Timothy P. Harrigan

In muscle force analysis, orientations and moment arms of the muscles about a joint provide essential coefficients in the equilibrium equations. For the determination of these parameters, several experimental techniques, including geometric measurement, tendon-joint displacement measurement and direct load measurement, are available. Advantages and disadvantages associated with each of the techniques are reviewed and compared based on our extensive experience.

Most existing stress analyses of the skeleton which consider cancellous bone assume that it can be modelled as a continuum. In this paper we develop a criterion for the validity of this assumption. The limitations of the continuum assumption appear in two areas: near biologic interfaces, and in areas of large stress gradients. These limitations are explored using a probabilistic line scanning model for density measurement, resulting in an estimate of density accuracy as a function of line length which is experimentally verfied. Within three to five trabeculae of an interface, a continuum model is suspect. When results as predicted using continuum analyses vary by more than 20–30% over a distance spanning three to five trabeculae, the results are suspect.

A three-dimensional non-linear finite element analysis of a cemented femoral component in which the component was partially debonded from the cement mantle was used to assess the effects of debonding on stresses in the cement. Three cases of partial cement-metal debonding were modelled with debonding of the proximal portion of the implant down to a horizontal plane which was 35, 62.5, or 82.5 mm below the prosthesis collar. Each situation was studied under loads simulating both gait and stairclimbing. Also, complete debonding between the implant and the surrounding cement mantle was modeled for loads simulating gait. Under stair climbing loads with partial cement-metal debonding, hoop stresses of 13–18 MPa were observed in the cement at the cement-metal interface at the proximal postero-medial corner of the implant. Similarly, in stair climbing, the maximum principal stresses in the cement were also adjacent to the proximal postero-medial region of the implant. These stresses were compressive and increased from 15 MPa with fully bonded interfaces to 48 MPa with debonding down to 82.5 mm below the prosthesis collar. Under gait loads, complete debonding caused high compressive stresses up to 34.9 MPa in the cement distal to the prosthesis tip. Thus, cement failure subsequent to prosthesis debonding is likely in the proximal region in a partially debonded implant due to stair climbing loads and is likely below the prosthesis tip in a fully debonded implant due to gait loading.

In the field of elastography, biological tissues are conveniently assumed to be purely elastic solids. However, several tissues, including brain, cartilage and edematous soft tissues, have long been known to be poroelastic. The objective of this study is to show the feasibility of imaging the poroelastic properties of tissue-like materials. A poroelastic material is a material saturated with fluid that flows relative to a deforming solid matrix. In this paper, we describe a method for estimating the poroelastic attributes of tissues. It has been analytically shown that during stress relaxation of a poroelastic material (i.e., sustained application of a constant applied strain over time), the lateral-to-axial strain ratio decreases exponentially with time toward the Poisson's ratio of the solid matrix. The time constant of this variation depends on the elastic modulus of the solid matrix, its permeability and its dimension along the direction of fluid flow. Recently, we described an elastographic method that can be used to map axial and lateral tissue strains. In this study, we use the same method in a stress relaxation case to measure the time-dependent lateral-to-axial strain ratio in poroelastic materials. The resulting time-sequenced images (poroelastograms) depict the spatial distribution of the fluid within the solid at each time instant, and help to differentiate poroelastic materials of distinct Poisson's ratios and permeabilities of the solid matrix. Results are shown from finite-element simulations.

In order to study initial mechanisms of failure in cemented femoral total hip components, an anatomically accurate three-dimensional linear finite element model was constructed and verified against experimental strain measurements in the cement mantle. Good agreement was found between predicted and measured strains. The likelihood of failure initiation due to cement-prosthesis debonding and crack initiation at voids was studied for loading conditions simulating both one-legged stance and stair climbing. The "out of plane" forces involved in stair climbing appear to be the greatest threat to the fixation of total hip replacements. In stair climbing, cement-prosthesis debonding and pore crack initiation were probable in the proximal anteromedial region of the cement mantle, and near the distal tip of the implant. The proximal stresses in stair climbing were higher than the distal stresses in either stair climbing or one-legged stance.

It remains unclear whether adjacent vertebral body fractures are related to the natural progression of osteoporosis or if adjacent fractures are a consequence of augmentation with bone cement. Experimental or computational studies have not completely addressed the biomechanical effects of kyphoplasty on adjacent levels immediately following augmentation. This study presents a validated two-functional spinal unit (FSU) T12-L2 finite element model with a simulated kyphoplasty augmentation in L1 to predict stresses and strains within the bone cement and bone of the treated and adjacent nontreated vertebral bodies. The findings from this multiple-FSU study and a recent retrospective clinical study suggest that changes in stresses and strains in levels adjacent to a kyphoplasty-treated level are minimal. Furthermore, the stress and strain levels found in the treated levels are less than injury tolerance limits of cancellous and cortical bone. Therefore, subsequent adjacent level fractures may be related to the underlying etiology (weakening of the bone) rather than the surgical intervention.

Elastography typically measures and images the normal strain component along the insonification/compression axis, i.e., in the axial direction. We have recently shown that, by using interpolation and cross-correlation methods of transversely displaced RF echo segments, it is possible to measure and image displacement and strain transversely to the beam with good precision. This enables the estimation and imaging of all three principal normal strain components. Generally, motion in a direction other than that in which strain is estimated may result in decorrelation noise, severely corrupting the estimates. Therefore, a correction method is applied to correct the displacement and strain estimates for decorrelating motion. In this paper, we show how corrected displacement estimates can also be used to estimate and image the shear strain components. This may allow us to identify regions of decorrelation noise in the normal strain measurement that are due to shear strain. Shear strain estimates provide supplementary information, which can characterize different tissue elements based on their mobility. In the case of breast lesions, low mobility is related to malignancy. Following an in vivo case, we show with 2D simulations how assessment of tumor mobility can be achieved with shear strain estimation.

A human head finite element model (HHFEM) was developed to study the effects of a blast to the head. To study both the kinetic and kinematic effects of a blast wave, the HHFEM was attached to a finite element model of a Hybrid III ATD neck. A physical human head surrogate model (HSHM) was developed from solid model files of the HHFEM, which was then attached to a physical Hybrid III ATD neck and exposed to shock tube overpressures. This allowed direct comparison between the HSHM and HHFEM. To develop the temporal and spatial pressures on the HHFEM that would simulate loading to the HSHM, a computational fluid dynamics (CFD) model of the HHFEM in front of a shock tube was generated. CFD simulations were made using loads equivalent to those seen in experimental studies of the HSHM for shock tube driver pressures of 517, 690 and 862 kPa. Using the selected brain material properties, the peak intracranial pressures, temporal and spatial histories of relative brain–skull displacements and the peak relative brain–skull displacements in the brain of the HHFEM compared favorably with results from the HSHM. The HSHM sensors measured the rotations of local areas of the brain as well as displacements, and the rotations of the sensors in the sagittal plane of the HSHM were, in general, correctly predicted from the HHFEM. Peak intracranial pressures were between 70 and 120 kPa, while the peak relative brain–skull displacements were between 0.5 and 3.0 mm.

Several technologies can be used for measuring strains of soft materials under high rate impact conditions. These technologies include high speed tensile test, split Hopkinson pressure bar test, digital image correlation and high speed X-ray imaging. However, none of these existing technologies can produce a continuous 3D spatial strain distribution in the test specimen. Here we report a novel passive strain sensor based on poly(dimethyl siloxane) (PDMS) elastomer with covalently incorporated spiropyran (SP) mechanophore to measure impact induced strains. We have shown that the incorporation of SP into PDMS at 0.25 wt% level can adequately measure impact strains via color change under a high strain rate of 1500 s-1 within a fraction of a millisecond. Further, the color change is fully reversible and thus can be used repeatedly. This technology has a high potential to be used for quantifying brain strain for traumatic brain injury applications.

