[Micro-tensile bond strength to sclerotic dentin in non-carious cervical lesions]

OBJECTIVE

This in vitro study was to evaluate the micro-tensile bond strength (microTBS) of three adhesives to sclerotic dentin in non-carious cervical lesions.

METHODS

The maxillary premolars extracted due to periodontitis and with non-carious cervical lesions were collected. The non-carious, natural cervical sclerotic lesions were bonded with a total-etching adhesive Scotchbond Multi-Purpose, a two-step self-etching adhesive Contax, and an all-in-one self-etching adhesive Adper Prompt L-Pop. Artificially prepared wedge-shaped lesions were also made in sound premolars and bonded with the same adhesives as the controls. MicroTBS of these three adhesives was measured.

RESULTS

MicroTBS of Scotchbond and Contax to sclerotic dentin was significantly lower than to normal dentin. But microTBS of Adper Prompt L-Pop to normal dentin was significantly lower than to sclerotic dentin. MicroTBS to sclerotic dentin was Scotchbond 46.805 MPa, Adper Prompt L-Pop 39.045 MPa, and Contax 29.852 MPa.

CONCLUSIONS

In sclerotic dentin the microTBS was decreased because of the inferior micro-morphology of resin tags. Adhesives with low pH value might bond to sclerotic dentin effectively.

Part one: The restoration of non-vital teeth: structural, biological, and micromechanical issues in maintaining tooth longevity

This manuscript will review research from 1967 to the present to find the best evidence for the reconstruction of non-vital teeth. The paper will review the contention that non-vital teeth are "more brittle" by analyzing the relevant physical properties of vital versus non-vital teeth; describe the structures of the tooth used to manage stress and strain; describe the formation of dentinal cracks and propagation of fracture planes through dentin; and analyze forces placed on human teeth and their effects on the tooth in Part One. In Part Two, the paper will describe the strength of various dowel and core designs relative to strength, retention, and durability and describe the results of testing various dentin bonding materials in strength and retention of dowels in non-vital teeth. The review will conclude with recommendations as to the materials and techniques in specific clinical situations best capable of single tooth reconstruction to ensure tooth longevity together with the scientific basis for their adoption.

Load-induced stresses in photoelastic primary canines with facial restorations

PURPOSE

The purpose of this study was to photoelastically compare the stresses generated by loads on primary canines with facial restorations of different stiffness.

METHODS

Composite photoelastic models of a typical maxillary primary canine were fabricated using individual simulant materials for enamel, dentin, periodontal ligament, and alveolar bone. Models were made with identical facial preparations included either near the cementoenamel junction or at 2 mm incisally. The model teeth were restored using the following materials of disparate elastic moduli: (1) high modulus, hybrid composite (Herculite XRV); and (2) lower modulus compomer (Dyract). Three replications of each type of restored tooth were fabricated. Simulated masticatory forces were applied on the cusp tip and the cingulum of each tooth model. The resulting stress patterns were observed and recorded photographically in the field of a circular polariscope.

RESULTS

Prior to load application, similar low-level, polymerization-induced shrinkage stresses were localized at the preparation margins of all models. Under both incisal and cingular loading, the higher modulus hybrid composite tended to concentrate stress along the gingival and proximal margins more than did the lower modulus compomer. This effect was more pronounced with cingular than with incisal loading. For both preparations and restoratives, higher stresses were produced by the cingulum loading. No significant differences were observed within each group of replicated restored models.

CONCLUSION

These results suggest that, regardless of preparation height, lower modulus compomer restoration of facial lesions in primary maxillary canines may reduce stress production by occlusal forces.

Light guiding in biological tissue due to scattering

For a description of light propagation in biological tissue it is usually assumed that tissue is a random medium. We report a pronounced light guiding effect in cubes of human dentin that cannot be described by this standard model. Monte Carlo simulations which consider the microstructure of dentin are performed and successfully compared to experiments. Contrary to explanations so far, we show that light guiding is due to scattering by the tissue's microstructure. Exploiting this concept, light can be guided in arbitrary directions or locations without involving reflections or wave effects.

Shear stress facilitates tissue-engineered odontogenesis

Numerous studies have demonstrated the effect of shear stress on osteoblasts, but its effect on odontogenic cells has never been reported. In this study, we focused on the effect of shear stress on facilitating tissue-engineered odontogenesis by dissociated single cells. Cells were harvested from the porcine third molar tooth at the early stage of crown formation, and the isolated heterogeneous cells were seeded on a biodegradable polyglycolic acid fiber mesh. Then, cell-polymer constructs with and without exposure to shear stress were evaluated by in vitro and in vivo studies. In in vitro studies, the expression of both epithelial and mesenchymal odontogenic-related mRNAs was significantly enhanced by shear stress for 2 h. At 12 h after exposure to shear stress, the expression of amelogenin, bone sialoprotein and vimentin protein was significantly enhanced compared with that of control. Moreover, after 7 days, alkaline phosphatase activity exhibited a significant increase without any significant effect on cell proliferation in vitro. In vivo, enamel and dentin tissues formed after 15 weeks of in vivo implantation in constructs exposure to in vitro shear stress for 12 h. Such was not the case in controls. We concluded that shear stress facilitates odontogenic cell differentiation in vitro as well as the process of tooth tissue engineering in vivo.

Age-related changes in hardness and modulus of elasticity of dentine

OBJECTIVES

Little knowledge has been clarified about the relationship between the morphological and physical changes of dentine during aging. The purpose of this study was to clarify the modulus of elasticity and hardness related to the morphological changes of dentine by aging using a transmitted light microscope (TLM) and a nano-hardness tester (NHT).

METHODS

Aged human molars and young third molars were used. The dentine morphology was observed under a TLM. The hardness and Young's modulus of elasticity related to the morphologic study were evaluated with an NHT.

RESULTS

The thickness of mantle dentine and globular dentine of aged teeth were less than that of young teeth. Transparent dentine was observed only underneath the attrition of young teeth. Reactionary tertiary dentine formed and a "dark zone" was found at the junction between physiologic secondary and reactionary dentine only in aged teeth. At the mantle dentine, hardness and modulus of elasticity of aged dentine were higher than those of young dentine. The reactionary dentine in aged teeth and newly developed secondary dentine in young teeth demonstrated lower modulus of elasticity and hardness than those of other circumpulpal dentine. Relatively low modulus of elasticity and hardness were observed at the zone between secondary and reactionary dentine.

CONCLUSIONS

Changes in dentine due to aging resulted in transformation of morphological features causing changes to their hardness and modulus of elasticity at the explicit areas such as the increase of hardness and modulus of elasticity at mantle dentin and the reduction of these properties at the "dark zone" that found in aged teeth.

Phase function measurements on nonspherical scatterers using a two-axis goniometer

We present a two-axis goniometer for measuring the phase function of scattering media with an angular resolution of about 0.2 deg having 12 decades of dynamic range and covering almost the full solid angle. The setup is evaluated with polystyrene spheres and with perpendicularly and obliquely illuminated thin glass cylinders. The scattering pattern and its intensity distribution are in excellent agreement with analytical theory. A multiple scattering configuration composed of two parallel cylinders is also examined. Finally, the phase function of dentin slabs is measured and its dependence on the dental microstructure is discussed.

