A description of the process of fracture repair can be found in many orthopaedic surgery textbooks', but a careful evaluation of the literature easily shows important differences in opinions about the aspects and phases of this process [1-3].

The gradual evolution of ideas about the biology of fracture healing, which took place over the course of the past 30 years, and the enormous increase in knowledge about the regulation of bone cell activity have gradually extended interest from a cellular basis to a molecular basis and to the genetics of bone consolidation: "...consolidation needs much more than osteoblasts" [4].

In light of current knowledge, bone consolidation must be considered as a complex recruitment and cellular differentiation process, led by local mediators that send particular physical and chemical signals to the cells.

A fracture is a break in the continuity of a bone (and a disruption of the blood supply to the bone) and the healing process begins as soon as the bone is broken, provided that the fundamental principles of fracture treatment (reduction and immobilization) are respected.

Fracture repair must be considered a regenerative process rather than a healing process, because the injured part is replaced by the formation of new bone tissue (callus) instead of scar tissue. The callus formed outside the bone is termed external callus; the callus in the medullary cavity is termed internal callus.

If a fracture is not displaced and stable, only a cast or a brace may be necessary to maintain immobilization (with a small amount of interfragmentary motion): in this case bone callus forms "under natural conditions". For unstable or displaced fractures operative treatment with an internal or external fixation is required. Operative treatment modifies the process of fracture repair: in this case a different kind of process (callus formation "after operative treatment") is observed.

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