Sternoclavicular joint

Articular surfaces

The sternoclavicular joint is a connection of three articular surfaces; the sternal end of the clavicle, the clavicular notch of the manubrium of sternum and the superior surface of the first costal cartilage.

The clavicular and sternal joint surfaces are convex and concave, respectively. These curvatures are found in the vertical plane. In the horizontal plane the articular surface of the clavicle is slightly concave while that of the sternum is convex, thus forming a sellar or saddle joint. The inferior aspect of both the clavicle and sternum, as well as the intervening joint space, rest on the first costal cartilage.

The radius of these respective curvatures differs, making the articular surfaces incongruent. The sternal end of the clavicle is also larger in size than the clavicular notch of the sternum, thus the medial end of the clavicle juts out superiorly above the upper border of the manubrium.

Joint congruency is enhanced somewhat by the presence of a fibrocartilaginous intra-articular disc. This disc lies between the clavicular and sternal surfaces, completely separating the joint into two compartments. The disc is held in place by several attachments, which strongly anchor it to the posterosuperior margin of clavicular articular surface, the costal cartilage of first rib, and to the joint capsule. The disc is the thinnest in its central part, which may perforate later in life.

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Ligaments and joint capsule

The joint is surrounded by a fibrous joint capsule which is thickened on its anterior and posterior aspects but loose superiorly and inferiorly. The articular surfaces are lined with fibrocartilage.

Due to the lack of bony congruence, joint stability is provided by two sets of ligaments and the intra-articular disc. The ligaments are divided into;

• Intrinsic ligaments; anterior and posterior sternoclavicular ligaments
• Extrinsic ligaments; interclavicular and costoclavicular ligaments

The broad anterior sternoclavicular ligament runs from the anterosuperior surface of the sternal end of the clavicle to the anterosuperior surface of the manubrium and adjacent part of the first costal cartilage. This provides strong reinforcement to the anterior aspect of the joint.  Similarly, the posterior sternoclavicular ligament covers the posterior aspect of the joint surfaces. Whilst also being broad, it is weaker than its anterior counterpart.

The interclavicular ligament emerges from the deep cervical fascia and connects the sternal ends of both clavicles, spanning the jugular notch. It strengthens the superior aspect of the joint capsule. The costoclavicular ligament unites the superior surface of the first rib and its costal cartilage with the inferomedial aspect of the clavicle. This strong thick ligamentous band consists of two sheaths (laminae) which differ in the direction of their fibers. The anterior lamina courses superolaterally, while the posterior fibers are directed superomedially, forming an inverted cone or cruciate shape. The costoclavicular ligament reinforces the inferior aspect of the joint, acting to limit clavicle elevation and anteroposterior movement.

Innervation

The sternoclavicular joint is innervated from two sources; superficially by the medial supraclavicular nerve (C3-C4; cervical plexus) and deeply by the nerve to subclavius (C5-C6; brachial plexus).

Blood supply

Blood supply to the sternoclavicular joint comes from branches of the suprascapular and internal thoracic arteries.

Movements

Due to the shape of its articular surfaces, the sternoclavicular joint is classified as a saddle joint. However, functionally, it has the features of a ball-and-socket joint, being a multiaxial joint with three degrees of freedom (mean active RoM values);

1. Elevation – depression (40°)
2. Protraction – retraction (35°)
3. Axial rotation (20-40°)

Sternoclavicular (SC) joint movements use the lateral end of the clavicle as a reference point. So during SC elevation the lateral end of the clavicle elevates, while during depression the lateral end of the clavicle lowers. In protraction the lateral end of the clavicle moves anteriorly while in retraction it moves posteriorly.

During elevation the lateral end of clavicle elevates while the medial end depresses. Movement at the medial end requires accessory roll and slide movements, occurring between the clavicle and the intra-articular disc, and between the disc and the sternum. During depression the opposite occurs. These movements occur around a fulcrum located at costoclavicular ligament. With regards to the range of motion (RoM), the lateral end of the clavicle will have a greater range of motion. It can elevate by approximately 10cm, and depress by 3cm, giving between 35 - 60° angular range. Elevation motion is limited by tension of the costoclavicular ligament and by the tone of the subclavius muscle, while depression is limited by the articular disc and tension of the interclavicular ligament.

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In protraction the lateral end of the clavicle moves forwards (anteriorly) while the medial end, together with articular disc, moves posteriorly. In retraction the opposite occurs. These movements also occur around an axis running through the center of the costoclavicular ligament. Protraction is limited by the costoclavicular and anterior sternoclavicular ligaments, while posterior sternoclavicular and costoclavicular ligaments limit retraction.

Axial rotation describes the clavicle rolling anteriorly or posteriorly around its longitudinal axis. This passive movement happens indirectly, whereby the scapular rotation motion is transmitted to the sternoclavicular joint via the coracoclavicular ligament. The range of motion ranges from 20° to 40°, depending on the position of the clavicle. The greatest rotation is achieved when the clavicle is moved posteriorly.

Abduction of the arm into a position of full elevation requires movement to occur at the sternoclavicular joint. Here, abduction at the glenohumeral joint is accompanied by lateral scapula rotation. The lateral scapula rotation is concurrently translated into the clavicle, resulting in clavicular elevation and axial rotation. The sternoclavicular joint takes a closed packed position when the clavicle is maximally (posteriorly) rotated, as occurs during maximum arm elevation and full scapular rotation. While the open (resting) position occurs when the arm is resting by the side. The capsular pattern is described as pain at extreme range of movement, especially during horizontal adduction and full elevation of the arm. The sternoclavicular joint permits the accessory movement of rolling and polyaxial gliding (translation).

Muscles acting on the sternoclavicular joint

No muscles have immediate action on the sternoclavicular joint. Instead movement at this joint relies primarily on the motion of the scapula and the pectoral (shoulder) girdle.

Several muscles have attachment to the clavicle and thus influence movement of the SC joint. These are the subclavius, deltoid, pectoralis, trapezius and sternocleidomastoid muscles. Of note here is the subclavius muscle, whose actions involve pulling the clavicle towards the sternoclavicular intra-articular disc and sternum. This functions to depresses the lateral end of clavicle and stabilizes the clavicle during movements of the pectoral girdle.

Major blood vessels pass directly posterior to the sternoclavicular joints. These are the aortic arch, superior vena cava, and their respective branches.