Exceptional contact elasticity of human enamel in nanoindentation test

Chronic kidney disease compromises structural and mechanical properties of maxillary cortical bone in a rat model

PURPOSE

This study aimed to investigate the effects of chronic kidney disease (CKD) on the structural and mechanical properties of the maxillary and mandibular cortical bone.

METHODS

The maxillary and mandibular cortical bones from CKD model rats were used in this study. CKD-induced histological, structural, and micro-mechanical alterations were assessed using histological analyses, micro-computed tomography (CT), bone mineral density (BMD) measurements, and nanoindentation tests.

RESULTS

Histological analyses indicated that CKD caused an increase in the number of osteoclasts and a decrease in the number of osteocytes in the maxilla. Micro-CT analysis revealed that CKD induced a void volume/cortical volume (%) increase, which was more remarkable in the maxilla than in the mandible. CKD also significantly decreased the BMD in the maxilla. In the nanoindentation stress-strain curve, the elastic-plastic transition point and loss modulus were lower in the CKD group than that in the control group in the maxilla, suggesting that CKD increased micro fragility of the maxillary bone.

CONCLUSIONS

CKD affected bone turnover in the maxillary cortical bone. Furthermore, the maxillary histological and structural properties were compromised, and micro-mechanical properties, including the elastic-plastic transition point and loss modulus, were altered by CKD.

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.

Functional non-uniformity of periodontal ligaments tunes mechanobiological stimuli across soft- and hard-tissue interfaces

The bone-periodontal ligament-tooth (BPT) complex is a unique mechanosensing soft-/hard-tissue interface, which governs the most rapid bony homeostasis in the body responding to external loadings. While the correlation between such loading and alveolar bone remodelling has been widely recognised, it has remained challenging to investigate the transmitted mechanobiological stimuli across such embedded soft-/hard-tissue interfaces of the BPT complex. Here, we propose a framework combining three distinct bioengineering techniques (i, ii, and iii below) to elucidate the innate functional non-uniformity of the PDL in tuning mechanical stimuli to the surrounding alveolar bone. The biphasic PDL mechanical properties measured via nanoindentation, namely the elastic moduli of fibres and ground substance at the sub-tissue level (i), were used as the input parameters in an image-based constitutive modelling framework for finite element simulation (ii). In tandem with U-net deep learning, the Gaussian mixture method enabled the comparison of 5195 possible pseudo-microstructures versus the innate non-uniformity of the PDL (iii). We found that the balance between hydrostatic pressure in PDL and the strain energy in the alveolar bone was maintained within a specific physiological range. The innate PDL microstructure ensures the transduction of favourable mechanobiological stimuli, thereby governing alveolar bone homeostasis. Our outcomes expand current knowledge of the PDL's mechanobiological roles and the proposed framework can be adopted to a broad range of similar soft-/hard- tissue interfaces, which may impact future tissue engineering, regenerative medicine, and evaluating therapeutic strategies. STATEMENT OF SIGNIFICANCE: A combination of cutting-edge technologies, including dynamic nanomechanical testing, high-resolution image-based modelling and machine learning facilitated computing, was used to elucidate the association between the microstructural non-uniformity and biomechanical competence of periodontal ligaments (PDLs). The innate PDL fibre network regulates mechanobiological stimuli, which govern alveolar bone remodelling, in different tissues across the bone-PDL-tooth (BPT) interfaces. These mechanobiological stimuli within the BPT are tuned within a physiological range by the non-uniform microstructure of PDLs, ensuring functional tissue homeostasis. The proposed framework in this study is also applicable for investigating the structure-function relationship in broader types of fibrous soft-/hard- tissue interfaces.

Biomechanical Modelling for Tooth Survival Studies: Mechanical Properties, Loads and Boundary Conditions-A Narrative Review

Recent biomechanical studies have focused on studying the response of teeth before and after different treatments under functional and parafunctional loads. These studies often involve experimental and/or finite element analysis (FEA). Current loading and boundary conditions may not entirely represent the real condition of the tooth in clinical situations. The importance of homogenizing both sample characterization and boundary conditions definition for future dental biomechanical studies is highlighted. The mechanical properties of dental structural tissues are presented, along with the effect of functional and parafunctional loads and other environmental and biological parameters that may influence tooth survival. A range of values for Young's modulus, Poisson ratio, compressive strength, threshold stress intensity factor and fracture toughness are provided for enamel and dentin; as well as Young's modulus and Poisson ratio for the PDL, trabecular and cortical bone. Angles, loading magnitude and frequency are provided for functional and parafunctional loads. The environmental and physiological conditions (age, gender, tooth, humidity, etc.), that may influence tooth survival are also discussed. Oversimplifications of biomechanical models could end up in results that divert from the natural behavior of teeth. Experimental validation models with close-to-reality boundary conditions should be developed to compare the validity of simplified models.

