When the bone is fractured, the blood from the torn vessels will be rushed around and between the fracture and form a clot like substance called haematoma. The fractured ends will be necrosed for few millimetres and this will initiate the healing process through sensitization of the precursor cells.
In the granulation tissue stage, the precursor cells will be differentiated into blood vessels, osteoblasts and fibroblasts. This formation leads to soft granulation tissue between the fracture fragments. The initial blood clots will be removed by macrophages and the granulation tissue is formed.
The granulation tissue is differentiated into osteoblasts which is mineralized with calcium salts and forms the callus. The callus is also called woven bone. It is the first sign of bone union which is seen in the x-ray. The callus is replaced with a typical lamellar structure to form a mature bone. The callus formation is slower in adults than in children and the remodelling of callus to mature bone will take one or more years.
The next stage is the modelling in which the bone gets its normal shape and strength through normal physical activity and stress on the bone. The factors which affect the bone healing is age (bone in children unite faster than adult), type of fracture and the blood supply to the fractured fragments. The scaphoid bone heals slowly or not at all, due to its lack of blood supply. The duration of immobilization is one of the factors in bone healing. The fractured ribs and the scapula bone don't need immobilization for healing.
The stage of haematoma takes less than 1 week, the stage of granulation tissue takes 2-3 weeks, the stage of callus takes 1-3 months, the stage of remodelling takes 1-2 years and finally the stage of modelling take more years.
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