The articulations of the cartilages of the true ribs with the sternum are arthrodial joints, with the exception of the first, in which the cartilage is directly united with the sternum, and which is, therefore, a synarthrodial articulation. The ligaments connecting them are:
The Articular Capsules (capsulæ articulares; capsular ligaments).The articular capsules surround the joints between the cartilages of the true ribs and the sternum. They are very thin, intimately blended with the radiate sternocostal ligaments, and strengthened at the upper and lower parts of the articulations by a few fibers, which connect the cartilages to the side of the sternum.
The Radiate Sternocostal Ligaments (ligamenta sternocostalia radiata; chondrosternal or sternocostal ligaments).These ligaments consist of broad and thin membranous bands that radiate from the front and back of the sternal ends of the cartilages of the true ribs to the anterior and posterior surfaces of the sternum. They are composed of fasciculi which pass in different directions. The superior fasciculi ascend obliquely, the inferior fasciculi descend obliquely, and the middle fasciculi run horizontally. The superficial fibers are the longest; they intermingle with the fibers of the ligaments above and below them, with those of the opposite side, and in front with the tendinous fibers of origin of the Pectoralis major, forming a thick fibrous membrane (membrana sterni) which envelopes the sternum. This is more distinct at the lower than at the upper part of the bone.
FIG. 315 Sternocostal and interchondral articulations. Anterior view. (See enlarged image)
The Interarticular Sternocostal Ligament (ligamentum sternocostale interarticulare; interarticular chondrosternal ligament).This ligament is found constantly only between the second costal cartilages and the sternum. The cartilage of the secondrib is connected with the sternum by means of an interarticular ligament, attached by one end to the cartilage of the rib, and by the other to the fibrocartilage which unites the manubrium and body of the sternum. This articulation is provided with two synovial membranes. Occasionally the cartilage of the third rib is connected with the first and second pieces of the body of the sternum by an interarticular ligament. Still more rarely, similar ligaments are found in the other four joints of the series. In the lower two the ligament sometimes completely obliterates the cavity, so as to convert the articulation into an amphiarthrosis.
The Costoxiphoid Ligaments (ligamenta costoxiphoidea; chondroxiphoid ligaments).These ligaments connect the anterior and posterior surfaces of the seventh costal cartilage, and sometimes those of the sixth, to the front and back of the xiphoid process. They vary in length and breadth in different subjects; those on the back of the joint are less distinct than those in front.
Synovial Membranes.There is no synovial membrane between the first costal cartilage and the sternum, as this cartilage is directly continuous with the manubrium. There are two in the articulation of the second costal cartilage and generally one in each of the other joints; but those of the sixth and seventh sternocostal joints are sometimes absent; where an interarticular ligament is present, there are two synovial cavities. After middle life the articular surfaces lose their polish, become roughened, and the synovial membranes apparently disappear. In old age, the cartilages of most of the ribs become continuous with the sternum, and the joint cavities are consequently obliterated.
Interchondral Articulations (articulationes interchondrales; articulations of the cartilages of the ribs with each other) (Fig. 315).The contiguous borders of the sixth, seventh, and eighth, and sometimes those of the ninth and tenth, costal cartilages articulate with each other by small, smooth, oblong facets. Each articulation is enclosed in a thin articular capsule, lined by synovial membrane and strengthened laterally and medially by ligamentous fibers (interchondral ligaments) which pass from one cartilage to the other. Sometimes the fifth costal cartilages, more rarely the ninth and tenth, articulate by their lower borders with the adjoining cartilages by small oval facets; more frequently the connection is by a few ligamentous fibers.