A model is developed in which osteoblasts can sense the strains applied to a small region of bone through electrical coupling between adjacent cells. The stress-generated potentials within bone are assumed to occur through streaming potentials, and the coupled network of osteocytes is assumed to act in a manner similar to the classical cable model for nerve cells. In a one-dimensional model, the linear poroelastic equations for motion of the fluid are solved analytically for sinusoidally varying imposed strains, and the streaming potentials are predicted from the fluid flow. The changes in the osteocyte and osteoblast transmembrane potential are given by an analytical solution to the governing equations, and the dependence of the transmembrane potential changes (TPC) on position, loading rate, manner of loading (compression versus bending), and on the degree of cellular coupling is discussed. The model correctly predicts the rate dependence of remodelling established by other investigators. The influence of the electrical parameters within the model indicate that further study of the cellular coupling in bone can yield important new information on bone remodelling.

The loading parameters in the canine hip were determined from multiple studies, involving the collection of kinematic and force plate data in vivo joint reaction force from an instrumented hip replacement prosthesis, and in vivo femoral cortical bone strain gauge data in different dogs. In the middle of the stance phase of gait the canine femur was flexed 110 degrees with respect to the pelvis and formed a 20 degree angle relative to the floor. At this point in the gait cycle, a line passing from the superior to the inferior aspect of the pubic symphysis was parallel to the floor. The joint reaction force measurements showed that the net force vector during midstance was directed inferiorly, posteriorly, and laterally, with a peak magnitude of up to 1.65 times the body weight. A torsional moment of 1.6 N m is exerted about the femoral shaft. In vivo strain data showed that during gait peak compressive strains of -300 to -502 microstrain were produced on the medial aspect of the femoral cortex and peak tensile strains of +250 to +458 midstrain were produced on the femoral cortex. At the midstance phase of gait, principal cortical bone strains were rotated up to 29 degrees relative to the long axis of the femur, suggesting torsional loads on the femur. These data in combination provide valuable insights on the loading parameters of the canine hip which can be used in future applications of the canine as a model for evaluating mechanically based phenomena such as bone ingrowth and remodeling or hip prostheses.

The origin of unstable bone remodelling simulations using strain-energy-based remodelling rules was studied mathematically in order to assess whether the unstable behavior was due to the mathematical rules proposed to characterize the processes, or to the numerical approximations used to exercise the mathematical predictions. A condition which is necessary for the stability of a strain-energy-based remodelling theory was derived analytically using the calculus of variation. The analytical result was derived using a simple elastic model which consists of a long beam loaded by an axial force and a bending moment. This loading situation mimics the coupling between local density and global density distributions seen in vivo. A condition necessary for a stable remodelling scheme is arrived at, but the conditions necessary to guarantee a stable remodelling scheme are not. In this remodelling scheme, the elastic modulus is proportional to volumetric density raised to an exponent n, and the microstructural stimulus is taken as the strain energy density divided by volumetric density raised to an exponent m. In order for a remodelling scheme to be stable in this loading situation, m must be greater than n. Finite-difference time-stepping is used to verify the predictions of the analytical study. These numerical studies appear to confirm the analytical studies. Physiologic interpretation of the behavior found with n>m indicates that this type of unstable behavior is unlikely to be observed in vivo. Since numerical approximations are not made in deriving this stability condition, we conclude that the mathematical rules proposed to characterize bone remodelling based on strain energy density should meet this condition to be relevant to physiologic bone remodelling.

Modeling human body response to dynamic loading events and developing biofidelic human surrogate systems require accurate material properties over a range of loading rates for various human organ tissues. This work describes a technique for measuring the shear properties of soft biomaterials at high rates of strain (100–1000 s−1) using a modified split Hopkinson pressure bar (SHPB). Establishing a uniform state of stress in the sample is a fundamental requirement for this type of high-rate testing. Input pulse shaping was utilized to tailor and control the ramping of the incident loading pulse such that a uniform stress state could be maintained within the specimen from the start of the test. Direct experimental verification of the stress uniformity in the sample was obtained via comparison of the force measured by piezoelectric quartz force gages on both the input and the output sides of the shear specimen. The technique was demonstrated for shear loading of silicone gel biosimulant materials and porcine brain tissue. Finite element simulations were utilized to further investigate the effect of pulse shaping on the loading rate and rise time. Simulations also provided a means for visualization of the degree of shear stress and strain uniformity in the specimen during an experiment. The presented technique can be applied to verify stress uniformity and ensure high quality data when measuring the dynamic shear modulus of soft biological simulants and tissue.

Abstract Adaptive bone remodelling simulations which use finite element analysis can potentially aid in the design of orthopedic implants and can provide examples which test specific bone remodelling hypotheses in a quantitative manner. By concentrating on remodelling algorithms in which the geometry is fixed but the tissue stiffness changes based on strain energy density, we have predicted stability conditions for bone remodelling and we have tested the applicability of these conditions using numerical simulations. The stability requirements arrived at using a finite element formulation are similar to the requirements arrived at in an earlier analytical study. In order to test the stability conditions, we have developed an Euler backward time stepping technique which uses the derivation for stability. These simulations arrived at solutions which were impossible using Euler forward time stepping as applied in this study. Cases in which a simplified version of the derived Euler backward method are unstable or marginally stable have also been seen, but when the Euler backward method is applied using the full derived matrices, no instabilities are apparent. The results of the stability tests indicate that the converged density distributions in the examples studied are stable. Although a priori conditions which ensure stability are not found, a test for stability is provided, given an assumed density distribution.

Bone remodeling has been viewed both as a process which adapts bone tissue to the mechanical environment at each point in the structure, and as a process which optimally adjusts the tissue distribution within bones to bear the loads placed on them. We have developed a connection between these two views of bone remodeling, in a restricted sense. We start with a remodeling rate equation based on strain energy density. We then define an indicator function which is a weighted sum of total strain energy and a measure of bone mass, and we show that finding bone density distributions in which the remodeling rate equation predicts no changes with time is the same as finding density distributions in which the indicator function is insensitive to small changes in density. The set point in the remodeling rate equation corresponds to a parameter in the indicator function whith determines the relative importance of bone mass and strain energy in the optimization indicator function. We have not assessed whether the density distributions which make the density rate of change zero are actually local or global minima for the indicator function in this study, but a related study shows that there is a single unique minimum for the indicator function developed here, implying that a unique solution exists for the bone remodeling rate equations considered in this study.

Aseptic loosening is the most common long-term complication of cemented total hip arthroplasties (THA). The functional longevity of these implants depends on the bone-cement interface. The influence of cement injection pressure, type of cement, ambient temperature, chilling of the monomer, and centrifugation of cement-on-cement intrusion depth was investigated in specimens of bovine cancellous bone. In order to validate the bovine model for comparative purposes relative to use in man, a linear relationship between human and bovine cancellous bone was first demonstrated for various porosities and cement intrusion depth. Three cements (Low Viscosity Cement [LVC, Simplex-P, and Palacos) were intruded at three different pressures (20, 40, and 60 PSI) at the same ambient temperature and relative humidity into commercially prepared plugs of bovine cancellous bone. Cement intrusion depth was proportional to injection pressure for all three cements, but was significantly different for each cement at a given pressure. At 20, 40, and 60 PSI, Palacos had a cement intrusion depth of 1.4, 2.4, and 2.8 mm respectively, while the figures for Simplex- P were 2.2, 4.2, and 5.0 mm, and for LVC were 8.0,12.0, and 14.6 mm. Ambient temperature had an inverse relationship with cement intrusion depth for all three cements given the same experimental conditions. Chilling the monomer increased the intrusion of Simplex-P to 5.8,8.2, and 12.7 mm at 20,40, and 60 PSI injection pressure respectively. Simplex-P intrusion depth was not modified by cement centrifugation at any of the three injection pressures tested. Cement intrusion depths obtained with three popular commercial cements were compared with respect to pressures, usually generated in the operating room, and prepared in various conditions in a bovine cancellous bone. In an experimental system, the cement intrusion depth obtained in conditions usually achieved in the operating room (60 PSI pressure, 22° ambient temperature, and 65% relative humidity) is greater than the remaining cancellous bone bed in the prepared human femoral canal. Cement centrifugation does not change cement intrusion depth. Notwithstanding the limitations of the experiment, the information seems clinically relevant.

