Finite element analysis of different restorative materials in primary teeth restorations

AIM

The purpose of this finite element analysis (FEA) study is to evaluate and compare the stress distributions at the primary molars and restorative materials according to the material used.

MATERIALS AND METHODS

A total of 12 3D models of Class II cavities in primary molars plus one control model were analysed. Study design: Three-dimensional FEA was used to compare stress distribution on enamel, dentin and restoration surfaces of cavities.

STATISTICS

Stresses occurring under occlusal forces were compared with the von Mises criterion.

RESULTS

The highest von Mises stress values at the enamel and restoration of restored tooth 84 were computed. On the basis of these results, all materials were ranked on enamel stress as: flowable composite resin (FCR)> compomer > resin modified glass ionomer cement (RMGIC) > giomer composite resin (GCR) > hybrid composite resin (HCR) > amalgam. Moreover, ranking of materials on restoration stress was FCR < compomer < RMGIC < GCR < amalgam < HCR.

CONCLUSION

A restorative material with appropriate elasticity module, able to balance stress concentrations, should be used to increase the survival rate of both the hard tissue of the tooth and the restoration material.

Comparative study of the measurement of enamel demineralization and remineralization using transverse microradiography and electron probe microanalysis

Transverse microradiography (TMR) and electron probe microanalysis (EPMA) are commonly used for characterizing dental tissues. TMR utilizes an approximately monochromatic X-ray beam to determine the mass attenuation of the sample, which is converted to volume percent mineral (vol%min). An EPMA stimulates the emission of characteristic X-rays from a variable volume of sample (dependent on density) to provide compositional information. The aim of this study was to compare the assessment of sound, demineralized, and remineralized enamel using both techniques. Human enamel samples were demineralized and a part of each was subsequently remineralized. The same line profile through each demineralized lesion was analyzed using TMR and EPMA to determine vol%min and wt% elemental composition and atomic concentration ratio information, respectively. The vol%min and wt% values determined by each technique were significantly correlated but the absolute values were not similar. This was attributable to the complex ultrastructural composition, the variable density of the samples analyzed, and the nonlinear interaction of the EPMA-generated X-rays. EPMA remains an important technique for obtaining atomic ratio information, but its limitations in determining absolute mineral content indicate that it should not be used in place of TMR for determining the mineral density of dental hard tissues.

Assessment of Er:YAG laser for cavity preparation in primary and permanent teeth: a scanning electron microscopy and thermographic study

PURPOSE

Most studies of cavity preparation using Er:YAG lasers have employed permanent teeth. This study's purpose was to compare the cutting efficiency of an Er:YAG laser versus diamond burs in primary and permanent teeth in order to measure thermal effects on the pulp and evaluate lased surfaces using scanning electron microscopy (SEM).

METHODS

A total of 80 primary and permanent teeth were used. Crater depths and mass loss were measured after delivering laser pulses at varying energies onto sound or carious enamel or dentin using the Key-3 laser. Control samples were cut using diamond burs in an air turbine handpiece. Thermal changes were measured using miniature thermocouples placed into the pulp chamber. Lased surfaces were evaluated using SEM.

RESULTS

Laser ablation crater-like defects were deeper in dentin than enamel at the same pulse energy. Greater ablation rates for dentin and enamel and significantly more efficient removal of carious tooth structure by laser was present in primary teeth. Temperature rises in the pulp did not exceed the 5.5 degrees Celsius threshold in any teeth during laser ablation.

CONCLUSIONS

The Er:YAG laser is an efficient device for cavity preparations in primary teeth, with no unacceptable increases in temperature detected in this model.

Influence of self-made saliva substitutes on tribological characteristics of human enamel

This paper describes the results of tests on the influence of human saliva and its substitutes on tribological characteristics of friction pairs. Each pair consists of enamel and one of the following materials: ceramics, the Meridian B2 dental composite, the GK dental amalgam, and Ti-6Al-4V titanium alloy. The saliva substitutes used were prepared using pyrophosphates, xanthan gum, and mucins dissolved in a saline buffer. The results of the tribological tests show that the values of the parameters under investigation (coefficient of friction and linear wear) were different from each other. Some similarity was observed between the evaluated level of wear characteristics after the friction process in the environment of human saliva and that in the environment of one of the mucins tested. Microscopic observations of the surfaces of the enamel samples after friction revealed varied forms of tribological wear.

