Clavicle
The clavicle, also known as the collarbone or keybone, is a slender S-shaped bone that connects the shoulder blade to the sternum. It’s the only long bone in the human body positioned horizontally and measures approximately 6 inches (15 cm) in length.
Structure Creates Functional Support
Your clavicle forms a crucial part of the shoulder girdle. This distinctive bone has a double curve that resembles an elongated letter S. At its rounded medial end (the part closer to your neck), it connects with the manubrium of the sternum at the sternoclavicular joint. The flattened lateral end joins with the acromion process of the scapula at the acromioclavicular joint.
The bone’s unique structure includes two main regions. The medial region curves forward and outward, making up about two-thirds of the shaft. The lateral region curves backward to meet the shoulder blade. This design isn’t random—it provides the perfect balance of strength and flexibility needed for arm movement.
Medial End Anchors to Chest
The sternal end of the clavicle has a quadrangular shape with an articular surface that extends to its inferior aspect. This design creates a stable connection with both the sternum and the first costal cartilage, anchoring the entire shoulder complex to the thorax.
Lateral End Connects to Shoulder
The acromial end is flat from top to bottom and bears a facet that articulates with the shoulder. The surrounding area provides attachment points for the joint capsule. Its anterior border curves inward while the posterior border curves outward, creating attachment surfaces for important muscles.
Shaft Provides Muscle Attachment
The shaft divides into medial and lateral regions with specific borders and surfaces. The anterior border of the lateral region is concave and gives origin to the deltoid muscle. The posterior border, convex in shape, attaches to the trapezius muscle. On the inferior surface, you’ll find the trapezoid line and conoid tubercle, which connect to ligaments that secure the collarbone to the coracoid process of the scapula.
Development Shows Unique Timeline
The clavicle has a fascinating developmental story. It’s the first bone to begin ossification during embryonic development (weeks 5-6 of gestation), yet it’s one of the last to complete the process—typically between ages 21-25. This extended timeline reflects the bone’s complex formation through both intramembranous ossification (lateral end) and endochondral ossification (medial end).
The bone contains a medullary cavity in its medial two-thirds and consists of spongy cancellous bone surrounded by a compact bone shell. It’s classified as a dermal bone, with origins from elements once attached to the skull.
Variation Reflects Individual Differences
The clavicle shows more variation than most long bones. It sometimes contains a branch of the supraclavicular nerve passing through it. Men typically have longer, larger clavicles than women. Research measuring 1,000 individuals found length differences between age groups 18-20 and 21-25 of about 6mm for males and 5mm for females.
Left clavicles tend to be longer but weaker than right clavicles. In rare cases, the collarbones may be partially or completely absent in a condition called cleidocranial dysostosis.
Functions Support Movement and Protection
Your clavicle serves several critical functions:
It acts as a rigid support from which your scapula and arm hang, keeping your upper limb away from your chest to maximize arm movement range. Like a flexible crane, it allows your shoulder blade to move freely on your thoracic wall.
The collarbone covers the cervicoaxillary canal, protecting the vital nerves and blood vessels that supply your arm. It also transmits physical impacts from your upper limb to your axial skeleton, helping to distribute force during falls or collisions.
Muscles Create Dynamic Connections
Several muscles attach to the clavicle, including the deltoid and trapezius on the superior surface, the subclavius on the inferior surface, and the pectoralis major along the anterior border. The sternocleidomastoid and sternohyoid muscles connect to the posterior border medially, while the trapezius attaches laterally.
These muscular connections create a dynamic system that allows for the complex movements of your shoulder and arm.
Clinical Significance Highlights Vulnerability
The clavicle is the most commonly fractured bone in the human body. Breaks typically occur from falls onto the shoulder, falls with an outstretched hand, or direct hits to the collarbone. The junction between the two curvatures represents the weakest point and the most common fracture site.
Other clinical issues include acromioclavicular dislocations (“AC separation”), sternoclavicular dislocations, and degenerative conditions like osteolysis.
Medical professionals use the mid-clavicular line—a vertical line drawn from the middle of the collarbone—as an important reference point during physical examinations. This line helps in describing the cardiac apex beat, evaluating an enlarged liver, and locating the gallbladder.
Evolutionary History Shows Ancient Origins
The clavicle first appeared in primitive bony fish, where it associated with the pectoral fin alongside another bone called the cleithrum. Early tetrapods maintained this arrangement but added a diamond-shaped interclavicle between the base of the clavicles.
Through evolutionary history, the cleithrum disappeared in reptiles, while the interclavicle persists in most modern reptiles and monotremes but is absent in marsupials and placental mammals. Many mammals have reduced or absent clavicles to allow greater scapular freedom, beneficial for fast-running animals.
Fossil evidence from hominins like Australopithecus afarensis provides clues about the evolution of the human shoulder. The shape and position of the human clavicle may date back 3-4 million years, contributing to our unique upright posture and arm mobility.
In dinosaurs, the clavicles fused in some theropods to form a furcula—the equivalent of a wishbone. This structure persists in modern birds, where the clavicles and interclavicle have fused into a single Y-shaped bone.
Your collarbone isn’t just a simple strut—it’s a remarkable piece of evolutionary engineering that enables the distinctive movements that make us human.
Citations:
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This article needs additional citations for verification. (July 2009) |
The clavicle, collarbone, or keybone is a slender, S-shaped long bone approximately 6 inches (15 cm) 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 is a palpable bone and, in people who have less fat in this region, the location of the bone is clearly visible. 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 fractured bone. It can easily be fractured by impacts to the shoulder from the force of falling on outstretched arms or by a direct hit.
| Clavicle (collarbone) | |
|---|---|
 Human collarbone | |
 Collarbone (shown in red) | |
| Details | |
| Identifiers | |
| Latin | clavicula |
| MeSH | D002968 |
| TA98 | A02.4.02.001 |
| TA2 | 1168 |
| FMA | 13321 |
| Anatomical terms of bone | |
English
Alternative forms
- clavicula
Etymology
Borrowed from Middle French clavicule, from
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