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CHAPTER 5Morphology of Permanent Molars

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CHAPTER 5Morphology of Permanent Molars CHAPTER Morphology of Permanent Molars Topics5 covered within the four sections of this chapter B. Type traits of maxillary molars from the lingual include the following: view I. Overview of molars C. Type traits of maxillary molars from the A. General description of molars proximal views B. Functions of molars D. Type traits of maxillary molars from the C. Class traits for molars occlusal view D. Arch traits that differentiate maxillary from IV. Maxillary and mandibular third molar type traits mandibular molars A. Type traits of all third molars (different from II. Type traits that differentiate mandibular second first and second molars) molars from mandibular first molars B. Size and shape of third molars A. Type traits of mandibular molars from the buc- C. Similarities and differences of third molar cal view crowns compared with first and second molars B. Type traits of mandibular molars from the in the same arch lingual view D. Similarities and differences of third molar roots C. Type traits of mandibular molars from the compared with first and second molars in the proximal views same arch D. Type traits of mandibular molars from the V. Interesting variations and ethnic differences in occlusal view molars III. Type traits that differentiate maxillary second molars from maxillary first molars A. Type traits of the maxillary first and second molars from the buccal view hroughout this chapter, “Appendix” followed Also, remember that statistics obtained from by a number and letter (e.g., Appendix 7a) is Dr. Woelfel’s original research on teeth have been used used within the text to denote reference to to draw conclusions throughout this chapter and are the page (number 7) and item (letter a) being referenced with superscript letters like this (dataA) that Treferred to on that appendix page. You can tear out the refer to data presented at the end of this chapter. perforated Appendix pages to facilitate study and mini- mize page turns. Other appendix pages will be referred to throughout this chapter. SECTION I OVERVIEW OF MOLARS OBJECTIVES This section is designed to prepare the learner to • From a selection of all teeth, select and separate perform the following: out all molars. • Describe the functions of molars. • Divide a selection of all molars into maxillary and • List class traits common to all molars. mandibular. • List arch traits that can be used to distinguish max- illary from mandibular molars. 120 Scheid_Chap05.indd 120 9/6/2010 7:52:34 PM Chapter 5 | Morphology of Permanent Molars 121 A. GENERAL DESCRIPTION In the adult dentition, first molars are distal to OF MOLARS second premolars. The permanent first molars are located near the center of each arch, anteroposteriorly. Study a cast of all permanent teeth, or Figure 5-1, while This is one reason that their loss is so devastating to learning the position of molars in the arch. There are 12 arch continuity (allowing movement and tipping of the permanent molars: six maxillary and six mandibular. teeth on either side). They are the largest and strongest The six permanent molars in each arch are the first, sec- teeth in each arch. The second molars are distal to the ond, and third molars on either side of the arch. They first molars, and the third molars are distal to the sec- are the sixth, seventh, and eighth teeth from the mid- ond molars. Said another way, in the complete adult line. Using the Universal Numbering System, the max- dentition, the mesial surface of the first molar contacts illary molars are Numbers 1, 2, and 3 for the right third, the distal surface of the second premolar, the mesial second, and first molars and Numbers 14, 15, and 16 surface of the second molar contacts the distal of the for the left first, second, and third molars, respectively. first molar, and the mesial surface of the third molar The mandibular molars are Numbers 17, 18, and 19 for contacts the distal of the second molar. The third molar the left third, second, and first molars and Numbers 30, is the last tooth in the arch, and its distal surface is not 31, and 32 for the right first, second, and third molars, in contact with any other tooth. respectively. PERMANENT TEETH MAXILLARY Anterior teeth Incisors Canine Canine C e rs ntr iso al inc Lateral incisors P Premolars Premolars o s h t t Canines e e r i e o t r r 1st Premolars t o e i r e e t t 2nd Premolars h s 3 14 o P 1st Molars Molars Molars 2 15 2nd Molars 1 3rd Molars 16 RIGHT LEFT 3rd Molars 32 17 2nd Molars Molars 31 18 Molars 1st Molars P o 2nd Premolars s 30 19 t e r 1st Premolars h i t o e r e t Canines t e e Lateral r t io h incisors r Premolars te Premolars s o P Canine Central incisors