bluuebunny/crossref_metadata_2025 huggingface dataset:
Structural Evaluation of Ceramic Femoral Heads: Effect of Taper Friction, Support Conditions and Trunnion Compliance
Authors: Dragoni E. , Andrisano A. O.
Published: February 2008

The outcome of a nonlinear finite element stress analysis of ceramic heads for artificial hip joints is presented. The analysis mainly covers the influence of taper friction, support conditions and trunnion modulus of elasticity on the hoop stress distribution at the surface of the head bore. The paper quantifies how much the maximum tensile stress decreases with increasing frictional coefficient, with stiffening of the support and with stiffening of the trunnion material. An appreciable rise of the maximum tensile upon unloading of the head is also shown for the case of cup support. The computational results are found in close correspondence with photoelastic measurements of taper pressures and encourage the use, for preliminary design purposes, of an approximate theoretical model retrieved from the literature.
Relationship Between Muscle Force and Stiffness in the Whole Mammalian Muscle: A Simulation Study
Authors: Cholewicki Jacek , McGill Stuart M.
Published: February 2008

Several types of analyses in biomechanics require estimates of both muscle force and stiffness. Simulations were performed using the two-state cross-bridge Bond Distribution-Moment muscle model of Zahalak (1981), together with other parameters for passive elasticity and tendon compliance, to estimate instantaneous stiffness and to compare these estimates with the wide range of values reported in the literature. While the relatively simple cross-bridge theory appears to approximate the stiffness of skinned muscle fibers, the stiffness of a complete muscle-tendon unit become complex and non-linear due to relative changes in muscle-tendon length and interaction with activation and length dependent passive elastic components. It would appear that the variability in muscle stiffness values reported in the literature can be explained with the D-M approach.
Modeling Interstitial Flow in an Artery Wall Allows Estimation of Wall Shear Stress on Smooth Muscle Cells
Authors: Wang D. M. , Tarbell J. M.
Published: February 2008

The arterial media is modeled as a periodic array of cylindrical smooth muscle cells residing in a matrix comprised of proteoglycan and collagen fibers. Using Brinkman’s model to describe transmural flow through such a fibrous media, we calculate the effective hydraulic permeability of the media and the wall shear stress on smooth muscle cells. Two interesting results are obtained: first, the wall shear stress on smooth muscle cells is on the order of 1 dyne/cm2, which is in the range known to affect endothelial cells in vitro; second, the flow resistance due to smooth muscle cells is not negligible compared to the resistance due to the fiber matrix.
Optimization Technique for the Calculation of In Vitro Three-Dimensional Vertebral Motion
Authors: Shea M. , Edwards W. T. , White A. A. , Hayes W. C.
Published: February 2008

A method for the calculation of translations and Eulerian rotations of an orthogonal axis system with respect to a fixed reference is described with application to the measurement of position in a vertebral motion segment. Kinematic equations were derived to compute the three-dimensional motion of a moving vertebra relative to an adjacent fixed body, without the requirement of a direct physical link between the two bodies. For this calculation, the quadratic error of the lengths of six position vectors was minimized to obtain a mathematically optimal estimate of the translations and rotations. Tests with a rigid model resulted in mean maximum overall system errors of 2.8 percent for the measurement of translation (translations less than 3.5 mm) and 6.1 percent for the measurement of rotations (rotations less than 10 deg) limited by transducer accuracy. The mathematical techniques presented for the quantitative description of rigid body motion, based on the measurement of three reference vectors, may be extended to a broad range of kinematic problems.
Energy-Shunting Hip Padding System Attenuates Femoral Impact Force in a Simulated Fall
Authors: Robinovitch S. N. , Hayes W. C. , McMahon T. A.
Published: February 2008

Recent studies suggest that hip padding systems reduce the incidence of hip fractures during falls. However, no data exist on the force attenuating capacity of hip pads under realistic fall impact conditions, and thus it is difficult to compare the protective merit of various pad designs. Our goal is to design a comfortable hip padding system which reduces femoral impact force in a fall below the mean force required to fracture the elderly cadaveric femur. In pursuit of this objective, we designed and constructed a hip pad testing system consisting of an impact pendulum and surrogate human pelvis. We then developed a hip pad containing a shear-thickening material which allows for shunting of the impact energy away from the femur and into the surrounding soft tissue. Finally, we conducted experiments to assess whether the surrogate pelvis accurately represents the impact behavior of the human female pelvis in a fall, and to determine whether our energy-shunting pad attenuates femoral impact force in a fall more effectively than seven available padding systems. We found the surrogate pelvis accurately represented the human female pelvis in regional variation in soft tissue stiffness, total effective stiffness and damping, and impact force attenuation provided by trochanteric soft tissues. We also found that our padding system attenuated femoral impact force by 65 percent, thereby providing two times the force attenuation of the next best system. Moreover, the energy-shunting pad was the only system capable of lowering femoral impact force well below the mean force required to fracture the elderly femur in a fall loading configuration. These results suggest that the force attenuating potential of hip pads which focus on shunting energy away from the femur is superior to those which rely on absorbing energy in the pad material. While these in-vitro results are encouraging, carefully designed prospective clinical trials will be necessary to determine the efficacy of these approaches to hip fracture prevention.
Hindered Transport of Macromolecules Through a Single Row of Cylinders: Application to Glomerular Filtration
Authors: Drumond M. C. , Deen W. M.
Published: February 2008

