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The aim of this study was to evaluate the stress induced by clamps made of different materials on mandibular first molar teeth using finite element analysis. The tooth model to be used in the study was created using micro-CT scanner and the rubber dam clamp model was created in three dimensions (3D) using Solidworks and applied to a finite element tooth model of an intact mandibular first molar. The size of the clamp opening and the force to be applied were calculated according to the buccolingual distances of the tooth. The contact areas of the clamps on the tooth were determined and subjected to force. The stress distribution patterns and maximum von Mises stresses were calculated and compared against these forces. The most stress against these forces occurred in the enamel tissue on the distal side where the clamp was in contact with the tooth. The stress value in the stainless steel clamp (191.63 MPa) was approximately 85% higher than the clamp made of polyethylene ketone (103.85 MPa). The stress in the dentin and pulp tissue of the tooth was negligible. Clinicians should examine the enamel tissue below the equatorial line of the tooth in detail before clamp application. The forces exerted by stainless steel clamps can cause damage to the enamel tissue of the tooth or progression of an existing enamel crack.

Background: The band and loop space maintainer is used to maintain the missing space of deciduous molars which are lost early. When the second deciduous molar is lost prematurely, the stress on the first permanent molar during different degrees of development may vary when it is the abutment. The design and use of the space maintainer may also lead to damage of the loop. The purpose of this article is to use the finite element method to study the stress on the first permanent molar and the loop with or without occlusal contact, with the first permanent molar of four different degrees of development serving as the abutment. We aimed to guide the clinical design and use of the space maintainer., Methods: We developed finite element models of the mandibular first permanent molar and the band and loop space maintainer, and simulated alveolar bone, periodontal ligament (PDL), enamel and dentin. The four developmental stages were 1/2 (I), 2/3 (II), 3/4 (III) and full development (IV). Ansys Workbench was used to analyze the effects of root development and occlusal contact between the loop and the opposite jaw on abutment teeth and the loop. Abutment teeth were statically loaded vertically and obliquely with a force of 70 N. The loop was statically loaded vertically with a force of 14 N. The stress on all structures and the displacement trends of the loop were calculated., Results: The stress on enamel, dentin, PDL and alveolar bone were similar, and the concentration was consistent. But if there was occlusal contact, the loop produced maximum displacement at the near middle edge of contact with the anterior teeth. When the loop was in occlusal contact with the opposing occlusal tooth, the peak value of the equivalent stress on the space maintainer under vertical load was: group I > group IV > group III > group II, and the maximum principal stress peak change was: group I > group III > group II > group IV. The change of the equivalent stress peak value of the loop under oblique load was: group I > group III > group IV > group II, and the maximum principal stress peak change was: group III > group I > group II > group IV. When the loop was not in occlusal contact with the opposing occlusal tooth, the peak value of the equivalent stress on the space maintainer under vertical load was: group IV > group I > group II > group III, and the maximum principal stress peak change was: group IV > group I > group II > group III. The change of the equivalent stress peak value of the space maintainer under oblique load was: group I > group IV > group II > group III, and the maximum principal stress peak change was: group I > group IV > group II > group III., Conclusions: Our results suggested that whenever possible, choosing the teeth with nearly complete root development as the abutment of the space maintainer is advisable. The design and use of the band and loop space maintainer should avoid occlusal contact with the occlusal teeth to prevent deformation of the loop., Competing Interests: The authors declare that they have no competing interests., (© 2024 Shi et al.)

Dogs are animals with strong bite force. This strong bite mechanism has led to significant changes in the skeletal system such as fossa masseterica. It can be thought that one side is used more than the other side in chewing and is related to the preference of using the same side's hand, eye and foot. In the study, directional asymmetry and fluctuating asymmetry, which occurs as a result of chewing asymmetry, were examined on the first molar teeth and the fossa masseterica in 85 dog mandibles including a wide diversity of morphotypes. The association of high PC1 values for directional asymmetry with a pronounced cranial index, as evident in breeds like Pekingese, Pomeranian and Bulldog, indicates a potential evolutionary or selective breeding trend favouring brachycephaly. On the contrary, guardian breeds like the German shepherd and Bernese mountain dog, which typically require strong jaws for their roles, showcased reduced PC1 values, which might be related to their functional morphology. Similarly, the PCA results for the first molar teeth shape variations also highlighted the influence of cranial shape, with boxer dogs displaying notably higher PC1 values. The fluctuating asymmetrical distributions provided valuable insights into individualistic variations. Interestingly, no specific breed distribution trend was observed for these asymmetries, indicating a more individual-based variation rather than breed-based. It is essential to note that while these results provide valuable insights, further studies are required to understand the underlying causes better. Factors like genetic variations, developmental processes, dietary habits and external environmental factors could play pivotal roles in these observed morphological differences., (© 2024 Wiley‐VCH GmbH. Published by John Wiley & Sons Ltd.)

Shuotheriids are Jurassic mammaliaforms that possess pseudotribosphenic teeth in which a pseudotalonid is anterior to the trigonid in the lower molar, contrasting with the tribosphenic pattern of therian mammals (placentals, marsupials and kin) in which the talonid is posterior to the trigonid 1-4 . The origin of the pseudotribosphenic teeth remains unclear, obscuring our perception of shuotheriid affinities and the early evolution of mammaliaforms 1,5-9 . Here we report a new Jurassic shuotheriid represented by two skeletal specimens. Their complete pseudotribosphenic dentitions allow reidentification of dental structures using serial homology and the tooth occlusal relationship. Contrary to the conventional view 1,2,6,10,11 , our findings show that dental structures of shuotheriids can be homologized to those of docodontans and partly support homologous statements for some dental structures between docodontans and other mammaliaforms 6,12 . The phylogenetic analysis based on new evidence removes shuotheriids from the tribosphenic ausktribosphenids (including monotremes) and clusters them with docodontans to form a new clade, Docodontiformes, that is characterized by pseudotribosphenic features. In the phylogeny, docodontiforms and 'holotherians' (Kuehneotherium, monotremes and therians) 13 evolve independently from a Morganucodon-like ancestor with triconodont molars by labio-lingual widening their posterior teeth for more efficient food processing. The pseudotribosphenic pattern passed a cusp semitriangulation stage 9 , whereas the tribosphenic pattern and its precursor went through a stage of cusp triangulation. The two different processes resulted in complex tooth structures and occlusal patterns that elucidate the earliest diversification of mammaliaforms., (© 2024. The Author(s), under exclusive licence to Springer Nature Limited.)

Objectives: Restored teeth undergo more damage than intact teeth. Therefore, the scientific investigation of their mechanical and physical behaviour under varying oral conditions is vital. The current study is to numerically investigate the stresses on a class-II mesio-occluso-distal (MOD) restored molar due to thermal and thermomechanical stimuli with varying input properties such as coefficient of thermal expansion and elastic properties. This is performed to optimise the dental restoration material, thereby reducing the stresses and failure of the restoration., Methods: An upper molar was scanned using μ-CT for segmenting and modelling the enamel and dentine. A class-II MOD cavity was then prepared on the model, after which non-manifold meshing was generated. The coefficient of thermal expansion (CTE) and elastic modulus (E) properties of the restoration were varied from 20 × 10 -6 ° C -1 to 55 × 10 -6 ° C -1 and 5 GPa-20 GPa, respectively. After the material properties and boundary conditions were set for the finite element (FE) analysis, the thermal and thermomechanical loading analyses were performed to demonstrate the influence of input parameters on the stress. The maximum values of principal stresses on the restoration-enamel junction and the restoration were evaluated. The results were statistically processed using analysis of variance, response surface methodology (RSM) and optimisation analysis to estimate the most optimum inputs for minimising principal stresses., Results: The study reveals that the location of principal stress occurs at the restoration-enamel junction (REJ) and the restoration changes based on the composite material value of E and CTE due to thermal and thermomechanical stimuli. The REJ showed higher principal stress than restoration during the application of both thermal and thermomechanical stimuli, making it more vulnerable to fracture and failure. Moreover, the study showed non-linear variations in the values and locations of principal stresses due to thermal and thermomechanical stimuli with the change in the property of the restoration composite used. Finally, this study derived an optimised restorative value for CTE and E due to the application of thermal and simultaneous thermal and mechanical stimuli., Conclusion: This study highlights the importance of choosing the suitable material properties of the restoration composite by dental clinicians to repair a large class MOD cavity. The findings from this study also suggest that the difference in the values of E and CTE in a dental restoration composite when compared with the enamel causes a lack of uniformity in mechanical and thermal properties, thereby forming stress concentrations at the interfaces. The study establishes two optimised CTE and E values for the MOD restoration composite as 25 × 10 -6 ° C -1 and 20 GPa and 37 × 10 -6 ° C -1 and 5 GPa, respectively., Competing Interests: Declaration of competing interest The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper., (Copyright © 2023 The Authors. Published by Elsevier Ltd.. All rights reserved.)

