Inelastic deformation and microcracking process in human dentin

The exploration of a compound cone-beam CT contrast agent for diagnosis of human extracted cracked tooth

Objectives

This study aims to investigate the use of sodium iodide (NaI), dimethyl sulfoxide (DMSO), ethyl alcohol, and ethyl acetate as cone-beam CT (CBCT) contrast agents for diagnosing cracked teeth. The optimal delay time for detecting the number of crack lines beyond the dentino-enamel junction (Nd), the number of cracks extending from the occlusal surface to the pulp cavity (Np), and the depth of the crack lines was explored.

Methods

14 human extracted cracked teeth were collected, 12 were used for enhanced scanning, and 2 were used for exploring the characteristic of crack lines. The teeth were scanned in 3 CBCT enhanced scanning (ES) modes: ES1 using meglumine diatrizoate (MD); ES2 using NaI and DMSO, ES3 using NaI, DMSO, ethyl alcohol and ethyl acetate. Three delay times (15mins, 30mins, and 60mins) were set for scanning. Nd, Np, and depth of crack lines were evaluated.

Results

There were totally 24 crack lines on 12 cracked teeth. Nd was 10 in ES1 at 60mins, 24 in ES2 at 60mins and 24 in ES3 at 15mins. Np was 1 in ES1 at 60mins, 10 in ES2 at 60mins and 21 in ES3 at 60mins, and there were significantly different among them (p < 0.01). The average depth presented on ES3 was significantly deeper than ES1 and ES2 (p < 0.01).

Conclusion

NaI, DMSO, ethyl alcohol and ethyl acetate show potential as contrast agents for enhanced CBCT scanning in diagnosis of cracked teeth and their depth in vivo. A delay time of 15 min is necessary to confirm the existence of crack lines, while a longer delay time is required to ascertain if these crack lines extend to the pulp cavity.

Compressive Strength Testing of Glass-Fibre-Reinforced Tooth Crown Tissues After Endodontic Treatment

The objective of this study was to compare the effects of using short and continuous fibres for repairing compression-induced tooth crown damage. Human teeth were used for the study. They were upper medial incisors and maxillary first premolars lost due to periodontal causes. The teeth were divided into two groups with Hahnenkratt and short glass fibres. Teeth compressive strength tests were carried out. Then micro-CT imaging of the teeth and their fractures obtained after compression was performed. The teeth restored with Hahnenkratt's glass fibre posts showed higher compressive strength than the teeth restored using the EverX Posterior material. The tooth's most weakened and sensitive point after endodontic treatment was the cervical area of the tooth. All cracks were parallel to the root canal.

Novel bioactive dental restorations to inhibit secondary caries in enamel and dentin under oral biofilms

OBJECTIVE

To provide the first review on cutting-edge research on the development of new bioactive restorations to inhibit secondary caries in enamel and dentin under biofilms. State-of-the-art bioactive and therapeutic materials design, structure-property relationships, performance and efficacies in oral biofilm models.

DATA, SOURCES AND STUDY SELECTION

Researches on development and assessment new secondary caries inhibition restorations via in vitro and in vivo biofilm-based secondary caries models were included. The search of articles was carried out in Web of Science, PubMed, Medline and Scopus.

CONCLUSIONS

Based on the found articles, novel bioactive materials are divided into different categories according to their remineralization and antibacterial biofunctions. In vitro and in vivo biofilm-based secondary caries models are effective way of evaluating the materials efficacies. However, new intelligent and pH-responsive materials were still urgent need. And the materials evaluation should be performed via more clinical relevant biofilm-based secondary caries models.

CLINICAL SIGNIFICANCE

Secondary caries is a primary reason for dental restoration failures. Biofilms produce acids, causing demineralization and secondary caries. To inhibit dental caries and improve the health and quality of life for millions of people, it is necessary to summarize the present state of technologies and new advances in dental biomaterials for preventing secondary caries and protecting tooth structures against oral biofilm attacks. In addition, suggestions for future studies are provided.

Biomechanical perspectives on dentine cracks and fractures: Implications in their clinical management

OBJECTIVES

The present review discussed the biomechanical properties of cracks and fractures in crown and root dentine and attempted to explain why cracked teeth and vertical root fractures are so frequent despite the existence of multiple crack toughening mechanisms in dentine. The implications of this knowledge were used to justify how these defects are managed clinically.

