The fracture behaviour of human and pig molar cusps

Analysis of dental chipping for identifying and diagnosing tooth fracture patterns in osteological series

OBJECTIVE

To develop a specific methodology for identifying dental chipping and determining its temporal occurrence in past populations.

DESIGN

The analysed sample comprised of 2191 human teeth from various Bronze Age on the Iberian Peninsula (Argar culture, 1900-1450 cal BC). Among these, 471 chipped teeth were identified. Chipping was examined using various microscopic techniques (digital three-dimensional, optical, and confocal), focusing on distribution, morphology, position in the tooth, extent of damage, and post-chipping antemortem modifications (PCAM).

RESULTS

The distribution and morphology of the chips enabled the identification chipping mechanism of the chipping, providing valid criteria to distinguish between antemortem and postmortem chipping. Microscopic analyses of the chipping segments-edges, sidewalls, surface, and surrounding area-facilitated determination of the time the chip ocurred (antemortem: recent, less recent, or not recent).

CONCLUSIONS

While experimental studies provide valuable insights into chipping mechanisms, many criteria may not be applicable to past populations because of the presence of PCAM. The lack of PCAM in some Argaric teeth suggests that previous studies may have underestimated the prevalence of chipping in past populations.

On edge chipping in molar teeth from blunt occlusal contact

Edge chipping is a leading failure mode in dental teeth. Virtually all chipping studies are limited to Vickers indentation on polished cusps of molar teeth. Such works are here extended to spherical contact. Occlusal loads are applied on the tooth's central fossa or a polished cusp using ball radii ranging from 0.4 to 5.16 mm. The chip dimensions are characterized by h/Dm and D/Dm, where h, D and Dm denote indent distance, chip size and tooth crown diameter. For the fossa loading, h/Dm, D/Dm and the least chipping force Pch are virtually independent of ball radius r for r < ≈ 4 mm. In this range, h/Dm and D/Dm lie between ≈0.30 to 0.36 and 0.51 to 0.69, respectively, while Pch equals ≈1330 N. For r > ≈ 4 mm, the failure occurs by debonding of enamel sectors from the dentin core. In the case of cusp loading, h/Dm < ≈ 0.3 while D/Dm and Pch vary with r. For relatively small h or large r, the failure occurs as soon as radial cracks initiate under the loading point. For a load applied near a cusp tip, the failure occurs by enamel debonding. Finally, the present work is easily extendable to fossil teeth of hominins and apes as well as prosthetic teeth. The morphological features obtained in such studies should provide quantitative means to assess the relationships between chip dimensions, chipping force and diet characteristics.

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

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

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

OBJECTIVES

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

DESIGN

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

RESULTS

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

CONCLUSIONS

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

Dentin hardness differences across various mammalian taxa

Differences in dentin microstructure have been used as a tool for dietary reconstruction; however, the extent that diet is associated with this aspect of dental morphology has yet to be empirically tested. We conducted microhardness tests of mammalian dentin sections, hypothesizing that species with adaptations to particularly hard diets would have softer dentin, owing to a higher proportion of soft intertubular dentin. Species adapted to abrasive diets, in contrast, should have harder dentin, resulting from a higher proportion of hypermineralized peritubular dentin. We examined molar dentin hardness in ten mammalian taxa with durophagous diets, abrasive diets, and a comparative "control" group of mechanical generalists. Samples included six primate taxa and four non-primate species representing various dietary regimes. Our results reveal significant variation among taxa in overall hardness, but the data do not distinguish between hard and abrasive diets. Several taxa with generalized (i.e., mechanically diverse) diets resemble each other in exhibiting large variance in hardness measurements and comparably soft dentin. The high variation in these species appears to be either a functional signal supporting the niche variation hypothesis or indicate the absence of sustained unidirectional selective pressure. A possible phylogenetic signal of dentin hardness in the data also holds promise for future systematic investigations.

