bluuebunny/crossref_metadata_2025 huggingface dataset:
Development and Evaluation of a Musculoskeletal Model of the Elbow Joint Complex
Authors: Gonzalez R. V. , Hutchins E. L. , Barr R. E. , Abraham L. D.
Published: February 2008

This paper describes the development and evaluation of a musculoskeletal model that represents human elbow flexion-extension and forearm pronation-supination. The length, velocity, and moment arm for each of the eight musculotendon actuators were based on skeletal anatomy and joint position. Musculotendon parameters were determined for each actuator and verified by comparing analytical moment-angle curves with experimental joint torque data. The parameters and skeletal geometry were also utilized in the musculoskeletal model for the analysis of ballistic (rapid-directed) elbow joint complex movements. The key objective was to develop a computational model, guided by parameterized optimal control, to investigate the relationship among patterns of muscle excitation, individual muscle forces, and to determine the effects of forearm and elbow position on the recruitment of individual muscles during a variety of ballistic movements. The model was partially verified using experimental kinematic, torque, and electromyographic data from volunteer subjects performing both isometric and ballistic elbow joint complex movements. This verification lends credibility to the time-varying muscle force predictions and the recruitment of muscles that contribute to both elbow flexion-extension and forearm pronation-supination.
Mechanical Properties of Tendons: Changes With Sterilization and Preservation
Authors: Smith C. W. , Young I. S. , Kearney J. N.
Published: February 2008

Tendon allografts are commonly used to replace damaged anterior cruciate ligaments (ACL). Some of the sterilization and preservation techniques used by tissue banks with tendon allografts are thought to impair the mechanical properties of graft tissues. The tensile mechanical properties of porcine toe extensor tendons were measured using a dynamic testing machine following either freezing, freeze-drying, freezing then irradiation at 25 kGy (2.5 MRad), freeze-drying then irradiation, or freeze-drying then ethylene oxide gas sterilization. There was a small but significant difference in Young’s modulus between the frozen group (0.88 GPa ± 0.09 SD) and both the fresh group (0.98 GPa ± 0.12 SD) and the frozen irradiated group (0.97 GPa ± 0.08 SD). No values of Young’s modulus were obtained for the freeze-dried irradiated tendons. The ultimate tensile stress (UTS) of the freeze-dried irradiated group (4.7 MPa ± 4.8 SD) was significantly different from both the fresh and the frozen irradiated groups, being reduced by approximately 90 percent. There were no significant changes in UTS or Young’s modulus between any of the other groups. If irradiation is to be used to sterilize a tendon replacement for an ACL it must take place after freeze-drying to maintain mechanical properties.
Biomechanical Model for the Myogenic Response in the Microcirculation: Part I—Formulation and Initial Testing
Authors: Lee Shouyan , Schmid-Scho¨nbein Geert W.
Published: February 2008

The pressure dependent or myogenic contraction of arterioles is one of the most fundamental control mechanisms of microvascular perfusion. While many experimental observations have been obtained on the myogenic response, no generally accepted biomechanical model has been formulated. A novel biomechanical theory is proposed based on two fundamental assumptions: the arteriolar wall exhibits viscoelastic properties before and during myogenic contractions, and the contraction is achieved by a pressure dependent change of reference length. The formulation of the model and its application to different experimental procedures on microvascular smooth muscle in the literature is presented. The model describes closely a broad spectrum of steady and unsteady pressure dependent diameter variations of arterioles under a pressure dependent stimulus.
Large Curvature Effect on Pulsatile Entrance Flow in a Curved Tube: Model Experiment Simulating Blood Flow in an Aortic Arch
Authors: Naruse T. , Tanishita K.
Published: February 2008

We measured the velocity profiles of pulsatile entrance flow in a strongly curved tube using a laser-Doppler anemometer in order to simulate blood flow in the aortic arch under various conditions, i.e., a ratio of tube to curvature radius of 1/3, Womersley parameters of 12 and 18, and peak Dean number up to 1200. Axial isovelocity contours of the cross-section showed the potential vortex to be near the entrance, and with the maximum velocity there being skewed towards the inner wall; thereafter shifting towards the outer wall. During the deceleration phase, reverse axial flow occurred near the inner wall, and a region of this flow extended downstream. The large curvature contributes to the enhancement of the secondary flow and flow reversal, which elevates the wall-shear stress oscillations. The location of elevated wall-shear oscillations corresponds to the vessel wall region where atherosclerotic formation frequently occurs; thereby indicating that both the large curvature and pulsatility play key roles in formation of localized atherosclerotic lesions.
Application of a Mechanical Filter to an Artificial Heart
Authors: Arabia M. , Pistella F.
Published: February 2008

A mechanical filter developed to improve the performances of an artificial heart whose electrical motor has a reciprocating motion is presented. The filter phases the mechanical load strongly reducing its inertial component (which is of the same order of magnitude as the useful load). The analysis reported for a prototype developed by us shows that when its rate is equal to the first resonant frequency of the filter, a reduction of about 50 percent for the maximum value of the torque due to inertial and friction forces is obtained.
The Measurement of Temperature With Electron Paramagnetic Resonance Spectroscopy
Authors: Eckburg J. J. , Chato J. C. , Liu K. J. , Grinstaff M. W. , Swartz H. M. , Suslick K. S. , Auteri F. P.
Published: February 2008