DATA, SOURCES AND STUDY SELECTION

Literature search was conducted on PubMed, Web of Science, and Scopus for a narrative review on fracture mechanics of crown and root dentine as well as the clinical management of cracked teeth and teeth with vertical root fracture.

CONCLUSIONS

Although dentine is tougher and less brittle than enamel, it's facture toughness is considerably lower than most ductile metals. Because the initiation toughness of dentine is very low, cracks initiate from incipient damage under low stress While crack toughening mechanisms exist that enable dentine to resist crack extension, these mechanisms are often inadequate for protecting dentine from crack propagation that ultimately leads to catastrophic failure. Additional factors such as ageing also reduces the resistance of dentine to crack growth. Because dentine cracks are eventually filled with bacteria biofilms upon exposure to oral fluids, they enable rapid bacteria ingress into the dental pulp via open dentinal tubules. To date, treatment options for cracked teeth are limited. While most teeth with vertical root fracture are recommended for extraction, new strategies have been reported that appeared to achieve short-term success in preserving these teeth.

CLINICAL SIGNIFICANCE

Current strategies for the management for dentine cracks and fractures are limited and their long-term effectiveness remain uncertain. Understanding the characteristics, toughening mechanism and weakening factors of tooth cracks is helpful in designing better treatment.

Effect of thickness and CAD-CAM material on fatigue resistance of endodontically treated molars restored with occlusal veneers

PURPOSE

To evaluate the influence of the thickness and type of computer-aided design and computer-aided manufacturing (CAD-CAM) material on the fatigue resistance and failure mode of endodontically treated teeth (ETT) restored with occlusal veneers (OV).

MATERIALS AND METHODS

Seventy-five (N = 75) ETT were restored with Herculite XRV in the endodontic access. Five experimental groups (n = 15) were tested. Four groups had two different thicknesses (0.6-0.7 mm or 1.4-1.6 mm) and two different CAD-CAM materials: zirconia-reinforced lithium-silicate (LS/Celtra Duo) and composite resin (RC/Cerasmart). The fifth group (control) did not have occlusal veneers. All the specimens were subjected to accelerated fatigue (5 Hz frequency) with an occlusal load increasing up to 1800 N and 131,000 cycles. The number of cycles was recorded when the machine stopped or at the completion of the test. Fatigue resistance was analyzed using the Kaplan-Meier survival test (95% significance level, log-rank post hoc pairwise comparisons). The samples were categorized according to failure mode. The CAD-CAM materials were examined through scanning electron microscopy (SEM) and energy dispersive X-ray spectroscopy (EDS).

RESULTS

No differences were found between the thicknesses, regardless of the type of the CAD-CAM material. The thick LS OV outperformed the RC and control groups. The thin RC OV and control groups showed a higher percentage of repairable and possibly repairable failures than the other groups. LS was more homogeneous under SEM, and the EDS analysis detected Si and Zr, but not Li.

CONCLUSIONS

A larger thickness did not improve the resistance of the CAD-CAM materials. Thick LS showed a higher cumulative survival rate to fatigue than the RC and control groups. The direct composite alone (control) survived similarly to the experimental groups, except for the thick LS.

Diagnosis of in vivo vertical root fracture using deep learning on cone-beam CT images

Objectives

Evaluating the diagnostic efficiency of deep learning models to diagnose vertical root fracture in vivo on cone-beam CT (CBCT) images.

Materials and methods

The CBCT images of 276 teeth (138 VRF teeth and 138 non-VRF teeth) were enrolled and analyzed retrospectively. The diagnostic results of these teeth were confirmed by two chief radiologists. There were two experimental groups: auto-selection group and manual selection group. A total of 552 regions of interest of teeth were cropped in manual selection group and 1118 regions of interest of teeth were cropped in auto-selection group. Three deep learning networks (ResNet50, VGG19 and DenseNet169) were used for diagnosis (3:1 for training and testing). The diagnostic efficiencies (accuracy, sensitivity, specificity, and area under the curve (AUC)) of three networks were calculated in two experiment groups. Meanwhile, 552 teeth images in manual selection group were diagnosed by a radiologist. The diagnostic efficiencies of the three deep learning network models in two experiment groups and the radiologist were calculated.

