Coupled Nanomechanical and Raman Microspectroscopic Investigation of Human Third Molar DEJ.

The dentino-enamel junction (DEJ) connects enamel, that covers the outer surface of a tooth, to a thicker underlying dentin. The DEJ is a critical interface that permits joining these materials that have widely dissimilar mechanical properties. AFM-based nanoindentation and Raman microspectroscopy were used to define the width and composition of human molar DEJ. Indentation elastic modulus and hardness of enamel, dentin, and DEJ were determined along lines of indents made at 2 µm intervals across the DEJ. Indents made at maximum loads at each end of the indent lines were used to make visible markers allowing Raman microspectroscopy at 1 µm intervals across the DEJ, while using the nanoindent markers for orientation and location. Functional DEJ width estimates were made based on results from nanoindentation and Raman microspectroscopy. DEJ width estimates ranged from 4.7 (±1.2) µm to 6.1 (±1.9) µm based on hardness and 4.9 (±1.1) µm to 6.9 (±1.9) µm based on modulus. DEJ width based on Raman peak intensity variations were 8.0 (±3.2) µm to 8.5 (±3.1) µm based on the phosphate peak, and 7.6 (±3.2) µm to 8.0 (±2.6) µm for C-H stretching mode. These estimates are in the range of DEJ width estimates reported using nanoindentation.

Mechanical properties of mineralized collagen fibrils as influenced by demineralization.

Dentin and bone derive their mechanical properties from a complex arrangement of collagen type-I fibrils reinforced with nanocrystalline apatite mineral in extra- and intrafibrillar compartments. While mechanical properties have been determined for the bulk of the mineralized tissue, information on the mechanics of the individual fibril is limited. Here, atomic force microscopy was used on individual collagen fibrils to study structural and mechanical changes during acid etching. The characteristic 67 nm periodicity of gap zones was not observed on the mineralized fibril, but became apparent and increasingly pronounced with continuous demineralization. AFM-nanoindentation showed a decrease in modulus from 1.5 GPa to 50 MPa during acid etching of individual collagen fibrils and revealed that the modulus profile followed the axial periodicity. The nanomechanical data, Raman spectroscopy and SAXS support the hypothesis that intrafibrillar mineral etches at a substantially slower rate than the extrafibrillar mineral. These findings are relevant for understanding the biomechanics and design principles of calcified tissues derived from collagen matrices.

Effects of polar solvents on the fracture resistance of dentin: role of water hydration.

Although healthy dentin is invariably hydrated in vivo, from a perspective of examining the mechanisms of fracture in dentin, it is interesting to consider the role of water hydration. Furthermore, it is feasible that exposure to certain polar solvents, e.g., those found in clinical adhesives, can induce dehydration. In the present study, in vitro deformation and fracture experiments, the latter involving a resistance-curve (R-curve) approach (i.e., toughness evolution with crack extension), were conducted in order to assess changes in the constitutive and fracture behavior induced by three common solvents-acetone, ethanol and methanol. In addition, nanoindentation-based experiments were performed to evaluate the deformation behavior at the level of individual collagen fibers and ultraviolet Raman spectroscopy to evaluate changes in bonding. The results indicate a reversible effect of chemical dehydration, with increased fracture resistance, strength, and stiffness associated with lower hydrogen bonding ability of the solvent. These results are analyzed both in terms of intrinsic and extrinsic toughening phenomena to further understand the micromechanisms of fracture in dentin and the specific role of water hydration.

Amelogenin-guided crystal growth on fluoroapatite glass-ceramics.

The formation of aligned fibrous apatite crystals in enamel is predominantly attributed to the involvement of amelogenin proteins. We developed a model to study interactions of matrix proteins with apatite mineral in vitro and tested the hypothesis that amelogenin solubility affects the ability to induce protein-guided mineralization. Crystal growth experiments were performed on fluoroapatite (FAP) glass-ceramics in mineralizing solutions containing recombinant full-length amelogenin (rH174) at different concentrations. Using atomic force microscopy, we observed that mineral precipitated randomly on the substrate, but also formed thin layers (height, 10 nm) on FAP within 24 hrs. This growth pattern was unaffected when 0.4 mg/mL of rH174 was added. In contrast, crystals grew on FAP at a rate up to 20 times higher, at 1.6 mg/mL protein. Furthermore, nanospheres and mineral bound specifically to FAP and aligned in strings approximately parallel to the c-axis of FAP, leading us to the conclusion that amelogenin proteins indeed control direction and rate of growth of apatite in enamel.