Objective: This study aims to investigate the biomechanical behavior and reveal the force transmission patterns of the human masticatory system through advanced three-dimensional finite element (FE) models., Methods: The FE model was constructed according to the medical images of a healthy male adult. It contains full skull structures, detailed temporomandibular joints (TMJs) with discs, complete dentitions, masticatory muscles, and related ligaments. Several static bite scenarios were simulated to demonstrate the effects of bite positions and muscle force recruitments on the force transmission patterns., Results: Molar occlusal surfaces are the primary force transmission region for clenching. Sensitivity analysis demonstrated that the stiffness of the bite substance would not alter the force transmission patterns but could affect the maximum contact stresses on the discs and the occlusal surfaces. During the unilateral clenching tasks, the high-stress region on the discal surfaces shifted ipsilaterally. The presence or absence of the molar cushions would significantly affect the biomechanical response of the masticatory system., Significance: FE analysis is an effective way of investigating biomechanical responses involving complicated interactions. Enriching the static analysis of the masticatory system with a detailed model can help understand better how the forces were transmitted and the significance of TMJs during the clenching process., Competing Interests: Declaration of Competing Interest None., (Copyright © 2023. Published by Elsevier B.V.)

Background and Objective: During orthodontic treatment, the figure-of-eight ligature and the physiological occlusion play an important role in providing anchorage effects. However, their effects on reaction forces of tooth and stress state in periodontal ligament (PDL) have not been quantitatively evaluated yet. In this study, we presented a finite element analysis process for simulating posterior molar ligature and direct occlusion during orthodontics in order to quantitatively assess their anchorage effects., Methods: A high precision 3D biomechanical model containing upper and lower teeth, PDL, brackets and archwire was generated from the images of computed tomographic scan and sophisticated modelling procedures. The orthodontic treatment of closing the extraction gap was simulated via the finite element method to evaluate the biomechanical response of the molars under the conditions with or without ligation. The simulations were divided into experimental and control groups. In the experimental group, orthodontic force of 1 N was first applied, then direct occlusal forces of 3 and 10 N were applied on each opposite tooth. While in the control group, occlusal forces were applied without orthodontic treatment. The tooth displacement, the stress state in the PDL and the directions of the resultant forces on each tooth were evaluated., Results: In the case of molars ligated, the maximum hydrostatic stress in the molars' PDL decreases by 60%. When an initial tooth displacement of several microns occurs in response to an orthodontic force, the direction of the occlusal force changes simultaneously. Even a moderate occlusal force (3 N per tooth) can almost completely offset the mesial forces on the maxillary teeth, thus to provide effective anchorage effect for the orthodontics., Conclusions: The proposed method is effective for simulating ligation and direct occlusion. Figure-of-eight ligature can effectively disperse orthodontic forces on the posterior teeth, while a good original occlusal relationship provides considerable anchorage effects in orthodontics., Competing Interests: Declaration of Competing Interest The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper., (Copyright © 2022 Elsevier B.V. All rights reserved.)

Objectives: Enamel wear is a common occurrence that may lead to tooth failure. Beyond reducing enamel thickness, wear exposes different regions of enamel microstructure to various types of stresses. This work was aimed at elucidating the effect of enamel wear on enamel morphology and tooth resilience in human molar teeth undergoing large-scale contact., Methods: Intact/polished molar cusps were indented with a hard disk/ball. The unloaded specimens were sectioned longitudinally or transversely, and the damage examined by optical and scanning electron microscopy. The onset of cracks at the dentin horn apex was determined by a FEM stress analysis that modeled a cusp as truncated, conical enamel shell supported by dentin., Results: The damage consisted of radial and cylindrical cracks growing under the contact, sparsely distributed radial cracks in the enamel shell region, and cracking from tufts at the dentin horn apex (T A cracks). The damage under the contact circle exhibited shear deformation zones. These zones helped relieve contact stresses, absorb energy, contain damage, and solicit cylindrical cracks in order to avoid growth of wear-sensitive cone cracks. Enamel tufts provided stress shielding while Hunter-Schreger Bands helped maintain this benefit by enforcing a collaborative cracking. The T A cracks were deemed a primary cause for tooth failure. The FEA predicted well the onset of these cracks. Making use of in vivo enamel wear data, the analysis showed that cusp failure might routinely occur at old age., Significance: The results provided new information on the response of the interior part of enamel to contact loading. This included the resistant effects of the waviness of enamel rods, interrods and tufts. The unique microstructure of enamel gave rise to shear bands under the contact circle that helped relieve contact stresses, absorb energy and contain damage. Enamel interrods solicited cylindrical cracking in order to avoid the wear-sensitive cone cracks. The onset of cracks at the dentin horn apex (T A cracks) was deemed critical to a tooth survival. The FEA showed that this cracking mode might routinely occur at old age due to natural enamel wear. Finally, the occlusal force needed to initiate the T A cracks in intact cusps can help explain the natural design logic involving the maximum bite force of hominid species., (Copyright © 2022 Elsevier Ltd. All rights reserved.)

Dentistry is confronted with the functional and aesthetic consequences that result from an increased prevalence of misaligned and discrepant dental occlusal relations in modern industrialised societies. Previous studies have indicated that a reduction in jaw size in response to softer and more heavily processed foods during and following the Industrial Revolution (1,700 CE to present) was an important factor in increased levels of poor dental occlusion. The functional demands placed on the masticatory system play a crucial role in jaw ontogenetic development; however, the way in which chewing behaviours changed in response to the consumption of softer foods during this period remains poorly understood. Here we show that eating more heavily processed food has radically transformed occlusal power stroke kinematics. Results of virtual 3D analysis of the dental macrowear patterns of molars in 104 individuals dating to the Industrial Revolution (1,700-1,900 CE), and 130 of their medieval and early post-medieval antecedents (1,100-1,700 CE) revealed changes in masticatory behaviour that occurred during the early stages of the transition towards eating more heavily processed foods. The industrial-era groups examined chewed with a reduced transverse component of jaw movement. These results show a diminished sequence of occlusal contacts indicating that a dental revolution has taken place in modern times, involving a dramatic shift in the way in which teeth occlude and wear during mastication. Molar macrowear suggests a close connection between progressive changes in chewing since the industrialization of food production and an increase in the prevalence of poor dental occlusion in modern societies., Competing Interests: The authors have declared that no competing interests exist.

Development has often been viewed as a constraining force on morphological adaptation, but its precise influence, especially on evolutionary rates, is poorly understood. Placental mammals provide a classic example of adaptive radiation, but the debate around rate and drivers of early placental evolution remains contentious. A hallmark of early dental evolution in many placental lineages was a transition from a triangular upper molar to a more complex upper molar with a rectangular cusp pattern better specialized for crushing. To examine how development influenced this transition, we simulated dental evolution on "landscapes" built from different parameters of a computational model of tooth morphogenesis. Among the parameters examined, we find that increases in the number of enamel knots, the developmental precursors of the tooth cusps, were primarily influenced by increased self-regulation of the molecular activator (activation), whereas the pattern of knots resulted from changes in both activation and biases in tooth bud growth. In simulations, increased activation facilitated accelerated evolutionary increases in knot number, creating a lateral knot arrangement that evolved at least ten times on placental upper molars. Relatively small increases in activation, superimposed on an ancestral tritubercular molar growth pattern, could recreate key changes leading to a rectangular upper molar cusp pattern. Tinkering with tooth bud geometry varied the way cusps initiated along the posterolingual molar margin, suggesting that small spatial variations in ancestral molar growth may have influenced how placental lineages acquired a hypocone cusp. We suggest that development could have enabled relatively fast higher-level divergence of the placental molar dentition., Competing Interests: The authors declare no competing interest., (Copyright © 2021 the Author(s). Published by PNAS.)

In the course of a lifetime the crowns of teeth wear off, cementum thickens and the pulp closes-in or may stiffen. Little is known about how these changes affect the tooth response to load. Using a series of finite element models of teeth attached to the jawbone, and by comparing these to a validated model of a 'young' pig 3-rooted tooth, the effects of these structural changes were studied. Models of altered teeth show a stiffer response to mastication even when material properties used are identical to those found in 'young' teeth. This stiffening response to occlusal loads is mostly caused by the thicker cementum found in 'old' teeth. Tensile stresses associated with bending of dentine in the roots fall into a narrower distribution range with lower peak values. It is speculated that this is a possible protective adaptation mechanism of the aging tooth to avoid fracture. The greatest reduction in lateral motion was seen in the bucco-lingual direction. We propose that greater tooth motion during mastication is typical for the young growing animal. This motion is reduced in adulthood, favoring less off-axis loading, possibly to counteract natural bone resorption and consequent compromised anchoring., (Copyright © 2021. Published by Elsevier Inc.)