To investigate the effects of a clinical lytic defect on the structural response of human thoracolumbar functional spinal unit. A novel CT-compatible mechanical test system was used to image the deformation of a T12-L1 motion segment and measure the change in strain response under compressive loads ranging from 50 to 750 N. A lytic lesion (LM) with cortex involvement (33% by volume) was introduced to the upper vertebral body and the CT experiments were repeated. Finite element models, established from the CT volumes, were used to investigate the defect's effects on the structural response and the state of principal and shear stresses within the affected and adjacent vertebrae. The lytic lesion resulted in severe loss of the vertebral structural competence, resulting in significant, non-linear, and asymmetric increase in the experimentally measured strains and computed stresses within both vertebrae (p < 0.01). At the cortex, the tensile strains were significantly increased, while compressive strains significantly decreased, (p < 0.05). Both the vertebral bone and cortex regions adjacent to the defect showed significant increase in computed compressive, tensile, and shear stresses (p < 0.01). Changes in stress and strain distribution within the affected and adjacent vertebral bone and the experimentally observed bulging and buckling of the vertebral cortices suggested that initiation of catastrophic vertebral failure may occur under load magnitudes encountered in daily living. Although the effect of LM on the global deformation of the spine was well-predicted, our results show that FE predictions of local strain changes must be carefully assessed for clinical relevance. © 2016 Orthopaedic Research Society. Published by Wiley Periodicals, Inc. J Orthop Res 34:1808-1819, 2016.

The assessment of Parkinson’s disease currently relies on the history of the present illness, the clinical interview, the physical examination, and structured instruments. Drawbacks to the use of clinical ratings include the reliance on real-time human vision to quantify small differences in motion and significant inter-rater variability due to inherent subjectivity in scoring the procedures. Rating tools are semi-quantitative by design, however, in addition to significant inter-rater variability, there is inherent subjectivity in administering these tools, which are not blinded in clinical settings. Sophisticated systems to quantify movements are too costly to be used by some providers with limited resources. A simple procedure is described to obtain continuous quantitative measurements of movements of people with Parkinson’s disease for objective analysis and correlation with visual observation of the movements.•Inexpensive accelerometers are attached to the upper and lower extremities of patients with Parkinson’s disease and related conditions to generate a continuous, three-dimensional recorded representation of movements occurring while performing tasks to characterize the deficits of Parkinson’s disease.•Movements of the procedure are rated by trained examiners live in real-time and later by videotapes.•The output of the instrumentation can be conveyed to experts for interpretation for diagnostic and therapeutic purposes.

Abstract To determine whether the strain patterns produced in the femoral cortex after uncemented femoral arthroplasty are influenced by the fit of the component and whether these patterns are different from those of cemented components, cortical surface strains of cadaveric femurs subjected to loads simulating single‐limb stance were measured before and after the insertion of uncemented, collared, straight‐stemmed femoral components. The effects of press fit, loose fit, and precise fit of the components were evaluated and were contrasted to the strain patterns occurring after insertion of cemented femoral components. Strains varied markedly, depending on the fit of the stem of the uncemented femoral component within the isthmus. Nearly normal patterns of femoral strain were produced when the femoral stem was fit precisely at the isthmus, and the proximal femoral strains were similar to those of the intact state. In contrast, press fit and loose fit at the isthmus altered the strain patterns. The proximal medial axial strains were significantly reduced with press fit, to a mean of 39% of normal (p &lt; 0.05), and increased with loose fit, to a mean of 141% of normal (p &lt; 0.05). The prostheses fixed with cement showed a mean reduction in proximal medial axial strains to 33% of normal, which was comparable with press fit uncemented components even though the collar was well seated. Thus, our findings indicated that, in the immediate postoperative period, femoral strain patterns can be influenced by the fit of an uncemented component within the isthmus and by the use of cement.

Abstract The usefulness of the push‐out test as an indicator of interface strength was evaluated using finite element models of intact and partially failed cylindrical push‐out specimens loaded against a rigid annular support. The irregular stress distributions that were found in intact specimens depended more on interface conditions at the loading fixture than on a 35% increase in interface area. The maximum stress at the interface was a tensile stress. Critical energy release rates for interface failure were calculated for flawed specimens in which flaw size was either 10 or 100 μm, and for boundary conditions at the loading fixture that were either fixed or slipping in the radial direction. The critical energy release rates depended heavily on the support boundary conditions. Thus, the results of parametric push‐out tests can be reasonably compared only for specimens that are very similar in geometry and that are loaded in very carefully controlled fixtures.

Conditions which can guarantee the global stability and uniqueness of the solution to a bone remodeling simulation are derived using a specific rate equation based on strain energy density. We modeled bone tissue as isotropic with a constant Poisson ratio and the elastic modulus proportional to volumetric density of calcified tissue raised to the power n. Our remodeling rate equation took the rate of change of volumetric hard tissue density as proportional to the difference between a stimulus (strain energy density divided by volumetric density taken to the power m) and a set point. In previous studies we defined state variables which are conjugate to the remodeling stimulus, and the function which acts as a variational indicator for the remodeling stimulus. In this study, we use the properties of this variational indicator to establish the stability and the uniqueness of the solution to the remodeling rate equations for all possible density distributions. We show that the solution is the global minimum of a weighted sum of the total strain energy and the integral of density to the power m over the remodeling elements. These results are proven for n < m, and we show that taking n > m will eliminate the possibility that a unique solution exists.

Bone remodeling in vertebrates is widely quoted as a process which optimizes the use of structural material, subject to mechanical requirements. In vertebrates, bone remodeling continues throughout life and tends to preserve the structure of a particular bone over decades of life. This implies that the processes which remodel bone are stable, at least over a time period spanning many years. Many recent numerical simulations of bone remodeling have used rate equations which have not been carefully assessed for stability and their ability to produce an optimal structure. In this study, we re-state the conditions necessary for stability of a particular bone remodeling rate equation derived in a related study, and we investigate whether the rate equation used can produce an optimal structure. Within the context of a finite element discretization, we show that this rale equation does not produce a structure optimized with respect to density. By making a simple modification to the stable remodeling rate equation, we show that the remodeling stimulus used can produce an optimal structure if the state variable manipulated is density taken to a power. We conclude that if bone is a stable, self-optimizing structure, there are specific requirements for the point-by-point rate of change of bone density in response to mechanical stress. The implications of these requirements for simulations ot adaptive bone remodeling are discussed.

Transcranial focused ultrasound stimulation (tFUS) is a noninvasive neuromodulation technique, which can penetrate deeper and modulate neural activity with a greater spatial resolution (on the order of millimeters) than currently available noninvasive brain stimulation methods, such as transcranial magnetic stimulation (TMS) and transcranial direct current stimulation (tDCS). While there are several studies demonstrating the ability of tFUS to modulate neuronal activity, it is unclear whether it can be used for producing long-term plasticity as needed to modify circuit function, especially in adult brain circuits with limited plasticity such as the thalamocortical synapses. Here we demonstrate that transcranial low-intensity focused ultrasound (LIFU) stimulation of the visual thalamus (dorsal lateral geniculate nucleus, dLGN), a deep brain structure, leads to NMDA receptor (NMDAR)-dependent long-term depression of its synaptic transmission onto layer 4 neurons in the primary visual cortex (V1) of adult mice of both sexes. This change is not accompanied by large increases in neuronal activity, as visualized using the cFos Targeted Recombination in Active Populations (cFosTRAP2) mouse line, or activation of microglia, which was assessed with IBA-1 staining. Using a model (SONIC) based on the neuronal intramembrane cavitation excitation (NICE) theory of ultrasound neuromodulation, we find that the predicted activity pattern of dLGN neurons upon sonication is state-dependent with a range of activity that falls within the parameter space conducive for inducing long-term synaptic depression. Our results suggest that noninvasive transcranial LIFU stimulation has a potential for recovering long-term plasticity of thalamocortical synapses in the postcritical period adult brain.

A new system of femoral cement pressurization is presented that attempts to produce sustained and elevated cement pressure. Five paired cadaver femora were pressurized with Simplex cement, and PFC femoral components were inserted. One of each pair was pressurized with the new system, and 1 was pressurized with an existing device. Pressure was recorded at the proximal and distal levels. After curing, all 10 specimens were sectioned at similar levels and subjected to push-out testing to failure. Specimens that achieved higher pressures and longer duration of pressures tended to show higher levels of failure (not statistically proven on 10 specimens). Cementing techniques that use higher pressurization of cement are recommended. It is mandatory, however, that the femur be thoroughly cleaned of fat before the application of these techniques to avoid a fat embolism syndrome.