The slit diaphragms of renal glomerular capillaries form an ultrafiltration barrier which may be approximated as a row of cylindrical fibers of macromolecular dimensions. To describe the hindered transport of plasma proteins and other macromolecules through this barrier, we developed an approximate hydrodynamic model for spherical, Brownian particles passing through a row of infinitely long cylinders. The selectivity of the slit diaphragm was assessed by computing concentration profiles for a wide range of molecular sizes for Pe ≤ 1, where Pe is a Peclet number based on the cylinder radius. The sieving coefficient for the slit diaphragm was computed as θSD = CB/CO, where CO was the average concentration at a specified distance upstream from the cylinders (corresponding to the location of the basement membrane), and CB was the concentration far downstream (corresponding to Bowman’s space). The results of previous experimental sieving studies using rats could be accounted for approximately by postulating a wide distribution of spacings between the fibers of the slit diaphragm. Comparing the results for θSD with calculations for a model of the glomerular basement membrane suggests that the slit diaphragm is by far the more size-restrictive part of the overall barrier.
Separation Surfaces for Laminar Flow in Branching Tubes—Effect of Reynolds Number and Geometry
Authors: Carr R. T. , Kotha S. L.
Published: February 2008

Flow visualization experiments of Newtonian laminar flow through branching tubes have been performed to identify the shape of the separation surface or flow divider. The influences of Reynolds number, flow fraction into the side branch, and branch geometry on the separation surface shape have been considered. The shapes presented in this paper are formed by the intersection of the separation surface with the cross section of the parent tube. At low Re the separation surfaces are curved in a convex manner, bulging away from the opening of the side branch. Increasing Re causes the surface to become concave. At Re > 194 the surfaces can become closed for Q* > 0.3. The branch angle has no noticeable effect on the separation surface shape. The side to parent branch diameter ratio, Db/Dp, has a strong influence at low Re. As Re increases the diameter effect diminishes. Previous studies have shown no difference between the separation surfaces of T and Y type junctions at low Re. At Re = 194 there is a marked difference between the separation surfaces of T and Y bifurcations.
Analysis of Thermal Injury Process Based on Enzyme Deactivation Mechanisms
Authors: Xu Yunsheng , Qian Renzhang
Published: February 2008

Based upon the analysis of enzyme-catalyzed reactions occurring in living tissue, a model of thermal injury process is presented in which the fraction of denatured enzyme protein was taken as the indicator of thermal damage degrees. The results from this model describe the dependence of thermal damage on exposure time and temperature elevation.
An Analytical Model to Study Blunt Impact Response of the Rabbit P-F Joint
Authors: Li Xiaowei , Haut Roger C. , Altiero Nicholas J.
Published: February 2008

While mechanisms of post-traumatic osteoarthrosis are largely unknown, excessive stresses and strains generated in articular cartilage and the underlying bone may play a role. In this manuscript a technique is described for studying the impact response of a diarthrodial joint. A mathematical model of the rabbit PF joint indicated that contact pressures predicted by a quasi-static plane strain linear elastic model compared well with experimental data when Poisson’s ratio and Young’s modulus of the cartilage were 0.49 and 2 MPa, respectively. This value for the elastic modulus compared well with that obtained from elastic analysis of short-time indentation experiments on cartilage from a previous study. The model analysis also suggested that surface fissuring of patellar cartilage occurs near areas where shear stresses and tensile strains are high. Impact location on the patella significantly influenced the distributions of shear stress along the bone-cartilage interface and tensile strains in the cartilage. These results may help explain some of the mechanisms of initial tissue damage reported elsewhere. Limited experimental data are presented here but the value of such mathematical models for estimation of material properties and for analysis of damage creation is clearly demonstrated.
Near-Boom Oil-Slick Instability Criterion in Viscous Flows and the Influence of Free-Surface Boundary Conditions
Authors: Sundararaghavan H. , Ertekin R. C.
Published: February 2008

