Probabilistic finite element analysis was used to determine whether there is a statistically significant relationship between maximum principal strain orientations and orthotropic material stiffness orientations in a primate cranium during mastication. We first sought to validate our cranium finite element model by sampling in-vivo strain and in-vivo muscle activation data during specimen mastication. A comparison of in vivo and finite element predicted (i.e. in silico) strains was performed to establish the realism of the FEM model. To the best of our knowledge, this thesis presents the world’s only complete in-vivo coupled with in-vitro validation data set of a primate cranium FEM. Our results indicate that a validated FEM of a Cebus apella cranium was achieved. Giving collaborating anthropologists, biologists, and engineers the confidence that these models have sufficient accuracy to address the research questions pertaining to cranial structure morphology. Probabilistic finite element analysis design was then utilized to determine the dependence of maximum principal strain orientations on material stiffness orientations in particular craniofacial regions during mastication. It was discovered that the maximum principal strain orientations are more dependent on loading conditions and/or the shape of and location in the cranium rather than the material stiffness orientation of a particular region. It was also uncovered that the material stiffness orientations are not developed in a way that is optimal for feeding biomechanics from the perspective of minimization of total elastic strain energy. Results from this research will provide insights into the co-evolution of bone morphology and material properties in the facial skeleton.

Walkers provided stability to the elderly but cannot assist a person from sitting to standing. The objective of this project is to present the design and analysis of a lift assist walker. This report discusses the design and analysis of a collapsible lift assist walker capable of lifting a patient up to 250 lbs. from seated to standing in under 10 seconds. The designed walker utilized a two stage scissor mechanism with a gas spring assisted embedded linear actuator.

An ongoing debate concerning Neandertal ecology is whether or not they utilized long range weaponry. The anteroposteriorly expanded cross-section of Neandertal humeri have led some to argue they thrusted their weapons, while the rounder cross-section of Late Upper Paleolithic modern human humeri suggests they threw their weapons. We test the hypothesis that Neandertal humeri were built to resist strains engendered by thrusting rather than throwing using finite element models of one Neandertal, one Early Upper Paleolithic (EUP) human and three recent human humeri, representing a range of cross-sectional shapes and sizes. Electromyography and kinematic data and articulated skeletons were used to determine muscle force magnitudes and directions during three positions of spear throwing and three positions of spear thrusting. Maximum von Mises strains were determined at the 35% and 50% cross-sections of all models. During throwing and thrusting, von Mises strains produced by the Neandertal humerus fell roughly within or below those produced by the modern human humeri. The EUP humerus performed similarly to the Neandertal, but slightly poorer during spear thrusting. This implies the Neandertal and EUP human humeri were just as well adapted at resisting strains during throwing as recent humans and just as well or worse adapted at resisting strains during thrusting as recent humans. We also did not find any correlation between strains and biomechanical metrics used to measure humeral adaptation in throwing and thrusting (retroversion angle, Imax/Imin, J). These results failed to support our hypothesis and suggest they were capable of using long distance weaponry.

The humerus of a racing horse Thoroughbred is highly susceptible to stress fractures at a characteristic location as a result of cyclic loading. The propensity of a Thoroughbred to exhibit humeral fracture has made equines useful models in the epidemiology of stress fractures. In this study, a racing Thoroughbred humerus was simulated during training using a 3D finite element model in conjunction with a bone remodeling algorithm. Nine muscle forces and two contact forces were applied to the 3-dimensional finite element model, which contains four separate load cases representing fore-stance, mid-stance, aft-stance, and standing. Four different training programs were incorporated into the model, which represent Baseline Layup and Long Layup training programs along with two newly implemented programs for racing, which have an absence of a layup period, last a period of 24 weeks, and a race once every four weeks. Muscle and contact forces were rescaled for all load cases to simulate dirt, turf, and synthetic track surfaces. Bone porosity, damage, and BMU activation frequency were examined at the stress fracture site and compared with a control location called the caudal diaphysis. It was found that race programs exhibited similar remodeling patterns between each other. Damage at the stress fracture site and caudal diaphysis was reduced during all training programs for the turf and synthetic track surfaces with respect to the dirt track surface. Key findings also included changes in bone remodeling at the stress fracture site and caudal diaphysis as a result of turf and synthetic track surfaces. This model can serve as a framework for further studies in human or equine athletes who are susceptible to stress fractures.

