The Permanent Mandibular Molars I The Permanent Mandibular Molars

A. Introduction:

1. The permanent mandibular molars are the three most posterior teeth in each lower quadrant. Like their maxillary counterparts, they are named first (six-year) molar, second (twelve-year) molar, and third molar (wisdom tooth). They are the largest and strongest teeth in the mandibular arch. Mandibular molar crowns are much larger than those of mandibular premolars in all dimensions except occlusogingivally, where they are slightly shorter. Their general size normally decreases from first molar through third molar.

2. The mandibular molars function with the maxillary molars in grinding, and their form, root structure, and bone support are suited to this role.

3. A review of the features which serve to differentiate mandibular and maxillary molars includes:

a. Crowns which are wider mesiodistally than buccolingually.

b. Crowns which are rectangular or pentagonal from the occlusal aspect.

c. Crowns which are rhomboidal and inclined to the lingual, from a proximal aspect.

d. The presence of four or five major cusps, of which there are always two lingual cusps of approximately the same size.

e. The presence of two roots in most cases.

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

a. Arch position - The initial permanent tooth to erupt, the mandibular first molar is located sixth from the midline, and distal to the second deciduous molar. Hence it is not a succedaneous tooth. Because of its normal eruption time, it is often called a "six year" molar. The first molars are also thought of as the cornerstones of occlusion in the mandibular arch.

It shares a mesial contact with the deciduous second molar for approximately five years, until that tooth is replaced by the second premolar. There is no distal contact, until eruption of the permanent second molar occurs at about age twelve.

b. Universal number: Mandibular right first molar - #30 Mandibular left first molar - #19

c. General form and function - The first molar is the largest and strongest tooth in the lower arch. It normally exhibits five functional cusps, and two well developed roots.

The crown is wider mesiodistally than buccolingually, and, in fact, the me-siodistal dimension is greater than that of any tooth in the mouth. The crown is relatively short occlusocervically, the only dimension which is normally less than that of the teeth anterior to it. It displays a trapezoidal outline from the buccal and lingual, and exhibits a rhomboidal form from either proximal aspect. From the occlusal, the general outline is pentagonal.

In mastication, it functions with the other molars in grinding.

Development Table: (Mandibular First Molar)*

Initiation of calcification at birth

Completion of enamel 2 1/2 to 3 years

Eruption 6 to 7 years

Completion of root 9 to 10 years

Buccal aspect:

a. General considerations - The buccal is the largest lateral surface of not only the mandibular first molar, but of any tooth in the mouth. It is trapezoidal in outline, with the greatest mesiodistal width at the occlusal. At least portions of all five cusps are visible from this aspect.

b. Mesial outline - The mesial outline is slightly concave from the contact area cervically, and convex occlusal to the contact. The height of contour of the mesial outline is located at the junction of the occlusal and middle thirds.

c. Distal outline - The distal margin is generally more convex than the mesial outline. In the occlusal portion, it is more rounded, and cervical to the contact area it is straight to slightly convex, as compared to the concavity of the mesial margin. The height of contour is found at a slightly more cervical location than that of the mesial margin, but it is still at the junction of the occlusal and middle thirds.

d. Cervical outline - The cervical line exhibits slight, but regular curvature apically, and sometimes displays a sharply pointed projection over the bifurcation area.

e. Occlusal outline - The occlusal margin is wider than the outline at the cervical. It is divided into three portions by two grooves, as they pass onto the buccal surface. They are termed buccal (mesiobuccal) groove, and distobuccal groove. The mesio-occlusal and disto-occlusal slopes of three cusps are present in the occlusal outline. The mesiobuccal and distobuccal cusp tips are relatively blunt, while the distal cusp is normally lower, and somewhat sharper than the other two.

f. Other considerations - The buccal surface itself is divided into threi portions by the two grooves, and these three sections decrease in size posteriorly. The convex buccal cusp ridges of the three cusps are the most prominent features of the occlusal portion of each section, and they are separated by the occlusocervical concavities containing the two grooves.

