Permanent Posterior Teeth
Learning Objectives
• Demonstrate the correct location of each permanent posterior tooth on a diagram and a patient.
• Define and pronounce the key terms when discussing the permanent posterior teeth.
New Key Terms
Bicuspid (bi-kus-pid)
Bifurcated (bi-fer-kay-ted)
Cusp planes (kusp), ridges
Cusp of Carabelli (kusp kare-ah-bell-ee), groove
Dilaceration (di-las-er-ay-shun)
Fluting (fos-ah) (plural, fossae, fos-ay): central, triangular
Fossa (fos-ah) (plural, fossae, fos-ay): central, triangular
Furcation (fer-kay-shin), crotches
Groove: central, marginal, triangular (kusp-al)
Inclined cuspal planes (kusp-al)
Molars (mo-lers): mulberry, (mull-bare-ee), peg
Occlusal (ah-kloo-zl) developmental pits, table
Ridge: oblique (obleek), transverse (trans-vers), triangular
Root fusion (try-fer-kay-ted)
Trifurcated (try-fer-kay-ted)
Permanent Posterior Teeth
The permanent posterior teeth include the premolars and molars (Figure 17-1, see Figure 2-4 15-2). The crown of each has an occlusal surface as its masticatory surface, bordered by the raised marginal ridges that are located on both the distal surface and mesial surface (Figure 17-2). The occlusal surface also has two or more cusps. Some anatomists liken a cusp to a gothic pyramid, with four cusp ridges descending from each cusp tip. Between these cusp ridges are sloping areas, or four inclined cuspal planes. These planes are named by combining the names of the two cusp ridges that are between them. Some inclined planes are functional and thus involved in the occlusion of the teeth (see Chapter 20).
The occlusal surface of permanent posteriors creates an inner occlusal table bordered by the marginal ridges (Figure 17-3). There are also triangular ridges, which are cusp ridges that descend from the cusp tips toward the central part of the occlusal table (Figure 17-4). They are so named because the slopes of each side of the ridge are inclined in a way that resembles two sides of a triangle. Thus, the triangular ridges are specifically named for the cusps to which they belong. Additionally present on many posteriors is a transverse ridge, a collective term given to the joining of two triangular ridges crossing the occlusal table transversely, or from the labial to the lingual outline.
Each shallow and wide depression on the occlusal table is a fossa, (plural, fossae). One type of fossa on posteriors, the central fossa, is located at the convergence of the cusp ridges in a central point, where the grooves meet. Another type of fossa is the triangular fossa, which appears to have a triangular shape at the convergence of the cusp ridges, and is associated with the termination of the triangular grooves (discussed next). Sometimes located in the deepest parts of the fossae are occlusal developmental pits; each pit is a sharp pinpoint depression where two or more grooves meet.
Developmental grooves, or primary grooves, are also found on the occlusal table. The developmental grooves on each different posterior tooth type are located in the same place and mark the junction between the developmental lobes. The grooves are sharp, deep, V-shaped linear depressions. The most prominent developmental groove on posteriors is the central groove, which generally travels mesiodistally and separates the occlusal table buccolingually.
Other developmental grooves are marginal grooves, which cross the marginal ridges and serve as a spillway, allowing food to escape during mastication. Finally, there are triangular grooves that separate a marginal ridge from the triangular ridge of a cusp, and at their terminations form the triangular fossae.
In contrast, supplemental grooves, or secondary grooves, appear as shallower, more irregular linear depressions (Figure 17-5). Supplemental grooves branch from the developmental grooves, but these grooves are not always present in the same pattern on the occlusal table of each different tooth type. In general, the more posterior a tooth is located in the dental arch, the more supplemental grooves are present, such that the occlusal table appears more wrinkled.
When examined from the buccal and lingual views, the crown outline of the posteriors is trapezoidal, or four-sided with only two parallel sides (not including the occlusal surface cusp form of posteriors). Thus, the longer of the two parallel sides is toward the occlusal aspect. This arrangement is quite important in the functioning of the teeth.