An electron paramagnetic resonance (EPR) technique, potentially suitable for in vivo temperature measurements, has been developed based on the temperature response of nitroxide stable free radicals. The response has been substantially enhanced by encapsulating the nitroxide in a medium of a fatty acid mixture inside a proteinaceous microsphere. The mixture underwent a phase transition in the temperature range required by the application. The phase change dramatically altered the shape of the EPR spectrum, providing a highly temperature sensitive signal. Using the nitroxide dissolved in a cholesterol and a long-chain fatty acid ester, we developed a mixture which provides a peakheight ratio change from 3.32 to 2.11, with a standard deviation of 0.04, for a temperature change typical in biological and medical applications, from 38 to 48°C. This translated to an average temperature resolution of 0.2°C for our experimental system. The average diameter of the nitroxide mixture-filled microspheres was ≈2 μm. Therefore, they are compatible with in vivo studies where the microspheres could be injected into the microvasculature having a minimum vessel diameter of the order of 8 μm. This temperature measuring method has various potential clinical applications, especially in monitoring and optimizing the treatment of cancer with hyperthermia. However, several problems regarding temperature and spatial resolution need to be resolved before this technique can be successfully used to monitor temperatures in vivo.
Numerical Simulation of Oil-Droplet Cleavage by Surfactant
Authors: He Xiaoyi , Dembo Micah
Published: February 2008

We present numerical computations of the deformation of an oil-droplet under the influence of a surface tension gradient generated by the surfactant released at the poles (the Greenspan experiment). We find this deformation to be very small under the pure surface tension gradient. To explain the large deformation of oil droplets observed in Greenspan’s experiments, we propose the existence of a phoretic force generated by the concentration gradient of the surfactant. We show that this hypothesis successfully explains the available experimental data and we propose some further tests.
A Mathematical Model for the Morphology of the Pulmonary Acinus
Authors: Denny E. , Schroter R. C.
Published: February 2008

A computational method is proposed for the construction of a three-dimensional space-filling model of an acinar ventilatory unit. Its geometry consists of truncated octahedra arranged in a cuboidal block. The ducts and alveoli are formed by opening specific common faces between polyhedra. The branching structure is automatically computed using algorithms solely to maximise the number of alveoli and minimise the average path lengths; it is not formed with reference to published experimental data. Properties of the model such as the total alveolar and ductal volumes, the distribution of individual path lengths to the alveolar sacs, and the average number of ducts per generation are calculated. The predicted morphology of the model compares well with published data for rat lungs.
A Single Integral Finite Strain Viscoelastic Model of Ligaments and Tendons
Authors: Johnson G. A. , Livesay G. A. , Woo S. L-Y. , Rajagopal K. R.
Published: February 2008

A general continuum model for the nonlinear viscoelastic behavior of soft biological tissues was formulated. This single integral finite strain (SIFS) model describes finite deformation of a nonlinearly viscoelastic material within the context of a three-dimensional model. The specific form describing uniaxial extension was obtained, and the idea of conversion from one material to another (at a microscopic level) was then introduced to model the nonlinear behavior of ligaments and tendons. Conversion allowed different constitutive equations to be used for describing a single ligament or tendon at different strain levels. The model was applied to data from uniaxial extension of younger and older human patellar tendons and canine medial collateral ligaments. Model parameters were determined from curve-fitting stress-strain and stress-relaxation data and used to predict the time-dependent stress generated by cyclic extensions.
Microdamage and Osteocyte-Lacuna Strain in Bone: A Microstructural Finite Element Analysis
Authors: Prendergast P. J. , Huiskes R.
Published: February 2008

Damage accumulation in living tissues occurs when the rate of damage formation is greater than the rate of damage repair. For very large increases in the loading rate of bones, this can result in “stress fractures” due to the growth and coalescence of fatigue related microdamage. At lower increases of loading rates, the damage accumulation process is halted because there is time for adaptive bone-remodeling to occur in response to the new load. However, it is not known if there is a relationship between microdamage and bone remodeling per se. One hypothesis for the control of bone remodeling is that osteocytes sense strains and mediate osteoblastic and osteoclastic activity. The purpose of this study was to investigate whether damage generates strains which may trigger bone remodeling. If this were true, then accumulative damage would cause adaptive bone remodeling. This study applies the methods of finite element analysis to determine the effect of observed damage mechanisms on the proposed sensors of remodeling in Haversian bone. Individual lamellae are modeled and osteocyte-lacunae are included in a generalized plane strain geometric representation. It is predicted that microdamage alters the local deformation behavior around lacunae, and that the changes increase as microdamage accumulates. Hence, if damage accumulates in a bone, it could be sensed as a change in strain at a microstructural level. The results give theoretical support to the experimental studies that have shown a correlation between microdamage and the initiation of resorption as a first step in bone remodeling.
Drag Reduction in Heavy Oil
Authors: Storm D. A. , McKeon R. J. , McKinzie H. L. , Redus C. L.
Published: February 2008