Buccal (mesiobuccal) groove - This groove is located in a concavity between the convex buccal cusp ridges of the mesiobuccal and distobuccal cusps. From the occlusal outline, it extends straight cervically to a point about midway between the gingival and occlusal margins, but a little to the mesial of center in the mesiodistal dimension. It most often terminates in a buccal pit, but may fade out, or bifurcate into two angular grooves which themselves fade out after a short distance. The groove, as well as the pit, are sometimes deep and fissured.

Distobuccal groove - This groove is located in a concavity between the buccal ridge convexities of the distobuccal and distal cusps. It crosses the occlusal outline close to the distal margin, and then extends cervically and slightly distally to terminate at a point in the middle third near the distobuccal line angle. It normally ends in a distobuccal pit, but sometimes simply fades out.

Buccogingival ridge - As in the maxillary arch, this structure is not found on all first molar specimens. When present, it appears as a mesiodistal convexity in the cervical third of the buccal surface. It is usually more prominent in its mesial portion

Again, like the maxillary molars, there may be a shallow concavity which extends mesiodistally in the middle third. More specifically, it is located just occlusal to the buccogingival ridge in the area of termination of the two buccal grooves.

The height of contour is located in the cervical third. Lingual aspect:

a. General considerations - The lingual surface is also roughly trapezoidal in outline, with the longer parallel side of the trapezoid at the occlusal. Since the crown is widest mesiodistally at the buccal, and its mesial and distal surfaces taper somewhat toward the lingual, portions of both proximal surfaces can be seen from this aspect. The lingual surface is, in fact, generally smaller than the buccal surface.

b. Mesial outline - The mesial outline is' convex occlusal from the contact area, which is the crest of curvature, and is located at the junction of the occlusal and middle thirds. From the contact area cervically, it is concave.

c. Distal outline - The entire distal margin is convex, but especially so from the contact area occlusally, which is the outline of the distal portion of the distal cusp. The height of contour of the distal outline is also at the junction of the occlusal and middle thirds.

d. Cervical margin - The cervical line is shorter mesiodistally, and is located at a more occlusal level, than on the buccal surface. It is usually irregular and nearly straight, although it can display a pointed projection in the bifurcation area.

e. Occlusal margin - The occlusal outline is usually broken by the lingual groove passing onto the lingual surface. The mesiolingual and distolingual cusps, and a small portion of the distal cusp are visible from this aspect. The outline of the mesiolingual cusp is slightly wider than that of the distolingual cusp. The two lingual cusp tips are more pointed than the buccal cusp tips, and they are approximately equal in height.

f. Other considerations - The lingual cusp ridges of the two lingual cusps are convex, with the shallow concavity containing the lingual groove lying between them in the occlusal third. Except for the concavity of the lingual groove, the occlusal and middle thirds of the surface are generally convex, but there is normally a flattened area in the center of the cervical third. Lingual groove - This groove crosses from the occlusal surface onto the lingual surface slightly to the distal of center, extends cervically, and terminates in the occlusal third near its junction with the middle third. It usually fades out, but on rare occasions it ends in a lingual pit. Occasionally, the groove is confined to the occlusal surface and does not pass onto the lingual surface at all. It is shallower than either of the buccal grooves, and is seldom fissured on the lingual surface.

The lingual height of contour is located in the middle third.

Mesial aspect:

a. General considerations - Although the mesial surface is said to be rhom-boidal, it is not a perfect rhomboid, since the surface is wider at the cervical than at the occlusal. From the mesial and distal aspects, the crown displays the lingual inclination which is unique to the mandibular posterior teeth. Only the two mesial cusps are visible from this aspect.

b. Buccal outline - The buccal margin is usually convex from gingival to occlusal, but is most convex at the cervical third crest of curvature, especially if the tooth has a buccocervical ridge. On some specimens there is a slight concavity in the middle third of the outline.

c. Lingual outline - The lingual outline is straight, or slightly convex, from the cervical margin to the height of contour in the middle third. It is quite convex occlusal to the height of contour.

d. Cervical margin - The CEJ may be either relatively straight, or slightly curved occlusally, but it is always located at a more occlusal level on the lingual side.

e. Occlusal margin - This margin is concave, and is composed of the mesial marginal ridge outline which is confluent with the mesial cusp ridges of the mesiobuccal and mesiolingual cusps. A mesial marginal groove is usually present.

f. Other considerations - A flattened, or slightly concave area, which is often triangular in form, is centrally located in the gingival third of the surface. It is not consistently as deep or as extensive as the mesial concavity of the maxillary first premolar.

