| Angle’s class Ⅱ division 1 malocclusion is a common disease in orthodontic clinic,the characteristics of which are protrusion of maxilla and maxillary incisors,retrusion of mandible occasionally,deep overbite and overjet of anterior teeth,and protrusion of profile.As a result,the occlusal function,facial apperance,and psychologic status of these paients couble be badly affected.Once these paients passed the puberty spurt period,their camouflage treatment usually requires premolar extraction and en-masses retraction of anterior teeth to improve the profile and achieve the normal occlusion.Clinically,alveolar bone remodeling secondary to tooth movement has gradually attracted the attention of scholars in recent years.Generally,the anterior teeth can be retracted adjunct with tip or torque movements with different biomechanics,but it is unknown whether there are differences of alveolar bone remodeling secondary to different retraction methods.Furthermore,even though external apical root resorption induced by orthodontic treatment is the most common iatrogenic sequelae,its risk factors are still equivocal.Moreover,compared to traditional two-demensional imaging tools,cone-beam computed tomography(CBCT) is commended for its accuracy and reliability.So,with CBCT technology the present study is aimed to research the retraction movements of maxillary incisors in angle’s class Ⅱ division 1 teenagers,analysis and compare alveolar bone remolding,and investigate external apical root resorption of incisors and its contribute factors.Objective 1.To qualitatively and quantitatively research the retraction movements of maxillary incisors.2.To analysis and compare alveolar bone remolding secondary to different kinds of retraction method of maxillary incisors.3.To investigate external apical root resorption and its contribute factors during retraction of maxillary incisors.Methods Forty teenagers with angle’s class Ⅱ division 1 malocclusion who met the inclusion criterias were selected,including 11 boys and 29 girls,and their mean start age and treatment duration were 13.90 and 2.19 years,respectively.Three-demensional softwares were utilized to anlysis the horizontal,vertical,and labialingual movements of upper incisors in CBCT images.Maxillary incisors were classified into two categories,namely retraction adjunct with tip group(R&Tp) and retraction adjunct with torque group(R&Tq),according to the movement direction of root apex.Later,tooth movements and its related alveolar bone thickness and height changes were analysed and compared between two groups.Finally,external apical root resorption and its contribute factors were investigated through correlation and regression analyses.Statistical analyses were performed with SPSS 20.0 software.Results 1.Tooth measurements: 66 and 94 of 160 maxillary incisors were included in R&Tp and R&Tq group,respectively.Maxillary central incisor showed a higher proportion in R&Tp group,whereas in R&Tq group,maxillary lateral incisor accounted for a higher proportion.The crown of teeth in R&Tp group was more flaring than that in R&Tq group before treatment,but teeth in both groups were intensively retracted and uprighted after treatment.In R&Tp group,the retraction distance of incisal egde and retraction angle of tooth were 5.86 mm and 19.70°,respectively,and the retraction distance of R1 and R2 points were 1.52 and 0.54 mm,respectively,yet R3 and root apex of incisor traveled labially.In vertical direction,the distance between incisal edge and the palatal plane increased by 1.24 mm,but the rest landmarks of incisor exhibited decreased distance to the palatal plane.In R&Tq group,the retraction distance of incisal egde and retraction angle of tooth were 4.86 mm and 9.14°,respectively,and the retraction distance of R1,R2,R3,and root apex were 2.98,2.58,2.20 and 1.98 mm,respectively.In vertical direction,all landmarks of incisor showed a decreased distance to the palatal plane,and the height reduction increased from 0.17 mm at incisal edge to 1.79 mm at root apex.By comparsion,the retraction distance of incisal egde and retraction angle of tooth in R&Tp group were significantly larger than those in R&Tq group,whereas it showed a opposite tendency of the retraction and intrusion distance of R1 and R2 points,and there were no statistically significant difference about the intrusion distance of R3 and root apex.2.Alveolar bone measurements: basically,both groups revealed the same alveolar bone height and thickness before treatment.In R&Tp group,labial alveolar bone thickness significantly decreased from L1 to L3 after treatment,with alveolar bone thickness reduced most at L3,and palatal alveolar bone thickness significantly decreased at P1.In R&Tq group,alveolar bone thickness significantly but slightly increased at L1,L2 and L3 in labial side but significantly decreased at P1,P2 and P3 in palatal side,with the highest level of thickness reduction at P1.Total alveolar bone thickness decreased at T1 in R&Tp group and at all levels in R&Tq group.Alveolar ridge height in two groups dropped both in the labial and palatal sides,with a more prominent extent in the palatal side,and ridge height dropped most in palatal side of the teeth in R&Tq group.Meanwhile,alveolar bone defect exhibited in 66 incisors or 41.25% of the total sample,including bone fenestration at 18 sites and bone dehiscence at 51 sites.Worse more,4 teeth in R&Tp group simultaneously exhibited fenestration in labial root apex area and dehiscence in palatal side,and in R&Tq group,severe dehiscence occurred in 11 teeth.Chi-square test showed that alveolar bone defect in R&Tp group was mostly the bone fenestration,while alveolar bone defect in R&Tq group was more manifested as the bone dehiscence.3.External apical root resorption(EARR): all 160 maxillary incisors suffered EARR,ranging from 0.09 to 4.46 mm.The overall frequency of root resorption of less than 2.00 mm was 91.9%,whereas 4.4% of the total sample suffered root resorption more than 3.00 mm,and only 2 teeth suffered root resorption more than 4.00 mm.The statistical analysis did not find significant differences in root resorption regarding the gender,and there were no significant correlations between EARR and initial tooth length,retraction method,initial age and treatment duration of paients.Multiple linear regression analyses showed that risk factors of EARR were: root intrusion,root approximation to the cortical plate,and root retraction.Conclusion 1.When maxillary incisors in teenagers with angle’s class Ⅱ division 1 malocclusion are intended to have en masse retraction,maxillary central incisor takes retraction adjunct with tip movement as clinical manifestation,whereas maxillary laternal incisor takes retraction adjunct with torque movement as clinical manifestation.2.Maxillary incisors in R&Tp and R&Tq groups were intensively retracted and uprighted.In both groups,the movement of tooth was a combination of translation,rotation and intrusion.3.The labial root apex area and the palatal alveolar ridge area of alveolar bone in the R&Tp group and the cervical and middle area of incisor root in palatal side of alveolar bone in the R&Tq group are high-risk areas for alveolar bone resorption when maxillary incisors are intended to have en masse retraction.4.EARR of most maxillary incisors is less than 2.00 mm,and it is rare for severe resorption.EARR does not have a sexual dimorphism and are not correlated to initial age,treatment duration and retraction method of incisor.Root intrusion,root approximation to the cortical plate,and root retraction are risk factors of EARR. |
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