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RESEARCH PAPERS

J Biomech Eng. 1980;102(3):181-189. doi:10.1115/1.3149571.

A specialists meeting on “The Role of Fluid Mechanics in Atherogenesis” was held August 24–25, 1978, at The Ohio State University. This meeting was a followup to a similar meeting held in 1974 [1, 2]. The present status of our knowledge of the importance of fluid mechanics in the initiation and progression of arterial lesions is summarized on the basis of the experimental data presented at the meeting; no attempt is made to provide a comprehensive review of the relevant literature. Three basic aspects are addressed: firstly, the localization of arterial lesions; secondly, the local hemodynamics of arterial segments with a high predilection to the development of lesions; and thirdly, the interaction of hemodynamic factors with the arterial wall. The many unresolved questions, apparently conflicting experimental data and areas in need of future research on the role of fluid mechanics in atherogenesis are identified specifically.

Commentary by Dr. Valentin Fuster
J Biomech Eng. 1980;102(3):190-193. doi:10.1115/1.3149572.

This paper describes the results of an investigation on the mechanical properties of canine anterior cruciate ligaments. A total of 38 ligaments were tested. It is shown that the completely reversible (elastic) range of strain is limited to 14 percent elongation, corresponding to an applied load of 200 N. Within this range each specimen was tested at different strain rates varying from 0.12 percent/s to 220 percent/s and it is demonstrated that the mechanical behavior of the ligaments is not sensitive to strain rate in the range investigated. After completion of tests in the reversible range, of strain ten ligaments were frozen and similar tests were performed after thawing. It is shown that freezing produces alterations of the mechanical properties. The ligaments become more rigid than when they are tested in fresh conditions. From room temperature up to 45C, the load-elongation relationship is not significantly dependent upon test temperature.

Commentary by Dr. Valentin Fuster
J Biomech Eng. 1980;102(3):194-198. doi:10.1115/1.3149573.

A prosthesis for a cruciate ligament of the knee involves the problem of implanting a short and small spring for which the load-deformation response cannot be duplicated by any single man-made material. This work presents a model of the elastic behavior of a two material composite prosthesis made of high-strength fibers spirally wound around a soft elastic core. At each end of the core, the fibers are attached to a pulling device. Under a tension load, the fibers exert a pressure on the core which deforms radially, permitting the elongation of the prosthesis. This allows the achievement of large deformation while both the fibers and the core remain in the elastic domain. The high strength of the spring is provided by the high yield strength of the fibers. The results show the influence of the design variables on the deformation of the prosthesis.

Commentary by Dr. Valentin Fuster
J Biomech Eng. 1980;102(3):199-207. doi:10.1115/1.3149574.

This paper reports on the application of a “postulate-based” control method for multi-axis artificial arm control. This method uses shoulder muscle EMG’s as control sites, but, unlike previous techniques, the theory is the first that can be rigorously defined in terms of musculoskeletal anatomy, EMG muscle-force relationships, EMG transmission characteristics, muscle recruitment, limb dynamics and normal motion constraints. The control theory results in a deterministic, mathematically expressible set of controller equations, which use the vector of natural limb torques estimated by shoulder EMG signals and a “constraint” for input. The output of the controller equations is a vector of prosthetic torques to be applied to the artificial limb. We report on the implementation of the theory up to the point of laboratory feasibility trials of actual simultaneous above-elbow amputee control of elbow flexion and humeral rotation. Implementation of the theory required: 1) deviation of the controller equations from Newton’s dynamic equations of motion into controller form in conformity with the postulate theory; 2) development of a methodology for estimating natural musculoskeletal torques from EMG signals; 3) hardware and software for experimental testing with actual closed loop amputee control of the prosthesis; and 4) a methodology for evaluating the performance of the prosthesis relative to both alternative prosthetic systems and the natural arm. These tasks were completed and simultaneous multiple-axis control of a prosthetic arm was accomplished by both amputee and nonamputee subjects. Key questions of control compatibility, naturalness, stability, and performance evaluation relative to other prostheses and the natural arm were addressed. Various problems are discussed with the conclusion that this method, despite some difficulties, holds great promise as a practical rehabilitation tool.

