Non-carious dental disorders such as erosive tooth hard tissue loss have been increasingly observed in recent years, especially in industrialized countries. The prevalence of clearly visible defects is about 30% (SCHLUETER & LUKA 2018). Causes include a changed lifestyle with increased intake of acidic foods and drinks, more stress and more reflux, but also the better education and training of people in the oral health sector that has improved detection.
It is important to note that erosive disorders are multifactorial. Figure 1 shows an overview of the various etiological factors that must be considered. However, knowledge of the erosive potential of a range of products is essential. This knowledge provides a basis for advising patients on how to avoid tooth damage, and for assessing the risks from diet and medication when working towards a diagnosis of erosion-related conditions.
Demineralization involves release of calcium, hydroxyl- and phosphate ions from hard tissue. In caries, acid produced by plaque bacteria causes subsurface demineralization but, in erosion, demineralization affects the tooth surface and results from contact with exogenous acidic substances. Potentially erosive substances includenot only foods and drinks but other products that are taken into the mouth, such as medications. Unfortunately, prediction of erosive potential is not straightforward. The capacity of a simple acidic solution to dissolve dental hard tissue depends on its pH (LARSEN & NYVAD 1999), but the products of interest are rarely simple solutions. Products may also contain substances that adsorb to the tooth surface and inhibit the demineralization process by interfering with the release of mineral ions. These inhibitory substances include various natural peptides and polymers.
It follows from the above discussion that the erosive potential of a particular product cannot be determined from a single property, such as a low pH, but has to be measured using a reliable procedure. The aim of this research was to provide such data, using a consistent in vitro methodology. A variety of drinks, foods and stimulants as well as medications and mouthwashes were tested for their erosive effects on human tooth enamel. Besides providing guidance to the erosive risk presented by products that are introduced into the mouth, the tables presented here should be helpful on the sometimes-difficult path to a correct diagnosis of erosion-related oral conditions.
Two secondary aims of the research were: (1) to evaluate the possible usefulness of deliberately adding calcium or phosphate to liquids as a means of reducing erosive potential; (2) to test the effect of different temperatures on erosive potential.
Some data were published earlier (LUSSI, JOÃO-SOUZA ET AL. 2019).
Materials and methods
Outline of experimental procedure
Specimens of human dental enamel were prepared to obtain a flat, polished surface suitable for Vickers microhardness measurements. These specimens were immersed in human saliva to create a pellicle, to simulate in vivo conditions. After measuring baseline microhardness, the specimens were exposed to the test products and the microhardness measured again. The difference in microhardness between baseline and post-exposure measurements was calculated and used as a measure of erosive potential.
Preparation of enamel specimens
From a pool of extracted teeth, 1684 caries-free human premolars and 300 deciduous molars with no cracks were selected. After the crowns of all teeth were separated from the roots, the buccal or lingual sites were ground flat under water-cooling on a LaboPol-21 rotary polishing machine (Struers, Ballerup, Denmark). Groups of six enamel slabs were embedded with resin (Paladur, Bad Homburg, Germany) in a two-part metal mold consisting of thick and thin plane-parallel components. Once the hardening process of the resin was complete, the thinner metallic mold (200 micro-m thick) was removed. The outer 200 micro-m of enamel, now standing proud, was serially polished on the polishing machine under constant cooling using silicon carbide paper discs with descending grain sizes. Thereafter, the enamel slabs were taken out of the larger mold, and polished for 1 min with 3 micro-m diamond abrasive on a DP-Mol polishing cloth (Struers). After each polishing step, the resin disks with the embedded teeth were rinsed and sonicated for 2 min in tap water. These preparation steps wore away exactly 200 micro-m enamel (the thickness of the thin metallic mold) in the center of the enamel window.
Just before the experimental procedures, the resin disks were further polished with a 1 micro-m diamond abrasive for 1 min (LaboPol-6, DP-Mol Polishing, DP-Stick HQ; Struers, Copenhagen, Denmark), which ensured the removal of possible remnants from storage.
