|SECTION IV||TYPE TRAITS THAT DIFFERENTIATE MANDIBULAR SECOND FROM FIRST MOLARS|
This section prepares the reader to perform the following:
- Describe the type traits that can be used to distinguish the permanent mandibular first molar from the mandibular second molar.
- Describe and identify the buccal, lingual, mesial, distal, and occlusal surfaces for all mandibular molars.
- Assign a Universal number to mandibular molars present in a mouth (or on a model) with complete dentition. If possible, repeat this on a model with one or more mandibular molars missing.
- Holding a mandibular molar, determine whether it is a first or a second, and right or left. Then, assign a Universal number to it.
Mandibular first and second molars have specific traits that can be used to distinguish one from the other. As you study the mandibular first and second molars, hold the crowns up and roots down as they are positioned in the mouth, and refer to Appendix page 8 while making the following comparisons.
When considering relative differences in cusp size, it is important to distinguish between cusp height and cusp size. Cusp height refers to the relative length of each cusp tip: whether it is longer or shorter, best appreciated when viewing a tooth from the buccal, lingual or proximal. Cusp size refers to the relative area of each cusp: whether it is bigger or smaller, best appreciated when viewing a tooth from the occlusal.
Refer to Figure 5-4 for similarities and differences between mandibular first and second molars.
1. Crown Proportions (Buccal View)
For both types of mandibular molars, the crowns are wider mesiodistally than high cervico-occlusally but more so on the larger first molars.D
2. Taper of Mandibular Molars (Buccal View)
There is proportionally more tapering of the crown from the contact areas to the cervical line on mandibular first molars than on second molars because of the bulge of the distal cusp (Appendix 8g). Therefore, the crown of the second molar appears to be wider at the cervix than on the first molar.
Also, from the buccal view, the occlusal outlines of both the first and second mandibular molars taper shorter cervically from mesial to distal since the cusps get shorter from mesial to distal (Appendix 7d). This distal taper combined with a greater distal crown outline bulge beyond the root, especially on mandibular first molars, results in the perception that the crown is tipped distally on its roots.
3. Number and Relative Height of Mandibular Molar Buccal Cusps and Grooves That Separate Them (Buccal View)
Most mandibular first molars have five cusps, and most mandibular second molars have four cusps. The mandibular first molar has the largest mesiodistal dimension of any tooth.E,F It most often has five cusps: three buccal cusps (named mesiobuccal, distobuccal, and distal) and two lingual cusps (named mesiolingual and distolingual) (Fig. 5-5). However, the smallest distal cusp may be absent (about a fifth of the time according to Woelfel’s research),G so do not be surprised when you see a mandibular first molar with only four cusps (Fig. 5-6).
When considering the relative height of the most common five-cusped mandibular first molar, the lingual cusps are generally longer than buccal cusps, and both lingual and buccal cusps get shorter toward the distal. Therefore, the mesiolingual cusp is the longest, followed by the distolingual, then the mesiobuccal cusp followed by the distobuccal, and finally, when present, the shortest distal cusp (located on the buccal surface near the distobuccal angle). Relative to other buccal cusps on this tooth, the mesiobuccal cusp is the largest, widest, and highest,H although the distobuccal cusp may be sharper than the mesiobuccal cusp.I
When a handheld mandibular first molar is oriented vertically and viewed from the buccal, the mesiolingual cusp, which is the longest cusp, is just visible behind the mesiobuccal cusp, and the shorter distolingual cusp tip is just visible behind the shorter distobuccal cusp. The shortest distal cusp is also visible almost like a buccal extension of the marginal ridge. This is clearly seen in Figure 5-7 and in the first molars in Figure 5-4. Even though the lingual cusps are higher than buccal cusps when viewing a handheld tooth with the root axis held vertically, the lingual cusp tips in the mouth are at a lower level than the buccal cusps due to the lingual tilt of the root axis in the mandible, creating the mediolateral curve (of Wilson) illustrated previously in Figure 1-49.
