The deciduous dentition is made up of primary teeth in humans. These teeth are shed and then replaced by permanent successors. This process of shedding the deciduous teeth and replacement by the permanent teeth is called exfoliation. Exfoliation begins 2 or 3 years after the deciduous root is completely formed. At this time the root begins to resorb at its apical end, and resorption continues in the direction of the crown until the entire root is resorbed and the tooth finally falls out.
The first deciduous teeth to erupt, about 8 months after birth, are the mandibular central incisors. The maxillary central incisors usually erupt about a month later. As in the permanent teeth, the primary mandibular teeth usually erupt before the maxillary. The following is an approximate eruption schedule of the deciduous teeth (see Fig. 5-7).
1. The deciduous anteriors are smaller than their permanent successors in both their crown and root proportions. Deciduous molars are wider mesiodistally than the permanent premolars, which will take their places.
2. The roots of deciduous anterior teeth appear longer and more slender proportionately when compared to permanent teeth. All permanent teeth have much longer roots but the crowns of deciduous teeth are so short that proportionately their roots appear to be long and slender.
4. The cervical ridge of enamel at the cervical third of the anterior crown labially and lingually is much more prominent in deciduous dentition. These bulky ridges extend out from the very narrow cervical necks of the teeth.
5. The buccocervical ridges on the deciduous molars are much more pronounced, especially on first molars. These cervical prominences give deciduous crowns a bulbous appearance and accentuate the narrow cervical portion of deciduous roots.
6. The buccal and lingual surfaces of deciduous molars taper occlusally above the cervical curvatures much more so than the permanent molar buccal and lingual surfaces. This results in a much narrower occlusal table of the occlusal surface buccolingually.
7. The roots of the deciduous molars when compared to permanent molars are also proportionately longer and more narrow because the deciduous crowns are so short. These roots also flare apically to allow room for the permanent teeth to develop between them (Fig. 16.2).
The importance of deciduous teeth cannot be stressed enough. These teeth are extremely important for the proper development of the muscles of mastication, the formation of the bones of the jaws, and the eventual location, alignment, and occlusion of the permanent teeth. Indeed, the succedaneous teeth develop as buds from the deciduous tooth buds.
The deciduous teeth maintain a place for the permanent teeth. It is their function to allow for bone growth of the dental arches. As the bone continues to grow, the deciduous teeth develop spaces between them called primate spaces.
The spaces between the deciduous canines and first molars and those between the first and second molars are called leeway spaces. They allow an extra margin of space for the eruption of the permanent canine, and the first and second premolars. Leeway space is necessary because several offsetting factors are present. First, the mesiodistal measurement of the two permanent premolars combined is less than the sum of the mesiodistal measurements of the deciduous molars. Although this allows extra room for the premolars, the permanent canine requires more room than the deciduous canine. Second, bone growth allows for leeway space, but this is offset by the phenomenon of mesial drift. The first permanent molar tends to move mesially; thus the amount of space reserved for the permanent premolars is shortened. If a deciduous molar is prematurely lost or a decayed interproximal space is not restored, a permanent molar pushes into this space and blocks out the premolar. Little, if any, extra space is available (see Fig. 16-3).
In addition, the resorption of the deciduous roots helps guide their erupting permanent replacements into the proper location. The succedaneous teeth follow the resorbing root through the bone until the deciduous tooth exfoliates from a lack of root anchorage. When a deciduous tooth exfoliates, its permanent replacement can often be seen directly underneath it. Sometimes a thin layer of gum may be covering it; usually it is not completely impacted with bone.
A deciduous central incisor’s mesiodistal diameter is greater than its cervicoincisal length, whereas a permanent central incisor’s cervicoincisal length is greater than its mesiodistal diameter. No mamelons are visible on the deciduous tooth.
From the proximal aspects (Fig. 16-7; see Fig. 16-4, D and E) the crown appears wide in relation to its total length. Because of its short length, the labiolingual measurements make the crown appear thick, even at the incisal third. The mesiocervical curvature is greater than the distal curvature.
A lateral incisor’s crown is smaller than a central incisor’s crown in all dimensions, except that the cervicoincisal length is greater than its mesiodistal width. In all other ways, it appears similar to a central incisor. The root appears much longer in proportion to the crown when compared with the central (Fig. 16-8).
The root of a deciduous maxillary incisor appears constricted at its cervical third. It is twice as long as the crown and tapers evenly toward a blunt apex. A mesial concavity is on the root surface, but the distal surface is generally convex. The lateral incisor surface is longer and more tapered than that of the central incisor.
Mamelons or grooves may be visible in a labial view of a deciduous mandibular incisor (Fig. 16-10). The crown appears wide in comparison with its permanent successor. The mesial and distal sides of the crown taper evenly from the contact areas. The root may be two to three times the height of the crown. It is very narrow and is also conical in shape.
From the mesial aspect the incisal ridge is centered over the root. The labial and lingual cervical contours are quite convex, much more so than those of the permanent mandibular incisors. Cervical curvature is greater on the mesial side than on the distal side (Fig. 16-12; see Fig. 16-9, D and E).
The mandibular lateral incisors (Fig. 16-14) are wider and longer than the central incisors, and their cingula are more developed. Labiolingually the lateral incisors are also wider. There is a tendency for the incisal ridge to slope distally, and its distal margin is more rounded.
