| Teeth with incomplete root formation could complete root development during the period of orthodontic treatment with no root lengths losing. Many studies reported that incomplete root formation have a high resistance to root resorption induced by orthodontic treatment. Some researchers hypothesized that with regard to immature teeth, force to move the teeth may speed up the rate of growth of the teeth. During the treatment, root shorten and root elongation may happen at the same time. DSP which is secreted and synthesized by odontoblasts is one of the most important NCPs in dentine. The expression of DSP is related to state of odontoblasts and dentine mineralization. The aim of the study is to evaluate the root dentin formation and mineralization during orthodontic tooth movement.The study consists of two parts:1. The effects of orthodontic force on DSP expression in dentine of young rats.Objective: To investigate the effects of orthodontic force on DSP expression in dentine during tooth development. Methods: Standardised orthodontic tooth movement was performed in six groups of SD male rats aged 5 weeks. A Ni-Ti-closed coil spring was inserted between the upper incisors and the upper right first molar to move the right first molar mesially while the contra-lateral side molar was kept intact to serve as internal control. Animals were sacrificed for histological evaluation at 1, 3, 7, 14, and 21 days after orthodontic tooth movement. Immunohistochemisical staining revealed the distribution and expression of DSP in maxillary first molar during orthodontic tooth movement. Results: Expressions of DSP were detected in dentinal tubules, predentin and odontoblast, especially at the cusped area. Immunoreactions for DSP in crown dentine were strongest at the 14th day, but relatively weak at the 21st day. Immunoreactions for DSP in root dentine were weaker than those in the crown, and increased in a time dependent manner. Statistical analysis showed that the immunoreactions of DSP in the experimental group were significantly stronger than in normal control rats. Conclusion: At later development stages of immature permanent teeth, moderate orthodontic force may promote the activation of odontoblasts and increase the expression of DSP in dentine, thereby accelerating the dentine mineralization.2. The effects of orthodontic force on dentine formation of young rats.Objective: To investigate the effects of orthodontic force on dentine formation in young rats. Methods: Standardised orthodontic tooth movement was performed in six groups of SD male rats aged 5 weeks. To mark the areas of dentin formation during experiment, we gave each animal an intraperitoneal inject of tetracycline 30 mg/ kg at the onset of the experiment and one day before termination of the experiment. Animals were sacrificed for histological evaluation at 1, 3, 7, 14, and 21 days after orthodontic tooth movement. Results: At the 1st and 3rd day, only one tetracyling strip could be observed in both experimental groups and the control groups. At the 7th, 14th and 21st day, two tetracyling strips could be observed. And the distance between tetracycline stripes increased in a time dependent manner. But statistical analysis showed no significantly differences between the experimental groups and the control groups. Conclusion: At later development stages of immature permanent teeth, moderate orthodontic force may not accelerate the dentine formation. However, the development of the young permanent teeth is not affected during the orthodontic tooth movement. |
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