J Biomech Eng. 2005;127(1):1-8. doi:10.1115/1.1835346.

The level of structural detail that can be acquired and incorporated in a finite element (FE) analysis might greatly influence the results of microcomputed tomography (μCT)-based FE simulations, especially when relatively large bones, such as whole vertebrae, are of concern. We evaluated the effect of scanning and reconstruction voxel size on the μCT-based FE analyses of human cancellous tissue samples for fixed- and free-end boundary conditions using different combinations of scan/reconstruction voxel size. We found that the bone volume fraction (BV/TV) did not differ considerably between images scanned at 21 and 50 μm and reconstructed at 21, 50, or 110 μm (−0.5% to 7.8% change from the 21/21 μm case). For the images scanned and reconstructed at 110 μm, however, there was a large increase in BV/TV compared to the 21/21 μm case (58.7%). Fixed-end boundary conditions resulted in 1.8% [coefficient of variation (COV)] to 14.6% (E) difference from the free-end case. Dependence of model output parameters on scanning and reconstruction voxel size was similar between free- and fixed-end simulations. Up to 26%, 30%, 17.8%, and 32.3% difference in modulus (E), and average (VMExp), standard deviation (VMSD) and coefficient of variation (COV) of von Mises stresses, respectively, was observed between the 21/21 μm case and other scan/reconstruction combinations within the same (free or fixed) simulation group. Observed differences were largely attributable to scanning resolution, although reconstruction resolution also contributed significantly at the largest voxel sizes. All 21/21 μm results (taken as the gold standard) could be predicted from the 21/50 (radj2=0.91–0.99;p<0.001), 21/110 (radj2=0.58–0.99;p<0.02) and 50/50 results (radj2=0.61–0.97;p<0.02). While BV/TV, VMSD, and VMExp/σz from the 21/21 could be predicted by those from the 50/110 (radj2=0.63–0.93;p<0.02) and 110/110 (radj2=0.41–0.77;p<0.05) simulations as well, prediction of E, VMExp, and COV became marginally significant (0.04<p<0.13) at 50/110 and nonsignificant at 110/110 (0.21<p<0.70). In conclusion, calculation of cancellous bone modulus, mean trabecular stress, and other parameters are subject to large errors at 110/110 μm voxel size. However, enough microstructural details for studying bone volume fraction, trabecular shear stress scatter, and trabecular shear stress amplification (VMExp/σz) can be resolved using a 21/110 μm, 50/110 μm, and 110/110 μm voxels for both free- and fixed-end constraints.

Commentary by Dr. Valentin Fuster
J Biomech Eng. 2005;127(1):9-14. doi:10.1115/1.1835347.

The main purpose of this work is to discuss the ability of finite element analyses, together with an appropriate anisotropic fracture criterion, to predict the ultimate load and type of fracture in bones and more specifically in the proximal femur. We show here that the use of a three-dimensional anisotropic criterion provides better results than other well-known isotropic criteria. The criterion parameters and the anisotropic elastic properties were defined in terms of the bone tissue microstructure, quantified by the apparent density and the so-called “fabric tensor”, whose spatial distributions were obtained by means of an anisotropic remodeling model able to capture the main features of the internal structure of long bones. In order to check the validity of the results obtained, they have been compared with those of an experimental work that analyzes different types of fractures induced in the proximal femur by a static overload.

Commentary by Dr. Valentin Fuster
J Biomech Eng. 2005;127(1):15-24. doi:10.1115/1.1835348.

The human facet joint capsule is one of the structures in the lumbar spine that constrains motions of vertebrae during global spine loading (e.g., physiological flexion). Computational models of the spine have not been able to include accurate nonlinear and viscoelastic material properties, as they have not previously been measured. Capsules were tested using a uniaxial ramp-hold protocol or a haversine displacement protocol using a commercially available materials testing device. Plane strain was measured optically. Capsules were tested both parallel and perpendicular to the dominant orientation of the collagen fibers in the capsules. Viscoelastic material properties were determined. Parallel to the dominant orientation of the collagen fibers, the complex modulus of elasticity was E* =1.63MPa, with a storage modulus of E′ =1.25MPa and a loss modulus of: E″ =0.39MPa. The mean stress relaxation rates for static and dynamic loading were best fit with first-order polynomials: B(ε)=0.1110ε−0.0733 and B(ε)=−0.1249ε+0.0190, respectively. Perpendicular to the collagen fiber orientation, the viscous and elastic secant moduli were 1.81 and 1.00 MPa, respectively. The mean stress relaxation rate for static loading was best fit with a first-order polynomial: B(ε)=−0.04ε−0.06. Capsule strength parallel and perpendicular to collagen fiber orientation was 1.90 and 0.95 MPa, respectively, and extensibility was 0.65 and 0.60, respectively. Poisson’s ratio parallel and perpendicular to fiber orientation was 0.299 and 0.488, respectively. The elasticity moduli were nonlinear and anisotropic, and capsule strength was larger aligned parallel to the collagen fibers. The phase lag between stress and strain increased with haversine frequency, but the storage modulus remained large relative to the complex modulus. The stress relaxation rate was strain dependent parallel to the collagen fibers, but was strain independent perpendicularly.

