11.11
The Thoracic Cage: Sternum
The thoracic or rib cage forms the body's thorax (chest) portion. Its primary function in the body is to protect vital organs in the thoracic cavity,…
The sternum is a T-shaped flat bone in the anterior midline of the thoracic wall that connects to the rib cage. It protects the thoracic organs and attaches to different muscles.
It is formed by the fusion of three bones: the manubrium, body, and xiphoid process.
The manubrium attaches to the clavicles at the clavicular notch and articulates with the first two pairs of ribs.
The body joins the manubrium at the sternal angle. Its notched sides articulate with the costal cartilages of the second to the seventh pair of ribs.
The xiphoid process fuses with the body at the xiphisternal joint. Primarily cartilaginous, it is ossified after the age of forty. It is located at the ninth thoracic vertebrae level, where it binds to the diaphragm and rectus abdominis muscles. The heart lies below this joint.
During cardiopulmonary resuscitation or CPR, the hands are placed mid-sternum while compressing the chest. This prevents any physical thrust that can break the xiphoid process and push it into the heart or liver, which can cause severe bleeding.
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Q1: What are the three main parts of the sternum?
The sternum consists of three fused bones: the manubrium, body, and xiphoid process. The manubrium is the broader superior portion where the first rib pair attaches. The body is the elongated central section articulating with ribs 3–7. The xiphoid process is the inferior cartilaginous tip that gradually ossifies with age after forty.
Q2: Where does the sternum attach to the rib cage?
The sternum connects to the thoracic cage through multiple articulation points. The manubrium attaches to the first two rib pairs at the clavicular notch and sternal angle. The body articulates with costal cartilages of ribs 2–7 along its notched sides. The xiphoid process fuses with the body at the xiphisternal joint, located at the ninth thoracic vertebrae level.
Q3: What is the sternal angle and why is it anatomically important?
The sternal angle is the slightly bent junction where the manubrium and body meet. The second rib attaches at this landmark. Since the first rib is hidden behind the clavicle, the sternal angle and second rib serve as essential palpable landmarks for identifying and counting the lower ribs during physical examination.
Q4: What protective functions does the sternum provide?
The sternum protects vital thoracic organs including the heart and lungs as part of the thoracic cage structure. It forms the anterior midline of the thoracic wall and connects to muscles including the diaphragm and rectus abdominis. The heart lies directly below the xiphisternal joint, making proper sternum anatomy critical during medical procedures like CPR.
Q5: Why is xiphoid process fracture dangerous during CPR?
During cardiopulmonary resuscitation, hands are placed mid-sternum to avoid fracturing the xiphoid process. If broken, the xiphoid process can be thrust into the heart or liver, causing severe internal bleeding. Proper hand placement prevents this injury while allowing effective chest compression to circulate blood during cardiac emergencies.
Q6: How does the xiphoid process change with age?
The xiphoid process is primarily cartilaginous early in life, making it flexible and vulnerable to injury. Gradually with age, it undergoes ossification, becoming bone. This process is typically complete after age forty, transforming the xiphoid process from a soft cartilaginous structure into a rigid bony landmark.
Q7: What anatomical landmarks help identify the sternum during physical examination?
The jugular notch is a shallow U-shaped depression at the top of the manubrium, easily felt at the anterior base of the neck between the medial clavicle ends. The clavicular notches are shallow depressions on the manubrium's superior-lateral margins marking the sternoclavicular joints. These landmarks allow clinicians to locate and palpate the sternum accurately during examination.