Ultrastructure and nanomechanical properties of cementum dentin junction

The attachment between cementum and dentin has been given several definitions and nomenclature, including: interzonal layer, intermediate cementum, collagen hiatus, Hopewell-Smith's hyaline layer, and more commonly, cementum-dentin junction (CDJ). Understanding the attachment of two structurally dissimilar hard tissues such as cementum and dentin defined by a junction may provide information necessary to engineer functionally graded materials that can be used for efficient tooth restorations in clinical dentistry and other bioengineering applications. Hence, in this study, as a first step toward understanding the CDJ using a biomechanical approach, it was hypothesized that the CDJ between cementum and dentin is a wide zone with mechanical properties significantly lower than the neighboring tissues. The structure of the CDJ was studied using an atomic force microscope (AFM), and site-specific mechanical response of the three regions; cementum, CDJ, and dentin were determined using an AFM-nanoindenter under dry and wet conditions. The AFM results of the CDJ demonstrated a valley under dry conditions and a peak under wet conditions. The magnitude of the depth of the valley was approximately the same as the height of the peak of the CDJ, ranging from 10 to 40 microm. The nanomechanical properties under dry conditions indicated no significant difference (p > 0.05) in elastic modulus and hardness of the CDJ (Er = 17.5 +/- 2.7 GPa, H = 0.6 +/- 0.1 GPa) and cementum (Er = 18.7 +/- 2.5 GPa, H = 0.6 +/- 0.1 GPa). The mechanical properties of the CDJ were significantly lower (p < 0.05) than dentin (Er = 19.9 +/- 2.9 GPa, H = 0.6 +/- 0.1 GPa) under dry conditions. However, under more relevant hydrated conditions, the mechanical properties of CDJ (Er 3.0 +/- 0.7 GPa, H = 0.1 +/- 0.0 GPa) were significantly lower (p < 0.05) than those of cementum (Er 6.8 +/- 1.9 GPa, H = 0.2 +/- 0.1 GPa) and dentin (Er 9.4 +/- 2.3 GPa, H = 0.3 +/- 0.1 GPa). Based on the results from this study, it can be concluded that the CDJ can be regarded as a wide zone containing large quantities of proteins including collagen that contribute to hydration and significantly reduce mechanical properties, compared with the adjacent hard tissues, cementum, and dentin. The lower mechanical properties of the CDJ may make it possible for it to redistribute occlusal loads to the alveolar bone.     

Local properties of a functionally graded interphase between cementum and dentin

The study of natural interfaces may provide information necessary to engineer functionally graded biomaterials for bioengineering applications. In this study, the mechanical, structural, and chemical composition variations associated with a region between cementum and dentin were studied with the use of nanoindentation, microindentation, optical microscopy, and Raman microspectroscopy techniques. Three-millimeter-thick transverse sections (N = 5) were obtained from the apical one-third of the roots of sterilized human molars. The samples were ultrasectioned at room temperature with the use of a diamond knife and an ultramicrotome. Longitudinal ground sections of 100 microm thickness were prepared and stained with von Kossa stain to determine the mineralized regions within the molar roots. Raman microspectroscopy was used to determine the relative inorganic content, mainly apatite (PO4(3-)nu1 mode at 960 cm(-1)) and organic content, mainly collagen (C--H stretch at 2940 cm(-1)) between cementum and dentin bulk tissues. The microindentation and nanoindentation results indicated a gradual transition in hardness from cementum to dentin over a width ranging from 100 to 200 microm. However, the variation in hardness data for cementum and dentin by nanoindentation was larger (0.62 +/- 0.21, 0.77 +/- 0.14 GPa) than from microindentation (0.49 +/- 0.03, 0.69 +/- 0.07 GPa). Within the 100 to 200 microm region there was a 10 to 50 microm fibrillar hydrophilic cementum-dentin junction (CDJ) with mechanical properties significantly lower than either the cementum or the dentin side of CDJ. Light microscopy revealed a 100 to 200 microm translucent region between cementum and dentin. Raman microspectroscopy results showed a variation in organic and inorganic composition 80 to 140 microm wide. It was concluded that a morphologically and biomechanically different CDJ lies within a wider cementum-dentin interphase. Hence, cementum, dentin, and the interphase can be classified as a functionally graded dental tissue within the root of a tooth.     

