Young's modulus of peritubular and intertubular human dentin by nano-indentation tests

The mechanical, wear, antibacterial properties and biocompatibility of injectable restorative materials under wet challenge

OBJECTIVES

To evaluate the mechanical, wear, antibacterial properties, and biocompatibility of injectable composite materials.

METHODS

Two injectable composite resins (GU and BI), one flowable composite resin (FS), and one flowable compomer (DF), in A2 shade, were tested. Mechanical properties were tested via three-point bending test immediately after preparation and after 1-day, 7-day, 14-day, and 30-day water storage. Under water-PMMA slurry immersion, specimens were subjected to a 3-body wear test (10,000 cycles) against stainless steel balls, while the roughness, wear depth, and volume loss were recorded. After 1-day and 3-day MC3T3-E1 cell culture, cell viability was evaluated with CCK-8 test kits, while the cell morphology was observed under CLSM and SEM. Antibacterial properties on S. mutans were assessed via CFU counting, CLSM, and SEM observation. SPSS 26.0 was used for statistical analysis (α = 0.05).

RESULTS

The mechanical properties were material-dependent and sensitive to water storage. Flexural strength ranked GU > FS > BI > DF at all testing levels. Three nanocomposites had better wear properties than DF. No significant difference on 1-day cell viability was found, but DF showed significantly lower cell proliferation than nanocomposites on 3-day assessment. GU and FS had more favourable cell adhesion and morphology. CFU counting revealed no significant difference, while FS presented a slightly thicker biofilm and BI showed relatively lower bacteria density.

CONCLUSIONS

Injectable nanocomposites outperformed the compomer regarding mechanical properties, wear resistance, and biocompatibility. The tested materials presented comparable antibacterial behaviours. Flowable resin-based composites' performances are affected by multiple factors, and their compositions can be attributed.

CLINICAL SIGNIFICANCE

A profound understanding of the mechanical, wear, and biological properties of the restorative material is imperative for the clinical success of dental restorations. The current study demonstrated superior properties of highly filled injectable composite resins, which imply their wider indications and better long-term clinical performances.

Is dentin analogue material a viable substitute for human dentin in fatigue behavior studies?

This study aimed to compare the fatigue performance of a lithium disilicate ceramic cemented on different substrates (human dentin and glass fiber-reinforced epoxy resin - GFRER), treated with different types of conditioning (CTR - without surface conditioning; HF5 - 5% hydrofluoric acid; HF10 - 10% hydrofluoric acid; H3PO4 - phosphoric acid 37%; SAND - sandblasting with aluminum oxide). The occlusal surface of human molars (DENT group) (n = 15) was ground for dentin exposure and the root portion was cut, then the dentin slice (2.0 mm thick) was conditioned with 37% phosphoric acid and a dual-curing dental adhesive was applied. The GFRER in a round-rod format was cut into discs (Ø = 10 mm, 2.0 mm thick). Lithium disilicate glass ceramic blocks (IPS e.max CAD, Ivoclar, Schaan, Liechtenstein) were shaped into a cylinder format and cut, resulting in 90 discs (Ø = 10 mm, 1.5 mm thick). The substrate materials of each group were etched according to the groups and the ceramic was etched with 5% hydrofluoric acid for 30 s. A silane coupling agent was applied over the cementation surface in ceramic and GFRER surfaces and a dual cement was used for cementation (ceramic/GFRER or dentin). The disc/disc sets were submitted to thermocycling (25,000 cycles + storage for 6 months), and then tested in step-wise accelerated cyclic fatigue test. The failure pattern and topography were analyzed and the roughness and contact angle were measured before and after surface treatment. The DENT group presented the lowest load to failure values and number of cycles to failure in fatigue (637.33 N; 118.333), showing no statistical similarity with any of the other tested groups (p < 0.05). The topographic analysis showed that all proposed surface treatments modified the substrate surface when compared to the CTR group. All of the fractographical inspections demonstrated failure by radial crack. Considering the roughness analysis, the post-etched DENT group showed similar roughness to all groups of GFRER materials with their surface treated, except for SAND, which showed greater roughness and statistically different from the other groups. The DENT group (49.5) showed statistically different post-conditioning contact angle values from the HF10 group (96.5) and similar to the other groups. The glass fiber-reinforced epoxy resin was not able to simulate the results presented by the human dentin substrate when cemented to lithium disilicate regarding fatigue failure load and number of cycles for failure, regardless of the surface treatment. Lithium disilicate cemented on dentin analogue overestimates the load values for fatigue failure.

Comparison of Dentin Microstructure and Its Correlation to the Direction of Fracture Line in Mandibular Molars of Young and Older Individuals: In Vitro Study

INTRODUCTION

The aim of the study was to assess the presence and extent of sclerotic dentin and to study its impact on the direction of fracture lines in extracted mandibular first molars of young (20-44 years) and older age groups (45 and older).

METHODS

Extracted permanent mandibular first molars were collected along with the related demographic details. A total of 40 teeth were included in this study, 20 each from young age group (YA group) (20-44 years) and older age group (OA group) (45-70 years). All molars were decoronated, and the sectioned mesial roots were embedded in acrylic blocks. They were subjected to vertical force in a universal testing machine. Fractured roots were then examined under the stereomicroscope at ×8 magnification to determine the direction and pattern of the fracture line. The roots were then sectioned and evaluated at ×10 and ×20 magnification to assess the dentin microstructure and its correlation with the direction of the fracture line. Statistical analysis was done by using χ2 test (P < .05).

RESULTS

A greater incidence and degree of sclerotic dentin were found in the OA group as compared with the YA group, which was statistically significant. The sclerotic dentin was distributed predominantly mesiodistally and the fracture line propagated buccolingually in both young and older groups, which was statistically significant (P < .05).

CONCLUSIONS

The presence of sclerotic dentin mesiodistally may impede crack propagation in this direction for both young and older age groups, causing the fracture line to extend buccolingually in coronal third of the root.

Microstructural features, physicο-mechanical properties, and wear behavior of dental translucent polychromic multilayer zirconia of hybrid composition prepared by milling technology

OBJECTIVE

The present study determined the mechanical properties and the wear behavior, as results of the micro(nano)structure, of the enamel, transition, and dentine layers, which comprise the polychromic multilayer zirconia materials of hybrid composition fabricated by milling technology.

MATERIALS AND METHODS

Prismatic blocks were fabricated from two commercial pre-sintered dental polychromic multilayer zirconia materials of hybrid composition, IPS e.max ZirCAD Prime (medium and high translucency, from the dentine to the incisal layer) and 3D Pro ML (translucency gradient, from the dentine to the incisal layer) by milling technique, and then, cut into 3 distinct parts to separate the enamel, transition, and dentine layers. The samples were sintered, thermally treated (similarly to the glazing procedure), and polished for characterization. Their microstructure, mechanical properties (determined by nanoindentation and microhardness), and wear behavior (evaluated by scratch test), were examined.

