This stage lasts for about 4-12 weeks. In this stage, the granulation tissue differentiates further and creates osteoblasts. These cells lay down an intercellular matrix which soon becomes impregnated with calcium salts. This results in formulation of the callus, also called woven bone. The callus is the first sign of union visible on X-rays, usually 3 weeks after the fracture (Fig-2.4). The formation of this bridge of woven bone imparts good strength to the fracture. Callus formation is slower in adults than in children, and in cortical bones than in cancellous bones