| SECTION VI | TERMINOLOGY USED TO DESCRIBE THE MORPHOLOGY OF A TOOTH |
A. MORPHOLOGY OF AN ANATOMIC CROWN
Teeth are made up of many rounded elevations, ridges, depressions, and grooves. Specific tooth structures that occur with some frequency on teeth within a class have been assigned specific names. To identify the following anatomic structures, reference will be made to representative drawings of various teeth seen in figures throughout this book.
1. Elevations: Pointed Cusps and Linear Ridges
a. Cusps: Cusp Names and Numbers
A cusp is a pyramidal elevation with a peak called a cusp tip. Cusps are located on the occlusal surfaces of molars and premolars and on the incisal surfaces of canines. Canines have one cusp, premolars normally have two or three cusps, and most molars have from three to five cusps. On teeth with multiple cusps, each cusp is named according to its location on the tooth. For example, on a two-cusped premolar, the two cusps are named after the surface adjacent to each cusp: a buccal cusp and a lingual cusp. Three-cusped premolars have one buccal and two lingual cusps, and the two lingual cusps are named after the adjacent line angles, that is, mesiolingual cusp and distolingual cusp. A four-cusped molar has four cusps named after the adjacent line angles: mesiobuccal, distobuccal, mesiolingual, and distolingual. A three-cusped maxillary molar has two buccal cusps (mesiobuccal and distobuccal) and one lingual cusp. On a five-cusped molar, the three buccal cusps are called mesiobuccal, distobuccal, and the smallest distal cusp. Refer to Figure 1-21 for examples of cusp names on posterior teeth with two, three, four, and five cusps.
FIGURE 1-21. Cusp names on teeth having two, three, four, and five cusps, viewed from the occlusal and buccal views. Notice that the cusps are named after the adjacent surface or line angle EXCEPT on five-cusped mandibular first molars with three buccal cusps. On five-cusped molars, the two larger buccal cusps are named mesiobuccal and distobuccal cusps, as on the four-cusped molar, but the smallest cusp is called the distal cusp.
b. Cusp Ridges
Many cusps can be thought of as having four cusp ridges (linear prominences of enamel) converging toward the cusp tip. These four ridges form the shape of a four-sided pyramid with rounded surfaces. If you draw a line along the greatest linear bulge of each of these four ridges, the lines would intersect at the cusp tip indicated by the “X” on Figure 1-22. On this example of a buccal cusp on a premolar, three of the ridges are named after the circumferential tooth surface they extend toward: the more subtle buccal ridge extends onto the buccal surface, the mesial cusp ridge extends from the cusp tip toward the mesial surface, and the distal cusp ridge extends from the cusp tip toward the distal surface. The fourth ridge extends from the cusp tip toward the faciolingual middle of the tooth and is called a triangular ridge.
FIGURE 1-22. The four-cusped ridges of the buccal cusp of a two-cusped premolar have a somewhat pyramidal design (actually, a pyramid with rounded sides called a gothic pyramid). The cusp ridges are numbered 1 to 4 and converge at the cusp tip (at the “X”). Ridge 1 is the mesial cusp ridge of the buccal cusp; ridge 2 is the distal cusp ridge of the buccal cusp; ridge 3 is the more subtle buccal ridge of the buccal cusp; and 4 is the triangular ridge of the buccal cusp. (Courtesy of Drs. Richard W. Huffman and Ruth Paulson.) Cusp ridges on the single cusp of a maxillary canine also have a mesial cusp ridge labeled 1 and distal cusp ridge labeled 2, the same as on the premolar, but 5 is the labial ridge of the canine cusp (similar to a buccal ridge on a premolar), and 6 is the lingual cusp ridge.
The buccal ridges that run cervico-occlusally on the buccal surfaces of premolars or molars are often the least distinct of the four ridges that emanate from the cusp tip, although they may be more prominent on some types of teeth (Fig. 1-23). Lingual cusps do not normally have prominent lingual ridges running cervico-occlusally from the cusp tips.
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FIGURE 1-23. A rounded buccal ridge can be seen on the buccal surface of many premolars.