Commentary by Dr. Valentin Fuster


J Biomech Eng. 2005;127(1):25-31. doi:10.1115/1.1835349.

Background: The development of endoluminal stents from polymeric materials requires an understanding of the basic mechanical properties of the polymer and the effects of manufacturing and sterilization on those properties. Methods: Pure poly(L-lactide) (PLLA) and PLLA containing varying amounts of triethylcitrate (TEC) as a plasticizer (5-10-15%) were studied. The specimens were solution-cast and CO2 laser-cut. Specimen dimensions were adapted to the strut size of polymeric vascular stents. The properties of the PLLA micro-specimens were assessed before and after sterilization (EtO cold gas, H2O2-plasma, beta- and gamma-irradiation). Tensile tests, and creep and recovery tests were carried out at 37°C. Additionally the thermal and thermo-mechanical characteristics were investigated using dynamic-mechanical analysis (DMA) and differential scanning calorimetry (DSC). Results: The results showed the dramatic influence of the plasticizer content and sterilization procedure on the mechanical properties of the material. Laser cutting had a lesser effect. Hence the effects of processing and sterilization must not be overlooked in the material selection and design phases of the development process leading to clinical use. Altogether, the results of these studies provide a clearer understanding of the complex interaction between the laser machining process and terminal sterilization on the primary mechanical properties of PLLA and PLLA plasticized with TEC.

Commentary by Dr. Valentin Fuster

TECHNICAL PAPERS: Fluids/Heat/Transport

J Biomech Eng. 2005;127(1):32-38. doi:10.1115/1.1835350.

Microscale liquid droplets could act as the SARS carriers in air when released from an infected person through breathing, coughing, or sneezing. In this study, a dynamic model has been built to quantitatively investigate the effect of the relative humidity on the transport of liquid droplets in air using coupled mass transfer and momentum equations. Under higher relative humidity, the exhaled liquid droplets evaporate slowly. Larger droplets fall faster, which could reduce the probability of the droplets inhalation. This may be one of the most important factors that influence the SARS transmission in air.

Commentary by Dr. Valentin Fuster
J Biomech Eng. 2005;127(1):39-45. doi:10.1115/1.1835351.

A two-component laser Doppler anemometer was used to determine the velocity of aqueous flow in the region from 0.25 to 2.5 diameters downstream of a collapsible tube while the tube was executing vigorous repetitive flow-induced oscillations. The Reynolds number for the time-averaged flow was 10,750. A simultaneous measurement of the pressure at the downstream end of the tube was used to align all the results in time at sixty locations in each of the two principal planes defined by the axes of collapse of the flexible tube upstream. The raw data of seed-particle velocity were used to create a periodic waveform for each measured velocity component at each location by least-squares fitting of a Fourier series. The results are presented as both velocity vectors and interpolated contours, for each of ten salient instants during the cycle of oscillation. In the plane of the collapse major axis, the dominant feature is the jet which emerges from each of the two tube lobes when it collapses, but transient retrograde flow is observed on both the central and lateral edges of this jet. In the orthogonal, minor-axis plane, the dominant feature is the retrograde flow, which during part of the cycle extends over the whole plane. All these features are essentially confined to the first 1.5 diameters of the rigid pipe downstream of the flexible tube. These data map the temporal and spatial extent of the highly three-dimensional reversing flow just downstream of an oscillating collapsed tube.

Commentary by Dr. Valentin Fuster
J Biomech Eng. 2005;127(1):46-59. doi:10.1115/1.1835352.