RESULTS

The produced materials had a homogeneous and dense nanostructure, where the grain size decreased from the enamel to dentine layer. The mechanical properties decreased from the dentine to enamel layer. However, the three layers manifested similar dynamic friction coefficient.

CONCLUSION

The differences in the above properties in the three layers negligibly influenced the wear behavior of the entire multilayer zirconia material.

CLINICAL SIGNIFICANCE

The properties of dental restorations produced from polychromic multilayer zirconia of hybrid composition by milling technology (i.e., strong, non-fragile, and esthetic materials), anticipate good performance in oral cavity.

Correlative Atomic Force Microscopy and Raman Spectroscopy in Acid Erosion of Dentin

Physical properties and chemical composition are fundamentally defining and interconnected surface characteristics. However, few techniques are able to capture both in a correlative fashion at the same sample location and orientation. This is especially important for complex materials such as dentin, which is an inner tooth structure and is a heterogeneous, composite inorganic-organic material with open channels (tubules) that extend toward the tooth pulp. Here, a combined microscope system consisting of an atomic force microscope and a confocal Raman spectrometer was used to study the correlative physical and chemical properties of human dentin. The local hardness of dentin was highly correlated with the Raman signal ratio of inorganic to organic material, and this was enhanced in the peritubular regions of dentin. When the samples were etched with citric acid, Young's modulus, hardness, and inorganic-to-organic material ratio decreased significantly, collagen fibrils on the surface were exposed, the peritubular regions were removed, and the tubule diameters increased. Thus, the combined atomic force microscopy (AFM)-Raman approach allows for comprehensive and correlative physical-chemical analysis of material surfaces and will be invaluable for evaluating oral therapeutic strategies.

Properties of dentin, enamel and their junction, studied with Brillouin scattering and compared to Raman microscopy

OBJECTIVE

Dentin, enamel and the transition zone, called the dentin-enamel junction (DEJ), have an organization and properties that play a critical role in tooth resilience and in stopping the propagation of cracks. Understanding their chemical and micro-biomechanical properties is then of foremost importance. The aim of this study is to apply Brillouin microscopy on a complex biological structure, that is, the DEJ, and to compare these results with those obtained with Raman microscopy.

DESIGN

Both techniques allow noncontact measurements at the microscopic scale. Brillouin microscopy is based on the interaction between acoustic phonons and laser photons and gives a relation between the frequency shift of the scattered light and the stiffness of the sample. Raman spectra contain peaks related to specific chemical bonds.

RESULTS

Comparison of the Brillouin and Raman cartographies reveals correlations between mechanical and chemical properties. Indeed, the shapes of the phosphate content and stiffness curves are similar. The two spectroscopies give compatible values for the mean distance between two tubules, i.e., 4-6 µm. Moreover, for the first time, the daily cross striations of enamel could be studied, indicating a relationship between the variation in the phosphate concentration and the variation in the rigidity within the enamel prisms.

CONCLUSIONS

We demonstrate here the possibility of using Brillouin scattering microscopy to both study complex biological materials such as the enamel-dentin junction and visualize secondary structures. Correlations between the chemical composition and mechanical properties could help in better understanding the tissue histology.

Micromechanical interlocking structure at the filler/resin interface for dental composites: a review

Dental resin composites (DRCs) are popular materials for repairing caries or dental defect, requiring excellent properties to cope with the complex oral environment. Filler/resin interface interaction has a significant impact on the physicochemical/biological properties and service life of DRCs. Various chemical and physical modification methods on filler/resin interface have been introduced and studied, and the physical micromechanical interlocking caused by the modification of fillers morphology and structure is a promising method. This paper firstly introduces the composition and development of DRCs, then reviews the chemical and physical modification methods of the filler/resin interface, mainly discusses the interface micromechanical interlocking structures and their enhancement mechanism for DRCs, finally give a summary on the existing problems and development potential.

Which dentine analogue material can replace human dentine for crown fatigue test?

OBJECTIVE

To seek dentine analogue materials in combined experimental, analytical, and numerical approaches on the mechanical properties and fatigue behaviours that could replace human dentine in a crown fatigue laboratory test.

METHODS

A woven glass fibre-filled epoxy (NEMA grade G10; G10) and a glass fibre-reinforced polyamide-nylon (30% glass fibre reinforced polyamide-nylon 6,6; RPN) were investigated and compared with human dentine (HD). Flexural strength and elastic modulus (n = 10) were tested on beam-shaped specimens via three-point bending, while indentation hardness (n = 3) was tested after fracture. Abutment substrates of G10, RPN and HD were prepared and resin-bonded with monolithic lithium disilicate crowns (n = 10), then subjected to wet cyclic loading in a step-stress manner (500 N initial load, 100 N step size, 100,000 cycles per step, 20 Hz frequency). Data were statistically analysed using Kruskal-Wallis one-way ANOVA followed by post-hoc comparisons (α = 0.05). Survival probability estimation was performed by Mantel-Cox Log-Rank test with 95% confidence intervals. The fatigue failure load (FFL) and the number of cycles until failure (NCF) were evaluated with Weibull statistics. Finite Element Models of the fatigue test were established for stress distribution analysis and lifetime prediction. Fractographic observations were qualitatively analysed.

RESULTS

The flexural strength of HD (164.27 ± 14.24 MPa), G10 (116.48 ± 5.93 MPa), and RPN (86.73 ± 3.56 MPa) were significantly different (p < 0.001), while no significant difference was observed in their flexural moduli (p = 0.377) and the indentation hardness between HD and RPN (p = 0.749). The wet cyclic fatigue test revealed comparable mean FFL and NCF of G10 and RPN to HD (p = 0.237 and 0.294, respectively) and similar survival probabilities for the three groups (p = 0.055). However, RPN promotes higher stability and lower deviation of fatigue test results than G10 in Weibull analysis and FEA.

SIGNIFICANCE

Even though dentine analogue materials might exhibit similar elastic properties and fatigue performance to human dentine, different reliabilities of fatigue on crown-dentine analogues were shown. RPN seems to be a better substrate that could provide higher reliability and predictability of laboratory study results.