Anomalous erosion patterns on the cheek teeth of babirusa (genus Babyrousa)

Two types of anomalous erosive wear, designated “Wear” and “Groove”, were found in the molar teeth of adult babirusa (genus Babyrousa Perry, 1811) species from Buru or the Sula Islands and Sulawesi, Indonesia. A survey of 590 specimens of babirusa crania and mandibles from international museums and private collections revealed their presence in 20 of 187 specimens from Buru or the Sula Islands, in 99 of 391 specimens from Sulawesi, and in 4 of 31 subfossil specimens from South Sulawesi. “N_Wear” comprised a discrete indentation or almost circular erosion of the enamel and underlying dentine in a molar tooth; “W_Wear” comprised tooth indentations of irregular shape larger than 7 mm × 7 mm in size; “E_Wear” comprised extensive tooth indentation. “Wear” anomalies were found on 10% of specimens from Buru or the Sula Islands and on 17% of all Sulawesi crania and mandibles. It was hypothesised that tooth grinding of the abrasive seeds of the wild banana (Musa balbisiana Colla) initiated this anomaly. Groove was a deep, oval-shaped cleft cut into the enamel and dentine between two molar teeth. Groove anomalies were evident in 3% of Buru or the Sula Islands specimens and in 12% of all Sulawesi specimens. No conclusion was reached regarding the initiator of the Groove anomaly.

The influence of storage temperature on fracture behavior of cryopreserved teeth-An in vitro study

Objectives

Cryopreservation is discussed as a viable method of preserving teeth for determined autogenous tooth transplantation. Unchanged physical properties of hard tooth tissues are crucial for functional healing. Due to different thermal expansion coefficients of enamel and dentin or the crystallization process, the freezing process may lead to crack formation, which could adversely impact the long‐term prognosis of the teeth.

Material and methods

Twenty third molars (n = 20) were frozen slowly using a conservative cryopreservation protocol and stored at −80°C (group 1) and −196°C (group 2). After a storage time of 2 weeks, the samples were thawed to a temperature of +36°C and embedded in polymethyl methacrylate blocks. Cyclic loading was carried out using a spherical steel test specimen with 50,000 mechanical load cycles, followed by load to failure testing for determination of critical load.

Results

No significant difference in the first load drop could be detected during the load to failure test under different storage conditions. The values until fracture correlated very closely in contralateral tooth pairs, which emphasizes the importance of crown geometry in load to failure tests.

Conclusions

Conclusions: Cryopreservation, specifically the storage temperature, does not appear to have a significant effect on the physical properties of tooth transplants.

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

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

Effects of food properties on chewing in pigs: Flexibility and stereotypy of jaw movements in a mammalian omnivore

Chewing is a rhythmic oral behavior that requires constant modifications of jaw movements in response to changes in food properties. The food-specific kinematic response is dependent on the potential for kinematic flexibility allowed by morphology and modulation of motor control. This study investigates the effects of food toughness and stiffness on the amplitude and variability of jaw movements during chewing in a typical omnivorous mammalian model (pigs). Jaw movements were reconstructed using X-ray Reconstruction Of Moving Morphology (XROMM) and kinematic data associated with the amplitude of jaw pitch (opening-closing) and jaw yaw (mediolateral rotation) were extracted for each cycle. Between-food differences were tested for the amplitude of jaw movements during each phase of the gape cycle, as well as in their respective within-food variability, or stereotypy, as indicated by coefficients of variation. With increasing toughness, jaw pitch amplitude is decreased during fast close, larger and more stereotyped during slow close, smaller but more variable during slow open, and more variable during fast open. In addition, when chewing on tougher foods, the amplitude of jaw yaw during slow close only increases in a subset of individuals, but all become less variable (i.e., more stereotyped). In contrast, increasing food stiffness has no effect on the amplitude or the variability of jaw pitch, whereas jaw yaw increases significantly in the majority of individuals studied. Our data demonstrate that food stiffness and toughness both play a role in modulating gape cycle dynamics by altering the trajectory of jaw movements, especially during the slow-close phase and tooth-food-tooth contact, albeit differently. This highlights how a generalist oral morphology such as that of pigs (e.g., bunodont teeth lacking precise occlusion, permissive temporomandibular joint allowing extensive condylar displacements in 3 dimensions) enables organisms to not only adjust chewing movements in their amplitude, but also in their variability.