Evaluation of a new modulus mapping technique to investigate microstructural features of human teeth.

Teeth contain several calcified tissues with junctions that provide interfaces between dissimilar tissues. These junctions have been difficult to characterize because of their small size. In this work a new technique using a combination of atomic force microscopy (AFM) and a force-displacement transducer was used to simultaneously study the surface topography and map mechanical properties of the junctions and adjacent hard tissues. Prepared specimens from human third molars were scanned by an AFM piezo-tube in contact mode. To measure the dynamic viscoelastic properties of the material a small sinusoidal force was superimposed on the contact force and the resulting displacement amplitude and the phase shift between the force and amplitude were measured. This force modulation technique was used to map the local variation of nanomechanical properties of intertubular dentin, peritubular dentin, enamel, dentin-enamel junction (DEJ) and peritubular-intertubular dentin junction (PIJ). This new technique allowed us to measure the widths of these junctions in addition to local variation in dentin and enamel without causing plastic deformation to the material and with 2 orders of magnitude increase in spatial resolution compared with previous studies that used discrete nanoindentation techniques. Due to the ability to analyze the sample line-by-line, the distribution functions associated with the width of the DEJ and PIJ were conveniently obtained for specific intratooth locations. The data suggested, for three third molar specimens, a DEJ width of 2-3 microm with full-width half-maximum (FWHM) of 0.7 microm and PIJ width of 0.5-1.0 microm with 0.3 microm FWHM. The intertubular dentin storage modulus variation was between 17 and 23 GPa with a mean value of 21 GPa. The range of storage modulus for enamel near the DEJ was between 51 and 74 GPa with a mean value of 63 GPa.

Micro-Raman spectroscopic investigation of dental calcified tissues.

The purpose of this study was to determine if dental calcified junctions (DEJs/CDJs) in human teeth contain different compositional phases compared to the adjacent dental calcified tissues. Peak positions and intensities were determined from micro-Raman spectra for PO(3-) (4) and the C--H modes and compared among the mineralized tissues and their junctions. Values of width were determined from the intersections of intensity regression lines through the junctions and in the adjacent tissues. The peaks were measured in 1-microm steps along a l00-microm line across the junction. High-resolution analysis revealed that PO(3-) (4) band peaks for dentin, the DEJ, enamel, the CDJ, and cementum were at the same position (959 cm(-1)), while for the C--H stretching mode a significant shift of 4.6 cm(-1) was found between enamel, the DEJ, and dentin. The mean width of the DEJ was 7.6 (+/- 2.8) microm using the PO(3-) (4) band and 8.6 (+/- 3.6) microm using the C--H stretching mode. Across the DEJ, the mineral content monotonically decreased from enamel to dentin while the organic component monotonically increased. The DEJ width was in agreement with prior nanoindentation studies. No width estimate was possible for the CDJ because the compositional differences between cementum and dentin were small.

Ultrastructure and nanomechanical properties of cementum dentin junction.

