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FOREWORD

J Biomech Eng. 1993;115(4B):451-452. doi:10.1115/1.2895522.
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Abstract
Topics: Biomechanics
Commentary by Dr. Valentin Fuster

RESEARCH PAPERS

J Biomech Eng. 1993;115(4B):453-459. doi:10.1115/1.2895523.

Changes in the mechanical properties of a blood vessel when it remodels itself under stress are reviewed. One of the recent findings about blood vessels is the rapidity of tissue remodeling when the blood pressure is changed. When the tissue structure and material composition remodel, the zero-stress state of the vessel changes. The mechanical properties change also in the remodeling process. If the elastic behavior is expressed in terms of a pseudo-elastic strain-energy function, then the constants in the function will change in the course of the remodeling. With all these changes taking place, the scope of constitutive equations broadens: it should now include a mass-and-structure growth-stress relationship as well as a stress-strain-relationship. To obtain the mass-and-structure growth-stress relationship, one must be able to determine the mechanical properties of the different layers of the vessel wall, as well as the chemical composition and morphology. For the blood vessels, new methods of mechanical testing must be introduced. A key thought is to use bending of the blood vessel wall. By bending, different layers of the vessel wall are subjected to different stresses, leading to equations that can be used to solve the inverse problem of determining the stress-strain law from measured stress and strain. In vitro and in vivo experiments and theoretical prospectives are presented.

Commentary by Dr. Valentin Fuster
J Biomech Eng. 1993;115(4B):460-467. doi:10.1115/1.2895525.

A survey of some of the advances made over the past twenty years in understanding diarthrodial joint biomechanics is presented. Topics covered in this review include: biotribology (i.e., friction, lubrication and wear of diarthrodial joints); contact area determinations; stereophotogrammetric rendering of articular surfaces; deformational field analysis using canonical problems; and finite element formulations for both infinitesimal and finite deformations of diphasic materials and precise anatomic surfaces. Suggestions are made for future research directions as well.

Commentary by Dr. Valentin Fuster
J Biomech Eng. 1993;115(4B):468-473. doi:10.1115/1.2895526.

Ligaments and tendons serve a variety of important functions in maintaining the structure of the human body. Although abundant literature exists describing experimental investigations of these tissues, mathematical modeling of ligaments and tendons also contributes significantly to understanding their behavior. This paper presents a survey of developments in mathematical modeling of ligaments and tendons over the past 20 years. Mathematical descriptions of ligaments and tendons are identified as either elastic or viscoelastic, and are discussed in chronological order. Elastic models assume that ligaments and tendons do not display time dependent behavior and thus, they focus on describing the nonlinear aspects of their mechanical response. On the other hand, viscoelastic models incorporate time dependent effects into their mathematical description. In particular, two viscoelastic models are discussed in detail; quasi-linear viscoelasticity (QLV), which has been widely used in the past 20 years, and the recently proposed single integral finite strain (SIFS) model.

Commentary by Dr. Valentin Fuster
J Biomech Eng. 1993;115(4B):474-480. doi:10.1115/1.2895527.
Abstract
Topics: Modeling , Cartilage
Commentary by Dr. Valentin Fuster
J Biomech Eng. 1993;115(4B):481-488. doi:10.1115/1.2895528.

Studies on the elastic properties of arterial walls which have been done for the past two decades are surveyed briefly. After several in vitro and in vivo experimental methods and clinical techniques for the measurements of the mechanical behavior of arterial walls have been reviewed, data obtained of the basic characteristics of the arterial wall, including wall incompressiblity and anisotropy, are discussed. The author then reviews constitutive laws proposed for the description of stress-strain relationships of arterial walls and methods for the parametric expression of pressure-diameter data, and shows data on the effects of aging and vascular diseases on arterial mechanics. Finally, residual stress in the arterial wall is discussed.

Commentary by Dr. Valentin Fuster
J Biomech Eng. 1993;115(4B):489-496. doi:10.1115/1.2895529.

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

Commentary by Dr. Valentin Fuster
J Biomech Eng. 1993;115(4B):497-502. doi:10.1115/1.2895530.

