Chemical and histological changes during development of artificial caries

Imaging of demineralized enamel in intact tooth by epidetected stimulated Raman scattering microscopy

Stimulated Raman scattering microscopy (SRS) was deployed to quantify enamel demineralization in intact teeth. The surfaces of 15 bovine-enamel blocks were divided into four equal-areas, and chemically demineralized for 0, 8, 16, or 24 h, respectively. SRS images (spectral coverage from ∼850 to 1150  cm  -  1) were obtained at 10-μm increments up to 90  μm from the surface to the dentin-enamel junction. SRS intensities of phosphate (peak: 959  cm  -  1), carbonate (1070  cm  -  1), and water (3250  cm  -  1) were measured. The phosphate peak height was divided by the carbonate peak height to calculate the SRS-P/C-ratio, which was normalized relative to 90  μm (SRS-P/C-ratio-normalized). The water intensity against depth decay curve was fitted with exponential decay. A decay constant (SRS-water-content) was obtained. Knoop-hardness values were obtained before (SMHS) and after demineralization (SMHD). Surface microhardness-change (SMH-change) [  (  SMHD  -  SMHS  )    /  SMHS] was calculated. Depth and integrated mineral loss (ΔZ) were determined by transverse microradiography. Comparisons were made using repeated-measures of analysis of variance. For SRS-P/C-ratio-normalized, at 0-μm (surface), sound (0-h demineralization) was significantly higher than 8-h demineralization and 24-h demineralization; 16-h demineralization was significantly higher than 24-h demineralization. For SRS-water-content, 24-h demineralization was significantly higher than all other demineralization-groups; 8-h demineralization and 16-h demineralization were significantly higher than 0-h demineralization. SRS-water-content presented moderate-to-strong correlation with SMH-change and weak-to-moderate correlation with depth. These results collectively demonstrate the potential of using SRS microscopy for in-situ chemical analysis of dental caries.

Objective and quantitative assessment of caries lesion activity

OBJECTIVES

Evaluate the ability of objectively measured specular reflection, roughness, and fluorescence change during dehydration to assess caries lesion activity.

METHODS

One hundred ninety-five ground/polished 3 × 3 × 2 mm sound human enamel specimens were divided into three groups and demineralized using a multispecies microbial caries model for 3, 6, or 9 days; and then remineralized with 1100 ppm-F as NaF solution for 10 days using a pH-cyclic model. Reflection (amplitude: %), roughness (Ra: μm), fluorescence change during dehydration (ΔQ: %×mm²), and microfocus computed tomography [μ-CT: lesion volume (μm³)] were measured for sound, demineralized and remineralized enamel. The surface was hydrated and fluorescence images were acquired at 1 s intervals for 10 s (ΔQ₁₀). During image acquisition, surface was dehydrated with continuous compressed air. Changes-in-ΔQ per second (ΔQ_D: %×mm²/sec) at 5 (ΔQ_D5) and 10 s (ΔQ_D10) were obtained.

RESULTS

Reflection decreased from sound to demineralized groups (p < 0.0001); remineralized groups were higher than demineralized groups (p < 0.001), but not different from sound (p > 0.32). Roughness increased from sound to demineralized groups (p < 0.0001) and remineralized groups were also higher than sound (p < 0.0001). ΔQ₁₀, ΔQ_D5 and ΔQ_D10 increased from sound to demineralized groups (p < 0.0001), and remineralized groups decreased compared to demineralized groups (p < 0.05), but was higher than sound (p < 0.0001). The correlations of μ-CT with reflection, roughness, and ΔQ₁₀ were -0.63, 0.71, and 0.82, respectively (p < 0.0001).

CONCLUSIONS

Reflection, roughness and ΔQ could distinguish between sound and demineralized enamel. Reflection and ΔQ were able to distinguish between demineralized and remineralized enamel.

CLINICAL SIGNIFICANCE

Determination of caries activity, whether a lesion is active or inactive, is an essential and critical component of caries diagnosis. However, especially for enamel lesions, it is difficult to estimate without longitudinal follow-up. Reflection, roughness and fluorescence change during dehydration have the potential to measure caries lesion activity at a-single-appointment.