Results

In manual selection group, ResNet50 presented highest accuracy and sensitivity for diagnosing VRF teeth. The accuracy, sensitivity, specificity and AUC was 97.8%, 97.0%, 98.5%, and 0.99, the radiologist presented accuracy, sensitivity, and specificity as 95.3%, 96.4 and 94.2%. In auto-selection group, ResNet50 presented highest accuracy and sensitivity for diagnosing VRF teeth, the accuracy, sensitivity, specificity and AUC was 91.4%, 92.1%, 90.7% and 0.96.

Conclusion

In manual selection group, ResNet50 presented higher diagnostic efficiency in diagnosis of in vivo VRF teeth than VGG19, DensenNet169 and radiologist with 2 years of experience. In auto-selection group, Resnet50 also presented higher diagnostic efficiency in diagnosis of in vivo VRF teeth than VGG19 and DensenNet169. This makes it a promising auxiliary diagnostic technique to screen for VRF teeth.

Effects of composite resin core level and periodontal pocket depth on crack propagation in endodontically treated teeth: An extended finite element method study

STATEMENT OF PROBLEM

Preserving teeth with radicular cracks with or without a periodontal pocket is an alternative to extraction. However, an effective protocol for the restoration of radicular cracks is lacking.

PURPOSE

The purpose of this study was to examine the composite resin core level and periodontal pocket depth effects on stress distribution, maximum von Mises stress, and crack propagation in endodontically treated teeth by using the extended finite element (FE) method.

MATERIAL AND METHODS

Four 3-dimensional models of a cracked endodontically treated mandibular first molar were constructed: PP2C2 (periodontal pocket depth, 2 mm; composite resin core level, 2 mm below the canal orifice level); PP2C4 (periodontal pocket depth, 2 mm; composite resin core level, 2 mm below the crack level); PP4C2 (periodontal pocket depth, 4 mm; composite resin core level, 2 mm below the canal orifice level); and PP4C4 (periodontal pocket depth, 4 mm; composite resin core level, 2 mm below the crack level). The crack initiation was at the same level in all models. A static 700-N load was applied to the models in a vertical direction.

RESULTS

The highest stress in dentin was observed in PP2C2, whereas PP2C4 exhibited the lowest stress and least crack propagation. Stress was high in the dentin and supporting bone. No reduction in crack propagation was observed in the PP4 models, regardless of the composite resin core level.

CONCLUSIONS

The periodontal pocket depth (2 mm and 4 mm) and composite resin core level (2 mm below the crack level and 2 mm below the canal orifice level) affected stress concentration in dentin, resulting in different patterns of crack propagation in the FE models.

Does Low-Taper Root Canal Shaping Decrease the Risk of Root Fracture? A Systematic Review

Minimal root-canal preparation has been suggested to reduce the risk of root fracture, but as a result, satisfactory cleaning and shaping do not take place. Large-scale taper instrumentation can contribute to removing infected tissue; however, it may weaken the tooth structure. The aim of this systematic review is to evaluate whether root-canal shaping with low-taper instruments decreases the risk of root fracture, compared to high-conicity shaping. A search was performed on Ovid MEDLINE, PubMed, and the Web of Science. The inclusion criteria were: studies comparing the root fracture resistance of endodontically treated teeth, shaped with low- and high-conicity taper instruments, in human trials, and via in vitro study. The review includes all types of endodontically treated teeth, with various instrument tapers. The scientific search engines produced 328 results. Only 20 of the results were evaluated after screening. Based on the articles analyzed, it is not clear whether a taper difference can determine differences in root fracture resistance. No randomized controlled trial (RCTs) with long follow-ups have been published to date. It must also be taken into account that the in vitro studies do not consider the numerous differences that there are between in vitro and clinical evaluation. The review was registered on the PROSPERO website, with the protocol number CRD42020151451.

Comparison of diagnosis of cracked tooth using contrast-enhanced CBCT and micro-CT

OBJECTIVES

To evaluate the diagnostic accuracy using sodium iodide (NaI) and dimethyl sulfoxide (DMSO) as contrast agent in cone beam computed tomography (CBCT) scanning, and compare this with micro-CT.

METHODS

18 teeth were cracked artificially by soaking them cyclically in liquid nitrogen and hot water. After pre-treatment with artificial saliva, the teeth were scanned in four modes: CBCT routine scanning without contrast agent (RS); CBCT with meglumine diatrizoate (MD) as contrast agent (ES1); CBCT with NaI + DMSO as contrast agent (ES2); and micro-CT (mCT). The number of crack lines was evaluated in all four modes. Depth of crack lines and number of cracks presented from the occlusal surface to the pulp cavity (Np) in ES2 and micro-CT images were evaluated.