Booms are often used to contain oil spills prior to various oil removal techniques. Under certain conditions, the oil droplets can leave the oil slick and enter the water. A simple balance of hydrodynamic forces on a droplet results in an instability criterion which determines whether the droplet will be swept past the boom or not. For viscous flows, it is shown here that the instability criterion consists of a term proportional to the pressure gradient along the boom, as in the potential-flow case, and a term that is inversely proportional to the Reynolds number, although the magnitude of this new term is found to be very small. The solution of viscous flow past an oil boom is obtained using the fractional-step method in a curvilinear coordinate system and the instability criterion is estimated. The influence of the approximate free-surface conditions, such as the rigid-lid no-slip, rigid-lid free-slip, and the exact free-surface condition, the instability criterion is also investigated. The different approximations of free-surface conditions are shown to influence the pressure distributions, thus resulting in different neutral stability curves.
An Analytical Approach to Understanding the “Pressure Gain” Combustor
Authors: Janus M. C. , Richards G. A. , Gemmen R. S. , Johnson E. K.
Published: February 2008

Although pulse combustion has been successfully utilized in various commercial applications, one potential application yet to reach the market is the pressure gain gas turbine (PGGT). A PGGT would incorporate a pulse combustor rather than the typical steady-flow combustor to increase system efficiency and decrease pollutant emissions. The distinctive advantage of pulse combustion is its ability to achieve a stagnation “pressure gain” from inlet to exit. A primary concern with pressure gain combustion development, however, is the lack of understanding as to how a combustor should be designed to achieve a pressure gain. While significant progress has been made in understanding the fundamental controlling physics of pulse combustor operation, little research has been aimed at understanding and predicting whether a given system will produce pressure gain. The following paper proposes a simple framework which helps to explain how a pulse combustor achieves a stagnation pressure gain from inlet to exit. The premise behind the framework is that pressure gain can be achieved by closely approximating a constant volume combustion process, is closely approximated by matching the resonant and operating frequencies of the system. The framework is primarily based upon results from a one-dimensional method-of-characteristics model.
Borehole Failure Resulting From Formation Integrity (Leak-Off) Testing in Upper Marine Sediments Offshore
Authors: Wojtanowicz A. K. , Zhou D.
Published: February 2008

This paper presents the results of a theoretical study, supported with the finite element analysis, into potential loss of external integrity around a casing shoe resulting from leak-off testing (LOT) in upper marine sediments (UMS). Three types of possible failures from LOTs were considered: vertical fracture, horizontal fracture, and a channel outside cemented annulus. It is proved in the paper that vertical fracture is the most unlikely failure of the three. The other two types of failure can be distinguished by different values of propagation pressures. Although horizontal fractures are initiated at low pressure in the plastic zone around the wellbore, they cannot propagate beyond the plastic zone until wellbore pressures exceed overburden pressures. Annular channels, on the other hand, may propagate upwards at pressures lower than overburden pressure. The paper shows that these channels are initiated at pressures equal to the contact stress between cement and rock and their propagation pressures are on average 3.5-fold greater than contact stress. It is also explained how to identify the UMS with high risk of annular channeling during LOTs.
Pipe Bending and Running Forces in Medium to High-Curvature Wells Using Finite Element Analysis
Authors: Seibi A. C. , Al-Shabibi A. M.
Published: February 2008

The present paper describes the running process in horizontal wells and studies the effect of some factors on running forces required to push pipes through curved holes with short to medium radii of curvatures. Estimation of the running forces was performed using a general-purpose finite element program called ANSYS. The effect of pipe bending stiffness, hole radius of curvature, and hole clearance are investigated. Finite element results showed that the pipe bending stiffness becomes insignificant for medium curvatures (i.e., radius of curvature greater than 80 m). It was also found that the running force at the kick-off point (k.o.p) increases as the radius of curvature shortens (severe doglegs) and as the pipe stiffness increases. In addition, FE results revealed that the effect of hole clearance on the running force is negligible.
Modeling of Multiphase Wax Deposition
Authors: Elphingstone G. M. , Greenhill K. L. , Hsu J. J. C.
Published: February 2008

The environmental conditions typically encountered in deepwater petroleum operations lead to a number of production problems. The cold environment promotes particle formation and deposition of paraffinic compounds in a waxy crude system. These solids can build up inside flowlines and significantly decrease production rates. The wax deposition scale-up technique used for single phase has been applied to multiphase flow conditions. The resulting semi-empirical model is used to simulate multiphase transport of waxy crude oil through integration with a commercial steady-state pipeline simulation package. The wax model is a semi-empirical representation of the wax deposition phenomena. The model incorporates the effects of diffusion and shear through the wax deposition tendency. The key parameters are first measured in a laboratory flow loop.
A Two-Dimensional Numerical Study of Heat Transfer in a Finned Tube Assembly During Axisymmetric Dehumidification
Authors: Rosario L. , Rahman M. M.
Published: February 2008

