2. In the second stage, as the inflammation subsides, the dead tissue at the bone ends is removed and stem cells begin to organize into a cellular matrix. The stem cells proliferate in the bone marrow cavity and evolve into cartilage cells and soft fracture callus (a matrix of fibrous tissue, cartilage and woven bone) in the outer and inner surface of the bone. During this period, dead bone is continually replaced with new cartilage cells. The gap between the sleeves of new bone is invaded by embryonic tissue (mesenchymal elements), which specializes to form a bridge of tough connective tissue (viscoelastic fibrocartilage). This occurs over the following seven to ten days.

3. In the third stage of healing, which begins approximately three to four weeks after fracture, there is an increase in soft callus that fills the marrow cavity and forms along the outside of the bone. Hardening (calcification) of the fibrocartilage begins at each end of the fracture or sides of the void and sweeps toward the center.

4. In the fourth stage, which typically begins six to eight weeks after fracture, the callus gradually calcifies through osteoblastic (osteoblasts are bone forming cells) action, increasing the stability of the site. As calcification proceeds, ducts (vessels) invade the mineralized fibrocartilage, removing it by breakdown and absorption (chondroclasis) and replacing it with fiberous bone. This calcified callus begins to remodel toward the end of this stage, and continues to convert into lamellar bone in the fifth stage.

5. This calcified callus continues to convert into lamellar bone (mature bone, refers to bone cortical (hard outside) and cancellous (soft inside) bone) during the fifth stage, over a period of two months to two years. Voids in cancellous bone heal in substantially the same manner and stages as cortical bone, except that cancellus bone does not have a bone marrow cavity, so bone healing occurs along the matrix of internal beams that comprise its structure. Cancellous bone heals in approximately one-third less time than cortical bone, primarily because cancellous bone is more vascularized than cortical bone.

Non-unions are fractures or voids that have not healed within nine months and have shown no sign of healing for the prior three months. In non-unions, the normal process of calcification fails to take place. The gap remains occupied by fibrocartilage and/or fibrous tissue and vascular penetration cannot proceed. Some of the factors that may contribute to slow union or non-union include patient characteristics, such as the age, gender, health conditions, weight and smoking habits.