Changes Of The Impacted Mandibular Third Molars After Orthodontic Treatment

Objective: It is controversial whether the mandibular impacted third molars must be extracted before carrying out an orthodontic treatment.The purposes of this study is to analyze the post-treatment status of the impacted mandibular third molars to confirm the influence factors leading to its position changes resulted from different treatment plan in different malocclusion,and to provide the evidence for extracting the third molar or not at the beginning of treatment.Methods: 81 impacted third molars of 41(41/238)completed orthodontic patients were preliminarily analyzed based on the measuring of pre-and post-treatment panoramic radiographs.The third molar located unilateral mandibular dentition was defined as an independent sample.All patients were divided into adolescent group and adult group.The changes of the tilt angle and eruption space were calculated compared with the anchorage(minimum,medium and maximum)in extraction group and both the crowding(no,mild and moderate)and the intermaxillary traction(no,II,?)in non-extraction group respectively.SPSS21.0 software was used to analyze the measurement results,and P < 0.05 indicated that the difference was statistically significant.Results: Due to the limited number of cases,only 74 teeth were finally statistically analyzed in this study.1.In tooth extraction cases,the average change value of tilt angle of mandibular third molars before and after treatment was 6.80?in the maximum anchorage group,0.63?in the medium anchorage group,and 9.05?in the minimum anchorage group.The difference before and after treatment of the minimum anchorage group was statistically significant(P<0.05).The average change of eruption space before and after treatment of mandibular third molars were 0.29 in the maximum anchorage group,0.31 in the medium anchorage group,and 0.27 in the minimum anchorage group,Among them,the difference before and after treatment in the medium anchorage group was statistically significant(P<0.05).2.In the non-extraction patients without crowding,the average change of tilt angle of mandibular third molars before and after treatment was-13.00?in the non-elastics group,9.63?in the class elastics? group,and-9.83?in the class elastics? group.There was no statistically significant difference in tilt angle between the three groups before and after treatment(P>0.05).The average change of eruption space before and after treatment of mandibular third molars was non-elastics 0.12 in the non-elastics group,0.12 in the class ? elastics group,and 0.08 in the class elastics? group.The difference before and after treatment in the class ?elastics group was statistically significant(P<0.05).3.In the non-extraction patients with mild crowding,the average change of tilt angle of mandibular third molars before and after treatment was 8.25?in the non-elastics group and 0.02?in the class ?elastics group.However,the differences before and after treatment and between the two groups were not statistically significant(P>0.05).The average change of eruption space before and after treatment of mandibular third molars was 0.05 in the non-elastics group and 0.20 in the class elastics? group.The difference before and after treatment in the class elastics? group was statistically significant(P<0.05).4.Further upright impacted mandibular third molar and the increase of the eruption space can effectively reduce the difficulty score of extraction and increase the alveolar bone mass between the second molar and the third molar.Conclusions: After the completion of orthodontic treatment,the impaction status of the mandibular third molar can be significantly improved in tooth extraction patients with minimum and medium anchorage and tooth non-extraction patients with class ?elastics.It effectively reduces the difficulty of tooth extraction and facilitates the preservation of extraction sites of alveolar bone.Therefore,it is not necessary to removing the impacted mandibular third molars before orthodontic treatment for the patients with above-mentioned treatment plan.