The contact area is round to slightly ovoid in shape, and located slightly to the buccal at the middle and occlusal third junction.

The height of contour of the mesial surface is associated with the contact area at the junction of the occlusal and middle thirds.

Distal aspect:

a. General considerations - The distal surface is similar in outline to the mesial, and is also wider buccolingually at the cervical than at the occlusal It is, however, generally smaller than the mesial surface, especially in the buccolingual dimension.

b. Buccal margin - The buccal margin is similar to that of the mesial aspect, except the concavity that is sometimes present in the middle third is not as evident when it is present.

c. Lingual outline - The lingual outline is comparable to that of the mesial aspect.

d. Cervical outline - The cervical line is relatively straight, although it may curve occlusally to a slight degree. As on the mesial surface, it is normally located at a more cervical level at its buccal extremity than at the lingual.

e. Occlusal outline - The occlusal outline is shorter than on the mesial, but similarly concave. The distal marginal ridge is notched by the distal marginal groove, which is shorter than the mesial marginal groove, and located to the lingual of center. The marginal ridge is sometimes difficult to separate from the outline of the distal cusp in the buccal portion of the occlusal outline. Therefore, the height of the distal marginal ridge is somewhat dependent on the prominence of the distal cusp. Normally, the ridge is located at a more cervical level than on the mesial, but the height is also more variable because of its relation to the distal cusp. The tip of the distal cusp is located toward the buccal, occlusal to the contact area, f. Other considerations - Since the distal surface is shorter occluso-gingivally than the mesial, more of the occlusal surface may be seen. The distal cusp is the most prominent feature of this view, but portions of all five cusps are visible.

More of the buccal surface is likewise visible, since it converges toward the distal, resulting in a narrower buccolingual dimension. Because of the convergence of the buccal surface, the distobuccal groove is also visible from this aspect.

The distal contact area is located similarly to the mesial contact area. It is larger, since it contacts the second molar, as contrasted to the mesial contact with the second premolar. It is ovoid in outline, wider buccolingually than occlusocervically.

7. Occlusal aspect:

a. General considerations - The occlusal form is roughly pentagonal in shape. The distal portion of the buccal outline tapers toward the lingual, to create the fifth side of the outline. The crown is wider mesiodistally than buccolingually, and it is widest mesiodistally toward the buccal, and widest buccolingually toward the mesial.

b. Buccal outline - The buccal outline is separated into three sections by the two buccal grooves. The relative length of the three portions decreases distally, so that the mesiobuccal is longest, distobuccal next, and the distal is shortest. The buccal line angles are quite rounded, especially when compared to those of anterior teeth and premolars.

c. Lingual outline - The lingual margin is divided into two slightly convex portions by the lingual groove. The mesial portion is slightly the longer of the two.

d. Mesial outline - The mesial outline is divided into two approximately equal segments by the mesial marginal groove.

e. Distal outline - The distal is the shortest of the four margins, and consists of two convexities, separated by the distal marginal groove.

f. Boundaries - The occlusal table is bounded proximally by the two marginal ridges, and on the buccal and lingual by the mesial and distal cusp ridges of the five cusps.

8. Components of the Occlusal Table:

a. Cusps - There are normally five cusps, all of which are functional, although the distal cusp is much smaller than the others. Despite its name, the distal cusp is grouped with the MB and DB cusps as one of the three "buccal" cusps. However, from the buccal or occlusal aspects, the reason for this grouping is evident.

i. Mesiobuccal cusp - The mesiobuccal is the bulkiest cusp, and the longest of the three buccal cusps, although rather blunt and rounded,. The MB cusp has four cusp ridges which are described as follows:

Buccal cusp ridge - The buccal cusp ridge extends cervically from the cusp tip about halfway down the buccal surface. Lingual cusp ridge - The lingual cusp ridge extends lingually to end at the mesial portion of the central groove. It is the longest and most prominent of the four ridges.

Mesial cusp ridge - This cusp ridge extends mesially to the mesio-bucco-occlusal point angle area.

