Dentino-enamel Junction Research Articles

Increased Matrix Metalloproteinase-2 and Bone Sialoprotein Response to Human Coronal Caries

Background: It has been suggested that host matrix metalloproteinase-2 (MMP-2) present in dentin may be involved in caries progression, however, its response to caries is not known. Bone sialoprotein (BSP) has been implicated in dentin mineralization and MMP-2 modulation. Objective: To identify and compare the distribution of MMP-2 and BSP in healthy human coronal dentin and those with early caries. Methods: Freshly extracted 3rd molars and premolars with and without early caries were fixed, demineralized and subjected to immunohistochemistry using a monoclonal anti-MMP-2 antibody and monoclonal anti-BSP antibody with an avidin-biotin complex method. Immunoreactivity was visualized with 3,3′-diaminobenzidine substrate and observed under light microscopy. Results: Immunohistochemical analysis revealed that MMP-2 and BSP are not detected in the tubule lumens of healthy dentin. However, intense immunoreactivity for MMP-2 and BSP was detected in association with the full length of the caries-affected dentinal tubules. The MMP-2 and BSP at the dentino-enamel junction appeared unaltered. Conclusion: The results indicate that MMP-2 and BSP may be actively secreted by odontoblasts in response to carious insult. MMP-2 and BSP accumulation in the caries-affected dentinal tubules may indicate their potential involvement in the host defense mechanism which results in calcification of regions affected by the carious process.

Influence of tooth-surface hydration conditions on optical coherence-tomography imaging

ObjectivesThis study examined the influence of tooth-surface hydration conditions on optical coherence tomography (OCT) imaging. MethodsEmitted light was coupled in a single-mode fibre-optic Michelson interferometer, and delivered to a sample and a reference mirror mounted on a linearly translating galvanometer driven by a triangular voltage waveform at a fringe-modulation frequency of 1kHz. Backscattered light from the sample was coupled back to the system, digitised and used to create two-dimensional images together with beam scanning. OCT imaging of the occlusal surfaces of 10 extracted human teeth was performed soon after mounting (‘wet’ condition), immediately after air blowing for 10s (‘air-blow’ condition), and at intervals thereafter (‘1-min’, ‘5-min’ and ‘10-min’ conditions, respectively). ResultsFor the wet condition, three distinct peaks indicated the water surface, tooth surface and dentino-enamel junction. Backscattered light was detected at levels above noise, possibly causing grainy OCT images. By contrast, two distinct peaks indicating the tooth surface and dentino-enamel junction were observed for the air-blow and 10-min conditions, with greater signal intensities for the latter. The intensity was lower for the air-blow condition than the wet condition. The signal intensities decreased during storage in air at room temperature. ConclusionTooth-substrate hydration conditions thus appeared to influence time domain-OCT imaging.

Bond Strength of Newer Dentin Bonding Agents in Different Clinical Situations

Clinical Relevance. The bond strengths of adhesive systems are affected by the amount of moisture on the bonding surface of the tooth.SUMMARY. This study compared the tensile bond strengths of different adhesive systems to different dentinal substrate conditions. The adhesive systems used were Adper Single Bond 2 total etch, XP Bond total etch, and two all-in-one adhesives, Adper Easy One and Xeno V. Ninety-six intact human premolars extracted on periodontal or orthodontic grounds were collected and stored in 0.9% physiologic saline for less than four weeks. Teeth were carefully cleaned to remove the soft tissue remnants. The occlusal surfaces were ground until the level of the dentinoenamel junction, exposing superficial dentin using a low-speed diamond disc. A standard smear layer was produced by wet sanding the dentin surface with 600-grit silicon carbide sandpaper for 40 seconds. The 96 specimens were embedded in autopolymerizing acrylic resin and divided into four groups of 24 each based on the dentin bonding agents used. Each group consisted of 24 specimens, which were further divided into two subgroups of 12 specimens each, moist dentin and dentin air-dried for 10 seconds. The regions of interest for adhesion were demarcated in each sample using adhesive tape, with a 4-mm–diameter hole isolating the peripheral region, and each adhesive system was employed. Four adhesive systems, two total etch and two self-etching adhesive, were applied to different dentinal substrates as per the manufacturers' instructions. Following the adhesive application, the specimens were restored with composite material using a metallic mold measuring 5 mm in diameter and 5 mm in height to keep the material in cylindrical form and were light cured per manufacturers' instructions. After being stored for 24 hours in distilled water at room temperature, the specimens were thermocycled for 550 cycles at temperatures ranging from 5°C to 60°C with a dwell time of 15 seconds in each bath and a transfer time of five seconds. The tensile bond strengths were determined using a Universal loading machine (Lloyd Universal Testing Machine) at a cross-head speed of 1 mm/min. On moist dentin, total-etch adhesives showed higher bond strength than did the all-in-one adhesives. Under the dry dentin condition, XP Bond exhibited significantly higher bond strength than did the Adper Single Bond 2 and two all-in-one adhesives, Adper Easy One and Xeno V.

