bluuebunny/crossref_metadata_2025 huggingface dataset:
Application of the u-p Finite Element Method to the Study of Articular Cartilage
Authors: Wayne Jennifer S. , Woo Savio L.-Y. , Kwan Michael K.
Published: March 2008

The finite element method using the principle of virtual work was applied to the biphasic theory to establish a numerical routine for analyses of articular cartilage behavior. The matrix equations that resulted contained displacements of the solid matrix (u) and true fluid pressure (p) as the unknown variables at the element nodes. Both small and large strain conditions were considered. The algorithms and computer code for the analysis of two-dimensional plane strain, plane stress, and axially symmetric cases were developed. The u-p finite element numerical procedure demonstrated excellent agreement with available closed-form and numerical solutions for the configurations of confined compression and unconfined compression under small strains, and for confined compression under large strains. The model was also used to examine the behavior of a repaired articular surface. The differences in material properties between the repair tissue and normal cartilage resulted in significant deformation gradients across the repair interface as well as increased fluid efflux from the tissue.
A Combined Heat Clearance Method for Tissue Blood Flow Measurement
Authors: Abramovich-Sivan Sarit , Benary Vili , Kaspi Thalia , Akselrod Solange
Published: March 2008

Tissue Blood Flow is measured by applying a combined procedure of two independent approaches based on heat clearance: the Pulse Decay Method and the Continuous Method. The Pulse Method allows absolute assessment of tissue BF with no need for calibration, and can be applied only if the tissue BF is steady during the period of measurement. On the other hand, the Continuous Method enables the observation of rapid changes in tissue BF, and can be applied under non steady-state conditions. Using the combined method, a continuous quantitative measurement of transient changes in tissue BF can be obtained. For this purpose, we have developed two experimental systems consisting of independent electronic units: a Pulse Unit and a Continuous Unit. A micro-computer with dedicated software controls the operation of the electronic units and calculates tissue BF on-line. In vitro measurements are performed and demonstrate the reliability of the methods. In vivo measurements in rat brain tissue are also performed and include physiological and pharmacological changes of local tissue BF. The results of the two heat clearance methods correlate well with tissue BF values measured by a third independent method, the Hydrogen Clearance Method.
The Effect of Angle and Flow Rate Upon Hemodynamics in Distal Vascular Graft Anastomoses: An In Vitro Model Study
Authors: Keynton R. S. , Rittgers S. E. , Shu M. C. S.
Published: March 2008

A steady flow, in vitro model of distal arterial bypass graft junctions was used to examine the effects of junction angle and flow rate on the local velocity field. Three test sections were fabricated from Plexiglas™ tubing having anastomotic junction angles of either 30, 45, or 60 deg. Flow visualization revealed velocity profiles skewed toward the outer wall with a flow split around a clear stagnation point along the outer wall. Laser Doppler anemometry [LDA] measurements confirmed a distinct stagnation point at the outer wall and both reverse and forward shear were detected immediately upstream and downstream, respectively, of this site. Axial velocities and shear rates along the outer wall were higher than along the inner wall and occurred in the junction angle order: 45, 60, and 30 deg. This study clearly identified changes in wall shear which varied with the anastomotic angle and flow rate.
Pulsatile Non-Newtonian Flow Characteristics in a Three-Dimensional Human Carotid Bifurcation Model
Authors: Perktold K. , Resch M. , Florian H.
Published: March 2008

Numerical analysis of flow phenomena and wall shear stresses in the human carotid artery bifurcation has been carried out using a three-dimensional geometrical model. The primary aim of this study is the detailed discussion of non-Newtonian flow velocity and wall shear stress during the pulse cycle. A comparison of non-Newtonian and Newtonian results is also presented. The applied non-Newtonian behavior of blood is based on measured dynamic viscosity. In the foreground of discussion are the flow characteristics in the carotid sinus. The investigation shows complex flow patterns especially in the carotid sinus where flow separation occurs at the outer wall throughout the systolic deceleration phase. The changing sign of the velocity near the outer sinus wall results in oscillating shear stress during the pulse cycle. At the outer wall of the sinus at maximum diameter level the shear stress ranges from −1.92 N/m2 to 1.22 N/m2 with a time-averaged value of 0.04 N/m2. At the inner wall of the sinus at maximum diameter level the shear stress range is from 1.16 N/m2 to 4.18 N/m2 with a mean of 1.97 N/m2. The comparison of non-Newtonian and Newtonian results indicates unchanged flow phenomena and rather minor differences in the basic flow characteristics.
Sedimentation of a Suspension in a Centrifugal Field
Authors: Lueptow R. M. , Hu¨bler W.
Published: March 2008