Osteoarthritis (OA) is a degenerative condition of articular cartilage that affects more than 25 million people in the US. Joint injuries, like anterior cruciate ligament (ACL) tears, can lead to OA due to a change in articular cartilage loading. Gait analysis combined with knee joint finite element modeling (FEM) has been used to predict the articular cartilage loading. To predict the change of articular cartilage loading during gait due to various ACL injuries, a tibiofemoral FEM was developed from magnetic resonance images (MRIs) of a 33 year male, with no prior history of knee injuries. The FEM was validated for maximum contact pressure and anterior tibial translation using cadaver knee studies. The FEM was used to model gait of knees with an intact ACL, anteromedial (AM) bundle injury, posterolateral (PL) bundle injury, complete ACL injury, AM deficiency, PL deficiency, complete ACL rupture, as well as a bone-patellar tendon-bone (BPTB) graft. Generally, the predicted maximum contact pressure and contact area increased for all the ACL injuries when compared to intact ACLs. While an increase in maximum contact pressure and contact area is an indication of an increased risk of the development of OA, the percent of increase was typically small suggesting that walking is a safe activity for individuals with ACL injuries.

Long-duration spaceflight has been shown to have significant, lasting effects on the bone strength of astronauts and to contribute to age-related complications later in life. The microgravity environment of space causes a decrease in daily mechanical loading, which signals a state of disuse to bone cells. This affects the bone remodeling process, which is responsible for maintaining bone mass, causing an increase in damage and a decrease in density. This leads to bone fragility and decreases overall strength, posing a risk for fracture. However, there is little information pertaining to the timeline of bone loss and subsequent fracture risk. This study used finite element analysis to model the human femur, the bone most adversely affected by spaceflight, and to simulate the environments of Earth preflight, a six-month mission on the International Space Station, and one year on Earth postflight. Changes in the properties of cortical and trabecular bone in the femoral neck were measured from the simulations, and used to provide evidence for high fracture risk and to predict when it is most prominent. It was found that a risk for fracture is extremely evident in the femoral neck in both cortical and trabecular bone. Cortical bone in the inferior neck exhibited high magnitudes of damage, while the superior neck suffered the greatest increases in damage that proceeded to increase upon return to Earth. The density of trabecular bone decreased the most significantly and was not fully recovered in the following year. While it is still unclear exactly when these changes cause the greatest risk for fracture, it is possible that they will add to and advance the onset of medical complications such as osteoporosis. Additionally, the results of this study support the claim that the current countermeasure of inflight exercise is insufficient in sustaining bone mass and preserving skeletal health. The effects of long-duration spaceflight on bone health should continue to be investigated especially if future missions are to last as long as one to three years.

<p>Cellular structural mechanics play a key role in homeostasis by transducing mechanical signals to regulate gene expression and by providing adaptive structural stability for the cell. The alteration of nuclear mechanics in various laminopathies and in natural aging can damage these key functions. Arterial endothelial cells appear to be especially vulnerable due to the importance of shear force mechanotransduction to structure and gene regulation as is made evident by the prominent role of atherosclerosis in Hutchinson-Gilford progeria syndrome (HGPS) and in natural aging. Computational models of cellular mechanics may provide a useful tool for exploring the structural hypothesis of laminopathy at the intracellular level. This thesis explores this topic by introducing the biological background of cellular mechanics and lamin proteins in arterial endothelial cells, investigating disease states related to aberrant lamin proteins, and exploring computational models of the cell structure. It then presents a finite element model designed specifically for investigation of nuclear shear forces in arterial endothelial cells. Model results demonstrate that changes in nuclear material properties consistent with those observed in progerin-expressing cells may result in substantial increases in stress concentrations on the nuclear membrane. This supports the hypothesis that progerin disrupts homeostatic regulation of gene expression in response to hemodynamic shear by altering the mechanical properties of the nucleus.</p>

