The clavicle, collarbone, or keybone is a slender, S-shaped long bone approximately long[The Shoulder Complex. In: Prentice WE. eds. Principles of Athletic Training: A Guide to Evidence-Based Clinical Practice, 17e
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/ref> that serves as a strut between the scapula 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.
Structure
The collarbone is a thin doubly curved long bone that connects the human arm to the torso of the body.
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| Right clavicle—from below, and from above |
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| Left clavicle—from above, and from below |
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.
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, it 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.
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the anterior border is concave forward and gives origin to the deltoid muscle.
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the posterior border is convex and gives attachment to the trapezius muscle.
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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 (bone marrow) in its medial two-thirds.
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 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.
Functions
The collarbone serves several functions:
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It serves as a rigid support from which the scapula and free limb are suspended; an arrangement that keeps the upper limb away from the thorax so that the arm has maximum range of movement. Acting as a flexible, crane-like strut, it allows the scapula to move freely on the thoracic wall.
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the cervicoaxillary canal, it protects the neurovascular bundle that supplies the upper limb.
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Transmits physical impacts from the upper limb to the axial skeleton.
Muscle
Muscles and ligaments that attach to the collarbone include:
! Attachment on collarbone !! Muscle/Ligament !! Other attachment |
| | deltoid tubercle, anteriorly on the lateral third |
| | posteriorly on the lateral third |
| | subclavian groove |
| | [[conoid tubercle]] |
| | [[trapezoid line]] |
| | medial third (rounded border) |
| | superiorly, on the medial third |
| | inferiorly, on the medial third |
| | lateral third
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Clinical significance
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.
Other animals
The clavicle first appears as part of the skeleton in primitive Osteichthyes, where it is associated with the pectoral fin; they also have a bone called the cleithrum. In such fish, the paired clavicles run behind and below the gills on each side, and are joined by a solid symphysis on the fish's underside. They are, however, absent in Chondricthyes and in the vast majority of living bony fish, including all of the .
The earliest retained this arrangement, with the addition of a diamond-shaped interclavicle between the base of the clavicles, although this is not found in living . The cleithrum disappeared early in the evolution of , and is not found in any living , but the interclavicle is present in most modern reptiles, and also in . In modern forms, however, there are a number of variations from the primitive pattern. For example, and lack clavicles altogether (although crocodilians do retain the interclavicle), while in , they form part of the armoured plastron.[
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The interclavicle is absent in and . In many mammals, the clavicles are also reduced, or even absent, to allow the scapula greater freedom of motion, which may be useful in fast-running animals.[
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Though a number of fossil hominin (humans and chimpanzees) clavicles have been found, most of these are mere segments offering limited information on the form and function of the pectoral girdle. One exception is the clavicle of AL 333x6/9 attributed to Australopithecus afarensis which has a well-preserved sternal end. One interpretation of this specimen, based on the orientation of its lateral end and the position of the deltoid attachment area, suggests that this clavicle is distinct from those found in extant (including humans), and thus that the shape of the human shoulder dates back to less than . However, analyses of the clavicle in extant primates suggest that the low position of the scapula in humans is reflected mostly in the curvature of the medial portion of the clavicle rather than the lateral portion. This part of the bone is similar in A. afarensis and it is thus possible that this species had a high shoulder position similar to that in modern humans.
In dinosaurs
In , the main bones of the pectoral girdle were the scapula (shoulder blade) and the coracoid, both of which directly articulated with the clavicle. The clavicle was present in saurischian dinosaurs but largely absent in ornithischian dinosaurs. The place on the scapula where it articulated with the humerus (upper bone of the forelimb) is called the glenoid. The clavicles fused in some theropod dinosaurs to form a furcula, which is the equivalent to a wishbone.[Martin, A.J. (2006). Introduction to the Study of Dinosaurs. Second Edition. Oxford, Blackwell Publishing. pg. 299-300. .]
In birds, the clavicles and interclavicle have fused to form a single Y-shaped bone, the furcula or "wishbone" which evolved from the clavicles found in coelurosaurian theropods.
Additional media
See also
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Costal tuberosity of clavicle
External links
