Osteology of thorax

Question 1

Describe the structure of the sternum.

Answer 1

• The sternum consists of an uppermost manubrium, an intermediate body and a small lowermost xiphoid process.
• The manubrium and body are connected by a secondary cartilaginous joint.
• Like the ribs, most of the sternum is made of cancellous bone which throughout life is filled with haemopoietic, red marrow.
• The manubrium is a flat four-sided bone broader above than below.
• Its upper margin is concave, the jugular notch, on which lies the adherent interclavicular ligament.
• The two layers of the investing layer of deep cervical fascia are attached to the anterior and posterior borders of the notch.
• Each upper angle of the manubrium is scooped out into a concavity for the sternoclavicular joint, an atypical synovial joint.
• Below this the lateral border is excavated for the first costal cartilage, which articulates here by a primary cartilaginous joint.
• At the inferior angle is a small facet for articulation (synovial) with the upper part of the second costal cartilage.
• Between the first and second costal cartilages the internal intercostal muscle is attached to the lateral border.
• The anterior surface is mainly covered by the attachment of pectoralis major but also gives attachment to the tendon of sternocleidomastoid.
• Sternohyoid and sternothyroid are attached to the upper part of the posterior surface.
• The manubrium forms the anterior wall of the superior mediastinum.
• The body of the sternum has articular facets along its lateral border for the lower part of the second costal cartilage and for the third to seventh costal cartilages, all these articulations being synovial joints.
• Between the costal facets the lateral border gives attachment to the anterior intercostal membrane and the internal intercostal muscle.
• Pectoralis major arises widely from the anterior surface almost to the midline, and transversus thoracis arises from the posterior surface low down. * From the posterior surface weak sternopericardial ligaments pass to the fibrous pericardium.
• From the posterior margin of the lower end of the sternal body the xiphoid process (xiphisternum) projects downwards for attachment of the linea alba. It articulates with the body by a symphysis which usually ossifies in middle age.

Question 2

Describe the structure of a typical rib.

Answer 2

• A typical rib from the middle of the series has a head, a neck and a shaft; a tubercle projects posteriorly from the lateral end of the neck.
• The head is bevelled by two articular facets that slope away from a dividing ridge. The lower facet is vertical; it articulates with its own vertebra and the upper, sloping facet articulates with the vertebra above.
• Each makes a synovial joint, and the cavities are separated by a ligament attached to the ridge on the head and to the intervertebral disc.
• The neck is flattened, with a sharp crest forming the upper border.
• The two laminae of the superior costotransverse ligament are attached to the crest.
• The neck of the rib inclines backwards when traced from head to tubercle.
• The tubercle shows two facets. The smooth medial facet makes a synovial joint with the transverse process of its own vertebra; upper ribs have slightly convex, and lower ribs have flat, facets.
• The rough lateral facet gives attachment to the lateral costotransverse ligament.
• The shaft slopes down and back to the angle and there twists forward in its characteristic curvature.
• Anterior to the angles the shafts of ribs are curved to form arcs of circles whose diameters increase from above downwards.
• On account of torsion at their angles, the heads of ribs lie at a higher level than their anterior ends; the degree of torsion is most marked in the seventh to ninth ribs.
• The upper border of a rib is blunt. It gives attachment in its whole length to the external and from the angle forwards to the internal intercostal muscles.
• The lower border of the shaft is sharp and hangs down to produce a well-marked costal groove on the inner surface.
• The external intercostal muscle arises from the sharp lower border and the internal intercostal is attached to the costal groove.
• The transverse thoracic group of muscles are attached to the inner surface of the shaft.
• The neurovascular bundle lies between this layer and the internal intercostal muscle, deep to the costal groove.
• The anterior end of the rib is excavated into a concave fossa which is plugged by the costal cartilage in an immovable primary cartilaginous joint.

Question 3

Describe the structure of the atypical first rib.

Answer 3

• The first rib is the strongest, broadest, flattest and most curved rib. Its superior surface is part of the root of the neck; its inferior (pleural) surface is part of the roof of the thoracic cavity.
• The neck slopes upwards, backwards and laterally from the head to the tubercle.
• Hence if the rib is laid on the table, the head and anterior extremity both touch the surface, but laid upside down the head lies above the table; this enables right and left sides to be identified.
• The plane of the shaft is at 45˚to the horizontal, with the tubercle the most posterior and highest part of the rib.
• The head is small and carries a single facet for the synovial joint it makes with the upper part of the body of T1 vertebra.
• The sympathetic trunk (or cervicothoracic ganglion) lies in contact with the anterior border of the neck alongside the head.
• Lateral to it the first posterior intercostal vein and then the superior intercostal artery lie in contact, and more lateral still the first thoracic nerve lies in front of the neck.
• The cervical dome of the pleura and the apex of the lung hold these vessels and nerves against the front of the neck of the rib.
• The rib broadens at the junction of neck and shaft, where a prominent tubercle projects back to form the most posterior convexity of the rib. It is a fusion of tubercle and angle.
• Medially it has a convex facet for a corresponding concavity on the first transverse process.
• The lateral prominent part of the tubercle receives the lateral costotransverse ligament.
• The undersurface of the shaft, crossed obliquely by the small first intercostal nerve and vessels, is covered by adherent parietal pleura.
• The external and internal intercostal muscles are attached together to the outer rim of this surface.
• The outer border is blunt between the tubercle and subclavian groove and the first digitation of serratus anterior is attached to the anterior part of this blunt area. The concave internal border of the shaft gives attachment to the suprapleural membrane in front of the subclavian groove.
• The upper surface of the shaft is grooved obliquely at its greatest lateral convexity. Called the groove for the subclavian artery, it lodges the lower trunk of the brachial plexus as well, behind the artery.
• The fibres in contact with the rib are all T1, the C8 fibres lying above them.
• Between the groove and the tubercle the large quadrangular area of the upper surface gives attachment to scalenus medius.
• At the front of the groove the inner border is projected into a small spur, the scalene tubercle. The tendon of scalenus anterior is attached to the tubercle and extends along a triangular impression on the upper surface which may be more discernable than the tubercle itself.
• Anterior to the attachment of scalenus anterior is another groove on the upper surface of the shaft in which the subclavian vein lies.
• The anterior end of the shaft expands into a concavity for the first costal cartilage. From the upper surface of this junction subclavius arises from both bone and cartilage.

Question 4

Describe the atypical eleventh rib.

Answer 4

The eleventh rib has a head, with a single facet for T11 vertebra, a short neck, no tubercle, a slight angle and a shallow costal groove.

Question 5

Describe the atypical twelfth rib.

Answer 5

The twelfth rib varies in length. The head has a single facet for T12 vertebra. A short constriction forms a neck which passes imperceptibly into the shaft. There is no tubercle, no angle and no costal groove. The inner surface of the shaft faces slightly upwards.