Determination of Caries Lesion Activity: Reflection and Roughness for Characterization of Caries Progression

Caries lesion progression is difficult to determine with visual and tactile examinations. The hypothesis of this study was that reflection and roughness measurements could determine caries progression. Ground/polished sound human enamel specimens were analyzed at baseline (sound) and after two four-day demineralization periods for reflection using optical reflectometry (ORef) and for roughness using optical surface profilometry (SPro). Specimens were demineralized using a microbial- Streptococcus mutans aries model. Comparisons among the periods for ORef and SPro were performed using repeated measures analysis of variance. Two-sample t-tests were used for differences in transverse microradiography. The integrated mineral loss and depth of the four-day demineralization period were significantly smaller than those for the eight-day demineralization period ( p<0.01). With increased demineralization time, reflection was significantly decreased and roughness was significantly increased ( p<0.01). Correlation between ORef and SPro was moderate ( r=-0.63). Both reflection and roughness can be characterized for nondestructive longitudinal assessment of caries lesion progression.

Pilot clinical study to assess caries lesion activity using quantitative light-induced fluorescence during dehydration

This study aimed to evaluate the ability of quantitative light-induced fluorescence (QLF) to assess caries lesion activity using visual examination (VE) as the gold standard. Twenty-four visible white spot lesions on buccal surfaces were examined from 23 children, ages 9 to 14 years. At baseline, the surface was hydrated with water, and thereafter, it was dehydrated with continuous compressed air during image acquisition. QLF images were acquired at 0 (baseline), 5, and 15 s. QLF variables [ QLF V : fluorescence loss ( ? F ), lesion size (S), ? Q : ? F × S ] was recorded. Changes-in- QLF V per second ( ? QLF V ) were determined: ? QLF V = ( QLF VN ? QLF V Baseline ) / N , where N indicates dehydration time. One experienced dentist conducted VE independently using a dental unit’s light, compressed air, and explorer.

Development of caries adjacent to composite restorations after exposure to dentifrices with different fluoride concentrations

Objective To evaluate the development of recurrent caries after exposure to fluoride dentifrices with different concentrations. Material and method: 48 samples of bovine incisors (4x4mm2) were exposed to pH cycling for 7 days before the preparation of the cavities (2mm deep). The samples were restored with a microhybrid resin composite. Then, the samples were exposed to thermal cycling (350 cycles) and they were randomly allocated into 4 treatment groups (n = 12): Group A - non-fluoridated dentifrice (negative control); Group B - 500 ppm dentifrice; Group C - 750 ppm dentifrice; group D - 1100 ppm dentifrice (positive control). The samples were treated with solutions of each dentifrice (9.6 ml water/1.6 g dentifrice) for 60 seconds and then were immersed in demineralizing (3 h) and remineralizing (2 h) solutions 3 times a day. Next, the samples were immersed in a remineralizing solution for 18 hours. Then, the blocks were sectioned for examination of the length of the outer caries lesion, using polarized light microscopy. The ANOVA parametric test complemented by the Tukey test with a confidence level of 95%, were used in the statistical analysis. Result: A smaller lesion length was observed in the group treated with the fluoride concentration of 1100 ppm F, but there were no differences between toothpastes with fluoride concentrations of 500 and 750 ppm F. Conclusion: The use of fluoride dentifrices (1100 ppm) reduces the development of caries adjacent to dental restorations.