For posteriors, the height of contour, or crest of curvature, for the crown’s buccal surface is in the cervical third, and the lingual surface is in the middle or occlusal third (Figure 17-6). When compared with
Clinical Considerations for Posterior Teeth
The complex pit and groove patterns on the occlusal surface of posteriors can put them at an increased risk of caries (Figure 17-7). This susceptibility is due to increased dental biofilm retention and the weakness of enamel forming the walls of the pits and grooves (see Figure 12-4, A). All pits and grooves must be checked for decay with an explorer and mirror. Clinicians need to be aware of these pit and groove patterns on posteriors when they examine dentitions to assess the patient’s caries risk level. All pits and grooves must be checked for decay with an explorer and mirror. Light-induced devices that measure changes in laser fluorescence of hard tissue allow dental professionals to better diagnose early lesions in pits and grooves. Posteriors with deep pit and groove patterns, but without incipient decay, should have enamel sealants applied as soon as they erupt.
anteriors, the posteriors are wider labiolingually than mesiodistally, except for the mandibular molars.
In another comparison with anteriors, the contact areas of each of the posteriors is wider, usually located to the buccal of center, and is nearer the same level on each proximal surface (see Figure 15-10). In addition, on each proximal surface is a cementoenamel junction (CEJ) curvature that is less pronounced on the posteriors than on the anteriors. In fact, the CEJ is often quite straight for posteriors.
Like anterior teeth, multirooted premolars and molars originate as a single root on the base of the crown. This part on these posterior teeth is considered the root trunk. The cervical cross section of the root trunk initially follows the form of the crown. However, the root of a posterior tooth divides from the root trunk into the correct number of root branches for its tooth type, either two (bifurcated) or three (trifurcated) (see Figure 6-21).
Permanent Premolars
General Features
Permanent premolars are the most anteriorly placed posteriors in the permanent dentition (Figure 17-8, Table 17-1). Each dental arch has four premolars, two to each quadrant.
TABLE 17-1
Anatomical Information on Permanent Premolars
| MAXILLARY FIRST PREMOLAR | MAXILLARY SECOND PREMOLAR | MANDIBULAR FIRST PREMOLAR | MANDIBULAR SECOND PREMOLAR | |
| Universal number | #5 and #12 | #4 and #13 | #21 and #28 | #20 and #29 |
| General crown features | Occlusal table with marginal ridges and cusps, with tips, ridges, inclined planes, grooves, fossae, pits; buccal ridge | |||
| Specific crown features | Larger than second, with buccal cusp longer of two, long central groove | Smaller than first, two cusps same length, short central groove, no mesial surface features like first, increased supplemental grooves | Smaller than second, smaller lingual cusp of two, mesial surface features | Larger than first, usually three cusps: Y groove pattern or two cusps: H or U groove pattern, increased supplemental grooves |
| Mesial and distal contact | Just cervical to the junction of occlusal and middle thirds | |||
| Distinguishing right from left | Longer mesial cusp slope, mesial features: marginal groove, developmental depression, deeper CEJ curvature | Lingual cusp offset to the mesial | Shorter mesial cusp slope, mesiolingual groove, deeper mesial CEJ curvature | Distal marginal ridge more cervically located, thus more occlusal surface visible from distal view |
| General root features | Proximal root concavities | |||
| Specific root features | Bifurcated with root trunk; elliptical on cross section | Single-rooted; elliptical on cross section | Single-rooted; ovoid or elliptical on cross section | |
There are two types of premolars: first premolar and second premolar. One of each type is present in each quadrant of each dental arch. The first is closer to the midline at the fourth position from it. The second is next to the first premolar and is in the fifth position from the midline. Both types are distal to the permanent canine and mesial to the permanent first molar when full eruption of the permanent dentition has occurred. Permanent premolars are succedaneous, because they replace the primary first and second molars.
As posterior teeth, premolars have a shorter crown than anterior teeth. The buccal surface is rounded and has a prominent vertical buccal ridge in the center of the crown (Figure 17-9). Two buccal developmental depressions are noted on each side of the buccal ridge. The buccal ridge of premolars is similar to the labial ridge of the canines and may be related to the increased development of the middle buccal lobe. The height of contour, or crest of curvature, of the crown buccally is in the cervical third, as in anterior teeth. Lingually, the height of contour for premolars is in the middle third.
An older term for a premolar was bicuspid, because of the usual presence of two cusps on the occlusal surface, one more cusp than in the canines. However, the mandibular second premolar frequently has three cusps. Thus, the name premolar is more widely used because these teeth are located anterior to the molars.