The Reduction in Fatigue Crack Growth Resistance of Dentin with Depth

The fatigue crack growth resistance of dentin was characterized as a function of depth from the dentino-enamel junction. Compact tension (CT) specimens were prepared from the crowns of third molars in the deep, middle, and peripheral dentin. The microstructure was quantified in terms of the average tubule dimensions and density. Fatigue cracks were grown in-plane with the tubules and characterized in terms of the initiation and growth responses. Deep dentin exhibited the lowest resistance to the initiation of fatigue crack growth, as indicated by the stress intensity threshold (ΔKth ≈ 0.8 MPa•m0.5) and the highest incremental fatigue crack growth rate (over 1000 times that in peripheral dentin). Cracks in deep dentin underwent incremental extension under cyclic stresses that were 40% lower than those required in peripheral dentin. The average fatigue crack growth rates increased significantly with tubule density, indicating the importance of microstructure on the potential for tooth fracture. Molars with deep restorations are more likely to suffer from the cracked-tooth syndrome, because of the lower fatigue crack growth resistance of deep dentin.

Localization of MMP-2, MMP-9, TIMP-1, and TIMP-2 in human coronal dentine

ObjectivesMatrix metalloproteinases (MMPs) and their inhibitors (TIMPs) play important roles in dentine formation, caries progression and hybrid layer degradation. This study tested the hypothesis that the distribution and concentrations of MMP-2, MMP-9, TIMP-1 and TIMP-2 are different at different depths of human coronal dentine, including odontoblasts. MethodsProtein localization was performed using immunohistochemistry. Co-localization of the MMPs and their inhibitors was conducted using immunofluorescence double labelling. Protein concentrations were measured by ELISA and gelatinolytic potential was assessed with gelatine zymography. ResultsMMP-2 was the main gelatinase in dentine and was concentrated in the odontoblasts, deep dentine and the dentinoenamel junction. TIMP-2 was co-localized with MMP-2 mainly in the odontoblasts but its concentration was low. Both MMP-9 and TIMP-1 showed a decreasing distribution from the deep to the superficial dentine layers; however, the concentration of TIMP-1 was much higher than that of MMP-9. The gelatinolytic potential of dentine protein extracts decreased gradually from deep to superficial dentine. ConclusionsThe concentrations and distribution patterns of MMP-2, MMP-9, TIMP-1 and TIMP-2, and the gelatinolytic potential of dentine matrix are variable along different dentine depths. Thus, differential collagen degradation potentials may be expected depending upon the depth in which dentine is exposed.

Why Does Enamel in Klk4-Null Mice Break above the Dentino-Enamel Junction?

Background: The enamel layerof kallikrein 4 (Klk4)-null mice has a normal thickness and a decussating pattern of enamel rods, but it contains residual enamel proteins, is less highly mineralized, and fractures in its deepest part just above the dentino-enamel junction (DEJ). The plane of fracture is puzzling because the deepest enamel is deposited earliest and, through the action of the secretory stage enamel protease (Mmp20), is the most mature part of the enamel layer at the time of the onset of Klk4 expression. Objectives: To characterize the planes of fracture in Mmp20- and Klk4-null mice and to localize Klk4 expression in developing teeth. Methods:Klk4- and Mmp20-null mice were sacrificed at 7 weeks and their mandibular incisors were characterized by scanning electron microscopy. Klk4^+/^lac^Z mice were mated with Klk4^+/^lac^Z mice. Offspring were genotyped by polymerase chain reaction. Klk4^+/⁺, Klk4^+/^lac^Z, and Klk4^lac^Z/^lac^Z (null) littermates on postnatal days 5, 8, 11, and 14 were processed for β-galactosidase histochemistry. Results: The enamel layer fractures at the DEJ in Mmp20-null mice, and fractures occur in enamel above the DEJ in Klk4-null mice. Klk4 is not expressed by secretory-stage ameloblasts, murine odontoblasts beneath the secretory stage, or maturation-stage ameloblasts. Klk4 is specifically expressed by transition and maturation-stage ameloblasts. Conclusions: The breakage of enamel near the DEJ in Klk4-null mice is not due to a failure of odontoblasts to express Klk4, but it relates to a progressive hypomineralization of enamel with depth.

