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The sternoclavicular joint is where the clavicle, or collarbone, meets the sternum, or breastbone. As the skeleton tends to be symmetrical, there are two sternoclavicular joints which mirror each other, one on the left and one on the right. A capsule surrounds the joint, and fibrous bands, known as ligaments, help to strengthen it. Dislocation of the sternoclavicular joint is rare because of the stability provided by these tough ligaments, with fracture of the clavicle being more likely to happen instead.
Providing the only bony connection between the upper arms and the torso, the sternoclavicular joint is involved in moving the arms, especially during actions such as throwing a ball. In common with other synovial joints, the sternoclavicular joint is enclosed by a capsule, lined with a thin layer of tissue known as a synovial membrane. This membrane produces fluid which lubricates the joint. The articulating surfaces of the clavicle and sternum are covered in fibrous cartilage which helps to make them smoother and more resilient during joint movement.
Inside the sternoclavicular joint, there is a flat disc made of fibrous cartilage called the articular disc. The disc acts as a shock absorber for forces traveling along the clavicle. It lies between the articulating bony surfaces and joins to the capsule at the sides.
The sternoclavicular joint is what is known as a saddle joint, which allows movement in two different directions. This enables the clavicle to move backward and forward and to lift up and down. The joint is well supported by its ligaments and the overall range of motion is fairly limited.
A particularly strong ligament called the costoclavicular ligament joins the clavicle to the cartilage of the first rib, making the joint quite stable. Occasionally, perhaps when playing sports, dislocation occurs when a person lands with one shoulder on the ground and the other shoulder is struck from above. The end of the clavicle becomes separated from the joint, being pushed down and to the front. Signs include pain and swelling of the joint, and treatment typically includes a combination of pain relief and resting the joint in a sling. In more complex cases surgery may be required to repair the joint.
Sometimes a condition occurs in young people which mimics dislocation of the sternoclavicular joint. Near the end of the clavicle is an area of cartilage called a growth plate. The growth plate does not develop into bone until one's teenage years are over and, if broken, it can resemble a sternoclavicular dislocation. A doctor will be able to tell the difference and most cases of growth plate fracture heal without any special treatment.