On the evolutionary advantage of multi-cusped teeth

A hallmark of mammalian evolution is a progressive complexity in postcanine tooth morphology. However, the driving force for this complexity remains unclear: whether to expand the versatility in diet source, or to bolster tooth structural integrity. In this study, we take a quantitative approach to this question by examining the roles of number, position and height of multiple cusps in determining sustainable bite forces. Our approach is to use an extended finite-element methodology with due provision for step-by-step growth of an embedded crack to determine how fracture progresses with increasing occlusal load. We argue that multi-cusp postcanine teeth are well configured to withstand high bite forces provided that multiple cusps are contacted simultaneously to share the load. However, contact on a single near-wall cusp diminishes the strength. Location of the load points and cusp height, rather than cusp number or radius, are principal governing factors. Given these findings, we conclude that while complex tooth structures can enhance durability, increases in cusp number are more likely to be driven by the demands of food manipulation. Structural integrity of complex teeth is maintained when individual cusps remain sufficiently distant from the side walls and do not become excessively tall relative to tooth width.

The Influence of Force Direction on the Fracture Pattern and Fracture Resistance of Canine Teeth in Dogs

Biomechanical studies of the elongated canine tooth of animals are few, and thus our understanding of mechanical and physical properties of animal teeth is limited. The objective of the present study was to evaluate the influence of force direction on fracture resistance and fracture pattern of canine teeth in an ex vivo dog cadaver model. Forty-five extracted canine teeth from laboratory beagle dogs were standardized by hard tissue volume and randomly distributed among 3 force direction groups. The teeth were secured within a universal testing machine and a load was applied at different directions based on testing group. The maximum force to fracture and the fracture pattern classification were recorded for each tooth. After correcting for hard tissue cross-sectional area in a multivariate analysis, no significant difference in the amount of force required for fracture was apparent between the different force direction groups. However, the influence of force direction on fracture pattern was significant. The results of this study may allow the clinician to educate clients on possible causal force directions in clinically fractured teeth and, thus, help prevent any contributing behavior in the future.

Mechanics analysis of molar tooth splitting

A model for the splitting of teeth from wedge loading of molar cusps from a round indenting object is presented. The model is developed in two parts: first, a simple 2D fracture mechanics configuration with the wedged tooth simulated by a compact tension specimen; second, a full 3D numerical analysis using extended finite element modeling (XFEM) with an embedded crack. The result is an explicit equation for splitting load in terms of indenter radius and key tooth dimensions. Fracture experiments on extracted human molars loaded axially with metal spheres are used to quantify the splitting forces and thence to validate the model. The XFEM calculations enable the complex crack propagation, initially in the enamel coat and subsequently in the interior dentin, to be followed incrementally with increasing load. The fracture evolution is shown to be stable prior to failure, so that dentin toughness, not strength, is the controlling material parameter. Critical conditions under which tooth splitting in biological and dental settings are likely to be met, however rare, are considered.

Role of multiple cusps in tooth fracture

The role of multiple cusps in the biomechanics of human molar tooth fracture is analysed. A model with four cusps at the bite surface replaces the single dome structure used in previous simulations. Extended finite element modelling, with provision to embed longitudinal cracks into the enamel walls, enables full analysis of crack propagation from initial extension to final failure. The cracks propagate longitudinally around the enamel side walls from starter cracks placed either at the top surface (radial cracks) or from the tooth base (margin cracks). A feature of the crack evolution is its stability, meaning that extension occurs steadily with increasing applied force. Predictions from the model are validated by comparison with experimental data from earlier publications, in which crack development was followed in situ during occlusal loading of extracted human molars. The results show substantial increase in critical forces to produce longitudinal fractures with number of cuspal contacts, indicating a capacity for an individual tooth to spread the load during mastication. It is argued that explicit critical force equations derived in previous studies remain valid, at the least as a means for comparing the capacity for teeth of different dimensions to sustain high bite forces.

