Anatomy of ribs and costal cartilages
RIBS or Costa
The ribs are 12 pairs of elastic arches. They articulate posteriorly with the vertebral column and form the greater part of the thoracic skeleton.
The first seven pairs are connected to the sternum by costal cartilages,and are referred to as the true ribs.
The remaining five are the so-called false ribs: the cartilages of the eighth to tenth usually join the superjacent costal cartilage, whereas the eleventh and twelfth ribs, which
are free at their anterior ends, are sometimes termed the ‘floating' ribs
The ribs are separated by the intercostal spaces, which are deeper in front and between the upper ribs. The latter are less oblique than the lower ribs; obliquity is maximal at the ninth rib and decreases to the twelfth.
Ribs increase in length from the first to seventh, and thereafter diminish to the twelfth. They decrease in breadth downwards; in the upper ten, the greatest breadth is anterior.
The first two and last three ribs present special features, whereas the remainder conform to a common plan.
Typical rib
A typical rib has a shaft with anterior and posterior ends. The anterior, costal, end has a small concave depression for the lateral end of its cartilage.
The shaft has an external convexity and is grooved internally near its lower border, which is sharp, whereas its upper border is rounded. The posterior, vertebral, end has a head, neck and tubercle.
The head presents two facets, separated by a transverse crest. The lower and larger facet articulates with the body of the corresponding vertebra, its crest attaching to the intervertebral disc above it.
The neck is the flat part beyond the head, anterior to the corresponding transverse process. Its upper border is the sharp crest of the neck, its lower border rounded. The tubercle, which is more prominent in upper ribs, is postero external at the junction of the neck and shaft and is divided into
medial articular and lateral non-articular areas.
The articular part bears a small, oval facet for the transverse process of the corresponding vertebra. The non articular area is roughened by ligaments.
The shaft is thin and flat and has external and internal surfaces, and superior and inferior borders. It is curved, bent at the posterior angle (5–6 cm from the tubercle). The smooth internal surface is marked by a costal groove, bounded below by the inferior border and terminates anteriorly at the junction of the middle and anterior thirds of the shaft, anterior to which the groove is absent.
Attachments and relations.
A radiate ligament is attached along the anterior border of the head and an intra-articular ligament is attached to the crest of the head. The anterior surface of the head is related to costal pleura. The posterior surface of the neck gives attachment to the costo transverse ligament. The superior
costo transverse ligament is attached to the crest of the neck.
The articular area of the tubercle in the succeeding three or four ribs it is almost flat, and faces down, back and slightly medially. The lateral costotransverse ligament is attached to the non-articular area.
From the second to the tenth ribs, the distance between angle and tubercle increases. The internal intercostal muscle is attached to the costal groove on the internal surface, and separates the bone and the intercostal neurovascular bundle. The innermost intercostal is attached to the superior rim of the groove. The external intercostal muscle is attached to the sharp inferior costal border.
First rib
Most acutely curved and usually
shortest, the first rib is broad and flat,
its surfaces are superior and inferior,
and its borders are internal and
external. It slopes obliquely down and
forwards to its sternal end. The
obliquity of the first ribs accounts for
the appearance of pulmonary and pleural apices in the neck. The head of the first rib is small and round. It bears an almost circular facet, and articulates with the body of the first thoracic vertebra. The neck is rounded and ascends postero laterally. The tubercle, wide and prominent, medially, an oval facet articulates with the transverse process of the first thoracic vertebra.
At the tubercle, the rib is bent, and so the angle and tubercle coincide. The superior
surface of the flattened shaft is crossed obliquely by two shallow grooves, separated by a slight ridge, which usually ends at the internal border as a small pointed projection, the scalene tubercle, to which scalenus anterior is attached. The groove anterior to the scalene tubercle forms a bed for the subclavian vein, The subclavian artery pass in the groove behind the tubercle. Behind this, scalenus medius is attached as far as the costal tubercle. The supra pleural membrane, which covers the cervical dome of the pleura, is attached to the internal border. The inferior surface is smooth.
The anterior end is larger than in any other rib.
Second rib
The second rib is twice the length of the first rib, and has a similar curvature. The non-articular area of the tubercle is small. The angle is slight and near the tubercle. The external surface of the shaft is convex and supero laterally. The internal surface, smooth and concave, faces infero medially and there is a short costal groove posteriorly.
The lower parts of the first and second digitations of serratus anterior are attached to the external surface. Scalenus posterior and serratus posterior superior are attached to the outer lip in front of the angle.
Tenth, eleventh and twelfth ribs The tenth rib has a single facet on its head that may articulate with the intervertebral disc above,
in addition to the upper border of the tenth thoracic vertebra near its pedicle. The ninth and tenth ribs are usually united anteriorly by a fibrous joint. However, the tenth rib may be free, in which case it is pointed like the eleventh and twelfth ribs.
The eleventh and twelfth ribs each have one large, articular facet on the head, but no neck or tubercle. Their pointed anterior ends are tipped with cartilage. The eleventh rib has a slight angle and shallow costal groove. The twelfth rib has neither, is much shorter and slopes cranially at its vertebral end. The internal surfaces of both ribs face slightly upwards, more so in the twelfth
Costal cartilages
Costal cartilages are the persistent, unossified anterior parts of the cartilaginous models in which the ribs develop.
They are flat bars of hyaline cartilage that extend from the anterior ends of the ribs, and contribute greatly to thoracic mobility and elasticity. The upper seven pairs join the
sternum; the eighth to tenth articulate with the lower border of the cartilage above; the lowest two have free, pointed ends in the abdominal wall. They increase in length from the first to the seventh, and then decrease to the twelfth.
In old age the costal cartilages tend to ossify superficially, lose their pliability and become brittle.
Rib fractures
Elastic recoil of the ribs which suspend the sternum may explain the rarity of sternal fractures.Despite their pliability, the ribs are much more frequently broken, the middle ribs being the most vulnerable. Because traumatic stress is often the result of compression of the thorax, the usual site of fracture is just in front of the angle, which is the weakest point of the rib. Direct impact may fracture a rib at any point; the ends of the broken bone may be driven inwards, and potentially may injure thoracic or upper abdominal viscera.