When there are three buccal cusps on the mandibular first molar, there are two buccal grooves: the mesiobuccal and shorter distobuccal. The longer mesiobuccal groove separates the mesiobuccal cusp from the distobuccal cusp, and the shorterJ distobuccal groove separates the distobuccal cusp from the distal cusp. Both of these grooves are likely to extend onto the buccal surface and may end in a pit that is sometimes a site of decay (Fig. 5-7). Six of the mandibular first molars in Figure 5-4 have pits at the end of the mesiobuccal groove, and a seventh has a filling, and one of the mandibular first molars has a pit at the end of the distobuccal groove; can you find it?
The mandibular second molar most often has four cusps: two buccal and two lingual (Fig. 5-5 lower drawings). The cusp heights are in the same order as for the four larger cusps of the mandibular first molar: the mesiolingual is longest, then the distolingual, mesiobuccal, and smallest distobuccal. As on the mandibular first molar, the lingual cusp tips are visible from the buccal view of a handheld tooth behind the shorter buccal cusps (seen in most teeth in Fig. 5-4). As on the mandibular first molar, the mesiobuccal cusp is often wider mesiodistally than the distobuccal cusp.K
Mandibular second molars have only one buccal groove that separates the mesiobuccal from the distobuccal cusp. This buccal groove may end on the buccal surface in a pit that is sometimes a site of decay (seen in 2 of the 10 second molars in Fig. 5-4). There is no distobuccal groove as on the five-cusped first molar.
4. Proximal Contacts of Mandibular Molars (Buccal View)
Both types of mandibular molars (in fact, all molars) normally have the mesial proximal contact located more occlusally than the distal. The mesial contact is close to the junction of the middle and occlusal thirds of the crown while the distal contact is located more cervically, in the middle third (near the middle of the tooth cervico-occlusally). This difference in proximal contact height can be seen in most mandibular molars in Figure 5-4.
5. Cervical Lines of Mandibular Molars (Buccal View)
The cervical lines of both mandibular first and second molars are often nearly straight across the buccal surface with little curve. On mandibular molars, cementum often covers the root bifurcation surface,1 but sometimes there is a point of enamel that dips down nearly into the root bifurcation (Fig. 5-8). There may even be a dipping down of enamel on both the buccal and the lingual surfaces, and these extensions may meet in the root bifurcation.2,3 An area with these enamel extensions may contribute to a deep gingival sulcus since cementum is required for the periodontal ligament to attach the tooth root to the bone in a healthy periodontium.
6. Roots of Mandibular Molars (Buccal View)
The roots of a mandibular first molar are more widely separated than on the seconds, so the root bifurcation is closer to the cervical line and the root trunks are relatively shorter compared to seconds. This considerable divergence is evident when the curved mesial root bows out beyond the mesial crown outline and then curves back distally placing its apex in line with the mesiobuccal groove of the crown (seen on Fig. 5-9). The mesial root is twisted, so it may be possible to see the distal surface of this root from the buccal view (seen on Fig. 5-9 and on some first molars in Fig. 5-4). The more pointed apex of the straighter distal root often lies distal to the distal crown outline.
The tapered, pointed roots of mandibular second molars are less widely separated, or more parallel, than on first molars (Appendix 8f), so the root trunk is often longer than that of the mandibular first molar. Often, the apices of both roots are directed toward the centerline of the tooth, similar in shape to the handle of pliers, and a buccal root depression extends occlusally from the furcation to the cervical line (Fig. 5-10 and on 2 of the 10 mandibular molars in Fig. 5-4; see if you can find them), or both roots may curve distally.
Refer to Figure 5-11 for similarities and differences between mandibular molars from the lingual view.
1. Number and Relative Size of Mandibular Cusps and the Lingual Groove (Lingual View)
Since the lingual cusps of both types of mandibular molars are both slightly longer (and more pointedN) than the buccal cusps, only the two longer lingual cusps are visible from the lingual aspect (not evident in Fig. 5-11 because of the camera angle). The mesiolingual cusp is most often slightly longer and wider than the distolingual cusp.M
A lingual groove separates the mesiolingual from the distolingual cusp, but it is unlikely to extend onto the lingual surface.
2. Narrower Crown Lingually on Mandibular Molars (Lingual View)
As with most teeth, mandibular first and second molar crowns taper from buccal to lingual and thus are narrower mesiodistally in the lingual half, permitting more view of the mesial and distal surfaces from the lingual view than from the buccal.
