The healing process of a bone fracture begins immediately when the severed blood vessels in the bone form a fracture hematoma, a type of blood clot, says HowStuffWorks. This keeps the bone stable and lines up the damaged edges so that they can mend as thoroughly as possible.
The fracture hematoma also stops the flow of blood to the broken ends of the bone and causes them to die, says HowStuffWorks. Inflammation and swelling set in, and the body removes the dead tissue while capillaries grow into the fracture hematoma.
In a few days the fracture hematoma toughens and forms a soft callus, says HowStuffWorks. Fibroblasts start making collagen, a type of protein found in connective tissue and bone. After this, chondroblasts make fibrocartilage, which turns the soft callus into a tougher fibrocartilaginous callus, which connects the two pieces of bone. This callus lives for about three weeks until it is replaced by the bone callus, which is created by cells called osteoblasts. The bone callus lasts for three or four months and further stabilizes the broken bone.
After this stage, the body sets up the position of the bone inside the flesh, makes a hard callus between the two pieces and cleans up any bits of dead bone, says HowStuffWorks. Still, it is some months before the bone is truly healed. The final stage of healing is largely done by the osteoblasts and osteoclasts.