Remarkable resilience of teeth

A Gradient Enamel-Mimetic Composite via Crisscross Assembly of Aligned Hybrid Nanowires for Excellent Mechanical Performance

Materials with excellent comprehensive mechanical properties (e.g., strength and toughness, stiffness and damping, fatigue et al.) are highly desirable for engineering applications, while it is still challenged for design. Tooth enamel is a typical biomaterial with outstanding mechanical properties that originate from its multiscale and gradient structure. Some composites with enamel-like multiscale structures are successfully synthesized, but mimicking the gradient structure of tooth enamel is still difficult to realize. Here, an enamel analog is fabricated with a gradient structure similar to inner enamel based on the crisscross assembly of aligned hybrid nanowires through a magnetic-assisted freeze casting and subsequent mechanical compression strategy. The gradient enamel-mimetic composites exhibited high strength and toughness surpassing the natural tooth enamel, and simultaneously high stiffness and damping comparable to those of enamel, as well as high fatigue resistance. The interface reinforcement of gradient structure, crystal/amorphous and organic/inorganic, fundamentally accounted for high mechanical performance. The gradient design strategy provides an avenue for the engineering of structural materials with excellent mechanical properties.

Determination of gender and stature from crown dimensions of primary teeth: An Odontometric study

BACKGROUND

Crown dimensions of teeth have been positively correlated with stature and gender. These odontometric values can aid significantly in the forensic identification of individuals in case of mass disasters as teeth are the least affected structures during such calamities. There is a dearth of studies investigating crown dimensions as a tool in forensic identification, especially in the pediatric population.

AIM

The aim of the study was to assess the correlation between stature, gender, and crown dimensions (occlusogingival [OG] height and mesiodistal [MD] width) in a pediatric population and to derive mathematical equations to predict stature and gender using these crown dimensions.

MATERIALS AND METHODS

A cross-sectional study was conducted among 1057 South Indian children in the age group of 3-6 years. Impressions of the maxillary dentition were made, and casts were obtained. A Vernier caliper was used to measure the OG height and MD width of teeth 51, 52, 53, 61, 62, and 63. A measuring tape was used to measure the stature of the children, and the gender was noted. These data were subjected to statistical analysis to obtain the results.

RESULTS

Male children had a significantly higher mean MD width and OG height with respect to teeth 52, 53, and 62. A statistically significant positive correlation was seen in teeth 62 between stature and crown dimensions indicating an increase in stature with an increase in these crown dimensions.

CONCLUSIONS

A positive correlation was affirmed between the crown dimensions of teeth 52, 53, and 62, stature and gender of the children. Mathematical equations were also successfully derived using these crown dimensions to predict the stature and gender of children.

Ameloblastin binding to biomimetic models of cell membranes - A continuum of intrinsic disorder

OBJECTIVE

A 37-residue amino acid sequence corresponding to the segment encoded by exon-5 of murine ameloblastin (Ambn), AB2 (Y67-Q103), has been implicated with membrane association, ameloblastin self-assembly, and amelogenin-binding. Our aim was to characterize, at the residue level, the structural behavior of AB2 bound to chemical mimics of biological membranes using NMR spectroscopy.

DESIGN

To better define the structure of AB2 using NMR-based methods, recombinant 13C- and 15N-labelled AB2 (*AB2) was prepared and data collected free in solution and with deuterated dodecylphosphocholine (dPC) micelles, deuterated bicelles, and both small and large unilamellar vesicles.

RESULTS

Amide chemical shift and intensity perturbations observed in 1H-15N HSQC spectra of *AB2 in the presence of bicelles and dPC micelles suggest that a region of *AB2, S6-E36 (murine Ambn S68 - E98), associates with the membrane biomimetics. A CSI-3 analysis of the NMR chemical shift assignments for *AB2 free in solution and bound to dPC micelles indicated the peptide remains disordered except for the adoption of a short, 12-residue α-helix, F10-G21 (murine Ambn F72-G83). In dPC micelles, the NOE NMR data was void of patterns characteristic of long-lived helical structure indicating this helix was transient in nature.

CONCLUSIONS

A continuum of intrinsic disorder in the membrane-bound state may be responsible for ameloblastin's ability to dynamically interact with multiple partners at the same site during amelogenesis.

Role of amelogenin phosphorylation in regulating dental enamel formation

Amelogenin (AMELX), the predominant matrix protein in enamel formation, contains a singular phosphorylation site at Serine 16 (S16) that greatly enhances AMELX's capacity to stabilize amorphous calcium phosphate (ACP) and inhibit its transformation to apatitic enamel crystals. To explore the potential role of AMELX phosphorylation in vivo, we developed a knock-in (KI) mouse model in which AMELX phosphorylation is prevented by substituting S16 with Ala (A). As anticipated, AMELXS16A KI mice displayed a severe phenotype characterized by weak hypoplastic enamel, absence of enamel rods, extensive ectopic calcifications, a greater rate of ACP transformation to apatitic crystals, and progressive cell pathology in enamel-forming cells (ameloblasts). In the present investigation, our focus was on understanding the mechanisms of action of phosphorylated AMELX in amelogenesis. We have hypothesized that the absence of AMELX phosphorylation would result in a loss of controlled mineralization during the secretory stage of amelogenesis, leading to an enhanced rate of enamel mineralization that causes enamel acidification due to excessive proton release. To test these hypotheses, we employed microcomputed tomography (µCT), colorimetric pH assessment, and Fourier Transform Infrared (FTIR) microspectroscopy of apical portions of mandibular incisors from 8-week old wildtype (WT) and KI mice. As hypothesized, µCT analyses demonstrated significantly higher rates of enamel mineral densification in KI mice during the secretory stage compared to the WT. Despite a greater rate of enamel densification, maximal KI enamel thickness increased at a significantly lower rate than that of the WT during the secretory stage of amelogenesis, reaching a thickness in mid-maturation that is approximately half that of the WT. pH assessments revealed a lower pH in secretory enamel in KI compared to WT mice, as hypothesized. FTIR findings further demonstrated that KI enamel is comprised of significantly greater amounts of acid phosphate compared to the WT, consistent with our pH assessments. Furthermore, FTIR microspectroscopy indicated a significantly higher mineral-to-organic ratio in KI enamel, as supported by µCT findings. Collectively, our current findings demonstrate that phosphorylated AMELX plays crucial mechanistic roles in regulating the rate of enamel mineral formation, and in maintaining physico-chemical homeostasis and the enamel growth pattern during early stages of amelogenesis.

Managing Cracked Teeth with Root Extension: A Prospective Preliminary Study Using Biodentine™ Material

Purpose

The authors of this study proposed an innovative approach involving the use of Biodentine™ material as an intraorifice barrier in cracked teeth with root extension to promote internal crack sealing, preventing the possibility of microinfiltration and apical crack propagation.

Materials and Methods

The dental records of 11 patients with 12 posterior cracked teeth with root extension were included with a precise protocol performed by a senior endodontist. The treatment protocol included pulp diagnosis, crack identification using a dental operating microscope (DOM), endodontic treatment, placing a Biodentine™ as an intraorifice barrier, and immediate full-coverage restoration. The effectiveness of the treatment was assessed at two intervals, 6 months, and 1-3 years posttreatment, evaluating clinical, radiographic, and tomographic aspects. The treatment was deemed successful if there were no indications of radiolucency, sinus tracts, edema, or periodontal pockets associated with the crack line.

