| SECTION VI | MAXILLARY AND MANDIBULAR THIRD MOLAR TYPE TRAITS |
OBJECTIVES
After studying this section, the reader should be able to perform the following:
- List the type traits that are unique to all third molars that can be used to distinguish them from first or second molars.
- From a selection of all types of molars, select the mandibular and maxillary third molars and assign each a Universal number.
- In mouths (or casts) of mandibular and maxillary arches with only one or two molars per quadrant, identify which molars are present and which are absent based on crown anatomy and position in the arch. (Remember that molars can change positions in the arch if they drift forward after the extraction of a first or second molar or are moved during orthodontic treatment, so arch position should not be the only way to confirm which molars are present.)
A. NUMBER AND LOCATION OF THIRD MOLARS
Most often, there are four third molars in the mouth, one at the distal position in each quadrant. However, nearly one fifth of the population may have one or more of their third molars congenitally missing (they never developed).BB The mesial surfaces of third molars contact the distal surfaces of second molars, but the distal surfaces of third molars are not in proximal contact with any other tooth. In ideal alignment of teeth between arches, maxillary third molars are UNIQUE since they bite against (occlude with) only one tooth, the mandibular third molar; all other teeth EXCEPT mandibular central incisors have the potential for occluding with two teeth.
B. SIZE AND SHAPE OF THIRD MOLARS
Both maxillary and mandibular third molars vary considerably in size, but they are, on average, the shortest teeth in the mouth. Mandibular thirds are the shortest of all mandibular teeth,CC and maxillary thirds are the shortest of all permanent teeth. The roots of third molars are, on average, shorter than the roots of the first or second molars,DD and their root trunks are proportionally longer than the root trunks of the firsts and seconds.EE
C. TYPE TRAITS OF MAXILLARY THIRD MOLARS THAT ARE SIMILAR TO FIRST AND SECOND MOLARS
Crowns: Although the crown of a maxillary third molar may exhibit great variance in size and shape and look like no other tooth, its crown may resemble a maxillary first molar (complete with fifth cusp of Carabelli) or second molar (without the fifth cusp and perhaps without the distolingual cusp). Many maxillary thirds have the same relative cusp size from largest to smallest as on first and second molars: the largest and longest mesiolingual cusp is followed by the mesiobuccal cusp, which is wider and usually longer than the distobuccal cusp, followed by the smallest distolingual cusp (if present). There could be an oblique ridge (but it may be poorly developed or even absent) (Fig. 5-46). From the occlusal view, the maxillary third molar crown outline may taper from buccal to lingual, being narrower on the lingual side, and may taper from mesial to distal, being considerably larger faciolingually in its mesial half due to a prominent mesiobuccal cervical ridge and large mesiolingual cusp. Also, the buccal surface can be distinguished from the lingual because the buccal surface is relatively more flat.
FIGURE 5-46. Occlusal views of mandibular and maxillary third molars. The buccal surfaces of all of these third molars are facing up. Observe the wrinkled occlusal anatomy of these third molars, and try to recognize the similarities to first and second molars in each respective arch. For example, most of the maxillary third molars are wider buccolingually than mesiodistally in contrast to the mandibular third molars, whose greater dimension is mesiodistally.
Roots: Maxillary third molars usually have three roots: mesiobuccal, distobuccal, and lingual as on the first and second molars, although they may be so fused together that they are difficult to distinguish.
D. TYPE TRAITS OF MANDIBULAR THIRD MOLARS THAT ARE SIMILAR TO MANDIBULAR FIRST AND SECOND MOLARS
Crowns: Although the crown of a mandibular third molar may exhibit great variance in size and shape and look like no other tooth (as seen in Fig. 5-44), its crown could resemble a four-cusped mandibular second molar or a five-cusped mandibular first molar. For example, the occlusal outlines of many mandibular third molar crowns are somewhat rectangular, wider mesiodistally than buccolingually.FF The relative cusp size from largest to smallest may be the same as on first and second molars: the mesiolingual is largest (and longest) followed by the distolingual, then the mesiobuccal is wider and longer than the distobuccal, and, when present, the distal cusp is the smallest. Also, from the occlusal view, the crown of the four-cusped type tapers from mesial to distal and from buccal to lingual (but only slightly).EE
FIGURE 5-44. Unusual third molars. Six unusual mandibular third molars, many with extra cusps or roots that may be due to the fusion of third molars with adjacent extra (fourth) molars.
Roots: Mandibular third molars usually have two roots: mesial and distal as on mandibular first and second molars, but they may be fused together.
E. TYPE TRAITS THAT DIFFERENTIATE THIRD MOLARS FROM FIRST AND SECOND MOLARS
Although third molars may have a number of traits in common with first or second molars, they all have certain type traits in common that set them apart from the first and second molars in their arches. Maxillary third molars have the greatest morphologic variation in shape of all teeth, which makes a general description difficult. The crown may have only one cusp or as many as eight.11 Sometimes, the form of the third molar crowns are so irregular that it is difficult to distinguish identifiable cusps. Mandibular third molars may have one or more extra roots.