Early caries imaging and monitoring with near-infrared light

Enamel is highly transparent in the near infrared (NIR); therefore, this region of the electromagnetic spectrum is suited ideally for the development of new optical diagnostic tools for the detection and imaging of early dental caries. This article discusses the NIR optical properties of sound and demineralized dental enamel and the potential use of polarization sensitive optical coherence tomography and NIR transillumination for the imaging of dental caries.

Factorial analysis of variables influencing stress in all-ceramic crowns

OBJECTIVE

The objective of this investigation was to evaluate the relative contribution of variables in the crown-cement-tooth system that can influence magnitude of maximum principal stress in all-ceramic crowns. METHODS.: Factorial analysis was performed to calculate the coefficients of main and interactive effects of seven variables on maximum stress distribution in a series of finite element models of an axisymmetric stylized ceramic crown-cement-tooth system. Variables investigated at two levels were selected for their clinical relevance and included those relating to crown material and geometry (thickness and cuspal incline), cement modulus and thickness, supporting tooth core, and position at which the occlusal load was applied.

RESULTS

The average principal stress for all combinations of all variables was 135.1 MPa but stress magnitude ranged from 73.7 to 214.0 MPa. Crown material and thickness are of primary importance in stress magnitude but other variables (cement modulus, load position, and supporting tooth core) also contribute to the stress magnitude. Interactions between these variables can have an important influence, particularly since the stress in the crown is not necessarily sensitive to the same factors for all crown material systems.

SIGNIFICANCE

Comprehensive evaluation of stress in a crown of a crown-cement-tooth system must integrate the influence of single variables and their interactions.

Fibromodulin-deficient Mice Display Impaired Collagen Fibrillogenesis in Predentin as Well as Altered Dentin Mineralization and Enamel Formation

To determine the functions of fibromodulin (Fmod), a small leucine-rich keratan sulfate proteoglycan in tooth formation, we investigated the distribution of Fmod in dental tissues by immunohistochemistry and characterized the dental phenotype of 1-day-old Fmod-deficient mice using light and transmission electron microscopy. Immunohistochemistry was also used to compare the relative protein expression of dentin sialoprotein (DSP), dentin matrix protein-1 (DMP 1), bone sialoprotein (BSP), and osteopontin (OPN) between Fmod-deficient mice and wild-type mice. In normal mice and rats, Fmod immunostaining was mostly detected in the distal cell bodies of odontoblasts and in the stratum intermedium and was weaker in odontoblast processes and predentin. The absence of Fmod impaired dentin mineralization, increased the diameter of the collagen fibrils throughout the whole predentin, and delayed enamel formation. Immunohistochemistry provides evidence for compensatory mechanisms in Fmod-deficient mice. Staining for DSP and OPN was decreased in molars, whereas DMP 1 and BSP were enhanced. In the incisors, labeling for DSP, DMP 1, and BSP was strongly increased in the pulp and odontoblasts, whereas OPN staining was decreased. Positive staining was also seen for DMP 1 and BSP in secretory ameloblasts. Together these studies indicate that Fmod restricts collagen fibrillogenesis in predentin while promoting dentin mineralization and the early stages of enamel formation. (J Histochem Cytochem 54:525-537, 2006)

Early evolution of vertebrate skeletal tissues and cellular interactions, and the canalization of skeletal development

The stratigraphically earliest and the most primitive examples of vertebrate skeletal mineralization belong to lineages that are entirely extinct. Therefore, palaeontology offers a singular opportunity to address the patterns and mechanisms of evolution in the vertebrate mineralized skeleton. We test the two leading hypotheses for the emergence of the four skeletal tissue types (bone, dentine, enamel, cartilage) that define the present state of skeletal tissue diversity in vertebrates. Although primitive vertebrate skeletons demonstrate a broad range of tissues that are difficult to classify, the first hypothesis maintains that the four skeletal tissue types emerged early in vertebrate phylogeny and that the full spectrum of vertebrate skeletal tissue diversity is explained by the traditional classification system. The opposing hypothesis suggests that the early evolution of the mineralized vertebrate skeleton was a time of plasticity and that the four tissue types did not emerge until later. On the basis of a considerable, and expanding, palaeontological dataset, we track the stratigraphic and phylogenetic histories of vertebrate skeletal tissues. With a cladistic perspective, we present findings that differ substantially from long-standing models of tissue evolution. Despite a greater diversity of skeletal tissues early in vertebrate phylogeny, our synthesis finds that bone, dentine, enamel and cartilage do appear to account for the full extent of this variation and do appear to be fundamentally distinct from their first inceptions, although why a higher diversity of tissue structural grades exists within these types early in vertebrate phylogeny is a question that remains to be addressed. Citing recent evidence that presents a correlation between duplication events in secretory calcium-binding phosphoproteins (SCPPs) and the structural complexity of mineralized tissues, we suggest that the high diversity of skeletal tissues early in vertebrate phylogeny may result from a low diversity of SCPPs and a corresponding lack of constraints on the mineralization of these tissues.

Nanoindentation hardness of mineralized tissues

A series elastic and plastic deformation model [Sakai, M., 1999. The Meyer hardness: a measure for plasticity? Journal of Materials Research 14(9), 3630-3639] is used to deconvolute the resistance to plastic deformation from the plane strain modulus and contact hardness parameters obtained in a nanoindentation test. Different functional dependencies of contact hardness on the plane strain modulus are examined. Plastic deformation resistance values are computed from the modulus and contact hardness for engineering materials and mineralized tissues. Elastic modulus and plastic deformation resistance parameters are used to calculate elastic and plastic deformation components, and to examine the partitioning of indentation deformation between elastic and plastic. Both the numerical values of plastic deformation resistance and the direct computation of deformation partitioning reveal the intermediate mechanical responses of mineralized composites when compared with homogeneous engineering materials.

Finite element analysis of stress distribution of 2 different tooth preparation designs in porcelain-fused-to-metal crowns

PURPOSE

The aim of this clinical simulation study was to investigate the effect of anatomic and nonanatomic occlusal preparation design on stress distribution in different metal-ceramic crowns and tooth and bone.

MATERIALS AND METHODS

For the finite element analysis method, a 2-dimensional mathematical model of a mandibular second premolar tooth and its supporting tissues was used. The analysis was performed by using a structural analysis program. Four groups were designed: gold-palladium alloy/anatomic occlusal preparation (Au-Pd/A), Au-Pd alloy/nonanatomic (flat) occlusal preparation (Au-Pd/N), nickel-chromium alloy/anatomic occlusal preparation (Ni-Cr/A), and Ni-Cr alloy/nonanatomic occlusal preparation (Ni-Cr/N). A distributed type load of 400 N (total) was applied to the centric stop points on the tip of the buccal cusp and on the central developmental groove in centric occlusion to all types of restorations.

