When a bone breaks the most important thing is the state of the periosteum, the membrane covering the bone. This is where all of the cells that participate in the healing of bone originate. Fractures of bones are very common; they happen in both the elderly and the young. Once a fracture is healed, the bone is typically stronger then it was prior to the injury.

Generalised steps in the healing of a simple fracture

1. After a fracture, the first thing that forms is a haematoma, similar to that seen elsewhere in the body. This is a clot of blood from all the bleeding that accompanies a fracture.
2. Necrotic bone is then demolished by osteoclasts, and the haematoma by macrophages.
3. Granulation tissue comes from the periosteum and endosteum as the haemotoma is demolished.
4. After this, pluripotent cells migrate into the granulation tissue. These cells become chrondrocytes and later osteocytes, that produce cartilage and bone respectively. The structure surrounding the fracture site is now slightly harder, this is a provisional callus.
5. The area can be called a proper callus as time goes on, and more and more woven bone is made by the osteoblasts.
6. This woven bone is initially remodelled into lamellar bone.
7. With time, the bone is remodelled over the next few months, and the callus becomes smaller, as the trabeculae are formed along lines of stress.