The Permanent Mandibular Premolars I Permanent Mandibular Premolars

A. Introduction:

1. As a review of the distinguishing characteristics of premolars, it will be recalled that:

a. The mandibular first premolar is smaller in general size than the second premolar.

b. The mandibular premolars' buccolingual and mesiodistal crown dimensions are approximately equal.

c. The mandibular premolars may have more than two cusps, and lingual cusps are less prominent than those of the maxillary premolars. The single buccal cusp is always more prominent than any lingual cusp.

d. When viewed from either proximal, the mandibular premolars' crown profile tilts toward the lingual.

e. The mandibular premolars are normally single rooted.

2. The two mandibular premolars do not resemble each other nearly as much as do their maxillary counterparts. They are transitional teeth, with the first premolar reflecting the transition from the canine, and the second premolar showing the change toward the molars. The first premolar has a diminutive lingual cusp which is normally non-functional, so that its morphology and role in mastication parallel those of the canine. The second premolar most often has two lingual cusps, and more closely approximates a small molar in structure and function. The slope of the marginal ridges is also transitional. The first premolar exhibits a slope which is more similar to that of anterior teeth, and the second premolar displays a more horizontal angulation like other posterior teeth.

B. Permanent Mandibular First Premolar: 1. General characteristics:

a. Arch position - The mandibular first premolar is the fourth tooth from the midline in each mandibular quadrant of the permanent dentition. It has a mesial contact with the mandibular canine, and a distal contact with the mandibular second premolar. It is a succedaneous tooth, replacing the deciduous first molar.

b. Universal number:

Mandibular right first premolar - #28 Mandibular left first premolar - #21

c. General form and function - Although the first premolar has two cusps, like most premolars, only the buccal cusp is functional, thus allying it most closely with the canine.

From the occlusal aspect, the diamond shaped outline of the first premolar resembles that of the canine. From either the facial or lingual aspects, this tooth exhibits a pentagonal form, while the proximal surfaces are rhomboi-dal in outline.

In summary, the mandibular first premolar actually is closer in form and masticatory function to a canine than to the other premolar in the mandibular arch. Only in relative size, and appearance from the facial aspect, does this tooth resemble the adjacent second premolar.

Development Table: (Mandibular First Premolar)*

Initiation of calcification 1 3/4-2 years

Completion of enamel 5-6 years

Eruption 10-11 years

Completion of root 12-13 years

Buccal aspect:

a. General considerations - The pentagonal outline from the buccal aspect is similar to the facial form of both the canine and second premolar. The buccal surface itself is convex both occlusogingivally and mesiodistally. The occlusocervical dimension is shorter than that of any anterior tooth, but longer than the teeth posterior to it.

b. Mesial margin - The mesial margin is slightly concave from the contact area to the cervical line. The height of contour, at the contact area, is in the middle third.

c. Distal margin - The distal outline is similar to the mesial margin, only a little shorter.

d. Cervical outline - The cervical line presents an even, slightly convex curve toward the apex. The depth of curvature is less than that of the anterior teeth.

e. Occlusal outline - The pointed buccal cusp tip divides the occlusal outline into two portions, the mesio-occlusal and disto-occlusal slopes, or mesial and distal cusp ridges. The mesio-occlusal slope is normally shorter, thus leaving the cusp tip slightly offset toward the mesial.

f. Other considerations - The buccal ridge, representing the middle buccal lobe, is the most prominent portion of the buccal surface. Mesiobuccal and distobuccal developmental depressions are usually present, but imbrication lines are not normally found.

From this aspect, it is apparent that the contact areas are located at approximately the same level. The marginal ridges display a similar arrangement. This feature is unique to the mandibular first premolar, since on other permanent posterior teeth the distal marginal ridge and contact area are found at a more cervical level. In some specimens of this tooth, the mesial structures are even more cervically located than those on the distal.

The buccal height of contour is found in the cervical third.