Results

The study observed remarkably positive outcomes during the follow-up period, which spanned from 1 to 3 years. All the cracked teeth (100%) remained asymptomatic, meaning they were free of pain or discomfort. Furthermore, these teeth were in occlusal function. Both radiographic and tomographic assessments revealed the absence of bone loss along the crack line. This outcome signifies that the treatment effectively prevented further deterioration of the surrounding bone.

Conclusions

Integrating advanced biomaterials and conservative restorative techniques has paved the way for innovative approaches in dental care. This protocol suggests a proactive step for managing cracked teeth with root extension. It addresses both biological aspects by sealing internal cracks and mechanical aspects by preventing crack progression, thereby improving these teeth' prognosis and long-term survival.

Nanocrystal residual strains and density layers enhance failure resistance in the cleithrum bone of evolutionary advanced pike fish

Failure-resistant designs are particularly crucial for bones subjected to rapid loading, as is the case for the ambush-hunting northern pike (Esox lucius). These fish have slim and low-density osteocyte-lacking bones. As part of the swallowing mechanism, the cleithrum bone opens and closes the jaw. The cleithrum needs sufficient strength and damage tolerance, to withstand years of repetitive rapid gape-and-suck cycles of feeding. The thin wing-shaped bone comprises anisotropic layers of mineralized collagen fibers that exhibit periodic variations in mineral density on the mm and micrometer length scales. Wavy collagen fibrils interconnect these layers yielding a highly anisotropic structure. Hydrated cleithra exhibit Young's moduli spanning 3-9 GPa where the yield stress of ∼40 MPa increases markedly to exceed ∼180 MPa upon drying. This 5x observation of increased strength corresponds to a change to brittle fracture patterns. It matches the emergence of compressive residual strains of ∼0.15% within the mineral crystals due to forces from shrinking collagen layers. Compressive stresses on the nanoscale, combined with the layered anisotropic microstructure on the mm length scale, jointly confer structural stability in the slender and lightweight bones. By employing a range of X-ray, electron and optical imaging and mechanical characterization techniques, we reveal the structure and properties that make the cleithra impressively damage resistant composites. STATEMENT OF SIGNIFICANCE: By combining structural and mechanical characterization techniques spanning the mm to the sub-nanometer length scales, this work provides insights into the structural organization and properties of a resilient bone found in pike fish. Our observations show how the anosteocytic bone within the pectoral gridle of these fish, lacking any biological (remodeling) repair mechanisms, is adapted to sustain natural repeated loading cycles of abrupt jaw-gaping and swallowing. We find residual strains within the mineral apatite nanocrystals that contribute to forming a remarkably resilient composite material. Such information gleaned from bony structures that are different from the usual bones of mammals showcases how nature incorporates smart features that induce damage tolerance in bone material, an adaptation acquired through natural evolutionary processes.

Bioinspired rational design of bi-material 3D printed soft-hard interfaces

Durable interfacing of hard and soft materials is a major design challenge caused by the ensuing stress concentrations. In nature, soft-hard interfaces exhibit remarkable mechanical performance, with failures rarely happening at the interface. Here, we mimic the strategies observed in nature to design efficient soft-hard interfaces. We base our geometrical designs on triply periodic minimal surfaces (i.e., Octo, Diamond, and Gyroid), collagen-like triple helices, and randomly distributed particles. A combination of computational simulations and experimental techniques, including uniaxial tensile and quad-lap shear tests, are used to characterize the mechanical performance of the interfaces. Our analyses suggest that smooth interdigitated connections, compliant gradient transitions, and either decreasing or constraining strain concentrations lead to simultaneously strong and tough interfaces. We generate additional interfaces where the abovementioned toughening mechanisms work synergistically to create soft-hard interfaces with strengths approaching the upper achievable limit and enhancing toughness values by 50%, as compared to the control group.

Exploring the influence of placing bi-directional E-glass fibers as protective layer under a CAD-CAM resin composite on the fracture pattern

OBJECTIVES

To investigate the influence of the presence and position of bidirectional E-glass fibers under a CAD-CAM resin composite on the fracture pattern evaluated both after quasi-static mechanical loading and after fatigue.

METHODS

Rectangular specimens (10 mm-long, 5 mm-large and 4.2 mm-thick) were prepared and divided into four groups (n = 30/group). The control group (C-Group) consisted of a 4.2 mm-thick layer of monolithic CAD/CAM resin composite resin (Cerasmart, GC). In the 3 other groups including the placement of a fiber layer (F-Groups), the CAD/CAM resin composite layer was reduced to 3-, 2- and 1-mm thickness (F3-, F2- and F1-Groups, respectively). Two bonded layers of bidirectional E-glass FRC (Dentapreg, ADM A.S.) were bonded underneath and a light-curable resin composite base (Gaenial Posterior, GC) was then added to reach a total thickness of 4.2 mm for all samples. In each group, half of the specimens (n = 15) were submitted to quasi-static mechanical loading to failure in a universal testing machine. The other half (n = 15) was subjected to cyclic isometric stepwise loading until failure or completion of 105000 cycles (5000 cycles at 500 N, followed by five stages of 20000 cycles at 750 N, 1000 N, 1250 N, 1500 N, and 1750 N). The data were analyzed by Weibull statistics for quasi-static loading, and by the Kaplan-Meier product limit estimation procedure after fatigue. All fractured specimens were studied using light and electron microscopy techniques, and the types of fracture were determined.

RESULTS

For quasi-static mechanical loading, significant differences were observed for Weibull modulus and characteristic strength between groups, with values ranging from 10.8 to 22.4 for the former and from 2336.6 to 2974.7 for the latter. Also, survival after stepwise fatigue revealed statistically significant differences between groups (p < 0.05), the lowest values of cycles before failure being observed for F1-Group - Median = 61223 (50415; 65446) - as compared to the other groups - C-Group: Median = 89005 (86189; 98195); F3-Group: Median = 85198 (77279; 87860); F2-Group: Median = 89306 (87454; 97024). Both in quasi-static loading and after fatigue, the observation of fracture modes revealed major differences. While all fractures were vertical (split) in C-Group, the majority of the specimens in F-Groups presented some degree of horizontal deflection of the crack. In all deviated fractures, fractographic analysis confirmed a toughening effect of the fiber layer.

SIGNIFICANCE

The present in vitro work tends to show that the fracture pattern of CAD-CAM resin composites is favorably affected by the presence and position of an underlying bidirectional E-glass fiber layer. The placement of E-glass fibers under a CAD-CAM resin composite may therefore represent an interesting strategy to reduce the risk of catastrophic restoration failure, which could be integrated in the development of the new generation of indirect materials, possibly in 3D-printing approaches.

