Sternal Angle: Definition, Palpation, and Clinical Significance

What is a Sternal Angle?

The sternal angle, also known as the angle of Louis, is a crucial anatomical landmark located at the junction of the manubrium and the body of the sternum, serving as a key reference point for identifying the second rib and other vital thoracic structures.

Anatomical Definition

The sternal angle is a slight, palpable, transverse ridge or cartilaginous joint formed where the superior portion of the sternum, known as the manubrium, articulates with the main body of the sternum. This articulation forms a symphysis, allowing for slight movement during respiration. Anatomically, it is located at the level of the T4-T5 intervertebral disc posteriorly, making it a critical reference point for dividing the mediastinum and identifying various internal structures.

Palpation and Identification

Identifying the sternal angle is straightforward and a fundamental skill for healthcare professionals and those studying human anatomy.

• Locate the Suprasternal Notch: Begin by palpating the top of the sternum, just below the neck, where you will feel a depression known as the suprasternal (or jugular) notch.
• Move Inferiorly: Slide your fingers directly down from the suprasternal notch along the anterior surface of the sternum.
• Identify the Ridge: Approximately 5 centimeters (2 inches) below the notch, you will feel a distinct horizontal ridge or prominence. This is the sternal angle.
• Find the Second Rib: Once the sternal angle is identified, move your fingers laterally (to the side) from the angle. The first palpable rib you encounter directly lateral to the sternal angle is always the second rib. This is because the first rib is often obscured by the clavicle.

Clinical and Anatomical Significance

The sternal angle holds immense importance in anatomy, clinical practice, and medical imaging due to its consistent position relative to several vital thoracic structures:

• Identification of the Second Rib: As mentioned, it is the most reliable landmark for finding the second costal cartilage and, consequently, the second rib. From this point, all other ribs can be accurately counted. This is critical for physical examinations and surgical planning.
• Level of Tracheal Bifurcation: The trachea (windpipe) divides into the right and left main bronchi (carina) at the level of the sternal angle. This is vital for understanding lung anatomy and pathologies.
• Beginning and End of the Aortic Arch: The ascending aorta ends, and the aortic arch begins at the level of the sternal angle. The aortic arch then terminates at this level, becoming the descending aorta.
• Bifurcation of the Pulmonary Trunk: The pulmonary trunk, carrying deoxygenated blood from the heart to the lungs, also bifurcates into the right and left pulmonary arteries at this level.
• Superior Limit of the Inferior Mediastinum: The sternal angle marks the arbitrary horizontal plane that divides the mediastinum (the central compartment of the thoracic cavity) into superior and inferior parts. This plane extends posteriorly to the T4-T5 intervertebral disc.
• Passage of the Thoracic Duct: The thoracic duct, a major lymphatic vessel, crosses from the right side of the vertebral column to the left at this level.
• Clinical Procedures: It serves as a crucial landmark for procedures such as needle insertions, chest tube placements, and identifying the correct intercostal spaces for auscultation (listening to heart and lung sounds). For instance, the second intercostal space, located just below the second rib, is a common site for listening to heart sounds.

Relevance in Exercise Science and Kinesiology

While primarily an anatomical landmark, the sternal angle has indirect relevance in exercise science and kinesiology:

• Postural Assessment: Although not a primary indicator, the alignment of the sternum and rib cage, which includes the sternal angle, contributes to overall postural assessment. Deviations in thoracic posture (e.g., excessive kyphosis) can subtly alter the perceived position or prominence of the sternal angle relative to other landmarks.
• Breathing Mechanics: The sternal angle's articulation allows for slight movement during inspiration and expiration. Understanding the mobility of the sternum and rib cage is fundamental to analyzing efficient breathing patterns and identifying restrictions that might impact athletic performance or daily function. Proper diaphragmatic breathing and accessory muscle recruitment influence the movement of the sternum.
• Anatomical Reference for Movement Analysis: For advanced analysis of trunk movements, core stability exercises, or even specific stretches targeting the thoracic spine, having a precise understanding of anatomical landmarks like the sternal angle helps kinesiologists and trainers accurately describe and assess client movements and provide targeted cues. It aids in visualizing the internal structures that are influenced by external movements.

Conclusion

The sternal angle, or angle of Louis, is far more than just a bony prominence. It is a critical anatomical crossroads in the human thorax, serving as a reliable external landmark for locating the second rib and numerous vital internal structures, including the bifurcation of the trachea and pulmonary trunk, and the beginning and end of the aortic arch. Its consistent anatomical relationship makes it indispensable for clinical diagnosis, surgical planning, and a foundational point of reference for anyone studying human anatomy, exercise science, or health.