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Accepted Manuscripts

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research-article  
Leszek Pstras, Karl Thomaseth, Jacek Waniewski, Italo Balzani and Federico Bellavere
J Biomech Eng   doi: 10.1115/1.4036258
The Valsalva maneuver consisting in a forced expiration against closed airways is one of the most popular clinical tests of the autonomic nervous system function. When properly performed by a healthy subject, it features four characteristic phases of arterial blood pressure and heart rate variations, based on the magnitude of which the autonomic function may be assessed qualitatively and quantitatively. In patients with some disorders or in healthy patients subject to specific conditions, the pattern of blood pressure and heart rate changes during the execution of the Valsalva maneuver may, however, differ from the typical sinusoidal-like pattern. Several types of such abnormal responses are well known and correspond to specific physiological conditions. In this paper we use our earlier mathematical model of the cardiovascular response to the Valsalva maneuver to show that such pathological responses may be simulated by changing individual model parameters or adding new parameters with a clear physiological meaning. The simulation results confirm the adaptability of our model and its usefulness for diagnostic or educational purposes.
TOPICS: Pressure, Blood, Modeling, Cardiovascular system, Hemodynamics, Simulation results, Physiology, Nervous system
research-article  
Andrea R Westervelt, Michael Fernandez, Michael House, Joy Vink, Chia-Ling Nhan-Chang, Ronald Wapner and Kristin M. Myers
J Biomech Eng   doi: 10.1115/1.4036259
Preterm birth is the leading cause of childhood mortality, and can lead to health risks in survivors. The mechanical function of the uterus, fetal membranes, and cervix have dynamic roles to protect the fetus during gestation. To understand their mechanical function and relation to preterm birth, we built a 3-dimensional parameterized finite element model of pregnancy. This model is generated by an automated procedure that is informed by maternal ultrasound measurements. A baseline model at 25 weeks of gestation was characterized, and to visualize the impact of cervical structural parameters on tissue stretch we evaluated the model sensitivity to: (1) anterior uterocervical angle, (2) cervical length, (3) posterior cervical offset, and (4) cervical stiffness. We found that cervical tissue stretching is minimal when the cervical canal is aligned with the longitudinal uterine axis and a softer cervix is more sensitive to changes in the geometric variables tested.
TOPICS: Canals, Ultrasound, Biological tissues, Finite element analysis, Finite element model, Membranes, Stiffness, Ultrasonic measurement, Health risk assessment
research-article  
Pengbo Li, Craig Kreikemeier-Bower, Wanchuan Xie, Vishal Kothari and Benjamin S. Terry
J Biomech Eng   doi: 10.1115/1.4036260
A wireless medical capsule for measuring the contact pressure between a mobile capsule and the small intestine lumen was developed. Two pressure sensors were used to measure and differentiate the contact pressure and the small intestine intraluminal pressure. After in vitro tests of the capsule, it was surgically placed and tested in the proximal small intestine of a pig model. The capsule successfully gathered and transmitted the pressure data to an ex vivo receiver. The measured pressure signals in the animal test were analyzed in the time and frequency domains and a mathematic model was presented to describe the different factors influencing the contact pressure. A novel signal process method was applied to isolate the contraction information from the contact pressure. The result shows the measured contact pressure was 1.08±0.08 kPa, and the small intestine contraction pressure’s amplitude and rate were 0.29±0.046 kPa and 12 min-1. Moreover, the amplitudes and rates of pressure from respiration and heartbeat were also estimated. The successful preliminary evaluation of this capsule implies it could be used in further systematic investigation of small intestine contact pressure on a mobile capsule-shaped bolus.
