Relationship Between Changes In Thickness Of Alveolar Bone And Movement Of Maxillary Central Incisor

Objective:Cause by the plenty of new techniques to help the orthodontists to control the anchorage,it seems like that the orthodontists can move teeth to anywhere they wanted.So a crucial question is that where is the safety boundary on palatal alveolar during the anterior teeth retraction.But exploration of this question does not mean that the orthodontists should prevent any dehiscences and fenestrations.Actually,it should be taken more consideration which how many or what kind of periodontal bony defect is safe(means that no influence on the resistibility of periodontal diseases and no influence on the adaptability of the masticatory force)or self-cure and the relationship between the movement of teeth and the changes in its periodontal bone mass.Finally,orthodontists will extend their insight about the safe boundary for tooth movement.The object of this study was to quantify the relationship between the changes in the palatal alveolar bone supporting and the tooth movement by using the three-dimensional(3D)cone-beam computed tomography(CBCT)superimposition technique.Subjects and methods:Totally 32 female cases were brought into this study.Each of 32 patients had two images taken in pre-and post-treatment(T1 and T2).A co-ordinate system was constructed using two planes defined by four anatomical landmarks(ELSA,left and right porion,dorsum foramen magnum)located by an orthodontist(ELSA co-ordinate).A second co-ordinate system was constructed using four anatomical landmarks that are corrected using a novel numerical optimization algorithm for any landmark location operator error using information from eight landmarks.The optimization algorithm minimizes the relative distance and angle between the known fixed points in the two images to find the correction.Then the third co-ordinate system(PNS co-ordinate)was constructed only using two anatomical landmarks(anterior nasal spine and posterior nasal spine)that base on the second co-ordinate system.In the PNS co-ordinate,the following items were calculated between pre-and post-treatment: 1)the three dimensional movement of apex of root and incisor point of left and right maxillary center incisor;2)the changes in length of long axis of left and right maxillary center incisor and its tilt angle in sagittal plane;3)the rotation angle of incisor edge on transverse plane;4)set the sagittal plane that included the most area of maxillary center tooth as the operate plane,then set the line through the root apex and parallel the line that through the ANS and the PNS as the reference line to measure the height and area of the labial/palatal alveolar bone,using the ratio(area/height)as the average thickness of labial/palatal alveolar bone in pre-or post-treatment(LAATpre、LAATpost、PAATpre、PAATpost);5)using the value from the movement of incisor point on y-axis minus the movement of root apex on y-axis to evaluated the tooth movement type(retraction associated with more torque or more tip movements).Then set the changes in PAAT,changes in length of long axis,growth length of PNS-ANS per year(PNS_ANS/y)and growth length of PNS-Ifn(PNS_Ifn/y)per year as the dependent variable separately to perform the multiple linear regression analysis by “Stepwise” method in SPSS software(Chicago,Ill).Only performed the logistic regression analysis on the dehiscence,because of the small numbers of fenestrations(<5).Set “0” represent the no dehiscence or dehiscence cure or dehiscence maintain(the defect increase less than 2mm),and “1” represent the new dehiscence or dehiscence exacerbation(the defect increase more than 2mm),performed the binary logistic regression analysis by “Forward: LR” method in SPSS software.The significate level on variable bring into was 0.05 while on variable remove was 0.1 in all regression analysis performance.Because that the PNS co-ordinate system was a right-hand Cartesian coordinate system,some characters in it will describe follow: 1)use the absolute value to representthe position difference(T2-T1)on x-axis of incisor point and apex,it means the displacement of landmark on x-axis,denoted as “|Incisorx|” and “|Apexx|”;2)about