Clavicle

The clavicle, commonly known as the collarbone or keybone, is a slender, S-shaped long bone that forms an essential component of the shoulder girdle. Positioned horizontally above the first rib, it serves as a structural strut between the scapula and the sternum, enabling the upper limb to move freely while maintaining mechanical stability. As one of the most palpable bones in the body, its outline is particularly visible in individuals with minimal subcutaneous fat. The term clavicula, Latin for “little key,” reflects the bone’s rotational movement during shoulder abduction. It is also the most frequently fractured bone, owing to its exposed position and role in transmitting forces from the upper limb to the axial skeleton.

Structure

The clavicle is a doubly curved long bone that links the upper limb to the trunk. It can be divided into three major regions: the medial end, the lateral end, and the shaft.
Medial end (sternal end)The medial end is quadrangular and articulates with the clavicular notch of the manubrium of the sternum to form the sternoclavicular joint. The articular surface extends to its inferior side, allowing connection with the first costal cartilage. This end forms the more robust portion of the clavicle and contributes significantly to the bone’s function as a strut.
Lateral end (acromial end)The lateral end is flat and bears a small facet that articulates with the acromion of the scapula, creating the acromioclavicular joint. Surrounding this area are attachments for the joint capsule, with the anterior border concave forward and the posterior border convex backward.
ShaftThe shaft demonstrates two distinct regions:

• Medial (sternal) region: This section comprises roughly two-thirds of the shaft and curves laterally and anteriorly. It is the longest portion of the clavicle.
• Lateral (acromial) region: The lateral third is the widest yet thinnest part of the bone. Its anterior border is concave and provides origin for the deltoid muscle, while its convex posterior border gives attachment to the trapezius muscle. On the inferior surface lie the trapezoid line and the conoid tubercle, which serve as attachment points for the trapezoid and conoid ligaments—components of the coracoclavicular ligament that anchor the clavicle to the scapula.

Overall, the clavicle’s curvature and regional specialisation reflect its mechanical roles in force distribution and mobility of the upper limb.

Development

The clavicle is notable for beginning ossification earlier than any other bone—during the fifth to sixth week of embryogenesis—yet completing its development much later, typically between 21 and 25 years of age. It is formed through both intramembranous ossification (laterally) and endochondral ossification (medially), making it a hybrid bone developmentally.
Its internal structure consists of cancellous (spongy) bone enclosed by a compact bone shell, with a medullary cavity present in the medial two-thirds. Two primary ossification centres, medial and lateral, fuse during growth, and the periosteal collar of compact bone develops from surrounding fascial tissue. The clavicle is considered a dermal bone derived from skeletal elements originally associated with the skull.

Variation

The clavicle shows significant variation among individuals, more so than most long bones. Differences may occur in length, curvature, and robustness. In males it is typically longer and larger than in females. Age-related studies have observed measurable differences in clavicle length among young adults.
The left clavicle is usually slightly longer and weaker than the right. Rarely, the bone may be pierced by a branch of the supraclavicular nerve. Genetic conditions such as cleidocranial dysostosis can lead to partial or complete absence of the clavicles. Additionally, approximately one quarter of people possess a levator claviculae muscle, arising from cervical vertebrae and attaching to the clavicle.

Functions

The clavicle performs several critical mechanical and protective roles:

• Support of the upper limb: Acting as a strut, it holds the scapula laterally, ensuring that the arm has maximum range of motion away from the thorax.
• Mobility: Its flexibility allows the scapula to move freely across the thoracic wall, improving upper-limb mobility.
• Protection: By covering the cervicoaxillary canal, the clavicle safeguards major neurovascular structures supplying the upper limb.
• Force transmission: It channels mechanical forces from the upper limb to the axial skeleton, enabling efficient load transfer.

Muscles and ligaments attached to the clavicle include the deltoid, trapezius, pectoralis major, sternocleidomastoid, and components of the coracoclavicular ligament, among others.

Clinical Significance

The clavicle holds various clinical landmarks and is commonly involved in trauma.
Midclavicular lineA vertical line drawn through the midpoint of the clavicle provides an anatomical reference for physical examination, aiding in assessment of the heart’s apex beat, liver enlargement, and gallbladder location.
Clavicle fracturesFractures are common due to falls on the shoulder, impact on an outstretched hand, or direct blows. The typical fracture site is the junction between the bone’s medial and lateral curvatures—its structurally weakest point. The medial fragment may be displaced superiorly by the sternocleidomastoid muscle, occasionally threatening the overlying skin.
Acromioclavicular separationInjuries to the acromioclavicular joint may occur from direct trauma to the shoulder, disrupting ligaments and altering clavicular alignment.

Comparative Anatomy

The clavicle has a diverse evolutionary history. It first appeared in early bony fish (Osteichthyes), where paired clavicles formed part of the shoulder girdle alongside a bone called the cleithrum. In many modern fish species, particularly teleosts, clavicles are absent.
Early tetrapods possessed clavicles and an interclavicle, although the latter is missing in living amphibians. Reptiles and monotremes retain the interclavicle, while crocodilians lack clavicles entirely and turtles incorporate clavicular structures into the plastron.
Among mammals, clavicles vary considerably depending on locomotive strategy. Fast-running species often reduce or lose the clavicle to enhance limb mobility. Fossil hominin clavicles provide insight into early human shoulder morphology. Specimens such as the Australopithecus afarensis clavicle from AL 333 offer clues about scapular position and limb function, suggesting that aspects of modern human shoulder configuration may have deep evolutionary roots.