Finite Element Analysis of Orthodontic Relapse in Different Maxillary Arch Form

Abstract Background: Orthodontic relapse is fairly common; however, the mechanisms between relapse and the dental arch form remain unclear. The purpose of our study was to establish three-dimensional (3D) finite element models of different dental arch forms after orthodontic treatment and to analyze the states of different arches applied with various sagittal forces.Methods: By calculating the equations of different dental arch forms and combining them with a full maxillary arch (14 teeth), 3D finite element models of square, oval, and tapered dental arches were established; they were designed to be subjected to anterior lingual, posterior mesial, and combined forces, respectively.Results: The von Mises stress and displacement of teeth under different forces were calculated for each loading scenario. Under the different forcing scenarios, all incisors had irregularity trends, and the inclination and intrusion of the canines were increased, and the premolars had a tendency to buccal or lingual crown tipping or even intrusion in our study. The tapered arch was the most stable and had the smallest displacement and von Mises stress, followed by the ovoid arch; the most unstable arch was the square arch.Conclusions: To achieve a stable orthodontic effect, a tapered or ovoid arch, rather than a square arch, should be chosen as the final outcome of treatment.

Microstructural and mechanical recovery of bone in ovariectomized rats: The effects of menaquinone-7

The loss of bone quantity and quality in postmenopausal female patients can be a problem for dental treatment. A sufficient intake of nutrients such as calcium, magnesium, and vitamins D and K is likely correlated with the mechanical properties of bone. In particular, vitamin K2, also called menaquinone (MK), inhibits bone loss in postmenopausal women. Here we demonstrate the microstructural and mechanical properties of bone recovery in ovariectomized (OVX) rats during MK-7 administration. Bilateral ovariectomy and a sham operation were performed on 14-week-old female SPF Wistar rats. MK-4 and -7 were orally administered at 30 mg/kg daily for 12 weeks. The femur was used for the 3-point bending test and microstructural analysis of the cancellous bone by micro-CT, and the mandibular cortical bone for the evaluation of mechanical properties on a nanoscale. Micro-computed tomography revealed irregular trabecular architecture, hollow marrow cavities, and sparse trabecular bone in the femurs of the OVX group. Trabecular bone structure analysis showed that the MK-7 group had greater bone volume per tissue volume (BV/TV) and a higher trabecular number than the OVX group. The bulk-scale 3-point bending test did not allow the mechanical properties between OVX and OVX/MK7 groups to be discerned, yet at the smallest level, the elastic-plastic transition point of the nanoindentation stress-strain curve of the mandibular cortical bone was higher in the MK-7 group than in the OVX group. These findings suggest that MK-7 enables bone microstructural and mechanical recovery in the OVX model.

High-resolution mechanical mapping of the adhesive-dentin interface: The effect of co-monomers in 10-methacryloyloxydecyl dihydrogen phosphate

The presence of 10-methacryloyloxydecyl dihydrogen phosphate (MDP) at the adhesive-dentin interface enables ionic binding to calcium salts, which results in rigid nano-layering within the submicron scale resin-dentin interdiffusion zone. MDP has been used with additional co-monomers, such as hydroxyethyl methacrylate (HEMA) and/or 4-methacryloyloxyethyl-trimellitic acid (4-MET), mainly to enhance the chemical bonding properties. However, the use of co-monomers may compromise the rigidity of the adhesive-dentin interface. In this study, we use high-resolution mechanical mapping across the interface to discern the in situ mechanical properties of each target region at the nanoscale. Visualization by modulus mapping demonstrated that HEMA increases the diffusion properties of MDP into dentin structures. However, the rigidity of the adhesive-dentin interface indicated by the storage modulus was markedly lower in MDP containing HEMA than in MDP containing 4-MET. Dynamic indentation testing revealed that the bonding layer was more deformable in the presence of HEMA. Moreover, the presence of MDP in the bonding layer might also increase the deformability because the polymerization linearity allows a large degree of viscoelasticity. These factors also diminish the rigidity of the adhesive-dentin interface. Within the limitations of this study, our findings demonstrated that 4-MET is a better co-monomer than HEMA in MDP-based dental adhesives. Modulus mapping and nanoindentation are introduced as new tests for the adhesive-dentin interface to address queries about the effectiveness of dental adhesives.

Nanotechnology in dentistry: Present and future perspectives on dental nanomaterials

OBJECTIVES

The number of dental nanomaterials has increased significantly over the past years. A variety of commercial dental nanomaterials are available and researched. Nevertheless, how these nanomaterials work, what makes them special and whether they are superior to traditional dental materials is not always clear to dentists and researchers. The objective of this review paper is, therefore, to give an overview of the principles of nanomaterials and basic research and applications of dental nanomaterials.

METHODS

The fundamentals of materials science of nanomaterials as well as their advantages and disadvantages are elaborated. The most important dental nanomaterials are discussed. This is mainly based on a survey of the literature and a review of the most frequently cited scientific papers in the international peer reviewed journal Dental Materials over the past five years. The developments of commercial dental nanomaterials as well as aspects of their clinical use are considered in this review.

RESULTS

Nanomaterials have unique structures and properties that distinguish them from other materials. The journal Dental Materials is the journal with the highest numbers of articles and citations on the subject of dental nanomaterials. The most frequently reported dental nanomaterials are nanocomposites, nanoparticles, antimicrobial nanomaterials and bio-mineralization systems. Hallmarks of dental nanomaterials include a set of unique properties and challenges in the preparation of these materials.

SIGNIFICANCE

By understanding the physical principles of dental nanomaterials, their strengths, limitations and their specific benefits will be better appreciated. Dental nanomaterials have potential for the future but currently do not always exhibit superior properties, for example in clinical situations.