bluuebunny/crossref_metadata_2025 huggingface dataset:
Numerical Simulation for the Propagation of Nonlinear Pulsatile Waves in Arteries
Authors: Ma X. , Lee G. C. , Wu S. G.
Published: March 2008

The behavior of nonlinear pulsatile flow of incompressible blood contained in an elastic tube is examined. The theory takes into account the nonlinear convective terms of the Navier-Stokes equations. The motion of the arterial wall is characterized by a set of linearized differential equations. The region bounded by the flexible arterial wall is mapped into a fixed area in which numerical discretization takes place. The finite element method (Galerkin weighted residual approach) is used for the solution of this nonlinear system. The results obtained are pressure distribution, velocity profile, flow rate and wall displacements along the elastic tube (20 cm long).
Numerical Simulation of Steady Flow in a Two-Dimensional Total Artificial Heart Model
Authors: Kim S. H. , Chandran K. B. , Chen C. J.
Published: March 2008

In this paper, a numerical simulation of steady laminar and turbulent flow in a twodimensional model for the total artificial heart is presented. A trileaflet polyurethane valve was simulated at the outflow orifice while the inflow orifice had a trileaflet or a flap valve. The finite analytic numerical method was employed to obtain solutions to the governing equations in the Cartesian coordinates. The closure for turbulence model was achieved by employing the k-ε-E model. The SIMPLER algorithm was used to solve the problem in primitive variables. The numerical solutions of the simulated model show that regions of relative stasis and trapped vortices were smaller within the ventricular chamber with the flap valve at the inflow orifice than that with the trileaflet valve. The predicted Reynolds stresses distal to the inflow valve within the ventricular chamber were also found to be smaller with the flap valve than with the trileaflet valve. These results also suggest a correlation between high turbulent stresses and the presence of thrombus in the vicinity of the valves in the total artificial hearts. The computed velocity vectors and turbulent stresses were comparable with previously reported in vitro measurements in artificial heart chambers. Analysis of the numerical solutions suggests that geometries similar to the flap valve (or a tilting disk valve) results in a better flow dynamics within the total artificial heart chamber compared to a trileaflet valve.
Measurement of Steady-Flow Instability and Turbulence Levels in Dacron Vascular Grafts
Authors: Shombert D. G.
Published: March 2008

Fluid dynamic properties of Dacron vascular grafts were studied under controlled steady-flow conditions over a Reynolds number range of 800 to 4500. Knitted and woven grafts having nominal diameters of 6 mm and 10 mm were studied. Thermal anemometry was used to measure centerline velocity at the downstream end of the graft; pressure drop across the graft was also measured. Transition from laminar flow to turbulent flow was observed, and turbulence intensity and turbulent stresses (Reynolds normal stresses) were measured in the turbulent regime. Knitted grafts were found to have greater pressure drop than the woven grafts, and one sample was found to have a critical Reynolds number (Rc) of less than one-half the value of Rc for a smooth-walled tube.
On the Subclavian Steal Syndrome In Vitro Studies
Authors: Rodkiewicz C. M. , Centkowski J. , Zajac S.
Published: March 2008

An elastic model of the arterial system has been used in which a specially designed pumping unit simulated the heart action. Physiological pressures and normal geometry, size, and flow distribution together with the normal cardiac output and use of prosthetic heart valves are the features of the system. Atherosclerosis was simulated by introducing blockages of known cross-section at specific sites of predilection. It has been shown that, for some specific occlusion magnitude in the left or right subclavian, or in the brachycephalic arteries, the stagnant no blood flow condition will appear in the left vertebral, or the right vertebral, or right common carotid, or the right internal carotid arteries. For larger occlusions the blood flow in these arteries reverses its direction, i.e., the “steal syndrome” appears. It is shown that besides the known single steal syndrome there exists also a double steal syndrome, i.e., blood reverses its flow direction simultaneously in two arteries, both on the right side of the arterial system. This blood is taken from the circle of Willis, which at the same time is significantly supplemented by the increased blood flow through the other arteries leading into the circle of Willis.
A Model for Estimating Corneal Stiffness Using an Indenter
Authors: Carnell Peter H. , Vito Raymond P.
Published: March 2008

A finite element model that simulates indentation of the cornea was generated to examine the feasibility of using indentation data to determine mechanical properties. A layered model which included geometric nonlinearities was necessary to characterize the indentation process. Results indicate that a secant modulus could be determined by measuring indenter force and contact area.
Cellular Response of Mouse Oocytes to Freezing Stress: Prediction of Intracellular Ice Formation
Authors: Toner M. , Cravalho E. G. , Karel M.
Published: March 2008

