Terahertz pulsed imaging study to assess remineralization of artificial caries lesions
David ChurchleyR.J.M. LynchFrank LippertJennifer Susan O’Bryan EderJesse AltonCarlos González‐Cabezas
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We compare terahertz-pulsed imaging (TPI) with transverse microradiography (TMR) and microindentation to measure remineralization of artificial caries lesions. Lesions are formed in bovine enamel using a solution of 0.1 M lactic acid/0.2% Carbopol C907 and 50% saturated with hydroxyapatite adjusted to pH 5.0. The 20-day experimental protocol consists of four 1 min treatment periods with dentifrices containing 10, 675, 1385, and 2700 ppm fluoride, a 4-h/day acid challenge, and, for the remaining time, specimens are stored in a 50:50 pooled human/artificial saliva mixture. Each specimen is imaged at the focal point of the terahertz beam (data-point spacing = 50 μm). The time-domain data are used to calculate the refractive index volume percent profile throughout the lesion, and the differences in the integrated areas between the baseline and post-treatment profiles are used to calculate ΔΔZ((THz)). In addition, the change from baseline in both the lesion depth and the intensity of the reflected pulse from the air/enamel interface is determined. Statistically significant Pearson correlation coefficients are observed between TPI and TMR/microindentation (P < 0.05). We demonstrate that TPI has potential as a research tool for hard tissue imaging.To investigate the mechanism of remineralization, artificial (HEC) lesions in bovine enamel and etched bovine enamel were remineralized in a pH-stat controlled system at 25, 37 and 50 °C. In all experiments 3 cm2 of demineralized enamel was immersed in 15 ml of a solution containing 1.5 mM Ca, 0.9 mM PO4 at pH = 7.0. The mineral deposition was followed by monitoring the alkali uptake and the changes of the calcium and phosphate concentration in solution. From the analytical data it was concluded firstly that in all cases hydroxyapatite precipitated. Secondly it was shown that the remineralization of both lesions and etched enamel was a second order chemical reaction. The rate of lesion remineralization is primarily determined by the diffusion in the surface layer pores, as indicated by the value of the activation energy (0.38 eV). For etched enamel the activation energy is 0.70 eV suggesting a surface reaction controlled process. Since the mechanism of remineralization of lesions and etched enamel is completely different it is dangerous to extrapolate results obtained with etched enamel to the situation of the clinically interesting remineralization of white spots.
Tooth Remineralization
Tooth enamel
Human tooth
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Blue light, especially from LED devices, is a tool very frequently used in dental procedures. However, the investigations of its effects on dental enamel are focused primarily on enamel demineralization and fluoride retention. Despite the fact that this spectral region can inhibit enamel demineralization, the consequences of the irradiation on demineralized enamel are not known. For this reason, we evaluated the effects of blue LED on enamel remineralization. Artificial lesions formed in bovine dental enamel samples by immersion in undersaturated acetate buffer were divided into three groups. In the first group (DE), the lesions were not submitted to any treatment. In the second (RE), the lesions were submitted to remineralization. The lesions from the third group (LED+RE) were irradiated with blue LED (455nm, 1.38W/cm2, 13.75J/cm2 and 10s) before the remineralization. Cross-sectional microhardness was used to assess mineral changes induced by remineralization under pH-cycling. The mineral deposition occurred preferably in the middle portion of the lesions. Specimens from group RE showed higher hardness value than the DE ones. On the other hand, the mean hardness value of the LED+RE samples was not statistically different from the DE samples. Results obtained in the present study show that the blue light is not innocuous for the dental enamel and inhibition of its remineralization can occur.
Tooth Remineralization
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A rapid and flexible model system has been developed to study human enamel behaviour under conditions of oscillating pH similar to those experienced in the mouth. The model uses realistic time intervals and operates at in vivo temperatures (35-37 degrees C). Mineral loss or gain is measured quantitatively on a volume basis. Results indicate that for single 1-day periods with three 20-min exposures to acid, 1% v/v of enamel mineral was lost. These results, together with the histological appearance of the tissue, are in close agreement with previous studies of enamel caries.
Cycling
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To study the effect of several fluorides on resistance to erosion of beverage on enamel and remineralization of etched enamel.The enamel slabs were prepared from human retained primary teeth. After treated with 0.1%NaF solution, fluoride Protector and Bifluorid 12 respectively, they were immersed at intervals in beverage. The SMH were measured before and after experiments. Morphological changes of enamel surface were observed by SEM.The SMH of all enamel slabs descended obviously after exposed to beverage. The difference between control group and test groups was significant statistically. The variable degree etch caused by beverage were found by SEM. The SMH of etched enamel increased significantly after treated with fluoride, and the Bifluorid 12 showed the highest significant increase.The treatment of enamel with topical fluoride can enhance the resistance of enamel to acid etch of soft drinks and the remineralization of enamel following exposed to beverage. This study recommends the Bifluorid 12 is an efficacious fluoride in remineralization of etched enamel.
