Two-Phase Theory of Osteogenesis

Two-Phase Theory of Osteogenesis

Two basic processes occur on transplanting bone from one area to another in the same indlvlduall-l”: The first process that leads to bone regeneration arises initially from transplanted cells in the graft. that proliferate and
form new osteoid. The amount of bone regeneration during this phase depends on the amount of transplanted bone cells that survive the grafting procedure. Obviously, when the graft is first removed from the body, the blood supply has been severed. Thus (he cells in the bone graft depend on diffusion of nutrients from the surrounding. graft bed (i.e., tlie area where the graft is placed) for survival. A considerable amount of cell death occurs during  the grafting procedure, and this first phase of bone regeneration
may not lead to an impressive amount ‘of bone regeneration when considered alone. Still, this phase is responsible for the formation of most of the new bone. The more viable cells that can be successfully transplanted
with the graft, the more bone that will form. , The graft bed also undergoes changes that lead to a second phase of bone regeneratlon.beglnrung in the second week. Intense angiogenesis and fibroblastic proliferation
from the graft bed begin after grafting; and osteogenesis from host connective tissues soon begins. Fibroblasts and other mesenchymal cells differentiate into osteoblasts and begin to lay down new bone. Evidence
shows that a protein (or proteins) found in the bone induces these reactions in the surrounding soft tissues of the graft bed.6•7 This second phase is also responsible for the orderly incorporation of the graft into the host bed
with continued resorption, replacement, and remodeling.irnn, -ne svstem mounts against “foreign” grafts is primarily a cell-mediated response by ‘l-lymphocytes. It may not occur imrucdiatpIy, however, and \n the early period the incorporation of a bone graft into the host may appear to
be progressing normally. The length of this latent period depends on the Similarity between the host and the recipient. The more similar they are (antigenically), the longer an immunologic reaction may take to appear. This type of immunologic reaction is the most common reason for rejection of hearts, kidneys, and other organs transplant- “ed to another individual. Tissue-typing procedures, in which a donor and recipient are genetically compared for similarities before transplantation, are currently commonplace for organ transplantation but never for bone grafts. Because of the immunologic rejection of transplants between individuals or between species, methods have been devised to improve the success of grafting procdures in these instances. Two basic approaches are used clinically: The first is the suppression of the host individual’s immune response.  suppression with various medications is most commonly used in organ transplant  patients. This approach is not used routinely in oral and
maxillofacial surgical bone grafting procedures because of the potential complications from immuno suppression. Another approach that has been used extensively in oral arid maxillofacial surgical procedures is the alteration of the antigenicity of the graft so that the host’s immune response
will not be stimulated. Several methods of treating grafts ‘have been used.rincluding boiling, deproteinizjng, rnerthiolating, freezing, freeze-drying, irradiating, and dry heating. All of these methods, potentially helpful for use in bone grafts, are obviously not helpful in organ transplants.’

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