Tooth acellular extrinsic fibre cementum incremental lines in humans are formed by parallel branched Sharpey's fibres and not by its mineral phase.

In humans, the growth pattern of the acellular extrinsic fibre cementum (AEFC) has been useful to estimate the age-at-death. However, the structural organization behind such a pattern remains poorly understood. In this study tooth cementum from seven individuals from a Mexican modern skeletal series were analyzed with the aim of unveiling the AEFC collagenous and mineral structure using multimodal imaging approaches. The organization of collagen fibres was first determined using: light microscopy, transmission electron microscopy (TEM), electron tomography, and plasma FIB scanning electron microscopy (PFIB-SEM) tomography. The mineral properties were then investigated using: synchrotron small-angle X-ray scattering (SAXS) for T-parameter (correlation length between mineral particles); synchrotron X-ray diffraction (XRD) for L-parameter (mineral crystalline domain size estimation), alignment parameter (crystals preferred orientation) and lattice parameters a and c; as well as synchrotron X-ray fluorescence for spatial distribution of calcium, phosphorus and zinc. Results show that Sharpey's fibres branched out fibres that cover and uncover other collagen bundles forming aligned arched structures that are joined by these same fibres but in a parallel fashion. The parallel fibres are not set as a continuum on the same plane and when they are superimposed project the AEFC incremental lines due to the collagen birefringence. The orientation of the apatite crystallites is subject to the arrangement of the collagen fibres, and the obtained parameter values along with the elemental distribution maps, revealed this mineral tissue as relatively homogeneous. Therefore, no intrinsic characteristics of the mineral phase could be associated with the alternating AEFC incremental pattern.

Incremental lines in human cellular cementum: A histological study.

OBJECTIVES: Human cellular cementum has incremental lines that demarcate individual cementum lamellae. The structural and functional details of the lines remain poorly understood. This study was designed to examine human cellular cementum using light microscopy, scanning electron microscopy, and contact microradiography and to elucidate the ultrastructure of incremental lines and their significance in cellular cementogenesis. METHODS: Longitudinal paraffin and ground sections of human mandibular molars were prepared. Paraffin sections were stained with hematoxylin, or hematoxylin and eosin, or impregnated with silver. Hematoxylin-stained sections were observed via scanning electron microscopy using NaOH maceration. Silver-impregnated sections were further stained with hematoxylin. Hematoxylin-stained ground sections were examined using contact microradiography. RESULTS: The incremental lines were found to be collagen fibril-poor layers. The outer area of each cementum lamella consisted of highly mineralized fibrils involved in constructing an alternating lamellar structure, whereas the inner area consisted of irregularly arranged, less highly mineralized, fibrils. The incremental lines corresponded with the innermost sites of the inner area. CONCLUSIONS: Based on the obtained findings, we suggest that cellular cementogenesis progresses as follows. (1) Cementoblasts alternate between low-to high-activity states. (2) In the earliest low-activity stage, cementoblasts generate poorly mineralized, fibril-poor, incremental lines. (3) As cementoblasts recover activity, fibril-organization and mineralization advance in the cementum. (4) In the high-activity stage, cementoblasts reach full activity and construct the highly mineralized, alternating lamellar structure. (5) Cementoblasts revert back to the low-activity stage. (6) The above processes are repeated, thus, alternately generating the incremental lines and cementum lamellae.

Teeth resorption at cement - enamel junction (CEJ) - Microscopy analysis.

The cement - enamel junction (CEJ) gains a growing interest in clinical dentistry, due to an increasing number of idiopathic tooth resorption leading to the inevitable loss of permanent dentition. Therefore, in this research study, we investigated healthy and under resorption teeth to verify junctions' morphology in relation to the resorption process, its initiation, and propagation. Using light and scanning electron microscopy we examined the three types of CEJ: 1) coronal cementum, 2) abutment) and 3) the gap between cementum and enamel. With energy-dispersive X-ray spectroscopy (EDX) we analyzed the Ca and P concentration along the CEJ border. The study indicated the strict correlation between the gap junction and tooth resorption, due to the dentine exposure to the oral environment.