TOPICS: Pressure, Design, Biomedicine, Signals, Surgery, Pressure sensors
research-article  
Jeffrey M Mattson and Yanhang Zhang
J Biomech Eng   doi: 10.1115/1.4036261
Elastin and collagen fibers are the major load-bearing extracellular matrix (ECM) constituents of the vascular wall. Arteries function differently than veins in the circulatory system, however as a result from several treatment options veins are subjected to sudden elevated arterial pressure. It is thus important to recognize the fundamental structure and function differences between a vein and an artery. Our research compared the relationship between biaxial mechanical function and ECM structure of porcine thoracic aorta and inferior vena cava. Our study suggests that aorta contains slightly more elastin than collagen due to the cyclical extensibility, but vena cava contains almost four times more collagen than elastin to maintain integrity. Furthermore, multiphoton imaging of vena cava showed longitudinally oriented elastin and circumferentially oriented collagen that is recruited at supraphysiologic stress, but low levels of strain. However in aorta, elastin is distributed uniformly and the primarily circumferentially oriented collagen is recruited at higher levels of strain than vena cava. These structural observations support the functional finding that vena cava is highly anisotropic with the longitude being more compliant and the circumference stiffening substantially at low levels of strain. Overall, our research demonstrates that fiber distributions and recruitment should be considered in addition to relative collagen and elastin contents. Also, the importance of accounting for the structural and functional differences between arteries and veins should be taken into account when considering disease treatment options.
TOPICS: Aorta, Fibers, Stress, Anisotropy, Bearings, Cardiovascular system, Diseases, Imaging, Accounting, Pressure
research-article  
Chi Zhu, Jung-Hee Seo, Hani Bakhshaee and Rajat Mittal
J Biomech Eng   doi: 10.1115/1.4036262
A computational framework consisting of a one-way coupled hemodynamic-acoustic method and a wave-decomposition based post-processing approach is developed to investigate the biomechanics of arterial bruits. This framework is then applied to studying the effect of the shear wave on the generation and propagation of bruits from a modeled stenosed artery. The blood flow in the artery is solved by an immersed boundary method (IBM) based incompressible flow solver. The sound generation and propagation in the blood volume is modeled by the linearized perturbed compressible equations, while the sound propagation through the surrounding tissue is modeled by the linear elastic wave equation. A decomposition method is employed to separate the acoustic signal into a compression/longitudinal component (curl free) and a shear/transverse component (divergence free), and the sound signals from cases with and without the shear modulus are monitored on the epidermal surface and are analyzed to reveal the influence of the shear wave. The results show that the compression wave dominates the detected sound signal in the immediate vicinity of the stenosis whereas the shear wave has more influence on surface signals further downstream of the stenosis. The implications of these results on cardiac auscultation are discussed.
TOPICS: Biomechanics, Computational methods, Signals, Shear waves, Waves, Acoustics, Compression, Hemodynamics, Shear modulus, Flow (Dynamics), Elastic waves, Shear (Mechanics), Biological tissues, Blood, Blood flow
research-article  
Ryan D.M. Packett, Philip Brown, Gautam S.S. Popli and F. Scott Gayzik
J Biomech Eng   doi: 10.1115/1.4036215
Tissue cooling is a viable therapy for multiple conditions and injuries, and has been applied to the brain to treat epilepsy and concussions, leading to improved long-term outcomes. To facilitate the study of temperature reduction as a function of various cooling methods, a thermal brain phantom was developed and analyzed. The phantom is composed of a potassium-neutralized, superabsorbent co-polymer hydrogel. The phantom was tested in a series of cooling trials using a cooling block and 37 deg. water representing non-directional blood flow ranging up to 6 GPH, a physiologically representative range based on the prototype volume. Results were compared against a validated finite difference (FD) model. Two sets of parameters were used in the FD model; one set to represent the phantom itself and a second set to represent brain parenchyma. The model was then used to calculate steady state cooling at a depth of 5 mm for all flow rates, for both the phantom and a model of the brain. This effort was undertaken to 1. validate the FD model against the phantom results and 2. evaluate how similar the thermal response of the phantom is to that of a perfused brain. The FD phantom model showed good agreement with the empirical phantom results. Furthermore the empirical phantom agreed with the predicted brain response within 2.5% at physiological flow, suggesting a biofidelic thermal response. The phantom will be used as a platform for future studies of thermally mediated therapies applied to the cerebral cortex.
