Accepted Manuscripts

Mattia Lui, Savino Martino, Mario Salerno and Maurizio Quadrio
J Biomech Eng   doi: 10.1115/1.4044029
The blood flow dynamics in a stenosed, subject-specific carotid bifurcation is numerically simulated using Direct Numerical Simulation (DNS) and Reynolds-averaged Navier--Stokes (RANS) equations closed with turbulence models. The former is meant to provide a term of comparison for the RANS calculations, that include classic two-equations models ($k-\epsilon$ and $k-\omega$) as well as a transitional three-equations eddy-viscosity model ($k_T-k_L-\omega$). Pulsatile inlet conditions based on in-vivo ultrasound measurements of blood velocity are used. The blood is modelled as a Newtonian fluid, and the vessel walls are rigid. The main purpose of this work is to highlight the problems related to the use of classic RANS models in the numerical simulation of such flows. The time-averaged DNS results, interpreted in view of their finite-time averaging error, are used to demonstrate the superiority of the transitional RANS model, which is found to provide results closer to DNS than those of conventional models. The transitional model is shown to possess better predictive capabilities in terms of turbulence intensity, temporal evolution of the pressure along the cardiac cycle, and the oscillatory shear index (OSI). Indeed, DNS brings to light the locally transitional or weakly turbulent state of the blood flow, which presents velocity and pressure fluctuations only in the post-stenotic region of the internal carotid artery during systole, while the flow is laminar during diastole.
TOPICS: Turbulence, Modeling, Vessels, Blood flow, Reynolds-averaged Navier–Stokes equations, Pressure, Flow (Dynamics), Computer simulation, Blood, Dynamics (Mechanics), Fluctuations (Physics), Shear (Mechanics), Fluids, Eddies (Fluid dynamics), Viscosity, Ultrasonic measurement, Bifurcation, Errors, Cardiac cycle, Carotid arteries
Ana C. Estrada, Kyoko Yoshida, Samantha A. Clarke and Jeffrey W. Holmes
J Biomech Eng   doi: 10.1115/1.4044030
A wide range of emerging therapies from surgical restraint to biomaterial injection to tissue engineering aim to improve heart function and limit adverse remodeling following myocardial infarction (MI). We previously showed that longitudinal surgical reinforcement of large anterior infarcts in dogs could significantly enhance systolic function without restricting diastolic function, but the underlying mechanisms for this improvement are poorly understood. The goal of this study was to construct a finite-element model that could match our previously published data on changes in regional strains and left ventricular function following longitudinal surgical reinforcement, then use the model to explore potential mechanisms for the improvement in systolic function we observed. The model presented here, implemented in FEBio, matches all the key features of our experiments, including diastolic remodeling strains in the ischemic region, small shifts in the end-diastolic pressure-volume relationship (EDPVR), and large changes in the end-systolic pressure-volume relationship (ESPVR) in response to ischemia and to patch application. Detailed examination of model strains and stresses suggests that longitudinal reinforcement reduces peak diastolic fiber stretch and systolic fiber stress in the remote myocardium and shifts those peaks away from the endocardial surface by reshaping the left ventricle (LV). These finding could help guide the development of novel therapies to improve post-MI function by providing specific design objectives.
TOPICS: Stress, Surgery, Patient treatment, Pressure, Fibers, Biomaterials, Design, Tissue engineering, Myocardium, Finite element model
Ifaz T. Haider, Michael Baggaley and W. Brent Edwards
J Biomech Eng   doi: 10.1115/1.4044034
Understanding the structural response of bone during locomotion may help understand the etiology of stress fracture. This can be done in a subject-specific manner using finite element (FE) modelling, but care is needed to ensure that modelling assumptions reflect the in-vivo environment. Here, we explored the influence of loading and boundary conditions (BC), and compared predictions to previous in-vivo measurements. Data were collected from a female participant who walked/ran on an instrumented treadmill while motion data were captured. Inverse dynamics of the leg (foot, shank and thigh segments) was combined with a musculoskeletal (MSK) model to estimate muscle and joint contact forces. These forces were applied to an FE model of the tibia, generated from computed tomography. Eight conditions varying loading/BCs were investigated. We found that modelling the fibula was necessary to predict realistic tibia bending. Applying joint moments from the MSK model to the FE model was also needed to predict torsional deformation. During walking, the most complex model predicted deformation of 0.5° posterior, 0.8° medial, and 1.4° internal rotation, comparable to in-vivo measurements of 0.5°-1°, 0.15°-0.7°, and 0.75°-2.2°, respectively. During running, predicted deformations of 0.3° posterior, 0.3° medial, and 0.5° internal rotation somewhat underestimated in-vivo measures of 0.85°-1.9°, 0.3°-0.9°, 0.65°-1.72°, respectively. Overall, these models may be sufficiently realistic to be used in future investigations of tibial stress fracture.