3. Cervical Line of Mandibular Molars (Lingual View)
The cervical line on the lingual surface is relatively straight (mesiodistally) but may rarely dip cervically toward the root bifurcation between the mesial and distal roots as may also be seen on the buccal surface.
4. Roots of Mandibular Molars (Lingual View)
On mandibular first molars, the root trunk appears longer on the lingual than on the buccal side because the cervical line is more occlusal in position on the lingual than on the buccal surface. Both roots are narrower on the lingual side than they are on the buccal side, and the mesial root is twisted making it possible to see the mesial surface of the mesial root (seen on five mandibular first molars in Fig. 5-11).
On both first and second mandibular molars, the lingual surface of the root trunk has a depression between the bifurcation and the cervical line just as on the buccal root trunk.
For proper orientation, as you study each trait, hold the crown so that the root axis line is in a vertical position as seen in Figure 5-12.
1. Crown Shape and Tilt (Proximal Views)
Recall that both types of mandibular molar crowns are relatively shorter cervico-occlusally and wider faciolingually (Appendix 7h) and that the crowns of both types of mandibular molars are tilted lingually from the root trunk (Fig. 5-13). Remember that this lingual tilt is an arch trait characteristic of all mandibular posterior teeth and is nature’s way of shaping them to fit beneath and lingual to the maxillary buccal cusps (recall Fig. 1-60).
2. Number and Relative Size of Mandibular Molar Cusps (Proximal Views)
Mandibular molar cusps (and marginal ridges) are shorter on the distal than on the mesial, so much of the occlusal surface and all cusp tips (including the distal cusp when present on mandibular first molars) can be seen from the distal aspect (as seen in Fig. 5-14 and on the distal surfaces of most mandibular molars in Fig. 5-12). Since the lingual cusps of both mandibular first and second molars are longer and more pointed than the buccal cusps, the longer triangular ridges of these lingual cusps are evident from this distal view. From the mesial aspect, only the mesiobuccal and mesiolingual cusps are visible, and most of the occlusal anatomy is hidden behind the prominent mesial marginal ridge (Fig. 5-13).
3. Crest (Height) of Contour of Mandibular Molars (Proximal Views)
The buccal crest of contour is actually formed by a buccal cervical ridge that runs mesiodistally near the cervical line (Fig. 5-15). This cervical ridge is more prominent on mandibular second molars than on firsts. As with all molars (and premolars), the convex buccal crest of contour or greatest bulge is in the cervical third (Fig. 5-13), whereas the lingual crest of curvature is in the middle third. This difference in crests of curvature heights is useful to distinguish buccal from lingual surfaces on molars.
4. Taper Toward Distal of Mandibular Molars (Proximal Views)
On both types of mandibular molars, the crown is narrower in the distal third than in the mesial third. Therefore, from the distal aspect, some of the lingual and the buccal surfaces can be seen (demonstrated clearly on the distal views of mandibular second molars in Fig. 5-12). Proximal contact areas may be seen as flattened areas (facets) caused from wear due to the rubbing of adjacent teeth during functional movements of the jaws. On mandibular first molars, the distal contact is centered on the distal surface cervical to the distal cusp.
5. Cervical Lines of the Mandibular Molars When Comparing Proximal Views
Mesial cervical lines on both first and second molars slope occlusally from buccal to lingual and curve very slightly toward the occlusal surface.O The distal cervical lines curve less and sometimes are so slightly curved that they are nearly straight.
6. Marginal Ridge Heights of Mandibular Molars When Comparing Proximal Views
Differences in mesial and distal marginal ridge heights are apparent on handheld teeth when viewing the crown from the lingual and rotating the tooth, first just enough in one direction to see the mesial marginal ridge height, and then enough in the opposite direction to compare it to the distal marginal ridge height. On mandibular molars, as with all posterior teeth (EXCEPT the mandibular first premolar), mesial marginal ridges are more occlusally located than distal marginal ridges so that from the mesial view, the triangular ridges of the cusps are mostly hidden from view (compare a mesial view in Fig. 5-13 with a distal view in Fig. 5-14).
Mesial marginal ridges are concave buccolingually. Distal marginal ridges of the first molar are short and V shaped, located just lingual and distal to the distal cusp (Fig. 5-14).