In partial liquid ventilation (PLV), perfluorocarbon (PFC) acts as a diffusion barrier to gas transport in the alveolar space since the diffusivities of oxygen and carbon dioxide in this medium are four orders of magnitude lower than in air. Therefore convection in the PFC layer resulting from the oscillatory motions of the alveolar sac during ventilation can significantly affect gas transport. For example, a typical value of the Péclet number in air ventilation is Pe∼0.01, whereas in PLV it is Pe∼20. To study the importance of convection, a single terminal alveolar sac is modeled as an oscillating spherical shell with gas, PFC, tissue and capillary blood compartments. Differential equations describing mass conservation within each compartment are derived and solved to obtain time periodic partial pressures. Significant partial pressure gradients in the PFC layer and partial pressure differences between the capillary and gas compartments (PC-Pg) are found to exist. Because Pe≫1, temporal phase differences are found to exist between PC-Pg and the ventilatory cycle that cannot be adequately described by existing non-convective models of gas exchange in PLV. The mass transfer rate is nearly constant throughout the breath when Pe≫1, but when Pe≪1 nearly 100% of the transport occurs during inspiration. A range of respiratory rates (RR), including those relevant to high frequency oscillation (HFO)+PLV, tidal volumes (VT) and perfusion rates are studied to determine the effect of heterogeneous distributions of ventilation and perfusion on gas exchange. The largest changes in PCO2 and PCCO2 occur at normal and low perfusion rates respectively as RR and VT are varied. At a given ventilation rate, a low RR-high VT combination results in higher PCO2, lower PCCO2 and lower (PC-Pg) than a high RR-low VT one.

Commentary by Dr. Valentin Fuster
J Biomech Eng. 2005;127(1):60-66. doi:10.1115/1.1835353.

Stenosis-induced thrombosis and abandonment of the hemodialysis synthetic graft is an important cause of morbidity and mortality. The graft vascular circuit is a unique low-resistance shunt that has not yet been systematically evaluated. In this study, we developed a mathematical model of this circuit. Pressure losses (ΔPs) were measured in an in vitro experimental apparatus and compared with losses predicted by equations from the engineering literature. We considered the inflow artery, arterial and venous anastomoses, graft, stenosis, and outflow vein. We found significant differences between equations and experimental results, and attributed these differences to the transitional nature of the flow. Adjustment of the equations led to good agreement with experimental data. The resulting mathematical model predicts relations between stenosis, blood flow, intragraft pressure, and important clinical variables such as mean arterial blood pressure and hematocrit. Application of the model should improve understanding of the hemodynamics of the stenotic graft vascular circuit.

Commentary by Dr. Valentin Fuster
J Biomech Eng. 2005;127(1):67-84. doi:10.1115/1.1835354.