Thermal response of a dental tissue induced by femtosecond laser pulses

This paper reports a theoretical and experimental study for thermal transport in a thin slice of human tooth induced by a 120 fs, 800 nm pulse laser at a repetition rate of 1 kHz. The surface reflectivity of enamel and the convection heat transfer coefficient were determined using an inverse heat transfer analysis. Instead of a fully three-dimensional modeling, two simplified two-dimensional (2D) planar and axisymmetric heat conduction models were proposed to simulate the temperature fields. The temperature responses obtained from the 2D planar and axisymmetric model agree well with the experimental measurements. On the other hand, the one-dimensional (1D) result significantly differs from the 2D axisymmetric one, suggesting that care should be taken when a 1D thermal model is considered for estimating temperature response.

Opportunistic feeding strategy for the earliest old world hypsodont equids: evidence from stable isotope and dental wear proxies

Background

The equid Hippotherium primigenium, with moderately hypsodont cheek teeth, rapidly dispersed through Eurasia in the early late Miocene. This dispersal of hipparions into the Old World represents a major faunal event during the Neogene. The reasons for this fast dispersal of H. primigenium within Europe are still unclear. Based on its hypsodonty, a high specialization in grazing is assumed although the feeding ecology of the earliest European hipparionines within a pure C₃ plant ecosystem remains to be investigated.

Methodology/Principal Findings

A multi-proxy approach, combining carbon and oxygen isotopes from enamel as well as dental meso- and microwear analyses of cheek teeth, was used to characterize the diet of the earliest European H. primigenium populations from four early Late Miocene localities in Germany (Eppelsheim, Höwenegg), Switzerland (Charmoille), and France (Soblay). Enamel δ¹³C values indicate a pure C₃ plant diet with small (<1.4‰) seasonal variations for all four H. primigenium populations. Dental wear and carbon isotope compositions are compatible with dietary differences. Except for the Höwenegg hipparionines, dental microwear data indicate a browse-dominated diet. By contrast, the tooth mesowear patterns of all populations range from low to high abrasion suggesting a wide spectrum of food resources.

Conclusions/Significance

Combined dental wear and stable isotope analysis enables refined palaeodietary reconstructions in C₃ ecosystems. Different H. primigenium populations in Europe had a large spectrum of feeding habits with a high browsing component. The combination of specialized phenotypes such as hypsodont cheek teeth with a wide spectrum of diet illustrates a new example of the Liem’s paradox. This dietary flexibility associated with the capability to exploit abrasive food such as grasses probably contributed to the rapid dispersal of hipparionines from North America into Eurasia and the fast replacement of the brachydont equid Anchitherium by the hypsodont H. primigenium in Europe.

How the tooth got its stripes: patterning via strain-cued motility

We hypothesize that a population of migrating cells can form patterns when changes in local strains owing to relative cell motions induce changes in cell motility. That the mechanism originates in competing rates of motion distinguishes it from mechanisms involving strain energy gradients, e.g. those generated by surface energy effects or eigenstrains among cells, and diffusion-reaction mechanisms involving chemical signalling factors. The theory is tested by its ability to reproduce the morphological characteristics of enamel in the mouse incisor. Dental enamel is formed during amelogenesis by a population of ameloblasts that move about laterally within an expanding curved sheet, subject to continuously evolving spatial and temporal gradients in strain. Discrete-cell simulations of this process compute the changing strain environment of all cells and predict cell trajectories by invoking simple rules for the motion of an individual cell in response to its strain environment. The rules balance a tendency for cells to enhance relative sliding motion against a tendency to maintain uniform cell-cell separation. The simulations account for observed waviness in the enamel microstructure, the speed and shape of the 'commencement front' that separates domains of migrating secretory-stage ameloblasts from those that are not yet migrating, the initiation and sustainment of layered, fracture-resistant decussation patterns (cross-plied microstructure) and the transition from decussating inner enamel to non-decussating outer enamel. All these characteristics can be correctly predicted with the use of a single scalar adjustable parameter.

