Demineralization does not occur or progress when the oral pH is maintained at or above what is known as the critical pH. The oral environment remains stable when the salivary pH remains above 6.5. Critical pH is a dynamic number dependent on salivary calcium and phosphate levels. When the pH drops below 5.5, and there are insufficient salivary concentrations of calcium and phosphorus to counteract the process, enamel begins losing valuable calcium and phosphorus ions on a microscopic level. As hard tooth structures are repeatedly exposed to lowered pH levels for protracted periods of time the risk for both caries and erosion increase.36
Enamel is the hardest substance in the body. Ninety-six percent of enamel is hydroxyapatite, an inorganic crystalline form of calcium phosphate. Water and organic material comprise the remaining 4%. When enamel is fortified with fluoride, hydroxyapatite becomes fluorapatite, which can withstand pH values as low as 4.5.36
Dentin is different than enamel and is 70% hydroxyapatite. The remaining 30% is organic material and water, with 90% as the protein, collagen. Dentin hydroxyapatite crystals are 30 times smaller than the enamel crystals, making dentin softer. The root surface is composed of dentin covered with a thin layer of cementum. The critical pH for root surfaces is higher than pH 6. This creates a significant challenge as more people keep their teeth throughout life. Over time, root exposure can occur as a direct result of periodontal disease, tooth misalignment, parafunctional habits, bruxism, or vigorous oral hygiene habits.36
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