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Y., Walsh,  J. R., Diller,  K. R., Brand,  J. J., and Aggarwal,  S. J., 2001, “ Algae Permeability to Me2SO from −3°C to 25°C,” Cryobiology, CRYBAS42, pp. 286–300.4ayCRYBAS0011-2240Walsh, J. R., Diller, K. R., and Brand, J. J., 2004, “Measurement and Simulation of Water and Methanol Transport in Algal Cells,” J. Biomech. Eng., 126 , pp. 167–179.Macias-Garza,  F., Bovik,  A. C., Diller,  K. R., and Aggarwal,  J. K., 1988, “ Digital Reconstruction of Three-Dimensional Serially Sectioned Optical Images,” IEEE Trans. Acoust., Speech, Signal Process., IETABA36, pp. 1067–1075.ieaIETABA0096-3518Kim,  N. K., Aggarwal,  S. J., Bovik,  A. C., Dille,  K. R., and Aggarwal,  J. K., 1989, “ Stereoscopic Analysis of Shape Changes in Solanum Tuberosa Slices under Osmotic Shock,” Eur. J. Cell Biol., EJCBDN48, pp. 21–24.4kjEJCBDN0171-9335Kim,  N. H., Bovik,  A. C., Aggarwal,  S. J., Diller,  K. R., and Aggarwal,  J. K., 1990, “ Automated Three-dimensional Analysis of Stereo-microscopic Images,” J. Microsc., JMICAR158, pp. 275–284.jmiJMICAR0022-2720Bartels,  K., Bovik,  A. C., Aggarwal,  S. J., and Diller,  K. R., 1993, “ The Analysis of Biological Shape Changes from Multi-Dimensional Dynamic Images,” J. Comput. Med. Imaging Graphics, ZZZZZZ17, pp. 89–99.Körber,  C., and Scheiwe,  M. W., 1983, “ Observation on the Non-planar Freezing of Aqueous Salt Solutions,” J. Cryst. Growth, JCRGAE61, pp. 307–316.jcrJCRGAE0022-0248Körber,  C., Scheiwe,  M. W., and Wöllhover,  K., 1983, “ Solute Polarization during Planar Freezing of Aqueous Salt Solutions,” Int. J. Heat Mass Transfer, IJHMAK26, pp. 1241–1253.ijhIJHMAK0017-9310Kourosh,  S., Crawford,  M. E., and Diller,  K. R., 1990, “ Microscopic Study of Coupled Heat and Mass Transport during Unidirectional Solidification of Binary Solutions. I. Thermal Analysis,” Int. J. Heat Mass Transfer, IJHMAK33, pp. 29–38.ijhIJHMAK0017-9310Kourosh,  S., Diller,  K. R., and Crawford,  M. E., 1990, “ Microscopic Study of Coupled Heat and Mass Transport during Unidirectional Solidification of Binary Solutions. II. Mass Transfer Analysis,” Int. J. Heat Mass Transfer, IJHMAK33, pp. 39–53.ijhIJHMAK0017-9310Hayes, L. J., Chang, H. J., and Diller, K. R., 1992, “Coupled Heat and Mass Transfer During Dendritic Solidification in Tissue Freezing,” in Macroscopic and Microscopic Heat and Mass Transfer in Biomedical Engineering, edited by K. R. Diller and A. Shitzer, ICHMT Press, Belgrade, pp. 327–336.Vemuri,  B. C., Diller,  K. R., Davis,  L. S., and Aggarwal,  J. K., 1983, “ Image Analysis of Solid-Liquid Interface Morphology in Freezing Solutions,” Pattern Recogn., PTNRA816, pp. 51–61.ptmPTNRA80031-3203Vemuri,  B. C., Diller,  K. R., and Aggarwal,  J. K., 1984, “ A Model for Characterizing the Motion of the Solid-Liquid Interface in Freezing Solutions,” Pattern Recogn., PTNRA817, pp. 313–319.ptmPTNRA80031-3203Ross,  D. C., and Diller,  K. R., 1978, “ The Therapeutic Effects of Postburn Cooling,” J. Biomech. Eng., JBENDY100, pp. 149–152.jbyJBENDY0148-0731Hlatky, M. A., Cravalho, E. G., Diller, K. R., and Huggins, C. E., 1973, “Response of the Microcirculation to Freezing and Thawing,” ASME 73-WA/BIO-16, 8 pp.Henriques,  F. C., 1947, “ Studies of Thermal Injury. V. The Predictability and Significance of Thermally Induced Rate Processes Leading to Irreversible Epidermal Injury,” Arch. Pathol., ZZZZZZ43, pp. 489–502.2n7ZZZZZZ0363-0153Aggarwal,  S. J., Shah,  S. J., Diller,  K. R., and Baxter,  C. R., 1989, “ Fluorescence Digital Microscopy of Interstitial Macromolecular Diffusion in Burn Injury,” Comput. Biol. Med., CBMDAW19, pp. 245–261.cbdCBMDAW0010-4825Aggarwal,  S. J., Diller,  K. R., Blake,  G. K., and Baxter,  C. R., 1994, “ Burn Induced Alterations in Vasoactive Function of the Peripheral Cutaneous Microcirculation,” J. Burn Care Rehabil., ZZZZZZ15, pp. 1–12.58mZZZZZZ0273-8481Funk,  W., and Intaglietta,  M., 1983, “ Spontaneous Arteriolar Vasomotion,” Prog. Appl. Microcirc., ZZZZZZ3, pp. 66–82.Gourgouliatos,  Z. F., Welch,  A. J., Diller,  K. R., and Aggarwal,  S. J., 1990, “ Laser-Irradiation-Induced Relaxation of Blood Vessels In Vivo,” Lasers Surg. Med., LSMEDI10, pp. 524–532.lsmLSMEDI0196-8092McGrath, J. J., 1987, “Temperature-controlled Cryogenic Light Microscopy–An Introduction to Cryomicroscopy,” in The Effects of Low Temperature on Biological Systems, edited by B. W. W. Grout and G. J. Morris, Edward Arnold Publishers, London, pp. 234–256.Diller, K. R., 1988, “Cryomicroscopy,” in Low Temperature Biotechnology: Emerging Applications and Engineering Contributions, edited by J. J. McGrath and K. R. Diller, ASME, New York, pp. 347–362.

Commentary by Dr. Valentin Fuster
J Biomech Eng. 2005;127(1):85-97. doi:10.1115/1.1835355.

Subablative thermotherapy is frequently used for the treatment of joint instability related diseases. In this therapy, mechanically deformed collagenous tissues are thermally shrunk and the stability of the tissue is re-established. In this research, the thermal damage fields generated by three different clinical heating modalities (monopolar and bipolar radio frequency and Ho:YAG laser) are compared numerically using finite element analysis. The heating rate dependent denaturation characteristics of collagenous tissues are incorporated into the model using experimental data from in vitro experimentation with rabbit patellar tendons. It is shown that there are significant differences among the thermal damage profiles created by these modalities, explaining the main reason for the discrepancies reported in the literature in terms of the efficacy and safety of each modality. In the complementary paper, the accuracy of the model presented here is verified by in vitro experimentation with a model collagenous tissue and by quantifying the denaturation-induced birefringence change using Optical Coherence Tomography and Magnetic Resonance Imaging.