TOPICS: Deformation, Boundary-value problems, Finite element model, Modeling, Fracture (Process), Rotation, Stress, Fracture (Materials), Bone, Finite element analysis, Dynamics (Mechanics), Muscle, Musculoskeletal system, Computerized tomography
Technical Brief  
Hyeon Lee, William D. Campbell, Kelcie M. Theis, Margaret E. Canning, Hannah Y. Ennis, Robert L. Jackson and R. Reid Hanson
J Biomech Eng   doi: 10.1115/1.4044031
Fresh and frozen cartilage samples of the fetlock, carpus, and stifle were collected from 12 deceased horses. Half were measured immediately following extraction, and half were frozen for seven days and then measured. Seven indentations (various normalized displacements) were implemented with an indention rate of 0.1mm/s. Solid phase aggregate modulus (Ea), hyperelastic material constant (a), and fluid load fraction (F') of equine articular cartilage were assessed using the Ogden hyperelastic model. The properties were statistically compared in various joints (fetlock, carpus, and stifle), and between fresh and frozen samples using various statistical models. There was no statistical difference between the fetlock and carpus in the aggregate modulus (p=0.5084), while both were significantly different from the stifle (fetlock: p=0.0017 and carpus: p=0.0406). For the hyperelastic material constant, no statistical differences between joints were observed (p=0.3310). For the fluid load fraction, the fetlock and stifle comparison showed a difference (p=0.0333), while the carpus was not different from the fetlock (p=0.1563) or stifle (p=0.3862). Comparison between the fresh and frozen articular cartilage demonstrated no significant difference among the joints in the three material properties: p=0.9418, p=0.7031, and p=0.9313 for the aggregate modulus, the hyperelastic material constant, and the fluid load fraction, respectively.
TOPICS: Cartilage, Fluids, Stress, Materials properties
Benjamin C. Marchi, Ellen M. Arruda and Rhima Coleman
J Biomech Eng   doi: 10.1115/1.4044032
Articular cartilage focal defects are common soft tissue injuries potentially linked to osteoarthritis development. Although several defect characteristics likely contribute to osteoarthritis, their relationship to local tissue deformation remains unclear. Using finite element models with various femoral cartilage geometries, we explore how defects change cartilage deformation and joint kinematics assuming loading representative of the maximum joint compression during the stance phase of gait. We show how defects, in combination with location-dependent cartilage mechanics, alter deformation in affected and opposing cartilages, as well as joint kinematics. Small and average sized defects increased maximum compressive strains by approximately 50% and 100%, respectively, compared to healthy cartilage. Shifts in the spatial locations of maximum compressive strains of defect containing models were also observed, resulting in loading of cartilage regions with reduced initial stiffnesses supporting the new, elevated loading environments. Simulated osteoarthritis (modeled as a global reduction in mean cartilage stiffness) did not significantly alter joint kinematics, but exacerbated tissue deformation. Femoral defects were also found to affect healthy tibial cartilage deformations. Lateral femoral defects increased tibial cartilage maximum compressive strains by 25%, while small and average sized medial defects exhibited decreases of 6% and 15%, respectively, compared to healthy cartilage. Femoral defects also affected the spatial distributions of deformation across the articular surfaces. These deviations are especially meaningful in the context of cartilage with location-dependent mechanics, leading to increases in peak contact stresses supported by the cartilage of between 11-34% over healthy cartilage.