Finally, in addition to the cusps, the occlusal surface of a premolar, similar to all posteriors, has marginal ridges, triangular ridges, developmental grooves, and occlusal developmental pits. The boundaries of the occlusal surface, the marginal ridges and cusp ridges, form an inner occlusal table.
Clinical Considerations for Premolars
Premolars function to assist the molars in grinding food during mastication, because of their broad occlusal surface and their prominent cusps. Premolars also assist the canines in piercing and tearing food with those cusps. These teeth, along with the canines, also help maintain the height of the lower third of the vertical dimension of the face and support the facial muscles, especially those muscles in the corners of the mouth. Thus, the premolars are involved in esthetics and speech, less so than the anteriors but more so than the molars.
Single, permanent premolars can be extracted in each quadrant during orthodontic therapy to improve dental arch spacing. If a premolar has been extracted, the distinctive pit and groove patterns on the occlusal surface will help in identifying the remaining premolars when the arch space from the extraction is lost if the remaining molars are not restored. However, orthodontic therapy tends instead to include expansion of the jaw, if needed, instead of removing premolars to retain a more natural rounded curved shape to the arches. If extraction is unavoidable, first premolars are usually extracted more often than second premolars. Additionally, they present difficulties in instrumentation of the root because they have proximal root concavities, especially on the mesial of the maxillary first premolar.
In addition, most premolars usually have one root, except for the permanent maxillary first premolar, which has two roots. Whether one or two roots are present, they have proximal root concavities
Permanent Maxillary Premolars
General Features
Both types of maxillary premolars resemble each other more than do the mandibular premolars. A maxillary first premolar is larger than a maxillary second premolar, but, in contrast, a mandibular first premolar is smaller than a mandibular second premolar. Both maxillary premolars erupt earlier than the mandibular premolars.
The crown of a maxillary premolar is shorter occlusocervically than that of a maxillary canine, but is slightly longer than that of a molar. The crown outline from the proximal aspect is trapezoidal, or four-sided with only two parallel sides, similar to all maxillary posterior teeth.
The crown is also centered over the root and shows no lingual inclination, which is unlike the mandibular premolars or other mandibular posteriors. They also have a greater buccolingual width than mesiodistal width compared with the mandibular premolars or other mandibular posterior teeth when viewed from the occlusal. The outline for both maxillary premolars is somewhat hexagonal, with six sides, and almost oval compared with the rounder mandibular premolars.
Both maxillary premolars have two cusps of almost equal size. In contrast, the mandibular premolars can have more than two cusps, but any lingual cusps are always smaller. The cusps of all premolars are centered over the long axis of the tooth from either proximal view. The maxillary premolars are composed of four developmental lobes: three buccal and one lingual.
Additionally, the roots of the maxillary premolars are shorter than the maxillary canine’s roots, but the root length is the same as that of the molars. The roots show slight lingual and distal inclination. The roots on cervical cross section are elliptical, or an elongated oval, which may be slightly altered by proximal root concavities.
Clinical Considerations for Maxillary Premolars
The roots of maxillary premolars may penetrate the anterior part of the maxillary sinus as a result of accidental trauma or during tooth extraction because of the close relation of these roots to the sinus walls (see Figure 11-21). In addition, the discomfort of sinusitis can be mistakenly interpreted as tooth-related (stemming from the maxillary premolar), and vice versa. Thus, radiographic study of the tooth or maxillary sinus and other diagnostic tests are necessary to determine the true cause of the discomfort.
Permanent Maxillary First Premolars #5 and #12
Specific Overall Features (Figure 17-10)
Permanent maxillary first premolars erupt between 10 and 11 years of age (root completion between ages 12 to 13). These teeth erupt distal to the primary maxillary canines or their arch space, and thus are the succedaneous replacements for the primary maxillary first molars.
The crown of a maxillary first premolar has an angular shape with sharp outlines compared with a maxillary second premolar’s more rounded shape. The tooth’s two cusps are also sharply defined, with the buccal cusp usually about 1 mm higher than the lingual cusp. The tooth appears bent mesially when viewed from the occlusal compared with the second premolar of the same arch. The central groove on the occlusal surface is also longer on the maxillary first premolar than on the second.