Successful protocols for cryopreservation of living cells can be designed if the physicochemical conditions to preclude intracellular ice formation (IIF) can be defined. Unfortunately, all attempts to predict the probability of IIF have met with very limited success. In this study, an analytical model is developed to predict ice formation inside mouse oocytes subjected to a freezing stress. According to the model, IIF is catalyzed heterogeneously by the plasma membrane (i.e., surface catalyzed nucleation, SCN). A local site on the plasma membrane is assumed to become an ice nucleator in the presence of the extracellular ice via its effects on the membrane. This interaction is characterized by the contact angle between the plasma membrane and the ice cluster. In addition, IIF is assumed to be catalyzed at temperatures below -30° C by intracellular particles distributed throughout the cell volume (i.e., volume catalyzed nucleation, VCN). In the present study, these two distinctly coupled modes of IIF, especially SCN, are applied to various experimental protocols from mouse oocytes. Excellent agreement between predictions and observations suggests that the proposed model of IIF is adequate.
Biaxial Mechanical Properties of Passive Right Ventricular Free Wall Myocardium
Authors: Sacks M. S. , Chuong C. J.
Published: March 2008

The biaxial mechanical properties of right ventricular free wall (RVFW) myocardium were studied. Tissue specimens were obtained from the sub-epicardium of potassium-arrested hearts and different stretch protocols were used to characterize the myocardium’s mechanical response. To assess regional differences, we excised tissue specimens from the conus and sinus regions. The RVFW myocardium was found to be consistently anisotropic, with a greater stiffness along the preferred (or averaged) fiber direction. The anisotropy in the conus region was more pronounced than in the sinus region. A comparison with studies of left ventricle (LV) midwall myocardium revealed that, 1) the fiber direction stiffnesses are greater in the RVFW than in the LV, 2) the degree of anisotropy is greater in the RVFW than in the LV.
A Design Procedure for Asymptotically Optimal Dynamic Compensators
Authors: Shimizu Kiyotaka , Suzuki Masakazu , Kato Misao
Published: March 2008

This paper is concerned with a design method for optimizing dynamic compensators of Pearson’s type. Optimal parameter matrices are obtained by use of a parameter matching technique and an arbitrary pole placement technique. The controlled system has the optimal LQ modes and the modes with arbitrarily quick damping. The presented compensator works as the optimal regulator with observer and performs about the same control as the optimal regulator. And it is designed not in two steps; observer, regulator, but in one step; optimization of output feedback gain without considering any state estimation.
An Experimental Thermal Time-Constant Correlation for Hydraulic Accumulators
Authors: Pourmovahed A. , Otis D. R.
Published: March 2008

A thermal time-constant correlation based on experimental data is presented for gas-charged hydraulic accumulators. This correlation, along with the thermal timeconstant model, permits accurate prediction of accumulator thermodynamic losses and the gas pressure and temperature history during compression or expansion. The gas is treated as a real gas, and all properties are allowed to vary with both pressure and temperature. The correlation was developed from heat transfer data obtained with a 2.5 liter piston-type accumulator charged with nitrogen gas. Both horizontal and vertical orientations were studied. The experiments covered the range, 2.6×108<Ra*<9.5×1010, 0.77<L/D<1.50, and 0.71<T*<1.0. The gas pressure was varied between 1.0 and 19.5 MPa.
Dynamic Analysis of a Variable Displacement Pump
Authors: Schoenau G. J. , Burton R. T. , Kavanagh G. P.
Published: March 2008

The ability of a variable displacement pump to respond to a control signal is a critical factor in assessing the dynamic performance of the circuit in which the pump is located. The need for a comprehensive dynamic response model of the pump is necessary if new techniques for control are to be realized. This paper presents a mathematical model, based on fluid mechanics considerations, of a variable displacement pump modulated by a hydraulic control signal. Many of the coefficients in the model depend on the pump model. In this study, a Vickers No. PVB5 pump is used. The describing equations are complex, nonlinear, and comprehensive in the initial model. Some nonlinear terms are simplified using linear approximations without significantly affecting accuracy. The model is subjected to a simulated pressure control signal and the output of the swash plate rotary displacement compared to an experimentally generated displacement time trace. The model and the experimental results show a good correlation.
Fourier-Based Optimal Control of Nonlinear Dynamic Systems
Authors: Nagurka M. L. , Yen V.
Published: March 2008

A method for generating near optimal trajectories of linear and nonlinear dynamic systems, represented by deterministic, lumped-parameter models, is proposed. The method is based on a Fourier series approximation of each generalized coordinate that converts the optimal control problem into an algebraic nonlinear programming problem. Due to its inverse dynamic nature, the method avoids many of the numerical difficulties typically encountered in solving standard optimal control problems. Furthermore, the method is easy to implement, capable of handling various types of constraints, and quite effective for solving non-bang-bang type control problems. The results of computer simulation studies compare favorably to optimal solutions obtained by closed-form analyses and/or by other numerical schemes.
Sensitivity Analysis of the Discrete-to-Continuous Dynamic System Transformation
Authors: Fassois S. D. , Eman K. F. , Wu S. M.
Published: March 2008

