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research-article  
Ganesh Thiagarajan, Mark T Begonia, Mark Dallas, Nuria Lara, JoAnna Scott and Mark L Johnson
J Biomech Eng   doi: 10.1115/1.4039982
The determination of the elastic modulus of bone is important in studying the response of bone to loading and is determined using a destructive 3-point bending method. Reference Point Indentation (RPI) offers a nondestructive alternative to determine the elastic modulus. Using the RPI measurements we have developed a numerical analysis procedure using the Oliver-Pharr (O/P) method to estimate the indentation elastic modulus. Two methods were used to determine the area function: (1) Oliver-Pharr (O/P) and (2) Geometric. The indentation modulus of PMMA calculated by the O/P (3.49-3.68 GPa) and Geometric (3.33-3.49 GPa) methods were similar to values in literature (3.5-4 GPa). In a study using femurs from C57Bl/6 mice of different ages and genders the 3-point bending modulus was lower than the indentation modulus indicating that the indentation modulus represents the elastic modulus at the site of the indentation whereas the 3-point bending modulus takes into account the architecture of the bone. In femurs from 4-5 mo. old TOPGAL mice we found that the indentation modulus from the Geometric (5.61±1.25 GPa) and O/P (5.53±1.27 GPa) methods was higher than the 3-point bending modulus (5.28±0.34 GPa). In females, the indentation modulus from the Geometric (7.45±0.86 GPa) and O/P (7.46±0.92 GPa) methods was also higher than the 3-point bending modulus (7.33±1.13 GPa).
TOPICS: Bone, Elastic moduli, Numerical analysis
research-article  
Calvin Kuo, Jake A. Sganga, Michael G. Fanton and David Camarillo
J Biomech Eng   doi: 10.1115/1.4039987
Wearable sensors embedded with inertial measurement units have become commonplace for the measurement of head impact biomechanics, but individual systems often suffer from a lack of measurement fidelity. While some researchers have focused on developing highly accurate, single sensor systems, we have taken a parallel approach in investigating optimal estimation techniques with multiple noisy sensors. In this work, we present a novel sensor network methodology that utilizes multiple skin patch sensors arranged on the head, and combines their data to obtain a more accurate estimate than any individual sensor in the network. Our methodology visually localizes subject-specific sensor transformations, and based on rigid body assumptions, applies estimation algorithms to obtain a minimum mean squared error estimate. During mild soccer headers, individual skin patch sensors had over 100% error in peak angular velocity magnitude, angular acceleration magnitude, and linear acceleration magnitude. However, when properly networked using our visual localization and estimation methodology, we obtained kinematic estimates with median errors below 20%. While we demonstrate this methodology with skin patch sensors in mild soccer head impacts, the formulation can be generally applied to any dynamic scenario, such as measurement of cadaver head impact dynamics using arbitrarily placed sensors.
TOPICS: Measurement units and standards, Kinematics, Sensors, Errors, Skin, Sensor networks, Dynamics (Mechanics), Biomechanics, Algorithms
research-article  
Raghu N. Natarajan, Kei Watanabe and Kazuhiro Hasegawa
J Biomech Eng   doi: 10.1115/1.4039989
Purpose. Examine the biomechanical effect of material properties, geometric variables and anchoring arrangements in a segmental pedicle screw with connecting rods spanning the entire lumbar spine using finite element models. This will help (1) to determine the variables with the greatest effect on spine kinematics and stresses in instrumentation, (2) to compare the multi-directional stability of the spinal instrumentation and (3) to determine less-rigid fixation systems that may reduce adjacent segment disc disease. Methods. A lumbar spine finite element model was used to analyze the biomechanical effects of different materials used for spinal rods (TNTZ or Ti or CoCr), varying diameters of the screws and rods (5 mm and 6 mm), and different fixation techniques (multi-level or intermittent). Results. The results based on the range of motion and stress distribution in the rods and screws revealed that differences in properties and variations in geometry of the screw-rod moderately affect the biomechanics of the spine. Further the spinal screw-rod system was least stable under the lateral bending mode. Stress analyses of the screws and rods revealed that the caudal section of the posterior spinal instrumentation was more susceptible to fatigue failure. Although CoCr screws and rods provided the greatest spinal stabilization, these constructs were susceptible to fatigue failure. Conclusion. The findings of the present study suggest that a posterior instrumentation system with a 5-mm screw-rod diameter made of Ti or TNTZ is advantageous over CoCr instrumentation system.
