The Evaluation Of Mandibular Dental And Basal Arch Forms Of Skeletal Class Ⅲ Malocclusion Adults With Various Vertical Facial Patterns

ObjectiveThe purpose of this study was to investigate the relationships and differences of mandibular dental and basal arch forms of skeletal classⅢmalocclusion adults with hypodivergent,normodivergent,and hyperdivergent facial patterns using cone-beam computed tomography to provide references for clinical diagnosis and treatment.Materials and methodsStrictly according to the inclusion criteria,lateral cephalograms and CBCT images were obtained from 48 subjects（mean age 22）with skeletal classⅢmalocclusion in orthodontic department of Dalian Stomatological Hospital.The subjects were divided into hypodivergent（<29°）,normodivergent（29°～39°）,and hyperdivergent（>39°）groups by their mandibular plane angles.The ratio of males to females in each group was1:1.Invivo dental5.2 software was applied for 3D volume rendering,and a three-dimensional reference system was established to record the coordinates of the dental arch（FA）and basal bone（RC）marks from the right to the left mandibular second molar.We measured the widths and lengths of dental arch and basal arch,and calculate thirteen ratio variables which can represent arch forms and the relationships between the arches.The x and y coordinates for the mean FA and RC points of each group were inputted into mathematical software Matlab?7.5（R2011b）to generate the best-fitting curve that represented the arch using the fourth degree polynomial equation:f（x）=ax 4+bx3+cx2+dx+e.The dental and basal arch dimensions were compared among the three groups by one-way anova（one-way analysis of variance）and LSD（Least-Significant Difference）t-test.In each group,the difference of dental arch and basal arch was analysed by independent sample t-test.Pearson correlation analysis was used to evaluate the correlation of dental and basal arch widths independently in each group.P-value<0.05was considered statistically significant.Results1.Comparisons of mandibular dental arch widths（DW）,DW3（P<0.01）,DW5,DW6 and DW7（P<0.05）in the hypodivergent group were significantly greater than those in the normodivergent group and the hyperdivergent group.As for dental arch depths（DD）,DD4、DD5、DD6、DD7（P<0.011）in the hypodivergent group were significantly greater than those in the hyperdivergent group.No statistically significant differences in mandibular basal arch widths（BW）among the three groups were found,but the average values of BW3,BW4,BW5,BW6,BW7 in the hypodivergent group were highest.2.Comparisons of mandibular dental arch forms,the ratio of DW3 and DW6（DW3/DW6）in the hypodivergent group was significantly larger than those in the normodivergent group and the hyperdivergent group（P<0.05）.As for mandibular basal arch forms,the ratio of BW3 and BD3（BW3/BD3）was significantly smaller in the hyperdivergent group than those in the hypodivergent group（P<0.05）.The ratio of BD3 and BD6 in the hyperdivergent group was significantly highest among the three groups（P<0.05）,and was significantly different between the normodivergent （P<0.05）or the hypodivergent group（P<0.01）.3.Significant positive correlations were observed between the mandibular dental and basal arch widths and in all three groups（P<0.05）,except the second premolar region in the hypodivergent group.And Pearson’s correlation coefficients in the normodivergent group were highest among the three groups.The correlation coefficients of canine segment in the three groups were lesser than those of molar segment.Pearson’s correlation coefficients between DW3 and BW3 showed a strong correlation in the normodivergent group（r=0.713）,a moderate correlation in the hyperdivergent group（r=0.564）and the hypodivergent group（r=0.533）.But Pearson’s correlation coefficients between DW6 and BW6 showed a strong correlation in all groups（r>0.63）.4.Statistically significant differences were found between mandibular dental and basal arch widths in all three groups except the second molar region（P<0.01）.5.For the x coordinates value of FA and RC points,the coefficients of variation（C·V）of the mandibular canine segment were greater than those of the molar segment (C·V_FA43=7.78,C·V_FA33=7.86,C·V_FA46=5.65,C·V_FA3=6.27, C·V_RC43=7.61,C·V_RC33=8.10,C·V_RC46=6.20,C·V_RC36=6.99).6.The results of best-fit curves showed that the anterior segments of dental and basal arch forms of the low angle group were more flatter.Conclusions1.There are associations between the mandibular dental arch dimensions of skeletal classⅢmalocclusion adults and the vertical facial pattern.DW3,DW5,DW6 and DW7 in the hypodivergent group were significantly greater than those in the normodivergent group and the hyperdivergent group.2.The mandibular dental and basal arch forms of skeletal classⅢmalocclusion adults are related to vertical facial patterns.Compared with the normodivergent and hyperdivergent groups,the anterior segment of dental arch form of the hypodivergent group is more flatter.Compared with the hypodivergent group,the anterior segment of basal arch form of the hyperdivergent group is more narrower.3.Significant positive correlations are observed between the mandibular dental and basal arch widths,except the second premolar region in the hypodivergent group.And the correlation coefficients are affected by vertical facial patterns,which are greater in the normodivergent group than the hypodivergent and hyperdivergent group.The correlation coefficients of canine segment in the three groups were lesser than those of molar segment.Pearson’s correlation coefficients of the molar region show a strong correlation in all groups.Vertical facial patterns play a role in correlation coefficients of the canine region.DW3 and BW3 are strongly correlated in the normodivergent group,and moderate correlated in the hyperdivergent and hypodivergent group.The individual differences are found to be significantly greater in the canine region.4.Not only significant positive correlations but also significant differences were found between mandibular dental and basal arch widths of skeletal classⅢmalocclusion adults.Clinical wire bending of molar region can refer to the dental arch dimension.During orthodontic treatment,it essential to custom arch wires for a patient’s mandibular dental arch shape.