A sensitivity analysis of the discrete-to-continuous transformation used in the “indirect” modeling of continuous-time systems from sampled experimental data is presented. For the transformation of poles it is shown that small errors in the discrete-time domain may yield large errors in corresponding continuous-time parameters, such as time constants, natural frequencies, and damping factors, if very fast sampling is used. An important consequence of this phenomenon is the introduction of large errors in the modal parameter estimates of the lower frequency modes of multiple degree-of-freedom systems. In order to alleviate this problem appropriate sensitivity specifications, leading to a lower bound for the allowable values of the sampling period, are introduced. The transformation of residues by the step approximation method is also examined, and, the results of the analysis are finally used for the development of guidelines for the appropriate selection of the sampling period so that the transformation sensitivity be confined within prespecified design limits.
Experiment With Vibrational Control of a Laser Illuminated Thermochemical System
Authors: Fakhfakh J. , Bentsman J.
Published: March 2008

Vibrational control is an open-loop control technique which utilizes zero mean parametric excitations to modify the behavior of dynamical systems in a desired manner. A potential applicability of vibrational control to laser illuminated thermochemical systems has been recently demonstrated analytically by Bentsman and Hvostov (1988). This paper presents experiments with vibrational stabilization of a laser illuminated thermochemical reaction that support the previous findings. A rectangular wave oscillating incident laser power is shown experimentally to induce asymptotically stable operating regimes with averages located at initially unstable steady states as predicted by vibrational control theory. Hence, vibrational control is demonstrated to be a feasible stabilizing strategy for laser induced reactions that needs no on-line measurements and complex actuators.
A Class of Discrete-Time Stabilizing Controllers for Flexible Mechanical Systems
Authors: Shamsa K. , Flashner H.
Published: March 2008

A class of multivariable discrete-time control laws for flexible mechanical systems with colocated actuators and sensors is characterized. Stability of the proposed control law is derived in terms of energy dissipation properties of the system and is independent of physical parameters of the plant. The control law is applicable to linear, as well as nonlinear mechanical systems with any number of vibrational modes. It is therefore unaffected by modal truncation. The validity of the proposed approach is demonstrated by two examples.
A State-Space Approach to the Synthesis of Random Vertical and Crosslevel Rail Irregularities
Authors: Fries R. H. , Coffey B. M.
Published: March 2008

Solution of rail vehicle dynamics models by means of numerical simulation has become more prevalent and more sophisticated in recent years. At the same time, analysts and designers are increasingly interested in the response of vehicles to random rail irregularities. The work described in this paper provides a convenient method to generate random vertical and crosslevel irregularities when their time histories are required as inputs to a numerical simulation. The solution begins with mathematical models of vertical and crosslevel power spectral densities (PSDs) representing PSDs of track classes 4, 5, and 6. The method implements state-space models of shape filters whose frequency response magnitude squared matches the desired PSDs. The shape filters give time histories possessing the proper spectral content when driven by white noise inputs. The state equations are solved directly under the assumption that the white noise inputs are constant between time steps. Thus, the state transition matrix and the forcing matrix are obtained in closed form. Some simulations require not only vertical and crosslevel alignments, but also the first and occasionally the second derivatives of these signals. To accommodate these requirements, the first and second derivatives of the signals are also generated. The responses of the random vertical and crosslevel generators depend upon vehicle speed, sample interval, and track class. They possess the desired PSDs over wide ranges of speed and sample interval. The paper includes a comparison between synthetic and measured spectral characteristics of class 4 track. The agreement is very good.
New Mathematical Definition and Calculation of Axial Rotation of Anatomical Joints
Authors: Miyazaki Shinji , Ishida Akimasa
Published: March 2008

In the field of joint kinematics, clinical terms such as internal-external, or medical-lateral, rotations are commonly used to express the rotation of a body segment about its own long axis. However, these terms are not defined in a strict mathematical sense. In this paper, a new mathematical definition of axial rotation is proposed and methods to calculate it from the measured Euler angles are given. The definition is based on the integration of the component of the angular velocity vector projected onto the long axis of the body segment. First, the absolute axial rotation of a body segment with respect to the stationary coordinate system is defined. This definition is then generalized to give the relative axial rotation of one body segment with respect to the other body segment where the two segments are moving in the three-dimensional space. The well-known Codman’s paradox is cited as an example to make clear the difference between the definition so far proposed by other researchers and the new one.
Human Head Dynamic Response to Side Impact by Finite Element Modeling
Authors: Ruan J. S. , Khalil T. , King A. I.
Published: March 2008