To model centrifugal sedimentation of biological suspensions, the time history of sedimentation of particles in a centrifugal field was considered for two geometries: a tube and a cylindrical container. The Kynch theory for batch gravitational settling in Cartesian coordinates based on mass conservation was extended to include a centrifugal sedimentation force, cylindrical coordinates, and the Hawksley-Vand hindered settling model. The resulting quasi-linear partial differential equation was solved by the method of characteristics. The combination of radial dependence of the sedimentation force and cylindrical geometry in the centrifugal case results in several differences in the time-position history diagram of the sedimentation process compared to the gravitational case. First, instead of a region of uniform concentration equal to the initial concentration, a region of concentration that is continuously decreasing with time results. Second, in the region of particle accumulation, curved constant concentration contours result instead of straight lines. Finally, a secondary shock that is dependent upon the initial concentration and the radius ratio of the rotating vessel appears in the centrifugal case. The time history of the concentration for a particle suspension with an initial concentration typical of blood is presented.
Finite Element Stress Analysis of a Push-Out Test Part 1: Fixed Interface Using Stress Compatible Elements
Authors: Shirazi-Adl A.
Published: March 2008

In this first part of a two-part paper, interelement stress compatible finite elements are developed and used to perform the stress analysis of a push-out test with a fixed interface. In the formulation, the required continuity of some of the stresses along either a specific interface or all interelement interfaces is enforced by a penalty procedure. The model is axisymmetric and consists of two cylinders attached to each other through the interface. Various relative material properties and boundary conditions are simulated in order to examine their effects on the interface stresses. Both loadings of axial compression force and axial torque are considered. The predicted results exhibit identical interelement stresses and displacements even when highly dissimilar materials are used. They also exhibit a complex state of interface stresses which depend on the geometry, material arrangement, boundary conditions, and loading. The variation of the shear stress is often highly nonuniform and the radial normal stresses are likely to be large. The present results, therefore, disagree with the common assumptions made in the pull-out tests in the orthopaedic applications. Finally, stress analysis of a number of possible testing configurations could lead to the design of an optimal pull-out test which maximizes the usefulness of the measured results in terms of the interface bond strength and factors affecting it.
Failure Analysis of a Beam-Column Under Oblique-Eccentric Loading: Potential Failure Surfaces for Cervical Spine Trauma
Authors: Dai Q. G. , Liu Y. K.
Published: March 2008

For a cantilever beam-column with one end built-in and the free end subjected to an oblique-eccentric arbitrary concentrated force, general formulas to produce failure were derived. The original generalized uniform solution to the oblique-eccentric buckling problem was obtained. The Secant formula and Euler’s formula were proved to be specific cases in this general solution. The load ratio, F/aE, was derived as functions of the force acting direction, α, the slenderness ratio, L/r, as well as the eccentricity ratio, ec/r2. Material and buckling failures aspects were combined in a uniform structural failure analysis. Safe regions for the load ratio, F/aE, were visualized in the three-dimensional (F/aE)-α-(L/r) space with the eccentricity ratios, ec/r2, as a parameter. The column failure factor, kL, was shown to be a key index controlling both aspects of failure as well as the orientation of the second stiff est region. The angle αE = tan−1 (2L/πe) for kL = π/2 is the singular point for both strength and buckling failure, and αII = tan−1 (2L/3e) for kL = 0 is the upper bound of the second stiffest region. The feasible domain of the second stiffest region is bounded by αE and αII both of which are only functions of geometrical properties. The implications of these analyses for the experimental validation of cervical spine trauma are discussed.
A Composite Micromechanical Model for Connective Tissues: Part I—Theory
Authors: Ault H. K. , Hoffman A. H.
Published: March 2008

A micromechanical model has been developed to study and predict the mechanical behavior of fibrous soft tissues. The model uses the theorems of least work and minimum potential energy to predict upper and lower bounds on material behavior based on the structure and properties of tissue components. The basic model consists of a composite of crimped collagen fibers embedded in an elastic glycosaminoglycan matrix. Upper and lower bound aggregation rules predict composite material behavior under the assumptions of uniform strain and uniform stress, respectively. Input parameters consist of the component material properties and the geometric configuration of the fibers. The model may be applied to a variety of connective tissue structures and is valuable in giving insight into material behavior and the nature of interactions between tissue components in various structures. Application of the model to rat tail tendon and cat knee joint capsule is described in a companion paper [2].
Secondary Flow in the Human Common Carotid Artery Imaged by MR Angiography
Authors: Caro C. G. , Dumoulin C. L. , Graham J. M. R. , Parker K. H. , Souza S. P.
Published: March 2008

The blood flow in arteries affects both the biology of the vessels and the development of atherosclerosis. The flow is three dimensional, unsteady, and difficult to measure or to model computationally. We have used phase-shift-based magnetic resonance angiography to image and measure the flow in the common carotid arteries of a healthy human subject. There was curvature of the vessels and thin-slice dynamic flow imaging showed evidence of the presence of secondary motions. Flexing the cervical spine straightened the vessels and reduced the asymmetry of the flow.
The Effect of Storage on the Biomechanical Behavior of Articular Cartilage—A Large Strain Study
Authors: Kwan M. K. , Hacker S. A. , Woo S. L.-Y. , Wayne J. S.
Published: March 2008