TOPICS: Biomechanics, Finite element model, Lumbar spine, Screws, Rods, Instrumentation, Fatigue failure, Spinal pedicle screws, Spine mechanics, Disks, Diseases, Geometry, Stress, Kinematics, Stability, Stress analysis (Engineering), Stress concentration, Materials properties
Technical Brief  
Stephen A. Schwaner, Alison M. Kight, Robert N. Perry, Marta Pazos, Hongli Yang, Elaine C. Johnson, John C. Morrison, Claude F. Burgoyne and C. Ross Ethier
J Biomech Eng   doi: 10.1115/1.4039998
Glaucoma is the leading cause of irreversible blindness and involves the death of retinal ganglion cells (RGCs). Although biomechanics likely contributes to axonal injury within the optic nerve head (ONH), leading to RGC death, the pathways by which this occurs are not well understood. While rat models of glaucoma are well-suited for mechanistic studies, the anatomy of the rat ONH is different from the human, and the resulting differences in biomechanics have not been characterized. The aim of this study is to describe a methodology for building individual-specific finite element (FE) models of rat optic nerve heads. This method was used to build three rat ONH FE models and compute the biomechanical environment within these ONHs. Initial results show that rat ONH strains are larger and more asymmetric than those seen in human ONH modeling studies. This method provides a framework for building additional models of normotensive and glaucomatous rat ONHs. Comparing model strain patterns with patterns of cellular response seen in studies using rat glaucoma models will help us to learn more about the link between biomechanics and glaucomatous cell death, which in turn may drive development of novel therapies for glaucoma.
TOPICS: Modeling, Biomechanics, Finite element analysis, Finite element model, Patient treatment, Wounds, Anatomy
research-article  
Eoin McEvoy, Gerhard A. Holzapfel and Patrick McGarry
J Biomech Eng   doi: 10.1115/1.4039947
While the anisotropic behaviour of the complex composite myocardial tissue has been well characterized in recent years, the compressibility of the tissue has not been rigorously investigated to date. In the first part of this study we present experimental evidence that passive excised porcine myocardium exhibits volume change. Under tensile loading of a cylindrical specimen, a volume change of 4.1±1.95% is observed at a peak stretch of 1.3. Confined compression experiments also demonstrate significant volume change in the tissue (loading applied up to a volumetric strain of 10%). In order to simulate the multi-axial passive behaviour of the myocardium a nonlinear volumetric hyperelastic component is combined with the well-established Holzapfel-Ogden anisotropic hyperelastic component for myocardium fibres. This framework is shown to describe the experimentally observed behaviour of porcine and human tissues under shear and biaxial loading conditions. In the second part of the study a representative volumetric element (RVE) of myocardium tissue is constructed to parse the contribution of the tissue vasculature to observed volume change under confined compression loading. Simulations of the myocardium microstructure suggest that the vasculature cannot fully account for the experimentally measured volume change. Additionally, the RVE is subjected to six modes of shear loading to investigate the influence of micro-scale fibre alignment and dispersion on tissue-scale mechanical behaviour.
TOPICS: Compressibility, Anisotropy, Modeling, Experimental analysis, Myocardium, Biological tissues, Fibers, Compression, Shear (Mechanics), Simulation, Composite materials, Engineering simulation, Mechanical behavior, Microscale devices
research-article  
Hamza Atcha, Chase Davis, Nicholas R Sullivan, Tim D. Smith, Sara Anis, Waleed Z. Dahbour, Zachery R Robinson, Anna Grosberg and Wendy Liu
J Biomech Eng   doi: 10.1115/1.4039949
Mechanical cues play a critical role in regulating the behavior of many cell types, particularly those that experience substantial mechanical stress within tissues. Devices that impart mechanical stimulation to cells in vitro have been instrumental in helping to develop a better understanding of how cells respond to mechanical forces. However, these devices often have constraints, such as cost and limited functional capabilities, that restrict their use in research or educational environments. Here, we describe a low-cost method to fabricate a uniaxial cell stretcher that would enable widespread use, and facilitate engineering design and mechanobiology education for undergraduate students. The device is capable of producing consistent and reliable strain profiles through the use of a servo motor, gear, and gear rack system. The servo motor can be programmed to output various waveforms at specific frequencies and stretch amplitudes by controlling the degree of rotation, speed, and acceleration of the servo gear. Furthermore, the stretchable membranes are easy to fabricate and can be customized, allowing for greater flexibility in culture well size. We used the custom-built stretching device to uniaxially strain macrophages and cardiomyocytes, and found that both cell types displayed functional and cell shape changes that were consistent with previous studies using commercially available systems. Overall, this uniaxial cell stretcher provides a more cost-effective alternative to study the effects of mechanical stretch on cells, and can therefore be widely used in research and educational environments to broaden the study and pedagogy of cell mechanobiology.