Transporting heavy crude oil by pipeline requires special facilities because the viscosity is so high at normal field temperatures. In some cases the oil is heated with special heaters along the way, while in others the oil may be diluted by as much as 30 percent with kerosene. Commercial drag reducers have not been found to be effective because the single-phase flow is usually laminar to only slightly turbulent. In this work we show the effective viscosity of heavy oils in pipeline flow can be reduced by a factor of 3–4. It is hypothesized that a liquid crystal microstructure can be formed so that thick oil layers slip on thin water layers in the stress field generated by pipeline flow. Experiments in a 1 1/4-in. flow loop with Kern River crude oil and a Venezuela crude oil BCF13 are consistent with this hypothesis. The effect has also been demonstrated under field conditions in a 6-in. flow loop using a mixture of North Sea and Mississippi heavy crude oils containing 10 percent brine.
Leak Detection in Liquid Subsea Flowlines With no Recorded Feed Rate
Authors: Dinis J. M. , Wojtanowicz A. K. , Scott S. L.
Published: February 2008

This paper presents a statistical method to detect leaks in subsea liquid flowlines when the inlet flow rate measurements are unavailable and conventional mass balance techniques cannot be used. Presently, the only method used in these situations is to “predict” the inlet rate from a steady-state reservoir/wellbore model for use in a transient pipeline simulator. The purpose of this work was to examine the utility of a pressure loss model which requires only routine production data. This technique is derived from the statistical theory of estimation and testing, and treats fluctuations due to turbulence, transients, and measurement errors as statistical noise. This method compares two mean values of the pipeline resistance coefficient, R; the new updated value representing the potential leak and the old value being R without leak. Precision of the new method was demonstrated using the 9460-ft long, (3.64 in. i.d.) flowloop at LSU, with flowrates up to 10,000 bbl/day of water, and pressures up to 2400 psi. The leak was simulated by an orifice which discharged the water from the flowline into a shut-in-well. An environmental (wellhead) pressure up to 2200 psi was applied to simulate hydrostatic pressure in the deep-sea environment. The results presented in this paper show how detection of small leaks relates to the number of R measurements, leak position, and statistical noise resulting from flow rate-pressure fluctuations, transients, and measurement uncertainties.
Theoretical Study of Mode Localization Phenomenon in Bottom-Hole Assemblies in Oil-Well Drilling
Authors: Yigit A. S. , Al-Ansary M. D. , Khalid M.
Published: February 2008

The mode localization phenomena in bottom-hole assemblies (BHA) used in oil-well drilling is investigated. A fully gaged stabilizer model is shown to produce weak coupling between segments of drillcollars separated by stabilizers. It is observed that in a weakly coupled BHA, small disorder in span length can cause strong mode localization. For the first time, it is demonstrated that gravity induces a stiffness disorder, which in turn causes strong mode localization in an inclined wellbore, even though no other disorder is present. The effect of localized modes for failure is presented by examining the maximum bending stresses with and without mode localization. The results show that design and operation of drillstrings should consider the possibility of mode localization for an accurate prediction of dynamic behavior.
Improving the Performance of an Existing Air-Cooled Binary Geothermal Power Plant: A Case Study
Authors: Kanog˘lu M. , C¸engel Y. A.
Published: February 2008

An existing air-cooled binary geothermal power plant in northern Nevada is studied. The current performance of the plant is analyzed with an emphasis on the effects of seasonal climate changes. Two potential sites have been identified to improve the performance of the plant. Northern Nevada has a dry climate, particularly in hot summer months, and the temperature of cooling air can be decreased considerably by evaporative cooling. When the air temperature is decreased to the wet-bulb temperature, the decrease in the condenser temperature is determined to increase the power output by up to 29 percent. The required amount of water for this case is calculated to be about 200,000 tons per yr. Several parametric studies are performed by simulating the operation of the plant with an equation solver with built-in thermophysical property functions. It is determined that the net power output of the plant can be increased by 2.8 percent by optimizing the maximum pressure in the cycle. Also, replacing the existing working fluid isobutane by other commonly used binary fluids such as butane, R-114, isopentane, and pentane do not produce as much of an improvement in the plant performance as operating with isobutane at the optimum maximum pressure. Therefore, isobutane appears to be the best choice for this power plant.
Improvements of a Molten Carbonate Fuel Cell Power Plant via Exergy Analysis
Authors: Braun R. J. , Gaggioli R. A. , Dunbar W. R.
Published: February 2008

A proposed molten carbonate fuel cell power plant design, intended for commercial production by the end of the 1990s and developed under the auspices of the U.S. Department of Energy, the Gas Research Institute, and Energy Research Corporation, has been analyzed with exergy and pinch analysis. The commercial production units, targeted for dispersed power generation markets, are based on an existing demonstration molten carbonate fuel cell power plant design which was demonstrated from 1996–1997. Exergy analysis of the commercial plant design shows the overall, second-law system efficiency to be 53 percent. The principal inefficiency, 17 percent of the total, lies in the catalytic combustor. Another major inefficiency is the stack loss, 14 percent. Heat transfer accounts for approximately 6 percent of the loss. System reconfigurations, incorporating a steam cycle with reheat (System I) and a gas turbine cycle (System II), both with revised heat exchanger networks, for significant improvement are proposed and evaluated. The second-law system efficiency is raised to 66 percent in System I and to 70 percent for System II.
Steady Flow in an Aneurysm Model: Correlation Between Fluid Dynamics and Blood Platelet Deposition
Authors: Bluestein D. , Niu L. , Schoephoerster R. T. , Dewanjee M. K.
Published: February 2008

