The sternoclavicular (SC) joint is the connection between the sternum (breastbone) and the clavicle (collarbone). Also called the medial clavicular joint (less commonly the functional ball and socket joint), it is the only bony hinge from the shoulder girdle to the trunk skeleton. It is secured by various ligaments that give it the necessary stability, at the same time it has a low mobility.
What is the sternoclavicular joint?
The SC joint allows movement in two axes that are perpendicular to each other. Therefore, two so-called degrees of freedom are possible. The directions of motion of the sternoclavicular joint allow for directional raising and lowering, as well as forward and backward movement of the shoulder. Since the two joint surfaces are shaped like a riding saddle, the sternoclavicular joint is classified as a saddle joint. It is thus comparable to the middle joint of the thumb. The two joint surfaces are curved inward and outward (convex/concave), respectively. However, the articular surface of the clavicle is significantly larger than that of the sternum. These two areas are separated from each other by an articular disc (discus articularis), which in turn is fused to the joint capsule. The disc divides the gap between the articular surfaces into two closed chambers and consists of fibrocartilage and firm connective tissue. The two articular surfaces themselves are also encased in such fibrocartilage to ensure their congruency.
Anatomy and structure
Spatially, the sternoclavicular joint is located above the sternum toward the neck. It protrudes beyond the upper border of the sternum, making it highly visible and easy to palpate externally. The SC joint ensures that the collarbone can rotate on its own axis. When the clavicle is fractured, displacement often occurs, but this has insignificant functional effects. Despite its relative clumsiness, the sternoclavicular joint receives a great deal of use. For example, it has to work with every major movement of an arm or the shoulder girdle. Although osteoarthritis of the SC joint is rare, it causes pain early on. They become noticeable when an arm is rotated and raised laterally more than 80 degrees. In this case, the clavicle in the sternoclavicular joint begins to rotate beyond normal. In individual cases, the joint space is also depleted, causing joint parts to rub against each other painfully. Resulting swellings of the SC joint and the adjacent costoclavicular joints occur relatively frequently, even in young women, and cause very uncomfortable pain. Here doctors not infrequently diagnose rheumatic diseases. However, this pain can often be treated well with simple means such as heat or electrotherapy. Possible, but not uncontroversial among experts, is the use of steroids for pain relief. Alternatively, however, suitable muscle or tendon tissue can be implanted to replace the damaged clavicular joint head if necessary. This lies on the articular surface of the sternum and is very susceptible to external irritation.
Function and tasks
The clavicle bears its name thanks to a borrowing from Latin. There, clavicle means “little key,” which, according to ancient lore, could also have something to do with the shape of this bone structure. In humans, the clavicle is between twelve and fifteen centimeters long. It has an S-shape. The end of the clavicle facing the center of the body is called the extremitas sternalis (facing the sternum). Its articular surface is round. The other end, Extremitas acromialis (pointing toward the shoulder level) is flattened in a saddle shape. It is connected to the scapula by the so-called acromioclavicular joint. The most important muscle of this area is the deltoid muscle, which due to its strength roughens the surface of the bone. It is in turn connected to the so-called subclavian muscle. A conspicuous feature is a hole on the underside of the middle section, which provides space for a large blood vessel to supply oxygen and nutrients to the clavicle bone. The clavicle is the bone in humans that breaks second most often. About 15 percent of all fractures involve the clavicle. Causes are often falls on the shoulder or directly the clavicle. In rare cases, the clavicle breaks in a fall onto the outstretched arm.
Diseases
The typical displacements following such a fracture are manifested by a noticeable step formation, an apparently longer arm, and occasionally an unusual head posture. Expressions of congenital disease may include an underdeveloped or even absent clavicle. After fractures or other injuries, the clavicle may be partially or completely removed. The partial claviculectomy is realized in many cases directly at the sternoclavicular joint. It is usually necessary due to prolonged instability of the clavicle or in cases of osteoarthritis. In many cases, only a small part of the clavicle close to the joint is removed. If the entire bone is removed, instabilities in the shoulder area and loss of function of the shoulder and the respective arm may follow. This is usually preceded by malignant bone tumors, which, however, rarely occur on the clavicle as a whole. Metastases practically do not occur here. Sometimes chronic bone infections or complicated bone fractures are occasions for a complete claviculectomy. Complete removal of the clavicle is risky and often associated with complications. Infections and vein injuries can occur. If these are overcome, removal of the clavicle is followed by relatively tolerable limitations in daily life. Sometimes the clavicle is also removed as a bone substitute to reconstruct parts of the humerus. This involves flipping the clavicle into the glenoid cavity and then shortening and reconnecting it to the remaining humerus.
