Effect of Acidic Solution Viscosity on Enamel Erosion

Abstract

The objective of this in vitro study was to investigate the effects of viscosity changes of different acidic solutions on dental erosion. Bovine enamel samples (n = 240, Ø = 3 mm) were embedded in acrylic resin and were allocated to 30 groups (n = 8). Citric acid (CA) and phosphoric acid (PA) solutions at pH 2.5, 3, and 3.5 were prepared in de-ionized water (titratable acidity to pH 5.5: 31 ± 0.6 mmol OH^-/l). The kinetic viscosities of the acidic solutions were adjusted to 1.5, 3, 6, 12, and 24 mm²/sec by the addition of hydroxypropyl cellulose (HPC) at different concentrations. Solutions were pumped over the enamel surface from a reservoir with a drop rate of 1 mL/min. Each specimen was eroded for 10 min at 20°C. Erosion of enamel surfaces was measured by profilometry. Data were analyzed by analyses of variance and logarithmic regression analyses (p < 0.05). Enamel loss was dependent on viscosity, pH, and the kind of acid. The regression analyses showed that higher viscosity caused lower enamel erosion for both acids and all pH levels. Dental erosion is dependent not only on chemical factors of the acid, like pH and acid type, but also on acid viscosity.

Keywords

tooth erosion acid cellulose pH enamel viscosity

Introduction

Dental erosion is defined as the pathologic, chronic, localized loss of dental hard tissue that is chemically etched away from the tooth surface by acid and/or chelation without bacterial involvement (ten Cate and Imfeld, 1996). Numerous chemical factors, such as pH, titratable acidity, degree of saturation, kind of acid, and chelating properties, have been identified to influence the erosive potential of acids (West et al., 2000; Eisenburger and Addy, 2003; Lussi et al., 2004, 2012; Attin et al., 2005). Thereby, the frequency and duration of acid contact might determine the development and progression of erosive lesions. However, compared with the chemical factors, limited information exists about the effects of the physical or physico-chemical aspects of the acid on dental erosion. It is known that enamel and dentin erosion increased with increasing velocity of the acid flow due to a faster ion exchange and clearance of dissolution products (Eisenburger and Addy, 2003; Shellis et al., 2005; Wiegand et al., 2007; Attin et al., 2012). Moreover, the thermodynamic properties of acidic solutions might influence their adhesiveness to and displacement of tooth surfaces (Ireland et al., 1995), which in turn might also affect their erosive capability.

To the authors’ best knowledge, there is no information available so far about a relation between the viscosity of acidic solutions and their erosive potential. Recent studies suggest that the erosion-inhibiting effects of polymers added to acidic solutions might be related not only to the formation of an erosion-protective layer on the surface, but also to a modification of the viscosity of the drink (Barbour et al., 2005b; Beyer et al., 2010).

Therefore, the aim of this in vitro study was to investigate the effects of viscosity changes of different acidic solutions on dental erosion. The null hypothesis was that the erosivity of citric acid and phosphoric acid at different pH values is not affected by their viscosity.

Materials & Methods

Specimen Preparation

Specimens were obtained from intact bovine incisors of 2- to 3-year-old cattle. The crowns were separated from roots and stored in 0.5% thymol solution for a maximum of 6 mos at 5°C until used. In total, 240 cylindrical enamel specimens (diameter, 3 mm) were prepared by means of a water-cooled trephine bur and were embedded in acrylic resin blocks (diameter, 6 mm; Paladur, Heraeus Kulzer, Hanau, Germany). The enamel surfaces were then ground and fine-ground with water-cooled discs (1200-, 2400-, and 4000-grit; waterproof silicon carbide paper, Struers, Erkrath, Germany). The specimens were randomly allocated to 30 groups of n = 8 specimens each.

