A broken bone heals by making new bone, not a scar. The process takes weeks to months and needs a good blood supply and a stable fracture.
Hematoma and inflammation
Torn vessels bleed and form a clot around the break within hours. Inflammatory cells clear debris and release signals that call in repair cells.
Soft callus
Over the first two to three weeks, new blood vessels grow in and cells lay down cartilage and fibrous tissue that bridge the gap. The fracture becomes less painful but is not yet strong.
Hard callus
Over the next weeks, the soft callus is replaced by immature woven bone. On X-ray the callus looks like a cloudy collar around the fracture, and the bone can usually bear more load.
Remodeling
For months to years, the bulky callus is reshaped into compact, organized bone along the lines of stress. Children remodel faster and can correct some angulation.
What slows healing
Smoking, diabetes, poor nutrition, steroids, infection, a poor blood supply and too much movement at the fracture all delay healing or cause nonunion.
At the bedside
After splinting or casting, check pulses, color, sensation and movement beyond the injury, because swelling in the first days can compromise circulation. Pain out of proportion to the injury can signal compartment syndrome.
Related guides
Written by Making Synapses. Topic reference: OpenStax, Anatomy and Physiology 2e (no text adapted).