[医药卫生]口腔解剖生理学之国外ppt--下颌第二磨牙

DESCRIPTIVE ANATOMY OF MANDIBULAR SECOND MOLARProf. A. ElsahnInitial calcification 2 3 yearsCompletion of crown 7 8 yearsEruption 11 13 yearsCompletion of roots 14 15 yearsProf. A. ElsahnCHRONOLOGY Its disto-buccal cusp is larger than that found in the lower first molar. Prof. A. ElsahnGENERAL CHARACTERISTICS This molar is the seventh tooth from the middle line. The crown is usually smaller than the first molar in the same mouth. o It has four cusps.1 234567DDML LBB4321 The cervical ridge is less pronounced than on the first molars in most instances, but a prominent bulge cervically on the mesio-buccal cusp is common. Prof. A. ElsahnGENERAL CHARACTERISTICS There is only one buccal groove.BD ML The roots are as long (or may be longer), less broad bucco-lingually than on the first molar and are less widely separated or more parallel. Prof. A. ElsahnGENERAL CHARACTERISTICS The root trunk is slightly longer than on the first molar.BTHE CROWN The crown appears to be wider at the cervix than the first molar (less taper toward the cervix below the contact areas), due to absence of a distal cusp. Prof. A. ElsahnBUCCAL ASPECT As in the first molar,the crown is much larger mesiodistally than cervico-occlusally.CCO ODD MMesial outline:Is nearly straight from the cervical line to the mesial contact area. Distal outline:Is convex from the cervical line to the distal contact area.Prof. A. ElsahnBUCCAL ASPECTTHE CROWNM DCONTACT AREASMESIAL Is at about the junction of the middle and occlusal thirds.DISTAL Is at the center of the middle third.Prof. A. ElsahnBUCCAL ASPECTTHE CROWN Both contact areas are at slightly lower level, especially in the distal area than those of the first molar.Prof. A. ElsahnBUCCAL ASPECTTHE CROWNOCCLUSAL OUTLINE The two buccal cusps (mesio-buccal and disto-buccal) and the tips of the two lingual cusps (mesio-lingual and disto-lingual) are visible from the buccal side because the lingual cusps are slightly longer. Prof. A. ElsahnBUCCAL ASPECTTHE CROWNDM The buccal developmental groove separates the two buccal cusps, and it may end in a pit on the middle of the buccal surface. The two buccal cusps are about equal mesio-distally but the mesio-buccal is usually wider.Prof. A. ElsahnBUCCAL ASPECTTHE CROWNM DCERVICAL LINEIt may point sharply toward the root bifurcation. Prof. A. ElsahnBUCCAL ASPECTTHE CROWNIt is nearly straight.THE ROOTS There is a depression on the root trunk from the cervical line to the bifurcation.Prof. A. ElsahnBUCCAL ASPECT The root trunk is short (but slightly longer than on lower first molar). The roots are usually closed together and nearly parallel.Prof. A. ElsahnBUCCAL ASPECTTHE ROOTS The roots (mesial and distal roots) vary in their length. They may be as long or shorter than those of the first molar. The roots may spread as much as those of the first molar or fused for all or part of their length.Prof. A. ElsahnBUCCAL ASPECTTHE ROOTS The roots taper apically and are more pointed than those of the first molar. Both roots are flattened mesio-distally and the mesial root is slightly longer than the distal. Prof. A. ElsahnBUCCAL ASPECTTHE ROOTS Both roots usually are inclined distally more than those of the first molar. DM Therefore, the cervix of the tooth is wider mesio-distally than that of the first molar. Prof. A. ElsahnLINGUAL ASPECTThe crown The crown is slightly narrower lingually than buccally due to less lingual convergence. M D The mesial and distal curvatures on the crown that describe the contact areas are more noticeable from the lingual aspect.Prof. A. ElsahnLINGUAL ASPECTThe crown Only the lingual cusps are visible. The lingual cusps are more pointed than the buccal cusps. The mesiolingual cusps is slightly wider and longer than the distolingual.LINGUAL ASPECTThe crown MDProf. A. Elsahn The cervical line is irregular mesio-distally, sometimes dropping cervically on the root bifurcation. Prof. A. ElsahnLINGUAL ASPECTThe crown The lingual developmental groove separates the two lingual cusps. It is a short groove, usually end in the occlusal third and is never pitted.THE ROOTS The proximal surfaces of the roots converge lingually to a slight degree, therefore, little of the mesial or distal surfaces may be seen lingually. Prof. A. ElsahnLINGUAL ASPECT The short root trunk has a depression between the cervical line and the bifurcation.THE CROWN Geometrically, the crown is similar to the first molar; rhomboidal in shape with lingual inclination, short cervico-occlusally, and wide bucco-lingually. Prof. A. ElsahnMESIAL ASPECT OCBL Buccally, the cervical ridge is in most cases less pronounced although many of them show a prominent bulge cervically on the mesio-buccal cusp only. Prof. A. ElsahnMESIAL ASPECTTHE CROWN Occlusal to the cervical ridge, the buccal outline slants lingually.L B Lingually, the crest of curvature of the crown is about in the center of the crown and is lingual to the root due to the lingual inclination of the crown. Prof. A. ElsahnMESIAL ASPECTTHE CROWN L B The tip of the mesio-lingual cusp is usually in line with the lingual outline of the root (due to lingual tilting). Prof. A. ElsahnMESIAL ASPECTTHE CROWNoOcclusally, the mesio-lingual cusp (which is the longest of the cusps) is slightly longer than the mesio-buccal cusp.L BThe mesial marginal ridge is sharply concave (often V-shaped), and is not always crossed by a marginal ridge groove.The occlusal surface is usually more constricted bucco-lingually.MESIAL ASPECTTHE CROWNProf. A. ElsahnL B The mesial contact area is at about the junction of the occlusal and middle thirds cervico-occlusally and near the junction of the buccal and middle third bucco-lingually.Prof. A. ElsahnMESIAL ASPECTTHE CROWN L BOC It runs occlusally from buccal to lingual in stra