Distal cusp ridge - It extends distally to the buccal groove.

ii. Distobuccal cusp - Except for the distal, the distobuccal cusp is the smallest of the cusps, and it has a rounded tip. The DB cusp has four cusp ridges which are described as follows:

Buccal cusp ridge - The buccal cusp ridge extends cervically from the cusp tip about halfway the width of the buccal surface.

Lingual cusp ridge - The lingual cusp ridge extends mesiolingually to the area of the central pit.

Mesial cusp ridge - This cusp ridge extends mesially to the buccal groove. Distal cusp ridge - It extends distally to the distobuccal groove. The four inclined planes of both the MB and DB cusps are named similarly to those of other posterior teeth. The inclined planes of the three buccal cusps are all functional, while only the buccal two are functional on the lingual cusps.

iii. Mesiolingual cusp - Along with the DL, the ML cusp is the longest and sharpest of the cusps, and it is second in size to the MB cusp. There are four cusp ridges which are described as follows:

Buccal cusp ridge - The buccal cusp ridge extends from the cusp tip distobuccally to end at the mesial portion of the central groove.

Lingual cusp ridge - The lingual cusp ridge extends cervically about halfway down the lingual surface.

Mesial cusp ridge - This cusp ridge extends mesially to the mesiolinguo-occlusal point angle area.

Distal cusp ridge - It extends distally to end at the lingual groove.

iv. Distolingual cusp - The DL cusp is quite sharp, but is slightly smaller in size than the mesiolingual cusp. The DL cusp has four cusp ridges, which are described as follows:

Buccal cusp ridge - The buccal cusp ridge extends from the cusp tip mesiobuccally to end in the area of the junction of the distobuccal groove and the distal portion of the central groove.

Lingual cusp ridge - It extends in a cervical direction to the middle third of the lingual surface.

Mesial cusp ridge - The mesial cusp ridge extends mesially to the lingual groove.

Distal cusp ridge - It extends distally to the distolinguo-occlusal point angle area.

v. Distal cusp - The distal cusp is much the smallest and shortest of the five cusps, but is relatively sharp. Its four cusp ridges are described as follows:

Buccal cusp ridge - The buccal cusp ridge runs in a cervical direction, and occupies much of the area surrounding the distobuccal line angle.

Lingual cusp ridge - It extends mesiolingually to end in the distal pit area. Compared to other triangular ridges of posterior teeth, it is short and poorly defined.

Mesial cusp ridge - The mesial cusp ridge extends from the cusp tip mesiobuccally to the distobuccal groove.

Distal cusp ridge - It forms the buccal portion of the distal border of the occlusal surface, and extends in a lingual direction rather than distally.

b. Cusp comparison:

i. Relative cusp length (height) from highest to lowest: The mesio-lingual and distolingual cusps are approximately the same height, followed by the mesiobuccal, distobuccal, and distal cusps.

ii. Relative cusp size (bulk) from largest to smallest: The mesiobuccal cusp is the largest cusp, followed in diminishing size by the mesiolingual, distolingual, distobuccal, and distal cusps.

c. Transverse ridges - There are no transverse ridges on the occlusal surface of the mandibular first molar.

d. Marginal ridges - The two marginal ridges are named mesial and distal marginal ridges, and enclose those limits of the occlusal surface.

e. Fossae - There are three recognizable fossae on the occlusal table, with the central fossa encompassing by far the largest area.

i. Central fossa - As the name implies, this fossa is located in the central portion of the occlusal table. It is somewhat circular in shape, and the largest and deepest of the three fossae. It is bounded by the triangular ridges of the four major cusps, as well as the distal cusp ridges of the MB and the ML cusps and the mesial cusp ridges of the DB and DL cusps.

ii. Mesial triangular fossa - The mesial triangular fossa has a location and limits similar to the same fossa on other posterior teeth. It is deeper and more distinct than the distal triangular fossa. Its boundaries include the mesial marginal ridge, the triangular ridges of the two mesial cusps, and the mesial cusp ridges of the two mesial cusps.

iii. Distal triangular fossa - Again, this fossa has a location similar to its counterparts on other posterior teeth. It is the shallowest and least distinct of the three occlusal fossae on this tooth. It is bounded by portions of the distal cusp and distal marginal ridge, as well as the triangular ridges of the D and DL cusps.

f. Pits and grooves - The occlusal surface of the first molar has the most complex groove pattern of any of the mandibular molars.

i. Central pit - The central pit is located in the central fossa, and is the deepest pit on the occlusal surface. It is situated midway mesiodistally, and more than halfway from buccal to lingual. It is at the junction of three primary developmental grooves:

Mesiobuccal (Buccal) groove - This groove extends from the central pit buccally onto the buccal surface. In its most lingual portion, it is confluent with the mesial portion of the central groove.