RESULTS

The results demonstrated that shear stresses in the dentin tissues and restorations in Au-Pd/A and Ni-Cr/A were similar. The shear stresses within the restorations in Au-Pd/N and Ni-Cr/N were similar.

CONCLUSION

Anatomic occlusal preparation designs were advantageous in stress distribution in the dentin tissue. Nonanatomic occlusal preparation designs were found to be advantageous in the stress amount and distribution in the porcelain structure. Occlusal preparation designs and restorative materials showed no differences in stress distribution and amount in the pulp tissue and bone tissues.

An investigation of the shear bond strength of compomer restorative material to enamel and dentine

The aim of this study was to investigate the effect of one step adhesives on the shear bond strength of a compomer restorative material to both enamel and dentine. Human extracted teeth were used for the study. Ten samples were prepared for both enamel and dentine specimens for each of the five groups: Tooth, no etch, Prime and Bond NT (P+B NT); tooth, Non-Rinse Conditioner (NRC), P+B NT; tooth, NRC, Prime and Bond 2.1 (P+B 2.1); tooth, etch, P+B NT; tooth, etch, P+B 2.1. The specimens were subjected to bond testing. The shear bond strength was measured using an Inston 1193 testing machine using a cross head speed of 1 mm/minute. The specimens were tested to destruction. The results show that for the enamel specimens the highest bond strength was recorded for those specimens subjected to Etch, P+B 2.1 (22.1 MPa) and Etch P+B NT (20.0 MPa). The groups of specimens which did not undergo etching had very low bond strengths ranging from 11.4 MPa for NRC, P+B 2.1, 8.5 MPa for NRC P+B NT to 6.9 MPa for P+B NT. For the dentine specimens, for all of the groups, the shear bond strengths were low. Those groups subjected to etching produced the highest values of 7.9 MPa for NRC P+B 2.1 with the lowest value of 6.1 MPa for NRC P+B NT. These bond strengths were significantly lower than those achieved for bonding to enamel. Prime and Bond NT and Prime and Bond 2.1, used in conjunction with acid etching, produce satisfactory bond strengths of compomer restorative material to enamel. Bond strengths to dentine were low.

Nanoindentation study of human premolars subjected to bleaching agent

Bleaching of teeth is gaining popularity due to cosmetic reasons. However, the effect it has on teeth is still largely unknown. This paper seeks to evaluate the effect of a bleaching agent, 30% hydrogen peroxide, on the nanomechanical properties of dentin and enamel using the nanoindentation technique. The Young's modulus and hardness obtained from nanoindentation before and after bleaching were compared. Five newly extracted human premolars were used. Nanoindentation was first done on the sliced enamel and dentin regions to determine their mechanical properties. One batch of samples was kept in Hank's balanced salt solution as control while the other was bleached in 30% hydrogen peroxide for 24h. The same number of nanoindentations was then done near the previously indented regions for both the control and bleached samples and the results compared. Using paired sample t-tests with alpha=0.05, it was found that there were no significant differences in both the Young's modulus and hardness of dentin and enamel kept in control. However, the mechanical properties of the bleached dentin were significantly decreased. For intertubular dentin, the mean hardness decreased by 29-55% and the mean Young's modulus decreased by 19-43%. For enamel, the mean hardness decreased by 13-32% while the mean Young's modulus decreased by 18-32%. The exact mechanism by which hydrogen peroxide affects the dentin and enamel has yet to be fully elucidated. However, it is observed to have an undermining effect on the nanomechanical properties of teeth.

Thermographic investigation of contemporary resin-containing dental materials

OBJECTIVES

To measure the temperature rise induced during visible light curing of modern resin-containing dental materials and the effect of dentine sections in reducing this temperature rise.

METHODS

A variety of newly introduced resin-containing materials were investigated, including flowable, packable and conventional hybrid composites, as well as a compomer and a resin modified glass ionomer material. The resin was packed into polytetrafluoroethylene (PTFE) moulds and cured for 40s. Temperature rises on the undersurface of the curing resin were measured using the Thermovision 900 infra-red scanning system. In the second part of the study, extracted, caries free teeth were sectioned into dentine disks of three thicknesses (0.7, 1.4 and 1.9 mm). Composite samples were overlaid by the disks and the insulating effect of dentine measured.

RESULTS

The maximum temperature increases were: 43.1 degrees C (flowable composite), 32.8 degrees C (conventional composite), 32.8 degrees C (RMGI), 23.3 degrees C (compomer) and 22.4 degrees C (packable composite).

CONCLUSIONS

There was a quantifiable amount of heat generated in resin-containing material during light curing. Dentine sections were good thermal insulators that significantly reduced temperature rises associated with resin composite photocuring.

The effect of EmdogainR on ectopic bone formation in tubes of rat demineralized dentin matrix

BACKGROUND

Emdogain (EMD) is made from enamel matrix proteins (EMPs) from the tooth germ of swine and propylene glycol alginate (PGA) as a matrix. The function of EMD is known to differentiate cells of the dental follicle into cementoblasts. However, little is known about the effect of EMD on mesenchymal cells in other tissue.

OBJECTIVE

The purpose of this study was to investigate whether EMD has the ability to induce hard tissue when applied with or without demineralized dentin matrix.

METHODS

Half of the dentin tubes prepared from rat incisors were demineralized by treatment with 0.6 N hydrochloric acid for 3 h. EMD or PGA was injected into the demineralized or non-demineralized dentin tubes, which were then transplanted into rectus abdominis muscles. Untreated dentin tubes were also transplanted as a control. Animals were killed at 7, 14 and 21 days after the implantation.

RESULTS

Non-demineralized dentin tubes with or without EMD or PGA did not form any hard tissue. In the demineralized group, chondrogenesis in the PGA groups occurred earlier than in the EMD groups. The expression of vascular endothelial growth factor (VEGF) mRNA in the demineralized group with PGA at day 14 was the highest. The expression of osteopontin and osteocalcin mRNAs was higher in all groups at 21 days compared with 7 or 14 days.

CONCLUSION

These results suggest that neither EMD nor PGA has the ability to induce hard tissue and that EMPs contained within EMD might aggregate on the dentin surface and inhibit the effect of the demineralized dentin matrix.

Ultrastructural examination of dentin using focused ion-beam cross-sectioning and transmission electron microscopy

Focused ion-beam (FIB) milling is a commonly used technique for transmission electron microscopy (TEM) sample preparation of inorganic materials. In this study, we seek to evaluate the FIB as a TEM preparation tool for human dentin. Two particular problems involving dentin, a structural analog of bone that makes up the bulk of the human tooth, are examined. Firstly, the process of aging is studied through an investigation of the mineralization in 'transparent' dentin, which is formed naturally due to the filling up of dentinal tubules with large mineral crystals. Next, the process of fracture is examined to evaluate incipient events that occur at the collagen fiber level. For both these cases, FIB-milling was able to generate high-quality specimens that could be used for subsequent TEM examination. The changes in the mineralization suggested a simple mechanism of mineral 'dissolution and reprecipitation', while examination of the collagen revealed incipient damage in the form of voids within the collagen fibers. These studies help shed light on the process of aging and fracture of mineralized tissues and are useful steps in developing a framework for understanding such processes.

