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IN THIS VOLUME


Bioengineering

2003;():1-3. doi:10.1115/IMECE2003-42907.

A probabilistic, transient, three-phase model of chemical transport through human skin has been developed to assess the relative importance of uncertain parameters and processes during chemical exposure assessments and transdermal drug delivery. Penetration routes through the skin that were modeled include the following: (1) intercellular diffusion through the multiphase stratum corneum; (2) aqueous-phase diffusion through sweat ducts; and (3) oil-phase diffusion through hair follicles. Uncertainty distributions were developed for the model parameters, and a Monte Carlo analysis was performed to simulate probability distributions of mass fluxes through each of the routes for a hypothetical scenario of chemical transport through the skin. At early times (60 seconds), the sweat ducts provided a significant amount of simulated mass flux into the bloodstream. At longer times (1 hour), diffusion through the stratum corneum became important because of its relatively large surface area. Sensitivity analyses using stepwise linear regression were also performed to identify model parameters that were most important to the simulated mass fluxes at different times.

Topics: Modeling , Skin
Commentary by Dr. Valentin Fuster
2003;():5-6. doi:10.1115/IMECE2003-42950.

The efficacy of radiofrequency ablation (RFA) treatments depends on the ability to ablate tumors completely while minimizing the damage to healthy tissue. Tissue cooling due to blood flow is an important factor affecting the size and shape of the ablation lesion. In this paper a new methodology for finite element modeling of the coupled electrical-thermal-flow process during RFA is presented. Our formulation treats heat losses due to blood flow explicitly rather than approximating the collective effects of blood vessles as a heat sink. Numerical models were compared to in vitro models using egg whites to simulate human tissue and a straight cylinder filled with a saline solution to simulate blood. Asymmetric burns were obtained close to the simulated blood vessels. Numerical results closely match the in vitro models.

Commentary by Dr. Valentin Fuster
2003;():7-8. doi:10.1115/IMECE2003-43049.

Isothermal drying and glass transition of solutions and films have drawn considerable attention from many industries. We here explore the feasibility of modifying the isothermal drying and vitrification kinetics of carbohydrate solutions in order to ensure the stability and quality of their ingredients. Modulated Differential Scanning Calorimetry experiments with isothermally dried trehalose and trehalose/dextran solutions were performed and the glass transition kinetics have been determined. Three distinct drying regimes were observed. With isothermal, isobaric drying at 0%RH, it was indeed possible to reach the glassy state for a trehalose and a trehalosedextran system. With the addition of high molecular weight sugars, the glass transitions of isothermally dried carbohydrate solutions can be accelerated as a function of dextran mass ratio in the sample.

Topics: Temperature , Drying , Glass
Commentary by Dr. Valentin Fuster
2003;():9-10. doi:10.1115/IMECE2003-43125.

A mathematical model is developed to describe the transport phenomena and electrochemical reaction kinetics during amperometric measurement of superoxide radical concentration using ZnO-polymer nanocomposite sensor. This model assumes a logarithmic normal distribution for the nanoparticles immobilized in the polymer matrix and an empirical relation for the diffusion coefficient of superoxide radicals as a function of pore volume fraction. A kinetic with secondary order rate constant is used to represent the electrochemical reactions of electron transfer from the superoxide radicals to nanoparticles. The predicted results include the effect of diffusion coefficient on concentration and electrical conductivity.

Commentary by Dr. Valentin Fuster
2003;():11-12. doi:10.1115/IMECE2003-43136.

The goal of this work is to compare the in vitro drug (Nimodipine) release rate from three different types (matrix, reservoir, and combination types) of intravitreal implants using a biodegradable polymer (PLGA). The matrix implants were prepared by a solvent cast method and the reservoir implants were fabricated by using a pellet press. The combination implants were a mixed type of matrix and reservoir implants. Each implant was placed in a vial with 7mL of Phosphate Buffered Saline (PBS) containing 0.4 g/L of Bovine Serum Albumin (BSA) and 0.5 mL aliquots were removed for the drug assay for 25 days. The sample was analyzed to determine the concentration using High Performance Liquid Chromatography (HPLC). Over 20 days, the average steady state release rate in vitro is 13.05±6.17, 2.66±1.98, 8.85±6.36 μg/day for matrix, reservoir, and combination implants, respectively.

Topics: Drugs , PLGA
Commentary by Dr. Valentin Fuster
2003;():13-14. doi:10.1115/IMECE2003-43169.

Treatment of vascular skin lesions, such as port wine stain (PWS) birthmarks, uses laser energy to thermally damage the hypervascular regions in the dermis. Delivery of sufficient laser energy to the dermis can be confounded by the more superficial epidermal melanin layer, a broadband optical absorber. Laser pulses directed at skin lesions may result in excessive heating and scarring of the epidermis, resulting also in limiting the energy available to treat the dermal lesion. Cryogen spray cooling is used to selectively cool the epidermis, allowing the use of higher laser fluences which cause thermal damage in the deeper lesion, while preventing hypertrophic scarring of the epidermis. In order to optimize cryogen cooling and laser pulse parameter for treatment, the depth profile of the epidermal melanin layer and vascular lesion must be known. We use a photoacoustic probe to determine this depth profile. A Q-switched Nd:YAG laser emitting 532 nm light pulses of 4 ns duration is used to excite acoustic waves in epidermal melanin and hemoglobin in the vascular lesions via a miniature probe incorporating a piezoelectric detector. We used acoustic propagation time to determine the spatial relationship of the absorbing structures, thereby giving the necessary means to determine cryogen spurt and laser pulse timing. We present a finite difference thermal model along with data from tissue phantoms showing the effectiveness of the photoacoustic method.

Commentary by Dr. Valentin Fuster
2003;():15-16. doi:10.1115/IMECE2003-43212.

Currently many methods of drying are under investigation for the anhydrous preservation of mammalian cells, including convective drying (Bhowmick et al., 2003), vacuum drying (Elliott et al., 2002), room temperature desiccation cabinet drying (Acker et al., 2002), and freeze-drying (Wolkers et al., 2001). Independent of the drying method, cells are typically in an aqueous suspension at the onset. Generally the change in ‘system’ weight (cells and extracellular solution) is measured gravimetrically and this moisture loss in the system is assumed to equate to the loss of moisture within the suspended cells. This approach assumes that the intra- and extra cellular water contents are equivalent and that the dried sample is homogenous. These assumptions are not always valid. Oftentimes cells are seen to pool towards the middle of a droplet and macroscopic heterogeneities of moisture content within the droplet are sometimes visible by eye. Although important advances in mammalian cell preservation have been made using this approach, a convenient method of determining localized water content within such samples would advance the state-of-the-art in drying technologies. Because sugars are a common excipient used in drying, the current study investigates the spatial distribution of moisture content in dried sugar matrices using pyranine as a fluorescent probe of the local water environment. The pyranine molecule has two fluorescent states, emitting at 511 nm (green) and 440 nm (blue). Using calibration curves based on ratiometric measurements of blue to green pyranine fluorescence emissions the effect of drying substrate on moisture content uniformity is explored in pyranine-doped sucrose solutions.

Commentary by Dr. Valentin Fuster
2003;():17-18. doi:10.1115/IMECE2003-43264.

In this research, continuum theory (macroscopic enthalpy model) for multi-component phase change was applied to study the freezing process of bulk sample (cell suspension in a flat blood bag), coupled with the investigation of microscopic mass transfer across cell membrane. Numerical simulation results indicated that distributions of temperature and solute concentration inside the sample are not uniform during the cooling process. As a consequence, the degrees of cell dehydration are different at various locations.

Topics: Heat , Freezing , Cryonics , Water
Commentary by Dr. Valentin Fuster
2003;():19-20. doi:10.1115/IMECE2003-43690.

Numerical simulations of blood and water flow and oxygen transport in a computational model of an intravenous membrane oxygenator including moving boundaries are presented. The simulations are compared to an analytical transport model which is validated by comparing its result to experimental data reported in the literature. Good agreement is found between numerical, analytical and experimental results.

Topics: Membranes , Oxygen
Commentary by Dr. Valentin Fuster
2003;():21-29. doi:10.1115/IMECE2003-43825.

Early detection is considered to be the best defense against breast cancer and imaging plays a very important role in screening and in the diagnosis of symptomatic women. Infrared thermal imaging of skin temperature changes caused by a malignant tumor in breast is a rapidly developing detection modality with potential for functional detection. Knowledge and control of environmental factors which affect the skin temperature can reduce misinterpretations and false diagnosis associated with infrared imaging. A bio heat transfer based numerical model was utilized to study the energy balance in healthy and malignant breasts subjected to low velocity forced convection in a wind tunnel. Existing estimates of metabolic heating rates and previous measurements of temperature distributions along the radial direction in a region intersecting a known tumor and a comparable region in the healthy breast of the same patient were used to estimate the blood perfusion rates for the tumor. A simplified structural and thermal model was used for representing the changes within and around the tumor. Steady state temperature distributions on the skin surface of the breasts were obtained by numerically solving the conjugate heat transfer problem. Parametric studies on the influences of the airflow on the skin thermal expression of tumors were performed. It was found that the presence of tumor may not be clearly shown due to the irregularity of the skin temperature distribution induced by the flow field. Image processing techniques could be employed to eliminate the effects of the flow field and thermal noise and significantly improve the thermal signature of the tumor on the skin surface.

Commentary by Dr. Valentin Fuster
2003;():31-32. doi:10.1115/IMECE2003-43044.

Pharmaceutical bone growth stimulation holds promise for prevention and treatment bone disorders, and the enhancement of fracture healing. Bone growth hormones have begun to have limited clinical use, but can illicit adverse side effects. Recent studies have shown that short peptides (less than 15 amino acids) derived from the protein sequence of Vitamin D Binding Protein (DBP), can enhance bone formation (osteogenesis). These peptides may have potential as controllable bone growth stimulators without the adverse side effects and cost of bone growth hormones. Rats, injected every other day for two weeks with DBP-based peptide fragments ranging from 3 to 13 amino acids in length, were euthanized and the tibias and femurs were scanned by peripheral quantitative computerized tomography (pQCT) to determine bone density and cross-sectional geometric properties. The bones were then tested in three-point bending to determine strength and bending modulus. Injection of DBP-based peptides over only a 2-week period resulted in significant (p<0.05) increases in bone density and material properties in the experimental rat bones in comparison to controls injected with saline. The short length of these effective peptides suggests their use not only in systemic injections but also as clinically convenient pills taken orally for pharmaceutically induced bone growth stimulation.

Commentary by Dr. Valentin Fuster
2003;():33-34. doi:10.1115/IMECE2003-43048.

An improved method for manufacturing prosthetic bones is examined. We are developing a new improved method for designing and manufacturing prosthetic bones that have a porous interior core covered by a solid outer shell, more closely matching the morphology of natural bone. The new method is compatible with a wide variety of materials, including polymers, metals, composites, and biodegradable scaffold materials. Use of biodegradable scaffold material holds the potential for eventual bone regeneration within and throughout the prosthesis. Regardless of the material selection, this improved type of prosthesis is expected to more closely mimic the overall material and structural properties of natural bone, including shape, strength, weight, and weight distribution. By fabricating prosthetic bones that duplicate the material and structural properties of natural bone, implants could be made to operate as precision replacements, feeling and functioning exactly like natural bone. In addition to improving patient comfort, these new prostheses are expected to reduce the occurrence of unnatural secondary wear patterns caused by current style prosthetic bones that function in unnatural fashions due to their non-matching material and structural properties.

Commentary by Dr. Valentin Fuster
2003;():35-36. doi:10.1115/IMECE2003-43054.

Researchers have tried to assess the morphological and mechanical characteristics of bone using various methods with a limited success (Mow and Hayes, 1997, and Cowin, 2001). This may be due to the complexity for documentation and characterization of architecture by the anisotropic and nonhomogeneous characteristic of the bone. The bones of the shoulder joint complex are particularly deprived of information on their mechanical and morphological properties. Very little is known for example on the mechanics and morphology of the glenoid of the shoulder. The present work presents an attempt to identify a proper method for the measurement and characterization of the morphology and mechanics of bone, particularly in the glenoid. Our results show that trabecular lines are directed at 82.15° ± 7.98° relative to the glenoid articular surface, young’s modulus obtained from CT scan was 230 ± 8 MP, and the elastic tensors obtained from cadaver were 326 ± 78 MPa for E2222 , 144 ± 22 MPa for E1111 =E3333 , 49 ± 7 MPa for E3311 , 59 ± 9 MPa for E1122 =E2233 , 51 ± 8 MPa for E1313 , and 70 ± 10 MPa for E1212 =E2323 . These results were consistent with Wolf’s Law and were in agreement with results reported in literatures (Anglin et al., 1999, Frich et al., 1998, and Mansat et al., 1998).

Topics: Elasticity
Commentary by Dr. Valentin Fuster
2003;():37-38. doi:10.1115/IMECE2003-43116.

Biglycan (Bgn) is a small leucine-rich proteoglycan (SLRP) that is enriched in bone and other skeletal connective tissues and is responsible, in part, for the regulation of postnatal skeletal growth (Bianco, 1990). Mice lacking Bgn display reduced skeletal development and a lower peak bone mass that leads to age-dependent osteopenia (Xu, 1998). We hypothesized that mechanical loading could reverse the skeletal phenotype of Bgn knockout mice. To test this hypothesis, we determined the effects of treadmill running on the geometric, mechanical and mineral properties of Bgn deficient mice bones. After sacrifice, femora and tibiae were tested in 4 point bending and cross-sectional geometric properties and bone mineral parameters were measured. Exercise was able to partially reverse the skeletal phenotype of the Bgn knockouts by increasing both the geometric and mechanical properties of the tibiae to values equal to or greater than those of wild type control mice.

Commentary by Dr. Valentin Fuster
2003;():39-40. doi:10.1115/IMECE2003-43134.