The aim of this study is to investigate the accuracies and the agreements of the 3D Endo software, conventional CBCT software Romexis Viewer at three voxel sizes, and the EAL ProPex Pixi in endodontic length measurements. Three hundred and twenty-nine root canals in 120 intact human extracted molars were accessed. The actual lengths (AL) and electronic lengths (EL) were measured using the ruler and electronic apex locator (EAL), respectively. Teeth were scanned using the CBCT at different voxel sizes (0.075, 0.10, and 0.15 mm). Root canal lengths were measured using 3D Endo with proposed length (3D-PL) by software, corrected length (3D-CL), Romexis Viewer. The Fisher's exact test, paired t-test and Bland-Altman plots were calculated to detect the agreements of the four methods with AL measurements. The ProPex Pixi measurements obtained the highest accuracy in the range of ± 0.5 mm. There was agreement between the 3D-PL and the 3D-CL with AL measurements at voxel size of 0.15 mm and at voxel size of 0.10 mm, respectively. The CBCT Romexis Viewer measurements agreed with AL at three voxel sizes. The conventional CBCT measurements using Romexis Viewer and dedicated software did not reach to the 100% accuracy in the range of ± 0.5 mm.

Objectives: One role of dental pulp is in the upkeep and maintenance of dentine. Under wear, odontoblasts in the pulp deposit tertiary dentine to ensure the sensitive internal dental tissues are not exposed and vulnerable to infection. It follows that there may be an adaptive advantage for increasing molar pulp volume in anthropoid primate taxa that are prone to high levels of wear. The relative volume of dental pulp is therefore predicted to covary with dietary abrasiveness (in the sense of including foods that cause high degrees of wear)., Materials and Methods: We examined relatively unworn lower second molars in pairs of species of extant hominoids, cebids, and pitheciids that vary in the abrasiveness of their diet (n = 36). Using micro-CT scans, we measured the percent of tooth that is pulp (PTP) as the ratio of pulp volume to that of the total volume of the tooth., Results: We found that in each pair of species, the taxa that consume a more abrasive diet had a significantly higher PTP than the closely related taxa that consume a softer diet., Conclusions: Our results point to an adaptive mechanism in the molars of taxa that consume abrasive diets and are thus subject to higher levels of wear. Our results provide additional understanding of the relationship between dental pulp and diet and may offer insight into the diet of extinct taxa such as Paranthropus boisei or into the adaptive context of the taurodont molars of Neanderthals., (© 2021 Wiley Periodicals LLC.)

Mammalian teeth have to sustain repetitive and high chewing loads without failure. Key to this capability is the periodontal ligament (PDL), a connective tissue containing a collagenous fibre network which connects the tooth roots to the alveolar bone socket and which allows the teeth to move when loaded. It has been suggested that rodent molars under load experience a screw-like downward motion but it remains unclear whether this movement also occurs in primates. Here we use synchroton micro-computed tomography paired with an axial loading setup to investigate the form-function relationship between tooth movement and the morphology of the PDL space in a non-human primate, the mouse lemur (Microcebus murinus). The loading behavior of both mandibular and maxillary molars showed a three-dimensional movement with translational and rotational components, which pushes the tooth into the alveolar socket. Moreover, we found a non-uniform PDL thickness distribution and a gradual increase in volumetric proportion of the periodontal vasculature from cervical to apical. Our results suggest that the PDL morphology may optimize the three-dimensional tooth movement to avoid high stresses under loading., (Copyright © 2020. Published by Elsevier Inc.)

Objectives: Carabelli is a nonmetric dental trait variably expressed as a small pit to a prominent cusp in the maxillary molars of modern humans. Investigations on the occurrence and expression rates of this trait have been conducted extensively, tracing its origin to genetic sources. However, there remains a lack of understanding about its potential role in chewing. In this study, we examine molar macrowear with the aim of reconstructing Carabelli trait occlusal dynamics occurring during chewing., Methods: We have examined 96 deciduous and permanent maxillary molars of children and young adults from Yuendumu, an Australian Aboriginal population that was at an early stage of transition from a nomadic and hunter-gatherer way of life to a more settled existence. We apply a well-established method, called Occlusal Fingerprint Analysis, which is a digital approach for analyzing dental macrowear allowing the reconstruction of jaw movements required to produce wear pattern specific to each tooth., Results: Carabelli trait slightly enlarges the surface functional area, especially in those molars where this feature is expressed in its cuspal form and it is closer to the occlusal plane. Moreover, the highly steep contact planes would also indicate that Carabelli wear areas contribute to increasing the shearing abilities of the occluded teeth, which are particularly important when processing fibrous and tough foods., Conclusions: The macrowear analysis suggests that Carabelli trait in the Aboriginal people from Yuendumu slightly enhanced occlusion and probably played some functional role during mastication. Future biomechanical and microwear analyses could provide additional information on the mechanical adaptation of Carabelli trait in modern human dentition., (© 2020 Wiley Periodicals LLC.)

When regenerative endodontic procedures (REPs) are performed on immature teeth diagnosed with pulp necrosis and apical periodontitis, various healing patterns occur. Furthermore, infected immature teeth with endodontic disorders often exhibit some remnant pulp and apical tissue. Therefore, this study investigated the impact of remnant healthy or fully functional pulp and apical tissue on healing patterns after REPs. Simulated REPs were performed on non-infected immature rat molars with different amounts of remnant pulp and apical tissue. Healing patterns in these teeth were assessed after 28 days. Teeth with 0.81-0.91 mm of remnant pulp healed with pulp-like tissue, dentin, and osteodentin-like dentin-associated mineralized tissue (OSD-DAMT); teeth with 0.60-0.63 mm of remnant pulp healed with pulp-like tissue and OSD-DAMT; teeth with 0.13-0.43 mm of remnant pulp healed with periodontal ligament (PDL)-like tissue, OSD-DAMT, and cementum-like dentin-associated mineralized tissue (CEM-DAMT); and teeth with disorganization of pulp and apical tissues at 0.15-0.38 mm beyond the root apex healed with PDL-like tissue, CEM-DAMT, and intracanal bone (IB). Loss of Hertwig's epithelial root sheath was observed with IB formation. These results showed that four distinct healing patterns occurred after REPs, depending on the preoperative amount of remnant healthy pulp and apical tissue.

MicroRNAs (miRNAs) exhibit strong potential clinical application owing to their extensive regulation and flexible delivery properties. MicroRNA-31 (miR-31) is an evolutionarily conserved miRNA expressed during tooth development, and it is highly expressed in mouse incisor epithelium. The specific role of miR-31 in odontogenesis has not been elucidated comprehensively, and the aim of the present study was to investigate its activity. Our results showed that miR-31 suppressed LS8 cell proliferation by inhibiting the cell cycle at the G1/S transition. Mutation of Special AT-rich sequence-binding protein 2 (SATB2) gene is responsible for human SATB2-associated syndrome (SAS), which is often accompanied by dental abnormities. Here, it was identified as a direct target of miR-31 in LS8 cells and a promoter of cell proliferation. The expression and distribution of SATB2 in mouse molars and incisors were explored using immunofluorescence, which showed strong signals in the nuclei of incisor epithelial cells and weak signals in the cytoplasm of molar epithelial cells. Moreover, rescue experiments demonstrated that Satb2 could mitigate the inhibitory effect of miR-31 on cell proliferation by promoting the expression of CDK4. Collectively, our results suggested that miR-31 regulates dental epithelial cell proliferation by targeting Satb2, highlighting the biological importance of miR-31 in odontogenesis., Competing Interests: Declaration of competing interest Authors declare no conflict of interest., (Copyright © 2020 Elsevier Inc. All rights reserved.)

One fundamental character common to pancrustaceans (Crustacea and Hexapoda) is a mineralized rigid exoskeleton whose principal organic components are chitin and proteins. In contrast to traditional research in the field that has been devoted to the structural and physicochemical aspects of biomineralization, the present study explores transcriptomic aspects of biomineralization as a first step towards adding a complementary molecular layer to this field. The rigidity of the exoskeleton in pancrustaceans dictates essential molt cycles enabling morphological changes and growth. Thus, formation and mineralization of the exoskeleton are concomitant to the timeline of the molt cycle. Skeletal proteinaceous toolkit elements have been discovered in previous studies using innovative molt-related binary gene expression patterns derived from transcriptomic libraries representing the major stages comprising the molt cycle of the decapod crustacean Cherax quadricarinatus. Here, we revisited some prominent exoskeleton-related structural proteins encoding and, using the above molt-related binary pattern methodology, enlarged the transcriptomic database of C. quadricarinatus. The latter was done by establishing a new transcriptomic library of the cuticle forming epithelium and molar tooth at four different molt stages (i.e., inter-molt, early pre-molt, late pre-molt and post-molt) and incorporating it to a previous transcriptome derived from the gastroliths and mandible. The wider multigenic approach facilitated by the newly expanded transcriptomic database not only revisited single genes of the molecular toolkit, but also provided both scattered and specific information that broaden the overview of proteins and gene clusters which are involved in the construction and biomineralization of the exoskeleton in decapod crustaceans., (Copyright © 2020 The Authors. Published by Elsevier Inc. All rights reserved.)