Uncemented femoral total hip components rely entirely on contact with the prepared femur for their initial fixation. The contact areas and stresses between a straight tubular bone and a metal cylindrical prosthesis 12.5 cm long and 13 mm in diameter were calculated in a finite element model which includes uniform diametral gaps varying from 20 to 500 microns, using transverse loads from 100 to 2000 N. Frictionless three-dimensional contact elements were used between the bone and the prosthesis. Contact stresses were high and irregular in all cases, and the contact areas were small. Two regions of contact were apparent for lower loads and larger gaps. A third region of contact occurred near the distal tip of the implant at higher loads. This region of contact markedly increased the contact stresses at the distal tip of the prosthesis. A 20 microns overlap between bone and implant was modelled to assess a slight interference fit. The contact stress distribution in this case was markedly different from the stress distribution with a 20 microns diametral gap. The data collectively indicates that gaps of less than 20 microns between bone and implant can substantially change contact stress distributions.

An examination of femoral bone-prosthesis interface behavior under different load types is undertaken using finite-element analysis. Three-dimensional finite-element models are made of two designs of hip prostheses after implantation in a femur. Femoral geometry was determined by computed tomography scans. The models were loaded in one-legged stance and stairclimbing configurations. The implants were modeled as both smooth surfaced and porous coated. The amount of contact and the relative motion between bone and implant were calculated. It is shown that torsional loads such as occur during stairclimbing contribute to larger amounts of implant micromotion than does stance loading. Contact at the bone-prosthesis interface is more dependent on load type than on implant geometry or surface coating type.

A low-cost quantitative continuous measurement of movements in the extremities of people with Parkinson‘s disease, a structured motor assessment administered by a trained examiner to a patient physically present in the same room, utilizes sensors to generate output to facilitate the evaluation of the patient. However, motor assessments with the patient and the examiner in the same room may not be feasible due to distances between the patient and the examiner and the risk of transmission of infections between the patient and the examiner. Therefore, we propose a protocol for the remote assessment by examiners in different locations of both (A) videos of patients recorded during in-person motor assessments and (B) live virtual assessments of patients in different locations from examiners. The proposed procedure provides a framework for providers, investigators, and patients in vastly diverse locations to conduct optimal motor assessments required to develop treatment plans utilizing precision medicine tailored to the specific needs of each individual patient. The proposed protocol generates the foundation for providers to remotely perform structured motor assessments necessary for optimal diagnosis and treatment of people with Parkinson's disease and related conditions.

The local material stiffness of tissues is a well-known indicator of pathology, with locally stiffer tissue related to the possible presence of an abnormal growth in otherwise compliant tissue. Elastography is a non-invasive technique for measuring displacement distributions in loaded tissues within a medical imaging context. From these measured displacement fields, estimated for local strain have been made using well-studied techniques, but the calculation of elastic modulus has been difficult. In this study we show a method for estimating local tissue elastic modulus that gives numerically stable and robust results in test cases, and that is numerically efficient. The method assumes the tissue is isotropic and it requires an independent estimate of tissue Poisson's ratio, but the method reaches a stable result when the estimated Poisson's ratio is in error, and the resulting estimates are not very sensitive to the assumed value.

The effect of a capacitively coupled electrical field on bone ingrowth into titanium fiber mesh porous-surfaced canine total hip arthroplasties (THAs) was investigated in a double-blind experiment. The electrical field was induced by an external source delivering a 60-kHz 5–6-V peak-to-peak sinusoid voltage through skin electrodes. No significant increase in the ingrowth of bone into the porous coating occurred at the end of six weeks of electrical stimulation. The amount of bone that grew into the porous surface, the areal density of bone within the available pore space, and the extent of the prosthesis surface area with bone ingrowth or apposition were not significantly different in the control and stimulated groups. This particular form of electrical stimulation does not improve bone ingrowth into porous-surfaced canine THAs by six weeks.

Estimation of the regional mechanical properties of the cardiac muscle has been shown to play a crucial role in the detection of cardiovascular disease. Current echocardiography-based cardiac motion estimation techniques, such as Doppler Myocardial Imaging (DMI), are limited due to angle dependence. By contrast, elastography, a method designed and used for the detection of tumors, measures displacement and strain by comparing echoes before and after (not during) a deformation and thus is not angle-dependent. Therefore, the feasibility of cardiac elastography to provide reliable and reproducible displacement and strain estimates from multiple sonographic views was recently demonstrated utilizing RF data from a normal human heart in vivo. In this paper, we demonstrate this technique utilizing 2D B-scan data in a patient with a known myocardial infarction. Envelope-detected sonographic data was used to estimate regional wall motion and deformation. Displacement and strain estimates were obtained in both non-infarcted, normally contracting and infarcted regions. By obtaining cine-loop and M-Mode elastograms from both regions, the ischemic regions could be identified. In conclusion, elastography may be a clinically viable method for detection of abnormalities of regional wall motion throughout the cardiac cycle.

Mild traumatic brain injury (mTBI) has recently been shown to include deficits in cognitive function that have been correlated to changes in tissue within regions of the white matter in the brain. These localized regions show decreased anisotropy in water diffusivity, which are thought to be related to local mechanical damage. However, a specific link to mechanical factors and tissue changes in these regions has not been made. This study is an initial attempt at such a correlation. A human head finite element model, verified against experimental data under simulated blast loading conditions, was used to estimate strains within regions in the brain that are correlated to functional deficits. Strain values from the most anterior and posterior extent of the corpus callosum (the rostrum and the splenium), the right and left anterior and posterior limb of the internal capsule (ALIC and PLIC), and the left cingulum bundle were calculated under frontal blast loading at overpressure intensities below those typically known to cause injury. Strain peaks of approximately 1 percent were noted in regions associated with cognitive brain injury, indicating that loading conditions which involve higher pressures could raise strains to significant levels.

Fluid pressure may stimulate osteolysis near screw holes in joint arthroplasty components. We developed a generalized in vitro model of a polyethylene liner and metal backing with a screw hole to investigate whether implant design factors influence local fluid pressure. We observed an order of magnitude of variation in the peak screw hole pressure (from 16.0 and 163 kPa) under clinically relevant loading conditions. Of the implant factors investigated, the surface finish of the metallic base plate had the greatest effect on peak screw hole fluid pressures; the thickness of the polyethylene liner, as well as the gap between the liner and the base plate, were also significant design variables. Our data suggest that unpolished metal base plates, thick polyethylene liners, and tight conformity between the liner and the metal base plate will all contribute to significantly reduced peak screw hole fluid pressures in joint arthroplasty.

Previously we had developed an articulated human body model to simulate the kinematic response to the external loadings, using CFDRC’s CoBi implicit multi-body solver. The anatomy-based human body model can accurately account for the surface loadings and surface interactions with the environment. A study is conducted to calibrate the joint properties (for instance, the joint rotational damping) of the articulated human body by comparing its response with those obtained from the PMHS test under moderate loading conditions. Additional adjustments in the input parameters also include the contact spring constants for joint stops at different joint locations. By comparing the computational results with the real scenarios, we fine tune these input parameters and further improve the accuracy of the articulated human body model. In order to simulate the effect of a C4 explosion on a human body in the open field, we employ a CFD model with a good resolution and the appropriate boundary treatment to obtain the blast loading condition on the human body surface more accurately. The numerical results of the blast simulation are shown to be comparable to the test data. With the interface to apply the blast pressure loading from the CFD simulation on the articulated human body surface, the articulated human body dynamics due to the C4 explosions are modeled and the simulation results are shown to be physiological reasonable.

