Biomechanical Characterization and Modeling of Human TMJ Disc

Temporomandibular joint (TMJ) disorder affects over 10 million people in the US each year. The signs and symptoms of temporomandibular joint disorders (TMDs) include limited mouth opening, clicking and locking of the jaw, and significant pain in the craniofacial region. In patients who seek treatment for TMDs, over 70% have TMJ disc displacement related to disc degeneration. The TMJ disc is interposed between the mandible condyle and the glenoid fossa of the temporal bone. The disc reduces contact stresses within the joint and provides lubrication to the joint. It is generally believed that pathological mechanical loadings, such as sustained jaw clenching or malocclusion, trigger a cascade of molecular events that lead to TMJ disc degeneration. A deeper understanding of the biomechanics, i.e. mechanical environment and effect on the nutrient environment, could lead to developments in TMD diagnosis and management. Therefore, the objective of this research study was to determine the mechanical and transport properties of the human TMJ disc and begin to model joint biomechanics using patient specific finite element models. Our central hypothesis is that sustained mechanical loading can alter solute transport and nutrient levels in the TMJ disc as well as mechanical function resulting in disc derangement and degeneration. The TMJ disc is a large, avascular structure. The nutrients required by disc cells for maintaining disc health are supplied by synovial fluid at the margins of the disc as well as nearby blood vessels through diffusion. The first study investigated the effect of mechanical strain on small solute diffusion in human TMJ discs using the electrical conductivity method. From ion exchange theory, electrical conductivity is proportional to diffusivity assuming the tissue is uncharged. The results indicated that mechanical strain significantly impeded solute diffusion through the disc, which was consistent with our similar porcine study. Female conductivity was higher than male conductivity and was affected more by mechanical strain. This study suggested that female TMJ disc tissue could have slight differences in composition and porosity responsible for differences in electrical conductivity. In addition to investigating diffusion under mechanical strain, the charged matrix of the TMJ disc was investigated by determining the fixed charge density (FCD) of human TMJ discs. The FCD of cartilaginous tissues has been shown to contribute to high osmotic swelling pressure responsible for mechanical properties in addition to electrokinetic effects such as streaming potential. The fixed charge density was determined using a two point electrical conductivity approach and correlated to the glycosaminoglycan (GAG) content. The FCD in the TMJ disc was most similar to annulus fibrosis tissues found in the intervertebral disc of the spine. This study suggested that the TMJ disc is similar to other fibrocartilage tissues, with the dense collagen matrix possibly contributing a more significant role in mechanical loading than GAG content. Finally, the human TMJ disc was characterized for viscoelastic tensile properties under incremental stress relaxation tests. The disc exhibited a linear stress response to incremental strain. The instantaneous and relaxed modulus values were lower than values found for human patellar tendon, human supraspinatus tendon and porcine TMJ discs studied previously. In addition, the disc did not exhibit an anisotropic response to loading as found in similar studies; nor were there differences in male and female results. This study suggested that the human TMJ disc (cadaver age ~69) likely exhibits a lower tensile modulus than young porcine TMJ discs (\u3c1 year), although in this study no age effects were determined from the cadaver age range between 58 to 82 years. The results of these studies are being used to construct a patient specific finite element model. The human material properties are being combined with patient specific anatomy from MRI/CT scans. In the future, these models may be used by the TMJ research community to track and monitor the progression of TMDs

Impact Of Laser Powder Bed Fusion Process Defects On Mechanical Properties Of Ti6Al4V Mandible Implants

DissertationEach year millions of patients’ quality of life is improved through surgical procedures involving medical implanted devices. The need for new implants, treatments and prostheses, as well as prolonging the life span of current implants has increased; the global prosthetics and orthotics market size is expected to reach $11.7 billion by 2025, as indicated in Healthcare Market Report (2020). Additive manufacturing (AM) was implemented in the medical field fairly recently. Despite the enormous contribution medical devices have made to the public health, there is a fear of possible liability exposure in the event of device malfunction or failure. Efficient quality control of implants produced by new AM technologies is an important task for suppliers in order to be in full compliance with existing regulations and certification of such implants. If any defects occur, implant strength will directly influence the part’s mechanical properties and performance, leading to the redistribution of stress and change in displacements affecting attached bone tissue and mineral matrix of the bone, resulting in implant failure. For wide applications in the medical industry, it is crucial that AM implants comply with international standards with regard to their mechanical properties. Three point bending tests (TPB) were carried out in this work on AM Ti6Al4V ELI specimens. TPB is a common tool used to characterize bone material properties and mechanical performance of biomaterials. Powder bed fusion is the unique AM method to produce metal objects with complex geometries and internal structures; it permits the manufacture of complex-shaped functional 3D objects such as customized implants. The benefits of AM in bone reconstruction using metal alloys are unquestionable in terms of customization of implants and production time. Comprehensive analysis of the laser powder bed fusion (LPBF) process together with functional anatomy biomechanics of the human mandible was done in this work. Some case studies on defects found in LPBF implants were evaluated. Based on biomechanics of the human mandible, LPBF Ti6Al4V ELI samples were designed. Experiments and numerical simulations of samples with sizes and placements of artificial pores were done. All samples were tested perpendicular to the vertical building direction and showed no signs of failure at a single loading pattern. Defects were designed and induced in the additive manufacturing of test samples of titanium, with different size and placement. Results indicate that defects of 1000 μm×300 μm×210 μm and 1000 μm×500 μm×420 μm at various depth to the neutral axis had no significant outcome on the mechanical performance of the samples with size of 100 mm 15 mm 2.5 mm when it was tested statically at loading of 800, 900 and 1500 N, representing a maximum biting force. This approach is a promising method of setting up a critical pore size to failure tolerance for AM implants with some defects