Life on earth is regulated by biological rhythms, some of which oscillate with a circadian, monthly or lunar cycle. Recent research suggests that there is a near weekly biorhythm that may exert an influence on human skeletal growth. Evidence for the timing of this biorhythm is retained in tooth enamel as the periodicity of Retzius lines. Studies report that Retzius periodicity (RP) relates to adult human stature and enamel thickness. Adult human stature is sexually dimorphic, and so is enamel thickness of maxillary third molars (M3) but not mandibular M3. Yet, previous studies report sex differences in RP are apparent in some populations but not others, and it is unknown if dimorphism in enamel thickness relates to RP. To further our understanding of this biorhythm we analysed sex-related variation in RP and its relationship with enamel thickness in a sample of M3's (n = 94) from adults in Northern Britain. Results reveal RP was significantly higher in our sample of female molars compared to those of males, which is consistent with the previously reported correlation between the biorhythm and adult stature. The RP of maxillary M3 related to sex differences in enamel thickness, but this relationship was not present in mandibular M3. Our results support previous findings suggesting that this biorhythm is sexually dimorphic and provide the first evidence that RP may be one factor influencing sex differences in enamel thickness. Our study also shows that correlations between RP and enamel thickness appear to be most readily detected for tooth types with sufficiently wide ranges of enamel thickness variation, as is the case for maxillary but not mandibular M3. Achieving a sufficient sample size was critical for detecting a sex difference in periodicity., Competing Interests: Declaration of Competing Interest The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper., (Copyright © 2020 Elsevier Inc. All rights reserved.)

Age at lower first molar (M 1 ) emergence is a commonly used proxy for inferring life-history scheduling in fossil primates, but its utility is dependent on knowing to what extent extant populations vary in this datum and how this variation correlates with the scheduling of life-history variables. Here, we address the first of these issues among extant chimpanzees. While age at M 1 emergence has been documented in several live individuals from the Kanyawara population of Pan troglodytes schweinfurthii in Uganda, it has been estimated for only one individual of Pan troglodytes verus, based on a deceased animal from the Taï Forest in Côte d'Ivoire. To further explore interpopulation variation in this variable in chimpanzees, and using dental histology, we calculated ages at death for two wild-shot individuals of P. t. verus with erupting M 1 , both collected in Liberia during the mid-1950s, and estimated ages at M 1 emergence from the ages at death. The overall range for these two individuals is ∼4.2-4.6 yr, compared with an age of ∼3.7 yr for the individual from the Taï Forest, and <2.5-3.3 yr for the several individuals of P. t. schweinfurthii. While the absolute range of ∼2 yr in these samples combined is little greater than in captive chimpanzees, the disparity between the samples of P. t. schweinfurthii and P. t. verus is striking, although it cannot be determined if this disparity represents a subspecies difference or simply population differences expressed in two different subspecies. While life-history data are unavailable for the population to which the Liberian chimpanzees belonged, the difference in M 1 emergence ages between these individuals and those from Kanyawara still suggests caution when attempting even broad life-history inference in fossil apes and hominins based on age at M 1 emergence., Competing Interests: Conflict of interest The authors declare no conflicts of interest in the research., (Copyright © 2020 Elsevier Ltd. All rights reserved.)

Dental enamel is a structure that is formed as a result of the regular functioning of ameloblasts. The knowledge of the patterns of enamel secretion allows an analysis of their disruptions manifested in pronounced additional accentuated lines. These lines represent a physiological response to stress experienced during enamel development. The aim of this study was to assess the occurrence of accentuated lines in the tooth enamel of autistic boys. The width of the neonatal line and the periodicity of the striae of Retzius were also assessed. The study material consisted of longitudinal ground sections of 56 primary teeth (incisors and molars): 22 teeth from autistic children and 34 teeth from the control group. The Mann-Whitney U test indicates that the accentuated lines were found significantly more often in autistic children (Z = 3.03; p = 0.002). No differentiation in the rate of enamel formation and in the rate of regaining homeostasis after childbirth were found. The obtained results may indicate a higher sensitivity of autistic children to stress factors, manifested in more frequent disturbances in the functioning of ameloblasts or may be a reflection of differences in the occurrence of stress factors in the first years of life in both analyzed groups.

Animal experiments are essential for the elucidation of biological-cellular mechanisms in the context of orthodontic tooth movement (OTM). So far, however, no studies comparatively assess available mouse models regarding their suitability. OTM of first upper molars was induced in C57BL/6 mice either via an elastic band or a NiTi coil spring for three, seven or 12 days. We assessed appliance survival rate, OTM and periodontal bone loss (µCT), root resorptions, osteoclastogenesis (TRAP + area) and local expression of OTM-related genes (RT-qPCR). Seven days after the elastic bands were inserted, 87% were still in situ, but only 27% after 12 days. Survival rate for the NiTi coil springs was 100% throughout, but 8.9% of the animals did not survive. Both methods induced significant OTM, which was highest after 12 (NiTi spring) and 7 days (band), with a corresponding increase in local gene expression of OTM-related genes and osteoclastogenesis. Periodontal bone loss and root resorptions were not induced at a relevant extent by neither of the two procedures within the experimental periods. To induce reliable OTM in mice beyond 7 days, a NiTi coil spring is the method of choice. The elastic band method is recommended only for short-term yes/no-questions regarding OTM.

This study aims to analyse the stress distributions and initial displacements of teeth during the space closing stage through a three-dimensional finite element method. Computed tomography images of a patient were used to reconstruct the detailed teeth and alveolar bone, and brackets with stainless steel archwire were modelled according to the orthodontic prescriptions. The second premolars and first molars were chosen as the anchorages in the model 6-force, with buccal tubes attached to the second molars in the model 6-force-7, and the second molars as additional anchorages in the model 7-force. The results indicated that a movement of lingual lateral inclination occurred on the incisors during the retraction, and the frictional force between the teeth and the archwire significantly reduced the stress on the teeth and periodontal structures. Graphical abstract Malocclusion is one of the most common issue in dentistry with high prevalence and orthodontic treatment need. The extraction of first premolar teeth was normally needed at the beginning of the treatment. And the straight wire appliance together with the sliding mechanics was used for space closure at the end of the treatment. However, side effects like root resorption also found after the surgery. Biomechanically, the stress distributions and initial displacements of teeth during space closing stage might be a crucial factor contributed to those undesirable side effects. And different selections of anchorages might alter the biomechanical environment during the treatment. Thus, the purpose of the current study was to analyse the stress distributions and initial displacements, with the different anchorage selections, of teeth during space closing stage through 3D finite element method.

Cementum is a bone connective tissue that provides a flexible attachment for the tooth to the alveolar bone in many mammalian species. It does not undergo continuous remodelling, unlike non-dental bone, which combined with its growth pattern of seasonal layering makes this tissue uniquely suitable as a proxy for tracking changes in body repair investment throughout an animal´s life. We tested functional and sexual selection hypotheses on the rate of cementum deposition related to the highly polygynous mating strategy of red deer. We used a sample of 156 first lower molars from wild Scottish red deer of known age between 1 and 17 years old, approximately balanced by sex and age class. Cementum deposition on the inter-radicular pad increased with age at a constant average rate of 0.26 mm per year, with no significant differences between sexes. Cementum deposition was independent of (i) tooth wear, other than that associated with age, and (ii) enamel and dentine micro-hardness. The results partially supported the hypothesis that the main function of cementum is the repositioning of the tooth to maintain opposing teeth in occlusion. However, teeth that had more wear or males´ teeth that had faster rates of tooth wear than those of females did not present the expected higher rates of cementum deposition., Competing Interests: The authors have declared that no competing interests exist.

Antemortem enamel chipping in living and fossil primates is often interpreted as evidence of hard-object feeding (i.e., 'durophagy'). Laboratory analyses of tooth fracture have modeled the theoretical diets and loading conditions that may produce such chips. Previous chipping studies of nonhuman primates tend to combine populations into species samples, despite the fact that species can vary significantly in diet across their ranges. Chipping is yet to be analyzed across population-specific species samples for which long-term dietary data are available. Here, we test the association between enamel chipping and diet in a community of cercopithecid primates inhabiting the Taï Forest, Ivory Coast. We examined fourth premolars and first molars (n = 867) from naturally deceased specimens of Cercocebus atys, Colobus polykomos, Piliocolobus badius,Procolobus verus, and three species of Cercopithecus. We found little support for a predictive relationship between enamel chipping and diet across the entire Taï monkey community. Cercocebus atys, a dedicated hard-object feeder, exhibited the highest frequencies of (1) chipped teeth and (2) chips of large size; however, the other monkey with a significant degree of granivory, Co. polykomos, exhibited the lowest chip frequency. In addition, primates with little evidence of mechanically challenging or hard-food diets-such as Cercopithecus spp., Pi. badius, and Pr. verus-evinced higher chipping frequencies than expected. The equivocal and stochastic nature of enamel chipping in the Taï monkeys suggests nondietary factors contribute significantly to chipping. A negative association between canopy preference and chipping suggests a role of exogenous particles in chip formation, whereby taxa foraging closer to the forest floor encounter more errant particulates during feeding than species foraging in higher strata. We conclude that current enamel chipping models may provide insight into the diets of fossil primates, but only in cases of extreme durophagy. Given the role of nondietary factors in chip formation, our ability to reliably reconstruct a range of diets from a gradient of chipping in fossil taxa is likely weak., (Copyright © 2020 Elsevier Ltd. All rights reserved.)