The mesial and distal cusp ridges are also known as cusp slopes or cusp arms. They are most evident when viewing teeth from the facial or lingual aspect where they can be seen as inclined ridges that converge toward the cusp tip to form an angle (seen in green on a buccal cusp of a premolar and on a buccal cusp of a molar in Fig. 1-24). For some teeth, the sharpness or bluntness of a cusp angle can be an defining trait. These ridges are more difficult to discern when viewing teeth from the occlusal, denoted in green on the two cusps of a premolar in Figure 1-25.
FIGURE 1-24. Cusp ridges (cusp slopes) are labeled on the buccal cusp of a mandibular premolar and on the mesiobuccal cusp of a four-cusped mandibular molar.
FIGURE 1-25. On this two-cusped maxillary premolar, the mesial and distal cusp ridges of the buccal and lingual cusps are shaded green. The triangular ridges of the buccal and lingual cusps are shaded blue and together are called a transverse ridge. The buccal ridge of the buccal cusp is shaded green.
Triangular ridges are located on the major cusps of posterior teeth. Each triangular ridge extends from a cusp tip toward the depression (sulcus) near the middle of the occlusal surface faciolingually, most easily identified when viewing a proximal surface as on Figure 1-26, but also evident when viewing the occlusal surface as on Figure 1-25. When a triangular ridge from a buccal cusp joins with a triangular ridge from a lingual cusp, these two ridges together form a longer ridge called a transverse ridge. A transverse ridge crosses the occlusal surface of posterior teeth in a more or less buccolingual direction, running between the buccal and lingual cusps on a premolar (seen from an occlusal view and a proximal view in Figs. 1-25 and 1-26) or connecting the buccal and lingual cusps that are lined up across from one another on a molar (seen on a mandibular molar in Fig. 1-27 and on the two-cusped premolar). An oblique ridge is found only on maxillary molars. It crosses the occlusal surface obliquely (diagonally) and is made up of one ridge on the mesiolingual cusp joining with the triangular ridge of the distobuccal cusp (seen in Fig. 1-27 on the maxillary molar). According to Ash,1 the ridge of the mesiolingual cusp that forms the lingual half of the oblique ridge is the distal cusp ridge of the mesiolingual cusp.
FIGURE 1-26. When seen from the mesial view on this maxillary two-cusped premolar, the outlines of two triangular ridges form a “V” shape and join at the depth of the occlusal sulcus to form one transverse ridge.
FIGURE 1-27. Three posterior teeth show transverse and oblique ridges. A. Two triangular ridges on a two-cusped premolar form one transverse ridge. B. Two pairs of triangular ridges on a mandibular molar join to form two transverse ridges. C. One pair of triangular ridges on a maxillary molar is aligned buccolingually and forms one transverse ridge in blue, and another pair of ridges is aligned obliquely (diagonally) to form an oblique ridge in purple.
The single cusp of many canines may also have four ridges emanating from its cusp tip (Fig. 1-22): a mesial cusp ridge and a distal cusp ridge, a labial ridge similar to a buccal ridge running cervicoincisally from the cusp tip, and sometimes a fourth ridge called a lingual ridge that extends lingually toward the cervical bulge (cingulum). These ridges can be prominent on maxillary canines (Fig. 1-28A and B).
FIGURE 1-28. A. On this maxillary canine, a labial ridge can be seen running from the cusp tip cervically along the labial surface. B. On this maxillary canine, a prominent lingual ridge is visible running from the cusp tip cervically to the cingulum.
c. Marginal Ridges and Cingulum
On the lingual of all anterior teeth, a cingulum [SING gyoo lum] (plural cingula) is the prominence or bulge in the cervical third of the lingual surface of the crown (incisors and canines) (seen on the lingual view in Fig. 1-29 and seen as a prominence in the cervical third of the crown on the proximal view in Fig. 1-30). On anterior teeth, mesial and distal marginal ridges form the mesial and distal borders of the lingual surface, and these ridges converge toward a rounded elevation or bulge in the cervical third called a cingulum, as seen on an incisor in Figures 1-29 and 1-30. When distinguishing a mesial from a distal marginal ridge on anterior teeth, it can be useful to remember that the mesial marginal ridge is normally longer than the distal. When determining which marginal ridge is longer, think of the length of a marginal ridge as extending from the incisoproximal line angle to its junction with the cingulum (as on Fig. 1-29 where the mesial marginal ridge appears slightly longer than the distal marginal ridge).