TOPICS: Biomechanics, Cartilage, Knee, Deformation, Osteoarthritis, Kinematics, Biological tissues, Compression, Finite element model, Stiffness, Wounds, Soft tissues, Stress
Zheng Li, Ye Chen, Siyuan Chang and Haoxiang Luo
J Biomech Eng   doi: 10.1115/1.4044033
We present a novel reduced-order glottal airflow model that can be coupled with the three-dimensional (3D) solid mechanics model of the vocal fold tissue to simulate the fluid-structure interaction (FSI) during voice production. This type of hybrid FSI models have potential applications in the estimation of the tissue properties that are unknown due to patient variations and/or neuromuscular activities. In this work, the flow is simplified to a one-dimensional (1D) momentum equation based model incorporating the entrance effect and energy loss in the glottis. The performance of the flow model is assessed using a simplified yet 3D vocal fold configuration. We use the immersed-boundary method based 3D FSI simulation as a benchmark to evaluate the momentum based model as well as the Bernoulli-based 1D flow models. The results show that the new model has significantly better performance than the Bernoulli models in terms of prediction for the vocal fold vibration frequency, amplitude, and phase delay. Furthermore, the comparison results are consistent for different medial thicknesses of the vocal fold, subglottal pressures, and tissue material behaviors, indicating that the new model has better robustness than previous reduced-order models.
TOPICS: Flow (Dynamics), Simulation, Vibration, Fluid structure interaction, Vocal cords, Biological tissues, Momentum, Solid mechanics, Delays, Robustness, Oscillating frequencies, Energy dissipation, Air flow
Daisuke Yoshino and Masaaki Sato
J Biomech Eng   doi: 10.1115/1.4044046
Blood pressure is important factor in both maintaining body homeostasis as well as in its disruption. Vascular endothelial cells (ECs) are exposed to varying degrees of blood pressure, and therefore play an important role in these physiological and pathological events. However, the effect of blood pressure on endothelial cell functions remains to be elucidated. In particular, we do not know how ECs sense and respond to changes in hydrostatic pressure. Here, we found that exposure to hydrostatic pressure causes an early actomyosin-mediated contraction of ECs without a change in cell morphology. This response could be caused by water efflux from the ECs following exposure to hydrostatic pressure. Although only a limited study, these findings do explain a part of the mechanism through which ECs sense and respond to pressure.
TOPICS: Dynamics (Mechanics), Pressure, Endothelial cells, Hydrostatic pressure, Blood, Water, Physiology
Lei Shi, Wang Yao, Yu Gan, Lily Zhao, William Eugene McKee, Joy Vink, Ronanld Wapner, Christine Hendon and Kristin Myers
J Biomech Eng   doi: 10.1115/1.4043977
The cervix is essential to a healthy pregnancy as it must bear the increasing load caused by the growing fetus. Preterm birth is suspected to be caused by the premature softening and mechanical failure of the cervix. The objective of this paper is to measure the anisotropic mechanical properties of human cervical tissue using indentation and video extensometry. The human cervix is a layered structure, where its thick stromal core contains preferentially aligned collagen fibers embedded in a soft ground substance. The fiber composite nature of the tissue provides resistance to the complex 3-dimensional loading environment of pregnancy. In this work, we detail an indentation mechanical test to obtain the force and deformation response during loading which closely matches in vivo conditions. We postulate a constitutive material model to describe the equilibrium material behavior to ramp-hold indentation, and we use an inverse finite element method based on genetic algorithm optimization to determine best-fit material parameters. We report the material properties of human cervical slices taken at different anatomical locations from women of different obstetric backgrounds. In this cohort of patients, the anterior internal os (the area where the cervix meets the uterus) of the cervix is stiffer than the anterior external os (the area closest to the vagina). The anatomic anterior and posterior quadrants of cervical tissue are more anisotropic than the left and right quadrants. There is no significant difference in material properties between samples of different parities (number of pregnancies reaching viable gestation age).