This paper presents the analysis of two-dimensional heat transfer in an annular finned tube assembly during the process of dehumidification. All possible fin surface conditions, namely, dry, fully wet, and partially wet, have been studied and fin efficiency under these conditions have been modeled. Computations have been carried out using a control volume-based finite-difference method and compared with past one-dimensional analytical studies and available experimental data. The parameters that influenced the heat transfer rate in the finned tube structure are ratio of fin and wall thermal conductivities, ratio of fin thickness to fin pitch, ratio of wall thickness to fin pitch, ratio of fin length to fin pitch, cold fiuid Biot number, ambient Blot number, the relative humidity and dry bulb temperature of the incoming air, and the cold fluid temperature inside the coil. It was found that the heat transfer increased with increment in both dry bulb temperature and the relative humidity of the air. The fin efficiency changed rapidly with relative humidity under partially wet condition. The results suggest that coil performance can be very significantly altered by the condensation phenomenon on the fin surface and designs with dry fin data may not be adequate. The present results are expected to be very useful for the design of dehumidifier (cooling) coils for air conditioning applications.
Optimal Planning of a Cascade-Type Multistage Refrigeration System for a Beverage Plant
Authors: Shiba T. , Ito K. , Yokoyama R. , Sakashita S. , Himura Y.
Published: February 2008

An optimal planning method is presented for a cascade-type multistage refrigeration system. Heat exchange areas of evaporator, condenser, and beverage cooler are determined optimally so as to minimize the annual total cost and input energy consumption subject to constraints concerning annual equipment operation. This problem is considered as a multiobjective optimization one, and a discrete set of Pareto optimal solutions is derived numerically by a weighting method. Through a numerical study, it is investigated how the heat exchange areas influence the long-term economics and energy conservation. Cascade-type multistage refrigeration systems are compared with single-stage systems.
Analysis of the Inverse Problem of Freezing and Thawing of a Binary Solution During Cryosurgical Processes
Authors: Budman Hector , Shitzer Avraham , Dayan Joshua
Published: February 2008

An integral solution for a one-dimensional inverse Stefan problem is presented. Both the freezing and subsequent thawing processes are considered. The medium depicting biological tissues, is a nonideal binary solution wherein phase change occurs over a range of temperatures rather than at a single one. A constant cooling, or warming, rate is imposed at the lower temperature boundary of the freezing/thawing front. This condition is believed to be essential for maximizing cell destruction rate. The integral solution yields a temperature forcing function which is applied at the surface of the cryoprobe. An average thermal conductivity, on both sides of the freezing front, is used to improve the solution. A two-dimensional, axisymmetric finite element code is used to calculate cooling/warming rates at positions in the medium away from the axis of symmetry of the cryoprobe. It was shown that these cooling/warming rates were always lower than the prescribed rate assumed in the one-dimensional solution. Thus, similar, or even higher, cell destruction rates may be expected in the medium consistent with existing in vitro data. Certain problems associated with the control of the warming rate during the melting stage are discussed.
Blood Flow Velocity Profiles in Pulmonary Branch Arteries in Lambs
Authors: Katayama H. , Henry G. W. , Lucas C. L. , Ha B. , Ferreiro J. I. , Frantz E. G.
Published: February 2008

We studied the detailed profiles of blood flow in the right and left pulmonary arteries using 20 MHz pulsed Doppler ultrasound equipment in a lamb model. Fourteen lambs aged four to six weeks were selected. In six lambs, monocrotaline pyrrole was injected parenterally to create pulmonary hypertension (PH group). Eight other lambs served as unaltered controls (control group). The blood flow velocities were sampled in 1mm increments along the anterior—posterior axis of the branch arteries. The maximum velocity of the forward flow in the left pulmonary artery was higher than that in the right pulmonary artery in the control group (71.7 ± 15.9cm/s vs 60.2 ± 13.5; p < 0.05). The fastest backward flow was located at the posterior position of the vessel in the right pulmonary artery in the control group. No significant bias in location was shown in the left pulmonary artery. Using indices of P90, acceleration time, P90*AcT, the velocity waveforms in the PH group were compared with those in the control group. In the left pulmonary artery, every index in the control group showed a significantly greater value that in the PH group. On the other hand, no significant differences were found between either group in the right pulmonary artery.
Effects of Particle Concentration on the Partitioning of Suspensions at Small Divergent Bifurcations
Authors: Ditchfield R. , Olbricht W. L.
Published: February 2008

Experimental results are reported for the low Reynolds number flow of a suspension of spherical particles through a divergent capillary bifurcation consisting of a straight tube of circular cross-section that splits to form two tubes of equal diameter. The partitioning of particles between the downstream branches of the bifurcation is measured as a function of the partitioning of total volume (particles + suspending fluid) between the branches. Two bifurcation geometries are examined: a symmetric Y-shaped bifurcation and a nonsymmetric T-shaped bifurcation. This experiment focuses on the role of hydrodynamic interactions between particles on the partitioning of particles at the bifurcation. The particle diameter, made dimensionless with respect to the diameter of the branch tubes, ranges from 0.4 to 0.8. Results show that hydrodynamic interactions among the particles are significant at the bifurcation, even for conditions where interactions are unimportant in the straight branches away from the bifurcation. As a result of hydrodynamic interactions among particles at the bifurcation, the partitioning of particles between the branches is affected for particle volume fractions as small as 2 percent. The experimental results show that the effect of particle volume fraction is to diminish the inhomogeneity of particle partitioning at the bifurcation. However, the magnitude of this effect depends strongly on the overall shape of the bifurcation geometry, and, in particular on the angles between the branches.
The Conformity of a Soft Contact Lens on the Eye
Authors: Funkenbusch GuoMing T. , Benson Richard C.
Published: February 2008