Shear bond strength and fracture analysis of human vs. bovine teeth

Purpose

To evaluate if bovine enamel and dentin are appropriate substitutes for the respective human hard tooth tissues to test shear bond strength (SBS) and fracture analysis.

Materials and Methods

80 sound and caries-free human erupted third molars and 80 freshly extracted bovine permanent central incisors (10 specimens for each group) were used to investigate enamel and dentine adhesion of one 2-step self-etch (SE) and one 3-step etch and rinse (E&R) product. To test SBS the buccal or labial areas were ground plane to obtain appropriate enamel or dentine areas. SE and E&R were applied and SBS was measured prior to and after 500 thermocycles between +5 and +55°C. Fracture analysis was performed for all debonded areas.

Results

ANOVA revealed significant differences of enamel and dentin SBS prior to and after thermocycling for both of the adhesives. SBS- of E&R-bonded human enamel increased after thermocycling but SE-bonded did not. Bovine enamel SE-bonded showed higher SBS after TC but E&R-bonded had lower SBS. No differences were found for human dentin SE- or E&R-bonded prior to or after thermocycling but bovine dentin SE-bonded increased whereas bovine dentine E&R-bonded decreased. Considering the totalized and adhesive failures, fracture analysis did not show significances between the adhesives or the respective tooth tissues prior to or after thermocycling.

Conclusion

Although SBS was different on human and bovine teeth, no differences were found for fracture analysis. This indicates that solely conducted SBS on bovine substrate are not sufficient to judge the perfomance of adhesives, thus bovine teeth are questionnable as a substrate for shear bond testing.

Osteopetrosis, osteopetrorickets and hypophosphatemic rickets differentially affect dentin and enamel mineralization

Osteopetrosis (OP) is an inherited disorder of defective bone resorption, which can be accompanied by impaired skeletal mineralization, a phenotype termed osteopetrorickets (OPR). Since individuals with dysfunctional osteoclasts often develop osteomyelitis of the jaw, we have analyzed, if dentin and enamel mineralization are differentially affected in OP and OPR. Therefore, we have applied non-decalcified histology and quantitative backscattered electron imaging (qBEI) to compare the dental phenotypes of Src(-/-), oc/oc and Hyp(-/0) mice, which serve as models for OP, OPR and hypophosphatemic rickets, respectively. While both, Src(-/-) and oc/oc mice, were characterized by defects of molar root formation, only oc/oc mice displayed a severe defect of dentin mineralization, similar to Hyp(-/0) mice. Most importantly, while enamel thickness was not affected in either mouse model, the calcium content within the enamel phase was significantly reduced in oc/oc, but not in Src(-/-) or Hyp(-/0) mice. Taken together, these data demonstrate that dentin and enamel mineralization are differentially affected in Src(-/-) and oc/oc mice. Moreover, since defects of dental mineralization may trigger premature tooth decay and thereby osteomyelitis of the jaw, they further underscore the importance of discriminating between OP and OPR in the respective individuals.

Characterization of tooth structure and the dentin-enamel zone based on the Stokes-Mueller calculation

This is the first study of dentin-enamel zone (DEZ) identification with tooth structure characterization based on the optical Stokes-Mueller measurement. Stokes vectors of a cross-sectional tooth slice were measured using various polarization inputs. The direction of the DEZ is different in enamel and dentin structures; therefore, the Stokes profiles can specifically characterize the structures based on the DEZ. This optical method, using polarimetry, provides a useful tool for characterizing tooth.

Hierarchical flexural strength of enamel: transition from brittle to damage-tolerant behaviour

Hard, biological materials are generally hierarchically structured from the nano- to the macro-scale in a somewhat self-similar manner consisting of mineral units surrounded by a soft protein shell. Considerable efforts are underway to mimic such materials because of their structurally optimized mechanical functionality of being hard and stiff as well as damage-tolerant. However, it is unclear how different hierarchical levels interact to achieve this performance. In this study, we consider dental enamel as a representative, biological hierarchical structure and determine its flexural strength and elastic modulus at three levels of hierarchy using focused ion beam (FIB) prepared cantilevers of micrometre size. The results are compared and analysed using a theoretical model proposed by Jäger and Fratzl and developed by Gao and co-workers. Both properties decrease with increasing hierarchical dimension along with a switch in mechanical behaviour from linear-elastic to elastic-inelastic. We found Gao's model matched the results very well.

