口腔解剖生理学之国外下颌第三磨牙.ppt

DESCRIPTIVE ANATOMY OF MANDIBULAR THIRD MOLARProf. A. ElsahnInitial calcification8 – 10 yearsCompletion of crown 12 – 16 yearsEruption 17 – 21 yearsCompletion of roots18 – 25 yearsProf. A. ElsahnCHRONOLOGY¡The lower third molar is the last tooth in the dental arch and the eighth from the middle line. Prof. A. ElsahnGENERAL CHARACTERISTICS¡It is the shortest of the mandibular teeth.21534678¡It shows variations in size, form, number of cusps and number and form of roots.Prof. A. ElsahnGENERAL 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. ElsahnGENERAL CHARACTERISTICS¡In many cases, it is larger than the lower second molar.Prof. A. ElsahnGENERAL 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. ElsahnBUCCAL ASPECTThe distal outline is markedly convex.MD¡The crest of contour mesially and distally is located a little more than half the distance from cervical line to cusp tips.Prof. A. ElsahnBUCCAL ASPECTTHE CROWN¡The crown is wider at the contact area mesio-distally than at the cervix.DM¡In the four-cusp type; the molar is small and shows two buccal cusps only.Prof. A. ElsahnBUCCAL ASPECTTHE 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. ElsahnBUCCAL ASPECTTHE CROWN¡The buccal surface is very convex.THE ROOTS¡This molar usually show two roots, one mesial and one distal.Prof. A. ElsahnBUCCAL ASPECTMD¡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. ElsahnBUCCAL ASPECTTHE ROOTS¡These roots are usually shorter than those of first or second molars.111.6 ::¡The roots are more inclined distally than those of the first and second molars.Prof. A. ElsahnBUCCAL ASPECTTHE ROOTSMMDD¡The roots may be separated or fused for all or part of their length.Prof. A. ElsahnBUCCAL ASPECTTHE 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. ElsahnBUCCAL ASPECTTHE ROOTSTHE CROWN¡The proximal sides of the crown converge slightly lingually.Prof. A. ElsahnLINGUAL ASPECT¡The lingual surface is also very convex, but more than the buccal surface. Prof. A. ElsahnLINGUAL ASPECTTHE CROWN¡The lingual cusps are longer than the buccal cusps.¡The mesio-lingual is larger than the disto-lingual.DM¡From the mesial aspect, the tooth resembles the second molar except in dimensions and the more bulbous crown.Prof. A. ElsahnMESIAL 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. ElsahnMESIAL ASPECT¡The buccal cusp tips are closer to the lingual cusp tips than on first and second molars, giving a small occlusal table. LB¡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. ElsahnDISTAL ASPECTsecond 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. ElsahnOCCLUSAL 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. ElsahnOCCLUSAL ASPECTM DLB¡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. ElsahnOCCLUSAL 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. Elsahn544333222111¡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. ElsahnVARIATIONS。