The Effect Of Different Extraction Methods On Eruption Tendency Of Mandibular Third Molar In Patients With Angle Class Ⅱ Malocclusion

Objectives: To compare the effect of different extraction methods on the eruption angle and the eruption space of mandibular third molar in patients with Angle Class II malocclusion according to the panoramic radiographs,and to further explore the correlation between tooth extraction and the eruption tendency of mandibular third molars.Materials and methods: Based on the inclusive standards,the subjects were selected and collected from orthodontic department at Dalian city Stomatological hospital-affiliated hospital of Dalian medical university from September 2015 to December 2017,which contained 60 patients(24 males,36 females),diagnosed to be Angle Class II malocclusion,with an average of age 13.48 ± 1.43 years old.Then all the patients were divided into three groups,respectively,the non-extraction group,the extraction of mandibular first premolar group and the extraction of mandibular second premolar group(20 cases in each group,including 113 mandibular third molars).All patients were treated with MBT technique.The panoramic radiographs were taken before and after the treatment,and were performed by the same radiologist with the German digital panoramic camera ORTHOPHOS XG5 in accordance with the same standard.Then the changes of eruption angle and eruption space of mandibular third molars were measured with the Winceph9 measurement software in panoramic radiograph. Measurements are as follows(Take the right mandibular third molar for example):M3-M1: The upper right angle of the long axis of the mandibular third molar intersecting with the long axis of the first molar;M3-M2: The upper right angle of the long axis of the mandibular third molar intersecting with the long axis of the second molar;M3-OP: The lower right angle of the long axis of the mandibular third molar intersecting with the occlusal plane;M3-MP: The lower right angle of the long axis of the mandibular third molar intersecting with the mandibular plane;M3-PP: The lower right angle of the long axis of the mandibular third molar intersecting with the palate plane;M3-FH: The lower right angle of the long axis of the mandibular third molar intersecting with the orbital ear plane;α: The lower right angle of the tangent of mandibular third molar occlusal surface intersecting with the tangent of lower first and second molar occlusal surfaces.A: The maximum width of the crown of mandibular third molar;B: The distance from the farthest point of mandibular second molar along occlusal plane to the leading edge of mandible ramus;B/A: The ratio of the distance from the farthest point of mandibular second molar along occlusal plane to the leading edge of mandible ramus and the maximum width of the crown of the mandibular third molar;Using SPSS21.0 statistical analysis software to process and analyze the measurement data.One-way ANOVA was used to analyze the age before treatment and the number of mandibular third molars in each group.The measurement indexes of before and after treatment in the same group were analyzed by two-sample t-test.The measurement indexes of different groups were analyzed by two independent samples t-test.Results: 1.There was no statistical significance in the age before treatmen and the number of mandibular third molars in the non extraction group,the extraction of the first premolar group and the extraction of the second premolar group(P<0.05).2.After treatment,the eruption angle of mandibular third molar decreased and the eruption space of mandibular third molar increased in the non extraction group,the extraction of the first premolar group and the extraction of the second premolar group(P<0.05).3.Before and after treatment,the changes of eruption angle and eruption space of the mandibular third molar in each group were compared,Among them,the changes of in the extraction of the second premolar group showed the greatest,followed by the extraction of the first premolar group,the non extraction group showed the smallest(P<0.05).Conclusions: Extraction treatment or non-extraction treatment can make the eruption space of mandibular third molar increased and the eruption angle of mandibular third molar decreased in growth and development of patients with Angle Class II malocclusion.The improvement of the eruption space and the eruption angle of mandibular third molar in extraction treatment is more obvious than that in non-extraction treatment.In particular,the removal of the second premolar can significantly increase the eruption space of mandibular third molar and improve the eruption angle of the mandibular third molar.