| Abstract | The primary aim of orthodontic treatment is maintaining a functional dentition, including chewing, aesthetics and comfort. Under the influence of an orthodontic force, the pressure side of the tooth root will experience bone resorption and bone formation will take place on the tension side. The experience of the orthodontist determines the nature and the length of the treatment. Although some typical movements and treatment plans are known, the reaction to the treatment is different for every patient and the treatment plan is adjusted regularly. Most existing models describing tooth movement are based on an empirical bone remodeling function in which the biological activity in the periodontal ligament and the alveolar bone is not taken into account. This study presents an onset to a mechanobiological model for bone formation in orthodontic tooth movement. Orthodontic tooth movement is achieved by the process of repeated alveolar bone resorption on the pressure side and new bone formation on the tension side. The bone remodeling is done by osteoclasts and osteoblasts, who coordinate their actions by communicating by means of the RANKL-RANK-OPG signalling pathway. Together with the alveolar bone remodeling, extensive remodeling of the periodontal ligament (PDL) takes place to ensure the attachement of the tooth root to the alveolar bone. This process is regulated by periodontal ligament fibroblasts, which have a high proliferation rate and are highly active cells, mainly due to the continued mechanical stimulation from occlusal contacts. The amount of tooth movement is regulated by the cellular respons to a mechanical stimulus. Both osteocytes, present in the alveolar bone, and fibroblasts, present in the periodontal ligament, have a high sensitivity for mechanical signals. However, since osteocytes are present in the stiffer alveolar bone, in contrast to the PDL, they are subjected to a much smaller strain. Therefore, this model only takes into account the reaction of fibroblasts to the applied force. Fibroblasts respond to mechanical stretching by producing the osteogenic growth factor TGF-β and by upregulating the production of nitric oxide. The developped model consists of 15 non-linear partial differential equations describing the concentration of various cells, growth factors, cytokines and matrix-components. The PDL has a large concentration of fibroblasts and its precursors, mesenchymal stem cells. The bone mainly consists of osteocytes and has a smaller concentration of osteoblasts and osteoclasts. The production of RANKL and OPG by osteoblasts is determined by the levels of NO in the bone. The osteogenic differentiation of mesenchymal stem cells into osteoblasts is regulated by TGF-β, present in an active and a latent form. Multinucleated osteoclasts are formed through the fusion of hematopoietic stem cells, which are present in the vascular matrix. Vascular endothelial cells are responsible for the formation of bloodvessels. |
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