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Sternum

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The sternum, commonly referred to as the breastbone, is a flat, elongated bone located in the central part of the chest. It forms the anterior portion of the thoracic cage and serves as an attachment site for the ribs and clavicles.[1] The sternum consists of three parts: the manubrium, body (or gladiolus), and xiphoid process. Together, these components create a protective framework for vital organs such as the heart and lungs.

Location

The sternum is located in the midline of the chest, extending from the base of the neck to the upper abdomen.[8] It lies anterior to the heart and directly beneath the skin and superficial tissues, articulating with the clavicles superiorly and the costal cartilages of the ribs laterally.[4]

Structure and Anatomy

The sternum is a flat bone forming the anterior midline of the thoracic cage. It consists of three main parts: the manubrium, body (gladiolus), and xiphoid process, connected by cartilaginous joints that may ossify with age.[6]

Manubrium

  • Description:
    The manubrium is the broad, upper part of the sternum. It is quadrangular in shape and wider than the other parts.
  • Anatomical Features:
    • Jugular Notch (Suprasternal Notch): A concave depression on the superior border, palpable at the base of the neck.
    • Clavicular Notches: Located on either side of the jugular notch, these articulate with the clavicles to form the sternoclavicular joints.
    • Costal Notches: The first costal notches on each side articulate with the first pair of ribs. A partial notch for the second rib is shared with the body.
  • Articulations:
    • Superiorly with the clavicles.
    • Laterally with the first ribs.
    • Inferiorly with the body of the sternum at the manubriosternal joint.

Body (Gladiolus)

  • Description:
    The body is the longest part of the sternum, forming the middle section.[7] It is flat and slightly curved, with a narrow superior end and wider inferior end.
  • Anatomical Features:
    • Costal Notches: Articulates with the costal cartilages of ribs 2 through 7 on each side.
    • Transverse Ridges: Represent lines of fusion from the four sternebrae, which are the embryological segments of the sternum.
  • Articulations:
    • Superiorly with the manubrium at the manubriosternal joint (forming the sternal angle or angle of Louis).
    • Inferiorly with the xiphoid process at the xiphisternal joint.

Xiphoid Process

  • Description:
    The xiphoid process is the smallest and most variable part of the sternum, located at the inferior end. It is cartilaginous at birth but ossifies with age.
  • Anatomical Features:
    • Shape: Can vary (pointed, forked, or rounded).
    • Position: Lies at the level of the T10 vertebra.
  • Articulations:
    • Superiorly with the body of the sternum at the xiphisternal joint.

Articulations of the Sternum

  • Sternoclavicular Joints:
    • Formed by the articulation of the clavicles with the clavicular notches on the manubrium.
    • These are synovial joints that allow movement of the clavicle.[5]
  • Costosternal Joints:
    • Articulations between the costal cartilages of ribs 1–7 and the sternum.
    • The first rib forms a synchondrosis, while ribs 2–7 form synovial joints.
  • Manubriosternal Joint:
    • A symphysis (fibrocartilaginous joint) connecting the manubrium and body.
    • Forms the sternal angle, a palpable landmark for rib counting.
  • Xiphisternal Joint:
    • A synchondrosis between the body and xiphoid process.

Shape and Curvature

  • The sternum is slightly convex anteriorly and concave posteriorly.
  • Its broad, flat surface provides a protective shield for underlying organs.

Blood Supply and Innervation

Development

  • The sternum develops from six paired cartilage segments called sternebrae, which fuse during early life.
  • Ossification occurs in a craniocaudal direction, with the xiphoid process ossifying last.

Function

Protection of Vital Organs
The sternum serves as a protective shield for critical structures within the thoracic cavity, including the heart, lungs, and major blood vessels like the aorta and vena cava.[3]

Attachment for Ribs and Clavicles

  • The sternum provides articulation points for the costal cartilages of the first seven ribs, forming the anterior part of the rib cage.
  • It also articulates with the clavicles, forming the sternoclavicular joints, which are essential for shoulder movement and stability.

Support for Thoracic Cage
Acting as a central anchor, the sternum helps maintain the structural integrity of the thoracic cage, allowing for efficient respiration and protecting internal organs from mechanical forces.

Muscle Attachment
The sternum provides attachment points for various muscles, including the pectoralis major, sternocleidomastoid, and rectus abdominis, which are involved in movements of the upper body and respiration.

Facilitation of Breathing
Through its articulation with the ribs and costal cartilages, the sternum allows the rib cage to expand and contract during breathing, aiding in efficient ventilation.

Clinical Significance

The sternum plays a critical role in thoracic anatomy, making it relevant in various clinical contexts:

  • Fractures:
    • Sternum fractures often result from direct trauma, such as in motor vehicle accidents or blunt chest injuries.[2] These fractures can be associated with underlying cardiac or pulmonary injuries.
  • Sternal Puncture:
    • The sternum is a common site for bone marrow aspiration or biopsy due to its accessibility and proximity to the marrow-rich medullary cavity.
  • Surgical Access:
    • Median sternotomy, an incision through the sternum, is frequently performed in cardiac surgeries to access the heart and great vessels.
  • Sternal Deformities:
    • Conditions like pectus excavatum (sunken chest) or pectus carinatum (protruding chest) can impact thoracic function and appearance, sometimes requiring corrective surgery.
  • Sternal Infections (Osteomyelitis):
    • Postoperative infections, particularly after sternotomy, can lead to sternal osteomyelitis, necessitating prompt medical or surgical treatment.
  • Congenital Variations:
    • Anomalies such as a sternal foramen (a small hole in the sternum) are typically asymptomatic but can pose risks during procedures like acupuncture.

References

  1. Rohen, J. W., Yokochi, C., & Lutjen-Drecoll, E. (2021). Color Atlas of Anatomy: A Photographic Study of the Human Body (9th ed.). Lippincott Williams & Wilkins. ISBN 9781975151348.
  2. Marieb, E. N., & Hoehn, K. (2019). Human Anatomy & Physiology (11th ed.). Pearson. ISBN 9780134580999.
  3. Snell, R. S. (2019). Clinical Anatomy by Regions (10th ed.). Wolters Kluwer. ISBN 9781496345646.
  4. Drake, R. L., Vogl, W., & Mitchell, A. W. M. (2020). Gray’s Anatomy for Students (4th ed.). Elsevier. ISBN 9780323393041.
  5. Moore, K. L., Agur, A. M. R., & Dalley, A. F. (2018). Essential Clinical Anatomy (6th ed.). Wolters Kluwer. ISBN 9781496369659.
  6. Ellis, H., Logan, B. M., & Dixon, A. K. (2010). Human Sectional Anatomy: Atlas of Body Sections, CT, and MRI Images (4th ed.). CRC Press. ISBN 9780340985167.
  7. Grant, J. C. B., & Basmajian, J. V. (1989). Grant’s Atlas of Anatomy (9th ed.). Williams & Wilkins. ISBN 9780683080158.
  8. Standring, S. (2016). Gray’s Anatomy: The Anatomical Basis of Clinical Practice (41st ed.). Elsevier. ISBN 9780702052309.