FIGURE 1-29. The mesial and distal marginal ridges are shaded red, and the cingulum is shaded green on the lingual surface of a maxillary incisor. If you think of the length of a marginal ridge as running from the proximal incisal line angles to the cingulum, you can see that this mesial marginal ridge is slightly longer than the distal marginal ridge.
FIGURE 1-30. This maxillary canine demonstrates that the cingulum bulge in green is located in the cervical third of the lingual surface. One visible marginal ridge is shaded red.
On posterior teeth, marginal ridges form the mesial and distal borders of the occlusal surface. The mesial marginal ridge on a premolar is shaded red in Figure 1-31.
FIGURE 1-31. Occlusal view of a two-cusped premolar shows the difference between the occlusal outline of the tooth (the black outline surrounding the entire tooth from this view) and the smaller red occlusal table (or occlusal chewing surface) bounded by six ridges: the two cusp ridges of the buccal cusp (green), the two cusp ridges of the lingual cusp (green), and the two marginal ridges (red).
d. Occlusal Table Outline versus Crown Outline
When viewing posterior teeth from the occlusal view, it is important to distinguish the entire crown outline of the tooth from the occlusal table of that tooth. The occlusal crown outline is the outer outline of the entire tooth crown from the occlusal view, whereas the occlusal table is the occlusal surface that is bounded by the continuous cusp ridges and marginal ridges. On the premolar in Figure 1-31, the occlusal table is bounded by a mesial marginal ridge joined with the mesial and distal cusp ridges of the buccal cusp, then the distal marginal ridge, and the cusp ridges of the lingual cusp.
This would be a good time to refer to Figure 1-32 and perform the learning exercise to test your knowledge of cusp ridges.
FIGURE 1-32. Identify the ridges numbered on this maxillary premolar.
LEARNING EXERCISE
The diagram in this Figure 1-32 the ridges seen from the occlusal view that bound the occlusal table of a two-cusped premolar. Name each ridge next to its corresponding number. (Note that ridges labeled 1, 3, 4, 5, 6, and 7 form a continuous outline on the occlusal surface. The area inside of this line is called the occlusal table.)
e. Other Bulges and Ridges
Other bulges or ridges can be seen on the cervical third of certain teeth facially or lingually. On the facial surface of permanent molars, the ridge or bulge running mesiodistally in the cervical one third of the facial surface of a crown is called the cervical ridge. This ridge forms the greatest bulge on the buccal surface, which is known as the crest of curvature (or height of contour) (Fig. 1-33A and B). This ridge is most evident on mandibular second molars where the occlusal outline of the mesiobuccal cusp appears to bulge (Fig. 1-33C).
FIGURE 1-33. A. On this mandibular first molar from the buccal view, a buccal cervical ridge is the rounded prominence located in the cervical third of the crown. B. From the proximal view, the buccal cervical ridge forms the greatest bulge or crest of curvature in the cervical third of this molar. C. From the occlusal view on this four-cusped mandibular second molar, the buccal cervical ridge forms a prominent bulge on the mesiobuccal outline.
Mamelons are three small bulges or tubercles on the incisal edges of newly erupted incisors (Fig. 1-34). Usually, mamelons are not evident on adult dentition since they are worn off after the tooth comes into functional contact with its opposing teeth. If you have the opportunity, observe a 7-year-old smile to see these mamelons on newly erupted permanent incisors. When mamelons remain on the incisors of an adult, it is because maxillary and mandibular anterior teeth do not touch together to wear away the enamel. When a patient desires, the dentist can reduce the mamelons to make the incisal edge more uniformly curved.
FIGURE 1-34. Three distinct unworn mamelons are evident on the incisal edge of this mandibular incisor.
