FT-Raman Spectroscopy Study of the Remineralization of Microwave-Exposed Artificial Caries.

Dental caries is a microbially mediated disease that can result in significant tooth structure degradation. Although the preponderance of lesions is treated by surgical intervention, various strategies have been developed for its noninvasive management. Here, we use a novel approach for noninvasive treatment based on killing Streptococcus mutans with high-frequency microwave energy (ME). The rationale for this approach is based on modulating the pH of caries to a physiological state to enable spontaneous tooth remineralization from exogenous sources. In the present study, after demonstrating that ME kills >99% of S. mutans in planktonic cultures, 8 enamel slabs were harvested from a single tooth. Baseline mineral concentration at each of 12 points per slab was obtained using Fourier transform (FT)-Raman spectroscopy. Surface demineralization was subsequently promoted by subjecting all samples to an S. mutans acidic biofilm for 6 d. Half of the samples were then exposed to high-frequency ME, and the other half were used as controls. All samples were next subjected to a remineralization protocol consisting of two 45-min exposures per 24-h period in tryptic soy broth followed by immersion in a remineralizing solution for the remaining period. After 10 d, samples were removed and cleaned. FT-Raman spectra were again obtained at the same 12 points per sample, and the mineral concentration was determined. The effect of the remineralization protocol on the demineralized slabs was expressed as a percentage of mineral loss or gain relative to baseline. The mineral concentration of the microwave-exposed group collectively approached 100% of baseline values, while that of the control group was in the order of 40%. Differences between groups were significant (P = 0.001, Mann-Whitney U test). We concluded that killing of S. mutans by ME promotes effective remineralization of S. mutans-demineralized enamel compared with controls.

Oxygen inhibition in dental resins.

Oxygen inhibits free radical polymerization and yields polymers with uncured surfaces. This is a concern when thin layers of resin are being polymerized, or in circumstances where conventional means of eliminating inhibition are inappropriate. In this study, we tested the hypothesis that viscosity, filler content, and polymerization temperature modify oxygen diffusion in the resin or the reactivity of radical species, and affect the degree of conversion near the surface. Confocal Raman micro-spectroscopy was used to measure monomer conversion from the surface to the bulk of cured resins. Increased viscosity was shown to limit oxygen diffusion and increase conversion near the surface, without necessarily modifying the depth of inhibition. The filler material was shown to increase, simultaneously, oxygen diffusivity and the viscosity of the resin, which have opposite effects on conversion. Polymerization at a temperature above approximately 110 degrees C was shown to eliminate oxygen inhibition.

High-pressure infrared and FT-Raman investigation of a dental composite.

Composite resins are often used as filling materials on load-bearing surfaces of teeth. As masticatory stresses can be high, here, we study the effect of pressure on the behaviour of a dental composite. Using a polymerized wafer, the IR and FT-Raman spectra of a zirconia-containing proprietary composite (Z100, 3M, Minneapolis, MN, USA) were recorded. The high-pressure IR spectra were also recorded. Band assignments were made for the main peaks of both organic and inorganic components. Breaks in the pressure dependences (dv/dP) of the organic components were found at 22 kbar. Different pressure dependences for different vibrational modes of inorganic components were also observed. These data suggest that the network structure of the composite is compacted under high pressure and that both the atomic distance and bonding angles in the network are altered.

A photoacoustic FTIRS study of the chemical modifications of human dentin surfaces: I. Demineralization.

Acids are used to modify the structure and composition of dentin surfaces to improve bonds formed with resins. The purpose of this work is to investigate such chemical modifications using the surface-sensitive technique photoacoustic Fourier transform infrared spectroscopy (PA-FTIRS). Spectra of acid-treated samples (citric, maleic, nitric, and phosphoric at pH = 1.0) were recorded at various time intervals. Analysis of these spectra indicates a gradual increase in sample surface area with treatment time. A decrease of the bands associated with calcium hydroxyapatite (HAP) and carbonate apatite inherent to the mineral phase of dentin are also observed. A comparison of spectra of samples treated for 2 min with each acid also reveals that maleic and phosphoric acids remove more HAP than citric acid. We conclude that citric acid may cause the formation of precipitates at the etching front which inhibit etching.

A photoacoustic FTIRS study of the chemical modifications of human dentin surfaces: II. Deproteination.

This work focuses on the chemical alterations occurring on the dentin surface on treatment with sodium hypochlorite (NaOCl), a known deproteinating agent. In addition, sequential treatment with both acid and NaOCl are characterized. Modifications are evaluated using photoacoustic FTIR (PA-FTIRS), a surface-sensitive spectroscopy. Spectra of NaOCl-treated dentin samples show a slow and heterogeneous removal of its organic phase, leaving calcium hydroxyapatite and carbonate apatite unchanged. Spectra of this deproteinated surface resemble those of apatites synthesized at low temperatures and having very high-surface areas. A combined sequential 2-min treatment of dentin with both maleic acid and NaOCl indicates that this treatment can produce a surface region which is neither significantly demineralized nor deproteinated. This sequential treatment can be used to remove the smear layer and restore the dentin surface to its natural composition.

