Clavicle fracture: why the middle third breaks
The clavicle (collarbone) carries force from the arm to the trunk, and it most often breaks in the middle third of its shaft (the long stretch between its ends).
Remember
Force travelling up the arm is carried along the clavicle, and the narrow middle third, which no strong ligament anchors, takes the bending load, so it is where the bone most often breaks.
The clavicle (collarbone) is a slender, S-shaped bone that links the breastbone (sternum) to the shoulder blade (scapula). It acts as a strut: a bone that carries force from the arm to the trunk. After a fall onto the shoulder or an outstretched hand, that force runs along the bone.
Its two ends are anchored by ligaments, which are tough fibrous bands that join bone to bone. The costoclavicular ligament holds the breastbone end, and the very strong coracoclavicular ligament (a tough band joining the clavicle to the shoulder blade) holds the shoulder end.
The shaft is the long stretch between the ends, and several muscles attach to it. It is mainly a site for muscle attachment, and it is among the narrowest parts of the bone, with no strong ligament of its own. The unsupported middle third takes the bending load and gives way.
The clavicle can be divided arbitrarily into thirds. On that split, about 80% of fractures sit in the middle third, about 15% in the lateral (shoulder-side) third and about 5% in the medial (breastbone-side) third. The medial third is the rarest site, and a break there usually follows great force.
Key facts
- Commonest site
- the middle third of the shaft, about 80% of clavicle fractures.
- Other thirds
- the lateral third accounts for about 15% and the medial third about 5%.
- Why the middle
- the middle third of the shaft is narrow, with no strong ligament of its own, and it takes the bending load.
- Usual mechanism
- a fall onto the shoulder or an outstretched hand.
- Rarest site
- the medial third, where a break usually follows great force.
Exam traps
- The coracoclavicular ligament does not explain a break in the middle of the clavicle, because this very strong band suspends the arm from the lateral end of the bone and reinforces that end rather than loading the shaft.
- The middle of the clavicle is not the thickest part of the bone: it is among the narrowest, it breaks because it is slim and unsupported, and a thicker region would resist bending better.
- Muscle pull describes the aftermath of a fracture rather than its cause, because the break itself comes from force passing along the bone in a fall, not from a muscle contracting.
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