The Dental Erosion Process

Constituent
Enamel (vol %)
Dentin (vol %)
Mineral
91.3
48.7
Organic material (protein and lipid)
5.3
29.9
Water
3.4
21.4

2.2.2 Dentin

Dentin differs radically from enamel in that about 30 % of the tissue is made up of organic matrix (Table 2.1), of which about 90 % is the fibrous, insoluble protein collagen. The remainder consists of a variety of proteins and carbohydrates and a small amount of lipid. The mineral crystals are platelike in form rather than ribbon-like as in enamel and are also much smaller and thinner: approximately 30 nm wide, 3 nm thick [5].
During dentinogenesis, many of the crystals are deposited within the collagen fibres, so are intimately associated with the sub-fibrils, while the remaining crystals are deposited between the fibres. The proportion of crystals within the fibres is between 25 and 80 % [6].
Of the overall porosity (about 21 vol%: Table 2.1), about 6.5 vol% is associated with the tubules, which run from the enamel-dentin junction to the pulp chamber. As these are wider and more closely packed towards the pulp, they occupy a greater proportion of the inner dentin (22 vol%) than of the outer dentin (1 vol%). Within the intertubular dentin, the average porosity is about 15 vol%. The interfibrillar regions are probably less porous than the intertubular regions, because of the close packing of organic and inorganic material. It is unlikely that the pores of dentin show much directionality because the crystals are very small and platelike.

2.3 Chemistry of Dental Mineral

Dental minerals are forms of a sparingly soluble calcium phosphate known as hydroxyapatite, which in its pure form has the formula Ca10(PO4)6(OH)2. An important property of dental mineral is the solubility, which determines whether a given solution will allow dissolution to proceed, and the concept requires a few words of introduction.
When a sparingly soluble solid such as hydroxyapatite is immersed in water, it will begin to dissolve. If there is an excess of solid and a limited volume of water, dissolution will not continue indefinitely but will eventually cease. At this point, the system is in equilibrium and the solution is said to be saturated. Analysis of the saturated solution enables the solubility of the solid to be determined. The fundamental thermodynamic solubility is defined in terms of the chemical activity of the dissolved solid and is a constant for a given temperature. In this chapter, a more practical definition of solubility will be used, namely, the gravimetric solubility, which is the concentration (mass per unit volume) of dissolved solid in solution.
Solutions in which the concentration of dissolved solid is less than in a saturated solution are undersaturated and solutions with a higher concentration are supersaturated. Undersaturated solutions can support dissolution of the solid but not precipitation, while supersaturated solutions support precipitation but not dissolution.
In the dental tissues, the mineral contains a number of impurity ions, which take the place of ions in the hydroxyapatite structure. Thus, Ca2+ ions can be replaced by Na+ or Mg2+ ions; 
$$ {\mathrm{PO}}_4^{3-} $$
ions can be replaced by 
$$ {\mathrm{CO}}_3^{2-} $$
ions and OH ions by 
$$ {\mathrm{CO}}_3^{2-} $$
or F ions [5]. In most of these cases, the impurity ion has a different charge or size to the ion it is replacing. This results in misfits in the crystal lattice which disturb the crystal structure and in turn make the mineral chemically less stable: in other words more soluble.
Table 2.2 shows that the major impurities in both dentin and enamel mineral are carbonate, magnesium and sodium. In relation to the calcium and phosphate concentrations (i.e. the total mineral), dentin contains more carbonate and magnesium than enamel and is also much less well crystallised. In Fig. 2.1, the curves represent the equilibrium concentration of relevant solids over a range of pH from neutral to the low values typical of erosive products. The higher the concentration at any particular pH, the greater the solubility. The figure shows that enamel is slightly more soluble than pure hydroxyapatite but dentin is much more soluble. The figure also shows that the gravimetric solubility of these solids increases as the pH of the solution decreases.

Table 2.2

Principal inorganic constituents of dentin and enamel (percent dry weight) [5]
Constituent
Enamel
Dentin
Ca
36.6
26.9
P
17.7
13.2
CO3
3.2
4.6
Na
0.7
0.6
Mg
0.4
0.8
Cl
0.4
0.06
K
0.04
0.02
A316956_1_En_2_Fig1_HTML.gif
Fig. 2.1

Solubility diagram for solids relevant to erosion. Lines represent equilibrium solubilities over the pH range 2.5–7.5. Solubilities for fluorapatite are given for the lowest and highest fluoride concentrations in the products studied by Lussi et al. [7]
Figure 2.1 also shows solubility of an additional solid, fluorapatite, which is structurally close to hydroxyapatite but in which all the OH ions are replaced by F. Because the F ion has the same charge as the OH ion and is slightly smaller, this substitution (unlike those discussed above) results in a more stable crystal structure and hence reduces solubility. The solubility of fluorapatite depends on the fluoride concentration in solution, so its solubilities are given for a range of fluoride concentrations found in representative erosive products [7].