Structure, chemical composition and mechanical properties of human and rat cementum and its interface with root dentin

This work seeks to establish comparisons of the physical properties of rat and human cementum, root dentin and their interface, including the cementum-dentin junction (CDJ), as a basis for future studies of the entire periodontal complex using rats as animal models. In this study the structure, site-specific chemical composition and mechanical properties of cementum and its interface with root dentin taken from 9- to 12-month-old rats were compared to the physiologically equivalent 40- to 55-year-old human age group using qualitative and quantitative characterization techniques, including histology, atomic force microscopy (AFM), micro-X-ray computed tomography, Raman microspectroscopy and AFM-based nanoindentation. Based on results from this study, cementum taken from the apical third of the respective species can be represented as a woven fabric with radially and circumferentially oriented collagen fibers. In both species the attachment of cementum to root dentin is defined by a stiffness-graded interface (CDJ/cementum-dentin interface). However, it was concluded that cementum and the cementum-dentin interface from a 9- to 12-month-old rat could be more mineralized, resulting in noticeably decreased collagen fiber hydration and significantly higher modulus values under wet conditions for cementum and CDJ (E(rat-cementum)=12.7+/-2.6 GPa; E(rat-CDJ)=11.6+/-3.2 GPa) compared to a 40- to 55-year-old human (E(human-cementum)=3.73+/-1.8 GPa; E(human-CDJ)=1.5+/-0.7 GPa). The resulting data illustrated that the extensions of observations made from animal models to humans should be justified with substantial and equivalent comparison of data across age ranges (life spans) of mammalian species.     

The tooth attachment mechanism defined by structure, chemical composition and mechanical properties of collagen fibers in the periodontium

In this study, a comparison between structure, chemical composition and mechanical properties of collagen fibers at three regions within a human periodontium, has enabled us to define a novel tooth attachment mechanism. The three regions include, (1) the enthesis region: insertion site of periodontal ligament (PDL) fibers (collagen fibers) into cementum at the root surface, (2) bulk cementum, and (3) the cementum-dentin junction (CDJ). Structurally, continuity in collagen fibers was observed from the enthesis, through bulk cementum and CDJ. At the CDJ the collagen fibers split into individual collagen fibrils and intermingled with the extracellular matrix of mantle dentin. Under wet conditions, the collagen fibers at the three regions exhibited significant swelling suggesting a composition rich in polyanionic molecules such as glycosaminoglycans. Additionally, site-specific indentation illustrated a comparable elastic modulus between collagen fibers at the enthesis (1-3 GPa) and the CDJ (2-4 GPa). However, the elastic modulus of collagen fibers within bulk cementum was higher (4-7 GPa) suggesting presence of extrafibrillar mineral. It is known that the tooth forms a fibrous joint with the alveolar bone, which is termed a gomphosis. Although narrower in width than the PDL space, the hygroscopic CDJ can also be termed as a gomphosis; a fibrous joint between cementum and root dentin capable of accommodating functional loads similar to that between cementum and alveolar bone. From an engineering perspective, it is proposed that a tooth contains two fibrous joints that accommodate the masticatory cyclic loads. These joints are defined by the attachment of dissimilar materials via graded stiffness interfaces, such as: (1) alveolar bone attached to cementum with the PDL; and (2) cementum to root dentin with the CDJ. Thus, through variations in concentrations of basic constituents, distinct regions with characteristic structures and graded properties allow for attachment and the load bearing characteristics of a tooth.     