The transplantation of stored shell osteochondral allografts is a potentially useful alternative to total joint replacements for the treatment of joint ailments. The maintenance of normal cartilage properties of the osteochondral allografts during storage is important for the allograft to function properly and survive in the host joint. Since articular cartilage is normally under large physiological stresses, this study was conducted to investigate the biomechanical behavior under large strain conditions of cartilage tissue stored for various time periods (i.e., 3, 7, 28, and 60 days) in tissue culture media. A biphasic large strain theory developed for soft hydrated connective tissues was used to describe and determine the biomechanical properties of the stored cartilage. It was found that articular cartilage stored for up to 60 days maintained the ability to sustain large compressive strains of up to 40 percent or more, like normal articular cartilage. Moreover, the equilibrium stress-strain behavior and compressive modulus of the stored articular cartilage were unchanged after up to 60 days of storage.
A General Method of Determining the Frequency-Dependent Propagation Coefficient and Characteristic Impedance of an Artery in the Presence of Reflections
Authors: Bertram C. D. , Greenwald S. E.
Published: March 2008

An iterative method of calculating propagation parameters at harmonics of heart rate for a uniform vascular segment from a combination of four arterial waveform measurements is presented. Measurements of blood pressure, vascular diameter, and blood flow-rate may be combined arbitrarily provided only that at least one measurement of pressure and one of flow-rate be included; the requirement of four measurements implies at least two measurement sites along the vessel. The analysis is thus a generalization of those associated with previous methods of determining propagation parameters, allowing for instance relaxation of the requirement of equal spacing in the three-point method. Results are presented for the propagation of an impulse along a rubber tube when the measurements are pressure at two sites, flowrate and diameter.
Advanced Spectral Estimators for Detailed Blood Flow Studies
Authors: Bharath A. A. , Kitney R. I.
Published: March 2008

Recent publications have emphasized the relationship between the spectrum of the backscattered acoustic signal, beam geometry, and flow patterns in the measurement of blood flow by Doppler ultrasound. On this basis, we believe that in the future more importance will be placed on analyzing various characteristics of the spectral shape rather than absolute parameters of measurement, such as the mean frequency. The potential of this approach for extracting more information from the raw Doppler signal is introduced by considering the Spectral Broadening Index (SBI). We explain the use of the SBI parameter for measuring flow angle under restricted flow conditions. This is done by using an analytic/computational model for prediction of the spectral broadening effect. By simulation study, the performance of various spectral estimators for determining the SBI from finite Doppler signal segments is evaluated.
Hemodynamic Patterns in Two Models of End-to-Side Vascular Graft Anastomoses: Effects of Pulsatility, Flow Division, Reynolds Number, and Hood Length
Authors: White S. S. , Zarins C. K. , Giddens D. P. , Bassiouny H. , Loth F. , Jones S. A. , Glagov S.
Published: March 2008

Flow behavior in models of end-to-side vascular graft anastomoses was studied under steady and pulsatile flow conditions. Models were constructed to simulate geometries employed in experimental studies on intimal thickening in a canine model. Reynolds numbers, division of flow in the outflow tracts and the pulsatile waveform employed were taken from measurements obtained in the canine model. Flows in the scaled-up, transparent models were visualized with white, neutrally buoyant particles which were photographed under laser illumination and also recorded on video tape under bright incandescent light. Strong, three-dimensional helical patterns which formed in the anastomotic junction were prominent features of the flow fields. Regions of low wall shear, oscillatory wall shear and long particle residence time were identified from the flow visualization experiments. Comparisons with the limited qualitative data available on intimal thickening in vascular graft anastomoses suggest a relation between localization of vascular intimal thickening and those surfaces experiencing low shear and long particle residence time.
Dispersion and Uptake in a Coiled Tube
Authors: Kaye S. R. , Kavafyan P. , Schroter R. C.
Published: March 2008

We have investigated the dispersion characteristics of a highly soluble tracer gas in a coiled tube lined with wet gelatine. Our experiments indicate that absorption and desorption of the gas at the wall dominates tracer dispersion along the tube. The results confirm previous experiments in a branching geometry indicating that the influence of secondary flows on dispersion is reduced if the tracer is very soluble in the wall.
Elementary Mechanics of the Endothelium of Blood Vessels
Authors: Fung Y. C. , Liu S. Q.
Published: March 2008

