Human enamel erosion in constant composition citric acid solutions as a function of degree of saturation with respect to hydroxyapatite

The objective of this study was to investigate human enamel erosion under constant composition conditions, as a function of solution degree of saturation (DS) with respect to hydroxyapatite. The experimental conditions were relevant to the initial stages of enamel erosion by soft drinks. Nanoindentation was used to compare enamel surface softening caused by a control mineral water and two citric acid solutions with DS = 0.000 and DS = 0.032, both having pH 3.30. Enamel hardness and reduced elastic modulus were measured after 0, 30, 60, 120, 300 and 600 s exposure. A statistically significant change in enamel hardness was detected after 30 s exposure to both citric acid solutions, indicating that nanoindentation is extremely sensitive to the initial stages of erosion. There was a statistically significant difference between the mechanical properties of enamel exposed to the two citric acid solutions after 30, 60 and 120 s. At these times, the solution with DS = 0.000 caused twice as much enamel softening as that with DS = 0.032. This demonstrates that it may be possible to design a soft drink with a low erosive potential and a good taste by a small change in DS, at a typical drink pH.     

Scanning and contact microradiographic study of the effect of degree of saturation on the rate of enamel demineralization

The effect of degree of saturation with respect to hydroxyapatite (DSHA) on enamel section demineralization was studied by contact and scanning microradiography (CMR and SMR). Two aspects were studied: (1) the effect of different values of DSHA at constant pH, and (2) the effect of constant DSHA with variable pH. In the pH range (2.5-4.5) and time scale (0-120 h) studied, the DSHA was more important in determining the rate of enamel lesion progression than was the pH.     

Human enamel dissolution in citric acid as a function of pH in the range 2.30< or =pH< or =6.30--a nanoindentation study

The objective of this study was to investigate the dissolution of human enamel in citric acid solutions over a wide range of pH. The in vitro conditions are considered to be relevant to soft drink-induced enamel erosion. Nanoindentation was used to investigate changes in the nanomechanical properties of polished enamel surfaces after exposure to citric acid solutions. Solutions used had 38.1 mmol l-1 citric acid and pH greater than 2.3 but less than 6.3 (2.30 < or = pH < or = 6.30). Samples were exposed to rapidly stirred, constant composition solutions for 120 s. Statistically significant changes in enamel hardness and reduced elastic modulus were observed after exposure to all solutions. There was an approximately linear dependence of enamel hardness on solution pH for 2.90 < or = pH < or = 6.30. Below pH 2.90, enamel is thought to have reached the lowest possible hardness value. The reduction in enamel dissolution caused by an increase in pH of a soft drink is likely to be small. Product modification to reduce the erosive potential of drinks may require additional methods such as addition of calcium salts.     

The kinetics of dissolution of tooth enamel--a constant composition study

The kinetics of dissolution of powdered bovine enamel and of human enamel, both untreated and extracted with either hypochlorite or chloroform, has been studied using a constant solution composition technique in undersaturated solutions of calcium phosphate (total molar calcium concentration, TCa = 0.3 to 13.1 X 10(-3) mol L-1, total molar phosphate, Tp = 0.18 to 7.9 X 10(-3) mol L-1) at an ionic strength of 0.15 mol L-1, and pH = 4.5. The kinetic equations describing the dissolution reactions suggest a surface dislocation mechanism, and the presence of fluoride ion markedly retarded the reaction. For human enamel, a fluoride level of only 0.5 ppm reduced the rate of dissolution ten-fold. In contrast, the dissolution of hydroxyapatite, HAP, is best interpreted in terms of a polynucleation process.     

Enamel demineralization in primary and permanent teeth

Although enamel demineralization is important for our understanding of caries formation, no consensus has been reached regarding the possible differences in susceptibility of primary and permanent enamel. We used the constant composition (CC) technique to investigate the acid-induced demineralization of these tissues at a relative undersaturation with respect to hydroxyapatite (HAP) of 0.902, pH = 4.5, and ionic strength = 0.15 mol L(-1). The demineralization rates showed significant differences, primary enamel having the greater susceptibility to dissolution during an initial linear stage: 1.5 +/- 0.5 x 10(-10) mol mm(-2) min(-1) compared with 2.6 +/- 0.5 x 10(-11) mol mm(-2) min(-1) for permanent enamel. During the reactions, we observed nanosized crystallites which attached to the enamel surfaces or escaped into the bulk solution. These nanosized crystallites were kinetically protected against further dissolution, even though the solutions remained undersaturated. It is hypothesized that they may contribute to the remarkable mechanical and dynamic characteristics of enamel.     