Deciphering load attenuation mechanisms of the dentin-enamel junction: Insights from a viscoelastic constitutive model

A considerable material discontinuity between the enamel and dentin might jeopardize the tooth's mechanical durability over time without the attenuation of the dentin-enamel junction (DEJ). However, the critical loading transmission mechanism at the DEJ remains understudied. This study aimed to define the extent and effective width of the DEJ, along with its mechanical competence. The presence of DEJ interphase layer was identified using a motif analysis based on the ion beam-transmission electron microscopy coupled with nanoindentation modulus mapping. For each region, nanoindentation load-displacement curves were recorded and mathematically analyzed using an appropriate viscoelastic constitutive model. The time-course of indenter penetration (creep) behavior of the tooth tissues can be mathematically approximated by the Kelvin-Voigt model in series, which determined the visco-contribution to the overall mechanical responses. Therefore, the elastic-plastic contribution can be distinguished from the overall mechanical responses of the tooth after subtracting the visco-contributions. During the loading period, the enamel behavior was dominated by elastic-plastic responses, while both the dentin and DEJ showed pronounced viscoelastic responses. The instantaneous modulus of the DEJ, which was measured by eliminating viscoelastic behavior from the raw load-displacement curve, was almost double that of the dentin. The DEJ was stiffer than the dentin, but it exhibited large viscoelastic motion even at the initial loading stage. This study revealed that the load attenuation competence of the DEJ, which involves extra energy expenditure, is mainly associated with its viscoelasticity. The mathematical analysis proposed here, performed on the nanoindentation creep behavior, could potentially augment the existing knowledge on hard-tissue biomechanics. STATEMENT OF SIGNIFICANCE: In this study, we undertake a rigorous mechanical characterization of the dentin-enamel junction (DEJ) using an advanced nanoindentation technique coupled with a pertinent viscoelastic constitutive model. Our approach unveils the substantial viscoelastic contribution of the DEJ during the initial indentation loading phase and offers an elaborate delineation of the DEJ interphase layer through sophisticated image analysis. These insights significantly augment our understanding of tooth durability. Importantly, our innovative mathematical analysis of creep behavior introduces a novel approach with profound implications for future research in the expansive field of hard-tissue biomechanics. The pioneering methodologies and findings presented in this work hold substantial potential to invigorate progress in biomaterials research and fuel further explorations into the functionality of biological tissues.

Proteomic Analyses Discern the Developmental Inclusion of Albumin in Pig Enamel: A New Model for Human Enamel Hypomineralization

Excess albumin in enamel is a characteristic of the prevalent developmental dental defect known as chalky teeth or molar hypomineralization (MH). This study uses proteomic analyses of pig teeth to discern between developmental origin and post-eruptive contamination and to assess the similarity to hypomineralized human enamel. Here, the objective is to address the urgent need for an animal model to uncover the etiology of MH and to improve treatment. Porcine enamel is chalky and soft at eruption; yet, it hardens quickly to form a hard surface and then resembles human teeth with demarcated enamel opacities. Proteomic analyses of enamel from erupted teeth, serum, and saliva from pigs aged 4 (n = 3) and 8 weeks (n = 2) and human (n = 4) molars with demarcated enamel opacities show alpha-fetoprotein (AFP). AFP expression is limited to pre- and perinatal development and its presence in enamel indicates pre- or perinatal inclusion. In contrast, albumin is expressed after birth, indicating postnatal inclusion into enamel. Peptides were extracted from enamel and analyzed by nano-liquid chromatography-tandem mass spectrometry (nanoLC-MS/MS) after tryptic digestion. The mean total protein number was 337 in the enamel of all teeth with 13 different unique tryptic peptides of porcine AFP in all enamel samples but none in saliva samples. Similarities in the composition, micro-hardness, and microstructure underscore the usefulness of the porcine model to uncover the MH etiology, cellular mechanisms of albumin inclusion, and treatment for demarcated opacities.

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.

The biomechanics of tooth strength: testing the utility of simple models for predicting fracture in geometrically complex teeth

Teeth must fracture foods while avoiding being fractured themselves. This study evaluated dome biomechanical models used to describe tooth strength. Finite-element analysis (FEA) tested whether the predictions of the dome models applied to the complex geometry of an actual tooth. A finite-element model was built from microCT scans of a human M3. The FEA included three loading regimes simulating contact between (i) a hard object and a single cusp tip, (ii) a hard object and all major cusp tips and (iii) a soft object and the entire occlusal basin. Our results corroborate the dome models with respect to the distribution and orientation of tensile stresses, but document heterogeneity of stress orientation across the lateral enamel. This implies that high stresses might not cause fractures to fully propagate between cusp tip and cervix under certain loading conditions. The crown is most at risk of failing during hard object biting on a single cusp. Geometrically simple biomechanical models are valuable tools for understanding tooth function but do not fully capture aspects of biomechanical performance in actual teeth whose complex geometries may reflect adaptations for strength.

Dental topography of the Oligocene anthropoids Aegyptopithecus zeuxis and Apidium phiomense: Paleodietary insights from analysis of wear series

Fossil primate dietary inference is enhanced when ascertained through multiple, distinct proxies. Dental topography can be used to assess changes in occlusal morphology with macrowear, providing insight on tooth use and function across the lifespans of individuals. We measured convex Dirichlet normal energy-a dental topography metric reflecting occlusal sharpness of features such as cusps and crests-in macrowear series of the second mandibular molars of two African anthropoid taxa from ∼30 Ma (Aegyptopithecus zeuxis and Apidium phiomense). Wear was quantified via three proxies: occlusal dentine exposure, inverse relief index, and inverse occlusal relief. The same measurements were calculated on macrowear series of four extant platyrrhine taxa (Alouatta, Ateles, Plecturocebus, and Sapajus apella) to provide an analogical framework for dietary inference in the fossil taxa. We predicted that Ae. zeuxis and Ap. phiomense would show similar patterns in topographic change with wear to one another and to extant platyrrhine frugivores like Ateles and Plecturocebus. The fossil taxa have similar distributions of convex Dirichlet normal energy to one another, and high amounts of concave Dirichlet normal energy 'noise' in unworn molars-a pattern shared with extant hominids that may distort dietary interpretations. Inverse relief index was the most useful wear proxy for comparison among the taxa in this study which possess disparate enamel thicknesses. Contrary to expectations, Ae. zeuxis and Ap. phiomense both resemble S. apella in exhibiting an initial decline in convex Dirichlet normal energy followed by an increase at the latest stages of wear as measured by inverse relief index, lending support to previous suggestions that hard-object feeding played a role in their dietary ecology. Based on these results and previous analyses of molar shearing quotients, microwear, and enamel microstructure, we suggest that Ae. zeuxis had a pitheciine-like strategy of seed predation, whereas Ap. phiomense potentially consumed berry-like compound fruits with hard seeds.

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.

Fatigue and wear of human tooth enamel: A review

Human tooth enamel must withstand the cyclic contact forces, wear, and corrosion processes involved with typical oral functions. Furthermore, unlike other human tissues, dental enamel does not have a significant capacity for healing or self-repair and thus the longevity of natural teeth in the oral environment depends to a large degree on the fatigue and wear properties of enamel. The purpose of this review is to provide an overview of our understanding of the fatigue and wear mechanisms of human enamel and how they relate to in vivo observations of tooth damage in the complex oral environment. A key finding of this review is that fatigue and wear processes are closely related. For example, the presence of abrasive wear particles significantly lowers the forces needed to initiate contact fatigue cracking while subsurface fatigue crack propagation drives key delamination wear mechanisms during attrition or attrition-corrosion of enamel. Furthermore, this review seeks to bring a materials science and mechanical engineering perspective to fatigue and wear phenomena. In this regard, we see developing a mechanistic description of fatigue and wear, and understanding the interconnectivity of the processes, as essential for successfully modelling enamel fatigue and wear damage and developing strategies and treatments to improve the longevity of our natural teeth. Furthermore, we anticipate that this review will stimulate ideas for extending the lifetime of the natural tooth structure and will help highlight where our understanding is too limited and where additional research into fatigue and wear of human tooth enamel is warranted.