Effect of Kallikrein 4 Loss on Enamel Mineralization: COMPARISON WITH MICE LACKING MATRIX METALLOPROTEINASE 20

Enamel formation depends on a triad of tissue-specific matrix proteins (amelogenin, ameloblastin, and enamelin) to help initiate and stabilize progressively elongating, thin mineral ribbons of hydroxyapatite formed during an appositional growth phase. Subsequently, these proteins are eradicated to facilitate lateral expansion of the hydroxyapatite crystallites. The purpose of this study was to investigate changes in enamel mineralization occurring in mice unable to produce kallikrein 4 (Klk4), a proteinase associated with terminal extracellular degradation of matrix proteins during the maturation stage. Mice lacking functional matrix metalloproteinase 20 (Mmp20), a proteinase associated with early cleavage of matrix proteins during the secretory stage, were also analyzed as a frame of reference. The results indicated that mice lacking Klk4 produce enamel that is normal in thickness and overall organization in terms of layers and rod/inter-rod structure, but there is a developmental defect in enamel rods where they first form near the dentinoenamel junction. Mineralization is normal up to early maturation after which the enamel both retains and gains additional proteins and is unable to mature beyond 85% mineral by weight. The outmost enamel is hard, but inner regions are soft and contain much more protein than normal. The rate of mineral acquisition overall is lower by 25%. Mice lacking functional Mmp20 produce enamel that is thin and structurally abnormal. Relatively high amounts of protein remain throughout maturation, but the enamel is able to change from 67 to 75% mineral by weight during maturation. These findings reaffirm the importance of secreted proteinases to enamel mineral acquisition.

Bonding efficiency of contemporary adhesives to the dentinoenamel junction zone

The aim of this study was to investigate the bonding of dental adhesives to the dentinoenamel junction (DEJ) zone. Bonding of four adhesives [two etch-and-rinse adhesives (ERAs) and two self-etching adhesives (SEAs)] to enamel, dentin, and the DEJ zone was evaluated using a micro-shear test. Based on the measured bond strengths of dentin/enamel and on the area percentages of dentin in the DEJ zone, predicted bond strengths for the DEJ zone were calculated and compared with those measured. The DEJ zone was analyzed, using scanning electron microscopy, after conditioning, resin infiltration, and debonding. Regardless of the adhesive, bond strengths were significantly influenced by substrates, exhibiting the following order of bond strength (strongest to weakest): dentin >DEJ zone > enamel. The predicted values of the DEJ zone for the ERA groups were significantly higher than the measured values. Analysis of the ERA specimens using scanning electron microscopy showed distinct etching textures of enamel and dentin, which outlined the DEJ and increased the adhesive thickness at the enamel side of the DEJ. Those characteristics could not be detected in the SEA groups. The DEJ zone displayed bond properties that were stronger than enamel but weaker than dentin and therefore may be considered as transitional bond properties. Enamel/dentin within the DEJ zone might bond more weakly to ERAs than its counterpart of the bulk tissue. The presence of the DEJ in the bond area might compromise the bonding efficiency of ERAs.

Microstructure of Mineralized Tissues in Human Primary Teeth

The aim of this study was to analyze the structural characteristics of the mineralized dental tissues--enamel, dentin and cementum--in primary teeth and to correlate the histological aspects observed in function of the dental type--single-rooted or multi-rooted. Eighteen human primary noncarious teeth were sectioned in facial-lingual (single-rooted) and mesio-distal direction (multi-rooted). One to three samples from each tooth were obtained. The samples were prepared by the ground technique and analyzed under light microscopy at different magnifications. A quantitative and descriptive analysis of the morphology of the mineralized tissues was performed. Spindles, tufts and lamellae were consistently observed mainly in the occlusal surface of the primary molars. The scalloped pattern of the dentinoenamel junction was not always present. The same was seen for zones of interglobular dentin. Dead tracts in dentin and tertiary dentin were observed mainly in single-rooted teeth below areas of dental attrition. Areas of cellular and acellular cementum were observed in the two dental types. Primary teeth have some structural peculiarities and these should be investigated concerning the clinical repercussion.