The following third molar traits can be used to differentiate a third molar from a first or second molar:
- Normally, third molars are smaller than first or second molars in the same mouth (Fig. 5-45). A common EXCEPTION is the five-cusped third molar, which may have a crown somewhat larger and more bulbous than the second molar.
FIGURE 5-45. Dental stone casts of maxillary and mandibular teeth (facial view) showing the decrease in size of molars from first to third molar that is typical in most people. (Model courtesy of Ms. Colleen Seto.)
- Third molar crowns are bulbous with fatter peripheral contours.
- Occlusal tables of third molars are relatively smaller than on first and seconds since the buccal cusp tips are closer to the lingual cusp tips.
- Occlusal surfaces of third molars are quite wrinkled due to numerous supplemental grooves and ridges (Fig. 5-46). Recall that many second molars have more supplemental grooves than first, but thirds are the most wrinkled.
- Roots of both maxillary and mandibular third molars are noticeably shorter than on firsts or secondsDD,GG so third molars have a small root-to-crown ratio compared to first and second molars in the same mouth (Fig. 5-47).
FIGURE 5-47. Buccal views of maxillary and mandibular third molars. Notice the relatively short roots with long root trunks and many fused roots. Also, note the similarity of the third molars with the first and second molars within their respective arches such as three (sometimes fused) roots on maxillary molars but only two roots on mandibular molars and two buccal cusps on all maxillary molars but two and often three buccal cusps (with a smallest distal cusp) on mandibular molars.
- Roots are frequently fused together for most of their length resulting in very long root trunks with the furcation located only a short distance from the apices of the roots (Fig. 5-47).
- Roots are pointed, crooked, and frequently curve distally in the apical third.
F. SHOULD ALL THIRD MOLARS BE EXTRACTED?
Third molars, known to many as wisdom teeth, have gotten a bad reputation for not serving any function, having soft enamel, readily decaying, and causing crowding of the anterior teeth and other dental problems. The truth is that the posterior location of third molars in the mouth makes it more difficult to keep them clean, and their wrinkled, fissured occlusal surfaces make them more prone to developing decay than other teeth. Further, mandibular third molars often erupt so far distally that there may not be room to completely erupt, which compromises the health of the surrounding tissue (gingiva), so dentists often suggest that these teeth be removed to prevent future problems. Inflammation of the tissue around these teeth (called pericoronitis) can be a cause of acute pain and spread of infection, resulting in the need for gingival surgery or extraction. This infection is even more likely to occur if the flap of tissue overlying the erupting third molar, called an operculum, becomes irritated (Fig. 5-48). However, it is not true that third molars have soft enamel, are useless, or should be routinely extracted. If the dental arches are of sufficient length to permit full eruption of third molars and a person’s oral hygiene is good, third molars can function for a lifetime without problems. Also, healthy third molars can serve as the posterior attachment (abutment) when replacing lost or missing first or second molars.
FIGURE 5-48. An operculum is a flap of tissue that may cover the crown of the most posterior, erupting mandibular molar (especially when there is no room for it to erupt completely). This flap is subject to irritation and infection surrounding the crown known as pericoronitis. (Photo courtesy of Carl Allen, D.D.S., M.S.D.)
Some oral surgeons recommend that when third molars have to be extracted, they be removed at an early age (under 25 years old) to facilitate an easier, less traumatic removal and a quicker, more comfortable recovery period than if they were extracted later in life.5 If these third molars are extracted before the roots are completely formed, the pulp within the open ends of the root canal apices may be clearly visible (Fig. 5-49).
FIGURE 5-49. The apical foramen of these two roots on a mandibular molar is quite large because this tooth was extracted before the roots had a chance to completely form. When it was extracted, the pulp tissue could be seen within these openings.
LEARNING EXERCISE
Suppose a patient just had all of his or her permanent teeth extracted and you were asked to find tooth 15 from among a pile of thirty-two extracted teeth on the oral surgeon’s tray because you wanted to evaluate a lesion on the root of that molar that had been seen on the radiograph. Before reading the recommended steps below, write out your plan. You have done this before in previous chapters, so you should be able to do it. How might you go about it?
After writing your list, compare it to the following:
- From a selection of all permanent teeth (extracted teeth or tooth models), select only the molars (based on class traits).
- Determine whether each molar is maxillary or mandibular (based on arch traits). You should never rely on only one characteristic difference between teeth to name them; rather, make a list of many traits that suggest the tooth is a maxillary molar, as opposed to only one trait that makes you think it belongs in the maxillary arch. This way, you can play detective and become an expert at recognition at the same time.
- If you determine that the tooth is maxillary, position the root up; if it is mandibular, position the root down.
- Use traits for each surface to identify the buccal surface. This will permit you to view the tooth as though you were looking into a patient’s mouth.
- Next, using type traits, determine the type of molar you are holding (first, second, or third).
- Finally, determine which surface is the mesial. While viewing the molar from the facial and picturing it within the appropriate arch (upper or lower), the mesial surface can be positioned toward the midline in only one quadrant, the right or left.
- Once you have determined the quadrant, assign the appropriate Universal number for the molar in that quadrant. For example, the second molar in the upper left quadrant is tooth 15.
Stay updated, free dental videos. Join our Telegram channel
VIDEdental - Online dental courses
