Fractures are indeed a vital cause of death, particularly in aging populations. The repair of bone fracture is a slow and yet complex procedure that involves regeneration of new bone tissue and absorption of old bone scaffolds.From the early stages of fracture healing, many cell types are involved. Both systemic and non systemic factors act upon the process of transition from early callus tissue to new bone. Differentiation and proliferation of many cellular types like chondrocytes, osteoblasts e.t.c are influenced by genetic background, systemic hormones, sensitivity to mechanical input, drug administration, other disorders, cytokines and many more factors. Consequently fracture healing is declined by aging, so delayed union of fractures is most common in elderly patients
Fractures are thoroughly classified according to the external force that induces the damage of bone continuity. They can be due to external violence and pressure on the bone, continuous pressures in the area, while a very unique form of fractures is caused by pathologic conditions such as osteoporosis. Most of the times the process of bone healing has no distinct phases. In the early stages there is an increased blood supply causing a hematoma in the area of fracture. This is observed in experimental models of rodents fracture and it is the very first stage of bone healing. Simultaneously there is an increased surrounding by macrophages and the broken ends become necrotic. Osteoclasts then remove the dead bone tissue and osteoblasts produce new bone, parallel with fibroblasts that produce fibrous tissue in soft tissue injuries of the area. Therefore a growing firm mass, or callus, is created and that is the early beginning of ossification of the osteoid.
At this point two levels of growth callus are observed: subperiosteal callus that forms the outside of injured bone and endosteal callus. The pH in this stage is increased and calcium hydroxapatite crystals are deposited. After 6 weeks (in normal conditions) the gaps are filled and bone regains mechanical strength, but only after 1 year does bone remodeling finish with normal bone microarchitecture to be restored.