Nano- and micromechanical properties of dentine: Investigation of differences with tooth side

The soft zone in dentine beneath the dentino-enamel junction is thought to play an important role in tooth function, strain distribution and fracture resistance during mastication. Recently reported asymmetry in mechanical properties with tooth side may point at a basic property of tooth function. The aim of our study was to test if this asymmetry was reflected in the nano- and micromechanical properties of dentine.We investigated the mechanical properties of dentine on the buccal and lingual side of nine extracted human teeth using nano- and microindentation. Properties were analysed on the natural log scale, using maximum likelihood to estimate the parameters. Two-sided 0.05-level likelihood ratio tests were used to assess the influences of surface (buccal versus lingual) and dentine depth, measured from the DEJ in crown dentine and from the CDJ in root dentine.Results showed the well known gradual increase in mechanical properties with increasing distance from the DEJ. Coronal dentine showed higher elastic modulus and hardness on the lingual side of teeth for all measurements, while root dentine was harder on the buccal side. Due to the subtlety of these effects and the small number of teeth studied, results failed to reach statistical significance. Results suggest that dentine nano- and micromechanical properties vary with tooth side in agreement with recent literature using macroscopic methods. They also reveal that buccal–lingual ratios of hardness are in opposite directions in crown and root dentine, suggesting compensatory functions.

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.

Effect of Hydration on the Strain Gradients in Dental Hard Tissues after Heat and Cold Application

Introduction This study was aimed to investigate the thermal strain gradients on dental hard tissues to localized heat and cold applications, applied to assess the vitality of pulp. The role of hydration on the thermal strain distribution within the enamel, dentin, and dentino-enamel junction (DEJ) was examined by using a digital moiré interferometry. Methods Extracted bovine incisors were prepared, and high-frequency diffraction gratings were replicated on one surface. Heat (120°C–140°C) and cold (–50°C) stimuli were applied on the external surface, and the strain patterns were recorded and analyzed for the first 3 seconds. The specimens were tested under fully hydrated and partially dehydrated conditions (72 hours at 24°C, 60% relative humidity). Results Distinct thermal strain gradient was observed in the enamel, dentin, and DEJ after the application of heat and cold stimuli. Application of both heat and cold resulted in significantly higher strains in the partially dehydrated teeth than in the fully hydrated teeth ( P < .05). There was only a marginal increase in strains at the location of application of stimuli in hydrated teeth. The DEJ in both the fully hydrated and partially dehydrated teeth showed the highest strains. Conclusions There was a marked difference in the thermal strain gradients within dental hard tissues after the application of heat/cold stimuli, depending on the level of tissue hydration. The findings from this study highlighted the role of free water and structural characteristics of enamel, dentin, and DEJ in dissipating the thermal strains in the tooth.

Influence of application parameters on bond strength of an “all in one” water-based self-etching primer/adhesive after 6 and 12 months of water aging

Our aim was to evaluate the influence of different application parameters on microtensile bond strength (microTBS) of an "all in one" water-based self-etching primer/adhesive to dentin. Extracted human third molars had their crowns transversally sectioned next to the dentinoenamel junction. Prompt-L-Pop adhesive was applied using five modes: (1) following the manufacturer's instructions (MI); (2) after 36% H(3)PO(4) etching (15 s) (AG); (3) after H(3)PO(4) etching and 5% NaOCl (2 min) with constant agitation (HG); (4) as a double layer (DL); and (5) for double the application time (DT). Resin build-ups were constructed with Tetric Ceram. After storage for 24 h, 6 months, or 12 months in water at 37 degrees C, the specimens were vertically sectioned into beams (cross-sectional areas, 1 mm(2)). Each beam was tested in an Instron Machine at 0.5 mm/min. Analysis of variance and Student-Newman-Keuls were used (P < 0.05). Bonded interfaces were examined by scanning electron microscopy (SEM) and transmission electron microscopy (TEM). The highest microTBS values were obtained when AG was used, and the lowest when MI was employed. The HG, DL, and DT modes resulted in similar microTBS values. Water storage reduced microTBS in all groups except the MI group. SEM and TEM revealed hybrid layer and resin tag formation in all groups. Applied after AG, HG, DL, and DT, Prompt L-Pop adhesive provided adequate bond strength to dentin after 24 h of water storage. Bond strength decreased when specimens were stored in distilled water for 6 or 12 months.