An improved understanding of the stress distribution in and around the hip joint may provide important information regarding the relationship between altered pelvic and acetabular geometry and development of hip osteoarthritis, as well as point to improved diagnostic methods and analysis of surgical treatment. It is very difficult to accurately assess how changes in pelvic geometry affect the stress and strain distribution of the joint in an experimental setting. The finite element (FE) method provides an alternative approach for study of hip joint mechanics. Although FE models of the pelvis have been developed, validation by direct comparison with subject-specific experimental measurements has not been performed. In addition, previous models have utilized over-simplified bone geometry and homogeneous material properties. The objectives of this study were to 1) develop and validate a FE model of the pelvis using subject-specific measurements of bone geometry as well as location-dependent cortical thickness and trabecular bone elastic modulus, and 2) assess the sensitivity of the subject-specific FE model to changes in material properties and cortical thickness.

Commentary by Dr. Valentin Fuster
2003;():41-42. doi:10.1115/IMECE2003-43171.

We model trabecular bone as an idealized periodic structure made of open cubic cells and represent it as a linear elastic couple stress continuum. We find couple stress moduli for such material using displacement and traction boundary conditions. Several different loadings are applied to obtain these couple stress constants: uniaxial extension (or tension), hydrostatic deformation (or pressure), applied curvature (or bending moment), and twist (or torsion).

Topics: Stress , Bone , Modeling
Commentary by Dr. Valentin Fuster
2003;():43-44. doi:10.1115/IMECE2003-42900.

A simple and accurate perfused tissue phantom would be a useful tool for biomedical research. The phantom described here is an agar-saline model that can be easily constructed. Space between finely diced pieces of agar is perfused with saline. A rotator Boekel-Orbitron® is used to perfuse the agarsaline mixture, mimicking the non-directional movement of blood. Dye injected into the phantom records the volumetric movement of saline for a set time period on the Orbitron. These data are used to determine perfusion estimates in units of g/s/mL. Experimental results can be appropriately scaled and additional modifications can be better mimic specific in-vivo conditions.

Commentary by Dr. Valentin Fuster
2003;():45-46. doi:10.1115/IMECE2003-43127.

A three-dimensional musculoskeltal model of the human knee is being developed to investigate the forces placed on the four major knee ligaments as a result of muscle contraction. This computational knee model includes the femur, tibia, and patella, seven major muscles, four knee ligaments, and the patellar tendon. A constrained forward dynamic simulation is performed by using EMG data to estimate muscle forces. The ligament forces estimated by the model are verified by means of a robotic-cadaver experimental setup in which the knee flexion angle of a cadaver specimen was controlled by a robot. Estimated muscle forces were dynamically applied to the cadaver knee as it was flexed/extended through a specified range of motion. The ligament strain calculated by the model is compared to the strain measured experimentally and to values found in literature to verify the accuracy of the model.

Topics: Force , Dynamic models , Knee
Commentary by Dr. Valentin Fuster
2003;():47-48. doi:10.1115/IMECE2003-43674.

The need for transferring a disabled individual from a wheelchair to another location and vice versa cannot be underemphasized, given the numerous day-to-day activities that may require relocation of the person and the psychological factors involved. While there are some devices currently available on the market that aid in transferring the handicapped individuals from one location to another, few are available for transferring them from a wheelchair to an automobile. Vehicular lifts, which is general require modification of the vehicle, and automobiles custom modified to accommodate wheelchairs are some examples of devices that facilitate transfer of individuals. However, these devices are in general suitable for one vehicle only and relatively expensive. This paper present the design of a device that will allow an individual to transfer himself/herself easily and safely from a wheelchair to automobile or another location and vice versa, with minimal assistance and at a relatively lower cost. Presented are the design and test results from a prototype. The results demonstrate that the intended specifications are satisfied and that this relatively low-cost design is likely to deliver a satisfactory performance and provide individuals in wheelchairs with more freedom to travel. Because of the easy usbility, less dependence on assistants, and ability for a larger range of vehicles to be used, the design may also provide feeling of independence.

Commentary by Dr. Valentin Fuster
2003;():49-50. doi:10.1115/IMECE2003-43841.

Siphoning is commonly associated with shunts, a treatment used in patients suffering from Hydrocephalus. Siphoning is known to have a negative effect on intracranial dynamics. In this paper, a model will be developed for the intracranial dynamics in the presence of siphoning based on the assumption of pulsatile intracranial dynamics as a forced non-linear van der Pol oscillator. This non-linear model is used to simulate how various degrees of siphoning can effect intracranial dynamics and create an understanding of painful side effects, such as vascular headaches. The model suggests that vascular headaches are due to an increase in the amplitude of arterial pulsations caused by the force exerted on the cranium by siphoning rather than due to volume shifts of cerebrospinal fluid (CSF).

Commentary by Dr. Valentin Fuster
2003;():51-52. doi:10.1115/IMECE2003-43997.

To adhere an artificial retinal implant onto the epiretinal surface of the eye, our group has designed retinal microtacks. The microtacks were fabricated by first bonding titanium foil to a silicon wafer; the tacks were then machined using an ultra-high precision micromilling technique with a 150-μm end-mill. The versatility of the micromilling technique allowed a partially conical, tapered tip to be added to the Ti tack, which created a sharp point. The Ti tacks average overall length and width were measured to be within 1% and 6%, respectively, of the design. Additionally, the grip width, stop thickness, and the tip taper angle of the Ti tacks were within 3%, 9%, and 4%, respectively, of the design.

Topics: Manufacturing
Commentary by Dr. Valentin Fuster
2003;():53-54. doi:10.1115/IMECE2003-41113.

2D and 3D multi-physics experiment-based nonlinear models with fluid-structure interactions (FSI) and structure-structure interactions (SSI) are introduced to model blood flow and stress/strain distributions in stenotic arteries with lipid pools. Material properties for the vessel and plaque are based on experimental measurements and information available in the literature (Huang et. al., 2001; Tang et. al., 2001). The Navier-Stokes equations are used as the governing equations for the fluid. Mooney-Rivlin models are used for both arteries and lipid cores. A well-tested finite element package ADINA is used to solve the models to perform flow and stress/strain analysis. Our results indicate that artery plaque stress/strain distributions are affected considerably (50%–400% or even more) by vessel material properties, stenosis severity and eccentricity, tube axial pre-stretch, pressure conditions, lipid core material property, size, position and geometry, and fluid-structure and structure-structure (vessel wall and lipid core) interactions. Differences in model assumptions and controlling factor specifications must be taken into consideration when interpreting the significance of computational results.

Commentary by Dr. Valentin Fuster
2003;():55-56. doi:10.1115/IMECE2003-41148.

The influence of fluid dynamics in atherogenesis has been intensively studied by many researchers (Caro et al., 1971, Giannoglou et al., 2002, Perktold et al., 1991, Qiu and Tarbell, 2000). It is widely believed that the atherosclerosis development and progression are affected by many risk factors, such as, static pressure, wall shear stress, blood viscosity flow velocity and geometry of the artery. Amongst those, static pressure plays a very important role. The objective of this work is to numerically analyze the blood flow in curved arteries with or without the presence of atherosclerotic plaque and to reveal how does the pressure drop along the inner wall of the arteries depend on the pressure drop along the inner wall of the arteries depend on the geometry of artery, plaque size and Reynolds number. A three-dimensional mathematical model is used and the finite element method is applied to spatial variables for solving the differential equations numerically. Computations are carried out with various values of physiological parameters, such as the angle of the curved artery, the size of plaque and Reynolds number. The numerical results show the pressure drop pattern along the inner wall of the curved arteries with or without plaque.

Commentary by Dr. Valentin Fuster
2003;():57-58. doi:10.1115/IMECE2003-41281.

Some results from a series of physiological flow experiments in a model of an atherosclerotic carotid bifurcation are presented. The flow model exactly replicates the lumen of the plaque excised intact from a patient with severe carotid atherosclerosis. Flow visualization (FV) and particle image velocimetry (PIV) are employed as the tools for this study. The complex internal geometry of the diseased artery combined with the pulsatile input flows gives rise to complex flow patterns. The flow fields are highly three-dimensional and chaotic with details varying from cycle to cycle. These flow patterns also include internal jets, three-dimensional shear layers, numerous separation/recirculation zones and stagnation lines. The vorticity and streamline maps confirm this complex and three-dimensional nature of the flow. Planar streamline maps show the three-dimensional flow by the multiple sources/sinks throughout the model. Wall shear stresses (WSS) are estimated to range form about −7 Pa to 34 Pa at the stenotic neck over time with the peak at peak systolic. These WSS also exhibit chaotic behavior during pulsatile flow cycles.

Commentary by Dr. Valentin Fuster
2003;():59-60. doi:10.1115/IMECE2003-42728.

The present study is devoted to the investigation of the pulsatile blood flow within the first few vessels of the Left Coronary Artery (LCA) vasculature of an anatomically accurate porcine coronary tree. Transient computational fluid dynamics simulations were performed under realistic pulsatile volume inflow boundary conditions. The numerical results have provided a comprehensive collection of information regarding the haemodynamics within the LCA and its major branches, namely the Left Anterior Descending (LAD) and the Left Circumflex (LCX) arteries. The underlying principle of developing computational techniques, which would eventually allow for the realistic simulation of the vascular haemodynamics of patients, lies on the capacity of such tools for predictive diagnostics and non-invasive, hence simulation-based, surgical planning.

Commentary by Dr. Valentin Fuster
2003;():61-62. doi:10.1115/IMECE2003-42766.

The pulsatile blood flow field in a patient-specific pathology of a large Abdominal Aortic Aneurysm (AAA) is being simulated, both pre and post interventionally. The anatomies of the aortic wall and blood lumen have been derived by digitized Computerized Tomography (CT) scans. Three dimensional unsteady computational fluid dynamics simulations have provided a comprehensive collection of quantitative information on the haemodynamics and the flow features that present themselves in both the temporal and spatial spaces. The focus lies on alterations in the haemodynamics triggered by the interventional procedure itself, which consists of the endoluminal introduction of a stent-graft. Significant information may also be deduced concerning the hydrodynamic loading of such implants. Computational tools of this nature, along with the non-invasive CT or Magnetic Resonance (MR) aortic imaging techniques, could enable an objective assessment of the possible effects of any interventional scenario in a virtual noninvasive environment both proximally and distally to the diseased region.

Commentary by Dr. Valentin Fuster
2003;():63-64. doi:10.1115/IMECE2003-42770.

Stents have proven very effective in opening the lumens of blocked and diseased arteries, leading to an increased quality of life for thousand of patients. Due to their success, stents have grown into a $1.5 billion dollar industry, but unfortunately still suffer from failure rates of 20–30% in the first year. Many of these failures can be traced back to restenosis or thrombosis of the stented arteries, a problem which conventional self-expanding or balloon-expanded stents have not proved effective in combating. Mathematical and experimental research shows that stents create adverse flow conditions and increase the stresses found around the implants, and trials of designs intended to reduce these effects have proven effective in combating restenosis. The goal of this research was to investigate mathematically design considerations for an improved stent that can reduce these negative effects. This was accomplished through the construction of a onedimensional numerical model for the fluid mechanics of the artery that was implemented using FEA and a combination of WENO and Runge-Kutta methods. The output from this model was compared with solutions from the literature and with in-vitro experimental results. Based on these tests it was concluded that the model accurately predicted the behavior of the pressure waves in a vessel. These numerical models were then used to evaluate several proposed designs. The pressure wave reflection was found to be controlled entirely by the design of the stent ends; mid-length variations in stent compliance provided no change in the model behavior. Also, a region of gradual transition between the low stiffness of the artery and the increased stiffness of the stent, while useful for reducing wall stresses, proved ineffective in reducing the magnitude of the reflected pressure waves. The best design for minimizing pressure wave reflection was found to be one that minimized total stent length.

Commentary by Dr. Valentin Fuster
2003;():65-66. doi:10.1115/IMECE2003-42840.

Blood flow in arteries is known to be closely related to atherosclerosis. Presence of recirculation zones, and low, high, and oscillatory wall shear stresses have been suggested to be important fluid dynamic factors causing development and progress of atherosclerosis. Our study was motivated to develop fluid mechanical indices between residence time of blood particles in arteries and atherosclerosis. In rigid models of stenosed arteries with 75% area reduction, trajectories of blood particles were numerically computed and used to determine local volumetric residence time (VRT) of platelets. The motion of particles in the model artery was computed by considering viscous drag forces between blood particles and presolved transient flow field from computational fluid dynamics (CFD). Many cardiac cycles were considered in the computation to reflect temporally accumulative characteristics of VRT in the recirculation zones. Our results showed that VRT in the recirculation zone was relatively low in the first cardiac cycle. However it increased in the subsequent cycles as more particles were trapped in the same zone. The results suggested that VRT contour calculated in the present study would be an effective indicator of the presence of atherosclerosis.

Topics: Blood
Commentary by Dr. Valentin Fuster
2003;():67-68. doi:10.1115/IMECE2003-42952.

Estimation of the wall shear stress distribution in stenotic carotid arteries is important for assessing risk of stroke. Since there are no reliable experimental methods to determine wall shear stress distributions, realistic patient-specific computational fluid dynamics models are constructed from medical images. Anatomical and physiologic data are obtained from multiple image modalities including 3D rotational angiography, contrast-enhanced magnetic resonance angiography, carotid duplex ultrasound and phase-contrast magnetic resonance. These images are used to construct patient-specific finite element grids and to solve the incompressible Navier-Stokes equations under physiological pulsatile flow conditions. The detailed knowledge of the carotid hemodynamics derived from these models can be used to enhance our understanding of the relationship between flow patterns and symptoms, and ultimately risk of stroke. This methodology can also be used to correllate flow patterns with the outcome of endovascular procedures such as angioplasty and stenting.

Commentary by Dr. Valentin Fuster
2003;():69-70. doi:10.1115/IMECE2003-43065.

Thrombosis in an arteriovenous hemodialysis access graft is a major cause of graft failure. Changes in flow features due to increasing resistance at the venous anastomosis may promote the development of thrombosis. A three-dimensional computational fluid dynamics model was developed to analyze flow at the arterial anastomosis of a PTFE brachial-brachial arteriovenous access graft. The geometry was obtained from contrast-enhanced magnetic resonance images. A surface mesh was extracted using Amira software, and the final volume mesh was generated by Tgrid (Fluent, Inc). The simulation was carried out for steady flow conditions using Fluent CFD software with low-Reynolds number k-ω turbulence model. Small areas of recirculation can be seen in the area of the bifurcation, and results show a pressure difference between the proximal artery and the graft that is consistent with reported values.