Preparation of Acidic Solutions

Citric acid (CA) and phosphoric acid (PA) solutions at pH levels of 2.5, 3.0, and 3.5 were used. CA solutions at pH 2.5, 3.0, and 3.5 were obtained by the addition of 2.81, 3.3, and 4.17 g/L (Fluka, Buchs, Switzerland), respectively, to de-ionized water. PA solutions at the respective pH values were obtained by the addition of 30.5, 109.3, and 110.3 mL/L, respectively from a 3 mol/L stock solution (Merck, Darmstadt, Germany) to de-ionized water. To compare the respective acidic solutions under the same conditions, we chose the titratable acidity (TA) of 31 mmol OH^-/l to pH 5.5, which was dictated by pH 2.5 CA solution having a possible minimum titratable acidity of 31 mmol OH^-/L to pH 5.5. The titratable acidities were measured with a titrator (Metrohm, Herisau, Switzerland).

The kinetic viscosities of the acidic solutions were adjusted to 1.5, 3, 6, 12, and 24 mm²/sec, respectively, by the addition of hydroxypropyl cellulose (HPC, Grade LM, HPC, Nippon Soda Company, Tokyo, Japan) at different concentrations. The viscosities were checked with a viscometer (Becker Research Equipment, Göttingen, Germany), which has been described in detail in a previous study (Seeliger et al., 2010). By adding 35.8 g HPC to 1,000 mL of the respective acid solution, we obtained a viscosity of 34 ± 1 mm²/sec. These solutions were then diluted with the respective acid until the targeted viscosity was reached. After adjusting the viscosities, we then re- measured the titratable acidities and the pH levels of the respective acidic solutions and, if necessary, added drops of NaOH or HCl for final adjustment.

The viscosities were chosen based on the results of a preliminary test, which aimed to determine the viscosities of commercially available potentially erosive drinks. Twenty-five potentially erosive beverages marketed in Switzerland were chosen, and the viscosities of the beverages were measured 5 times for each beverage by means of the above-mentioned viscometer. The viscosities of the drinks ranged from 0.91 to 58.65 mm²/sec, with only a few drinks exhibiting a very high viscosity (≥ 20 mm²/sec, Table).

Table.

