Effect of Defect on Elastic/Plastic and Creep Behavior of Bone: A Finite Element Study

Three-dimensional structure of trabecular bone can be modeled by 2D or 3D Voronoi structure. The effect of missing cell walls on the mechanical properties of 2D honeycombs is a first step towards understanding the effect of local bone resorption due to osteoporosis. In patients with osteoporosis, bone mass is lost first by thinning and then by resorption of the trabeculae [1]. Furthermore, creep response is important to analyze in cellular solids when the temperature is high relative to the melting temperature. For trabecular bone, as body temperature (38 °C) is close to the denaturation temperature of collagen (52 °C), trabecular bone creeps [1]. Over the half of the osteoporotic vertebral fractures that occur in the elderly, are the result of the creep and fatigue loading associated with the activities of daily living [2]. The objective of this work is to understand the effect of missing walls and filled cells on elastic-plastic behavior of both regular hexagonal and non-periodic Voronoi structures using finite element analysis. The results show that the missing walls have a significant effect on overall elastic properties of the cellular structure. For both regular hexagonal and Voronoi materials, the yield strength of the structure decreased by more than 60% by introducing 10% missing walls. In contrast, the results indicate that filled cells have much less effect on the mechanical properties of both regular hexagonal and Voronoi materials.

Plaque and Shear Stress Distribution in Human Coronary Bifurcations: a Multi-Slice Computed Tomography Study

It is generally accepted that early atherosclerosis develops in low shear-stress (SS) regions such as the outer wall of arterial bifurcations and the inner bend of curved vessels (1). However, in clinical practice, it is common to observe atherosclerotic plaques at the flow-divider, or carina, of coronary bifurcations (2). Plaques at the carina are more frequently found in symptomatic patients, and may represent a more advanced stage of atherosclerosis. The carina is located in a region which is exposed to high SS. We hypothesize that if plaques are located in atheroprotective high SS regions, they have grown circumferentially from the atherogenic low SS regions.

Computational Model of Interstitial Transport in the Rat Brain Using Diffusion Tensor Imaging

In spite of the high therapeutic potential of macromolecular drugs, it has proven difficult to apply them to recovery after injury and treatment of cancer, Parkinson’s disease, and other neurodegenerative diseases. One barrier to systemic administration is low capillary permeability, i.e., the blood-brain and blood-spinal cord barrier. To overcome this barrier, convection-enhanced delivery (CED) infuses agents directly into tissue to supplement diffusion and increase the distribution of large molecules in the brain [1,2]. Predictive models of distribution during CED would be useful in treatment optimization and planning. To account for large infusion volumes, such models should incorporate tissue boundaries and anisotropic tissue properties.

CFD Modeling of Transient Flow Phenomena in an Axial Flow VAD

A ventricular assist device (VAD) effectively relieves the workload from a native heart, which has been weakened by disease, and increases blood flow supplied to the body to maintain normal physiologic function. The device must be able to operate over a wide range of conditions. Designed to operate at a single, best-efficiency operating point, it must frequently perform at off-design conditions due to a fluctuating flow rate demanded by the human body and a time varying flow within the pump, due to the beating of the native heart. The design and optimization of a blood pump is a challenging and complex process. Pump design equations are used to estimate the initial dimensions of the pump regions. Computational fluid dynamics (CFD) analyses are then performed to optimize the blood flow path according to specific design criteria under steady flow conditions [1].

Developing an Active Ankle Foot Orthosis Based on Shape Memory Alloy Actuators

This paper is on development of an active ankle foot orthosis (AAFO). This device will fill the gap in the existing research aimed at helping patients with drop foot muscle deficiencies as well as rehabilitation activities. Drop foot patients are unable to lift their foot because of reduced or no muscle activity around the ankle. The major causes of drop foot are severing of the nerve, stroke, cerebral palsy and multiple sclerosis. There are two common complications from drop foot. First, the patient cannot control the falling of their foot after heel strike, so that it slaps the ground on every step. The second complication is the inability to clear the toe during swing. This causes the patients to drag their toe on the ground throughout the swing.

Effects of Glycerol on the Thermal Properties of Phosphate Buffered Saline and Porcine Liver at Subzero Temperatures

Improvements in the prediction of thermal behavior during cryosurgery and cryopreservation can help improve the outcome of these cryobiological applications. The accuracy of the models depends on numerous factors including the kinetics and energy release during phase change phenomena and knowledge of thermal properties. Furthermore, connecting the thermal properties to crystalline, amorphous, and other phases adds an important mechanistic dimension that can also improve and direct an outcome. However, insufficient data for thermal properties in the subzero domain result in reliance on property estimations based usually upon tabulated water-ice data or weight averaged values from known materials primarily in temperature ranges above −40 °C [1]. This study focused on expanding the thermal properties database for both solutions and tissues. Results for Phosphate Buffered Saline (PBS) and porcine liver with glycerol at subzero temperatures (−150 ∼ 0 °C) are reported. The shifting of thermal property values due to sample crystallization, amorphous phase transition, and melting is discussed.

Simulation-Guided Stimulation for Paretic Ankle Muscles During Stroke Gait

Stroke is the leading cause of long-term adult disability in the U.S. Neuronal damage in the brain results in impaired muscle coordination which induces asymmetric and abnormal walking patterns. Muscle-actuated forward dynamic simulation of walking patterns of healthy young adults has elucidated unique and synergistic roles of the uniarticular and biarticular plantarflexors. Neptune and colleagues (2001) reported that soleus delivers energy to the trunk, gastrocnemius accelerates the leg forward, and both contribute significantly to vertical support of the center of mass [1]. In a simulation of post-stroke hemiparetic gait, Higginson et al. (2006) observed that non-paretic muscles mimicked the function of healthy muscles, while paretic ankle plantarflexor function was limited and required supplemental effort by hip and knee extensors [2].

Tissue Swelling During Extended Material Testing of Ligaments

Material testing is often used to characterize the mechanical properties of biological tissue and to understand the specific effects of treatments and pathologies on mechanical behavior. To have confidence in results from material testing, it is important that the test environment is repeatable between samples and that tests are performed in an environment that mimics physiological conditions.

Resultant Carpal Kinematics With Respect to Muscle Force Application

Purpose: The purpose of this study was to investigate the kinematics of seven carpal bones during a simulated active (tendon-driven) and passive (externally-assisted) motion of the wrist.

Stress Dependent Collagen Fibril Diameter Distribution in Human Aortic Valves

The mechanical properties of collagenous tissues are known to depend on a wide variety of factors, such as the type of tissue and the composition of its extracellular matrix. Relating mechanical roles to individual matrix components in such a complex system is difficult, if not impossible. However, as collagen is the main load bearing component in connective tissues, the relation between collagen and tissue biomechanics has been studied extensively in various types of tissues. The type of collagen, the amount and type of inter- and intramolecular covalent cross-links and collagen fibril morphology are involved in the tissues mechanical behavior (Beekman et al., 1997; Parry et al., 1978; Avery and Bailey, 2005). From literature it is known that the the collagen fibril diameter distribution can be directly related to the mechanical properties of the tissue. In particular, the diameter distribution of collagen fibrils is largely determined by the tissues requirement for tensile strength and elasticity (Parry et al., 1978).