|SECTION V||TYPE TRAITS THAT DIFFERENTIATE MAXILLARY SECOND FROM FIRST MOLARS|
OBJECTIVES FOR SECTION V
This section prepares the reader to perform the following:
- Describe the type traits that can be used to distinguish the permanent maxillary first from second molar.
- Describe and identify the buccal, lingual, mesial, distal, and occlusal surfaces for all maxillary molars.
- Assign a Universal number to maxillary molars present in a mouth (or on a model) with complete dentition. If possible, repeat this on a model with one or more maxillary molars missing.
- Holding a maxillary molar, determine whether it is a first or a second and right or left. Then assign a Universal number to it.
Examine a maxillary first and second molar as you read. Hold the roots up and the crowns down, with the two somewhat parallel buccal roots closest to you.
1. Number and Relative Height and Size of Cusps on Maxillary Molars and Associated Grooves (Buccal View)
In review, both first and second maxillary molars most often have four prominent cusps: mesiobuccal, distobuccal, mesiolingual, and the smaller (in area) distolingual cusp (best seen on an occlusal view in Fig. 5-24A). On some maxillary second molars, the smaller distolingual cusp may be missing resulting in only three cusps: mesiobuccal, distobuccal, and lingual (Fig. 5-24B). On some maxillary first molars, there may be a small fifth cusp (of Carabelli) seen on its lingual surface.
The relative heights of the four prominent cusps of four-cusped maxillary molars are the longest mesiolingual cusp, followed by the mesiobuccal, distobuccal, and the shortest distolingual cusp (if present). Therefore, from the buccal view, the two buccal cusps are prominently visible, but the mesiolingual cusp tip may also be visible because it is longer than the buccal cusps (and even the short distolingual cusp tip might be visible because the lingual cusps are positioned slightly to the distal of the buccal cusps) (Fig. 5-25). On both types of maxillary molars, but especially on maxillary second molars, the mesiobuccal cusp is somewhat wider than the distobuccal cusp (Fig. 5-26).S A buccal groove lies between the buccal cusps, but this groove is unlikely to be fissured and form decay on the buccal surface.
2. Crown Proportion, Shape, and Taper (Buccal View)
Both maxillary first and second molars are wider mesiodistally than high occlusocervically, similar to mandibular molars. The maxillary first molar is normally the widest tooth in the upper arch. The crown height tapers shorter on the distal than on the mesial because the distobuccal cusps are shorter than mesiobuccal cusps. This trait is most helpful in distinguishing mesial from distal and therefore rights from lefts. Maxillary molar crowns may appear to tip distally relative to its root trunk (Fig. 5-25).
3. Proximal Contacts of Maxillary Molars (Buccal View) (Same for All Molars)
On maxillary (and mandibular) first and second molars, the distal contact area is normally located more cervically than the mesial contact (Fig. 5-25). Mesial contacts are located near the junction of occlusal and middle thirds, whereas distal contacts are in the middle third of the crown. A summary of the location of the proximal contacts of maxillary and mandibular molars is found in Table 5-3, and a summary for all adult teeth (except thirds) is presented in Table 5-4.
|TABLE 5-3||Molars: Location of Proximal Contactsa (Proximal Crest of Curvature) Seen Best from Buccal|
aGeneral learning guidelines:
1. For molars, the mesial and distal contacts are closer to the middle of the tooth and are more nearly at the same level than on premolars or anterior teeth.
2. Distal proximal contacts are slightly more cervical than mesial contacts.
Source: Brand RW, Isselhard DE. Anatomy of orofacial structures. 6th ed. St. Louis, MO: C.V. Mosby, 1998.
|TABLE 5-4||Summary of Location of Proximal Contacts on all Teetha (Proximal Crests of Curvature) Seen Best from Facial View|
aGeneral learning guidelines:
1. On any adult tooth, distal proximal contacts are more cervical than mesial contacts EXCEPT for mandibular central incisors, where the mesial and distal contacts are at the same height, and mandibular first premolars, where the mesial contact is more cervical than the distal.
2. Proximal contacts become more cervical when moving from anterior to posterior teeth.
a. For anterior teeth, most contacts are in the incisal third EXCEPT the distal of maxillary lateral incisors and canines, which are likely in the middle third.
b. For posterior teeth, the mesial and distal contacts are closer to the middle of the tooth and are more nearly at the same level.
3. No proximal contact is cervical to the middle of the tooth.
4. Roots of Maxillary Molars from the Buccal View
There are three roots on both types of maxillary molars: a mesiobuccal, distobuccal, and a lingual root. The three roots are close to the same length,T but the lingual (palatal) root is the longest, followed by the mesiobuccal root, and then the shortest distobuccal root. All three are usually visible from the buccal view (Fig. 5-25).