Cellular network across cementum and periodontal ligament elucidated by FIB/SEM tomography.

Cementocytes in cementum form a lacuna-canalicular network. However, the 3D ultrastructure and range of the cementocyte network are unclear. Here, the 3D ultrastructure of the cementocyte network at the interface between cementum and periodontal ligament (PDL) was investigated on the mesoscale using FIB/SEM tomography. The results revealed a cellular network of cementocytes and PDL cells. A previous histomorphological study revealed the osteocyte-osteoblast-PDL cellular network. We extended this knowledge and revealed the cementum-PDL-bone cellular network, which may orchestrate the remodeling and modification of periodontal tissue, using a suitable method for imaging of complex tissue.

Microstructure, micromorphology, and fractal geometry of hard dental tissues: Evaluation of atomic force microscopy images.

Determining surface topography of different tissues of the molar tooth with novel analytical methods has opened new horizons in dental surface measurements which characterize tooth surface quality in dentistry. Studying surface topological measurements and comparing surface morphology of hard tissue of the molar tooth are the ultimate goals of the present study. Ten molar teeth have been chosen for investigating their surface characteristics through image processing techniques. The power spectral density (PSD) and fast Fourier transform algorithms of every molar tooth containing enamel, dentin, and cementum have determined that the characterization of surface profiles is possible. As can be seen, PSD along with fractal dimensions leads to good results for teeth surface topography. Moreover, PSD angular plot assures appropriate description of surface.

An unusual disordered alveolar bone material in the upper furcation region of minipig mandibles: A 3D hierarchical structural study.

Teeth are subjected to compressive loads during mastication. Under small loads the soft tissue periodontal ligament (PDL) deforms most. However when the loads increase and the PDL is highly compressed, the tooth and the alveolar bone supporting the tooth, begin to deform. Here we report on the structure of this alveolar bone in the upper furcation region of the first molars of mature minipigs. Using light microscopy and scanning electron microscopy (SEM) of bone cross-sections, we show that this bone is hypermineralized, containing abundant small pores around 1-5 µm in diameter, lacunae around 10-20 µm as well as larger spaces. This bone does not possess the typical lamellar motif or other repeating structures normally found in cortical or trabecular mammalian bone. We also use high resolution focused ion beam scanning electron microscopy (FIB-SEM) in the serial surface mode to image the 3D organization of the demineralized bone matrix. We show that the upper furcation bone matrix has a disordered isotropic structure composed mainly of individual collagen fibrils with no preferred orientation, as well as highly staining material that is probably proteoglycans. Much larger aligned arrays of collagen fibers - presumably Sharpey's fibers - are embedded in this material. This unusual furcation bone material is similar to the disordered material found in human lamellar bone. In the upper furcation region this disordered bone comprises almost all the volume excluding Sharpey's fibers. We surmise that this most unusual bone type functions to resist the repeating compressive loads incurred by molars during mastication.

The relation between structural, rugometric and fractal characteristics of hard dental tissues at micro and nano levels.

Human tooth exhibits a structure of a mixture of inorganic hydroxyapatite nanocrystals and organic phases. The aim of this study is to investigate different tissues of human canine teeth surface along with the micro structure parameters of each tissue. X-ray diffraction (XRD) is used to study the amorphous or crystalline nature of each tissue with different mineral compositions and crystalline structures where the highest crystalline quality is related to enamel. The surfaces are also examined by energy-dispersive X-ray spectrometry. Moreover, crystalline quality factor is carried out to estimate the crystallinity of the tissues. Also, based on the basic Scherrer equation, the Williamson-Hall equation is applied to extend the formula for the XRD. Enamel and cementum tissues of a typical human tooth, which look similar, are composed of a large variety of wide lines with different widths through Raman spectra analysis. In addition, the applied scanning electron microscopy extracts similar morphology for all tissues with round granular structures which are denser in the cementum. Atomic force microscopy is finally used for investigation of micro-morphologies of the different tissues and the results are compared with the fractal analysis which ends to the bifractal and anisotropic nature of enamel and cementum along with monofractal and isotropic nature of dentin.

