In some parts of the surface enamel, and particularly in teeth of the primary dentition, the enamel is covered by crystals which are not organized as rods, but the directions of the individual crystals are oriented perpendicular to the surface. This layer is called aprismatic enamel12 and can present problems when etching enamel for sealing/bonding procedures (see below).
Fig. 1.6 Schematic illustration of three-dimensional arrangement of enamel crystallites within the rods (prisms) resulting from their formation by ameloblasts. Note the in the prism periphery, the crystal orientation changes abruptly, resulting in enlarged intercrystalline spaces in the prism boundaries.45
Fig. 1.7 Mandibular front teeth of a 6-year-old child, with erupting permanent first incisors. They appear more yellowish than the adjacent more opaque deciduous teeth, owing to the color of the underlining dentin. * protuberances.
NOTE
Enamel is the hardest tissue in the human body; however, it is still soluble in acid with a pH below 5.5. The inorganic content of enamel is hydroxyapatite (HAP), fluoride hydroxyapatite (FHAP), carbonate-modified hydroxyapatite (CHAP), and magnesium-modified hydroxyapatite (MHAP). FHAP is less soluble than HAP, which is less soluble than CHAP or MHAP.
Fig. 1.8 Scanning electron microscopic view of an enamel surface showing developmental defects as Tome's process pits, large enough for microorganisms to shelter in them.
The Dentin–Pulp Organ
The dentin and the pulp (see Fig. 1.1) are closely related developmentally and functionally. The odontoblasts, which are the cells responsible for the formation of the dentin, are separated from the pulp cells only by a cell-free zone.
In contrast to the enamel, dentin continues to be formed after crown formation is complete. This is called secondary dentin formation, which over time results in reduction of the size of the pulp chamber.
The dentin consists of about 70 wt% inorganic material, 18 wt% organic material, and 12 wt% water.12 As in the enamel, the inorganic material consists of HAP crystals (20nm in length, <20nm in width, and 3.5nm in thickness) which are smaller than those in enamel. As in enamel, the ions in dentin HAP can also be substituted by other ions, for example, fluoride. About 90% of the organic material consists of collagen. The structure of dentin includes dentinal tubules holding the odontoblast process, surrounded by the periodontoblastic spaces, the peritubular dentin, and the intertubular dentin. The mineral content varies in these different parts of the dentin, with the highest mineral level in the peritubular dentin. Dentin is a vital tissue that reacts to a stimulus such as caries by further dentin formation, in particular tubular sclerosis but also reparative dentin (see Chapter 3).
The pulp consists of 25 wt% organic material and 75 wt% water. The organic content is connective tissue cells (fibroblasts), fibers (collagenous in nature), and ground substances (proteoglycans and fibronectin).12 Arterioles and venules enter and leave the pulp through the apical foramen and accessory root canals. The pulp is richly vascular; however, this changes with age. The nerves follow the course of the blood vessels and often a triad of artery, vein, and nerves is found scattered around the pulp. Extensions of nerve fibers in the pulp are seen along with the odontoblast process in the dentin.
Sensations in the pulp and in the dentin are limited to pain reactions irrespective of the factor initiating the reaction. Pulpal pain is usually dull, throbbing and lasts for some time, dentinal pain is sharp, stabbing, and short-lived.
The Cementum
Cementum made by cementoblasts is the least mineralized of the three dental hard tissues, consisting of about 65 wt% HAP/FHAP or other impure forms of HAP. As with dentin, the majority of the organic matrix (~23%) is composed of collagen. Cementum is a part of the attachment apparatus of the tooth to the alveolar bone. Cementum plays no major role in caries disease as it is often abraded at predilection sites in elderly patients.
NOTE
In contrast to enamel, dentin is a vital tissue, with less inorganic content, and is therefore more soluble in acid than enamel. Cementum often abrades before caries initiates.
Saliva
Saliva Production, Salivary Glands
Saliva is produced mainly by three large pairs of glands: the parotid glands, the submandibular glands, and the sublingual glands (Fig. 1.9). The amount of saliva secreted per day is 0.7–1.5L.20 Without stimulation, an average of 0.25mL per minute is produced, while in stimulated conditions an average 0.7mL per minute is produced. The saliva covers all surfaces in the mouth with a thin film. The parotid gland secretes thin, watery saliva rich in amylase (an enzyme that breaks down starch into sugar). The submandibular glands secrete viscous, slimy saliva rich in mucin (a protein lubricant that also protects body surfaces). The sublingual glands produce viscous saliva. Without stimulation, two-thirds of the total saliva is secreted by the submandibular glands. Some 50% of stimulated saliva is secreted by the parotid glands and 35% comes from the submandibular glands. On viewing reflected light, one will notice that the floor of the mouth is always wet. About 10% of the daily volume of saliva comes from the minor salivary glands in the tongue, lips, and palate.
Fig. 1.9 Overview showing the location and names of the three large glands producing >90% of the daily production of saliva.
Function of Saliva
More than 99% of saliva is water, the rest is electrolytes and organic components including proteins, glycoproteins, and enzymes. The functions of saliva concerning caries are related to all three types of constituent.
The water in the saliva contributes to the following:
• Rinsing effect of the mouth (clearance rate)
• Solubilization of food substances
• Facilitation of bolus formation
• Facilitation of food and bacterial clearance
• Dilution of detritus
• Lubrication of oral soft tissues
• Facilitation of mastication, swallowing, and speech
The electrolytes have the following functions:
• Maintaining supersaturated calcium and phosphate concentrations in saliva with regard to HAP
• Neutralization of acid by buffering actions