The root of the deciduous mandibular lateral incisor is longer, narrower, and more tapered than that of the central. It also is less blunt at the apex. The lateral has a distal longitudinal groove and a mesial depression running lengthwise.
A primary canine is bulkier than the primary incisors in every aspect. The crown is more constricted at the cervix in relation to its mesiodistal width and more convex on its mesial and distal surfaces. The facial lobes are well developed, and a sharp cusp is evident. The root is about twice as long as the crown and more slender than that of its permanent successor.
From the lingual view (Fig. 16-17; see Fig. 16-15, B) the mesial and distal marginal ridges, incisal ridges, and cingulum are all very pronounced. A tubercle may extend from the cusp tip to the lingual ridge. The lingual ridge extends from the cusp tip to the cingulum and divides the lingual surface into mesiolingual and distolingual fossae.
Compared with a maxillary canine, the labial surface of a mandibular canine (Fig. 16-21; see Fig. 16-20, A) is much flatter, with shallow developmental grooves. The distal cusp ridge is longer than that of a maxillary canine. The root is long, narrow, and almost twice the length of the crown, although it is shorter and more tapered than that of a maxillary canine.
The most obvious difference between the maxillary and mandibular canines is the presence of a slight concavity called the lingual fossa. Instead of two lingual fossae, one is present. The lingual surface (Fig. 16-22; see Fig. 16-20, B) is less prominent than that of a maxillary canine, and the crown converges lingually so that it is narrower on the lingual side than on the labial.
The incisal ridge (Fig. 16-24; see Fig. 16-20, C) is straight and centers over the crown labiolingually. The lingual surface shows a definite tapering toward the cingulum. The labial surface from this aspect presents a flat surface with a slight convexity, whereas the lingual surface presents a flattened surface that is slightly concave.
The roots of the deciduous canines are almost twice as long as their crowns, are thicker than the roots of the incisors, and their apices are more blunt. The mandibular root is slightly shorter than the maxillary root and is more tapered. Both have roots that taper lingually and apically. They are triangular in cross section.
The deciduous maxillary first molar is a blend of premolar and molar. It does not resemble any other tooth, deciduous or permanent. This is the most atypical of all maxillary molars. However, like all other maxillary molars, it is wider buccolingually than mesiodistally.
The mesiolingual cusp is the longest and sharpest cusp on this tooth. The distolingual cusp is small and rounded, if present at all. A type of deciduous maxillary first molar has only three cusps: one lingual and two buccal.
Mesial aspect (Fig. 16-28; see also Fig. 16-25, D)
The buccolingual measurement of a deciduous maxillary first molar at the cervical third is greater than the same measurement at the occlusal third. This is true of all molar teeth but is more evident on the deciduous teeth. The mesiolingual cusp is more pronounced and longer in size than the mesiobuccal cusp. The most obvious difference between the deciduous and permanent molars is that the deciduous first molars have an extreme convexity in the cervical third of the buccal surface (buccocervical ridge). This convexity appears to be overdeveloped when compared with that of the permanent teeth. It is a major characteristic of the deciduous maxillary first molars. The cervical line curves slightly toward the occlusal side.
The crown appears to be narrower distally than mesially. The distobuccal cusp is more developed than the distolingual cusp, which is not always present. The cervical convexity (buccocervical ridge) on the buccal surface does not continue onto the distal surface.
The occlusal view (Fig. 16-30; see Fig. 16-25, C) shows that the crown converges in a lingual direction so that the occlusal table appears triangular. The crown may have three or four cusps. If four are present, then two are on the buccal side and two are on the lingual. If three cusps develop, only one is lingual.
The occlusal surface is similar to that of the permanent molars except that the occlusal table is smaller in comparison. On the three-cusp form, only a central and a mesial pit (no distal pit) is evident, and an oblique ridge often unites the mesiolingual with the distofacial cusps. The central groove connects the two fossae—the central fossa and the mesial triangular fossa. The buccal developmental groove is well developed and divides the two buccal cusps occlusally. The mesial, mesiofacial triangular, mesial marginal, and mesiolingual triangular grooves originate in the mesial pit. The distal, facial, and mesial developmental grooves radiate from the central pit.
The three fossae on four-cusp form are mesial, central, and distal. A small pit is usually present in each fossa. Grooves originating at the distal pit are the distofacial triangular, the distolingual, and the distal marginal grooves. An oblique ridge runs from the distobuccal cusp to the mesiolingual cusp.
Maxillary first molars have three roots: two buccal and one lingual. They are long, slender, and very flared. The lingual root is longer and more curved and tips back buccally at the apex. The mesiobuccal root is the next longest; the distobuccal is the shortest and straightest. The root trunk becomes trifurcated immediately above the cervical line. The root trunk is proportionately small when compared with the length of the roots. Each root has a single root canal.
A deciduous maxillary second molar resembles a permanent maxillary first molar, although it is much smaller. From the buccal view (Fig. 16-32; see Fig. 16-31, A) two equal-sized buccal cusps with a buccal groove between them are visible. As on a deciduous first molar, the crown is narrow at its cervix, compared with its mesiodistal measurement at the contact area. A deciduous second molar is much larger than a deciduous first molar both in crown and root formation. The two buccal cusps are about equal in size. How is this different from the cusps of a deciduous first molar?