A low-cost quantitative structured office measurement of movements in the extremities of people with Parkinson's disease [1,2] was performed on people with Parkinson's disease, multiple system atrophy, and age-matched healthy volunteers. Participants underwent twelve videotaped procedures rated by a trained examiner while connected to four accelerometers [1,2] generating a trace of the three location dimensions expressed as spreadsheets [3,4]. The signals of the five repetitive motion items [1,2] underwent processing to fast Fourier [5] and continuous wavelet transforms [6]. The dataset [7] includes the coding form with scores of the live ratings [1,2], the raw files [3], the converted spreadsheets [4], and the fast Fourier [5] and continuous wavelet transforms [6]. All files are unfiltered. The data also provide findings suitable to compare and contrast with data obtained by investigators applying the same procedure to other populations. Since this is an inexpensive procedure to quantitatively measure motions in Parkinson's disease and other movement disorders, this will be a valuable resource to colleagues, particularly in underdeveloped regions with limited budgets. The dataset will serve as a template for other investigations to develop novel techniques to facilitate the diagnosis, monitoring, and treatment of Parkinson's disease, other movement disorders, and other nervous and mental conditions. The procedure will provide the basis to obtain objective quantitative measurements of participants in clinical trials of new agents.

Deformation in soft biological connective tissue is intimately coupled to interstitial fluid movement, tissue solute concentration, and electrical fields. The role of deformation and of the associated electrical and chemical gradients in connective tissue physiology is currently unknown, and is of major interest in the study of growth, healing, and remodelling in the tissue. A quantitative study aimed at assessing relevant physiologic effects in connective tissue awaits accurate physical analysis. This paper reviews the available poroelasticity formulations with specific emphasis on establishing physically relevant continuum variables and on methods for including coupling between electrical or chemical gradients and deformations, then proposes a new set of state variables useful for engineering analysis of the tissue. Since solid and fluid components intermingle on a molecular scale, phase boundaries do not exist and thus a porosity cannot be defined. Further, in a material which includes fixed charge groups, which must be balanced by mobile species to maintain electroneutrality, the physics inside the tissue are not amenable to standard analysis. Relevant continuum state variables can be defined relative to a reference medium, however. The chemical, electrical, and mass transfer potentials of a differential element are defined by the potentials of a medium into which an excised differential element can be immersed at constant strain without transferring energy. Given this new definition of state variables, an energy differential is written, and a new incremental equation of state is derived based on a definition of coenergy.

A finite element model (FEM) of the human head attached to a Hybrid III FEM neck was developed to study the effects of blast loading on the brain. Simulations of blast loading to this Human Head Finite Element Model (HHFEM) were generated by creating a computational fluid dynamics (CFD) model of the HHFEM headform in a shock tube. Three different driver pressure loading conditions from experimental testing of the Human Surrogate Head Model (HSHM) were simulated by this model. The pressure time histories at each grid point of the CFD headform were used as inputs to the HHFEM. Brain/cerebral spinal fluid (CSF) and CSF/skull boundary conditions along with different brain material models were considered. The Kelvin-Maxwell material model and a low friction surface-to-surface interface were found to best replicate conditions seen in experimental testing of the HSHM. Deformations in the anterior and posterior locations of the brain varied from 0.5–0.9 mm and intracranial pressures at those locations were between 32 and 55 kPa.

Single-phase porous materials contain multiple components that intermingle up to the ultramicroscopic level. Although the structures of the porous materials have been simulated with agent-based methods, the results of the available methods continue to provide patterns of distinguishable solid and fluid agents which do not represent materials with indistinguishable phases. This paper introduces a new agent (hybrid agent) and category of rules (intra-agent rule) that can be used to create emergent structures that would more accurately represent single-phase structures and materials. The novel hybrid agent carries the characteristics of system’s elements and it is capable of changing within itself, while also responding to its neighbours as they also change. As an example, the hybrid agent under one-dimensional cellular automata formalism in a two-dimensional domain is used to generate patterns that demonstrate the striking morphological and characteristic similarities with the porous saturated single-phase structures where each agent of the “structure” carries semi-permeability property and consists of both fluid and solid in space and at all times. We conclude that the ability of the hybrid agent to change locally provides an enhanced protocol to simulate complex porous structures such as biological tissues which could facilitate models for agent-based techniques and numerical methods.

Severities and types of under-body blast lumbar injuries maybe associated with loading severity and amount of torso-borne mass, such as personal protective equipment. The objective of this study was to delineate these effects using a high-fidelity pelvis-lumbar spine finite element model (FEM). Geometries of the FEM wa s reconstructed from computed tomography scans and scaled to 50th percentile male. Hexagonal solid elements were used for majority of the FEM, except shell elements for cortical shells and endplates and nonlinear springs for ligaments. Material properties were obtained from in-house high-rate bulk and shear testing when available. Pelvis accelera tion loadings were obtained from full-body Hybrid-III FEM. Simulations were conducted with high and low pelvis accelerations, with and without torso-borne mass. Results found loading modes in the spine progressively changes from flexion, to compression, and extension from upper to mid- and lower level resulted in an S shaped deform ation, indicating change of injury mechanisms along the spine. Localized spine deformation decoupled the torso-borne mass from the high-rate pelvis acceleration during the initial stage (20-30ms). Under high pelvis acceleration, the spine may fail before the torso mass is engaged. Under low severity and lateral stage, motion of torso ma ss needs to be considered.

Since the 19th century, underwater explosions have posed a significant threat to service members. While there have been attempts to establish injury criteria for the most vulnerable organs, namely the lungs, existing criteria are highly variable due to insufficient human data and the corresponding inability to understand the underlying injury mechanisms. This study presents an experimental characterization of isolated human lung dynamics during simulated exposure to underwater shock waves. We found that the large acoustic impedance at the surface of the lung severely attenuated transmission of the shock wave into the lungs. However, the shock wave initiated large bulk pressure-volume cycles that are distinct from the response of the solid organs under similar loading. These pressure-volume cycles are due to compression of the contained gas, which we modeled with the Rayleigh-Plesset equation. The extent of these lung dynamics was dependent on physical confinement, which in real underwater blast conditions is influenced by factors such as rib cage properties and donned equipment. Findings demonstrate a potential causal mechanism for implosion injuries, which has significant implications for the understanding of primary blast lung injury due to underwater blast exposures.

A low-cost quantitative structured office measurement of movements in the extremities of people with Parkinson's disease [1,2] was performed on participants with Parkinson's disease and multiple system atrophy as well as age- and sex-matched healthy participants with typical development. Participants underwent twelve videotaped procedures rated by a trained examiner while connected to four accelerometers [1,2] generating a trace of the three location dimensions expressed as spreadsheets [3,4]. The signals of the five repetitive motion items (3.4 Finger tapping, 3.5 Hand movements, 3.6 Pronation-supination movements of hands, 3.7 Toe tapping, and 3.8 Leg agility) [1] underwent processing to fast Fourier [5] and amor and bump continuous wavelet transforms [6], [7], [8], [9], [10], [11], [12], [13]. Images of the signals and their transforms [4], [5], [6] of the five repetitive tasks of each participant were randomly expressed as panels on an electronic framework for rating by 35 trained examiners who did not know the source of the original output [14]. The team of international raters completed ratings of the signals and their transforms independently using criteria like the scoring systems for live assessments of movements in human participants [1,2]. The raters scored signals and transforms for deficits in the sustained performance of rhythmic movements (interruptions, slowing, and amplitude decrements) often observed in people with Parkinson's disease [15], [16], [17], [18], [19], [20]. Raters were first presented the images of the signals and transforms of a man with multiple system atrophy as a test and a retest in a different random order. After the raters completed the assessments of the man with multiple system atrophy, they were presented random test and retest panels of the images of signals and transforms of ten participants with Parkinson's disease who completed a single rating session. After the raters completed the assessments of the participants with Parkinson's disease who completed one set of ratings, they were presented random test and retest panels of the images of signals and transforms of (A) ten participants with Parkinson's disease and (B) eight age- and sex-match healthy participants with typical development who completed two rating session separated by a month or more [15], [16], [17], [18], [19], [20]. The data provide a framework for further analysis of the acquired information. Additionally, the data provide a template for the construction of electronic frameworks for the remote analysis by trained raters of signals and transforms of rhythmic processes to verify that the systems are operating smoothly without interruptions or changes in frequency and amplitude. Thus, the data provide the foundations to construct electronic frameworks for the virtual quality assurance of a vast spectrum of rhythmic processes. The dataset is a suitable template for solving unsupervised and supervised machine learning algorithms. Readers may utilize this procedure to assure the quality of rhythmic processes by confirming the absence of deviations in rate and rhythm. Thus, this procedure provides the means to confirm the quality of the vast spectrum of rhythmic processes.

