The biomechanical function of periodontal ligament fibres in orthodontic tooth movement

Orthodontic tooth movement occurs as a result of resorption and formation of the alveolar bone due to an applied load, but the stimulus responsible for triggering orthodontic tooth movement remains the subject of debate. It has been suggested that the periodontal ligament (PDL) plays a key role. However, the mechanical function of the PDL in orthodontic tooth movement is not well understood as most mechanical models of the PDL to date have ignored the fibrous structure of the PDL. In this study we use finite element (FE) analysis to investigate the strains in the alveolar bone due to occlusal and orthodontic loads when PDL is modelled as a fibrous structure as compared to modelling PDL as a layer of solid material. The results show that the tension-only nature of the fibres essentially suspends the tooth in the tooth socket and their inclusion in FE models makes a significant difference to both the magnitude and distribution of strains produced in the surrounding bone. The results indicate that the PDL fibres have a very important role in load transfer between the teeth and alveolar bone and should be considered in FE studies investigating the biomechanics of orthodontic tooth movement. © 2014 McCormack et al

The biomechanical role of the chondrocranium and sutures in a lizard cranium

The role of soft tissues in skull biomechanics remains poorly understood. Not least, the chondrocranium, the portion of the braincase which persists as cartilage with varying degrees of mineralization. It also remains commonplace to overlook the biomechanical role of sutures despite evidence that they alter strain distribution. Here, we examine the role of both the sutures and the chondrocranium in the South American tegu lizard Salvator merianae. We use multi-body dynamics analysis (MDA) to provide realistic loading conditions for anterior and posterior unilateral biting and a detailed finite element model to examine strain magnitude and distribution. We find that strains within the chondrocraniumare greatest during anterior biting and are primarily tensile; also that strain within the cranium is not greatly reduced by the presence of the chondrocraniumunless it is given the same material properties as bone. This result contradicts previous suggestions that the anterior portion (the nasal septum) acts as a supporting structure. Inclusion of sutures to the cranium model not only increases overall strain magnitudes but also leads to a more complex distribution of tension and compression rather than that of a beam under sagittal bending

Masticatory biomechanics in the rabbit : a multi-body dynamics analysis

Acknowledgement We thank Sue Taft (University of Hull) for the µCT-scanning of the rabbit specimen used in this study. We also thank Raphaël Cornette, Jacques Bonnin, Laurent Dufresne, and l'Amicale des Chasseurs Trappistes (ACT) for providing permission and helping us capture the rabbits used for the in vivo bite force measurements at la Réserve Naturelle Nationale de St Quentin en Yvelines, France.Peer reviewedPublisher PD

Inclusion of periodontal ligament fibres in mandibular finite element models leads to an increase in alveolar bone strains

Alveolar bone remodelling is vital for the success of dental implants and orthodontic treatments. However, the underlying biomechanical mechanisms, in particular the function of the periodontal ligament (PDL) in bone loading and remodelling, are not well understood. The PDL is a soft fibrous connective tissue that joins the tooth root to the alveolar bone and plays a critical role in the transmission of loads from the tooth to the surrounding bone. However, due to its complex structure, small size and location within the tooth socket it is difficult to study in vivo. Finite element analysis (FEA) is an ideal tool with which to investigate the role of the PDL, however inclusion of the PDL in FE models is complex and time consuming, therefore consideration must be given to how it is included. The aim of this study was to investigate the effects of including the PDL and its fibrous structure in mandibular finite element models. A high-resolution model of a human molar region was created from micro-computed tomography scans. This is the first time that the fibrous structure of the PDL has been included in a model with realistic tooth and bone geometry. The results show that omission of the PDL creates a more rigid model, reducing the strains observed in the mandibular corpus which are of interest when considering mandibular functional morphology. How the PDL is modelled also affects the strains. The inclusion of PDL fibres alters the strains in the mandibular bone, increasing the strains in the tooth socket compared to PDL modelled without fibres. As strains in the alveolar bone are thought to play a key role in bone remodelling during orthodontic tooth movement, future FE analyses aimed at improving our understanding and management of orthodontic treatment should include the fibrous structure of the PDL

Effect of marker position and size on the registration accuracy of HoloLens in a non-clinical setting with implications for high-precision surgical tasks

Acknowledgments: We are grateful to Mike Whyment for the purchase of the holographic headset used in this study and to Rute Vieira and Fiona Saunders for their advice on statistics. We would also like to thank Denise Tosh and the Anatomy staff at the University of Aberdeen for their support. This research was funded by The Roland Sutton Academic Trust (RSAT 0053/R/17) and the University of Aberdeen (via an Elphinstone Scholarship, IKEC Award and Medical Sciences Honours project funding). Funding: This study was funded by The Roland Sutton Academic Trust (RSAT 0053/R/17) and the University of Aberdeen (via an Elphinstone Scholarship, IKEC Award and Medical Sciences Honours project funding).Peer reviewedPublisher PD

Comparative cranial biomechanics in two lizard species: impact of variation in cranial design

