Caries Process, Prevention, and Management: Demineralization/Remineralization
Course Number: 714
Course Contents
Enamel
Enamel is the most mineralized tissue of the body, forming a very hard, thin, translucent layer of calcified tissue that covers the entire anatomic crown of the tooth. Enamel’s remarkable hardness is due to its composition, which is primarily inorganic. Approximately 95% of enamel consists of calcium and phosphate ions that combine to form strong hydroxyapatite crystals (Ca10(PO4)6(OH)2). These hydroxyapatite crystals are highly organized, giving enamel with its strength and resistance to mechanical wear.
Figure 1. Enamel Structure and Composition
Hydroxyapatite crystals are dynamic and can incorporate trace minerals into its crystal lattice. These ions can be negatively charged, such as fluoride or carbonate, or positively charged, such as sodium, zinc, strontium, or potassium. The concentration and type of these trace minerals influence the solubility of enamel. For example, the presence of fluoride in the crystal structure strengthens enamel and reduces its solubility, making the enamel more resistant to demineralization. On the other hand, the incorporation of carbonate increases enamel solubility, making it more susceptible to acid dissolution. It has been shown that the outer layers of enamel are richer in fluoride and poorer in carbonate than the inner layers, making the enamel surface more resistant to acid dissolution than the deeper layers. This gradient in mineral content plays a critical role in the enamel’s ability to resist environmental challenges, such as acid exposure from dietary sources.
Approximately 1% to 2% of enamel consists of organic materials, primarily enamelins, which are specialized proteins that have a high affinity for binding to hydroxyapatite crystals, facilitating enamel mineralization and organization. The remainder of enamel’s composition is water, which accounts for approximately 4%.
The inorganic, organic, and water components of enamel are highly organized: Millions of carbonated hydroxyapatite crystals are arranged in long, thin prisms (or rods) with diameters ranging from 4 to 8 µm.7,8 These rods are aligned perpendicular to the dento-enamel junction (DEJ), which separates enamel from the underlying dentin. Viewed in cross-section, these rods appear as keyhole-shaped structures, with each rod surrounded by a sheath made of enamelin proteins. The interrod enamel, or interrod cement, lies between the rods and has the same crystal composition, but with different orientations of the crystals.
Figure 2. Enamel rod cross-section
The number of enamel rods varies depending on the tooth, with estimates ranging from 5 million in the lower lateral incisor to 12 million in the upper first molar. These structural features are critical for the mechanical properties of enamel, as they contribute to its hardness and resistance to mechanical wear. However, minute spaces, or pores, exist between the rods, where crystals are absent. These spaces contribute to enamel’s permeability, enabling fluid movement and diffusion within the enamel, which is vital for processes such as remineralization and demineralization. However, these pores also lead to variations in density and hardness, making enamel more prone to demineralization, especially when the oral pH becomes acidic due to the presence of cariogenic bacteria.
When the pH of the oral environment drops below a critical threshold (typically around pH 5.5), demineralization occurs, where calcium and phosphate ions are leached from the enamel, weakening the structure. This process is exacerbated by the acidic byproducts of bacterial metabolism, particularly lactic acid produced by acidogenic bacteria during the fermentation of dietary carbohydrates.