Chemical and structural challenges in remineralization of dental enamel lesions

The aim of the present paper was to identify some chemical and structural factors which may prevent a full remineralization of caries lesions and to study whether it is possible to overcome such obstacles. Samples of powdered enamel apatite were equilibrated with solutions metastably supersaturated with respect to enamel hydroxyapatite and fluorapatite. After 10 min and 60 min of equilibration at 20 degrees C the suspensions were centrifuged and the calcium and phosphate concentrations and the pH were determined in the supernatant. In parallel studies, 50 75-microns-thick sections of 27 fluorotic teeth of a severity of 5-7 according to Thylstrup and Fejerskov's classification were examined by microradiography and in polarized light using distilled water, and Thoulett's media or seen dry in air. Five obstacles inhibiting remineralization were identified: 1) Although remineralizing solutions or saliva are supersaturated with respect to enamel apatite the total amount of calcium and phosphate dissolved in it is small, so that after precipitation of the dissolved mineral only 1/20,000-1/30,000 of the volume of the mineralizing solution is occupied by mineral. 2) The concentration gradients from the mineralizing solution into the enamel is small, which indicates a slow diffusion into and out of the lesion. 3) The uptake of calcium and phosphate by the enamel apatite crystals is so rapid that the aqueous phase within the pores can be presumed to be only marginally supersaturated in the deeper parts of the lesion. 4) The surface layer of the enamel lesions was found to be a serious obstacle to remineralization so that a subsurface area remains hypomineralized after exposure to salivary remineralization even for a lifetime. 5) Nucleation of new apatite crystals to substitute lost crystals is an unsolved problem.     

Impact of modified acidic soft drinks on enamel erosion

OBJECTIVE: To evaluate the enamel erosive potential of modified acidic soft drinks under controlled conditions in an artificial mouth. MATERIALS AND METHODS: From each of 144 bovine incisors one enamel sample was prepared. Labial surfaces of the samples were ground flat, polished and covered with adhesive tape, leaving an exposed area. The samples were distributed among four (A-D) groups for treatment with A: Coca-Cola, B: Sprite; C: Sprite light, D: orange juice. Either 1.0 mmol l(-1) calcium (Ca) or a combination (comb.) of 0.5 mmol l(-1) calcium plus 0.5 mmol l(-1) phosphate plus 0.031 mmol l(-1) fluoride was added to the beverages. Samples of each group were subdivided into three subgroups (-original; -Ca and -comb.) for treatment with original and modified drinks. De- and remineralization cycles were based on a standard protocol described earlier. Surface loss of the specimens was determined using profilometry after test procedure. RESULTS: In all subgroups, loss of enamel was observed. The enamel loss recorded for the samples rinsed with original Sprite and original orange juice was significantly higher compared with all other solutions (P = 0.001). Lowest enamel loss was recorded for the original Coca-Cola group (P = 0.001). With the exception of Coca-Cola, demineralization with the modified beverages led to significantly lower losses compared with the respective original solutions. CONCLUSION: Modification of the test soft drinks with low concentrations of calcium or a combination of calcium, phosphate and fluoride may exert a significant protective potential with respect to dental erosion.     

Kinetics of dissolution and growth of calcium fluoride and effects of phosphate

The rate of growth of pure calcium fluoride crystals is controlled by a surface polynuclear mechanism when the supersaturation is less than 4.4. The surface free energy is found to be 120 mJ/m2. The dissolution process is also controlled by a surface process. Both of these processes are very strongly inhibited by phosphate ions. Calcium fluoride-like materials contaminated with phosphate are formed when calcium fluoride is precipitated in phosphate-containing solutions or suspensions. The physical and chemical properties of these materials have been investigated and compared with the corresponding properties of pure calcium fluoride. The former dissolve much faster than pure calcium fluoride in solutions containing phosphate, but an inhibitory effect is still shown. It is suggested that the calcium fluoride-like material formed on dental enamel during treatment of enamel with acidified solutions of high fluoride content is a phosphate-containing calcium fluoride.     