TOPICS: Anisotropy, Biological tissues, Materials properties, Fibers, Stress, Deformation, Composite materials, Mechanical properties, Equilibrium (Physics), Finite element methods, Optimization, Failure, Genetic algorithms, Mechanical testing
Danè Dabirrahmani, Desmond Bokor, Thomas Tarento, Shahrulazua Ahmad and Richard Appleyard
J Biomech Eng   doi: 10.1115/1.4043969
As the use of glenoid suture anchors in arthroscopic and open reconstruction, for instability after Bankart lesions of the shoulder, increases, an emerging problem has been the incidence of glenoid rim fractures through suture drill holes. Very little is known regarding the effect of the Hill-Sachs lesion on the glenoid's susceptibility to fracture and how drill hole location can further affect this. This study used finite element modelling techniques to investigate the risk of fracture of the glenoid rim in relation to variable sized Hill-Sachs defects impacting on the anterior glenoid edge with suture anchor holes placed in varying positions. The distribution of Von Mises (VM) stresses and the Factor of Safety (FOS) for each of the configurations were calculated. The greatest peak in von Mises (VM) stresses was generated when the glenoid was loaded with a small Hill-Sachs lesion. The VM stresses were lessened and the FOS increased (reducing likelihood of failure) with increasing size of the Hill-Sachs lesion. Placement of the suture drill holes at 2mm from the glenoid rim showed the highest risk of failure; and when combined with a medium sized Hill-Sachs lesion, which matched the central line of the drill holes, a potentially clinically significant configuration was presented. The results of this study are useful in assisting the surgeon in understanding the interaction between the Hill-Sachs lesion size and the placement of suture anchors with the purpose of minimising the risk of subsequent rim fracture with new injury.
TOPICS: Maintenance, Fracture (Materials), Fracture (Process), Risk, Drills (Tools), Stress, Failure, Wounds, Arthroscopy, Modeling, Safety engineering, Finite element analysis
Review Article  
Olufunmilayo Adebayo, Derek Holyoak and Marjolein C H van der Meulen
J Biomech Eng   doi: 10.1115/1.4043970
Osteoarthritis (OA) is a degenerative joint disease that affects millions of people worldwide, yet its disease mechanism is not clearly understood. Animal models have been established to study disease progression by initiating OA through modified joint mechanics or altered biological activity within the joint. However, animal models often do not have the capability to directly relate the mechanical environment to joint damage. This review focuses on a novel in vivo approach based on controlled, cyclic tibial compression to induce OA in the mouse knee. First, we discuss the development of the load-induced OA model, its different loading configurations, and other techniques used by research laboratories around the world. Next, we review the lessons learned regarding the mechanobiological mechanisms of load-induced OA and relate these findings to the current understanding of the disease. Then, we discuss the role of specific genetic and cellular pathways involved in load-induced OA progression and the contribution of altered tissue properties to the joint response to mechanical loading. Finally, we propose using this approach to test the therapeutic efficacy of novel treatment strategies for OA. Ultimately, elucidating the mechanobiological mechanisms of load-induced OA will aid in developing targeted treatments for this disabling disease.
TOPICS: Stress, Osteoarthritis, Diseases, Joint mechanics, Knee, Biological tissues, Compression, Damage
Omid Amili, Robroy MacIver and Filippo Coletti
J Biomech Eng   doi: 10.1115/1.4043939
This study explores the optimal LVAD cannula outflow configuration in a patient-specific replica of the aorta. The volumetric velocity field is measured using phase-contrast magnetic resonance imaging under a physiologically relevant steady flow. The effect of the LVAD outflow graft insertion site and anastomosis angle on the transport of embolic particles to cranial vessels is studied by solving the particle equation of motion for spheres in the range of 0.1-1.0 mm using the measured 3D velocity field. Results show that for a given aorta anatomy, it is possible to design the cannula graft location and terminal curvature so that the probability of embolic transport to the cranial vessels is significantly minimized. This is particularly important since the complex flow pattern in each cannula case affects the embolic trajectories differently, and hence the common assumption that particles distribute by the volumetric flow division does not hold.
TOPICS: Flow (Dynamics), Particulate matter, Ventricular assist devices, Magnetic resonance imaging, Vessels, Aorta, Outflow, Anatomy, Equations of motion, Design, Probability
Yonghui Qiao, Jianren Fan, Ying Ding, Professor Kun Luo and Ting Zhu
J Biomech Eng   doi: 10.1115/1.4043881
The impact of left subclavian artery (LSA) coverage during thoracic endovascular aortic repair (TEVAR) on the circulatory system is not fully understood. Here we coupled single-phase non-Newtonian model and fluid-structure interaction (FSI) technique to simulate blood flow in an acute type B aortic dissection. Additionally, three-element Windkessel model was implemented to reproduce physiological pressure waves, where a new workflow was used to determine model parameters with the absence of measured data. Simulations were carried out in three models to demonstrate the efficacy of TEVAR with the LSA coverage; case A: pre-TEVAR aorta; case B: post-TEVAR aorta with the disappearance of LSA; case C: post-TEVAR aorta with virtually adding LSA. Results show that the blood flow through the compressed true lumen is only 8.43%, which may lead to ischemia in related organs. After TEVAR, the wall pressure on the stented segment increases and blood flow in the supra-aortic branches and true lumen is improved. Meantime, the average deformation of the aorta is significantly reduced due to the implantation of the stent-graft. After virtually adding LSA, significant changes in the distribution of blood flow and two indices based on wall shear stress are observed. Moreover, the movement of residual false lumen becomes stable which could contribute to patient recovery. Overall, this study quantitatively evaluates the efficacy of TEVAR for acute type B aortic dissection and demonstrates that the coverage of LSA has a considerable impact on the important hemodynamic parameters.