Laminar and turbulent numerical simulations of steady flow in an aneurysm model were carried out over Reynolds numbers ranging from 300 to 3600. The numerical simulations are validated with Digital Particle Image Velocimetry (DPIV) measurements, and used to study the fluid dynamic mechanisms that characterize aneurysm deterioration, by correlating them to in vitro blood platelet deposition results. It is shown that the recirculation zone formed inside the aneurysm cavity creates conditions that promote thrombus formation and the viability of rupture. Wall shear stress values in the recirculation zone are around one order of magnitude less than in the entrance zone. The point of reattachment at the distal end of the aneurysm is characterized by a pronounced wall shear stress peak. As the Reynolds number increases in laminar flow, the center of the recirculation region migrates toward the distal end of the aneurysm, increasing the pressure at the reattachment point. Under fully turbulent flow conditions (Re = 3600) the recirculation zone inside the aneurysm shrinks considerably. The wall shear stress values are almost one order of magnitude larger than those for the laminar cases. The fluid dynamics mechanisms inferred from the numerical simulation were correlated with measurements of blood platelet deposition, offering useful explanations for the different morphologies of the platelet deposition curves.
Separation of Arterial Pressure Waves Into Their Forward and Backward Running Components
Authors: Pythoud F. , Stergiopulos N. , Meister J.-J.
Published: February 2008

A new separation technique has been developed to determine the forward and backward running arterial pressure wave components. It takes into account friction as well as nonlinear effects due to convective acceleration and to the pressure dependence of the arterial compliance. The new method is a combination of two methods treating friction and nonlinearities separately. The method requires the measurements of pressure and flow at one location as well as the knowledge of the area-pressure relationship. The validity of the method was tested by a simulation experiment in which the forward and backward waves were known a priori. It was shown that the new method is significantly more accurate in the predictions of the forward and backward waves when compared to the classical method assuming linearity and no dissipation. The new wave separation method was also applied to simulated aortic waves for (a) a healthy subject and (b) a subject with decreased compliance. Comparison with the classical linear method showed that neglecting nonlinearities leads to an overestimation of the forward and backward pressure wave amplitudes. The errors, however, were in the order of 5 to 10 percent. We concluded that, for most clinical purposes, the improvement using the nonlinear method is of the same magnitude as experimental errors, and thus the linear method would suffice.
Numerical Investigation of Steady Flow in Proximal and Distal End-to-Side Anastomoses
Authors: Henry F. S. , Collins M. W. , Hughes P. E. , How T. V.
Published: February 2008

Steady flows in model proximal and distal end-to-side bypass anastomoses were simulated numerically. The predictions were compared to whole field measurements of the flow in in vitro models, and were shown to match well the general features of the measured flows. The predictions confirmed that the flows in end-to-side anastomoses are complex and three dimensional, and contain areas that could allow long residence times. Careful examination of the predictions revealed certain features of the flows not seen easily in the experiments. Shear stress and pressure on the vessel walls were predicted, and areas known to be prone to intimal hyperplasia were shown to correspond to areas of high spatial gradient of shear stress. Two anastomosis angles, 30 and 45 deg, were considered, and it was shown that the more acute angle may have some benefit in terms of the levels of shear gradients and the power required to drive the flow through the anastomosis.
Classification of Pulsating Flow Patterns in Curved Pipes
Authors: Tada Shigeru , Oshima Shuzo , Yamane Ryuichiro
Published: February 2008

The fully developed periodic laminar flow of incompressible Newtonian fluids through a pipe of circular cross section, which is coiled in a circle, was simulated numerically. The flow patterns are characterized by three parameters: the Womersley number Wo, the Dean number De, and the amplitude ratio β. The effect of these parameters on the flow was studied in the range 2.19 ≤ Wo ≤ 50.00, 15.07 ≤ De ≤ 265.49 and 0.50 ≤ β ≤ 2.00, with the curvature ratio δ fixed to be 0.05. The way the secondary flow evolved with increasing Womersley number and Dean number is explained. The secondary flow patterns are classified into three main groups: the viscosity-dominated type, the inertia-dominated type, and the convection-dominated type. It was found that when the amplitude ratio of the volumetric flow rate is equal to 1.0, four to six vortices of the secondary flow appear at high Dean numbers, and the Lyne-type flow patterns disappear at β ≥ 0.50.
The Influence of Shape on the Stresses in Model Abdominal Aortic Aneurysms
Authors: Elger D. F. , Blackketter D. M. , Budwig R. S. , Johansen K. H.
Published: February 2008