TOPICS: Rotation, Servomechanisms, Stress, Servomotors, Engineering design, Biological tissues, Gears, Membranes, Shapes, Education, Undergraduate students
research-article  
Faisal Ahmed, Marmar Mehrabadi, Zixiang Liu, Gilda Barabino and Dr. Cyrus K Aidun
J Biomech Eng   doi: 10.1115/1.4039897
Cytoplasmic viscosity-dependent margination of red blood cells (RBC) for flow inside micro-channels was studied using numerical simulations and the results were verified with microfluidic experiments. Wide range of suspension volume fractions or hematocrits were considered in this study. Lattice Boltzmann method for fluid phase coupled with Spectrin Link method for RBC membrane deformation was used for accurate analysis of cell margination. RBC margination behavior shows strong dependence on the internal viscosity of the RBCs. At equilibrium, RBCs with higher internal viscosity marginate closer to the channel wall and the RBCs with normal internal viscosity migrate to the central core of the channel. Same margination pattern has been verified through experiments conducted with straight channel microfluidic devices. Segregation between RBCs of different internal viscosity is enhanced as the shear rate and the hematocrit increases. Stronger separation between normal RBCs and RBCs with high internal viscosity is obtained as the width of a high aspect ratio channel is reduced. Overall, the margination behavior of RBCs with different internal viscosities resembles with the margination behavior of RBCs with different levels of deformability. Observations from this work will be useful in designing microfluidic devices for separating the sub-populations of RBCs with different levels of deformability that appear in many hematologic diseases such as sickle cell disease, malaria or cancer.
TOPICS: Viscosity, Microfluidics, Erythrocytes, Diseases, Membranes, Microchannels, Lattice Boltzmann methods, Shear rate, Design, Cancer, Computer simulation, Equilibrium (Physics), Flow (Dynamics), Deformation, Separation (Technology), Fluids
research-article  
Dan Tudor Zaharie and Andrew Phillips
J Biomech Eng   doi: 10.1115/1.4039894
The pelvic construct is an important part of the body as it facilitates the transfer of upper body weight to the lower limb and protects a number of organs and vessels in the lower abdomen. In addition, the importance of the pelvis is highlighted by the large mortality rates associated with pelvic trauma. Although computational models of the pelvis have been used to assess its structure or behaviour under loading, no attempt has been made to develop a model using a structural mechanics approach as opposed to a continuum mechanics approach. This study presents a mesoscale structural model of the pelvic construct and the joints and ligaments associated with it. Shell elements were used to model cortical bone, while truss elements were used to model trabecular bone and the ligaments and joints. The finite element model was subjected to an iterative optimisation process based on a strain driven bone adaptation algorithm. The bone model was adapted to a number of common daily living activities (walking, stair ascent, stair descent, sit-to-stand and stand-to-sit) by applying onto it both joint and muscle loads derived using a musculoskeletal modelling framework. The cortical thickness distribution and trabecular architecture of the adapted model were compared qualitatively with computed tomography scans and models developed in previous studies showing good agreement. The developed structural model enables a number of applications such as fracture modelling, design and additive manufacturing of frangible surrogates.