Twisted plywood structure of an alternating lamellar pattern in cellular cementum of human teeth

Human cellular cementum was examined by scanning electron microscopy to elucidate the manner of the alternate lamellar pattern forming the cellular cementum. Specimens were demineralized, trimmed with a freezing microtome, and treated by NaOH-maceration. This procedure was chosen to avoid artifacts in the fibril arrangement, and to study the fibrous architecture in detail. For comparison, non-demineralized, polished and HCl-etched specimens were also prepared. In the NaOH-macerated specimens, the lamellar pattern of the cellular cementum conformed to the twisted plywood principle of bone lamellation with a periodic rotation of matrix fibrils resulting in an alternating lamellar pattern. In contrast, matrix fibrils were irregularly arranged without indication of rotation of matrix fibrils in the polished and etched specimens. Our results suggest that polishing and etching procedures cause damage to fibrils and fibril arrangement.     

Nanoindentation studies of titanium single crystals.

Titanium single crystal planes of different atomic density have been reported to show different oxidation characteristics. The differences in oxide characteristics have further been demonstrated to lead to differences in osteoblast attachment. Investigations of the preferred crystallographic planes of titanium for osteoblast attachment can be used to optimize the surfaces of single crystal and polycrystalline titanium implants for anchoring various prostheses. Nanoindentation techniques were used to determine mechanical properties of two crystallographic planes of titanium of different atomic density. Modulus of elasticity of 128 +/- 10 GPa was obtained for polycrystalline titanium and 123 +/- 5 and 124 +/- 6 GPa for the basal plane and pyramidal planes, respectively. The variation of modulus with crystal orientation was not greater than the statistical variation in the data. Surface hardness values were 2.1 +/- 0.1 GPa for the polycrystalline sample and 1.6 +/- 0.1 and 1.9 +/- 0.1 GPa, respectively, for the basal and pyramidal planes. Curves of hardness as a function of depth (0-2000 nm) obtained from electrochemically polished surfaces showed a sharp increase at shallow depths and may reflect changes caused by oxidation of the titanium surfaces.     

Alteration of dentin–enamel mechanical properties due to dental whitening treatments

The mechanical properties of dentin and enamel affect the reliability and wear properties of a tooth. This study investigated the influence of clinical dental treatments and procedures, such as whitening treatments or etching prior to restorative procedures. Both autoclaved and non-autoclaved teeth were studied in order to allow for both comparison with published values and improved clinical relevance. Nanoindentation analysis with the Oliver–Pharr model provided elastic modulus and hardness across the dentin–enamel junction (DEJ). Large increases were observed in the elastic modulus of enamel in teeth that had been autoclaved (52.0 GPa versus 113.4 GPa), while smaller increases were observed in the dentin (17.9 GPa versus 27.9 GPa). Likewise, there was an increase in the hardness of enamel (2.0 GPa versus 4.3 GPa) and dentin (0.5 GPa versus 0.7 GPa) with autoclaving. These changes suggested that the range of elastic modulus and hardness values previously reported in the literature may be partially due to the sterilization procedures. Treatment of the exterior of non-autoclaved teeth with Crest Whitestrips?, Opalescence? or UltraEtch? caused changes in the mechanical properties of both the enamel and dentin. Those treated with Crest Whitestrips? showed a reduction in the elastic modulus of enamel (55.3 GPa to 32.7 GPa) and increase in the elastic modulus of dentin (17.2 GPa to 24.3 GPa). Opalescence? treatments did not significantly affect the enamel properties, but did result in a decrease in the modulus of dentin (18.5 GPa to 15.1 GPa). Additionally, as expected, UltraEtch? treatment decreased the modulus and hardness of enamel (48.7 GPa to 38.0 GPa and 1.9 GPa to 1.5 GPa, respectively) and dentin (21.4 GPa to 15.0 GPa and 1.9 GPa to 1.5 GPa, respectively). Changes in the mechanical properties were linked to altered protein concentration within the tooth, as evidenced by fluorescence microscopy and Fourier transform infrared spectroscopy.     