Tooth Diversity Underpins Future Biomimetic Replications

Although the evolution of tooth structure seems highly conserved, remarkable diversity exists among species due to different living environments and survival requirements. Along with the conservation, this diversity of evolution allows for the optimized structures and functions of teeth under various service conditions, providing valuable resources for the rational design of biomimetic materials. In this review, we survey the current knowledge about teeth from representative mammals and aquatic animals, including human teeth, herbivore and carnivore teeth, shark teeth, calcite teeth in sea urchins, magnetite teeth in chitons, and transparent teeth in dragonfish, to name a few. The highlight of tooth diversity in terms of compositions, structures, properties, and functions may stimulate further efforts in the synthesis of tooth-inspired materials with enhanced mechanical performance and broader property sets. The state-of-the-art syntheses of enamel mimetics and their properties are briefly covered. We envision that future development in this field will need to take the advantage of both conservation and diversity of teeth. Our own view on the opportunities and key challenges in this pathway is presented with a focus on the hierarchical and gradient structures, multifunctional design, and precise and scalable synthesis.

Mammalian tooth enamel functional sophistication demonstrated by combined nanotribology and synchrotron radiation FTIR analyses

The teeth of limbed vertebrates used for capturing and processing food are composed of mineralized dentine covered by hypermineralized enamel, the hardest material organisms produce. Here, we combine scanning probe microscopy, depth sensing, and spectromicroscopy (SR-FTIR) to characterize the surface ultrastructural topography, nanotribology, and chemical compositions of mammal species with different dietary habits, including omnivorous humans. Our synergistic approach shows that enamel with greater surface hardness or thickness exhibited a more salient gradient feature from the tooth surface to the dentino-enamel junction (DEJ) one that corresponds to the in situ phosphate-to-amide ratio. This gradient feature of enamel covering softer dentine is the determining factor of the amazingly robust physical property of this unique biomaterial. It provides the ability to dissipate stress under loading and prevent mechanical failure. Evolutionary change in the biochemical composition and biomechanical properties of mammalian dentition is related to variations in the oral processing of different food materials.

Elucidating the role of keratin 75 in enamel using Krt75tm1Der knock-in mouse model

Keratin 75 (K75) was recently discovered in ameloblasts and enamel organic matrix. Carriers of A161T substitution in K75 present with the skin condition Pseudofollicullitis barbae. This mutation is also associated with high prevalence of caries and compromised structural and mechanical properties of enamel. Krt75^tm1Der knock-in mouse (KI) with deletion of Asn159, located two amino acids away from KRT75^A161T, can be a potential model for studying the role of K75 in enamel and the causes of the higher caries susceptibility associated with KRT75^A161T mutation. To test the hypotheses that KI enamel is more susceptible to a simulated acid attack (SAA), and has altered structural and mechanical properties, we conducted in vitro SAA experiments, microCT, and microhardness analyses on 1st molars of one-month-old WT and KI mice. KI and WT hemimandibles were subjected to SAA and contralateral hemimandibles were used as controls. Changes in enamel porosity were assessed by immersion of the hemimandibles in rhodamine, followed by fluorescent microscopy analysis. Fluorescence intensity of KI enamel after SSA was significantly higher than in WT, indicating that KI enamel is more susceptible to acid attack. MicroCT analysis of 1st molars revealed that while enamel volumes were not significantly different, enamel mineral density was significantly lower in KI, suggesting a potential defect of enamel maturation. Microhardness tests revealed that in KI enamel is softer than in WT, and potentially less resilient to damages. These results suggest that the KI enamel can be used as a model to study the role of K75 in enamel.

Modeling ameloblast-matrix interactions using 3D cell culture

The distinct morphology adopted by ameloblasts during amelogenesis is highly stage specific and involved intimately with the development of a hierarchical enamel microstructure. The molecular mechanisms that govern the development of an elongated and polarized secretory ameloblast morphology and the potential roles played by the enamel matrix proteins in this process are not fully understood. Thus far, the in vitro models that have been developed to mimic these early cell-matrix interactions have either been unable to demonstrate direct morphological change or have failed to adapt across ameloblast cell lines. Here, we use a recently established 3D cell culture model to examine the interactions between HAT-7 cells and the major enamel matrix proteins, amelogenin and ameloblastin. We demonstrate that HAT-7 cells selectively respond to functional EMPs in culture by forming clusters of tall cells. Aspect ratio measurements from three-dimensional reconstructions reveal that cell elongation is 5-times greater in the presence of EMPs when compared with controls. Using confocal laser scanning microscopy, we observe that these clusters are polarized with asymmetrical distributions of Par-3 and claudin-1 proteins. The behavior of HAT-7 cells in 3D culture with EMPs is comparable with that of ALC and LS-8 cells. The fact that the 3D model presented here is tunable with respect to gel substrate composition and ameloblast cell type highlights the overall usefulness of this model in studying ameloblast cell morphology in vitro.

On the evolution of the morphology and resilience of molar cusps in fossil hominid teeth

Teeth play an important role in evolutionary studies due to their good preservation and direct link to diet. The present work makes use of a previously generated database on molar teeth of fossil hominids which consists of cuspal enamel thickness d_c, dentin horn angle φ and section width D, all measured on a given histological tooth section. These data are here interpreted with the aid of "fracture stress" Q_F = P_F/D² and geological age t, where P_F is the occlusal force needed to cause cusp failure as determined from d_c and φ. Q_F is virtually a constant in non-hominins ("apes") while monotonically increasing toward present time in hominins. These two trends intersect at t = t_s = 4.5 (0.11) mya, a value similar to other divergence estimates. Q_F was fitted with a function f(t) which is proportional to (d_c/D)². The monotonic variation of Q_F and in turn d_c/D with t contrasts the more complex behavior generally characterizing other physical entities of fossil hominids. The increase in d_c/D in hominins promotes tooth resilience and in turn life span. Finally, it is suggested that P_F provides an upper bound to the maximum bite force produced by the jaw structure.

Hunter-Schreger Band configuration in human molars reveals more decussation in the lateral enamel of 'functional' cusps than 'guiding' cusps

OBJECTIVES

Enamel prism decussation, which manifests as Hunter-Schreger Bands (HSB), is considered a mechanism to mitigate crack propagation. During the chewing cycle, the 'functional' cusps that are involved in Phase II crushing and grinding experience more complex patterns of stress than do those that 'guide' the molars into occlusion (Phase I). This study examines HSB configuration in the lateral enamel of human molars to identify potential differences between these cusps as predicted from their functional distinctions.

DESIGN

Measurements were recorded from scanning electron micrographs of sections through the mesial cusps of unworn permanent molars. For each section, HSB packing density and the relative thickness of decussated enamel were quantified in the cuspal and middle segments of lateral enamel over the guiding and functional cusps.