An axisymmetric elastic shell deformation model has been created to predict the conformity of a soft contact lens when pressed against the eye. Regions of contact and gap may be predicted, and the nonuniform reaction pressure between the lens and eye may also be found. This is important for issues like abrasion and comfort. Bending, membrane and transverse shear loads within the lens are also computed. Commercial soft contact lenses and a representative eye shape are used for the examples. We find that the uniformity of loading against the eye is strongly affected by the degree to which the lens is shaped to fit the eye, and relatively unaffected by the thickness of the lens.
On the Neuro-Muscular Control of Trochlean Joints
Authors: Balthazar Muriel , Mountian Irina , Willems Pierre Y.
Published: February 2008

This paper presents a simplified dynamical model for the control of one-degree-of-freedom synovial joints considered as pure trochlean joints. This model considers the joint dynamics, the dynamics of the corresponding muscles and their calcium balance dynamics, as well as position and force feedbacks provided by the spindles and the Golgi tendon organs. Delays in the transmission of information are also taken into account as they proved to be of critical importance for the dynamical behavior of the considered systems. The linearized version of this model, which is valid for a rather wide range of movements, also allows us to investigate the stability of the system, as well as its stability robustness with respect to the feedback gains. Further, particular behaviors such as tremor are described.
Automated Finite Element Analysis of Excised Human Femora Based on Precision-QCT
Authors: Merz Beat , Niederer Peter , Mu¨ller Ralph , Ru¨egsegger Peter
Published: February 2008

The creation of 3-D finite element (FE) models of bone and implant-bone systems is a labor-intensive task due to the need to model different cases and variations, to perform patch tests and to account for the nonhomogeneous material properties with an acceptable amount of work. In this study, we developed an interface between a precision QCT and a FE system and applied it to a specimen bone. With the new method, the time necessary for model generation was reduced substantially. Furthermore, the elastic properties for each element were automatically derived from the corresponding CT-values. In order to demonstrate the importance of taking into account inhomogeneity, a comparison between the nonhomogeneous model and a homogeneous, “averaged” model was performed.
Factors Affecting the Pullout Strength of Cancellous Bone Screws
Authors: Chapman J. R. , Harrington R. M. , Lee K. M. , Anderson P. A. , Tencer A. F. , Kowalski D.
Published: February 2008

Screws placed into cancellous bone in orthopedic surgical applications, such as fixation of fractures of the femoral neck or the lumbar spine, can be subjected to high loads. Screw pullout is a possibility, especially if low density osteoporotic bone is encountered. The overall goal of this study was to determine how screw thread geometry, tapping, and cannulation affect the holding power of screws in cancellous bone and determine whether current designs achieve maximum purchase strength. Twelve types of commercially available cannulated and noncannulated cancellous bone screws were tested for pullout strength in rigid unicellular polyurethane foams of apparent densities and shear strengths within the range reported for human cancellous bone. The experimentally derived pullout strength was compared to a predicted shear failure force of the internal threads formed in the polyurethane foam. Screws embedded in porous materials pullout by shearing the internal threads in the porous material. Experimental pullout force was highly correlated to the predicted shear failure force (slope = 1.05, R2 = 0.947) demonstrating that it is controlled by the major diameter of the screw, the length of engagement of the thread, the shear strength of the material into which the screw is embedded, and a thread shape factor (TSF) which accounts for screw thread depth and pitch. The average TSF for cannulated screws was 17 percent lower than that of noncannulated cancellous screws, and the pullout force was correspondingly less. Increasing the TSF, a result of decreasing thread pitch or increasing thread depth, increases screw purchase strength in porous materials. Tapping was found to reduce pullout force by an average of 8 percent compared with nontapped holes (p = 0.0001). Tapping in porous materials decreases screw pullout strength because the removal of material by the tap enlarges hole volume by an average of 27 percent, in effect decreasing the depth and shear area of the internal threads in the porous material.
Axisymmetric Finite Element Analysis of a Debonded Total Hip Stem With an Unsupported Distal Tip
Authors: Norman T. L , Saligrama V. C. , Hustosky K. T. , Gruen T. A. , Blaha J. D.
Published: February 2008