Nonhomogeneous distributions of strains are simulated and utilized to determine two potential errors in the measurement of cardiac strains. First, the error associated with the use of single-plane imaging of myocardial markers is examined. We found that this error ranges from small to large values depending on the assumed variation in stretch. If variations in stretch are not accompanied by substantial regional changes in ventricular radius, the associated error tends to be quite small. However, if the nonuniform stretch field is a result of substantial variations in local curvature from their reference values, large errors in stretch and strain occur. For canine hearts with circumferential radii of 2 to 4 cm, these errors in stretch may be as great as 30 percent or more. Moreover, gradients in stretch may be over- or underestimated by as much as 100 percent. In the second part of this analysis, the influence of random measurement errors in the coordinate positions of markers on strains computed from them is studied. Arrays of markers covering about 16 cm2 of ventricular epicardium are assumed and nonuniform stretches imposed. The reference and deformed positions of the markers are perturbed with Gaussian noise with a standard deviation of 0.1 mm, and then strains are computed using either homogeneous strain theory or a nonhomogeneous finite element method. For the strain distributions prescribed, it is found that the finite element method reduces the error resulting from noise by about 50 percent over most of the region. Accurate measurements of cardiac strain distributions are needed for correlation with and validation of realistic three-dimensional stress analyses of the heart. Moreover, with the advent of increasingly effective noninvasive methods to measure cardiac deformation such as magnetic resonance imaging, the use of nonhomogeneous strain analysis to determine more accurate strain distributions has increasing clinical significance.

Commentary by Dr. Valentin Fuster
J Biomech Eng. 1993;115(4B):503-509. doi:10.1115/1.2895531.

A novel experimental method of producing and observing the active motion of polymorphonuclear leukocytes (PMNs) using a micropipette technique has been recently developed (Usami et al., 1992). The present paper develops a quantitative theory for the chemoattractant gradients and cell locomotion observed in these experiments. In previous experimental methods (e.g., the Boyden chamber, the Zygmond chamber and the Dunn chamber) for study chemotaxis of leukocytes, fibroblasts, and PMNs, the exact nature of the concentration gradient of the chemoattractant is unknown. The cells may themselves modify the local gradient of the chemoattractant. In experiments using the micropipette, an internal source of chemoattractant provides well-defined boundary and initial conditions which allow the computation of the chemoattractant concentration gradient during the active locomotion of the PMNs. Since the cell completely fills the pipette lumen, convection is limited to the motion of the cells themselves. In coordinates moving with cell, it is assumed that diffusion is the only mechanism of mass transport of the chemoattractant (fMLP). Computations of the fMLP concentration during locomotion of the cell were carried out for a range of rates of fMLP binding by the receptors expressed on the front face of the cell membrane. The results show that the front face of the cell is subjected to increasing fMLP concentration during the cell motion. The sequence of events involve receptor binding of fMLP, signal transduction, polymerization of the cell cytoskeleton at the membrane of the front face, spatially dependent adhesion to the pipette wall, and localized contraction of the cytoskeleton. This sequence of events leads to the steady locomotion of the leukocytes in the micropipette. The computation of the distribution of the fMLP concentration during cell locomotion with constant velocity in micropipette experiments shows that the cell is exposed to increasing concentration of fMLP. This suggests that chemotaxis maybe induced by temporal gradient of an attractant.

Commentary by Dr. Valentin Fuster
J Biomech Eng. 1993;115(4B):510-514. doi:10.1115/1.2895532.

The endothelium, once thought to be a passive, non-thrombogenic barrier, is now recognized as being a dynamic participant in vascular biology and pathobiology. Part of its dynamic nature is due to the influence of the mechanical environment imposed by the hemodynamics of the vascular system. Over the past two decades much has been learned about the influence of hemodynamics on the vascular endothelium. This has been in part through in vivo experiments; however, in the past 15 years a number of laboratories have turned to the application of in vitro cell culture systems to investigate the influence of flow and cyclic stretch on the biology of vascular endothelium. Taken together these studies demonstrate that flow and the associated shear stress modulate both endothelial cell structure and function. Cell culture studies employing cyclic stretch provide similar evidence. Furthermore, these effects of mechanical environment extend to the gene expression level, with there being a differential regulation of mRNA. A critical question is how does an endothelial cell recognize the mechanical environment in which it resides and, having done so, how is this transduced into the changes in structure and function observed? Although our knowledge of the recognition events remains limited, studies on signal transduction in response to a mechanical stimulus indicate that many of the second messengers known to be triggered by chemical agonists also are involved in transducing a mechanical signal. Over the past 20 years our understanding of the importance of the influence of the mechanical environment imposed by the hemodynamics of the system on vascular endothelial biology, both in the regulation of the normal biology of blood vessels and as a determinant of the distribution and development of atherosclerotic lesions, has grown immensely; however, there is still much to be learned.

Commentary by Dr. Valentin Fuster
J Biomech Eng. 1993;115(4B):515-519. doi:10.1115/1.2895533.

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

Commentary by Dr. Valentin Fuster
J Biomech Eng. 1993;115(4B):520-527. doi:10.1115/1.2895534.