Shear strength of the composite bond to etched porcelain.

The shear bond strength of composite resin to porcelain was investigated to optimize variables for bonding porcelain laminate veneers. Scanning electron microscopy was initially used to examine the surface configuration of porcelain prepared under various conditions. A factorial experiment was undertaken to determine the effects of three different bonding methods on both etched and non-etched porcelain. Composite resin was bonded to the porcelain groups using (a) unfilled resin, (b) silane, and (c) silane with dentin adhesive. The results indicated a significant difference in shear bond strength for the three bonding groups, depending on the porcelain surface condition. For the unetched samples, significant differences in bond strength were obtained for all three bonding conditions. However, for the etched group, there were no differences between the silane and silane-with-dentin-adhesive groups. Porcelain etching significantly increased bond strength across all three bonding methods and was the main contributor to the obtained values.

An FT-Raman spectroscopic investigation of dentin and collagen surfaces modified by 2-hydroxyethylmethacrylate.

Although 2-hydroxyethylmethacrylate (HEMA) is commonly used for adhesive bonding to dentin, its role in promoting adhesion is not completely understood. Here, we use FT-Raman spectroscopy to elucidate further the nature of the interaction of HEMA with dentin. Ground dentin was exposed to 2.5% (w/w) nitric acid, washed, dried in air, and treated with HEMA. The samples were then sequentially washed with distilled water, with FT-Raman spectra being obtained after different wash times. Hydroxyapatite and bovine type I collagen were similarly treated with HEMA except for the acid exposure. The FT-Raman spectra of these samples were also recorded. The spectra of HEMA-treated water-washed dentin and collagen revealed the following changes: (1) The band intensities of HEMA absorbed on dentin and collagen decreased with increasing wash times (2) the nu(C=O) and nu(CCO) modes of HEMA at 1718 and 607 cm-1, respectively, either disappeared or decreased after extensive washing; (3) the nu (C=C) (1640 cm-1) and delta (=CH2), (1403 cm-1) bands exhibited minor variations in band position and relative intensity. These results demonstrate that HEMA interacts with dentin both physically and chemically. The chemical interaction can be interpreted by either hydrogen bonding or the formation of a new bond to the ester group of HEMA.

A tapping mode AFM study of collapse and denaturation in dentinal collagen.

OBJECTIVES: Tapping mode atomic force microscopy (AFM) was used to investigate the surface changes of collagen exposed to phosphoric acid treatment. We focus on denaturation and collapse following demineralization and exposure to air. METHODS: Unpolished dentin disks, obtained from freshly extracted human molars, were etched in 37% phosphoric acid for 15s, rinsed ultrasonically and gently blotted with soft paper; the specimens were then continuously observed using tapping mode AFM. RESULTS: Immediately after the removal of bulk water, the surface consisted of a porous network of banded collagen fibrils, having periodicities of 67nm. After approximately 8min of subsequent air-drying, the spacing between fibrils was lost, and the surface was observed to consist of a dense array of closely spaced fibrils. The banding periodicity was still observable. SIGNIFICANCE: The air drying of etched dentin results in the collapse of the collagen network, but not in the denaturation of the collagen fibrils. This study indicates that collapse and denaturation are separate phenomena. It further shows that water loss occurs rapidly, and disrupts the native conformation of the collagen network. This would have adverse effects on adhesion.

Risk of respiratory exposure of dental personnel to amalgam alternatives.

Although the use of alternatives to dental amalgam is increasing, the possible hazard associated with their occupational exposure has received inadequate attention. The purpose of this study is to use available toxicological and environmental information in a qualitative risk assessment to address potential health hazards associated with exposure to these materials by dental personnel. The members of dental profession should be aware of risk due to long-term exposure to dental materials.

Factors affecting the future need for dental manpower in Canada and Quebec.

During the past decade, dental faculties in North America have reduced class sizes due to a perceived oversupply of dentists. Several schools have been closed outright, and others have been threatened with closure. These actions may have a negative impact on the future supply of dentists. The current beliefs with regards to the oversupply of dentists have inadequately accounted for the dramatic demographic and epidemiologic changes that are occurring in North America. Major changes in population distribution and disease trends point to an increased need for adult dental services in the future. Therefore, models for dental manpower needs should integrate these data to avoid a potential shortage of dental health care personnel in the future.

[Computer-assisted creation and fabrication of ceramic inlays and veneers]. / Conception et fabrication assistées par ordinateur d'incrustations et facettes en céramique.

The application of computers in restorative dentistry make it possible to fabricate ceramic restorations. The dental CAD/CAM system CEREC (Siemens AG Co.) is available to produce inlays and veneers in the dental office in a single appointment.

Clinical evaluation of two posterior composite resins: two-year results.

