Inspiration from the Wear of Human Teeth - Dental Biotribology - page 163

8.1.4 Bioactive Self-Protection and Repair Capacity

In general, saliva has a pH of 7 (neutral); however, corrosive agents such as acids
can be introduced into the mouth [ 12 ]. The mouth of a person with a particularly
acidic diet could be at pH 3, and regurgitated gastric acid has pH 1.2. Acidic drinks
contain a range of different acids, which can range from pH 1-6. Increased acidity
in the mouth has been clearly shown to initially decrease both the hardness and the
elastic modulus of enamel [ 13 , 14 ] and then result in pathological wear of teeth.
There has been a worldwide monumental increase in the consumption of soft
drinks, fruit juices, and sport drinks, and this trend will likely increase in the future,
rather than slow down [ 15 ]. This implies that the exposure of teeth to an acidic
environment is increasing. However, human saliva plays an important role in the
antierosion properties of human teeth. The saliva is thought to (1) protect the enamel
surface against acid attack because it is a buffer to acids produced in plaque and
(2) also provide a matrix for the remineralization [ 16 , 17 ] because it supplies
calcium and phosphate ions to remineralize enamel. That is, due to the existence
of saliva in the mouth, human teeth have self-protection and a repair capacity to
some degree.
When the teeth are exposed to an acidic oral environment, human saliva could
dilute the acidic agents introduced into the mouth so as to reduce their erosion to
teeth. Moreover, the presence of even a minute fi lm of salivary pellicle (thickness of
100-500 nm) can protect the exposed materials' surface from the acid, thereby
preventing its removal during the next friction phase. So the protective properties of
saliva are extremely signifi cant for minimizing the corrosive effects of acids on
teeth and restorations.
Human teeth possess a certain self-repair capacity, and remineralization is the
foundation of the bioactive self-repair. Erosive substance loss of enamel is a dynamic
process with demineralization and remineralization periods [ 18 ]. In the initial stage,
the softening of enamel occurs due to partial demineralization of the surface. At this
very early stage of the process, when the pH of saliva returns to neutrality, reminer-
alization will occur on the acid-eroded enamel surface, as the remaining tissue
could act as a scaffold [ 19 ], leading to a bioactive self-repair. Only at a more
advanced stage is the mineral of the outer enamel totally lost, and then self-repair is
not possible, while the remaining softened enamel beneath the lost hard tissue is
still probably remineralizable. Saliva, the most important component of the chem-
istry of the human mouth, also plays an extremely important role in the remineral-
ization repair of teeth. It has been accepted that saliva is a source of inorganic ions
necessary for the remineralization because it supplies calcium and phosphate ions
to build blocks [ 20 ]. Hence, an enamel surface softened by acidic beverages could
be rehardened by the subsequent exposure to saliva.
It should be noted that the chemical composition, amount, and fl ow velocity of
human whole saliva vary with aging. As a result, a signifi cant effect of age on the
bioactive self-protection and repair capacity of human teeth should exist.