Commentary by Dr. Valentin Fuster
2003;():71-72. doi:10.1115/IMECE2003-43099.

Endovascular occlusion of cerebral aneurysms with bare platinum detachable coils is now recognized as preferable to surgical clipping (ISUIA Group, 2003, ISAT Group, 2002, Bavinzski et al, 1995, Thornton et al, 2002). Dependent on coil packing density (the ratio of the coil volume deposited in an aneurysm to that of the aneurysm volume), aneurysm location, size and neck width, coil compaction with recanalization of the aneurysm remains in the long-term a major concern. The aneurysm neck size is reported to be the main predictor for aneurysm recanalization (Fernandez-Zubillaga et al, 1994). The forces exerted on the coil mass at the aneurysm neck due to blood pulsatility are larger for wide neck aneurysms as compared to small neck aneurysms (Bavinzski et al, 1995). However, impingement forces have not been evaluated. We evaluated the force impinging on the aneurysm neck in a simplified aneurysm (basilar top) geometry utilizing the impulse-momentum equation and Womersley’s flow. Maximum impingement force as a function of aneurysm neck to parent lumen diameter ratio varies as a sigmoid curve. Analysis of the hemodynamic forces affecting coil compaction in cerebral aneurysms shows that the coil mass at the aneurysm neck may be subjected to cyclic impulse impingement due to redirection of blood momentum. Orientation of the aneurysm neck and the main axis of the aneurysm in relation to the oncoming parent vessel flow may help clinicians predict the risk of coil compaction and the location of subsequent aneurysm recanalization.

Commentary by Dr. Valentin Fuster
2003;():73-74. doi:10.1115/IMECE2003-43107.

Endovascular repair (EVAR) has emerged as an alternative, less-invasive surgical technique for the treatment of patients diagnosed with abdominal aortic aneurysms (AAAs). The anatomical pathway of blood flow in the abdominal aorta is restored by the implantation of an endovascular graft (EVG), effectively depressurizing the aneurysm and initiating a remodeling process of the diseased aorta. The short-term results of endovascular grafting are promising, but its long-term success has been compromised by the occurrence of graft migration and detachment, which induce endoleaks or incomplete occlusion of the aneurysm from the blood circulation. The forces induced by the blood as it flows through the graft are believed to be a factor of probable cause in the partial detachment from its proximal and distal anchoring points and the migration of the graft downstream. The purpose of this study is to utilize analytical tools to provide an estimation of the forces required to secure the graft proximally when relying only on stresses induced by the flow.

Commentary by Dr. Valentin Fuster
2003;():75-76. doi:10.1115/IMECE2003-43121.

Early intimal thickening (IT) in the carotid artery bifurcation has been associated with multiple hemodynamic and biophysical factors including low wall shear stress (WSS) and high oscillatory shear index (OSI) (Ku et al., 1995), as well as abnormal intramural stress and strain. While WSS conditions have been widely shown to affect vascular biology and arterial wall self-regulation, the near-wall localization of critical blood particles, such as monocytes and platelets, also plays a significant role in atherosclerotic lesion formation and general IT (Ross, 1993). In this study, we hypothesize that locations of elevated monocyte and platelet interactions with reactive or activated vascular surfaces, due to injury or endothelial dysfunction, are highly susceptible to IT initialization and progression in the carotid artery bifurcation (CAB). To assess the potential role of platelet-and monocyte-wall interactions, as well as other WSS-based hemodynamic wall parameters (HWPs), experimentally validated particle-hemodynamic simulations have been conducted for a representative CAB geometry. Areas of significant particle interactions with the vascular surface have been identified by a validated near-wall residence time (NWRT) model for monocytes and platelets, which encapsulates regions of near-wall particle stasis and/or elevated concentrations. Actual early intimal thickening has been assumed consistent with the observations of Masawa et al. (1994a & b) (Fig. 1).

Commentary by Dr. Valentin Fuster
2003;():79-80. doi:10.1115/IMECE2003-43145.

This study analyzes the pressure-flow characteristics during the peristaltic pumping of power law fluids in an axi-symmetric non-uniform distensible tube. The analyzed geometry is of a diverging shape that is common in several biological flow conduits, especially in mammals. Using the Fourier series, the dimensionless wall coordinates for sinusoidal, triangular, trapezoidal, and square wave forms are obtained to simulate wall movement. Equations expressing the pressure-flow rate relationship for different wall shapes are developed from the wave equation. Pressure-flow and velocity plots are obtained by solving the equations numerically. The results indicate that there is significant difference in pressure-flow relationship between Newtonian and non-Newtonian fluid. Also, the maximum flow rate can be achieved when the wall movement follows a square wave form.

Commentary by Dr. Valentin Fuster
2003;():81-82. doi:10.1115/IMECE2003-43148.

The present study focuses on developing basal to near hyperemic flow through the entrance region of a deployed stent in a coronary artery segment. Stents that are presently available in market differ significantly in design. Hence, there is a need to optimize its design such that the magnitude of wall shear stress is within physiologic limit, thus minimizing the patho-physiological effects. For near hyperemic flow, the analysis showed a 20 fold increase in the positive values of wall shear stress at the stent wires exposed to the blood flow. Further, at the void next to the entrance, the wall shear stress was an order of magnitude lower than the values typically observed in similar downstream regions.

Commentary by Dr. Valentin Fuster
2003;():83-84. doi:10.1115/IMECE2003-43364.

Local hemodynamics and wall shear stress (WSS) impact healing of implanted expanded polytetrafluoroethylene (ePTFE) vascular grafts. Since adverse outcomes occur shortly after implantation, we documented the effect of local hemodynamics on short-term healing. We implanted a control or a stenotic ePTFE graft with a 60 μm internodal porosity into the abdominal aorta of 15–20 kg juvenile male baboons. At one month we harvested all grafts using in situ pressure perfusion. We interrogated the local hemodynamics with aid of computational fluid dynamics. We constructed three different geometric grids of the vascular grafts. We defined the first grid based on the material specifications of the graft material. We constructed the second grid by outlining the contours of the graft from the sectioned and stained histologic slides. Lastly, we created the third grid by capturing the graft contours from axial sections of the in vivo grafts acquired with a 1.5 T Phillips® MRI. Using volumetric flow rate and PC-MRI data, we performed steady state and pulsatile simulations. We then correlated the calculated wall shear stress and the measured pseudointima formation. The results demonstrated that high wall shear stress fails to inhibit intimal thickening during short term graft healing.

Commentary by Dr. Valentin Fuster
2003;():85-86. doi:10.1115/IMECE2003-43484.

Magnetic Resonance Imaging has the potential to become the new gold standard in the management of cardiovascular disease, particularly in the assessment of arterial lesions and post-intervention restenosis. NiTi stents exhibit superior MRI compatibility and allow direct MR imaging through the stented area. In the present study, pulsatile in-vitro flow measuremnts were obtained by Magnetic Resonance Phase-Velocity mapping (MR-PVM) through a nickel-titanium alloy stent with different grades of stenosis: a) 75%, 90% (symmetric) and 50% (asymmetric). The results showed clear visibility of the lumen through the stenosis and good correlation of the phase velocity maps through the stent/stenosis when compared with independent calculations of flow. The detection and evaluation of instent restenosis using these techniques could provide a non-invasive means of screening for further management of patients.

Commentary by Dr. Valentin Fuster
2003;():87-88. doi:10.1115/IMECE2003-55065.

Blood circulation as well as body fluids distribution undergoes significant adaptation during and after space flight. Much study on physiological changes under weightlessness has been performed since the early days of the space program. In particular, cardiovascular research in conjunction with the space shuttle program has included diverse physiological functions affected by the nervous system such as heart rate, blood pressure, hormone release, and respiration. The altered cardiac output due to adaptation during flight and deconditioning after the flight will impact blood circulation in the human body. Especially, this altered blood supply in the brain and consequent oxygen supply to certain parts of the brain will make non-negligible impact on long-duration flight. To assess the impact of changing gravitational forces on human space flight, it will be essential to quantify the flow characteristics in the brain under varying gravity conditions. Analysis of blood circulation in brain as well as other parts of human body requires the capability to analyze flow in large arteries and capillaries.

Commentary by Dr. Valentin Fuster
2003;():89-90. doi:10.1115/IMECE2003-55545.

Evans blue dye (EBD) was injected into the carotid arteries of three anesthetized pigs and allowed to circulate for 90 minutes. At the conclusion of the 90-minute period, the animals were sacrificed and injection casts of the infrarenal aorta and iliac-femoral arteries were prepared. The casts with their surrounding arteries were removed and immersed in fixative. After fixation, the EBD-stained vessels were separated from the casts, which were used to construct computational meshes for simulation of the flow fields and wall shear stress distributions that had existed in the casted regions during the experiments. The inlet flow waves and flow partitions were based on flow measurements performed during each experiment. Based on a conceptual model of the relation between shear stress nonuniformity and permeability increase, the spatial and angular variation of the gradient of the time-average shear stress at the walls of the external iliac arteries was found from the computational fluid dynamic simulations for each experiment. Using affine transformations, the gradient and time-average shear stress results, and the EBD optical density distributions, were mapped to a common template, allowing pixel-by-pixel correlations of the hemodynamic stress parameters and local permeability. The results suggest that both shear stress gradient and time-average shear play a role in determining vascular permeability to macromolecules.

Commentary by Dr. Valentin Fuster
2003;():91-92. doi:10.1115/IMECE2003-41156.

An integrated computational-experimental method was developed to characterize the global nonlinear elastic stress-strain behavior of a beating rat heart. This method combines finite element (FE) simulation with the experimental end-diastolic cavity pressure- balloon volume relationship of left ventricle (LV) to characterize the deformation resistance of a beating heart. In the FE simulations, the hyperelastic Ogden strain energy potential was used and geometric nonlinearity was also considered. The elastic moduli for the ex-vivo rat heart obtained through the study vary from 0.003 to 0.577 MPa.

Commentary by Dr. Valentin Fuster
2003;():93-105. doi:10.1115/IMECE2003-41184.

This paper addresses a comparative study of structural analyses for flat and curved mechanical heart valve prostheses. The same fluid force computed by the fluid mechanics analysis for the blood flow passing through the leaflets of a bileaflet mechanical heart valve prosthesis is used to both flat and curved mechanical heart valve prostheses for comparison. This fluid force is applied to both mechanical heart valve prostheses for the rigid body dynamics analyses to confirm the kinematic and dynamic characteristics of leaflet motions, the structural mechanics analyses for deformed leaflets of both flat and curved mechanical heart valve prostheses are executed to give quite different stress and deflection results even though they have the almost same kinematic and dynamic characteristics.

Commentary by Dr. Valentin Fuster
2003;():107-108. doi:10.1115/IMECE2003-41408.

Vibro-acoustography is a new noncontact imaging method based on the radiation force of ultrasound. We extend this new technique for tissue characterization of arterial vessels by vibration techniques. In this paper a theoretical model for vibration analysis of arterial vessel with tissue is developed. Experimental studies were carried out on a silicone rubber tube embedded in a cylindrical gelatin phantom of larger radius, which simulates a large artery and the tissue body. The fundamental mode is well excited by the radiation force of ultrasound. The fundamental frequency was measured 81.8 Hz for a tube-phantom structure that is quite close to our theoretical prediction of 83.3 Hz.

Commentary by Dr. Valentin Fuster
2003;():109-110. doi:10.1115/IMECE2003-42947.

It has been clinically proven that vascular reconstructions tend to restenose within a relatively short period of time. Intimal hyperplasia and smooth muscle proliferation appear to be promoted by the altered intramural stress distributions at the distal anastomosis of the artery-graft junction. This paper examines the pressure induced stresses and strains in a simulated artery and bypass graft junction. Numerical and experimental methods were used to determine both the magnitude and location of the stresses and strains. A Finite Element package and silicon models were used for the in vitro analysis. Initial numerical analysis involved the modeling of a cylinder with homogenous material properties, followed by the modeling of a homogenous graft artery junction under static pressures. These experimental results were then used to validate the numerical model.

Topics: Pressure , Stress , Junctions
Commentary by Dr. Valentin Fuster
2003;():111-112. doi:10.1115/IMECE2003-43102.

Abdominal Aortic Aneurysms (AAAs) are localized enlargements of the aorta. If untreated, AAAs will grow irreversibly until rupture occurs. Ruptured AAAs are usually fatal and are a leading cause of death in the United States, killing 15,000 per year (National Center for Health Statistics, 2001). Surgery to repair AAAs also carries mortality risks, so surgeons desire a reliable tool to evaluate the risk of rupture versus the risk of surgery.

Commentary by Dr. Valentin Fuster
2003;():113-114. doi:10.1115/IMECE2003-43141.

Flexure is a major deformation mode of the aortic valve (AV) leaflet during valve function [1]. The structure of the AV must thus allow for low flexural rigidity yet also be of sufficient strength to support large tensile loads during valve closure. It well known that the AV is a heterogeneous structure uniquely adapted to provide efficient valve function. Yet, unlike the trilayered central leaflet region, the commissural region commissural region of the AV leaflet (Fig. 1) is composed of only a single layer of collagen fibers. The effect of this structural difference in the AV leaflets flexural rigidity and hence its function is unknown. Also, knowledge of the flexural rigidity of the AV leaflets can be used as design criteria in fabricating bioprosthetic and tissue engineered heart valve considering the fact that the commissural region is known to be a major site for porcine bioprosthetic failure [2]. Yet, to our knowledge the mechanical properties of the AV leaflet in the commissural area have never been measured, due to its small size (3–5 mm).

Topics: Valves , Stiffness
Commentary by Dr. Valentin Fuster
2003;():115-116. doi:10.1115/IMECE2003-41221.

Cell adhesion to postcapillary vascular endothelium in the fluid dynamic environment is an important event in many physiological and pathological processes, e.g. leukocyte emigration is critical for the successful host defense against tissue injury and infection. Since cell-blood interaction is strongly coupled with cell adhesion in the microchannel, we have developed an integrated mathematical model of cell adhesion, deformation, and rolling. Cells are modeled as elastic solids interacting with the flat wall of a micro-channel. The ligand-receptor bonds between the cell and micro-channel wall are simulated as many simple springs. The formation and dissociation rates of bonds are characterized by a reversible kinetic model (Dembo et al., 1988).