TOPICS: Pressure, Deformation, Maintenance, Simulation, Waves, Computational fluid dynamics, Engineering simulation, Cardiovascular system, Hemodynamics, stents, Workflow, Fluid structure interaction, Physiology, Aorta, Blood flow, Shear stress
Technical Brief  
Kevin Sunderland, Qinghai Huang, Charlie Strother and Jingfeng Jiang
J Biomech Eng   doi: 10.1115/1.4043868
The objective of this study was to use image-based CFD simulation techniques to analyze the impact that multiple closely spaced IAs of the supra-clinioid segment of the ICA have on each other's hemodynamic characteristics. The vascular geometry of fifteen (15) subjects with 2 IAs were gathered using a 3D clinical system. Two groups of computer models were created for each subject's vascular geometry: both IAs present (Model A) and after removal of one IA (Model B). Models were separated into two groups based on IA separation: tandem (one proximal and one distal) and tandem (aneurysms directly opposite on a vessel). Simulations using a pulsatile velocity waveform were solved by a commercial CFD solver. Proximal IAs altered flow into distal IAs (5 of 7), increasing flow energy and spatial-temporally averaged wall shear stress (STA-WSS: 3-50\% comparing Model A to B) while decreasing flow stability within distal IAs. Thus, proximal IAs may ``protect" a distal aneurysm from destructive remodeling due to flow stagnation. Among adjacent IAs, the presence of both IAs decreased each other's flow characteristics, lowering WSS (Model A to B) and increasing flow stability: all changes statistically significant (t-test p < 0.05). A negative relationship exists between the mean percent change in flow stability in relation to adjacent IA volume and ostium area. Closely spaced IAs impact hemodynamic alterations onto each other concerning flow energy, stressors and stability. Understanding these alterations may improve clinical management of closely-spaced IAs.
TOPICS: Aneurysms, Carotid arteries, Flow (Dynamics), Stability, Simulation, Computational fluid dynamics, Geometry, Hemodynamics, Vessels, Shear stress, Computers, Separation (Technology)
Amy R Lewis, Will Robertson, Elissa J Phillips, Paul N Grimshaw and Marc Portus
J Biomech Eng   doi: 10.1115/1.4043869
The anthropometries of elite wheelchair racing athletes differ to the generic, able-bodied anthropometries commonly used in computational biomechanical simulations. The impact of using able-bodied parameters on the accuracy of simulations involving wheelchair racing is currently unknown. In this study, athlete-specific mass segment inertial parameters of five elite wheelchair athletes were calculated using dual-energy X-ray absorptiometry scans. These were compared against commonly used anthropometrics parameters of data presented in the literature. A computational biomechanical simulation of wheelchair propulsion assessed the sensitivity of athlete-specific mass parameters using Kruskal-Wallis analysis, Mann-Whitney U analysis and Spearman correlations. Substantial between-athlete body mass distribution variances (thigh mass < 14.6% total body mass), and between-limb asymmetries (<62.4%; 3.1 kg) were observed. Compared to non-athletic able-bodied anthropometric data, wheelchair racing athletes demonstrated greater mass in the upper extremities (up to 3.8% total body mass), and less in the lower extremities (up to 9.8% total body mass). Computational simulations were sensitive to individual body mass distribution, with simulation outputs increasing by up to 12.5% when measured segment masses were 14.3% greater than the generic counterpart. These data suggest non-athletic, able-bodied mass segment inertial parameters are inappropriate for analysing elite wheelchair racing motion.
