口腔解剖生理学之国外下颌第三磨牙

,Click to edit Master title style,Click to edit Master text styles,Second level,Third level,Fourth level,Fifth level,*,单击此处编辑母版标题样式,单击此处编辑母版文本样式,第二级,第三级,第四级,第五级,*,*,单击此处编辑母版标题样式,单击此处编辑母版文本样式,第二级,第三级,第四级,第五级,*,*,口腔解剖生理学之国外下颌第三磨牙,Initial calcification,8 10 years,Completion of crown,12 16 years,Eruption,17 21 years,Completion of roots,18 25 years,Prof. A. Elsahn,CHRONOLOGY,The lower third molar is the last tooth in the dental arch and the eighth from the middle line.,Prof. A. Elsahn,GENERAL CHARACTERISTICS,It is the shortest of the mandibular teeth.,2,1,5,3,4,6,7,8,It shows variations in size, form, number of cusps and number and form of roots.,Prof. A. Elsahn,GENERAL CHARACTERISTICS,This tooth is a very variable tooth, but,less,so than upper third molar.,The crown of this tooth in most cases,resembles that of the lower second,molars,in the number of cusps (four cusps) and sometimes,resembles the crown of the lower first molar,(five cusps) or it may bear little resemblance to either.,Prof. A. Elsahn,GENERAL CHARACTERISTICS,In many cases, it is larger than the lower second molar.,Prof. A. Elsahn,GENERAL CHARACTERISTICS,The third molar may be small or large.,THE CROWN,The mesial and distal outlines of the crown are generally as those of all lower molars.,Prof. A. Elsahn,BUCCAL ASPECT,The distal outline is markedly convex.,M,D,The crest of contour,mesially and distally is located a little more than half the distance from cervical line to cusp tips.,Prof. A. Elsahn,BUCCAL ASPECT,THE CROWN,The crown,is wider at the contact area,mesio-distally,than at the cervix.,D,M,In the four-cusp type; the molar is small and shows two buccal cusps only.,Prof. A. Elsahn,BUCCAL ASPECT,THE CROWN,The buccal cusps are short and rounded.,The,mesio-buccal,cusp is the,widest,and usually,highest,of the two or three buccal cusps.,Prof. A. Elsahn,BUCCAL ASPECT,THE CROWN,The buccal surface is,very convex,.,THE ROOTS,This molar usually show two roots, one mesial and one distal.,Prof. A. Elsahn,BUCCAL ASPECT,M,D,The root to crown ratio is smaller than that of first and second molar (1.6 to 1 compared to 1.8 to 1).,Prof. A. Elsahn,BUCCAL ASPECT,THE ROOTS,These roots are usually shorter than those of first or second molars.,1,1,1.6,:,:,The roots,are,more inclined,distally,than those of the first and second molars.,Prof. A. Elsahn,BUCCAL ASPECT,THE ROOTS,M,M,D,D,The roots may be separated or fused for all or part of their length.,Prof. A. Elsahn,BUCCAL ASPECT,THE ROOTS,If the roots are separated, the root trunk is long and the roots are usually more pointed at the apex than on other molars.,Prof. A. Elsahn,BUCCAL ASPECT,THE ROOTS,THE CROWN,The proximal sides of the crown,converge,slightly lingually.,Prof. A. Elsahn,LINGUAL ASPECT,The lingual surface,is also,very convex, but more than the buccal surface.,Prof. A. Elsahn,LINGUAL ASPECT,THE CROWN,The lingual cusps,are,longer,than the buccal cusps.,The mesio-lingual,is,larger than,the disto-lingual.,D,M,From the mesial aspect, the tooth resembles the second molar,except in dimensions,and the,more bulbous,crown.,Prof. A. Elsahn,MESIAL ASPECT,The mesial root,is tapering more from cervix to apex and its apex is usually more pointed than that of the second molar.,Prof. A. Elsahn,MESIAL ASPECT,The buccal cusp tips,are closer to the lingual cusp tips than on first and second molars, giving a small occlusal table.,L,B,From the aspect, the tooth is also similar to that of second molar,except for size,absence of the distal contact area,and the more pointed apex of the distal root.,Prof. A. Elsahn,DISTAL ASPECT,second molar,The occlusal aspect is similar to that of the second or first mandibular molar according to the type of the third molar (four-cusp type or five-cusp type).,Prof. A. Elsahn,OCCLUSAL ASPECT,The occlusal outline,is,rectangular,or,oval,mesio-distally but,more rounded,than that of lower first or second molars and the distal side is much smaller bucco-lingually.,Prof. A. Elsahn,OCCLUSAL ASPECT,M,D,L,B,The occlusal surface has,irregular groove,pattern with,numerous supplemental grooves,and,pits,that produce a,wrinkled appearance, which is a constant finding.,Prof. A. Elsahn,OCCLUSAL ASPECT,VARIATIONS,It is,very variable,tooth but less so than upper third molar.,Crown may show three, four, five or multi-cusped variations.,Most of the,anomalies,have large crown portion than normal.,The roots,of these,oversize,third molars may be short and poorly developed.,In contrary, most of the,anomalies,of upper third molar are,undersized,.,Prof. A. Elsahn,5,4,4,3,3,3,2,2,2,1,1,1,The occlusal surface may be very,irregular,with many,supplemental grooves,.,The,underdeveloped,and excessively distally curved roots indicate that the tooth is a lower third molar.,This tooth may have one or more,extra roots,(multi-rooted forms).,Prof. A. Elsahn,VARIATIONS,人有了知识，就会具备各种分析能力，,明辨是非的能力。,所以我们要勤恳读书，广泛阅读，,古人说“书中自有黄金屋。,”通过阅读科技书籍，我们能丰富知识，,培养逻辑思维能力；,通过阅读文学作品，我们能提高文学鉴赏水平，,培养文学情趣；,通过阅读报刊，我们能增长见识，扩大自己的知识面。,有许多书籍还能培养我们的道德情操，,给我们巨大的精神力量，,鼓舞我们前进,。,