Commentary by Dr. Valentin Fuster
2003;():117-118. doi:10.1115/IMECE2003-41469.

Transverse mechanical properties of mammalian cardiac myocytes, was determined by using atomic force microscopy (AFM). The AFM can be used as a nano-indentation device allowing transverse stiffness measurements to be conducted on biological cells in a physiological environment. This enables real-time biomechanical and physiological processes to be monitored with nano-scale resolution. Cellular mechanical properties were determined by indenting the cell’s body, and analyzing the indentation data with classical infinitesimal strain theory (CIST). This calculation was accomplished by modeling the AFM probe as a blunted cone. The blunted cone geometry fits the AFM force indentation data well and was used to calculate the apparent elastic modulus of the cardiac myocyte body. The mechanical properties of male 344 x Brown Norway F1 hybrid (F344×BN) rat cells was measured and an apparent elastic modulus of 35.1 ± 0.7 kPa (n = 53) was calculated. Further studies are being conducted on myocytes isolated from aged hearts to determine whether age effects cardiac mechanical properties at the level of the single myocyte.

Topics: Elastic moduli
Commentary by Dr. Valentin Fuster
2003;():119-120. doi:10.1115/IMECE2003-42394.

The human ear is often regarded as a paragon of mechanical engineering. To understand how the hearing system works, scientists have proposed detailed models of its specific aspects—the transfer of acoustic energy from the atmosphere to the tympanic membrane via the external ear; the coupling of the tympanic membrane to the oval window of the cochlea via ossicles; the resultant fluidic oscillations in the cochlear ducts; the formation of traveling waves in the basilar membrane of the cochlea; the mechanical stimulation of inner hair cells by the basilar membrane; and the consequential transduction of nerve impulses. Scientists have also proposed models to explain the phenomenon of enhancement of the traveling waves in the basilar membrane by synchronized co-contraction in the length of outer hair cells (OHCs). Although it is unrealistic that any OHC would contract in length without expanding in diameter, the models proposed by other analysts have so far incorporated the longitudinal contraction of OHCs only, suggesting that the impact of any diametric expansion of OHCs would be relatively trival. Here we show that the basilar membrane would behave like a Beam-Column system, which may be significantly influenced by the diametric expansion of OHCs.

Topics: Ear
Commentary by Dr. Valentin Fuster
2003;():121-122. doi:10.1115/IMECE2003-42905.

The area of cell spreading has been proven to be critical to a cell’s fate, and direct physical connections between integrins, cytoskeleton, and the nucleus have been found [1]. We investigate the cell adhesion-cytoskeleton-nucleus mechanical pathway using experimental and theoretical tools. We measured changes in adhesion area and corresponding elliptical projected nuclear stretches caused by disruption of cell adhesions with trypsin. For our analysis, we treated the nucleus as an incompressible, neo-Hookean material. Using our measurements of the two-dimensional nuclear stretches, we calculated the stretch in the normal direction. We then estimated the strains of the nuclear deformation, and used these to estimate the cytosolic/cytoskeletal pressure and tangential forces that act upon the nucleus. Comparing our estimates of the internal forces acting on the nucleus to measurements of cell surface traction force in the literature, both are on the same order of magnitude.

Topics: Deformation
Commentary by Dr. Valentin Fuster
2003;():123-124. doi:10.1115/IMECE2003-42948.

Present biologic models envision organisms behave like the character ‘Topsy’ in Gone with the Wind; they “just grew.” Modeled of Lego© -like components, the individual structures are linked together as if they are automobile parts that are manufactured at different plants and assembled at some central factory. For the most part, hexahedral finite element meshes are used to model structures. When tetrahedral modeling is used, no account is made of the different mechanical properties that are inherent in triangulated structures, (trusses), that make the structures behave very differently than hexahedral-based models.

Commentary by Dr. Valentin Fuster
2003;():125-126. doi:10.1115/IMECE2003-43022.

A recent study showed that exposure to hypotonic conditions increased chondrocyte surface area up to 234% by stretching the folded membrane, which may reduce the chondrocyte’s ability to deform under load. The goal of this study was to determine the effect of hypo-osmolarity on chondrocyte survival from load-induced injury. Bovine cartilage explants were incubated in either isotonic or hypotonic pH-buffered solution for 20 minutes, loaded with cyclic confined compression at 5MPa for 1 hour, and then assessed for cell viability using cell vital dyes and for pericellular matrix (PCM) using an type VI collagen antibody. Cell death in loaded explants was significantly greater than that of non-loaded controls (p<0.001). However, explants loaded in the hypotonic solution showed significantly greater cell death than those loaded in the isotonic solution. An increase of dead cells with flatten PCM were located in the superficial zone. Our findings suggest that hypo-osmolarity decreases the ability of chondrocytes in articular cartilage to survive from load-induced injury.

Topics: Stress , Wounds , Cartilage
Commentary by Dr. Valentin Fuster
2003;():127-128. doi:10.1115/IMECE2003-43047.

This study investigated the effect of cyclic mechanical stretching on the collagen gene expression and protein synthesis of human patellar tendon fibroblasts (HPTFs). We hypothesized that cyclic mechanical stretching of HPTFs would increase collagen synthesis via transforming growth factor-beta 1 (TGF-β1). To test the hypothesis, the tendon fibroblasts were cultured on microgrooved surfaces of silicone dishes under serum-free conditions. The cells were subjected to cyclic uniaxial stretching with a constant frequency and duration (0.5Hz, 4hr), and one of three stretching magnitudes (no stretch, 4%, and 8%) followed by 4 hours of rest. It was found that the gene and protein expression of both collagen type I and TGF-β1 were significantly increased in a stretching-magnitude dependent manner, whereas collagen type III gene and protein levels were not significantly changed. The exogenous addition of antibody to TGF-β1 eliminated the stretching-induced increase in collagen type I protein synthesis. The results therefore confirmed our working hypothesis and suggest that mechanical stretching of tendon fibroblasts can lead to matrix remodeling by modulating the collagen production of tendon fibroblasts, a process at least particially mediated by TGF-β1.

Topics: Tendons , Fibroblasts
Commentary by Dr. Valentin Fuster
2003;():129-130. doi:10.1115/IMECE2003-43293.

Our recent investigations on human brain tumor (glioma) cell micro and nanodynamics via AFM methodologies have shown that brain tumor invadopodia (malignant cytostructural cell extensions with sensory, motility, and invasive characteristics extended by tumor cells into their environment) can assume specific geometries based on cell plating density and the location/distance of neighboring cells indicating strong cell sensing and signaling mechanisms between malignant cells and their surroundings. In certain occasions, cancer cell processes (extensions) have been found to be highly directional measuring more than 80 μm while invading neighboring cells by following a connecting straight path. Moreover, strong chemical gradients are suggested to influence the growth and motility of cell processes allowing for gradual adjustments of the direction of the invasive tumor extension. In response to external signals, tumor cell invadopodia develop micron-sized side-ligaments that follow the chemical gradients in their neighborhood and assist the reorientation of their main intrusive elements.

Commentary by Dr. Valentin Fuster
2003;():131-132. doi:10.1115/IMECE2003-43446.

DNA-crosslinked polyacrylamide gels are polymeric electrolytes by virtue of the fact that DNA is negatively charged in an aqueous solution. As such, their mechanical properties can be altered by electrophoretic and electro-osmotic effects. Hybridization of single-stranded DNA with single-stranded sections of the crosslinks provides a novel means of altering gel mechanical properties. As a step toward exploring this means of altering gel mechanical properties, we report here on a study of the use of electrophoresis to introduce single stranded DNA into DNA crosslinked gels. Changes in elastic properties of the gel, before and after electrophoresis, were measured.

Topics: Electrophoresis , DNA
Commentary by Dr. Valentin Fuster
2003;():133-134. doi:10.1115/IMECE2003-55020.

This paper deals with the transit of a neutrophil through narrow capillary segments. In the segments are modeled by arc-shaped constrictions arranged in a straight pipeline. Initially the spherical cell deforms in transit through the first constriction. We focus on the transit time to the deformed cell through the second constriction. We investigate the effects of segment geometry on the transit time. Finally, it is shown that the transit time is independent of the shape of the first segment so long as it is organized by the entrance cell radius.

Topics: Pipelines , Geometry , Shapes
Commentary by Dr. Valentin Fuster
2003;():135-136. doi:10.1115/IMECE2003-41417.

Traditional minimally-invasive endovascular procedures like coiling are inadequate for the effective treatment of may wide-necked, fusiform aneurysms and arteriovenous fistulae (AVF) in the brain. An endovascular approach of placing a covered microstent across the intracranial aneurysm neck and AVE rent is promising. Unlike bare stents (stent without cover), covered microstents will have the capability of completely preventing blood flow into the aneurysm cavity of AVF rent. The strategy will restore normal vessel morphology and blood flow.

Commentary by Dr. Valentin Fuster
2003;():137-138. doi:10.1115/IMECE2003-41465.

The flow through a compliant-walled channel or tube is analogous to human systems, such as blood vessels (Holt, 1969), the trachea during forced expiration (Elad and Kamm, 1989), and, with some limitations, the glottis (Conrad, 1980). Typical studies involving collapsible channels have used a rigid channel with a longitudinal section of the channel wall replaced with a deformable structure, such as a membrane. Numerical and analytical studies of the flow and accompanying wall deformation have used 1-dimensional or lumped-parameter models (e.g., Shapiro, 1977; Cancelli and Pedley, 1985) as well as detailed flow and structural models using the finite element method (e.g., Rast, 1994; Luo and Pedley, 1995, 1996; Shim and Kamm, 2002; Cai and Luo, 2003).

Commentary by Dr. Valentin Fuster
2003;():139-140. doi:10.1115/IMECE2003-43069.

A three-dimensional dynamic computational model was developed for the dual purposes of predicting and replicating joint loading generated by a five-axis dynamic knee simulator. The model was verified through an analog knee that was constrained for accurate modeling and instrumented to directly measure joint forces. The verified model was then used to generate control profiles to the five axes of the simulator for replication of desired joint loading. Reproduction of a walking profile is demonstrated.

Commentary by Dr. Valentin Fuster
2003;():141-142. doi:10.1115/IMECE2003-43483.

Cardiac synchronized magnetic resonance imaging of flowfields has suffered due to the relatively long acquisition times required. We developed a rapid MRI approach, BRISK PCA (Block Regional Interpolation Scheme for k-space Phase Contrast Angiography) which was simulated here using data generated by computational fluid dynamics to investigate the role of interpolation and segmentation on the accuracy and efficiency of the method. BRISK differs from other sparse sampling schemes in that the sampling rate is a function of the position in k-space and interpolation is used to generate data points not directly acquired. Combined with conventional segementation, this allows more efficient use of time, resulting in rapid acquisitions with good spatial and temporal resolution. FAST (Fourier AcquiSition in Time) is a similar sparse sampling strategy that varies the segmentation factor, rather than the sampling rate, as a function of k-space position. BRISK and FAST can be performed in nearly equally scan times. However, deviation from ideal in the FAST data was highly dependant on the starting phase of the flow waveform, while BRISK was immune to such variation. Simulations showed that BRISK (up to segmentation factor 5) and FAST 5 retained excellent axial-velocity accuracy, but the accuracy of FAST was variable and dependent on waveform characteristics.

Commentary by Dr. Valentin Fuster
2003;():143-144. doi:10.1115/IMECE2003-43835.

In this paper, a computational study was carried out to investigate the CSF dynamics in the spinal cavity. A theoretical and computational model was developed to simulate the pulsatile CSF flow and the deformation of the spinal cavity that results from transmission of the CSF pulse wave from the cranial cavity. Under a pulsatile pressure gradient, the velocity profile of the flow is blunt for large Womersley numbers, while for small Womersley numbers the velocity profile is parabolic. The phase relationship between the pressure and the velocity is also related to Womersley number. This is the first step to understand the pulsatile dynamics of the CSF in the spinal cavity and will help explain the cause of related diseases and improve the clinical treatment.

Commentary by Dr. Valentin Fuster
2003;():145-146. doi:10.1115/IMECE2003-41395.

A new method for imaging and detecting modal shapes of vessels is introduced. Theory is developed that predicts the measured velocity is proportional to the value of the mode shape at the focal point of the ultrasound beam. Experimental a cylindrical gel phantom of large radius. This model simulates approximately a large artery and the surrounding body. The fundamental frequency was measured 83 Hz for the tube-phantom system. At this frequency the ultrasound transducer was scanned across the vessel plane with velocity measurement at one single point on the vessel and on the phantom by laser. The images obtained show clearly the interior tube and the modal shape of the tube.

Commentary by Dr. Valentin Fuster
2003;():147-148. doi:10.1115/IMECE2003-41521.

The necessity of robust temperature packaging is critical as more biotechnology products are being brought to market globally. This paper will outline the steps necessary to develop temperature sensitive packaging. Development of temperature sensitive packaging is a complex process, especially for new products. The steps to develop robust packaging start and end with good design control. There are many regulating bodies that can give guidance on this topic, but the majority of the work remains with the company. Well defining the design requirements dramatically shortens the design time of any packaging. Acceptance of any developed packaging must include process verification that all product release criteria are still met at the end of the product shelf life. Once the package has been tested, and meets all of the design requirements, validation testing can begin. Validation consists of testing the package in its final configuration, against worst case scenarios. A successful temperature sensitive package would hold the required temperature range for greater than the duration of the shelf life. All validation testing must incorporate high and low temperature challenges to simulate the year round shipping that the product will face. Following these steps will aid in development of a robust package that will protect your product during transit to the final destination.

Commentary by Dr. Valentin Fuster
2003;():149-150. doi:10.1115/IMECE2003-42047.

DNA hybridization on microarrays is enhanced with the use of a pulsed source-sink system, a design that is based on the occurrence of chaotic advection. Experimental results demonstrate increased signal levels in faster times, an increase in signal-to-noise ratios, and a decrease in signal variability.

Topics: Flow (Dynamics) , DNA
Commentary by Dr. Valentin Fuster
2003;():151-155. doi:10.1115/IMECE2003-42109.