Presence of a small abdominal aortic aneurysm (AAA) often presents a difficult clinical dilemma—a reparative operation with its inherent risks versus monitoring the growth of the aneurysm, with the accompanying risk of rupture. The risk of rupture is conventionally believed to be a function of the AAA bulge diameter. In this work, we hypothesized that the risk of rupture depends on AAA shape. Because rupture is inevitably linked to stress, membrane theory was used to predict the stresses in the walls of an idealized AAA, using a model which was axisymmetric and fusiform, with the ends merged into straight open-ended tubes. When the stresses for many different shapes of model AAAs were examined, a number of conclusions became evident: (i) maximum hoop stress typically exceeded maximum meridional stress by a factor of 2 to 3 (ii) the shape of an AAA had a small effect on the meridional stresses and a rather dramatic effect on the hoop stresses, (iii) maximum stress typically occurred near the inflection point of a curve drawn coincident with the AAA wall, and (iv) the maximum stress was a function—not of the bulge diameter—but of the curvatures (i.e. shape) of the AAA wall. This last result suggested that rupture probability should be based on wall curvatures, not on AAA bulge diameter, Because curvatures are not much harder to measure than bulge diameter, this concept may be useful in a clinical setting in order to improve prediction of the likelihood of AAA rupture.
Bolus Contaminant Dispersion for Oscillatory Flow in a Curved Tube
Authors: Jiang Yahong , Grotberg James B.
Published: February 2008

The dispersion of a bolus of soluble contaminant in a curved tube during volume-cycled oscillatory flows is studied. Assuming a small value of δ (the ratio of tube radius to radius of curvature), the Navier-Stokes equations are solved by using a perturbation method. The convection-diffusion equation is then solved by expanding the local concentration in terms of the cross-sectionally averaged concentration and its axial derivatives. The time-averaged dimensionless effective diffusivity, 〈Deff/D〉, is calculated for a range of Womersley number α and different values of stroke amplitude A and Schmidt number Sc, where D is the molecular diffusivity of contaminant. For the parameter values considered, the results show that axial dispersion in a curved tube is greater than that in a straight tube, and that it has a local maximum near α = 5 for given fixed values of Sc = 1, A = 5 and δ = 0.3. Finally it is demonstrated how the time history of concentration at a fixed axial position can be used to determine the effective diffusivity.
A Parametric Study of a Side Airbag System to Meet Deflection Based Criteria
Authors: Ha˚land Yngve , Pipkorn Bengt
Published: February 2008

A side airbag system comprising a 12 liter bag to cover the BioSid chest and the abdomen down to the arm rest level, and 75 mm of padding to cover the pelvic/thigh area was evaluated by a series of sled tests at two different velocities, 10 m/s and 12 m/s. The initial bag (over) pressure was varied from 0 to 80 kPa and the bag ventilation area was varied from zero to 1500 mm2. Compressed air was used to fill the bag. It was found that the ventilation of the bag reduced the maximum chest deflection by 30 percent and the maximum viscous criterion, VC, by 50 percent (comparison was made with the same bag without ventilation). A suitable initial bag (over) pressure was found to be about 40 kPa, when the loading of the abdomen was also taken into consideration. The results indicate that the chest deflection is proportional to the door average velocity (during the first 20 ms of deflection) to the power of about 2 and that the VC is proportional to the same velocity to the power of about 4. It was also found that a 12 liter ventilated side airbag resulted in 30–40 percent lower chest deflection and about 60 percent lower VC than 50 mm of chest padding (Ethafoam 220).
Influence of Elastic Nonlinearity on Arterial Anastomotic Compliance
Authors: Schajer G. S. , Green S. I. , Davis A. P. , Hsiang Y. N.-H.
Published: February 2008

This study illustrates how the highly nonlinear elastic behavior of artery wall material can cause unusual structural characteristics that do not occur with a linear-elastic material. An example mathematical model of an end-to-end anastomosis successfully predicts the experimentally observed area of elevated elastic compliance, called the “Para-anastomotic Hypercompliant Zone” (PHZ). The elastic hypercompliance is shown to occur because the anastomosis locally restricts the arterial diameter, thus forcing the adjacent material to remain in a lower strain, and correspondingly a lower stiffness, part of its non-linear stress-strain curve. Elevated elastic compliance can be avoided by locally matching both the arterial diameter and the elastic compliance within the physiological pressure range.
Fluid Dynamics of a Partially Collapsible Stenosis in a Flow Model of the Coronary Circulation
Authors: Siebes Maria , Campbell Charles S. , D’Argenio David Z.
Published: February 2008

The influence of passive vasomotion on the pressure drop-flow (ΔP-Q) characteristics of a partially compliant stenosis was studied in an in vitro model of the coronary circulation. Twelve stenosis models of different severities (50 to 90 percent area reduction) and degrees of flexible wall (0 to 1/2 of the wall circumference) were inserted into thin-walled latex tubing and pressure and flow data were collected during simulated cardiac cycles. In general, the pressure drop increased with increasing fraction of flexible wall for a given flow rate and stenosis severity. The magnitude of this effect was directly dependent upon the underlying stenosis severity. The diastolic ΔP-Q relationship of severe, compliant models exhibited features of partial collapse with an increase in pressure drop at a decreasing flow rate. It is concluded that passive vasomotion of a normal wall segment at an eccentric stenosis in response to periodic changes in intraluminal pressure causes dimensional changes in the residual lumen area which can strongly affect the hemodynamic characteristics of the stenosis during the cardiac cycle. This mechanism may have important implications for the onset of plaque fracture and the prediction of the functional significance of a coronary stenosis based on quantitative angiogram analysis.
Quasisteady Behavior of Pulsatile, Confined, Counterflowing Jets: Implications for the Assessment of Mitral and Tricuspid Regurgitation
Authors: Grimes R. Y. , Pulido G. A. , Levine R. A. , Yoganathan A. P.
Published: February 2008

