Cellular events, recapitulating embryogenesis. After initiation of woven bone formation, deposition of parallelfibered bone ensues. These two primary types of bone repair the defect within weeks; thereafter, the formation of perivascularly arranged lamellar bone takes place with simultaneous resorption of the two primary bone types. This substitution, which gives strength to the bone, may take months to years, depending upon the size of defect. Finally, a structural rearrangement of trabeculae in response to function (Wolff law) takes place. Healing of fractures of long bones may often be characterized by a callus formation in the initial stages, where woven bone is mixed with cartilage, resulting in a clinically as well as radiologically visible thickening of the fracture site. This callus will disappear along with the osseous maturation.