Commentary by Dr. Valentin Fuster
J Biomech Eng. 1980;102(3):208-213. doi:10.1115/1.3149575.

An apparatus is described which is capable of measuring the load-displacement characteristics as well as determining the loci of centers of rotation of articulating joints. The apparatus employs a floating head which permits the axis of rotation to coincide with the position of minimum joint resistance, thus eliminating non-physiological rotations at the joint. The coupled loads are also measured, thus providing an accurate description of the joint structural characteristics. Data taken from a freshly amputated human ankle joint is presented.

Commentary by Dr. Valentin Fuster
J Biomech Eng. 1980;102(3):214-220. doi:10.1115/1.3149576.

The deformation occurring in the articular cartilage covering the human femoral head has been measured both when the femoral head is loaded in its natural acetabulum and when the cartilage is loaded with a small indentor. The results indicate that the material response is substantially different in these two situations. In the intact joint the cartilage deformation is substantially greater in older joints, but the response of cartilage to loading with an indentor does not change significantly with age. Theoretical elastic models of the cartilage behavior in these two situations were analyzed. For old cartilage which is idealized as an elastic material the increased deformation which is observed in the intact joint can be attributed to changes in Poisson’s ratio, though in the real material increased fluid flux under load is the more probable cause.

Commentary by Dr. Valentin Fuster
J Biomech Eng. 1980;102(3):221-229. doi:10.1115/1.3149577.

A miniature catheter suitable for clinical use based on the principle of the continuously everting tube has been developed and tested extensively in dogs. The 1-mm-dia flexible tube can advance up a narrow tortuous blood vessel 30 cm beyond the tip of the conventional catheter to which it is attached. A slippery hydromer coating combined with a U-shaped cross section enables the tube eversion to be accomplished at an acceptable operating pressure. The new system will provide access to previously inaccessible regions of the body, and has the potential for clinical use in embolizing selected vessels, providing highly localized chemotherapy, and sampling body fluids. The catheter can advance both with and against the blood flow in arteries and veins, and is awaiting clinical trials.

Commentary by Dr. Valentin Fuster
J Biomech Eng. 1980;102(3):230-233. doi:10.1115/1.3149578.

Strain measurement was performed on loaded fresh human cadaveric proximal femurs before and after total hip femoral component implantation. Changes in the femurs’ strain state associated with loading through stainless steel and titanium femoral components were recorded. The role of proximal femur remodeling in femoral component loosening is discussed along with the likely effect of femoral component elastic modulus on the remodeling process.

Commentary by Dr. Valentin Fuster
J Biomech Eng. 1980;102(3):234-239. doi:10.1115/1.3149579.

An analysis is presented of the deformation of a homogeneous, isotropic, elastic half space subjected to a constant radial strain in a circular area on the boundary. Explicit analytic expressions for the normal and radial displacements and the shear stress on the boundary are used to interpret experiments performed on inflated pig lungs. The boundary strain was induced by inflating or deflating the lung after bonding a flexible disk to the lung surface. The prediction that the surface bulges outward for positive boundary strain and inward for negative strain was observed in the experiments. Poisson’s ratio at two transpulmonary pressures was measured, by use of the normal displacement equation evaluated at the surface. A direct estimate of Poisson’s ratio was possible because the normal displacement of the surface depended uniquely on the compressibility of the material. Qualitative comparisons between theory and experiment support the use of continuum analyses in evaluating the behavior of the lung parenchyma when subjected to small local distortions.

Commentary by Dr. Valentin Fuster
J Biomech Eng. 1980;102(3):240-246. doi:10.1115/1.3149580.

