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RESEARCH PAPERS

J Biomech Eng. 1990;112(1):1-8. doi:10.1115/1.2891121.

A second order, linear oscillator transfer function model is fit to the measured transfer function relating the abduction-adduction rotation of the first finger to the applied moment. Nearly constant isometric contractions of the first palmar and dorsal interossei are maintained by the subjects during the measurements. The stiffness and damping components of the identified models increase significantly with increasing isometric contraction when compared to those recorded under relaxed contraction. Muscle fatigue causes the natural frequency, damping ratio and stiffness of the joint rotation to decrease under full isometric contraction, and it causes the natural frequency and stiffness to increase when the muscles are relaxed.

Commentary by Dr. Valentin Fuster
J Biomech Eng. 1990;112(1):9-14. doi:10.1115/1.2891133.

The human subtalar joint was modelled as a quasi-linear second-order underdamped system to simulate sudden inversion motion of the foot relative to the shank. The model was fed with experimental data obtained from six subjects on a specially constructed apparatus. A total of 35 deg inversion was produced on the tested leg rapidly enough (lasting less than 40 ms) in order to ensure that the protective muscles are not activated. The parameters of the joint were evaluated and the following ranges were obtained at 35 deg inversion: elastic stiffness 14–52 Nm rad-1 , damping coefficient 1.4–2.9 Nms rad-1 , and natural frequency 78–125 Hz. The effects on the test parameters of weight bearing amount, foot dominance, and protective footwear were studied on one subject.

Commentary by Dr. Valentin Fuster
J Biomech Eng. 1990;112(1):15-21. doi:10.1115/1.2891120.

Ilizarov proposes the use of a special circular fixation device for treatment of bone defects and nonunions or for limb lengthening. Supposition was that the success of this method has to do with the specific mechanical behavior of the device. This behavior is a result of the configuration of the fixation elements. Therefore, a mechanical study of the sensitivity of the circular compression and distraction device (CDD) to configuration parameters was performed. The CDD was found to exhibit a nonlinear stiffness behavior, in particular under axial load. This may be favorable for the induction and tolerance of bone formation. Among the different parameters tested the ring radius was the most important with respect to stiffness. In general, the stiffness of the CDD allows adjustment during postoperative management. In magnitude, it is equal or lower when compared to other fixator types.

Commentary by Dr. Valentin Fuster
J Biomech Eng. 1990;112(1):22-28. doi:10.1115/1.2891122.

Mechanical fracture is believed to be a primary reason for loss of fixation at the bone-cement-implant interface. In addition to the expected cracks at the bone-cement interface, cracks are also observed to be formed at voids and inclusions within the cement. An analytical solution is presented for cracks emanating from circular voids or elastic inclusions under uniaxial tension using the solution for a single dislocation as a Green’s function. Stress intensity factors are calculated for arbitrary orientations of the cracks, and for varying relative stiffnesses of the inclusion and the matrix, to determine the most favorable combination of parameters for crack growth.

Commentary by Dr. Valentin Fuster
J Biomech Eng. 1990;112(1):29-37. doi:10.1115/1.2891123.

The deformations of orthodontic appliances used for space closure are large so that any mathematical analysis will require a nonlinear approach. Existing incremental finite element and finite difference numerical methods suffer from excessive computational effort when analyzing these problems. An accurate segmental technique is proposed to handle these difficulties in an extremely efficient fashion. The segmental technique starts by assuming that an orthodontic appliance is composed of a number of smaller segments, the ends of which undergo small relative rotation. With an appropriate choice of local coordinate system the equilibrium equations for each segment are linearized and solved in a straightforward manner. The segments are then assembled using geometric and force compatibility relations similar to the transfer matrix method. Consequently, the original nonlinear boundary value problem is solved as a sequence of linear initial value problems which converge to the required boundary conditions. As only one segment need be considered at a time, the computations can be performed accurately and efficiently on a PC type computer. Although an iterative solution is used to match the boundary conditions, the time required to solve a given problem ranges from a few seconds to a couple of minutes depending on the initial geometric complexity. The accuracy of the segmental technique is verified by comparison with an exact solution for an initially curved cantilever beam with an end load. In addition, comparisons are made with existing experimental and numerical results as well as with a new set of experimental data. In all cases the segmental technique is in excellent agreement with the results of these other studies.

Commentary by Dr. Valentin Fuster
J Biomech Eng. 1990;112(1):38-45. doi:10.1115/1.2891124.