Distobuccal groove - The distobuccal groove extends in a distobuccal direction from the central pit onto the buccal surface. In its most lingual area, it is confluent with the distal portion of the central groove. Lingual groove - The lingual groove extends from the central pit lin-gually onto the lingual surface.

ii. Mesial pit - The mesial pit is situated halfway buccolingually in the deepest area of the mesial triangular fossa. It is not as deep as the central pit. This pit is the junction of four developmental grooves.

Central groove (Mesial portion) - The mesial portion of the central groove extends mesiobuccally from the central pit a short distance, via the me-siobuccal groove, and then after their separation, continues in a mesial direction to the mesial pit.

Mesiobuccal triangular groove - This groove is similar to the same groove as it was described for the maxillary molars.

Mesiolingual triangular groove - It is also similar to the same groove as it was described for the maxillary molars.

Mesial marginal groove - From the mesial pit, this groove crosses the mesial marginal ridge in a mesial direction.

iii. Distal Pit - The distal pit is located midway buccolingually in the depth of the distal triangular fossa. It is not so deep as the central or mesial pits. It is the union of three developmental grooves:

Central groove (Distal portion) - From the distal pit, this groove passes mesiobuccally to become confluent with the distobuccal groove.

Distolingual triangular groove - This groove extends from the distal pit toward the distolingual line angle, where it fades out. Distal marginal groove - It extends distally from the distal pit over the distal marginal ridge.

Note: The central groove in entirety extends from the mesial pit to the distal pit, and includes its mesial portion, its distal portion, and segments of the mesiobuccal and distobuccal grooves.

Roots:

a. The mandibular first molar has a root trunk which bifurcates to form mesial and distal roots (branches). Both roots are widest buccolingually, and both may have developmental depressions on the mesial and distal root surfaces, termed longitudinal grooves, or root concavities. The two root branches are usually about the same length, but if one root is slightly longer, it invariably is the mesial root which is favored.

Normally, the two roots have some distal angulation, although occasionally they are nearly straight from the root trunk apically.

i. Mesial root - The mesial branch is the widest and strongest of the two roots. It curves mesially from the cervical line to the middle third, and then angles slightly distally to the apex. Its buccal and lingual surfaces are convex throughout their length, while the mesial and distal surfaces are flattened to concave, depending on the presence and prominence of longitudinal grooves.

ii. Distal root - The distal branch is generally smaller and weaker than the mesial root. It is usually straight, but on occasion it curves mesially or distally in the apical third. Normally, only the mesial root surface has a longitudinal groove. The buccal and lingual surfaces are convex throughout their length.

b. Mid root section - In cross section at this level, both roots are wider buccolingually, and the mesial root has a larger outline. The outline is convex buccally and lingually on both roots. Mesially and distally, the outline is flattened or concave, depending on the presence and prominence of longitudinal grooves.

10. Variations and Anomalies:

a. The first molar exhibits few developmental anomalies. However, on rare occasions, the crown may lack a distal cusp.

b. Mulberry molar - The mulberry molar, along with Hutchinson's incisor, are a consequence of congenital syphilis. On the first molars, the cusps are more centrally positioned on the occlusal table, creating a gnarled appearance.

c. Occasionally, the first molar exhibits three roots, when the mesial root has buccal and lingual branches.

Permanent Mandibular Second Molar: 1. General characteristics:

a. Arch position - The second molar is the seventh tooth from the midline in each mandibular quadrant. The mesial contact is shared with the permanent mandibular first molar, while distal contact with the permanent third molar occurs if and when that tooth erupts. It is also known as the "twelve year" molar, due to its normal time of eruption.

b. Universal number:

Mandibular right second molar - #31 Mandibular left second molar - #18

c. General form and function - The second molar resembles the first molar in many respects, although it is more symmetrical, and smaller in all dimensions. It has the least complicated occlusal design of any molar. Normally only four cusps are present, and thus there is no distobuccal groove, and no distal cusp.

