How does a broken bone mend itself?
Most tissues heal with a scar, but a broken bone is rebuilt as new bone, and the cast only holds things still while the body does the work.
▶ Start the storyA broken bone mends itself by building a temporary patch and then slowly turning that patch into new bone. The body does this on its own: a plaster cast or an operation may help hold the pieces in place, but a fracture ultimately heals through the body's own processes, mainly directed by the , the thin membrane that covers the bone.
Within hours of the break, blood vessels at the site constrict to stop the bleeding, and blood cells form a clot called a . That clot acts as a template for what comes next. Inflammation peaks by 24 hours and is complete by about a week.
About a week after the fracture, cells of the begin to multiply and change. They build a mass of new tissue, the fracture , which bridges the gap between the broken ends. Callus formation peaks around day 14. Eventually the and cartilage of the callus are replaced by , restoring most of the bone's original strength.
Hours
Clot () forms
1 day to 1 week
Inflammation peaks, then completes
7 to 9 days
Periosteal cells multiply
Day 14
formation peaks
3 to 4 weeks
can begin
3 to 5 years
may be complete
Then comes the quiet phase. can begin as early as three to four weeks after the fracture and may take three to five years to complete. The is swapped for compact bone, and the lump of is reshaped until it closely duplicates the bone's original shape and strength.
That is the remarkable thing about bone: unlike many other tissues, which close a wound with a scar, bone aims to produce new bone without a scar, because a scar would be a structural weakness.
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Recap
Clot, callus, then years of remodelling: bone repairs itself with new bone, not a scar.
💡 A trick to remember it · Think of scaffolding going up, then a temporary bridge, then a stone mason slowly carving the final arch.
Surprising fact · Remodelling can take three to five years to complete.
Sources (1)
No source, no claim. Every fact in this lesson (14 claims) cites at least one of these.