Macrophage response and surface analysis of dental cementum after treatment with high intensity focused ultrasound.

OBJECTIVE: To investigate effects of HIFU on macrophage phenotype, surface micro-topography and nano-scale surface mechanical properties of dental cementum. MATERIALS AND METHODS: Root discs (2 mm thickness) were cut apical to CEJ and sectioned into quadrants. HIFU setup with bowl-shaped piezo ceramic transducer submerged in a water tank was used for exposure on each specimen for 15 s, 30 s or 60 s. The specimens of the control group were left without any HIFU exposure. HIFU was generated with a continuous sinusoidal wave of 120Vpp amplitude, 250 KHZ resonance-frequency and highest ultrasonic pressure of ∼10 bar at the focus. Specimens for SEM were viewed, and micro-topography characterization performed, using AFM and Ra parameter and surface area (SA) calculated by specialized SPM surface analysis software. For nano-indentation testing, experiments were carried out using AFM. Macrophage cell isolation and culturing was performed on cementum to receive the HIFU treatment at different time periods. Raman spectroscopy were scanned to create spectra perpendicular to the cementum substrate to analyze generation of standard spectra for Raman intensity ratio of hydroxyapatite normalized to the peaks ν1 960 cm-1. Data was expressed as means ± standard deviations and analyzed by one-way ANOVA in term of Ra, SA, H and Er. Different points for fluorescence intensity ratio were analyzed by Raman using Wilcoxon rank sum test. RESULTS: HIFU exposure at 60 s removed the smear layer and most of cementum appeared smoothened. AFM characterisation, showed a slight decrease in the irregularity of the surface as exposure time increased. Intact macrophages can be identified in control and all experimental HIFU groups. The level of fluorescence for the control and HIFU 15 and 30 s were low as compared to HIFU 60 s. CONCLUSION: If HIFU can be successfully implemented, it may be a possible alternative to current methods used in periodontal therapy to achieve smooth root surfaces.

Inhibition of Stat3 signaling pathway decreases TNF-α-induced autophagy in cementoblasts.

Autophagy is a self-digestive process that eliminates impaired or aged proteins and potentially toxic intracellular components to maintain homeostasis. We previously demonstrated that TNF-α played a critical role in cementoblast differentiation, mineralization and apoptosis; however, the effect of TNF-α on cementoblast autophagy has remained unclear. In this study, an elevated immunofluorescence signal of LC3B and autophagic vacuoles, autophagosomes and autolysosomes were detected under TNF-α stimulation in OCCM-30 cells. Autophagy-related genes and proteins, Beclin-1, LC3A and Atg-5, were significantly upregulated by TNF-α in a time- and concentration-dependent manner. During this process, the activity of Stat3 was dramatically enhanced and when the activity of Stat3 was blocked by either a specific chemical inhibitor or siRNA transfection before TNF-α stimulation, the TNF-α-induced upregulation of autophagy-related genes and proteins was strongly inhibited. Our results suggest that TNF-α induced autophagy in cementoblasts was dependent, or partially dependent on the activity of Stat3 signaling pathway.

Preservation of root cementum: a comparative evaluation of power-driven versus hand instruments.