The endothelium lining human arteries is a continuum of endothelial cells. The flowing blood imposes a shear stress on the endothelium. To compute the internal stress in the endothelium, we use two alternative hypotheses: 1) The cell content is fluid-like so that at steady-state it has no shear stress. 2) The cell content is solid-like. Under hypothesis No. 1, the membrane tension in the upper cell membrane grows in the direction opposite to the blood flow at a rate equal to the blood shear stress. At the junction of two neighboring cells the membrane tension in the downstream cell is transmitted partly to the basal lamina, and partly to the upstream cell. The transmission depends on the osmotic or static pressure difference between the cell and blood. If the static pressure difference is zero, the tension in the upper cell membrane will accumulate upstream. At other values of static pressure, the cell membrane tension may increase, decrease, or fluctuate along the vessel depending on the inclination of the side walls of the cells at the junctions. To determine the sidewall inclinations, we propose to use the complementary energy theorem. Under hypothesis No. 2, the cell content can bear shear, which tends to reduce the cell membrane tension; but the cell membrane tension accumulation phenomenon discussed above remains valid. These results are used to analyze the interaction of the cell membrane and cell nucleus; and the effect of turbulences in the flow on causing large fluctuations in cell membrane tension and vertical oscillations of the nuclei. The implication of tensile stress on the permeability of the cell membrane is discussed. We conclude that for the study of mass transport and stress fibers in the endothelial cells, one should consider the interaction of neighboring endothelial cells as a continuum, and shift attention from the shear stress in the blood to the principal stresses in the cells.
Mechanical Performance of External Fixators With Wires for the Treatment of Bone Fractures—Part II: Wire Tension and Slippage
Authors: Delprete C. , Gola M. M.
Published: March 2008

The work shows correct procedures needed in order to gather reliable data from measurement of displacements versus axial load in a laboratory mounting of the Ilizarov external fixator. The mechanism of settling after load cycling is investigated. Detension under load is a major problem of wires. By means of vibration frequency measurements, tests on single wire allow determination of reduction in wire tension due to transverse loading: it is found that, almost independently from the amount of clamp tightening, the tension reaches a lower limit related only to the transverse load and not related to pretension. It is shown that, for higher clamp tightening torques, wire detension must be attributed to permanent plastic deformation of the wires; moreover, it is shown that the unavoidable errors in the spacing of the tensioned wires lead to marked decrease of their stiffness under transverse load.
The Volume of Motion: Introduction, Derivation, and an Application Comparing Various Spinal Fixation Devices
Authors: Crisco J. J.
Published: March 2008

Range of motion (ROM), the displacement between two limits, is one of the most common parameters used to describe joint kinematics. The ROM is a one-dimensional parameter, although the motion at many normal and pathological joints is three-dimensional. Certainly, the ROM yields vital information, but an overall measure of the three-dimensional mobility at a joint may also be useful. The volume of motion (VOM) is such a measure. The translational VOM is the volume defined by all possible ROMs of a point on a rigid body. The rotational VOM, although its interpretation is not as tangible as the translational VOM, is a measure of the three-dimensional rotational mobility of a rigid body. The magnitude of the VOM is proportional to mobility; the VOM is a scaler, which does not contain any directional information. Experimental determination of the VOM is not practical since it would require applying loads in an infinite number of directions. The mathematical derivation given here allows the VOM to be calculated, with the assumption of conservative elasticity, from the resultant displacements of three distinct load vectors of equal magnitude. An example of the VOM is presented in the comparison of the biomechanical stabilizing potential of various spinal fixation devices.
Measurement of the Impulsive Bone Motion by Skin-Mounted Accelerometers
Authors: Kim W. , Voloshin A. S. , Johnson S. H. , Simkin A.
Published: March 2008

A measurement system was designed to investigate longitudinal wave propagation through the lower extremity generated from foot strikes. The principal goal of the design was to eliminate measurement time lag and amplitude reduction, such that the acceleration measured by Skin Mounted Accelerometer—SMA is equal to the actual acceleration of the bone measured by Bone Mounted Accelerometer—BMA. For accurate dynamic measurement, it is important that the gain and phase of the measurement system are as close as possible to a constant and zero, respectively, for the frequency range being covered. An in vitro experiment was carried out to simultaneously measure skin and bone accelerations. The obtained information was used for identification of a linear spring/damper model representing the interface between the BMA and the SMA. The present work showed that the SMA overestimated the BMA by 12 percent in the signals between 15–30 Hz.
Mechanics of Active Contraction in Cardiac Muscle: Part I—Constitutive Relations for Fiber Stress That Describe Deactivation
Authors: Guccione J. M. , McCulloch A. D.
Published: March 2008

Constitutive relations for active fiber stress in cardiac muscle are proposed and parameters are found that allow these relations to fit experimental data from the literature, including the tension redeveloped following rapid deactivating length perturbations. Contraction is driven by a length-independent free calcium transient. The number of actin sites available to react with myosin is determined from the total number of actin sites (available and inhibited), free calcium and the length history-dependent association and dissociation rates of two Ca2+ ions and troponin as governed by a first-order, classical kinetics, differential equation. Finally, the relationship between active tension and the number of available actin sites is described by a general cross-bridge model. Bridges attach in a single configuration at a constant rate, the force within each cross-bridge varies linearly with position, and the rate constant of bridge detachment depends both on position and time after onset of contraction. In Part II, these constitutive relations for active stress are incorporated in a continuum mechanics model of the left ventricle that predicted end-systolic transmural strain distributions as observed experimentally.
Mechanics of Active Contraction in Cardiac Muscle: Part II—Cylindrical Models of the Systolic Left Ventricle
Authors: Guccione J. M. , Waldman L. K. , McCulloch A. D.
Published: March 2008

