Study Of Chin Morphology In Skeletal Class ? Subjects Of Hyperdivergent With Different Dentition Stage

Objectives: Chin morphology of hyperdivergent skeletal class ? malocclusion were compared with hyperdivergent skeletal class?malocclusion by using the method of cephalometric in each dentition stage, to analyze the specific features and development tendency of chin morphology in hyperdivergent skeletal class ? malocclusion.Methods:120 subjects were selected from those who sought orthodontic treatment at the Orthodontic Department in Hospital of Stomatology, Hebei Medical University, from 2009 to October 2015. They were divided into three groups according to age and dentition stage: mixed dentition group(7-10 years old), young permanent dentition group(11-14 years old), mature permanent dentition group(18 years old and over). Then each group was divided into class?malocclusion group and class ? malocclusion group. The average age of each group, consisted of 20 cases(10 male and 10 female), were as follows: class?of mixed dentition 8.99 years old, class ? of mixed dentition 8.74 years old; class?of young permanent dentition 12.17 years old, class ? of young permanent dentition 12.30 years old; class?of mature permanent dentition 24.61 years old, class ? of mature permanent dentition 21.86 years old.Cephalograms were analyzed for all subjects with Win Ceph 8.06 software. Based on 17 landmarks, 28 linear and angle measurements were selected and analyzed to study the vertical and sagittal development of craniomaxillofacial skeleton, as well as chin morphology. The data were analyzed with SPSS 21.0 statistical analysis software: 1) studying the effect of gender on the chin development in each dentition stage with factorial analysis of two factors; 2) comparing class?and class ? in each dentition stage with group t test or Wilcoxon rank sum test.Results:1 There were no gender differences in the measurement items of mixed dentition group. However, in young permanent dentition and mature permanent dentition group, both chin height and chin projection showed gender differences(P<0.05). Whether in class?or in class ?, the two measurements were higher in male than in female.2 Compared to class?group of mixed dentition, subjects in class ? group had greater chin height as well as chin protrusion, and the difference values were 0.16 mm and 0.21 mm(P<0.05). Wilcoxon rank sum test results showed that class ? group of mixed dentition had smaller ANB angle and mentolabial furrow depth than class?group(P<0.05).3 Through comparing two groups of young permanent dentition, class ? group had greater SNB angle, chin height, in which the difference values were 4.38° and 0.90 mm(P<0.05), as well as smaller genial angle and L1-MP angle, with 2.25° and 3.72° difference(P<0.05). Moreover, by using Wilcoxon rank sum test, chin protrusion of class ? was greater than class?, while ANB angle, chin depression, chin curvature and mentolabial furrow depth were smaller than class?(P<0.05).4 Compared to class?group of mature permanent dentition, subjects in class ? group had greater SNB angle and chin height, in which the difference values were 6.54° and 1.76 mm(P<0.05). While chin depression, chin curvature and L1-NB distance were smaller than class?, with 0.41 mm, 2.04° and 1.52 mm difference(P<0.05). Meanwhile, chin protrusion of class ? was greater than class?, and ANB angle, genial angle, mentolabial furrow depth, L1-NB angle, L1-MP angle were smaller than class?with Wilcoxon rank sum test(P<0.05).Conclusion:1 There are gender differences in the chin development of skeletal class ? subjects of hyperdivergent.2 Sagittal and vertical excessive development of chin in skeletal class ? subjects of hyperdivergent appears in mixed dentition. Then, in young permanent dentition, mental excessive development becomes more apparent, and lower incisors appear compensative changes. All of these are further aggravated in mature permanent dentition.3 In permanent dentition, the chin of skeletal class ? subjects of hyperdivergent shows common characteristics such as small chin angle, large chin protrusion and small chin curvature.4 Whenever in mixed dentition and permanent dentition, the shape of lower lip is upright with high tension.