Mechanical measurements of intrinsic membrane material properties are used to characterize the defect in hereditary spherocyte membrane at a continuum level. The value of the surface elastic shear modulus is two-thirds as large as normal values, and the value of the yield shear resultant is one-third as large as normal values. The viscosity of the surface above the elastic-plastic transition appears normal. Under similar geometric conditions, the force required to fragment a hereditary spherocyte is about one-third as large as the force required to fragment a normal cell.

Commentary by Dr. Valentin Fuster
J Biomech Eng. 1980;102(3):247-251. doi:10.1115/1.3149581.

A new approach is presented for studying the vascular response to hemodynamic stress. A laser doppler anemometer is used to make velocity measurements very near the walls of human arterial casts; these measurements are then correlated with the histology of the artery from which the cast was made. Several illustrative results are given which suggest that the velocity profiles along the outer walls of aortic bifurcations may be significantly determined by the longitudinal variation of cross-sectional area. The shapes of these profiles were qualitatively different for each cast studied. In one specimen, the location of initial lipid deposits appeared to correlate with flow acceleration.

Commentary by Dr. Valentin Fuster
J Biomech Eng. 1980;102(3):252-257. doi:10.1115/1.3149582.

The constitutive equation with stress-dependent coefficients for laminated composite is derived and employed for iterative determination of myocardial fiber’s stiffness equation Ef = Kσf + C from myocardial strip’s stiffness equation Es = Ks σs + Cs . The strip’s stiffness constants Ks and Cs are estimated by the least-square curve fitting of the stress-strain data experimentally obtained from uniaxially stretching of strips of left ventricular heart wall excised from seven canine hearts. The values of K and C computed at selected fiber orientations across the thickness of the strip and using three, five, and ten-layer approximations are reported.

Commentary by Dr. Valentin Fuster
J Biomech Eng. 1980;102(3):258-264. doi:10.1115/1.3149583.

Recording the washout of indicator (for example, heat, radio-labeled dissolved gas, etc.) transiently introduced into tissue allows the estimation of tissue blood perfusion rate. Analysis of the washout data requires a material balance which appropriately accounts for all transport mechanisms and sources and sinks of the given indicator. From that balance one may perform a sensitivity analysis which specifies the susceptibility of the perfusion estimate to experimental errors in any of the pertinent parameters and variables. The sensitivity analysis is based on the normalized partial derivatives of tissue indicator concentration with respect to the experimental variables. The results indicate that the estimation of the tissue blood perfusion rate is highly sensitive to errors in the concentration of the diffusible indicator which dominate, by two orders of magnitude or more, the errors attributed to other parameters. For typical experimental conditions, the errors in the perfusion estimate due to the various parameters are shown to vary considerably, according to the sensor position and time of measurement. Based on this type of analysis, one may specify optimal temporal and spatial domains for the parameter estimation in order to minimize error propagation. The optimal time domains are shown to differ from those used in typical indicator washout analyses for estimating tissue perfusion rate.

Commentary by Dr. Valentin Fuster
J Biomech Eng. 1980;102(3):265-273. doi:10.1115/1.3149584.

Highly localized long-term measurements of tumor transport properties are performed with pairs of adjacent, parallel single-strand optic fibers embedded in a laboratory animal. External illumination and light sensing equipment detect local concentration of a fluorescent indicator initially injected into the venous system. Under appropriate conditions, blood flow per unit volume, capillary permeability, and uptake of fluorescent drugs can be determined from the fluorescence time history. The observation volume has a characteristic dimension on the order of 0.5 mm. Chronic tests in 20 animals monitored for 1–2 weeks have demonstrated that the technique is practical, the tumor histology is acceptable, and the measured local tissue uptake is comparable to average values in the literature. The technique is also suitable for acute experiments using a miniature fiber optic needle probe.

Commentary by Dr. Valentin Fuster

DISCUSSIONS

Commentary by Dr. Valentin Fuster

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