There is much variation in the shapes of the three roots. On maxillary first molars, the more blunt mesiobuccal root and distobuccal root are often well separated (Appendix 8j). Both buccal roots often bend distally, or they may bend in such a way that they look like pliers handles. In some of these teeth, the mesiobuccal root may bow out mesially in the cervical half before it curves toward the distal, placing its apex distal to the line of the buccal groove on the crown. Root trunks are relatively long with the furcation (in this case a trifurcation denoting a split of three cusps) often near the junction of the cervical and middle thirds.
In contrast, on maxillary second molars, roots are often less curved and less separated and more nearly parallel so the root trunks are usually longer than on firsts. Both buccal roots often bend toward the distal in their apical third. Find the two maxillary second molars in Figure 5-26 that are exceptions.
Refer to Figure 5-27 for similarities and differences.
1. Relative Size and Taper of Maxillary Molars (Lingual View)
On the crowns of most maxillary first molars, little or no mesial or distal crown surface is visible from the lingual view (except in the cervical third) since these teeth may be as wide (or even wider) in the lingual half compared to the buccal half due to a relatively wide distolingual cusp. This is an EXCEPTION (along with the three-cusped mandibular second premolar) to the normal crown taper toward the lingual for most other posterior teeth. Maxillary second molars are narrower in the lingual half due to the relatively smaller or nonexistent distolingual cusp so some of the mesial or distal surfaces may be visible from the lingual view. The lingual outline of the crown on both types of maxillary molars tapers cervically to join the single palatal root (seen clearly in the maxillary molars in Fig. 5-27).
2. Number and Relative Size of Lingual Cusps and Lingual Groove on Maxillary Molars (Lingual View)
On maxillary first molars, there are two well-defined cusps on the lingual surface, the larger mesiolingual cusp and the smaller, but still sizable, distolingual cusp. The mesiolingual cusp is almost always the longest and largest (in area) cusp on any maxillary molar (seen on most first molars in Fig. 5-27). Additionally, over two thirds of maxillary first molars have a small depression or fifth cusp (cusp of Carabelli) located on the lingual surface of the mesiolingual cuspU (Appendix 8i). This cusp was named after the Austrian dentist who described it, Georg von Carabelli (1787–1842). This cusp varies greatly in shape but is considered to be a nonfunctioning cusp because it is usually about 2 to 3 mm shorter than the mesiolingual cusp tip. Examples of a large cusp of Carabelli (or a slight depression in the same area) are seen from an occlusal view in Figure 5-28.
There are two types of maxillary second molars based on the number of cusps: four or three. Most maxillary second molars have four cusps: two buccal and two lingual cusps. The mesiolingual cusp is normally considerably wider than the distolingual cusp, compared to maxillary first molars where the size of the lingual cusps is closer to the same size. Over one third of maxillary second molars have only three cusps, which is called a tricuspid form (Fig. 5-24B).V On these teeth, the distolingual cusp is missing, so it has just one large lingual cusp and two buccal cusps. It would be rare, but not impossible, to find a fifth cusp (of Carabelli) on maxillary second molars.
On the lingual surfaces of both first and second maxillary molars with two lingual cusps, there is a lingual groove seen separating the mesiolingual and distolingual cusps. This lingual groove may line up with the longitudinal depression on the lingual surface of the lingual root. On three-cusped maxillary second molars with one large lingual cusp, there is no distolingual cusp so there is no lingual groove. See if you can identify the several maxillary second molars with only one lingual cusp in Figure 5-27.
3. Roots of Maxillary Molars (Lingual View)
On both the maxillary first and second molars, the broad, longest lingual rootW does not appear curved when seen from the lingual view, but it does taper apically to a blunt or rounded apex. On maxillary first molars, there is usually a longitudinal root depression on the lingual side of the lingual root (seen aligned with the lingual groove of the crown on many lingual roots of maxillary first molars in Fig. 5-27). The buccal roots often spread out far enough that they are visible behind the lingual root from this view, especially on first molars.
Refer to Figure 5-29 for similarities and differences of maxillary molars from the proximal views.
1. Crown Shape of Maxillary Molar Cusps (Proximal Views)
Like on crowns of mandibular molars, maxillary molar crowns are broader buccolingually than occlusocervically from the proximal view (Appendix 7h).
2. Number and Relative Size of Maxillary Molar Cusps (Proximal Views)
Recall that the cusps of the maxillary first molar from longest to shortest are mesiolingual, mesiobuccal, distobuccal, and distolingual, followed by the functionless fifth cusp (of Carabelli) if present. This fifth cusp is located on the lingual surface of the largest mesiolingual cusp, but is 2 to 3 mm shorter than the height of that cusp. Therefore, from the distal view, the distobuccal cusp and smallest distolingual cusp are prominent in the foreground, while the cusp tips of the longer mesiobuccal and longest mesiolingual cusp can be seen behind them. In contrast, from the mesial view, only the longer mesiolingual and mesiobuccal cusps are visible. From the mesial or distal views, if the fifth cusp (of Carabelli) is prominent, it might be visible projecting out from lingual surface of the mesiolingual cusp (Fig. 5-29 drawing). From the proximal views, the crown of the maxillary second molar looks much like that of the first molar, but there is no fifth cusp, and about one third of the time, there may be no distolingual cusp.