High-resolution PS-OCT of Enamel Remineralization

Previous studies have demonstrated that Polarization Sensitive Optical Coherence Tomography (PS-OCT) can be used to image the remineralization of early artificial caries lesions. However, the depth resolution of the imaging system employed in those previous studies was limited and the outer surface structure of the lesions were not resolved as clearly as desired. The purpose of this study was to repeat the earlier remineralization study using a broadband light-source of higher resolution to determine if there can be improved resolution of the remineralized surface zones of the lesions. An all polarization-maintaining fiber based PS-OCT system operating at 1310-nm was used to acquire polarization resolved images of bovine enamel surfaces exposed to a demineralizing solution at pH-4.9 followed by a fluoride containing remineralizing solution at pH-7.0 containing 2-ppm fluoride. The structure of the surface zones could be clearly resolved using PS-OCT in the samples that underwent remineralization. The PS-OCT measurements indicated a significant (p<0.05) reduction in the integrated reflectivity between the severity of the lesions that were exposed to the remineralization solution and those that were not. The lesion depth and mineral loss were also measured with polarized light microscopy and transverse microradiography after sectioning the enamel blocks. These results show that PS-OCT can be used to non-destructively monitor the remineralization potential of anti-caries agents.

Caries and fluoride processes

OBJECTIVES

The purpose of the work was to establish the level and source of salivary fluoride, whether it could interact with tooth mineral and whether it was able to effect calcium re-acquisition and remineralisation.

METHODS

Fluoride in saliva and in solution was measured by electrode, calcium by complexometric titration and phosphate colourimetrically-to measure salivary fluoride, its uptake by mineral and the effects of such low levels on calcium and phosphate levels in solution in contact with suspensions of mineral hydroxyapatite.

RESULTS

Fluoride levels in saliva were low but could interact with hydroxyapatite. Such levels caused apatite crystallite growth and preferential acquisition of calcium by calcium-deficient apatite.

CONCLUSIONS

Salivary fluoride contents rose with increasing water fluoride levels. It was acquired by hydroxyapatite mineral. Topical fluoride was stored on oral tissues. Even these low fluoride contents could cause mineral crystallite growth with preferential calcium uptake. On a mineral area basis these effects were caused by quite low fluoride uptakes. These findings do not support reliance upon large fluoride uptakes by tooth enamel as evidence for commensurate caries reductions.

In vitro inhibition of caries-like lesions with fluoride-releasing materials

The aim of this study was to compare the in vitro caries inhibition of various resin-based materials. Class V cavities were prepared in twenty-five freshly extracted human premolar teeth which were then restored with glass-ionomer cement (Chemfill II), compomer (Compoglass F, Dyract AP) and composite resin (Tetric Ceram and Z 100). The teeth were submerged in an acid gel for 6 weeks. Each specimen was sectioned. These sections were left in water for 24 hours, and then examined using polarized light microscopy. The lesion consisted of two parts, the outer surface lesion and the cavity wall lesion. There was no significant difference in the body depth of the outer lesion and in the depth of the wall lesion among teeth restored with Compoglass F, Dyract AP and Chemfill II (P > 0.05). There was a significant difference between those restored with Z 100 and Tetric Ceram (P < 0.05). The length of the wall lesion for the teeth restored with Chemfill II was significantly smaller than that in the remaining groups (P < 0.05). The length of the wall lesion for teeth restored with Tetric Ceram and Z 100 was significantly higher than in the remaining groups (P < 0.05). These results suggest that composite materials and compomer provide less caries inhibition than glass-ionomer cements.

Effect of pulsed Nd:YAG laser irradiation on acid demineralization of enamel and dentin

OBJECTIVE

The purpose of this study was to investigate the effect of Nd:YAG laser irradiation on the acid demineralization of enamel and dentin by spectrophotometry. A mechanism of acquired acid resistance is also proposed.

SUMMARY BACKGROUND DATA

The ability of Nd:YAG laser irradiation to the enhanced resistance to artificial caries formation is still controversial.

METHODS

A pulsed Nd:YAG laser at 1.064-micron wavelength was used to irradiate the human enamel and dentin samples from 20 extracted human molars at the parameters of 1, 2, and 3 W and 20 pps for a total of 9 sec after painting with black ink. Samples were then subjected to 2 microliters of 0.1 M lactic acid solution (pH 4.8) for 24 h at 36 degrees C. The parts per million (ppm) of calcium ion (Ca2+) dissolved in each solution was determined by atomic absorption spectrophotometry and the morphological changes were also investigated by scanning electron microscopy (SEM).