The second molar complements the other molars in their grinding function.

Development Table: (Mandibular Second Molar)*

Initiation of calcification 2 1/2 to 3 years

Completion of enamel 7 to 8 years

Eruption 11 to 13 years

Completion of root 14 to 15 years

The second molar is so similar to the first molar that mostly contrasts with that tooth will be made. Buccal aspect:

a. General considerations - The buccal surface is trapezoidal like that of the first molar, but is shorter occlusogingivally, and narrower mesiodistally.

b. Mesial margin - The mesial margin is similar to the first molar's. It is convex in the occlusal portion, and concave in the cervical portion.

c. Distal margin - Again, this outline is similar to that of the first molar. It is generally convex, and more so than the mesial margin.

d. Cervical margin - The cervical line normally has little curvature like that of the first molar, but some specimens may exhibit a sharp dip over the bifurcation area.

*From Wheeler, R.C.: Dental Anatomy, Physiology and Occlusion (5th ed.; Philadelphia: W.B. Saunders Company, 1974).

e. Occlusal margin - This margin is separated into two nearly equal halves by the buccal groove. The two buccal cusps, the mesiobuccal and distobuccal cusps, are about equal in length as are their cusp outlines.

f. Other considerations - The buccal groove breaks the occlusal outline at about its mesiodistal midpoint. It extends cervically to the middle third, where it normally terminates in a buccal pit, but may just fade out. There is no distobuccal groove. M

The buccal cusp ridges of the two buccal cusps form occlusocervical convexities on either side of the concave area which contains the buccal groove.

When present, the buccogingival ridge is similar to that of the first molar, and is most prominent in its mesial portion.

There is normally no mesiodistal concavity in the middle third like some first molars exhibit, but some specimens do show a little flattening in this area.

The height of contour is located in the cervical third.

4. Lingual aspect:

a. General considerations - The lingual surface is also trapezoidal in outline. It is also shorter occlusocervically, and narrower mesiodistally than the first molar.

b. Mesial, distal, and cervical outlines are similar to those of the lingual m surface of the first molar.

' c. Occlusal outline - The occlusal outline is divided approximately in half by the lingual groove. Only the two lingual cusps are visible. D d. Other considerations - The lingual groove crosses the occlusal outline onto the lingual surface, and fades out in the occlusal third near its junction with the middle third.

Since the mesial and distal surfaces of the second molar crown converge only slightly toward the lingual, little, if any, of these surfaces is visible from the lingual aspect.

The height of contour in the middle third and other surface contours are similar to those of the first molar. There is often a centrally located concavity in the cervical third, which corresponds to the flattened area of the first molar.

5. Mesial aspect:

The mesial aspect is similar to the first molar except:

a. It is smaller in general size and is more convex in all directions.

b. The cervical outline is straighter, but like the first molar is more cervically positioned on the buccal as compared to the lingual.

c. The mesial contact area is definitely ovoid, when compared to the first molar's round or slightly ovoid mesial contact.

6. Distal aspect:

The distal aspect is comparable to the first molar except:

a. There is no distal cusp contour, and no distobuccal groove.

b. Since there is no distal cusp, the buccal surface shows much less convergence toward the distal. Consequently, the distal surface is about the same size as the mesial surface, and only a little of the cervical third of the buccal surface is visible.

c. The contact area is centered on the surface both buccolingually and occlusogingivally. It is wider buccolingually than occlusocervically, but is more irregular in its configuration.

7. Occlusal aspect:

a. General considerations - The occlusal table of most second molars is rectangular in shape, but the distal outline is more rounded, when compared to the slightly rounded mesial half. Even though the occlusal table itself is rectangular, the tooth outline from this aspect bulges at the me-siobuccal. This is due to the greater prominence of the mesial portion of the cervical height of contour, which is visible because of the lingual inclination of the crowns of mandibular posterior teeth. The design of the occlusal table and its anatomy are the simplest of any first or second molar.

b. Buccal outline - The buccal outline exhibits a single convexity, which is usually greater toward the mesial.

c. Lingual margin - The lingual outline is slightly shorter than the buccal, and is divided into two equal segments by the lingual groove.

d. Mesial margin - Mesially, the outline presents two portions, separated by the mesial marginal groove.

e. Distal margin - The distal is slightly shorter than the mesial margin, and displays more convexity.

f. Boundaries - The occlusal table is bounded by the two marginal ridges, along with the mesial and distal cusp ridges of all four cusps.