7. Roots and Root Depressions of Mandibular Molars (Proximal Views)
The mesial root of both first and second mandibular molars is longer and broader buccolingually than the shorter more pointed distal root. Therefore, from the distal view, the longer, broader mesial root is usually visible behind the shorter, narrower and more pointed distal root, and from the mesial view, the narrower distal root is hidden behind the wider, longer mesial root (seen in Fig. 5-16 and on all but one mandibular second molar from the distal view in Fig. 5-12). Further, the mesial root of the mandibular first molar is larger (broader buccolingually with a wider apex) than the mesial root of the mandibular second molar.
ROOT DEPRESSIONS: On both mandibular first and second molars, the mesial root usually has a deep longitudinal depression on its mesial surface and an even deeper longitudinal depression on its distal surface. These depressions indicate the likelihood of two root canals in this broad root, one buccal and one lingual (as seen in cross-section views in Fig. 5-17). Infrequently, this mesial root might even be divided into two separate roots called a mesiobuccal and mesiolingual root3 (seen in the partially divided mesial root of two mandibular first molars in Fig. 5-12). On the distal root, the likelihood of a longitudinal depression on the distal surface is variable and the mesial surface may be more evident, but this root most often has only one root canal. Notice that the root surfaces with the deeper depressions face one another: the distal surface of the mesial root and the mesial surface of the distal root.
To follow this description, the tooth should be held in such a position that the observer is looking exactly along the root axis line. Because of the lingual inclination of the crown, more of the buccal surface should be visible than the lingual surface when the tooth is properly held in this position, similar to mandibular premolars. Refer to Figure 5-18 and Appendix page 8 for similarities and differences of mandibular molars from the occlusal view.
1. Relative Size of Cusps of Mandibular Molars (Occlusal View)
As stated earlier, the mandibular first molar crown has the largest mesiodistal dimension of any tooth in the mouth. To review, most mandibular first molars have five cusps: three on the buccal (mesiobuccal, distobuccal, and the smallest distal cusp closest to the distal marginal ridge) and two on the lingual (mesiolingual and distolingual). Most mandibular second molars have four cusps: two on the buccal (mesiobuccal and distobuccal) and two on the lingual (mesiolingual and distolingual). On most mandibular first and second molars, the mesiobuccal cusp is larger in area than the distobuccal, and the mesiolingual cusp is larger in area than the distolingual (Fig. 5-19), and the fifth distal cusp, when present on first molars, is the smallest in area. This is consistent with the taper of mandibular molar crowns narrower toward the distal (Appendix 7c).
2. Outline Shape and Taper of Mandibular Molars (Occlusal View)
As stated previously, both types of mandibular molars are wider mesiodistally than faciolingually.Q The mandibular second molar outline is roughly a four-sided rectangle, whereas on the five-cusped first molar, the widest portion of the tooth may be located in the middle third on the prominent buccal bulge of its distobuccal cusp, so the outline would be more like a five-sided pentagon (Appendix 8e and Fig. 5-20).
The crown outlines of both types of mandibular molars taper lingually, so they have more bulk mesiodistally in the buccal half than in the lingual half (recall Appendix 7b). Mandibular second molar crown outlines (and outlines of four-cusped first molars) are wider buccolingually on the mesial half than on the distal half (Appendix 7c) due primarily to the prominent buccal cervical ridge on the mesiobuccal cusp outline. This ridge is a consistent landmark useful for identifying the mesiobuccal cusp on the otherwise nearly symmetrical mandibular second molar in Figure 5-19B. This mesiobuccal cervical ridge is evident on most mandibular second molars in Figure 5-18. On five-cusped mandibular first molars, even when the outline is pentagon shaped due to the prominent distobuccal cusp bulge, there is still an appearance of tapering narrower from mesial to distal since the width buccolingually through the mesiobuccal cusp (in the mesial third) is wider than through the distal cusp (in the distal third) (Fig. 5-20).
3. Ridges of Mandibular Molars (Occlusal View)
On both first and second mandibular molars, the triangular ridges of the mesiobuccal and mesiolingual cusps join to form one transverse ridge on the mesial half of the crown, and the triangular ridges of the distobuccal and distolingual cusps form a second transverse ridge on the distal half of the crown (Fig. 5-21).