Objectives: Here we describe the case of an ectopic maxillary third molar (M 3 ), preventing the eruption of the M 2 , in the individual H3 of the hominin hypodigm of level TD6.2 of the Early Pleistocene site of Gran Dolina (Sierra de Atapuerca, Spain)., Materials and Methods: The fossil remains from the TD6.2 level of the Gran Dolina site (about 170 specimens) are assigned to Homo antecessor. Different geochronological methods place these hominins in the oxygen isotopic stage 21, between 0.8 and 0.85 million years ago (Ma). The immature individual H3 is represented by an almost complete midface (ATD6-69), preserving various teeth in situ. We used high-resolution microtomograhy (mCT) to investigate the abnormal position of the left M 3 , virtually reconstruct M 2 , and M 3 as well as assessing the development stage of these. Finally, we compare this case with extinct and extant populations., Results: Based on the identified signs, we suggest that individual H3 suffered from a unilateral impaction of the M 2 as a result of the ectopic position of the developing M 3 ., Discussion: We conclude that the most likely etiology for the ectopic position of the M 3 is the lack of space in the maxilla. We discuss possible contributing factors, such as morphometric aspects of the maxilla and the early mineralization of the M 3 , to support the M 2 impaction. Finally, due to the early age at death of this individual we did not identify any secondary lesion associated with the M 2 impaction., (© 2020 Wiley Periodicals, Inc.)

Odontoblasts act as dentin formation and sensory receptors. Recently, it was reported that transient receptor potential ankyrin (TRPA) 1, TRP vanilloid (TRPV) 4 and pannexin 1 (PANX-1) play important roles in odontoblast sensory reception. However, it is not known when odontoblasts begin to possess a sense reception function. The aim of this study was to clarify the development of odontoblasts as sense receptors. Sections of mandibular first molars from postnatal day (PN) 0 to PN12 Wistar rats were prepared for hematoxylin-eosin staining. Immunohistochemically, we used anti-dentin sialoprotein (DSP), anti-TRPA1, anti-TRPV4, anti-PANX-1, and anti-neurofilament (NF) antibodies. In addition, we investigated TRPA1 and TRPV4 expression by reverse transcriptional quantitative polymerase chain reaction (RT-qPCR). At PN0, undifferentiated odontoblasts showed no immunoreaction to anti-DSP, anti-TRPA1, anti-TRPV4, or anti-PANX-1 antibodies. However, immunopositive reactions of these antibodies increased during odontoblast differentiation at PN3 and PN6. An immunopositive reaction of the anti-NF antibody appeared in the odontoblast neighborhood at PN12, when the odontoblasts began to form root dentin, and this appeared later than that of the other antibodies. By RT-qPCR, expression of TRPA1 at PN6 was significantly lower than that at PN0 (p < 0.05) and PN3 (p < 0.01). Expression of TRPV4 at PN6 was significantly lower than that at PN0 (p < 0.01) and PN3 (p < 0.01). The results of this study suggest that odontoblasts may acquire sensory receptor function after beginning to form root dentin, when TRPA1, TRPV4, PANX-1 channels, and nerve fibers are completely formed.

Experimental research on living beings faces several obstacles, which are more than ethical and moral issues. One of the proposed solutions to these situations is the computational modelling of anatomical structures. The present study shows a methodology for obtaining high-biofidelity biomodels, where a novel imagenological technique is used, which applies several CAM/CAD computer programs that allow a better precision for obtaining a biomodel, with highly accurate morphological specifications of the molar and tissues that shape the biomodel. The biomodel developed is the first lower molar subjected to a basic chewing simulation through the application of the finite element method, resulting in a viable model, able to be subjected to various simulations to analyse molar biomechanical characteristics, as well as pathological conditions to evaluate restorative materials and develop treatment plans. When research is focused in medical and dental investigation aspects, numerical analyses could allow the implementation of several tools commonly used by mechanical engineers to provide new answers to old problems in these areas. With this methodology, it is possible to perform high-fidelity models no matter the size of the anatomical structure, nor the complexity of its structure and internal tissues. So, it can be used in any area of medicine., Competing Interests: The authors declare that they have no conflicts of interest., (Copyright © 2020 Rosa Alicia Hernández-Vázquez et al.)

This study compared the fatigue performance and the stress distribution of endodontically treated molars restored with endocrowns obtained with lithium disilicate glass-ceramic or a polymer-infiltrated ceramic network, both processed by CAD-CAM, and direct composite restorations. Forty-eight human mandibular molars were randomly assigned into 03 groups (n = 16) and restored with endocrowns (LD - lithium disilicate glass-ceramic or PICN - polymer-infiltrated ceramic network) or with direct composite restorations. Fatigue testing followed a step-stress approach (initial maximum load of 200 N and 5000 cycles, incremental step load of 200N and 10,000 cycles/step, being the specimens loaded until failure or to a maximum of 135,000 cycles at 2800 N). The fatigue failure load and number of cycles until failure were recorded and statistically analyzed. Fractographic and finite element (FEA) analyzes were conducted as well. There were no differences in fatigue failure load, number of cycles until fracture and mean survival probabilities among groups. However, indirect endocrowns had higher mechanical structural reliability, and LD restorations lasted more time before start to failing. FEA showed that the stress concentration in tooth tissues was higher for the resin composite, followed by PICN and LD in a decreasing order. Almost all fractures were restricted to the restorative material (without tooth involvement), and origins were identified at occlusal surface. The type of restoration did not influence the fatigue failure load, number of cycles until fracture and mean survival probabilities of the restorative strategies. Despite that, the mechanical structural reliability of endocrowns, especially those made of lithium disilicate, was higher and lasted more time before start to failing., (Copyright © 2019 Elsevier Ltd. All rights reserved.)

The accurately establishment of the eruptive pathway is of vital importance. The mechanisms governing tooth eruption pathway remain little known. This study is to elucidate the roles of Semaphorin 3A (Sema 3A) in mouse tooth eruptive pathway. C57BL/6 mice (11-13 and 15-17 days after birth) were chosen to observe eruptive pathway of mouse lower first molar. Expressions of Sema 3A and its receptor neuropilin 1 and plexin A1 were detected. Osteoclasts were identified by TRAP staining. Co-localization of Sema 3A and osteoclast maker CD68 was detected by double immunofluorescence staining. Picrosirius red staining was applied to observe collagen fibers during mucosal penetration phase. In vitro, Bone marrow-derived macrophages (BMMs) were prepared from 4 week C57BL/6 mice to observe the effect of Sema 3A on the differentiation of BMMs into osteoclasts by TRAP staining. Expressions of Sema 3A was observed by immunofluorescence and western blotting. At osseous eruption phase, many TRAP-positive multi-nucleated cells were distributed around occlusal alveolar bone. The positive expressions of Sema 3A were observed in the multi-nucleated cells. Fluorescence double staining showed that Sema 3A and CD68 were co-expressed in osteoclasts. Its receptor neuropilin 1 and plexin A1 were also found in osteoclasts. In vitro, Sema3A negatively regulated osteoclast differentiation. At mucosal penetration, occlusal alveolar bone had been completely resorbed and collagen fires were gradually degraded for eruptive pathway. Similar positive expressions of Sema 3A and its receptor neuropilin 1 and plexin A1 were also found in the mucosal penetration pathway. Sema 3A gets involved in the establishment of mouse tooth eruptive pathway by modulating osteoclast activity. Sema3A should be considered as a novel nervous agent or a potential biomarker for mouse tooth eruptive pathway.

This research uses quasi-static nanoindentation and nanoscratching to quantify human tooth deformation as a function of enamel rod and dentin tubule orientations at the nanoscale. Nanoindentation tests were performed on enamel and dentin to determine elastic modulus, hardness, and observe fracture. Additionally, nanoscratch tests were performed to determine pileup geometry and parameters such as recovery, scratch hardness, and scratch roughness. In enamel, it was found that nanofiber orientation gives rise to unique microcrack propagation and nanofiber behavior that affect these properties. For dentin, densification and organic content affect these properties., (Copyright © 2019 Elsevier Ltd. All rights reserved.)

