Occlusal Aspect
From the occlusal aspect, this tooth resembles the permanent first molar (see Figures 3-24 and 3-26, 3). It is somewhat rhomboidal and has four well-developed cusps and one supplemental cusp: mesiobuccal, distobuccal, mesiolingual, distolingual, and fifth cusps. The buccal surface is rather flat with the developmental groove between the cusps less marked than that found on the first permanent molar.
Developmental grooves, pits, oblique ridge, and others are almost identical. The character of the "mold" is constant.
The occlusal surface has a central fossa with a central pit, a well-defined mesial triangular fossa, just distal to the mesial marginal ridge, with a mesial pit at its center. A well-defined developmental groove called the central groove is also at the bottom of a sulcus, connecting the mesial triangular fossa with the central fossa. The buccal developmental groove extends buccally from the central pit, separating the triangular ridges, which are occlusal continuations of the mesiobuccal and distobuccal cusps. Supplemental grooves often radiate from these developmental grooves.
The oblique ridge is prominent and connects the mesio-lingual cusp with the distobuccal cusp. Distal to the oblique ridge, the distal fossa is found, which harbors the distal developmental groove. The distal groove has branches of supplemental grooves within the distal triangular fossa, which is rather indefinitely outlined just mesial to the distal marginal ridge.
The distal groove acts as a line of demarcation between the mesiolingual and distolingual cusps and continues on to the lingual surface as the lingual developmental groove. The distal marginal ridge is as well developed as the mesial marginal ridge. It will be remembered that the marginal ridges are not developed equally on the primary maxillary first molar.
MANDIBULAR FIRST MOLAR
The mandibular first molar does not resemble any of the other teeth, deciduous or permanent. Because it varies so much from all others, it appears strange and primitive (Figure 3-27).
Buccal Aspect
From the buccal aspect, the mesial outline of the crown of the primary mandibular first molar is almost straight from the contact area to the cervix, constricting the crown very little at the cervix (Figure 3-28, A).
Figure 3-27 Primary mandibular first molars. This tooth has characteristics unlike those of any other tooth in the mouth, primary or permanent. 1, Buccal aspect. 2, Lingual aspect. 3, Occlusal aspect.
Figure 3-28 Three rare specimens of the primary mandibular first molars. A, Buccal aspect. B, Mesial aspect. These three specimens have their roots intact with little or no resorption showing. They enable the viewer to observe the actual shape and size of the mesial and distal roots. The mesial root is broad, curved, and long, with fluting down the center. This makes for tremendous anchorage. The distal root is much shorter, but it is heavy and also curved. It does its share in bracing the crown, being in partnership with the mesial root during the process.
The outline describing the distal portion, however, converges toward the cervix more than usual, so that the contact area extends distally to a marked degree (see Figures 3-21, C and 3-27, 1).
The distal portion of the crown is shorter than the mesial portion, with the cervical line dipping apically where it joins the mesial root.
The two buccal cusps are rather distinct, although no developmental groove is evident between them. The mesial cusp is larger than the distal cusp. A developmental depression dividing them (not a groove) extends over to the buccal surface.
The roots are long and slender, and they spread greatly at the apical third beyond the outline of the crown.
The buccal aspect emphasizes the strange, primitive look of this tooth. The primary first mandibular molar from this angle impresses one with the thought of the possibility that at some time in the dim past, a fusion of two teeth ended in a strange single combination. That thought seems particularly apropos when a well-formed specimen of the tooth in question is located—one with its roots intact, showing no evidences of decalcification.
From the buccal aspect, if a line is drawn from the bifurcation of the roots to the occlusal surface, the tooth will be evenly divided mesiodistally. However, the mesial portion represents a tooth with a crown almost twice as tall as the distal half, and the root is again a third as long as the distal one. Two complete teeth are represented, but their dimensions differ considerably (see Figures 3-21, C and 3-27, 1).
Lingual Aspect
The crown and root converge lingually to a marked degree on the mesial surface (see Figures 3-22, C and 3-27, 2). Distally, the opposite arrangement is true of both crown and root. The distolingual cusp is rounded and suggests a developmental groove between this cusp and the mesiolin-gual cusp. The mesiolingual cusp is long and sharp at the tip, more so than any of the other cusps. The sharp and prominent mesiolingual cusp (almost centered lingually but in line with the mesial root) is an outstanding characteristic found occlusally on the primary first mandibular molar. It is noted that the mesial marginal ridge is so well developed that it might almost be considered another small cusp lingually. Part of the two buccal cusps may be seen from this angle.
