The Animal Experimental Research On Rapid Orthodontic Tooth Movement Of Whole Anterior Teeth Aided By Alveolar Surgery

Dentognathic deformity is one of three common oral diseases definited by WHO. In particular,maxillary protrusion is the most common deformity in Guangdong province,China.Many orthodontic appliances and techniques have been widely used for patients during last one handend years.So far,orthodontic treatment takes a long time because the tooth has to move slowly in order to assure the health of both the tooth and its periodontal tissue.The speed of the tooth movement is ultimately 1mm per month with traditional method.However,shorter period of the treatment is always expected by the patients.For many years,methods to accelerate tooth movement while avoiding irreversible damage to the patient's pulp,root and periodontal tissue have been explored.In 1998,Eric Liou employed the principles of distraction osteogenesis and bone seam distraction osteogenesis in his clinical study of rapid distal movement of the canine.In 2002,Kisnisci proposed'dentoalveolar distraction osteogenesis'(DAD) and reported the clinical application study.Several Chinese investigators have established experimental animal model successfully of accelerate tooth movement based on these two concepts above mentioned.Also,they have studied the changes happened to pulp tissues,periodontal tissues and the roots after rapid tooth movement. But all of these studies focused on individual tooth movement.Whether it is possible to move the whole anterior teeth at one stretch,with the help of alveolar surgery and the rigid distracter,remains a question.In 2000,Castello proposed the concept of contractionosteogenesis(CO).Under the condensing force on the maxillary bone seam,there was bone absorption and the maxillary was consequently shortened.In 2004,Li Yong et al proved that the mandibule was contractable on experimental animal model.Therefore,if the anterior teeth are moved rapidly with the help of rigid distractor and dentoalveolar surgery,it seems possible that the maxillary will also be distracted.If so,how about the bone healing? One of the aims of this study is to explore it.To establish the experimental animal modelEighteen beagles were used in this study,with all their maxillary canines extracted.Then the bottom of extraction fossa was grinded and removed to the nasal floor with a ball drill.The palate mucoperiosteal flap was reflected from the alveolar crest to the apical region to expose the cortical bone around the six anterior teeth.The cortex of palatal bone was grinded and removed about 6 mm with a ball drill and the spongy substance was retained.The palatal arc cuts stoped at extract fossa.The palate mucoperiosteal flap was sutured after corticotomy.After that a custom-made intraoral, rigid,tooth-borne distraction device was used.Then,a microimplant was placed into the crest between two upper incisors,and another one into the palatal bone in the middle of two point of upper first molars' distal crest.The beagles were divided into two groups randomly.Stress to Group A at 0.5mm/d for 12 days continuously,and stress to Group B at 0.75mm/d for 8 days continuously were applied.All beagles were to be killed at the end of stress application,two weeks after fixation,and four weeks after fixation respectively.X-ray examination was performed before retracted force application or execution.All animals survived during experiment period and their soft and bony tissues were normal in appearance.Clinical examination showed a definite retraction of maxillary anterior teeth.All the anterior teeth showed reversed overbite and overjet, and the maxillary anterior teeth tipped clockwise slightly.Tooth and radiograph measurement indicate that the anterior teeth were moved 4.55±0.10mm on average, while the posterior teeth were moved 1.36±0.11mm.The anterior teeth tipped distally 5.28±1.05 degree.The average skeleton movement was 1.30±0.60mm.There is no significant statistical difference between the Group A and Group B.And there are no apparent complications,such as root resorption,loosening of tooth,bone necrosis and so on.Periapical films showed that all of the anterior teeth had a blunt root apex with wider PDL space.And the shadow of worm appearance on the root compression side indicated root resorption.The results indicate that the technique of rapid orthodontic tooth movement of whole anterior teeth aided by alveolar surgery can accelerate the speed of tooth movement without causing significant anchorage losses.At the same time,it can retract the maxilla without any complications.Histologic examination after rapid tooth movementAfter animals were executed,the two upper third incisors were separated to prepare for H and E dying and scanning electronic microscope(SEM) examination.The microphotograph of sections stained with H and E showed the distal periodontal membrane space was narrowed and the alveolar was resorpted,while the mesial periodontal membrane space was widened and the alveolar was deposited. There were a decreased cell constitution and hyalinization zone in PDL appeared in A1,but not that in group B.Histologic images also suggested obviously blood vessel dilatation and congestion in pulp tissue at the end of distraction with significant inflammatory cell infiltration.After retention for 4 weeks,most of the congestion in pulp tissue of group A had disappeared except for a small part of infiltrated inflammatory cells.On the other hand,in group B the cell constitute decreased and fibro-component of pulp tissue increased,indicating a fibroid degeneration.From the H and E images,it was observed that there was a considerable root resorption extending into the dentin