the position difference(T2-T1)on y-axis of incisor point and apex,the negative value means move forward and the positive value means move backward,denoted as “Incisory”and “Apexy”;3)about the position difference(T2-T1)on z-axis of incisor point and apex,the negative value means move down(intrusion),positive value means move up(extrusion),denoted as “Incisorz” and“Apexy”;4)aboutthe angle of incisor edge between pre-andpost-treatment,its range is 0 to 90 degrees,denoted as Rotation_angle;5)about the angle of long axis of tooth between pre-and post-treatment,its range is-90 to 90 degrees,the negative value means tilt backward(palatal side),the positive value means tilt forward(labial side),denoted as Tilt_angle;6)about the value of “Incisory-Apexy”,the negative value means tilt forward and the positive value means tilt forward;7)about the absolute value of “Incisory-Apexy”,the value more large means the tooth more close to retraction associated tip movement,denoted as|Incisory-Apexy|.Because that all involved cases without any enamel slices and other endodontic treatment,the changes in root length during treatment can be represented by the changes in long axis of tooth.Result:The LAAT had nosignificant difference between pre-and post-treatment.While the PAAT had high significant difference(P <0.0001)between pre-and post-treatment,it decreased about 1mm.The results of multiple linear analysis will showed following(about the value of independent variable,negative value means decrease and positive value means increase;use R2 to evaluate the goodness of fit;use the square of semipartial correlation,Pa2,to evaluate the importance of each dependent variable): the changes in PAAT during treatment =-0.50×PAATpre-1.42×Apexy+0.33×Apexz-1.90×Incisorz,finished by 4 steps,R2 =43.15%,the dominant dependent variable was “PAATpre”(Pa2 =21.34%)and second was “Apexz”(Pa2 =12.61%),indicated that: the resorption is the dominant process on the palatal alveolar bone during teeth retraction;stand up and intrusion of tooth will decrease the sagittal resorption;more thick palatal alveolar bone need more resorption to trigger its bone apposition;changes in root length during treatment =-0.15×|Apexx|+0.87×Apexz-0.8×Incisorz-1.47×(Incisory-Apexy),finished by 6 steps,R2 =75.25%,the dominant dependent variable was “|Apexx|”(Pa2 =57.32%)and second was “Incisorz”(Pa2 =47.89%),indicated that retraction associated with tip movements and extrusion will increase the risk of root resorption;the growth length of PNS-ANS pre year during treatment =0.34-0.15×Apexy-0.04×|Incisory-Apexy|,finished by 3 steps,R2 =46.85%,the dominant dependent variable was “Apexy”(Pa2 =41.54%)and second was “|Incisory-Apexy|”(Pa2 =5.75%);the growth length of PNS-Ifn pre year during treatment =1.10-0.04×Incisory-0.05×starting age,the dominant dependent variable was “starting age”(Pa2 =17.33%)and second was “Incisory”(Pa2 =9.85%),indicated that maybe intra-maxilla orthodontic force will influence the growth of maxilla.The results of binary logistic analysis showed following: the odds ratio of new palatal dehiscence or defect exacerbation =EXP(0.41×|Incisory-Apexy|-0.48×Apexz),finished by 2 steps,overall predict percentage correct was 78.13%,indicated that per 1mm intrusion of apex will decreasethe risk of new palatal dehiscence or defect exacerbation 9%,and per 1 mm intrusion of incisor point will increase the risk of new palatal dehiscence or defect exacerbation 1.5 fold;the odds ratio of new labial dehiscence or defect exacerbation =EXP(-0.70-2.02×PNS_ANS/y),finished by 1 step,overall predict percentage correct was 73.44%,indicated that per 1mm increase of growth length of PNS-ANS will decrease the risk of new labial dehiscence or defect exacerbation 87%.Conclusions: the results of present study indicated that during anterior teeth retraction: 1)the ability of remodeling on labial alveolar bone was highly stronger than palatal;2)bodily intrusion of the tooth will decrease the sagittal resorption and the risk of the dehiscence on the palatal alveolar bone;3)incline and extrusion of the tooth will increase the risk of localized root resorption that cause by the increased risk of thrust against the labial/lingual cortical plate.