Recent extensive field prospecting conducted in the Upper Miocene of Lebanon resulted in the discovery of several new fossiliferous localities. One of these, situated in the Zahleh area (Bekaa Valley, central Lebanon) has yielded a particularly diverse vertebrate fauna. Micromammals constitute an important part of this assemblage because not only do they represent the first Neogene rodents and insectivores from Lebanon, but they are also the only ones from the early Late Miocene of the Arabian Peninsula and circumambient areas. Analyses of the murines from Zahleh reveal that they belong to a small-sized early Progonomys, which cannot be assigned to any of the species of the genus hitherto described. They are, thereby, shown to represent a new species: Progonomys manolo. Morphometric analyses of the outline of the first upper molars of this species suggest a generalist and omnivorous diet. This record sheds new light onto a major phenomenon in the evolutionary history of rodents, which is the earliest dispersal of mice. It suggests that the arrival of murines in Africa got under way through the Levant rather than via southern Europe and was monitored by the ecological requirements of Progonomys.

Orthodontic treatment is widely used to correct irregular teeth and/or jaw discrepancies to improve oral function and facial aesthetics. However, it is frequently associated with enamel damage that include chipping, demineralisation, and white spot formation. So far, current bonding systems that can maintain shear bond strengths (SBS) suitable for clinical performance are unable to limit enamel demineralisation, adhesive remnants and damage caused on removal of brackets after treatment. This study reports a novel "safe enamel etch" clinically viable procedure that was accomplished via application of novel etchant pastes developed with β-tricalcium phosphate and monocalcium phosphate monohydrate powders mixed with citric acid (5 M) or phosphoric acid (37% PA) to yield BCA and BPA etchants respectively. Although enamel etched with clinically used PA gel yielded higher SBS than the BCA/BPA etchants, it exhibited greater adhesive remnants with evidence of enamel damage. In contrast, the experimental etchants resulted in unblemished enamel surfaces with zero or minimal adhesive residue and clinically acceptable SBS. Furthermore, the BPA etchant caused lower enamel decalcification with extensive calcium-phosphate precipitation. The study conclusively showed that BPA facilitated in vitro enamel adhesion without detrimental effects of the aggressive PA gel with potential for remineralisation and saving time at the post-debonding step.

In our previous work, we established an in vivo coronal pulp regeneration model in which biodegradable hydrogel-made scaffolds carrying rat bone marrow mesenchymal stem cells (BM-MSCs) were implanted in the coronal pulp chamber of pulpotomized rat maxillary first molars. In this study, we investigated the in vivo fate of LacZ-labeled BM-MSCs in our coronal pulp regeneration model. BM-MSCs were nucleofected with pVectOZ-LacZ plasmid encoding β-galactosidase 1 day before implantation, and the LacZ-transfected BM-MSCs were implanted into the pulpotomized pulp chamber with biodegradable preformed scaffold-hydrogel constructs. Empty vector was used as a control. After 3 and 14 days, the molars were retrieved and subjected to β-galactosidase staining. At 3 days, β-galactosidase-expressing cells with a round profile were located mainly around the scaffold. At 14 days, when the pulp-like tissue had been generated, the majority of β-galactosidase-expressing cells were detected under the newly formed dentin bridge-like structure, where nestin-expressing odontoblast-like cells were arranged. Immunoreactivity for dentin sialoprotein, a marker of mature odontoblasts, was strongly detected under the original dentin. No β-galactosidase staining was observed in the control group. Thus, we demonstrated that BM-MSCs survived for 2 weeks after implantation and colonized within the site of potential cytodifferentiation. Our findings indicated that BM-MSCs could differentiate into cells involved in mineralized tissue formation in the functionally relevant region., (Copyright © 2019 The Authors. Published by Elsevier B.V. All rights reserved.)

In humans, the incisors and molars have distinct functions during mastication, analogous to the two main types of handgrip, the precision and power grips. In the present study, we investigated cortical activation and masticatory muscle activity during incisal and molar biting via simultaneous functional magnetic resonance imaging and electromyogram (EMG) recordings. We conducted recordings in 15 healthy adult participants while they performed incisal and molar biting tasks at three step-wise force levels using two custom-made splints. Regarding the results of the ROI analysis, we found a significantly stronger positive linear correlation between the blood oxygenation level dependent signal and EMG activity during molar biting than incisal biting, which was particularly prominent in the primary sensorimotor cortex and the cerebellum. We also found a significantly stronger negative linear correlation during incisal biting than molar biting, which was particularly prominent in the rostral cingulate motor area, superior frontal gyrus, and caudate nucleus. These findings indicate that molar biting enables powerful chewing: brain activity in several brain areas related to motor function was increased with increasing bite force levels, while incisal biting enables fine motor control: brain activity in several brain areas related to motor control was increased with reduced bite force levels.

Objectives: In humans it has been shown that abrasive particles in the diet result in increased tooth wear and less intense chewing behavior, both of which decrease chewing efficiency. This behavioral response may also exist in non-human primates as a means to reduce the wear effect of dust-laden food. Here we tested whether the periodical occurrence of abrasive dust particles in the diet of Western chimpanzees affects tooth wear and reduces chewing efficiency., Materials and Methods: We measured fecal particle size of undigested food matter as an indicator of chewing efficiency in 13 Western chimpanzees of the Taï National Park (Ivory Coast) before (wet), after (wet) and during a dust-rich (dry) period. Moreover, feeding data were compiled for a further 12 chimpanzees and matched to three-dimensional surface texture data measured on two molar facets of 26 skulls of the same population., Results: Fecal particles were larger during the dry period, indicating a reduced chewing efficiency compared to wet periods; age and sex did not have an effect. Concomitantly, dust led to an increase of abrasive wear evidenced by smaller texture features and higher density of fine furrows on wear facets., Discussion: Our findings show that a periodical increase in dust loads on foods places a dietary-physiological stress on the digestive system in chimpanzees. We suggest that the impact of extrinsic abrasive particles from globally acting periodical dust-laden winds may affect evolutionary fitness. Further studies are required to elucidate this relationship in other non-human primates and fossil hominins., (© 2019 Wiley Periodicals, Inc.)

The aim of this study was to investigate the fracture behaviour of fissural dental enamel under simulated occlusal load in relation to various interacting factors including fissure morphology, cuspal angle and the underlying material properties of enamel. Extended finite element method (XFEM) was adopted here to analyse the fracture load and crack length in tooth models with different cusp angles (ranging from 50° to 70° in 2.5° intervals), fissural morphologies (namely U shape, V shape, IK shape, I shape and Inverted-Y shape) and enamel material properties (constant versus graded). The analysis results showed that fissures with larger curved morphology, such as U shape and IK shape, exhibit higher resistance to fracture under simulated occlusal load irrespective of cusp angle and enamel properties. Increased cusp angle (i.e. lower cusp steepness), also significantly enhanced the fracture resistance of fissural enamel, particularly for the IK and Inverted-Y shape fissures. Overall, the outcomes of this study explain how the interplay of compositional and structural features of enamel in the fissural area contribute to the resistance of the human tooth against masticatory forces. These findings may provide significant indicators for clinicians and technicians in designing/fabricating extra-coronal dental restorations and correcting the cuspal inclinations and contacts during clinical occlusal adjustment.

Background: Le Fort III advancement and/or distraction involve osteotomies and dysjunction in the region of the maxillary tuberosity in proximity to the maxillary posterior tooth buds. The purpose of this study was to determine the effect of early conventional Le Fort III advancement and/or distraction on development and eruption of the maxillary posterior permanent molars., Methods: A retrospective review of patients diagnosed with syndromic craniosynostosis, who underwent early Le Fort III or early midface distraction and late surgical intervention, was analyzed., Results: In the early conventional Le Fort III surgery group, 93 percent of maxillary second molars and 28 percent of maxillary first molars experienced a disturbance in eruption. In the early distraction group, 82 percent of maxillary second molars and 20 percent of maxillary first molars experienced a disturbance in eruption. In the control group, the late conventional Le Fort III and the late distraction groups, only 26 percent of maxillary second molars and none of maxillary first molars experienced a disturbance in eruption., Conclusions: A common disruption seen postoperatively in the early Le Fort III and distraction groups was displacement of the second molars. The majority of the displaced tooth buds were located in the maxillary sinus. Overall, the early Le Fort III surgery groups experienced more frequent disturbances for both first and second molars, with the common sequela of displacement in the maxillary sinus, leading to the question of whether presurgical planning in cases of early intervention to address midface retrusion should include extraction/enucleation of the second molar tooth buds., Clinical Question/level of Evidence: Therapeutic, III.

