Study Of Third Molar's Anatomy

The third molars are the teeth which develop and erupt last in the human permanent teeth,usually after 18-year old.When they were erupting,the impaction often happen because of the lack of space.The impacted third molars and surrounding tissues were hard to clear,so caries, pericoronitis and other diseases often appeared. However, there are a certain proportion of third molars which could erupt normally and exercise the masticatory function.In recent years, more third molars are being preserved and playing more roles because of the development of prosthetic and transplantation. Whether removal or retention, dentists need to understand the anatomy and the law of eruption of the third molars.This study is carried on according to above purposes.Part one Literature review about molars'anatomy (2000~2009)Materials and methods Chinese literatures in CNKI database and English or Chinese literatures in Medline database of the past 10 years (2000.1.1~2009.12.31) about the molars'anatomy were searched .The subjects and methods in literatures were classified and analyzed.Results One hundred and thirty three literatures were included, including 66 Chinese articale and 67 English articles. (1) The numbers of articles containing the study on maxillary first, second and third molar were 56, 36 and 18 respectively. The numbers of articles containing the study on mandibular first, second and third molar were 43, 53 and 30 respectively. (2) Sample sizes of most molar were less than 200.(3) Methods used in the study of the external morphology in vitro contain naked eye observation, measurement, magnification observation,radiological methods and apical staining,and the number of articles were 50,17,7,4 and 2 respectively.Methods used in the study of the internal morphology in vitro contain cleared teeth,radiological methods,cross-section,chamber observation and measurement and endodontic probe, and the number of articles were 42,27,20,14 and 7 respectively.Conclusion (1)The literatures about the anatomy of third molars was less than the first and second molars.(2) Sample sizes of most molar were less than 200.(3) The main methods to study the molars'anatomy in vitro are naked eye observation and cleared teeth.Part two The survey on eruption situation of third molarsMaterials and methods Digital panoramic radiographs were screened from those shot in the department of radiology of affiliated stomatological hospital of Nanjing medical university from 2008.9.1 to 2009.8.31. Radiographs with clear image and four third molars were included. Those were excluded as following:(1)third molars were microdonts(2) apical foramens of third molars were not closed (3) second or third molars had large defects(4)second molars were missing.Six hundreds digital panoramic radiographs were randomly selected from those complying with above standards.The eruption status, impacted direction of third molars were judged and recorded.The relationship between third molars'eruption status was analyzed . Results (1) Proportion of impaction The proportion of impacted maxillary and mandibular third molars is 19.58%(235/1200) and 59.25%(711/1200).The proportion of impacted mandibular third molars was higher than maxillary(χ2=39.67,p =0.000).(2) Impacted direction The numbers of vertical,horizontal,mesial,distal and other impaction in maxillary third were 159(67.66%), 8(3.40%), 30(12.77%), 29(12.34%) and 9(3.83%) respectively.The numbers of vertical,horizontal,mesial and other impaction in mandibular third were 222(31.22%), 230(32.35%), 248(34.88%) and 11(1.55%). (3) Symmetry Maxillary third molars(18 and 28) in 13.50%(81/600) radiographs were symmetrical impaction, 74.33%(446/600) were symmetrical non-impaction and 12.17%(73/600) were asymmetric.Mandibular third molars(38 and 48) in 52.67%(313/600) radiographs were symmetrical impaction,33.66%(202/600) were symmetrical non-impaction and 14.17%(85/600) were asymmetric.(4)Correspondence Left third molars(28 and 38) in 17.17%(103/600) radiographs were corresponding impaction, 39.83%(239/600) were corresponding non-impaction and 43.00%(258/600) were non-corresponding.Right third molars(18 and 48) in 17.00%(102/600) radiographs were corresponding impaction,. 36.67%(220/600) were corresponding non-impaction and 46.33%(278/600) were non- corresponding.Conclusion (1) The proportion of impacted mandibular third molars was higher than maxillary (2) Vertical impacted and mesial impacted were the most common type in maxillary and mandibular third molars .(3) In more than 85% radiographs,third molars in the same jaw had similar eruption and showed strong symmetry.