Callus formation is a critical part of bone healing that occurs during the reparative phase of fracture healing. During this phase, a soft callus composed of fibrous tissues and cartilage replaces the blood clot that formed in the inflammatory stage. This callus holds the pieces of fractured bone together, but it is not strong enough to be used in the way that bone would be used. The formation of this soft callus occurs somewhere between 2 and 6 weeks after the fracture occurs. Eventually, as healing continues, the soft callus develops into a hard callus, which is actually bone but still softer than regular bone. Bone remodeling is a lifelong process that happens over and over in the body, and it also happens after a fracture occurs. During this process, the callus goes through the remodeling process, and special cells called osteoclasts chip away at the old bone, while other cells called osteoblasts form new bone in its place. The cavities are eventually filled by bone that is produced by osteoblasts, which causes the bony union to generate while also restoring the Haversian systems, which are formed in an axial direction.