N. Schlueter

Dental erosion--an overview with emphasis on chemical and histopathological aspects

The quality of dental care and modern achievements in dental science depend strongly on understanding the properties of teeth and the basic principles and mechanisms involved in their interaction with surrounding media. Erosion is a disorder to which such properties as structural features of tooth, physiological properties of saliva, and extrinsic and intrinsic acidic sources and habits contribute, and all must be carefully considered. The degree of saturation in the surrounding solution, which is determined by pH and calcium and phosphate concentrations, is the driving force for dissolution of dental hard tissue. In relation to caries, with the calcium and phosphate concentrations in plaque fluid, the ‘critical pH’ below which enamel dissolves is about 5.5. For erosion, the critical pH is lower in products (e.g. yoghurt) containing more calcium and phosphate than plaque fluid and higher when the concentrations are lower. Dental erosion starts by initial softening of the enamel surface followed by loss of volume with a softened layer persisting at the surface of the remaining tissue. Dentine erosion is not clearly understood, so further in vivo studies, including histopathological aspects, are needed. Clinical reports show that exposure to acids combined with an insufficient salivary flow rate results in enhanced dissolution. The effects of these and other interactions result in a permanent ion/substance exchange and reorganisation within the tooth material or at its interface, thus altering its strength and structure. The rate and severity of erosion are determined by the susceptibility of the dental tissues towards dissolution. Because enamel contains less soluble mineral than dentine, it tends to erode more slowly. The chemical mechanisms of erosion are also summarised in this review. Special attention is given to the microscopic and macroscopic histopathology of erosion.

Effect of Fluoride Compounds on Enamel Erosion in vitro: A Comparison of Amine, Sodium and Stannous Fluoride

The aim of the study was to evaluate the relevance of cations in different fluoride compounds for their effectiveness as anti-erosive agents. Human enamel samples underwent a de- and re-mineralisation procedure for 10 days. Erosive demineralisation was performed with 0.05 Mcitric acid (pH 2.3) 6 × 2 min daily followed by immersion in the test solution 6 × 2 min each. Test solutions were: SnCl2 (815 ppm Sn; pH 2.6), NaF (250 ppm F; pH 3.5), SnF2 (250 ppm F, 809 ppm Sn; pH 3.5), amine fluoride (AmF, 250 ppm F; pH 3.5), AmF/NaF (250 ppm F; pH 4.3), and AmF/SnF2 (250 ppm F, 390 ppm Sn; pH 4.2). In the control group no fluoridation was performed. Mineral content was monitored by longitudinal microradiography. Finally, scanning electron microscopy was performed. The highest erosive mineral loss was found in the control group (48.0 ± 17.1 µm). Mineral loss was nearly completely inhibited by AmF/SnF2 (5.7 ± 25.1 µm; p ≤ 0.001) and SnF2 (–3.8 ± 14.4 µm; p ≤ 0.001) treatments. Groups treated with SnCl2 (17.6 ± 19.5 µm; p ≤ 0.001) and NaF (13.2 ± 21.7 µm; p ≤ 0.001) showed a decrease in erosive mineral loss, AmF (41.6 ± 16.0 µm) and AmF/NaF (27.7 ± 28.4 µm) had no significant effect on erosion progression. The results indicate considerable differences between the fluoride compounds tested. Treatment with solutions containing SnF2 was most effective.

Methods for the Measurement and Characterization of Erosion in Enamel and Dentine

The advantages, limitations and potential applications of available methods for studying erosion of enamel and dentine are reviewed. Special emphasis is placed on the influence of histological differences between the dental hard tissue and the stage of the erosive lesion. No method is suitable for all stages of the lesion. Factors determining the applicability of the methods are: surface condition of the specimen, type of the experimental model, nature of the lesion, need for longitudinal measurements and type of outcome. The most suitable and most widely used methods are: chemical analyses of mineral release and enamel surface hardness for early erosion, and surface profilometry and microradiography for advanced erosion. Morphological changes in eroded dental tissue have usually been characterised by scanning electron microscopy. Novel methods have also been used, but little is known of their potential and limitations. Therefore, there is a need for their further development, evaluation, consolidation and, in particular, validation.

Effect of Titanium Tetrafluoride and Sodium Fluoride on Erosion Progression in Enamel and Dentine in vitro

Our aim was to investigate the effect of TiF4 solutions on mineral loss on enamel and dentine in vitro. Samples were fluoridated 1 × 5 min per day with 1.64% w/v TiF4 or 2.2% w/v NaF solutions, each with a pH of 1.2, and then subjected to a cyclic de- and remineralization procedure for 5 days. Demineralization was performed for 6 × 10 min per day with citric acid (pH 2.3). In controls no fluoridation was performed. Mineral content was determined by longitudinal microradiography. Enamel mineral loss was markedly reduced by both fluoride solutions, but TiF4 was significantly more effective than NaF: cumulative mineral loss on day 3 was 61.7 ± 15.0 µm in the NaF and 34.2 ± 13.1 µm in the TiF4 group (p ≤ 0.001) compared with 121.0 ± 27.0 µm in the control group. Dentine mineral loss ceased after both TiF4 and NaF applications (cumulative mineral loss on day 5 in controls: 61.0 ± 17.0 µm, in the TiF4 group: 15.4 ± 13.4 µm and in the NaF group: 21.8 ± 11.8 µm). Both TiF4 and NaF application reduced mineral loss both on enamel and dentine, which could open new possibilities for a symptomatic therapy of erosions.