The role of enamel proteins in protecting mature human enamel against acidic environments: a double layer force spectroscopy study

Characterisation of the electrostatic properties of dental enamel is important for understanding the interfacial processes that occur on a tooth surface and how these relate to the natural ability of our teeth to withstand chemical attack from the acids in many soft drinks. Whereas, the role of the mineral component of the tooth enamel in providing this resistance to acid erosion has been studied extensively, the influence of proteins that are also present within the structure is not well understood. In this paper, we report for the first time the use of double-layer force spectroscopy to directly measure electrostatic forces on as received and hydrazine-treated (deproteinated) enamel surfaces in solutions with different pH to determine how the enamel proteins influence acid erosion surface potential and surface charge of human dental enamel. The deproteination of the treated samples was confirmed by the loss of the amide bands (~1,300-1,700 cm(-1)) in the FTIR spectrum of the sample. The force characteristics observed were found to agree with the theory of electrical double layer interaction under the assumption of constant potential and allowed the surface charge per unit area to be determined for the two enamel surfaces. The values and, importantly, the sign of these adsorbed surface charges indicates that the protein content of dental enamel contributes significantly to the electrostatic double layer formation near the tooth surface and in doing so can buffer the apatite crystals against acid attack. Moreover, the electrostatic interactions within this layer are a driving factor for the mineral transfer from the tooth surface and the initial salivary pellicle formation.

The role of property gradients on the mechanical behavior of human enamel

In this study, the mechanical design principles of human enamel were evaluated using a hybrid experimental and computational approach. Nanoindentation was applied to evaluate the load-depth response of human enamel, and Vickers indentations were used to assess the damage behavior. An elastic-plastic numerical model was then developed to analyze the stress and strain distribution about the indentations, and to characterize energy dissipation about indents in three locations including inner, middle and outer enamel. Results confirm that enamel exhibits a gradient in its mechanical behavior. Outer enamel has a limited potential for energy dissipation by inelastic deformation, indicating that the ability of outer enamel to resist fracture is low. While inner enamel, the region close Dentin Enamel Junction (DEJ), possesses less resistance to penetration deformation, it has a much higher capacity to dissipate energy by inelastic deformation than outer enamel. The computational simulations identified that the gradients in mechanical properties of human enamel promote resistance to penetration, energy dissipation and mitigation of fracture, all critical performance requirements of human teeth.

Abrasive, silica phytoliths and the evolution of thick molar enamel in primates, with implications for the diet of Paranthropus boisei

BACKGROUND

Primates--including fossil species of apes and hominins--show variation in their degree of molar enamel thickness, a trait long thought to reflect a diet of hard or tough foods. The early hominins demonstrated molar enamel thickness of moderate to extreme degrees, which suggested to most researchers that they ate hard foods obtained on or near the ground, such as nuts, seeds, tubers, and roots. We propose an alternative hypothesis--that the amount of phytoliths in foods correlates with the evolution of thick molar enamel in primates, although this effect is constrained by a species' degree of folivory.

METHODOLOGY/PRINCIPAL FINDINGS

From a combination of dietary data and evidence for the levels of phytoliths in plant families in the literature, we calculated the percentage of plant foods rich in phytoliths in the diets of twelve extant primates with wide variation in their molar enamel thickness. Additional dietary data from the literature provided the percentage of each primate's diet made up of plants and of leaves. A statistical analysis of these variables showed that the amount of abrasive silica phytoliths in the diets of our sample primates correlated positively with the thickness of their molar enamel, constrained by the amount of leaves in their diet (R(2) = 0.875; p<.0006).