Fracture behavior of human molars

Despite the durability of human teeth, which are able to withstand repeated loading while maintaining form and function, they are still susceptible to fracture. We focus here on longitudinal fracture in molar teeth-channel-like cracks that run along the enamel sidewall of the tooth between the gum line (cemento-enamel junction-CEJ) and the occlusal surface. Such fractures can often be painful and necessitate costly restorative work. The following study describes fracture experiments made on molar teeth of humans in which the molars are placed under axial compressive load using a hard indenting plate in order to induce longitudinal cracks in the enamel. Observed damage modes include fractures originating in the occlusal region ('radial-median cracks') and fractures emanating from the margin of the enamel in the region of the CEJ ('margin cracks'), as well as 'spalling' of enamel (the linking of longitudinal cracks). The loading conditions that govern fracture behavior in enamel are reported and observations made of the evolution of fracture as the load is increased. Relatively low loads were required to induce observable crack initiation-approximately 100 N for radial-median cracks and 200 N for margin cracks-both of which are less than the reported maximum biting force on a single molar tooth of several hundred Newtons. Unstable crack growth was observed to take place soon after and occurred at loads lower than those calculated by the current fracture models. Multiple cracks were observed on a single cusp, their interactions influencing crack growth behavior. The majority of the teeth tested in this study were noted to exhibit margin cracks prior to compression testing, which were apparently formed during the functional lifetime of the tooth. Such teeth were still able to withstand additional loading prior to catastrophic fracture, highlighting the remarkable damage containment capabilities of the natural tooth structure.

Dental evidence for the diets of Plio-Pleistocene hominins

Diet is fundamental to the interaction between an organism and its environment, and is therefore an important key to understanding ecology and evolution. It should come as no surprise then that paleoanthropologists have put a great deal of effort into reconstructing the diets of Plio-Pleistocene hominins. Most of this effort has focused on teeth; these durable parts of the digestive system are usually the most commonly preserved elements in vertebrate fossil assemblages. In this article, I review much of this work. Tooth size, occlusal morphology, enamel thickness, and microstructure provide evidence for the physical properties of the foods to which a species was adapted. Dental microwear can offer insights into the properties of foods that an individual ate on a day-to-day basis. Taken together, these lines of evidence can offer important insights into early hominin food choices and adaptations. New methods of analysis and theoretical perspectives are improving our understanding of the diets of Australopithecus, Paranthropus, and early Homo, and promise further progress long into the future.

Mechanics of longitudinal cracks in tooth enamel

A study is made of longitudinal "channel" cracking in tooth enamel from axial compressive loading. The cracks simulate those generated in the molar and premolar teeth of humans and animals by natural tooth function. Contact loading tests are made on extracted human molars with hard and soft indenting plates to determine the evolution of such cracks with increasing load. Fracture is largely stable, with initial slow growth followed by acceleration as the cracks approach completion around an enamel side wall. A simple power law relation expresses the critical load for full fracture in terms of characteristic tooth dimensions-base radius and enamel thickness-as well as enamel toughness. Extended three-dimensional finite element modeling with provision for growth of embedded cracks is used to validate this relation. The cracks leave "fingerprints" that offer valuable clues to dietary habits, and provide a basis for a priori prediction of bite forces for different animals from measured tooth dimensions.

Fracture in teeth: a diagnostic for inferring bite force and tooth function

Teeth are brittle and highly susceptible to cracking. We propose that observations of such cracking can be used as a diagnostic tool for predicting bite force and inferring tooth function in living and fossil mammals. Laboratory tests on model tooth structures and extracted human teeth in simulated biting identify the principal fracture modes in enamel. Examination of museum specimens reveals the presence of similar fractures in a wide range of vertebrates, suggesting that cracks extended during ingestion or mastication. The use of 'fracture mechanics' from materials engineering provides elegant relations for quantifying critical bite forces in terms of characteristic tooth size and enamel thickness. The role of enamel microstructure in determining how cracks initiate and propagate within the enamel (and beyond) is discussed. The picture emerges of teeth as damage-tolerant structures, full of internal weaknesses and defects and yet able to contain the expansion of seemingly precarious cracks and fissures within the enamel shell. How the findings impact on dietary pressures forms an undercurrent of the study.