The multi-scale meso-mechanics model of viscoelastic dentin

Human dentin is a hierarchical material with multi-level micro-/nano-structures, consisting of tubule, perti-tubular dentin (PTD) and intertubular dentin (ITD) as the major constituents at microscale; and the PTD and ITD are further composed of collagen and hydroxyapatite (HAp) crystals with different volume fractions at nanoscale. In most cases, the HAp is considered as elastic while the collagen as viscoelastic material. It is of great significance to study the hierarchical structure and viscoelasticity of human dentin to understand the mechanical properties of dentin for further development of restorative materials. Based on this, this paper focuses on multiscale modeling of the elastic properties and dynamic viscoelastic response of dentin and establishes a bottom-up micromechanics model from nano-to macro-scale. In order to study the nanostructural effect on the viscoelastic behavior of hierarchical structures, the homogenization theories of random platelets composites (HTRPC) and the locally-exact homogenization theory (LEHT) are introduced for the homogenization of heterogeneous materials of microstructures at different levels. The HTRPC, based on Eshelby Inclusion theory, is used to predict the effective modulus of PTD and ITD. The LEHT is a method for homogenizing multiphase dentin characterized by repeated unit cells (RUCs). The resulting predictions are in very good agreement with several experimental data from the literature. In addition, the results of nanostructrual effect on dentin show that the viscoelasticity of dentin is majorly contributed by collagen and the HAp greatly provide the strength and hardness of dentin. Furthermore, the ageing effect on dentin's viscoelasticity is considered from the proposed multiscale micromechanics model. It is demonstrated that the ageing effect is much more influential in affecting the loss moduli of dentin than the storage.

3D-Printed Polymer-Infiltrated Ceramic Network with Antibacterial Biobased Silver Nanoparticles

This work aimed at the antimicrobial functionalization of 3D-printed polymer-infiltrated biomimetic ceramic networks (PICN). The antimicrobial properties of the polymer-ceramic composites were achieved by coating them with human- and environmentally safe silver nanoparticles trapped in a phenolated lignin matrix (Ag@PL NPs). Lignin was enzymatically phenolated and used as a biobased reducing agent to obtain stable Ag@PL NPs, which were then formulated in a silane (γ-MPS) solution and deposited to the PICN surface. The presence of the NPs and their proper attachment to the surface were analyzed with spectroscopic methods (FTIR and Raman) and X-ray photoelectron spectroscopy (XPS). Homogeneous distribution of 13.4 ± 3.2 nm NPs was observed in the transmission electron microscopy (TEM) images. The functionalized samples were tested against Gram-positive (Staphylococcus aureus) and Gram-negative (Pseudomonas aeruginosa) bacteria, validating their antimicrobial efficiency in 24 h. The bacterial reduction of S. aureus was 90% in comparison with the pristine surface of PICN. To confirm that the Ag-functionalized PICN scaffold is a safe material to be used in the biomedical field, its biocompatibility was demonstrated with human fibroblast (BJ-5ta) and keratinocyte (HaCaT) cells, which was higher than 80% in both cell lines.

Principles of elastic bridging in biological materials

Load-bearing biological materials employ specialized elastic bridging regions to connect material parts with substantially different properties. While such bridging regions emerge in diverse systems of biological systems, their functional-mechanical origins are yet disclosed. Here, we hypothesize that these elastic bridging regions evolved primarily to minimize the near-interface stress effects in the biological material and, supported by experiments and simulations, we develop a simple theoretical model for such stress-minimizing bridging modulus. Our theoretical model describes well extensive experimental data of diverse biomechanical systems, suggesting that despite their compositionally distinct bridging regions, they share a similar mechanical adaptation strategy for stress minimization. The theoretical model developed in this study may directly serve as a design guideline for bio-inspired materials, biomedical applications, and advanced interfacial architectures with high resilience to mechanical failure. STATEMENT OF SIGNIFICANCE: Biological materials exhibit unconventional structural-mechanical strategies allowing them to attain extreme load-bearing capabilities. Here, we identify the strategy of biological materials to connect parts of distinct elastic properties in an optimal manner of stress minimization. Our findings are compatible with broad types of biological materials, including biopolymers, biominerals, and their bio-composite combinations, and may promote novel engineering designs of advanced biomedical and synthetic materials.

Structure-function relationships in dog dentin

Investigations into teeth mechanical properties provide insight into physiological functions and pathological changes. This study sought to 1) quantify the spatial distribution of elastic modulus, hardness and the microstructural features of dog dentin and to 2) investigate quantitative relationships between the mechanical properties and the complex microstructure of dog dentin. Maxillary canine teeth of 10 mature dogs were sectioned in the transverse and vertical planes, then tested using nanoindentation and scanning electron microscopy (SEM). Microstructural features (dentin area fraction and dentinal tubule density) and mechanical properties (elastic modulus and hardness) were quantified. Results demonstrated significant anisotropy and spatial variation in elastic modulus, hardness, dentin area fraction and tubule density. These spatial variations adhered to a consistent distribution pattern; hardness, elastic modulus and dentin area fraction generally decreased from superficial to deep dentin and from crown tip to base; tubule density generally increased from superficial to deep dentin. Poor to moderate correlations between microstructural features and mechanical properties (R² = 0.032-0.466) were determined. The results of this study suggest that the other constituents may contribute to the mechanical behavior of mammalian dentin. Our results also present several remaining opportunities for further investigation into the roles of organic components (e.g., collagen) and mineral content on dentin mechanical behavior.

Micromechanics of dental cement paste

Biodentine is a calcium silicate/calcium carbonate/zirconium dioxide/water-based dental replacement biomaterial, significantly outperforming the stiffness and hardness properties of chemically similar construction cement pastes. We here report the first systematic micromechanical investigation of Biodentine, combining grid nanoindentation with ultrasonic testing and micromechanical modeling. Histograms of nanoindentation-probed hardness and elastic modulus, comprising more than 5700 values each, are very well represented by the superposition of three log-normal distributions (LNDs). Most of the data (74%) belong to the intermediate LND, representing highly dense calcite-reinforced hydration products with on-average more than 60GPa elastic modulus and 3GPa hardness. The remaining data refer, on the one hand, to lower density hydration products, and on the other hand, to single-micron-sized unhydrated clinker and zirconium-dioxide inclusions. Micromechanical homogenization of these three material phases delivers elastic properties of the overall cement paste material, which significantly exceed those probed by more than 300 ultrasonic tests performed in the kHz and MHz regime. This indicates the presence of micro-defects, which slightly weaken the otherwise highly optimized biomaterial system.