Lingual aspect:

a. General considerations - The lingual surface lacks the prominent occlusocervical ridge that is present on the facial, and it is consistently convex in all directions. Most of the buccal half of the occlusal table is visible from this aspect, because of the small lingual cusp, and the inclination of the crown toward the lingual. All lingual dimensions are less than those of the buccal surface. The lingual surface is much narrower mesiodistally, due to the lingual convergence of the mesial and distal surfaces. This feature allows a view of most of the mesial and distal surfaces from this aspect.

b. Mesial and distal margins - These margins are similar to those of the buccal surface, only much shorter, because the lingual surface itself is shorter.

c. Cervical outline - The CEJ is slightly convex toward the apical.

d. Occlusal outline - The actual occlusal outline is that of the buccal cusp, because of its much greater height. For instructive purposes, however, the outline of the much smaller lingual cusp will be described. The lingual cusp tip is very short, but may be sharp. The disto-occlusal slope is slightly longer than the mesio-occlusal slope, thus the cusp tip is offset to the mesial, like the buccal cusp. Since both cusp tips are mesially offset, the portion of the crown distal to the transverse ridge is larger than the portion mesial to it.

e. Other considerations - The lingual height of contour is located in the middle third.

One of the landmark features of this tooth, and its mesial and lingual aspects in particular, is the mesiolingual developmental groove. This groove originates in the mesial pit of the occlusal surface and crosses onto the mesial surface near the mesiolingual line angle. It normally fades out at about the junction of the cervical and middle thirds. It is visible from this aspect because of the convergence of the mesial surface toward the lingual.

Mesial aspect:

a. General considerations - The mesial surface is roughly rhomboidal in form, although this shape is more difficult to visualize than it is on other mandibular posterior teeth, since the lingual outline is so short.

b. Buccal margin - The buccal outline is generally convex, with the height of contour in the gingival third. As is typical of all mandibular posterior teeth, this outline is greatly inclined toward the lingual.

c. Lingual margin - The lingual margin is less convex and much shorter than the buccal outline. The crest of curvature is close to the occlusal limit of the lingual margin, but since the margin is so short, it is in the middle third of the crown.

d. Cervical outline - The cervical line is curved slightly toward the occlusal.

e. Occlusal outline - The occlusal outline reveals the buccal portion of the transverse ridge sloping at approximately a 45° angle, which is nearly paralleled by the outline of the mesial marginal ridge at a more cervical level.

Since the crown profile from this aspect is inclined toward the lingual, the buccal cusp tip is located over the center of the root.

f. Other considerations - The previously described mesiolingual developmental groove is fully visible on the mesial surface near the lingual margin.

The contact area is located toward the buccal, in the middle third. It is circular to somewhat ovoid in shape.

The height of contour of the mesial surface is located in the middle third, in association with the contact area.

6. Distal aspect:

The basic outline and anatomy of the distal surface is similar to the mesial surface, with a few exceptions:

a. There is no distolingual developmental groove, but there is a distal marginal groove.

b. The distal surface is a little shorter occlusocervically, and it is wider buccolingually than the mesial surface.

c. The cervical line curvature is slightly less.

d. The contact area is similarly shaped, but occupies a slightly broader area, since it approximates the second premolar, which is a larger tooth than the canine. Its location in both dimensions is similar to that of the mesial surface.

e. The distal marginal ridge does not show quite as steep a slope toward the lingual.

7. Occlusal aspect:

a. General considerations - From the occlusal aspect, the crown profile is more or less rhomboidal, or diamond shaped, with a notch in the mesial outline at the mesiolingual developmental groove.

Due to the mesial offset of both cusp tips and thus the transverse ridge, the distal portion of the tooth is larger than the mesial.

Because of the lingual inclination of the crown of this tooth, much of the buccal surface, but almost none of the lingual surface, may be seen from the occlusal aspect.

b. Buccal outline - The buccal margin is rather uniformly convex, with well described, though rounded, MB and DB line angles. The outline of the buccal ridge is the most prominent feature of this margin.

c. Lingual outline - This margin is also convex, but is much shorter than the buccal. Mesially, it ends near the mesiolingual developmental groove.

d. Mesial outline - The mesial margin is slightly convex to nearly straight. The only exception is the concave offset near the mesiolingual line angle, where the mesiolingual developmental groove traverses it.

e. Distal outline - The distal outline is more regularly convex than the mesial outline.

f. Boundaries - The occlusal surface is bounded by the mesial and distal marginal ridges proximally. The boundaries on the buccal and lingual include the mesial and distal cusp ridges of both cusps.