The dynamic response of the human head to side impact was studied by 2-dimensional finite element modeling. Three models were formulated in this study. Model I is an axisymmetric model. It simulated closed shell impact of the human head, and consisted of a single-layered spherical shell filled with an inviscid fluid. The other two models (Model II and III) are plane strain models of a coronal section of the human head. Model II approximated a 50th percentile male head by an outer layer to simulate cranial bone and an inviscid interior core to simulate the intracranial contents. The configuration of Model III is the same as Model II but more detailed anatomical features of the head interior were added, such as, cerebral spinal fluid (CSF); falx cerebri, dura, and tentorium. Linear elastic material properties were assigned to all three models. All three models were loaded by a triangular pulse with a peak pressure of 40 kPa, effectively producing apeak force of 1954 N (440 lb). The purpose of this study was to determine the effects of the membranes and that of the mechanical properties of the skull, brain, and membrane on the dynamic response of the brain during side impact, and to compare the pressure distributions from the plane strain model with the axisymmetric model. A parametric study was conducted on Model II to characterize fully its response to impact under various conditions. It was found that: (a) The membranes affected the dynamic response of the brain significantly, the fundamental frequency of the brain was 72 Hz with membranes and 49 Hz without them: (b) Significant variations in the pressure distribution were obtained as a result of assigning different material properties to the skull and brain; (c) The normal variation of pressure from compression at the pole (impact side) to tension at the antipole (opposite to the impact side) was disrupted by the membrane and a complex distribution of pressure was found.
Fracture Prediction for the Proximal Femur Using Finite Element Models: Part II—Nonlinear Analysis
Authors: Lotz J. C. , Cheal E. J. , Hayes W. C.
Published: March 2008

In Part I we reported the results of linear finite element models of the proximal femur generated using geometric and constitutive data collected with quantitative computed tomography. These models demonstrated excellent agreement with in vitro studies when used to predict ultimate failure loads. In Part II, we report our extension of those finite element models to include nonlinear behavior of the trabecular and cortical bone. A highly nonlinear material law, originally designed for representing concrete, was used for trabecular bone, while a bilinear material law was used for cortical bone. We found excellent agreement between the model predictions and in vitro fracture data for both the onset of bone yielding and bone fracture. For bone yielding, the model predictions were within 2 percent for a load which simulated one-legged stance and 1 percent for a load which simulated a fall. For bone fracture, the model predictions were within 1 percent and 17 percent, respectively. The models also demonstrated different fracture mechanisms for the two different loading configurations. For one-legged stance, failure within the primary compressive trabeculae at the subcapital region occurred first, leading to load transfer and, ultimately, failure of the surrounding cortical shell. However, for a fall, failure of the cortical and trabecular bone occurred simultaneously within the intertrochanteric region. These results support our previous findings that the strength of the subcapital region is primarily due to trabecular bone whereas the strength of the intertrochanteric region is primarily due to cortical bone.
Nonlinear Dynamic Behavior of the Human Knee Joint—Part I: Postmortem Frequency Domain Analyses
Authors: Dortmans L. , Jans H. , Sauren A. , Huson A.
Published: March 2008

Characteristic results of postmortem experiments on five knee-joint specimens are reported. The experiments were performed to investigate the applicability of a local linearization technique that would make it possible to describe the dynamic behavior of the joint in terms of transfer functions. The results indicate that the stiffness of the bracing wires, attached to muscle tendons to create a static equilibrium position, can be accounted for when determining the stiffness of the joint. Besides the static equilibrium configuration, the magnitude of the dynamic load and the type of dynamic load applied to the joint can be shown to have their influence. As the influence of the dynamic load is significant, it has to be concluded that in essence the knee joint has to be regarded as a nonlinear system, making application of a Local Linearization Technique questionable. However, when the magnitude of the dynamic load is included as an additional measurement parameter, an indication can be obtained about the behavior of the joint and the degree of nonlinearity.
Normal Contact of Elastic Spheres With Two Elastic Layers as a Model of Joint Articulation
Authors: Eberhardt A. W. , Lewis J. L. , Keer L. M.
Published: March 2008

An analytical model of two elastic spheres with two elastic layers in normal, frictionless contact is developed which simulates contact of articulating joints, and allows for the calculation of stresses and displacements in the layered region of contact. Using various layer/layer/substrate combinations, the effects of variations in layer and substrate properties are determined in relation to the occurrence of tensile and shear stresses as the source of crack initiation in joint cartilage and bone. Vertical cracking at the cartilage surface and horizontal splitting at the tidemark have been observed in joints with primary osteoarthritis. Deep vertical cracks in the calcified cartilage and underlying bone have been observed in blunt trauma experiments. The current model shows that cartilage stresses for a particular system are a function of the ratio of contact radius to total layer thickness (a/h). Surface tension, which is observed for a/h small, is alleviated as a/h is increased due to increased load, softening and/or thinning of the cartilage layer. Decreases in a/h due to cartilage stiffening lead to increased global compressive stresses and increased incidence of surface tension, consistent with impact-induced surface cracks. Cartilage stresses are not significantly affected by variations in stiffness of the underlying material. Tensile radial strains in the cartilage layer approach one-third of the normal compressive strains, and increase significantly with cartilage softening. For cases where the middle layer stiffness exceeds that of the underlying substrate, tensile stresses occur at the base of the middle layer, consistent with impact induced cracks in the zone of calcified cartilage and subchondral bone. The presence of the superficial tangential zone appears to have little effect on underlying cartilage stresses.
Thermal Property Measurements on Biological Materials at Subzero Temperatures
Authors: Bai Xuemei , Pegg David E.
Published: March 2008

