Articular surfaces

The articular surfaces of the sternoclavicular joint are covered by fibrocartilage, an anomaly for a synovial joint. The surface upon the manubrium sternii sits at 45 degrees to the vertical. The surface on the medial end of the clavicle extends a short distance onto its inferior surface. This permits articulation with the first costal cartilage that partially sits within the inferior end of the joint.

The articular surfaces are far from congruous:

• the clavicular surface is larger and typically only 50% of its surface will lie in direct apposition to the sternal surface; the rest projects above the anterior, superior and posterior margin of the manubrium.
• the clavicular surface is mildly convex in a vertical plane and concave in a horizontal plane; the sternocostal surface has a reciprocal shape but with a far deeper concavity

The interposition of the fibrocartilaginous disc increases congruity.