The authors regret that they mistakenly published a prior version of Appendix 1 in supplementary materials. Please refer to the attached Appendix 1 “Revised coding form for a low-cost quantitative continuous measurement of movements in the extremities of people with Parkinson's disease” by Gregory Neal McKay, Timothy P. Harrigan, James Robert Brašić in supplementary materials for the correct Appendix 1 for this article. The authors would like to apologize for any inconvenience caused. Download .docx (.03 MB) Help with docx files Feasibility of virtual low-cost quantitative continuous measurement of movements in the extremities of people with Parkinson’s diseaseMethods XVol. 11PreviewA low-cost quantitative continuous measurement of movements in the extremities of people with Parkinson‘s disease, a structured motor assessment administered by a trained examiner to a patient physically present in the same room, utilizes sensors to generate output to facilitate the evaluation of the patient. However, motor assessments with the patient and the examiner in the same room may not be feasible due to distances between the patient and the examiner and the risk of transmission of infections between the patient and the examiner. Full-Text PDF Open Access

The internal parameters in bone remodelling theories often are not clearly related to the bony structure which results from the simulations in which they are implemented. For a restricted class of bone remodelling theories, we have previously found a connection between overall structural optimization and the parameters within a continuum-level remodelling rule. In this study, we assess whether a simplified analytical formula based on structural optimization can predict the behaviour of a large-scale finite element bone remodelling simulation. The analytical formula predicts when bone will remain around an intramedullary implant. The predictions of the formula are borne out in the numerical results. This leads to a physical interpretation of one of the two parameters in the remodelling rule used. The results also show some characteristics which are clinically relevant. This study extends earlier results due to Huiskes for internal remodelling around intramedullary implants by using a different, numerically stable remodelling algorithm based on optimization. The study also shows a direct practical application of the optimizing remodelling theory the authors have developed previously.

Strains and pressures in the brain are known to be influenced by rotation of the head in response to loading. This brain rotation is governed by the motion of the head, as permitted by the neck, due to loading conditions. In order to better understand the effect neck characteristics have on pressures and strains in the brain, a human head finite element model (HHFEM) was attached to two neck FEMs: a standard, well characterized Hybrid III Anthropometric Test Device neck FEM; and a high fidelity parametric probabilistic human FEM neck that has been hierarchically validated. The Hybrid III neck is well-established in automotive injury prevention studies, but is known to be much stiffer than in vivo human necks. The parametric FEM is based on CT scans and anatomic data, and the components of the model are validated against biomechanical tests at the component and system level. Both integrated head-neck models were loaded using pressure histories based on shock tube exposures. The shock tube loading applied to these head models were obtained using a computational fluid dynamics (CFD) model of the HHFEM surface in front of a 6 inch diameter shock tube. The calculated pressure-time histories were then applied to the head-neck models. The global head rotations, pressures, brain displacements, and brain strains of both head-neck models were compared for shock tube driver pressures from 517 to 862 kPa. The intracranial pressure response occurred in the first 1 to 5 msec, after blast impact, prior to a significant kinematic response, and was very similar between the two models. The global head rotations and the strains in the brain occurred at 20 to 100 msec after blast impact, and both were approximately two times higher in the model using the head parametric probabilistic neck FEM (H2PN), as compared to the model using the head Hybrid III neck FEM (H3N). It was also discovered that the H2PN exhibited an initial backward and small downward motion in the first 10 ms not seen in the H3N. The increased displacements and strains were the primary difference between the two combined models, indicating that neck constraints are a significant factor in the strains induced by blast loading to the head. Therefore neck constraints should be carefully controlled in studies of brain strain due to blast, but neck constraints are less important if pressure response is the only response parameter of primary interest.

In Burkina Faso, cultivation operations (tillage, sowing, weeding, fertilization, etc.) are mainly carried out by women and young people. Among them, sowing is a particular constraint that determines the success or failure of production. In this country, maize is grown by 78% of producers in the rainy season. In order to improve production, a simple seeder that can be made by local craftsmen was designed as part of the Appropriate Scale Mechanization Consortium (ASMC) project and evaluated with SR21 maize seed in the Koumbia region. It has been harnessed by two oxen. The equipment is evaluated on a plowed plot and on minimum tillage plot. Seed dimensions and distribution disc characteristics were measured. The characteristics of sowing, the traction force and the labor times were measured and compared with those of manual sowing. The results indicate that the sowing time is 3.6 to 3.8 h ha-1, i.e., 8 to 10 times faster than manual practice. The traction force is 22.6 kgf (226 N). That is available for oxen hitch even with one animal. Seed calibration can improve tool performance. The tool has great potential for increasing production if the other production inputs are assured.

Abstract : A finite element model of a representative 50th percentile male torso has been created by researchers at the Johns Hopkins University Applied Physics Laboratory. The components of this detailed Human Torso Finite Element Model (HTFEM) include the heart, lungs, liver, stomach, intestinal mass, kidneys as well as the thoracic skeletal structure system. The detailed components of the torso provide relevant internal geometries, material differences and boundary conditions to study the propagation of a blast pressure wave through the thoracic region. Injury due to blast has largely been predicted using the Bowen curves, which are based on experiments of various animal species exposed to air blast that provide a biological response to blast. LS-DYNA, a dynamic finite element modeling tool is used to simulate the complex system response of the HTFEM to an open air blast event. LS-DYNA's enhanced version of the CONWEP blast model will be used to load the HTFEM. Loading conditions representing the overpressure and positive phase duration as defined in existing injury curves adapted from Bowen's lethality model are applied to the HTFEM. These simulations will explore HTFEM response to peak overpressures in the range of 400-800 kPa and positive phase durations in the range of 2.0 to 4.5 ms. The temporal pressure plots show organ response for the various loading conditions. The HTFEM can be used as a tool used to examine the blast effects on the human torso and to aid in the design of personal protective equipment (PPE).

In the field of elastography, biological tissues are typically assumed to be purely linear elastic solids. However, several tissues including brain, cartilage and edematous soft tissues, have long been known to be poroelastic. The authors recently developed a method to estimate the local Poisson's ratio in linear elastic solids (see Ultrasound in Medicine and Biology, vol. 24, no. 8, p. 1183-99, 1998). In the current study the authors use the same method to measure the time-dependent effective Poisson's ratio in poroelastic materials. The resulting time-sequenced poroelastograms show the spatial distribution of the fluid within the solid at each time instant and give insight into the Poisson's ratio of the solid and its permeability to the fluid. Results were obtained from both finite-element simulations and experimental poroelastic phantoms.

A simple theoretical model for the role of strain energy density in the initial mineralization of soft tissues is presented and used to derive a limit of the allowable strain in tissue engineered biomaterials. The model incorporates the mechanical energy in calcified tissue due to time-varying loads into the more commonly used energetic arguments for mineralization. By using the Voight (equal-strain) and Reuss (equal-stress) composite material models to relate the volumetric density of calcified tissue to overall material modules, two models were developed to assess the effect of an imposed overall material strain on mineralization. A rate equation based on strain energy was used to model the kinetics of mineralization, and the stability of the rate equation was assessed, leading to a limit on overall material strain based on the specific energy for mineralization of soft tissues. The result depended on the stiffness of the material in series with the mineralizing tissue. Taking the stiffness of the material in series with the tissue as infinite lead to a prediction of critical strain for mineralization in the calcifying biological tissue which was the same on the Reuss and Voight models. The interaction of this theoretical model with biological factors and some clinical implications of the model are discussed.

Estimation of the mechanical properties of the cardiac muscle has been shown to play a crucial role in the detection of cardiovascular disease. Elastography was recently shown feasible on RF cardiac data in vivo. In this paper, the role of elastography in the detection of ischemia/infarct is explored with simulations and in vivo experiments. In finite-element simulations of a portion of the cardiac muscle containing an infarcted region, the cardiac cycle was simulated with successive compressive and tensile strains ranging between -30% and 20%. The incremental elastic modulus was also mapped uisng adaptive methods. We then demonstrated this technique utilizing envelope-detected sonographic data (Hewlett-Packard Sonos 5500) in a patient with a known myocardial infarction. In cine-loop and M-Mode elastograms from both normal and infarcted regions in simulations and experiments, the infarcted region was identifed by the up to one order of magnitude lower incremental axial displacements and strains, and higher modulus. Information on motion, deformation and mechanical property should constitute a unique tool for noninvasive cardiac diagnosis.