The self-heated thermistor technique was used to measure the thermal conductivity and thermal diffusivity of biomaterials at low temperatures. Thermal standards were selected to calibrate the system at temperatures from −10°C to −70°C. The thermal probes were constructed with a convection barrier which eliminates convection inside liquid samples of low viscosity, without affecting the conductivity and diffusivity results. Using this technique, the thermal conductivity and diffusivity of two organ perfusates (HP5 and HP5 + 2M glycerol), one kidney phantom (a low ionic strength gel), as well as rabbit kidney cortex have been measured from −10°C to −70°C.
Controlled Freezing of Nonideal Solutions With Application to Cryosurgical Processes
Authors: Budman H. M. , Dayan J. , Shitzer A.
Published: March 2008

Success of a cryosurgical procedure, i.e., maximal cell destruction, requires that the cooling rate be controlled during the freezing process. Standard cryosurgical devices are not usually designed to perform the required controlled process. In this study, a new cryosurgical device was developed which facilitates the achievement of a specified cooling rate during freezing by accurately controlling the probe temperature variation with time. The new device has been experimentally tested by applying it to an aqueous solution of mashed potatoes. The temperature field in the freezing medium, whose thermal properties are similar to those of biological tissue, was measured. The cryoprobe temperature was controlled according to a desired time varying profile which was assumed to maximize necrosis. The tracking accuracy and the stability of the closed loop control system were investigated. It was found that for most of the time the tracking accuracy was excellent and the error between the measured probe temperature and the desired set point is within ±0.4°C. However, noticeable deviations from the set point occurred due to the supercooling phenomenon or due to the instability of the liquid nitrogen boiling regime in the cryoprobe. The experimental results were compared to those obtained by a finite elements program and very good agreement was obtained. The deviation between the two data sets seems to be mainly due to errors in positioning of the thermocouple junctions in the medium.
One-Dimensional Computer Analysis of Oscillatory Flow in Rigid Tubes
Authors: Donovan F. M. , Taylor Bruce C. , Su M. C.
Published: March 2008

The dynamic characteristics of catheter-transducer systems using rigid tubes with compliance lumped in the transducer and oscillatory flow of fluid in rigid tubes were analyzed. A digital computer model based on one dimensional laminar oscillatory flow was developed and verified by exact solution of the Navier-Stokes Equation. Experimental results indicated that the damping ratio and resistance is much higher at higher frequencies of oscillation than predicted by the one dimensional model. An empirical correction factor was developed and incorporated into the computer model to correct the model to the experimental data. Amplitude of oscillation was found to have no effect on damping ratio so it was concluded that the increased damping ratio and resistance at higher frequencies was not due to turbulence but to two dimensional flow effects. Graphs and equations were developed to calculate damping ratio and undamped natural frequency of a catheter-transducer system from system parameters. Graphs and equations were also developed to calculate resistance and inertance for oscillatory flow in rigid tubes from system parameters and frequency of oscillation.
6R Instrumented Spatial Linkages for Anatomical Joint Motion Measurement—Part 2: Calibration
Authors: Kirstukas S. J. , Lewis J. L. , Erdman A. G.
Published: March 2008

The six-revolute-joint instrumented spatial linkage (6R ISL) is often the measurement system of choice for monitoring motion of anatomical joints. However, due to tolerances of the linkage parameters, the system may not be as accurate as desired. A calibration algorithm and associated calibration device have been developed to refine the initial measurements of the ISL’s mechanical and electrical parameters so that the measurement of six-degree-of-freedom motion will be most accurate within the workspace of the anatomical joint. The algorithm adjusts the magnitudes of selected linkage parameters to reduce the squared differences between the six known and calculated anatomical position parameters at all the calibration positions. Weighting is permitted so as to obtain a linkage parameter set that is specialized for measuring certain anatomical position parameters. Output of the algorithm includes estimates of the measuring system accuracy. For a particular knee-motion-measuring ISL and calibration device, several interdependent design parameter relationships have been identified. These interdependent relationships are due to the configuration of the ISL and calibration device, the number of calibration positions, and the limited resolution of the devices that monitor the position of the linkage joints. It is shown that if interdependence is not eliminated, then the resulting ISL parameter set will not be accurate in measuring motion outside of the calibration positions, even though these positions are within the ISL workspace.
An Evolutionary Wolff’s Law for Trabecular Architecture
Authors: Cowin S. C. , Sadegh A. M. , Luo G. M.
Published: March 2008