The attachment between cementum and dentin has been given several definitions and nomenclature, including: interzonal layer, intermediate cementum, collagen hiatus, Hopewell-Smith's hyaline layer, and more commonly, cementum-dentin junction (CDJ). Understanding the attachment of two structurally dissimilar hard tissues such as cementum and dentin defined by a junction may provide information necessary to engineer functionally graded materials that can be used for efficient tooth restorations in clinical dentistry and other bioengineering applications. Hence, in this study, as a first step toward understanding the CDJ using a biomechanical approach, it was hypothesized that the CDJ between cementum and dentin is a wide zone with mechanical properties significantly lower than the neighboring tissues. The structure of the CDJ was studied using an atomic force microscope (AFM), and site-specific mechanical response of the three regions; cementum, CDJ, and dentin were determined using an AFM-nanoindenter under dry and wet conditions. The AFM results of the CDJ demonstrated a valley under dry conditions and a peak under wet conditions. The magnitude of the depth of the valley was approximately the same as the height of the peak of the CDJ, ranging from 10 to 40 microm. The nanomechanical properties under dry conditions indicated no significant difference (p > 0.05) in elastic modulus and hardness of the CDJ (Er = 17.5 +/- 2.7 GPa, H = 0.6 +/- 0.1 GPa) and cementum (Er = 18.7 +/- 2.5 GPa, H = 0.6 +/- 0.1 GPa). The mechanical properties of the CDJ were significantly lower (p << 0.05) than dentin (Er = 19.9 +/- 2.9 GPa, H = 0.6 +/- 0.1 GPa) under dry conditions. However, under more relevant hydrated conditions, the mechanical properties of CDJ (Er 3.0 +/- 0.7 GPa, H = 0.1 +/- 0.0 GPa) were significantly lower (p << 0.05) than those of cementum (Er 6.8 +/- 1.9 GPa, H = 0.2 +/- 0.1 GPa) and dentin (Er 9.4 +/- 2.3 GPa, H = 0.3 +/- 0.1 GPa). Based on the results from this study, it can be concluded that the CDJ can be regarded as a wide zone containing large quantities of proteins including collagen that contribute to hydration and significantly reduce mechanical properties, compared with the adjacent hard tissues, cementum, and dentin. The lower mechanical properties of the CDJ may make it possible for it to redistribute occlusal loads to the alveolar bone.

Optical spectroscopy and imaging of the dentin-enamel junction in human third molars.

A 351-nm laser excitation source was used to perform autofluorescence microscopy of dentin, enamel, and the dentin-enamel junction (DEJ) to obtain information regarding their morphology and spectral characteristics. The emission spectra of these calcified dental tissues were different from one another, and this enabled the DEJ to be imaged and dimensionalized. The DEJ displayed sharp and clearly delineated borders at both its enamel and dentin margins. The dentinal tubules and the enamel prisms appeared to terminate abruptly at the DEJ. The median DEJ width was 10 microm, ranging from 7 to 15 microm, and it did not appear to depend on intratooth position.

The functional width of the dentino-enamel junction determined by AFM-based nanoscratching.

The dentino-enamel junction (DEJ) constitutes a structurally unique interphase uniting two mineralized tissues with very different matrix composition and physical properties. Its excellent biomechanical properties have drawn interest as a biomimetic model for joining dissimilar materials. In order to characterize the functional width of the DEJ, nanoscratching experiments were performed on human third molars. Friction coefficients of enamel, of dentin, and at the DEJ were obtained with a nanoscratch tester attached to an atomic force microscope (AFM). Normal loads in the range of 50 to 600 microN were applied to a spherical diamond indenter (r = 10 microm), which was driven 10 microm across the sample surface, recording the lateral force. Imaging with an AFM facilitated exact positioning of the scratches. The friction coefficient of intertubular dentin was 0.31 +/- 0.05, significantly above the coefficient of enamel of 0.14 +/- 0.02. The increased friction of dentin is attributed to the higher content of organic phases. Scratches performed across the interphase between enamel and dentin showed a sharp monotonic change in the friction coefficient. The average width of the slope between the friction coefficients of dentin and enamel was 2.0 +/- 1.1 microm and is assumed to represent the functional width of the dentino-enamel junction. The effect of the scalloped structure of the DEJ on its functional width as determined by mechanical testing is discussed.

Nanomechanical properties of hydrated carious human dentin.

Most restorative materials are bonded to caries-affected dentin that has altered structure. We tested the hypothesis that hydrated dentin of the transparent zone did not have increased hardness or elastic modulus. Nanoindentation by modified AFM was used to determine site-specific elastic modulus and hardness for components of hydrated dentin from 8 carious and non-carious human teeth. Indentations in intertubular dentin were made at intervals from pulp through the affected layers (subtransparent, transparent, and discolored zones). The values of intertubular dentin increased slightly from near the pulp into the transparent zone, then remained constant or decreased slightly through transparent dentin (E, 18.3 GPa; H, 0.8 GPa; confirming the hypothesis), and decreased markedly through the discolored region. Peritubular dentin values were unaltered in transparent dentin, and intratubular mineral had values between those of normal peritubular and intertubular dentin. Superficial areas contained distorted tubules without peritubular dentin or intratubular mineral.

Mechanical properties of human dental enamel on the nanometre scale.