This literature review will confirm prior work in the use of locomotive airbag technologies for vehicle or pedestrian collision mitigation, and to focus planned activities and tasks for this research. The state of the art in relevant technologies has been summarized to assess the feasibility of this technology and identify critical model challenges for supporting impact simulations. The literature review did not reveal any currently deployed locomotive airbag solutions. In patent literature, external airbag technology has been described for mitigation of crashes between railcars and motor vehicles, but no meaningful analysis of feasibility has been discussed in detail in scientific or professional literature. Therefore, it appears that although crash mitigation technology using airbags in front of locomotives has been conceptualized, it has not yet been rigorously engineered or implemented.

There are well over 100,000 total hip replacements done per year in the U.S., and over 250,000 per year worldwide. Extending the service life of these implants is a high priority due to the large number of patients and the increasing use of joint replacement in younger patients. In an older population, total hip replacements usually outlast the patient, due to advanced age and the relatively low physical demands placed on the prosthesis. Younger, more active, and heavier patients often require revision of their total hip due to progressive implant loosening and the pain which results. Revised total hips do not last as long as the original, and a younger patient can sometimes require more than three revisions. In a standard total hip replacement, a steel prosthesis is fixed to the thigh bone using polymethyl methacrylate bone cement. The fatigue characteristics of the bone cement, and of the prosthesis-cement interface have been identified as the critical areas which initiate failure in this structure. In this study, the stresses within the cement and at the cement-metal interface have been studied using a large-scale linear finite element analysis. Also, to assess the effects of cement-prosthesis debonding, a 3D contact study was conducted which assessed the effect of several different areas of debonding between the prosthesis and the cement. The results of finite element analysis showed that the most likely sites for failure initiation are in the proximal antero-medial region and at the distal prosthesis tip. The loading situation which appeared to put the interface in the most danger for failure is that encountered in stair climbing. The stresses during normal walking did not seem as critical. Simulation of cement-metal debonding showed that a drastic increase in cement stresses occurred under both gait and stair climbing loads, and that stair climbing loads also produced much higher stresses with debonding. The effect of pores which often occur in bone cement was assessed using an analytical elasticity solution for a spherical void in an infinite medium, which allowed a calculation of the maximum tensile stress at the surface of a pore. These stresses were sufficient to initiate fractures near the distal tip of the implant in many cases, and near the proximal medial region of the implant in stair climbing. This study concurs to a remarkable degree with a study of well-functioning total hip replacements recovered at autopsy. The initiating failure events seen in the retrieved femoral components matched closely with the predicted areas of failure initiation. The conclusions of the study were that the cement-prosthesis interface should be strengthened, porosity in bone cement should be minimized, and that total hip patients should not use their prosthetic hip when climbing stairs.

The skull-brain complex is typically modeled as an integrated structure, similar to a fluid-filled shell. Under dynamic loads, the interaction of the skull and the underlying brain, cerebrospinal fluid, and other tissue produces the pressure and strain histories that are the basis for many theories meant to describe the genesis of traumatic brain injury. In addition, local bone strains are of interest for predicting skull fracture in blunt trauma. However, the role of skull flexure in the intracranial pressure response to blunt trauma is complex. Since the relative time scales for pressure and flexural wave transmission across the skull are not easily separated, it is difficult to separate out the relative roles of the mechanical components in this system. This study uses a finite element model of the head, which is validated for pressure transmission to the brain, to assess the influence of skull table flexural stiffness on pressure in the brain and on strain within the skull. In a Human Head Finite Element Model, the skull component was modified by attaching shell elements to the inner and outer surfaces of the existing solid elements that modeled the skull. The shell elements were given the properties of bone, and the existing solid elements were decreased so that the overall stiffness along the surface of the skull was unchanged, but the skull table bending stiffness increased by a factor of 2.4. Blunt impact loads were applied to the frontal bone centrally, using LS-Dyna. The intracranial pressure predictions and the strain predictions in the skull were compared for models with and without surface shell elements, showing that the pressures in the mid-anterior and mid-posterior of the brain were very similar, but the strains in the skull under the loads and adjacent to the loads were decreased 15% with stiffer flexural properties. Pressure equilibration to nearly hydrostatic distributions occurred, indicating that the important frequency components for typical impact loading are lower than frequencies based on pressure wave propagation across the skull. This indicates that skull flexure has a local effect on intracranial pressures but that the integrated effect of a dome-like structure under load is a significant part of load transfer in the skull in blunt trauma.

In this technical note a sufficiency condition is established for the stability of a strain-energy-based bone remodeling theory in the special case of a beam loaded by an axial force and a bending moment. In a previous report the same condition was shown to be a necessary condition for stability in the same situation. The remodeling scheme is one characterized by a remodeling stimulus equal to the strain energy density divided by the bulk or apparent density raised to an exponent m as well an elastic modulus proportional to bulk or apparent density raised to an exponent n. In order for a remodeling scheme to be stable for an elastic beam loaded by an axial force and a bending moment, it is established that the condition that m must be greater than n is not only necessary, but also sufficient.

Due to the relatively large number of exposures to IED blasts in Iraq and Afghanistan, the mechanical effects of blast waves impinging on the head have become an area of high interest. The ways in which the physical aspects of blast loading can cause injury are controversial in some respects but a general consensus is forming that much of the knowledge from closed head injury due to blunt trauma can be applied to injury mechanisms in blast loading [2]. In particular, sudden head rotations are known to be significant, as these are connected to high strains in brain tissue, in much the same way that rotations applied to a jar full of gelatin can induce deformations in the gelatin. High strains induced by sudden rotations of the head are known to be significant in developing injuries to the white matter of the brain [3, 4]. This study used a coupled fluid-structure finite element model to assess the effects of blast over pressure on translation and rotation of the head due to blast wave exposure. A finite element model of a 5th percentile Hybrid-III dummy was used which was supplied by Livermore Scientific Technlogy Corp. (LSTC) as part of a license for LS-Dyna. The ALE formulation for fluid-structure coupling was used [1]. The sudden increase in head velocity is significant, but the sudden increases in rotation rates are small compared to those for some impacts. The increases in velocity and rotation rate due to the passage of the blast wave on the head of the dummy occur over a few milliseconds, and rotation becomes significant only after the neck loads become active, 15 to 20 milliseconds after blast wave impingement. These results indicate that efforts to modify the rotational velocity of the head in the reaction to a blast wave should act within 15 to 20 milliseconds of the blast.

Flammable materials such as gasoline, ethanol, and diesel fuel are commonly transported in bulk via rail. In many cases, pockets of vapor can be generated inside the tank that can present a hazard if spilled during a collision or other catastrophic accident. Vapor conditions above the Lower Explosive Limit (LEL) if exposed to an external ignition source can result in an explosion or fire. Alternately, residual vapors within a tank present an explosion hazard if not properly vented or inerted prior to maintenance activities. This paper summarizes a generalized study of hazards associated with flammable liquids using computation fluid dynamics (CFD) to predict vapor conditions within a tank or following a spill. The analysis was verified in laboratory testing using scaled tank geometries. A demonstration case was developed using diesel fuel in a locomotive fuel tank. Typical road locomotives carry 3000–5000 gal of diesel fuel during normal operation. As the locomotive consumes fuel, large volumes are available for vapor generation within the tank. In a post-collision scenario, under ambient temperatures over the flash point of the fuel, the vapor that vents to the atmosphere presents a significant fire hazard. Further, flammable mists can be generated by the sprays that develop due to fuel leaks from a moving train. Studies of accident cases over a 10 year period indicated that a fire occurred in 80% of the accidents in which fuel was spilled. A CFD analysis was applied to the geometry associated with a locomotive fuel tank. The analysis models the two phase flow using the “volume of fluid” formalism in Fluent, and using a user defined diesel fuel evaporation algorithm. The tank and environmental parameters included fuel volume, fuel temperature, and air flow within the tank, and critical values of vapor content, temperature and velocity were plotted. The analysis predicted ignition of the external vapor cloud at temperatures relevant to a spill in a summer environment in the southwest, and propagation of the flame into the fuel tank. Laboratory testing confirmed the analysis: Once ignited, a flame propagated into the tank, causing an explosion and fire. The analysis methods developed can be applied to a variety of geometries and fluids, providing a basis for full scale testing. The overall intent of the analysis is to aid in the development of fire mitigation approaches for fuel and flammable material transport that would be practical for railroad use.