RESULTS

No clear trend from first to third molars in HSB configuration was found in either jaw. In maxillary molars, the functional cusp displays higher HSB packing density in the cuspal and middle segments, and relatively thicker decussated enamel in the cuspal segment than does the guiding cusp. In mandibular molars, the functional cusp displays higher HSB packing density in the middle segment than does the guiding cusp, but no difference in relative thickness was found between them. Enamel of mandibular molars shows weaker decussation than maxillary molars.

CONCLUSIONS

The results suggest that guiding cusps are intrinsically more susceptible to crack propagation than functional cusps in human permanent molars. Structural factors such as enamel decussation should be considered when interpreting enamel chipping patterns in dietary contexts.

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.

Role of the Mineral in the Self-Healing of Cracks in Human Enamel

Human enamel is an incredibly resilient biological material, withstanding repeated daily stresses for decades. The mechanisms behind this resilience remain an open question, with recent studies demonstrating a crack-deflection mechanism contributing to enamel toughness and other studies detailing the roles of the organic matrix and remineralization. Here, we focus on the mineral and hypothesize that self-healing of cracks in enamel nanocrystals may be an additional mechanism acting to prevent catastrophic failure. To test this hypothesis, we used a molecular dynamics (MD) approach to compare the fracture behavior of hydroxyapatite (HAP) and calcite, the main minerals in human enamel and sea urchin teeth, respectively. We find that cracks heal under pressures typical of mastication by fusion of crystals in HAP but not in calcite, which is consistent with the resilience of HAP enamel that calcite teeth lack. Scanning transmission electron microscopy (STEM) images of structurally intact ("sound") human enamel show dashed-line nanocracks that resemble and therefore might be the cracks healed by fusion of crystals produced in silico. The fast, self-healing mechanism shown here is common in soft materials and ceramics but has not been observed in single crystalline materials at room temperature. The crack self-healing in sound enamel nanocrystals, therefore, is unique in the human body and unique in materials science, with potential applications in designing bioinspired materials.

Modifications of the Dental Hard Tissues in the Cervical Area of Occlusally Overloaded Teeth Identified Using Optical Coherence Tomography

Background and objectives: Occlusal overloads produce a series of manifestations in teeth, especially attrition and non-carious cervical lesions (NCCL). Optical Coherence Tomography (OCT) can highlight and evaluate tooth lesions. The aim of this study was to examine the changes of dental hard tissues in the cervical area because of occlusal overload by macroscopic examination and using in vitro Swept Source OCT examination. Materials and Methods: The study was performed on 21 extracted teeth with occlusal trauma. After macroscopic and OCT examination, the 2D OCT images were transformed into 3D images using ImageJ software. Statistical analysis of macroscopic and OCT images was performed with Statistical Package for Social Sciences. Results: On 21 teeth, 88 cervical lesions (cracks) were identified. Upper premolars with an occlusal Smith and Knight tooth wear score of 2 had the most NCCL. Statistical analysis revealed significant differences in the median widths of cervical lesions between teeth with score 1 and score 3. Additionally, we obtained statistically significant differences in median widths between the buccal and oral surfaces. Conclusions: These cracks can be considered precursors of NCCL. NCCL can be located on dental surfaces in the cervical area other than the buccal one. A 3D reconstruction of OCT images emphasized that cracks are located especially at enamel level, evolving towards the enamel-dentin junction, with multiple ramifications.

Biomineralized Materials as Model Systems for Structural Composites: 3D Architecture

Biomineralized materials are sophisticated material systems with hierarchical 3D material architectures, which are broadly used as model systems for fundamental mechanical, materials science, and biomimetic studies. The current knowledge of the structure of biological materials is mainly based on 2D imaging, which often impedes comprehensive and accurate understanding of the materials' intricate 3D microstructure and consequently their mechanics, functions, and bioinspired designs. The development of 3D techniques such as tomography, additive manufacturing, and 4D testing has opened pathways to study biological materials fully in 3D. This review discusses how applying 3D techniques can provide new insights into biomineralized materials that are either well known or possess complex microstructures that are challenging to understand in the 2D framework. The diverse structures of biomineralized materials are characterized based on four universal structural motifs. Nacre is selected as an example to demonstrate how the progression of knowledge from 2D to 3D can bring substantial improvements to understanding the growth mechanism, biomechanics, and bioinspired designs. State-of-the-art multiscale 3D tomographic techniques are discussed with a focus on their integration with 3D geometric quantification, 4D in situ experiments, and multiscale modeling. Outlook is given on the emerging approaches to investigate the synthesis-structure-function-biomimetics relationship.

Cross-Linking of Apatite-Gelatin Nanocomposites as the Basis for Dentine Replacement Materials

A novel approach for the production of a bioinspired dentine replacement material is introduced. An apatite-gelatin nanocomposite material was cross-linked with various cross-linkers. These nanocomposites have a high resemblance to mammalian dentine regarding its composition and properties. A precipitation reaction was used to produce apatite-gelatin nanocomposites as starting materials. Cross-linking of the gelatin has to be performed to produce dentine-like and thus tough and robust apatite-gelatin nanocomposites. Therefore, the efficacy of various protein cross-linkers was tested, and the resulting materials were characterized by scanning electron microscopy, transmission electron microscopy, powder X-ray diffraction, and EXAFS as well as CHNS analysis and tested for their mechanical performance using Vickers hardness measurements as well as for their dissolution stability in EDTA. Especially glutaraldehyde, proanthocyanidins, and transglutaminase gave promising results with hardness values of up to 63 HV0.2. To further improve the material properties, we combined the effective cross-linker transglutaminase with casein, which led to an improved interconnection between the single nanocomposite platelets. By doing so, a cross-linked composite was obtained, which shows even higher hardness values than does human dentine, at 76 HV_0.2. The combination of apatite-gelatin nanocomposites with an effective cross-linker resulted in a bioinspired material with composition and properties close to those of human dentine.

Asymmetric Mass Transport through Dense Heterogeneous Polymer Membranes: Fundamental Principles, Lessons from Nature, and Artificial Systems

Many organisms rely on directional water transport schemes for the purpose of water retention and collection. Directional transport of water and other fluids is also technologically relevant, for example to harvest water, in separation processes, packaging solutions, functional clothing, and many other applications. One strategy to promote mass transport along a preferential direction is to create compositionally asymmetric, multi-layered, or compositionally graded architectures. In recent years, the investigation of natural and artificial membranes based on this design has attracted growing interest and allowed researchers to develop a good understanding of how the properties of such membranes can be tailored to meet the demands of particular applications. Here a summary of theoretical works on mass transport through dense asymmetric membranes, comprehensive reviews of biological and artificial membranes featuring this design, and a discussion of applications, remaining questions, and opportunities are provided.

Anthroengineering: an independent interdisciplinary field

In recent decades, funding agencies, institutes and professional bodies have recognized the profound benefits of transdisciplinarity in tackling targeted research questions. However, once questions are answered, the previously abundant support often dissolves. As such, the long-term benefits of these transdisciplinary approaches are never fully achieved. Over the last several decades, the integration of anthropology and engineering through inter- and multidisciplinary work has led to advances in fields such as design, human evolution and medical technologies. The lack of formal recognition, however, of this transdisciplinary approach as a unique entity rather than a useful tool or a subfield makes it difficult for researchers to establish laboratories, secure permanent jobs, fund long-term research programmes and train students in this approach. To facilitate the growth and development and witness the long-term benefits of this approach, we propose the integration of anthropology and engineering be recognized as a new, independent field known as anthroengineering. We present a working definition for anthroengineering and examples of how anthroengineering has been used. We discuss the necessity of recognizing anthroengineering as a unique field and explore potential novel applications. Finally, we discuss the future of anthroengineering, highlighting avenues for moving the field forward.