2.4 Acids and Demineralisation

The acids responsible for erosion (Table 2.3) may be intrinsic or extrinsic in origin. The intrinsic acid is hydrochloric acid, the principal component of gastric juice. This comes into contact with teeth when gastric juice is regurgitated, either as an occasional occurrence or more frequently, as in gastro-oesophageal reflux disorder. Extrinsic acids reach the mouth via two routes. Certain industries, e.g. battery production, are associated with vapours of strong acids such as sulphuric acid, which attack the teeth after inhalation and dissolution in the saliva. Of far greater importance at population level are acidic components of products intended for human consumption: soft drinks, fruit juices, acidic fruits and vegetables, some alcoholic drinks, some vitamin supplements and medications. The acids found in foods may be metabolic products of fruits or vegetables (malic, tartaric, citric acids) or products of bacterial fermentation (acetic, lactic).

Table 2.3

Acids associated with dental erosion
Acid
Occurrence
Intrinsic acid
Hydrochloric acid
Gastric juice reflux
Extrinsic acids
Sulphuric acid, chromic acid
Vapours associated with battery production
Phosphoric acid
Cola
Acetic acid
Vinegar, pickles
Lactic acid
Cheese, yoghurt, wine, fermented cabbage (e.g. sauerkraut)
Malic acid
Apples, grapes, wine
Tartaric acid
Grapes, tamarind, wine
Citric acid
Citrus fruits
Ascorbic acid
Vitamin C supplements
All of these acids provide hydrogen ions (H+) which dissolve dental mineral. Taking hydroxyapatite as an example for this process, the reaction is:


$$ \begin{array}{l}{\mathrm{Ca}}_{10}{\left({\mathrm{PO}}_4\right)}_6{\left(\mathrm{O}\mathrm{H}\right)}_2+\left(2+3x+2y+z\right){\mathrm{H}}^{+}\to 10{\mathrm{Ca}}^{2+}+x{\mathrm{H}}_3{\mathrm{PO}}_4^0\\ {}\kern0.36em +y{\mathrm{H}}_2{\mathrm{PO}}_4^{-}+z{\mathrm{H}\mathrm{PO}}_4^2+2{\mathrm{H}}_2\mathrm{O}\end{array} $$
(2.1)

$$ \left(x+y+z=6\right) $$

Here, the fully dissociated phosphate anion, 
$$ {\mathrm{PO}}_4^{3-} $$
, is omitted because its concentration is exceedingly low. The proportions of the other forms of phosphate (x, y, z) depend on pH. The reaction for dental mineral is similar but also involves the conversion of carbonate ions to carbon dioxide and water:


$$ {\mathrm{CO}}_3^{2-}+2{\mathrm{H}}^{+}\to {\mathrm{CO}}_2\left(\mathrm{gas}\right)+{\mathrm{H}}_2\mathrm{O} $$
(2.2)

Hydrochloric and sulphuric acids are strong acids: i.e. at all pH values they are fully dissociated into H+ ions and Cl or 
$$ S{O}_4^{2-} $$
ions. The remaining acids in Table 2.3 are weak acids. At low pH, they consist almost entirely of undissociated acid. As the pH increases, the acids dissociate progressively. Each molecule of acid may provide one H+ (acetic, lactic), two H+ (malic, tartaric) or three H+ (citric, phosphoric). The dissociation reactions for tartaric acid are:


$$ {\mathrm{H}}_2{\mathrm{Tartrate}}^0\rightleftharpoons {\mathrm{H}\mathrm{Tartrate}}^{-}+{\mathrm{H}}^{+}\left({\mathrm{pK}}_a=3.04\right) $$
(2.3a)

$$ {\mathrm{H}\mathrm{Tartrate}}^{-}\rightleftharpoons {\mathrm{Tartrate}}^{2-}+{\mathrm{H}}^{+}\left({\mathrm{pK}}_a=4.37\right) $$
(2.3b)

The pH values at which dissociation occurs is determined by the acid dissociation constant(s), Ka, which are given after the above equations as the negative logarithms (pKa). The dissociation process is illustrated graphically in Fig. 2.2. Weak acids, because they dissociate progressively, act as buffers, so can resist changes in pH. When pH equals a pKa, the buffering strength is at a maximum. It is considered that there is effective buffering over the pH range pKa ± 1. Polybasic acids can therefore buffer over a wide pH range: for instance, citric acid, with pKa of 3.13, 4.74 and 6.42, is a good buffer over the pH range 2.1–7.4.