Viscosity and pH Values of the Products Tested in the Preliminary Study

Potentially Erosive Beverage	Brand Name	Company	Viscosity [mm²/s]	pH
Soft Drinks
Coke	Coca-Cola	Coca Cola HBC Schweiz AG, Switzerland	1.05 ± 0.03	2.44
Iced tea	MIGROS Iced Tea Zitrone	Migros, Bischofszell Nahrungsmittel AG, Zürich, Switzerland	1.32 ± 0.04	3.16
Tonic water	Schweppes - Indian Tonic Water	Schweppes Holdings Limited Proizvodi Coca-Cola HBC- Serbia A.D. Zemun, Serbia	0.91 ± 0.04	2.42
Syrup with tap water	MIGROS Syrup Orange	Migros-Genossenschafts-Bund, Zürich, Switzerland	1.17 ± 0.03	2.96
Energy Drink
Energy drink	Red Bull	Red Bull GmbH, Austria	1.60 ± 0.06	3.39
Juices
Apple spritzer	obi Apfelschorle	Migros-Genossenschafts-Bund, Zürich, Switzerland	1.92 ± 0.14	3.42
Orange juice	Sarasay Florida Orange	Bischofszell Nahrungsmittel AG, Zürich, Switzerland	2.47 ± 0.13	3.75
Orange juice	Hohes C	Eckes-Granini Group GmbH, Germany	1.73 ± 0.02	3.74
Orange juice + Ca	Hohes C Orange with Calcium	Eckes-Granini Austria GmbH, Austria	2.15 ± 0.15	3.97
Carrot juice	Coop Biotta Bio Karottensaft	Coop, Switzerland	1.81 ± 0.13	4.2
Peach nectar	Michel Pfirsichnektar	Rivella AG Alkoholfreie Getränke, Switzerland	7.46 ± 0.59	3.69
Tomato juice	Sarasay Nectar Tomatensaft	Migros-Genossenschafts-Bund, Zürich, Switzerland	10.68 ± 0.89	4.03
Smoothie	Anna’s Best Smoothie Ananas-banane-orangen Smoothie	Bischofszell Nahrungsmittel AG, Zürich, Switzerland	22.50 ± 4.61	3.61
Alcoholic Drinks
Lemon beer	Cardinal Lemon Zitronen Bier	Feldschlösschen Getränke AG, Switzerland	0.96 ± 0.04	3.07
Beer	Feldschlösschen Original Bier	Feldschlösschen Getränke AG, Switzerland	0.99 ± 0.08	4.42
White wine	Chasselas Romand Weisswein	Schenk SA, Switzerland	1.53 ± 0.03	3.58
Vodka	Wodka Ice-Lemon Rachmaninoff	St. Ambrosius Sektkellerei, Germany	1.12 ± 0.04	3.03
Cocktail	Palm Beach Exotic – Cocktail	Rheinberg Kellerei GmbH, Germany	1.45 ± 0.05	2.74
Lemon liqueur	Limoncello	SCHUÜWO Trink-Kultur, Switzerland	3.46 ± 0.22	5.27
Liqueur	Grand Marnier Likör	MARNIER LAPOSTOLLE, France	5.18 ± 0.06	4.21
Cointreau	Cointreau	E REMY MARTIN CO., France	4.40 ± 0.14	5.5
Rum	Malibu	Twelve Islands Shipping CO, Spain	2.51 ± 0.10	4.61
Milk Products
Yogurt drink	MIGROS Joghurt Drink	Estavayer Lait SA, Switzerland	7.51 ± 0.46	4.16
Lassi	Sélection Joghurtgetränk with Mango	Migros-Genossenschafts-Bund, Zürich, Switzerland	58.65 ± 2.11	4.07
Buttermilk	MIGROS Saure Buttermilch Nature	Migros-Genossenschafts-Bund, Zürich, Switzerland	8.69 ± 0.26	4.51
Reference Drink
Water		Zentrum für Zahnmedizin der Universität Zürich, Switzerland	0.85 ± 0.07	7.92

For comparison, the viscosity of tap water was also tested.

Experimental Procedure

From each specimen, 5 baseline profiles were recorded with a stylus profilometer (Perthometer S2; Mahr, Göttingen, Germany) with a distance of 50 µm between profiles. To ensure an exact repositioning of the samples during and after the experimental procedure, we equipped the profilometer and the samples with a custom-made jig (Wiegand et al., 2009).

All experiments were performed at a controlled room temperature of 20°C. The temperatures of the respective acidic solutions were also adjusted to 20 to 21°C, since viscosity is highly dependent on the temperature of a liquid (Bourne, 2002).

The respective acidic solutions were dropped from a reservoir into an inclined (45°) channel which was made from polyvinyl chloride (8 mm in length) at a flow rate of 1 mL/min, which directed the acidic solutions over the surface of an enamel specimen (Fig. 1). The specimens were eroded for 10 min at a constant room temperature (20°C), providing a total amount of 10 mL flow on each sample. Then, the specimens were rinsed with distilled water, and profilometry was performed again. Enamel wear was calculated by custom-made software (4D Client, custom-designed software; University Zürich, Switzerland), comparing the baseline profiles with the respective profiles after erosion.

Data Analysis

Descriptive statistics (mean, SD) were computed, and normality of the data was checked with Kolmogorov-Smirnov and Shapiro-Wilks tests. Since normal distribution was found in all groups (p > 0.05), data were analyzed by three-way analysis of variance (ANOVA), the factors being acid type, pH, and viscosity. Since significant interactions between and among the experimental factors were observed, the data were split according to the acid and analyzed further by two-time two-way ANOVA. Again, significant interactions were found, so the data were split according to pH, and six-time one-way ANOVA followed by Scheffeé’s post hoc tests was applied. Additionally, a regression model was used to study the relation between viscosity and enamel erosion. The level of significance was set at α = 0.05.