8. Components of the Occlusal Surface:

a. Cusps - There are two cusps, a buccal and a lingual, with the buccal being much larger and the only functional cusp.

i. Buccal cusp - The buccal cusp tip is offset to the mesial and located toward the buccal portion of the occlusal surface, but since the crown is so inclined lingually, it is also located approximately over the long axis of the tooth. The cusp has four cusp ridges and four inclined planes, which are named exactly like those of the maxillary premolars. Since this is a mandibular tooth, all four of the inclined planes of the buccal cusp are functional.

ii. Lingual cusp - The lingual cusp is very small, usually no more than half the height of the buccal cusp. Sometimes it is more of a tubercle than a true cusp. Like the buccal cusp, it is offset to the mesial. It also has four cusp ridges and four inclined planes named like those of the maxillary premolars. Since this cusp itself is normally nonfunctional, none of its inclined planes is considered to be functional.

b. Transverse ridge - As in the maxillary premolars, the buccal and lingual triangular ridges of the tooth meet in the central groove area to form a transverse ridge. The buccal triangular ridge is considerably larger and longer than the lingual, thus comprising a greater portion of the transverse ridge. The central groove, which separates the two component triangular ridges, is sometimes so indistinct in the first premolar, that the two triangular ridges appear to be continuous.

c. Marginal ridges - The mesial marginal ridge slopes from buccal to lingual at a 45° angle, while the marginal ridges of other posterior teeth are roughly horizontal. In fact, it more closely resembles the angulation of the marginal ridges of anterior teeth, especially the canine. The distal marginal ridge is longer, more prominent, and does not exhibit quite as steep a slope toward the lingual.

d. Fossae - The two irregular depressions on the occlusal surface are designated as the mesial and distal fossae. They correspond to the mesial and distal triangular fossae of other posterior teeth, and are bounded by the transverse ridge, the marginal ridges, and the mesial and distal cusp ridges of the two cusps. The mesial fossa is roughly linear in shape, while the distal fossa is larger and more or less circular.

e. Pits and grooves - The two pits on the occlusal surface are found in the deepest portions of the two fossae.

i. Mesial pit - The mesial pit is located just distal to the mesial marginal ridge, at about the midpoint buccolingually on the occlusal table. It is normally the point of union of three primary grooves.

Central groove - The central groove extends mesiodistally between the two pits.

Mesiobuccal triangular groove - This groove is similar in location to that of the maxillary premolars.

Mesiolingual developmental groove - This unique groove has been previously described from mesial and lingual aspects. On the occlusal surface, it angles mesiolingually from the mesial pit, where it crosses over the mesial marginal ridge onto the mesial surface near the mesiolingual line angle.

On rare occasions, a mesial marginal groove also originates in the mesial pit.

ii. Distal pit - The distal pit is positioned in the deepest portion of the distal fossa, similar to the location of the mesial pit, and is the junction of four primary grooves:

Central groove

Distal marginal groove

Distolingual triangular groove

Distobuccal triangular groove

a. The root is normally single, fairly straight, and its outline tapers from the cervical line to a relatively sharp apex.

b. The root length is slightly less than that of the mandibular second premolar, and considerably less than the lower canine.

c. It is wider buccolingually than mesiodistally. The buccal and lingual surfaces are convex, while the mesial and distal surfaces are slightly convex to flat, and converge toward the lingual. Root concavities occur only occasionally.

d. In a midroot cross-section, the outline is roughly ovoid and wider buccolingually than mesiodistally. It is slightly wider at the buccal than at the lingual.

10. Variations and anomalies:

a. The crown of this tooth exhibits wide variation in form. Some of the more common differences include:

i. An absence of the ML developmental groove.

ii. Two parallel ML developmental grooves.

iii. The size of the lingual cusp, which can range from complete absence to nearly as large as the buccal cusp.

iv. The continuity of the transverse ridge, as it is affected by the depth of the central groove.

v. The location of the lingual cusp mesiodistally. Although normally slightly offset to the mesial, the lingual cusp tip can be centered mesiodistally, or even offset to the distal.

b. The root, on rare occasions, may display a bifurcation, thus creating buccal and lingual root branches.