CONCLUSIONS/SIGNIFICANCE

The need to resist abrasion from phytoliths appears to be a key selective force behind the evolution of thick molar enamel in primates. The extreme molar enamel thickness of the teeth of the East African hominin Paranthropus boisei, long thought to suggest a diet comprising predominantly hard objects, instead appears to indicate a diet with plants high in abrasive silica phytoliths.

Development of fluorapatite cement for dental enamel defects repair

In order to restore the badly carious lesion of human dental enamel, a crystalline paste of fluoride substituted apatite cement was synthesized by using the mixture of tetracalcium phosphate (TTCP), dicalcium phosphate anhydrous (DCPA) and ammonium fluoride. The apatite cement paste could be directly filled into the enamel defects (cavities) to repair damaged dental enamel. The results indicated that the hardened cement was fluorapatite [Ca(10)(PO(4))(6)F(2), FA] with calcium to phosphorus atom molar ratio (Ca/P) of 1.67 and Ca/F ratio of 5. The solubility of FA cement in Tris-HCl solution (pH = 5) was slightly lower than the natural enamel, indicating the FA cement was much insensitive to the weakly acidic solutions. The FA cement was tightly combined with the enamel surface, and there was no obvious difference of the hardness between the FA cement and natural enamel. The extracts of FA cement caused no cytotoxicity on L929 cells, which satisfied the relevant criterion on dental biomaterials, revealing good cytocompatibility. In addition, the results showed that the FA cement had good mechanical strength, hydrophilicity, and anti-bacterial adhesion properties. The study suggested that using FA cement was simple and promising approach to effectively and conveniently restore enamel defects.

Pseudogenization of the tooth gene enamelysin (MMP20) in the common ancestor of extant baleen whales

Whales in the suborder Mysticeti are filter feeders that use baleen to sift zooplankton and small fish from ocean waters. Adult mysticetes lack teeth, although tooth buds are present in foetal stages. Cladistic analyses suggest that functional teeth were lost in the common ancestor of crown-group Mysticeti. DNA sequences for the tooth-specific genes, ameloblastin (AMBN), enamelin (ENAM) and amelogenin (AMEL), have frameshift mutations and/or stop codons in this taxon, but none of these molecular cavities are shared by all extant mysticetes. Here, we provide the first evidence for pseudogenization of a tooth gene, enamelysin (MMP20), in the common ancestor of living baleen whales. Specifically, pseudogenization resulted from the insertion of a CHR-2 SINE retroposon in exon 2 of MMP20. Genomic and palaeontological data now provide congruent support for the loss of enamel-capped teeth on the common ancestral branch of crown-group mysticetes. The new data for MMP20 also document a polymorphic stop codon in exon 2 of the pygmy sperm whale (Kogia breviceps), which has enamel-less teeth. These results, in conjunction with the evidence for pseudogenization of MMP20 in Hoffmann's two-toed sloth (Choloepus hoffmanni), another enamel-less species, support the hypothesis that the only unique, non-overlapping function of the MMP20 gene is in enamel formation.

Anticaries potential of commercial fluoride rinses as determined by fluoridation and remineralization efficiency

OBJECTIVE

The objective of this work was to compare the anticaries potential of several currently marketed fluoride-containing mouthrinse products using two in vitro approaches: 1) fluoride uptake studies of demineralized human enamel samples after exposure to rinse products; and 2) microhardness studies of sound enamel samples after exposure to the rinse products and demineralizing agents.

METHODS

Four currently marketed rinse products, formulated at 100 ppm F, were evaluated in fluoride uptake studies relative to a negative (water) rinse control (Study 1). The same rinse products were evaluated in microhardness studies (Study 2) against a positive control, ACT Anticavity rinse, which is formulated with 225 ppm F and carries the ADA Seal of Acceptance as an effective anticavity mouthrinse. Test products included ACT Total Care rinse (pH = 6.34), Listerine Total Care rinse (pH = 3.57), Crest Pro-Health for Me rinse (pH = 3.33), and Crest Pro-Health Complete rinse (pH = 3.43).