A316956_1_En_2_Fig2_HTML.gif
Fig. 2.2

Diagram illustrating ionisation of tartaric acid. As pH rises, tartaric acid dissociates into HTartrate ions and this in turn into tartrate2− ions
The strength of buffering is related to the total concentration of acid, and it can be quantified in different ways, using different techniques of titration with a base such as sodium hydroxide. The buffer capacity measures the strength of buffering at the pH of the solution. The titratable acidity measures the amount of buffering between the pH of the solution and some defined higher pH, usually 5.5 or 7.0. The titratable acidity to pH 5.5 is preferable, mainly because the pH region between 5.5 and 7.0 is of little or no interest to erosion.
A possibly important property of weak acids which form anions with more than one negative charge is that they are capable of forming ionic complexes with cations such as Ca2+. Complexes are stabilised by formation of chemical bonds rather than by simple electrostatic attraction. One type of complex is formed by chelation, in which formation of coordinate bonds between two or more negative anionic groups and the Ca2+ ion results in a ring structure (see Fig. 2.3 for an example). Chelation will remove calcium ions from solution and reduce their concentration, but they could also speed up the process of mineral solution more directly. Chelating anions can bind to Ca2+ ions at the surface of the solid, thus weakening bonds holding the Ca2+ in place and causing them to be solubilised [6, 8].

A316956_1_En_2_Fig3_HTML.gif
Fig. 2.3

An example of chelation: a schematic diagram of the tartrate-calcium complex. The arrows symbolise coordinate bonds, formed by sharing of electrons from the COO groups with the calcium ion
The possible roles of buffering and chelation in erosion will be discussed later.

2.5 Erosion of the Tooth Surface

When an erosive solution comes into contact with a tooth, the surface starts to dissolve. Simultaneously, acid diffuses into the tissue and begins to demineralise the tissue beneath the surface [9].

2.5.1 Enamel

Acid diffusing into the narrow pores between the crystals results in partial loss of mineral, increased porosity and reduction of mechanical strength of the outer layer of enamel, which is hence referred to as the ‘softened layer’ [9] (Fig. 2.4). Even after partial demineralisation, the pores within the enamel are still extremely narrow so that the acidic solution can only diffuse inwards for a short distance before becoming saturated with respect to enamel mineral, thus losing its erosive capacity. Consequently, the softened layer produced by an average challenge is no more than a few micrometres thick [9]. The high degree of orientation of the pores in enamel means that there is a gradient of mineral content within the softened layer, content being least at the outer surface and increasing towards the unaffected enamel [9].

A316956_1_En_2_Fig4_HTML.jpg
Fig. 2.4

Scanning electron micrograph of the surface of a specimen of enamel which has been exposed to 0.3 % citric acid (pH 3.2) for 20 min. Etching of the surface revealing the profiles of the prisms. At the outer surface, the crystals are more completely demineralised than those deeper in the softened layer
Intra-oral measurements show that drinking an erosive beverage causes the pH at tooth surfaces to fall for a few minutes [10]. A single such challenge from acid is unlikely to cause the loss of surface enamel. However, after more prolonged erosion, or after repeated challenges, the outermost enamel eventually becomes completely demineralised, causing a loss of surface profile. Acidic foods could also produce this effect, because the contact time with the teeth would be longer than for drinks and also because the mixing effect of chewing would accelerate demineralisation. However, no measurements of tooth-surface pH during mastication of acidic foods have been made. During prolonged erosion, the overall rate of mineral loss from enamel becomes constant a few minutes after the initial contact [11].

2.5.2 Dentin

The erosion of dentin follows a different pattern [9]. Whereas erosion of enamel eventually causes loss of the surface tissue, erosion of dentin leaves behind a persistent layer of demineralised collagenous matrix [12] (Fig. 2.5). With continuing exposure to acid, this layer becomes thicker, which means that inward diffusion of acid and outward diffusion of mineral end products, between the surface and the demineralisation front, are slower: the overall rate of demineralisation therefore slows down as erosion proceeds [11]. Because of their great solubility and small size, the dentin crystals are completely dissolved over a short distance, so that there is only a narrow zone of partially demineralised dentin between the unaffected dentin and the demineralised outer layer. The intrafibrillar domains are demineralised more slowly than the intrafibrillar regions because of diffusion inhibition by the collagen fibrils [6]. Peritubular dentin is attacked first, then the intertubular dentin: a sequence observed both at the surface (Fig. 2.6) and at the interface between sound dentin and the demineralised surface layer (Fig. 2.5).