RESULTS

The lowest mean Ca2+ ppm was recorded in the samples irradiated at 3 W, in those by irradiated at 2 W and 1 W. The unlased samples showed the highest Ca2+ ppm. SEM observation showed that in the lased areas, the smear layer was partially melted and the underlying primary enamel or dentin seemed to be thermally degenerated.

CONCLUSIONS

The results of this study suggested that melted smear layer and underlying enamel or dentin degenerated thermally by the heat treatment of Nd:YAG laser might play a major role to enhance resistance to artificial caries-like formation.

Acquired acid resistance of dental hard tissues by CO2 laser irradiation

OBJECTIVE

This investigation was performed to evaluate the effect of CO2 laser irradiation on the acquired acid resistance of dental hard tissues to artificial caries-like formation and the ultrastructure of lased areas was morphologically investigated in vitro.

SUMMARY BACKGROUND DATA

In recent years, many studies have been carried out to evaluate the effect of the CO2 laser on the enhanced resistance to artificial caries formation of enamel. However, a limited number of papers concerning its effect on dentin have been published.

METHODS

A CO2 laser of 10.6 microns wavelength was used to irradiate human enamel and dentin samples from 20 extracted human molar teeth, and samples were subjected to 2 microliters of 0.1 M lactic acid solution (pH 4.8) for 24 h at 36 degrees C. The parts per million (ppm) of calcium ion (Ca2+) dissolved in each solution was determined by atomic absorption spectrophotometry and the morphological changes were investigated by scanning electron microscopy (SEM).

RESULTS

The lowest mean Ca2+ ppm was recorded in the samples irradiated at 3 W, followed by 2 W, 1 W, and unlased samples. SEM observation showed that the lased areas were melting with solidification of the smear layer. Even after acid demineralization, the lased surfaces were almost unchanged.

CONCLUSIONS

The results of this study suggested that CO2 laser irradiation could sufficiently melt and solidify the enamel and dentin surfaces and thus enhance resistance to artificial caries-like formation.

Changes in the permselectivity of human teeth during caries attack

Previous studies have shown that enamel permselectivity can influence fluid composition within caries lesions during de- and remineralization. The permselectivity of human enamel, cementum, and dentin sections was examined, in a microwell model, by measurement of the membrane potential developed by KCl diffusion while the sections were immersed in solutions simulating resting (pH = 5.6) and cariogenic plaque fluid (pH = 4.8). In a second experiment, the effects of charged compounds (phytate and Zonyl-FSC) on the tooth permselectivity were examined. The average membrane potentials (+/- SD) in "resting plaque" solution were: sound enamel, 18.9 +/- 3.2 mV, n = 66; dentin, 0.9 +/- 9.2 mV, n = 59; and cementum, -0.8 +/- 8.2 mV, n = 42, with a positive sing indicating cation selectivity. The average membrane potentials became more negative in "cariogenic plaque" solution for all types of sections: sound enamel, 5.2 +/- 2.1 mV, n = 46; dentin, -8.1 +/- 7.4 mV, n = 45; and cementum, -14.3 +/- 8.0 mV, n = 34. In lesion enamel sections, the membrane potential was reduced from the non-lesion wells in both types of test solutions, while phytate treatment caused an increase of approximately 10 mV in potential (increased cation selectivity) in every enamel well in either "resting" or "cariogenic" solution. Treatment of enamel sections with Zonyl-FSC caused the membrane potential to become more negative in both test solutions, with many of the wells showing anion selectivity in the cariogenic "plaque-like" solution. However, the changes in enamel membrane potentials induced by Zonyl-FSC slowly increased toward the initial values after treatment, while the effects of the phytate pre-treatment persisted. Most dentin sections treated with phytate also showed an increase in potential after phytate treatment; however, Zonyl-FSC seemed to have little effect on the membrane potential of dentin. The results of this study suggest that modification of tooth permselectivity by surface-active agents may be a viable method of decreasing the rate of caries progression.

