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e healing with scar fibrous tissue instead of bone.
     It is not uncommon to find only fibrous granulation 'tissue centrally in extraction sockets that earlier contained teeth affected by severe periodontitis, which were extracted by conventional methods. Such sites make fixture placement difficult or impossible. Consequently, the recommendation is always to remove any residual (pathological) soft tissue by thorough curettage of the bony walls and close the area primarily. Ideally, this should be done by bringing in fresh periosteum to promote healing by first intention.
     Little is known about the length of time that should pass between the surgical removal of pathologically altered tissues and the installation of fixtures to give them optimal healing circumstances. Unloaded fixtures have become successfully osseointegrated after installation in dog sockets preceded by immediate extraction of healthy teeth. Again, however, human bone heals at a slow rate. Waiting 9 to 12 months generally proves to be the safest and most worthwhile alternative especially if the patient' s long - term prognosis of the treatment is taken into account.
     Irradiation
     In his doctoral thesis on Co gamma irradiation to the rabbit tibia, Jacobsson concluded that bone healing was temporarily depressed even after a single dose of 5 Gy. At 15 Gy, a significant depression of early osteogenesis was observed. However, if this dose had been given l year earlier, the bone forming capacity would have been improved by a factor of 2.5 relative to immediate preoperative irradiation trauma of the same magnitude. Consequently, an osseous recovery after irradiation was observed. These results were 'all related to healing bone. Mature bone was relatively resistant to irradiation up to a single dose of 40 Gy, ie, remodeling continued at a normal rate and no vascular changes were observed after the irradiation.
     Clinical trials with fixtures installed in previously irradiated maxillary, frontal, and temporal bone to support facial prostheses after combined surgical and radiological treatment have yielded positive results. Jacobsson et al reported on 35 fixtures inserted 9 months to 37 years after irradiation treatment with 25 to 86 Gy. With follow-up times from implant insertion of 15 to 44 months, only 5 fixtures (14.3%) were Lost.
     On the basis of these investigations, treatment with fixtures and facial or dental prostheses after irradiation appears no longer to be an unattainable objective, provided the bone is allowed a long enough recovery period after the irradiation trauma. Moreover, irradiated patients frequently are the most needy ones.
     The effects of treatment with cytotoxic drugs have not been studied.
     Present attempts to revitalize irradiated bone by treatment with hyperbaric oxygen prior to the installation of fixtures have elicited positive preliminary results. Further documentation is, however, needed.
     Perioperative factor for fixture installation
     Implant properties.
     It is the surgeon' s responsibility to choose implants that will maximize the possibility of osseointegration. The properties of such implants and their interfacial reactions with the host (bone) tissues ha本论文由英语论文网提供整理，提供论文代写，英语论文代写，代写论文，代写英语论文，代写留学生论文，代写英文论文，留学生论文代写相关核心关键词搜索。