4. Fossae of Mandibular Molars (Occlusal View)
There are three fossae on both types of mandibular molars: the largest central fossa (approximately in the center of the tooth), a smaller mesial triangular fossa (adjacent to the mesial marginal ridge), and the smallest distal triangular fossa (adjacent to the distal marginal ridge; it is very small on second molars). These fossae are shaded red in Figure 5-19A (on a second molar) and Figure 5-22A (on a first molar). There may be a pit at the intersection of grooves in the deepest portion of any of these fossae.
5. Grooves of Mandibular Molars (Occlusal View)
The pattern of major grooves on mandibular second molars is simpler than that on first molars. It is made up of three major grooves: a central groove running mesiodistally, plus a buccal and a lingual groove (Fig. 5-19A). The central groove starts in the mesial triangular fossa, passes through the central fossa, and ends in the distal triangular fossa. Its mesiodistal course is straighter than on mandibular first molars. The buccal groove separates the mesiobuccal and distobuccal cusps and extends onto the buccal surface. The lingual groove separates the mesiolingual and the distolingual cusps but does not usually extend onto the lingual surface. The buccal and lingual grooves line up to form an almost continuous groove running from buccal to lingual that intersects with the central groove in the central fossa (Fig. 5-19B). The resultant groove pattern resembles a cross (or +).
Major grooves on most mandibular first molars separate five cusps instead of four, so the pattern is slightly more complicated (Fig. 5-22). As on second molars, the central groove passes from the mesial triangular fossa through the central fossa to the distal triangular fossa. The central groove may be more zigzag or crooked in its mesiodistal course. The lingual groove starts at the central groove in the central fossa and extends lingually between the mesiolingual and the distolingual cusps, but does not normally extend onto the lingual surface. Instead of one buccal groove, the mandibular first molar has two. Like the buccal groove on mandibular second molars, the mesiobuccal groove separates the mesiobuccal and distobuccal cusps. It starts in or just mesial to the central fossa (Fig. 5-22A and B) and extends from the buccal groove onto the buccal surface. This groove may be nearly continuous with the lingual groove, or it may not join with it. The distobuccal groove, UNIQUE to the first molar, starts at the central groove between the central fossa and the distal triangular fossa and extends between the distobuccal and the distal cusps often onto the buccal surface.
On both first and second mandibular molars, marginal ridge grooves are more likely to be present on mesial marginal ridges than distal marginal ridges.R Some mandibular first molars in Figure 5-18 have mesial marginal ridge grooves, but fewer have distal marginal ridge grooves.
On mandibular first and second molars, there may be supplemental (secondary, extra, minor) grooves, more likely on mandibular second molars than first molars.4 These grooves provide important escape ways for food as it is crushed. Supplemental ridges are located between supplemental and major grooves and serve as additional cutting blades. Grooves and ridges increase efficiency of food crushing.
6. Proximal Contact Areas and Embrasures of Mandibular Molars (Occlusal View)
The mesial and distal contact areas (crests of curvature) of mandibular first and second molars are normally slightly buccal to the middle of the tooth (Fig. 5-22B). Therefore, when molars are in contact with adjacent teeth, the lingual embrasure space is usually longer and larger than the space buccal to the contact. This becomes clinically relevant when restoring proximal surface contours on posterior teeth since access is better through the larger lingual embrasure than through the smaller buccal embrasure.
Also, mesial contacts are usually slightly buccal in position relative to distal contacts. Mesial contact areas on five-cusped mandibular first molars are slightly buccal to the center buccolingually, whereas distal contacts are just lingual to the distal cusp (Fig. 5-22). On mandibular second molars, mesial contacts are near the junction of the middle and buccal thirds, whereas the distal contacts are just buccal to the center buccolingually (Fig. 5-19B).
Take a break and find a small flashlight and possibly a small mirror to perform this exercise.
Clean your hands, and using the flashlight, visually examine your mouth to determine if your mandibular first molars have four or five cusps and if your second molars have four cusps. Also, try to determine if your first molars are larger than your seconds and if your seconds are larger than your thirds (which may be difficult to see). Then, if you can look into the mouth of a friend or family member without putting your hands in their mouth, look at their mandibular molars and answer the same questions.