These costo-vertebral articulations (French: articulations costo-vertébrales)consist of two sets: the capitular (costo-central): i.e., the articulation of the head of the rib with the vertebrae and the costo-transverse, or the articulation of the tubercle (of each of the first ten ribs) with the transverse process of the lower of the two vertebrae, with which the head of the rib articulates: i.e., the one bearing its own number, as the first rib with the first thoracic vertebra, the second rib with the second thoracic vertebra, and so on.
The Capitular (Costo-central) Articulation
It is a very perfect joint, into the formation of which the head of the rib and two vertebrae, with the intervertebral disc between them, enter. In the case of the first, tenth, eleventh, and twelfth ribs, it is formed by the head of the rib articulating with a single vertebra.
The ligaments are:
- Articular capsule.
The articular capsule consists of short, strong, woolly fibers, completely surrounding the joint, which are attached to the bones and intervertebral substances, a little beyond their articular margins.
At its upper part, it reaches through the intervertebral foramen toward the back of the bodies of the vertebrae, being strengthened here by fibers which at intervals connect the anterior with the posterior longitudinal ligaments. The lower fibers extend downward nearly to the demi-facet (costal pit) of the rib below; behind, it is continuous with the neck ligament, and in front is overlaid by the radiate.
The interarticular ligament consists of short, strong fibers, closely interwoven with the outermost ring of the intervertebral disc, and attached to the transverse ridge separating the articular facets on the head of the rib. It completely divides the articulations into two parts, but does not brace the rib tightly to the spine, being loose enough to allow a moderate amount of rotation on its own axis. There is no interarticular ligament in the costo-vertebral joints of the first, tenth, eleventh, and twelfth ribs.
The radiate (or stellate) ligament, a thickening of the anterior part of the capsule, is the most striking of all, and consists of bright, pearly-white fibers attached to the anterior surface, and upper and lower borders of the neck of the rib, a little way beyond the articular facet; from this they radiate upward, forward, and downward, so as to form a continuous layer of distinct and sharply defined fibers.
The middle fibers run straight forward to be attached to the intervertebral disc; the upper ascend to the lower half of the lateral surface of the vertebra above, and the lower descend to the upper half of the vertebra below. The radiate ligament is overlapped on the vertebral bodies by the lateral (short) vertebral ligaments.
In the case of the first, tenth, eleventh, and twelfth ribs, each of which articulates with one vertebra, the ligament is not quite so distinctly radiate, but even in these the ascending fibers reach the vertebra above that with which the rib articulates.
The synovial membranes consist of two closed sacs which do not communicate: one above, and the other below, the interarticular ligament. In the case of the first, tenth, eleventh, and twelfth articulations, there is but one synovial membrane, as these joints have no interarticular ligament.
The arterial supply is from the intercostal arteries, the twigs piercing the radiate and capsular ligaments.
The nerve supply comes from the anterior primary branches of the intercostal nerves.
These joints approach most nearly in their movements to the condylarthroses.
The movements are ginglymoid in character, consisting of a slight degree of elevation and depression around an obliquely horizontal axis corresponding with the interarticular ligament; there is also a slight amount of forward and backward gliding; and a slight degree of screwing or rotatory movement is also possible. There is a considerable difference in the degree of mobility of the different ribs, for while the first rib is almost immobile except in a very deep inspiration, the mobility of the others increases from the second to the last; the two floating ribs being the most mobile of all. The head of the rib is the most fixed point of the costal arch, and upon it the whole arch rotates; the interarticular ligament allows only a very limited amount of flexion and extension (i. e., elevation and depression), and of gliding. Gliding is checked by the radiate ligament.
In inspiration, the rib is elevated, and glides forward in its socket, too great elevation being checked not only by the ligaments, but also by the overhanging upper edge of the cavity itself. In expiration, the rib is depressed, and glides backward in its cavity.
The Costo-transvehse Articulation
Class. - Diarthrosis.
Subdivision. - Arthrodia.
This joint is formed by the tubercle of the rib articulating with the anterior part of the tip of the transverse process. The eleventh and twelfth ribs are devoid of these joints, for the tubercles of these ribs are absent, and the transverse processes of the eleventh and twelfth thoracic vertebrae are rudimentary.
The ligaments of the union are:
- Articular capsule. Neck ligament.
- Tubercular ligament. Costo-transverse ligaments.
The articular capsule forms a thin, loose, fibrous envelope to the synovial membrane. Its fibers are attached to the bones just beyond the articular margins, and are thickest below, where they are not strengthened by any other structure. It is connected medially with the neck ligament, above with the costo-transverse, and laterally with the tubercular (posterior costo- transverse) ligaments. The eleventh and twelfth ribs are unprovided with costo- transverse capsules.
The neck ligament [lig. colli costae] (middle costo-transverse, or interosseous ligament), consists of short fibers passing between the back of the neck of the rib and front of the transverse process, with which the tubercle articulates. It extends from the capsule of the capitular joint to that of the costo-transverse. It is best seen on horizontal section through the bones. In the eleventh and twelfth ribs this ligament is rudimentary.
The tubercular ligament (posterior costo-transverse) is a short but thick, strong, and broad ligament, which extends laterally and upward from the extremity of the transverse process to the non-articular surface of the tubercle of the corresponding rib. The eleventh and twelfth ribs have no posterior ligament.
The (superior) costo-transverse ligament is a strong, broad band of fibers which ascends laterally from the crest on the upper border of the neck of the rib, to the lower border of the transverse process above. A few scattered posterior fibers pass upward and medially from the neck to the transverse process. The costo-transverse ligament is subdivided into a stronger anterior portion (anterior costo-transverse ligament) best seen from the front, and a weaker posterior portion (posterior costo-transverse ligament). Its medial border bounds the foramen through which the posterior branches of the inter- costal vessels and nerves pass. To the lateral border is attached the thin aponeurosis covering the external intercostals. Its anterior surface is in relation with the intercostal vessels and nerve; the posterior with the longissimus dorsi. The first rib has no (superior) costo-transverse ligament.
The synovial membrane is a single sac.
The arterial and nerve supplies come from the posterior branches of the intercostal arteries and nerves.
The movements which take place at these joints are limited to a gliding of the tubercle of the rib upon the transverse process. The exact position of the facet on the transverse process varies slightly from above downward, being placed higher on the processes of the lower vertebrae. The plane of movement in most of the costo-transverse joints is inclined upward and backward in inspiration, and downward and forward in expiration. The point round which these movements occur is the head of the rib, so that the tubercle of the rib gUdes upon the transverse process in the circumference of a circle, the center of which is at the capitular joint.