Property variations in the prism and the organic sheath within enamel by nanoindentation

Atomic force microscopy (AFM) combined with nanoindentation technique was used to definitely, site-specifically, test the nanomechanical properties, including nanohardness and elastic modulus, of the isolated domains within single enamel, the prisms and the surrounding sheaths, of mature human maxillary third molars. In this way, it is for the first time that evident differences of nanomechanical properties were revealed between these domains. The nanohardness and elastic modulus of the sheaths were about 73.6% and 52.7% lower than those of the prisms, respectively. Measuring the residual impressions with AFM supported the similar conclusion. The variations of mechanical properties in these domains are considered to be mainly relative to their different component and fibrils arrangement.     

Micro/nanoscale mechanical and tribological characterization of SiC for orthopedic applications

Micro/nanomechanical and tribological characterization of SiC has been carried out. For comparison, measurements on SiC, CoCrMo, Ti-6Al-4V, and stainless steel have also been made. Hardness and elastic modulus of these materials were measured by nanoindentation using a nanoindenter. The nanoindentation impressions were imaged using an atomic force microscope (AFM). Scratch, friction, and wear properties were measured using an accelerated microtribometer. Scratch and wear damages were studied using a scanning electron microscope (SEM). It is found that SiC exhibits higher hardness, elastic modulus, scratch resistance as well as lower friction with fewer and smaller debris particles compared to other materials. These results show that SiC possesses superior mechanical and tribological properties that make it an ideal material for use in orthopedic and other biomedical applications.     

Mechanical properties of human bone surrounding plateau root form implants retrieved after 0.3-24 years of function

Bone remodeling, along with tissue biomechanics, is critical for the clinical success of endosseous implants. This study evaluated the long-term evolution of the elastic modulus (GPa) and hardness (GPa) of cortical bone around human retrieved plateau root form implants. Thirty implant-in-bone specimens showing no clinical failure were retrieved from patients at different in-vivo times (0.3 to ～24 years) due to retreatment needs. After dehydration, specimens were embedded in methacrylate-based resin, sectioned along the bucco-lingual long axis and fixed to acrylic plates and nondecalcified processed to slides with ～50 μm in thickness. Nanoindentation testing was carried out under wet conditions on bone areas within the first three plateaus. Indentations (n = 120 per implant total) were performed with a maximum load of 300 μN (loading rate: 60 μN/s) followed by a holding and unloading time of 10 s and 2 s, respectively. Elastic modulus (E, GPa) and hardness (H, GPa) were computed. Both E and H values presented increased values as time in vivo elapsed (E: r = 0.84; H: r = 0.78). Significantly higher values for E and H were found after 5 years in vivo (p < 0.001). Maxillary or mandibulary arches or positioning did not affect mechanical properties, nor did implant surface treatment on the long-term bone biomechanical response (E: p ≥ 0.09; H: p ≥ 0.3). This work suggests that human cortical bone around plateau root form implants presents an increase in elastic modulus and hardness during the first 5 years following implantation and presents stable mechanical properties thereafter. ? 2012 Wiley Periodicals, Inc. J Biomed Mater Res Part B: Appl Biomater, 2012.     

