The clavicle (collarbone) is classified as a long bone. This might seem counterintuitive given its relatively short, curved shape compared to obvious long bones like the femur or humerus, but anatomists categorize it this way because it is longer than it is wide and connects two joints at either end. The clavicle is also one of the most unusual bones in the human body: it’s the first bone to start forming in a developing embryo and the very last to finish growing, sometimes not fully fusing until age 27.
Why the Clavicle Counts as a Long Bone
Bones are sorted into five main types based on their shape: long, short, flat, irregular, and sesamoid. Long bones are defined not by absolute size but by having a shaft that’s longer than it is wide, with distinct ends that form joints. The clavicle fits this description. It has a slightly curved shaft running horizontally across the upper chest, with a joint at each end.
That said, the clavicle breaks a few rules that most long bones follow. Typical long bones like the femur or radius have a hollow central cavity filled with marrow, called a medullary cavity. The clavicle lacks this. Instead, it has a hard outer shell surrounding an inner core of spongy bone tissue (cancellous bone). This structure makes it somewhat of an outlier in its category, and some anatomy references describe it as a “modified long bone” for this reason.
How the Clavicle Develops Differently
Most long bones grow through a process where a cartilage model is gradually replaced by bone tissue. The clavicle takes a different path. It forms primarily through intramembranous ossification, meaning bone tissue develops directly from connective tissue membranes rather than from a cartilage template. This is the same process that forms the flat bones of the skull, making the clavicle unusual among long bones.
The clavicle’s development timeline is remarkable. Two bone-forming centers appear in the shaft around the sixth week of embryonic life, making it the first bone in the body to begin hardening. Yet the growth plate at the inner (medial) end doesn’t fully close until much later in life. While most bones in the hand finish fusing around age 18, the medial end of the clavicle typically completes fusion by age 27. This extended growth window is why the clavicle is sometimes used in forensic science to help estimate a young person’s age.
What the Clavicle Does
The clavicle acts as a rigid strut that holds the shoulder joint away from the chest, giving the arm its full range of motion. Without it, the shoulder would collapse inward. It connects the arm to the rest of the skeleton at two points: the sternoclavicular joint (where it meets the breastbone) and the acromioclavicular joint (where it meets the shoulder blade). The sternoclavicular joint is the only direct bony connection between the entire upper limb and the trunk of the body.
Movement at the clavicle occurs in three planes, and the integrity of this motion is essential for normal arm function. Any disruption to how the clavicle moves will automatically limit the range of motion of the arm, particularly the ability to raise it out to the side. The sternoclavicular joint serves as a fulcrum for the entire shoulder girdle, while the acromioclavicular joint allows smaller but crucial adjustments during overhead movements.
Beyond its mechanical role, the clavicle serves as a protective roof over important blood vessels and nerves running between the neck and arm. The subclavian vein sits remarkably close to the underside of the bone near its inner end, averaging less than 5 millimeters away. The subclavian artery and the brachial plexus (the nerve network supplying the arm) run about 2 centimeters behind the bone at the midshaft. These structures fan further away as they move toward the shoulder, sitting at least 4.5 centimeters from the bone near the shoulder blade.
Where Clavicle Fractures Happen
The clavicle is one of the most commonly broken bones in the body, and where it breaks follows a consistent pattern. About 69% of clavicle fractures occur in the middle third of the bone. This is the thinnest section and the point where the bone’s two natural curves transition, creating a structural weak spot. Another 28% of fractures happen in the outer third, near the shoulder. Fractures of the inner third, closest to the breastbone, are rare at roughly 3% of cases.
The high rate of midshaft fractures relates directly to the clavicle’s anatomy. It sits just under the skin with minimal soft tissue padding, and forces from a fall on an outstretched hand or a direct blow to the shoulder travel along its length and concentrate at the narrowest point. The proximity of major blood vessels near the inner end of the bone is one reason fractures in that region, though uncommon, receive careful medical attention.