3D-Printed Polymer-Infiltrated Ceramic Network with Biocompatible Adhesive to Potentiate Dental Implant Applications

The aim of this work was to prepare and characterize polymer-ceramic composite material for dental applications, which must resist fracture and wear under extreme forces. It must also be compatible with the hostile environment of the oral cavity. The most common restorative and biocompatible copolymer, 2,2-bis(p-(2'-2-hydroxy-3'-methacryloxypropoxy)phenyl)propane and triethyleneglycol dimethacrylate, was combined with 3D-printed yttria-stabilized tetragonal zirconia scaffolds with a 50% infill. The proper scaffold deposition and morphology of samples with 50% zirconia infill were studied by means of X-ray computed microtomography and scanning electron microscopy. Samples that were infiltrated with copolymer were observed under compression stress, and the structure's failure was recorded using an Infrared Vic 2DTM camera, in comparison with empty scaffolds. The biocompatibility of the composite material was ascertained with an MG-63 cell viability assay. The microtomography proves the homogeneous distribution of pores throughout the whole sample, whereas the presence of the biocompatible copolymer among the ceramic filaments, referred to as a polymer-infiltrated ceramic network (PICN), results in a safety "damper", preventing crack propagation and securing the desired material flexibility, as observed by an infrared camera in real time. The study represents a challenge for future dental implant applications, demonstrating that it is possible to combine the fast robocasting of ceramic paste and covalent bonding of polymer adhesive for hybrid material stabilization.

Case Report: A Novel COL1A1 Missense Mutation Associated With Dentineogenesis Imperfecta Type I

Background: Osteogenesis imperfecta (OI) is a clinical and genetic disorder that results in bone fragility, blue sclerae and dentineogenesis imperfecta (DGI), which is mainly caused by a mutation in the COL1A1 or COL1A2 genes, which encode type I procollagen. Case Report: A missense mutation (c.1463G > C) in exon 22 of the COL1A1 gene was found using whole-exome sequencing. However, the cases reported herein only exhibited a clinical DGI-I phenotype. There were no cases of bone disease or any other common abnormal symptom caused by a COL1A1 mutation. In addition, the ultrastructural analysis of the tooth affected with non-syndromic DGI-I showed that the abnormal dentine was accompanied by the disruption of odontoblast polarization, a reduced number of odontoblasts, a reduction in hardness and elasticity, and the loss of dentinal tubules, suggesting a severe developmental disorder. We also investigated the odontoblast differentiation ability using dental pulp stem cells (DPSCs) that were isolated from a patient with DGI-I and cultured. Stem cells isolated from patients with DGI-I are important to elucidate their pathogenesis and underlying mechanisms to develop regenerative therapies. Conclusion: This study can provide new insights into the phenotype-genotype association in collagen-associated diseases and improve the clinical diagnosis of OI/DGI-I.

Alumina particle air-abrasion and aging effects: Fatigue behavior of CAD/CAM resin composite crowns and flexural strength evaluations

The aim of this study was to characterize the flexural strength and elastic modulus of CAD/CAM resin composite material and to evaluate the influence of different surface treatments and storage conditions on the fatigue behavior of bonded composite crowns. Bars (flexural strength, n= 30; elastic modulus, n= 5) (1.2 × 4 × 12 mm) were produced for three-point bending test and CAD/CAM milled crowns (n= 5) (thickness= 1 mm) adhesively cemented to an epoxy resin substrate for fatigue tests. Bars and crowns were randomly allocated into two "surface treatments": no surface treatment (CTRL) and air-abrasion with 110 μm Al₂O₃ particles (AlOx); while the crowns were also subdivided into "aging condition" (baseline - storage for 24 h to 7 days, and aging - storage for 150 days + 25,000 thermal cycles). The three-point bending test was performed according to ISO 6872 and the luted crowns were subjected to step-stress fatigue test (initial load of 200 N; step-size of 50 N; 10,000 cycles per step; 20 Hz). Complementary analysis by Stereomicroscopy and Field Emission Scanning Electron Microscopy (FE-SEM) were performed. The flexural strength and fatigue data were submitted to statistical tests (α= 0.05). The results showed that air-abrasion reduces the flexural strength and the characteristic strength of the resin composite, without modifying its elastic modulus or its structural reliability (Weibull Modulus). Air-abrasion did not influence the fatigue behavior of the cemented crowns. Notwithstanding, a decrease in the survival rate was observed after 445,000 cycles (2,400 N) when subjected to aging at both the CTRL or AlOx conditions. FE-SEM micrographs of the crowns showed that alumina particle air-abrasion treatment can modify the topography of its treated inner surface. Therefore, air-abrasion with alumina powder introduces defects onto the surface of the CAD/CAM resin composite material, decreasing the flexural strength, but without changing its elastic modulus and reliability. Adhesive cementation onto an epoxy resin substrate prevented an influence of the introduced defects on the fatigue performance of the resin composite restoration. Nevertheless, the fatigue behavior may be damaged by aging regimen.

The role of lateral branches on effective stiffness and local overstresses in dentin

The 3D microstructure of dentinal tissue, the main tissue of the tooth, is the subject of an increasingly comprehensive body of knowledge. The relationship between this microstructure and the mechanical behaviour of dentinal tissue remains, nonetheless, under question. This article proposes an original SEM analysis of dentin microstructure, accounting for lateral branches, and a mechanical model based on these findings. An interesting observation is that lateral branches have a dense collar, as do tubules. The diameter of these branches as well as a percentage area are quantified all along the depth of a dentin sample. We use these unprecedented data to build an orthotropic homogenized model of dentin. The heterogeneities of microstructure are taken into account using level-set functions. The results reveal that the lateral branches slightly influence the global homogenized elastic behavior of the dentin tissue, albeit creating stress concentration areas that are highly influenced by the inclination of the traction with respect to the tubule and branches.

Human dentin characteristics of patients with osteogenesis imperfecta: insights into collagen-based biomaterials

Osteogenesis imperfecta (OI), also known as "brittle bone disease", is a rare genetic disorder of the skeleton, whose most benign form I corresponds to autosomal dominant mutations in the genes encoding type I collagen (COLA1, COLA2). Several associated skeletal manifestations are often observed but, surprisingly, while dentin defects often reflect genetic bone disorders, about half of OI patients have no obvious oral manifestations. Here, we investigated the collagen, mineral and mechanical properties of dentin from deciduous teeth collected from patients with mild form of OI and displaying no obvious clinical signs of dentinogenesis imperfecta. For the first time, an increase in the hardness of OI dentin associated with an increase in mineral content compared to healthy patients was reported. In addition, OI altered the tissue characteristics of the dentin-enamel junction but the interfacial gradient was preserved. The impact of changes in molecular structure due to mutations in OI was assessed by Raman microspectroscopy. Our results highlighted a change in the hydroxyproline-proline ratio in direct association with collagen mineralization. Our findings suggest that the evaluation of teeth could be an important aid for mild types of OI that are often difficult to diagnose clinically and provide experimental evidence that hydroxyproline content should be considered in future studies on collagen-based biomaterials.