RESULTS

Study 1-Samples treated with any of the fluoride-containing rinses showed significantly higher (p < 0.05) levels of fluoride uptake than the negative (water) control. Two of the products (Crest Pro-Health for Me and Crest Pro-Health Complete) showed significantly higher (p < 0.05) levels of fluoride uptake into demineralized enamel than the other marketed rinses (Listerine Total Care and ACT Total Care). Study 2-Samples treated with the same two rinse products (Crest Pro-Health For Me and Crest Pro-Health Complete) showed significantly lower mineral loss than the other rinse products, as well as the positive control.

CONCLUSION

Results of these in vitro studies indicate that the Crest mouthrinse products evaluated here are capable of providing significantly better fluoridation of demineralized enamel, as well as significantly better protection against the initiation and progression of demineralization, compared to the other marketed fluoride-containing mouthrinse products tested.

The mechanical anisotropy on a longitudinal section of human enamel studied by nanoindentation

Nanoindentation has been widely used for probing the mechanical properties of tooth, especially for characterizing its complex hierarchical structures. Previous studies have confirmed the anisotropic mechanical behaviors caused by the alternated orientations of enamel rods and the alignment of fibril-like hydroxyapatite crystals, but the longitudinal section of enamel, which was composed of parallel-arranged rods, was regarded as a homogeneous continuum as always. In this study, nanoindentation combined with SEM was carried out with the indenter rotating on the longitudinal section of enamel to evaluate the relativity between the nano-mechanical properties and the orientation of indentation impressions. It has been shown that the enamel presented different elastic modulus and hardness with different angles of indenter on its longitudinal section, and its anisotropy was also confirmed by the remarkable asymmetric morphologies of impressions. We observed that the parallel arrangement of crystal fibrils and enamel rods might trigger the expansion of the micro-cracks in preferred orientation, and result in scalene triangle indentation impressions, altering contact areas as well as inconsistent mechanical behaviors. Consequently, it is considered that the longitudinal sections of enamel should be modeled as anisotropic.

Dental enamel: genes define biomechanics

Regulated gene expression assembles an extracellular proteinaceous matrix to control biomineralization and the resultant biomechanical function of tooth enamel. The importance of the dominant enamel matrix protein, amelogenin (Amel); a minor transiently expressed protein, dentin sialoprotein (Dsp); an electrogenic sodium bicarbonate cotransporter (NBCe1); the timely removal of the proteinaceous matrix by a serine protease, Kallikrein-4 (Klk4); and the late-stage expression of Amelotin (Amtn) on enamel biomechanical function were demonstrated and measured using mouse models.

Effect of 10% sodium ascorbate and 10% alpha-tocopherol in different formulations on the shear bond strength of enamel and dentin submitted to a home-use bleaching treatment

This in vitro study assessed the shear bond strength of human enamel and dentin submitted to a bleaching treatment with 10% carbamide peroxide and treatment with antioxidant agents containing 10% alpha-tocopherol and 10% sodium ascorbate formulated in solution and gel. Sixty human dental enamel slabs (E) and 60 human dental dentin slabs (D) were randomly divided into six groups (n = 10). Groups E1 and D1 were negative control groups and the bleaching agent was not applied. The bleaching agent was applied daily for two-hours on the dental slabs of all the other groups and, during the remaining 22 hours, the specimens were stored in an artificial saliva solution for a total of 14 days. Groups E2 and D2 were positive control groups and they only received application of the bleaching agents. Antioxidant agents were applied in Groups E3 and D3 (10% sodium ascorbate solution), E4 and D4 (10% alpha-tocopherol solution), E5 and D5 (10% sodium ascorbate gel) and E6 and D6 (10% alpha-tocopherol gel) for two hours. Cylinders were made with microhybrid resin composite and a total-etch adhesive system for shear bond strength tests. These tests were performed in a universal testing machine at a speed of 0.5 mm/minute to obtain the values in MPa. ANOVA (p > 0.05) showed no significant differences among groups E4, E5, E6 and E1. However, groups E3, E5 and E6 presented statistically similar values to group E2. The Kruskal-Wallis test showed no significant differences among D1 and all the other experimental groups; the same values occurred with D2, which did not differ from the experimental groups. Antioxidant treatment with 10% alpha-tocopherol solution was the only effective agent to revert the oxidizing effects of the bleaching treatment on enamel.