Models of contracting ventricular myocardium were used to study the effects of different assumptions concerning active tension development on the distributions of stress and strain in the equatorial region of the intact left ventricle during systole. Three models of cardiac muscle contraction were incorporated in a cylindrical model for passive left ventricular mechanics developed previously [Guccione et al. ASME Journal of Biomechanical Engineering, Vol. 113, pp. 42-55 (1991)]. Systolic sarcomere length and fiber stresses predicted by a general “deactivation” model of cardiac contraction [Guccione and McCulloch, ASME Journal of Biomechanical Engineering, Vol. 115, pp. 72-81 (1993)] were compared with those computed using two less complex models of active fiber stress: In a time-varying “elastance” model, isometric tension development was computed from a function of peak intracellular calcium concentration, time after contraction onset and sarcomere length; a “Hill” model was formulated by scaling this isometric tension using the force-velocity relation derived from the deactivation model. For the same calcium ion concentration, the sarcomeres in the deactivation model shortened approximately 0.1 μm less throughout the wall at end-systole than those in the other models. Thus, muscle fibers in the intact ventricle are subjected to rapid length changes that cause deactivation during the ejection phase of a normal cardiac cycle. The deactivation model predicted rather uniform transmural profiles of fiber stress and cross-fiber stress distributions that were almost identical to those of the radial component. These three components were indistinguishable from the principal stresses. Transmural strain distributions predicted at end-systole by the deactivation model agreed closely with experimental measurements from the anterior free wall of the canine left ventricle.
A Semilinear State and Parameter Estimation Algorithm for Inverse Hyperthermia Problems
Authors: Liauh C.-T. , Roemer R. B.
Published: March 2008

An improved state and parameter estimation algorithm has been developed which decreases the total computational time required to accurately reconstruct complete hyperthermia temperature fields. Within this improved iterative estimation algorithm, if the change in the unknown perfusion parameters is small a linear approximation scheme is implemented in which the old Jacobian matrix (the sensitivity matrix) is used, instead of recalculating the new Jacobian matrix for the next iteration. In the hyperthermia temperature estimation problem the relationship between the temperature and the blood perfusion based on the bioheat transfer equation is generally nonlinear. However, the temperature can be approximated as a linear function of the blood perfusion over a certain range thus allowing this improved approach to work. Results show that if the temperature is approximated as a linear (or quasi-linear) function of the blood perfusion, the linearizing approach considerably reduces the CPU time required to accurately reconstruct the temperature field. The limiting case of implementing this approach is to calculate the Jacobian matrix for each iteration, which is identical to the approach used in the original nonlinear algorithm. Critical values of determining whether or not there is a need to recalculate the new Jacobian matrix during the iterations are presented for several inverse hyperthermia temperature estimation problems.
Three-Body Segment Dynamic Model of the Human Knee
Authors: Tu¨mer Sami Turgut , Engin Ali Erkan
Published: March 2008

In this paper, a two-dimensional, three-body segment dynamic model of the human knee is introduced. The model includes tibio-femoral and patello-femoral articulations, and anterior cruciate, posterior cruciate, medial collateral, lateral collateral, and patellar ligaments. It enables one to obtain dynamic response of the knee joint to any one or combination of quadriceps femoris, hamstrings, and gastrocnemius muscle actions, as well as any externally applied forces on the lower leg. A specially developed human knee animation program is utilized in order to fine tune some model parameters. Numerical results are presented for knee extension under the impulsive action of the quadriceps femoris muscle group to simulate a vigorous lower limb activity such as kicking. The model shows that the patella can be subjected to very large transient patello-femoral contact force during a strenuous lower limb activity even under conditions of small knee-flexion angles. The results are discussed and compared with limited data reported in the literature.
A B-Spline Least-Squares Surface-Fitting Method for Articular Surfaces of Diarthrodial Joints
Authors: Ateshian G. A.
Published: March 2008

The B-spline least-squares surface-fitting method is employed to create geometric models of diarthrodial joint articular surfaces. This method provides a smooth higher-order surface approximation from experimental three-dimensional surface data that have been obtained with any suitable measurement technique. Akima’s method for surface interpolation is used to provide complete support to the B-spline surface. The surface-fitting method is successfully tested on a known analytical surface, and is applied to the human distal femur. Applications to other articular surfaces are also shown. Results show that this method is precise, highly flexible, and can be successfully applied to a large variety of articular surface shapes.
Finite Element Analysis of the Temperature Field Around Two Adjacent Cryo-Probes
Authors: Weill Anne , Shitzer Avraham , Bar-Yoseph Pinhas
Published: March 2008