TOPICS: Weight (Mass), Stairs, Structural analysis, Stress, Trusses (Building), Continuum mechanics, Fracture (Materials), Algorithms, Bone, Design, Fracture (Process), Modeling, Optimization, Structural mechanics, Computerized tomography, Finite element model, Muscle, Shells, Vessels, Additive manufacturing, Musculoskeletal system
research-article  
Ziwen Fang, Allison N Ranslow, Patricia De Tomas, Allan Gunnarsson, Tusit Weerasooriya, Sikhanda Satapathy, Kimberly A. Thompson and Reuben Kraft
J Biomech Eng   doi: 10.1115/1.4039895
The development of a multiaxial failure criterion for trabecular skull bone has many clinical and biological implications. This failure criterion would allow for modeling of bone under daily loading scenarios that typically are multiaxial in nature. Some yield criteria have been developed to evaluate the failure of trabecular bone, but there is a little consensus among them. To help gain deeper understanding of multiaxial failure response of trabecular skull bone, we developed 30 microstructural finite element models of porous porcine skull bone and subjected them to multiaxial displacement loading simulations that spanned three-dimensional stress and strain space. High-resolution micro-computed tomography (microCT) scans of porcine trabecular bone were obtained and used to develop the meshes used for finite element simulations. In total, 376 unique multiaxial loading cases were simulated for each of the 30 microstructure models. Then, results from the total of 11,280 simulations were used to develop three-dimensional yield surfaces in strain space. The resulting yield surfaces capture both the geometric and material nonlinearities in the response that are characteristic of porous trabecular bone. From this study, we characterized the overall response of the microstructural failure response with an equation for a parallelepiped that describes the multiaxial failure behavior of porcine trabecular skull bone very well.
TOPICS: Bone, Engineering simulation, Simulation, Failure, Finite element model, Stress, Resolution (Optics), Finite element analysis, Modeling, Displacement
research-article  
C.P.L. Paul, Kaj S. Emanuel, Idsart Kingma, Albert J. van der Veen, Roderick M. Holewijn, Pieter-Paul A. Vergroesen, Peter M. van de Ven, Margriet G. Mullender, Marco N. Helder and Theodoor H. Smit
J Biomech Eng   doi: 10.1115/1.4039890
Intervertebral disc (IVD) degeneration is described by loss of height and hydration. However, in the first stage of IVD degeneration this loss has not yet occurred. In the current study, we use an ex vivo degeneration model to quantify changes in the IVDs mechanical behavior. We assess whether these changes, characterized by stretched-exponential fitting, qualify as markers for early degeneration. Enzymatic degeneration of healthy lumbar caprine IVDs was induced by injecting 100uL of Chondroïtinase ABC (Cabc) into the nucleus. A no-intervention and PBS injected group were used as controls. IVDs were cultured in a bioreactor for 20 days under diurnal, simulated-physiological loading conditions. Disc deformation was continuously monitored. Changes in disc height recovery behavior were quantified using stretched-exponential fitting. Disc height, histological sections and water- and GAG-content measurements were used as gold standards for the degenerative state. Cabc injection caused significant GAG loss from the nucleus and had detrimental effects on poro-elastic mechanical properties of the IVDs. These were progressive over time, with a propensity towards more linear recovery behavior. On histological sections, both PBS en Cabc injected IVDs showed moderate degeneration. A small GAG loss yields significant changes in IVD recovery behavior, which were quantified with stretched-exponential fit parameters. Studies on early disc degeneration and biomaterial engineering for DDD could benefit from focusing on IVD biomechanical behavior rather than height, as marker for early disc degeneration.
TOPICS: Mechanical behavior, Intervertebral discs, Disks, Fittings, Water, Physiology, Bioreactors, Deformation, Biomaterials, Biomechanics, Mechanical properties
Editorial  
Craig J. Goergen and Corey P. Neu
J Biomech Eng   doi: 10.1115/1.4039879
N/A
TOPICS: Engineering education
research-article  
Amanda Wach, Linda McGrady, Mei Wang and M. Barbara Silver-Thorn
J Biomech Eng   doi: 10.1115/1.4039816
Recent designs of ankle-foot orthoses (AFOs) have been influenced by the increasing demand for higher function from active individuals. The biomechanical function of the individual and device is dependent upon the underlying, and as yet unevaluated, mechanical characteristics of the AFO. Prior mechanical testing of AFOs has primarily focused on rotational stiffness to provide insight into expected functional outcomes; mechanical characteristics pertaining to energy storage and release have not yet been investigated. A pseudo-static bench testing method is introduced to characterize compressive stiffness, device deflection, and motion of AFOs of various designs. AFOs, donned over a surrogate limb, were compressively loaded at different joint angles to simulate the foot-shank orientation during various sub-phases of stance. In addition to force-displacement measurements, motion analysis of each AFO strut, proximal, and supramalleolar segments were analyzed. Although similar compressive stiffness values were observed for AFOs designed to reduce ankle motion, the corresponding strut deflection mechanisms differed based on the respective fabrication material. For example, strut deflection of carbon-fiber AFOs resemble column buckling deflection. Expanded clinical test protocols to include quantification of AFO deflection and rotation during subject use may provide additional insight into design and material effects on performance and functional outcomes, such as energy storage and release and protection of painful or sensitive limb structures. Understanding the behavior of these devices will improve orthotic prescription and future design development.
