Sternocostal joints

Articular surfaces

The sternochondral joint is the articulation between two articular surfaces; the costal notches located along the lateral border of the sternum and the corresponding sternal ends of the first seven costal cartilages.

When the sternum is examined from an anterior view, the sternal costal notches appear as shallow, concave depressions. In the sagittal plane, the surface area of the superior costal notches is larger compared to the inferior ones. The shape of the costal notches also varies; the superior ones are ellipsoid or round, while the inferior ones gradually become more rectagular and irregular quadrilaterals. The seventh costal notch is the deepest out of all of them, with its articular surfaces forming an acute angle. The margins of the costal grooves are smooth overall, with small, irregular portions dotted throughout.

The sternal ends of the costal cartilages have a large and convex, almost semiround shape in the coronal plane. In the sagittal plane, they follow a similar pattern to the corresponding sternal costal notches in terms of size, surface area and shape. The two exceptions are the first and seventh costal cartilages; the former is only slightly convex and significantly less round compared to the rest, while the latter has a pointed shape. These matching characteristics facilitate the accommodation between the costal notches and costal cartilages, allowing them to fit like a lock and key.

The second sternochondral joint is distinct compared to the rest. The sternal articulation is a demifacet rather than one continuous articular surface. This is caused by the presence of the manubriosternal joint of the sternum. The superior articular surface on the manubrium of sternum is bevelled in the frontal plane, pointing inferolaterally. The inferior articular surface on the body of sternum is a circular depression in the shape of a crescent. It points superolaterally in the frontal plane. The sternal end of the second costal cartilage matches the shape of the demifacet, forming a joint that is divided into two compartments by an intraarticular ligament.

A similar situation takes place in the seventh sternochondral joint. It is also a demifacet due to the presence of the xiphisternal joint, exhibiting almost identical articular surface characteristics to the second sternochondral joint. However, the seventh sternochondral joint consists of a single joint cavity because it lacks an intraarticular ligament.

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Joint capsule

The second to seventh sternochondral joints are synovial joints. They are surrounded by a thin fibrous capsule, which is reinforced by the surrounding sternochondral ligaments. The fibrous capsule is lined by a synovial membrane which secretes viscous synovial fluid that acts as a lubricant.

Overall, the second to seventh sternochondral joints all contain joint cavities. However, the cavities are thin and usually disappear with increasing age, especially in the inferior sternochondral joints. In such cases, the obliterated joint cavities are replaced by intraarticular fibrocartilage. The exception are the joint cavities of the second sternochondral joint, which remain open even in advanced age. The articular surfaces of all sternochondral joints are lined by fibrocartilage.

In contrast to its neighbours, the first sternochondral joint is classified as a primary cartilaginous joint (symphysis) rather than a synovial joint. Therefore, the cartilage is united with the corresponding costal notch on the sternum with no intervening joint cavity, exhibiting almost no movement. In addition, the seventh sternochondral joint can also be symphysitic in certain individuals due to a tendency of the inferior sternochondral joints to be more cartilaginous.

Learn more about the general features of the synovial joints by exploring articles, diagrams, videos and quizzes.

Ligaments

The thin fibrous capsules of the sternochondral joints are supported by several ligaments; radiate sternochondral, xiphichondral and intraarticular sternochondral ligaments.

There are two sets of broad, short and thin radiate sternochondral ligaments; anterior and posterior. The anterior ligaments extend between the anterior surface of the sternal ends of the costal cartilage and the anterior margins of the corresponding costal notches of the sternal body. The posterior ligaments connect the identical, but posterior counterparts. Therefore, the radiate sternochondral ligaments strengthen the sternochondral joints directly, both anteriorly and posteriorly. The fibers of those ligaments  spread out over the sternal surfaces, connecting with the ones from the opposite side. While doing so, they intermingle with tendinous fibers from pectoralis major, forming the sternal membrane. This criss-crossed, thick and fibrous membrane envelopes the sternum, supporting all of its joints, especially inferiorly where it is the strongest.