Commentary by Dr. Valentin Fuster


J Biomech Eng. 2005;127(1):98-107. doi:10.1115/1.1846072.

The dual Euler angles method has been proposed as an alternative approach to describe the general spatial human joint motion. In this study, the dual Euler angles method was applied to study the three-dimensional motion of the ankle complex. The methodology for obtaining dual Euler angles of the ankle complex was developed by using a “Flock of Birds” electromagnetic tracking device. The repeatability of the methodology was studied based on the intertester and intratester variability analysis. Finally kinematic coupling characteristics of the ankle complex during dorsiflexion–plantarflexion, eversion–inversion, and abduction–adduction were analyzed according to the parameters of the dual Euler angles.

Commentary by Dr. Valentin Fuster
J Biomech Eng. 2005;127(1):108-113. doi:10.1115/1.1835357.

It is believed that nurses risk the development of back pain as a consequence of sudden loadings during tasks in which they are handling patients. Forward dynamics simulations of sudden loads (applied to the arms) during dynamic lifting tasks were performed on a two-dimensional whole-body model. Loads were in the range of −80 kg to 80 kg, with the initial load being 20 kg. Loading the arm downwards with less than that which equals a mass of 20 kg did not change the compressive forces on the spine when compared to a normal lifting motion with a 20 kg mass in the hands. However, when larger loads (40 kg to 80 kg extra in the hands) were simulated, the compressive forces exceeded 13 000 N (above 3 400 N is generally considered a risk factor). Loading upwards led to a decrease in the compressive forces but to a larger backwards velocity at the end of the movement. In the present study, it was possible to simulate a fast lifting motion. The results showed that when loading the arms downwards with a force that equals 40 kg or more, the spine was severely compressed. When loading in the opposite direction (unloading), the spine was not compressed more than during a normal lifting motion. In practical terms, this indicates that if a nursing aide tries to catch a patient who is falling, large compressive forces are applied to the spine.

Commentary by Dr. Valentin Fuster
J Biomech Eng. 2005;127(1):114-122. doi:10.1115/1.1835358.

Active joint torques are the primary source of power and control in dynamic walking motion. However the amplitude, rate, timing and phasic behavior of the joint torques necessary to achieve a natural and stable performance are difficult to establish. The goal of this study was to demonstrate the feasibility and stable behavior of an actively controlled bipedal walking simulation wherein the natural system dynamics were preserved by an active, nonlinear, state-feedback controller patterned after passive downhill walking. A two degree-of-freedom, forward-dynamic simulation was implemented with active joint torques applied at the hip joints and stance leg ankle. Kinematic trajectories produced by the active walker were similar to passive dynamic walking with active joint torques influenced by prescribed walking velocity. The control resulted in stable steady-state gait patterns, i.e. eigenvalue magnitudes of the stride function were less than one. The controller coefficient analogous to the virtual slope was modified to successfully control average walking velocity. Furture developments are necessary to expand the range of walking velocities.

Commentary by Dr. Valentin Fuster
J Biomech Eng. 2005;127(1):123-133. doi:10.1115/1.1846070.

This work describes the design and capabilities of the Purdue Knee Simulator: Mark II and a sagittal-plane model of the machine. This five-axis simulator was designed and constructed to simulate dynamic loading activities on either cadaveric knee specimens or total knee prostheses mounted on fixtures. The purpose of the machine was to provide a consistent, realistic loading of the knee joint, allowing the kinematics and specific loading of the structures of the knee to be determined based on condition, articular geometry, and simulated activity. The sagittal-plane model of the knee simulator was developed both to predict the loading at the knee from arbitrary inputs and to generate the necessary inputs required to duplicate specified joint loading. Measured tibio-femoral compressive force and quadriceps tension were shown to be in good agreement with the predicted loads from the model. A controlled moment about the ankle-flexion axis was also shown to change the loading on the quadriceps.

Topics: Force , Stress , Knee , Machinery
Commentary by Dr. Valentin Fuster


J Biomech Eng. 2005;127(1):134-147. doi:10.1115/1.1835359.