Fundamental mechanics of tooth fracture and wear: implications for humans and other primates

Until recently, there had been little attempt in the literature to identify and quantify the underlying mechanics of tooth durability in terms of materials engineering concepts. In humans and most mammals, teeth must endure a lifetime of sustained occlusal mastication-they have to resist fracture and wear. It is well documented that teeth are resilient, but what are the unique features that make this possible? The present article surveys recent materials engineering research aimed at addressing this fundamental question. Elements that determine the mechanics and micromechanics of tooth fracture and wear are analysed: at the macrostructural level, the geometry of the enamel shell and cuspal configuration; and at the microstructural level, interfacial weakness and property gradients. Inferences concerning dietary history in relation to evolutionary pressures are discussed.

Confocal Raman data analysis of tufts and spindles at the human dentin-enamel junction

OBJECTIVE

The aim of this article is to analyze the chemical mapping of tufts and spindles of the human dental enamel using confocal Raman microscopy measuring length, structuration and composition of spindles and tufts.

DESIGN

we used Raman diffusion, based on the interaction between photons and optic phonons, to reveal chemical bound. Adult molars were selected and longitudinally sectioned. Areas of 120 * 120 μm were scanned near the dentin-enamel junction and grooves. Spectra were collected and phosphate and proteins peak intensities images were reconstructed, related to HPA concentration. Images of Phosphate (PO₄^3-, 960 cm^-1) and protein (CH, 2800/3000 cm^-1) intensities have been reconstructed. K-mean cluster has been calculated to compare centroid spectra from enamel, dentin and tuft or spindle.

RESULTS

intensity profile revealed spindles as less mineralized areas than enamel, from 5 to 10 µm large. In the groove of molar, long tufts were found, more than 150 µm.

CONCLUSIONS

Confocal Raman microscopy is a very interesting tool to characterize chemically secondary structure of enamel. The size of a tuft in the groove allows us make the hypothesis that they could play a role in long term resilience of mechanical stress.

Establishment of optimal variable elastic modulus distribution in the design of full-crown restorations by finite element analysis

To establish optimal elastic modulus distribution throughout the entire all-ceramic crown, aiming at improvement of the mechanical properties of the restoration as well as the adhesive interface, seven 3D models of mandibular first premolars of zirconia monolithic and bilayer crowns and lithium disilicate monolithic and bilayer crowns were constructed. The elastic modulus distribution of 8-layer crown A referred to human enamel, B was calculated by a genetic algorithm (GA) to minimize the principle stresses on the crown, and C minimized the shear stresses at the cementing lines. After applying a static load of 600 N, the maximum principle stresses were calculated and analyzed by finite element analysis (FEA). Group C were found to have the lowest peak shear stress at the cementing line and moderate peak tensile stress in the crown. Introduction of the modified elastic modulus distribution from human enamel into the entire all-ceramic crown reinforces the mechanical properties of the whole restoration as well as the adhesive interface against chipping and debonding.

Introduction: biopolymers and biocomposites

Biopolymers and biocomposites are an exciting class of ubiquitous materials. Interest in these materials has been driven in part by their biocompatibility/biodegradability, sustainability, potentially low-cost, renewability, being environmental benign, among other properties. These fascinating materials come in a range of forms from the DNA and RNA that is essential to life to the cellulose and collagen that mechanically reinforce tissues and as hybrid organic–inorganic composites like teeth. Herein, we summarize some aspects of the two classes of materials biopolymer and biocomposites, exploring specific examples while pointing to potential monomer sources, neoteric post-extraction modification and processing conditions. This lays the foundation to the following more specific chapters while illustrating the breadth of these material classes.

Crystal misorientation correlates with hardness in tooth enamels

The multi-scale hierarchical structure of tooth enamel enables it to withstand a lifetime of damage without catastrophic failure. While many previous studies have investigated structure-function relationships in enamel, the effects of crystal misorientation on mechanical performance have not been assessed. To address this issue, in the present study, we review previously published polarization-dependent imaging contrast (PIC) maps of mouse and human enamel, and parrotfish enameloid, in which crystal orientations were measured and displayed in every 60-nm-pixel. By combining those previous results with the PIC maps of sheep enamel presented here we discovered that, in all enamel(oid)s, adjacent crystals are slightly misoriented, with misorientation angles in the 0°-30° range, and mean 2°-8°. Within this limited range, misorientation is positively correlated with literature hardness values, demonstrating an important structure-property relation, not previously identified. At greater misorientation angles 8°30°, this correlation is expected to reverse direction, but data from different non-enamel systems, with more diverse crystal misorientations, are required to determine if and where this occurs. STATEMENT OF SIGNIFICANCE: We identify a structure-function relationship in tooth enamels from different species: crystal misorientation correlates with hardness, contributing to the remarkable mechanical properties of enamel in diverse animals.

On the vital role of enamel prism interfaces and graded properties in human tooth survival

Teeth of omnivores face a formidable evolutionary challenge: how to protect against fracture and abrasive wear caused by the wide variety of foods they process. It is hypothesized that this challenge is met in part by adaptations in enamel microstructure. The low-crowned teeth of humans and some other omnivorous mammals exhibit multiple fissures running longitudinally along the outer enamel walls, yet remain intact. It is proposed that inter-prism weakness and enamel property gradation act together to avert entry of these fissures into vulnerable inner tooth regions and, at the same time, confer wear resistance at the occlusal surface. A simple indentation experiment is employed to quantify crack paths and energetics in human enamel, and an extended-finite-element model to evaluate longitudinal crack growth histories. Consideration is given as to how tooth microstructure may have played a vital role in human evolution, and by extension to other omnivorous mammals.

Beyond Description: The Many Facets of Dental Biomechanics

Teeth lie at the interface between an animal and its environment and, with some exceptions, act as a major component of resource procurement through food acquisition and processing. Therefore, the shape of a tooth is closely tied to the type of food being eaten. This tight relationship is of use to biologists describing the natural history of species and given the high instance of tooth preservation in the fossil record, is especially useful for paleontologists. However, correlating gross tooth morphology to diet is only part of the story, and much more can be learned through the study of dental biomechanics. We can explore the mechanics of how teeth work, how different shapes evolved, and the underlying forces that constrain tooth shape. This review aims to provide an overview of the research on dental biomechanics, in both mammalian and non-mammalian teeth, and to synthesize two main approaches to dental biomechanics to develop an integrative framework for classifying and evaluating dental functional morphology. This framework relates food material properties to the dynamics of food processing, in particular how teeth transfer energy to food items, and how these mechanical considerations may have shaped the evolution of tooth morphology. We also review advances in technology and new techniques that have allowed more in-depth studies of tooth form and function.