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

Commentary by Dr. Valentin Fuster
J Biomech Eng. 1993;115(4B):528-533. doi:10.1115/1.2895535.

The basic concepts employed in formulating models of the process of stress adaptation in living bone tissue are reviewed. A purpose of this review is to define and separate issues in the formulation of bone remodeling theories. After discussing the rationale and objective of these models, the concepts and techniques involved in the modeling process are reviewed one by one. It is concluded that some techniques will be more successful than others in achieving the goals of computational bone remodeling. In particular, rationale is given for the preference of surface bone remodeling approaches over internal bone remodeling approaches, and for interactive multi-scale level, rather than mono-scale level, computational strategies.

Topics: Stress , Bone , Modeling
Commentary by Dr. Valentin Fuster
J Biomech Eng. 1993;115(4B):534-542. doi:10.1115/1.2895536.

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

Commentary by Dr. Valentin Fuster
J Biomech Eng. 1993;115(4B):543-548. doi:10.1115/1.2895537.

Scanning acoustic microscopy (SAM) provides the means for studying the elastic properties of a material at a comparable level of resolution to that obtained by optical microscopy for structural studies. SAM is nondestructive and permits observation of properties in the interior of materials which are optically opaque. Two modes of ultrasonic signals have been used in a Model UH3 Scanning Acoustic Microscope (Olympus Co., Tokyo, Japan) as part of a continuing study of the microstructural properties of bone. The pulse mode, using a single narrow pulse in the range of 30 MHz to 100 MHz, has been used to survey the surface and interior of specimens of human and canine femoral compact cortical bone at resolutions down to approximately 30μm. To obtain more detailed information at significantly higher resolution, the burst mode, comprised of tens of sinusoids, has been used at frequencies from 200 MHz to 600 MHz. This has provided details of both human and canine single osteons (or haversion systems) and osteonic lamellae at resolutions down to approximately 1.7μm, well within the thickness of a lamella as viewed in a specimen cut transverse to the femoral axis.

Commentary by Dr. Valentin Fuster
J Biomech Eng. 1993;115(4B):549-554. doi:10.1115/1.2895538.

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

Commentary by Dr. Valentin Fuster
J Biomech Eng. 1993;115(4B):555-557. doi:10.1115/1.2895539.

Twenty-years ago groups from California to Massachusetts were actively involved in the development of an artificial heart. From biomaterials development to biomedical power sources, the supporting industry and spin-off benefit was broad indeed. Young people were seeking careers in biomedical engineering and science. The National Institutes of Health was supporting artificial heart research at $10 to $12 million dollar levels. Groups at Andros, Inc. (now Baxter Novacor) and Stanford, Thoratec, Penn State and the Hershey Medical Center, Cleveland Clinic and the Division of Artificial Organs, the University of Utah, the Texas Heart Institute and the Baylor College of Medicine, Thermal Electron Corporation, and many more were the source of research and breakthrough development of pumps and systems for artificial hearts. We reported on performance criteria for an artificial heart pump at the First Biomechanics Symposium in 1973 [1]. By the beginning of the decade of the 90’s, thousands of presentations had been made and manuscripts written reporting significant progress in the development of artificial heart pumps and systems. The Heart, Lung and Blood Institute of the National Institutes of Health was supporting an artificial heart contract research and development program at a level of $6 million dollars in 1991 [2]. Broad basic research grant activity also continues. The National Institutes of Health’s artificial heart program received renewed support from the Institute of Medicine’s special review in 1991 [3]. In December of 1992, the 16th Annual Cardiovascular Science and Technology Conference attracted over 500 attendees. This annual conference has provided a continuing forum for an update on progress in artificial heart development.

Commentary by Dr. Valentin Fuster
J Biomech Eng. 1993;115(4B):558-561. doi:10.1115/1.2895540.

Dramatic advances have been made in the last two decades in the diagnosis and treatment of coronary artery disease. The development of open heart surgical techniques for bypassing occluded arteries made quantitative diagnostic techniques more important. Computer enhanced angiographic methods, together with measurements using tomography, ultrasound and magnetic resonance imaging have greatly improved the precision of the diagnosis. A more complete understanding of coronary mechanics and control has enabled physicians to better interpret the significance of geometric information and to supplement this information with functional assessment of stenosed arteries. Finally, traditional bypass surgery is now supplemented with closed-chest techniques such as balloon angioplasty. Biomedical engineers have been involved in all of these developments. This paper will review these developments and attempt to identify remaining questions.

Commentary by Dr. Valentin Fuster
J Biomech Eng. 1993;115(4B):562-568. doi:10.1115/1.2895541.