8. Components of the occlusal table:

a. Cusps - There are normally four cusps on the mandibular second molar, all of which are functional.

i. The cusps are termed mesiobuccal, distobuccal. mesiolingual, and distolingual. and are fairly symmetrical in their position on the occlusal surface.

ii. They are more nearly equal in size than the cusps of the first molar. Even so, the mesiobuccal cusp is normally the largest, while the distolingual cusp is normally the smallest, but its size may vary the most.

iii. The buccal cusp ridges of the buccal cusps and the lingual cusp ridges of the lingual cusps are similar to the cusp ridges of the same four cusps on the first molar. The lingual cusp ridges of the buccal cusps meet the buccal cusp ridges of the lingual cusps to form two transverse ridges. Inclined planes, and whether they are functional or not, compare directly with the first molar. The mesial and distal cusp ridges of all cusps either meet each other (example: the mesial cusp ridge of the distobuccal cusp meets the distal cusp ridge of the mesiobuccal cusp in the buccal groove area), or they extend to the point angles (example: The mesial cusp ridge of the mesiobuccal cusp extends to the mesiobucco-occlusal point angle).

b. Marginal ridges - There are two marginal ridges which are similar to, and named the same, as those of other posterior teeth.

c. Transverse ridges - The two transverse ridges are formed by the union of the lingual cusp ridges of the buccal cusps and the buccal cusp ridges of the lingual cusps in the central groove area.

d. Fossae - The three fossae are named and located similar to those of the first molar, although the central fossa is more regular in shape.

e. Pits and grooves - Unlike the first molar, the major groove pattern is almost symmetrical, with the central groove and the buccal and lingual grooves combining to form a cross pattern, the intersection of which is in the central pit. There are often more supplemental grooves on the second molar, however.

i. Central pit - The central pit is aptly named, because it is located centrally on the occlusal surface. It is the deepest of the three pits, and is formed by the junction of three developmental grooves:

Buccal groove - The buccal groove extends buccally from the central pit onto the buccal surface.

Lingual groove - The lingual groove extends lingually from the central D pit onto the lingual surface.

Central groove - It extends between the mesial and distal pits in a straight line which passes through the central pit. The central groove has mesial and distal portions separated by the central pit.

ii. Mesial pit - The mesial pit is not as deep as the central pit, and is located midway buccolingually in the depth of the mesial triangular fossa. It is formed by the junction of four developmental grooves: Central groove (Mesial portion) Mesiobuccal triangular groove Mesiolingual triangular groove Mesial marginal groove

These grooves are located similarly to those of other posterior teeth. This is also true of the grooves which form the distal pit.

iii. Distal pit - The distal pit resembles the mesial pit in depth and relative location, and is the junction of four developmental grooves:

Central groove (Distal portion)

Distobuccal triangular groove

Distolingual triangular groove

Distal marginal groove

The root structure of the second molar is similar to that of the first molar, with the following exceptions:

a. Usually the roots are shorter, but there is more variation, and they can occasionally even be longer.

b. The two branches are closer together, and thus their partial or total fusion is more common.

c. They usually have a greater distal angulation than the roots of the first molar.

10. Variations and Anomalies:

a. Crown anomalies are uncommon, although five-cusp specimens are occasionally seen.

b. Root anomalies are more common, and may be manifested in fused roots or irregular curvatures.

D. Permanent Mandibular Third Molar:

1. General characteristics:

a. Arch position - The mandibular third molar occupies the eighth, and last, position from the midline in each mandibular quadrant. The mesial contact area is shared with the permanent second molar, and there is no distal contact. It is also known as the lower "wisdom tooth."

b. Universal number: Mandibular right third molar - #32 Mandibular left third molar - #17

c. General form and function - The mandibular third molars are extremely variable in size and shape of both their crown and root. However, they have one feature which remains constant in the midst of all the variation. Almost always, the mesiodistal dimension of the crown is greater than the bucco-lingual dimension, just the opposite of the maxillary third molar's dimensions.