A tapered femoral total hip stem with a debonded stem-cement interface and an unsupported distal tip subjected to constant axial load was evaluated using two-dimensional (2D) axisymmetric finite element analysis. The analysis was performed to test if the mechanical condition suggest that a “taper-lock” with a debonded viscoelastic bone cement might be an alternative approach to cement fixation of stem type cemented hip prosthesis. Effect of stem-cement interface conditions (bonded, debonded with and without friction) and viscoelastic response (creep and relaxation) of acrylic bone cement on cement mantle stresses and axial displacement of the stem was also investigated. Stem debonding with friction increased maximum cement von Mises stress by approximately 50 percent when compared to the bonded stem. Of the stress components in the cement mantle, radial stresses were compressive and hoop stresses were tensile and were indicative of mechanical taper-lock. Cement mantle stress, creep and stress relaxation and stem displacement increased with increasing load level and with decreasing stem-cement interface friction. Stress relaxation occur predominately in tensile hoop stress and decreased from 1 to 46 percent over the conditions considered. Stem displacement due to cement mantle creep ranged from 614 μm to 1.3 μm in 24 hours depending upon interface conditions and load level.
Kinematics and Dynamic Stability of the Locomotion of Post-Polio Patients
Authors: Hurmuzlu Yildirim , Basdogan Cagatay , Stoianovici Dan
Published: February 2008

The study reported in this article was conducted to propose a set graphical and analytical tools and assess their clinical utility by analyzing gait kinematics and dynamics of polio survivors. Phase-plane portraits and first return maps were used as graphical tools to detect abnormal patterns in the sagittal kinematics of post-polio gait. Two new scalar measures were introduced to assess the bilateral kinematic symmetry and dynamic stability of human locomotion. Nine healthy subjects and seventeen post-polio patients were involved in the project. Significant increases in the knee extension and ankle plantar flexion of post-polio patients were observed during the weight acceptance phases of their gait. Polio patients also exhibited highly noticeable excessive hip flexion during the swing phase of their ambulation. Using the proposed symmetry measure, we concluded that post-polio patients walked less symmetrically than normals. Our conclusion, however, was based on the bilateral symmetry in the sagittal plane only. Finally, we observed that post-polio patients walked significantly less stably than normals. In addition, weaknesses in lower extremity muscles of polio patients were found to be an important factor that affected stable ambulation.
A Device for the Measurement of Pedicle Screw Moments by Means of Internal Strain Gauges
Authors: Smith Tait S. , Yerby Scott A. , McLain Robert F. , McKinley Todd O.
Published: February 2008

Pedicle screws are commonly used in spinal reconstruction, and failure of pedicle screws due to bending is a significant clinical problem. To measure the moments typically placed on pedicle screws in situ we instrumented 7 mm Cotrel-Dubousset (CD) pedicle screws with internally mounted strain gauges. The screws were designed to measure flexion-extension moments at a single cross-section as dictated by strain gauge placement. It is possible to measure moments of up to 12 Nm at any location along the length of the screw by constructing transducers with varying strain gauge placements. These transducers are capable of measuring moments at points located within the vertebra including the pedicle, which is where failure usually occurs clinically. Transducer output was both linear and reproducible. These transducers are being used to investigate the load transfer characteristics between the pedicle screw and the vertebra. This technique could be applied to investigations of load sharing in reconstruction plates, lag-screws, and cross-locked intra-medullary nails.
A New Approach to Mechanical Testing and Modeling of Biological Tissues, With Application to Blood Vessels
Authors: Brossollet L. J. , Vito R. P.
Published: February 2008

The collection and processing of data from mechanical tests of biological tissues usually follow classical principles appropriate for studying engineering materials. However, difficulties specific to biological tissues have generally kept such methods from producing quantitative results for statistically-oriented studies. This paper demonstrates a different approach linking testing and data reduction with modern statistical tools. Experimental design theory is used to minimize the detrimental effects of collinearity on the stability of the parameters in constitutive equations. The numerical effects of time-dependent biasing factors such as viscoelasticity are reduced by randomizing the order of collection of data points. Some of the parameters of the model are allowed to vary from specimen to specimen while the others are computed once from a database of designed experiments on several specimens. Finally, a new self-modeling algorithm based on principal component analysis is used to generate uncorrelated parameters for a model that is linear in its specimen-dependent parameters. The method, associated with a recently published complementary energy formulation for vascular mechanics, is illustrated with biaxial canine saphenous vein data. Results show that three specimen-dependent linear parameters are enough to characterize the experimental data and that they can be repeatedly estimated from different data sets. Independently collected biaxial inflation data can also be predicted reasonably well with this model.
A Three-Dimensional Finite Element Method for Large Elastic Deformations of Ventricular Myocardium: I—Cylindrical and Spherical Polar Coordinates
Authors: Costa K. D. , Hunter P. J. , Rogers J. M. , Guccione J. M. , Waldman L. K. , McCulloch A. D.
Published: February 2008