Mitral and tricuspid regurgitation create turbulent jets within the atria. Clinically, for the purpose of estimating regurgitant severity, jet size is assumed to be proportional to peak jet flow rate and regurgitant volume. Unfortunately, the relationship is more complex because the determinants of jet size include interactions between jet pulsatility, jet momentum, atrial width, and the velocity of ambient atrial counterftows. These effects on fluorescent jet penetration were measured using an in vitro simulation. Both steady and pulsatile jets were driven into an opposing counterflow velocity field peak jet length (Ljp) measurements made as a function of (1) peak orifice velocity (Ujp), (2) the time required for the jet to accelerate from zero to peak velocity and begin to decelerate (Tjp), (3) jet orifice diameter (Dj), (4) counterflow velocity (Uc), and (5) counterflow tube diameter (Dc). A compact mathematical description was developed using dimensional analysis. Results showed that peak jet length was a function of the counterflow tube diameter, the ratio of peak jet to counterflow momentum, (Mjp/Mc) = (Ujp2Dj2)/(Uc2Dc2), and a previously undescribed jet pulsatility parameter, the pulsatility index (PI), PI = Dc2/(TjpUjpDj). For the same jet orifice flow conditions, jet penetration decreased as chamber diameter decreased, as the jet PI increased, and as the momentum ratio decreased. These interactions provide insight into why regurgitant jet size is not always a good estimate of regurgitant severity.
Flow Input Waveform Effects on the Temporal and Spatial Wall Shear Stress Gradients in a Femoral Graft-Artery Connector
Authors: Kleinstreuer C. , Lei M. , Archie J. P.
Published: February 2008

Employing a validated finite volume code, a computer-aided design of the distal end of a femoral graft-artery junction has been considered to simulate transient three-dimensional blood flow for various flow input waveforms. The study relies on the hypothesis that large sustained wall shear stress gradients play a major role in the rapid recurrence of intimal hyperplasia plus atheroma after bypass surgery, leading to early graft failure. Two new dimensionless parameters have been introduced to correlate flow waveform characteristics with the severity of nonuniform hemodynamics and hence the potential risk for restenosis. The transient and, more importantly, the time-averaged wall shear stress gradient distributions shown, map out the junction areas which are still susceptible to restenosis, especially the toe region. Future geometric modifications will further reduce disturbed flow patterns and hence the probability of graft failure.
Fracture of Articular Cartilage
Authors: Chin-Purcell Michele V. , Lewis Jack L.
Published: February 2008

Crack formation and propagation is a significant element of the degeneration process in articular cartilage. In order to understand this process, and separate the relative importance of structural overload and material failure, methods for measuring the fracture toughness of cartilage are needed. In this paper, two such methods are described and used to measure fracture properties of cartilage from the canine patella. A modified single edge notch (MSEN) specimen was used to measure J, and a trouser tear test was used to measure T, both measures of fracture toughness with units of kN/m. A pseudo-elastic modulus was also obtained from the MSEN test. Several potential error sources were examined, and results for the MSEN test compared with another method for measuring the fracture parameter for urethane rubber. Good agreement was found. The two test methods were used to measure properties of cartilage from the patellae of 12 canines: 4–9 specimens from each of 12 patellae, with 5 right-left pairs were tested. Values of J ranged from 0.14–1.2 kN/m. J values correlated with T and were an average of 1.7 times larger than T. A variety of failure responses was seen in the MSEN tests, consequently a grade of 0 to 3 was assigned to each test, where 0 represented a brittle-like crack with minimal opening and 3 represented plastic flow with no crack formation. The initial cracks in 12/82 specimens did not propagate and were assigned to grade 3. The method for reducing data in the MSEN test assumed pseudo-elastic response and could not be used for the grade 3 specimens. Stiffness did not correlate with J. Neither J nor T was statistically different between right-left pairs, but varied between animals. The test methods appear useful for providing a quantitative measure of fracture toughness for cartilage and other soft materials.
The Relationship Between Loading Conditions and Fracture Patterns of the Proximal Femur
Authors: Yang K. H. , Shen K.-L. , Demetropoulos C. K. , King A. I. , Kolodziej P. , Levine R. S. , Fitzgerald R. H.
Published: February 2008

In an attempt to test the hypothesis of spontaneous hip fracture, seven pairs of femurs, with ages ranging from 59 to 90, were tested under two loading conditions designed to simulate muscular contraction. Simulated iliopsoas contraction produced femoral neck fractures at an average normalized ultimate load of 5.2 ± 0.8 times body weight. Simulated gluteus medius contraction produced sub-/inter-trochanteric fractures at an average normalized ultimate load of 4.1 ± 0.6 times body weight. The average ultimate load for all specimens was 3040 ± 720 N. Fracture patterns produced by both loading conditions were clinically relevant. The results from this study suggest that abnormal contraction produced by major rotator muscles could induce hip fracture.
Design and Assessment of an Adaptive Intermittent Cervical Traction Modality With EMG Biofeedback
Authors: Lee M. Y. , Wong M. K. , Tang F. T. , Chang W. H. , Chiou W. K.
Published: February 2008