OBJECTIVES: The purpose of this study was to evaluate the effects of three distinct periodontal treatment methods in comparison with hand instrumentation on residual cementum of periodontal diseased teeth. Cementum can influence the activities of periodontal cells and may play an important regulatory role in periodontal treatment. The ideal method for periodontal therapy involves removal of biofilm, calculus and endotoxin while preserving root cementum. MATERIAL AND METHODS: Forty-eight caries free, single-rooted teeth in patients diagnosed with severe chronic periodontitis were treated using four different methods prior to extraction. The teeth were instrumented subgingivally at one approximal site either by hand curettes (HC), piezoelectric ultrasonic scalers (U), piezoelectric ultrasonic scalers following air polishing (U + AP) or air polishing (AP) alone. Following extraction of teeth, instrumented and non-instrumented sites were analysed with a dissecting microscope and SEM for measurement of the amount of and surface characteristics of residual cementum. RESULTS: The percentage of coronal cementum remaining following subgingival instrumentation was 84% for U, 80% for U + AP, 94% for AP and 65% for HC. Although subgingival instrumentation of apical portions of the cementum demonstrated 6% less retained cementum in comparison with coronal portions, the amount of retained cementum with AP was still significantly greater than with HC. SEM results found the smoothest root surfaces were produced by the HC followed by the AP, while root surfaces instrumented by U or U + AP presented grooves and scratches. CONCLUSIONS: This study demonstrated that AP was superior to U devices in preserving cementum, whereas HC were the most effective instruments in removing cementum.

Use of a continual sweep motion to compare air polishing devices, powders and exposure time on unexposed root cementum.

Low abrasive air polishing powders are a viable method for subgingival biofilm removal. This in vitro study evaluated the effects of air polishing using a standard tip on cementum following clinically recommended protocols. Forty-eight teeth were randomly divided into eight groups with six teeth per group. Teeth were treated using either a Hu-Friedy EMS or DENTSPLY Cavitron® air polishing device. One of three glycine powders (Air-flow 25 µm, Clinpro 45 µm, Clinpro+TCP 45 µm) or a sodium bicarbonate powder (NaHCO3  85 µm) was sprayed on cementum using a clinically relevant sweeping motion. Volume and depth of cementum removed after 5 and 90 s exposures were calculated. Surface texture was evaluated using SEMs taken following the last exposure. After 5 s exposures, neither unit nor powder had a substantial effect on volume loss or defect depth. After 90 s exposures, differences between powders existed only for the DENTSPLY unit (p < 0.0001). Pairwise comparisons for this unit revealed mean volume loss and maximum defect depth were greater for NaHCO3 85 µm than the glycine powders (p < 0.0001). The 90 s exposure produced greater mean volume loss and defect depth for all powders (p < 0.0001). SEM images revealed dentinal tubule exposure with all powders; however, exposed tubules were larger and more prevalent for NaHCO3 85 µm. Root surface loss was similar for glycine powders evaluated in this study. Differences in powder performance between units may be related to tip apertures and spray patterns. Additional research is needed to determine if cementum loss is greater than what occurs with conventional biofilm removal methods, such as curets and ultrasonic scalers.

Understanding External Cervical Resorption in Vital Teeth.

INTRODUCTION: The aim of this study was to investigate the 3-dimensional (3D) structure and the cellular and tissue characteristics of external cervical resorption (ECR) in vital teeth and to understand the phenomenon of ECR by combining histomorphological and radiographic findings. METHODS: Twenty-seven cases of vital permanent teeth displaying ECR were investigated. ECR diagnosis was based on clinical and radiographic examination with cone-beam computed tomographic imaging. The extracted teeth were further analyzed by using nanofocus computed tomographic imaging, hard tissue histology, and scanning electron microscopy. RESULTS: All examined teeth showed some common characteristics. Based on the clinical and experimental findings, a 3-stage mechanism of ECR was proposed. At the first stage (ie, the initiation stage), ECR was initiated at the cementum below the gingival epithelial attachment. At the second stage (ie, the resorption stage), the resorption invaded the tooth structure 3-dimensionally toward the pulp space. However, it did not penetrate the pulp space because of the presence of a pericanalar resorption-resistant sheet. This layer was observed to consist of predentin, dentin, and occasionally reparative mineralized (bonelike) tissue, having a fluctuating thickness averaging 210 µm. At the last advanced stage (ie, the repair stage), repair took place by an ingrowth and apposition of bonelike tissue into the resorption cavity. During the reparative stage, repair and remodeling phenomena evolve simultaneously, whereas both resorption and reparative stages progress in parallel at different areas of the tooth. CONCLUSIONS: ECR is a dynamic and complex condition that involves periodontal and endodontic tissues. Using clinical, histologic, radiographic, and scanning microscopic analysis, a better understanding of the evolution of ECR is possible. Based on the experimental findings, a 3-stage mechanism for the initiation and growth of ECR is proposed.