3. Crest of Curvature for Maxillary Molars (Proximal Views) (Same for All Posterior Teeth)
As on all posterior teeth, the crest of curvature of the buccal surface of both first and second maxillary molar crowns is in the cervical third, usually close to the cervical line. The crest of curvature of the lingual surface of the crown is more occlusal, usually in or near the middle third of the crown. However, the lingual crest of curvature may be located even more occlusally on teeth with a large fifth cusp (of Carabelli) (Fig. 5-30).
4. Taper Toward Distal of Maxillary Molars (Distal View)
From the distal view on both maxillary first and second molars, a little of the buccal surface and the lingual surface of the crown can be seen because the crown tapers toward the distal, so the crown is narrower buccolingually on the distal surface than on the mesial surface (seen on many distal views in Fig. 5-29).
5. Marginal Ridges of Maxillary Molars (Proximal Views)
As on mandibular molars, both first and second maxillary molars have a concave distal marginal ridge that is located more cervically than its mesial marginal ridge, permitting a better view of the occlusal surface (including the triangular ridges) from the distal view than from the mesial view. (Compare mesial and distal views in Fig. 5-29.) This marginal ridge height difference is very helpful in differentiating right from left sides.
Marginal ridge grooves are found more often on mesial marginal ridges than on distal marginal ridgesX and are more common on first molars than on seconds. Also, on the unworn marginal ridges of the maxillary molars, there may be one or more projections of enamel called tubercles. Like marginal ridge grooves, they are more common on mesial marginal ridges than on the distal, are more common on first molars than on secondsY, and are seen clearly on the mesial marginal ridges of the maxillary first and second molars in Figure 5-31.
6. Cervical Lines of Maxillary Molars (Proximal Views)
On both types of maxillary molars, the slight curvature of the mesial cervical line is greater than on the distal cervical line curvature, which is almost flatZ (compare Fig. 5-32A and B), but this difference may be hardly discernible since both proximal cervical lines often have very little curvature.
7. Roots and Root Depressions of Maxillary Molars (Proximal Views)
Roots visible from the mesial view: On both maxillary first and second molars from the mesial view, only two roots can be seen: the lingual root and the mesiobuccal root, which is considerably wider buccolingually and longer than the hidden distobuccal root (Fig. 5-32A). On first maxillary molars, the convex buccal outline of the mesiobuccal root often extends a little buccal to the crown outline, but the apex of this root is in line with the tip of the mesiobuccal cusp (Fig. 5-32A). The lingual outline of the mesiobuccal root is often more convex especially near the apex. The longest lingual root is bent somewhat like a curved banana (concave on its buccal surface and convex on its lingual surface), and it extends conspicuously beyond the crown lingually (Fig. 5-32B). On maxillary second molars, the lingual root is straighter and the roots spread out less than on first molars so that all three roots are usually confined within the crown width outline. Compare the differences in root flare in Figure 5-29.
Roots visible from the distal view: From the distal view of both maxillary first and second molars, you can see all three of the roots: the lingual root, the distobuccal root, and the longer, less pointed and wider the mesiobuccal root behind it (evident in Fig. 5-32B and on most distal views in Fig. 5-29).
Root depressions: On both maxillary first and second maxillary molars, the mesiobuccal root has a longitudinal depression on its mesial surface and possibly a depression on its distal surface dividing the root into buccal and lingual halves indicating the likelihood of two root canals within this mesiobuccal root: one buccal and one lingual. The distobuccal root is most often convex on its distal surface without a longitudinal depression and the chance of a mesial depression is variable (indicating the likelihood of only one root canal). There is often a slight concavity on the distal root trunk of maxillary first molars located between the distobuccal root and the cervical line.1,4 This concavity may be difficult to keep clean resulting in the buildup of calcified debris called calculus, which contributes to periodontal disease (see Fig. 5-32B). A summary of the presence and relative depth of root depressions for all teeth is presented in Table 5-5.
|TABLE 5-5||Summary of Presence and Relative Depth of Longitudinal Root Depressions (“Root Grooves”)a|
aGeneral learning guidelines:
1. Maxillary incisors are less likely to have root depressions.
2. All mandibular incisors, all canines, and premolars (EXCEPT maxillary first premolars) are likely to have deeper distal surface root depressions.