Permanent Mandibular Second Premolar: 1. General characteristics:

a. Arch position - In the permanent dentition, the mandibular second premolar is the fifth tooth from the midline in each lower quadrant. It has a mesial contact with the mandibular first premolar, and a distal contact with the permanent mandibular first molar. It is the succedaneous tooth for the deciduous mandibular second molar.

b. Universal number:

Mandibular right second premolar - #29 Mandibular left second premolar - #20

c. General form and function - The second premolar has a generally larger crown, and a slightly larger and longer root than the first premolar. The opposite arrangement is true for the crowns of maxillary premolars, where the first premolar is slightly larger.

From the facial aspect, the general shape of the crown is similar to that of the mandibular first premolar. However, from all other aspects, differences are apparent. There are two general forms of the mandibular second premolar, with the most common form exhibiting two lingual cusps, while the other type displays but one. The difference between the two types is primarily in occlusal form, since the other surface contours are similar.

Regardless of the number of lingual cusps, the occlusal table is most similar to that of a small molar. Consequently, this tooth functions in a grinding capacity with the molars, as contrasted to the first premolar which functions much like the canine.

Development Table (Mandibular Second Premolar)*

Initiation of calcification 2 1/4 to 2 1/2 years

Completion of enamel 6 to 7 years

Eruption 11 to 12 years

Completion of root 13 to 14 years

Buccal aspect:

a. The mandibular second premolar resembles the mandibular first premolar from the buccal, with the following exceptions:

i. The tooth is slightly larger, even though the tip of the buccal cusp is shorter and the occlusocervical dimension is a little less. Since the cusp tip is not so high, it is not as sharp and the mesio-occlusal and disto-occlusal slopes are not as inclined.

ii. The cusp tip is also centered mesiodistally, making the two slopes approximately equal in length.

Despite these slight differences, it is difficult to distinguish between the two mandibular premolars from this aspect.

b. The location and morphology of the structures of the facial surface, including height of contour, are also similar to those of the first premolar.

Lingual aspect:

a. General considerations - The lingual cusp or cusps, are better developed and higher in comparison to the first premolar. The lingual surface is generally smooth and convex.

b. The mesial, distal, and cervical outlines are similar to those of the first premolar, although the lingual surface is considerably wider mesiodistally, and longer occlusocervically.

c. Occlusal outline - The lingual cusps are higher, and as a consequence, much less of the occlusal surface can be seen from this aspect, when compared to the first premolar. The height of the lingual cusp(s) is still somewhat less than the buccal cusp height.

i. The three cusp type exhibits a mesiolingual and a distolingual cusp. Between the two lingual cusps a lingual groove extends a short distance onto the lingual surface. The mesiolingual cusp is wider and longer, while the distolingual cusp is smaller, but often is the sharper of the two. This arrangement leaves the lingual groove offset to the distal in the occlusal outline.

ii. The two cusp type displays a single lingual cusp. There is no lingual groove, but a depression is often found toward the distal portion of the surface. The single cusp is approximately the same height as the mesiolingual cusp of the three cusp type.

d. The height of contour of the lingual surface is found in the occlusal third of the crown.

5. Mesial aspect:

From the mesial aspect, the two mandibular premolars are similar, but much easier to differentiate than from the facial aspect. Dimensionally, the second premolar is wider buccolingually, bul-^Tghtly shorter occlusocervically. Structurally, the two premolars differ in the following ways:

a. The lingual inclination of the crown and of its buccal surface is not quite as great as on the first premolar. Consequently, the buccal cusp tip is not centered over the root, but rather is buccal of center. The buccal cusp tip is also shorter and less sharp.

b. Lingual cusps are more prominent than on the first premolar. In the three cusp type, the DL cusp is not visible from the mesial aspect.

; c. Occlusogingivally, the mesial surface is convex in the occlusal portion, J and concave in the gingival portion.

d. The contact area is located toward the buccal, at the junction of the occlusal and middle thirds. It is larger in size than the mesial contact of the first premolar. It is also roughly circular in outline.

e. The marginal ridge is more nearly horizontal, and much less of the occlusal surface is visible.

f. The landmark of the first premolar, the mesiolingual developmental groove, is absent on the second premolar, but there is normally a mesial marginal groove present.

g. The height of contour of the lingual margin is found in the occlusal third, a location which is unique to the mandibular second premolar.

h. The cervical line shows less depth in its occlusal curvature.

