Animal Experimental Study Of Mini-implant Anchorage For The Maxillary Anterior Teeth Intrusion

PURPOSE To establish the animal model for the maxillary incisors intrusion with the mini-implant as the anchorage and evaluate the effect of this treatment, investigated structure changes of the root surface;histological changes of the root and periodontal tissue ;osteoprotegerin (OPG) expression changes in the periodontal tissue when the teeth were persistently intruded, evaluated the effectiveness and safety of mimi-implant anchorage for the maxillary incisors intrusion,explored the biological mechanism of the teeth intrusion movement.METHODS 9 dogs were selected and randomly divided into 5 groups: I group was called the contral group ,contained 1 dog with no force given; II, III, IV, V group contained 2 dogs respectively.Mini-implants were inserted into the labial alveolar bone between the roots of maxillary the second and the third incisors on both sides,one hundred grams force was imposed on the first and the second incisors on both side.At the end of l,2,4,12weeks (activation for 4 weeks,then retention for 8 weeks after force removal) after the activation.X-ray examination were performed and the distance of tooth movement was measured. Then the animals were sacrificed ,the first and second incisor teeth on the one side were randomly selected to examine the surface of the cementum by using the scanning electron microscope.Furthermore, The teeth on the other side together with gums, alveolar bone tissue were completely cut to make the histological specimen. The histological changes of the root and periodontal tissue were observed by HE staining and expression of Osteoprotegrin (OPG) in periodontal ligament(PDL)was also inspected by using immunohistochemistry .RESULTS1. Amounts of intrusion were averagely obtained (2.83±0.28)mm by using the mini-implant as anchorage to intrude the incisors. Significantly less root resorption was observed in the X-ray.2. By using scanning electronic telescope (SEM), absorption lacunas were found in the root surface after force given. As the time went on, the numbers and areas of absorption lacunas increased and went deep into the dentin.As the intrusion went on,the areas of absorption expanded from the root tip to the neck and repair of the cementum occured at the resorbed areas after force removal.3. Histological examinations showed: 1 week after activation, compared with the contral group, cementum and alveolar bone of the root tip had some absorption lacuna. Hyalinization of the periodontal ligamen emerged at the local root area , crest of ridge is uneven,Osteoclasts were active,periodontal fiber was stretched. 2 weeks after activation, the extent and scope of absorption expanded from root apical and to neck , which coincided with the observed results of scanning electron microscopy.4 weeks after activation, absorption lacunas in alveolar bone and cementum were still evident,osteoclasts were active,hyalinization of the periodontal ligament was disapp -eared.After 12 weeks (activation for 4 weeks,then retention for 8 weeks after force removal),the surface of alveolar bone and cementum was significantly repaired,new bone formation and osteoblasts in apical area and crest of ridge were active.Periodontal ligament was arranged orderly .4. Expression of OPG in the alveolar bone and periodontal ligament tissues increased in the 1 weeek after force given, and reached maximum in the 2 weeks after activation, then dropped to normal level in the 4 weeks after activation. After 12 weeks (activation for 4 weeks,then retention for 8 weeks after force removal),it increased insignificant, there were no obvious difference between 4 weeks after activation and the contral group.CONCLUSION1. Mini-implant anchorage could be used as an efficient method to control maxillary anterior teeth intrusion.2. The root resorption occured during themaxillary anterior teeth intrusion, Such absorption could be repaired after force removal.3.The reconstruction of periodontal tissues mainly can be seen in the apical area and the alveolar crest. The rule of tooth movement and reconstruction of periodontal tissues under the vertical force was the same as under the horizontal force.4. OPG participated in the reconstruction of periodontal tissues.