Articular surfaces and ligaments
The SC joint is a saddle-type synovial joint that behaves functionally like a ball and socket. The large, bulbous sternal end of the clavicle sits above a much smaller clavicular notch on the superolateral corner of the manubrium, and the first costal cartilage completes the socket below. As at the AC joint, the surfaces are covered with fibrocartilage.
Articular disc
A complete fibrocartilaginous disc divides the joint into two separate synovial cavities. The disc is attached above to the upper posterior margin of the clavicle and below to the first costal cartilage. This attachment acts as a check strap that stops the clavicle being driven medially over the manubrium when a force is transmitted along it from the shoulder.
Ligaments
- Anterior and posterior sternoclavicular ligaments: thickenings of the capsule. The posterior ligament is the stronger and resists posterior displacement.
- Interclavicular ligament: crosses the jugular notch, linking the two clavicles and the top of the manubrium.
- Costoclavicular ligament: a short, strong band from the first rib and its cartilage to the impression on the underside of the medial clavicle. It is the chief stabiliser, limiting elevation and acting as the pivot for clavicular movement.
Movements
The SC joint moves whenever the scapula moves, because the clavicle is the strut holding the scapula away from the chest wall. The lateral end of the clavicle travels up and down with elevation and depression of the shoulder, and forwards and backwards with protraction and retraction, pivoting near the costoclavicular ligament. During full elevation of the arm, the clavicle also rotates backwards on its long axis. Movement at the SC joint is therefore essential for raising the arm above the head.
Relations
The structures behind the SC joint are what make its injuries dangerous. Sternohyoid and sternothyroid lie directly behind it, and deep to them are the great vessels at the root of the neck.
- Right side: the brachiocephalic trunk divides into the right subclavian and right common carotid arteries behind the right SC joint. The right brachiocephalic vein forms here.
- Left side: the left brachiocephalic vein begins behind the left SC joint and crosses to the right behind the manubrium. The left common carotid artery lies close behind.
- Deeper: the trachea, oesophagus, vagus and phrenic nerves lie close by in the root of the neck and superior mediastinum.
Sternocleidomastoid's sternal and clavicular heads attach in front of and above the joint, and pectoralis major arises from the front of the medial clavicle and manubrium.
Blood supply and innervation
The SC joint is supplied by branches of the internal thoracic and suprascapular arteries. It is innervated by the medial supraclavicular nerve, a branch of the cervical plexus (C3, C4), and by the nerve to subclavius.
Clinical relevance
SC joint dislocation is rare because the ligaments are strong, and the clavicle usually fractures first.
- Anterior dislocation is the commoner type. The medial end of the clavicle forms a visible lump in front of the manubrium. It is usually treated without surgery.
- Posterior dislocation is less common but can press on the great vessels, trachea and oesophagus, causing breathlessness, dysphagia or venous congestion of the arm. It needs CT to assess the relations and is reduced urgently, ideally with thoracic surgical support available.
- Physeal injury: the medial clavicular epiphysis is the last in the body to fuse, in the mid-twenties, so an apparent dislocation in a young adult may be a fracture through the growth plate.
- Septic arthritis of the SC joint is seen in intravenous drug users and people with diabetes or indwelling central lines.