Saliva stimulated by chewing paraffin wax (Fluka; Sigma-Aldrich Chemie GmbH, Munich, Germany), was collected in an ice-cooled tube at least 1 h after any intake of drink or food from a single healthy donor (B.M.) with neither caries nor periodontitis. She gave informed consent, and saliva collection was performed in accordance with the protocol approved by the University of Bern (Bern, Switzerland). Before the erosive challenge, enamel specimens were immersed in 20 ml of freshly collected human saliva for 3 h to form the salivary pellicle. The temperature was held constant at 37°C and the saliva was carefully shaken. After being rinsed with tap water for 50 s, with deionized water for 10 s, the specimens were dried for 5 s with oil-free air.
Tested dietary substances and medications
In the present study 226 various popular drinks, foods and stimulants, as well as medications and mouthwashes available in Switzerland were included (table 1). According to their constituents and applications, these agents are ordered as follows.
Group 1: Mineral water, soft drinks, lemonades
Group 2: Energy drinks, sports drinks
Group 3: Fruits, juices, smoothies
Group 4: Dairy products
Group 5: Tea, ice tea, coffee
Group 6: Alcoholic beverages
Group 7: Medication
Group 8: Sweets, candy, chewing gum
Group 9: Children's articles
Group 10: Miscellaneous
Group 11: Mouthwashes
The pH value of the liquids was measured using a standard electrode, which was always calibrated immediately before use.
Non-liquid products required processing before they could be tested. Fruits were crushed, and the pulps and seeds were removed by pressing through a sieve. Medications (tablets and powders) were dissolved in tap water according to the suggestions of the manufacturers. Confectionery was dissolved in deionized water (1.6 g in 10 ml) at 45°C while stirring and cooled again to 30°C for the experiment. The different teas were prepared as indicated on the respective products. Chewing gum, sweets etc. (12 g each) were ground in a mortar for 5 min and then added to artificial saliva resulting in the same volume for all experiments of 60 ml. The composition of the artificial saliva with a pH 7 was as follows: 0.213g CaCl2 2H2O, 0.738g KH2PO4, 0.381g NaCl,1.114g KCl, 2.20 g mucine (NEWBY ET AL. 2006).
The enamel specimens (total 1684 premolars and 300 deciduous molars) were exposed to test products as follows. They were individually placed in 60 ml (or g) of the appropriate solution under constant agitation (95 rpm) at 30°C (shaking bath Salvis; Renggli AG, Rotkreuz, Switzerland). After immersion for 2 min, the enamel samples were taken out of the solution, rinsed with tap water for 50 s, with deionized water for 10 s and then dried for 5 s with oil-free air.
Additional experiments aimed to investigate the influence of temperature, calcium and phosphate on softening of the enamel were performed (Table 2). In order to achieve the desired calcium or phosphate concentration in the sample solutions, a concentrated calcium chloride (20g CaCl2 2H2O in 100 ml) or potassium hydrogen phosphate solution (10g KH2PO4 in 100 ml) was prepared and the required amount pipetted into the sample solution.
Surface hardness measurements
Surface hardness was determined with a Vickers diamond (force 50 mN for 15 s; Fischerscope HM 2000 XYp; Helmut Fischer, Hünenberg, Switzerland). The Vickers hardness was automatically calculated from the dimensions of the impressions. A total of 6 baseline indentations were performed at intervals of 70 micro-m. After exposure to the test products, a further series of 6 post-exposure measurements were taken, right next to the previous counterparts. For each sample, the mean baseline and post-exposure hardness values were calculated. With this procedure, the variations in hardness within a tooth were taken into account (LUSSI, MEGERT ET AL. 2012). We used first 12 teeth per group and later 6 teeth per group as we had low standard deviations.
For each specimen, the change in hardness was calculated by subtracting the post-exposure hardness from the baseline hardness. A mean change in hardness was then calculated for the group of specimens exposed to each product. Numbers have been rounded according to the generally applicable rules.