Fracture micromechanics of human dentin: A microscale numerical model

In the present study, we investigate the effects of microstructural morphology and heterogeneity on the initiation and propagation of microcracks in dentin. We create 2D pre-cracked models of human dentin at the microscale level and use a brittle fracture framework of the phase-field method to analyze the crack growth. We discuss the influence of the microstructural features on crack deflection, microcracking, and uncracked ligament bridging through various regions in dentin. The results demonstrate that the difference between the critical energy release rates of peritubular (PTD) and intertubular dentin (ITD) has considerable impacts on microcracking. Our simulations reveal that tubules surrounded by PTDs play an important role in the crack deflection. Our results also indicate that the toughness of dentin increases from the inner to outer dentin. In conclusion, the findings in our study provide valuable insights into the fracture behavior in various regions of dentin.

Fracture Load of CAD/CAM Feldspathic Crowns Influenced by Abutment Material

In vitro studies investigating the mechanical properties of dental reconstructions use various materials to replicate prepared teeth. However, no uniform recommendation exists as to which material is most suitable for standardized testing. The purpose of this study was to identify a material that resembles human dentin in fracture load tests. Sixteen human teeth were scanned with an intraoral scanner to obtain copies of the original crown morphology and were then prepared for crowns. Replica dies of the prepared teeth including the root morphology were fabricated with a Computer-aided design and computer-aided manufacturing (CAD/CAM) system and divided into four groups: (A) reinforced composite (RC); (B) human dentin (HD); (C) polymethyl methacrylate (PM); and (D) hybrid ceramic (HC). Sixty-four feldspar ceramic crowns were designed with the biocopy mode, fabricated with a CAD/CAM system, luted on the dies, and then with the roots embedded in polymethyl methacrylate. Care was taken to position all specimens of the same morphology identically. Thermo-mechanical load cycling was performed in a chewing simulator followed by fractural loading of the crowns. A mixed effect linear model was fitted to the data, and pairwise contrasts were estimated on the marginal means and corrected for multiple testing according to Tukey (α = 0.05). The means for fracture load (N) were 2435 N (95% CI (2162, 2709)) for hybrid ceramic, 1838 N (95% CI (1565, 2112)) for reinforced composite, 1670 N (95% CI (1396, 1943)) for human tooth and 1142 N (95% CI (868, 1415)) for polymethyl methacrylate abutment materials. Post-hoc pairwise contrasts revealed a statistically significant (p < 0.05) difference among all groups except for reinforced composite and human dentin (p = 0.76). The results indicate that the mechanical properties of abutment dies play a significant role for a possible substitution of natural teeth in in vitro studies.

AN ALTERNATIVE TO PERIODIC HOMOGENIZATION FOR DENTIN ELASTIC STIFFNESS

Dentin, the main tissue of the tooth, is made of tubules surrounded by peri-tubular dentin (PTD), embedded in a matrix of inter-tubular dentin (ITD). The PTD and the ITD have different relative fractions of collagen and hydroxyapatite crystals. The ITD is typically less rigid than the PTD, which can be seen as a set of parallel hollow cylindrical reinforcements in the ITD matrix. In this paper, we extend Hashin and Rozen’s homogenization scheme to a nonuniform distribution of hollow PTD cylinders, determined from image analysis. We relate the transverse isotropic elastic coefficients of a Representative Elementary Volume (REV) of dentin to the elastic and topological properties of PTD and ITD. The model is calibrated against experimental data. Each sample tested is consistently characterized by Environmental Scanning Electron Microscopy (ESEM), nanoindentation and Resonant Ultrasound Spectroscopy (RUS), which ensures that macroscopic mechanical properties measured are correlated with microstructure observations. Despite the high variability of microstructure descriptors and mechanical properties, statistical analyses show that Hashin’s bounds converge and that the proposed model can be used for back-calculating the microscopic Poisson’s ratios of dentin constituents. Three-point bending tests conducted in the laboratory were simulated with the Finite Element Method (FEM). Elements were assigned transverse isotropic elastic parameters calculated by homogenization. The tubule orientation and the pdf of the ratio inner/outer tubule radius were determined in several zones of the beams before testing. The remainder of the micro-mechanical parameters were taken equal to those calibrated by RUS. The horizontal strains found experimentally by Digital Image Correlation (DIC) were compared to those found by FEM. The DIC and FEM horizontal strain fields showed a very good agreement in trend and order of magnitude, which verifies the calibration of the homogenization model. By contrast with previous studies of dentin, we fully calibrated a closed form mechanical model against experimental data and we explained the testing procedures. In elastic conditions, the proposed homogenization scheme gives a better account of microstructure variability than micro–macro dentin models with periodic microstructure.

Translation Approach for Dentine Regeneration Using GSK-3 Antagonists

The canonical Wnt/β-catenin signaling pathway is crucial for reparative dentinogenesis following tooth damage, and the modulation of this pathway affects the rate and extent of reparative dentine formation in damaged mice molars by triggering the natural process of dentinogenesis. Pharmacological stimulation of Wnt/β-catenin signaling activity by small-molecule GSK-3 inhibitor drugs following pulp exposure in mouse molars results in reparative dentinogenesis. The creation of similar but larger lesions in rat molars shows that the adenosine triphosphate (ATP)–competitive GSK-3 inhibitor, CHIR99021 (CHIR), and the ATP noncompetitive inhibitor, Tideglusib (TG), can equally enhance reparative dentine formation to fully repair an area of dentine damage up to 10 times larger, mimicking the size of small lesions in humans. To assess the chemical composition of this newly formed dentine and to compare its structure with surrounding native dentine and alveolar bone, Raman microspectroscopy analysis is used. We show that the newly formed dentine comprises equal carbonate to phosphate ratios and mineral to matrix ratios to that of native dentine, both being significantly different from bone. For an effective dentine repair, the activity of the drugs needs to be restricted to the region of damage. To investigate the range of drug-induced Wnt-activity within the dental pulp, RNA of short-term induced (24-h) molars is extracted from separated roots and crowns, and quantitative Axin2 expression is assayed. We show that the activation of Wnt/β-catenin signaling is highly restricted to pulp cells in the immediate location of the damage in the coronal pulp tissue with no drug action detected in the root pulp. These results provide further evidence that this simple method of enhancement of natural reparative dentinogenesis has the potential to be translated into a clinical direct capping approach.

