination findings.

Thoracic Procedures

Many procedures involving the chest wall require accurate identification of intercostal spaces.

The sternal angle provides a reliable starting point for determining these levels.

Chest Trauma

When evaluating chest trauma, the sternal angle can help establish the level of rib tenderness, deformity, or other thoracic wall findings.

Mediastinal Orientation

The sternal angle provides an external reference for the plane separating the superior and inferior mediastina.

This makes it useful when correlating physical examination, radiological anatomy, and cross-sectional imaging.

Respiratory Variation

Some internal structures associated with the sternal angle plane change position during respiration.

For example, the tracheal bifurcation can move with breathing, so surface relationships should be understood as approximate rather than absolutely fixed.

Age-Related Variation

The manubriosternal joint may become less mobile or ossify with age.

Changes in thoracic shape and posture can also alter the prominence and apparent level of the sternal angle.

Body Habitus

The ease of palpating the sternal angle varies with body habitus, muscular development, subcutaneous tissue thickness, thoracic deformity, and previous surgery.

In most individuals, however, the landmark remains identifiable by careful palpation.

Sternal Angle Versus Xiphisternal Joint

Feature	Sternal Angle	Xiphisternal Joint
Junction	Manubrium and sternal body	Sternal body and xiphoid process
Main rib landmark	Second costal cartilage	Not primary landmark for rib counting
Approximate vertebral relationship	T4-T5 disc plane	Lower thoracic level
Main surface use	Rib counting and mediastinal orientation	Inferior sternal and epigastric orientation

Key Surface Anatomy Features

Feature	Key Point
Alternative name	Angle of Louis
Anatomical junction	Manubriosternal joint
Surface appearance	Palpable transverse ridge
Rib landmark	Second costal cartilage and second rib
Primary practical use	Counting ribs and intercostal spaces
Approximate posterior level	T4-T5 intervertebral disc
Plane	Transverse thoracic plane
Mediastinal significance	Separates superior and inferior mediastina
Airway relationship	Tracheal bifurcation lies near this plane
Great vessel relationship	Beginning and end of aortic arch classically related to this level

Anatomical Importance

The sternal angle is one of the most important landmarks of thoracic surface anatomy. Its palpable ridge marks the manubriosternal junction and provides a reliable external reference for identifying the second costal cartilage and second rib.

Once the second rib is located, the remaining ribs and intercostal spaces can be counted inferiorly. This makes the landmark fundamental to cardiac and pulmonary examination and to accurate description of findings on the chest wall.

The sternal angle also corresponds approximately to the T4-T5 intervertebral disc plane and defines the transverse thoracic plane. This links a readily palpable surface landmark with major internal relationships, including the division between the superior and inferior mediastina, the tracheal bifurcation, and the aortic arch. Its combination of accessibility and deep anatomical significance makes the angle of Louis a central reference point in thoracic anatomy.