Most maxillary first premolars are bifurcated, having two root branches in the apical third, with a buccal root and a lingual (palatal) root. This is unlike the other premolars, which are single-rooted. Maxillary first premolars originate as a single root on the base of the crown, as do other premolars and anteriors; this part is considered the root trunk.
A cervical cross section of the root trunk follows the form of the crown. The root trunk usually makes up half the length of the entire root, and the root branches make up the other half. The roots are rounded overall and taper to sharp apices. The buccal root of this tooth is larger but not longer than the lingual root. A distinct mesial root concavity is present on the root trunk of the maxillary first premolar, extending from the contact area to the bifurcation. The mesial surface groove on the root puts this tooth possibly at an increased risk for periodontal disease because it allows an increased deposit level. The distal surface has a groove that is reduced in depth, creating a convex or flat surface.
This trunk can also have a root fusion, with little of the root bifurcated. If a single root is present, which occurs in 20% of the population, it is wider buccolingually than mesiodistally, the buccal and lingual surfaces are rounded, and the root is tapered to a blunt apex. In cross section, the root becomes kidney shaped. A single root also has a deep and wide mesial surface root concavity, which ranges from relatively shallow to deep enough to almost bifurcate the root. Three-rooted, or trifurcated teeth have been noted, with two buccal roots and a single lingual root.
The pulp cavity for a two-rooted tooth usually shows two pulp horns (one for each cusp) and two pulp canals (one for each root) (Figure 17-11). Even if there is only one undivided root, as for the maxillary second premolar, two pulp canals are usually found, although they often combine to form one apical foramen.
Buccal View Features
The crown of a maxillary first premolar is the widest mesiodistally of all the premolars (Figure 17-12). This tooth’s crown is wide at the level of the contact areas, becoming narrower at the CEJ, similar to the maxillary canine. The mesial contact with the maxillary canine is just cervical to the junction of the occlusal and middle thirds. The distal contact with the maxillary second premolar is the same, just cervical to the junction of the occlusal and middle thirds.
The mesial and distal outlines of the crown of the maxillary first premolar are almost straight from the contact areas to the CEJ, but the mesial outline is more rounded. Both these outlines converge more toward the cervical than they do on the maxillary second premolars. Imbrication lines and perikymata are found on the buccal surface, and these extend mesiodistally in the cervical third. The CEJ curvature of the tooth is evenly rounded toward the apex of the tooth and has less depth than on anterior teeth.
The buccal cusp of a maxillary first premolar is high and sharp, located slightly distal to the long axis of the tooth because the two cusp slopes of the buccal cusp are not equal in height. This tooth is the only tooth in the permanent dentition that has a buccal cusp with the mesial cusp slope longer than the distal cusp slope, which helps to distinguish the right maxillary first premolar from the left. This relationship of the cusp slopes normally exists upon eruption; attrition may change it. A bulge may be found occasionally on the buccal cusp of this tooth.
Lingual View Features
The lingual surface of the maxillary first premolar is rounded in all directions but is smaller than the buccal surface. The shorter lingual cusp is sharp but not as sharp as the buccal cusp and is offset toward the mesial. Thus, the cusp slopes of the lingual cusp are again not equal in length. From the lingual aspect, however, the mesial cusp slope is shorter than the distal cusp slope.
Proximal View Features
On the mesial surface of the crown of a maxillary first premolar, the mesial marginal ridge is present on the concave occlusal margin. A mesial marginal groove is also sometimes present (Figure 17-13). This developmental groove crosses the mesial marginal ridge and extends from the occlusal to the middle third of the crown, lingual to the contact area.
The mesial surface usually also has a mesial developmental depression located cervical to the contact area, across the CEJ, normally extending onto the root. On the root, the depression joins a deep developmental root concavity between the roots. The CEJ curvature is more occlusally located on the mesial than on the distal surface. All these prominent mesial features from the proximal view help to distinguish the right maxillary first premolar from the left.
The distal surface is similar to the mesial, except that it does not have a depression, and more of the occlusal surface shows because the distal marginal ridge is more cervically located than is the mesial marginal ridge. A distal marginal groove is sometimes located across the distal marginal ridge, but this distal groove is shallower than the groove on the mesial surface. Additionally, the CEJ curvature on the distal surface is not as deep cervically as the mesial.