Finally, perikymata [pear i KY mah tah] are the numerous, minute horizontal ridges on the enamel of newly erupted permanent teeth (Fig. 1-35). They form from the overlapping of layers of enamel laid down during tooth formation. These lines are closer together in the cervical third of the crown than in the incisal third. Perikymata are more prominent on the teeth of young people than on the teeth of older persons because perikymata, like mamelons, wear away from ongoing abrasion due to eating and even tooth brushing with abrasive toothpastes.
FIGURE 1-35. A. Perikymata are the small ridges visible on the labial surface of this incisor. B. Magnified cross section of enamel shows perikymata ridges on the tooth surface (on the right half). C. Higher magnification (220×) shows the enamel rods that make up enamel ending on the perikymata waves. (These scanning electron micrographs were provided by Dr. Ruth B. Paulson, Associate Professor Emeritus, Division of Oral Biology, the Ohio State University.)
2. Depressions and Grooves of an Anatomic Crown
An occlusal sulcus [SUL kuss] (plural sulci [SUL sye]) of a tooth is the broad V-shaped depression or valley on the occlusal surface of each posterior teeth running mesiodistally between the buccal and lingual cusps. The sulcus is formed by the sloping of the buccal and lingual triangular ridges that converge toward the developmental grooves located in the depth of the sulcus on each posterior tooth (Fig. 1-36). Although a sulcus is a linear depression, there is much variation to the anatomy within the sulcus of each tooth. For example, there are triangular and marginal ridges (discussed previously) and grooves (channels) within the sulcus, as well as multiple depressions, each called a fossa [FAH sah] (plural, fossae [FAH see].
FIGURE 1-36. This two-cusped premolar has a sulcus between the buccal and lingual cusps.
GROOVES: A groove is a linear channel often found between cusps in the depth of the sulcus and between ridges. They serve as important escape ways for food morsels when the teeth of the lower jaw move from side to side and forward against the upper teeth during chewing. Partially chewed food squirts out through grooves toward the tongue and cheeks. Each tooth has major developmental grooves, which are often consistent in location for teeth of the same type, and other minor, supplemental grooves, which can vary greatly from tooth to tooth. Developmental grooves separate cusps and other major portions of a tooth formed from the developmental lobes (described later in Section IX of this chapter). On most posterior teeth like the premolar in Figure 1-37A, a central groove is a developmental groove that separates the buccal from the lingual cusps and is located near the buccolingual center of the tooth sulcus. Other developmental grooves are named according to their location. For example, on mandibular molars with two buccal cusps, a buccal groove separates the mesiobuccal and distobuccal cusps and is likely to extend onto the buccal surface. On mandibular molars with three buccal cusps, there are two grooves separating the three buccal cusps, so they are called a mesiobuccal groove that separates the mesiobuccal cusp from the distobuccal cusp, and a distobuccal groove that separates the distobuccal cusp from the smallest distal cusp (Fig. 1-37B). Both of these grooves are likely to extend onto the buccal surface. Mandibular molars also have a lingual groove between the mesiolingual and distolingual cusp, but these grooves are not likely to extend onto the lingual surface. Most maxillary molars have a lingual groove that often extends onto the lingual surface between the mesiolingual and distolingual cusps, and a buccal groove between the mesiobuccal and distobuccal cusps that does not normally extend onto the buccal surface (Fig. 1-37).
FIGURE 1-37. A. This two-cusped premolar has developmental (major) and supplemental (extra) occlusal grooves. (Courtesy of Drs. Richard W. Huffman and Ruth Paulson.) B. Grooves are labeled on a mandibular four-cusped molar (upper left and center), a mandibular five-cusped molar (upper right) and a maxillary molar with four major cusps (lower row). The buccal, lingual, and central grooves are considered developmental grooves. The buccal groove extends between the two buccal cusps onto the buccal surface on mandibular molars, and the lingual groove extends between the two lingual cusps onto the lingual surface of maxillary molars. Five-cusped mandibular first molars have three buccal cusps, so there are two developmental grooves: a mesiobuccal groove between the mesiobuccal and distobuccal cusps and a distobuccal groove between the distobuccal and distal cusps.