TOPICS: Simulation, Biomechanics, Wheelchairs, Propulsion, X-rays
Jagjit Singh, Nitin Kumar Sharma, MD. Sarker, Saman Naghieh, Satbir Sehgal and Daniel X.B. Chen
J Biomech Eng   doi: 10.1115/1.4043870
The fracture properties of cortical bone are directly coupled to its complex hierarchical structure. The limited availability of bone material from many anatomic locations creates challenges for assessing the effect of bone heterogeneity and anisotropy on fracture properties. The small punch technique (SPT) was employed to examine the fracture behavior of cortical bone in terms of area under the curve values obtained from load-load point displacement behavior. Fracture toughness of cortical bone was also determined in terms of J-toughness values obtained using a compact tension (CT) test. Area under the curve values obtained from the small punch test were correlated with the J-toughness values of cortical bone. The effects of bone density and compositional parameters on area under the curve and J-toughness values were also analyzed using linear and multiple regression analysis. Area under the curve and J-toughness values are strongly and positively correlated. Bone density and %mineral content are positively correlated with both area under the curve and J-toughness values. The multiple regression analysis outcomes support these results. Overall, the findings support the hypothesis that area under the curve values obtained from small punch tests can be used to assess the fracture behavior of cortical bone.
TOPICS: Fracture (Materials), Bone, Fracture (Process), Testing, Fracture toughness, Regression analysis, Density, Stress, Anisotropy, Displacement, Performance, Tension
Amy A. Claeson, Edward J. Vresilovic, Brent L. Showalter, Alexander C. Wright, James C Gee, Neil R. Malhotra and Dawn M. Elliott
J Biomech Eng   doi: 10.1115/1.4043874
Nucleotomy is a common surgical procedure and is also performed in ex vivo mechanical testing to model decreased nucleus pulposus (NP) pressurization that occurs with degeneration. Here, we utilize magnetic resonance imaging (MRI) to study internal 3D annulus fibrosus (AF) deformations after partial nucleotomy and during axial compression by evaluating changes in internal AF deformation at reference loads (50N) and physiological compressive loads (~10% strain). Intact grade II L3-L4 discs before and after nucleotomy were subjected to identical mechanical testing and imaging protocols. Internal disc deformation fields were calculated by registering MR images captured in each loading state (reference and compressed) and each condition (intact and nucleotomy). Comparisons were drawn between the resulting three deformation states (intact at compressed load, nucleotomy at reference load, nucleotomy at compressed load) with regards to the magnitude of internal strain and direction of internal displacements. Under compressed load, internal AF axial strains averaged -18.5% when intact and -22.5% after nucleotomy. Deformations of intact discs under compressed load oriented in-plane, whereas deformations after nucleotomy oriented axially. For intact discs, in-plane components of displacements under compression loads were oriented radially outward and circumferentially. After nucleotomy, in-plane displacements oriented radially inward under reference load and were not significantly different from the intact state at compressed loads. Re-establishment of outward displacements after nucleotomy indicates increased axial loading restores the characteristics of internal pressurization. Results may have implications for the recurrence of pain, design of novel therapeutics, or progression of disc degeneration.
TOPICS: Disks, Annulus, Stress, Deformation, Magnetic resonance imaging, Compression, Mechanical testing, Imaging, Physiology, Design, Surgery
R. Matthew Miller, James R Thunes, Volker Musahl, Spandan Maiti and Richard E. Debski
J Biomech Eng   doi: 10.1115/1.4043872
Rotator cuff tears are a significant clinical problem previously investigated by unvalidated computational models that either use simplified geometry or isotropic elastic material properties to represent the tendon. The objective of this study was to develop an experimentally validated, finite element model of supraspinatus tendon using specimen-specific geometry and inhomogeneous material properties to predict strains in intact supraspinatus tendon. Three-dimensional tendon surface strains were determined at 60°, 70°, and 90° of glenohumeral abduction for articular and bursal surfaces of supraspinatus tendon during cyclic loading to serve as validation data. A finite element model was developed using the tendon geometry and inhomogeneous material properties to predict surface strains for loading conditions mimicking experimental loading conditions. Experimental strains were directly compared with computational model predictions to validate the model. Overall, the model successfully predicted magnitudes of strains that were within the experimental repeatability of 3% strain of experimental measures on both surfaces of the tendon. Model predictions and experiments showed the largest strains to be located on the articular surface (~8% strain) between the middle and anterior edge of the tendon. Importantly, the reference configuration chosen to calculate strains had a significant effect on strain calculations, and therefore must be defined with an innovative optimization algorithm. This study establishes a rigorously validated, specimen-specific computational model using novel surface strain measurements for use in investigating the function of the supraspinatus tendon and to ultimately predict the propagation of supraspinatus tendon tears based on the tendon's mechanical environment.