Canine Incisors FIGURE 5-1. Adult dentition with Anterior teeth the Universal numbers for molars MANDIBULAR highlighted in red. Scheid_Chap05.indd 121 9/6/2010 7:52:37 PM 122 Part 1 | Comparative Tooth Anatomy B. FUNCTIONS OF MOLARS 3. TAPER TO THE DISTAL FOR MOST MOLARS The permanent molars, like the premolars, (a) play a major role in the mastication of food (chewing and For both arches, molar crowns from the occlusal view grinding to pulverize) and (b) are most important in tend to taper distally, so that the distal third is narrower maintaining the vertical dimension of the face (prevent- buccolingually than the mesial third (Appendix 7c). ing the jaws from closing too far, which could reduce This taper may be less apparent on mandibular first the vertical dimension between the chin and the nose, molars where the tooth may be widest buccolingually resulting in a protruding chin and a prematurely aged in the middle third. Also, from the buccal (or lingual) appearance). They are also (c) important in maintain- views, the occlusal surfaces of all molars slope shorter ing continuity within the dental arches, thus keeping toward the cervix from mesial to distal (Appendix other teeth in proper alignment. Further, molars have 7d). This, along with the more cervical placement of (d) at least a minor role in esthetics or keeping the the distal marginal ridge, makes more of the occlusal cheeks normally full or supported. You may have seen surface visible from the distal aspect than from the someone who has lost all 12 molars (six upper and six mesial aspect (compare mesial to distal views in lower) and has sunken cheeks. Appendix page 7). The loss of a first molar is really noticed and missed by most people when it has been extracted. More than 4. HEIGHT (CREST) OF CONTOUR 80 mm2 of efficient chewing surface is gone; the tongue FOR ALL MOLARS feels the huge space between the remaining teeth; As with premolars, the height of contour on the buccal and during mastication of coarse or brittle foods, the of molars viewed from the proximal is in the cervical attached gingiva in the region of the missing molar third; on the lingual, it is most often in the middle third often becomes abraded and uncomfortable. Loss of six (Appendix 7e). or more molars could even lead to problems in the jaw joints (temporomandibular joints or TMJ). 5. CONTACT AREAS FOR ALL MOLARS C. CLASS TRAITS FOR MOLARS The contact areas on all molars viewed from the buccal Refer to Appendix page 7 while reading about the fol- (or lingual) are at or near the junction of the occlusal lowing class traits of all molars. and middle thirds mesially and are more cervical on the distal (in the middle third near the middle of the tooth) 1. CROWN SIZE FOR ALL MOLARS (Appendix 7f). Molars have an occlusal (chewing) surface with three to five cusps, and their occlusal surfaces are larger than D. ARCH TRAITS THAT DIFFERENTIATE the other teeth in their respective arches. They have MAXILLARY FROM MANDIBULAR broader occlusal surfaces than premolars, both facio- MOLARS lingually and mesiodistally.A The combined mesiodistal Compare extracted maxillary and mandibular molars width of the three mandibular molars in one quadrant and/or tooth models while reading about these dif- makes up over half of the mesiodistal dimension of ferentiating arch traits. Also refer to page 8 in the their quadrant. The maxillary molars constitute 44% of Appendix. their quadrant’s mesiodistal dimension, still a signifi- cant portion. In contrast, both mandibular and max- illary molar crowns are shorter occlusocervically than 1. CROWN OUTLINE TO DISTINGUISH all other adult crowns and are shorter occlusocervically MAXILLARY FROM MANDIBULAR MOLARS than mesiodistally (Appendix 7a). From the occlusal view, the crowns of mandibular molars are oblong: they are characteristically much 2. TAPER FROM BUCCAL TO LINGUAL wider mesiodistally than faciolingually.B This is just FOR MOST MOLARS the opposite of the maxillary molars, which have their From the occlusal view, molar crowns taper (get nar- greater dimension faciolingually. From the occlusal rower) from the buccal to the lingual. That is, the mesi- view, maxillary molars have a more square or twisted odistal width on the buccal half is wider than on the parallelogram shape. Mandibular molars have a some- lingual half (Appendix 7b), EXCEPT on maxillary first what rectangular shape, and on mandibular first molars, molars with large distolingual cusps, where crowns the outline may be pentagon shape (compare the outline actually taper narrower from lingual toward the buccal. shapes on Appendix 8k). Scheid_Chap05.indd 122 9/6/2010 7:52:38 PM Chapter 5 | Morphology of Permanent Molars 123 Table 5-1 MOLARS: GUIDELINES FOR DETERMINING NUMBER OF LOBES FOR MOLARS MOLAR NAME NO.
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