Ordinarily, the molars decrease in general size from first molar to third molar, but the crowns of third molars can range from very small to much larger than any other molar. When third molars do vary greatly from normal size, it is more common to find extra large mandibular third molars, and extra small maxillary third molars.

2. Development Table: (Mandibular Third Molar)*

Initiation of calcification 8 to 10 years

Completion of enamel 12 to 16 years

Eruption 17 to 21 years

Completion of root 18 to 25 years

3. Crown form - The crown form is so variable that only generalizations about two basic types will be made.

a. Type I - The type I crown resembles the permanent second molar. It has four cusps, and the same general contours and occlusal pattern.

b. Type II - This type resembles the permanent first molar with five cusps, a similar occlusal pattern, and comparable contours.

c. Third molars normally exhibit many more secondary grooves on the occlusal table, and it is even sometimes difficult to identify the primary grooves.

d. The most common third molar reveals a Type I crown in conjunction with two root branches. Other crown forms range from one-cusped dwarfs to six-cusped specimens.

a. Roots are also extremely variable in numbers, size, and curvatures. Single fused roots are common, as are two-rooted specimens similar to other man-

*From Wheeler, R.C.: Dental Anatomy, Physiology and Occlusion (5th ed.; Philadelphia: W.B. Saunders Company, 1974).

dibular molars. There may be as many as eight roots, and curvatures and fusions, or partial fusions, run the gamut.

b. Most often, root length is less than with other mandibular molars, regardless of the crown size. The most common root form reveals two short root branches. 5. Variations and Anomalies:

a. Third molars are often congenitallv missing or impacted.

b. The area immediately distal to the third molar may be the site of supernumerary teeth. When present, they are normally impacted.

c. Many of the numerous anomalies have been mentioned previously, but almost any structural anomaly conceivable is possible in third molars.

Mandibular Right First Molar

Mandibular Right Second Molar

BUCCAL

OCCLUSAL

LINGUAL

OCCLUSAL

Mid-Root Section

DISTAL

TYPE I

Mandibular Right Third Molar Four-Cusp type with Fused Root

MESIAL

TYPE II

Mandibular Right Third Molar Five-Cusp Type with Two Roots

BUCCAL

OCCLUSAL

LINGUAL

LINGUAL

OCCLUSAL

MESIAL

Mid-Root Section p§I mm o

Mid-Root Section

DISTAL

MESIAL

DISTAL

UNIT #9

I. Reading Assignment:

Unit # 9 (Pulp Cavities)

II. Specific Objectives:

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

A. Define any of the anatomical terms relating to the pulp, or select the correct definition, or application thereof, from a list, when given the term or a description or application of a term.

B. List the main functions of the pulp, or differentiate between them by selecting the correct response from a list, when given the function or any of its applications.

C. Describe, or choose the correct response from a list concerning the changes which occur in the pulp and pulp cavity due to development, aging, or pathology.

D. Differentiate between the various pulp sections by describing or selecting the correct response from a list regarding their feasibility by x-ray, or any of their advantages or disadvantages.

E. Identify from a diagram or written description of any of the common sections of the pulp cavity, which permanent tooth is being described or diagrammed.

F. Demonstrate a knowledge of the normal pulpal anatomy and morphology for all the individual permanent teeth by describing it, selecting the correct response from a list, or making comparisons among the permanent teeth, when given a description of the anatomical feature. Anatomy and morphology include numbers, locations, shapes, outlines, relative thickness and lengths of pulp cavities, pulp horns, pulp chambers, chamber floors, orifices, pulp canals, and apical foramina, in any of the common sections or views.

G. Demonstrate a knowledge of the commonly observed differences from normal pulpal morphology for any of the individual permanent teeth by describing them for any tooth or group of teeth, or by selecting the correct response from a list, when given the normal anatomy or the deviations from normal.

H. Demonstrate a knowledge of the anatomy and components of a normal maxillary molar triangle by identifying them from a diagram or description.

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

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