<p>Injection to the subcutaneous (SC) tissue is one of the preferred methods for drug delivery of pharmaceuticals, from small molecules to monoclonal antibodies. Delivery to SC has become widely popular in part thanks to the low cost, ease of use, and effectiveness of drug delivery through the use of auto-injector devices. However, injection physiology, from initial plume formation to the eventual uptake of the drug in the lymphatics, is highly dependent on SC mechanics, poroelastic properties in particular. Yet, the poroelastic properties of SC have been understudied. In this thesis, I present a two-pronged approach to understanding the poroelastic properties of SC. Experimentally, mechanical and fluid transport properties of SC were measured with confined compression experiments and compared against gelatin hydrogels used as SC-phantoms. It was found that SC tissue is a highly non-linear material that has viscoelastic and porohyperelastic dissipation mechanisms. Gelatin hydrogels showed a similar, albeit more linear response, suggesting a micromechanical mechanism may underline the nonlinear behavior. The second part of the thesis focuses on the multiscale modeling of SC to gain a fundamental understanding of how geometry and material properties of the microstructure drive the macroscale response. SC is composed of adipocytes (fat cells) embedded in a collagen network. The geometry can be characterized with Voroni-like tessellations. Adipocytes are fluid-packed, highly deformable and capable of volume change through fluid transport. Collagen is highly nonlinear and nearly incompressible. Representative volume element (RVE) simulations with different Voroni tesselations shows that the different materials, coupled with the geometry of the packing, can contribute to different material response under the different kinds of loading. Further investigation of the effect of geometry showed that cell packing density nonlinearly contributes to the macroscale response. The RVE models can be homogenized to obtain macroscale models useful in large scale finite element simulations of injection physiology. Two types of homogenization were explored: fitting to analytical constitutive models, namely the Blatz-Ko material model, or use of Gaussian process surrogates, a data-driven non-parametric approach to interpolate the macroscale response.</p>

Necessary for effective ambulation, head stability affords optimal conditions for the perception of visual information during dynamic tasks. This maintenance of head-in-space equilibrium is achieved, in part, by the attenuation of the high frequency impact shock resulting from ground contact. While a great deal of experimentation has been done on the matter during steady state locomotion, little is known about how head stability or dynamic visual acuity is maintained during asymmetric walking. In this study, fifteen participants were instructed to walk on a split-belt treadmill for ten minutes while verbally reporting the orientation of a randomized Landolt-C optotype that was projected at heel strike. Participants were exposed to the baseline, adaptation, and washout conditions, as characterized by belt speed ratios of 1:1, 1:3, and 1:1, respectively. Step length asymmetry, shock attenuation, high (impact) and low (active) frequency head signal power, and dynamic visual acuity scores were averaged across the first and last fifty strides of each condition. Over the course of the first fifty strides, step length asymmetry was significantly greater during adaptation than during baseline (p d =2.442). Additionally, high frequency head signal power was significantly greater during adaptation than during baseline (p d =1.227), indicating a reduction in head stability. Shock attenuation was significantly lower during adaptation than during baseline (p d =-0.679), and a medium effect size suggests that dynamic visual acuity was lower during adaptation than during baseline as well (p =0.052; d =0.653). When comparing the baseline and adaptation conditions across the last fifty strides, however, many of these decrements were greatly reduced. The results of this study indicate that the locomotor asymmetry imposed by the split-belt treadmill during the early adaptation condition is responsible for moderate decrements to shock attenuation, head stability, and dynamic visual acuity. Moreover, the relative reduction in magnitude of these decrements across the last fifty strides underscores the adaptive nature of the locomotor and visuomotor systems.

Total ankle replacement (TAR) implants are an effective option to restore the range of motion of the ankle joint for arthritic patients. An effective tool for analyzing these implants’ mechanical performance and longevity in-silico is finite element analysis (FEA). ABAQUS FEA was used to statically analyze the von Mises stress and contact pressure on the articulating surface of the bearing component in two newly installed fixed-bearing total ankle replacement implants (the Wright Medical INBONE II and the Exactech Vantage). This bearing component rotates on the talar component to induce primary ankle joint motion of plantarflexion and dorsiflexion. The stress response was analyzed on this bearing component since it is made of the least strong material in the implant assembly (ultra-high molecular weight polyethylene (UHMWPE). This bearing component commonly fails and is the cause for surgeon revisions. Six different FEA models for various gait percentages during stance (10%, 20%, 30%, 40%, 50%, and 60%) were created. They varied in magnitude of the compressive load and the ankle dorsiflexion/plantarflexion angle. This study captured the variation in stress magnitudes based on the portion of the stance phase. The results indicated that the stress distribution on the articulating surface increased as compressive load increased, and the largest magnitudes occurred at high dorsiflexion angles (15-30°). The von Mises stress and contact pressure tended to occur in regions where the thickness of the bearing was the least. Additionally, high contact pressures were examined in areas near the talar component's edge or at the bearing's edges. To the author’s knowledge, this is the first study available to the research community that analyzes the Vantage implant with FEA. This study lays an essential foundation for future researchers in presenting a thorough literature review of TAR and for a simple model setup to capture the stress distributions of two TAR implants. This study provides valuable information that can be beneficial to medical company designers and orthopedic surgeons in understanding the stress response of TAR patients.