A316956_1_En_2_Fig5_HTML.jpg
Fig. 2.5

Cross section of dentin exposed to 0.3 % citric acid, pH 3.2, for 20 min. Surface was polished using a graded series of diamond paste then viewed in a scanning electron microscope with a backscattered electron detector. In this mode, contrast from flat surfaces is due to variations in average atomic number, so areas with high mineral content appear lighter than areas with reduced mineral content. At top of field is a layer of demineralised dentin, with obliquely sectioned dentin tubules (as demineralised dentin is relatively soft, it cannot be polished completely flat and displays some topographic contrast). The junction between the dark demineralised and pale sound dentin is sharply defined. Preferential dissolution of peritubular dentin at and just beneath the junction between sound and demineralised dentin is clearly seen
A316956_1_En_2_Fig6_HTML.gif
Fig. 2.6

Polished surface of dentin exposed to 0.3 % citric acid solution, pH 3.2, for 20 min. Note the enlarged openings of the tubules and the absence of peritubular dentin. Tubule openings visible beneath the surface (arrows) are small because the peritubular dentin is intact at this level
Dentin mineral is more soluble than enamel mineral, so in theory should be more susceptible to erosion. However, in practice, the relative rate varies with pH. Erosion tends to be faster in enamel at low pH (< pH 3) [11], possibly because a high concentration of H+ ions promotes dissolution at the outermost enamel surface, whereas loss of acid from dentin is always hindered by the presence of collagen fibres. The variation in relative erosion rates of dentin and enamel with pH probably reflects the relative contributions of dissolution rate of the individual mineral crystals and of the rate of diffusion out of the respective tissue.

2.6 Factors Controlling Erosive Demineralisation

2.6.1 Chemical Factors

The rate of erosion is influenced by a variety of chemical properties of the erosive solution. To understand which factors are important, information from two complementary types of study is required. In both, standardised specimens are exposed to the solution under defined conditions of temperature and stirring rate for a preset time, and the extent of erosion is then measured by an appropriate technique, such as microhardness or profilometry. Experiments on defined solutions allow chemical variables such as pH or ionic concentrations to be controlled and manipulated as desired and solution variables can be studied over a wide range of values. These studies are free of possible interference from ingredients found in commercial products, so they need to be compared with tests on erosive potential of such products. Since both types of study are performed in vitro, neither takes into account the diverse effects of the oral environment, particularly saliva. Of course, in situ experiments can address this problem but, because of the greater variability of any experiments in humans, these require more replicates and are expensive. However, the available studies indicate that, although the rates of erosion in vitro and in situ differ considerably, tests on the same products under the two conditions place their erosive potentials in the same rank order [13]. Thus, in vitro tests seem to give reliable estimates of erosive potential.
In tests of erosive potential, statistical analysis is performed to establish the extent to which erosion is correlated with the chemical properties of the products. Some studies have employed simple bivariate tests, which assess the association between the extent of erosion (the dependent variable) and the individual properties of the test product (the independent variables) in turn. Multivariate tests, in which associations between the dependent and independent variables are tested simultaneously, are more informative because they take into account correlations between the independent variables. The results of tests on erosive potential are summarised in Table 2.4.

Table 2.4

Statistically significant bivariate and multivariate associations between properties of acidic products and erosion
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Tissue
Variable
Reference
Bivariate
Enamel
Degree of saturation with respect to hydroxyapatite
[7, 14]
Enamel
Degree of saturation with respect to fluorapatite
[7]
Enamel
pH
[14, 15]
Enamel
Buffer capacity
[14]
Enamel
Fluoride concentration
[15]
Enamel
Phosphate concentration
[14, 15]
Dentin
Buffer capacity
[15]
Dentin
Calcium concentration
[15]
Multivariate
Enamel
pH
[7, 16, 17]
Enamel
Buffer capacity
[7, 16, 17]
Enamel
Fluoride concentration
[7, 16, 17]
Nov 30, 2015 | Posted by in General Dentistry | Comments Off on The Dental Erosion Process
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