Bones of the Thoracic Wall

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Bones of the Thoracic Wall

• The osteocartilaginous thoracic cage is formed by part of the vertebral column (12 thoracic vertebrae and intervertebral discs); 12 pairs of ribs and costal cartilages, and the sternum. The ribs and the costal cartilages form the largest part of the thoracic cage.

The Thoracic Vertebrae (p. 33)

• Click here for detailed description.
• The special features of these vertebrae are: (1) they have facets on their bodies for articulation with the heads of ribs; (2) there are facets on their transverse processes for articulation with the tubercles of ribs, except for the inferior two or three ribs, and (3) they have long spinous processes.

The Costal Facets (p. 33)

• Thoracic vertebrae are unique in that they have facets on their bodies and transverse processes for articulation with ribs (T11 and T12 are exceptions).
• Two demifacets are located laterally on bodies of T2 to T9. The superior demifacet articulates with the head of its own rib and the inferior demifacet articulates with the head of the rib inferior to it.

• T1 has a single costal facet for the head of the firs rib and a small demifacet for the cranial part of the second rib.
• T10 has only one costal facet which is partly on its body and partly on its pedicle.
• T11 and T12 have only a single costal facet on their pedicles.

The Spinous Processes (p. 33)

• Spinous processes of T5 to T8 are nearly vertical and overlap the vertebrae like roof shingles.
• This arrangement covers the small intervals between the laminae of adjacent vertebrae, thereby preventing sharp objects such as a knife from penetrating the vertebral canal and injuring the spinal cord.
• The spinous processes of T1, T2, T11 and T12 are horizontal, and those of T3, T4, T9 and T10 slope inferiorly.

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The Ribs (pp. 34-6)

• There are usually 12 of these elongated flat bones on each side of the thorax. They form the largest part of the osteocartilaginous thoracic cage.
• The ribs curve anteriorly and inferiorly from the thoracic vertebrae.
• Each typical rib has a head, neck, tubercle, and shaft.

• The head of a rib is wedge-shaped and presents two articular facets for articulation with the numerically corresponding vertebra and the vertebra superior to it.
• These facets are separated by the crest of the head.

• The neck of a rib is the stout, flattened part located between the head and the tubercle.
• The neck lies anterior to the transverse process of the corresponding vertebra.
• Its superior border, called the crest of the neck, is sharp, whereas its inferior border is rounded.

• The tubercle of a rib is on the posterior surface at the junction of its neck and shaft and is most prominent on the superior ribs.
• The tubercles of most ribs have a smooth convex fact, which articulates with the corresponding transverse process of the vertebra, and a rough non-articular part for attachment of the lateral costotransverse ligament.
• The tubercles of the 8^th to 10^th ribs have flat facets for articulation with similar facets on the transverse processes of the vertebrae.

• The shaft (body) of a rib is thin, flat, and curved. Forming its largest part, the shaft has external and internal surfaces, thick and rounded superior borders, and thin, sharp inferior borders.
• A short distance beyond the tubercle, the shaft ceases to pass posteriorly and swings sharply anteriorly.
• The point of greatest curvature is the angle of the rib. Here the rib is both curved and twisted.
• The thorax of a person lying on his/her back is supported by the angle of the ribs and the spinous processes of the vertebrae.
• The costal groove (sulcus) and flange formed by the inferior border of the rib protect the intercostal nerve and vessels that accompany the rib.

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True Ribs (p. 35)

• The first seven (and sometimes the eighth) pairs of ribs are called true or vertebrosternal ribs because they are connected to the sternum by their costal cartilages.

False Ribs (p. 35)

• The 8^th to 12^th pairs of ribs are false or vertebrochondral ribs.
• Each of the 8^th to 10^th ribs is connected by its costal cartilage to the ribs superior to it.

• The 11^th and 12^th pairs of false ribs are often called vertebral or floating ribs because they are unattached anteriorly.
• They end in the muscles of the anterior abdominal wall.

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Atypical Ribs (pp. 35, 38)

• The 1^st, 2^nd, and 10^th to 12^th pairs of ribs are atypical.

The First Rib (pp 35, 38)

• This is the broadest and most curved of all the ribs. It is also the shortest of the true ribs.
• The first rib is clinically important because so many structures cross or attach to it.

• It is flat and has a prominent scalene tubercle on the internal border of its superior surface for the attachment of the scalenus anterior muscle.
• The subclavian vein crosses the first rib anterior to the scalene tubercle and the subclavian artery.
• The inferior trunk of the brachial plexus passes posterior to it.
• A distinct groove is formed by the subclavian vessels and brachial plexus on the superior surface of the first rib.

• This rib articulates with the body of the first thoracic vertebra. The prominent tubercle of the first rib articulates with the transverse process of this vertebra.
• The first rib is difficult to palpate due to the clavicle, but the first intercostal space can be felt just inferior to the clavicle.

