| Background:Orthodontic treatment with tooth extraction usually causes a large amount of retraction and torque changes in the anterior teeth,during which side effects may occur,such as gingival recession,root resorption and alveolar bone defects.There are many researches in regard to root resorption at home and abroad.Scholars generally believe that orthodontic treatment can cause different degrees of root resorption,but the severity of root resorption is mostly within the acceptable range,and the amount of resorption is obvious in the anterior teeth[1].The orthodontic treatment with tooth extraction often requires a large amount of retraction in anterior teeth,but the relationship between tooth movement and OIRR is still unclear in the current research.CAT often causes tipping movement of anterior teeth,and whether it can reduce the incidence and severity of OIRR compared with FAT remains to be studied.In terms of alveolar bone remodeling,CAT,whose mechanical system is different from FAT,has been developing rapidly in recent years.However,there is still a lack of research at home and abroad on whether its intermittent force is more conducive to the remodeling of alveolar bone then FAT.In 2019,Jiang Ting et al.[3]performed non-extraction orthodontic treatment with clear aligners on 30 adult patients.By measuring CBCT before and after treatment,they found that alveolar bone defects existed obviously in these patients congenitally,and non-extraction orthodontic treatment with clear aligners tended to increase alveolar bone defects.However,this study did not take traditional fixed appliance as a reference,so it is difficult to evaluate whether the alveolar bone remodeling caused by clear aligners is worse than that caused by fixed appliances.At present,there are few studies on invisible orthodontic extraction and alveolar bone remodeling at home and abroad.In 2021,Guo Yanming et al.[4]measured the CBCT before and after treatment of 60 patients with orthodontic extraction,and compared the alveolar bone remodeling in the anterior dental area after orthodontic treatment with different kind of orthodontic appliances.It is considered that there is no significant difference between clear aligners and fixed appliances in alveolar bone remodeling,but this study didn’t take tooth movement into consideration,so the trial design is not comprehensive enough and needs to be refined and perfected.CBCT can reconstruct the shape of alveolar bone in three dimensions,and show the cortex and cancellous bone well[5].It has been widely used in recent years[6,7].The use of CBCT to evaluate alveolar bone remodeling caused by tooth movement in three dimensions is helpful for orthodontists to assess the risk of tooth movement before treatment[8]。Objective:To study the root resorption rate,root resorption region and alveolar bone remodeling of the anterior teeth after orthodontic retraction treatment with fixed appliances and clear aligners.To investigate the relationship between the mode and amount of anterior tooth movement and the severity and location of root resorption,as well as the relationship between anterior tooth movement and alveolar bone remodeling,so as to provide guidance for the clinical design and monitoring of orthodontic extraction cases with clear aligners.Contents:By comparing root resorption,bone fenestration and dehiscence（alveolar bone defect）at different dental positions in skeletal Class I patients before and after CAT and FAT,we investigated whether the effect of clear aligners on periodontal hard tissue in adult patients is different from that of fixed appliances.This study is divided into three parts:Part 1:A comparative study of root resorption and alveolar bone remodeling in skeletal Class I patients before and after orthodontic treatment with clear aligners and fixed appliancesMethod:Thirty patients who received fixed appliances treatment and 25 patients who received clear aligners treatment were selected.Measuring the CBCT before and after treatment to observe the resorption of the anterior tooth root and alveolar bone remodeling.Chi-square test was used to compare the difference of root resorption rate in the two groups.Paired t test was used to analyze alveolar bone remodeling at different dental positions in the clear aligner group before and after treatment.Group t test was used to analyze the differences of root resorption and alveolar bone remodeling between the fixed appliance group and the clear aligner groups.P<0.05 was statistically significant.Result:The incidence of root resorption in the clear aligner group（64.43%）was significantly lower than that in the fixed appliance group（79.89%）（P<0.05）,the OIRR incidence of lateral incisor teeth was the highest in all groups,and the root length loss in the clear aligner group was significantly lower than that in the fixed appliance group（P<0.05）.The palatal alveolar bone height decreased significantly after orthodontic extraction cases with a large amount of anterior retraction（P<0.05）.However,there was no significant difference between the fixed appliance group and clear aligner group.The increase of labial alveolar bone height at the maxillary central incisor in the clear aligner group at 0.32±0.442 mm was significantly higher than that in the fixed appliance group at0.05±0.547 mm（P<0.05）,indicate clear aligners may be beneficial to reduce or avoid the occurrence of black triangle in the anterior region.After the anterior tooth retraction,labial alveolar bone was generally thickened and palatal alveolar bone was generally thinned.The labial alveolar bone thickness of maxillary incisor teeth at S1 in clear aligner group was higher than that in fixed appliance group,and palatine alveolar bone thickness of maxillary canine at S3 thickened（P<0.05）,suggesting that the anterior teeth were prone to tipping movement with clear aligner treatment.Part 2:The relationship between root resorption and tooth movement in skeletal Class I patients before and after treatment with clear aligners and fixed appliancesMethods:A total of 191 maxillary anterior teeth were included in this study.Three-dimensional reconstruction of pre-treatment（T0）and post-treatment（T1）CBCT was to determine sagittal and vertical apical movement.The volume and morphological changes of maxillary anterior teeth were measured to evaluate apical