RESULTS

There were 63 crack lines in all 18 teeth. 45 crack lines were visible on ES2 images as against four on the RS and ES1 images (p<0.05) and 37 on micro-CT images (p>0.05). Further, 34 crack lines could be observed on both ES2 and micro-CT images, and the average depth presented on ES2 images was 4.56 ± 0.88 mm and 3.89 ± 1.08 mm on micro-CT images (p<0.05). More crack lines could be detected from the occlusal surface to the pulp cavity on ES2 images than on micro-CT images (22 vs 11).

CONCLUSION

CBCT with NaI +DMSO as the contrast agent was equivalent to micro-CT for number of crack lines and better for depth of crack lines. NaI + DMSO could be a potential CBCT contrast agent to improve diagnostic accuracy for cracked tooth.

Effect of cervical lesion centered access cavity restored with short glass fibre reinforced resin composites on fracture resistance in human mandibular premolars- an in vitro study

AIM

The aim of this study was to evaluate the fracture resistance of cervical lesion centered access cavity restored with short glass fibre reinforced resin materials in human mandibular premolars.

MATERIALS AND METHODS

Sixty freshly extracted human mandibular premolars were collected and assigned to positive control group (G1- Intact teeth) and other experimental groups (G2, G3. G4, G5, G6), Traditional Access Cavity(TAC) and Cervical Lesion Centered Access Cavity(CLCAC). Following endodontic therapy, samples were grouped accordingly, G2-CLCAC without restoration (Negative Control), G3-CLCAC restored with conventional nano-hybrid composite (Tetric-N-Ceram), G4-TAC restored with short glass fibre reinforced resin composite (Ever-X Posterior), G5-CLCAC restored with short glass fibre reinforced resin composite (Ever-X Posterior), G6- CLCAC restored with individually formed unidirectional fibre-reinforced post (Everstick post) and short glass fibre-reinforced resin composite (Ever-X Posterior). The samples were thermocycled (35 °C/28s, 15 °C/2s, 35 °C/28s, 45 °C/2s/10,000 cycles) and cyclically fatigued(2,50,000 cycles/15-30N/2 Hz) and then subsequently loaded to failure. The mean load to fracture (Newtons) were statistically analyzed using one-way ANOVA and Tukey's post HOC test and failure mode analysis was also done.

RESULTS

The mean fracture resistance of the CLCAC design restored with fibre reinforced materials was higher when compared to the TAC design but the difference was not statistically significant. The negative control group (CLCAC without restoration) showed significantly the least fracture resistance (P < 0.05) when compared to all the other groups except for group 3 (CLCAC restored with conventional composites).

CONCLUSIONS

Within the limitations of the study, it can be concluded that short glass fibre reinforced resin composites improved the fracture resistance of endodontically treated mandibular premolars irrespective of the type of access cavity designs. Favourable fractures were seen more in cervical lesion centered access cavity restored with short glass fibre reinforced composite materials. Nevertheless, the applicability of this design in multirooted teeth, canal cleanliness efficacy, and long term clinical performance are yet to be explored to complement this new access design.

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.

Current perspectives on dental adhesion: (3) Adhesion to intraradicular dentin: Concepts and applications

Adhesion science is one of the greatest contributions to restorative dentistry. Adhesion not only established the current principles of tissue preservation, but also allowed for the production of more hermetic and long-lasting restorations. Although adhesive strategies are routinely used in most clinical situations, adhesion to root dentin is still a major challenge. The presence of humidity together with less intertubular dentin are factors that limit the adhesive potential of root dentin. This situation is more unfavorable in endodontically treated teeth prepared for prefabricated or custom-made intraradicular posts; these procedures may alter the mechanical properties of teeth by modifying the viable dentin surface for adhesion. Also, contaminants deposited on the dentin surface are difficult to remove through conventional techniques. Moreover, root canal morphology has a very unfavorable C-factor, bringing undesirable effects resulting from polymerization contraction of resin-based materials. However, the differences between coronal and root dentin are not a barrier for dentin adhesion. Standardization of procedures and care during clinical steps are fundamental to the success of adhesion to coronal or intraradicular dentin. Thus, it is essential to know the anatomy of the root structure, the factors that interfere with intraradicular adhesion, as well as the current adhesive materials and techniques.