Tin-containing fluoride solutions as anti-erosive agents in enamel: an in vitro tin-uptake, tissue-loss, and scanning electron micrograph study

Tin-containing fluoride solutions can reduce erosive tissue loss, but the effects of the reaction between tin and enamel are still not clear. During a 10-d period, enamel specimens were cyclically demineralized (0.05 M citric acid, pH 2.3, 6 x 5 min d(-1)) and remineralized (between the demineralization cycles and overnight). In the negative-control group, no further treatment was performed. Three groups were treated (2 x 2 min d(-1)) with tin-containing fluoride solutions (400, 1,400 or 2,100 ppm Sn2+, all 1,500 ppm F-, pH 4.5). Three additional groups were treated with test solutions twice daily, but without demineralization. Tissue loss was determined profilometrically. Energy-dispersive X-ray spectroscopy was used to measure the tin content on and within three layers (10 mum each) beneath the surface. In addition, scanning electron microscopy was conducted. All test preparations significantly reduced tissue loss. Deposition of tin on surfaces was higher without erosion than with erosion, but no incorporation of tin into enamel was found without demineralization. Under erosive conditions, both highly concentrated solutions led to the incorporation of tin up to a depth of 20 mum; the less-concentrated solution led to small amounts of tin in the outer 10 mum. The efficacy of tin-containing solutions seems to depend mainly on the incorporation of tin into enamel.

Efficacy of a Tin/Fluoride Rinse: a Randomized in situ Trial on Erosion

Concentrated tin- and fluoride-containing mouthrinses are effective erosion inhibitors in enamel and dentin. To test whether this is also true for solutions with lower concentrations, we conducted a randomized double-blind three-cell crossover in situ study with extra-orally performed erosive impacts (citric acid, 6 x 5 min/day) and an intra-oral rinsing protocol (1 x 30 sec/day) in 24 volunteers. The mouthrinses were a placebo, a NaF (500 ppm F-), and an amine fluoride (AmF)/NaF/SnCl2 mouthrinse (500 ppm F-, 800 ppm Sn2+). Compared with the placebo, the NaF mouthrinse reduced substance loss by 19% in enamel and 23% in dentin (p ≤ 0.01 each); the AmF/NaF/SnCl2 mouthrinse reduced this parameter by 67% in enamel and 47% in dentin (p ≤ 0.001 each). AmF/NaF/SnCl2 was significantly more effective than NaF in both tissues (p ≤ 0.01). The mouthrinse containing Sn and F exhibited good efficacy, even under severe erosive conditions.

Conventional and Anti-Erosion Fluoride Toothpastes: Effect on Enamel Erosion and Erosion-Abrasion

New toothpastes with anti-erosion claims are marketed, but little is known about their effectiveness. This study investigates these products in comparison with various conventional NaF toothpastes and tin-containing products with respect to their erosion protection/abrasion prevention properties. In experiment 1, samples were demineralised (10 days, 6 × 2 min/day; citric acid, pH 2.4), exposed to toothpaste slurries (2 × 2 min/day) and intermittently stored in a mineral salt solution. In experiment 2, samples were additionally brushed for 15 s during the slurry immersion time. Study products were 8 conventional NaF toothpastes (1,400–1,490 ppm F), 4 formulations with anti-erosion claims (2 F toothpastes: NaF + KNO3 and NaF + hydroxyapatite; and 2 F-free toothpastes: zinc-carbonate-hydroxyapatite, and chitosan) and 2 Sn-containing products (toothpaste: 3,436 ppm Sn, 1,450 ppm F as SnF2/NaF; gel: 970 ppm F, 3,030 ppm Sn as SnF2). A mouth rinse (500 ppm F as AmF/NaF, 800 ppm Sn as SnCl2) was the positive control. Tissue loss was quantified profilometrically. In experiment 1, most NaF toothpastes and 1 F-free formulation reduced tissue loss significantly (between 19 and 42%); the Sn-containing formulations were the most effective (toothpaste and gel 55 and 78% reduction, respectively). In experiment 2, only 4 NaF toothpastes revealed significant effects compared to the F-free control (reduction between 29 and 37%); the F-free special preparations and the Sn toothpaste had no significant effect. The Sn gel (reduction 75%) revealed the best result. Conventional NaF toothpastes reduced the erosive tissue loss, but had limited efficacy regarding the prevention of brushing abrasion. The special formulations were not superior, or were even less effective.

Investigation of the effect of various fluoride compounds and preparations thereof on erosive tissue loss in enamel in vitro.