absorption.Chi-square test and group t test were used to compare the difference of root resorption at different dental positions.Linear regression analysis was used to analyze the regression of root resorption at different positions and the type of actual apical movement,P<0.05 is considered statistically significant.Results:The incidence of OIRR in the clear aligner group（67.71%）was significantly lower than that in the fixed appliance group（85.26%）（P<0.05）,and the root resorption rate and severity in the clear aligner group were generally lower than those in the fixed appliance group.In the fixed appliance group,there was a regression relationship between the amount of maxillary lateral incisor retraction and the volume of root resorption in the nonretraction sector（P<0.05）,which was not observed in the clear aligner group.There was a regression relationship between the amount of apical intrusion and the amount of root resorption in the cervical and middle sectors of the lateral incisor in the fixed appliances group（P<0.05）,but no similar situation was observed in the clear aligner group.When apical retraction ranged from 0 to 3 mm,the volume loss of the root retraction sectors in the clear aligners group（-0.21±6.408mm3）was significantly lower than that in the fixed appliance group（3.37±4.726mm3）（P<0.05）.When apical intrusion ranged from 0 to 1.5 mm,the volume loss of the root apex in the clear aligners group（1.51±4.067 mm3）was significantly lower than that in the fixed appliance group（4.15±4.482 mm3）（P<0.05）.Part 3:The relationship between alveolar bone remodeling and tooth movement in skeletal Class I patients before and after treatment with clear aligners and fixed appliancesMethods:The study included 32 adult patients with skeletal ClassⅠ.These patients underwent extraction of first premolars and orthodontic treatment with upper incisor intrusion and retraction,16 of which treated with fixed orthodontics（mean age23.56±3.687）,and 16 treated with clear aligners（mean age 24.63±4.603 years）.CBCT was collected before and after treatment to measure alveolar bone defect volume,anterior alveolar bone height of mesial,distal,labial and palatal side,anterior alveolar bone thickness at different level of root apex,midst and cervix and movement of anterior tooth crown and root.Group t test and rank sum test were used to compare the difference in alveolar bone defect volume between different treatment groups.Multiple linear regression analysis was used to evaluate the relationship between different types/amount of tooth movement and the change in alveolar bone height/thickness.Results:Before treatment,alveolar bone defects mainly occurred in the labial side,and the incidence of bone fenestration was 23.96-26.04%,which was higher than that of dehiscences（5.21%）.After orthodontic treatment,the incidence of palatal dehiscences increased significantly,and the incidence of palatal dehiscences in the fixed appliance group was higher than that in the clear aligner group.However,there was no statistically significant difference in alveolar bone defect volume between the two groups during treatment.In terms of alveolar bone height,the labial alveolar bone height decreased by0.272mm and the palatal alveolar bone height increased by 0.617mm for every 1mm downward intrusion of anterior tooth apex in the fixed appliance group.In the clear aligner group,the height of palatal alveolar bone was decreased by 0.447mm for every1mm retraction of anterior tooth crown.In terms of alveolar bone thickness,in the fixed appliances,the apical alveolar bone thickness increased by 0.228mm for every 1mm retraction of anterior root apex.In the clear aligner group,the thickness of labial alveolar bone at cervical,middle and apical level was significantly correlated with apical retraction.the thickness of labial alveolar bone at cervical level increased by 0.077mm,that at middle level increased by 0.099mm,and that at apical level increased by 0.177mm for every 1mm retraction of anterior tooth apex.In addition,alveolar bone thickness was significantly correlated with apical intrusion and torque variation.Conclusions:1.In orthodontic treatment of tooth extraction cases,anterior tooth root resorption was mild and moderate,only a few teeth had severe resorption,and the incidence of lateral incisor root resorption was the highest in the two groups.The incidence of anterior tooth root absorption in the invisible group was 64.43%,which was significantly lower than that in the fixed group 79.89%.The mechanical system and good program design of the invisible appliance may be more beneficial to root health than the fixed appliance.However,compared with fixed appliance,invisible appliance has poor root control ability and is more prone to pendulum effect.More torque compensation or root control accessories need to be designed clinically.2.The research results showed that when orthodontic treatment for extraction patients involved a large number of anterior adduction and movement,invisible orthodontic treatment without bracket had less influence on apical absorption than fixed orthodontic treatment.Invisible treatment is more beneficial to root health of lateral incisor than fixed treatment with different mechanical systems.When the apical adduction amount was0-3mm,the amount of adduction in the anterior root of the invisible appliance group was significantly lower than that in the fixed appliance group.When the root tip pressure was0-1.5 mm,the amount of the 1/3 part of the anterior root tip in the invisible appliance group was significantly lower than that in the fixed appliance group.3.The height of labial alveolar bone of anterior teeth decreases when the anterior teeth are depressed by fixed correction.When invisible orthosis is applied to the anterior adducted teeth,the anterior teeth should be designed with the actual pressure of 0.5mm at least.At the same time,alveolar bone cannot be reconstructed to the same degree with tooth movement,and anterior adduction volume needs to be within a certain range.Both fixed and invisible orthotics will increase the incidence of palatal bone cracking.Among them,the mechanical system of light and intermittent force correction with invisible orthotics may be more conducive to the reconstruction of periodontal hard tissue. |
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