Mandibular bone remodeling after β-tricalcium phosphate transplantation: histological, immunohistochemical and ultrastructural aspects

This article presents the research results of the histological, immunohistochemical, and ultrastructural characteristics of bone-ceramic regenerate after β-tricalcium phosphate transplantation into an experimental defect in the rabbit mandible, since complete and high-quality regeneration of maxillofacial bones, its mechanisms and dynamics remain not fully understood, need clarification and detailing. Aim. To study in an experiment the dynamics of histological, immunohistochemical, and ultrastructural changes in the lower jaw bone after its traumatic injury with subsequent replacement of the defect with β-tricalcium phosphate. Methods. Experiments were conducted on 45 male rabbits aged 6-7 months, weighing 2.5-3.0 kg. 20 animals constituted the control group, and 20 the experimental group. Another 5 intact animals were used to study the normal structure of the bone tissue of the studied area of the mandible. The control group included animals with a bone tissue defect that healed under a blood clot. The experimental group consisted of rabbits where the bone defect was filled with β-tricalcium phosphate (β-TCP). Post-traumatic bone tissue status within the defect area was monitored for 84 days using the following methods: bone defect modeling, light-optical assessment of the histostructure of decalcified bone sections, immunohistochemical determination of the expression of markers CD34, Calcitonin, Ki-67, transmission electron microscopy. Results and conclusion. Implantation of the β-TCP material doesn`t affect the nature and dynamics of alterative changes in the native bone tissue of the mandibular alveolar part after trauma, but during the 2-3rd weeks of the experiment it causes a significant increase in the number and density of membranous osteogenesis foci on the periphery of the bone regenerate with their subsequent anastomosis and association with the bone trabeculae of the native cancellous bone. The realization of the osteoconductive potential of synthetic β-TCP granules occurs through the creation of conditions for active neovascularization, migration and cytodifferentiation of osteogenic cells, and these processes spread in a wave-like manner from the periphery to the deep zone of the regenerate. The osteoblasts ultrastructure,near the border with the native bone, indicates a limitation of their synthetic activity, while osteoblasts in the thickness of the ceramic-bone regenerate intensively synthesize amorphous and fibrous osteoid components. In the deep zone of the implant, the appearance of functioning microvessels prevents the formation of cartilage matrix foci and limits the degree of fibrosis, which is accompanied by the formation of islands of desmal osteogenesis. In 5 weeks after implantation of synthetic β-TCP in the peripheral areas of the bone-ceramic regenerate, intensive remodeling of the woven matrix into primitive bone plates occurs. Moreover, the restored osteons of the native bone adjacent to the implantation zone, as well as the structures of the restored periosteum, stimulate the appearance of mature osteocytes in the newly formed trabeculae of the regenerate. Implantation of synthetic calcium phosphate material significantly accelerates the spread of the bone remodeling wave from the periphery to the depths of the regenerate from 4 to 12 weeks of the experiment, but doesn`t ensure complete osseointegration of the implant at the cellular level and restoration of the quality of fibrillogenesis at the ultrastructural level, also doesn`t prevent partial fibrosis of the deep zone of the regenerate and doesn`t ensure complete restoration of the osteocyte lacunae-canal system.