To provide a quick overview of the erosive potential of the various products in the table, they were divided into 3 groups (Fig. 2). A product was classified as non-erosive (grade 0: horizontal arrow) if there was an increase in hardness or a decrease in hardness of < 2% after 2 min immersion in the test solutions. Erosive products (grade 1: one descending arrow) were those that reduced hardness by 2-15% after 2 min. Products with a highly erosive potential (grade 2: two descending arrows) were those that reduced hardness by more than 15% after 2 min.
Table 1 shows possible ingredients (sorted by importance concerning dental erosion) for each substance tested, the pH value, the hardness, the change in hardness after 2 min immersion (also in %) in the respective solutions. It is also mentioned whether a premolar or a deciduous molar was used. The tables are, as announced, in addition to the compilation of the erosivity of the products presented at that time, using the same measurement methods (LUSSI, JOÃO-SOUZA ET AL. 2019).
The results show that mineral water with their slightly acidic pH-values did not soften the enamel surface, which is correlated with their calcium content. Even with the addition of lemon flavor, this property did not change. If, on the other hand, citric acid was added and the pH value was significantly lowered down to 3.2 we found a significant softening of the enamel surface.
Most energy and sports drinks showed an erosive potential with pH values between 2.9 (Gatorade) and 3.9 (Isostar). Some of them contained casein or calcium and thus show virtually no softening. The softening of enamel by crushed blueberries (pH 3.7) was significantly greater when the latter were mixed with water. Without the addition of water, the hardness decrease was 11.7%, with the addition of water (1:1) it increased to 38.5%. Dilution resulted in a small decrease in pH, to 3.1, but the increase in erosion is more likely due to the reduced viscosity, which enables better ion exchange with the enamel surface . Mucosolvon cough syrup for children is another example of a highly viscous and acidic medication (pH 3.1) which provoked no softening if undiluted.
Carrot juice with a pH of 4.2 hardly softened the enamel surface. All dairy products were acidic (pH between 3.8 and 6.7) but showed no erosive potential. Wild berries yogurt, which had no added berries but only berry flavors, even showed a hardening of the enamel by 5%. Rosehip tea showed only together with hibiscus (pH around 3.1) a softening of the enamel surface, pure rosehip tea (pH 6.3) did not change the hardness of enamel.
In the case of alcoholic beverages, it is noticeable that all pure beers with pH values between 4.1 and 4.4 did not cause a change on the enamel surface hardness. However, addition of citric acid rendered them erosive. Cynar also has a pH of 4 but there was no softening. Some medications and sweets with pH-values as low as 2.7 showed a large erosive potential. A chewing gums also revealed an erosive potential. In our selection of frequently used children's items, there was no product that did not soften the enamel. However, the two types of honey tested have pH values of 3.6 and 4.2, respectively, but did not cause any hardness change.
Sauerkraut with a pH value of 3.8, on the other hand, showed a large erosive potential. Mouthwashes showed pH values between 3.3 (Listerine Smart Kidz) and 6.1 (Elmex Sensitive Professional). Some of them softened the hardness of enamel after 2 min. immersion by up to 6.5%.
In Table 2 the impact of temperature and the effects of addition of calcium and phosphate are noted. We measured less softening on enamel when the temperature was decreased from 37°C to 5°C. Orange juices produced decreases in hardness of up to 16.5 % at 37°C. When the temperature was lowered to 5°C the changes were lowered to 1.6 %. A decrease with lower temperatures was also measured with Smoothie, Red Bull and Coca Cola.
Addition of calcium decreased surfaces softening in all tested substances. The more calcium was added the smaller the surface softening. Addition of CaCl2 decreases the pH around 0.2 units due to chemical reasons. Even when the pH was not adjusted to the original value less surface softening was found. (Africa, Baobab and Pineapple Smoothie; Asia, Mangosteen and Passionfruit Smoothie, Orange juice hohes C ) Innocent Kiwi Wonder Smoothie showed even a slight increase of hardness after the addition of 0.1 % calcium (with adjustment of the pH). Softening of 17.2 % using Red Bull (pH 3.5) was measured without added calcium. With addition of 0.1% calcium the decrease in hardness was 1.4 %. The same was true when calcium was added to orange juices. Orange juice Hohes C had a decrease in hardness of 16.5 %, addition of calcium with adjusting to the same pH value of 3.8 lowered the decrease to 2.3 %. Addition of calcium to Coca-Cola with its pH of 2.6 hardly changed the softening.