The influence of fallback foods on great ape tooth enamel

Lucas and colleagues recently proposed a model based on fracture and deformation concepts to describe how mammalian tooth enamel may be adapted to the mechanical demands of diet (Lucas et al.: Bioessays 30 2008 374-385). Here we review the applicability of that model by examining existing data on the food mechanical properties and enamel morphology of great apes (Pan, Pongo, and Gorilla). Particular attention is paid to whether the consumption of fallback foods is likely to play a key role in influencing great ape enamel morphology. Our results suggest that this is indeed the case. We also consider the implications of this conclusion on the evolution of the dentition of extinct hominins.

Effect of wear on stress distributions and potential fracture in teeth

Finite element analysis is conducted on a tooth model with different degrees of wear. The model is taken as a hemispherical shell (enamel) on a compliant interior (dentin). Occlusal loading is simulated by contact with a flat or curved, hard or soft, indenter. Stress redistributions indicate that development of a wear facet may enhance some near-contact fracture modes (cone-ring cracks, radial-median cracks, edge-chipping), but have little effect on far-field modes (margin cracks). Contacts on worn surfaces with small, hard food objects are likely to be most deleterious, generating local stress concentrations and thereby accelerating the wear process. More typical contacts with larger-scale soft foods are unlikely to have such adverse effects. Implications concerning dietary habits of animals is an adjunct consideration in this work.

Hypocalcified type of amelogenesis imperfecta in a large family: clinical, radiographic, and histological findings, associated dento-facial anomalies, and resulting treatment load

OBJECTIVE

The purpose of this study was to report on the clinical, radiographic, and histological dental findings and the resulting treatment load in a five-generation family with amelogenesis imperfecta (AI).

MATERIAL AND METHODS

Thirteen affected and 15 unaffected individuals were examined clinically and radiographically. In addition, four exfoliated deciduous teeth were examined by scanning electron microscopy and microradiography.

RESULTS

The mode of inheritance of AI was autosomal-dominant. At eruption, most of the tooth enamel was yellow, lacking translucency, and prone to gradual loss in subjects with AI. Post-eruptive breakdown of enamel was extensive in accordance with the histological observations of hypomineralized and porous enamel. Extensive enamel loss and discoloration were observed in older affected individuals. The treatment need had been extensive: 76.2% of the total number of teeth present in affected individuals had been treated with partial or full coverage compared to 1.7% of the teeth in unaffected relatives. Unaffected individuals had more endodontically treated teeth than AI-affected relatives. Adjunctive findings, e.g. tooth agenesis, tooth impaction, pulp stones, enlarged follicular space, and taurodontism, were rare in both groups.

CONCLUSIONS

Affected family members had the hypocalcified type of AI, which is characterized by severe hypomineralization, extensive post-eruptive loss, and discoloration of the enamel. Adjunctive findings were rare. Individuals with the hypocalcified type of AI have an extensive restorative treatment load compared to unaffected relatives.

Analysis of fracture and deformation modes in teeth subjected to occlusal loading

An analysis of fracture and deformation modes in tooth enamel subjected to occlusal loading is presented. Several competing modes are identified: deformation by yield beneath the indenter; median cracking from the ensuing plastic zone and analogous radial cracking from the dentin-enamel junction along the load axis; and margin cracking from the cervical enamel-cement junction. The analysis, based on a simple model of tooth geometry, presents relations for the critical loads to initiate these damage modes within the enamel, and to drive ensuing cracks longitudinally around the tooth walls to failure. The relations are explicit in their dependence on characteristic tooth dimensions - enamel thickness and cuspal radius - and on material properties - modulus, hardness, toughness and strength. Provision is made to incorporate properties of the occlusal contact, whether from opposing dentition or intervening food particles. All these features are demonstrated on critical-load master diagrams. A characteristic feature of the damage evolution is the gradual evolution of each mode with increasing load, so that failure is generally a prolonged rather than abrupt event. This accounts for the remarkable damage tolerance of natural teeth. The equations may enable basic predictions of tooth responses for humans and animals under a variety of specified dietary and functional conditions.