Cranial morphology in lepidosaurs is highly disparate and characterised by the frequent loss or reduction of bony elements. In varanids and geckos, the loss of the postorbital bar is associated with changes in skull shape, but the mechanical principles underlying this variation remain poorly understood. Here, we sought to determine how the overall cranial architecture and the presence of the postorbital bar relate to the loading and deformation of the cranial bones during biting in lepidosaurs. Using computer-based simulation techniques, we compared cranial biomechanics in the varanid Varanus niloticus and the teiid Salvator merianae, two large, active foragers. The overall strain magnitude and distribution across the cranium were similar in the two species, despite lower strain gradients in V. niloticus. In S. merianae, the postorbital bar is important for resistance of the cranium to feeding loads. The postorbital ligament, which in varanids partially replaces the postorbital bar, does not affect bone strain. Our results suggest that the reduction of the postorbital bar impaired neither biting performance nor the structural resistance of the cranium to feeding loads in V. niloticus. Differences in bone strain between the two species might reflect demands imposed by feeding and non-feeding functions on cranial shape. Beyond variation in cranial bone strain related to species-specific morphological differences, our results reveal that similar mechanical behaviour is shared by lizards with distinct cranial shapes. Contrary to the situation in mammals, the morphology of the circumorbital region, calvaria and palate appears to be important for withstanding high feeding loads in these lizards

An assessment of the role of the falx cerebri and tentorium cerebelli in the cranium of the cat (Felis silvestris catus)

© 2018 The Author(s). The falx cerebri and the tentorium cerebelli are two projections of the dura mater in the cranial cavity which ossify to varying degrees in some mammalian species. The idea that the ossification of these structures may be necessary to support the loads arising during feeding has been proposed and dismissed in the past, but never tested quantitatively. To address this, a biomechanical model of a domestic cat (Felis silvestris catus) skull was created and the material properties of the falx and tentorium were varied for a series of loading regimes incorporating the main masticatory and neck muscles during biting. Under these loading conditions, ossification of the falx cerebri does not have a significant impact on the stress in the cranial bones. In the case of the tentorium, however, a localized increase in stress was observed in the parietal and temporal bones, including the tympanic bulla, when a non-ossified tentorium was modelled. These effects were consistent across the different analyses, irrespective of loading regime. The results suggest that ossification of the tentorium cerebelli may play a minor role during feeding activities by decreasing the stress in the back of the skull

Comparing the distribution of strains with the distribution of bone tissue in a human mandible: a finite element study

Comparative anatomy and experimental studies suggest that the mass and distribution of tissue within a bone is adapted to the strains the bone experiences during function. Finite element analysis is a powerful tool that can be used to investigate this since it allows the creation of hypothetical models with unadapted morphology. Here we use FE models of a human mandible with modified internal morphology to study the relationships between the gross distribution of bone tissue (i.e., the presence or absence of bone in a certain area), the variation of cortical bone thickness within the mandible and the distribution of strain magnitudes. We created one model in which all internal cavities were filled with cortical bone material and a second, hollow model with constant cortical bone thickness. In both the models, several load cases representing bites at different positions along the tooth row were applied and peak strain magnitudes across these load cases were calculated. The peak strain distributions in both models show striking similarities with the gross distribution of bone tissue and the variation of cortical thickness in the real mandible, but the correlation coefficients are rather low. These low coefficients could be explained by confounding factors and by the limited spectrum of load cases that were simulated. However, the correspondences we find between strain magnitude and bone tissue distribution suggest that models with altered internal geometry are useful in studying the mechanical adaptation of bone, especially in the absence of any in vivo strain data. Anat Rec, 2012. © 2012 Wiley Periodicals, Inc

Modeling the human mandible under masticatory loads: which input variables are important?

Finite element analyses (FEA) that have simulated masticatory loadings of the human mandible differ significantly with regard to their basic input variables such as material properties, constraints, and applied forces. With sensitivity analyses it is possible to assess how the choice of different input values and the degree of model simplification affect FEA results. However, published FEA studies are rarely accompanied by sensitivity analyses so that the robusticity of their results is impossible to assess. Here, we conduct a sensitivity analysis with an FE model of a human mandible to quantify the relative importance of several modeling decisions: (1) the material properties assigned to the cancellous bone tissue; (2) the inclusion or not of the periodontal ligament; (3) the constraints at the joints and bite point; and (4) the orientation of applied muscle forces. We study the effects of varying these properties by analysing the strain magnitudes and directions across the model surface. In addition, we perform a geometric morphometric analysis of the deformation resulting from the loading of each model. The results show that the effects of altering the different model properties can be significant and that most effects are potentially large enough to cause problems for the biological interpretation of FEA results. We therefore recommend that researchers conduct more sensitivity analyses than at present to assess the robusticity of their FEA results and their biological conclusions

Why do humans have chins? Testing the mechanical significance of modern human symphyseal morphology with finite element analysis

The modern human mandibular symphysis differs from those of all other primates in being vertically orientated and possessing a chin, but the functional significance of this unique morphology is not well understood. Some hypotheses propose that it is an adaptation to specific loads occurring during masticatory function. This study uses finite element analysis to examine these symphyseal loads in a model of a modern human mandible. By modifying the symphyseal cross-sectional form, the mechanical significance of the presence of the chin and symphyseal orientation is tested, and modern human and Neanderthal symphyseal cross-sections are compared with regard to their ability to withstand different loads. The results show that changes in symphyseal form have profound effects on the strains. The presence of a chin leads to lower symphyseal strains overall, whereas a vertical orientation of the symphysis results in higher strains under wishboning, but not under vertical bending in the coronal plane and dorsoventral shear. Compared to Neanderthals, the modern human symphysis shows higher strains during dorsoventral shear and wishboning, but is as effective as the Neanderthal symphysis in resisting vertical bending in the coronal plane and the loads resulting from simulated incision and unilateral molar biting. In general, the results of this study corroborate prior hypotheses about the mechanical effects of the human chin and vertical symphyseal orientation and support the idea that the relative importance of wishboning and vertical bending in the coronal plane might have played a role in the evolution of modern human symphyseal morphology