Two composite resins were evaluated for clinical acceptability as restorative materials in Class I and II cavities over a 2-year-period. In addition, dispersed phase alloy was evaluated for comparison. All restorations were assessed for anatomical form, marginal integrity and marginal leakage using modified United States Public Health Service (USPHS) criteria. At the 2-year recall, 96% of the composites and 100% of the amalgams were rated 'alpha' or 'bravo', using the parameters of assessment defined in this study. However, a significant number of shifts from the baseline, within the level of acceptability, had occurred. The two composites showed the greatest number of shifts for anatomical form and marginal leakage, while there were no differences between the composites and the amalgam for shifts in marginal integrity. Although a very high level of acceptability was determined for all of the materials, the frequency of rating change within categories for the composites was a cause of concern with regard to their long-term clinical use.

Absorption of iron by dentin: its role in discoloration.

This study investigated the effect of iron-containing mordants on the discoloration of human dentin. Dentin wafers with intact enamel borders were treated with one of several acid solutions containing iron. After exposure to aqueous sodium sulfide, color change was evaluated colorimetrically. X-ray photoelectron spectroscopy (XPS) and energy dispersive x-ray spectrometry (EDS) were used to probe surfaces for the presence of iron. Changes in surface morphology were evaluated by scanning electron microscopy (SEM). The results indicated that dentin treated by iron-containing solutions always discolored when the sodium-sulfide concentration exceeded 9.7 mM; variability in discoloration occurred below this level. XPS detected iron in enamel but not in dentin, while EDS detected iron in both enamel and dentin. Since XPS probes the upper atomic layers, these results indicate that acid-demineralized dentin absorbs iron; however, a water wash removes iron from the uppermost dentin surface, suggesting that the iron is physically absorbed. When exposed to sodium sulfide, at least some of the iron remaining in the bulk region is able to migrate to the surface to form iron-sulfide compounds. Based on SEM findings, discoloration is associated with the formation of a surface film on dentin.

Effect of high external pressures on the vibrational spectra of biomedical materials: calcium hydroxyapatite and calcium fluoroapatite.

Infrared and Raman spectra of the principal mineral component of human hard tissue, calcium hydroxyapatite, Ca10(PO4)6(OH)2, or HAP, and the analogous calcium fluoroapatite, Ca10(PO4)6F2, or FAP, have been recorded, using a diamond-anvil cell, at pressures ranging from ambient to 30 kbar. For FAP, the absence of any discontinuities in the slopes of the nu (cm-1) versus P (kbar) plots for the observed bands indicates that no pressure-induced structural transition occurs in this material throughout the pressure range investigated. For the internal vibrational modes of HAP, however, there are distinct breaks at approximately 20 kbar in the nu versus P plots, suggesting the occurrence of a structural change at this pressure. The OH stretching mode of HAP shifts to higher wave numbers with increasing pressure while the associated OH librational mode shifts in the opposite direction. The pressure-induced structural transition in HAP is reversible and occurs at approximately 22 kbar upon decompression. Further evidence for a structural change taking place at approximately 20 kbar was provided by a parallel pressure-tuning Raman study. Hydrogen bonding does not occur, or is very weak, in HAP.

Moisture-dependent renaturation of collagen in phosphoric acid etched human dentin.

We used atomic force microscopy (AFM) to investigate the effects of acidic and aqueous treatments on human dentin. Two basic points were determined: the first is the ability of AFM to discriminate the effect of phosphoric acid (pH approximately equal to 1) on polished dentin, and the second is the demonstrable effect of moisture on fibrous collagen structure. AFM images confirmed that the polishing process led to the removal of both smear layer and smear plugs. Our AFM study of undried dentin, which was then acid treated and kept moist, revealed substantial morphological changes at the dentin surface. Collagen fibers, having a characteristic periodicity of 67 nm, were imaged in situ for the first time; these structures were absent in dentin treated by phosphoric acid and subsequently vacuum dried, even after prolonged reimmersion in water. The AFM technique permitted us to demonstrate the important roles that moisture and etching play in the determination of the structure of collagen fibrils. Such structure may also play an important role in the diffusibility of subsequently applied dental adhesion systems.

Field emission in-lens SEM study of enamel and dentin.

This investigation used field emission in-lens scanning electron microscopy (FEISEM) for the study of tooth surfaces, with particular reference to adhesive bonding and acid conditioning. Dentin wafers with an intact enamel periphery were treated by either ethylenediaminetetraacetic acid (EDTA) (pH 7.4) or phosphoric acid (pH 0.7). The samples were then fixed, sequentially dehydrated in alcohol, and either air- or critical point-dried. After coating, surfaces were examined by FEISEM. For enamel, intraprismatic crystals were clearly recognizable, with the crystals showing both a longitudinal and parallel orientation to the long axis of the prisms. For dentin, the surface ultrastructure (mineral crystals and collagen banding) for the both untreated and treated samples was observed. Fine structures measuring on the order of 6 nm were also observed on samples treated by EDTA. We conclude that FEISEM can routinely provide high-resolution images of enamel and dentin, and that it has the capability of revealing the defined distribution of crystals and collagen fibers in dental tissues.