Over the past two centuries, mammalian chewing and related anatomical features have been among the most discussed of all vertebrate evolutionary innovations 1-3 . Chief among these features are two characters: the dentary-only mandible, and the tribosphenic molar with its triangulated upper cusps and lower talonid basin 3-5 . The flexible mandibular joint and the unfused symphysis of ancestral mammals-in combination with transformations of the adductor musculature and palate-are thought to have permitted greater mobility of each lower jaw, or hemimandible 6,7 . Following the appearance of precise dental occlusion near the origin of the mammalian crown 8,9 , therians evolved a tribosphenic molar with a craggy topography that is presumed to have been used to catch, cut and crush food. Here we describe the ancestral tribosphenic therian chewing stroke, as conserved in the short-tailed opossum Monodelphis domestica: it is a simple symmetrical sequence of lower tooth-row eversion and inversion during jaw opening and closing, respectively, enacted by hemimandibular long-axis rotation. This sequence is coupled with an eversion-inversion rotational grinding stroke. We infer that the ancestral therian chewing stroke relied heavily on long-axis rotation, including symmetrical eversion and inversion (inherited from the first mammaliaforms) as well as a mortar-and-pestle rotational grinding stroke that was inherited from stem therians along with the tribosphenic molar. The yaw-dominated masticatory cycle of primates, ungulates and other bunodont therians is derived; it is necessitated by a secondarily fused jaw symphysis, and permitted by the reduction of high, interlocking cusps 10-12 . The development of an efficient masticatory system-culminating in the tribosphenic apparatus-allowed early mammals to begin the process of digestion by shearing and crushing food into small boli instead of swallowing larger pieces in the reptilian manner, which necessitates a long, slow and wholly chemical breakdown. The vast diversity of mammalian teeth has emerged from the basic tribosphenic groundplan 13 .

The experimental determination of local bone deformations due to implant loading would allow for a better understanding of the biomechanical behavior of the bone-implant-prosthesis system as well as the influence of uneven force distribution on the onset of implant complications. The present study aimed at describing an innovative in vivo strain gauge measurement technique to evaluate implant-to-bone force transmission, assessing whether and how oral implants can transfer occlusal forces through maxillary bones. In vivo force measurements were performed in the maxillary premolar region of a male patient who had previously received a successful osseointegrated titanium implant. Three linear mini-strain gauges were bonded onto three different buccal cortical bone locations (i.e. coronal, middle, apical) and connected to strain measuring hardware and software. A customized screw-retained abutment was manufactured to allow for vertical and horizontal loading tests. As to the vertical load test, the patient was instructed to bite on a load cell applying his maximum occlusal force for 20 s and then recovering for 10 s to restore the bone unstrained state; the test was repeated 20 times consecutively. As regards the horizontal load test, the implant was subjected to a total of 20 load applications with force intensities of 5 and 10 kg. During the tests, the recorded signals were plotted in real time on a graph as a function of time by means of a strain analysis software. The described strain gauge measurement technique proved to be effective in recording the forces transmitted from osseointegrated implants to the cortical bone. Horizontal loads caused higher deformations of cortical bone than vertical biting forces; in both situations, the deformation induced by the force transferred from the implant to the bone progressively decreased from the coronal to the apical third of the alveolar ridge. At approximately 9 mm from the implant neck, the effect of occlusal force transmission through osseointegrated titanium implants was negligible if compared to the apical region., (Copyright © 2018 Elsevier Ltd. All rights reserved.)

The most desired target of orthodontic treatment is tooth movement as a result of application of efficient force system. In this study, effect of tooth loading is studied on temperature profile around the tooth at early stages of treatment. The basis of temperature variation is increase of cell number and activities in periodontium as a result of compression and tension of this layer. Highest cellular activities occur in the beginning of loading procedure and aim to reduce mechanical stress in the periodontium which finally ends up with orthodontic tooth movement during couple of years. To find out the correlation between temperature variation and the applied force, in vivo experiments are conducted on ten rats and temperature is measured in specific time periods. It is observed that temperature is higher in direction of the net force about 0.3℃. Next, numerical finite element analysis is carried out on the rat tooth model. Mechanical stress results show that regions with compressive stress have rather high temperature in the experiments. Mechanical stress on periodontium-bone interface is multiplied by a coefficient to simulate cellular activities on this boundary as a heat source and thermal analysis is carried out to obtain temperature profile. The thermo-mechanical coefficient is identified for each rat by imposing the experimental temperatures on numerical outputs. For assessment of a treatment efficiency and deduction of the applied force, temperatures could be measured experimentally and compared with the corresponding numerical analysis temperature result obtained by employing the thermo-mechanical coefficient found earlier for each rat., (Copyright © 2019 Elsevier Ltd. All rights reserved.)

Background: The mandibular advancement device (MAD) is widely used for obstructive sleep apnoea (OSA) treatment, and several studies have demonstrated its effectiveness. However, no comprehensive studies have yet examined the biomechanical safety of the MAD., Objectives: The objective of this study was to analyse the biomechanical effect of different protrusion positions of a MAD on the teeth and facial bones., Methods: The posterior restorative forces due to the stretched mandibular muscles were measured by pressure sensors attached to the experimental mandibular advancement device for mandibular protrusions of 10-70% of the maximum protrusion of the subject. A detailed three-dimensional biomechanical model of the study subject, constructed from computed tomography scans, was used in finite element analysis, with loading conditions calculated from the measured posterior restorative forces. The outcome measures were the principal stresses on the periodontal ligaments (PDL) and cancellous bone, and the pressure at the PDL surfaces. The measurements were used to analyse the risk of the tooth movement, tooth root resorption, and bone resorption., Results: The lowest and highest restorative forces occurred at 40% and 70% of maximum protrusion, respectively. The highest risk of tooth movement occurred at the mandibular molar teeth. The mandibular second molar teeth had the highest risks of root and bone resorption., Conclusions: Mandibular advancement at 70% of maximum protrusion induces risks of tooth root resorption and bone resorption. The mandibular second molars were subjected to the highest stresses. Stress on the teeth and facial bones was the lowest at 40% of maximum mandibular advancement., (© 2018 John Wiley & Sons Ltd.)

During orthodontic treatment, binding teeth, may change the topographically organized representation of teeth in the cerebral cortex. To test the hypothesis that experimental tooth movement (ETM) changes the somatotopy of an individual tooth arrangement in the somatosensory cortex, we examined the spatiotemporal features of cortical excitatory propagation in response to mechanical stimulation of the maxillary incisor or molar using optical imaging in late adolescent rats without or with ETM. The ETM models consisted of 1d, 3d, and 7d ETM in which a closed-coil spring was ligated between the maxillary first molar and incisors. In controls, incisor and molar mechanical stimulation evoked excitation in the rostral and dorsocaudal regions of the primary somatosensory cortex (S1), respectively. In addition, the secondary somatosensory cortex and insular oral region (S2/IOR) were also activated. Incisor stimulation-induced excitatory regions in S1 of 3d and 7d ETM shifted without changing the maximum excitatory area or peak amplitude; the incisor stimulation-responding region moved toward the dorsocaudal region, which responded to molar stimulation in the control. This shift in excitatory region was not observed in 1d ETM. One day after removal of the coil spring that was attached for 6 days, the excitatory region shift in S1 was recovered to the control region. On the other hand, 1d ETM exhibited facilitation of the excitatory area and peak amplitude upon molar stimulation, and the facilitation of excitatory propagation disappeared in 3d and 7d ETM. These results may explain the clinical finding that abnormal sensation temporally occurs during orthodontic treatment., (Copyright © 2018 Elsevier B.V. All rights reserved.)

Background: Maxillary skeletal expander (MSE) in combination with miniscrews was developed to overcome the drawbacks that may have resulted from the application of conventional rapid maxillary expander (RME). This research was conducted to analyze the difference of stress distribution of maxillary expansion using RME and MSE in the region of interests (ROIs): first molars (M1), palatal alveolar bones of M1, palatine sutures, zygomatic sutures, miniscrews, and their surrounding bones., Methods: A dry skull was scanned using CBCT and rendered into a three-dimensional (3D) model of craniomaxillary structures. The data analysis was done both visually and numerically., Result: The stress distributions in RME group were located at the palatal side of M1, mesial side of palatal alveolar of M1, pulp chamber of M1, and inferior cortex of palatine sutures. The stress distributions in the MSE group were located at the distopalatal cusp of M1, palatal side of palatal alveolar of M1, and inferior and superior cortex of palatine sutures. The stress distributions in zygomatic sutures on both groups were located at the zygomaticotemporal sutures, whereas in the miniscrews, the stress were located at the anterior miniscrews and palatal side of surrounding bones., Conclusions: There were significant differences of stress distribution of maxillary expansion measured in the ROIs in the craniomaxillary 3D model using RME and MSE.

The aim of this study was to evaluate stress distribution in the implants/components and bone tissue for splinted and nonsplinted prostheses with different lengths of implants using three-dimensional finite element analysis. Six models from the posterior maxillary area were used in simulations. Each model simulated three Morse taper implants of 4.0 mm diameter with different lengths, which supported metal-ceramic crowns. An axial load of 400 N and an oblique load of 200 N were used as loading conditions. Splinted prostheses exhibited better stress distribution for the implants/components, whereas nonsplinted prostheses exhibited higher stress in the first molar under axial/oblique loading. Implant length did not influence stress distribution in the implants/components. In cortical bone tissue, splinted prostheses decreased the tensile stress in the first molar, whereas nonsplinted prostheses were subjected to higher tensile stress in the first molar; implant length had no influence on stress distribution. Within the limitations of this study, we conclude that splinted prostheses contributed to better stress distribution in the implant/abutment and cortical bone tissue; however, the reduction in the implant length did not influence the stress distribution.