An intermittent cervical traction modality with closed-loop traction force control based on EMG biofeedback was developed and used for clinical study. This system consists of a EMG scanner, on-line self-adjusted traction force controller, audio/video alarm system, real time therapeutic status display, computer interface hardware, and control software. Twenty-four subjects with diagnosed cervical radiculopathy and muscle spasm symptom who were randomly divided into two groups served as subjects in this study. The control and experimental groups were treated with conventional open loop and new EMG biofeedback closed loop traction control protocols respectively. The results of this study indicate that the average reductions in paraspinal EMG signal during traction after 7 weeks treatment for experimental and control groups were 71 and 50 percent, respectively (p < 0.001). These results not only support the clinical use of intermittent, sitting traction to produce cervical paraspinal muscle relaxation, but also revealed that the average myoelectric activity of cervical paraspinal muscle during traction was reduced as traction force increased over the 7-week duration of traction treatment. Through EMG biofeedback traction force control, muscle injury, neck soreness, or pain after traction may be avoided.
In Vivo Observations of Hydraulic Stiffening in the Canine Femoral Head
Authors: Ochoa J. A. , Sanders A. P. , Kiesler T. W. , Heck D. A. , Toombs J. P. , Brandt K. D. , Hillberry B. M.
Published: February 2008

The role that intertrabecular contents and their boundary conditions have on the dynamic mechanical response of canine femoral heads was investigated in vivo. Femoral heads from paired intact hind limbs of canine specimens were subjected to a sinusoidal strain excitation, at physiologic frequencies, in the cranio-caudal direction. The fluid boundary conditions for the contralateral limbs were changed by predrilling through the lateral femoral cortex and into the femoral neck. The drilling procedure did not invade the head itself. This femoral head fluid boundary alteration reduced the stiffness by 19 percent for testing at 1 Hz. The results of this study demonstrate that fluid stiffening occurs in vivo as previously observed ex vivo.
Identification of the Mechanical Impedance at the Human Finger Tip
Authors: Hajian A. Z. , Howe R. D.
Published: February 2008

Rapid transients were applied to the outstretched human index finger tip, which resulted in motion primarily at the metacarpophalangeal (MCP) joint in extension and in abduction. A second-order linear model was fit to approximately 20 milliseconds of the force and displacement data to determine the effective mechanical impedance at the finger tip. Ranges of mass, damping, and stiffness parameters were estimated over a range of mean finger tip force (2–20 N for extension, 2–8 N for abduction). Effective translational finger tip mass for each subject was relatively constant for force levels greater than 6 N for extension, and constant throughout the abduction trials. Stiffness increased linearly with muscle activation. The estimated damping ratio for extension trials was about 1.7 times the ratio for abduction.
Identification and Correlation of Human Footfall Load Parameters Using Multivariate Analysis
Authors: Wilson J. F. , Rochelle R. D. , Bischoff J. E.
Published: February 2008

This research had two main objectives: to identify and quantify the multiple reaction parameters of human footfall load histories for 24 subjects; and to seek statistical correlations of the reaction parameters with two gaits: fitness walking and running, with two footfall surfaces: rigid and mat, and with two subject attributes: gender and arch index. These reaction parameters, measured with a force plate, include the subjects’ foot reaction forces in the three orthogonal directions and the particular features of these forces such as their duration, average values, peak values, and rates of loading. An automated data retrieval-software system evaluated these reaction parameters. The statistical correlations were made using principal component analysis (PCA), a method that projected the 13 identified footfall reaction parameters onto subsets of three or four parameters called principal components that contained most of the variance of the original thirteen. The results, among others, show couplings between the vertical and the peak medial load, but an uncoupling of the posterior–anterior loads with the loads in the other two directions.
The Effects of Cryopreservation on the Biaxial Mechanical Properties of Canine Saphenous Veins
Authors: Brossollet L. J. , Vito R. P.
Published: February 2008

The experimental and analytical methods presented in the companion paper [1] are used here to study the effects of cryopreservation on the in vitro biaxial random-elastic mechanical properties of canine saphenous veins. The properties of specimens tested in their physiological range of loadings immediately after thawing were not significantly different from their properties before cryopreservation. However, they stiffened significantly in the few hours following thawing. This effect was not observed for aging fresh specimens, nor for cyanide-poisoned specimens, indicating that the static tone of the venous smooth muscle may be affected by cryopreservation and thawing, but the elastin and collagen fibers are most likely unaffected.
A Re-examination of Calcium Activation in the Huxley Cross-Bridge Model
Authors: Zahalak G. I. , Motabarzadeh I.
Published: February 2008