Microradiographic study on the effects of mucin-based solutions used as saliva substitutes on demineralised bovine enamel in vitro

Sialic acids and proteins bound to mucins are known to form complexes with calcium, and this mechanism may hamper the remineralization of calcium-containing mucin-based saliva substitutes. Thus, the aim of this investigation was to evaluate the effects of adding various concentrations of calcium phosphate to self-made mucin-containing solutions on demineralised bovine enamel in vitro. Bovine specimens were prepared, embedded in epoxy resin, and polished to 4000 grit. Subsequently, the surfaces of the specimens were partially covered with nail varnish, thus serving as a control of sound enamel, and demineralised (37 degrees C; pH 5.0) for 14 (19 groups; n=10) or 28 days (three groups; n=9). After demineralization, the specimens were exposed to mucin-based solutions (30 g/l) with various saturations with respect to apatites containing 0.1 mM NaF, CaCl(2) (0-20 mM) and KH(2)PO(4) (0-52 mM) at two different pH values (5.5 or 6.5). A fluoride-free solution and the commercially available saliva substitute Saliva Orthana (Orthana, Kastrup, Copenhagen Denmark) served as controls. The differences in mineral loss (DeltaDeltaZ) between the values prior to (DeltaZ(Demin)) and after storage (DeltaZ(Effect)) in the various solutions were evaluated from microradiographs of thin sections (100 microm). The general linear model revealed a significant dependency of DeltaDeltaZ for calcium (P=0.006), but not for phosphate (P=0.081) or pH (P=0.114). DeltaZ(Effect) was only significantly reduced compared with DeltaZ(Demin) in the group with the highest saturation with respect to hydroxyapatite (P<0.05; t-test). In conclusion, mucin-based saliva substitutes with an adequate composition are able to remineralize bovine enamel in vitro.     

Prevention by means of fluoride of enamel erosion as caused by soft drinks and orange juice

Fluoride has been suggested to prevent erosion of the teeth, either after a topical treatment of the teeth or by addition of fluoride to the acidic drink. The main aim of the present study was to describe the dissolution of calcium fluoride in some soft drinks and orange juice and compare it with the amounts of calcium fluoride left on the enamel surfaces after a topical treatment. A further aim was to describe the dissolution of enamel in soft drinks and juice saturated for 3 days with solid calcium fluoride. Solid calcium fluoride was suspended in each of 10 soft drinks and orange juices and gently agitated for 72 h, after which the drinks were analyzed for calcium, phosphate and fluoride and pH was determined. To examine the erosion-preventive effect of the calcium fluoride-rich drink, intact teeth were exposed to the drinks with or without calcium fluoride. It was found that from 6 to 45 mg of calcium fluoride was dissolved per liter of drink. The more acidic the drink, the more calcium fluoride was dissolved, presumably due to HF formation. The teeth exposed to the soft drinks all showed erosion-like lesions. Very little effect of the 4-6 ppm ionic fluoride dissolved in the soft drinks was observed. In orange juice, however, the dissolved calcium fluoride established a saturation with respect to fluorapatite and consequently, the erosion-like lesion was replaced by a caries-like lesion. In conclusion, the acidic soft drinks are capable of dissolving considerable amounts of calcium fluoride and the erosion-preventive effect of even high fluoride concentrations is limited.     

Inhibition of hydroxyapatite dissolution by whole casein: the effects of pH, protein concentration, calcium, and ionic strength

Formulating drinks with reduced erosive potential is one approach for reducing dental erosion. In this study, whole casein was added to citric acid solutions representative of soft drinks, and the hydroxyapatite dissolution rate was assessed. Adding 0.02% (w/v) casein to acid solutions significantly reduced the hydroxyapatite dissolution rate by 51 +/- 4% at pH values of 2.80, 3.00, 3.20, 3.40, and 3.60, although the baseline dissolution rates of course varied as a function of pH. The protein concentration [0.002, 0.02, and 0.2% (w/v) casein] had no significant effect on dissolution inhibition. Adding both casein and calcium to citric acid resulted in a further reduction in the dissolution rate at low and intermediate calcium concentrations (5 and 10 mM) but not at higher calcium concentrations (20 and 50 mM). Ionic strength had no significant impact on the efficacy of casein. Casein also significantly reduced the hydroxyapatite dissolution rate when the hydroxyapatite was coated with a salivary pellicle. The reduction in dissolution rate is ascribed to firmly adsorbed casein on the hydroxyapatite surface, which stabilizes the crystal surface and inhibits ion detachment.     