TOPICS: Mechanical properties, Orthotics, Deflection, Stiffness, Struts (Engineering), Design, Energy storage, Performance, Testing, Buckling, Rotation, Manufacturing, Carbon fibers, Biomechanics, Displacement, Mechanical testing
research-article  
Grant/W Rowlands, Bryan Good, Steven Deutsch and Keefe B. Manning
J Biomech Eng   doi: 10.1115/1.4039822
Ventricular assist devices (VADs) are implanted in patients with a diseased ventricle to maintain peripheral perfusion as a bridge-to-transplant or as destination therapy. However, some patients with continuous flow VADs (eg. HeartMate II (HMII)) have experienced gastrointestinal bleeding, in part caused by the proteolytic cleavage or mechanical destruction of von Willebrand Factor (vWF), a clotting glycoprotein. In vitro studies were performed to measure the flow located within the HMII outlet cannula under both steady and physiological conditions using particle image velocimetry. Under steady flow, a mock flow loop was used with the HMII producing a flow rate of 3.2 L/min. The physiological experiment included a pulsatile pump operated at 105 BPM with a stroke volume of 50 mL and in conjunction with the HMII producing a total flow rate of 5.0 L/min. Velocity fields, Reynolds normal stresses (RNS), and Reynolds shear stresses (RSS) were analyzed to quantify the outlet flow's potential contribution to vWF degradation. Under both flow conditions, the HMII generated principal Reynolds stresses that are, at times, orders of magnitude higher than those needed to unfurl vWF, potentially impacting its physiological function. Under steady flow, principal RNSs were calculated to be approximately 500 Pa in the outlet cannula. Elevated Reynolds stresses were observed throughout every phase of the cardiac cycle under physiological flow with principal RSSs approaching 1500 Pa during peak systole. Prolonged exposure to these conditions may lead to Acquired von Willebrand Syndrome, which is accompanied by uncontrollable bleeding episodes.
TOPICS: Particulate matter, Ventricular assist devices, Outflow, Flow (Dynamics), Physiology, Stress, Pumps, Patient treatment, Cardiac cycle, Shear stress, Bridges (Structures)
research-article  
Rami M A Al-Dirini, Dermot O'Rourke, Daniel Huff, Saulo Martelli and Mark Taylor
J Biomech Eng   doi: 10.1115/1.4039824
Successful designs of total hip replacement (THR) need to be robust to surgical variation in sizing and positioning of the femoral stem. This study presents an automated method for comprehensive evaluation of the potential impact of surgical variability in sizing and positioning on the primary stability of a contemporary cementless femoral stem (Corail®, Depuy Synthes). A patient-specific finite element (FE) model of a femur was generated from computed-tomography (CT) images from a female donor. An automated algorithm was developed to span the plausible surgical envelope of implant positions constrained by the inner cortical boundary. The analysis was performed on four stem sizes: oversized, ideal (nominal) size and undersized by up to two stem sizes. For each size, Latin Hypercube sampling was used to generate models for 100 unique alignment scenarios. For each scenario, peak hip contact and muscle forces published for stair climbing were scaled to the donor's body weight and applied to the model. The risk of implant loosening was assessed by comparing the bone-implant micromotion/strains to thresholds (150µm and 7000µe) above which fibrous tissue is expected to prevail and the peri-prosthetic bone to yield, respectively. The risk of long-term loosening due to adverse bone resorption was assessed using bone adaptation theory. The range of implant positions generated effectively spanned the available intra-cortical space. The Corail® stem was found stable and robust to changes in size and position, with the majority of the bone-implant interface ...