A continuum model is proposed to describe the temporal evolution of both the density changes and the reorientation of the trabecular architecture given the applied stress state in the bone and certain material parameters of the bone. The data upon which the proposed model is to be based consist of experimentally determined remodeling rate coefficients and quantitative stereological and anisotropic elastic constant measurements of cancellous bone. The model shows that the system of differential equations governing the temporal changes in architecture is necessarily nonlinear. This nonlinearity is fundamental in that it stems from the fact that, during remodeling, the relationship between stress and strain is changing as the stress and strain variables themselves are changing. In order to preserve the remodeling property of the model, terms that are of the order strain times the changes in density and/or microstructural properties must be retained. If these terms were dropped, there would be no feedback mechanism for architectural adaptation and no adaptation of the trabecular architecture. There is, therefore, no linearized version of this model of the temporal evolution of trabecular architecture. An application of the model is illustrated by an example problem in which the temporal evolution of homogeneous trabecular architecture is predicted. A limitation of the proposed continuum model is the length scale below which it cannot be applied. The model cannot be applied in regions of cancellous bone where the trabecular bone architecture is relatively inhomogeneous or at a bone-implant interface.
Does Vibration Cause Poststenotic Dilatation In Vivo and Influence Atherogenesis in Cholesterol-Fed Rabbits?
Authors: Gow B. S. , Legg M. J. , Yu W. , Kukongviriyapan U. , Lee L. L.
Published: March 2008

Arterial post-stenotic dilatation (PSD) is a fusiform swelling immediately downstream to a stenosis. It is characterized by the presence of turbulent blood flow and wall vibration which has been claimed by others to be causal by producing structural weakening. We tested the hypothesis that vibration causes PSD in vivo by attaching electromagnetic and pneumatic vibrators to the aortic wall in chronic rabbits. We also observed whether mechanical vibration of the aorta in vivo influenced the distribution of oil-red-O lesions during one percent dietary cholesterol feeding. Low mass vibration gauges were developed to measure the vibration. Electromechanical vibrators having a ceramic magnet slug within a coil supplied with 50 Hz were glued to the aorta of chronic rabbits and the vibration maintained for an average of 8 weeks. Despite greater amounts of energy imparted to the wall there was no dilatation or difference in oil-red-O staining from the controls. Five weeks vibration at 100 Hz and an amplitude equal to the normal diameter pulse also produced no dilatation. We conclude that vibration does not cause PSD in vivo and suggest that its cause is likely to involve the vascular muscle stimulated by the effect of turbulent flow on the endothelium.
Numerical Simulation of Steady Flow in a Model of the Aortic Bifurcation
Authors: Thiriet M. , Pares C. , Saltel E. , Hecht F.
Published: March 2008

The governing equations of steady flow of an incompressible viscous fluid through a 3-D model of the aortic bifurcation are solved with the finite element method. The effect of Reynolds number on the flow was studied for a range including the physiological values (200≤Re≤1600). The symmetrical bifurcation, with a branch angle of 70 degrees and an area ratio of 0.8, includes a tapered transition zone. Secondary flows induced by the tube curvature are observed in the daughter tubes. Transverse currents in the transition zone are generated by the combined effect of diverging and converging walls. Flow separation depends on both the Reynolds number and the inlet wall shear.
Pulsatile Flow in a Constricted Channel
Authors: Tutty O. R.
Published: March 2008

A nonuniform channel is used as a simple model of a constricted arterial vessel. Flow patterns have been calculated for pulsatile flow with both sinusoidal and nonsinusoidal flow rates for a range of Reynolds number, Re, and Strouhal number, St. The results show that even for relatively low frequency flows a strong vortex wave will be generated with a complex wall shear stress distribution and peak values much greater than those found in steady or unsteady parallel flow. The vortex wave increases in strength with increasing Re and St, with its total length and wavelength independent of Re but inversely proportional to St. The form of the imposed flow rate is found to have an important effect on the flow and the shear stress distribution.
Chaotic Oscillations in a Simple Collapsible-Tube Model
Authors: Jensen O. E.
Published: March 2008

A steady flow through a segment of externally pressurized, collapsible tube can become unstable to a wide variety of self-excited oscillations of the internal flow and tube walls. A simple, one-dimensional model of the conventional laboratory apparatus, which has been shown previously to predict steady flows and multiple modes of oscillation, is investigated numerically here. Large amplitude oscillations are shown to have a relaxation structure, and the nonlinear interaction between different modes is shown to give rise to quasiperiodic and apparently aperiodic behavior. These predictions are shown to compare favorably with experimental observations.
Possible Sources of Discrepancy Between Sphygmomanometer Cuff Pressure and Blood Pressure Quantified in a Collapsible-Tube Analogue
Authors: Bertram C. D. , Butcher K. S. A.
Published: March 2008

This paper examines the assumption that the audible events detected as Korotkov sounds in sphygmomanometry occur when blood pressure equals arm-cuff pressure. Several effects that contribute to discrepancy between these pressures are quantified using an idealised arm-and-cuff system consisting of a thick-walled collapsible tube subject to external compression along a central part of its length. The effects studied are (1) transverse pressure difference, resulting from tissues sustaining a part of the external compression through (a) circumferential bending stiffness and (b) longitudinal curvature of the tensed localised neck at the site of initial collapse, (2) longitudinal pressure difference between upstream pressure and pressure at the collapse point due to both (a) viscous and (b) inertial pressure drop. These effects are found to compensate partially for each other; the pressure within the vessel at the collapse point is less than the cuff pressure, but is also less than the blood pressure at the upstream end of the cuff. All four of the contributing terms increase proportionally to the flow-rate raised to a power greater than one, except the viscous pressure drop. Owing to a progressive shortening of the collapsed neck as flow-rate increases, the viscous term is almost independent of the flow-rate. The overall discrepancy displays less flow-rate dependency and is smaller than some of the terms which contribute to it. This means that considerable accuracy is needed if measurements of the effects are to be used to correct the raw data on cuff pressure at the time of Korotkov sound emission so as to obtain an improved estimate of the blood pressure.
Blood Flow in a Brachial Artery Compressed Externally by a Pneumatic Cuff
Authors: Shimizu M.
Published: March 2008

