Study On Establishment Animal Model Of Root Resorption During Tooth Movement And The Self-repair Characteristics After Resorption

Root resorption is a common complication during the orthodontic tooth movement. Usually, it is not an easy thing for the patient to accept the orthodontic treatment with root resorption. For safety of orthodontic treatment, orthodontists must study on the mechanism of the resorption and the prevention for it. In this study, the root resorption model was established on adult male hybrid dogs'maxillary incisors by the way of continuous intrusive heave force engagement. The resorption on the model can be seen on X-ray periapical films. And the characteristic of the selt-repair after resorption was investigated by the observation of the process on the models.Objective:1. To develop the way of continuous heave intrusive force to be engaged on the dog's maxillary incisors.2. To establish the root resorption model on the dogs'maxillary incisors by the way of continuous intrusive heave force engagement. The resorption could be detected on the the X-ray periapical films with shortened root. 3. To investigate the characteristics of root self-repair after the resorption.Method:1. The development of the continuous heave intrusive force engagement way: Some orthodontic elastic appliances were chosen for this study, such as elastomeric ligatures and four-loop elastomeric chains with long, short, and no intervals respectively. They were randomly divided into two groups. One group was at 1 time stretched length, and the other group was at 0.5 time stretched length. The regularity of time-dependant force decay was analyzed in order to investigate whether the unvarying force could be engaged by the appliances. Spss12.0 statistical software was used for data processing and analysis.2. The establishment of the animal model of root resorption: In six adult hybrid male dog, continuous heave intrusive force 300g was imposed to the maxillary incisor teeth with mini-implant as anchorage. Then the occurrence and development of the resorption were observed by X-ray periapical films,gross anatomy and histological morphology.3. The study on the characteristics of root self-repair after resorping: In six adult hybrid male dog, continuous heave intrusive force 300g was imposed to the maxillary incisor teeth with mini-implant as anchorage. After 6 weeks, the force was removed and the process of selt-repair began. The animals were killed after 2, 4, 6 weeks'self-repair respectively. The repair in different time was compared on the the X-ray periapical films and histological sections.Results: 1. The development of the continuous heave intrusive force engagement way: For all kinds of elastic appliances, there was no significant difference in the regularity of time-dependant force decay between the two different tension conditions. During the first hour after the engagement, force decay was the fastest with the decaying rate 20%-35%.Then, the force degradation decreased. Two weeks after the engagement, the elastic force was nearly stable. At the end of the fourth week, elastic force remained at 40% - 55% of the initial force.2. The establishment of the animal model of root resorption:â‘ observation on the X-ray periapical film: Two weeks later, the periapical radiolucent areas appeared; Six weeks later, defective root resorption was found around root tip.â‘¡observation on the anatomy morphology: It was found that the root was incomplete with irregular absorbed section in the apex area.â‘¢observation on the histological sections: It was found that a large number of cells with monocytes or polyocytes focused in the area of root resorption.â‘£the crown-root ratio in X-ray periapical film: It was found that the crown-root ratio increased about 3-4% per week.3. The study on the characteristics of root self-repair after resorping: After removing the force two weeks, the periapical radiolucent areas still existed and the root tip was blunt. There were obvious absorption lacunas and a large number of cells with monocytes or polyocytes in the area of root resorption; Four weeks later, the absorption lacunas were less and shallow; Six weeks later, the absorption lacunas were disappeared basically and the morphology of root tip was normal, cells arranging regularly. The crown-root ratio increased about 1.5-2.5% per week.Conclusion: 1. The elastic force of orthodontic elastic appliances was with low decayi- -ng rate since one hour after the engagement. It was suggested that if the appliance would be applied for the continuous heave force engagement in our study, the initial force of the elastic appliance should be set according to the condition (300g) of the appliances after one hour's degradation.2. By the way of continuous heave intrusive force engagement, the animal model of root resorption can be established and the resorption can be found on X-ray film. It was suggested that the continuous heave intrusive force engagement can induce shorter root resorption during the orthodontic treatment.3. The absorption of root tip will not stop as soon as the force engagement was removed. As time going, the absorption of shortened root would stop gradually. However, the resorped part of the root could not be rehabilitated and the original root length could not be restored.Therefore, in this study, the animal model of root resorption was established successfully, on which the resorption could be detected with shortened root on X-ray periapical films. The study on the developing process of the resorption could be studied on the models. Through observing the self-repair after resorption, it was showed that the absorption of shortened root would not stop as soon as the force engagement was removed. It was suggested that it was not effective enough only to remove force engagement when the clinical root resorption was detected. To decrease the amount of root resorption, it is necessary to find a way to promote root restoration.