Induction
Once a bone has been broken the body's first response is called induction. When a bone breaks, the blood vessels inside of it also get severed. The Journal of Bone and Joint Surgery notes that this causes blood to quickly seep into the nearby tissue and begin to clot, forming what is called a fracture hematoma. This fracture hematoma helps hold the bone in place and also prevents further blood loss. At the same time the blood vessels within the broken bone constrict, cutting off blood flow. This causes the jagged edges of the bone (where the break occurred) to die, keeping them from cutting through the tissue. At the same time, the surrounding tissue swells as the body works to remove any dead or damaged tissue. Small blood vessels are made nearby (via the action of chemical signals that stimulate blood vessel growth) to make it easier for the body to clean up the area of the fracture.
Callus
The next step in the healing of the bone is the formation of the soft callus, according to an article published in the Journal of Bone and Joint Surgery. The soft callus serves as the scaffolding where new bone will be made to heal the break. It is made of a strong and tough protein called collagen, which is made by special cells called fibroblasts. Other nearby cells (called chondroblasts) then take this collagen and add onto it to form fibrocartilage, which is a tough connective tissue. This fibrocartilage callus lasts for about three weeks on average.
Bone Formation and Remodeling
Once the fibrocartilage callus has been made, the body will cover it with a layer of bone cells to harden and protect it. Then it will start to make the bone that will connect the two broken pieces. This mass is called the bone callus. Although the bone callus is made of bone tissue, it is not as strong as healthy bone. As a result, even once the two broken ends have been joined together by bone tissue, cells called osteoclasts will break some of the bone down to be replaced with new bone made by osteoblasts. This process turns the bone callus into the stronger material (called compact bone) that makes up healthy bones.