Micromanipulation systems find increasing applications in many fields. In these systems, operators carry out fine motion directly with the aid of the microscope. In order to free operators from eye-straining operations under the microscope we propose to present the 3D view to a TV or PC screen. Based on stereovision an analog time-division system, a subfield system and a digital field sequential system are synthesized, as described in this paper. The development of these systems is centered on how to eliminate the flicker and how to enhance the resolution of the video images. The experimental results show that the subfield system and the digital field sequential system are effective in addressing the problems associated with the micromanipulation.

Commentary by Dr. Valentin Fuster
2003;():157-158. doi:10.1115/IMECE2003-42404.

Compared with open surgery, laparoscopy results in significantly less pain, faster convalescence, and less morbidity. However, eye-hand dissociation, a two-dimensional field-of-view and fixed instrumentation with limited degrees of freedom contribute to a steep learning curve and demanding dexterity requirements for many laparoscopic procedures. One of the main limitations of laparoscopy is the fixed working envelope surrounding each trocar, often necessitating placement of multiple ports to accomodate changes in position of the instruments or laparoscope to improve visibility and efficiency. The placement of additional working ports contributes to post-operative pain and carries a small risk of bleeding or adjacent organ damage. In order to provide for greater flexibility of endoscopic viewing and instrument usage and to further reduce morbidity, a novel adjunct laparoscopic system has been developed consisting of a platform capable of supporting various laparoscopic tools which is secured magnetically to the abdominal wall and subsequently positioned within the abdominal cavity through surgeon-controlled, external magnetic couples on the patient’s abdomen. Using this technique, instruments such as miniature endoscopic cameras used to augment the surgical field of view and surgical retractors have been successfully evaluated in a dry laboratory as well as in porcine models, with several others currently under investigation. This document elaborates on the theoretical and empirical process which has led to anchoring designs optimized for size, strength and surgical compatibility, as well as the benefits, limitations and prospects for the use of incisionless, magnetically-coupled tooling in laparoscopic surgery.

Topics: Surgery
Commentary by Dr. Valentin Fuster
2003;():159-160. doi:10.1115/IMECE2003-43109.

A decubitus ulcer or bedsore is a pressure-induced ulceration of the skin occurring in persons confined to bed for long periods of time. Reduction of pressure over bony prominences is of primary importance to prevent and cure bedsores. For this purpose, specially designed mattresses can be used and/or the patient should be turned frequently to avoid ischemia of soft tissue. In addition to pressure, other principal factors causing bedsore are friction and shear forces. In this paper, we designed a new 5 degree of freedom bed mechanism that can be used to change the posture of pressure ulcer patients, which generates 7 motions including backrest elevation, kneerest elevation, lounge position, left and right rotation, trendelenberg and reverse trendelenberg motion, and straight elevation. Particularly, we focused on the synthesis of a backrest and seatrest assembly that can reduce sliding between the bed and the patient.

Commentary by Dr. Valentin Fuster
2003;():161-162. doi:10.1115/IMECE2003-43156.

This paper presents the design of a passive leg orthosis for gait correction of human subjects. During normal walk, the joints of a human leg, namely, the hip, knee, and ankle satisfy certain functional relationships. During post stroke and other neural disorders, the gait deviates substantially from this normal gait. Hence, there is a need to design rehabilitation machines for gait training. Our design targets the sagittal plane motion of the leg, i.e., it considers the hip and knee flexion and extension motions. The paper describes the details of the cam design and results of a demonstration prototype.

Topics: Design , Orthotics
Commentary by Dr. Valentin Fuster
2003;():163-164. doi:10.1115/IMECE2003-43158.

For retraining stroke victims, there is a need for rehabilitation devices, which can support the weight of leg during walking. Machines that gravity balance the leg are potentially very useful. This paper presents gravity balancing designs using non-zero free length springs. These designs are further optimized for spring connection points and parameters of the spring such as free length and stiffness.

Commentary by Dr. Valentin Fuster
2003;():165-166. doi:10.1115/IMECE2003-43442.

A planar waveguide (PWG) device has been developed for the detection of fluorescently labeled nucleic acid sequences immobilized or hybridized to the surface of a planar waveguide. Unlike current technologies requiring image gathering and reading capabilities or specially textured waveguide surfaces, this instrument uses simple glass slide based arrays, providing a numerical output in proportion to the fluorescent intensity recorded. The system consists of an optical waveguide (a glass microscope slide), an excitation light source, a photo detector, filters for select fluorescent emissions and the positioning array cassette. A data analysis algorithm is presented for interpretation of two dimensionally organized arrays. Based on our experimental evaluations we conclude that this sensing system shows promise as a simple and effective means to read fluorescent microarrays.

Topics: Waveguides
Commentary by Dr. Valentin Fuster
2003;():167-168. doi:10.1115/IMECE2003-43699.

Motion of macromolecules in flows is important to several disciplines such as DNA hybridization studies, self assembly of nanostructures, and transport of suspensions. The present study simulates the motion of macromolecular structures in linear shear flows. A molecular chain is modeled as a coarse-grained series of beads and springs. For a wide range flow conditions, the flow appears chaotic, where quasi-stable limit cycles are observed for several smaller ranges of flow conditions.

Topics: Motion , Shear flow , DNA
Commentary by Dr. Valentin Fuster
2003;():169-170. doi:10.1115/IMECE2003-42889.

The purpose of this study was to develop a 3D visulaization tool to investigate the contact mechanics at the object-digit interfaces and the force coordination among multiple digits during manipulation tasks. Five miniature 6-component force/torque transducers were used to record force and torque data at the tips of individual digits during 5-digit grasping, rotating, and lifting tasks. A digitizing device was used to obtain 3D coordinates of anatomical landmarks of the hand for the construction of a hand geometric model. Three dimensional force vectors and the centers of pressure were obtained using the force and torque data collected during the manipulation tasks and were displayed at the tips of the 5 digits. The visualization methods developed in this study could potentially provide an effective tool for the study of object-digit interaction mechanics and inter-digit force coordination during various manipulation tasks.

Topics: Visualization
Commentary by Dr. Valentin Fuster
2003;():171-172. doi:10.1115/IMECE2003-42942.

Wrist external fixator (WEF) is widely used in unstable distal radius fracture. The mechanical properties of WEF were thoroughly studied by many authors. The researches of external skeletal fixator include the stability of the frame structure, fatigue or failure model of pins, exploration of biomechanical characteristics of pin-bone interface, and the strength of pin clamps. The mechanical models [1–5] and cadaver bone [6] were frequently used to test the WEF. For most of the studies, the stiffness (or stability) of the construct was the criteria to evaluate the WEF. In these many studies, however, the mechanical contribution of pins was less studied. Due to the non-weight bearing property and prevention of injuries from the soft tissue of peri-wrist, the structure and elements of WEF are designed to be small and slender. It is hence important to find if all pins of WEF were necessary or proper designed. We tested the mechanical function of Traumafix WEF using the mechanical model, the cadaver model and in vivo patient model. The strains of the pins on the top surface were analyzed. The data showed that the load distribution of each pins were different from pins to pins, and from models. Our result suggests the distal pins of WEF can be smaller if the space of metacarpal bone is limited.

Commentary by Dr. Valentin Fuster
2003;():173-174. doi:10.1115/IMECE2003-42990.

The purpose of this study is to assess the kinematical changes in the flexion of the finger joints after MCP arthroplasty. Angular joint position in relation to its corresponding excursion was used to quantify the kinematics of the finger. The assessment used real time data acquisition and fresh-frozen cadaver hands. Several important benchmarks were considered in analyzing the data. First of all, the initiation of the digit was considered. When a tendon is pulled, motion is normally observed in the DIP first, followed by the PIP, and finally the MCP. This order should be maintained after the arthroplasty. For our kinematics study, it is also important to note any significant increase or decrease in the time of the initiation. Other benchmarks we considered include the starting and ending angle of the joints. The start and ending angle is important when studying the overall range of motion of the digit. The overall range of motion affects basic hand function and is one measure of successful arthroplasty. Excursion was also considered as an overall measure of how well the prosthesis mimics the original joint. Large changes in excursion could indicate that the original center of rotation has not been maintained.

Commentary by Dr. Valentin Fuster
2003;():175-176. doi:10.1115/IMECE2003-43043.

A number of neuropathologies such as Duchenne’s muscular dystrophy (DMD), cause disability in the upper extremity due to the loss of muscle strength. This will eventually prevent the individual from moving their arms in three-dimensional space so it has been proposed that a robotic orthosis could support and augment movement. This orthosis would need to accommodate the movement capabilities of the user. To accomplish this, knowledge of how movements are formed and controlled in the presence of neuromuscular disease needs to be determined. While the arm was supported in a floatation device, DMD subjects were asked to make pointing movements to several targets in the transverse plane. This was done from two start positions while torso movement was constrained and unconstrained. The hand trajectories formed while the torso was constrained were essentially straight but at a cost to the uni-modality of the hand velocity profile. In this configuration the velocity profile contains several phases of acceleration and deceleration producing a multi-modal profile. However, the additional degrees of freedom introduced in the unconstrained torso configuration were employed is such a manner as to produce a smooth uni-modal hand velocity profile.

Topics: Motion
Commentary by Dr. Valentin Fuster
2003;():177-178. doi:10.1115/IMECE2003-43095.

The purpose of this experiment is to create a mathematical model for the function of the annular pulleys of the human finger in flexion. We have assumed that the flexion moment of the middle and proximal phalanges occurs at the proximal and distal ends of the A-2 and A-4 pulleys. The amount of force generated is dependent on the angle of flexion at the adjacent joint, the tension in the tendon and the stiffness constant of the pulley.

Commentary by Dr. Valentin Fuster
2003;():179-180. doi:10.1115/IMECE2003-41100.

A new concept for cryosurgery control is presented in this report, a concept which has the potential to dramatically change the outcome of cryosurgery. Unlike other cryosurgery control techniques, which are based on controlling the thermal performance of the cryoprobe, this new concept is based on heating the treated tissue as a means of shaping the frozen region. The new controlling heater, which is termed a ‘cryoheater,’ is complimentary to the cryoprobe and can work with any cryosurgery cooling technique. In the current pilot study, the new cryoheater is studied in simulated cases of prostate cryosurgery.

Commentary by Dr. Valentin Fuster
2003;():181-182. doi:10.1115/IMECE2003-41102.

As part of an ongoing effort to characterize the mechanical behavior of biological tissues in the cryogenic temperature range, the current study focuses on the thermal expansion measurements of cryoprotective agents. This study focuses particularly on the upper part of the cryogenic temperature range, where the cryoprotectant behaves like low viscous liquid at all practical cooling rates. A new experimental apparatus for thermal expansion measurements is presented in this report. This report includes thermal expansion measurements of the cryoprotectant mixtures DP6 and VS55, and comparison with data from the literature on DMSO solutions.

Commentary by Dr. Valentin Fuster
2003;():183-184. doi:10.1115/IMECE2003-41357.

Cryosurgery is a clinical therapy aiming at destroying the target of diseased tissues through a controlled deep freezing and subsequent rewarming [1,2]. Applications of this treatment are quite wide in skin cancers, glaucoma, lung tumor etc. [3]-[10]. In contrast to the freezing therapy, heating of tumors has also long been proved to be an effective way of selectively killing the cells of cancerous tissues [11]-[13]. Clinical tests showed that heating the tumor to above a critical minimum temperature such as 42–43 °C for an extended period could effectively destroy the target. It was recently realized that freezing immediately followed by a rapid and strong heating of the target tissues would significantly improve the treatment effect [14]-[16]. Therefore, an apparatus thus developed will be of great importance in cancer clinics. But until now, most of the currently available cryoprobe systems are only capable of performing a single freezing function, in which the treated tissue is often let to naturally re-warm by simply switching off the apparatus. The first one and only commercially available cryoprobe system aiming at both freezing and heating therapy is perhaps Endocare Corporation’s Ar-He Cryoprobe system [14]. However, the highest tissue temperature for this system to achieve is about 0–20 °C [17], which is not high enough to thermally destroy the target tissues. Presently, there is a strong lack of freezing applicators with powerful heating functions for hyperthermia purposes. Without strong enough heating, tumors may still have a chance to regenerate. This is perhaps one of the critical reasons to impede the widespread of cryosurgery in destroying pathological tissues.

Commentary by Dr. Valentin Fuster
2003;():185-186. doi:10.1115/IMECE2003-41606.

Laser thermotherapy is a technique used for tumor treatment. It generates a local heating, causes thermal coagulation of living tissue and eliminates the tumor. Precise heating of tumor tissue with healthy minimum thermal injury to adjacent tissue is essential to thermotherapy. Understanding of heat transfer and optical-thermal interaction is important for control of temperature and design of thermotherapy. This study applies the Arrhenius damage model to describe the heat-induced change of optical properties. It calculates the distribution temperature, damage and optical-thermal response of bio-tissue during the laser treatment, and shows how these factors affect the effectiveness of laser thermotherapy. Similar research has been performed by Kim and coworkers [1996], Iizuka and coworkers [2000], and Whelan and coworkers [2000]. This study relaxes some conditions in previous investigations. It reveals the importance and the effect of size of the laser head.

Commentary by Dr. Valentin Fuster
2003;():187-188. doi:10.1115/IMECE2003-42269.

Investigators at Triangle Research and Development Corporation (TRDC) have developed materials and applications for enhanced thermal management using both micro-and macro-encapsulated phase change materials (PCMs) since 1983; including: microPCM coolants, coatings, composites, fibers and PCM apparel. The PECS™ (Personal Environmental Control System) was developed for microclimate body cooling beneath NBC (Nuclear-Biological-Chemical) protective clothing for the US NAVY and USMC (Colvin, 1997 and 2000). Similar cooling apparel using 3–4mm macroPCM COOLBEADS™ were developed for costumed characters at two theme parks as well as potential use by firemen (Colvin, 1998). The results for these programs were reported at the 2000 and 2002 ASME ICEME (Colvin, 2000 and 2002). Civilian apparel has included 3.5 lb and 5 lb vests, a 1.5 lb cowl as well as a 1-lb collar, which suggested the potential for the microclimate cooling of athletes. Extreme heat during outdoor sporting events can be a major problem for athletes. Competitive runners, who often generate 700–900 W/m2 , commonly deal with temperatures above 32.2°C (90°F) and a relative humidity greater than 80%. Natural cooling by evaporation and convection are often inadequate for a vigorously exercising athlete. Many athletes fatigue, drop out or have to wrap their chest and shoulders with ices and towels in order to finish the races. Ice and frozen gels, however, are uncomfortable, heavy, and can cause the blood vessels to constrict, thus restricting good blood circulation. Encapsulated PCMs can store 60 J/kg and the air spaces between the particles permit evaporation and convection as well as rapid thermal recharging. Development of an effective cooling collar could potentially permit competitive athletes to combat heat exhaustion and increase the body’s ability to dissipate heat.