The radiate sternochondral ligaments of the second sternochondral joint are quite distinct. Rather than attaching only to the body of sternum, both the anterior and posterior ligaments connect to the three parts of the sternum; superiorly to the manubrium, horizontally to the fibrocartilage of the manubriosternal joint and inferiorly to the body of sternum. The horizontal fibers collectively form the intraarticular sternochondral ligament, which extends to the sternal end of the second costal cartilage. This intraarticular ligament provides an extra support to the second sternochondral joint, but simultaneously restricts its movements. In certain individuals, the intraarticular sternochondral ligaments can also connect the third sternochondral joints with either the first or second sternochondral joints.

The xiphichondral ligaments reinforce only the seventh sternochondral joint. There are two such ligaments: anterior and posterior. The anterior one is more pronounced, connecting the anterior surface of the sternal end of the seventh costal cartilage to the anterior margin of the seventh sternal costal notch on the xiphoid process. The posterior xiphichondral ligament accomplishes the same task, but on the opposite (posterior) side.

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Innervation

The sternochondral joints are innervated by the intercostal nerves, which represent the anterior rami of spinal nerves T1 to T11.

Blood supply

Blood supply to the sternochondral joints comes from branches of the internal thoracic artery, which stems from the subclavian artery.

Movements

Due to the lack of a joint cavity and the presence of an intraarticular fibrocartilage, the first sternochondral joint is structurally and functionally classified as a primary cartilaginous joint, or synchondrosis. As a result, the joint is nonaxial and permits almost no movement. This aspect is very important during mechanical ventilation. As the ribs move up and down and their anterior ends elevate, this nonmoveable sternochondral joint helps to automatically move the sternum upwards and outwards (‘pump handle’ movement). Therefore, the first sternocostal joint helps to increase the anteroposterior diameter of the thorax during inspiration. The reverse happens during expiration.

The articular surfaces and the presence of a joint cavity structurally, classifies the remaining six sternochondral joints as planar synovial joints. Functionally, they permit only nonaxial, translational movements. This uniplanar motion involves a gliding, or sliding motion in a linear direction between the articular surfaces of the sternal costal notches and the corresponding sternal ends of the costal cartilages. The movement of the second sternochondral joint is even more limited by the intraarticular sternochondral ligament. This articular arrangement of the second to seventh sternochondral joints facilitates thoracic movements during mechanical ventilation.

As the ribs move up and down, and the sternum travels upwards and outwards (‘pump handle’ movement), the sternal ends of the costal cartilages glide superoinferiorly within the sternal costal notches. In turn, as the sixth and seventh ribs also move outward and laterally (‘bucket handle’ movement), their sternochondral joints permit the movement axis to pass through them, facilitating thoracic expansion. The reverse happens during expiration, when anteroposterior and transverse thoracic diameters are decreased.

Muscles acting on the sternochondral joints

No muscles have direct action on the sternochondral joints. Instead, their movements happen indirectly due to the motions of the true ribs, their costal cartilages and sternum.

Several muscles attach to the ribs, the most important ones being the anterolateral trunk muscles responsible for breathing. These include the intercostal muscles (external, internal, innermost), subcostal muscle, transversus thoracis, abdominal oblique (external, internal) muscles, transverse abdominis, rectus abdominis and quadratus lumborum. Working in unison, these muscles elevate or depress the ribs as needed during inspiration and expiration, respectively. This in turn moves the sternal end of the costal cartilage relative to the sternal costal notch.

The second group of muscles attaching to the ribs and causing movements at the sternochondral joints are the superficial (extrinsic) and deep (intrinsic) muscles of the back. Notable mentions include the serratus anterior and posterior muscles, iliocostalis, longissimus and levatores costarum. They have a secondary importance compared to the breathing muscles, but they also move the ribs during various trunk movements (extension, flexion, lateral flexion, rotation). As a result, the sternal ends of the costal cartilages are also moved at the sternochondral joints.