Approximal Surfaces
On approximal surfaces at least three macromorphological features can influence the development of caries and must be taken into consideration:
• The width and location of the approximal contact area. That is, approximal surfaces on tooth types with narrow contact points (front teeth) have less caries than approximal surfaces of tooth types with wide approximal surface contact areas (molar teeth).10,11
• The curvature of the approximal surfaces. Certain molars in both dentitions show a degree of concavity on the approximal surfaces.3
• The margino-segmental grooves (Fig. 1.3a) may contribute to an uneven contact with the adjacent tooth, and the grooves can be both fissurelike and groovelike.
The Cervical Enamel Line and the Roots
The cervical enamel line (Fig. 1.3b) is also termed the cemento-enamel junction and is the boundary line between the anatomical crown and the anatomical root complex.3 In patients with healthy gingiva, the line/junction is at the same level as the marginal gingiva. This line/junction is irregular and rough, so microorganisms can adhere easily to this area of the tooth.
Apart from some grooves on the roots of particular teeth, there are no macromorphological structures which promote caries development in the roots. Rather, the gingiva around the neck of the tooth promotes stagnation of microorganisms, eventually developing into plaque. In the case of gingival recession, new plaque stagnation areas are formed where root caries can develop.
NOTE
Caries usually develops in specific locations in the teeth: these are the occlusal surfaces, the approximal surfaces, and along the gingival margin.
Enamel
The enamel is formed by ameloblasts in three consecutive steps. Initially, the ameloblasts secrete proteins in such a way that the final form of the tooth is developed; simultaneously, a part of the protein is replaced by mineral. This is the secretory phase of amelogenesis.12 The majority of the protein is, however, replaced by mineral during the maturation stage of amelogenesis, which takes place over several years. The amelogenesis ends at the time for emergence of the tooth when the reduced ameloblast fuses with the epithelium cells. More details can be found in Mjör and Fejerskov.12
Fig. 1.4 Illustration showing the structure of a hydroxyapatite crystal. The smallest repeating entity of the crystal in enamel (arrow) has in its purest form the formula Ca5(PO4)3(OH).
Fig. 1.5 Scanning electron microscopic images of enamel rods (R) built up by crystals.
Chemical Composition and Structure of Apatite Crystals
The inorganic content of mature enamel amounts to 96%–97% by weight; the remainder is organic material and water. On the basis of volume around 86% is mineral, 12% is water, and 2% is organic material.12
Owing to its hardness, enamel is difficult to cut for histological examinations used to study its structure. Therefore different approaches have been considered to describe its nature. One way to do this is at the crystalline level. In material science, a crystal is a solid substance in which the atoms, molecules, or ions are arranged in an orderly repeating pattern extending in all three dimensions. The crystals made by the ameloblasts consist of calcium phosphate, and the smallest repeating entity of the crystals in enamel has, in its purest form, the formula Ca5(PO4)3(OH), which is termed hydroxyapatite (HAP) (Fig. 1.4). The crystals are approximately hexagonal in cross-section, with a diameter of ca. 40nm. The length of the crystals is difficult to assess, but today it is assumed that the length is between 100nm and 1000nm.13
At the chemical level, several substitutions of the ions in HAP can and do occur (resulting in impure forms of HAP)—for example, substitution with fluoride giving fluoride hydroxyapatite (FHAP); with carbonate, carbonate-modified hydroxyapatite (CHAP); and with magnesium, magnesium-modified hydroxyapatite (MHAP). Fluorapatite is a crystal where (nearly) all of the OH− ions in HAP are replaced by fluoride, and which has a lower solubility than HAP; this, however, is not that common in human enamel.14 More commonly, the OH− ions are only partially replaced by fluoride, and FHAP is formed. These crystals also have a lower solubility than HAP, which again has lower solubility than CHAP.15–17 These chemical conditions have great influence on the caries process and will be highlighted in Chapters 2 and 3.
The individual crystals are arranged in rods (or prisms) (Fig. 1.5) extending from the enamel–dentin junction to the surface, with an average diameter of about 4–5μm. The crystals in the rods all align in the same direction except at the periphery, where the crystals change direction from those in the core of the rod. Thus, the space between the crystals or intercrystalline spaces (also called the pore volume which is filled with air, water, or proteins) is larger at the periphery of the rod than at the core. As the periphery of one rod meets other peripheries of other rods, the pore volume between rods is relatively large and much larger than in the core of the rod (Fig. 1.6). This is important for caries formation as acid and other products more easily penetrate through areas of enlarged pore volume (see also Chapter 3).
Due to this uniform structure of the enamel with tightly packed crystals, light penetrates through the enamel and is reflected or absorbed in the dentin. Well mineralized, permanent enamel is translucent, and it is the underlying dentin which, eventually, gives the tooth its color (Fig. 1.7). If the pore volume in the enamel increases, the light is scattered and reflected in the enamel which results in a white color. Primary teeth (see Fig. 1.7), which show a greater pore volume than the erupting permanent enamel, appear therefore whiter than permanent teeth.
Macroscopically/clinically the enamel generally looks smooth and even (Figs. 1.3, 1.7); however, at high magnification the surface enamel is full of developmental defects such as pits, cracks, and fissures18,19 as well as normal anatomical features such as Tomes’ process pits corresponding to the head of the ameloblasts (Fig. 1.8). Thus, there