TOPICS: Finite element model, Tendons, Materials properties, Geometry, Optimization algorithms, Strain measurement
Mehdi Ramezanpour, Farhad Rikhtegar Nezami, Nahid Ramezanpour, Foad Kabinejadian, Mehdi Maerefat, Gerhard A. Holzapfel and Joseph Bull
J Biomech Eng   doi: 10.1115/1.4043873
Compliance mismatch between the graft and the host artery of an end-to-side (ETS) arterial bypass graft increases the intramural stress in the ETS graft-artery junction, and thus may compromise its long-term patency. The present study takes into account the effects of collagen fibers to demonstrate how their orientations alter the stresses. The stresses in a bypass graft, as a man-made bifurcation, are compared to those of its natural counterparts with different fiber orientations. The results indicate that the fiber orientation mismatch between the graft and the host artery may increase the stresses at both the heel and toe regions of the ETS anastomosis (the maximum principal stress at the heel and toe regions increased by 72 and 12%, respectively). Our observations, thus, propose that the mismatch between the collagen fiber orientations of the graft and the host artery, independent of the effect of the suture line, may induce aberrant stresses to the anastomosis of the bypass graft.
TOPICS: Fibers, Stress, Bifurcation, Junctions, Vessels
Dimitrios P. Sokolis, Andreas Bompas, Stavroula Papadodima and Stavros K. Kourkoulis
J Biomech Eng   doi: 10.1115/1.4043877
Our understanding of aortic biomechanics is customarily limited by lack of information on the axial residual stretches of the vessel in both humans and experimental animals that would facilitate the identification of its actual zero-stress state. The aim of this study was thus to acquire hitherto unreported quantitative knowledge of axial opening angle and residual stretches in different segments and quadrants of the human aorta according to age and gender. 23 aortas were harvested during autopsy from the aortic root to the iliac bifurcation and were divided into =12 segments and 4 quadrants. Morphometric measurements were taken in the excised/curled configuration of rectangular strips considered to be under zero-stress using image-analysis software to study the axial/circumferential variation of axial opening angle, internal/external residual stretch, and thickness of the aortic wall. The measured data demonstrated: (1) an axial opening angle peak at the arch branches, decreasing towards the ascending and to a near-constant value in the descending thoracic aorta, and increasing in the abdominal aorta; (2) the variation of residual stretches resembled that of opening angle, but axial differences in external residual stretch were more prominent; (3) wall thickness showed a progressive diminution along the vessel; (4) the highest opening angle/residual stretches were found in the inner quadrant and the lowest in the outer quadrant; (5) the anterior was the thinnest quadrant throughout the aorta; (6) age caused thickening but greatly reduced axial opening angle/residual stretches, without differences between males and females.
TOPICS: Aorta, Vessels, Stress, Biomechanics, Bifurcation, Computer software, Strips, Wall thickness, Arches
Technical Brief  
Thien-Khoi N. Phung, Christopher Waters and Jeffrey W. Holmes
J Biomech Eng   doi: 10.1115/1.4043876
Creating patient-specific models of the heart is a promising approach for predicting outcomes in response to congenital malformations, injury, or disease, as well as an important tool for developing and customizing therapies. However, integrating multi-modal imaging data to construct patient-specific models is a non-trivial task. Here, we propose an approach that employs a prolate spheroidal coordinate system to interpolate information from multiple imaging datasets and map those data onto a single geometric model of the left ventricle. We demonstrate the mapping of the location and transmural extent of post-infarction scar segmented from late gadolinium enhancement (LGE) MRI, as well as mechanical activation calculated from displacement encoding with stimulated echoes (DENSE) MRI. As a supplement to this article, we provide MATLAB and Python versions of the routines employed here for download from SimTK (https://simtk.org/projects/lvdatamap).
TOPICS: Magnetic resonance imaging, Imaging, Echoes, Performance, Diseases, Displacement, Encryption, Gadolinium, Matlab, Patient treatment, Wounds

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