Chemical and structural challenges in remineralization of dental enamel lesions

Abstract – The aim of the present paper was to identify some chemical and structural factors which may prevent a full remineralization of caries lesions and to study whether it is possible to overcome such obstacles. Samples of powdered enamel apatite were equilibrated with solutions metastably supersaturated with respect to enamel hydroxyapatite and fluorapatite. After 10 min and 60 min of equilibration at 20°C the suspensions were centrifuged and the calcium and phosphate concentrations and the pH were determined in the supernatant. In parallel studies, 50–75-μm-thick sections of 27 fluorotic teeth of a severity of 5–7 according to Thylstrup and Fejerskov's classification were examined by microradiography and in polarized light using distilled water, and Thoulett's media or seen dry in air. Five obstacles inhibiting remineralization were identified: 1) Although remineralizing solutions or saliva are supersaturated with respect to enamel apatite the total amount of calcium and phosphate dissolved in it is small, so that after precipitation of the dissolved mineral only 1/20 000-1/30 000 of the volume of the mineralizing solution is occupied by mineral. 2) The concentration gradients from the mineralizing solution into the enamel is small, which indicates a slow diffusion into and out of the lesion. 3) The uptake of calcium and phosphate by the enamel apatite crystals is so rapid that the aqueous phase within the pores can be presumed to be only marginally supersaturated in the deeper parts of the lesion. 4) The surface layer of the enamel lesions was found to be a serious obstacle to remineralization so that a subsurface area remains hypomineralized after exposure to salivary remineralization even for a lifetime. 5) Nucleation of new apatite crystals to substitute lost crystals is an unsolved problem.     

Enamel subsurface lesion remineralisation with casein phosphopeptide stabilised solutions of calcium, phosphate and fluoride

Casein phosphopeptide stabilised amorphous calcium phosphate (CPP-ACP) and amorphous calcium fluoride phosphate (CPP-ACFP) solutions have been shown to remineralise enamel subsurface lesions. The aim of this study was to determine the effect of ion composition of CPP-ACP and CPP-ACFP solutions on enamel subsurface lesion remineralisation in vitro. CPP-bound and free calcium, phosphate and fluoride ion concentrations in the solutions were determined after ultrafiltration. The ion activities of the free ion species present were calculated using an iterative computational program. The mineral deposited in the subsurface lesions was analysed using transverse microradiography and electron microprobe. CPP was found to stabilise high concentrations of calcium, phosphate and fluoride ions at all pH values (7.0-4.5). Remineralisation of the subsurface lesions was observed at all pH values tested with a maximum at pH 5.5. The CPP-ACFP solutions produced greater remineralisation than the CPP-ACP solutions at pH 5.5 and below. The mineral formed in the subsurface lesions was consistent with hydroxyapatite and fluorapatite for remineralisation with CPP-ACP and CPP-ACFP, respectively. The activity gradient of the neutral ion pair CaHPO(4)(0) into the lesion was significantly correlated with remineralisation and together with HF(0) were identified as important species for diffusion.     

An investigation of some food-approved polymers as agents to inhibit hydroxyapatite dissolution