Effect of polymerization modes and resin composite on the temperature rise of human dentin of different thicknesses: an in vitro study

This in vitro study evaluated the effect of different polymerization modes and the presence of resin composite on the temperature rise (TR) in human dentin of different thicknesses. For this purpose, 90 specimens were assigned to 30 groups (n=3): five polymerization modes (1-conventional; 2-soft-start; 3-high intensity; 4-ramp cure: progressive and high intensity; 5-high intensity with the tip of the light cure at a distance of 1.3 cm for 10 seconds and the tip leaned in the sample); two levels of resin composite presence (absence or presence of resin composite) and three dentin thicknesses (1, 2, 3 mm). During polymerization, temperature was measured by a digital laser thermometer (CMSS2000-SL/SKF). Three-way ANOVA and Tukey tests were performed. There were statistical differences in TR among polymerization modes, presence of resin composite and dentin thicknesses. Within the limits of this study, it can be concluded that 1) conventional and high intensity polymerization modes presented lower TR means, and it was statistically different from soft start, distanced tip and ramp curing polymerization modes; 2) the presence of resin composite showed a statistically significant reduction TR means and 3) the thicker the dentin, the less the temperature rise.

Pressure transmission across dentine in response to an external pressure: A laboratory study

PURPOSE

To determine whether transducers could provide a convenient method of measuring dentine permeability and to investigate the effects of removal of pulp tissue and perfusion time on the pressure increase across human dentine after low-pressure perfusion.

METHOD

Human premolar teeth that had been stored for 1-2 months were prepared for full crown preparations. The pulp tissue was removed from half the samples and all the coronal segments perfused with saline at 1.3kPa for varying times. An external pressure was then applied and the response within the pulp chambers recorded with transducers.

RESULTS

Extirpation of pulp tissue reduced the rate of rise of pressure inside the pulp chamber by approximately 50%. Perfusion at 1.3kPa for up to 1h had no effect on pressure rise, but 6h of perfusion produced a very significant increase.

CONCLUSION

Transducers can provide a relatively simple, convenient and clinically relevant measurement of dentine permeability. For stored coronal segments that have been perfused at physiological pressures, extirpation of pulp tissue and perfusion time have significant effects on pressure transmission across dentine.

Elastic anisotropy of bone and dentitional tissues

The calculation of the scalar compressive and shear anisotropy factors developed for single crystal refractory compounds has been adapted to the anisotropic elastic stiffness coefficients determined by a number of ultrasonic measurements of bone based on transverse isotropic symmetry. Later, this work was extended to include the ultrasonic measurements of bone based on orthotropic symmetry. Recently, the five transverse isotropic elastic constants for both wet and dry human dentin were determined using resonant ultrasound spectroscopy. The five transverse isotropic elastic constants for wet bovine enamel and dentin had been calculated based on modeling of ultrasonic wave propagation measurements and related data in the literature. The scalar compressive and shear anisotropy factors have been calculated from both these sets of data and are compared with a representative set from those published previously for both human and bovine bone and both fluoro- and hydroxyl-apatites.

Histological and immunohistochemical studies of tissue engineered odontogenesis

The successful regeneration of complex tooth structures based on tissue-engineering principles was recently reported. The process of this regeneration, however, remains poorly characterized. In this study, we have used histochemistry to examine the regeneration process of tissue engineered teeth in order to determine the cell types that give rise to these engineered tooth structures. Porcine third molar tooth buds were dissociated into single-cell suspensions and seeded onto a biodegradable polyglycolic acid polymer scaffold. Following varying periods of growth in rat hosts, the specimens were evaluated by histology and immunohistochemistry. Aggregates of epithelial cells were first observed 4-6 weeks after implantation. These aggregates assumed three different shapes: a natural tooth germ-like shape, a circular shape, or a bilayer-bundle. Based on the structure of the stellate reticulum in the dental epithelium, the circular and bilayer-bundle aggregates could be clearly classified into two types: one with extensively developed stellate reticulum, and the other with negligible stellate reticulum. The epithelial cells in the circular aggregates differentiated into ameloblasts. The continuous bilayer bundles eventually formed the epithelial sheath, and dentin tissue was evident at the apex of these bundles. Finally, enamel-covered dentin and cementum-covered dentin formed, a process most likely mediated by epithelial-mesenchymal interaction. These results suggest that the development of these engineered teeth closely parallels that of natural odontogenesis derived from the immature epithelial and mesenchymal cells.

Shear bond strength of adhesive systems to enamel and dentin. Thermocycling influence

OBJECTIVES

The purpose of the this study was to evaluate the influence of thermocycling on shear bond strength on bovine enamel and dentin surfaces of different adhesive systems.

METHODS

Thirty sound bovine incisors were sectioned in mesiodistal and inciso-cervical direction obtaining 60 incisal surfaces (enamel) and 60 cervical surfaces (dentin). Specimens were randomly assigned to 3 groups of equal size (n=40), according to the adhesive system used: I-Single Bond; II-Prime & Bond NT/NRC; III-One Coat Bond. After 24-h storage in distilled water at 37 degrees C, each main group was divided into two subgroups: A-specimens tested after 24 h storage in distilled water at 37 degrees C; B-specimens submitted to thermocycling (500 cycles). Shear bond strength tests were performed. Data were submitted to ANOVA and Tukey test.

RESULTS

Means (MPa) of different groups were: I-AE-16.96, AD-17.46; BE-21.60, BD-12.79; II-AE-17.20, AD-11.93; BE-20.67, BD-13.94; III-AE-25.66, AD-17.53; BE-24.20, BD-19.38.

SIGNIFICANCE

Thermocycling did not influence significantly the shear bond strength of the tested adhesive systems; enamel was the dental substrate that showed larger adhesive strength; One Coat Bond system showed the best adhesive strength averages regardless of substrate or thermocycling.

[The physiological and pathological role of some organic dentine and enamel structures]

The enamel is the toughest human tissue. The major component of the inorganic part is hydroxyl apatite (90-92%). The organic part of enamel is formed by proteins, proteoglycans and lipoids, and represents only 1-2% of the entire weight. The organic components are organized, forming histological structures like enamel lamellae, enamel rods sheaths, enamel spindles and tufts. The authors, with the aid of the scanning electron microscope and histochemical staining, have demonstrated that enamel lamellae presented a true histological structure, contrary to some opinions that consider this entities developmental failures or simple cracks. In the opinion of the authors, these lamellae confer elasticity to the enamel when exposed to lateral or tangential forces, or even torque. The lamellae are also considered pathways for the progression of dental caries. The dentine-enamel junction is another non-mineralized tooth-structure which functions like an elastic support for the tough enamel, opposing unfortunately a very low resistance in the face of dental caries progression. In such cases we talk about secondary enamel caries. The dentinal tubules with the organic structures inside are essential in maintaining the vitality of the dentine and enamel; but in pathological circumstances they represent pathways for pathological stimuli heading toward the pulp, and they are the weakest points in front of caries progression.

