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 is so hard because it is composed primarily of inorganic materials: Roughly 95% of enamel is calcium and phosphate ions combined to make up strong hydroxyapatite crystals. Hydroxyapatite crystals contain calcium and phosphate ions in the following proportions: Ca10 (PO4)6 OH2. Hydroxyapatite readily incorporates 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 concentrations of these trace minerals change the solubility of enamel. For example, the presence of fluoride in the crystal structure strengthens the structure and decreases solubility, while carbonate incorporation increases solubility. It has been found that hydroxyapatite crystals have more fluoride and less carbonate than crystals in the interior, making the outer surface less soluble than deeper layers of enamel.2,4,5
Approximately 1% to 2% of enamel is made up of organic materials, particularly enamel-specific proteins called enamelins, which have a high affinity for binding hydroxyapatite crystals. Water makes up the remainder of enamel, accounting for about 4% of its composition.
The inorganic, organic, and water components of enamel are highly organized: Millions of carbonated hydroxyapatite crystals are arranged in long, thin structures called rods that are 4 µm to 8 µm in diameter.2,4 Viewed in cross section, these rods appear as keyhole-shaped structures. It is estimated that the number of rods in a tooth ranges from 5 million in the lower lateral incisor to 12 million in the upper first molar. In general, rods extend at right angles from the dento-enamel junction (the junction between enamel and the layer below it called dentin) to the tooth surface. Surrounding each rod is a rod sheath made up of a protein matrix of enamelins. The area in between these rods is referred to as interrod enamel, or interrod cement. While it has the same crystal composition, crystal orientation is different, distinguishing rods from interrod enamel.2,4,5
Minute spaces exist where crystals do not form between rods. Typically called pores, they contribute to enamel’s permeability, which allows fluid movement and diffusion to occur, but they also cause variations in density and hardness in the tooth, which can create spots that are more prone to demineralization - the loss of calcium and phosphate ions - when the oral pH becomes too acidic.2,6
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