Objective: During orthodontic tooth movement (OTM), the speed of movement depends on the rate of bone turnover. In this study, we used a rat model to investigate the effect of 970 nm low-level laser therapy (LLLT) on OTM under different dose and frequency protocols., Methods: We first compared the OTM rates between the OTM only control and the OTM + LLLT group (1250 J/cm 2 ) in Experiment 1 and showed that LLLT significantly increased OTM. In Experiment 2, we employed 3 different LLLT protocols: the low-dose group and the high-dose group receiving 5 doses of 750 J/cm 2 and 15,000 J/cm 2 of LLLT every 3 days, respectively, and the early high-dose group which received 5 daily doses at 15,000 J/cm 2 at the beginning of the experiments. The OTM-only control group received no LLLT. Tooth movement rate was measured through sequential silicone impressions. MicroCT was also performed to evaluate bone de-mineralization rate. Bone histmorphometry was used to compare the bone turnover rate between LLLT group and control group. Finally, TRAP, Osteocalcin, and VEGF expression is evaluated by immunohistochemistry (IHC) in tissue sections., Results: When LLLT treatment was given every three days, both the 1250 J/cm 2 and 15,000 J/cm 2 groups showed significantly increased OTM compared to the control group. No significant difference was observed in the 750 J/cm 2 group, or in the early irradiation group, when compared with controls, although 750 J/cm 2 showed the same trend of accelerating OTM. The MicroCT result of rat maxilla demonstrated that LLLT increased bone remodeling and showed decreased bone mineral density and bone volume/total volume in the furcation areas of the maxillary first molars at the end of experiment. LLLT without OTM increased bone turnover as evidenced by fluorochrome incorporation. Immunohistochemistry analyses revealed high osteocalcin expression at later stages of OTM in the LLLT group, while VEGF expression was highly induced in the LLLT + OTM group at an early stage., Conclusion: Our results suggest that the 970 nm LLLT increases the rate of OTM in a dose-sensitive and frequency-dependent manner. Further animal and human studies are needed to determine the optimal timing and dosage of LLLT for OTM acceleration., (Copyright © 2018 Elsevier B.V. All rights reserved.)

Dental caries is one of the most common chronic diseases that affect human teeth. It often initiates in enamel, undermining its mechanical function and structural integrity. Little is known about the enamel demineralisation process caused by dental caries in terms of the microstructural changes and crystallography of the inorganic mineral phase. To improve the understanding of the carious lesion formation process and to help identify efficient treatments, the evolution of the microstructure at the nano-scale in an artificially induced enamel erosion region was probed using advanced synchrotron small-angle and wide-angle X-ray scattering (SAXS and WAXS). This is the first in vitro and time-resolved investigation of enamel demineralisation using synchrotron X-ray techniques which allows in situ quantification of the microstructure evolution over time in a simulated carious lesion. The analysis revealed that alongside the reduction of mineral volume, a heterogeneous evolution of hydroxyapatite (HAp) crystallites (in terms of size, preferred orientation and degree of alignment) could be observed. It was also found that the rate and direction of dissolution depends on the crystallographic orientation. Based on these findings, a novel conceptual view of the process is put forward that describes the key structural parameters in establishing high fidelity ultrastructure-based numerical models for the simulation of the enamel demineralisation process., Statement of Significance: Hydroxyapatite (HAp) crystallites in the enamel dissolve during dental caries although little is known about the structural-chemical relationships that control the dynamic demineralisation process. For the first time this work investigated the in situ evolution of nano-scale morphology and the spatial distribution of ultrastructural HAp crystallites of human enamel during demineralisation in simulated caries. Advanced synchrotron SAXS and WAXS techniques showed that the heterogeneous evolution of crystallites (size, preferred orientation and degree of alignment) could be attributed to crystallographic-orientation-dependent anisotropic dissolution. Hence we propose a novel conceptual schematic diagram to describe the demineralisation process. These findings have important implications for understanding the detailed mechanisms of enamel demineralisation and provide insight into potential enamel remineralisation that could restore structural integrity and function., (Copyright © 2018 Acta Materialia Inc. Published by Elsevier Ltd. All rights reserved.)

Rat molar eruption and occlusion data were compiled from several studies but several inconsistencies were found, rendering the planning of eruptional studies difficult and imprecise. Our aim was to measure eruption and occlusion days, as well as eruption velocity, in the upper and lower three molars from infancy to end of adolescence in the rat. A total of 19 male and female Wistar rats were scanned daily by micro-computed tomography (CT) from day 15 to 70. We measured the eruption of all maxillary and mandibular molars with reference points at the hard palate and mandibular canal at three stages: pre-emergent, pre-occlusal, and functional. Statistical analysis was performed with a mixed-model analysis of variance (ANOVA) and a Sidak post hoc test. The first molar erupts on average on day 17, the second molar on day 20, and the third molar on day 33. The eruption velocity of the first molar was the highest at 90.9 microns/day (standard error (se) = 12.80), followed by the second molar at 65.9 microns/day (se = 5.80), and the lowest was the third at 47.0 microns/day (se = 3.28), ( p < 0.001). On average, the pre-occlusal phase had the highest velocity at 97.2 microns/day (se = 1.72), the pre-emergent was lower at 84.9 (se = 2.29), and the functional was the lowest at 21.7 (se = 0.45), ( p < 0.001). The eruption rate decreased from the first to third molar and was also different between phases: the pre-occlusal phase had the highest rate, closely followed by the pre-emergent phase while the functional eruption rate was significantly lower than the other phases.

Fossil teeth are a primary source for inferring species development via evolutionary adaptation due to their linkage to feeding ecology and well perseverance. The main working tools in such studies are bite force analysis derived from jaw musculature and lever arms and morphogenetic based on enamel thickness and occlusal surface area. Despite progress made, quantitative correlation between predictions and behavior is still lacking. We studied histological sections in varieties of extracted premolar and molar human teeth. Sections corresponding to planes intersecting tips of primary cusps as well as more random planes were considered. The results revealed a unique, conclusive link between cuspal enamel thickness d c and dentin horn angle φ, a developmental parameter which contribution to tooth functioning has been overlooked. Naturally led by design principles of corbel arches, we examined the bending stress at the horn apex due to axial cuspal loading. The results show that this d c vs. φ relationship produces a constant force causing cusp fracture P F , making the latter a viable measure of tooth resilience. A preliminary study on published sections of extinct hominin teeth showed that their d c vs. φ behavior is consistent with modern humans albeit with varying P F . Scaling BF with P F enables direct estimate of bite force from measures of d c and φ in fossil teeth, achievable nondestructively from micro-computed tomography scans., Statement of Significance: The correspondence between cuspal enamel thickness and dentin horn angle in the postcanine row is a natural design here revealed for the first time. This correspondence yields constant force causing fracture at the horn apex, P F , making the latter a viable measure of tooth resilience. Scaling bite force (BF) with P F enables direct estimate of BF. The proposed mechanistic link between bite force and anatomical parameters d c and φ, expressed in a simple analytic form, offers direct, development-based expectation for examining evolutionary processes in hominins., (Copyright © 2018 Acta Materialia Inc. Published by Elsevier Ltd. All rights reserved.)

Purpose: The purpose of this study was to ascertain whether computer aided design/computer aided manufacturing (CAD/CAM) composite resin crowns have sufficient strength to withstand the bite force of the molar teeth. The null hypothesis was that the fracture strength of CAD/CAM composite resin crowns is lower than the average maximum bite force of the molar tooth., Methods: The crowns, which shape is the right maxillary first molar, were fabricated using four CAD/CAM blanks made of composite resins (Block HC: HC, KZR-CAD HR: HR, KZR-CAD HR2: HR2, Avencia Block: AVE) and one CAD/CAM blank made of lithium disilicate glass-ceramic (IPS e.max CAD: IPS), which was used as a control. Fracture strength of fabricated crowns bonded to metal abutment and biaxial flexural strength of the materials were evaluated., Results: The results of fracture strength test and biaxial flexural strength test showed different tendencies. The fracture strength of CAD/CAM composite resin crowns except HC ranged from 3.3kN to 3.9kN, and was similar to that of IPS (3.3kN). In contrast, biaxial flexural strength of CAD/CAM composite resins ranged from 175MPa to 247MPa, and was significantly lower than that of IPS (360MPa)., Conclusions: All CAD/CAM composite resin crowns studied presented about 3-4 times higher fracture strength than the average maximum bite force of the molar tooth (700-900N), which result leads to the conclusion that CAD/CAM composite resin crowns would have sufficient strength to withstand the bite force of the molar teeth., (Copyright © 2017 Japan Prosthodontic Society. Published by Elsevier Ltd. All rights reserved.)