Structural and mechanical evaluations of a topology optimized titanium interbody fusion cage fabricated by selective laser melting process

A topology optimized lumbar interbody fusion cage was made of Ti-Al6-V4 alloy by the rapid prototyping process of selective laser melting (SLM) to reproduce designed microstructure features. Radiographic characterizations and the mechanical properties were investigated to determine how the structural characteristics of the fabricated cage were reproduced from design characteristics using micro-computed tomography scanning. The mechanical modulus of the designed cage was also measured to compare with tantalum, a widely used porous metal. The designed microstructures can be clearly seen in the micrographs of the micro-CT and scanning electron microscopy examinations, showing the SLM process can reproduce intricate microscopic features from the original designs. No imaging artifacts from micro-CT were found. The average compressive modulus of the tested caged was 2.97+/-0.90 GPa, which is comparable with the reported porous tantalum modulus of 3 GPa and falls between that of cortical bone (15 GPa) and trabecular bone (0.1-0.5 GPa). The new porous Ti-6Al-4V optimal-structure cage fabricated by SLM process gave consistent mechanical properties without artifactual distortion in the imaging modalities and thus it can be a promising alternative as a porous implant for spine fusion.     

Mechanical properties and structure of the biological multilayered material system,Atractosteus spatula scales

During recent decades, research on biological systems such as abalone shell and fish armor has revealed that these biological systems employ carefully arranged hierarchical multilayered structures to achieve properties of high strength, high ductility and light weight. Knowledge of such structures may enable pathways to design bio-inspired materials for various applications. This study was conducted to investigate the spatial distribution of structure, chemical composition and mechanical properties in mineralized fish scales of the species Atractosteus spatula. Microindentation tests were conducted, and cracking patterns and damage sites in the scales were examined to investigate the underlying protective mechanisms of fish scales under impact and penetration loads. A difference in nanomechanical properties was observed, with a thinner, stiffer and harder outer layer (indentation modulus ～69 GPa and hardness ～3.3 GPa) on a more compliant and thicker inner layer (indentation modulus ～14.3 GPa and hardness ～0.5 GPa). High-resolution scanning electron microscopy imaging of a fracture surface revealed that the outer layer contained oriented nanorods embedded in a matrix, and that the nanostructure of the inner layer contained fiber-like structures organized in a complex layered pattern. Damage patterns formed during microindentation show complex deformation mechanisms. Images of cracks identify growth through the outer layer, then deflection along the interface before growing and arresting in the inner layer. High-magnification images of the crack tip in the inner layer show void-linking and fiber-bridging exhibiting inelastic behavior. The observed difference in mechanical properties and unique nanostructures of different layers may have contributed to the resistance of fish scales to failure by impact and penetration loading.     

Atomic force microscopy and nanoindentation characterization of human lamellar bone prepared by microtome sectioning and mechanical polishing technique

Surface topography, microstructure, and micromechanical properties of human lamellar bone were characterized by atomic force microscopy and nanoindentation. The lamellar bone surfaces were prepared by two different methods: microtome sectioning and mechanical polishing. The lamellar bone surfaces prepared by mechanical polishing revealed that thin lamellae formed depressions approximately 200 nm deep, whereas the surfaces prepared by microtome sectioning were flat. Atomic force microscopy surface topographic images at higher magnification showed differences between thick and thin lamellae in polished samples, but these differences were less pronounced in microtomed samples. Roughness measurements confirmed that there was a significant difference between thick (21.0 nm) and thin lamellae (8.3 nm) in polished samples (p < 0.001). The difference in surface roughness between thick (13.9 nm) and thin lamellae (12.7 nm) in microtomed sample was statistically insignificant (p = 0.74). Higher elastic modulus values were observed for thick lamella in microtomed samples compared with that of thin lamellae, whereas measured elastic modulus differences between thick and thin lamellae in polished samples were found to be statistically insignificant.     

Mechanical properties of novel calcium phosphate-mullite biocomposites

Herein, the results of systematic mechanical property measurements of pressureless sintered calcium phosphate (CaP)-mullite composites are discussed. Our experimental results demonstrated how the mullite addition (upto 30 wt%) influenced hardness, elastic modulus, strength and toughness properties of the composites. In assessing each of these fundamental material properties, either a range of load or a number of complimentary techniques were used to obtain reliable measure of mechanical properties. Importantly, the results of single edge V notch beam measurements revealed that a reliable toughness value of ~1.5 MPa m(0.5) could be obtained in composites containing 20 or 30 wt% mullite. Our results clearly illustrated that a combination of elastic modulus (~80 GPa), compressive strength of more than 350 MPa, three-point flexural strength of 70-80 MPa, hardness of 4-5 GPa were achievable with the investigated composites. Such a combination of material properties, in addition to modest toughness property appeared to indicate that CaP-mullite composites could be used as a biomaterial for hard tissue replacement.     