Results

Generally, PA caused higher erosion than CA. Mean enamel loss by erosion with CA and PA is presented in Figs. 2a and 2b. All one-way ANOVAs showed significant differences in enamel loss. Generally, Scheffé’s post hoc tests revealed highest loss for the acid with the lowest viscosity and lowest loss for the acid with the highest viscosity (Figs. 2a, 2b).

The logarithmic regression analysis revealed that enamel erosion decreased with increased viscosity (Fig. 3). Thereby, at pH 2.5, enamel losses decreased from 6.8 ± 0.6 µm (CA) and 14.7 ± 2.0 µm (PA) at viscosity 1.5 m²/sec to 5.0 ± 0.5 µm (CA) and 10.4 ± 1.2 µm (PA) at viscosity 24 mm²/sec. At pH 3, enamel losses decreased from 4.6 ± 0.6 µm (CA) and 7.9 ± 1.4 µm (PA) at viscosity 1.5 mm²/sec to 2.7 ± 0.3 µm (CA) and 5.6 ± 0.5 µm (PA) at viscosity 24 mm²/sec. At pH 3.5, enamel losses decreased from 2.6 ± 0.3 µm (CA) and 2.5 ± 0.5 µm (PA) at viscosity 1.5 mm²/sec to 2.0 ± 0.3 µm (CA) and 1.5 ± 0.2 µm (PA) at viscosity 24 mm²/sec.

Discussion

We have shown that the erosive potential of acids is dependent not only on various chemical properties, such as pH, but also on the viscosity of the acidic solution. Therefore, the null hypothesis is rejected.

To isolate the effect of viscosity only, we adjusted the tested acids (CA and PA) to the same pH values and the same titratable acidity. Titratable acidity is an important parameter for characterization of the erosive potential of different acidic solutions for the same pH levels. In the present study, the titratable acidity was chosen according to the native titratable acidity of citric acid at pH 2.5. We decided to measure the amount of base needed to raise the pH to 5.5, since a pH equal to or less than 5.5 is traditionally considered to be critical for enamel dissolution, although mineral loss may begin at an even higher pH (Birkhed, 1984).

The acids and the ranges of pH values were chosen according to our preliminary study and the composition of typically erosive beverages (Lussi et al., 2012). The tested increments of viscosities were determined according to the results of a preliminary test, which provided the data that viscosities of potentially erosive beverages range from 0.91 to 58.65 mm²/sec, with only a few drinks exhibiting a very high viscosity of ≥ 20 mm²/sec. Therefore, we decided to use the viscosities of 1.5, 3, 6, 12, and 24 mm²/sec.

We adjusted the viscosities by adding hydroxypropyl cellulose (HPC), which is a derivative of polysaccharide cellulose widely used in foods, drugs, and cosmetics (Heitfeld et al., 2008) and is a non-ionic, water-soluble, and pH-insensitive cellulose ether (Kadajji and Betageri, 2011).

The set-up of the present study allowed for a free flow of the acid drops over the surfaces of the enamel specimens, so that the viscosity of the liquid rather than any pump system determined the flow rate of the acid over the surface. Because the viscosity is directly related to the temperature of fluids (Kestin et al., 1978), not only were the experiments undertaken at a standardized temperature of 20ºC but also the temperature of the tested solutions was adjusted to 20ºC.

