The Effect Of Different Extraction Modes On The Morphology Of Chin Of Adolescents With Class â…¡ Division 1 Malocclusion

Chin is one of the characteristics of human facial appearance. From the aesthetic point of view, it is the important aesthetic reference under one-third of face. Harmonious relations among the nose, lips, chin are important indicators of the facial appearance. Good chin shape is considered not only a sign of facial beauty, but also a symbol of wisdom and charm. Much more protrusion or retrusion of chin, facial appearance will be affected. With the updated concept of orthodontic treatment, a good facial profile has become an important criterion.In this study, we investigated and compared differences of chin morphologic features between the classⅡdivision 1 malocclusion and normal adolescents. And we also evaluated the influence on chin morphology of classⅡdivision 1 malocclusion by using different extraction methods . There were three parts in this thesis: Part 1 Study the chin morphology in normal occlusion of adolescents groupAim: To study the chin morphology in normal occlusion of adolescents and select the representative parameters. Methods: By using lateral cephalogram measurements to acquire the average values of chin characteristics in normal group. Results: The representative parameters included: thickness of chin at root tip is 9.27mm; thickness of chin at B point is 9.42mm; thickness of chin at Po point is 11.15mm; height of chin is 36.61mm; proptosis degree of chin is 7.94mm; fossa of chin is 2.69mm; tilt angle of chin is 77.45°; curve of chin is 159.83°; thickness of soft tissue of chin is 11.52mm and depth of mentolabial furrow is 5.58mm. Conclusion:The morphology icaldata of chin of adolescents with normal occlusion between 11 to 14 years old. There are no differences for gender.Part 2 Study the chin morphologic characteristics in adolescents with classⅡdivision 1 malocclusion groupAim: Investigated and summarized the chin morphologic characteristics in adolecents with classⅡdivision 1 malocclusion and then selected the appropriate the measurement parameters. Methods: The average values of each measured category in experiment group(classⅡdivision 1 malocclusion) were calculated. One-way ANOVA were performed on each measured category to examine the significance of the differences between the 2 groups. Results:No significant differences were seen between two groups in mandibular skeletal measurements : thickness of chin at Po point, height of chin, tilt angle of chin, curve of chin, thickness of soft tissue of chin and depth of mentolabial furrow. The patients with ClassⅡdivisionⅠmalocclusion had significantly smaller means in thickness of chin at root tip and B point and proptosis degree of chin than the control group, whereas the mean of chin fossa increased. Conclusion: The morphologic of chin in adolecents with classⅡdivision 1 malocclusion accords with its etiopathogenisis, which mainly shows decreased proptosis degree of chin and increased fossa of chin.Part 3 The effect of different extraction modes on the morphology of chin of adolescents with ClassⅡDivision 1 malocclusionAim:To investigate the effects of extractions on the morphology of chin in adolescents with ClassⅡDivision 1 malocclusion .Methods:A total of 74 subjects were divided into 4 groups with extracting 4 first premolars (Group 1),extracting upper first premolars and lower second premolars (Group 2), extracting upper first premolars only(Group 3) and nonextraction (Control group), then pre- and post-treatment changes of chin were compared by cephalometric analysis. Statistical analysis was performed with SPSS14 software. No significant differences were found in height of chin and thickness of chin at Po point measurements in all groups. Proptosis degree of chin increased evidently in all groups. Fossa of chin, thickness of chin at B point and root tip, curve of chin, tilt angle of chin, and depth of mentolabial furrow measurements revealed significantly smaller in groups treated with bimaxillary extraction. But there were no significant difference in maxillary extraction group. Conclusion: The changes of morphology of chin is affacted by mandibular teeth extraction or not, but not obviously connect with the site of mandibular extraction.The orthodontists should choose proper extraction modes to achieve the best results.