Computational Simulation of Trabecular Bone Distribution around Dental Implants and the Influence of Abutment Design on the Bone Reaction for Implant-Supported Fixed Prosthesis

Computational modelling of trabecular bone distribution based on the remodelling process is a challenging issue. Up to now, most of bone remodelling models attempted to describe the remodelling process with noncemented implants of the hip joint. Few studies are published about remodelling processes around dental implants. This work presents a computational simulation of bone remodelling around dental implants from a biomechanical point of view. The model is based on the stimulation of bone remodelling by a local mechanical stimulus. Furthermore, this study investigates the reaction of the bone to different prosthetic abutment designs that are commonly used for implant-supported fixed prosthesis. The first part includes the investigation of the influence of abutment design on the bone behaviour at the cervical region of the implants that are used for implant-supported fixed prosthesis. The investigations cover three aspects: Experimental, numerical, and clinical. The experimental part deals with measuring the magnitude of implant micromotion in relation to the abutment design. The numerical part analyses the distribution of stresses and strains and their relation to the abutment design. The clinical part represents the final step for the validation of the experimental and numerical results. The probing depth is measured up to one-year after the placement of the abutments. The second part of the presented study deals with testing the sensitivity of the applied remodelling model to different mechanical conditions, e.g. varying boundary conditions, loading conditions, material properties, etc. The third part of this work deals with the simulation of remodelling processes during the healing phase by considering three healing intervals and different tissue layers by means of different mechanical properties at the bone-implant interface. In conclusion, this work demonstrates, in its first half, the reaction of the bone to the load distribution created by different abutment designs in implantsupported fixed prosthesis. In its second half, the present word describes a computational simulation of trabecular structure around dental implants based on the change of the apparent bone density as a function of the mechanical daily stimulus

The Bioarchaeology of the Lake St. Agnes Mound (16AV26) Site: Exploring Diet from Fragmentary Remains

The Lake St. Agnes Mound (16AV26) site, located in central Louisiana, is composed of two, temporally distinct burial components; one, a Coles Creek period component, at the base of the mound (~780-880 CE), and the other, a Plaquemine subperiod component, at its apex (~1400 CE). These burials, though heavily fragmented, commingled, and representing small sample sizes, are valuable for studying the transition to agriculture in the Lower Mississippi River Valley. It is now clear that for the Coles Creek period, maize was likely only a ceremonial crop rather than a staple food source (Kidder, 1993; Listi, 2011). The reliance on maize agriculture in Plaquemine times is inconsistent. An exploration into the diet of the two Lake St. Agnes burial components may illuminate how or if maize agriculture spread into this region of Louisiana. Both samples were assessed for the presence of dental caries, calculus, linear enamel hypoplasias, dental micro- and macrowear, and porotic cranial lesions. The results demonstrated few statistically significant differences between the samples. Both samples exhibited low levels of caries and linear enamel hypoplasia (~10%) but experienced higher rates of periodontal disease and porotic cranial lesions. What is suggested by these results is consistency over time in diet, rather than evidence for a dietary transition as would be expected with the adoption of agriculture. The variability within each sample regarding the dental micro- and macrowear is interpreted as both seasonal differences in the types of food available, as well as differences in the access, or preference towards certain types of foods, such as native garden crops over tougher, wild plants

An Experimental and Finite Element Study of the Protective Mechanisms of Sports Mouthguards

This work presents a broad investigation into the protective mechanisms and design specifications of sports mouthguards, motivated by the need to standardise testing of these personal protective devices. A mechanical model in the form of an artificial jaw was developed to allow impact testing of a range of mouthguards in situ. The protective capabilities of different design features were assessed, with thicker larger mouthguards performing best. Various forms of tooth fracture were investigated and the influence of the embedding method was linked to the type of fracture seen. A user study was also conducted that compared the same range of mouthguards investigated in the impact tests, but in term of user perceptions of comfort prior to fitting the mouthguard, after fitting the mouthguard and after mild exercise. The concept of familiarisation was found to have a significant influence on the perception of comfort. Clear trends could be seen with respect to exercise, user experience, and specific design features such as thin soft material and close fit. Static 3D and dynamic 2D FE modelling were carried out to investigate the protective mechanisms of idealised mouthguards in simple linear elastic models. The concepts of cushioning and support were defined and explored. Cushioning is the mechanisms by which soft layers reduce impact stresses. Support is the mechanism by which rigid layers share the load over neighbouring teeth. FE modelling showed that bi-layer and tri-layer guards can offer superior protection compared to monolayer guards by combining the requirements of cushioning and support. However, the optimum design parameters of mouthguards depend upon the type of load, and so there is no universal solution for all risks. Hence it is proposed to classify the requirements of any particular mouthguard application in terms of cushioning and support, based on a probability of impact characteristics occurring