Background : Many diseases that affect the mitral valve are accompanied by the proliferation or degradation of tissue microstructure. The early acoustic detection of these changes may lead to the better management of mitral valve disease. In this study, we examine the nonstationary acoustic effects of perturbing material parameters that characterize mitral valve tissue in terms of its microstructural components. Specifically, we examine the influence of the volume fraction, stiffness and splay of collagen fibers as well as the stiffness of the nonlinear matrix in which they are embedded. Methods and Results : To model the transient vibrations of the mitral valve apparatus bathed in a blood medium, we have constructed a dynamic nonlinear fluid-coupled finite element model of the valve leaflets and chordae tendinae. The material behavior for the leaflets is based on an experimentally derived structural constitutive equation. The gross movement and small-scale acoustic vibrations of the valvular structures result from the application of physiologic pressure loads. Material changes that preserved the anisotropy of the valve leaflets were found to preserve valvular function. By contrast, material changes that altered the anisotropy of the valve were found to profoundly alter valvular function. These changes were manifest in the acoustic signatures of the valve closure sounds. Abnormally, stiffened valves closed more slowly and were accompanied by lower peak frequencies. Conclusion : The relationship between stiffness and frequency, though never documented in a native mitral valve, has been an axiom of heart sounds research. We find that the relationship is more subtle and that increases in stiffness may lead to either increases or decreases in peak frequency depending on their relationship to valvular function.

Commentary by Dr. Valentin Fuster
J Biomech Eng. 2005;127(1):148-157. doi:10.1115/1.1835360.

We describe an experimental method and apparatus for the estimation of constitutive parameters of soft tissue using Magnetic Resonance Imaging (MRI), in particular for the estimation of passive myocardial material properties. MRI tissue tagged images were acquired with simultaneous pressure recordings, while the tissue was cyclically deformed using a custom built reciprocating pump actuator. A continuous three-dimensional (3D) displacement field was reconstructed from the imaged tag motion. Cavity volume changes and local tissue microstructure were determined from phase contrast velocity and diffusion tensor MR images, respectively. The Finite Element Method (FEM) was used to solve the finite elasticity problem and obtain the displacement field that satisfied the applied boundary conditions and a given set of material parameters. The material parameters which best fit the FEM predicted displacements to the displacements reconstructed from the tagged images were found by nonlinear optimization. The equipment and method were validated using inflation of a deformable silicon gel phantom in the shape of a cylindrical annulus. The silicon gel was well described by a neo-Hookian material law with a single material parameter C1=8.71±0.06 kPa, estimated independently using a rotational shear apparatus. The MRI derived parameter was allowed to vary regionally and was estimated as C1=8.80±0.86 kPa across the model. Preliminary results from the passive inflation of an isolated arrested pig heart are also presented, demonstrating the feasibility of the apparatus and method for isolated heart preparations. FEM based models can therefore estimate constitutive parameters accurately and reliably from MRI tagging data.

Commentary by Dr. Valentin Fuster
J Biomech Eng. 2005;127(1):158-165. doi:10.1115/1.1835361.

Biological tissues like intervertebral discs and articular cartilage primarily consist of interstitial fluid, collagen fibrils and negatively charged proteoglycans. Due to the fixed charges of the proteoglycans, the total ion concentration inside the tissue is higher than in the surrounding synovial fluid (cation concentration is higher and the anion concentration is lower). This excess of ion particles leads to an osmotic pressure difference, which causes swelling of the tissue. In the last decade several mechano-electrochemical models, which include this mechanism, have been developed. As these models are complex and computationally expensive, it is only possible to analyze geometrically relatively small problems. Furthermore, there is still no commercial finite element tool that includes such a mechano-electrochemical theory. Lanir (Biorheology, 24 , pp. 173–187, 1987) hypothesized that electrolyte flux in articular cartilage can be neglected in mechanical studies. Lanir’s hypothesis implies that the swelling behavior of cartilage is only determined by deformation of the solid and by fluid flow. Hence, the response could be described by adding a deformation-dependent pressure term to the standard biphasic equations. Based on this theory we developed a biphasic swelling model. The goal of the study was to test Lanir’s hypothesis for a range of material properties. We compared the deformation behavior predicted by the biphasic swelling model and a full mechano-electrochemical model for confined compression and 1D swelling. It was shown that, depending on the material properties, the biphasic swelling model behaves largely the same as the mechano-electrochemical model, with regard to stresses and strains in the tissue following either mechanical or chemical perturbations. Hence, the biphasic swelling model could be an alternative for the more complex mechano-electrochemical model, in those cases where the ion flux itself is not the subject of the study. We propose thumbrules to estimate the correlation between the two models for specific problems.