A finite element code was developed for the analysis of the temperature field around two adjacent cylindrical cryo-probes. The two-phase, two-dimensional Stefan problem is solved using a moving boundary approach and space-time finite elements. Solution of one-cryo-probe problem compared well with an existing analytic solution. The two-cryo-probes problem yielded reasonable results. The program simulated the nonsymmetric activation of two probes and the merging of the two freezing fronts in the case of symmetric activation.
Determination of Principal Reynolds Stresses in Pulsatile Flows After Elliptical Filtering of Discrete Velocity Measurements
Authors: Baldwin J. T. , Deutsch S. , Petrie H. L. , Tarbell J. M.
Published: March 2008

The purpose of this study was to develop a method to accurately determine mean velocities and Reynolds stresses in pulsatile flows. The pulsatile flow used to develop this method was produced within a transparent model of a left ventricular assist device (LVAD). Velocity measurements were taken at locations within the LVAD using a two-component laser Doppler anemometry (LDA) system. At each measurement location, as many as 4096 realizations of two coincident orthogonal velocity components were collected during preselected time windows over the pump cycle. The number of realizations was varied to determine how the number of data points collected affects the accuracy of the results. The duration of the time windows was varied to determine the maximum window size consistent with an assumption of pseudostationary flow. Erroneous velocity realizations were discarded from individual data sets by implementing successive elliptical filters on the velocity components. The mean velocities and principal Reynolds stresses were determined for each of the filtered data sets. The filtering technique, while eliminating less than 5 percent of the original data points, significantly reduced the computed Reynolds stresses. The results indicate that, with proper filtering, reasonable accuracy can be achieved using a velocity data set of 250 points, provided the time window is small enough to ensure pseudostationary flow (typically 20 to 40 ms). The results also reveal that the time window which is required to assume pseudostationary flow varies with location and cycle time and can range from 100 ms to less than 20 ms. Rotation of the coordinate system to the principal stress axes can lead to large variations in the computed Reynolds stresses, up to 2440 dynes/cm2 for the normal stress and 7620 dynes/cm2 for the shear stress.
An Experimental Study of Pulsatile Pipe Flow in the Transition Range
Authors: Einav S. , Sokolov M.
Published: March 2008

The study of pulsatile flows is relevant to many areas of applications. Typical applications include aerodynamics, biofluid mechanics, wind flows, and gas transport. Transition to turbulence during pulsatile flow is physiologically and clinically important. It has been suggested as a possible mechanism to enhance the transport of gases during high-frequency ventilation, may be related to valvular regurgitation and heart murmurs and to post stenotic dilatation and aneurysms. Measurements in a pulsatile pipe flow with a superimposed mean flow are reported. Data were taken in a water flow with mean Reynolds numbers in the range of 0 < Rem < 3000, oscillating Reynolds numbers of 0 < Reω < 4000, and Stokes parameter 7 < λ < 15. Velocity profiles of various phases of the flow, condition for flow reversal, and pressure losses were measured. The adequacy of a quasi-steady-state model is discussed. Condition for transition is determined by visually inspecting velocity signals at the centerline.
Collapse of Thin-Walled Elliptical Tubes for High Values of Major-to-Minor Axis Ratio
Authors: Ribreau C. , Naili S. , Bonis M. , Langlet A.
Published: March 2008

The topic of this study concerns principally representative models of some elliptical thin-walled anatomic vessels and polymeric tubes under uniform negative transmural pressure p (internal pressure minus external pressure). The ellipse’s ellipticity ko, defined as the major-to-minor axis ratio, varies from 1 up to 10. As p decreases from zero, at first the cross-section becomes somewhat oval, then the opposite sides touch in one point at the first-contact pressure pc. If p is lowered beneath pc, the curvature of the cross-section at the point of contact decreases until it becomes zero at the osculation pressure or the first line-contact pressure p1. For p<p1, the contact occurs along a straight-line segment, the length of which increases as p decreases. The pressures pc and p1 are determined numerically for various values of the wall thickness of the tubes. The nature of contact is especially described. The solution of the related nonlinear, two-boundary-values problem is compared with previous experimental results which give the luminal cross-sectional area (from two tubes), and the area of the mid-cross-section (from a third tube).
A Finite Difference Heat Transfer Analysis of a Percutaneous Transluminal Microwave Angioplasty System
Authors: Young L. A. , Boehm R. F.
Published: March 2008

