[Bone fracture and the healing mechanisms. Histological aspect of fracture healing. Primary and secondary healing].

It is quite important to understand the histological aspect of fracture healing. It is helpful in the clinical practice to recognize how fracture management induces biological reaction. Repair process is classified into primary and secondary fracture healing. Primary healing is direct bone repair without endochondral ossification, which includes cartilaginous callus formation. This pattern does not usually happen in the natural process of fracture healing. Bone remodeling initially occurs under the stable condition of fracture with rigid fixation and no gap formation. The key of this process is Haversian canal remodeling and reestablishment of blood vessels. Secondary healing is typically characterized by three overlapping stages: the initial inflammatory response, callus formation (soft and hard callus), initial bony union and bone remodeling. Most of fracture repair in the clinic follow this process, and callus formation is confirmed with X-ray. The key of this process is the appropriate stability of the fracture site to maintain biological healing response.