A simple model for enamel fracture from margin cracks

We present results of in situ fracture tests on extracted human molar teeth showing failure by margin cracking. The teeth are mounted into an epoxy base and loaded with a rod indenter capped with a Teflon insert, as representative of food modulus. In situ observations of cracks extending longitudinally upward from the cervical margins are recorded in real time with a video camera. The cracks appear above some threshold and grow steadily within the enamel coat toward the occlusal surface in a configuration reminiscent of channel-like cracks in brittle films. Substantially higher loading is required to delaminate the enamel from the dentin, attesting to the resilience of the tooth structure. A simplistic fracture mechanics analysis is applied to determine the critical load relation for traversal of the margin crack along the full length of the side wall. The capacity of any given tooth to resist failure by margin cracking is predicted to increase with greater enamel thickness and cuspal radius. Implications in relation to dentistry and evolutionary biology are briefly considered.

Fracture Modes in Human Teeth

The structural integrity of teeth under stress is vital to functional longevity. We tested the hypothesis that this integrity is limited by fracture of the enamel. Experiments were conducted on molar teeth, with a metal rod loaded onto individual cusps. Fracture during testing was tracked with a video camera. Two longitudinal modes of cracking were observed: median cracking from the contact zone, and margin cracking along side walls. Median cracks initiated from plastic damage at the contact site, at first growing slowly and then accelerating to the tooth margin. Margin cracks appeared to originate from the cemento-enamel junction, and traversed the tooth wall adjacent to the loaded cusp from the gingival to the occlusal surface. All cracks remained confined within the enamel shell up to about 550 N. At higher loads, additional crack modes—such as enamel chipping and delamination—began to manifest themselves, leading to more comprehensive failure of the tooth structure.

Crack propagation in teeth: a comparison of perimortem and postmortem behavior of dental materials and cracks

This study presents a new method for understanding postmortem heat-induced crack propagation patterns in teeth. The results demonstrate that patterns of postmortem heat-induced crack propagation differ from perimortem and antemortem trauma-induced crack propagation patterns. Dental material of the postmortem tooth undergoes dehydration leading to a shrinking and more brittle dentin material and a weaker dentin-enamel junction. Dentin intertubule tensile stresses are amplified by the presence of the pulp cavity, and initiates crack propagation from the internal dentin, through the dentin-enamel junction and lastly the enamel. In contrast, in vivo perimortem and antemortem trauma-induced crack propagation initiates cracking from the external surface of the enamel toward the dentin-enamel junction where the majority of the energy of the crack is dissipated, eliminating the crack's progress into the dentin. These unique patterns of crack propagation can be used to differentiate postmortem taphonomy-induced damage from antemortem and perimortem trauma in teeth.

Predicting failure in mammalian enamel

Dentition is a vital element of human and animal function, yet there is little fundamental knowledge about how tooth enamel endures under stringent oral conditions. This paper describes a novel approach to the issue. Model glass dome specimens fabricated from glass and back-filled with polymer resin are used as representative of the basic enamel/dentine shell structure. Contact loading is used to deform the dome structures to failure, in simulation of occlusal loading with opposing dentition or food bolus. To investigate the role of enamel microstructure, additional contact tests are conducted on two-phase materials that capture the essence of the mineralized-rod/organic-sheath structure of dental enamel. These materials include dental glass-ceramics and biomimicked composites fabricated from glass fibers infiltrated with epoxy. The tests indicate how enamel is likely to deform and fracture along easy sliding and fracture paths within the binding phase between the rods. Analytical relations describing the critical loads for each damage mode are presented in terms of material properties (hardness, modulus, toughness) and tooth geometry variables (enamel thickness, cusp radius). Implications in dentistry and evolutionary biology are discussed.

Inferences regarding the diet of extinct hominins: structural and functional trends in dental and mandibular morphology within the hominin clade

