Dental Anatomy 15-.pdf

Dental Anatomy Dr.Anas Hussein Lec:15 ‫ـــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــ...

Dental Anatomy Dr.Anas Hussein Lec:15 ‫ـــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــ‬ The Permanent Maxillary Molars Introduction: Permanent Molars: They are the largest, and most posteriorly located teeth in the mouth. They erupt distal to the deciduous dentition, and therefore are not succedaneous. In a normal eruption sequence for permanent teeth, molars are the initial, and the final teeth to emerge. There are 3 permanent molars per quadrant, 6 per arch, and 12 in total. From mesial to distal in each quadrant, they are named first, Second, and third molars. The first and second molars are also called six year molars, and twelve year molars, respectively, referring to their approximate time of eruption. The third molar, also known as the wisdom tooth, which is variable in its time of eruption. Third molars exhibit a wide range of crown shapes, sizes, contours, and root numbers and forms. Similar to the permanent canines which are cornerstones of each arch in position, form, and function, the permanent first molars are the cornerstones in the development of occlusion. This is due to their early eruption date, and location in the arch.  The molars' function in mastication is mainly grinding. They also function in esthetics and phonetics, but less than the teeth anterior to them. Their contribution to esthetics through muscle support and maintenance of the vertical dimension. Permanent Maxillary Molars They are the largest teeth in the maxillary arch. With shorter occlusogingival crown comparing to the teeth anterior to them, but much larger in all other dimensions. The first molar is the largest in size, while the second and third molars are smaller. The maxillary molars are differentiated from other permanent teeth, particularly mandibular molars in 1. Crowns are wider buccolingually than mesiodistally. Mandibular molars are wider in the mesiodistal dimension. 2. Presence of four cusps in most specimens, with the size of the two lingual cusps differs greatly. Some mandibular molars display four cusps, but the two lingual cusps are approximately equal in size. 3. The presence of an oblique ridge and a distolingual groove on the occlusal surface. Those structures are not found on the mandibular molars. 4. Rhomboidal or heart-shaped crowns from the occlusal aspect comparing with a rectangular or pentagonal outline in the mandibular molars. 5. Trapezoidal crowns from the mesial or distal aspect compared with rhomboidal and inclined to the lingual in a proximal view in the mandibular molars. 6. The presence of three root branches in most cases. Mandibular molars normally exhibit two roots. Permanent Maxillary First Molar General characteristics: Arch position - is the sixth tooth from the midline in each maxillary quadrant. It has a mesial contact with the second deciduous molar, until that tooth is exfoliated. The mesial contact is then shared with the permanent second premolar when it erupts at 12 years. There is no distal contact until the permanent second molar erupts at 12 years. Development Table: (Maxillary First Molar)* Initiation of calcification.................................... at birth Completion of enamel................................. 3 to 4 years Eruption....................................................... 6 to 7 years Completion of root.................................... 9 to 10 years. Buccal aspect: 1. General considerations – the crown is trapezoidal, with the longer parallel side at the occlusal. The buccal surface is larger than of the premolars, while it is shorter in occlusogingival dimension. Because of the rhomboidal crown form, a portion of the distal surface is visible from this aspect. Also the tips of all the major cusps are visible, except the distolingual cusp tip. 2. Mesial outline - The mesial outline is flat from the cervical margin occlusally to the contact area (at the junction of the occlusal and middle thirds). Occlusally from the contact area, the mesial margin is convex, and joins the occlusal margin in a slightly rounded, but well defined mesio-occlusal angle. 3. Distal outline - The entire distal outline is convex occlusogingivally, with the contact area in the middle third. The disto-occlusal angle is more rounded than the mesio-occlusal angle. 4. Cervical outline - The cervical line is slightly and irregularly curved apically with much less curvature comparing with anterior teeth or premolars. 