Mechanical Properties of Materials

In the oral environment, restorative and prosthetic materials and appliances are exposed to chemical, thermal and mechanical challenges. The mechanical properties of a material define how it responds to the application of a physical force. Recent advances in nanotechnology and 3D printing have rapidly spread, and manufacturers continuously develop new materials and solutions to provide high-quality dental care, with particular attention being paid to long-term follow-up. Restorative dentistry, prosthodontics, oral surgery, implants, periodontology and orthodontics are all involved in this continuing evolution. This Special Issue focuses on all the recent technology that can enhance the mechanical properties of materials used in all of the different branches of dentistry

Food mechanical properties and dietary ecology

Interdisciplinary research has benefitted the fields of anthropology and engineering for decades: a classic example being the application of material science to the field of feeding biomechanics. However, after decades of research, discordances have developed in how mechanical properties are defined, measured, calculated, and used due to disharmonies between and within fields. This is highlighted by “toughness,” or energy release rate, the comparison of incomparable tests (i.e., the scissors and wedge tests), and the comparison of incomparable metrics (i.e., the stress and displacement‐limited indices). Furthermore, while material scientists report on a myriad of mechanical properties, it is common for feeding biomechanics studies to report on just one (energy release rate) or two (energy release rate and Young's modulus), which may or may not be the most appropriate for understanding feeding mechanics. Here, I review portions of materials science important to feeding biomechanists, discussing some of the basic assumptions, tests, and measurements. Next, I provide an overview of what is mechanically important during feeding, and discuss the application of mechanical property tests to feeding biomechanics. I also explain how 1) toughness measures gathered with the scissors, wedge, razor, and/or punch and die tests on non‐linearly elastic brittle materials are not mechanical properties, 2) scissors and wedge tests are not comparable and 3) the stress and displacement‐limited indices are not comparable. Finally, I discuss what data gathered thus far can be best used for, and discuss the future of the field, urging researchers to challenge underlying assumptions in currently used methods to gain a better understanding between primate masticatory morphology and diet

Book of abstracts of the 18th International Symposium on Dental Morphology and the 3rd congress of the International Association of the Palaeodontology, 15-19.8.2022, Frankfurt, Germany

Book of abstracts of the 18th International Symposium on Dental Morphology and the 3rd congress of the International Association of the Palaeodontology, 15-19.8.2022, Frankfurt, German

Book of abstracts of the 18th International Symposium on Dental Morphology and the 3rd congress of the International Association of the Palaeodontology, 15-19.8.2022, Frankfurt, Germany

Book of abstracts of the 18th International Symposium on Dental Morphology and the 3rd congress of the International Association of the Palaeodontology, 15-19.8.2022, Frankfurt, German

Recent hominim cranial form and function

This thesis aims to assess if biting mechanics drives craniofacial morphology in recent hominins. To that end, a virtual functional morphology toolkit, that includes Finite Element Analysis (FEA) and Geometric Morphometrics (GM), is used to simulate biting, measure bite force and quantify deformations arising due to simulated biting in Homo sapiens and its proposed ancestral species, Homo heidelbergensis. Moreover, the mechanical significance of the frontal sinus and of the brow-ridge is also assessed in Kabwe 1 (a Homo heidelbergensis specimen). The frontal sinus is examined by comparing the mechanical performance in three FE models with varying sinus morphology. A similar approach is applied to the brow-ridge study. This approach relies on the assumption that FEA approximates reality. Thus, a validation study compares the deformations experienced by a real cranium under experimental loading with those experienced by an FE model under equivalent virtual loading to verify this assumption. A sensitivity analysis examines how simplifications in segmentation impact on FEA results. Lastly, the virtual reconstruction of Kabwe 1 is described.Results show that prediction of absolute strain magnitudes is not precise, but the distribution of regions of larger and smaller (i.e. pattern of) deformations experienced by the real cranium is reasonably approximated by FEA, despite discrepancies in the alveolus. Simplification of segmentation stiffens the model but has no impact on the pattern of deformations, with the exception of the alveolus. Comparison of the biting performance of Kabwe 1 and H. sapiens suggests that morphological differences between the two species are likely not driven by selection of the masticatory system. Frontal sinus morphogenesis and morphology are possibly impacted by biting mechanics in the sense that very low strains are experienced by this region. Because bone adapts to strains, the frontal sinus is possibly impacted by this mechanism. Lastly, biting mechanics has limited impact on brow-ridge morphology and does not explain fully the enormous brow-ridge of Kabwe 1. Hence, other explanations are necessary to explain this prominent feature