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

Commentary by Dr. Valentin Fuster
J Biomech Eng. 1993;115(4B):569-574. doi:10.1115/1.2895542.

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

Commentary by Dr. Valentin Fuster
J Biomech Eng. 1993;115(4B):575-581. doi:10.1115/1.2895543.

This paper examines the biomechanics of total knee arthroplasty as a treatment for arthritis and anterior cruciate ligament (ACL) reconstruction for repair of torn anterior cruciate ligaments of the knee. These are two of the most frequent reconstructive procedures for the knee joint. Functional testing of patients while performing various activities of daily living was used to study the relationship between the intrinsic biomechanics of the knee and function. The results of the study of patients following total knee replacement demonstrated a dynamic interaction between the posterior cruciate ligament and quadriceps function during stairclimbing. The study of patients with ACL-deficient knees demonstrated that loss of the anterior cruciate ligament can cause the avoidance of quadriceps contraction during activities when the knee is near full extension. Other studies demonstrated a relationship between tibiofemoral joint mechanics and patellofemoral mechanics. In addition, the importance of combined ligamentous laxity with higher than normal adduction moments during gait was examined in relationship to progressive degenerative changes to the medial compartment of the knee. In summary, functional testing such as gait analysis has proven to be an important basic research tool as well as extremely effective for clinical testing of new procedures and devices.

Commentary by Dr. Valentin Fuster
J Biomech Eng. 1993;115(4B):582-587. doi:10.1115/1.2895544.

Impact biomechanics is an area of research primarily associated with the protection of vehicular occupants. It is also concerned with the prevention of sports-related injuries and with injuries due to falls. It deals mainly with loads that have high rates of onset and with durations that are considerably less than one second. The objectives of impact biomechanics are to understand how a body region is injured in an impact and to seek means to minimize injury through environmental modifications. A secondary objective is to ensure that such environmental modifications do not become a secondary source of injury. Impact biomechanics can be broadly categorized into four areas. They are Mechanisms of Injury, Human Response to Impact, Human Tolerance to Impact, and Development of Human Surrogates for Impact Simulation. Each area plays a role in the design of automotive restraint systems and interior structures and in the understanding of how injury is caused so that effective countermeasures can be taken to minimize the injury. This paper describes advances made in all four areas of impact biomechanics over the last two decades. Areas of current research interest are also discussed. They point to the need for new data and further research work in impact biomechanics.

Commentary by Dr. Valentin Fuster
J Biomech Eng. 1993;115(4B):588-594. doi:10.1115/1.2895545.

Fluid dynamics research over the past twenty years has contributed immensely to our knowledge of atherosclerosis. The ability to detect localized atherosclerotic plaques using noninvasive ultrasonic methods was advanced significantly by investigations into the nature and occurrence of velocity disturbances created by arterial stenoses, and diagnosis of carotid bifurcation disease using a combination of ultrasonic imaging and Doppler measurement of blood velocity is now quite routine. Since atherosclerotic plaques tend to be localized at sites of branching and artery curvature and since these locations would be expected to harbor complex flow patterns, investigators postulated that fluid dynamics might play an initiating role in atherogenesis. Several fluid dynamic variables were proposed as initiating factors. Investigations were undertaken during the 1980s in which fluid dynamic model experiments with physiologic geometries and flow conditions were employed to simulate arterial flows and in which morphometric mapping of intimal thickness was performed in human arteries. Correlations between fluid dynamic variables and intimal thickness revealed that atherosclerotic plaques tended to occur at sites of low and oscillating wall shear stress; and these observations were reinforced by studies in a monkey model of atherosclerosis. Concomitantly, it was realized that arteries adapt their diameters so as to maintain wall shear stress in a narrow range of values around 15 dynes/cm2 , findings which were based both on observations of normal arteries and on animal studies in which flow rates were manipulated and arterial diameter adaptation was measured. Currently, a working hypothesis for the role of fluid dynamics in atherogenesis is that intimal thickening is a normal response to low wall shear stress, and this intimal thickening can develop into an early atherosclerotic plaque under certain circumstances such as excessive low density lipoprotein concentrations in blood.

Commentary by Dr. Valentin Fuster
J Biomech Eng. 1993;115(4B):595-601. doi:10.1115/1.2895546.