Topics: Cooling
Commentary by Dr. Valentin Fuster
2003;():189-190. doi:10.1115/IMECE2003-41666.

This paper provides a new technique to assist patients during gait rehabilitation and safety support during exercise. The device can also be used by patients who have experienced physical trauma, and in need of assistance so that they can regain the strength needed to support them during rehabilitation. The procedure results in an adjustable gait rehabilitation lifting system and has the ability to support the weight of the user. The apparatus can lift a patient from a sitting position in a wheel chair to a standing position and has the provisions to remove a percentage of the patient’s body weight and recognize subtle changes in elevation. The patient requiring gait rehabilitation would be free to traverse a flat plane and climb a number of stairs and at the same time does not impede free walking and prevents sudden fall of the patient.

Commentary by Dr. Valentin Fuster
2003;():191-192. doi:10.1115/IMECE2003-42913.

Degenerative osteoarthritis is recognized as the consequences of mechanical injuries. The abnormal impact force applied to articular cartilage would result in bone fracture or surface fissuring, and would cause the osteoarthritis [1,2]. The relation among the injury and impact energy was well studied. However, how the external energy attenuated to the internal joint is not carefully studied yet. The porcine knee joint was used as a biomechanical model for the simulation of human knee joint during impact loading. The objective of current study was to find the variation of kinetic characteristics between human and porcine knee joint during axial impact loading. Eight fresh-frozen knee joints from 10 month-old swine and seven cadaver human knee joints were used in the experiment. The mechanical responses such as forces and bending moment of knee joint, and the accelerations of femur was quantitatively analyzed. The results showed that the axial force response between human and porcine joints was similar, however, the anteroposterior shear, flexion bening moment and accelerations of these two joints were different.

Topics: Knee
Commentary by Dr. Valentin Fuster
2003;():193-194. doi:10.1115/IMECE2003-42978.

The objective of this study is to biomechanical quantify the intracranial displacement and pressure distributions associated with civilian projectiles to advance clinical understanding of the pathophysiological consequences of penetrating head injuries. A finite element head model was developed in an attempt to investigate the penetrating processes and brain injury mechanisms. Two geometrical shapes of projectiles (flat and pinpoint headed) were considered for penetration. They were modeled as rigid bodies (6.5 and 9 g) impacting at an initial velocity of 300 m/s. The head was modeled as a spherical skull with left and right hemispheres. Material properties and damage criteria for the skull and brain were based on literature. The penetration process was modeled with eroding contact surface method with LS-DYNA. Elements considered damaged were removed from further computation when the stress or strain reached their thresholds. Temporal displacement and pressure distributions are described. The effects of projectile type on the wounding pattern are discussed. The entry location responded with higher magnitudes of displacement than other locations (e.g., exit, mid brain). The flat head projectile penetration resulted in higher magnitudes of pressure and displacement than the pinpoint projectile in the entire skull-brain system. The finite element analysis provides a quantitative understanding of the localized intrinsic responses secondary to projectile penetration.

Commentary by Dr. Valentin Fuster
2003;():195-196. doi:10.1115/IMECE2003-43117.

Pain affects as many as 50 million Americans, with annual costs estimated as high as $90 billion. Unfortunately, the mechanism of injuries leading to persistent pain syndromes remain largely uncharacterized. A common painful injury results due from mechanical loading of nerve roots, which can occur for spinal injuries in both the low back and neck. Relationships have been demonstrated between tissue compression and behavioral hypersensitivity responses in animal models, with differential patterns of sensitivity depending on the nature of the mechanical insult (Colburn et al., 1999). Mechanical allodynia (MA) is an increased behavioral sensitivity to a non-noxious stimulus and is observed in the dermatome of the injured tissue. It can be measured by the frequency of paw withdrawals elicited by stimulation with normally non-noxious von Frey filaments. Allodynia is a clinical measure of sensitivity and, therefore, provides a useful gauge of nociceptive responses. Animal studies have shown that compression of neural structures initiates a variety of physiologic responses, including decreased electrical activity, increased edema formation, and increased endoneurial pressure in the region of compression (Lundborg et al., 1983; Olmarker et al., 1989, 1990; Pedowitz et al., 1992). While these studies document physiologic changes immediately following injury, they do not describe the temporal nature of these changes following tissue loading as they relate to pain behaviors. Moreover, despite this evidence of edema formation and increased endoneurial pressure locally in the nerve roots, no study has simultaneously documented local changes in nerve root geometry following compressive injury and how these changes may be linked to the onset and/or maintenance of pain-associated behaviors. Therefore, this study examines injury biomechanics for pain-behaviors in a radiculopathy (nerve root injury) model and temporally characterizes the local geometric changes in the nerve root for a series of postsurgical time points following compressive injury. While these results indicate that compression magnitude clearly modulates pain responses, the local nerve root swelling does not appear to directly drive behavioral changes. This suggests a complicated physiology for injury which likely contributes to the manifestation of pain. Findings are also presented for preliminary investigations into tissue rebound/recovery responses for varied mechanical insult magnitudes to begin to understand potential injury mechanisms leading to pain.

Commentary by Dr. Valentin Fuster
2003;():197-198. doi:10.1115/IMECE2003-43149.

The National Transportation Safety Board is furthering its accident investigation capabilities by implementing biomechanical tools and principles in its accident investigative process. Vehicle dynamics simulation is a commonly used investigative tool implemented to develop a complete understanding of how the vehicle moved and accelerated during an accident. Now the Safety Board is looking at both the reactions and actions of the occupants within the vehicle in response to the vehicle dynamics. This paper highlights two cases in which the principles and tools of biomechanics are applied to aid in the investigative process.

Commentary by Dr. Valentin Fuster
2003;():199-200. doi:10.1115/IMECE2003-43150.

This study examined the occupant kinematics during the rollover of a 15-passenger van. The main focus of the investigation was to determine possible kinematics that led to ejection, the time period of the potential ejections, and to determine injury causing kinematics or impacts. The time during which the ejections occurred was used to determine whether all the ejections could have occurred during the first overturn or if at least two overturns were necessary. The occupants were simulated in the estraint condition similar to the accident, all restrained in lap belts, and all restrained in lap/shoulder belts. The results of the simulation indicated that predicted injury levels were reduced and full ejection was eliminated when occupants were properly restrained with lap/shoulder belts. The simulation did not account for intrusion when determining injury.

Commentary by Dr. Valentin Fuster
2003;():201-202. doi:10.1115/IMECE2003-43454.

Eye injuries have been shown to be greater in the elderly population as a possible cause of this greater incidence. Three separate impact scenarios simulating a foam particle (30 m/s), a streering wheel (15 m/s), and an air bag (67 m/s), were applied to a finite element eye model in order to reveal the effects of aging on the eye when subjected to blunt trauma. The lens stiffness of the model was varied according to human lens stiffness values determined for each age group. The computational eye model demonstrated that increased risk was associated with the increasing stiffness of the lens, producing up to a 120% larger stress in the ciliary body.

Commentary by Dr. Valentin Fuster
2003;():203-204. doi:10.1115/IMECE2003-55585.

Recent studies suggest that there is increased risk to the pelvis and lower extremities for unbelted, front seat occupants when airbags deploy in frontal collisions. Among belted drivers, women and small adults are more likely to experience fractures of the knee-thigh-hip complex and lower leg. The occupant kinematics and impact mechanics for varying sized drivers under belted and unbelted conditions, with a deploying airbag, have not been well-investigated. The present study used occupant kinematic computer software (MADYMO) to investigate injury likelihood for the pelvis, femur and lower leg in simulations of FMVSS 208 test conditions (30 mph, rigid barrier, frontal crash) for a mid-size sedan with airbag deployment. The pelvic force criterion (PFC), femur force criterion (FFC), and Tibia index (TI) were calculated as injury predictors for 50th percentile male and 5th percentile female drivers, belted and unbelted, with variations in instrument panel angle and stiffness as well as hip abduction. The results indicated, most notably, that the unbelted 5th percentile female submarined beneath the airbag and experienced TI values that exceeded the current tolerance in nearly every unbelted simulation. Injury scores for the left leg were generally higher for both dummies, due to leg entrapment and the intruding floor pan. Hip abduction of 20 degrees led to excessive hip forces in the 50th percentile male. Seatbelts were effective at reducing injury measures in both dummies, most notably the TI score of the 5th percentile female.

Topics: Wounds
Commentary by Dr. Valentin Fuster
2003;():205-206. doi:10.1115/IMECE2003-55596.

Pelvic injuries due to lateral motor vehicle crashes continue to be a source of morbidity and mortality for accident victims as well as a serious problem for trauma surgeons and automotive safety engineers. In the present study, we sought to further explore the relationship between bone mineral density (BMD) measurements in the hip and pelvic fracture load and compression. We conducted experimental side impacts on intact lower torsos of female cadavers, building upon our previous work conducted on isolated bone-ligament structures. Significant linear relationships between pelvic fracture load/compression and total hip BMD emerged as further evidence that total hip bone mineral density may be a useful predictor of pelvic fracture risk. The presence of soft tissues increased resulting pelvic fracture loads as compared to those found in our previous isolated pelvic impacts.

Commentary by Dr. Valentin Fuster
2003;():207-208. doi:10.1115/IMECE2003-41615.

A multiobjective optimization was used to solve several failure problems of cemented total hip arthroplasty. The objective functions were three; the principal stress in the bone cement, the shear stress of stem/cement interface and cement/bone interface. In the results, the cement stress was reduced by 49%, the stem-cement stress was reduced by 68%, and the cement-bone stress was reduced by 43%, compared with the basic model.

Commentary by Dr. Valentin Fuster
2003;():209-210. doi:10.1115/IMECE2003-42418.

A three-dimensional finite element model was constructed of the distal tibia and fibula and a semi-constrained ankle prosthesis (Agility™ system). Contact elements were used at the interface between the talar component and the polyethylene liner and the proximal tibia and fibular were loaded in the in vertical direction. The minimal thickness of the polyethylene liner was varied from 3 mm to 8 mm in 1 mm increments. The results showed that the liner contact pressure in the sagittal plane mid-line decreased from 20 MPa to 14 MPa with increasing thickness while the medial edge contact pressure increased from 26 MPa to 30 MPa.

Commentary by Dr. Valentin Fuster
2003;():211-212. doi:10.1115/IMECE2003-42836.

This paper utilizes a 2nd generation proprioception device to evaluate active and passive proprioception in the shoulder and knee. The device is developed for this study is designed to be portable, inexpensive and easy to use.

Topics: Knee
Commentary by Dr. Valentin Fuster
2003;():213-214. doi:10.1115/IMECE2003-42925.

Muscle forces that compress the glenohumeral joint during midranges of motion may lead to increased translational forces in endrange positions, such as the apprehension position, where symptoms of anterior instability occur. The objective of this study was to quantify active stability provided by eight shoulder muscles in mid-range and end-range positions through muscle force vector analysis. Lines of action were derived from a standard geometric model and muscle force magnitudes were estimated with electromyography-based techniques. Resultant muscle force vectors were calculated by summing individual muscle force vectors. Compared to mid-range positions, lines of action of resultant force vectors were more anteriorly-directed in end-range positions. The deviation angle in the anterior direction was greatest (35°) and, consequently, stability was lowest in the apprehension position. Based on a sensitivity analysis, lines of action of resultant force vectors vary up to 6° within the population. In the apprehension position, muscle forces may promote anterior humeral head translation, predisposing the glenohumeral joint to anterior instability when other joint stabilizers are not functioning normally.

Topics: Stability
Commentary by Dr. Valentin Fuster
2003;():215-216. doi:10.1115/IMECE2003-42955.

The kinetics and kinematics of the knee joint during a variety of activities of daily living were studied in a group of total knee replacement (TKR) patients. The parameters examined were those needed to program a force-controlled knee simulator (axial and anterior-posterior (AP) forces, internal-external (IE) moment, and flexion angle). These parameters were calculated for walking, fast walking, stand-to-sit, sit-to-stand, bending, stair ascent and stair descent using body-fixed inertial sensors, a force platform, and estimates of muscles forces. Peak values for loading patterns were not significantly different from those for an age-matched control group. Axial forces were lower in comparison to published results for normal and TKR populations, due to slower cadences and conservative estimates of muscle forces. Peak posterior forces and IE moments were higher than published results. These patterns were combined to form a spectrum loading pattern, with the activities occurring in approximately the same ratios of relative frequency as reported in the literature. The spectrum pattern can be used to program a force-controlled knee simulator in order to apply more relevant loading patterns to knee implants.

Topics: Knee
Commentary by Dr. Valentin Fuster
2003;():217-218. doi:10.1115/IMECE2003-43026.

Cine phase contrast MRI, combined with a model-based rigid body tracking technique, was used to measure kinematics during a knee extension exercise in both knees of 16 anterior cruciate ligament (ACL) deficient surgical candidates and 16 uninjured subjects. A statistically significant increase in anterior tibial translation was observed in the ACL-deficient knees. Evidence of disruption to the screw-home mechanism was observed in the ACL-injured knees, although this was not a statistically significant result.

Commentary by Dr. Valentin Fuster
2003;():219-220. doi:10.1115/IMECE2003-43031.