High energy X-ray scattering quantification of in situ-loading-related strain gradients spanning the dentinoenamel junction (DEJ) in bovine tooth specimens

High energy X-ray scattering (80.7keV photons) at station 1-ID of the Advanced Photon Source quantified internal strains as a function of applied stress in mature bovine tooth. These strains were mapped from dentin through the dentinoenamel junction (DEJ) into enamel as a function of applied compressive stress in two small parallelepiped specimens. One specimen was loaded perpendicular to the DEJ and the second parallel to the DEJ. Internal strains in enamel and dentin increased and, as expected from the relative values of the Young’s modulus, the observed strains were much higher in dentin than in enamel. Large strain gradients were observed across the DEJ, and the data suggest that the mantle dentin-DEJ-aprismatic enamel structure may shield the near-surface volume of the enamel from large strains. In the enamel, drops in internal strain for applied stresses above 40MPa also suggest that this structure had cracked.

Variations in human DEJ scallop size with tooth type

Objective Recent literature suggests that the scalloped structure of the dentino-enamel junction (DEJ) is critical for DEJ stability. Aim of our study was to see if there are differences in scallop size and shape with tooth type. Methods Enamel of extracted permanent human teeth was demineralised using EDTA. After fixation and dehydration the scallops of the DEJ were investigated in a scanning electron microscope. Scallop area and shape (circularity) were measured for molars, premolars, canines and incisors. Results Scallop area showed main effects for tooth type and specimen, while, due to high variability in third molars, there was also an interaction effect (repeated measures two-way ANOVA, p < 0.05). Differences between tooth types were statistically significant, suggesting that posterior teeth showed larger scallops compared to anterior teeth. Differences in shape (circularity) were not statistically significant. Conclusion Our results suggest that teeth which are subject to higher masticatory loads (posterior teeth) show larger and more pronounced scallops. These findings might be of interest for improving other interfaces joining dissimilar materials.

Crack arrest within teeth at the dentinoenamel junction caused by elastic modulus mismatch

Enamel and dentin compose the crowns of human teeth. They are joined at the dentinoenamel junction (DEJ) which is a very strong and well-bonded interface unlikely to fail within healthy teeth despite the formation of multiple cracks within enamel during a lifetime of exposure to masticatory forces. These cracks commonly are arrested when reaching the DEJ. The phenomenon of crack arrest at the DEJ is described in many publications but there is little consensus on the underlying cause and mechanism. Explanations range from the DEJ having a larger toughness than both enamel and dentin up to the assumption that not the DEJ itself causes crack arrest but the so-called mantle dentin, a thin material layer close to the DEJ that is somewhat softer than the bulk dentin. In this study we conducted 3-point bending experiments with bending bars consisting of the DEJ and surrounding enamel and dentin to investigate crack propagation and arrest within the DEJ region. Calculated stress intensities around crack tips were found to be highly influenced by the elastic modulus mismatch between enamel and dentin and hence, the phenomenon of crack arrest at the DEJ could be explained accordingly via this elastic modulus mismatch.

Measurement of hydroxyapatite density and Knoop hardness in sound human enamel and a correlational analysis between them

Objective The aim of this study was to measure the hydroxyapatite (HAP) density and Knoop hardness (KHN) of enamel slabs and to analyse the relationship between them. Design Twenty enamel slabs (10 lingual sides and 10 buccal sides) were prepared and scanned with micro-CT. Tomographic images of each slab from dental cusp to dentinoenamel junction (DEJ) were reconstructed. On these three-dimensional (3D) images, regions of interest (ROIs) were defined at an interval of 50 μm, and the HAP density for each ROI was calculated. Then the polished surfaces were indented from cusp to DEJ at intervals of 50 μm with a Knoop indenter. Finally, the data were analysed with one-way ANOVA, Student's t-test, and linear regression analysis. Results The HAP density and KHN decreased from the dental cusp to DEJ. Both HAP density and KHN in the outer-layer enamel were significantly higher than those in the middle- or inner-layer enamel ( P < 0.05). The HAP density showed no significant difference between the buccal and lingual sides for enamel in the outer, middle and inner layers, respectively ( P > 0.05). The KHN in the outer-layer enamel of the lingual sides was significantly lower than that of the buccal sides ( P < 0.05); there was no significant difference between the lingual and buccal sides in the middle or inner layer. Linear regression analysis revealed a linear relationship between the mean KHN and the mean HAP density ( r = 0.87). Conclusion Both HAP density and KHN decrease simultaneously from dental cusp to DEJ, and the two properties are highly correlated.