人体主动脉瓣膜钙化组织的力学性质

目的采用纳米压痕测试方法,测量人体主动脉瓣取出物的钙化组织的材料力学性能。方法采集5名主动脉瓣狭窄患者的瓣叶取出物,选取钙化瓣叶进行纳米压痕测试,获得钙化组织弹性模量、硬度等材料力学参数。结果瓣叶钙化组织的弹性模量为(15.69±3.89) GPa,硬度为(0.59±0.15) GPa。结论通过纳米压痕测试方法得到瓣叶钙化组织的弹性模量和硬度,为瓣膜的生物力学研究提供实验数据参考。     

Influences of ionic concentration on nanomechanical behaviors for remineralized enamel

This study evaluates the influences of 8DSS peptide and ionic concentrations of simulated body fluid on remineralization behaviors. The polished enamel specimens were acid-demineralized, exposed briefly to 8DSS peptide solution, and then immersed into simulated body fluid (SBF) that favors mineral deposition. At various stages of treatment, nanohardness and elastic modulus were determined by nanoindentation. The results show that the nanomechanical properties of the acid-demineralized enamel were greatly improved as increasing the ionic concentrations of SBF due to the acceleration of mineral deposition. Additionally, the demineralized enamel, treated with 8DSS peptide and immersed into SBF×2 solution, possesses the highest values of nanohardness and elastic modulus resulting from the combinative effects of surface roughness, morphology, microstructure and crystallinity of the newly formed nanocomposite of calcium phosphate carbonate and hydroxyapatite. The formation of pores in the subsurface induced a reduction in the nanomechanical properties for the enamel subjected into SBF×3 solution.     

Human enamel rod presents anisotropic nanotribological properties

The AFM combined nanoindentation was performed to observe the ultrastructure of enamel rod from various section plans and positions while probing their mechanical and tribological properties of the area. The nanohardness and the elastic modulus of the head region of the enamel rods are significantly higher than that of the tail region and the axial-sectional plane. Both nanohardness and elastic modulus gradually decrease from enamel surface toward dentino-enamel junction. Such a variation correlates well with the decreasing trend of calcium composition from our element analysis. The friction coefficient and nanowear of the enamel showed an inversed trend to the hardness with respect to their relative topological position in the long axis of enamel rod toward DEJ. The relationship between the nanowear depth and the distance from the outer enamel surface to DEJ presented exponential function. The results presented clarify the basic nanomechanical and nanotribological properties of human enamel rods and provide a useful reference for the future development of dental restorative materials.     

Nano-mechanics of bone and bioactive bone cement interfaces in a load-bearing model