This contribution investigates the evolution of diet in the Pan-Homo and hominin clades. It does this by focusing on 12 variables (nine dental and three mandibular) for which data are available about extant chimpanzees, modern humans and most extinct hominins. Previous analyses of this type have approached the interpretation of dental and gnathic function by focusing on the identification of the food consumed (i.e. fruits, leaves, etc.) rather than on the physical properties (i.e. hardness, toughness, etc.) of those foods, and they have not specifically addressed the role that the physical properties of foods play in determining dental adaptations. We take the available evidence for the 12 variables, and set out what the expression of each of those variables is in extant chimpanzees, the earliest hominins, archaic hominins, megadont archaic hominins, and an inclusive grouping made up of transitional hominins and pre-modern Homo. We then present hypotheses about what the states of these variables would be in the last common ancestor of the Pan-Homo clade and in the stem hominin. We review the physical properties of food and suggest how these physical properties can be used to investigate the functional morphology of the dentition. We show what aspects of anterior tooth morphology are critical for food preparation (e.g. peeling fruit) prior to its ingestion, which features of the postcanine dentition (e.g. overall and relative size of the crowns) are related to the reduction in the particle size of food, and how information about the macrostructure (e.g. enamel thickness) and microstructure (e.g. extent and location of enamel prism decussation) of the enamel cap might be used to make predictions about the types of foods consumed by extinct hominins. Specifically, we show how thick enamel can protect against the generation and propagation of cracks in the enamel that begin at the enamel-dentine junction and move towards the outer enamel surface.

A novel approach to bite force measurements in a porcine model in vivo

A novel device for a bite force measurement system in a porcine model is described. A single polyethylene layer was vacuum-formed into a splint, and a force sensor was fitted on to the splint occlusal surface and seated with a silicone layer. This design enabled the measurement of bite forces on selected teeth in a large animal model with either natural dentition or single implant crowns and could be used in assessing information on biomechanical adaptation of the bone-implant interface to masticatory loads. Preliminary recordings of force values obtained during mastication in the premolar region (200-560 N) and various limiting factors of the technical design are discussed.

Swine teeth as potential substitutes for in vitro studies in tooth adhesion: A SEM observation

Animal teeth are routinely used in studies that test dental materials; however, no studies have yet proposed the use of swine teeth in laboratory tests. The purpose of this study was to compare the effects of acid etching on the dentine and enamel of human and swine teeth, aiming at the use of swine teeth as substitutes for human teeth in adhesion tests. Four human molars and four swine molars free of caries were used. For the study of enamel, two human and two swine teeth were longitudinally sectioned with a dental handpiece and a carborundum disk. For evaluation of dentine, the remaining teeth had the occlusal third of their crowns removed using a low speed diamond saw. All specimens underwent acid etching with 37% phosphoric acid for 30s. The etched surfaces were then washed with air/water spray for 10s. The specimens were then prepared for analysis in a Sterioscan 440 scanning electron microscope. It was observed a similar morphology in human and swine teeth, despite swine enamel was thinner than human enamel. Acid etching of swine enamel promoted a demineralization similar to that observed in human enamel in which the three types of Silverstone's patterns were observed. Acid etching of dentine exposed the opening of dentinal tubules in both swine and human teeth. Peritubular dentine was removed by phosphoric acid, while collagen fibrils appeared discernible in intertubular dentine. Thus, this study showed that enamel and dentine from swine and human teeth share many structural similarities after acid etching.

Enamel microstructure and microstrain in the fracture of human and pig molar cusps

The role of microstructure in enamel strain and breakage was investigated in human molar cusps and those of the pig, Sus scrofa. Rosette strain gauges were affixed to cusp surfaces (buccal human M3, n=15, and lingual pig M1, n=13), and a compressive load was applied to individual cusps using an MTS materials testing machine. Load and strain data were recorded simultaneously until cusp fracture, and these data were used to estimate enamel stresses, principal strains, and stiffness. Fractured and polished enamel fragments were examined in multiple planes using scanning electron microscopy (SEM). Human cusp enamel showed greater stiffness than pig enamel (P=0.02), and tensile stress at yield was higher (17.9 N/mm2 in humans versus 8.9 N/mm2 in pigs, P=0.06). SEM revealed enamel rod decussation in both human and pig enamel; however, only pig enamel showed a decussation plane between rod and inter-rod crystallites. Human inter-rod enamel was densely packed between rods, whereas in pig enamel, inter-rod enamel formed partitions between rows of enamel rods. Overall, human enamel structure enabled molar cusps to withstand horizontal tensile stress during both elastic and plastic phases of compressive loading. In contrast, pig cusp enamel was less resistant to horizontal tensile stresses, but appeared to fortify the enamel against crack propagation in multiple directions. These structural and biomechanical differences in cusp enamel are likely to reflect species-level differences in occlusal function.