6. Distal aspect:

The distal surface is similar to the mesial surface, except in the following ways:

a. The distal marginal ridge is more cervically placed than on the mesial, resulting in more of the occlusal surface being visible from this aspect, as well as a shorter surface occlusocervically.

b. In the three cusp type, the tips of both the mesiolingual cusp and the distolingual cusp are visible.

c. The contact area is similarly located, but because it is shared with the first molar, it is larger and somewhat ovoid, wider buccolingually than occlusocervically.

7. Occlusal aspect:

a. General considerations - The general shape of the crown from this aspect is more nearly square especially in the three cusp type, when compared to the first premolar. The convergence of the mesial and distal surfaces toward the lingual is not nearly so severe.

b. General groove pattern - The occlusal groove pattern is responsible for the names of the second premolar types. For example, the main groove pattern on the three cusp type takes the form of a "Y", and it is thus named Y type. The main groove pattern on the two cusp type resembles a "U" or "H", resulting in U type and H type second premolars. The Y type is the most common form of mandibular second premolar, and is present in the majority of cases. The two cusp types are less often seen, with H type specimens more common than U type.

i. General occlusal form - The outline of the Y type from the occlusal aspect is roughly square lingual to the buccal line angles, which are quite distinct. On an occasional specimen, the mesiodistal dimension of the crown is even greater through the lingual line angle area than it is through the buccal portion. The Y pattern of the occlusal table is formed by a combination of the central and lingual grooves.

ii. Cusps - The three cusps vary in height and size from largest to smallest, as follows: buccal cusp, mesiolingual cusp, and distolingual cusp. Each cusp exhibits four cusp ridges and four inclined planes, and they are named like those of other posterior teeth. The buccal cusp has four functional inclined planes, while the lingual cusps possess two functional inclined planes each. There is no transverse ridge on the Y type second premolar.

iii. Fossae - There are two fossae, the mesial and distal triangular fossae. Both are relatively shallow and irregular, but are more linear in form than the triangular fossae of the maxillary premolars. They are located just inside the marginal ridges, and are also bounded by two cusp ridges of each appropriate cusp. For example, the mesial triangular fossa is bounded by the mesial and lingual cusp ridges of the buccal cusp, and the buccal and mesial cusp ridges of the mesiolingual cusp, in addition to the mesial marginal ridge.

iv. Pits and grooves - There are three pits present on the occlusal table: Mesial pit - The mesial pit is located in the deepest portion of the mesial triangular fossa, which is about midway from buccal to lingual just inside the mesial marginal ridge. It is the point of union of the following four primary grooves:

Central groove - It extends from the mesial pit to the distal pit in a shallow "V" form.

Mesiolingual triangular groove

Mesiobuccal triangular groove

Mesial marginal groove

Distal pit - Its location in the depth of the distal triangular fossa is similar to that of the mesial pit, and it is also the point of union of four primary grooves: Central groove

Distolingual triangular groove

Distobuccal triangular groove

Distal marginal groove

Central pit - The central pit is the deepest of the three pits. It is located toward the lingual, and more than halfway from mesial to distal, since the ML cusp is wider than the DL cusp. The central pit is located along the central groove, where it joins the lingual groove, which itself exits the occlusal surface between ihe two lingual cusps. The central pit is thus the junction of two primary grooves.

Note: Some anatomists prefer to divide the central groove into two grooves as follows:

Mesial groove - The mesial groove extends distolingually from the mesial pit to the central pit.

Distal groove - The distal groove extends mesiolingually from the distal pit to the central pit.

However, this text will consider that there is but one groove, the central groove, with mesial and distal portions, rather than a division into two separate grooves, d. Two cusp types (U and H types)

i. General occlusal form - Two cusp type second premolars exhibit a rounded outline lingual to the buccal line angles, and the buccal line angles are themselves more rounded and less distinct than in the Y type. The mesial and distal surfaces may converge somewhat more toward the lingual, making the lingual portion narrower than the buccal, but the taper is never to the degree of the first premolar. The one lingual cusp is placed directly opposite the buccal cusp and their respective triangular ridges create a transverse ridge. These teeth do not have either a lingual groove or a central pit.