A three-dimensional Galerkin finite element method was developed for large deformations of ventricular myocardium and other incompressible, nonlinear elastic, anisotropic materials. Cylindrical and spherical elements were used to solve axisymmetric problems with r.m.s. errors typically less than 2 percent. Isochoric interpolation and pressure boundary constraint equations enhanced low-order curvilinear elements under special circumstances (69 percent savings in degrees of freedom, 78 percent savings in solution time for inflation of a thick-walled cylinder). Generalized tensor products of linear Lagrange and cubic Hermite polynomials permitted custom elements with improved performance, including 52 percent savings in degrees of freedom and 66 percent savings in solution time for compression of a circular disk. Such computational efficiencies become significant for large scale problems such as modeling the heart.
Aspects of the Mechanics of Lobed Liposomes
Authors: Pamplona D. C. , Calladine C. R.
Published: February 2008

Hotani has studied, by means of dark-field light microscopy, morphological transformations which unilamellar liposomes undergo when their interior volume decreases steadily with time as a consequence of osmosis. In a previous paper, we made a theoretical study of the initial buckling of an originally spherical vesicle into the observed oblate spheroidal shape; and we argued that some in-plane shear elastic stiffness is required—in addition to the well-known flexural stiffness of the lipid bilayer—in order to explain the observed phenomena. In the present paper, we consider a later stage in the chain of morphological transitions observed by Hotani, when a series of cudgel-shaped lobes have sprung out of a previously axisymmetric, biconcave-shaped vesicle. Specifically, we compare the observed shapes of such lobes with half of a series of “peanut”-shaped vesicles that are an equilibrium conformation of an initially spherical liposome under reduced internal volume. We find that the shapes do not match well. On the other hand, the observed lobe forms do match satisfactorily portions of “undulating tube” shapes which evolve from a hypothetical cylindrical vesicle, according to some simple calculations. In view of this agreement, we are led to propose that the formation of cudgel-shaped lobes requires some sliding of one lipid monolayer over another. This conflicts, of course, with the Love-Kirchhoff hypothesis which is normally invoked at the outset of analyses of lipid vesicles by means of classical thin-shell theory; but it is in accord with previous suggestions in the context of more obviously severe distortion of the lipid bilayer.
Pulsatile Albumin Transport in Large Arteries: A Numerical Simulation Study
Authors: Rappitsch G. , Perktold K.
Published: February 2008

Albumin transport in a stenosed artery configuration is analyzed numerically under steady and pulsatile flow conditions. The flow dynamics is described applying the incompressible Navier-Stokes equations for Newtonian fluids, the mass transport is modelled using the convection diffusion equation. The boundary conditions describing the solute wall flux take into account the concept of endothelial resistance to albumin flux by means of a shear dependent permeability model based on experimental data. The study concentrates on the influence of steady and pulsatile flow patterns and of regional variations in vascular geometry on the solute wall flux and on the ratio of endothelial resistance to concentration boundary layer resistance. The numerical solution of the Navier-Stokes equations and of the transport equation applies the finite element method where stability of the convection dominated transport process is achieved by using an upwind procedure and a special subelement technique. Numerical simulations are carried out for albumin transport in a stenosed artery segment with 75 percent area reduction representing a late stage in the progression of an atherosclerotic disease. It is shown that albumin wall flux varies significantly along the arterial section, is strongly dependent upon the different flow regimes and varies considerably during a cardiac cycle. The comparison of steady results and pulsatile results shows differences up to 30 percent between time-averaged flux and steady flux in the separated flow region downstream the stenosis.
Effect of Physiological Factors on Proximal Flow Convergence Upstream of an Incompetent Valve: An In-Vitro Study
Authors: Guenet F. S. A. , Walker P. G. , Doyle M. W. , Pohost G. M. , Yoganathan A. P.
Published: February 2008

The flow (Q) through regurgitant valves may be quantified by multiplying the area of an isovelocity contour (isovel) by its velocity. This was tested computationally and experimentally (using MRI), Q = 14 to 141 ml/s, using flat and conical orifice plates. Plotting Q versus isovelocity radius, a plateau was found which, for low flow, corresponded to the true Q. At higher flow or large confinement, Q was overestimated. For conical plates, angle correction worked at low Q but not at higher values due to the formation of separation regions. These converted the cone plate into a flat plate. MRI produced similar results at 57 ml/s in that Q was correct with no angle correction. At low flow, MRI was too noisy to produce a clear plateau consistently.
Sensitivity Analysis of One-Dimensional Heat Transfer in Tissue With Temperature-Dependent Perfusion
Authors: Davies C. R. , Saidel G. M. , Harasaki H.
Published: February 2008

Design criteria for implantable, heat-generating devices such as the total artificial heart require the determination of safe thresholds for chronic heating. This involves in-vivo experiments in which tissue temperature distributions are obtained in response to known heat sources. Prior to experimental studies, simulation using a mathematical model can help optimize the design of experiments. In this paper, a theoretical analysis of heat transfer is presented that describes the dynamic, one-dimensional distribution of temperature from a heated surface. Loss of heat by perfusion is represented by temperature-independent and temperature-dependent terms that can reflect changes in local control of blood flow. Model simulations using physiologically appropriate parameter values indicate that the temperature elevation profile caused by a heated surface adjacent to tissue may extend several centimeters into the tissue. Furthermore, sensitivity analysis indicates the conditions under which temperature profiles are sensitive to changes in thermal diffusivity and perfusion parameters. This information provides the basis for estimation of model parameters in different tissues and for prediction of the thermal responses of these tissues.
A Numerical Investigation of the Mechanics of Swelling-Type Intramedullary Hip Implants
Authors: Kalidindi S. R. , Ahmad P.
Published: February 2008