Tooth enamel
Human tooth
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The purpose of this study was to examine the influence of laser irradiation on remineralization of demineralized enamel. Twenty human premolar enamel slabs (3×5mm) were prepared for this study. The enamel slabs were ground down to about 400μm and immersed in 0.1M lactic acid buffer (3.0mM Ca, 1.8mM P, 1.0% carboxymethylcellulose, pH 4.5, 37°C) for 12 hours. Demineralized enamel slabs were irradiated with a Q-switch Nd-YAG laser at 50 J/cm2of total energy density. After laser irradiation, slabs were immersed in remineralizing solution (3.0mM Ca, 1.8mM P 1.0% carboxymethylcellose, 150mM NaCl, 3.0ppm F, pH 7.0, 37°C) for 10 days. The degree of remineralization was evaluated by microradiograph using calculated mineral content. Sectioned enamel slabs were also immersed in 0.5M HClO4solution for 1 minute to examine acid resistance of the remineralized enamel.There were signs of remineralization in both the lased and unlased areas, and no clearly difference was recognized between these areas. It appears that remineralization of enamel was not inhibited by laser irradiation. The acid resistance of the remineralized enamel was higher in the lased area than in the unlased area. However, the degree of remineralization corresponding to the mineral level was not significantly different between two areas. It is probable that the permeability of enamel was not decreased by laser irradiation.
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Ex vivo
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Remineralization of human enamel is an important issue in the prevention of caries. Fluoride has an important influence on the remineralization process. A great deal of research has been done into the role played by the method of application and by the concentration and type of fluoride. In the clinical study of this dissertation the process of remineralization of lesions in human enamel was longitudinally followed in vivo during a period of 3 months. Both the remineralization in the surface softened enamel lesion and the subsurface enamel lesion of human enamel were studied. In addition, attention was devoted especially to the influence of fluoride on this remineralization. The results indicated that the use of a 1,500 ppm fluoridated NaF toothpaste was more effective on the remineralization process than the use of a 50 pmm NaF mouthrinse. Important is the fluoride gradient in the enamel lesion, and in particular the fact that fluoride reaches the lesion front. This most likely accounts for the remineralization efficiency.
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The aim of this study was to examine the change in acid resistance of lased-remineralized enamel. Sixty-six enamel slabs were prepared from thirty-three human premolars. These enamel slabs were demineralized artificially. They were irradiated with a Q-switch Nd-YAG laser and remineralized with two remineralizing solutions, one with and the other without fluoride. To evaluate acid resistance, the enamel slabs were subsequently immersed in 0.1M lactic acid solution (pH 4.5) while stirring (139r.p.m. ±12; Mean±S.D.), and dissolved Ca2+ was measured as a function of time from 15min to 180min.The results were as follows:1) Laser irradiation increased acid resistance of demineralized enamel during short periods (up to 60min).2) Remineralization without fluoride increased acid resistance of demintralized enamel during short periods (up to 60min) regardless of laser irradiation.3) Remineralization with fluoride increased acid resistance of demineralized enamel during long periods (up to 180min) regardless of laser irradiation.
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pH-cycling and in situ studies have shown that fluctuations in de-/remineralisation conditions or in fluoride usage can lead to laminations inside enamel or dentine lesions. Layers with different mineral content are thought to reflect the history of fluoride administrations. Studying the dissolution properties of such lesions at various depths – using bulk specimens – is presumably hampered by limited diffusion of acids through the lesion pores. Therefore, in this study the acid susceptibility of enamel and dentine lesions and the underlying sound tissues was studied by exposing sections to acid buffers from the cut rather than from the external surface. Specimens were obtained from a previous study of the effects of high-fluoride (0, 1,000, 2,000, 3,000/5,000 ppm F) toothpastes on enamel and dentine de-/remineralisation. Sections were subjected to acid buffers for 3 and 7 days and the changes in mineral content were monitored by contact microradiography. For enamel lesions a significant difference in dissolution over depth was observed between the groups subjected to the different fluoride schemes. At 7 days a dose response was found between the different fluoride groups and the lesion parameters. In the no-fluoride group dissolution in the original lesion and the sound tissue were similar. All dentine lesions which had been treated with fluoride showed inhibition of dissolution, but the inhibition did not increase with higher fluoride concentrations. Deeper into the dentine tissue there was some protection, but it was not statistically significant. We conclude that penetration of fluoride through the lesion pores determines the dissolution pattern of a lesion at various depths.
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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’.
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