3D FEA of cemented steel, glass and carbon posts in a maxillary incisor

OBJECTIVES

A comparative study on the stress distribution in the dentine and cement layer of an endodontically treated maxillary incisor has been carried out by using Finite Element Analysis (FEA). The role of post and cement rigidity on reliability of endodontic restorations is discussed.

METHODS

A 3D FEM model (13,272 elements and 15,152 nodes) of a central maxillary incisor is presented. A chewing static force of 10 N was applied at 125 degree angle with the tooth longitudinal axis at the palatal surface of the crown. Steel, carbon and glass fiber posts have been considered. The differences in occlusal load transfer ability when steel, carbon and glass posts, fixed to root canal using luting cements of different elastic moduli (7.0 and 18.7 GPa) are discussed.

RESULTS AND SIGNIFICANCE

The more stiff systems (steel and carbon posts) have been evaluated to work against the natural function of the tooth. Maximum Von Mises equivalent stress values ranging from 7.5 (steel) to 5.4 and 3.6 MPa (respectively, for carbon posts fixed with high and low cement moduli) and to 2.2 MPa (either for glass posts fixed with high and low cement moduli) have been observed under a static masticatory load of 10 N. A very stiff post works against the natural function of the tooth creating zones of tension and shear both in the dentine and at the interfaces of the luting cement and the post. Stresses in static loading do not reach material (dentine and cement) failure limits, however, they significantly differ leading to different abilities of the restored systems to sustain fatigue loading. The influence of the cement layer elasticity in redistributing the stresses has been observed to be less relevant as the post flexibility is increased.

Outward fluid flow reduces inward diffusion of bacterial lipopolysaccharide across intact and demineralised dentine

OBJECTIVE

To examine the ability of outward fluid flow (OF) on resisting the inward diffusion of bacterial lipopolysaccharide (LPS) across the demineralised dentine (DD) in comparison with that across the intact dentine (ID).

DESIGN

Twenty ID discs were prepared from freshly extracted human third molars. After etching both dentine surfaces, hydraulic conductance (L(p)) of the dentine was measured. Ten dentine discs were then completely demineralised using 10% EDTA, and L(p) was re-measured. The diffusion of LPS through ID and DD was measured against the OF and compared to the non-outward flow (NF) (n = 5 for each group) at 0, 1, 4 and 8h. Longitudinal sections of ID and DD surfaces were observed under a scanning electron microscope (SEM).

RESULTS

The L(p) of DD was significantly higher than that of ID (independent t-test, p < 0.001). The application of OF and demineralisation significantly affected LPS diffusion (two-way ANOVA, p < 0.05). In addition, the effect of OF depended on dentine demineralisation. SEM images of ID showed intact dentinal tubules, whereas those of DD showed expanded collagen fibres and enlarged dentinal tubules.

CONCLUSIONS

The inward diffusion of LPS across DD differed from that of ID and the OF affected the inward diffusion of LPS. In the presence of the OF, the inward diffusion of LPS was reduced to near zero in both ID and DD. Nevertheless, when compared to that in the ID group, the OF produced the slightly greater effect to resist the inward LPS diffusion in the DD group.

[Effects of surface potentials of tooth hard tissue on bone remodeling in rabbit tibiae]

OBJECTIVE

To investigate the influence of surface potentials of tooth hard tissue on bone remodeling.

METHODS

After insured the surface potentials of human extracted teeth with electrochemical methods, teeth sections and artificial hydroxyapatite were implanted into 25 rabbits' tibiae. The rabbits were sacrificed at 1, 2, 4, 6 and 8 weeks after implantation, respectively. The bone regeneration was compared between opposite two sides (cathode side and anode side) of tooth sections using hematoxylin-eosin (HE) staining, tartrate-resistant acid phosphatase (TRAP) activity detecting and tetracycline tracing method.

RESULTS

Resorption lacunae was seen in the tibiae facing to the enamel anode and new bone density in the implant bed near the cathode of tooth samples was much higher than that near the anode, while the number of TRAP positive cells near the cathode was smaller than that near the anode (P < 0.01). The fluorescent area of tetracycline tracing near the cathode was larger than that near the anode (P < 0.05).

CONCLUSIONS

The cathode of tooth hard tissue (cementum) could improve or trigger new bone formation, while the other side, anode (enamel), could improve the bone resorption. This study suggests that tooth hard tissue's electrochemical characteristic might affect the remodeling of alveolar bone, and tooth supraeruption and the alveolar bone loss after tooth extraction might result from the redundant or lack of root electrochemical stimulation to bone.

Effect of Cyclosporin A on the Mineral Apposition Rate of Cementum and Dentin in Growing Rats

BACKGROUND

Since there is no direct information to verify whether cyclosporin A (CsA) can affect the mineralization of dental hard tissue, the formation of dentin and cementum in growing rats was recorded by labeling the mineral phase of these tissues with fluorochrome marker in this study.

METHODS

After the extraction of the right maxillary molars, 30 male 3-week-old Sprague-Dawley rats were assigned to two groups. Following a 2-week healing period, the experimental rats received 30 mg/kg CsA daily for 7 weeks, while the control rats received only mineral oil. The fluorescent markers, calcein and alizarin red, were given on alternate weeks for 7 weeks. At the end of study, the mandibles were obtained and undemineralized sections were processed. Serial sections, 8 microm thick, were cut for the entire distal roots of the first molars. Five central sections were selected to determine the mineral apposition of cellular cementum and dentin at the apex and middle of root, respectively.

RESULTS

The apposition rates of apical cellular cementum were significantly influenced by CsA therapy, occlusal function, and observation duration. However, the dentin apposition rates were significantly influenced by the observation intervals only.

CONCLUSIONS

In this study, CsA therapy and occlusal function significantly influenced the apposition rates of apical cementum, but not the rates of mid-root dentin. Our hypothesis that CsA can induce oral hard tissue alterations, as well as gingival overgrowth, is demonstrated.

Shear bond strength of experimental methacrylated beta-cyclodextrin-based formulations

Previous studies have shown that methacrylated beta-cyclodextrins (MCDs) can be used as comonomers in resin-based dental composites. These MCDs by virtue of having several polymerizable methacrylate groups and hydrophilic hydroxyl groups, may also promote bonding of dental composites to dentin. This study evaluated MCDs as adhesive comonomers, and optimized comonomer and polymerization initiator concentrations for maximum shear bond strength (SBS). Experimental MCD-based bonding formulations in acetone were prepared by mixing 33 mass fraction % MCDs with (10, 20, 30, 40, or 50) mass fraction % of 2-hydroxyethyl methacrylate (HEMA). The MCD/HEMA-based solutions were activated with varied amounts of camphorquinone (CQ) and ethyl 4-dimethylamino benzoate (4E). Samples for SBS were prepared by bonding a composite resin to acid-etched dentin surfaces of extracted human molars with the experimental bonding solutions. The specimens were immersed in 37 degrees C water for 24 h and bond strengths were determined in shear mode. With increasing HEMA concentration, the SBS values of MCD-bonding solutions increased to 16 MPa at a composition of 33% MCD, 30% HEMA, and 37% acetone by mass. Also, SBS values of MCD-bonding solutions varied as a function of the CQ and 4E concentrations and passed through a maximum SBS at 21 MPa, which was comparable to that of a commercial control. This preliminary study indicated that nonacidic MCD monomers could be used as an adhesion-promoting comonomer. Additional modification of MCDs having both polymerizable groups and anionic ligand groups, e.g., polymerizable acidic cyclodextrin derivatives should increase the SBS even further.

