This is a synovial joint, separated into two cavities by an intervening fibrocartilage. Although synovial, it is atypical since the bony surfaces are covered by fibrocartilage, not the usual hyaline variety. The articular surface on the manubrium sterni is set at an angle of 45° with the perpendicular, and is markedly concave from above downwards. The articular surface on the sternal end of the clavicle, flattened or slightly concave, is continued over the inferior surface of the shaft in a high percentage of cases, for articulation with the first costal cartilage. The sternal end of the clavicle projects well above the upper margin of the manubrial facet so that only about the lower half of the clavicular articular surface lies opposite the sternal articular facet.
Ligaments of sternoclavicular joint:
The capsule invests the articular surfaces like a sleeve. To this capsule the articular disc is attached, thus dividing the joint into two separate cavities. Rarely the disc is perforated. The disc is also firmly attached to the medial end of the clavicle above and behind, and to the first costal cartilage below, as though to restrain the sternal end of the clavicle from tilting upwards and being displaced medially as the weight of the arm depresses the acromial end. The capsule is thickened above and behind as the anterior and posterior sternoclavicular ligaments, and it is weaker at the front (anterior dislocation is commoner than posterior, though both are rare).
The interclavicular ligament joins the upper borders of the sternal ends of the two clavicles and is itself attached to the suprasternal (jugular) notch. The costoclavicular ligament binds the clavicle to the first costal cartilage and the adjacent end of the first rib, just lateral to the joint; it is an accessory ligament thereof. It is in two laminae (usually separated by a bursa), which are attached to the anterior and posterior lips of the ligamentous impression on the clavicle. The fibers of the anterior lamina run upwards and laterally, and those of the posterior lamina upwards and medially (these are the same directions as those of the external and internal intercostal muscles). The ligament is very strong and is the major stabilizing factor of the sternoclavicular joint.
Nerve supply of sternoclavicular joint:
The medial supraclavicular nerves (C3 and 4) from the cervical plexus give articular branches to the capsule and ligaments.
Movements of sternoclavicular joint:
The fulcrum of movements at this joint is not the sternal end of the clavicle, but the costoclavicular ligament. As the lateral end of the clavicle moves, its medial end moves in the opposite direction. This can be readily demonstrated by simple palpation (feel your own!). Elevate the acromial end by shrugging the shoulder; the sternal end moves down. Only in complete elevation of the acromial end, as in full abduction of the arm, when the medial end of the clavicle can be depressed no further, is the ligament put on full stretch. Depress the acromial end of the clavicle by drooping the shoulder; the sternal end moves up. Upward movement of the sternal end is halted by the interclavicular ligament, and especially by the intra-articular disc. Protrude the acromial end of the clavicle by hunching the shoulders forward; the sternal end moves back. Retract the acromial end by squaring the shoulders; the sternal end moves forward. The clavicle moves about the costoclavicular ligament like a see-saw in both the horizontal and the coronal planes. In shrugging the shoulder the clavicle moves on the disc; in protracting the scapula, the clavicle moves with the disc.
A further movement takes place at the sternoclavicular joint, namely rotation. Rotation of the clavicle is passive; there are no rotator muscles. It is produced by rotation of the scapula and transmitted to the clavicle through the coracoclavicular ligaments. Palpate the forward convexity of the clavicle. Flex the arm in the sagittal plane and continue upwards in this plane to full abduction above the head. Restore the arm and carry it back in the sagittal plane into full extension. The rotation of the clavicle can be easily felt, it amounts, in fact, to some 40°.
Elevation and depression of the acromial end of the clavicle, resulting in movements downwards and upwards respectively of the sternal end, cause movement between the clavicle and the disc. Forward and backward movements in the horizontal plane result in movements between the fibrocartilage and manubrium, the former moving with the sternal end of the clavicle. Similarly, in rotary movements (abduction of the arm above the head) the disc moves with the clavicle.
Stability of sternoclavicular joint:
The stability of the joint is maintained by the ligaments, most especially the costoclavicular ligament. It takes all strain off the joint, transmitting stress from clavicle to first costal cartilage. The latter is itself immovably fixed to the manubrium by a primary cartilaginous joint. Dislocation is unusual; the clavicle breaks in preference.