Optical and Mechanical Properties of Highly Translucent Dental Zirconia

The aim was to evaluate the translucency, opalescence, and fluorescence of highly translucent zirconia, lithium disilicate, and bovine teeth. One mm-thick specimens of five monolithic zirconia systems, two glass-ceramics, and bovine enamel/dentin were investigated. A spectrophotometer (Ci7600) was used to measure the CIELab color coordinates, and the translucency and opalescence values were obtained. For evaluating the fluorescence emission, the differences in spectral reflectance by the UV component of illumination were calculated. The microstructures of ceramic specimens were examined with a scanning electron microscope and the chemical compositions were determined with an X-ray fluorescence spectrometer. Mechanical properties were appraised with three-point bending strength, indentation fracture toughness, and Vickers hardness. Data were analyzed using a one-way ANOVA, followed by Tukey’s multiple comparison test (α = 0.05). A higher yttria content (5 mol%) significantly improved the translucency of zirconia ceramics, while they were less translucent than lithium disilicate (p < 0.05). Lowering the alumina content below 0.05 wt.% enhanced the translucency (p < 0.05), but a small amount of alumina was still required to obtain full densification. 0.05 wt.% Fe was used to increase the chroma of zirconia specimens without compromising their mechanical properties. The Er-containing zirconia specimen showed a maximal fluorescence emission at 430 nm. The degree of opalescence was affected by the microstructures of ceramic materials. The microstructure, incorporation of a secondary phase, and sintering behavior can have a strong impact on the final mechanical and optical properties of dental ceramics. Addition of small amounts of metal oxides can affect the translucency, opalescence or fluorescence qualities of zirconia

Stress Analyses of Retrograde Cavity Preparation Designs for Surgical Endodontics in the Mesial Root of the Mandibular Molar: A Finite Element Analysis-Part I

INTRODUCTION

The aim of this study was to investigate the influence of various apical preparation designs for surgical endodontics on stress concentrations in the mesial root of the mandibular molar under different experimental conditions using finite element analysis.

METHODS

We designed 2 apical preparation groups according to whether an isthmus was present or not. Each group contained 4 subgroups according to the size of the apical preparation. We constrained the displacement of all nodes at the base of the supporting bone and applied a force of 150 N to the vertical axis. We analyzed stress generation and concentrations numerically for the groups and subgroups.

RESULTS

In the subgroups, the von Mises and maximum principal stresses reduced gradually according to the enlargement of the prepared cavity. However, when the preparation extended excessively in the isthmus preparation groups, the situation reversed (ie, both von Mises and maximum principal stresses increased).

CONCLUSIONS

Within the limitations of this study, the apical preparation design influenced the distribution of stress concentration. Unlike the overall pattern in which stress decreased as the amount of apical preparation increased, stress increased when the amount of residual dentin was extremely thin.

A theory of biological composites undergoing plastic deformations

Natural biological composites such as bone, dentin, nacre and enamel exhibit anisotropic microstructures, giving rise to orientation-dependent mechanical properties. Although the mechanical properties of these materials have been studied extensively, there is limited progress on modeling the common features associated with the orientation-dependent plastic deformation of biological composites. In this study, we develop a continuum theory for elastic-viscoplastic deformations of anisotropic biological composites. The pressure-sensitive and plastically dilatant plastic flow is incorporated into the theory, and the plastic spin related to the kinematics of the underlying substructure during macroscopic plastic deformation is explicitly taken into account. A special set of constitutive equations are implemented in a finite element program. Furthermore, the material parameters have been calibrated and numerical simulations of elastic-plastic deformation in bone are performed. It is found that the theory can capture the major features of plastic deformation of biological composites. The numerical simulations are in good agreement with experiments, demonstrating that the model is capable of predicting the complex plastic deformation of bone.

Effect of cryopreservation of teeth on the structural integrity of dentin

The autotransplantation of teeth after cryopreservation has become an increasingly viable method for whole tooth replacement. While the immediate success rates are quite high, damage introduced by cryopreservation within the dentin or enamel could be detrimental to the durability of these teeth.

OBJECTIVE

to determine whether cryopreservation alters the microstructure of dentin or causes a reduction of its resistance to mechanical failures.