Atomic force microscopy (AFM) combined with a nano-indentation technique was used to reveal the structure and to perform site-specific mechanical testing of the enamel of third molars. Nano-indentations (size<500 nm) were made in the cusp area to measure the mechanical properties of single enamel rods at different orientations. The influence of etching on the physical properties was studied and etching conditions that did not significantly alter the plastic-elastic response of enamel were defined. Elasticity and hardness were found to be a function of the microstructural texture. Mean Young's moduli of 87.5 (+/-2.2) and 72.2 (+/-4.5) GPa and mean hardness of 3.9+/-0.3 and 3.3+/-0.3 GPa were measured in directions parallel and perpendicular to the enamel rods, respectively. Analysis of variance showed that the differences were significant. The observed anisotropy of enamel is related to the alignment of fibre-like apatite crystals and the composite nature of enamel rods. Mechanical properties were also studied at different locations on single enamel rods. Compared to those in the head area of the rods, Young's moduli and hardness were lower in the tail area and in the inter-rod enamel, which can be attributed to changes in crystal orientation and the higher content of soft organic tissue in these areas.

Mechanical properties of the dentinoenamel junction: AFM studies of nanohardness, elastic modulus, and fracture.

The dentinoenamel junction (DEJ) is a complex and poorly defined structure that unites the brittle overlying enamel with the dentin that forms the bulk of the tooth. In addition, this structure appears to confer excellent toughness and crack deflecting properties to the tooth, and has drawn considerable interest as a biomimetic model of a structure uniting dissimilar materials. This work sought to characterize the nanomechanical properties in the region of the DEJ using modified AFM based nanoindentation to determine nanohardness and elastic modulus. Lines of indentations traversing the DEJ were made at 1-2 microm intervals from the dentin to enamel along three directions on polished sagittal sections from three third molars. Nanohardness and elastic modulus rose steadily across the DEJ from bulk dentin to enamel. DEJ width was estimated by local polynomial regression fits for each sample and location of the mechanical property curves for the data gradient from enamel to dentin, and gave a mean value of 11.8 microm, which did not vary significantly with intratooth location or among teeth. Nanoindentation was also used to initiate cracks in the DEJ region. In agreement with prior work, it was difficult to initiate cracks that traversed the DEJ, or to produce cracks in the dentin. The fracture toughness values for enamel of 0.6-0.9 MPa . m(1/2) were in good agreement with recent microindentation fracture results. Our results suggest that the DEJ displays a gradient in structure and that nanoindenation methods show promise for further understanding its structure and function.

Local mechanical and optical properties of normal and transparent root dentin.

The mechanical and optical properties of healthy and transparent root dentin are compared using atomic force microscopy (AFM), micro-Raman and emission spectroscopies and fluorescence microscopy. The elastic modulus and hardness of intertubular and peritubular transparent and healthy dentin did not differ appreciably. The tubule filling material in the transparent zone, however, exhibited values between peritubular and intertubular dentin. Raman spectroscopy revealed a shift in the 1066 cm(-1) band to 1072 cm(-1) from normal to transparent intertubular dentin. The material filling the tubule lumen in transparent dentin showed an increase in frequency of the band near 1070 cm(-1) as well. The emission spectral characteristics under 351 nm photoexcitation indicate differences between normal and transparent intertubular dentin. A transition region of about 300 microm between normal and transparent dentin was identified. In this region the intertubular emission properties were the same as for normal dentin, but tubules were filled. The filling material had emission characteristics closer to the normal intertubular than to transparent intertubular dentin.

H(2)O Outgassing Properties of Fumed and Precipitated Silica Particles by Temperature-Programmed Desorption.

Temperature-programmed desorption was performed at temperatures up to 850 K on as-received fumed and precipitated silica particles. Physisorbed water molecules on both types of silica had activation energies in the range of 38-61 kJ/mol. However, the activation energies of desorption for chemisorbed water varied from approximately 80 to >247 kJ/mol for fumed silica, Cab-O-Sil-M-7D, and approximately 96 to 155 kJ/mol for precipitated silica, Hi-Sil-233. Our results suggest that physisorbed water can be effectively pumped away at room temperature (or preferably at 320 K) in a matter of hours. Chemisorbed water with high activation energies of desorption (>126 kJ/mol) will not escape silica surfaces in 100 years even at 320 K, while a significant amount of the chemisorbed water with medium activation energies (80-109 kJ/mol) will leave the silica surfaces in that time span. Most of the chemisorbed water with activation energies <126 kJ/mol can be pumped away in a matter of days in a good vacuum environment at 500 K. We had previously measured about 0.1-0.4 wt% of water in silica-reinforced polysiloxane formulations containing approximately 21% Cab-O-Sil-M-7D and approximately 4% Hi-Sil-233. Comparing present results with these formulations, we conclude that the adsorbed H(2)O and the Si-OH bonds on the silica surfaces are the major contributors to water outgassing from these types of silica-filled polymers. Copyright 2000 Academic Press.