Additional occlusal grooves that are not developmental grooves are called supplemental grooves. These small irregular (extra) grooves do not occur at the junction of the lobes or major portions of the tooth and do not occur at the same location on teeth of the same type, so these extra grooves are normally not assigned a specific name.
FOSSAE: Located at the mesial and distal ends of the central groove on each posterior tooth within the occlusal table and next to the mesial and distal marginal ridges, there are shallow depressions called a mesial fossa and distal fossa (sometimes called a mesial and distal triangular fossa) (Fig. 1-38). Fossa grooves (also called fossa developmental grooves or triangular fossa grooves) may be found within these fossae splitting off of the ends of the central groove directed toward the line angles of the tooth. These grooves can be named for the line angles of the tooth toward which they aim, for example, in Figure 1-38, the distobuccal developmental groove (more precisely called the distobuccal triangular or fossa groove) runs toward the distobuccal line angle of the tooth. A pit may form at the depth of a fossa where central groove joins the fossa grooves. For example, within the distal fossa on a premolar, there is a distal pit at the junction of the central groove with the distobuccal and distolingual fossa grooves (Fig. 1-38). Most molars and three-cusped premolars have an additional central fossa seen in Figure 1-39.
FIGURE 1-38. The mesial and distal fossae are outlined in red on this maxillary two-cusped premolar.
FIGURE 1-39. Fossae and pits are labeled on teeth with two, three, and four cusps. Two-cusped teeth have two fossae (a mesial and a distal fossa), while three- or four-cusped teeth also have a third fossa called a central fossa.
Many anterior teeth have a shallow, broad lingual fossa that is located on the lingual surface between the mesial and distal marginal ridges and just incisal to the cingulum (particularly on maxillary incisors, Fig. 1-40). The lingual ridge of some maxillary canines may divide the lingual surface into two fossae: a mesial fossa bounded by the mesial marginal ridge and the lingual ridge and a distal fossa bounded by the distal marginal ridge and the lingual ridge (Fig. 1-41).
FIGURE 1-40. The lingual surface of this maxillary lateral incisor shows the shallow lingual fossa and an adjacent lingual pit.
FIGURE 1-41. This maxillary canine has a lingual ridge that divides the lingual surface into a mesial fossa and a distal fossa.
Deep, defective pits and fissures may be found at the depth of fossae and grooves caused by the incomplete fusion of enamel during tooth development (at the white arrow in Fig. 1-42). Because it is very difficult to remove food debris from these inaccessible fissures, tooth decay (also called dental caries [CARE eez]) often begins in the deepest part of a fissure or pit (seen spreading out within dentin as the dark area between the two black arrows) and described in more detail in Chapter 10. These carious pits and fissures are most likely to be located in four areas: at the depth of deep grooves and pits on the occlusal surface of posterior teeth, in buccal grooves that extend onto the buccal surface of mandibular molars, in lingual grooves that extend onto the lingual surface of maxillary molars, and on the lingual surface of maxillary incisors where the lingual fossa joins the cingulum (Fig. 1-43).
FIGURE 1-42. A cross section of a mandibular molar shows an occlusal groove (white arrow), which actually has a fissure (crack-like fault) extending through the outer enamel and into the dentin. The black arrows show how the dental decay spreads out once it reaches softer dentin at the depth of this fissure.
FIGURE 1-43. Caries-prone pits and fissures are located in four places (arrows). A. The occlusal surfaces of posterior teeth. B. The lingual surfaces of maxillary molars. C. The buccal surfaces of mandibular molars. D. The lingual surfaces of maxillary incisors.