Insights into the reinforcement role of peritubular dentine subjected to acid dissolution

Human dentine is a mineralised dental tissue that consists of dentinal tubules surrounded by two distinct dentinal phases: peritubular dentine (PTD) and intertubular dentine (ITD). Dental caries, which manifests itself as a consequence of demineralisation, is one of the most common chronic diseases that affect the function of human teeth. Due to the difference in the packing density of crystallites, PTD and ITD exhibit different reaction rates to acid dissolution. The present study evaluates how the effective Young's modulus degrades and how the effective stress redistributes in demineralised human dentine as a result of incremental acid dissolution process. An analytical two-layer composite model is proposed and used for the effective Young's modulus calculation. 3D numerical representative volume elements (RVEs) with different variations in PTD fraction and dentinal tubule density are established to evaluate effective stress redistribution and examine the critical factors that can affect the mechanical performance. The models are then applied on an actual dentine bulk sample. The results reveal how PTD serves as a protection to ITD thus highlight the important role that PTD plays for the structural integrity of dentine. The obtained insights are crucial for advancing the understanding of a variety of natural and therapeutic effects from the mechanical perspective, e.g. the mechanical performance assessment of human dentine subject to complex dynamic processes of de- and re-mineralisation that can occur in human dental caries and dental treatments. It will ultimately inspire the biomimetic design towards strengthening the dentine and dentine-like materials.

Nanoscale chemical and mechanical heterogeneity of human dentin characterized by AFM-IR and bimodal AFM

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AFM-IR technique was used to detect the chemical heterogeneity of human dentin for the first time.

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The heterogeneity of mechanical properties of human dentin was explored by AFM AM-FM technique.

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A band at 1336 cm⁻¹ assigned to S Created by potrace 1.16, written by Peter Selinger 2001-2019 ]]>O stretching vibrations was found only in peritubular dentin.

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Peritubular dentin had a higher Young’s modulus (32.25 ± 4.67 GPa) than intertubular dentin.

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AFM-IR and AFM AM-FM are useful for understanding the mineral deposition mechanisms of dentin.

Human dentin, as an important calcified tissue in the body, plays significant roles in withstanding masticatory forces and has a complex hierarchical organization. Understanding the composition and ultrastructure of dentin is critical for elucidating mechanisms of biomineralization under healthy and pathological states. Here, atomic force microscope infrared spectroscopy (AFM-IR) and AFM-based amplitude modulation-frequency modulation (AM-FM) techniques were utilized to detect the heterogeneity in chemical composition and mechanical properties between peritubular and intertubular dentin at the nanoscale. AFM-IR spectra collected from peritubular and intertubular dentin contained similar vibrational bands in the amide regions (I, II and III), suggesting that collagen may exist in both structures. A distinctive band at 1336 cm⁻¹ indicative of SO stretching vibrations was detected only in peritubular dentin. AFM-IR imaging showed an uneven distribution of chemical components at different locations, confirming the heterogeneity of dentin. The Young’s modulus of peritubular dentin was higher, and was associated to a higher mineral content. This study demonstrated distinctive chemical and mechanical properties of peritubular dentin, implying the different development and mineralization processes between peritubular and intertubular dentin. AFM-IR is useful to provide compositional information on the heterogeneity of human dentin, helping to understand the mineral deposition mechanisms of dentin.

Anisotropic crack propagation and deformation in dentin observed by four-dimensional X-ray nano-computed tomography

Understanding the cracking behaviour of biological composite materials is of practical importance. This paper presents the first study to track the interplay between crack initiation, microfracture and plastic deformation in three dimensions (3D) as a function of tubule and collagen fibril arrangement in elephant dentin using in situ X-ray nano-computed tomography (nano-CT). A nano-indenter with a conical tip has been used to incrementally indent three test-pieces oriented at 0°, 45° and 70° to the long axis of the tubules (i.e. radial to the tusk). For the 0° sample two significant cracks formed, one of which linked up with microcracks in the axial-radial plane of the tusk originating from the tubules and the other one occurred as a consequence of shear deformation at the tubules. The 70° test-piece was able to bear the greatest loads despite many small cracks forming around the indenter. These were diverted by the microstructure and did not propagate significantly. The 45° test-piece showed intermediate behaviour. In all cases strains obtained by digital volume correlation were well in excess of the yield strain (0.9%), indeed some plastic deformation could even be seen through bending of the tubules. The hoop strains around the conical indenter were anisotropic with the smallest strains correlating with the primary collagen orientation (axial to the tusk) and the largest strains aligned with the hoop direction of the tusk. STATEMENT OF SIGNIFICANCE: This paper presents the first comprehensive study of the anisotropic nature of microfracture, crack propagation and deformation in elephant dentin using time-lapse X-ray nano-computed tomography. To unravel the interplay of collagen fibrils and local deformation, digital volume correlation (DVC) has been applied to map the local strain field while the crack initiation and propagation is tracked in real time. Our results highlight the intrinsic and extrinsic shielding mechanisms and correlate the crack growth behavior in nature to the service requirement of dentin to resist catastrophic fracture. This is of wide interest not just in terms of understanding dentin fracture but also can extend beyond dentin to other anisotropic structural composite biomaterials such as bone, antler and chitin.

Multiscale micromechanical modeling of the elastic properties of dentin

The paper focuses on multiscale modeling of the elastic properties of dentin. It is modeled as a hierarchical structure consisting of collagen fibers and hydroxyapatite particles at the lower level. Different concentrations of hydroxyapatite in this tissue correspond to peritubular and intertubular dentins. Then, the overall material is modeled as intertubular dentin matrix containing parallel cylindrical holes (the tubules) surrounded by layers of peritubular dentin. At each microstructural level, the model accounts for anisotropy of the constituents. The model predictions are compared with experimental data available in literature.

Gradual dehydration affects the mechanical properties and bonding outcome of adhesives to dentin

This study evaluated the effects of dehydration on the mechanical properties of adhesive resins and dentin, and on the microtensile bond strength (µTBS) of adhesives. Third molars were randomly bonded with Clearfil Mega Bond (MB) or Clearfil SE Bond 2 (SE). After water-storage (37°C; 24 h), µTBS was obtained in 'wet' (tested after 5 min of removal from storage) and 'dehydrated' (tested after 10, 15 min and 24 h) conditions by a universal tester (crosshead speed: 1 mm/min). Data were analyzed by two-way ANOVA and Duncan's test. Hardness (H), Elastic modulus (E) and weight-loss of dentin beams and adhesive-resin discs were also monitored over time and analyzed by one-way repeated measures ANOVA and Bonferroni's test (α=0.05). Significant differences in bond strength were observed for adhesives and for conditions. Except for dentin's E, dehydration caused significant gradual changes in the H, E and weight of adhesive resins and dentin (p<0.05).