The quadrupled semitendinosus-gracilis (ST-G) graft is rapidly becoming the graft of choice for orthopaedic surgeons when reconstructing the anterior cruciate ligament (ACL). During this procedure orthopaedic surgeons harvest the distal semitendinosus and gracilis tendons and use them to replace the ruptured ACL. Although people who undergo this procedure have good functional outcomes over the short-term, we do not know the effect that harvesting these two tendons has on neuromuscular function. The purpose of this study was to examine the effect that ACL reconstruction with an autologous ST-G had on musculotendinous morphology. The methods used in the study included digital reconstruction of knee musculature from magnetic resonance images (MRI). Marked reductions in muscle volume, cross-sectional area, and length were observed in the semitendinosus and gracilis when reassessed approximately 6 months following surgery (after the subjects had returned to sports participation). The subjects appeared to compensate for the diminished medial knee flexor function with the biceps femoris (a lateral muscle) and semimembranosus muscles. These findings may have important implications for joint loading, the long term health of the knee, and surgical decision-making.

Commentary by Dr. Valentin Fuster
2003;():221-222. doi:10.1115/IMECE2003-43035.

Approximately 250,000 anterior cruciate ligament (ACL) injuries occur in the United States each year. Most people cannot return to sports after an ACL injury without surgical intervention (Non-copers), but some can (Copers). Recent research suggests that the ability to cope with ACL injury is most likely related to neuromuscular function. The purpose of this study was to evaluate the neuromuscular control strategies of ACL deficient (ACL-D) Non-copers, ACL-D Copers, and people with uninjured knees using an established target-matching protocol, electromyography (EMG) of 10 muscles acting at the knee, and circular statistics methods. Thirty-two people (12 Non-Copers, 8 Copers, and 12 people without a history of knee injury) volunteered to participate in the study. The ACL-D subjects demonstrated diminished neuromuscular control when their muscle activity patterns were compared to those of the uninjured subjects. The key difference between the Copers and Non-copers was that Copers demonstrated better quadriceps control than the Non-copers. This study may have important implications for the treatment of people who sustain ACL injuries.

Commentary by Dr. Valentin Fuster
2003;():223-224. doi:10.1115/IMECE2003-43045.

In this paper we develop a neuro-muscular elasto-dynamic model of the human arm. The generality of the approach provides a method for relating effects of neural and muscular control on stress development in skeletal structures. The humerus is pinned parallel to the body and allowed to rotate in the plane. The radius and ulna are free to move in the sagittal plane. The bones are modeled as elastic elements, allowing large transverse deflections and elongations. Standard Hill-type models of the musculotendon actuators are used to generate the joint torques. Simulations are presented that correspond to both open and closed loop neural stimulus, and stresses and strains that result from both types of control are compared.

Commentary by Dr. Valentin Fuster
2003;():225-226. doi:10.1115/IMECE2003-43053.

To improve surgical procedures and rehabilitation protocols for injuries to the anterior cruciate ligament (ACL), the function of the ACL and ACL graft during in vivo activities must be understood. Robotic manipulators with a payload less than 500 N (low-payload) have been used to study joint and ligament function during application of external loading conditions (Fujie, 1993; Fujie, 2000). Robotic manipulators with a payload capacity of at least 1500 N (high-payload) will be needed to simulate high joint contact forces that have been estimated to be between 2 to 5 times body weight (Morrison, 1970; Escamilla, 1998). However, there is a trade-off between payload capacity and position and path repeatability. The objective of this study was to determine the effect of position and path repeatability of two high-payload robotic manipulators, (KUKA™ KR210 and FANUC™ S900W) used to apply external loads to diarthrodial joints and determine the corresponding joint kinematics and forces in the soft tissue structures.

Commentary by Dr. Valentin Fuster
2003;():227-228. doi:10.1115/IMECE2003-43055.

The effectiveness of ACL reconstruction for restoring normal joint mechanics is poorly understood, especially during physically demanding tasks. For this study, 3D kinematics of ACL-reconstructed and contralateral (uninjured) knees were assessed during down-hill running, using a unique high frame rate stereo-radiography system. Though anterior/posterior tibial translation was restored to normal levels, significant rotational abnormalities were identified in the ACL-reconstructed knees in both the coronal and transverse planes.

Commentary by Dr. Valentin Fuster
2003;():229-230. doi:10.1115/IMECE2003-43062.

In order to take advantage of the opportunities that dynamic Magnetic Resonance Imaging (d-MRI) offers to the study of in vivo joint mechanics, d-MRI compatible devices capable of producing joint loads replicating dynamic physiological activities are needed (Sheehan et al., 1999). The purpose of this research effort was to design, model and test a device for the expressed purpose of using d-MRI to study precise ankle joint dynamics during loaded pseudo-functional movements. The device adjusts to subject specific anthropometric measurements, allowing for the device’s axis of rotation to approximate the ankle’s transverse axis. By combining imaging data and the model of the device, the magnitude, direction and point of application of the force applied to the foot were calculated throughout the motion cycle, with an average error of .7 Nm. This allows for comparisons between the externally applied load and internal ankle joint kinematics to be made, which are essential determinants for in vivo estimates of forces within tendon and ligament. The next phase of this work will be to combine this device with fast-Phase Contrast MRI (fast-pc), a previously developed d-MRI technique for the quantification of 3D musculoskeletal motion, in order to create a complete tool for the noninvasive in vivo measurement of joint kinematics during a loaded dynamic functional task in both healthy and impaired ankles.

Commentary by Dr. Valentin Fuster
2003;():231-232. doi:10.1115/IMECE2003-43067.

Assessment of in vivo human cartilage loading generally requires computer modeling, since loads usually cannot be directly measured. The utility of these models for assessing knee behavior during complex activities has been limited by the relatively poor quality of experimental data on in vivo knee function. We have developed a method combining high-accuracy knee kinematics (from high-speed stereo-radiography) with subject-specific finite-element models to estimate in vivo cartilage contact pressures during stressful tasks. When applied to ACL reconstruction, significantly higher contact pressures were found in reconstructed knees as compared to the contralateral (uninjured) knees of the same individuals.

Commentary by Dr. Valentin Fuster
2003;():233-234. doi:10.1115/IMECE2003-43079.

This paper presents a method to calculate functional joint space during dynamic movement. This method combines high-speed biplane radiographic image data and three-dimensional (3D) bone surface data obtained from computed tomography (CT). Subjects were patients undergoing anterior cruciate ligament (ACL) reconstructive surgery. Three tantalum beads were implanted bilaterally into both the femur and tibia during surgery. CT scans were performed after bead implantation, and the CT slices were reconstructed into 3D solid figures, with the implanted beads identifiable within the stack of CT slices. Subjects were tested 6,12 and 24 months post surgery. Testing activities included downhill running on a treadmill and one-legged hopping onto a force plate. During testing, the stereo-radiographic imaging system collected images at 250 frames per second. Later, the implanted beads were identified in the x-ray images and tracked in 3D with an accuracy of 0.10 mm. The 3D bead location data were used to position the reconstructed solid bone figures in 3D space. In this way, the location of each bone surface was determined each instant. This method can be used to identify the regions of close contact between bones during dynamic motion, to calculate the surface area of subchondral bone within close contact, and to determine the changing position of the close contact area during dynamic activities. Using these techniques, comparisons can be made between subchondral bone motion in healthy and reconstructed joints and changes in dynamic joint space can be measured over time.

Commentary by Dr. Valentin Fuster
2003;():235-236. doi:10.1115/IMECE2003-43084.

Forces generated by muscle are transferred to bone via tendon. Since muscle force cannot be measured directly, computer modeling is a useful tool to enhance our understanding of normal and pathological movement. Hill-type muscle models have been used to estimate force based on information about a muscle’s architecture, activation and kinematics (Delp et al., 1995; Manal et al., 2002). Architectural parameters include optimal fiber length (l o m ), tendon slack length (l s t ), pennation angle (α), and maximum isometric force (Fmax ). In addition, musculotendon length (l mt ) and activation (a) are required inputs when estimating isometric muscle force (Equation I).

Fm = f(lmt, lom, lst, Fmax, α, a) (1)
Musculotendon length can be determined from MR images (Arnold et al., 2000), and activation recorded from EMGs (Manal, et al., 2002). Optimal fiber length and pennation angle can be measured experimentally (Murray, 2002), while Fmax can be estimated from the muscle’s physiologic cross-sectional area. Tendon slack length however cannot be measured readily, and therefore few estimates of l s t can be found in the literature. In this paper we present a numerical method for estimating tendon slack from subject specific muscle parameters and musculotendon lengths. An advantage of this method is that it yields subject specific estimates of tendon slack length.

Commentary by Dr. Valentin Fuster
2003;():237-238. doi:10.1115/IMECE2003-43090.

A non-invasive, non-contact methodology to estimate forces in the anterior cruciate ligament (ACL) in response to in vivo knee kinematics will allow surgical procedures and rehabilitation protocols for ACL injuries to be improved. During the last decade, intensive efforts have been made to quantify the forces in the ACL in vivo (Holden, 1994; Lundberg, 1997; Zacharias, 2001). With the use of these methods, valuable information on the forces experienced by the ACL has been obtained, however many of these methods were invasive, and involved direct contact with the ACL, which may affect the force measurements. It has been proposed at our research center that the forces in the ACL during activities of daily living and rehabilitation exercises can be estimated in a non-contact, non-invasive manner by reproducing in vivo kinematics in 6-degrees of freedom (DOF) on a cadaveric knee. Therefore, the specific aim of this study was to evaluate the feasibility of a non-invasive, non-contact methodology for estimating force in the ACL by reproducing average kinematics in 6-DOF degrees of freedom from one set of porcine knees (source) onto a separate set of porcine knees (target).

Commentary by Dr. Valentin Fuster
2003;():239-240. doi:10.1115/IMECE2003-43098.

In order to correctly diagnose and treat pathological knee joint mechanics we must be able to non-invasively quantify the 3D in vivo kinematics of this joint. Unfortunately, the majority of clinical diagnoses, for this joint, are based upon static 2D imaging. This is due to the fact that currently there is a scarcity of noninvasive measurement techniques that acquire 3D in vivo data dynamically. Thus, in vivo patellofemoral (PF) kinematic measurements typically compress a 3D time-dependent joint attitude to a static 2D representation. The purpose of this study was to investigate if patellar maltracking is limited to two dimensions, as assumed clinically, or if it is a complete six-degree of freedom problem. To do this, we quantified the 3D patellofemoral and tibiofemoral (TF) kinematics in both healthy individuals and those with suspected patellofemoral maltracking using fast-phase contrast magnetic resonance imaging, a technique developed in our previous work. Our data suggest that variations in kinematics were not confined to the standard axial plane measures (e.g. patellar tilt, patellar subluxation), but variations are exhibited in all six degrees of freedom. Therefore, future clinical diagnoses and interventions along with future research will be most effective if the measures used are broadened to include all six-degrees of freedom.

Commentary by Dr. Valentin Fuster
2003;():241-242. doi:10.1115/IMECE2003-43114.

The use of surface markers to calculate kinematics of diarthrodial joints is subject to error from relative motion between the markers on the skin and the underlying bones. It has been demonstrated that markers placed over the knee and hip joints could move 10–30 mm with respect to the skeleton which can have significant effects on the accuracy of calculated knee kinematics (Cappozzo et al., 1996, Manal et al., 2000). Several methods have been developed to calculate knee kinematics from marker locations and decrease error resulting from skin motion. The Helen Hayes method uses markers attached to the skin over anatomical landmarks and on wands to calculate the locations of the hip, knee, and ankle joints (Vaughan et al., 1999). The location and orientation of the thigh and shank segments can then be calculated based on anthropometric correlations. Other methods, such as that developed by the Cleveland Clinic, record the position of the femur and tibia using triads of markers attached to each segment (Manal et al., 2000). Additionally, a Point Cluster Technique was developed using clusters of markers attached to each segment to reduce error from skin motion (Andriacchi et al., 1998).

Topics: Kinematics , Motion , Skin , Knee
Commentary by Dr. Valentin Fuster
2003;():243-244. doi:10.1115/IMECE2003-43137.

The shoulder is one of the most complex and often injured joints in the human body. The inferior glenohumeral ligament (IGHL), composed of the anterior band (AB), posterior band (PB) and the axillary pouch, has been shown to be an important contributor to anterior shoulder stability (Turkel, 1981). Injuries to the IGHL of the glenohumeral capsule are especially difficult to diagnose and treat effectively. The objective of this research was to develop a methodology for subject-specific finite element (FE) modeling of the ligamentous structures of the glenohumeral joint, specifically the IGHL, and to determine how changes in material properties affect predicted strains in the IGHL at 60° of external rotation. Using the techniques developed in this research, an improved understanding of the contribution of the IGHL to shoulder stability can be acquired.

Commentary by Dr. Valentin Fuster
2003;():245-246. doi:10.1115/IMECE2003-43152.

Meniscal injuries frequently occur in combination with anterior cruciate ligament (ACL) tears. Tears in the menisci that occur acutely with ACL injury are located either medially or laterally, but in ACL deficient knees chronic instability most commonly leads to meniscal tears in the posterior horn of medial meniscus (Smith & Barrett, 2001). Levy et al. (1982) suggested that medial meniscus might act as a mechanical wedge preventing tibial anterior translation due to its firm attachment to the tibia at the posterior horn. Previous biomechanical studies infer the role of the meniscus through measurements of alternations in forces or knee motions (Shoemaker & Markolf, 1986; Allen, 2000), however no study has provided clear images of the motion or deformation of the meniscus itself. The objective of this study was to investigate the effects of anterior cruciate ligament resection, as well as partial and complete meniscectomies on knee kinematics in response to anterior loading. Application of loads during magnetic resonance (MR) imaging allows us to visualize the motion and deformation of the menisci, and therefore obtain a more clear understanding of its role in knee stability.

Commentary by Dr. Valentin Fuster
2003;():247-248. doi:10.1115/IMECE2003-43153.

A finite element model of the knee joint could be helpful in providing insight on mechanisms of injury, effects of treatment, and the role of mechanical factors in degenerative conditions. However, preparation of such a model involves many geometric simplifications and input of material properties, some of which are poorly understood. Therefore, a method to compare model predictions to actual behaviors under controlled conditions could provide confidence in the model before exploration of other loading scenarios. Our laboratory has developed a method to apply axial loads to the in vivo human knee during magnetic resonance imaging, resembling weightbearing conditions. Image processing algorithms may then be used to assess the three-dimensional kinematics of the tibia and femur during loading. A three-dimensional model of the tibio-menisco-femoral contact has been generated and the image-based kinematic boundary conditions were applied to investigate the distribution of stresses and strains in the articular cartilage and menisci throughout the loading period. In this study, our goal is to investigate the contact patterns during long term loading of up to twenty minutes in the healthy knee. Specifically, we assess the use of both elastic and poroelastic material properties in the cartilage, and compare model predictions to known loading conditions and images of tissue deformations.