Occlusal View Features
The outline of the occlusal surface of a maxillary first premolar is somewhat hexagonal, or six-sided, but is wider buccolingually than mesiodistally (Figure 17-14). The buccal ridge (or buccal cusp ridge of the buccal cusp, as discussed later) is prominent on the buccal margin, and the lingual margin of the occlusal outline is almost a semicircle. The mesial and distal margins are straight as they converge toward the lingual. Thus, the lingual part of the tooth is narrower mesiodistally than the buccal part. When the mesial marginal groove is prominent, it may create a dip in the mesial outline.
Occlusal Table Components
The buccal cusp of a maxillary first premolar is sharper and higher than the lingual cusp. The occlusal function of the buccal cusp involves only its lingual surface. Four buccal cusp ridges descend from the buccal cusp tip, each named for its location: buccal, lingual, mesial, and distal. Because this is the first occlusal table of a posterior tooth under discussion, this text provides specific details on each of the occlusal table features; this information can then be related to the occlusal tables of other posterior teeth.
The buccal cusp ridge of the buccal cusp extends cervically from the cusp tip on the buccal surface and corresponds to the buccal ridge. The lingual cusp ridge extends lingually from the buccal cusp tip to the central groove (also called the buccal triangular ridge or buccal part of the transverse ridge, as discussed later). The mesial cusp ridge of the buccal cusp extends mesially from the cusp tip to the mesiobucco-occlusal point angle. The distal cusp ridge extends distally from the buccal cusp tip to the distobucco-occlusal point angle.
Between the cusp ridges are four buccal-inclined cuspal planes, named for the two cusp ridges between which they lie: mesiobuccal, mesiolingual, distobuccal, and distolingual. However, only the mesiolingual- and distolingual-inclined cuspal planes function during occlusion.
The lingual cusp of the maxillary first premolar is rounder, less sharp, and shorter than the buccal cusp. This cusp is also located well to the lingual and offset to the mesial. Again, there are four lingual cusp ridges and four lingual-inclined cuspal planes similar to those associated with the buccal cusp, but all the lingual inclined cuspal planes are functional. This is because the entire lingual cusp functions during occlusion, unlike the buccal cusp.
Extending mesiodistally, across the occlusal table of the maxillary first premolar, is a long central groove, evenly dividing the tooth buccolingually. The central groove is a developmental groove that is sharply defined; it is deep and V-shaped. A few supplemental grooves appear irregular in shape and shallower, and they branch from the central groove. Thus, the occlusal surface is relatively smooth compared with the adjacent maxillary second premolar.
The lingual cusp ridge, which runs from the buccal cusp tip to the central groove, is also termed the buccal triangular ridge (Figure 17-15). The buccal cusp ridge of the lingual cusp is also termed the lingual triangular ridge, because it runs from the lingual cusp tip to the central groove. Perpendicular to the central groove is a transverse ridge, the collective term given to the joining of the buccal triangular ridge and the lingual triangular ridge.
The central groove of the maxillary first premolar also crosses to the mesial marginal ridge, which is shorter than the distal marginal ridge. Extending from the central groove, another developmental groove, the mesial marginal groove, crosses the mesial marginal ridge and travels onto the mesial surface of the tooth.
Descending down the slope of the buccal cusp, and just inside the distal and mesial marginal ridges, are two developmental grooves, the mesiobuccal triangular groove and the distobuccal triangular groove. Across the occlusal table, the lingual cusp also has two developmental grooves, the mesiolingual triangular groove, and the distolingual triangular groove.
Each of these triangular grooves ends in a triangular depression called a triangular fossa. These fossae are the deeper mesial triangular fossa, which surrounds the mesiobuccal triangular groove, and the shallower distal triangular fossa, which surrounds the distobuccal triangular groove.
The boundaries of the mesial triangular fossa are the mesial marginal ridge, the transverse ridge, and the mesial cusp ridges of the two cusps. The distal triangular fossa has boundaries similar to those of the mesial fossa in a mirror-image fashion. The deepest parts of these fossae are the occlusal developmental pits. These are termed the mesial pit and distal pit, respectively, and they are connected by the central groove.