(4) In more than 50% radiographs, third molars in the same side had similar eruption and showed some correspondence. Part three Observation and measurement of third molars'external morphologyMaterials and methods Third molars in vitro were collected in oral and maxillofacial surgery clinic of affiliated stomatology hospital of Nanjing medical university from 2008.6.4 to 2008.12.31. All integrated third molars extracted were included and those which were microdonts and with significant defects or filling materials were excluded.The external anatomy of four hundreds third molars(maxillary 206, mandibular 194) were observed and measured.The numbers of cusps and roots, curvature of the roots ,curvature direction and data such as the overall tooth length, crown height were recorded. Root length and crown-root ratio were computed based on measued data. Differences of external anatomy between genders were analyzed initially.Results (1) Cusp type In 206 maxillary third molars, the numbers of 2-cusps, 3-cusps, 4-cusps and multi-cusps were 32(15.53%), 95(46.12%), 62(30.10%) and 17(8.25%) respectively. In 194 mandibular third molars, the numbers of 4-cusps,5-cusps and multi-cusps were 49(25.26%),113(58.25%) and 32(16.49%) respectively.(2) Root type In maxillary third molar, the numbers of single-root, 2-roots, 3-roots and 4-roots were 125(60.68%),55(26.70%),24(11.65%) and 2(0.97%) respectively.In mandibular third molars, the numbers of single-root, 2-roots and 3-roots were 114(58.76%),73(37.63%)and 7(3.61%) respectively.(3) Root curvation In 315 maxillary roots, 118(37.46%) were not curved, 197(62.54%) were curved and most(61.42%) curved distally. In 281 mandibular roots, 128(45.55%) were not curved, 153(54.45%)were curved and most(58.17%) curved distally.(4) Measurement: The means of maxillary third molars'overall tooth length, crown length, mesiodistal crown diameter, buccolingual crown diameter, mesiodistal cervical diameter, and buccolingual cervical diameter were 17.60mm, 7.08mm, 8.94mm, 11.02mm, 7.16mm and 10.36mm respectively. The means of mandibular third molars'were 17.38mm, 6.91mm, 10.89mm, 10.27mm, 9.25mm and 9.11mm respectively. Root length and crown-root ratio could be computed based on above data.(5) Differences between Genders Crown length of both maxillary and mandibular third molars belong to male was longer than female(maxillary t=3.298,p=0.001;mandibular t=2.948, p=0.004). The proportions of root-type of mandibular third molars (χ2 =8.487,p =0.014) had significantly statistical difference between genders and the mean root number of males (1.60) was more than females(1.36).Other analyzed data had no significantly statistical difference.Conclusions (1) Three-cusps maxillary third molars and 5-cusps mandibular third molars were most common.(2) Single-root was most common in maxillary and mandibular third molars.(3) Most roots of third molars were curved and curved distally.(4) Data measured by vernier caliper could be used to compute other data about tooth's external anatomy.(5) Crown length of third molars belong to males were longer than females and mean root number was more than females.Part four Study of third molars'root canal systemMaterials and methods The samples were the four hundreds third molars from the part 3.The pulp access was prepared from occlusal surface and residual pulp was cleared.Then the pulp chambers were stained with carbon ink.After ink solidified,the samples were immersed in 5% hydrochloric acid for decalcification.After full decalcification,80%,90% and 100% ethanol solution were used for series dehydration. Finally,the samples were immersed in 99% methyl salicylate to become cleared teeth. The number of root canals, root canal type and occurrence of accessory canals were observed and judged. Data were processed using statistical analysis software and gender differences in the internal anatomy were analyzed initially.Result Because of excessive staining, incompelete decalcification and other reasons, some cleared teeth's internal anatomy could not be observed clearly.A total of 384 (maxillary 193, mandibular 191) cleared teeth were studied excluding samples mentioned above.(1) The number of root canals In maxillary third molars, 42 (21.76%) had single root canal, 59(30.57%)had 2 root canals, 74(38.34%) had 3 root canals, and 18(9.33%) had 4 root canals; In mandibular third molars, 12(6.28%) had single root canal, 126(65.97%) had 2 root canals, 45(23.56%) had 3 root canals, 7(3.66%) had 4 root canals and 1(0.53%) had 5 root canals. (2) Root canal type In 454 canals of maxillary third molars, the canal numbers of type 1-1 ,type1-2,type 2-1 and type others were 363(79.96%), 30(6.61%), 44(9.69%) and 17(3.74%) respectively; In 432 canals of mandibular third molars, the canal numbers of type 1-1 ,type1-2,type 2-1 and type others were 335(77.55%), 24(5.56%), 54(12.50%) and 19(4.39%) respectively.(3)Accessory canals In maxillary third molars, 61(31.61%) had accessory canals. In mandibular third molars, 43(22.51%) had accessory canals. (4) Gender differences The proportions of root canal numbers and prevalence of various accessory canals had no significantly statistical differences between genders.Conclusion Root canal anatomy of third molars had following characteristics: (1)The maxillary third molars usually had 3 root canals and the mandibular third molars usually had 2 root canals .(2) The root canals of third molars were usually type 1-1. (3) Accessory canals emerged in both maxillary and mandibular third molars. (4) The root canal anatomy studied had no significantly statistical differences between genders.