Effects of aging on the mechanical behavior of human dentin

An experimental study on the mechanical behavior of human dentin and the influence of age was conducted. Beams with rectangular cross-section were sectioned from the coronal dentin of virgin extracted molars (N = 76) that were obtained from (N = 70) patients between 17 and 80 years of age. The beams were loaded in either quasi-static 4-point flexure or 4-point flexural fatigue to failure and the stiffness, strength and fatigue properties were evaluated. In characterizing the fatigue response the beams were divided into two age groups that were regarded as young (17 < or = age < or = 30, mean +/- std. dev. = 25 +/- 5 years) and old (50 < or = age < or = 80, mean +/- std. dev. = 64 +/- 9 years) dentin. Results from monotonic loading showed that both the flexural strength and strain to fracture of dentin decreased significantly with age. The fatigue life of dentin increased with a reduction in cyclic stress amplitude and the fatigue strength of young dentin was greater than that of old dentin at all cyclic stress amplitudes. The endurance strength of young dentin (at 10(7) cycles) was approximately 44 MPa, whereas the old dentin exhibited an endurance strength of approximately 23 MPa. Based on differences in the mechanical behavior and microscopic features of the fracture surfaces from the young and old specimens, aging appears to result in an increase in both the rate of damage initiation and propagation in dentin.

Effects of Laser Irradiation on Tensile Strength of Bovine Dentin

OBJECTIVE

The purpose of the present study was to investigate the tensile strengths of dentin after laser irradiation using three kinds of dental lasers to elucidate the laser-irradiation effect on dentin properties.

BACKGROUND DATA

Different kinds of laser devices have been developed in dentistry. The characteristics of each laser are determined by its original wavelength; however, one common feature is to generate heat in irradiated tissues, and such heat possibly affects dentin collagen, which contributes to tensile strength of the tissues.

MATERIALS AND METHODS

Er:YAG, CO2, and diode (GaAlAs) lasers were used to irradiate bovine dentin. Subsequently, tensile test specimens were made from the irradiated dentin and tensile tests were conducted. The tensile strengths were analyzed using the paired-t test and Weibull analysis. Irradiated dentin was also observed transversally using light microscopy.

RESULTS

The tensile strengths of the lased dentin and the control group for the Er:YAG, CO2, and diode lasers were 73.1 and 78.5, 70.3 and 74.3, and 64.3 and 71.0 MPa, respectively. The tensile strength of the dentin had a tendency to decrease with laser irradiation. Weibull analysis indicated that the laser influence was different among the three kinds of laser apparatuses and seemed to correspond to the depths the laser beam reached, which were suggested by light microscopy observation.

CONCLUSION

Laser irradiation could possibly decrease dentin tensile strength, which suggests the importance of careful use of laser for hard tissue treatment, considering its energy-transforming characteristics.

Stimulation of reparative dentin formation by ex vivo gene therapy using dental pulp stem cells electrotransfected with growth/differentiation factor 11 (Gdf11)

Dental pulp progenitor/stem cells have the capacity to differentiate into odontoblasts and they provide a potential for dentin repair and regeneration by gene therapy. To develop a successful ex vivo gene therapy to induce reparative dentin formation rapidly and effectively after treatment of caries, we developed a three-dimensional pellet culture system of pulp cells electrotransfected with growth/differentiation factor 11 (Gdf11). The viability after electrotransfection was more than 85%, and the efficiency was about 70% as determined by flow cytometry. After 10 days of culture, the total amount of type I and type III collagen was 3-fold higher in the pEGFP-Gdf11-transfected pellet than in the control. Real-time RT-PCR analysis demonstrated that the expression of markers of odontoblast differentiation (alkaline phosphatase, dentin matrix protein 1 [Dmp1], dentin sialophosphoprotein [Dspp], enamelysin, and phosphate-regulating gene with homologies to endopeptidases on X-chromosome [Phex]) was increased in the pEGFP-Gdf11-transfected pellet compared with the control on day 14. On the basis of this in vitro evaluation, an in vivo investigation in the dog was performed. Autogenous transplantation of Gdf11-transfected cells cultured as a pellet on amputated pulp stimulated reparative dentin formation. Thus, Gdf11 gene therapy may be potentially used in endodontic treatment in dentistry.

Photomechanical investigations on the stress-strain relationship in dentine macrostructure

In this study photomechanical experiments were carried out to examine the relationship between macroscopic mechanical stress and strain gradients within the root dentine structure. Three-dimensional digital photoelasticity was used to study the stress distribution patterns in tooth models, while digital moire interferometry was used to study the strain gradients within the natural teeth. The stress analysis showed a distinct bending stress distribution, along faciolingual plane in the coronal and cervical regions of the tooth. There was a reduction in bending towards the apical third of the tooth model. The strain analysis displayed strain gradients in the axial (along the long axis of the tooth) and lateral (perpendicular to the long axis of the tooth) directions in dentine. There was a conspicuous reduction in strains from the cervical to the apical third of the root dentine. The root dentine displayed uniform distribution of normal strains. Although there was a steep increase in stresses from the inner core region to the outer surface of an isotropic tooth model, there were more uniform strain gradients in the natural dentine structure. It is apparent from these observations that complex organization of material properties facilitated distinct strain gradients in dentine structure during mechanical functions.

Selective Influence of Dentin Thickness upon Cytotoxicity of Dentin Contacting Materials

The effect of three dentin contacting materials on three-dimensional cultures of pulp-derived cells was evaluated in a dentin barrier test device. The test materials (Syntac Classic, Prompt L-Pop, Vitrebond) were applied on dentin disks of different thicknesses ranging from 100 to 500 microm. After 24 h of exposure with and without perfusion of the test chamber, cell survival was evaluated using the MTT assay and related to a nontoxic control material. Syntac Classic decreased cell activity significantly (p </= 0.0003), independently of the dentin thickness. For Prompt L-Pop and Vitrebond a significant influence of dentin thickness was found on the cell reaction. After exposure of the control material, photometric readings showed no dependency of the cellular reaction on dentin thickness (p > 0.05). It could be demonstrated that dentin acts as a barrier, decreasing the elicited cytotoxicity with increasing thickness. This effect is material related, showing little influence for the nontoxic or the glutaraldehyde containing material.

