This is the most important biological factor in the prevention of dental erosion. It starts acting even before the acid attack with an increase in salivary flow in response to visual or olfactory stimuli or to chewing, increasing the buffering system and diluting and clearing acids on tooth surfaces during the erosive challenge.2 The properties of saliva that influence dental erosion are:
Salivary Flow Rate: A low salivary flow rate due to xerostomia, dehydration, use of certain medications, salivary gland pathology, or when there are no stimuli to trigger a protective salivary response (such as when a patient is suffering with GERD) means that teeth are less protected during an acid attack.2,29 A high salivary flow rate, on the other hand, has a protective effect against acid, particularly because it has the ability to clear acids from teeth surfaces.
Saliva’s Chemical Composition and Buffering Capacity: A higher hydrogen bicarbonate content increases the capacity of saliva in neutralising and buffering acids to protect from erosion, while a low buffering capacity is strongly associated with increased erosion. In addition, saliva that is supersaturated with calcium and phosphate ions is more effective at maintaining the integrity of teeth by remineralising the hydroxyapatite in enamel, while saliva that is undersaturated with calcium and phosphate cannot replenish enamel’s mineral content.2,30 The degree of supersaturation of hydroxyapatite, fluorapatite and calcium fluoride also increases as saliva flow is stimulated and increases. It is also important to note that sites poorly bathed by saliva or mainly bathed with mucous saliva (which typically contains fewer mineralising ions) are more likely to show erosion when compared to sites protected by saliva that is serous in nature.31
The Acquired Pellicle
Saliva plays a role in the formation of this protein-based layer which forms within minutes on the surface of a tooth after its removal by toothbrushing, chemical dissolution, or prophylaxis. This barrier prevents the direct contact of an acid and the tooth’s surface, and can serve as a reservoir of remineralising electrolytes.32,33 This protective effect could be clearly visualised by a scanning electron microscopy study where the 2-hour formed pellicle was able to reduce erosion by an acidic beverage.34,35 The acquired pellicle also contains salivary mucins, proteins that have the capacity to increase enamel surface protection against demineralisation.2 The enzymatic composition of the pellicle also plays an important role: The presence of the enzyme carbonic anhydrase VI in the pellicle may protect against tooth erosion because it speeds the neutralisation of demineralising hydrogen ions on the tooth’s surface.35
The pellicle reaches its full thickness in 2 hours, but after this, there is further maturation that allows it to become most acid-resistant. If it is removed often due to factors such as excessive brushing, it will not be allowed to reach maximal thickness or maturation, and risk of erosion is higher.36
Tooth Position and Soft Tissues
Tooth position in the mouth can make it more or less susceptible to dental erosion. This is because different sites in the mouth are affected by variations in salivary flow and composition, or by soft tissues by soft tissues like the tongue. As such, facial surfaces of upper incisors have higher susceptibility to erosion because the exposure to saliva is lower, while lingual surfaces of lower teeth have lower erosion susceptibility because the exposure to protective saliva is higher.2 The most severe erosive lesions are typically found in the palatal surfaces of the upper teeth because of the abrasive effect of the tongue. It has been shown that the tongue is able to remove already softened enamel and dentine.37