ii. Cusps - The buccal cusp is larger and somewhat higher than the lingual cusp. The lingual cusp sometimes appears as an irregular convexity, rather than a distinct cusp, especially in the U type. The lingual cusp of the H type is larger and sharper than in the U type, and on both types it is offset to the mesial. Both buccal and lingual cusps have four cusp ridges, and four inclined planes, which are named like the same structures on other premolars. The buccal cusp features four functional inclined planes, while the lingual cusp has two.

iii. Fossae - The two cusp type also has two fossae. However, they are roughly circular in shape, and are termed mesial and distal fossa, both facts differing from the three cusp type. They are bounded by the transverse ridge, and appropriate marginal ridges and cusp ridges.

iv. Pits and grooves (U type) - The U pattern is formed by the central groove, portions of the two buccal triangular grooves, and the secondary grooves of the buccal cusp. The central groove extends from the mesial pit to the distal pit, and in the process forms the lower portion of the "U". It can thus be characterized as crescent shaped. The only two pits, the mesial and distal pits, are the point of union of the same four primary grooves previously described in the Y type.

v. Pits and grooves (H type) - The H pattern is formed by the central groove, portions of the four triangular grooves, and the secondary grooves of the buccal and lingual cusps. The central groove runs roughly in a straight line between the two pits, in contrast to the crescent shape of the U type. Pits, fossae, and grooves are named as in the U type.

Root:

a. The root is normally single, and tapers evenly to the apex which is relatively sharp. It often has a slight distal inclination in the apical third.

b. The root is slightly wider and longer than that of the first premolar.

c. The general shape of the root from all aspects, as well as in mid root cross section, is similar to that of the first premolar.

Variations and anomalies:

a. In three cusp type crowns, the comparative size of the ML and DL cusps is quite variable. The DL cusp ranges from a barely discernible bump to approximately the same size as the ML cusp.

b. Anomalies are rare, although a root bifurcation into buccal and lingual branches is sometimes seen.

c. The mandibular second premolars are, on occasion, congenitally missing. This phenomenon may occur either unilaterally or bilaterally.

d. Supernumerary teeth are sometimes observed in the mandibular premolar area.

Mandibular Right First Premolar

Mandibular Right First Premolar

Mandibular Right Second Premolar (Y-Type Shown In Four Aspects)

U-type

U-type

Y-type

LINGUAL

Y-type

LINGUAL

mm 0

Mid-Root Section

H - Type
DISTAL

I. Reading Assignment:

Unit # 7 (The Permanent Maxillary Molars)

II. Specific Objectives:

At the completion of this unit, the student will be able to:

A. List the appropriate age(s) concerning developmental chronology of the maxillary molars found in the development tables, or select the appropriate age(s) from a list, when given a certain developmental feature. The student should also be able to compare these ages among the maxillary molars.

B. Demonstrate a knowledge of the morphology of each surface of the crown, as well as the root, of each permanent maxillary molar by:

1. describing,

2. selecting the correct information from a list,

3. or interpreting a diagram to identify or name any of the following features:

a. Contours of any surface, or margin of any surface.

b. Structural entities such as grooves, pits, ridges, inclined planes, cusps, fossae, lobes, etc.

c. Height of contour and contact areas.

d. Relative dimensions and shape.

e. Any other surface features.

Furthermore, the student will be able to make comparisons of any of the above features among maxillary molars.

C. Make comparisons between permanent maxillary molars and other permanent teeth where appropriate, by selecting the correct response from a list.

D. Make comparisons in the general characteristics of the maxillary molars, including function, arch position, distinguishing features, etc., by describing them, or selecting the correct response from a list when given the tooth (teeth), or a description of the general characteristic(s).

E. Determine from a diagram or description whether a given permanent maxillary molar is first, second, or third, or right or left.

F. Determine the correct universal number or Palmer notation for a given diagram or description of any permanent maxillary molar.

G. Demonstrate a knowledge of any of the new terms in this unit by defining them, or selecting the correct definition, or application thereof, from a list, when given the term, or any of its applications.

H. Demonstrate a knowledge of any of the variations or anomalies in this unit by describing them, or selecting the correct response from a list, when given the particular tooth (teeth), the anomaly, or any of its features or applications.

The student is also responsible for any material which was to have been mastered in previous units.

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