Synergistic effects of VE-TPGS and riboflavin in crosslinking of dentine

OBJECTIVE

Effect of d-alpha-tocopheryl poly(ethyleneglycol)-1000-succinate (VE-TPGS) with riboflavin-5'-phosphate solution on crosslinking of dentine collagen was investigated to analyze collagen's structural integrity.

METHODS

VE-TPGS was added to RF-solution, at RF/VE-TPGS (w/w) ratios of 0.125/0.250 and 0.125/0.500. Demineralized dentine beams were used (10wt.% phosphoric acid), rinsed using deionized-water and analysed using ELISA (Human MMP2 ELISA; Human CTSK/Cathepsin-K for MMP2 and Cathepsin K analysis). AFM of dentine collagen-fibrils structure was done before and after dentine specimens' placement in mineralization solution and tested after 14days in artificial saliva/collagenase (AS/Co) solution. The specimens were tested after 24h in mineralization solution for surface/bulk elastic modulus. Nano-indentation was carried out for each specimen on intertubular-dentine with lateral spacing of 400nm. Reduced elastic-modulus and nano-hardness were calculated and collagen content was determined using hydroxyproline-assay. Micro-Raman were performed. TEM was carried out to study structural variations of dentine-collagen in artificial-saliva (collagenase). Data were presented as mean±standard deviation and analyzed by SPSS v.15, by analysis of variance.

RESULTS

Synergetic effect of VE-TPGS was observed with RF through higher structural integrity of dentine collagen-fibrils shown by TEM/AFM. Superior surface/bulk mechanical stability was shown by nano-indentation/mechanical testing. Improvement in collagenase degradation resistance for hydroxyproline release was observed and lower endogenous-protease release of MMP-2/Cathepsin-K. Raman-analysis analysed chemical interactions between RF and collagen confirming structural-integrity of collagen fibrils after crosslinking. After 24h mineralization, AFM showed mineral depositions in close association with dentine-collagen fibrils with RF/VE-TPGS formulations.

SIGNIFICANCE

Potential synergetic effect of RF/VE-TPGS was observed by reflection of higher structural integrity and conformational-stability of dentine-collagen fibrils.

Structure-Function Correlative Microscopy of Peritubular and Intertubular Dentine

Peritubular dentine (PTD) and intertubular dentine (ITD) were investigated by 3D correlative Focused Ion Beam (FIB)-Scanning Electron Microscopy (SEM)-Energy Dispersive Spectroscopy (EDS) tomography, tapping mode Atomic Force Microscopy (AFM) and scattering-type Scanning Near-Field Optical Microscopy (s-SNOM) mapping. The brighter appearance of PTD in 3D SEM-Backscattered-Electron (BSE) imaging mode and the corresponding higher grey value indicate a greater mineral concentration in PTD (~160) compared to ITD (~152). However, the 3D FIB-SEM-EDS reconstruction and high resolution, quantitative 2D map of the Ca/P ratio (~1.8) fail to distinguish between PTD and ITD. This has been further confirmed using nanoscale 2D AFM map, which clearly visualised biopolymers and hydroxyapatite (HAp) crystallites with larger mean crystallite size in ITD (32 ± 8 nm) than that in PTD (22 ± 3 nm). Correlative microscopy reveals that the principal difference between PTD and ITD arises primarily from the nanoscale packing density of the crystallites bonded together by thin biopolymer, with moderate contribution from the chemical composition difference. The structural difference results in the mechanical properties variation that is described by the parabolic stiffness-volume fraction correlation function introduced here. The obtained results benefit a microstructure-based mechano-chemical model to simulate the chemical etching process that can occur in human dental caries and some of its treatments.

The effect of microcracking in the peritubular dentin on the fracture of dentin

Dentin is a biocomposite possessing elegant hierarchical structure, which allows it to resist fracture effectively. Despite the considerable efforts to unravel the peculiar fracture behavior of dentin, the effect of microstructural features on the fracture process is largely unknown. In this study, we explore the interaction between the primary crack with crack tip located in intertubular dentin (ITD) and microcracking of peritubular dentin (PTD) ahead of the primary crack. A micromechanical model accounting for the unique composite structure of dentin is developed, and computational simulations are performed. It is found that the microcracking of PTD located in the crack plane in front of the primary crack tip can promote the propagation of the primary crack, increasing the propensity of coalescence of primary crack and microcracks nucleating in PTD. We show that the two-layer microstructure of dentin enables reduction in driving force of primary crack, potentially enhancing fracture toughness. The high stiffness of PTD plays a critical role in reducing the driving force of primary crack and activating microcracking of PTD. It is further identified that the microcracking of PTD arranged parallel to the crack plane with an offset could contribute to the shielding of primary crack.

The interrelationship of microstructure and hardness of human coronal dentin using reference point indentation technique and micro-Raman spectroscopy

OBJECTIVES

The aim of this paper is to determine the interrelationship between the microstructure - in terms of chemical composition and crystallinity - to the microhardness of coronal dentin.

METHODS

Dentin microhardness was tested by a novel reference point indenter and compared to the traditional Knoop hardness method. Micro-Raman spectroscopy was used to determine the chemical composition and crystallinity of dentin.

RESULTS

From the occlusal groove to the border of the coronal pulp chamber, dentin hardness decreased from superficial dentin (SD) to deep dentin (DD). Mineral/organic matrix ratios (phosphate/CH and phosphate/amide I) also decreased from SD to DD; however, this change was significant (P<0.05) in the phosphate/amide I ratio only. The phosphate/carbonate ratio decreased significantly by varying position from SD to DD. The degree of the crystallinity, as measured by the full width at half maximum (FWHM) of the peak at 960cm^-1, decreased significantly going from superficial to deep dentin.

SIGNIFICANCE

For the first time, the interrelationship between the microstructure and the mechanical properties of coronal dentin was determined by using the novel reference point indentation technique and micro-Raman spectroscopy. We hypothesize that the decrease in hardness from superficial to deep dentin can potentially be explained by decreased mineral content and increased carbonate content, which is also associated with decreased crystallinity. Collectively, there is a positive association between dentin hardness and mineral content and a negative association between dentin hardness and carbonate content.

UVA-activated riboflavin improves the strength of human dentin

The aim of this study was to evaluate the effects of UVA-activated riboflavin (UVA-RF) on the mechanical properties of non-demineralized human dentin. Dentin specimens obtained from 20 teeth were randomly divided into the following four groups: group 1 (control): no treatment, group 2 (low UVA-RF): specimens were exposed to UVA-RF for 10 min, group 3 (medium UVA-RF): specimens were exposed to UVA-RF for 30 min, and group 4 (high UVA-RF): specimens were exposed to UVA-RF for 60 min. Three-point flexural test and Raman spectroscopic analyses were performed. The mean flexural strengths (MPa) were 129.96, 128.96, 144.21, and 147.54, and the mean elastic modulus (GPa) were 8.59, 8.38, 10.21, and 9.87 for groups 1 to 4, respectively. Raman spectra showed chemical modifications of dental collagen under medium and high UVA-RF treatment. We conclude that medium and high UVA-RF increases the strength of non-demineralized human dentin by collagen crosslinking.

