Influence on the eruption of mandibular third molar after the extraction of second premolar or first molar during orthodontic treatment

Objective To estimate the influence on the eruption of mandibular third molar after extracting the mandibular second premolar or first molar.Methods 68 adolescent cases with Class Ⅰ malocclusion were studied by retrospective analytic method and divided into three groups:second premolar extraction group,first molar extraction group and non-extraction group.All the cases were finished with straight wire appliance and closed the space by closing loop method.then the third molar' angulation,mesio-disto crown width and eruption space were measured on panoramic radiographs pre and after treatment in each group by t-test.Results There was no significant changes of third molar angulation(RM3/LM3),eruption space(RS/LS) and mesio-disto crown width(Rratio/Lratio) in the non-extraction group(P 0.05).There was significant difference of RS,LS,Rratio and Lratio(P 0.05) in second premolar extraction group,but the change of RM3,LM3 have no statistical meaning(P0.05).In first molar extraction group,there was significant difference of RS,LS,Rratio and Lratio(P0.01),but the change of RM3,LM3 still have no statistical meaning.Conclusion extraction in Orthodontic can increase the third molar' eruption space,the eruption space increased obviously after extracting the first molar.But teeth extraction does not improve the third molar' angulation.improve the third molar' angulation.