Interactions of fluoride and non-fluoride agents with the caries process

Fundamental to the caries process and its inhibition is an understanding of the composition and structural relationships of dental mineral. These have received greater study in recent years, leading to a better understanding of the processes involved. Fluoride has been the most successful of the anti-caries agents to date, and many studies have concentrated on this ion. The anti-caries action of fluoride has been only partially explained by the early finding that fluoride-treated mineral was less soluble, and this criterion is now less widely accepted. The dissolutive process of caries is inhibited by fluoride, monofluorophosphate, trimetaphosphate, and zinc. However, only the first three of these show anti-caries activity. The presence of fluoride during in vivo and in vitro caries is conducive to the formation of an apparently intact surface zone. Current evidence is that this zone reforms during the caries process, thus acquiring fluoride and having larger crystallites compared with sound enamel. Trimetaphosphate also favors the formation of a surface zone. There is a clear beneficial involvement of fluoride, even at low levels, in the process of lesion remineralization. It is highly probable that this process results from re-growth of residual enamel crystallites rather than de novo precipitation of calcium phosphates. Levels of fluoride found in saliva can interact with dental mineral. Although zinc has been shown to adsorb upon apatite mineral and to restrict subsequent crystal growth, it does not appear to affect the action of fluoride, including remineralization, adversely. This may be due to the fact that the uptake of zinc is reversible.(ABSTRACT TRUNCATED AT 250 WORDS)

Enhancing the virulence of Streptococcus sobrinus in rats

The purpose of this study was to explore the virulence of strains of Streptococcus sobrinus that had been re-isolated from a desalivated rat. Furthermore, we wished to determine the influence of desalivation on the acidogenicity of fasting and sucrose-pulsed dental plaque in rats infected by strains of S. sorbrinus. Experimental groups were formed and infected as follows: Group 1, desalivated animals, S. sorbrinus ATCC27352; Group 2, intact animals, infected as in Group 1; Group 3, desalivated animals, S. sorbrinus re-isolated from a desalivated animal that had been infected with the ATCC 27352 strain for five weeks; Group 4, intact animals, infected as in Group 3; Group 5, desalivated animals, S. sobrinus re-isolated from a desalivated animal that had been infected with the ATCC27352 strain for ten weeks; Group 6, intact animals, infected as in Group 5; Group 7, intact animals, no infection. All animals were fed cariogenic diet 2000 and sucrose-sweetened drinking water ad libitum. After five weeks, desalivated and intact animals that had been infected with isolates obtained from desalivated animals had significantly higher (p less than 0.05) smooth-surface-caries scores than did intact and desalivated animals infected with cultures of the original stock ATCC strain. Moreover, fasting plaque pH values were statistically lower in desalivated animals (Groups 1 and 3) than in intact animals (Groups 2, 6, and 7) (Turkey analysis, p less than 0.05).(ABSTRACT TRUNCATED AT 250 WORDS)

Chemical events during tooth dissolution

Information on chemical changes during enamel dissolution has been collected from investigations on hydroxyapatite solubility, enamel solubility, artificial lesion formation, and natural caries. Although hydroxyapatite and enamel will ultimately dissolve in acid or during caries, compositional changes also occur. Most notably, there is a preferential dissolution of calcium, both from hydroxyapatite and from enamel, and of carbonate and magnesium from enamel. Root dentin yields substantial amounts of magnesium on acid attack. Fluoride may be involved in surface zone formation during attack, but an additional theory of coupled diffusion is described. Calcium-deficient mineral is produced during an acid attack, and this has lattice parameters and solubility behavior different from those of stoichiometric material. The interaction of fluoride produces a more stable lattice, resisting dissolution and favoring accretion, and tending to counteract the effects of carbonate and magnesium in forming mineral. The provision of fluoride, albeit at low levels, in plaque fluid is seen as being important in maintaining the net integrity of the tooth. More information is also needed on the role of the organic phase in tooth structures during caries and acid attack.