Permanent Maxillary Second Premolars #4 and #13
Specific Overall Features (Figure 17-16)
Permanent maxillary second premolars erupt between 10 to 12 years of age (root completion between ages 12 to 14). These teeth erupt distal to the permanent maxillary first premolars, and thus are the succedaneous replacements for the primary maxillary second molars.
A maxillary second premolar resembles a first premolar, except that its crown is less angular and more rounded. Additionally, more crown variations, especially in its occlusal surface anatomy, are noted in this tooth as compared with maxillary first premolars.
Unlike a maxillary first premolar, a maxillary second premolar usually has only a single root, but it may occasionally have two roots. The dimensions between the maxillary second and the first premolars are usually about the same overall, except for greater root length of the second. The mesial concavity is not as pronounced as in a first premolar. The pulp cavity of this tooth has two pulp horns and one single pulp canal (Figure 17-17).
Buccal View Features
The buccal cusp of a maxillary second premolar is neither as long nor as sharp as that of a maxillary first premolar (see Figure 17-16). All other features of the buccal surface of a maxillary second are similar to those of the first. Again, the mesial contact with a maxillary first premolar is just cervical to the junction of the occlusal and middle thirds on the buccal surface. The distal contact with a maxillary first molar is the same, just cervical to the junction of the occlusal and middle thirds.
Lingual View Features
All lingual surface features of a maxillary second premolar are similar to those of a maxillary first premolar. One noteworthy exception is that the lingual cusp is larger, almost the same height as the buccal cusp on a maxillary second premolar. In addition, the lingual cusp is slightly displaced to the mesial, which helps to distinguish the right maxillary second premolar from the left. In addition, less of the occlusal surface is seen from this view because the crown is longer on the lingual.
Proximal View Features
The mesial surface of a maxillary second premolar is similar to that of a maxillary first premolar, except that the cusps are closer to being the same size, and no mesial developmental depression is present on the crown and root. Instead, this area cervical to the contact area is rounder.
In addition, this tooth has no mesial marginal groove. Both the contact areas and mesial marginal ridge are more cervically located than those on a maxillary first premolar. The distal surface is the same as the mesial surface without any distal marginal groove, but the contact area is larger.
Occlusal View Features
The outline of the occlusal surface of a maxillary second premolar is more rounded and larger overall than that of a maxillary first premolar from the occlusal view. Thus, the hexagonal outline of the crown from the occlusal is more difficult to see.
Occlusal Table Components
The central groove is shorter on a maxillary second premolar than on a maxillary first premolar (Figure 17-18). This groove ends in a mesial pit and distal pit, which are closer together and thus more to the middle of the occlusal table.
A maxillary second premolar has numerous supplemental grooves radiating from the central groove. This gives the tooth a more wrinkled appearance compared with a maxillary first premolar. Other features and the overall anatomy of this tooth’s occlusal surface are similar to those of a maxillary first premolar.
Clinical Considerations for Maxillary Second Premolars
With premature loss of a primary maxillary second molar, the developing permanent maxillary first molar inclines and drifts mesially. The developing permanent maxillary second premolar is prevented from normal eruption because its arch leeway space is nearly closed (see Chapter 20). This situation can allow the maxillary second premolar to become impacted against the first molar. An impacted tooth is an unerupted or partially erupted tooth that is positioned against another tooth, bone, or even soft tissue, making complete eruption unlikely. Additionally, the leeway space can be compromised if the permanent maxillary second molar erupts before the maxillary second premolars, the arch perimeter is significantly shortened, and occlusal disharmony is likely to occur, as is with malocclusion. These problems may be prevented by careful evaluation of patients with mixed dentition and use of interceptive orthodontic therapy, such as space maintainers (see Figure 20-4).
Permanent Mandibular Premolars
General Features
Mandibular premolars do not resemble each other as much as do the maxillary premolars. In addition, a mandibular first premolar is smaller than a mandibular second premolar; in contrast, a maxillary first premolar is larger overall than the second premolar. Generally, both mandibular premolars erupt into the oral cavity later than do the maxillary premolars.
Quite distinct from maxillary premolars, the buccal outline of the crown of all mandibular premolars shows a strong lingual inclination when viewed from the proximal, similar to all mandibular posterior teeth. The permanent mandibular premolars also have an equal buccolingual and mesiodistal width when viewed from the occlusal, making the outline almost round. In addition, both types of premolars have a similar buccal outline of both the crown and root.
