Clavicle and Ribs — MRCS Part B Anatomy

The clavicle and the first rib, with the subclavian artery threading between them. The bone questions are about attachments and the fracture; the rib questions lead to the thoracic outlet.

The clavicle

The only long bone that lies horizontally, and the only bony strut between the upper limb and the trunk: a double curve, convex forward in its rounded medial two-thirds and concave forward in its flat lateral third. It articulates medially with the manubrium at the sternoclavicular joint and laterally with the acromion. Attachments: pectoralis major on the front of the medial part and sternocleidomastoid on top of it; deltoid on the front of the lateral part and trapezius behind; subclavius in the groove underneath; and on the under-surface the costoclavicular ligament medially and the conoid tubercle and trapezoid line laterally, for the two parts of the coracoclavicular ligament that hang the scapula from it. It ossifies first of all the bones, in membrane, and its medial end is the last epiphysis in the body to fuse.

It breaks at the junction of the middle and outer thirds, where the curves change and the bone is thinnest: the medial fragment is pulled up by sternocleidomastoid and the lateral fragment drops with the weight of the arm and is pulled in by pectoralis major. The subclavian vessels and the brachial plexus lie behind and below it, protected by subclavius, and are rarely injured.

The subclavian artery

The right arises from the brachiocephalic trunk behind the sternoclavicular joint, the left from the arch of the aorta, so the left has a course in the thorax first. Each arches over the first rib, grooving it behind the scalene tubercle, and becomes the axillary artery at the rib's outer border. Scalenus anterior divides it into three parts. The first part, medial to the muscle, gives the vertebral artery, the internal thoracic artery and the thyrocervical trunk (inferior thyroid, suprascapular and transverse cervical arteries). The second part, behind the muscle, gives the costocervical trunk (superior intercostal and deep cervical arteries). The third part, lateral to the muscle, gives the dorsal scapular artery in most people. So: three, one, one — or none, when the dorsal scapular comes from the transverse cervical. The subclavian vein runs in front of scalenus anterior, separated from the artery by the muscle, and the phrenic nerve lies on the front of the muscle between them.

Ribs

A typical rib (the third to the ninth) has a head with two facets for its own vertebral body and the one above, a neck, a tubercle with a facet for the transverse process of its own vertebra, an angle, and a shaft that is flattened and twisted, with a costal groove along the inside of its lower border sheltering the intercostal vein, artery and nerve in that order from above. The first, second, tenth, eleventh and twelfth are atypical. The ribs lengthen from the first to the seventh and shorten again below; the upper seven reach the sternum by their own cartilages (true ribs), the eighth to tenth join the cartilage above (false ribs), and the eleventh and twelfth end free in the muscles of the wall (floating ribs). The lower ribs slope more steeply, so the bucket-handle movement of the lower cage widens the chest while the pump-handle movement of the upper cage deepens it.

The first rib

Short, broad, flat and sharply curved, with its surfaces facing up and down rather than in and out. Its head has a single facet, for T1 only. On its upper surface is the scalene tubercle, on the inner border, where scalenus anterior inserts; in front of it is the groove for the subclavian vein, behind it the groove for the subclavian artery and the lower trunk of the brachial plexus, and behind that the rough area for scalenus medius. The neck is crossed, from medial to lateral, by the sympathetic trunk, the superior intercostal artery and the first thoracic nerve as it climbs to join the plexus. The rib therefore has one tubercle for its own transverse process behind, and the scalene tubercle on its upper surface: the answer to "how many" depends on which the examiner means, and it is worth saying so.

Cervical rib

An extra rib, or a fibrous band, from the seventh cervical vertebra, in about one person in 200, usually silent. When it presses on the lower trunk of the brachial plexus it gives pain and paraesthesia down the inner arm and into the little finger, with wasting of the small muscles of the hand (T1); when it presses on the subclavian artery it gives a cold, pale arm, a weakened radial pulse that fades on turning the head or raising the arm, a bruit above the clavicle, and post-stenotic dilatation of the artery with distal emboli. Symptoms usually start in early adult life. A neck radiograph shows the bony rib, and the band is found at operation.