5. Occlusal outline - The buccal groove is dividing the occlusal margin into two parts (buccal cusps which are the mesiobuccal cusp and the distobuccal cusp). The outline of the mesiobuccal cusp is wider, but the distobuccal cusp tip is sharper. The two buccal cusps are approximately the same height, and the mesiolingual cusp tip is visible between them. 6. Other considerations – the buccal groove occupies a shallow occlusogingival concavity, which ends in a buccal pit, or terminate by splitting into two grooves, which extend a short distance before fading out. 7. The three roots are visible and inclined distally, the lingual root is the longest and the two buccal roots are nearly equal in length. 8. The height of contour of the buccal surface is located in the cervical third. Lingual aspect: 1. General considerations – The lingual surface is trapezoidal with approximately the same mesiodistal width of the buccal surface. With more general convexity occlusogingivally than the buccal surface. 2. Mesial outline - The mesial outline is similar to the buccal aspect. 3. Distal outline - The distal margin is also similar to the buccal aspect, except it is shorter and the disto-occlusal angle is more rounded, since the DL cusp is much smaller than the DB cusp. 4. Occlusal margin - As on the buccal surface, the distolingual groove separates the occlusal margin into two unequal portions. The mesiolingual cusp which is longer and larger, but blunter than the distolingual cusp. The mesiolingual cusp is the largest and longest cusp on this tooth. 5. Other considerations - The distolingual groove originates on the occlusal surface, and crosses onto the lingual surface that terminates in a lingual pit. 6. On the lingual portion of the mesiolingual cusp is a tubercle or minicusp known as the cusp of Carabelli. A groove that separates the cusp of Carabelli from the mesiolingual cusp, is named the cusp of Carabelli groove. 7. The height of contour is located in the middle third of the lingual surface. Mesial aspect: 1. General considerations - The mesial surface exhibits a roughly trapezoidal shape that is wider at the cervical than at the occlusal, which is the reverse of the buccal and lingual surfaces. 2. Buccal margin - from the cervical line, the buccal outline is convex in the cervical third, then it is flat to slightly concave for a short distance in the middle third. From this point to the cusp tip, the outline is straight, or slightly convex. The height of contour is in the gingival third. 3. Lingual margin - The lingual outline is convex throughout its length, but may be irregular if the cusp of Carabelli is prominent. The height of contour is located in the middle third. 4. Cervical margin - The cervical line is shallow, and irregularly curved toward the occlusal. 5. Occlusal margin - The only cusps which are visible are the two mesial cusps. The outline of the mesial marginal ridge curves irregularly toward the cervical line. There is normally a mesial marginal groove notching the marginal ridge outline about midway along its length. 6. Other considerations -The mesial surface is wider at the cervical than at the occlusal, due to the general convergence of both the buccal and lingual surfaces toward the occlusal. 7. The contact area varies from round to somewhat ovoid, and is situated slightly to the buccal, at the junction of the occlusal and middle thirds. The occlusal half of the surface is convex, but there is usually a buccolingual flattening, or even a slight concavity, located cervical to the contact area. Distal aspect: 1. General considerations - the distal resembles the mesial surface, but with slightly smaller dimensions. The trapezoidal outline is not as pronounced either, because on the distal surface the cervical width is more nearly equal to the occlusal width. 2. Occlusal outline - The mesial cusp tips are visible projecting beyond the outline of the distal cusps. The distal marginal ridge is less prominent and dips farther cervically than on the mesial, thus allowing more of the occlusal surface to be seen. There is usually a distal marginal groove situated about midway along its extent. 3. Cervical outline - The cervical line reveals very little curvature occlusally, and may approach a straight line in some specimens. 4. Other considerations - Because of the crown's rhomboidal shape, much of the buccal surface can be seen from the distal. 5. The distal contact area is larger than the mesial contact area, and irregularly long buccolingually, and narrow occlusogingivally. It is located in the middle third, about midway between the buccal and lingual margins. 6. There may be a slight flattening or concavity in the cervical third, but when present it is never as pronounced as on the mesial