at the end of distraction of two groups.This root resorption extended from the cementoenamal junction to the root apex.But after retention for 4 weeks,we observed repaired cementum formation on the resorbed dentin surface.From high power lens,we saw many cementoblasts distributed on the repaired cementum.According to the SEM observation,there was a serious root resorption at the end of distraction.The root on the compression side appears stripping and introcession from the cementoenamal junction to the root apex.Comparatively the problem is more serious in group A than in group B.After retention for 4 weeks,the large stripping area became generally smaller and the deep introcession became shallow. From high power lens,we can see many remineralized tubercles at the floor of the absorption lacuna.But the extent and area of stripping still appears more serious in group A than group B.And there is significant difference between two groups from statistical results.The results indicate that applying stress at 0.5mm/d would make the root resorption more serious while applying stress at 0.75mm/d would make the pulp tissue degeneration more serious.Thus,we prefer the rate of 0.5mm/d taking consideration of the recoverability of root resorption.The healing of the palatal bone after rapid tooth movementWhen the cortex of palatal bone was grinded and removed,the cortex of buccal bone on the top of upper first maxillary root apex was also removed as the control side.Then all the animals were injected tetracycline before they were killed one and seven day.After sacrifice,samples were dissected and prepared for H and E dying and undecalcified sections.Clinical examination showed no apparent complications such as inflammation and necrosis,and the appliance was still fixed on the teeth.At the end of distraction, the gap on the palatal bone was contracted and filled with bright red granulation tissue,while there was no apparent granulation tissue on the buccal side except for a little fibro-tissue.After retention for 2 weeks,the granulation tissue on the palatal bone changed into solide elastic gel appearance,while a little reticulate neogenesis fossa could be observed on the buccal side.After retention for 4 weeks,the gap had bridged with the new-formed bone there was no sign of fracture,defect and angulation deformity. From the H and E images,we observed lots of newly formed bone trabecula and osteoid formation on both sides.During the process of repair,intramembranous ossification is obvious.From the results of the osteoid area with tetracycline fluorescent labeling,we could see that the extent of osteoid area in buccal side of group A2,A3,B3 were bigger than in the palatal side,while that of group A is generally bigger than group B after retention for 2 weeks.And there was significant difference between two groups from statistical results.Our results demonstrate that the healing process of palatal bone is intramembranous ossification as well as buccal bone.But the healing speed was lower than the buccal side for the reason of stress.The expression of OPG and RANKL during the healing of boneAfter the animals were killed,samples were prepared for immunity histochemistry and real-time PCR examination.From the result of immunity histochemistry,we observed that positive expression of OPG and RANKL protein was found in endochylema and cell membrane of osteoblast,osteoclast and mensenchymal cell.According to the results of RT-PCR,gene expression of OPG on the palatal side was increased significantly (p＜0.05) gradually,while that on the buccal side was upregulated significantly (p＜0.05) until the end of retention for 2 weeks and decreased after retention for 4 weeks.After retention for 2 weeks,the expression of OPG mRNA on buccal side was stronger than that of palatal side(p＜0.05).The gene expression of RANKL on the buccal side was the strongest at the end of distraction significantly(p＜0.05),and decreased gradually,but still at a high level.While on the palatal side,it was increased significantly(p＜0.05) until the end of retention for 2 weeks and decreased after retention for 4 weeks.Also,the expression on the buccal side was stronger than that of the palatal side except for the No.3 group.Analyzing the ratio of OPG and RANKL,we can see that on the buccal side the ratio upregulated until the end of retention for 2 weeks and decreased after retention for 4 weeks.While on the palatal side,the ratio did not increase until the end of retention for 4 weeks.And the expression on the buccal side was stronger than that of the palatal side except for the No.1 group.Several factors may contribute to the bone repair deficiency.OPG/RANKL system is one of those which play a role in this procession.The data collected from the present study suggest that the expression level of OPG/RANKL can be influenced by force stimulation.Yet,the accuracy adjustment mechanism still requires further experimental research.Above all,the animal experiment model for the first time to test the technique of rapid orthodontic tooth movement of whole anterior teeth aided by alveolar surgery was established.This technique proved to accelerate the speed of tooth movement, while reduce anchorage loss and maxillary length to some extent.But in the study, some kinds of root resorption and pulp tissue degeneration were detected.Thus, further research is suggested to verify the safety of the method in clinical application. Also,this study observes,for the first time,bone healing status under mechanic force. The results showed that the process of repaired is mainly realized through intramembranous ossification.The retraction force will affect the speed of bone healing and the expression level of OPG/RANKL.