Structure
The clavicle has two ends, a shaft with two curves, and an under-surface marked by ligament and muscle attachments.
Ends
- Sternal (medial) end: thick, rounded and roughly quadrangular, with a saddle-shaped facet for the clavicular notch of the manubrium and a smaller area below for the first costal cartilage.
- Acromial (lateral) end: flattened from above down, with a small oval facet facing laterally and slightly down for the acromion.
Shaft and borders
The medial two-thirds of the shaft are rounded and convex forwards; the lateral third is flat and concave forwards. The borders follow these curves.
- Anterior border: convex and rounded medially, where pectoralis major arises; concave and sharper laterally, where deltoid arises, sometimes from a small deltoid tubercle.
- Posterior border: smooth and concave medially, facing the neurovascular structures behind; convex and roughened laterally for trapezius.
Under-surface
Medially, a rough oval impression takes the costoclavicular ligament. In the middle third the subclavian groove lodges subclavius, and its edges give attachment to the clavipectoral fascia. Laterally, near the posterior border, the conoid tubercle takes the conoid ligament, and the trapezoid line runs forwards and laterally from it for the trapezoid ligament.
Articulations and ossification
The clavicle moves at both of its joints whenever the shoulder moves, and its development is unusual among long bones.
- Sternoclavicular joint: a saddle-type synovial joint with an articular disc, the only bony attachment of the upper limb to the axial skeleton. It is braced by anterior and posterior sternoclavicular ligaments, the interclavicular ligament and, most strongly, the costoclavicular ligament.
- Acromioclavicular joint: a plane synovial joint, often with an incomplete disc. The acromioclavicular ligament holds it horizontally; the coracoclavicular ligament gives vertical stability.
The clavicle is the first bone in the body to begin ossifying, from two primary centres in the shaft during the fifth and sixth weeks of fetal life, mostly by intramembranous ossification. The ends form in cartilage. A secondary centre at the sternal end appears in the late teens and fuses in the mid-twenties, making it the last epiphysis in the body to fuse; forensic age estimation uses it. A small acromial centre may appear and fuse quickly.
Relations
The clavicle is subcutaneous along its whole length, with major vessels and nerves close behind its medial two-thirds.
- Superficial: skin, platysma and the supraclavicular nerves (C3–C4), which cross the bone to supply the skin over the upper chest and shoulder.
- Behind the medial end: the brachiocephalic vein, and on the right the bifurcation of the brachiocephalic trunk.
- Behind and below the middle third: the subclavian vein, the subclavian artery and the trunks of the brachial plexus, separated from the bone by subclavius and the clavipectoral fascia.
- Below: the first rib, with the costoclavicular space between the two bones.
Clinical relevance
The clavicle is one of the most commonly fractured bones, and its shape explains both where it breaks and how the fragments move.
Fractures
Most clavicle fractures occur in the middle third, at the junction of the two curves, where the bone is thinnest and least supported by ligaments; lateral-third fractures are next, and medial fractures are rare. The usual cause is a fall on to the point of the shoulder. The medial fragment is pulled up by sternocleidomastoid; the lateral fragment drops with the weight of the arm and is drawn medially by pectoralis major, so the shoulder sags forwards and inwards. Most heal without surgery. Shortening, wide displacement and comminution raise the risk of non-union, and a lateral fracture behind the coracoclavicular ligament behaves like an acromioclavicular dislocation.
Other conditions
- Neurovascular injury: rare, because subclavius cushions the vessels and plexus, but possible with severely displaced fractures or during fixation.
- Numbness below a plate: injury to supraclavicular nerve branches during fixation.
- Posterior sternoclavicular dislocation: the medial end can press on the great vessels, trachea or oesophagus and needs urgent assessment.
- Birth injury: the clavicle is the bone most often fractured during delivery.
- Cleidocranial dysplasia: partial or complete absence of the clavicles, allowing the shoulders to be brought together in front.
On the specimen
On a dry clavicle the examiner pins the two ends, the two borders, the shaft and the conoid tubercle, and often asks which side the bone comes from.
To side a clavicle, hold it against your own shoulder as if it were yours: flat end lateral, rough under-surface with the conoid tubercle and subclavian groove downwards, and the medial two-thirds curving convexly forwards, away from you. The side it then fits is its side.
- Acromial end: the flat end with a small facet; do not confuse it with the bulky sternal end.
- Sternal end: the rounded, heavier end with a large saddle facet.
- Anterior border: the edge that is convex medially and concave laterally.
- Posterior border: the edge that is concave medially and convex laterally.
- Conoid tubercle: the rough knob on the under-surface near the posterior border, where the flat and rounded parts meet; the trapezoid line runs laterally from it.
- Shaft: the body between the ends; the change in curve marks the common fracture site.