Chemical gradients in human enamel crystallites

Dental enamel is a principal component of teeth1, and has evolved to bear large chewing forces, resist mechanical fatigue and withstand wear over decades2. Functional impairment and loss of dental enamel, caused by developmental defects or tooth decay (caries), affect health and quality of life, with associated costs to society3. Although the past decade has seen progress in our understanding of enamel formation (amelogenesis) and the functional properties of mature enamel, attempts to repair lesions in this material or to synthesize it in vitro have had limited success4-6. This is partly due to the highly hierarchical structure of enamel and additional complexities arising from chemical gradients7-9. Here we show, using atomic-scale quantitative imaging and correlative spectroscopies, that the nanoscale crystallites of hydroxylapatite (Ca5(PO4)3(OH)), which are the fundamental building blocks of enamel, comprise two nanometric layers enriched in magnesium flanking a core rich in sodium, fluoride and carbonate ions; this sandwich core is surrounded by a shell with lower concentration of substitutional defects. A mechanical model based on density functional theory calculations and X-ray diffraction data predicts that residual stresses arise because of the chemical gradients, in agreement with preferential dissolution of the crystallite core in acidic media. Furthermore, stresses may affect the mechanical resilience of enamel. The two additional layers of hierarchy suggest a possible new model for biological control over crystal growth during amelogenesis, and hint at implications for the preservation of biomarkers during tooth development.

Effects of Endodontic Access Cavity Design and Thermocycling on Fracture Strength of Endodontically Treated Teeth

Introduction

This study aimed to assess the fracture strength of endodontically treated mandibular molars with traditional endodontic access cavity (TEC) and truss endodontic access cavity (TREC) designs that were restored with composite resin and underwent thermocycling.

Methods

Sixty mandibular first and second molars were randomly divided into 6 groups (n=10) of intact controls without thermocycling (group 1), intact controls with thermocycling (group 2), TEC without thermocycling (group 3), TEC with thermocycling (TEC-TC, group 4), TREC without thermocycling (group 5) and TREC with thermocycling (TREC-TC, group 6). The root canals were then instrumented to #25,7% using nickel-titanium files and were filled with gutta-percha and AH26 sealer with lateral compaction technique. Access cavity was restored with Gradia composite. All teeth were then thermocycled for 480 cycles between 5°C and 55°C for 30 seconds and their fracture strength was measured in a universal testing machine with a round-end piston with 6 mm diameter at a speed of 1 mm/min. Data were analyzed using two-way and one-way ANOVA.

Results

Without thermocycling, the fracture strength of endodontically treated teeth with TREC designs had no significant difference with the control group (P>0.05). However, both TEC and TREC designs significantly decreased the fracture strength of endodontically treated teeth after thermocycling (P<0.05), such that minimum fracture strength was noted in TEC-TC group.

Conclusion

Under the conditions of this ex vivo study, TREC enhances the fracture strength of endodontically treated teeth under thermal stresses.

Nature's design solutions in dental enamel: Uniting high strength and extreme damage resistance

The most important demand of today's high-performance materials is to unite high strength with extreme fracture toughness. The combination of withstanding large forces (strength) and resistance to fracture (toughness), especially preventing catastrophic material failure by cracking, is of utmost importance when it comes to structural applications of these materials. However, these two properties are commonly found to be mutually exclusive: strong materials are brittle and tough materials are soft. In dental enamel, nature has combined both properties with outstanding success - despite a limited number of available constituents. Made up of brittle mineral crystals arranged in a sophisticated hierarchical microstructure, enamel exhibits high stiffness and excellent toughness. Different species exhibit a variety of structural adaptations on varying scales in their dental enamel which optimise not only fracture toughness, but also hardness and abrasion behaviour. Nature's materials still outperform their synthetic counterparts due to these complex structure-property relationships that are not yet fully understood. By analysing structure variations and the underlying mechanical mechanisms systematically, design principles which are the key for the development of advanced synthetic materials uniting high strength and toughness can be formulated. STATEMENT OF SIGNIFICANCE: Dental enamel is a hard protective tissue that combines high strength with an exceptional resistance to catastrophic fracture, properties that in classical materials are commonly found to be mutually exclusive. The biological material is able to outperform its synthetic counterparts due to a sophisticated hierarchical microstructure. Between different species, microstructural adaptations can vary significantly. In this contribution, the different types of dental enamel present in different species are reviewed and connections between microstructure and (mechanical) properties are drawn. By consolidating available information for various species and reviewing it from a materials science point of view, design principles for the development of advanced biomimetic materials uniting high strength and toughness can be formulated.

Enamel chipping in Taï Forest cercopithecids: Implications for diet reconstruction in paleoanthropological contexts

Antemortem enamel chipping in living and fossil primates is often interpreted as evidence of hard-object feeding (i.e., 'durophagy'). Laboratory analyses of tooth fracture have modeled the theoretical diets and loading conditions that may produce such chips. Previous chipping studies of nonhuman primates tend to combine populations into species samples, despite the fact that species can vary significantly in diet across their ranges. Chipping is yet to be analyzed across population-specific species samples for which long-term dietary data are available. Here, we test the association between enamel chipping and diet in a community of cercopithecid primates inhabiting the Taï Forest, Ivory Coast. We examined fourth premolars and first molars (n = 867) from naturally deceased specimens of Cercocebus atys, Colobus polykomos, Piliocolobus badius,Procolobus verus, and three species of Cercopithecus. We found little support for a predictive relationship between enamel chipping and diet across the entire Taï monkey community. Cercocebus atys, a dedicated hard-object feeder, exhibited the highest frequencies of (1) chipped teeth and (2) chips of large size; however, the other monkey with a significant degree of granivory, Co. polykomos, exhibited the lowest chip frequency. In addition, primates with little evidence of mechanically challenging or hard-food diets-such as Cercopithecus spp., Pi. badius, and Pr. verus-evinced higher chipping frequencies than expected. The equivocal and stochastic nature of enamel chipping in the Taï monkeys suggests nondietary factors contribute significantly to chipping. A negative association between canopy preference and chipping suggests a role of exogenous particles in chip formation, whereby taxa foraging closer to the forest floor encounter more errant particulates during feeding than species foraging in higher strata. We conclude that current enamel chipping models may provide insight into the diets of fossil primates, but only in cases of extreme durophagy. Given the role of nondietary factors in chip formation, our ability to reliably reconstruct a range of diets from a gradient of chipping in fossil taxa is likely weak.

Breaking crown dentine in whole teeth: 3D observations of prevalent fracture patterns following overload

Teeth with intact crowns rarely split or fracture, despite decades of cyclic loading and occasional unexpected overload. This is largely attributed to the presence of dentine, since cracking and fracture of enamel have been frequently reported. Dentine is similar to bone, comprising mineralised collagen fibres as a main constituent. Unlike cortical bone, however, where microcracking and damage arrest are essential for re/modelling and healing, dentine can neither remodel nor regenerate. This raises questions regarding the evolutionary benefits of toughening, leading to uncertainty whether cracks actually appear in dentine in situ. Here we study the notion that circumpulpal dentine is usually protected against, rather than damaged by severe overloads, even though it is not much more massive or stronger than it needs to be. To address this, we examined hydrated teeth still within whole jawbones of freshly-slaughtered skeletally mature pigs, mechanically loaded until fracture. Force displacement curves, optical and electron microscopy combined with 3D microstructural analysis by conventional micro-computed tomography (μCT) revealed mostly brittle fracture paths in circumpulpal crown dentine. Once overload cracks reach this mass of dentine they propagate rapidly along straight paths often parallel to the enamel flanks of the oblong shovel shaped premolars. We find infrequent signs of active toughening mechanisms with minimal crack diversion, ligament bridging and microcracking. When such toughening is seen, it mainly appears in softer dentine in the root, or near the dentine-enamel-junction (DEJ) in mantle dentine. We observed shear bands in overloaded circumpulpal dentine, due to mutual gliding of upper and lower segments. These shear bands are formed as periodic arrays of rotated dentine fragments. The 3D data consistently demonstrate the importance of the layered tooth structure, containing a stiff outer enamel shell, a soft sub-DEJ interlayer and a stiff circumpulpal dentine bulk, for deflecting cracks from splitting the tooth.