In summary, if you compare tooth morphology to a mountain range, the mountain peak would be the cusp tip. Ridges emanating from the mountain peak are like the cusp ridges and triangular ridges. The depression between the mountains is a valley, like the depressions between cusps is a sulcus. The dried river bed at the bottom of the valley is like a groove at the bottom of the sulcus. If the riverbed is cracked open, it is like a fissured groove. Where two rivers converge (as when grooves or fissures converge), the whirlpools and eddies may have formed a depression, like a fossa, possibly with a pit at its depth. Needless to say, it is difficult to define exactly where a mountain stops and the valley begins, just as it would be difficult to define exactly where a tooth cusp stops and a sulcus or fossa begins. Just realize that these terms are not precise, but that they are helpful when learning how to reproduce tooth form during construction of crowns and placement of fillings or when learning to finish and polish an existing filling.
B. EXTERNAL MORPHOLOGY OF THE ANATOMIC ROOT
Recall that the anatomic root is the part of a tooth that is covered with cementum. The apex of the root is the tip or peak at the end of the root, often with visible openings called apical foramina, where the nerves and blood vessels enter into the tooth pulp canals. The cervix [SUR viks] or neck of the tooth is the slightly constricted region surrounding the junction of the crown and the root (Fig. 1-44B).
FIGURE 1-44. A. Root anatomy on a single-rooted canine. B. Root anatomy of a bifurcated (split) root on a maxillary first premolar.
Some new terms apply to multirooted teeth (Fig. 1-44B). The root trunk or trunk base is the part of the root of a multirooted molar or two-rooted premolar next to the cementoenamel junction that has not yet split (like a stubby tree trunk before it gives off branches). The furcation [fur CAY shun] is the place on multirooted teeth where the root trunk divides into separate roots (called a bifurcation on two-rooted teeth and a trifurcation on three-rooted teeth). The furcal region or interradicular space is the region or space between two or more roots, apical to the furcation where the roots divide from the root trunk.
C. CERVICAL LINE (CEMENTOENAMEL JUNCTION OR CEJ) CURVATURE
When viewed from the mesial or distal aspect, the cervical line of a tooth curves (is convex) toward the incisal or occlusal surface (Fig. 1-44). In general, the amount of curvature is greater on the mesial surface than on the distal surface of the same tooth, and the amount of curvature is greatest for central incisors and diminishes in size for each tooth when moving distally around each quadrant (Table 1-2).
| TABLE 1-2 | Summary of Curvatures of the Cementoenamel Junction |
|
Proximal surfaces: mesial curvature vs. distal curvature |
Generally, teeth have a greater proximal cervical line curvature on the mesial than the distal. |
|
|
Proximal surfaces: anterior teeth vs. posterior teeth |
Proximal cervical line curvatures are greatest on the mesial surfaces of central incisors and, for most teeth, tend to get smaller when moving from the anterior teeth toward the last molar where there may be no curvature at all. |
|
|
Posterior teeth: facial vs. lingual surface |
On many posterior teeth, the cervical line is in a more occlusal position on the lingual than on the facial. |
In order to document the relative sizes of tooth crowns and roots, Dr. Woelfel studied a convenient sample of 4572 extracted teeth. His findings are presented in Table 1-7 at the end of this chapter. This table should not be memorized, but it can be useful when comparing the average dimensions of each tooth and in order to appreciate the wide range of dimensions for each tooth. A summary of the most important highlights of that data is presented in Table 1-3.
| TABLE 1-3 | Important Tooth Dimensions to Memorize |
|
Tooth with longest crown |
Mandibular canine (Woelfel research: maxillary incisor) |
|
Longest tooth overall |
Maxillary canine |
|
Longest root |
Maxillary canine |
|
Widest crown mesiodistally |
Mandibular first molar |
|
Widest crown buccolingually |
Maxillary first molar |
|
Narrowest crown mesiodistally |
Mandibular central incisor |
|
Greatest cervical line curve |
Mesial of maxillary incisor |
| TABLE 1-7 | Average Measurements on 4572 Extracted Teeth Obtained From Ohio Dentists during a Study by Dr. Woelfel and his First-Year Dental Hygiene Students of the Ohio State University College of Dentistry, 1974–1979 |
Size ranges are shown in TABLEs in each chapter.
Key for Tooth Surface Abbreviations: D, distal; DB, distobuccal; FL, faciolingually; L, lingual; M, mesial; MB, mesiobuccal; MD, mesiodistal.
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