Oral features in Apert syndrome: a histological investigation

The number of publications on the oral features in Apert syndrome is limited. The present study investigated dental tissues in Apert syndrome histologically, to determine the nature and extent of anomalies, to provide some insight into the nature of the condition, and to explain how observed anomalies may affect the dental management of individuals with Apert syndrome. Extracted primary and secondary teeth were collected from patients with Apert who had attended the Australian Craniofacial Unit, Adelaide, South Australia. The total study sample comprised 13 individuals, aged from 14 to 21 , with nine men and four women. A total of 40 teeth were available for histological examination (the number belonging to each individual varied from 2 to 5 per patient). The teeth were sectioned longitudinally, and one-half of each tooth underwent decalcification. Sections were stained with H&E for routine histological examination. Ground sections were prepared from undecalcified tooth halves. Histological assessment of the dental hard tissues revealed an intact enamel and dentinal structure but some irregularities were noted in the region of the dentino-enamel junction (DEJ), which could affect caries progression and also make dental management more difficult. This study identified histological anomalies of the DEJ of Apert syndrome teeth. An improved appreciation of the nature and extent of dental anomalies in Apert syndrome should assist clinicians when undertaking management of affected individuals.

Finite Element Stress Analysis of Diastema Closure with Ceramic Laminate Veneers

The purpose of this study was to use finite element stress analysis to examine the relative importance of variables such as porcelain laminate veneer (PLV) extensions, loading angle, and loading level for the case of feldspathic ceramic veneering of teeth to manage diastema. A 3D maxillary central incisor including its internal anatomy and morphology was constructed with ANSYS software for different extensions of PLV. Internal boundaries defining the dentinoenamel junction, the pulp-dentinal junction, the interface between the enamel-luting cement, and the porcelain-luting cement were well defined. The von Mises stresses distribution and stress intensity were analyzed on the free extension of PLV for varying extensions, various angulations (0 degrees , 30 degrees , and 60 degrees ) on the incisal edge, and for different loading levels (50, 150, and 250 N). The numerical values of stress were recorded. A significant difference in stress was observed. Increased stresses occurred with increased extensions, angulations, and loading levels. At 0 degrees angulation, compressive stresses were visualized in finite element analysis for various magnitudes of force. Higher stress values of 182 MPa and 211 MPa were obtained for the 2.5-mm extension in the mesial surface and in both proximal surfaces for 0 degrees angulation at 250 N magnitude of force. The stress occurring at 30 degrees and 60 degrees angulations was the combination of compressive and tensile stress. Higher values of 261 MPa and 232 MPa were observed when forces were applied on the mesial extension of the PLV and on both the proximal surfaces for 2.5 mm at 30 degrees , 250 N magnitude of force. A maximum stress value of 507 MPa was observed when PLV were increased in mesial width by 2.5 mm for 60 degrees angulation at 250 N magnitude of force. The extensions of PLV in diastema closure have more of an esthetic than functional consideration, but critical factors such as angulations and the loading level acting on the free extension of PLV are important.

Kinematic parameters inferred from enamel microstructure: new insights into the diet of Australopithecus anamensis

The dietary adaptations of Australopithecus anamensis are contentious, with suggestions that range from soft fruits to hard, brittle, tough, and abrasive foods. It is unlikely that all propositions are equally valid, however. Here we extend recent finite element (FE) analyses of enamel microstructure ( Shimizu and Macho, 2008) to enquire about the range of loading directions (i.e., kinematics) to which A. anamensis enamel microstructure/molars could safely be subjected. The rationale underlying this study is the observation that hard brittle foods are broken down in crush, while tough foods require shear. The findings are compared with those of Pan and Gorilla. Eighteen detailed FE models of enamel microstructure were created and analysed. The results highlight the uniqueness of A. anamensis dental structure and imply that mastication in this species included a greater shear component than in Pan, as well as a wider range of loading directions; it is similar to that in Gorilla in this respect. These findings are in accord with microwear studies (Grine et al., 2006a). Unlike either of the great apes, however, enamel microstructure of A. anamensis was found to be poorly equipped to withstand loading parallel to the dentino-enamel junction; such loading regimes are associated with mastication of soft fleshy fruits. This, together with broader morphological considerations, raises doubts as to whether A. anamensis was essentially a frugivore that expanded its dietary niche as a result of fluctuations in environmental conditions, e.g., during seasonal food shortages. Instead, it is more parsimonious to conclude that the habitual diet of A. anamensis differed considerably from that of either of the extant African great apes.