Blood flow in human brachial arteries, compressed by a pneumatic cuff for blood pressure measurement, is examined using several different noninvasive techniques. From the experimental results it is shown that, when arterial pressure distal to the cuff is always lower than cuff pressure, flow in the artery under the cuff becomes supercritical near the cuff downstream margin and no reflection occurs there and the reflected wave from the peripheral vascular system of the arm does not propagate beyond the cuff downstream margin. Therefore an unsteady transition from supercritical to subcritical flow occurs near the cuff downstream margin. When the peak value of distal arterial pressure exceeds cuff pressure, a reflection occurs near the cuff downstream margin. The reflection becomes stronger corresponding to decrement of the cuff pressure and the whole artery segment under the cuff inflates fully at systole. At diastole, however, the artery segment under the cuff does not collapse completely and, hence, the phenomenon becomes that of pressure wave propagation in a partially collapsed artery segment.
6R Instrumented Spatial Linkages for Anatomical Joint Motion Measurement—Part 1: Design
Authors: Kirstukas S. J. , Lewis J. L. , Erdman A. G.
Published: March 2008

Six-revolute-joint instrumented spatial linkages (6R ISLs) have become often-used devices to measure the complete six-degree-of-freedom motion of anatomical joints. Accuracy of motion measurement depends on ISL design and calibration technique. In this paper, a design process is outlined that uses computer graphics and numerical methods as aids in developing 6R ISLs that (i) physically assemble within the desired range of motion of the joint; (ii) do not collide with either the experimental apparatus or the subject joint; (iii) avoid singular linkage configurations that can cause forces to be applied to the joint; and (iv) measure selected anatomical motions most accurately. It is found that a certain subgroup of 6R linkages are suitable for accurate measurement of specific motions, and can be the basis for new ISL designs. General guidelines are developed that can assist in the generation of unique linkage designs for different anatomical joints. The design process is demonstrated in the creation of an ISL to measure knee motion.
Factors in the Initiation and Propagation of Aortic Dissections in Human Autopsy Aortas
Authors: Tiessen I. M. , Roach M. R.
Published: March 2008

Our goal was to evaluate physical factors in the initiation and propagation of dissections in human aortas. Aortic dissection was simulated in 21 open unpressurized human autopsy aortas by infusing dyed isotonic saline into the media at constant flow. Pressure during growth of the fluid filled cavity was monitored and correlated with volume to give pressure-volume curves, from which distensibility, peak pressure, and work data were calculated. Dissections in specimens occurred at a very high nonphysiological mean pressure of 79 ± 29 (SD) kPa (596 ± 214 mmHg). Age and tear depth had no significant effect on medial strength of human aortas but sex, location and atherosclerotic plaque formation did (p < 0.05). The mean pressure value for the female abdominal fibrous/calcified group (92 ± 30 kPa, 691 ± 222 mmHg) differed from that of the male abdominal fibrous/calcified group (69 ± 36 kPa, 508 ± 269 mmHg). We also observed a significant difference between the male thoracic fibrous calcified (80 ± 31 kPa, 601 ± 230 mmHg) and male abdominal fibrous/calcified (68 ± 36 kPa, 508 ± 269 mmHg) groups. Significant differences were observed between the female fatty/normal (75 ± 25 kPa, 562 ± 187 mmHg) and fibrous/calcified (91 ± 23 kPa, 685 ± 176 mmHg) groups of the thoracic aorta and between the male fatty/normal (86 ± 23 kPa, 650 ± 169 mmHg) and fibrous/calcified (68 ± 36 kPa, 508 ± 269 mmHg) groups of the abdominal aorta. Distensibility data showed no trends. It is probable that dissection does not occur spontaneously in the absence of a connective tissue disorder and/or surgical procedure. Atherosclerosis is not responsible for dissection initiation but may alter the propagation of dissection.
The Use of Robotics Technology to Study Human Joint Kinematics: A New Methodology
Authors: Fujie Hiromichi , Mabuchi Kiyoshi , Woo Savio L.-Y. , Livesay Glen A. , Arai Shinji , Tsukamoto Yukio
Published: March 2008