The Second Rib (p. 38)

• This rib has a curvature similar to the first rib, but it is thinner, much less curved, and is about twice as long as the first rib.
• It can easily be distinguished from the first rib by the presence of a broad, rough eminence, which is the tuberosity for the serratus anterior muscle.

The 10^th Rib (p. 38)

• This rib usually articulates with T10 vertebrae only.

The 11^th and 12^th Ribs (p. 38)

• These ribs are short, especially the 12^th pair.
• They are capped with a small costal cartilage, have a single facet on their heads, and have no neck or tubercle.
• The 11^th rib has an ill-defined angle and a shallow costal groove.
• The 12^th rib has neither of these features and may be shorter than the first rib.

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The Costal Cartilages (p. 39)

• These hyaline cartilage bars are more rounded than the ribs, and extend from the anterior ends of the ribs.

• The first seven, extend from the anterior ends of the ribs.
• These seven pairs (and sometimes the eighth) are connected with the sternum.

• The 8^th and 10^th pairs of costal cartilages articulate with the inferior border of the cartilage of the preceding rib, and the costal cartilages of the 11^th and 12^th ribs are pointed and end in the musculature of the anterior abdominal wall.
• The costal cartilages contribute significantly to the elasticity and mobility of the ribs.

The Costal Margins (p. 39)

• The medial ends of the seventh to tenth costal cartilages join to form a cartilaginous costal margin on each side. Together, these margins form the costal arch.
• The angle formed where the right and left costal margins converge at the inferior end of the body of the sternum is called the infrasternal angle. It is located at the xiphisternal joint, where the body of the sternum articulates with the xiphoid process.

Joints of the Ribs

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The Sternum

• The sternum (breastbone) is an elongated flat bone that resembles a short broadsword or dagger. It forms the middle part of the anterior wall of the thorax.
• The sternum (G. sternon, chest) consists of three parts: manubrium, body and xiphoid process.

The Manubrium (p. 41)

• The manubrium (L. handle), is the superior part of the sternum. It is located anterior to T3 and T4 vertebrae.
• It is wider and thicker than the other two parts of the sternum. Its narrow inferior end gives it a somewhat triangular shape
• Broad and thick superiorly, the manubrium slopes inferoanteriorly.

• The superior surface of the manubrium is indented by the jugular notch (suprasternal notch), which can be easily palpated.
• On each side of this notch there is an oval articular facet, called the clavicular notch, which articulates with the medial end of the clavicle.
• Just inferior to the clavicular notch, the costal cartilage of the first rib is fused with the lateral margin of the manubrium.
• This is a flexible but strong primary cartilaginous joint.

• The inferior border of the manubrium is oval and rough where it articulates with the body of the sternum at the manubriosternal joint.

• The manubrium and body lie in slightly different planes; hence, their junction forms a projecting sternal angle (angle of Louis).
• The bony landmark is located opposite the second pair of costal cartilages.
• The sternal angle is an important guide to the accurate numbering of the ribs.

The Body of the Sternum (p. 41)

• The body of the sternum is the longest of the three parts. It is located anterior to T5 to T9 vertebrae.
• It is longer, thinner, and narrower that the manubrium, but its width varies owing to the scalloping of its lateral borders by the costal notches.
• The body is broadest at the level of the fifth pair of sternocostal joints and then gradually tapers inferiorly.

• The anterior surface of the body is slightly concave from side to side.
• In young people four sternebrae are obvious. They articulate with each other at primary cartilaginous joints.
• The sternebrae begin to fuse from the inferior end between puberty and 25 years of age.
• In adults, the sternum is marked by three transverse ridges marking the lines of fusion of the sternebrae may be visible in a sternum from a young adult, but they are less distinct that the ridges on the anterior surface.

The Xiphoid Process (p. 41)

• This is a thin sword-shaped process (G. xiphos, sword) is the smallest and most variable part of the sternum. Although it is often pointed, the xiphoid process (xiphisternum) may be blunt, bifid, curved, or deflected to one side or anteriorly.
• The xiphoid is cartilaginous at birth and remains so until early childhood. It may begin to ossify during the third year of life and then it consists of a bony core surrounded by hyaline cartilage. However, ossification usually does not begin until much later.
• The xiphoid usually ossifies and unites with the body of the sternum around 40 years of age, but it may not be united even in very old people.

• The xiphoid process is an important landmark in the median plane for the following reasons: (1) its junction with the body of the sternum at the xiphisternal joint indicates the inferior limit of the thoracic cavity anteriorly and the site of the infrasternal angle; and (2) it is a midline pointer to the diaphragmatic surface of the liver, diaphragm, and inferior border of the hear.

The Sternal Joints

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