Analysis And Research Of Third Molar Erupting Age, Impaction And Congenital Missing

There are few researches about the erupting age of third molars except some articles reported by professor Jiang Yuan-chuan about 50 years ago. A long period from dental germ to eruption, many kinds of eruption styles, congenital missing, difficulty to establish normal occlusion relationships may all be the reasons. So, a lot of researches abandoned the study of the third molars. Even though, the erupting age of third molars is important to oral clinic.A great change has taken place since the reform and opening of China. People's living condition has risen significantly. The constitution of food has changed too. Most families in 1980s have only one child, who lives in a better condition than 50 years ago. Will the changes make some differences in the eruption of third molars?The aim of this cross-sectional study was to investigate the erupting age of third molars in modern people. After analyzing the collected data, impaction, missing condition of the third molars and the relationship between the third molars and the development of malocclusion was studied, hoping to provide some reference data to the research on the third molars.The first stage was carried out on 3831students including 1859 males and 1972 females from middle schools and universities divided into 13 groups. According to works reported by professor Jiang Yuan-chuan, the eruption patterns of the third molars can be divided into first class eruption and eruption. Two qualified doctors examined students and recorded the eruption condition after consistency checking. The results showed that the mean age for the eruption of male maxillary third molars was 20.09±1.57 while 20.56±2.16 in the mandible; the mean age for the eruption of female maxillary third molars was 19.98±1.51 while 19.84±1.57 in mandible. Female third molars erupt earlier than male (P<0.05). All third molars has erupted before 30 except congenital missing, impaction and retarded eruption.In the second stage we collected the data of 800 orthodontic patients including 288 males and 512 females whose age ranging from 13 to 23 in our department from 2004 to 2006. We can see whether the third molar germs were congenital missing or impacted in panoramic radiographs. The impaction can be divided into vertical impaction, horizontal impaction, mesioangular impaction, distoangular impaction and bucco-linguoangular impaction according to the long axis relationships of the second and third molars. The results showed that the constituent ratio of impaction in orthodontic patients was 45.55% in mesioangular impaction, 4.85% in distoangular impaction, 34.71% in vertical impaction, 12.50% in horizontal impaction and 2.37% in bucco-linguoangular impaction. There were no significant differences in the incidence of impacted third molars in maxillary, mandible and bimaxillary inχ~2 test (P>0.05). The difference of the incidence of impaction between maxillary and mandible was significant (P<0.01), which mean there were more impacted third molars in mandible. There were no significant sexual dimorphism in the incidence of congenital missing third molars in maxillary, mandible and bimaxillary (P>0.05). The incidence of congenital missing third molars between maxillary and mandible was significant (P<0.01) which mean there were more congenital missing third molars in maxillary no matter in males ,females or all subjects.The third stage was to determine the congenital missing of the third molar germs in panoramic radiographs. All the data was classified into three groups according to Angle's classification and analyzed by statistic analysis. The result showed that the incidence of congenital missing third molars was 32.63% by the number of people and 18.72% by the number of tooth germs. There were no differences between left and right sides in both maxillary and mandible while the incidence of congenital missing third molars was much higher in maxillary than in mandible (P<0.05). There is no significant difference in the incidence of third molar missing among classificationⅠ,Ⅱ,Ⅲin maxillary or mandible (P>0.05).Conclusions:1. The third molars in modern people erupt at a late age and many of them can't erupt normally. That may be the result of masticating organs degeneration in human evolution.2. The incidence of third molar impaction and congenital missing is much higher nowadays which can explain that fewer and fewer third molars can erupt normally and be the evidence of degeneration of masticating organs.3. There is no significant difference in the incidence of third molar germs missing among malocclusion patients. So we can say that there is no direct association between the third molar congenital missing and malocclusion.