TOPICS: Cooling, Brain, Phantoms, Patient treatment, Flow (Dynamics), Temperature, Hydrogels, Engineering prototypes, Biological tissues, Performance, Polymers, Potassium, Steady state, Water, Wounds, Physiology, Blood flow
research-article  
Joshua D. Roth, Stephen M. Howell and Maury L. Hull
J Biomech Eng   doi: 10.1115/1.4036147
Previous reports of tibial force sensors have neither characterized nor corrected errors in the computed contact location between the femoral and tibial components in total knee arthroplasty (TKA) that are theoretically caused by the curved articular surface of the tibial component. The objectives were to experimentally characterize there errors and to develop and validate an error correction algorithm. The errors were characterized by calculating the difference between the errors in the computed contact location when forces were applied normal to the tibial articular surface and those when forces were applied normal to the tibial baseplate. The algorithm generated error correction functions to minimize these errors and was validated by determining how much the error correction functions reduced the errors in the computed contact location caused by the curved articular surface. The curved articular surfaces primarily caused bias which ranged from 1.0 to 2.7 mm in regions of high curvature. The error correction functions reduced the bias in these regions to negligible levels ranging from 0.0 to 0.6 mm (p < 0.001). Bias in the computed contact locations caused by the curved articular surface of the tibial insert needs to be accounted for because it may inflate the computed internal-external rotation and anterior-posterior translation of femur on the tibia leading to false identifications of clinically undesirable contact kinematics (e.g. external rotation and anterior translation). Our novel error correction algorithm is an effective method to account for this bias to more accurately compute contact kinematics.
TOPICS: Errors, Knee, Arthroplasty, Algorithms, Kinematics, Rotation, Force sensors
research-article  
Ehsan Ban, Sijia Zhang, Vahhab Zarei, Victor H. Barocas, Beth A. Winkelstein and Catalin R. Picu
J Biomech Eng   doi: 10.1115/1.4036019
The spinal facet capsular ligament (FCL) is primarily comprised of heterogeneous arrangements of collagen fibers. This complex fibrous structure and its evolution under loading play a critical role in determining the mechanical behavior of the FCL. A lack of analytical tools to characterize the spatial anisotropy and heterogeneity of the FCL's microstructure has limited the current understanding of its structure-function relationships. Here, the collagen organization was characterized using spatial correlation analysis of its optically-obtain fiber orientation field. FCLs from the cervical and lumbar spinal regions were characterized in terms of their structure, as was the reorganization of collagen in stretched cervical FCLs. Higher degrees of intra- and inter-sample heterogeneity were found in cervical FCLs than in lumbar specimens. In the cervical FCLs, heterogeneity was manifested in the form of wavy patterns formed by collections of collagen fiber or fiber bundles. Tensile stretch, a common injury mechanism for the cervical FCL, significantly increased the spatial correlation length in the stretch direction, indicating an elongation of the observed structural features. Lastly, an affine estimation for the change of correlation lengths under loading was performed and gave predictions very similar to the actual values. These findings provide structural insights for multiscale mechanical analysis of the FCLs from various spinal regions and also suggest methods for quantitative characterization of complex tissue patterns.
TOPICS: Deformation, Fibers, Anisotropy, Biological tissues, Mechanical behavior, Elongation, Fault current limiters, Injury mechanisms
Technical Brief  
Nachiket M. kharalkar, Steven C. Bauserman and Jonathan W. Valvano
J Biomech Eng   doi: 10.1115/1.4026559
Effect of formalin fixation on thermal conductivity of the biological tissues is presented. A self-heated thermistor probe was used to measure the tissue thermal conductivity. The thermal conductivity of muscle and fatty tissue samples was measured before the formalin fixation and then 27 hours after formalin fixation. The results indicate that the formalin fixation does not cause a significant change in the tissue thermal conductivity of muscle and fatty tissues. In the clinical setting, tissues removed surgically are often fixed in formalin for subsequent pathological analysis. These results suggest that, in terms of thermal properties, it is equally appropriate to perform in vitro studies in either fresh tissue or formalin-fixed tissue.
TOPICS: Thermal conductivity, Biological tissues, Muscle, Probes, Surgery, Thermal properties

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