TOPICS: Biomechanics, Surgery, Robustness, Bone, Risk, Finite element analysis, Hip joint prostheses, Artificial limbs, Computerized tomography, Muscle, Algorithms, Biological tissues, Weight (Mass), Stability, Stairs
research-article  
Behzad Vafaeian, Samer Adeeb, Marwan El-Rich, Dornoosh Zonoobi, Abhilash R. Hareendranathan and Jacob L. Jaremko
J Biomech Eng   doi: 10.1115/1.4039827
Developmental dysplasia of the hip (DDH) in infants under 6 months of age is typically treated by the Pavlik harness (PH). During successful PH treatment, a subluxed/dislocated hip is spontaneously reduced into the acetabulum, and DDH undergoes self-correction. PH treatment may fail due to avascular necrosis (AVN) of the femoral head. An improved understanding of mechanical factors accounting for the success/failure of PH treatment may arise from investigating articular cartilage contact pressure (CCP) within a hip during treatment. In this study, CCP in a cartilaginous infant hip was investigated through patient-specific finite element (FE) modeling. We simulated CCP of the hip equilibrated at 90° flexion at abduction angles of 40°, 60° and 80°. We found that CCP was predominantly distributed on the anterior and posterior acetabulum, leaving the superior acetabulum (mainly superolateral) unloaded. From a mechanobiological perspective, hypothesizing that excessive pressure inhibits growth, our results qualitatively predicted increased obliquity and deepening of the acetabulum under such CCP distribution. This is the desired and observed therapeutic effect in successful PH treatment. The results also demonstrated increase in CCP as abduction increased. In particular, the simulation predicted large magnitude and concentrated CCP on the posterior wall of the acetabulum and the adjacent lateral femoral head at extreme abduction (80°). This CCP on lateral femoral head may reduce blood flow in femoral head vessels and contribute to AVN. Hence, this study provides insight into biomechanical factors potentially responsible for PH treatment success and complications.
TOPICS: Pressure, Finite element model, Vessels, Cartilage, Blood flow, Accounting, Simulation, Biomechanics, Finite element analysis, Modeling, Failure
research-article  
Joseph Chen, Bryn Brazile, Raj Prabhu, Sourav Patnaik, Robbin Bertucci, Hongjoo Rhee, Mark F. Horstemeyer, Yi Hong, Lakiesha N. Williams and Jun Liao
J Biomech Eng   doi: 10.1115/1.4039825
In this study, the damage evolution of liver tissue was quantified at the microstructural level under tensile, compression, and shear loading conditions using an interrupted mechanical testing method. To capture the internal microstructural changes in response to global deformation, the tissue samples were loaded to different strain levels and chemically fixed to permanently preserve the deformed tissue geometry. Tissue microstructural alterations were analyzed to quantify the accumulated damages, with damage-related parameters such as number density, area fraction, mean area, and mean Nearest Neighbor Distance (NND). All three loading states showed a unique pattern of damage evolution, in which the damages were found to increase in number and size, but decrease in NND as strain level increased. To validate the observed damage features as true tissue microstructural damages, more samples were loaded to the above-mentioned strain levels and then unloaded back to their reference state, followed by fixation. The most major damage-relevant features at higher strain levels remained after the release of the external loading, indicating the occurrence of permanent inelastic deformation. This study provides a foundation for future structure-based constitutive material modeling that can capture and predict the stress-state dependent damage evolution in liver tissue.
TOPICS: Shear (Mechanics), Biological tissues, Liver, Mechanical testing, Tension, Damage, Compression, Deformation, Stress, Modeling, Geometry, Density
research-article  
Pedro Garcia Carrascal, Javier Garcia, Jose Sierra Pallares, Francisco Castro Ruiz and Fernando Manuel Martín
J Biomech Eng   doi: 10.1115/1.4039676
In-stent restenosis ails many patients who have undergone stenting. When the stented artery is a bifurcation, the intervention is especially critical because of the complex stent geometry involved in these structures. Computational Fluid Dynamics (CFD) has demonstrated to be an effective approach when modelling blood flow behavior and understanding the mechanisms that underlie in-stent restenosis. These CFD models, however, require validation through experimental data in order to be reliable. It is with this purpose in mind that we performed Particle Image Velocimetry (PIV) measurements of velocity fields within flows through a simplified coronary bifurcation. Although the flow in this simplified bifurcation differs from the actual blood flow, it emulates the main fluid dynamic mechanisms found in hemodynamic flow. Experimental measurements for several stenting techniques under steady and unsteady flow conditions were performed. The test conditions were highly controlled and uncertainty was accurately predicted. The results obtained in this research consist of readily accessible, easy to emulate, detailed velocity fields and geometry. These results have been successfully used to validate our numerical model. This data can be used as a benchmark for further development of numerical CFD modelling through comparison of the main characteristics of the flow pattern.