The Cementocyte-An Osteocyte Relative?

Cementum is a mineralized tissue covering the tooth root that functions in tooth attachment and posteruptive adjustment of tooth position. During formation of the apically located cellular cementum, some cementoblasts become embedded in the cementoid matrix and become cementocytes. As apparently terminally differentiated cells embedded in a mineralized extracellular matrix, cementocytes are part of a select group of specialized cells, also including osteocytes, hypertrophic chondrocytes, and odontoblasts. The differentiation and potential function(s) of cementocytes are virtually unknown, and the question may be posed whether the cementocyte is a dynamic actor in cementum in comparable fashion with the osteocyte in the skeleton, responding to changing tooth functions and endocrine signals and actively directing local cementum metabolism. This review summarizes the literature with regard to cementocytes, comparing them to their closest "cousins," the osteocytes, where insights gained from osteocyte studies serve to inform the critical examination of cementocytes. The review identifies important unanswered questions about these cells regarding their origins, differentiation, morphology and lacuno-canalicular system, selective markers, and potential functions.

Microleakage of Self-Etch Adhesive System in Class V Cavities Prepared by Using Er:YAG Laser with Different Pulse Modes.

OBJECTIVE: In vitro studies evaluated cementum surface morphology and microleakage of three different energy density parameters of Erbium: Yttrium Aluminum Garnet (Er:YAG) laser compared with diamond bur preparation on class V cavities with self-etch adhesive system and composite resin restoration. MATERIAL AND METHODS: Standard class V cavities were prepared at cervical area below the cementoenamel junction (CEJ) in 80 extracted premolars, by using a diamond bur on the buccal surface. All teeth were randomly allocated into four groups: Group 1, diamond bur; Group 2, Er:YAG 50 mJ/15 Hz, 3.77 J/cm(2); Group 3, Er:YAG 75 mJ/15 Hz, 5.65 J/cm(2); and Group 4, Er:YAG 100 mJ/15 Hz, 7.53 J/cm(2). Five cavities from each group were evaluated by scanning electron microscopy (SEM). The 15 remaining cavities from each group were restored with self-etch adhesive and nano-hybrid composite. After thermocycling, all sample teeth were immersed in 0.2% methylene blue dye and sectioned buccolingually. Statistics were analyzed using the one way ANOVA and Mann-Whitney U tests with Bonferroni correction. RESULTS: The morphology showed micro-irregularities in the cementum surface of the laser group with the absence of a smear layer. The microstructure characteristics were increased surface roughness followed by increasing laser energy transmission. The Er:YAG laser groups were statistically significant, with less microleakage than the diamond bur group (p<0.05). There was statistically significant difference between the occlusal and gingival microleakage in all the groups (p<0.05). When the laser groups were compared, the lowest microleakage was achieved with energy density at 3.77 J/cm(2) on the occlusal and gingival cementum margin, which showed less microleakage than at energy densities of 5.65 and 7.53 J/cm(2) with Er:YAG laser. CONCLUSIONS: These observations indicate that the micro-irregularities of the cementum surface could facilitate the formation of a hybridization zone with a self-etch adhesive system. Therefore, the microleakage of Er:YAG laser irradiation was significantly decreased compared with diamond bur cavities.

Absolute polarity determination of teeth cementum by phase sensitive second harmonic generation microscopy.

