Inferring biological evolution from fracture patterns in teeth.

It is hypothesised that specific tooth forms are adapted to resist fracture, in order to accommodate the high bite forces needed to secure, break down and consume food. Three distinct modes of tooth fracture are identified: longitudinal fracture, where cracks run vertically between the occlusal contact and the crown margin (or vice versa) within the enamel side wall; chipping fracture, where cracks run from near the edge of the occlusal surface to form a spall in the enamel at the side wall; and transverse fracture, where a crack runs horizontally through the entire section of the tooth to break off a fragment and expose the inner pulp. Explicit equations are presented expressing critical bite force for each fracture mode in terms of characteristic tooth dimensions. Distinctive transitions between modes occur depending on tooth form and size, and loading location and direction. Attention is focussed on the relatively flat, low-crowned molars of omnivorous mammals, including humans and other hominins and the elongate canines of living carnivores. At the same time, allusion to other tooth forms - the canines of the extinct sabre-tooth (Smilodon fatalis), the conical dentition of reptiles, and the columnar teeth of herbivores - is made to highlight the generality of the methodology. How these considerations impact on dietary behaviour in fossil and living taxa is discussed.

Endovascular Treatment of Anterior Communicating Artery Aneurysms: A Single-Center Experience from a Developing Country.

Objective In recent years, endovascular methods have been developed to treat intracranial aneurysms. To date, results of endovascular treatment (EVT) for anterior communicating aneurysms (ACoAs) have never been investigated in Iran. Thus, we sought to assess the mid-term angiographic and clinical outcomes of patients with ACoAs who underwent EVT in a tertiary center. Materials and Methods Electronic health documents of patients with ACoAs who underwent EVT from March 2019 to July 2021 were retrospectively reviewed. Demographic and clinical characteristics of patients, procedural and clinical complications along with immediate and 12 months' postprocedural angiographic and clinical results were included in the analysis. Aneurysm occlusion status was classified based on the Raymond-Roy Occlusion Classification (RROC), and clinical outcomes were assessed using the modified Rankin Scale (mRS). Results Of 38 patients with 38 ACoAs, 32 patients (84.21%) presented with subarachnoid hemorrhage of whom 23 (60.52%) had ruptured ACoAs. EVT included simple coiling in 29 patients (76.32%), balloon-assisted coiling in 6 (15.79%), and stent-assisted coiling in 3 (7.89%). Immediate and 12-month postprocedural angiograms demonstrated complete/near-complete occlusion (RROC I and II) in 32 (84.21%) and 35 patients (97.22%), respectively. Periprocedural complications occurred in five patients (13.15%), and the mortality rate was 5.26%. Thirty-two patients (84.21%) had favorable outcomes (mRS 0-2) at the last follow-up. Conclusion EVT is a safe and beneficial procedure with favorable mid-term clinical and angiographic outcomes for ACoAs. Our results can lay the foundation for further studies in developing countries and are satisfactory enough for neurointerventionists to put EVT on the therapeutic agenda of ACoAs.

Fracture-resistant monolithic dental crowns.

OBJECTIVE: To quantify the splitting resistance of monolithic zirconia, lithium disilicate and nanoparticle-composite dental crowns. METHODS: Fracture experiments were conducted on anatomically-correct monolithic crown structures cemented to standard dental composite dies, by axial loading of a hard sphere placed between the cusps. The structures were observed in situ during fracture testing, and critical loads to split the structures were measured. Extended finite element modeling (XFEM), with provision for step-by-step extension of embedded cracks, was employed to simulate full failure evolution. RESULTS: Experimental measurements and XFEM predictions were self-consistent within data scatter. In conjunction with a fracture mechanics equation for critical splitting load, the data were used to predict load-sustaining capacity for crowns on actual dentin substrates and for loading with a sphere of different size. Stages of crack propagation within the crown and support substrate were quantified. Zirconia crowns showed the highest fracture loads, lithium disilicate intermediate, and dental nanocomposite lowest. Dental nanocomposite crowns have comparable fracture resistance to natural enamel. SIGNIFICANCE: The results confirm that monolithic crowns are able to sustain high bite forces. The analysis indicates what material and geometrical properties are important in optimizing crown performance and longevity.

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.

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.

Transverse fracture of canine teeth.

The critical conditions to effect transverse fracture in canine teeth of carnivores in lateral loading are analyzed. The teeth are modeled as tapered coaxial beams with uniformly thin enamel coats. A stress analysis is first carried out using beam theory, and stress intensity factors for inward propagating cracks at the location of maximum tensile stress along the lingual face are then determined. The fracture begins as arrested channel cracks within the enamel, followed by stable penetration around the tooth and into the dentin to the point of failure. Two- and three-dimensional finite element models are used to evaluate the full fracture evolution. The analysis yields an explicit scaling relation for the critical fracture load in terms of characteristic tooth dimensions, notably tooth height and base radius. The role of enamel, ignored in previous 'strength of materials' analyses, is shown to be important in determining the precursor crack equilibrium prior to full fracture. Implications concerning allometry are briefly discussed.

Effect of property gradients on enamel fracture in human molar teeth.

A model for the fracture of tooth enamel with graded elastic modulus and toughness is constructed using an extended finite element modeling (XFEM) package. The property gradients are taken from literature data on human molars, with maximum in modulus at the outer enamel surface and in toughness at the inner surface. The tooth is modeled as a brittle shell (enamel) and a compliant interior (dentin), with occlusal loading from a hard, flat contact at the cusp. Longitudinal radial (R) and margin (M) cracks are allowed to extend piecewise along the enamel walls under the action of an incrementally increasing applied load. A simple stratagem is deployed in which fictitious temperature profiles generate the requisite property gradients. The resulting XFEM simulations demonstrate that the crack fronts become more segmented as the property gradients become more pronounced, with enhanced propagation at the outer surface and inhibited propagation at the inner. Whereas the growth history of the cracks is profoundly influenced by the gradients, the ultimate critical loads required to attain full fractures are relatively unaffected. Some implications concerning dentistry are considered.

Role of tooth elongation in promoting fracture resistance.

A study is made of the role of tooth height on the resistance to side-wall longitudinal fracture under axial occlusal loading, building on earlier analyses for molar teeth with low dome-like ('bunodont') crown structures characteristic of primates and several other omnivorous mammals. The present study extends the analysis by considering molar teeth with an elongate columnar structure below the crown, more characteristic of grazing mammals. Extended finite element modeling is used to determine the evolution of longitudinal cracking, from initial growth to final failure. Experimental tests on sheep teeth confirm the predicted behavior of the longitudinal fracture mode, at least in its early stages. It is demonstrated that elongate tooth structures have a substantially increased resistance to longitudinal fracture, by restricting crack growth along the extended side walls. Biological implications concerning the adaptation of tooth structure to meet changes in the dietary habits of herbivores, and of some carnivores, are considered.