Characterisation of resin–dentine interfaces by compressive cyclic loading

The aims of this in vitro study were to evaluate the ultra-morphological changes in resin-dentine interfaces after different amounts of thermomechanical load (TML), and to determine the corresponding microtensile bond strengths (microTBS). Enamel/dentine discs with a thickness of 2 mm were cut from 24 human third molars and bonded with four adhesives involving different adhesion approaches: Syntac (Ivoclar Vivadent; used as multi-step etch-and-rinse adhesive), Clearfil SE Bond (Kuraray; two-step self-etch adhesive), Xeno III (Dentsply DeTrey; mixed all-in-one self-etch primer adhesive system), and iBond (Heraeus Kulzer; non-mixed all-in-one self-etch adhesive). The resin-dentine discs were cut into beams (width 2 mm; 2 mm dentine, 2 mm resin composite) and subsequently subjected to cyclic TML using ascending amounts of mechanical/thermal cycles (20 N at 0.5 Hz of mechanical load and 5-55 degrees C of thermal cycles: for 0/0, 100/3, 1,000/25, 10,000/250, 100,000/2,500 cycles). Loaded specimens were either cut perpendicularly in order to measure microTBS (n=20; crosshead speed: 1 mm/min) or were immersed in an aqueous tracer solution consisting of 50 wt% ammoniacal silver nitrate and processed for ultra-morphological nanoleakage examination using transmission electron microscopy (TEM). microTBS were significantly decreased by increasing amounts of TML for all adhesives (p<0.05). Bond strengths after 0 vs. 100,000 thermomechanical cycles were: Syntac: 41.3/30.1 MPa; Clearfil SE Bond 44.8/32.5 MPa; Xeno III 27.5/13.7 MPa; iBond 27.0/6.2 MPa. Relatively early, a certain amount of nanoleakage was observed in all groups by TEM, which was more pronounced for Xeno III and iBond. The incidence of nanoleakage remained stable or was even reduced with increasing load cycles for all adhesives except iBond, where exact failure origins were detected within the adhesive and at the top of the hybrid layer.

On a mathematical model of a human root dentin

OBJECTIVE

On the basis of recent experimental data, a new mathematical model that predicts creep in human root dentin has been developed.

METHOD

The chosen constitutive model comprises fractional derivatives of stress and strain and the restrictions on the coefficients that follow from the Clausius-Duhem inequality.

RESULTS

The four constants describing mechanical properties of the human dentin at constant temperature are calculated from a highly non-linear system involving Mittag-Leffler-type functions. Special attention is paid to thermodynamical restrictions that should be observed in determining parameters of the model from experimental results.

SIGNIFICANCE

The proposed model allows us to predict behavior of a human dentin in different load situations. Also it could be used for describing mechanical properties of dentin that are important in the development of 'dentin-like' restorative materials.

Micro-rotary fatigue of tooth–biomaterial interfaces

The bonding effectiveness of restorative materials to tooth tissue is typically measured statically. Clinically tooth/composite bonds are however subjected to cyclic sub-critical loads. Therefore, in vitro fatigue testing of dental adhesives should predict better the in vivo performance of adhesives. The objective of this study was to determine the fatigue resistance of two representative adhesives, a self-etch and an etch&rinse adhesive, bonded to enamel and dentin. Therefore, tooth/composite interfaces were cyclically loaded using a miniaturized version of a rotating beam fatigue testing device. Subsequently, the load at which 50% of the specimens fail after 10(5) cycles, was determined as the median micro-rotary fatigue resistance (microRFR). For both adhesives, the microRFR was about 30-40% lower than the corresponding micro-tensile bond strength (microTBS) to both enamel and dentin. Analysis of the fracture surfaces by Feg-SEM revealed typical fatigue fracture patterns. It is concluded that resin/tooth interfaces are vulnerable to progressive damage by sub-critical loads, with the 3-step etch&rinse adhesive being more resistant to fatigue than the 2-step self-etch adhesive.

Phase shifting speckle interferometry for determination of strain and Young's modulus of mineralized biological materials: a study of tooth dentin compression in water

Mineralized biological materials have complex hierarchical graded structures. It is therefore difficult to understand the relations between their structure and mechanical properties. We report the use of electronic speckle pattern-correlation interferometry (ESPI) combined with a mechanical compression apparatus to measure the strain and Young's modulus of root dentin compressed under water. We describe the optomechanical instrumentation, experimental techniques and procedures needed to measure cubes as small as 1 x 1 x 2 mm. Calibration of the method is performed using aluminum, which shows that the measurements are accurate within 3% of the compression modulus reported for standard aluminum 6061. Our results reveal that the compression moduli of root dentin from the buccal and lingual sides of the root are quite different from the moduli of the interproximal sides. Root dentin from interproximal locations is found to have an average modulus of 21.3 GPa, which is about 40% stiffer than root dentin from the buccal and lingual locations, found to have a modulus of 15.0 GPa. Our approach can be used to map deformations on irregular surfaces, and measure strain on wet samples of varying sizes. This can be extended to the study of other biological materials including bone and synthetic biomaterials.

An inhomogeneous and anisotropic constitutive model of human dentin

Dentin constitutes the major part of human tooth. It is composed of a large number of tubules with both variational radii and radially parallel pattern. In addition, peritubular dentin surrounds each tubule lumen and has a higher elastic modulus than the matrix of dentin, i.e. intertubular dentin. Considering the above microstructural characteristics, a micromechanics model is used in this paper to evaluate the overall elastic properties of dentin. Five independent effective elastic parameters in transverse isotropic elasticity matrix can be expressed analytically by the material parameters of peri- and intertubular dentin and the volume fraction of tubules. To determine the effectivity of this theoretical model, a finite element (FE) model simulating a longitudinal tooth slice in moire fringe testing of Wang and Weiner (J. Biomech. 31 (1998) 135) was performed. Furthermore, the FE model was developed incorporate modeling of variation of tubule's diameter and softer characteristic of intertubular dentin near the dentin-enamel junction and around the pulp chamber. It turned out that the isoline figure of longitudinal displacement by FE calculation has very similar patterns to the moire fringe results. However, the FE results of displacement by traditional stress-strain models which regard dentin as a homogeneous and isotropic material show an obviously different strain distributions as compared to published moire fringes results. Thus the inhomogeneous and anisotropic model developed in this paper more accurately reflects the true physical nature of human dentin.

Macrophages and mast cells control the neutrophil migration induced by dentin proteins

Dentin sialoprotein (DSP) and dentin phosphoprotein (DPP), the major dentin proteins, have been shown to induce neutrophil migration through release of IL-1beta, TNF-alpha, MIP-2, and KC. However, the sources of these mediators were not determined. Here, the roles of macrophages and mast cells (MC) in dentin-induced neutrophil accumulation were investigated. Peritoneal MC depletion or the enhancement of macrophage population increased DSP- and DPP-induced neutrophil extravasation. Moreover, supernatants from DSP- and DPP-stimulated macrophages caused neutrophil migration. The release of neutrophil chemotactic factor by macrophages was inhibited by dexamethasone or the supernatant of DSP-treated MC. Consistently, dexamethasone and the MC supernatant inhibited the production of IL-1beta, TNF-alpha, and MIP-2 by macrophages. This inhibitory activity of the DSP-stimulated MC was neutralized by anti-IL-4 and anti-IL-10 antibodies. These results indicate that dentin induces the release of the neutrophil chemotactic substance(s) by macrophages, which are down-modulated by MC-derived IL-4 and IL-10.