TOPICS: Flow (Dynamics), Computer simulation, Bifurcation, Computational fluid dynamics, stents, Geometry, Modeling, Blood flow, Uncertainty, Fluids, Particulate matter, Hemodynamics, Unsteady flow
Technical Brief  
Ahmed Ramadan, Connor Boss, Jongeun Choi, N. Peter Reeves, Jacek Cholewicki, John Popovich, Jr. and Clark J. Radcliffe
J Biomech Eng   doi: 10.1115/1.4039677
Estimating many parameters of biomechanical systems with limited data may achieve good fit, but there could be multiple parameter solutions that yield very similar model response. This results in lack of identifiability in the estimation problem. Therefore, we propose two systematic methods to select a sensitive parameter subset to be estimated, while fixing the remaining parameters to values obtained from preliminary estimation. The selection approach relies on identifying the parameter subset to which the measurement output is most sensitive. The proposed methods are based on the Fisher Information Matrix (FIM) and the Least Absolute Shrinkage and Selection Operator (LASSO). 95% confidence intervals of the parameter estimates were computed using model-based bootstrap. We presented an application of identifying a parametric model of a head rotational position-tracking test for human subjects. Our methods led to reduced model complexity (less number of parameters to be estimated), narrow confidence intervals of the parameter estimates, and accepted goodness of fit.
TOPICS: Biomechanics, Goodness-of-fit tests, Shrinkage (Materials)
Technical Brief  
James Funk, Roberto Quesada, Alexander Miles and Jeff Crandall
J Biomech Eng   doi: 10.1115/1.4039673
The inertial properties of a helmet play an important role in both athletic performance and head protection. In this study, we measured the inertial properties of 37 football helmets, a NOCSAE size 7 ¼ headform, and a 50th percentile male Hybrid III dummy head. The helmet measurements were taken with the helmets placed on the Hybrid III dummy head. The center of gravity and moment of inertia were measured about 6 axes (x, y, z, xy, yz, and xz), allowing for a complete description of the inertial properties of the head and helmets. Total helmet mass averaged 1834 ± 231 g, split between the shell (1377 ± 200 g) and the facemask (457 ± 101 g). On average, the football helmets weighed 41% ± 5% as much as the Hybrid III dummy head. The center of gravity of the helmeted head was 1.1 ± 3.0 mm anterior and 10.3 ± 1.9 mm superior to the center of gravity of the bare head. The moment of inertia of the helmeted head was approximately 2.2 ± 0.2 times greater than the bare head about all axes.
TOPICS: Inertia (Mechanics), Center of mass, Shells
Technical Brief  
Anup Pant, Syril Dorairaj and Rouzbeh Amini
J Biomech Eng   doi: 10.1115/1.4039679
Quantifying the mechanical properties of the iris is important, as it provides insight into the pathophysiology of glaucoma. Recent ex-vivo studies have shown that the mechanical properties of the iris are different in glaucomatous eyes as compared to normal ones. Notwithstanding the importance of the ex-vivo studies, such measurements are severely limited for diagnosis and preclude development of treatment strategies. With the advent of detailed imaging modalities, it is possible to determine the in-vivo mechanical properties using inverse finite element modeling. An inverse modeling approach requires an appropriate objective function for reliable estimation of parameters. In the case of the iris, numerous measurements such as iris chord length and iris concavity are made routinely in clinical practice. In this study, we have evaluated five different objective functions chosen based on the iris biometrics (in the presence and absence of clinical measurement errors) to determine the appropriate criterion for inverse modeling. Our results showed that in the absence of experimental measurement error, a combination of iris chord length and concavity can be used as the objective function. However, with the addition of measurement errors, the objective functions that employ a large number of local displacement values provide more reliable outcomes.
TOPICS: Finite element analysis, Modeling, Mechanical properties, Errors, Chords (Trusses), Parameter estimation, Imaging, Biometrics, Performance, Displacement

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