Many bioactive bone cements were developed for total hip replacement and found to bond with bone directly. However, the mechanical properties at the bone/bone cement interface under load bearing are not fully understood. In this study, a bioactive bone cement, which consists of strontium-containing hydroxyapatite (Sr-HA) powder and bisphenol-alpha-glycidyl dimethacrylate (Bis-GMA)-based resin, was evaluated in rabbit hip replacement for 6 months, and the mechanical properties of interfaces of cancellous bone/Sr-HA cement and cortical bone/Sr-HA cement were investigated by nanoindentation. The results showed that Young's modulus (17.6+/-4.2 GPa) and hardness (987.6+/-329.2 MPa) at interface between cancellous bone and Sr-HA cement were significantly higher than those at the cancellous bone (12.7+/-1.7 GPa; 632.7+/-108.4 MPa) and Sr-HA cement (5.2+/-0.5 GPa; 265.5+/-39.2 MPa); whereas Young's modulus (6.3+/-2.8 GPa) and hardness (417.4+/-164.5 MPa) at interface between cortical bone and Sr-HA cement were significantly lower than those at cortical bone (12.9+/-2.2 GPa; 887.9+/-162.0 MPa), but significantly higher than Sr-HA cement (3.6+/-0.3 GPa; 239.1+/-30.4 MPa). The results of the mechanical properties of the interfaces were supported by the histological observation and chemical composition. Osseointegration of Sr-HA cement with cancellous bone was observed. An apatite layer with high content of calcium and phosphorus was found between cancellous bone and Sr-HA cement. However, no such apatite layer was observed at the interface between cortical bone and Sr-HA cement. And the contents of calcium and phosphorus of the interface were lower than those of cortical bone. The mechanical properties indicated that these two interfaces were diffused interfaces, and cancellous bone or cortical bone was grown into Sr-HA cement 6 months after the implantation.     

Micromechanical bending of single collagen fibrils using atomic force microscopy

A new micromechanical technique was developed to study the mechanical properties of single collagen fibrils. Single collagen fibrils, the basic components of the collagen fiber, have a characteristic highly organized structure. Fibrils were isolated from collagenous materials and their mechanical properties were studied with atomic force microscopy (AFM). In this study, we determined the Young's modulus of single collagen fibrils at ambient conditions from bending tests after depositing the fibrils on a poly(dimethyl siloxane) (PDMS) substrate containing micro-channels. Force-indentation relationships of freely suspended collagen fibrils were determined by loading them with a tip-less cantilever. From the deflection-piezo displacement curve, force-indentation curves could be deduced. With the assumption that the behavior of collagen fibrils can be described by the linear elastic theory of isotropic materials and that the fibrils are freely supported at the rims, a Young's modulus of 5.4 +/- 1.2 GPa was determined. After cross-linking with glutaraldehyde, the Young's modulus of a single fibril increases to 14.7 +/- 2.7 GPa. When it is assumed that the fibril would be fixed at the ends of the channel the Young's moduli of native and cross-linked collagen fibrils are calculated to be 1.4 +/- 0.3 GPa and 3.8 +/- 0.8 GPa, respectively. The minimum and maximum values determined for native and glutaraldehyde cross-linked collagen fibrils represent the boundaries of the Young's modulus.     

Mechanical properties of the dentinoenamel junction: AFM studies of nanohardness, elastic modulus, and fracture

The dentinoenamel junction (DEJ) is a complex and poorly defined structure that unites the brittle overlying enamel with the dentin that forms the bulk of the tooth. In addition, this structure appears to confer excellent toughness and crack deflecting properties to the tooth, and has drawn considerable interest as a biomimetic model of a structure uniting dissimilar materials. This work sought to characterize the nanomechanical properties in the region of the DEJ using modified AFM based nanoindentation to determine nanohardness and elastic modulus. Lines of indentations traversing the DEJ were made at 1-2 microm intervals from the dentin to enamel along three directions on polished sagittal sections from three third molars. Nanohardness and elastic modulus rose steadily across the DEJ from bulk dentin to enamel. DEJ width was estimated by local polynomial regression fits for each sample and location of the mechanical property curves for the data gradient from enamel to dentin, and gave a mean value of 11.8 microm, which did not vary significantly with intratooth location or among teeth. Nanoindentation was also used to initiate cracks in the DEJ region. In agreement with prior work, it was difficult to initiate cracks that traversed the DEJ, or to produce cracks in the dentin. The fracture toughness values for enamel of 0.6-0.9 MPa . m(1/2) were in good agreement with recent microindentation fracture results. Our results suggest that the DEJ displays a gradient in structure and that nanoindenation methods show promise for further understanding its structure and function.