Influence of Dentin on the Effectiveness of Antibacterial Agents

The influence of dentin on the effectiveness of three antibacterial agents (triclosan, glutaraldehyde, NaOCl) on Streptococcus mutans, S. sobrinus, and Lactobacillus acidophilus was tested using the agar diffusion method with and without bovine dentin discs (200 mum and 500 mum thickness) placed between bacteria and test substances. The effect of 0.3% triclosan on all tester strains (100%) was reduced after passage through 500 mum dentin discs to 0% (L. acidophilus) and to 22% and 28% (S. mutans and S. sobrinus). Seal&Protect (Dentsply, Konstanz, Germany), a triclosan containing dental bonding agent, produced inhibition zones only against S. mutans, but no zone when applied on 200 mum dentin discs. The inhibition zones for 1% NaOCl and 5% glutaraldehyde against all tester strains were significantly increased up to 230% (glutaraldehyde) and 236% (NaOCl) when applied on dentin discs, compared to direct application (100%). Dentin may either decrease or increase the inhibitory effect of antibacterial agents.

The odontoblast as a sensory receptor cell? The expression of TRPV1 (VR-1) channels

Previous reports have shown the expression of several mechanosensitive ionic channels on the plasma membrane in odontoblasts, which are the cells responsible for dentin formation. The membrane characteristics of odontoblasts imply that they could play critical roles in the mechano-transduction of fluid displacement within dentinal tubules into the electrical cell signals, to carry dentin sensation to the central nervous system. However, the direct ionic mechanism underlying such a dentin nociceptive function remains unclear. In the present study, we investigated the expression of the transient receptor potential vanilloid subfamily member 1 (TRPV1) channel--which essentially contributes to the detection of pain sensation--in rat odontoblasts by immunohistochemical and nystatin perforated patch-clamp techniques. Immunohistochemical observation showed the localization of TRPV1-immunoreactions on the distal regions of odontoblast membranes. In the patch-clamp experiments, we observed capsaicin-induced inward currents that were inhibited by capsazepine, a TRPV1 channel antagonist. Our results indicate a significant expression of TRPV1 channels in odontoblasts, suggesting that odontoblasts may directly respond to noxious stimuli such as a thermal-heat stimulus, and point to the necessity for a reconsideration of the cellular mechanisms of dentin sensation based on the transmembrane ionic signals in odontoblasts.

The nano-hardness and elastic modulus of sound deciduous canine dentin and young premolar dentin--preliminary study

The purpose of this study was to compare the nano-hardness and elastic modulus among deciduous and permanent dentin, buccal and lingual sides, incisal, center and cervical areas, and outer, middle and inner layers. Three premolars and three deciduous canines were bucco lingually (BL) sectioned, and three deciduous canines were mesio-distally (MD) sectioned parallel to the long axis at the center of the tooth. Hardness (H), plastic hardness (PH) and Young's modulus (Y) were measured using a nano-indentation tester. The H, PH and Y values from the deciduous canine dentin were significantly lower than those from the premolar dentin at most sites. For deciduous canine dentin, the H and PH values of the MD sectioned dentin were significantly higher than those of the BL sectioned dentin in many layers of many areas. Generally deciduous canine dentin had H, PH and Y values that decreased from outer toward the inner layers and significant differences were obtained among the layers in many areas. For MD sectioned deciduous canine and BD sectioned premolar dentin, the H, PH and Y values of the cervical area were significantly lower than those of the incisal and center areas in many layers. It is possible that optimum bonding may require different treatments for deciduous and permanent dentin and perhaps also for different intratooth locations.

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.

Effects of pregnancy and lactation on the microhardness of rat incisor dentine and enamel

Pregnancy and lactation affect the rate of maternal dentine apposition and mineralization in the rat. In this study we have measured the effects of pregnancy and lactation on the degree of mineralization of dentine and enamel by using the microhardness method in rat incisors. At incisal sections, pregnant and lactation group enamel values were increased compared to that of a control group; the pregnancy and post-lactation period dentine values decreased slightly compared to the control group. At neck sections, pregnant and lactation group enamel values were decreased compared to that of the control group; the pregnancy and post-lactation period dentine values were also decreased slightly compared to the control group. At the post-lactation period neck section dentine mineralization was decreased compared with the incisal section. As a result, enamel and dentine layers of rat incisors are affected to varying degrees by these changes in mineralization levels during the maternal period.

Effect of traditional and alternative intracoronal bleaching agents on microhardness of human dentine

The purpose of this study was to compare the effect of traditional and alternative bleaching agents on microhardness of human dentine when used intracoronally. Thirty-six premolars were divided into six groups and bleaching agents were sealed into the pulp chambers as follows: group 1--distilled water (control), group 2--30% hydrogen peroxide solution, group 3--sodium perborate mixed with distilled water, group 4--sodium perborate mixed with 30% hydrogen peroxide solution, group 5--35% carbamide peroxide gel, group 6--35% hydrogen peroxide gel. Access cavities were sealed and the teeth were stored in distilled water at 37 degrees C. After 7 days, each tooth was sectioned at the cemento-enamel junction level and microhardness testing was carried out on dentine. The results showed that treatment with 35% hydrogen peroxide gel, 30% hydrogen peroxide solution and 35% carbamide peroxide gel reduced the microhardness of outer dentine to a small extent while treatment with sodium perborate mixed with water and sodium perborate mixed with 30% hydrogen peroxide solution did not significantly alter the microhardness of dentine.

Importance of mini-dumbbell specimen to access tensile strength of restored dentine: historical background and the future perspective in dentistry

OBJECTIVES

The development of adhesive resins to dentine enables resin restorations to be more durable. Several bond strength measurement techniques for measuring adhesion have been proposed. A standardised method is needed which produced higher and more consistent bond strength values which allow bond stability and the bonding mechanism to be studied. Our aim was to investigate mainly the adhesives and not the substrate.

DATA SOURCES

This review is based on the literature on an adhesive, 4-META/MMA-TBB resin and conditioners to modify dentine substrates. The latter is a very important topic for developing our understanding of the bonding to dentine. The objective of the review is to explain the efficacy of the mini-dumbbell specimen in measuring the tensile strength of resin to dentine and to analyze the resin to dentine interface. Both the adhesive and the substrate control the quality of hybridized dentine.

CONCLUSIONS

By creating an impermeable acid resistant barrier to both biological and chemical stimuli between the exposed dentine and the restored tooth surface we are able to protect exposed dentine from caries (infection) at the same time as protecting the pulp and preventing toothache. This barrier also helps maintain tooth vitality and for the purposes of this article is termed 'artificial enamel'. Microleakage free restorations are possible through the introduction of this 'impermeable artificial enamel' barrier.