Proanthocyanidins-Loaded Nanoparticles Enhance Dentin Degradation Resistance

Previous studies reported that grapeseed extract (GSE), which is rich in proanthocyanidins (PAs), improves the biodegradation resistance of demineralized dentin. This study aimed to investigate the effect of a new GSE delivery strategy to demineralized dentin through loading into biodegradable polymer poly-[lactic-co-glycolic acid] (PLGA) nanoparticles on the biodegradation resistance in terms of structural stability and surface/bulk mechanical and biochemical properties with storage time in collagenase-containing solutions. GSE-loaded nanoparticles were synthetized by nanoprecipitation at PLGA/GSE (w/w) ratios of 100:75, 100:50, and 100:25 and characterized for their morphological/structural features, physicochemical characteristics, and drug loading, entrapment, and release. Nanoparticle suspensions in distilled water (12.5% w/v) were applied (1 min) to demineralized dentin specimens by simulating pulpal pressure. The nanoparticle delivery was investigated by scanning electron microscopy (SEM)/transmission electron microscopy (TEM), and the GSE release from the delivered nanoparticles was further characterized. The variations in surface and bulk mechanical properties were characterized in terms of reduced elastic-modulus, hardness, nanoindentation testing, and apparent elastic-modulus with a storage time up to 3 mo. Hydroxyproline release with exposure to collagenase up to 7 d was estimated. An etch-and-rinse dentin adhesive was applied to investigate the morphology of the resin-dentin interface after nanoparticle delivery. Treatment with the GSE-loaded nanoparticles enhanced the collagen fibril structural resistance, reflected from the TEM investigation, and improved the biomechanical and biochemical stability of demineralized dentin. Nanoparticles having PLGA/GSE of 100:75 (w/w) showed the highest cumulative GSE release and were associated with the best improvement in biodegradation resistance. TEM/SEM showed the ability of the nanoparticles to infiltrate dentinal tubules' main and lateral branches. SEM revealed the formation of a uniform hybrid layer and well-formed resin tags with the presence of numerous nanoparticles located within the dentinal tubules and/or attached to the resin tag. This study demonstrated the potential significance of delivering collagen crosslinkers loaded into biodegradable polymer nanoparticles through the dentinal tubules of demineralized dentin on the biodegradation resistance.

Mechanical performance of polymer-infiltrated zirconia ceramics

OBJECTIVE

The aim of this study was to evaluate the microstructure and mechanical behavior of polymer-infiltrated zirconia ceramics as a function of pre-sintering temperature (1000-1150°C).

METHODS

Polymer-infiltrated zirconia ceramics were prepared by combining the porous zirconia networks and polymer through infiltration and polymerization. XRD was employed to determine phase structure. The microstructure and fracture mechanism were observed by SEM. Flexural strength and fracture toughness were measured by three-point bending method and single-edge-notched beam method, respectively. A nanoindentation system was employed to determine elastic modulus and hardness.

RESULTS

Different porosities and polymer contents can be obtained by tuning the pre-sintered temperature of zirconia ceramic precursors. Zirconia network porosity varies from 46.3% to 34.7% and the relevant polymer content ranges from 18.4wt.% to 12.3wt.% when the pre-sintered temperature is set from 1000°C to 1150°C. The flexural strength, fracture toughness, hardness, and elastic modulus values of the specimen pre-sintered at 1150°C are 240.9MPa, 3.69MPam^1/2, 3.1GPa, and 58.8GPa, respectively.

CONCLUSION

The pre-sintering temperature has a significant effect on the microstructure and mechanical properties of polymer-infiltrated zirconia ceramics and the optimal pre-sintering temperature is 1150°C.

CLINICAL SIGNIFICANCE

Specimen pre-sintered at 1150°C shows tooth-like mechanical properties, suggesting a promising restorative material in dental clinic. Moreover, the synthesis process is simple and can be easily performed in a prosthesis laboratory.

Stress distribution difference between Lava Ultimate full crowns and IPS e.max CAD full crowns on a natural tooth and on tooth-shaped implant abutments

The goal of this short communication is to present finite element analysis comparison of the stress distribution between CAD/CAM full crowns made of Lava Ultimate and of IPS e.max CAD, adhesively luted to natural teeth and to implant abutments with the shape of natural teeth. Six 3D models were prepared using a 3D content-creating software, based on a micro-CT scan of a human mandibular molar. The geometry of the full crown and of the abutment was the same for all models representing Lava Ultimate full crowns (L) and IPS e.max CAD full crowns (E) on three different abutments: prepared natural tooth (n), titanium abutment (t) and zirconia abutment (z). A static load of 400 N was applied on the vestibular and lingual cusps, and fixtures were applied to the base of the models. After running the static linear analysis, the post-processing data we analyzed. The stress values at the interface between the crown and the abutment of the Lt and Lz groups were significantly higher than the stress values at the same interface of all the other models. The high stress concentration in the adhesive at the interface between the crown and the abutment of the Lava Ultimate group on implants might be one of the factors contributing to the reported debondings of crowns.

A New Method Combining Finite Element Analysis and Digital Image Correlation to Assess Macroscopic Mechanical Properties of Dentin

A literature review points out a large discrepancy in the results of the mechanical tests on dentin that can be explained by stress and strain assessment during the tests. Errors in these assessments during mechanical tests can lead to inaccurate estimation of the mechanical properties of the tested material. On top of that, using the beam theory to analyze the bending test for thick specimens will increase these experimental errors. After summarizing the results of mechanical tests on dentin in the literature, we focus on bending tests and compare the stress assessment obtained by finite element analysis (FEA) and by beam theory application. We show that the difference between the two methods can be quite large in some cases, leading us to prefer the use of FEA to assess stresses. We then propose a new method based on coupling finite element analysis and digital image correlation (DIC) to more accurately evaluate stress distributions, strain distributions and elastic modulus in the case of a three-point bending test. To illustrate and prove the feasibility of the method, it is applied on a dentinal sample so that mean elastic modulus and maximum tensile stress are obtained (11.9 GPa and 143.9 MPa). Note that the main purpose of this study is to focus on the method itself, and not to provide new mechanical values for dentin. When used in standard mechanical testing of dentin, this kind of method should help to narrow the range of obtained mechanical properties values.