Enamel-inspired materials design achieving balance of high stiffness and large energy dissipation

Owing to the unique non-self-similar hierarchical microstructure, enamel achieves the balance of high stiffness and toughness, and in turn provides important ideas for the bio-inspired materials design. In this study, a multiscale numerical study has been conducted to investigate whether the property of high stiffness and large energy dissipation could be duplicated in engineering materials through certain material design principles. Motivated by the structure of enamel, the bio-inspired materials consisting of hard and soft phases were considered, and the designing parameters including the cross-sectional shape, volume fraction, and inclination angle of the reinforcement, and other three parameters related to the waviness of the reinforcement were taken into account. It was found that by employing the non-self-similar hierarchical structure, the designed composites exhibited the balance between stiffness and toughness, which has not been achieved in many engineering materials yet. Furthermore, the influences of the aforementioned designing parameters on the mechanical performance of the composites have been elucidated. The findings of this study have provided a guideline for designing bio-inspired composites achieving the balance between stiffness and toughness.

Enhancing Collagen Mineralization with Amelogenin Peptide: Towards the Restoration of Dentin

Mammalian teeth primarily consist of two distinct calcified tissues, enamel and dentin, that are intricately integrated by a complex and critical structure, the dentin-enamel junction (DEJ). Loss of enamel exposes the underlying dentin, increasing the risk of several irreversible dental diseases. This paper highlights the significance of utilizing the functional domains of a major enamel matrix protein, amelogenin, intrinsic to tooth enamel and the DEJ interface, to rationally design smaller bioinspired peptides for regeneration of tooth microstructures. Using this strategy, we designed a synthetic peptide, P26, that demonstrates a remarkable dual mineralization potential to restore incipient enamel decay and mineralization defects localized in peripheral dentin below the DEJ. As a proof of principle, we demonstrate that interaction between P26 and collagen prompts peptide self-assembly, followed by mineralization of collagen fibrils in vitro. P26-mediated nucleation of hydroxyapatite (HAP) crystals on demineralized dentin in situ significantly facilitates the recovery of mineral density and effectively restores the biomechanical properties of dentin to near-native levels, suggesting that P26-based therapy has promising applications for treating diverse mineralized tissue defects in the tooth.

Amelogenin phosphorylation regulates tooth enamel formation by stabilizing a transient amorphous mineral precursor

Dental enamel comprises interwoven arrays of extremely long and narrow crystals of carbonated hydroxyapatite called enamel rods. Amelogenin (AMELX) is the predominant extracellular enamel matrix protein and plays an essential role in enamel formation (amelogenesis). Previously, we have demonstrated that full-length AMELX forms higher-order supramolecular assemblies that regulate ordered mineralization in vitro, as observed in enamel rods. Phosphorylation of the sole AMELX phosphorylation site (Ser-16) in vitro greatly enhances its capacity to stabilize amorphous calcium phosphate (ACP), the first mineral phase formed in developing enamel, and prevents apatitic crystal formation. To test our hypothesis that AMELX phosphorylation is critical for amelogenesis, we generated and characterized a hemizygous knockin (KI) mouse model with a phosphorylation-defective Ser-16 to Ala-16 substitution in AMELX. Using EM analysis, we demonstrate that in the absence of phosphorylated AMELX, KI enamel lacks enamel rods, the hallmark component of mammalian enamel, and, unlike WT enamel, appears to be composed of less organized arrays of shorter crystals oriented normal to the dentinoenamel junction. KI enamel also exhibited hypoplasia and numerous surface defects, whereas heterozygous enamel displayed highly variable mosaic structures with both KI and WT features. Importantly, ACP-to-apatitic crystal transformation occurred significantly faster in KI enamel. Secretory KI ameloblasts also lacked Tomes' processes, consistent with the absence of enamel rods, and underwent progressive cell pathology throughout enamel development. In conclusion, AMELX phosphorylation plays critical mechanistic roles in regulating ACP-phase transformation and enamel crystal growth, and in maintaining ameloblast integrity and function during amelogenesis.

Determining primates bite force from histological tooth sections

OBJECTIVES

The ability to accurately estimate bite force (BF) in extant and fossil primates is valuable to biological anthropologists. BF is generally evaluated using complex jaw musculature and lever arm analyses employing numerous assumptions and requiring complete cranial morphology. Here, a simple method to determine BF from data measured on histological sections of fossil teeth is proposed.

METHODS

Published sections of molar teeth encompassing 27 different extinct and extant primates dating back to as early as 17 million years ago were examined. Focusing on the cusp region, the extracted data include characteristic enamel thickness d_c and dentin horn angle φ. The occlusal force needed to fracture a cusp, P_F , was determined from these variables with the aid of a finite element stress analysis similarly to a previous study on postcanine human teeth. The bite force was obtained by linking BF to P_F using a universal constant.

RESULTS

The measured variables d_c and φ are conclusively linked. This link produces a virtually constant fracture force P_F and in turn bite force BF for all cusps in the molar row. An explicit formula tying BF to d_c and φ was derived. For nonhominin taxa the bite force, molar crown area, and body mass are found to be intimately related. The case of hominins is more involved. The so determined BF is gender-averaged, with the bite force of males estimated to be ≈12% greater than that of females.

CONCLUSIONS

The use of "fracture mechanics" concepts from mechanics of materials facilitates determination of critical bite force in primates based on characteristic enamel thickness d_c and dentin horn angle φ as extracted from histological sections of molar teeth. This novel approach enables quantitative insight into the role played by crown area, body mass and bite force on evolutionary trends. The conclusive link between cuspal enamel thickness and dentin horn angle facilitates optimal food processing without hindering cusp resilience. The proposed approach may be extended to mammals having asymmetric cusp structures.

Nature-Inspired Nacre-Like Composites Combining Human Tooth-Matching Elasticity and Hardness with Exceptional Damage Tolerance

Making replacements for the human body similar to natural tissue offers significant advantages but remains a key challenge. This is pertinent for synthetic dental materials, which rarely reproduce the actual properties of human teeth and generally demonstrate relatively poor damage tolerance. Here new bioinspired ceramic-polymer composites with nacre-mimetic lamellar and brick-and-mortar architectures are reported, which resemble, respectively, human dentin and enamel in hardness, stiffness, and strength and exhibit exceptional fracture toughness. These composites are additionally distinguished by outstanding machinability, energy-dissipating capability under cyclic loading, and diminished abrasion to antagonist teeth. The underlying design principles and toughening mechanisms of these materials are elucidated in terms of their distinct architectures. It is demonstrated that these composites are promising candidates for dental applications, such as new-generation tooth replacements. Finally, it is believed that this notion of bioinspired design of new materials with unprecedented biologically comparable properties can be extended to a wide range of material systems for improved mechanical performance.