The sternoclavicular joint is a diarthrodial joint. The articular surface of the proximal end of the clavicle has a coronal convexity and a slight sagittal concavity, whereas that of the sternal manubrium has a coronal concavity and a slight sagittal convexity. The cartilage of the first rib also articulates with the clavicle. Both articular surfaces are covered with fibrocartilage, which is thicker on the clavicle than on the sternum. A circular meniscus extending from above the articular surface of the clavicle to the junction between the sternal manubrium and
FIGURE 4 Needle placement into an acromioclavicular joint.
FIGURE 5 Opacification of the acromioclavicular joint cavity. Arrow indicates a small superior synovial diverti-cle. Arrowheads indicate the superior and inferior portions of the joint meniscus.
the cartilage of the first rib separates the joint cavity into two compartments, the clavicular one being larger than the sternal one (Fig. 6) (4). Communications between these compartments are related to aging (5). The meniscus also acts as a checkrein that prevents the proximal end of the clavicle from medial subluxation. The joint space is anteriorly and inferiorly oriented. During the movements of the shoulder, both the end of the clavicle and the meniscus rotate around the longitudinal axis of the clavicle and simultaneously shift vertically on the steady surface of the sternum. In addition, the end of the clavicle also slightly rotates against the meniscus.
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