Microstructural evolution in external callus of human long bone

Accurate knowledge of bone fracture healing process is of clinical and theo retical importance in bone repair and regeneration, and biomineralization. It is well known that the histological healing occurs via the formation of hematoma, fibrocartilage, bony callus and bone modeling/remodeling. However , the detailed process from fracture to healing at the microstructural leve l remains unclear. In the present study, an evolutionary model of external callus is proposed, in which five representative stages are presented in te rms of the organization of collagen and minerals during the formation of bo ny callus. The first stage is the formation of loose, disordered collagen f ibrils, which is followed by mineralization on some of these individual mic rofibrils. Then the matrix is characterized by the fusion of mineralized in dividual fibrils into bundles. In the third stage, the absorption of disord ered matrix occurs. This is gradually replaced by ordered collagen in stage four. Finally, completely ordered mineralized tissue is formed. The proper sequence of the process plays an important role in deciding the success of healing. In addition to the common mineral phase of hydroxyapatite (HA), dicalcium p hosphate dihydrate (DCPD) phase was also found in early stage of healing, e specially in rapid healing (children's callus). It Vanished in the followin g process of healing. The deposition of DCPD is supposed to be brought abou t by some non-collagenous protein. (C) 2000 Elsevier Science S.A. All right s reserved.