Local surface strains in bone-fascicle-bone subunits from human patellar tendon and anterior and posterior cruciate ligaments were measured between markers using low-speed photography during low rate subfailure testing. A simple stress-strain relationship of the power form was found to describe the bone-to-bone responses up to four percent strain for all three tissue types examined. The regional material behavior were best fit using an inverted strain-stress relationship, however. The power model, fitted to the experimental data, conformed to the expected stress-strain relationship better than either the quadratic or cubic models. With few exceptions, for a given stress, the strains near the proximal and distal bone ends were not significantly different from each other, but were significantly higher than the strains in the tissue midregions. Local strain patterns generally varied among subunits from the same tissue.

Commentary by Dr. Valentin Fuster
J Biomech Eng. 1990;112(1):46-51. doi:10.1115/1.2891125.

This study was designed to determine the in situ strains, stresses, and loads in the medial collateral ligament (MCL) of skeletally immature and mature rabbits. Using a noncontact method, the magnitudes of the in situ strains were first determined as a function of knee flexion angle. The MCL was divided into three anatomical regions (anterior, middle, and posterior) across its width. For strain measurements, the variation of a gauge length in these regions was obtained in the intact knee at 60, 90, and 120 deg of flexion. Subsequently, all soft tissues around the knee were dissected away, leaving the femur-MCL-tibia (FMT) complex. The MCL was allowed to retract freely and the new length, called the zero length, was measured. From this, the in situ strains were determined. To obtain the stress-strain relationship of the FMT complex, the specimens were subjected to tensile testing. Knowing the in situ strains and the stress-strain relationship, the in situ stresses in the three anatomical regions of the MCL were determined as a function of knee flexion angle. Multiplying these stresses by 1/3 of the cross-sectional area and summing the loads thus calculated, the in situ loads of the MCL were obtained. Our data suggest that the in situ load in the MCL is not large within the range of knee flexion angles studied, i.e., 1.4 to 2.7 N for the skeletally immature animals and 3.0 to 5.8 N for the skeletally mature animals. An increase in the in situ load with skeletal maturation was demonstrated.

Commentary by Dr. Valentin Fuster
J Biomech Eng. 1990;112(1):52-62. doi:10.1115/1.2891126.

This paper reviews recent work aimed at deriving tractable constitutive relations for skeletal muscle from biophysical cross-bridge theories. Discussion is focused on a model proposed previously by the first author (the Distribution-Moment Model), which emphasizes the important role of the moments of the actin-myosin bond distribution function. The theory leads to a relatively simple third order state variable model for contraction dynamics in which the state variables are the three lowest order moments of the bond-distribution function; further, these three moments have simple macroscopic interpretations as muscle stiffness, force, and elastic energy. New results are presented on the formulation of a compatible model for excitation-contraction coupling, and this model requires the introduction of only one more state variable—the free calcium concentration.

Commentary by Dr. Valentin Fuster
J Biomech Eng. 1990;112(1):63-69. doi:10.1115/1.2891127.

Indentation tests were carried out on the forehead skin of volunteers under loading pressures of 0–5 KPa. The indentation was found to increase asymptomatically with loading pressure. In an attempt to develop a quantitative index for aging of the skin, its response to indentation loading was analyzed in reference to its glycosaminoglycan (GAG) containing ground substance and fibers’ network microstructure. An analytic model that considers the skin as an incompressible solid-fluid mixture was developed and utilized to simulate the skin’s indentation response. Following parameter fitting, the predicted results are in close agreement with the data. The model can thus serve as a tool for evaluating the effect of changes in the dermis components which accompany aging.

Commentary by Dr. Valentin Fuster
J Biomech Eng. 1990;112(1):70-74. doi:10.1115/1.2891128.

Inflation-extension experiments were carried out on segments of the descending thoracic aortas from 4 normotensive and 4 hypertensive dogs rendered hypertensive using either unilateral or bilateral renal artery constriction. Intravascular pressures up to 200 mm Hg and axial forces up to 200 g were used. The external diameter of the segment and the distance between two longitudinally spaced gage marks were recorded photographically at each pressure-force level combination. Dimensions in the undeformed configuration were measured at the end of the inflation-extension experiment. Data were analyzed for changes in geometry and force-deformation response. Results indicate that: 1. Under sustained hypertension the wall thickness in the undeformed configuration increases with a concurrent reduction in the in-situ longitudinal extension ratio. 2. This dual tissue response accomplishes substantial reductions in the circumferential and longitudinal stresses from the levels that would be reached at equivlaent pressures in the absence of these geometric changes. 3. At comparable intravascular pressures the extensibility in the circumferential direction is slightly greater for the hypertensive aortas as compared to normals. However, the stress-extension ratio relationship in the circumferential direction is similar in the two groups. 4. The stress-extension ratio relationship in the longitudinal direction indicates that the hypertensive aorta is stiffer than its normotensive counterpart.

Commentary by Dr. Valentin Fuster
J Biomech Eng. 1990;112(1):75-79. doi:10.1115/1.2891129.