This paper investigates mathematical relations between models of calcium activation kinetics and Huxley-type models of cross-bridge dynamics in muscle. It is found that different calcium-activation schemes lead to the same form of generalized Huxley rate equation with calcium activation (∂n/∂t)−v(∂n/∂x)=rf(α−n)−gn if it is assumed that calcium–troponin interaction rates are fast compared to the rates of transition associated with force-generating cross-bridge states. Calcium affects cross-bridge dynamics by modifying the bonding rate f, but does not affect the number of interacting cross bridges α or the unbonding rate g; this occurs through the appearance in the equation of an activation factor, r, which is a pure function of sarcoplasmic free calcium concentration. In particular, it is shown that both the “tight-coupling” and “loose-coupling” calcium-activation schemes introduced by Zahalak and Ma [1] lead to the same rate equation with the same activation factor; the difference between them appears in the calcium mass-balance equation. While both of these activation models can be made to fit simple twitch and force-velocity data equally well, experimentally observed load-dependent shifts in the free calcium concentration are compatible with the tight-coupling scheme, but not with loose coupling.
In-Vivo Measurements of Blood Flow Velocity Profiles in Canine Ilio-Femoral Anastomotic Bypass Grafts
Authors: Jones S. A. , Giddens D. P. , Loth F. , Zarins C. K. , Kajiya F. , Morita I. , Hiramatsu O. , Ogasawara Y. , Tsujioka K.
Published: February 2008

In-vivo velocity profiles were recorded with a 20 MHz, 80-channel pulsed Doppler ultrasound velocimeter in canine end-to-side ilio-femoral anastomotic grafts. The geometries were obtained from casts of the anastomotic region, and flow rates were measured with electromagnetic flow probes. Three cases reported here include a “standard” geometry, which was similar to previously studied in vitro models, a stenosed geometry, and a case with below average flow rate. Observed flow features include separation at the hood and toe, movement of the floor stagnation point, and skewed profiles in the proximal outflow segment. Out-of-plane curvature and lateral displacement of the anastomosis inlet appear to have a strong effect on the flow fields. In addition, compliance affects the instantaneous flow rates within the proximal and distal branches.
Inspiratory and Expiratory Steady Flow Analysis in a Model Symmetrically Bifurcating Airway
Authors: Zhao Y. , Brunskill C. T. , Lieber B. B.
Published: February 2008

Steady inspiratory and expiratory flow in a symmetrically bifurcating airway model was studied numerically using the finite element method (FIDAP). Flows of Reynolds number of 500 and 1000 during inspiration and a flow of Reynolds number of 500 during expiration were analyzed. Since the geometry of the bifurcation model used in this study is exactly the same as the model used in the experimental studies, the computed results were compared to the experimental findings. Results show that most of the important flow features that were observed in the experiment, such as the skewed velocity profiles in the daughter branches during inspiration and velocity peak in the parent tube during expiration, were captured in the numerical simulation. Quantitatively, the computed velocity profiles are in good agreement with the measured profiles. This comparison validates the computational simulations.
Finite Element Model of the Human Lower Cervical Spine: Parametric Analysis of the C4-C6 Unit
Authors: Yoganandan N. , Kumaresan S. , Voo L. , Pintar F. A.
Published: February 2008

In this study, a three-dimensional finite element model of the human lower cervical spine (C4-C6) was constructed. The mathematical model was based on close-up CT scans from a young human cadaver. Cortical shell, cancellous core, endplates, and posterior elements including the lateral masses, pedicle, lamina, and transverse and spinous processes, and the intervertebral disks, were simulated. Using the material properties from literature, the 10,371-element model was exercised under an axial compressive mode of loading. The finite element model response agreed with literature. As a logical step, a parametric study was conducted by evaluating the biomechanical response secondary to changes in the elastic moduli of the intervertebral disk and the endplates. In the stress analysis, the minimum principal compressive stress was used for the cancellous core of the vertebral body and von Mises stress was used for the endplate component. The model output indicated that an increase in the elastic modulii of the disk resulted in an increase in the endplate stresses at all the three spinal levels. In addition, the inferior endplate of the middle vertebral body responded with the highest mean compressive stress followed by its superior counterpart. Furthermore, the middle vertebral body produced the highest compressive stresses compared to its counterparts. These findings appear to correlate with experimental results as well as common clinical experience wherein cervical fractures are induced due to external compressive forces. As a first step, this model will lead to more advanced simulations as additional data become available.
Evaluating an Integrated Musculoskeletal Model of the Human Arm
Authors: Soechting J. F. , Flanders M.
Published: February 2008

A simplified model of the mechanical properties of muscle and of the musculoskeletal geometry was used to predict torques at the shoulder and elbow during arm movements in the sagittal plane. Subjects made movements to 20 targets spaced on the diameter of a circle centered on the initial location of the hand. Movement kinematics and the electromyographic (EMG) activity of nine shoulder and elbow muscles were recorded. Muscle force was predicted using rectified EMG activity as an input to a Hill-type model of muscle dynamics. The model also made simplifying assumptions about muscle geometry. Muscle force was then converted to torque and the individual muscle torques were weighted to provide the best fit to the joint torque computed from the kinematic data. The overall fit of the model was reasonably good, but the goodness of fit was not uniform over all movement directions. The results suggest that the assumptions about the musculo-skeletal geometry, the model of muscle dynamics, and muscles not included in the analysis all contributed to the error.