Dental erosion involves dissolution of the hydroxyapatite fraction of enamel and dentine, so agents that reduce the dissolution rate of hydroxyapatite could find application in food products aimed at reducing erosion. This study was performed to test some common food ingredients and additives for their effect on the dissolution rate of hydroxyapatite in a citric acid solution representative of soft drinks. Pyrophosphate, tripolyphosphate and a linear chain polyphosphate (average 25 phosphate units) significantly reduced the hydroxyapatite dissolution rate by 35, 46 and 64%, respectively. Xanthan gum and carboxymethylcellulose significantly reduced the hydroxyapatite dissolution rate by 29 and 16%, respectively. The protective effect may be ascribed to the binding of condensed phosphate or to the formation of an adsorbed layer of gum at the hydroxyapatite surface. Several other common food additives had no statistically significant effect on the hydroxyapatite dissolution rate. Polyphosphate exhibited a considerable persistence of action, causing a reduction in the dissolution rate for 3 h after treatment. Tripolyphosphate was slightly persistent, and pyrophosphate and xanthan gum did not exhibit a substantial persistence of action. A solution containing polyphosphate and xanthan gum reduced the hydroxyapatite dissolution rate by 70% and exhibited a similar persistence of action to the solution containing only polyphosphate. These compounds are suggested to have potential as erosion-reducing agents in soft drinks.     

Effect of mineral supplements to citric acid on enamel erosion

The aim of this study was to evaluate the effect of mineral supplements to citric acid (1%; pH 2.21) on enamel erosion under controlled conditions in an artificial mouth. From each of 156 bovine incisors one polished enamel sample was prepared. The samples were divided among 13 experimental groups (n=12). In group 1 citric acid only was used (control). In groups 2-10 either calcium, phosphate or fluoride in various low concentrations was admixed to the citric acid. In groups 11-13 the citric acid was supplemented with a mixture of calcium, phosphate and fluoride. For demineralisation the specimens were rinsed with the respective solution for 1 min, immediately followed by a remineralisation period with artificial saliva (1 min). The specimens were cycled through this alternating procedure five times followed by rinsing for 8 h with artificial saliva. The de- and remineralisation cycle was repeated three times for each specimen interrupted by the 8 h-remineralisation periods. Before and after the experiments, the specimens were examined using microhardness testing (Knoop hardness) and laser profilometry. Hardness loss and enamel dissolution was significantly higher for the controls as compared to the remaining groups. Significantly lowest hardness loss for all groups was recorded for group 12 with admixture of calcium, phosphate and fluoride to citric acid. The significantly highest enamel loss was recorded for the controls compared to all other samples. Groups 3 and 4 revealed significantly lower and higher tissue loss compared to the remaining groups (2-13), respectively. The other groups did not differ significantly from each other. Modification of citric acid with calcium, phosphate and fluoride exerts a significant protective potential with respect to dental erosion. However, with the low concentrations applied enamel dissolution could not be completely prevented.     

The demonstration of a dicalcium phosphate stabilizing factor in human saliva

Dicalcium phosphate dihydrate (DCPD) was equilibrated in H₂O, buffer solutions at pH 7.4, saliva samples and their ultrafiltrates and suspensions of cellular residue obtained from centrifiugation of whole saliva. The supernatants were analysed chemically for calcium phosphate and pH after 48 hr and the solids examined by X-ray diffraction. DCPD suspended in saliva remained unchanged in 18 out of 21 duct salivas and in 12 out of 26 samples of whole saliva, while the more basic calcium phosphate, octacalcium phosphate, was formed in 43 out of 47 ultrafiltrates and in water and buffer solutions. DCPD also remained unchanged in suspensions of salivary residue. If saliva was pretreated with ground enamel powder or synthetic hydroxyapatite prior to DCPD equilibrium, more basic calcium phosphate would form. It is concluded that saliva contains a DCPD stabilizing factor which is non-ultrafiltrable and which may be adsorbed on to enamel powder, synthetic hydroxyapatite and salivary cellular residue.     

Effects of inorganic orthophosphate and pyrophosphate on dissolution of calcium fluoride in water

Calcium and fluoride release from excess solid calcium fluoride was monitored for 15-30 min in aqueous solutions containing various concentrations of inorganic orthophosphate and pyrophosphate. Low concentrations of these ions (1-10 mumol/L) considerably inhibited the rate of dissolution of calcium fluoride. This inhibition was pH-dependent, being reduced at pH values below 5. It is suggested that a reduced calcium fluoride dissolution rate, in the presence of phosphate, can account for the relatively slow loss of calcium fluoride from dental enamel observed in recent clinical studies. It also appears that calcium fluoride coated with phosphate may provide a pH-controlled slow release of fluoride that may be of clinical significance and a major component of the cariostatic mechanism of topically applied fluoride.     