Robotics technologies have been modified to control and measure both the force and position of synovial joints for the study of joint kinematics. One such system was developed to perform kinematic testing of a human joint. A 6-axis articulated robotic manipulator with 6 degrees of freedom (DOF) of motion was designed and constructed; a mathematical description for joint force and position was devised; and hardware and software to control forces applied to the joint, as well as position of the joint, were developed. The new methodology was utilized to simulate physiological loading conditions and to perform an anterior-posterior (A-P) translation test on a human cadaveric knee. Testing showed that this new system can simulate complex loading conditions and also measure the resulting joint kinematics.
A Method for Measuring Tension in Small Ligaments: An Application to the Ligaments of the Wrist Carpus
Authors: Kristal P. , Tencer A. F. , Trumble T. E. , North E. , Parvin D.
Published: March 2008

A new technique has been devised for measuring the in situ tension in small ligaments. It is based on measuring the tension in an axially loaded flexible cable with pinned endpoints by deflecting the cable laterally and measuring its lateral load and deformation. Studies were performed in which nylon line and bone ligament bone preparations were placed in a materials tester and loaded in axial tension. Axial load as measured by lateral load and deformation was found to agree with the known load to within 8 percent. The method was sensitive to error in determination of ligament length, nonperpendicularity of the laterally applied load to the long axis of the ligament, and when used in situ, impingement of the ligament on a third bone causing bending. A device, consisting of an LVDT mounted to a rigid frame with its core rod connected to a load cell, was developed. The position of the core rod was controlled by a manual screw drive, and a hook on the other end of the core rod was used to deflect the ligament laterally. This device was applied to the study of tensions in five ligaments of the palmar wrist carpus, in seven cadaver specimens. Results showed that the radioscaphocapitate (RSC) and radiolunate (RL) ligaments had significantly greater tensions than the lunotriquetral (LT), the triquetrocapitate (TC), and scaphocapitate (SC) ligaments. For the four positions of the hand tested, neutral, 14 deg radial and 14 deg ulnar deviation, and 28 deg of extension, ligament tensions were found to be unaffected by position. In all positions tested, all ligaments had measurable tension, demonstrating the importance of ligaments in maintaining the integrity of the wrist carpus.
Biomechanics of Ocular Pneumoplethysmography
Authors: Chen Chaorong , Reed J. F. , Rice D. C. , Gee W. , Updike D. P. , Salathe E. P.
Published: March 2008

A mathematical analysis of ocular pneumoplethysmography is presented, based on the physiological, anatomical, and biomechanical properties of the eye. Ocular pneumoplethysmography is a clinical procedure involving elevation of intraocular pressure, by application of a suction cup to the sclera, to a level that exceeds ophthalmic artery systolic pressure. As decay in intraocular pressure is allowed, return of retinal artery pulsations indicates ophthalmic artery systolic pressure. We obtain a quantitative relationship between increase in intraocular pressure and applied vacuum, and compare the theoretical predictions with experiments on rabbits in which a variable descending vacuum was applied to bilateral scleral eyecups. The bilateral intraocular pressures were simultaneously recorded from cannulae in the respective vitreous bodies, and the pressures at which return of ocular pulsations were observed were correlated with the scleral vacuums. Regression lines were calculated for three serial determinations in each animal, with two groups of animals distinguished by the inner diameter of the eyecups used. The theoretical results indicate that the relationship between intraocular pressure increase and applied vacuum is independent of Young’s modulus, and depends primarily on the ratio of the diameter of the vacuum cup to the diameter of the eye.
Surfactant Effects on Fluid-Elastic Instabilities of Liquid-Lined Flexible Tubes: A Model of Airway Closure
Authors: Halpern D. , Grotberg J. B.
Published: March 2008

A theoretical analysis is presented predicting the closure of small airways in the region of the terminal and respiratory bronchioles. The airways are modelled as thin elastic tubes, coated on the inside with a thin viscous liquid lining. This model produces closure by a coupled capillary-elastic instability leading to liquid bridge formation, wall collapse or a combination of both. Nonlinear evolution equations for the film thickness, wall position and surfactant concentration are derived using an extended version of lubrication theory for thin liquid films. The positions of the air-liquid and wall-liquid interfaces and the surfactant concentration are perturbed about uniform states and the stability of these perturbations is examined by solving the governing equations numerically. Solutions show that there is a critical film thickness, dependent on fluid, wall and surfactant properties above which liquid bridges form. The critical film thickness, εc, decreases with increasing mean surface-tension or wall compliance. Surfactant increases εc by as much as 60 percent for physiological conditions, consistent with physiological observations. Airway closure occurs more rapidly with increasing film thickness and wall flexibility. The closure time for a surfactant rich interface can be approximately five times greater than an interface free of surfactant.
Airway Area by Acoustic Reflection: The Two-Microphone Method
Authors: Louis B. , Glass G. , Kresen B. , Fredberg J.
Published: March 2008

This report deals with noninvasive imaging of airway geometry based upon information contained in acoustic reflections measured at the mouth. Here we describe a new theoretical approach that enables development of a new miniaturized apparatus. Unlike the single-transducer systems used currently, this new strategy is based upon a two-transducer system that is a variant of that suggested originally by Shroeder (1967). We have developed, implemented, and tested computational algorithms necessary to reconstruct airway dimensions from acoustic reflection data using this two-transducer strategy.