Finite difference time domain (FDTD) techniques have been developed to calculate the specific absorption rates (SAR) in hyperthermia models. The University of Utah Hyperthermia Research Group has extended these numerical techniques for developing a percutaneous transluminal microwave angioplasty applicator, calculating the SARs produced by a high energy electromagnetic field to remove atherosclerotic plaques in blood vessels. The objective of this research was to derive a method for calculating the bioheat transfer in biological tissue surrounding a microwave angioplasty applicator. A hypothetical model was developed and observations are discussed based on the numerical results of this study. A limited analysis on the thermal effects of microwave pulsing was also completed. Preliminary numerical calculations yielded reasonable results. Experimental laboratory tests are required to validate the accuracy of the numerical results found in this study. Further development of this thermal analysis method may greatly assist in future studies of new applicator configurations to ensure safe atherosclerotic plaque removal and to predict the resulting thermal environments.
Effects of Rehydration State on the Flexural Properties of Whole Mouse Long Bones
Authors: Broz J. J. , Simske S. J. , Greenberg A. R. , Luttges M. W.
Published: March 2008

The effects of bone water content during dehydration and rehydration on the flexural properties of whole mouse femora were evaluated using three-point bending. The elastic and plastic flexural properties of the bones were determined on a dry mass normalized basis over dehydration times ranging from 0.25 to 48.0 hr; and (following complete dehydration) rehydration times ranging from 0.08 to 12.0 hr. Bones stored in physiological saline for times <1 hr served as the control group. As expected, dehydration produced increased stiffness and strength along with decreased ductility. Upon rehydration, a statistically significant linear dependence of mechanical properties on recovered free water was obtained for all parameters except the maximum load. Elastic mechanical properties comparable to the controls were regained at differing rates and levels of recovered water content; however, after 3 hr of rehydration there were no statistically significant differences with respect to the control values. The results of this study indicate that the original flexural properties of whole mouse femora are preserved by air dehydration and can be recovered using appropriate saline rehydration intervals.
Finite Element Models for Arterial Wall Mechanics
Authors: Simon B. R. , Kaufmann M. V. , McAfee M. A. , Baldwin A. L.
Published: March 2008

Arterial wall mechanics has been studied for nearly 200 years. This subject is of importance if we are to gain a fundamental understanding of this complex biological structure, as well as information needed to design prosthetics. Biomechanical arterial models continue to play an important role in the study of atherosclerosis, a disease of the arterial wall that is the chief cause of mortality and morbidity in the United States and the Western World. Over the past 20 years, the finite element model (FEM) has been used in a variety of ways to simulate the structural response of large arteries. Our purpose is to summarize the uses of FEMs in arterial mechanics. We will also indicate directions for future research in this area. A specialized FEM was described in the literature for the study of transport in the arterial wall, however the convection was not directly linked to arterial wall mechanics. In this paper special attention will be given to the development of FEMs based on the poroelastic view of arterial tissues which couple wall deformation, free tissue fluid motion, and associated transport phenomena in the arterial wall. In the future such models should provide fundamental quantitative information relating arterial wall mechanics and transport which may lead to a better understanding of both normal arterial physiology and atherogenesis.
Measuring the Mechanical Properties of Individual Human Blood Cells
Authors: Hochmuth Robert M.
Published: March 2008

The largest human blood cells—the red cells (erythrocytes) and white cells (leukocytes)—must undergo a significant amount of deformation as they squeeze through the smallest vessels of the circulation and the small openings between bone, vessel and tissue. This ability to deform in response to external forces shows that cells exhibit material behavior and behave as either elastic solids or viscous liquids. The question then is “how can we measure the deformation and flow of something as small as a blood cell and what kinds of constitutive equations describe cellular deformation”? In this paper the use of the micropipet to measure red cell and white cell, especially neutrophil, deformation will be described and the viscoelastic models used to describe the deformation behavior of red cell membrane and neutrophil cytoplasm will be discussed. Values for the elasticities of a red cell membrane subjected to shear, area expansion and bending will be given. The viscosity of red cell membrane in shear will also be discussed. Finally, the cortical tension of the neutrophil and the Newtonian and Maxwell models used to characterize its apparent viscosity will be discussed even though neither is wholly successful in describing the viscous behavior of the neutrophil. Thus, alternate models will be suggested.
From Structure to Process, From Organ to Cell: Recent Developments of FE-Analysis in Orthopaedic Biomechanics
Authors: Huiskes Rik , Hollister Scott J.
Published: March 2008

The introduction of finite element analysis (FEA) into orthopaedic biomechanics allowed continuum structural analysis of bone and bone-implant composites of complicated shapes (Huiskes and Chao, J. Biomechanics, Vol. 16, 1983, pp. 385–409). However, besides having complicated shapes, musculoskeletal tissues are hierarchical composites with multiple structural levels that adapt to their mechanical environment. Mechanical adaptation influences the success of many orthopaedic treatments, especiallly total joint replacements. Recent advances in FEA applications have begun to address questions concerning the optimality of bone structure, the processes of bone remodeling, the mechanics of soft hydrated tissues, and the mechanics of tissues down to the microstructural and cell levels. Advances in each of these areas, which have brought FEA from a continuum stress analysis tool to a tool which plays an ever-increasing role in the scientific understanding of tissue structure, adaptation, and the optimal design of orthopaedic implants, are reviewed.
A 20-Year Perspective on the Mechanical Properties of Trabecular Bone
Authors: Keaveny Tony M. , Hayes Wilson C.
Published: March 2008

