Long Bone Fractures and Healing (2/3)
Induction is the second stage of the healing process. It starts with the formation of a fracture haematoma and ends with the appearance of inflammatory cells approximately 48 hours from the impact. The third stage is inflammation. It begins with the influx of inflammatory cells and ends with the appearance of the bone and cartilage production. Stage four is known as the soft callus stage. It is characterised by the development of cartilage and bone tissue and is completed with a cessation of appreciable fracture motion. Stage five involves the conversion of the soft, largely chondroid, callus into woven bone via endochondral ossification. At the completion of this stage, the fracture is considered healed both clinically and radio-graphically. The fracture strength is regarded as directly proportional to the amount of the new bone produced. Bone remodelling, the final stage, is a conversion of the woven bone to the lamellar bone. Unwanted bone is removed, and both the medullary cavity and the bone geometry are restored.
The fracture healing process itself is very complicated. Although it all starts with stage one and two, the way it progresses through stages three to five depends on a variety of factors which influence different types of healing. McKibbin  in his review on biology of fracture healing in long bones identified four healing processes.
Primary callus response is the first and fastest type of healing. It is initiated in the majority of fractures. Large amounts of callus beneath the periosteum are produced, trying to bridge the bone ends of the fracture. This type of healing is very tolerant of interfragmentary movement and total rigidity. However if the bone gap is not bridged within two weeks, primary callus response is likely to fail.