Metal fluoride preparations are promising in view of protection against dental erosion. The aim of this study was to screen the effect fluoride preparations containing various polyvalent metal cations had on erosion progression under increasingly severe erosive conditions in human enamel in vitro. Specimens were subjected to a cyclic de- and remineralisation procedure. Demineralisation (experiment E1 and E2a: 6 × 2 min/day, experiment E2b: 6 × 5 min/day) was performed with 0.05 M citric acid (pH 2.3). Fluoridation directly followed demineralisation and was reduced stepwise (E1: 6 × 2 min/day, E2a and E2b: 2 × 2 min/day). The test solutions were: ZnF2 (E1), AmF/NaF/CuSO4 (E1), TiF4/AmF/Zn lactate (E1), TiF4 (E1, E2a), TiF4/AmF (E2a), TiF4/AmF/NaF (E2a), TiF4/AmF/ZnF2 (E1, E2a, E2b), AmF/NaF/SnCl2 (2,800 mg/l Sn2+; E1, E2a, E2b), AmF/NaF/SnCl2-1 (700 mg/l Sn2+; E2a, E2b), all 1,500 ppm F–, pH 4.5, and AmF/SnF2 (positive control, pH 4.2, 250 ppm F–; E1, E2a, E2b). In the negative control group, specimens were not fluoridated. Tissue loss was determined by profilometry after the last experimental day. Under mild conditions (E1), tin- and titanium-containing solutions were of marked efficacy and could nearly completely inhibit erosive tissue loss (reduction between 94.2 and 97.5%). With increasingly severe conditions (E2a, E2b), titanium lost its efficacy. Only the highly concentrated tin preparation (2,800 mg/l Sn2+) was able to reduce erosive tissue loss by 93.1%, even under severe conditions (E2b), with less frequent application of the experimental preparations. It should be considered for approaches for a symptomatic therapy of dental erosion.

Effects of toothbrushing force on the mineral content and demineralized organic matrix of eroded dentine

Eroded dentine has a complex histological structure, and its organic fraction becomes increasingly exposed as a result of the continual action of acids. The present study sought to investigate the effects of brushing forces up to 4 N on mineralized and organic dentine fractions. The study was a cyclic demineralization and remineralization experiment (carried out over 9 d). Erosion was performed with HCl (6 x 2 min d-1), pH 1.6. Samples exposed to erosion alone served as controls; test samples were eroded and brushed with a powered toothbrush (2 x 15 s d-1), applying forces of 2, 3, or 4 N. Samples were analysed (using profilometry and longitudinal microradiography) before and after the removal of superficial organic material with collagenase. Randomly selected samples were subjected to scanning electron microscopy. Demineralized organic material was present on all samples regardless of brushing force. Loss values (determined using profilometry) after erosion only, and after brushing with forces of 2, 3, and 4 N, were 11.7 +/- 5.1, 13.6 +/- 11.2, 30.7 +/- 19.0, and 25.5 +/- 20.3, respectively, before treatment with collagenase, and 111.7 +/- 11.6, 122.0 +/- 11.8, 121.9 +/- 15.7, and 123.0 +/- 12.0, respectively, after treatment with collagenase. Microradiography confirmed the results. Significant effects of brushing force were only found on the demineralized organic fraction, and mineral loss was unaffected. The notion that eroded dentine is particularly prone to abrasion should be reconsidered.

Efficacy of an Experimental Tin-F-Containing Solution in Erosive Tissue Loss in Enamel and Dentine in situ

The aim of this study was to evaluate the effects of an experimental tin-containing fluoride solution on erosive tissue loss in human enamel and dentine using a cyclic de- and remineralization model in situ. The study was a three-cell (7 days each) crossover design involving 20 healthy volunteers. Samples were mounted on buccal shields of mandibular mouth appliances and worn for 24 h, except during meals and oral hygiene. Samples were demineralized extraorally with 0.05 M citric acid (pH 2.3) for 6 × 5 min per day. Fluoride treatment was performed intraorally once per day for 30 s after the first demineralization. Three solutions were used: placebo (negative control); NaF solution (positive control, 1,000 mg/kg F, pH 4.5), and an experimental solution (1,900 mg/kg Sn as SnCl2, 500 mg/kg F as NaF, 500 mg/kg F as amine fluoride, pH 4.5). Tissue loss was determined profilometrically. In enamel, tissue loss was 33.6 ± 15.4 μm in the negative control group, 24.2 ± 9.2 μm (p ≤ 0.05) in the positive control group and 9.2 ± 3.4 μm (p ≤ 0.001) in the experimental solution group. The respective values for dentine were 47.8 ± 15.5 μm in the negative control group, 34.1 ± 9.3 μm (p ≤ 0.001) in the positive control group and 23.9 ± 6.4 μm (p ≤ 0.001) in the experimental solution group. The tin-containing solution was very effective in inhibiting erosive mineral in enamel in situ, even under severe conditions and with short application times, but was less effective in dentine.