We have reviewed highlights of the research in trabecular bone biomechanics performed over the past 20 years. Results from numerous studies have shown that trabecular bone is an extremely heterogeneous material—modulus can vary 100-fold even within the same metaphysis—with varying degrees of anisotropy. Strictly speaking, descriptions of the mechanical properties of trabecular bone should therefore be accompanied by specification of factors such as anatomic site, loading direction, and age. Research efforts have also been focused on the measurement of mechanical properties for individual trabeculae, improvement of methods for mechanical testing at the continuum level, quantification of the three-dimensional architecture of trabecular bone, and formulation of equations to relate the microstructural and continuum-level mechanical properties. As analysis techniques become more sophisticated, there is now evidence that factors such as anisotropy and heterogeneity of individual trabeculae might also have a significant effect on the continuum-level properties, suggesting new directions for future research. Other areas requiring further research are the time-dependent and multiaxial failure properties at the continuum level, and the stiffness and failure properties at the lamellar level. Continued research in these areas should enhance our understanding of issues such as age-related bone fracture, prosthesis loosening, and bone remodeling.
Fracture Mechanics of Bone
Authors: Melvin J. W.
Published: March 2008

This paper reviews the progress that has been made in applying the principles of fracture mechanics to the topic of fracture of long bones. Prediction of loading conditions which result in the propagation of fractures in bones has been of interest to the field of trauma biomechanics and orthopedics for over one hundred years. Independent verifications, by various investigators, of bone fracture mechanics parameters are reviewed and investigations of the effects of bone density and specimen thickness on the critical fracture mechanics parameters and of other factors such as critical crack length and plastic zone size in bovine femoral bone, and the effects of crack velocity on fracture mechanics parameters in bovine tibial bone are discussed. It took over ten years for the techniques of bone fracture mechanics to be applied to human compact bone, due primarily to geometric constraints from the smaller size of human bones. That work will be reviewed along with other continuing work to define the orientation dependence of the fracture mechanics parameters in bone and to refine the experimental techniques needed to overcome the geometric constraints of specimen size. A discussion is included of work still needed to determine fracture mechanics parameters for transverse and longitudinal crack propagation in human bone and to establish the effects of age on those parameters. Finally, a discussion will be given of how this knowledge needs to be extended to allow prediction of whole bone fracture from external loading to aid in the design of protective systems.
Simulation and Animation of Musculoskeletal Joint System
Authors: Chao E. Y. S. , Lynch J. D. , Vanderploeg M. J.
Published: March 2008

This paper describes the development of computer-based software for three-dimensional geometric data base of the human musculoskeletal system. Using a computer graphics workstation, a user of the software will interactively display detailed information about the muscles, tendons, ligaments, bone, and joint anatomy. This software will enable a wide range of health care workers to viualize complex physiological data. In addition to geometric and visual realism, this software will include kinematic relationships which allow the calculation and display of the motion and forces of the joints, muscles, and tendons. This will permit a user to interactively move joints or tendons and display the resulting motion of the surrounding tissues, as well as internal reactive forces and joint pressure distribution. A two-dimensional version of this software is currently being used for knee and hip osteotomy preoperative planning, total joint replacement prosthesis design and dimensional selection, and osteochondral allograft sizing and reconstruction using radiographic data.
Spinal Biomechanics
Authors: Pope Malcolm H. , Novotny John E.
Published: March 2008

The lumbar spine is a source of disability due to low back pain (LBP), yet the precise diagnosis is unknown in 80-90 percent of patients. The lifetime prevalence is 75 percent with a cost to the U.S. economy as high as 80 billion dollars. The problem is partly caused by mechanical overloading of the tissues and thus, there is some potential for both primary and secondary prevention. Biomechanical techniques have been effective in improving our understanding of the loading conditions leading to LBP, and in developing techniques for improved diagnosis and more effectual methods of treatment. Much progress has been made through the use of biomechanical models. Most models assume that the external moments are balanced by trunk musculature. Multiple muscle system models, employing agonist and antagonists, now are available to define 3D spine reaction forces. The static indeterminacy is taken care of either by simplification of the model or by linear or nonlinear optimization. Dynamic analysis has shown that vibrational and impact conditions (such as vehicle driving) can excite the natural frequency of the spine and lead to high spinal loadings. In vivo measurements have shown the resonant frequency of the lumbar spine to be 4–5 Hz and many vehicles excite those frequencies. New biomechanical techniques employing electromyography can estimate muscle load and muscle fatigue. Stereo photogrammetric techniques for establishing segmental kinematics have great potential for improving the diagnosis of spinal problems. These techniques are solidly based on prior in-vitro measurements of spinal kinematics. Mechanical fixation techniques, such as pedicle fixation, show great promise in improving the treatment of spinal problems. These have been extensively analyzed by both finite element techniques and in-vitro simulation so as to improve design as well as surgical technique.