Influence of phosphate was measured with orange juice only and showed no protective effect on softening of human enamel. The influences of Ca and P is also discussed in the study by Shellis and co-workers (Swiss Dental Journal, this volume).
The possible erosive potential was arbitrarily divided into 3 groups: no erosive potential, medium and distinct erosive potential. This classification does not consider the many other factors that need to be considered when assessing potential damage. However, the tables presented here are an important tool on the sometimes-difficult path to a correct diagnosis.
The discussion is structured according to the different possible factors involved in erosion or erosive tooth wear (Fig. 1).
Patient related factors
These factors include: eating and drinking habits, tooth cleaning, vomiting, saliva, pellicle, soft tissue, medication
Because of changing dietary habits, acid-related tooth damage has gained in importance today, although tooth decay is still the most common oral disease. In particular, the frequent consumption of erosion-promoting beverages and foods can lead to a progressive erosive process. Progressive erosive loss of enamel or cement can expose dentin tubules and thus cause hypersensitivity of the teeth (WEST ET AL. 2013).
In a study of 3187 healthy European citizens aged 18-35 years, the frequency of erosive tooth wear (ETW) and the diet of the subjects were recorded (BARTLETT ET AL. 2013). Rural residents showed more ETW than the urban population. While increased chewing gum had no effect on ETW, the intake of acidic drinks and fresh fruits was associated with a statistically significant increase in the occurrence of ETW (p<0.001). Regurgitation and vomiting were also associated with increased ETW (p<0.001).
An important finding was that a delay between breakfast and toothbrushing did not influence the occurrence of ETW. Even after a delay as long as 60 min, the same amount of ETW was found. Also, the frequency of brushing in the morning, at noon or in the evening had no significant influence on the prevalence of ETW. This epidemiological study showed that the recommendation to delay toothbrushing after eating can be abandoned. This recommendation was based on the concept that tooth surfaces which had suffered erosion during a meal could be remineralized if the surfaces were not subject to frictional forces (from toothbrushing or contact with tongue and cheeks) for a period. However, the short delays periods of 30-60 min sometimes suggested in the literature are mostly based on laboratory experiments using artificial saliva. Remineralization of tooth surfaces through natural saliva, to the extent that they are no longer vulnerable to cleaning forces, does not take place in a clinically relevant time, because natural saliva, in contrast to artificial saliva, contains proteins that inhibit remineralization. It therefore takes a long time (up to several weeks) for enamel to be remineralized (GARBEROGLIO & COZZANI 1979; GANSS ET AL.2007; LUSSI ET AL. 2014; O’TOOLE ET AL. 2017; STEIGER-RONAY ET AL. 2019 .; GANSS ET AL.2007). The meta-analysis by Hong and coworkers clearly confirmed that the recommendation to delayed toothbrushing after the consumption of erosive foodstuffs or beverages is not capable of preventing erosive enamel wear (HONG ET AL.2020).
After vomiting it is important to rinse the mouth with a non-erosive mouthwash (Table 1) or water. This dilutes the acid and thus protects against further dissolution of the tooth hard substance. Calculations from our original data when drinking a glass of orange juice showed a reduction of H+ ion concentration by a factor of 4 if rinsing the mouth was rinsed with water 5 min after intake and by a factor of 9 2 min after intake. When the oral cavity was rinsed immediately with water after the acid intake the reduction of H+ increased to a factor of 16 (LUSSI, VON SALIS-MARINCEK ET AL. 2012A).