Commentary by Dr. Valentin Fuster
J Biomech Eng. 2005;127(1):166-180. doi:10.1115/1.1835362.

Clinical studies have identified factors such as the stent design and the deployment technique that are one cause for the success or failure of angioplasty treatments. In addition, the success rate may also depend on the stenosis type. Hence, for a particular stenotic artery, the optimal intervention can only be identified by studying the influence of factors such as stent type, strut thickness, geometry of the stent cell, and stent–artery radial mismatch with the wall. We propose a methodology that allows a set of stent parameters to be varied, with the aim of evaluating the difference in the mechanical environment within the wall before and after angioplasty with stenting. Novel scalar quantities attempt to characterize the wall changes in form of the contact pressure caused by the stent struts, and the stresses within the individual components of the wall caused by the stent. These quantities are derived numerically and serve as indicators, which allow the determination of the correct size and type of the stent for each individual stenosis. In addition, the luminal change due to angioplasty may be computed as well. The methodology is demonstrated by using a full three-dimensional geometrical model of a postmortem specimen of a human iliac artery with a stenosis using imaging data. To describe the material behavior of the artery, we considered mechanical data of eight different vascular tissues, which formed the stenosis. The constitutive models for the tissue components capture the typical anisotropic, nonlinear and dissipative characteristics under supra-physiological loading conditions. Three-dimensional stent models were parametrized in such a way as to enable new designs to be generated simply with regard to variations in their geometric structure. For the three-dimensional stent–artery interaction we use a contact algorithm based on smooth contact surfaces of at least C1-continuity, which prevents numerical problems known from standard facet-based contact algorithm. The proposed methodology has the potential to provide a scientific basis for optimizing treatment procedures and stent geometries and materials, to help stent designers examine new stent designs “virtually,” and to assist clinicians in choosing the most suitable stent for a particular stenosis.

Commentary by Dr. Valentin Fuster
J Biomech Eng. 2005;127(1):181-185. doi:10.1115/1.1835363.

Evaluations of tendon mechanical behavior based on biochemical and structural arrangement have implications for designing tendon specific treatment modalities or replacement strategies. In addition to the well studied type I collagen, other important constituents of tendon are the small proteoglycans (PGs). PGs have been shown to vary in concentration within differently loaded areas of tendon, implicating them in specific tendon function. This study measured the mechanical properties of multiple tendon tissues from normal mice and from mice with knock-outs of the PGs decorin or biglycan. Tail tendon fascicles, patellar tendons (PT), and flexor digitorum longus tendons (FDL), three tissues representing different in vivo loading environments, were characterized from the three groups of mice. It was hypothesized that the absence of decorin or biglycan would have individual effects on each type of tendon tissue. Surprisingly, no change in mechanical properties was observed for the tail tendon fascicles due to the PG knockouts. The loss of decorin affected the PT, causing an increase in modulus and stress relaxation, but had little effect on the FDL. Conversely, the loss of biglycan did not significantly affect the PT, but caused a reduction in both the maximum stress and modulus of the FDL. These results give mechanical support to previous biochemical data that tendons likely are uniquely tailored to their specific location and function. Variances such as those presented here need to be further characterized and taken into account when designing therapies or replacements for any one particular tendon.

Commentary by Dr. Valentin Fuster


J Biomech Eng. 2005;127(1):186-192. doi:10.1115/1.1835364.

Laminectomy and facetectomy are surgical techniques used for decompression of the cervical spinal stenosis. Recent in vitro and finite element studies have shown significant cervical spinal instability after performing these surgical techniques. However, the influence of degenerated cervical disk on the biomechanical responses of the cervical spine after these surgical techniques remains unknown. Therefore, a three-dimensional nonlinear finite element model of the human cervical spine (C2–C7) was created. Two types of disk degeneration grades were simulated. For each grade of disk degeneration, the intact as well as the two surgically altered models simulating C5 laminectomy with or without C5–C6 total facetectomies were exercised under flexion and extension. Intersegmental rotational motions, internal disk annulus, cancellous and cortical bone stresses were obtained and compared to the normal intact model. Results showed that the cervical rotational motion decreases with progressive disk degeneration. Decreases in the rotational motion due to disk degeneration were accompanied by higher cancellous and cortical bone stress. The surgically altered model showed significant increases in the rotational motions after laminectomies and facetectomies when compared to the intact model. However, the percentage increases in the rotational motions after various surgical techniques were reduced with progressive disk degeneration.

Commentary by Dr. Valentin Fuster
J Biomech Eng. 2005;127(1):193-197. doi:10.1115/1.1835365.

