Clavicle

The clavicle, collarbone, or keybone is a slender, S-shaped long bone approximately 15 centimetres (6 in) long that serves as a strut between the shoulder blade and the sternum (breastbone). There are two clavicles, one on each side of the body. The clavicle is the only long bone in the body that lies horizontally. Together with the shoulder blade, it makes up the shoulder girdle. It receives its name from Latin clavicula 'little key' because the bone rotates along its axis like a key when the shoulder is abducted. The clavicle is the most commonly broken bone. It can easily be fractured by impacts to the shoulder from the force of falling on outstretched arms or by a direct hit.

Structure

The collarbone is a thin doubly curved long bone that connects the arm to the trunk of the body. Located directly above the first rib, it acts as a strut to keep the scapula in place so that the arm can hang freely. At its rounded medial end (sternal end), it articulates with the manubrium of the sternum (breastbone) at the sternoclavicular joint. At its flattened lateral end (acromial end), it articulates with the acromion, a process of the scapula (shoulder blade), at the acromioclavicular joint.

The rounded medial region (sternal region) of the shaft has a long curve laterally and anteriorly along two-thirds of the entire shaft. The flattened lateral region (acromial region) of the shaft has an even larger posterior curve to articulate with the acromion of the scapula. The medial region is the longest clavicular region as it takes up two-thirds of the entire shaft. The lateral region is both the widest clavicular region and thinnest clavicular region. The lateral end has a rough inferior surface that bears a ridge, the trapezoid line, and a slight rounded projection, the conoid tubercle (above the coracoid process). These surface features are attachment sites for muscles and ligaments of the shoulder.

It can be divided into three parts: medial end, lateral end, and shaft.

Medial end

The medial end is also known as the sternal end. It is quadrangular and articulates with the clavicular notch of the manubrium of the sternum to form the sternoclavicular joint. The articular surface extends to the inferior aspect for articulation with the first costal cartilage.

Lateral end

The lateral end is also known as the acromial end. It is flat from above downward. It bears a facet that articulates with the shoulder to form the acromioclavicular joint. The area surrounding the joint gives an attachment to the joint capsule. The anterior border is concave forward and the posterior border is convex backward.

Shaft

The shaft is divided into two main regions, the medial region, and the lateral region. The medial region is also known as the sternal region, it is the longest clavicular region as it takes up two-thirds of the entire shaft. The lateral region is also known as the acromial region, and is both the widest clavicular region and thinnest clavicular region.

Lateral region of the shaft

The lateral region of the shaft has two borders and two surfaces.

the anterior border is concave forward and gives origin to the deltoid muscle.

the posterior border is convex and gives attachment to the trapezius muscle.

the inferior surface has a ridge called the trapezoid line and a tubercle; the conoid tubercle for attachment with the trapezoid and the conoid ligament, part of the coracoclavicular ligament that serves to connect the collarbone with the coracoid process of the scapula.

Development

The collarbone is the first bone to begin the process of ossification (laying down of minerals onto a preformed matrix) during development of the embryo, during the fifth and sixth weeks of gestation. However, it is one of the last bones to finish ossification at about 21–25 years of age. Its lateral end is formed by intramembranous ossification while medially it is formed by endochondral ossification. It consists of a mass of cancellous bone surrounded by a compact bone shell. The cancellous bone forms via two ossification centres, one medial and one lateral, which fuse later on. The compact forms as the layer of fascia covering the bone stimulate the ossification of adjacent tissue. The resulting compact bone is known as a periosteal collar.

The collarbone has a medullary cavity (marrow cavity) in its medial two-thirds. It is made up of spongy cancellous bone with a shell of compact bone. It is a dermal bone derived from elements originally attached to the skull.

Variation

The shape of the clavicle varies more than most other long bones. It is occasionally pierced by a branch of the supraclavicular nerve. In males the clavicle is usually longer and larger than in females. A study measuring 748 males and 252 females saw a difference in collarbone length between age groups 18–20 and 21–25 of about 6 and 5 mm (0.24 and 0.20 in) for males and females respectively.

The left clavicle is usually longer and weaker than the right clavicle.

The collarbones are sometimes partly or completely absent in cleidocranial dysostosis.

The levator claviculae muscle, present in 2–3% of people, originates on the transverse processes of the upper cervical vertebrae and is inserted in the lateral half of the clavicle.

Clinical significance

Acromioclavicular dislocation ("AC Separation")

A vertical line drawn from the mid-clavicle called the mid-clavicular line is used as a reference in describing cardiac apex beat during medical examination. It is also useful for evaluating an enlarged liver, and for locating the gallbladder which is between the mid-clavicular line and the transpyloric plane.

Collarbone fracture

Clavicle fractures (colloquially, a broken collarbone) occur as a result of injury or trauma. The most common type of fractures occur when a person falls horizontally on the shoulder or with an outstretched hand. A direct hit to the collarbone will also cause a break. In most cases, the direct hit occurs from the lateral side towards the medial side of the bone. The most common site of fracture is the junction between the two curvatures of the bone, which is the weakest point. This results in the sternocleidomastoid muscle lifting the medial aspect superiorly, which can perforate the overlying skin.

Source: Wikipedia, The Free Encyclopedia · CC BY-SA 4.0