From the lingual aspect, the crown length mesially and distally is more uniform than it is from the buccal aspect. The cervical line is straighter.
Mesial Aspect
The most noticeable detail from the mesial aspect is the extreme curvature buccally at the cervical third (see Figure 3-28, B). Except for this detail, the crown outline of this tooth from this aspect resembles the mesial aspect of the primary second molar and that aspect of the permanent mandibular molars. In this comparison, the buccal cusps are placed over the root base, and the lingual outline of the crown extends out lingually beyond the confines of the root base.
Both the mesiobuccal cusp and the mesiolingual cusp are in view from this aspect, as is the well-developed mesial marginal ridge. Because the mesiobuccal crown length is greater than the mesiolingual crown length, the cervical line slants upward buccolingually. Note the flat appearance of the buccal outline of the crown from the crest of curvature of the buccal surface at the cervical third to the tip of the mesiobuccal cusp. All of the primary molars have flattened buccal surfaces above this cervical ridge.
The outline of the mesial root from the mesial aspect does not resemble the outline of any other primary tooth root. The buccal and lingual outlines of the root drop straight down from the crown and are approximately parallel for more than half their length, tapering only slightly at the apical third. The root end is flat and almost square. A developmental depression usually extends almost the full length of the root on the mesial side.
Distal Aspect
The distal aspect of the mandibular first molar differs from the mesial aspect in several ways. The cervical line does not drop buccally. The length of crown buccally and lingually is more uniform, and the cervical line extends almost straight across buccolingually. The distobuccal cusp and the disto-lingual cusp are not as long or as sharp as the two mesial cusps. The distal marginal ridge is not as straight and well defined as the mesial marginal ridge. The distal root is rounder and shorter and tapers more apically.
Occlusal Aspect
The general outline of this tooth from the occlusal aspect is rhomboidal (see Figure 3-27, 3). The prominence present mesiobuccally is noticeable from this aspect, which accents the mesiobuccal line angle of the crown in comparison with the distobuccal line angle and thereby emphasizes the rhom-boidal form.
The mesiolingual cusp may be seen as the largest and best developed of all the cusps, and it has a broad, flattened surface lingually. The buccal developmental groove of the occlusal surface divides the two buccal cusps evenly. This developmental groove is short, extending from between the buccal cusp ridges to a point approximately in the center of the crown outline at a central pit. The central developmental groove joins it at this point and extends mesially, separating the mesiobuccal cusp and the mesiolingual cusp. The central groove ends in a mesial pit in the mesial triangular fossa, which is immediately distal to the mesial marginal ridge. Two supplemental grooves join the developmental groove in the center of the mesial triangular fossa; one supplemental groove extends buccally and the other extends lingually.
The mesiobuccal cusp exhibits a well-defined triangular ridge on the occlusal surface, which terminates in the center of the occlusal surface buccolingually at the central developmental groove. The lingual developmental groove extends lin-gually from this point, separating the mesiolingual cusp and the distolingual cusp. Usually, the lingual developmental groove does not extend through to the lingual surface but stops at the junction of lingual cusp ridges. Some supplemental grooves immediately mesial to the distal marginal ridge in the distal triangular fossa join with the central developmental groove.
MANDIBULAR SECOND MOLAR
The primary mandibular second molar has characteristics that resemble those of the permanent mandibular first molar, although its dimensions differ (Figure 3-29).
Buccal Aspect
From the buccal aspect, the primary mandibular second molar has a narrow mesiodistal calibration at the cervical portion of the crown compared with the calibration mesio-distally on the crown at contact level. The mandibular first permanent molar, accordingly, is wider at the cervical portion (see Figures 3-21, D and 3-29, 1).
From this aspect also, mesiobuccal and distobuccal developmental grooves divide the buccal surface of the crown occlusally into three cuspal portions almost equal in size. This arrangement forms a straight buccal surface presenting a mesiobuccal, a buccal, and a distobuccal cusp. It differs, therefore, from the mandibular first permanent molar, which has an uneven distribution buccally, presenting two buccal cusps and one distal cusp.