Three-dimensional morphometry of the L6 vertebra in the ovariectomized rat model of osteoporosis: biomechanical implications.

This article summarizes the results of a three-dimensional study of changes in the morphology of the L6 rat vertebra at 120 days after ovariectomy (OVX), with estrogen replacement therapy used as a positive control. Synchrotron radiation microtomography was used to quantify the structural parameters defining trabecular bone architecture, while finite-element methods were used to explore the relationships between these parameters and the compressive elastic behavior of the vertebrae. There was a 22% decrease in trabecular bone volume (TBV) and a 19% decline in mean trabecular thickness (Tb.Th) with OVX. This was accompanied by a 150% increase in trabecular connectivity, a result of the perforation of trabecular plates. Finite-element analysis of the trabecular bone removed from the cortical shell showed a 37% decline in the Young's modulus in compression after OVX with no appreciable change in the estrogen-treated group. The intact vertebrae (containing its trabecular bone) exhibited a 15% decrease in modulus with OVX, but this decline lacked statistical significance. OVX-induced changes in the trabecular architecture were different from those that have been observed in the proximal tibia. This difference was a consequence of the much more platelike structure of the trabecular bone in the vertebra.

A micromechanics model of the elastic properties of human dentine.

A generalized, self-consistent model of cylindrical inclusions in a homogeneous and isotropic matrix phase was used to study the effects of tubule orientation on the elastic properties of dentine. Closed-form expressions for the five independent elastic constants of dentine were derived in terms of tubule concentration, and the Young's moduli and Poisson ratios of peri- and intertubular dentine. An atomic-force microscope indentation technique determined the Young's moduli of the peri- and intertubular dentine as approx. 30 and 15 GPa, respectively. Over the natural variation in tubule density found in dentine, there was only a slight variation in the axial and transverse shear moduli with position in the tooth, and there was no measurable effect of tubule orientation. It was concluded that tubule orientation has no appreciable effect on the elastic behaviour of normal dentine, and that the elastic properties of healthy dentine can be modelled as an isotropic continuum with a Young's modulus of approx. 16 GPa and a shear modulus of 6.2 GPa.

Effect of citric acid concentration on dentin demineralization, dehydration, and rehydration: atomic force microscopy study.

Most current dentin bonding procedures use acid etchants to partially demineralize the dentin structure and provide pathways for resin infiltration. This study determined the recession rates of peritubular dentin and intertubular dentin as a function of pH during demineralization in citric acid solutions (0.0005-2.5M) and the effects of dehydration and rehydration on the partially demineralized dentin. Polished dentin disks were prepared with an internal reference layer and were studied at specific intervals for citric acid etching between pH 1 and 3.4 in an atomic force microscope. Peritubular dentin etched rapidly and linearly with time until it could no longer be measured. The intertubular surface began etching at nearly the same rate, but then recession slowed for all concentrations and stabilized after recession of less than 1 microm for all but the pH 1 solution. The decrease in recession was attributed to the limitation of contraction of the demineralized collagen scaffold as long as it remained hydrated. Dehydration following etching resulted in significant collapse of the surface, changes in roughness, and a slight decrease in tubule diameter for samples etched for 30 min. Measurements could not be made of the collapse for low pH samples, because shrinkage stresses disrupted the integrity of the reference layer. On rehydration, the dehydrated surfaces underwent an expansion up to the level seen after etching and tubule diameters returned to the etched values. These results indicate that the collapse of demineralized matrix is almost totally recoverable on rehydration.

Viscoelastic properties of demineralized human dentin measured in water with atomic force microscope (AFM)-based indentation.