The absolute sign of local polarity in relation to the biological growth direction has been investigated for teeth cementum using phase sensitive second harmonic generation microscopy (PS-SHGM) and a crystal of 2-cyclooctylamino-5-nitropyridine (COANP) as a nonlinear optic (NLO) reference material. A second harmonic generation (SHG) response was found in two directions of cementum: radial (acellular extrinsic fibers that are oriented more or less perpendicular to the root surface) and circumferential (cellular intrinsic fibers that are oriented more or less parallel to the surface). A mono-polar state was demonstrated for acellular extrinsic cementum. However, along the different parts of cementum in circumferential direction, two corresponding domains were observed featuring an opposite sign of polarity indicative for a bi-polar microscopic state of cellular intrinsic cementum. The phase information showed that the orientation of radial collagen fibrils of cementum is regularly organized with the donor (D) groups pointing to the surface. Circumferential collagen molecules feature orientational disorder and are oriented up and down in random manner showing acceptor or donor groups at the surface of cementum. Considering that the cementum continues to grow in thickness throughout life, we can conclude that the cementum is growing circumferentially in two opposite directions and radially in one direction. A Markov chain type model for polarity formation in the direction of growth predicts D-groups preferably appearing at the fiber front.

Evaluation of the influence of three different temperatures on microleakage of two self-etch and one total-etch adhesives.

AIM: To evaluate the bonding temperature effect on dentin-restoration microleakage. The null hypothesis of the study is that the score of microleakage is identical among different adhesive bondings at different temperatures. MATERIALS AND METHODS: Ninety caries free maxillary premolars were selected. Class V cavities were prepared on the cemento enamel junction (CEJ) of the buccal sides with enamel margins on occlusal sides and cementum margins on gingival sides. The specimens were divided into 3 groups: G1, single bond adhesive + Z250 composite; G2, P90 adhesive + Filtek Silorane composite; and G3, Clearfil SE bond + Clearfil APX. All groups were divided into three subgroups based on the adhesive temperature: A-4°C; B-25°C; and C-40°C. After coating the specimens with nail polish 1 mm beyond the margin of the restorations, they were stored in 0.5% basic Fuchsin dye solution for 24 hours. The teeth then were buccolingually sectioned and observed under a stereomicroscope. RESULTS: There was no significant difference between microleakage of occlusal and gingival margins in each group. Clearfil SE bond and Adper single bond displayed lower microleakage than P90 adhesive at 4°C and 25°C. The most and least microleakage score for Adper single bond was at 40°C and 25°C respectively. Clearfil SE bond showed less microleakage at 25°C than 4°C and 40°C. CONCLUSION: Clearfil SE bond and Adper single bond displayed less microleakage at 25°C while there was no significant difference among for P90 adhesive microleakage at three temperatures.

Development of coronal cementum in hypsodont horse cheek teeth.

The horse is a grazing herbivore whose cheek teeth are hypsodon; that is, they possess long crowns that are completely covered by coronal cement at eruption. For elucidation of the sequential events in the formation of this coronal cementum in the mandibular horse cheek teeth, in the present study the lower 3rd permanent premolar teeth (PM4 ) from 3.5-, 4-, and 5-year-old horses were compared by using radiography, microcomputed tomography (Miro-CT), light microscopy (LM), and scanning electron microscopy (SEM). The present study clearly showed that prior to coronal cementogenesis tartrate-resistant acid phosphatase (TRAP)-positive odontoclasts resorbed on the enamel surface of the reserve crown in horse cheek tooth. Enamel resorption areas were relatively narrow, and started from the cuspal tips, and moved in the apical direction during tooth development. A primary cementum was initially deposited on the irregularly pitted enamel-cementum junction (ECJ) of the infolding and peripheral enamel. The infolding cementum filled grooves completely by the time of tooth eruption. On the other hand, in the peripheral cementum, the secondary and tertiary cementum layers were sequentially deposited on the primary cementum. These two cementum layers were sites for the insertion of the periodontal ligaments, and were continually laid down on the primary cementum coronally rather than apically throughout the life. The results of the present study suggest that the coronal cementum of horse cheek teeth is a multistructural and multifunctional tissue, meeting the requirements of its many different functions.

Scanning electron microscopic observations of fibrous structure of cemento-dentinal junction in healthy teeth.