The roots of the primary second molar from this angle are slender and long. They have a characteristic flare mesio-distally at the middle and apical thirds. The roots of this tooth may be twice as long as the crown.
The point of bifurcation of the roots starts immediately below the CEJ of crown and root.
Lingual Aspect
From the lingual aspect, two cusps of almost equal dimensions can be observed (see Figures 3-22, D and 3-29, 2). A short, lingual groove is between them. The two lingual cusps are not quite as wide as the three buccal cusps; this arrangement narrows the crown lingually. The cervical line is relatively straight, and the crown extends out over the root more distally than it does mesially. The mesial portion of the crown seems to be a little higher than the distal portion of the crown when viewed from the lingual aspect. It gives the impression of being tipped distally. A portion of each of the three buccal cusps may be seen from this aspect.
The roots from this aspect give somewhat the same appearance as from the buccal aspect. Note the length of the roots.
Mesial Aspect
From the mesial aspect, the outline of the crown resembles that of the permanent mandibular first molar (see Figures 3-23, D and 3-29, 3). The crest of contour buccally is more prominent on the primary molar, and the tooth seems to be more constricted occlusally because of the flattened buccal surface above this cervical ridge.
The crown is poised over the root of this tooth in the same manner as all mandibular posteriors; its buccal cusp is over the root and the lingual outline of the crown extending out beyond the root line. The marginal ridge is high, a characteristic that makes the mesiobuccal cusp and the mesi-olingual cusp appear rather short. The lingual cusp is longer, or higher, than the buccal cusp. The cervical line is regular, although it extends upward buccolingually, making up for the difference in length between the buccal and lingual cusps.
The mesial root is unusually broad and flat with a blunt apex that is sometimes serrated.
Distal Aspect
The crown is not as wide distally as it is mesially; therefore it is possible to see the mesiobuccal and distobuccal cusps from the distal aspect. The distolingual cusp appears well developed, and the triangular ridge from the tip of this cusp extending down into the occlusal surface is seen over the distal marginal ridge.
The distal marginal ridge dips down more sharply and is shorter buccolingually than the mesial marginal ridge. The cervical line of the crown is regular, although it has the same upward incline buccolingually on the distal as on the mesial.
The distal root is almost as broad as the mesial root and is flattened on the distal surface. The distal root tapers more at the apical end than does the mesial root.
Figure 3-29 Primary mandibular second molars. 1, Buccal aspect. 2, Lingual aspect. 3, Mesial aspect. 4, Occlusal aspect.
Occlusal Aspect
The occlusal aspect of the primary mandibular second molar is somewhat rectangular (see Figures 3-24, D and 3-29, 4). The three buccal cusps are similar in size. The two lingual cusps are also equally matched. However, the total mesio-distal width of the lingual cusps is less than the total mesiodistal width of the three buccal cusps.
Well-defined triangular ridges extend occlusally from each one of these cusp tips. The triangular ridges end in the center of the crown buccolingually in a central developmental groove that follows a staggered course from the mesial triangular fossa, just inside the mesial marginal ridge, to the distal triangular fossa, just mesial to the distal marginal ridge. The distal triangular fossa is not as well defined as the mesial triangular fossa. Developmental grooves branch off from the central groove both buccally and lingually, dividing the cusps. The two buccal grooves are confluent with the buccal developmental grooves of the buccal surface, one mesial and one distal, and the single lingual developmental groove is confluent with the lingual groove on the lingual surface of the crown.
Scattered over the occlusal surface are supplemental grooves on the slopes of triangular ridges and in the mesial and distal triangular fossae. The mesial marginal ridge is better developed and more pronounced than the distal marginal ridge. The outline of the crown converges distally. An outline following the tips of the cusps and the marginal ridges conforms to the outline of a rectangle more closely than does the gross outline of the crown in its entirety.
A comparison occlusally between the deciduous mandibular second molar and the permanent mandibular first molar brings out the following points of difference. In the deciduous molar the mesiobuccal, distobuccal, and distal cusps are almost equal in size and development. The distal cusp of the permanent molar is smaller than the other two. Because of the small buccal cusps, the deciduous tooth crown is narrower buccolingually, in comparison with its mesiodistal measurement, than is the permanent tooth.