Since the erosive enamel loss decreased with increasing viscosity, it can be assumed that the relative stickiness of the acid solutions with higher viscosity decreased the ion exchange and clearance of dissolution products. The clearance of dissolution products from the enamel surface is determined by the dissolution through a static liquid layer, which becomes partly saturated with respect to minerals dissolved from the underlying enamel (Shellis et al., 2005; Wiegand et al., 2007). At a higher viscosity, this static layer might be thicker and less undersaturated, so that enamel erosion decreased. However, it is not yet known if there is also any interaction between the HPC polymer and the enamel surface, which might influence the erosive potential of the HPC-modified acids. In previous studies, it was shown that polymer-modified citric acid solutions with a viscosity > 50 mPa/ sec were less erosive than the unmodified solution under stirring conditions, probably due to the formation of a polymer layer on the surface (Beyer et al., 2010, 2012). In this regard, we must also mention that the presence of the acquired salivary pellicle was not considered in the present study. The salivary pellicle is known to reduce erosive demineralization (Wiegand et al., 2008; Hannig et al., 2009), but might also affect the ability of liquids to adhere to enamel (Ireland et al., 1995). However, because we aimed to minimize the variables in this study and focus on viscosity, a salivary pellicle was not formed on the specimens’ surface.

Xanthan gum, pectins, chitosan derivatives, dextran, carrageenan, guar gum, and cellulose ethers [e.g., hydroxypropylmethyl cellulose (HPMC), hydroxypropyl cellulose (HPC), hydroxyethyl cellulose (HEC), and sodium carboxy methyl cellulose (Na-CMC)] as well as albumin, starch, or starch-based derivatives are natural water-soluble polymers and have been reported as inhibitors of dental erosion (West et al., 2004; Barbour et al., 2005a; Hooper et al., 2007; Beyer et al., 2010; Gracia et al., 2010). Therefore, the erosion reduction mechanism was described as related not only to the formation of the gel-like polymer layer on enamel surfaces but also to an increased viscosity, resulting in a reduced ion mobility and slower dissolution kinetics of polymer-modified citric acid solutions (Beyer et al., 2010). The authors also hypothesized that the enamel-protecting polymer layer was formed by the interactions between negatively charged carboxyl groups of polymers and calcium ions of the enamel surface and between negatively charged carboxyl groups of different polymer molecules in the presence of positively charged calcium ions by forming a chelate complex (Beyer et al., 2010). HPC used in the present study is known to be non-ionic, so a binding to the surface seems relatively unlikely (see Appendix).

In the present study, phosphoric acid generally caused higher enamel erosion than citric acid. This observation is in contrast to the results of previous studies, usually showing a higher erosive potential of citric acid, probably also due to the chelating properties of citric acid (Wiegand et al., 2007, 2008). It might be speculated that the free-flow mechanism of the respective acidic solutions did not provide enough time for the chelating ability of citric acid to become relevant. It must also be considered that different experimental conditions make a comparison between studies difficult, especially since the erosive potential of acids is influenced by temperature, concentration, exposure time, and/or exposure type. To the best of our knowledge, there is no study regarding the comparison of erosive potential of CA and PA at the same pH levels and the same titratable acidities, which might be responsible for greater erosion for PA. It must also be considered that bovine instead of human enamel was used in this study. In this regard, bovine enamel presents a higher susceptibility to erosion than does human enamel (Attin et al., 2007), probably due to differences in morphological, chemical, and physical properties. Although the differences between the 2 substrates must be considered in the interpretation of results obtained from any experiment with bovine tooth substrate (Yassen et al., 2011), the use of bovine specimens seems to be appropriate when relative rather than absolute differences are of interest (Wiegand and Attin, 2011).

Although the chemical factors (pH and kind of acid) had a higher influence on erosion than the physical aspect (viscosity), the present study demonstrated that the modeling of erosive conditions is very complex and not only dependent on chemical aspects, such as pH or titratable acidity, which are usually considered as erosion-determining factors.

Further studies are needed to show whether adding HPC to commonly consumed acidic beverages might influence their erosive potential.

Footnotes

Acknowledgements

The authors thank DKSH Switzerland Ltd. for providing the hydroxypropyl cellulose powders.

The authors declare no potential conflicts of interest with respect to the authorship and/or publication of this article.

A supplemental appendix to this article is published electronically only at .

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