Saliva, through its composition and flow rate, and through pellicle formation, is partly protective against acid and can weaken an erosive event. Variations in pellicle formation in the different areas of the dentition could be responsible for the different distribution of ETW. Surfaces with thicker pellicle formation (lingual surfaces) and more saliva accumulation usually show less ETW than teeth with thin pellicle, such as palatal surfaces of anterior teeth. In addition, the clearance of acid is better in the lower jaw. Some of our results suggest that modification of the pellicle may protect against erosion. Beers are acidic (pH around 4), but do not cause erosion and nor does Cynar, a liqueur made of artichokes and herbs, also with pH of 4 (Table 1). Neither of these drinks has a high calcium content to explain the lack of erosivity, so other factors, most likely peptides or proteins, must have an important protective effect, for example by modifying the pellicles or by adsorbing to the tooth surface. Protective proteins from sugar cane have recently been isolated and their beneficial effect on dental erosions has been demonstrated (PELA ET AL. 2022). Peptides and proteins are now included as a potential protective factor in the modified ETW scheme presented here (Fig. 1).
The abundance and flow rate of saliva can be reduced by radiotherapy of the head and neck area, or by some medications, including tranquilizers, anticholinergics, antihistamines, antiemetics and antiparkinsonian drugs. Patients with ETW should therefore always be asked about regular medications and their side effects clarified. The above-mentioned study of over 3000 Europeans showed that the probability of ETW is significantly increased by taking antidepressants (odds ratio of 4). Taking antidepressants may reduce salivation and thus to increased susceptibility to dental hard tissue erosion (BARTLETT ET AL. 2013). For erosive problems caused by medications, the treating family doctor or specialist should be consulted and efforts made to find possible alternatives with less effect on saliva secretion. It should be noted that prolonged and frequent tooth contact of medications or mouthwashes with low pH can directly cause erosion, or at least accelerate it (see Table 1).
These factors include: Acid type, pH, buffering, peptides and proteins, adhesion, calcium, fluoride, viscosity
The acid type, pH and buffer capacity of the food and beverages are inter-related and cannot be discussed separately. The pH is a very important, though not the only, factor that determines the erosive potential of a beverage or food. In table 1 one can find soft drinks, sports drinks, or even medicines that are acidic, but do not cause erosion. The development of ETW is often incorrectly attributed only to a low pH of consumed beverages and foods. In tooth decay, demineralization occurs when the pH falls below a ‚critical pH‘, which is defined as the pH of a liquid which is in equilibrium with a dental tissue of interest. At this pH value, the fluid is saturated and there is neither dissolution of the tooth nor precipitation of mineral. The critical pH is calculated from the concentrations of mineral ions in the liquid and from the solubility of the dental tissue. Dentine is more soluble than enamel, so the critical pH in relation to dentine is higher than that in relation to enamel. In caries, the relevant liquid is the "plaque fluid", which has an approximately constant electrolyte composition, so the critical pH in relation to enamel or dentine remains within a fairly narrow range (SHELLIS 2010). However, in dental erosion, the fluids surrounding the tooth contain a variety of dissolved substances and consequently a specific critical pH cannot be defined. Whether demineralization occurs depends not on the pH value itself, but on the degree of saturation of the dissolved minerals in the liquid that contacts the teeth. The principal relevant mineral ions are calcium and phosphate because they are components of dental minerals. If the concentrations of these mineral ions is too low, it is undersaturated with respect to the dental hard tissues, which can then start to dissolve. However, if the concentrations of these mineral ions in the liquid is high enough to produce saturation or even supersaturation, demineralization never occurs. For a technical discussion of these phenomena and other factors in erosion, see Barbour et al. (2011) and SHELLIS ET AL. (2014). The degree of saturation influences erosion at different pH values. Thus, at a low pH, it is possible that a high concentration of calcium counteracts erosion because the fluid is saturated or even oversaturated in terms of dental minerals. If only low calcium concentrations are present, erosive demineralization of the tooth substance can occur even at a higher pH value, as the fluid is undersaturated. Drinks such as calcium-added orange juiceKlicken oder tippen Sie hier, um Text einzugeben. (WEGEHAUPT ET AL. 2011) or foods like yogurt have a high calcium content, which protects against erosion. Addition of calcium, however, has its limits due to solution problems, taste changes and legal regulations.