This study presents a simulated analysis of Phased Compression Cardiac Assist Device (PCCAD) and evaluation of its applicability as a non-invasive temporary assist for a failing heart. The new technique is based on the chest pump mechanism for blood flow augmentation during external massage by phased compression of the abdominal and thoracic cavities. A semi-closed hydraulic system to simulate the systemic circulation was constructed; the system includes a left ventricle which functions according to the Starling principle and a pneumatic system which controls the pressures applied to the thoracic and abdominal cavities, in complete synchronization with the beating normal or failing heart. The possibility of manipulating the three pumps in series (venous, heart, and arterial) has been checked, and the principal parameters which effect the efficiency of the PCCAD were evaluated. This in-vitro analysis shows the high potential of a non-invasive temporary cardiac assist device. It points to the necessary measures one has to take in order to achieve good synchronization and to interfere externally with the augmentation of cardiac output or with the augmentation of root aortic pressure.

Commentary by Dr. Valentin Fuster
J Biomech Eng. 1990;112(1):80-87. doi:10.1115/1.2891130.

The predictions of the simplified Weinbaum-Jiji (WJ) bioheat transfer equation in one dimension are compared to those of the complete one-dimensional three-equation model that represented the starting point for the derivation of the WJ equation, as well as results obtained using the traditional bioheat transfer equation of Pennes [6]. The WJ equation provides very good agreement with the three-equation model for vascular generations 2 to 9, which are located in the outer half of the muscle layer, where the paired vessel diameters are less than 500 μm, under basal blood flow conditions. At the same time, the Pennes equation yields a better description of heat transfer in the first generation, where the vessels’ diameters are greater than 500 μm and ε, the vessels’ normalized thermal equilibration length, is greater than 0.3. These results were obtained under both normothermic and hyperthermic conditions. A new conceptual view of the blood source term in the Pennes equation has emerged from these results. This source term, which was originally intended to represent an isotropic heat source in the capillaries, is shown to describe instead the heat transfer from the largest countercurrent microvessels to the tissue due to small vessel bleed-off. The WJ equation includes this effect, but significantly overestimates the second type of tissue heat transfer, countercurrent convective heat transfer, when ε > 0.3. Indications are that a “hybrid” model that applies the Pennes equation in the first generation (normothermic) and first two to three generations (after onset of hyperthermia) and the Weinbaum-Jiji equation in the subsequent generations would be most appropriate for simulations of bioheat transfer in perfused tissue.

Commentary by Dr. Valentin Fuster
J Biomech Eng. 1990;112(1):88-92. doi:10.1115/1.2891131.

The objective of this study was to characterize in detail the secondary flow velocity patterns in an in vitro model of a human (adult) pulmonary artery with varying degrees of valvular pulmonic stenosis. A two-dimensional laser Doppler anemometer (LDA) system was used to map the flow fields in the main (MPA), left (LPA), and right (RPA) branches of the pulmonary artery model. The study was conducted in the Georgia Tech right heart pulse duplicator system. A pair of counter-rotating secondary flows were observed in each daughter branch in which the fluid moved outwardly along the side walls and then circled back inwardly toward the center of the vessel. For the case of the “normal” valve, the two counter-rotating secondary flows were symmetric about the centerline. The strength of secondary flows in the RPA was much stronger than in the LPA. However, as the pulmonic valve became more stenotic, the two counter-rotating secondary flows in both the LPA and RPA were no longer symmetric. In addition, the strength of secondary flows in both daughter branches increased with increasing degree of valvular stenosis. The increment in the LPA was, however, greater than in the RPA. The study demonstrates the importance of analyzing complex biological flows from a three-dimensional viewpoint.

Commentary by Dr. Valentin Fuster
J Biomech Eng. 1990;112(1):93-99. doi:10.1115/1.2891132.

A bolus-tracking magnetic resonance imaging (MRI) method has been employed to measure velocity profiles for oscillatory flow with and without a steady flow component as well as pulsatile flow in an axisymmetric tube model. A range of flow conditions within normal physiological limits was tested. The imaged velocity profiles were observed to be generally in accord with theoretical predictions. Instantaneous flow rates calculated from the MR images agreed well with those assessed using an ultrasonic flowmeter. Because MRI is noninvasive and poses few risks to subjects, this technique is potentially useful for studying vascular hemodynamics in vivo.

Commentary by Dr. Valentin Fuster

TECHNICAL BRIEFS

J Biomech Eng. 1990;112(1):100-103. doi:10.1115/1.2891118.
Abstract
Commentary by Dr. Valentin Fuster
J Biomech Eng. 1990;112(1):103-105. doi:10.1115/1.2891119.
Abstract
Topics: Stress , Knee
Commentary by Dr. Valentin Fuster

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