The objective of this study was to assess the precision and accuracy of a nonproprietary, optical three-dimensional (3D) motion analysis system for the simultaneous measurement of soft tissue strains and joint kinematics. The system consisted of two high-resolution digital cameras and software for calculating the 3D coordinates of contrast markers. System precision was assessed by examining the variation in the coordinates of static markers over time. Three-dimensional strain measurement accuracy was assessed by moving contrast markers fixed distances in the field of view and calculating the error in predicted strain. Three-dimensional accuracy for kinematic measurements was assessed by simulating the measurements that are required for recording knee kinematics. The field of view (190 mm) was chosen to allow simultaneous recording of markers for soft tissue strain measurement and knee joint kinematics. Average system precision was between ±0.004 mm and ±0.035 mm, depending on marker size and camera angle. Absolute error in strain measurement varied from a minimum of ±0.025% to a maximum of ±0.142%, depending on the angle between cameras and the direction of strain with respect to the camera axes. Kinematic accuracy for translations was between ±0.008 mm and ±0.034 mm, while rotational accuracy was ±0.082 deg to ±0.160 deg. These results demonstrate that simultaneous optical measurement of 3D soft tissue strain and 3D joint kinematics can be performed while achieving excellent accuracy for both sets of measurements.

Commentary by Dr. Valentin Fuster
J Biomech Eng. 2005;127(1):198-203. doi:10.1115/1.1835366.

Background: Osteoporosis in long bones involves loss of cortical thickness and of the trabecular microarchitecture. Deterioration and weakening of trabecular bone tissue during osteoporosis imposes greater physiological loads on the cortical shell. However, it is unclear whether trabecular bone significantly contributes to the strength of whole bones under non-physiological impact loads. Method of Approach: We hypothesize that trabecular tissue in epiphyses of long bones contributes to resisting and distributing impact loads. To test this hypothesis, we caused artificial trabecular bone loss in proximal femora of adult hens but did not alter the bone cortex. Subsequently, we compared the energy required to fracture the proximal part of femora with missing trabecular tissue with the energy required to fracture control femora, by means of a Charpy test. Results: Extensive loss of trabecular bone in hens (over 0.50 grams or ∼71% weight fraction) significantly reduced the energy required to fracture the whole proximal femur in mediolateral impacts (from ∼0.37 joule in controls to ∼0.20 joule after extraction of core trabecular tissue). Conclusions: These findings indicate that trabecular bone in the proximal femur is important for distributing impact loads applied to the cortex, and support the concept that in treating osteoporosis to prevent hip fractures, it is just as important to prevent trabecular bone loss as it is important to prevent loss of cortical thickness.

Commentary by Dr. Valentin Fuster
J Biomech Eng. 2005;127(1):204-207. doi:10.1115/1.1835367.

Transparent stereolithographic rapid prototyping (RP) technology has already demonstrated in literature to be a practical model construction tool for optical flow measurements such as digital particle image velocimetry (DPIV), laser doppler velocimetry (LDV), and flow visualization. Here, we employ recently available transparent RP resins and eliminate time-consuming casting and chemical curing steps from the traditional approach. This note details our methodology with relevant material properties and highlights its advantages. Stereolithographic model printing with our procedure is now a direct single-step process, enabling faster geometric replication of complex computational fluid dynamics (CFD) models for exact experimental validation studies. This methodology is specifically applied to the in vitro flow modeling of patient-specific total cavopulmonary connection (TCPC) morphologies. The effect of RP machining grooves, surface quality, and hydrodynamic performance measurements as compared with the smooth glass models are also quantified.

Commentary by Dr. Valentin Fuster


J Biomech Eng. 2005;127(1):208-209. doi:10.1115/1.1835985.

Ateshian,  G. A., and Wang,  H.-Q., 1995, “ A Theoretical Solution for the Frictionless Rolling Contact of Cylindrical Biphasic Articular Cartilage Layers,” J. Biomech., JBMCB528, pp. 1341–1355.jbiJBMCB50021-9290Kelkar,  R., and Ateshian,  G. A., 1999, “ Contact Creep of Biphasic Cartilage Layers,” ASME J. Appl. Mech., JAMCAV66, pp. 137–145.97dJAMCAV0021-8936McCutchen,  C. W., 1962, “ The Frictional Properties of Animal Joints,” Wear, WEARCJ5, pp. 1–17.weaWEARCJ0043-1648McCutchen,  C. W., 1975, “ An Approximate Equation for Weeping Lubrication, Solved With an Electrical Analogue,” Ann. Rheum. Dis., ARDIAO34, Supplement, pp. 85–90.andARDIAO0003-4967

Commentary by Dr. Valentin Fuster

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