The novel concept of swelling-type intramedullary hip implants that attain self-fixation by an expansion-fit mechanism resulting from controlled swelling of the implant (by absorption of body fluids) was examined in detail using a finite element model of the implant-femur system. Some of the potential advantages of this technique over traditional techniques include enhanced fixation, lower relative micromotions, improved bony ingrowth, and elimination of acrylic cement. The finite element model created in this study incorporated: (i) the major aspects of the three-dimensional geometry of the implant and femur, (ii) the anisotropic elastic properties of bone and implant materials and the changes in orientation of the principal axes of anisotropy along the length of the implant-femur system, (iii) a layer of cancellous bone between the implant and cortical bone in the proximal femoral region, and (iv) frictional sliding between the bone and implant. The model was used to study quantitatively the parametric influence of various material design variables on the micromotions and stress fields in the bone-swelling-type implant system. The results of the finite element analyses were used to establish material behavior goals and provide targets for a material development study.
Wall Shear Rate Distribution in an Abdominal Aortic Bifurcation Model: Effects of Vessel Compliance and Phase Angle Between Pressure and Flow Waveforms
Authors: Lee C. S. , Tarbell J. M.
Published: February 2008

The goal of this study was to determine how vessel compliance (wall motion) and the phase angle between pressure and flow waves (impedance phase angle) affect the wall shear rate distribution in an atherogenic bifurcation geometry under sinusoidal flow conditions. Both rigid and elastic models replicating the human abdominal aortic bifurcation were fabricated and the wall shear rate distribution in the median plane of the bifurcation was determined using the photochromic flow visualization method. In the elastic model, three phase angle conditions were simulated (+12, −17, −61 deg), and the results compared with those obtained in a similar rigid model. The study indicates a very low (magnitude close to zero) and oscillatory wall shear rate zone within 1.5 cm distal to the curvature site on the outer (lateral) wall. In this low shear rate zone, unsteadiness (pulsatility) of the flow greatly reduces the mean (time-averaged) wall shear rate level. Vessel wall motion reduces the wall shear rate amplitude (time-varying component) up to 46 percent depending on the location and phase angle in the model. The mean wall shear rate is less influenced by the wall motion, but is reduced significantly in the low shear region (within 1.5 cm distal to the curvature site on the outer wall), thus rendering the wall shear rate waveform more oscillatory and making the site appear more atherogenic. The effect of the phase angle is most noteworthy on the inner wall close to the flow divider tip where the mean and amplitude of wall shear rate are 31 and 23 percent lower, respectively, at the phase angle of −17 deg than at −61 deg. However, the characteristics of the wall shear rate distribution in the low shear rate zone on the outer wall that are believed to influence localization of atherosclerotic disease, such as the mean wall shear rate level, oscillation in the wall shear rate waveform, and the length of the low and oscillatory wall shear rate zone, are similar for the three phase angles considered. The study also showed a large spatial variation of the phase angle between the wall shear stress waveform and the circumferential stress waveform (hoop stress due to radial artery expansion in response to pressure variations) near the bifurcation (up to 70 deg). The two stresses became more out of phase in the low mean shear rate zone on the outer wall (wall shear stress wave leading hoop stress wave as much as 125 deg at the pressure-flow phase angle of −61 deg) and were significantly influenced by the impedance phase angle.
LDA Measurements of Velocities in a Simulated Collapsed Tube
Authors: Bertram C. D. , Godbole S. A.
Published: February 2008

A perspex (plexiglas) tube was locally deformed into an almost bi-lobar interior cross section, representative of the localized throat at the downstream end of a collapsed tube conveying a flow. The axial and transverse (parallel to the long axis of the deformed cross section) components of fluid velocity were measured in a dense rectangular grid of points covering the whole cross section, at 15 axial sites between one diameter upstream of and three diameters downstream of the center of the constriction. The Reynolds number based on undeformed tube diameter and mean velocity was 705. Results are presented both as surfaces showing the variation of each component over the cross section and as velocity vector profiles. The overall changes in velocity in the streamwise direction are presented in terms of the variation of the maximum and minimum of each component with axial position. Flow downstream of the throat consisted of two parallel side-jets with a broad region of reverse flow in between. This pattern persisted until beyond 2.5 diameters downstream, by which point transverse inflow at the top and bottom of the cross section had converted the side jets into a complete annulus of axial velocity surrounding a central deficit. Jet velocities and reverse flow disappeared relatively abruptly before three diameters downstream.