The mesial and distal contact areas of mandibular premolars are on nearly the same level. Similar CEJ curvatures are also found on both premolars. From each proximal view, both the crown outlines of mandibular premolars are rhomboidal, with four sides, having the opposite sides parallel, like all mandibular posteriors. The crowns thus incline lingually on their root bases, bringing the cusps into proper occlusion with their maxillary antagonists and the distribution of forces along their long axes.
Unlike the maxillary premolars, both of which have two cusps of almost equal size, the mandibular premolars can have more than two cusps; however, any lingual cusps are always smaller than the buccal cusp.
These teeth usually have a single root; the angulation of the roots of mandibular premolars may show slight distal inclination. The root on cervical cross section is either ovoid (egg-shaped) or elliptical (an elongated oval), shapes that may be slightly altered by the presence of proximal root concavities. These proximal root concavities are most frequently found on the mesial surface of the root.
Clinical Considerations for Mandibular Premolars
Both types of mandibular premolars can present difficulties during instrumentation due to narrow lingual surfaces combined with the lingual inclination of the crown, especially with subgingival placement. In addition, patients may have problems performing adequate home care because of the lingual inclination of the crown, which causes some patients to miss the associated lingual gingival tissue and clean only the occlusal surface with a toothbrush. The nearby tongue also makes home care and instrumentation more difficult on the lingual surface.
Permanent Mandibular First Premolars #21 and #28
Specific Overall Features (Figure 17-19)
Permanent mandibular first premolars erupt between 10 to 12 years of age (root completion between ages 12 to 13). These teeth erupt distal to the permanent mandibular canines, and thus are the succedaneous replacements for the primary mandibular first molars.
A mandibular first premolar resembles a mandibular canine in many more ways than it does a mandibular second premolar. This is true despite the fact that a premolar is smaller overall than a canine. However, the buccolingual width of this tooth is similar to that of a mandibular canine. Thus, a mandibular first premolar shows a transition in the dental arch from the canine to the molar-like second premolar.
A mandibular first premolar has a buccal cusp that is long and sharp and is the only functional cusp during occlusion, similar to a mandibular canine. The lingual cusp of a mandibular first premolar is usually small and nonfunctioning. The lingual cusp, then, is similar in appearance to the cingulum found on some maxillary canines, but it can vary considerably. Finally, the occlusal surface of the mandibular first premolar has a similar outline and slopes sharply to the lingual, and the mesiobuccal cusp ridge is shorter than the distobuccal cusp ridge, features similar to the mandibular canine.
A mandibular first premolar has a smaller and shorter root than does a mandibular second premolar, although it is closer to the length of a second premolar than to that of a mandibular canine. The buccal aspect of the root is more conical, but the lingual aspect is tapered. A deep groove may be found on the distal root surface. The tooth occasionally has a bifurcated root, with the root divided into buccal and lingual parts.
The pulp cavity of this tooth consists of two pulp horns and a single pulp canal (Figure 17-20). Each pulp horn is located within a cusp. The buccal pulp horn is more pronounced, and the lingual pulp horn is smaller and less significant.
Buccal View Features
The outline of the crown of a mandibular first premolar from the buccal is nearly symmetrical (see Figure 17-19). The middle developmental lobe is visibly well developed, resulting in a prominent buccal ridge and a large, pointed buccal cusp. In contrast, the buccal ridge is not as prominent as on a maxillary first premolar. Two buccal developmental depressions are often seen among the three buccal lobes. Imbrication lines are not usually found on the buccal surface.
The buccal cusp is also located slightly to the mesial of the center of the crown, again similar to a mandibular canine. Thus, the two cusp slopes of the premolar are not equal in length. The mesial cusp slope of the buccal cusp is shorter than the distal cusp slope, which helps to distinguish the right mandibular first premolar from the left.
The mesial outline of the mandibular first premolar is slightly concave from the mesial contact to the CEJ. The distal outline is rounder and shorter. Again, the mesial contact with the maxillary first premolar is just cervical to the junction of the occlusal and middle thirds. The distal contact with the maxillary first molar is the same, just cervical to the junction of the occlusal and middle thirds.
Lingual View Features
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VIDEdental - Online dental courses