INTRODUCTION: The cemento-dentinal junction (CDJ) is a structural and biologic link between cementum and dentin present in the roots of teeth. Conflicting reports about the origin, structure and composition of this layer are present in literature. The width of this junctional tissue is reported to be about 2-4 µm with adhesion of cementum and dentin by proteoglycans and by collagen fiber intermingling. AIM AND OBJECTIVE: The objective of this study is to observe and report the fibrous architecture of the CDJ of healthy tooth roots. MATERIALS AND METHODS: A total of 15 healthy teeth samples were collected, sectioned into halves, demineralized in 5% ethylenediaminetetraacetic acid, processed using NaOH maceration technique and observed under a scanning electron microscope. RESULTS: The CDJ appeared to be a fibril poor groove with a width of 2-4 µm. Few areas of collagen fiber intermingling could be appreciated. A detailed observation of these tissues has been presented.

Morphology and microhardness of dentin at the furcation area of mandibular molars.

INTRODUCTION: This study evaluated dentin morphology and microhardness at the furcation area of mandibular molars. METHODS: Tooth segments of mandibular molars were embedded in resin blocks and bisected longitudinally (n = 20), and the furcation area was delimited by tracing 2 orthogonal lines from the most concave point of the outer surface of the furcation toward the mesial and distal canal entrances. In half of the specimens, Knoop microhardness was measured in 2 directions in the furcation area: vertically, parallel to the orthogonal lines and the bisector of the angle formed by them; and horizontally, in the outer (close to the pulp chamber floor), inner (close to the cementum), and middle dentin layers. Data were analyzed statistically by one-way analysis of variance and Tukey-Kramer test (α = 0.05). The other half of the specimens were examined by scanning electron microscopy to evaluate dentin morphology and trajectory of the tubules. RESULTS: No statistically significant difference (P > .05) was found among the mesial (46.5 ± 6.4), central (47.3 ± 8.1), and distal (49.7 ± 6.5) orthogonal lines. The inner dentin layer (51.7 ± 2.5) was statistically similar to the middle (46.3 ± 2.9) (P > .05), which was similar to the outer layer (41.4 ± 2.4) (P > .05). The inner layer was significantly harder than the outer layer (P < .05). The morphologic analysis revealed a tendency of calcification of the tubules from the outer toward the inner layer. CONCLUSIONS: Dentin microhardness at the furcation area is uniform in its 3 vertical axes, but the inner dentin layer is harder than the outer layer. The dentinal tubules follow a centripetal direction toward the inner layer, in which dentin is much more mineralized.

Characterization of crocodile teeth: correlation of composition, microstructure, and hardness.

Structure and composition of teeth of the saltwater crocodile Crocodylus porosus were characterized by several high-resolution analytical techniques. X-ray diffraction in combination with elemental analysis and infrared spectroscopy showed that the mineral phase of the teeth is a carbonated calcium-deficient nanocrystalline hydroxyapatite in all three tooth-constituting tissues: Dentin, enamel, and cementum. The fluoride content in the three tissues is very low (<0.1 wt.%) and comparable to that in human teeth. The mineral content of dentin, enamel, and cementum as determined by thermogravimetry is 71.3, 80.5, and 66.8 wt.%, respectively. Synchrotron X-ray microtomography showed the internal structure and allowed to visualize the degree of mineralization in dentin, enamel, and cementum. Virtual sections through the tooth and scanning electron micrographs showed that the enamel layer is comparably thin (100-200 µm). The crystallites in the enamel are oriented perpendicularly to the tooth surface. At the dentin-enamel-junction, the packing density of crystallites decreases, and the crystallites do not display an ordered structure as in the enamel. The microhardness was 0.60±0.05 GPa for dentin, 3.15±0.15 GPa for enamel, 0.26±0.08 GPa for cementum close to the crown, and 0.31±0.04 GPa for cementum close to the root margin. This can be explained with the different degree of mineralization of the different tissue types and is comparable with human teeth.