METHODS

Third molars were obtained from young donors (18≤age≤30yrs) and subjected to a cryopreservation protocol involving storage for 10days in cryoprotectant solution at -196°C. After treatment, the mid-coronal dentin was characterized in terms of its elastic modulus, strength and fatigue behavior. Scanning electron microscopy and Raman spectroscopy were used to evaluate the microstructure and integrity of collagen after cryopreservation.

RESULTS

There was no significant difference in the elastic modulus or flexural strength between dentin from the cryopreserved and non-cryopreserved (control) teeth. However, the cryopreservation treatment caused a significant decrease in the fatigue strength of dentin with respect to the controls, with average reduction of nearly 20%. While there were no differences apparent in the collagen matrix or fracture surfaces between the cryopreserved and control groups, the microstructure of dentin from the cryopreserved teeth exhibited unique features and damage that appear to have caused the decrease in durability.

SIGNIFICANCE

Autotransplantation of cryopreserved teeth may be a viable option for whole tooth restorations, but hidden damage within the dentin could render these teeth more susceptible to mechanical failures by fatigue and fracture.

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.

Mesoscale porosity at the dentin-enamel junction could affect the biomechanical properties of teeth

In this paper, the 3D-morphology of the porosity in dentin is investigated within the first 350μm from the dentin-enamel junction (DEJ) by fluorescence confocal laser scanning microscopy (CLSM). We found that the porous microstructure exhibits a much more complex geometry than classically described, which may impact our fundamental understanding of the mechanical behavior of teeth and could have practical consequences for dental surgery. Our 3D observations reveal numerous fine branches stemming from the tubules which may play a role in cellular communication or mechanosensing during the early stages of dentinogenesis. The effect of this highly branched microstructure on the local mechanical properties is investigated by means of numerical simulations. Under simplified assumptions on the surrounding tissue characteristics, we find that the presence of fine branches negatively affects the mechanical properties by creating local stress concentrations. However, this effect is reduced by the presence of peritubular dentin surrounding the tubules. The porosity was also quantified using the CSLM data and compared to this derived from SEM imaging. A bimodal distribution of channel diameters was found near the DEJ with a mean value of 1.5-2μm for the tubules and 0.3-0.5μm for the fine branches which contribute to 30% of the total porosity (∼1.2%). A gradient in the branching density was observed from the DEJ towards the pulp, independently of the anatomical location. Our work constitutes an incentive towards more elaborate multiscale studies of dentin microstructure to better assess the effect of aging and for the design of biomaterials used in dentistry, e.g. to ensure more efficient bonding to dentin. Finally, our analysis of the tubular network structure provides valuable data to improve current numerical models.

Learning from evolutionary optimisation: what are toughening mechanisms good for in dentine, a nonrepairing bone tissue?

The main mass of material found in teeth is dentine, a bone-like tissue, riddled with micron-sized tubules and devoid of living cells. It provides support to the outer wear-resistant layer of enamel, and exhibits toughening mechanisms which contribute to crack resistance. And yet unlike most bone tissues, dentine does not remodel and consequently any accumulated damage does not 'self repair'. Because damage containment followed by tissue replacement is a prime reason for the crack-arresting microstructures found in most bones, the occurrence of toughening mechanisms without the biological capability to repair is puzzling. Here we consider the notion that dentine might be overdesigned for strength, because it has to compensate for the lack of cell-mediated healing mechanisms. Based on our own and on literature-reported observations, including quasistatic and fatigue properties, dentine design principles are discussed in light of the functional conditions under which teeth evolved. We conclude that dentine is only slightly overdesigned for everyday cyclic loading because usual mastication stresses may come close to its endurance strength. The in-built toughening mechanisms constitute an evolutionary benefit because they prevent catastrophic failure during rare overload events, which was probably very advantageous in our hunter gatherer ancestor times. From a bio-inspired perspective, understanding the extent of evolutionary overdesign might be useful for optimising biomimetic structures used for load bearing.

Compressive Residual Strains in Mineral Nanoparticles as a Possible Origin of Enhanced Crack Resistance in Human Tooth Dentin

The tough bulk of dentin in teeth supports enamel, creating cutting and grinding biostructures with superior failure resistance that is not fully understood. Synchrotron-based diffraction methods, utilizing micro- and nanofocused X-ray beams, reveal that the nm-sized mineral particles aligned with collagen are precompressed and that the residual strains vanish upon mild annealing. We show the link between the mineral nanoparticles and known damage propagation trajectories in dentin, suggesting a previously overlooked compression-mediated toughening mechanism.

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.