Critical pH in resting and stimulated whole saliva in groups of children and adults

Objective . The concentrations of calcium and phosphate in saliva have significant influence on the protective mechanisms of dental hard tissues within the oral environment. A lower calcium concentration means: 1) a lower thermodynamic driving force for hydroxyapatite precipitation at normal oral pH; 2) a higher driving force for hydroxyapatite dissolution at low pH; 3) a lower critical pH. The aims of this study were to: 1. determine the calcium and phosphate concentrations 2. calculate the critical pH for enamel and 3. determine the driving forces for demineralization and remineralization in a group of children and adults.
Methods . In this comparative study, calcium and phosphate concentrations were measured in the resting and stimulated saliva of child and adult volunteers using a spectrophotometric system used in routine blood analysis. Salivary flow rates were also measured in each group.
Results . The calcium concentrations were lower in children than adults, but the phosphate concentrations were not significantly different. The critical pH was significantly higher for children than adults in both resting and stimulated saliva. Therefore, the thermodynamic driving forces for; (1) demineralisation at low oral pH, is greater , and (2) for remineralisation at normal oral pH, is lower, in children compared to adults.
Conclusion . The results of this study show that from thermodynamic considerations alone, there is increased risk of demineralization in children compared with adults.     

A comparison of the influence of lanthanum and fluoride on de- and remineralization of bovine enamel in vitro

The influence of fluoride and lanthanum on enamel softening was compared by hardness measurements and by determination of the calcium content in the solution. The addition of lanthanum to an acetate buffer solution (0.05 mol/L, pH = 5.0) had no significant influence on the change of the indentation length and on the calcium release, compared with those of the control group. In the fluoride group, a moderate increase of the indentation length and a reduced calcium loss were observed. In a second experiment, surface-softened bovine enamel was treated with calcifying solutions (Ca/P ratio = 1.67). Differences in treatment were created by the addition of traces (2 ppm) of lanthanum, fluoride, or both. The acid resistance of the treated enamel was evaluated by two successive softening steps (0.1 mol/L acetate buffer, pH = 5.5). Alterations of the enamel surface were longitudinally followed with a hardness tester. The addition of fluoride, lanthanum, or both significantly improved the rehardening of surface-softened enamel, compared with that of the control group. A 2 x 2 factorial analysis of variance showed that lanthanum and fluoride had a highly significant main effect and a significant negative interaction effect on the rehardening of surface-softened enamel. Subsequent acid treatments revealed that only samples treated with a fluoride-containing solution were adequately protected from further dissolution. The enamel surfaces treated with a solution containing both lanthanum and fluoride were covered with an acid-resistant surface coating. SEM observation of this surface layer revealed the presence of globular and spherulite structures.     

The effect of adjacent dentine blocks on the demineralisation and remineralisation of enamel in vitro

The aim was to investigate interactions between enamel and dentine at low pH under conditions simulating those at the enamel-dentine junction. Sound enamel blocks were demineralised in acid-gel systems, at pH 4.6, either in isolation, next to one, or in the middle of two, abutting dentine blocks. The gels were initially infinitely undersaturated with respect to enamel. In a second study, enamel blocks containing pre-formed lesions were placed in acid-gel systems, at pH 5.0, either in isolation or next to dentine blocks. The systems were initially either partially or infinitely undersaturated. In the partially saturated systems, calcium and phosphate concentrations were representative of plaque fluid. In the first study, demineralisation of enamel next to one dentine block was reduced in inverse proportion to the distance from the dentine. Demineralisation of enamel between two dentine blocks was retarded markedly across the whole block. In the second study, in the partially saturated systems, enamel lesions next to dentine blocks remineralised, whereas those in isolation demineralised further. We suggest that diffusion of dissolved dentine mineral over the enamel in the infinitely undersaturated system was sufficient to reduce undersaturation, thus retarding demineralisation, and that in the partially saturated systems, dentine dissolution together with the added calcium phosphate caused remineralisation of enamel lesions. Fluoride released from dissolving dentine may have augmented these effects. Different rates of demineralisation in enamel and dentine, or enamel remineralisation with concurrent dentine demineralisation, enabled by differences in their solubilities, could help explain the progression of so-called 'hidden caries'.