Micro-Structure Changes In Rat Tooth Movement And Recurrence Process Under Different Force Values Through Micro-CT Dynamic Observation

In modern society, people become more and more concerned for their dental appearance and the number of people choosing to receive orthodontic treatment has been increasing gradually. However, during the process of the orthodontic treatment, various degrees of absorption may occur to tooth root due to the influence of both internal and external factors, some may even cause maladjustment to crown root ratio, tooth mobility or abscission, which will bring severe impact to patients’health. In the meanwhile, all the patients will suffer from various degrees of tooth relapse after orthodontic treatment which may potentially influence treatment effect. Hence, an effective measure is eagerly expected to enhance the stability for treatment effect and promote repair for root resorption. However, we have to fully understand the regular pattern for orthodontic tooth movement and recurrence, root resorption and repair and alveolar bone remodeling beforehand, so that we can take fully utilization of it for better and effective intervention.This paper provides an exploration for the regular pattern of the orthodontic tooth movement and relapse, root resorption and repair, alveolar bone changes through establishing rat tooth movement model under different force values effect, implementing Micro-CT for rat scanning in vivo, observing distance for tooth movement, relapse, tooth root resorption crater volume and micro-structure changes for bone trabecula around tooth root.The experiment is divided into two parts:1. Micro-structure changes in rat tooth movement process under different force values effect through Micro-CT dynamic observation.Objective:Observation for rat tooth movement distance, tooth root resorption crater volume and micro-structure changes for bone trabecula around tooth root under different force values through implementing Micro-CT dynamic observation.Method:15 sprague-dawley adult male rats at the age of 8 weeks is selected, with the left maxillary as the experimental side and the nickel titanium coil springs, fixed between the first molar and incisor where thrust augmentation is conducted respectively by 20g,50g and 100g for continuous 14 days to it. After that, rat scanning in vivo through Micro-CT on 0 day,3rd day,7th day,10th day and 14th day shall be conducted respectively once force values are loaded on; besides, the first molar, second molar and the surrounding alveolar bones at the left maxillary should be included for scanning. Furthermore, the 3D measurement system shall be used for scanned image processing after scanning is done, so as to measure tooth movement distance, root resorption crater volume and structure parameters for bone trabecula.Result:There are obvious tooth movements at the first three days after thrust augmentation is conducted; from the 3rd day to 10th day, movement distance is in a slow increase, with 20g and 100g stress group recovered for rapid movement 10 days later and tooth movement distance for 20g stress group is obviously greater than the other two groups with statistical differences (P<0.05) 7 days after thrust augmentation is conducted; 3 days after thrust augmentation is conducted, root resorption crater volume for each group has a slight increase and an obvious increase is occurred from the 3rd day to the 10th day, then slow increase is happened 10 days later; from the 7th day to the 14th day, root resorption crater volume for 100g stress group is obviously greater than the other groups with statistical differences (P< 0.05). With time for thrust augmentation pass by, bone volume fraction at the pressure side of alveolar bone is in gradually decrease and bone trabecula morphology becomes tiny and exiguous; besides, bone trabecula gap becomes decreasing first and broadening afterwards, with status changing from clintheriform to rhabditiform; furthermore, alveolar bone resorption is occurred with consistent changes happened within each stress group.Conclusion:Different orthodontic force values could all lead to tooth movement, which its regular pattern is reflected by fast-moving period, stable-moving period and fast-moving period, with greater tooth movement distance caused by light force than gravity force; in addition, different orthodontic force values could all cause root resorption, which the regular pattern is reflected by lag phase, rapid phase and stationary phase, with more root resorption caused by over-sized orthodontic forces; during the process of the tooth movement, the pressure side of alveolar bones could all arises reconstruction and absorption, with variable quantity less interrelated to force value degree.2. Micro-structure changes for rat tooth movement in relapse process under different force values through Micro-CT observationObjective:Micro-CT dynamic observation is implemented to conclude changes regulation for rat tooth movement relapse distance, root resorption crater volume and bone trabecula structure around alveolar bone under different force values in the process of the relapse after rat tooth movement is occurred.Method:The experiment pattern is as same as the pattern in Part 1. Micro-CT rat scanning in vivo is conducted on 0 day,3rd day,7th day, 10th day,14th day,28th day and 42th day respectively after thrust augmentation is moved away, with the same scope for scanning as in Part 1.3D measurement system is used for scanned image processing to measure tooth relapse distance, root resorption crater volume and bone trabecula structure parameters.Result:After appliance is removed,20g stress group shows a clear relapse at the first 3 days; for 50g and 100g stress group, relapse can be seen clearly at the first 7 days; thus, for 20g stress group, it has a quicker relapse than the other two groups with statistical difference (P<0.05), with movement distance becoming smaller gradually as time is increasing, then, complete relapse is occurred on the 28th day; in the first 3 days after appliance is removed, changes for root resorption crater volume are relatively small and become clearly decreased from 3rd day to 14th day; for 20g stress group after 14th day,50g and 100g stress groups after 28th day, changes for root resorption crater volume tend to be stable; during the relapse period after orthodontics is removed, bone volume fraction at side tension alveolar bone increases slowly; besides, bone trabecula morphology restores gradually with number increased; meanwhile, bone trabecula gap reduces slowly and retrieves from rhabditiform to clintheriform; furthermore, alveolar bone starts deposition and reconstruction, with consistent change tendency happened within each stress group.Conclusion:Relapse tendency can be occurred after orthodontic force is removed for all tooth movement caused by different force values. The first week is shown with very clear relapse; greater distance for tooth movement, faster relapse is happened. Root resorption caused by different force values is all provided with self-repairing capability, with stationary phase, fast-repairing period and stable period as its repairing rules. Repair of root resorption caused by light force shows an early stable period. In relapse process, after orthodontic treatment, side tension alveolar bone occurs reconstruction of new bone deposition, with changes less related to force value degree.