Commentary by Dr. Valentin Fuster
2003;():249-250. doi:10.1115/IMECE2003-43241.

A validated non-linear 3-D finite element model of human tibiofemoral joint was utilized to investigate the effect of constraint on tibial coupled internal-external and varus-valgus rotations on the passive joint response and force in ligaments under 100N drawer loads at different flexion angles. The model consisted of two bony structures and their articular cartilage layers, menisci and four principal ligaments. For the cruciate ligaments, the results showed that, in the fully unconstrained joint, ACL force decreased with flexion but remained as the primary ligament to resist the posterior femoral load throughout the range of flexion considered. A further significant decrease in ACL force with flexion angle was computed as the joint coupled rotations were constrained. As for PCL ligament, a minor contribution was at full extension under 100N anterior femoral load which further decreased as the coupled rotations were constrained. With joint flexion up to 90°, PCL force, however in contrast to ACL force, substantially increased in both constrained and unconstrained joints. Collateral ligaments, in the unconstrained joint at full extension, were the primary structures to resist the anterior femoral load but had negligible role in posterior-directed load. With joint flexion up to 90°, however, forces in collateral ligaments diminished. Similar trends were computed after fixing coupled tibial rotations with the exception of much greater LCL force and smaller MCL force at full extension under femoral anterior load and larger MCL force in flexion.

Topics: Force , Stress , Knee
Commentary by Dr. Valentin Fuster
2003;():251-252. doi:10.1115/IMECE2003-41133.

Described here is a novel tissue tester that can measure the tensile properties of monolayer embryonic epithelia specimens as small as 0.5mm by 0.3mm, something that had not been possible previously. The instrument is used to determine the uniaxial stress-strain characteristics of epithelium from early-stage embryos of the axolotl (Ambystoma mexicanum), a type of amphibian.

Commentary by Dr. Valentin Fuster
2003;():253-254. doi:10.1115/IMECE2003-41481.

This study describes a detailed methodology for modeling a three-dimensional radiofrequency ablation procedure using reconstructed porous tissue geometries. In this study, MRI images of a sectioned liver tissue containing arterial vessels are converted into a finite element mesh. An rf heat source in the form of a spherically symmetric Gaussian distribution, fit from a previously computed profile, is employed Computations of temperature rise were performed for transient rf procedures in the case where the tumor is located near the bifurcation point of a hepatic artery. Results demonstrate a significant effect due to convective cooling by the large vessels. Substantial asymmetries in the temperature profiles indicate ablation procedures that may achieve adequate tumor destruction in some regions, but that elevate the temperature only minimally in other regions, thereby permitting possible tumor recursion. These critical features of the temperature field are due to the directional nature of the arterial flow and are difficult to capture with models that treat perfusion with a scalar source term in the bioheat equation.

Commentary by Dr. Valentin Fuster
2003;():255-256. doi:10.1115/IMECE2003-43009.

Patients with diabetes mellitus (DM) suffer impaired lower urinary tract dysfunctions. The purpose of the current study was to evaluate the effects of DM on the passive biomechanical properties of the female rat urethra. DM was induced by injection of streptozotocin. Urethras were excised and mounted in an ex-vivo testing system. EDTA was added to the bath to inactivate smooth muscle. Continuous outer diameter measurements were made at proximal, middle, and distal portions of the urethra with a laser micrometer during stepwise increases of static, intraurethral pressure (0 to 20 mmHg). Compliance and beta stiffness were calculated from measured data. Healthy urethras served as controls. Statistical comparisons were made via ANOVA. The control tissue was most compliant proximally and decreased significantly along the length. This compliance gradient vanished with DM. A significant decrease in compliance and increase in beta stiffness was noted for 10 wk DM compared to controls. These findings suggest that DM has a large effect on the biomechanical properties of the urethra.

Commentary by Dr. Valentin Fuster
2003;():257-258. doi:10.1115/IMECE2003-43074.

Six-degree-of-freedom (DOF) robotic/universal force-moment sensor (UFS) testing systems have become very useful tools in the study of biomechanics of joints. (DeFrate 2001, Fujie 2000, Rudy 1996) Researchers have successfully utilized these systems to determine joint kinematics using force control algorithms. Recent interest in using these robotic systems to reproduce previously recorded motions of a diarthrodial joint during a clinical examination has emerged. However, reproducing these previously recorded kinematics using a robotic/UFS testing system poses several problems.

Topics: Motion , Robotics
Commentary by Dr. Valentin Fuster
2003;():259-260. doi:10.1115/IMECE2003-43086.

Hypobaric hypoxia produces pulmonary hypertension in mice which causes pulmonary vascular remodeling. To study the biomechanics of this process, mice were exposed to hypoxia for 0-(control), 10-, and 15-days. Using a pressurized arteriograph system, mechanical properties of the main pulmonary artery were measured and compared to the biological changes in the vessel wall measured histologically. 10- and 15-day hypoxic vessels were significantly stiffer when compared to 0-day vessels. This stiffness correlated with greater elastin and collagen content in the vessel wall.

Commentary by Dr. Valentin Fuster
2003;():261-262. doi:10.1115/IMECE2003-43092.

Disc degeneration is a multifactor phenomenon. It has been found that intervertebral disc (IVD) cells respond to such factors as pH, osmotic pressure, genetic factors, and mechanical loading (Guilak, 1999). During daily activities the human intervertebral disc is exposed to oscillatory hydrostatic loads that produce pressures >2MPa in vivo (Nachemson, 1964 and 1979). It is known that dynamic loads with critical frequencies close to that of the in vivo human spine resonant frequency (4–5 Hz) have a destructive effect on disc tissue properties (Pope, 1993). Whether this destructive effect is purely mechanical, due to loading magnification, or biological, affecting cell metabolism, is unknown. Previous work (Merryman, 2002) showed that there was no significant effect upon monolayer IVD cells loaded at 15Hz, while lower frequencies (1 and 8Hz) altered collagen synthesis compared to control. To address this issue, we developed a mechanically active culture system capable of delivering a wide range of loading frequencies and amplitudes of hydrostatic pressure to cultures of disc cells. Nucleus pulposus cells of pig discs were isolated and suspended in alginate beads. Alginate cultures were divided into 6 groups; five groups were exposed to cyclic pressures of frequencies 1, 3, 5, 8, and 10Hz with the same amplitude of 1MPa, and group 6 was the control group (no loading). Cultures of different groups were loaded for 3 days (30 minutes daily) in a hydraulic chamber filled with culture media. The effect of loading frequency on collagen metabolism among different groups was compared by measuring incorporated [3 H]-proline into collagen for medium and total extracts. The results indicated a poor synthesis rate and more degradation near the 5Hz frequency.

Topics: Disks
Commentary by Dr. Valentin Fuster
2003;():263-264. doi:10.1115/IMECE2003-42711.

Various articles suggest that the maximum release force for buckle according to Federal Motor Vehicle Safety Standards (FMVSS) 109 of 133 N is beyond the capability of a large percentage of our population [1, 2]. Inversion studies with a large male in a three-point production belt showed he could not open a side release buckle [3]. Numerous articles and patents reference the potential for entrapment of inverted occupants unable to release the seat belt buckle [4–11]. Various articles and patents discuss the problems associated with entrapment of individuals in fires, water or emergency situations or where the occupant is deprived of oxygen due to positional asphyxia [12]. While the use of seat belts has increased markedly over the years [13], investigations indicate that rollover accidents showed fatally injured occupants in their seats which were entrapped in their vehicle. The forces to release the buckles under full load of the inverted occupants were beyond the physical capacities of the occupants involved. Canadian motor vehicle safety standard 209 (CMVSS 209) requires that a buckle must release with a force of 133 N to the button with a restraining loop force of 666 N. About 80 % of driver’s could not release a buckle that requires 133 N of force on the button [1]. Females could exert about 80 N with their fingers when opening child restraint buckles [14]. Females were generally found to have about half the physical capacity to open buckles compared to males. The maximum buckle release force of 133 N is not found in literature. Dreyfuss in his book indicates various forces for females and males [15]. European standards require that latch plate be ejected, therefore side release buckles are not allowed.

Commentary by Dr. Valentin Fuster
2003;():265-266. doi:10.1115/IMECE2003-43061.

Government recommendations have been made to place children into the rear seating areas of motor vehicles in order to alleviate airbag hazards in frontal impact. In most moderate to severe rear impacts, however, the adult occupied front seats will “yield” or “collapse” into the rear seat area and thus pose another potential head and chest injury hazard to the rear seated children. Numerous factors or variables, each with a wide parameter range, influence whether or not an occupied collapsing front seat will result in engagement with the rear occupant, and whether that engagement is likely to cause injury to the rear-seated occupant. A combined experimental and analytical method, employing instrumented surrogates in a sled-buck test set-up, has been utilized to study the multivariable potential injury problem of the rear-seated child in rear impact. A 3 year-old H-III surrogate, seated in the built-in booster seat of a minivan, was used as the rear seat passenger in this study. Five tests were utilized. The experimental surrogate data from the test method is combined into a “polynomial response function” that expresses “injury levels” (i.e. HIC and chest G) as a function of the many variables, and allows for analytical “interpolation and extrapolation” at variable combinations and ranges not tested. Actual accident cases were compared with the biomechanical injury measures. The present study presents a methodology to delineate the biomechanics of injuries using multivariate analysis.

Topics: Biomechanics , Wounds
Commentary by Dr. Valentin Fuster
2003;():267-268. doi:10.1115/IMECE2003-43071.

To date, human responses in motor vehicle rollover accidents have been studied through the use of Hybrid III dummies in dolly vehicle rollover tests, quasi-static spit tests where the vehicle and occupant are rotated slowly about the rotation axis of the spit fixture, computer simulations and vehicle drop tests. To demonstrate human responses to dynamic rollover conditions more accurately we designed and built a fixture to accommodate a passenger compartment in a hoop structure that rotates as it translates. The rotational axis of the hoop structure is offset from the rotational axis of the passenger compartment to replicate vehicle center of gravity motion seen in dolly rollover tests. Testing showed the difference in restraint behavior depending upon whether the occupant was seated on the near (initially leading) or far side. It demonstrated that human and Hybrid III dummy neck response is very different. The human test subject received no injuries from diving into the roof of the passenger compartment even though this is the predicted injury mechanism reported in several technical papers.

Commentary by Dr. Valentin Fuster
2003;():269-270. doi:10.1115/IMECE2003-43076.

Experimental rollover tests have been criticized for their poor emulation of actual rollovers and for their lack of repeatability. We have designed and built a test fixture that overcomes both of these criticisms. The fixture holds a passenger compartment, weighted to match the inertia characteristics of a complete vehicle, or a complete vehicle at the appropriate pitch and yaw. The compartment is then rotated about its principal (longitudinal) axis through an arc that mimics the rolling motion of an entire vehicle. At the appropriate roll angle and falling velocity, the roof strikes a moving patch of concrete. The compartment is controlled throughout the sequence and is suspended after the impact, so that a sequence of impacts can be individually studied in separate tests. Initial tests have shown that we can achieve repeatable impacts. Test variables include pitch, yaw, roll rate and vehicle center of gravity motion (both lateral and vertical velocity). This test device addresses the various shortcomings of previous rollover tests, fixtures and the various static and drop tests of vehicles conducted to determine rollover performance.

Commentary by Dr. Valentin Fuster
2003;():271-272. doi:10.1115/IMECE2003-43077.

We have developed and used a repeatable roof strength survey tool to assess the force resistance characteristics of over 50 passenger car, SUV, pickup, and van roofs. In a rollover, the initial roof-to-ground contact typically fractures and/or separates the vehicle’s bonded windshield. Subsequent trailing-side roof-to-ground impacts apply lateral forces to the roof and its support pillars. In 1971 the National Highway Safety Bureau (NHSB) recognized this rollover sequence and proposed a Federal Motor Vehicle Safety Standard (FMVSS) that tested both sides of the roof in sequence. Our repeatable roof strength survey tool uses a hydraulic cylinder to pull the upper A-pillar, roof rail, windshield header intersection toward the rear of the opposite front door sill imitating the proposed 1971 test, but at a more realistic roll angle. It is used first on one side of the vehicle with the windshield intact before being repositioned on the other side after the fractured and separated windshield is removed and the test repeated. Tests on three vehicles of the same make, model, and model year have validated the repeatability of the test and protocol. Results from all the vehicles demonstrate that in the first side test, the strength of the roof is typically about half the strength recorded in a typical FMVSS 216 test, a further decrease in force resistance occurs after the windshield has failed, and similar elastic restoration of the deformed structure occurs on both sides.

Commentary by Dr. Valentin Fuster
2003;():273-274. doi:10.1115/IMECE2003-43104.

Recent statistics highlight the significant risk of serious and fatal injuries to occupants involved in rollover collisions due to excessive roof crush. The government has reported that in 2002, Sports Utility Vehicle rollover related fatalities increased by 14% to more than 2400 annually. 61% of all SUV fatalities included rollovers. [1] Rollover crashes rely primarily upon the roof structures to maintain occupant survival space. Frequently these crashes occur off the travel lanes of the roadway and, therefore, can include impacts with various types of narrow objects such as light poles, utility poles and/or trees. A test device and methodology is presented which allows for dynamic, repeatable rollover impact evaluation of complete vehicle roof structures with such narrow objects. These tests allow for the incorporation of Anthropomorphic Test Dummies (ATDs) which can be instrumented to measure accelerations, forces and moments to evaluate injury potential. High-speed video allows for detailed analysis of occupant kinematics and evaluation of injury causation. Criteria such as restraint performance, injury potential, survival space and the effect of roof crush associated with various types of design alternatives, countermeasures and impact circumstances can also be evaluated. In addition to presentation of the methodology, two representative vehicle crash tests are also reported. Results indicated that the reinforced roof structure significantly reduced the roof deformation compared to the production roof structure.

Topics: Design , Testing
Commentary by Dr. Valentin Fuster