• Primary molars have a broader and flatter interproximal contact than permanent molars.
Micromorphology
• The enamel surface is characterized by a largely aprismatic enamel surface (layer thickness 30–100 μm).
• The enamel prisms in the cervical area increase from the dentinoenamel junction toward the occlusal surface.
• The mineral content of the primary tooth enamel is lower than in the permanent dentition.
• In primary teeth the enamel formed postnatally is far less densely mineralized than the prenatal enamel mantle.
• The structure of primary tooth dentin is different than permanent tooth dentin: The dentinal tubules are larger, the peritubular dentin is more highly developed, and the mineral content of the intertubular dentin is lower than in the permanent dentition.
Fig 1-1 Morphologic differences between primary and permanent teeth.
The micromorphology is characterized by an aprismatic and irregular enamel structure (Fig 1-2). The proportion of organic constituents is higher than in permanent teeth, which explains poorer conditioning by the acid etch technique. The dentin structure also differs from that of permanent teeth (Fig 1-3): The mineral content is reduced, the distribution of dentinal tubules is more irregular, and the tubules are larger. This explains the faster progression of caries and the lower dentin adhesive values.3
Fig 1-2 Cross section of a primary (a) versus a permanent (b) tooth revealing enamel layer thickness. In the primary tooth, the enamel layer is very thin compared with the permanent tooth. (Photographs courtesy of Peter Schaller.)
Fig 1-3 Longitudinal section of a primary (a) versus a permanent (b) tooth. The size of the pulp cavity is much larger in the primary tooth, whereas the dentin layer between the enamel and the pulp is much thicker in the permanent tooth. (Photographs courtesy of Peter Schaller.)
MINERALIZATION AND ERUPTION TIMES
To understand disorders such as hypomineralization or dental fluorosis, we need to know exactly when primary and permanent teeth are mineralized (Tables 1-1 and 1-2). Furthermore, when assessing radiographs in the mixed dentition, it can be helpful to know when the dental crowns of the permanent premolars or molars should be visible so that any agenesis can be diagnosed. Table 1-3 shows the eruption times of the primary and permanent teeth. It should be noted that relatively wide variations in these timings are possible; those listed in the table should only serve as a guide.
TABLE 1-1 Mineralization times of the primary teeth4
| Tooth | Start of mineralization | End of mineralization | Root fully developed |
| Incisors | 3–5 months in utero | 4–5 months postnatal | 1.5–2 years |
| Canines | 5 months in utero | 9 months postnatal | 2.5–3 years |
| Primary first molar | 5 months in utero | 6 months postnatal | 2–2.75 years |
| Primary second molar | 6–7 months in utero | 10−12 months postnatal | 3 years |
TABLE 1-2 Mineralization times of the permanent teeth4
| Tooth | Start of mineralization | Crown fully developed | Root fully developed |
| Maxilla | |||
| Central incisor | 3–4 months | 4–5 years | 10 years |
| Lateral incisor | Up to 1 year | 4–5 years | 11 years |
| Canine | 4–5 months | 6–7 years | 13–15 years |
| First premolar | 1.5–1.75 years | 5–6 years | 13–15 years |
| Second premolar | 2–2.25 years | 6–7 years | 12–14 years |
| First molar | At birth | 2.5–3 years | 9–10 years |
| Second molar | 2.5–3 years | 7–8 years | 14–16 years |
| Third molar | 7–9 years | 12–16 years | 18–25 years |
| Mandible | |||
| Central incisor | 3–4 months | 4–5 years | 9 years |
| Lateral incisor | 3–4 months | 4–5 years | 10 years |
| Canine | 4–5 months | 6–7 years | 12–14 years |
| First premolar | 1.75–2 years | 5–6 years | 13 years |
| Second premolar | 2.25–2.5 years | 6–7 years | 13–14 years |
| First molar | At birth | 2.5–3 years | 9–10 years |
| Second molar | 2.5–3 years | 7–8 years | 14–15 years |
| Third molar | 8–10 years | 12–16 years | 18–25 years |