A large body of evidence implicates fluid dynamic forces in the genesis and progression of atherosclerosis, the leading cause of death in the United States. To understand the mechanism by which hemodynamics influences the disease process, and to identify the specific flow variable(s) responsible for its localization, it is essential to know the distribution of hemodynamic variables in susceptible regions of the vasculature. Vascular flow models have been used more than any other means to gain insight into the details of arterial hemodynamics. The first flow models were not very realistic. Our first attempt, reported at an early Biomechanics Symposium, was probably the most unrealistic of all: a “2-D branch” that was constructed to validate a 2-D computed flow field. Most of the first models were made of cylindrical tubes, and their geometry too only approximated that of real arteries. Much was learned about the fluid dynamics in branches and bends using such models, but measurements in them could be related only generally to the fluid dynamics in living vessels. Accordingly, we began to make flow field measurements in replicas prepared from human arteries. Others challenged their glassblowers and shops to make models more representative of real vessels. These flow-through casts and fabricated models were initially rigid and perfused with Newtonian fluids. Using these more realistic systems, we and others were able to demonstrate relationships between specific hemodynamic variables and localized arterial pathology. The fidelity of flow simulations today exceeds that of only a few years ago. We now perfuse compliant replicas as small as coronary diagonal branches with fluids whose rheology mimics blood. This level of fidelity is harder to justify for the present application than the switch from tubes to flow-through casts. There is no evidence that the disease has kept secrets from the rigid casts that will be exposed in compliant ones. Nonetheless, there is comfort in simulating the real world as faithfully as possible, and one never knows until one tries whether the next increment of reality will yield unexpected new insights.

Commentary by Dr. Valentin Fuster
J Biomech Eng. 1993;115(4B):602-610. doi:10.1115/1.2895547.

In this review we shall examine the current understanding of events that lead to the incipient formation of the early foam cell lesion in atherogenesis and its localization. Particular emphasis will be placed on the intimal transport mechanisms that lead to the growth of extracellular lipid liposomes in the intima, since there is now substantial evidence that this growth is the triggering event in the complex sequence of processes that leads to the recruitment of blood borne monocytes into the sub-endothelial intima and their subsequent conversion to macrophages. The role of the endothelium, intimal proteoglycans and internal elastic lamina (IEL) in modulating the transport of low density lipoproteins (LDL) in the subendothelial space will be analyzed and a new hypothesis for the co-localization of liposome formation, cellular level endothelial leakage and monocyte entry described. The possible modifications of LDL in the lipsomes that facilitate the conversion of monocytes into foam cells is summarized. We also discuss the fluid dynamic aspects of intimal transport and the relationship of fluid shear stress to the localization of cellular level endothelial leakage of LDL. The effect of fluid shear on other endothelial cell functions has been recently reviewed in [1].

Commentary by Dr. Valentin Fuster
J Biomech Eng. 1993;115(4B):611-616. doi:10.1115/1.2895548.

The aorta is the major blood vessel transporting blood pumped by the left ventricle to the systemic circulation. The tricuspid aortic valve at the root of the aorta provides a centralized flow with nearly uniform velocity profile into the ascending aorta. The aorta consisting of the ascending limb, the aortic arch, and the descending segment is a vessel of complex geometry including curvature in multiple planes, branches and bifurcation as well as taper. The understanding of the development of blood flow in this distensible vessel has been the subject of several theoretical as well as experimental investigations. Flow development in the aorta and in the branch vessels has been of interest in delineating the role of wall shear stresses on the etiology of atherosclerosis. In this paper, a review of the current status on our understanding of the complex flow dynamics in the aorta is presented. With the advent of transesophageal echocardiography and magnetic resonance velocity mapping, further evidence of the presence of secondary flows even in the descending aorta has been reported. The importance of the effect of secondary flow in the descending aorta on the perfusion of distal blood vessels (such as superior mesenteric and renal arterial branches) as well as in the iliac bifurcation is also included in the discussion.

Commentary by Dr. Valentin Fuster
J Biomech Eng. 1993;115(4B):617-621. doi:10.1115/1.2895549.

Thermal (or heat) shock phenomena have been observed in all organisms at the cellular level. They cause an acceleration in the rate of expression of specific genes (heat shock genes), resulting in an increase and accumulation of heat shock proteins in cells. The purpose of this study is to investigate the mechanisms of thermal shock from two different viewpoints: biothermal and biothermomechanical aspects. The former predicts more severe consequences on cells that the latter, whose thermal wave fronts are smoothed due to the coupling effects of thermoelasticity. In conclusion, it is the thermal wave propagation (the so-called “second sound” effect) which triggers a perturbation of normal gene expression. Thermotolerance is found to be inherited in the heat flux equation of the thermal wave model. The information obtained from this study can also be useful to therapeutical hyperthermia, preservation of organs and tissues, and laser and cryogenic surgery.

Commentary by Dr. Valentin Fuster

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