For hydroxyapatite [Ca10(PO4)6(OH)2], which is the type mineral for enamel, PO43- is an important ion for the equilibrium and solubility. The lower the pH of a solution, the smaller the concentration of PO43- (SHELLIS ET AL. 2014). However, at the low pH values common for erosion, the concentration of PO4 3- is extremely small (10 -10 -10 15 mmol/L) and for this reason has no clinical significance for the prophylaxis of erosion (Fig. 3). The concentration of calcium, on the other hand, does not vary with pH. It follows that the addition of calcium reduces the erosive potential, while the addition of phosphate has only a negligible effect. The results in Table 2 are in accordance with Shellis and co-workers (Swiss Dental Journal, this volume). They showed that calcium addition to soft drinks and other acidic products may reduce erosivity against enamel, if pH is not too low. Phosphate on the other hand does not reduce the erosive potential of a liquid against enamel and is omitted from the modified scheme (Fig 1).
Another missing factor in the new improved erosion scheme is "chelation". The importance of chelation (calcium-complex formation) is another property that has been and is still overestimated in connection with dental erosion. The reason is that erosion is associated with low-pH products. The anions of citric acid (contained in many juices) or phosphoric acid (contained in cola drinks) which have the highest affinities for calcium ions are the completely dissociated forms citrate3- and PO43- and at low pH these anions are present at such low concentrations (Fig 3) that this complexing effect is negligible (SHELLIS ET AL. 2014).
If an acid has a high buffering capacity, it takes longer to be neutralized by saliva, which leads to a higher risk of softening of the tooth surface (SHELLIS ET AL. 2013).The greater the buffer capacity, as measured, for example, by the amount of acid-neutralizing base needed to increase the pH to 7, the more erosive this substance will be compared to other acidic foods – provided that the other parameters are the same.
Depending on the concentration, the fluoride content of the drink or food seems to have a certain protective effect concerning dental erosion but compared to the protection against caries this protection is very small. Fluoride enrichment of food and beverages to avoid tooth erosion is not advisable because of the possible side effects of fluoride at the high concentration required to protect against erosion (LUSSI, BUZALAF ET AL. 2019)
The influence of adhesion of components of a drink to tooth surfaces is still little researched. This property could well influence the erosive potential, as more adherent substances would promote a longer contact time on the teeth and thus a longer lasting erosive effect. Adhesion and viscosity appear to be correlated. Several studies have shown that a high viscosity has a protective effect because the supply of H+ ions to the interface with the tooth is impaired must (AYKUT-YETKINER ET AL. 2013, 2014; JAGER ET AL. 2012). Brain Licker, which is offered as a roll-on liquid candy, is a good example of this. In the original version, it softened the enamel surface by 4.4% (Table 1). If it was diluted in a ratio of 1:1 with artificial saliva, the difference was 30%, with dilution 1:2 the softening even increased to 42.8%. With the Giant Candy Spray Super Sour, these values increased from 23% to 57% and then to 58% with the same dilution series (results not shown). This property must be seen in the context of other parameters promoting erosion, such as pH or buffer capacity (CARVALHO ET AL. 2017).
The temperature of drinks and food must also be considered, as it has an influence on the degree of saturation. Higher temperatures accelerate the chemical reaction rate, which can lead to faster erosive dissolution of teeth (BARBOUR ET AL. 2006).
In summary, an exact anamnesis, early diagnosis, and the determination of etiological factors on the nutritional and patient side combined with adequate prophylaxis measures are essential to avoid erosive dental hard tissue erosion or to stop its progression. As the above discussion demonstrates, many factors could influence erosion potential and their interactions are likely to be complex, so accurate prediction of the erosive properties of ingested products is at present elusive. Hence, reliance must be placed on empirical data, as presented in the tables here, to assess nutritional and patient-related etiological factors in ETW.