Noise and vibrations in locomotive cabs can significantly affect crew performance and cause long-term ailments, such as hearing loss, fatigue, and low back pain. Methods to reduce noise and vibrations have been implemented for the high frequency range but resulted in low frequency resonances. These resonances can exacerbate low frequency vibrations (&lt;0.5 Hz), which can cause motion sickness. In addition, a tonal noise exists in the 50 to 200 Hz frequency range, which is more annoying than broadband noise, and which is not addressed in current noise reduction methods based on A-weighted noise metrics. To reduce vibration, the innovative approach proposed here will consider isolating only the floor of the cab rather than the whole cab as was previously reported in the literature. The isolation is achieved using nonlinear springs and dampers that provide isolation at high frequencies while avoiding resonances at low frequencies. The smaller inertia of the floor, controls, and crew, as compared to the entire cab, makes the necessary components much less expensive. To reduce the tonal noise in the range 50 to 200 Hz, active noise control is used in the vicinity of the crew seats. Analyses have shown that this new approach is very promising, and demonstrations are planned for mockups of locomotive cabs.

Aims: The purpose of this study was to assess the effect of changes in peripheral attachment on stresses and displacements at the liner-shell interface. Methods: Three dimensional þnite element models were constructed of two acetabular cup designs for a liner with a 32 mm inner diameter, a liner thickness of 5 mm, and a shell thickness of 4 mm. An additional set of models was constructed with a 28 mm head diameter, corresponding to a liner thickness of 7 mm. 16 sequential quasistatic loading steps were used to describe the stance phase of a patientOs gait cycle. Results: Changes in the design had a larger insuence on the backside relative motion during the gait cycle than load magnitude. However, changes in the design had a smaller insuence on the backside contact stress, von Mises stress, or radial extrusion into screw holes. Reduction in head size from 32 to 28 mm diameter resulted in a slight decrease in screw hole extrusion. Conclusions: In this study, changes in the acetabular cup design, including screw hole placement and increased peripheral interlocking, were shown to decrease relative motion at the liner-shell interface, but the peak liner-shell contact stresses, backside von Mises stresses, and radial screw hole extrusion were less signiþcantly changed.

Aims: The purpose of this study was to assess the effect of gaps between the polyethylene liners and the metal acetabular shells used in two generations of acetabular component design. Methods: Finite element models were developed for two generations of acetabular component. The three variables assessed were: design (Mark I versus II); liner thickness (5, 8, and 11 mm); and gap size (0., 0.1, and 0.3 mm). 16 sequential quasistatic loading steps, coupled with sexion/extension of the femoral head, were used to describe the stance phase of a patientOs gait cycle. Results: Gaps of less than 0.1 mm between the acetabular liner and the supporting metal shell will close under loading typical for gait, but only for smaller Ðsized acetabular cups. A gap of 0.1 mm seems to be at the edge of the range where rim loading, versus dome loading, occurs. Gaps of 0.1 to 0.3 mm between a polyethylene acetabular liner and the metal shell can produce suid pumping of approximately 100 to 250 microliters in each gait cycle. Conclusions: The changes from the 1st to the 2nd generation of this acetabular component led to important advantages. Indeed, due to an improvement of the liner conformity and the locking mechanism, backside micromotion, suid volume displaced, liner stresses and liner-shell contact stresses were strongly diminished.

In Elastography, strain estimation has been shown to be far more reliable compared to the elastic properties obtained using reconstruction techniques. In this study, to make the method less sensitive to noise in the experimental data and inspired by the clinical practice of palpation (i.e., the use of sequential finger loading), we investigated the effect of using several different smaller quasi-static load cases (instead of a one-time load on the whole boundary), with the error indicator taken as the sum of the errors from each load case. This increased the ratio of measurements to the fitted parameters, which made the method less sensitive to random errors. To demonstrate this effect, we calculated displacements from a two-dimensional, quadrilateral, plane-strain, finite-element model of a 40-by-40 mm region containing a cylindrical inclusion (7 mm in diameter) three-times stiffer than the background. The ratio of nodal pressures was chosen to produce approximately 0.75% strain. Known amounts of random displacement errors were then added at a signal-to-noise ratio varying from 60 dB to 20 dB. Elastic modulus reconstructions using the noisy displacement results from a single, total-boundary, pressure load (as is typically applied in elastography) were compared to reconstructions using data from nine smaller-width loading cases, and the reconstructed modulus distributions were compared to the original model parameters. It was found that in the cases of 60 dB and 40 dB the multiple loading cases resulted in noise reduction in the modulus reconstruction by at least a twofold compared to the single-loading case, at the expense of a 'shadowing' effect (i.e., erroneous modulus estimates) underneath the inclusion that could be eliminated by using larger loading areas for the individual loading cases. Finally, at 20 dB both the large single-load and combined, smaller five-load cases failed to accurately reconstruct the modulus of the inclusion; depicting thus a fundamental limit on the reconstruction method.

Cette etude a eu pour objectif d’evaluer l’etat corporel, la force et la vitesse de traction de bœufs de trait afin d’evaluer leurs conditions d’utilisation et proposer des pistes d’amelioration de la traction animale. Pour ce faire, deux villages ont ete identifies dans trois provinces de la Region des Hauts-Bassins (Houet, Tuy et Kenedougou), soit au total 6 villages. Dans chaque village, 8 paires de bœufs de trait en situation de travail ont ete selectionnees aupres des producteurs prealablement inclus dans l’enquete de base du projet mecanisation agricole appropriee (ASMC-USAID), soit au total 96 bœufs de trait. Nos resultats revelent que 75 % des bœufs de la Region des Hauts-Bassins avait une force de traction pouvant realiser le labour et les autres operations culturales. Cependant, la vitesse de traction a ete inferieure a la norme et a varie autour de 0,64 ± 0,03 m/s. Quant a la note d’etat corporel (NEC), 71,9 % des bœufs de trait ont obtenu des valeurs comprises entre 3 et 4. Des efforts doivent etre entrepris a l’echelle de la Region pour que la totalite des bœufs de trait ait une bonne NEC et une force adequate pour le labour afin d’impacter positivement les productions agricoles.

Abstract Increasing attention has been given in recent years to using magnetic resonance imaging (MRI) to assess skeletal condition. This method of imaging is sensitive to the marrow/bone interface and is therefore attractive for characterizing trabecular bone, which is the bone tissue most seriously affected by osteoporosis or bone loss due to disuse or weightlessness. With proper processing and analysis, the information gained from MRI scans has the potential to provide measures of trabecular bone architecture rather than just bulk density or mass, but the extent to which this is true has not been definitively documented to date. Recent studies by Chung et al. (1993) and Jergas et al. (1995) have examined the relationships among mechanical properties and MRI signals, with encouraging results. More specifically, the effective transverse relaxation time T2* and its reciprocal (transverse relaxation rate) have been the main parameters used in previous studies because T2* is believed to be an indicator to some degree of trabecular architectural features. The present study addressed these same questions but a different protocol for imaging and specimen preparation has been used, and a larger number of specimens were tested.

This project was aimed at making predictions of bone mechanical properties from non-invasive DXA and MRI measurements. Given the bone mechanical properties, stress calculations can be made to compare normal bone stresses to the stresses developed in exercise countermeasures against bone loss during space flight. These calculations in turn will be used to assess whether mechanical factors can explain bone loss in space. In this study we assessed the use of T2(sup *) MRI imaging, DXA, and fractal dimensional analysis to predict strength and stiffness in cancellous bone.