Using an atomic force microscope (AFM) with an attachment specifically designed for indentation, we measured the mechanical properties of demineralized human dentin under three conditions: in water, in air after desiccation, and in water after rehydration. The static elastic modulus (E(h)r = 134 kPa) and viscoelastic responses (tau(epsilon) = 5.1 s and tau(sigma) = 6.6 s) of the hydrated, demineralized collagen scaffolding were determined from the standard linear solid model of viscoelasticity. No significant variation of these properties was observed with location. On desiccation, the samples showed considerably larger elastic moduli (2 GPa), and a hardness value of 0.2 GPa was measured. Upon rehydration the elastic modulus decreased but did not fully recover to the value prior to dehydration (381 kPa).

Dentin demineralization: effects of dentin depth, pH and different acids.

OBJECTIVES: This investigation sought to determine: 1) if dentin demineralization rates are proportional to acid concentration for demineralization in phosphoric acid (10% or 1.76 M, 0.025 M, 0.0001 M, with pH = 0.95, 2.0, 4.0 respectively); 2) if the etching characteristics are independent of dentin depth; and 3) if the etching characteristics for phosphoric acid were comparable to those for citric acid over a similar pH range. METHODS: Highly polished dentin disks from freshly extracted, non-carious, third molars were prepared with a reference layer. Samples were prepared from either superficial or deep coronal dentin. The samples were etched for periods of up to 30 min using phosphoric acid solutions (pH = 0.95, 2.0, 4.0) in a wet cell of an atomic force microscope (AFM). Depth changes with respect to the reference layer were determined for the intertubular and peritubular dentin to quantify structural changes. The results were compared with similar studies using citric acid (pH = 1.0, 2.15 or 3.4). Etching characteristics were statistically compared using 2-way repeated measures ANOVA at p < 0.05 and the Tukey's multiple comparison test. RESULTS: The relation between time and recession for peritubular dentin was initially linear. The intertubular dentin recession started rapidly but then reached a plateau within a very short interval for etching solutions at pH = 2.0 and 4.0. At the highest concentration, the recession decreased with time, but a clear plateau was not established. There was no statistical difference between peritubular etching rates of superficial and deep dentin surfaces with phosphoric acid at any concentration. There was also no difference in the intertubular dentin recession at the location of the plateau that depended on dentin depth. Etching rates increased dramatically with decreased pH for both phosphoric and citric acids, but were higher for citric acid than for phosphoric acid. SIGNIFICANCE: The AFM allowed quantification of changes during etching of wet dentin. Peritubular dentin etching rates increased with decreasing pH, as expected, but changes were not linear and were different for the two acids studied over a similar pH range. Intertubular dentin surface recession was small and plateaued for low concentrations. The peritubular etching rate and intertubular dentin recession did not depend on dentin depth.

The dentin substrate: structure and properties related to bonding.

OBJECTIVES: Dentin is a vital, hydrated composite material with structural components and properties that vary with location. These variations are reviewed along with alterations by physiological and pathological changes that allow classification into various forms of dentin. Structural characteristics and mechanical properties are reviewed and the limitations of our understanding of structure-property relationships for normal and modified forms of dentin are discussed with respect to their impact on dentin bonding. Recent progress in methods available to study dentin and its demineralization are emphasized with their promise to increase our understanding of dentin properties and structure. DATA SOURCES: Recent microstructural studies, focusing on scanning electron microscopy, atomic force microscopy and X-ray tomographic microscopy are included. A review of fundamental studies with emphasis on microstructurally sensitive methods, and prior reviews of basic mechanical properties are included with discussion of their correlation to composition and structure. STUDY SELECTION AND CONCLUSIONS: Emphasis in this work was placed on the major structural components of the tissue, including the collagen based organic matrix and its mineral reinforcement, the distribution of these components and their microstructural organization as related to mechanical properties and response to demineralization. Little information is included on biochemical and developmental studies or on non-collagenous proteins and other organic components for which limited understanding is available with respect to their role in structure-property relations and influence on bonding. In spite of the fact that the complexity of dentin precluded a comprehensive review, it is clear that local structural variations influence properties and impact nearly all preventive and restorative dental treatments. Much more work is needed in order to understand differences between vital and non-vital dentin, and dentin from extracted teeth. Although our knowledge is rudimentary in certain areas, increasingly sophisticated methods of studying dentin should provide the necessary information to model structure-property relations, optimize dentin bonding, and improve many aspects of preventive and restorative dentistry.