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At least 721 records · Page 40Linked to original sources
PubMed2005

Numerical simulation techniques to study the structural response of the human chest following median sternotomy.

BACKGROUND: The optimal closure technique of median sternotomy remains controversial. The objective of this study was to analyze the structural response of the separated sternum using computer-based numerical discretization techniques, such as finite element methods. METHODS: Thoracic computer tomographic scans (2.5-mm slices) were segmented, analyzed by image processing techniques, and transferred into a three-dimensional finite element model. In a first approach a linear elastic material model was used; neglecting nonlinear and damage effects of the bones. The influence of muscles and tendons was disregarded. Nonlinear contact conditions were applied between the two sternal parts and between fixation wires and sternum. The structural response of this model was investigated under normal breathing and asymmetric leaning on one side of the chest. Displacement and stress response of the segmented sternum were compared regarding two different closure techniques (single loop, figure-of-eight). RESULTS: The obtained results revealed that for the normal breathing load case the single loop technique is capable of clamping the sternum sufficiently, assuming that the wires are prestressed. For asymmetric loading conditions, such as leaning on one side of the chest, the figure-of-eight loop can substantially reduce the relative longitudinal displacement between the two parts compared with the single loop. CONCLUSIONS: The application of numerical simulation techniques using complex computer models enabled the determination of structural behavior of the chest regarding the influence of different closure techniques. They allowed easy and fast modifications and therefore, in contrast to a real physical model, in-depth parameter studies.

Biomechanical Phenomena↗
PubMed2005

Rapid modulation of Ca2+ uptake in human jejunal enterocytes.

The active metabolite of D vitamin, 1,25(OH)2D3, has been suggested to promote acute uptake of calcium through the intestinal lining in cell lines and murine models. In this study, the effects of D vitamin on the cytoplasmic Ca2+ of single human jejunal enterocytes, obtained with LOC-I-GUT technique, was analyzed in vivo in a fluorometric system using fura-2 as the Ca2+-sensing probe. Vitamin-promoted acute Ca2+ influx exhibited dual kinetics, indicating initial release from intracellular Ca2+ pools and fast entry from the extracellular space. Furthermore, providing a chemical clamp of membrane potential close to 0 mV did not activate voltage-sensitive calcium channels in the cellular membrane, neither was the hormone-induced Ca2+ influx affected by verapamil. This advocates that voltage-operated channels like L-type Ca2+ channels do not participate in the process of Ca2+ uptake. In fact, the existence of calcium-release-activated-calcium channels (I(CRAC)) was implied by the findings that irreversible depletion of intracellular Ca2+ stores by thapsigargin promoted Ca2+ entry. In the thapsigargin-treated enterocytes, D vitamin lost its ability to promote calcium entry indicating an important role for intracellular store-operated Ca2+ stores in the acute effects of 1,25(OH)2D3.

Adult↗
PubMed2005

Does consciousness really collapse the wave function? A possible objective biophysical resolution of the measurement problem.

An analysis has been performed of the theories and postulates advanced by von Neumann, London and Bauer, and Wigner, concerning the role that consciousness might play in the collapse of the wave function, which has become known as the measurement problem. This reveals that an error may have been made by them in the area of biology and its interface with quantum mechanics when they called for the reduction of any superposition states in the brain through the mind or consciousness. Many years later Wigner changed his mind to reflect a simpler and more realistic objective position which appears to offer a way to resolve this issue. The argument is therefore made that the wave function of any superposed photon state or states is always objectively and stochastically changed within the complex architecture of the eye in a continuous linear process initially for most of the superposed photons, followed by a discontinuous nonlinear collapse process later for any remaining superposed photons, thereby guaranteeing that only final, measured information is presented to the brain, mind or consciousness. An experiment to be conducted in the near future may enable us to simultaneously resolve the measurement problem and also determine if the linear nature of quantum mechanics is violated by the perceptual process.

Animals↗
PubMed2006

Phantom reflexes: muscle contractions at a frequency not physically present in the input stimuli.

In the motor system, the periodic stimulation of one Ia-afferent input produces reflex muscle contractions at the input frequency. However, we observed that when two Ia monosynaptic reflex-afferent inputs are involved the periodic muscle contractions may occur at a frequency physically not present in the afferent inputs even when these inputs are sub-threshold. How can the muscles respond with such phantom reflex contractions at a frequency physically absent in the sub-threshold Ia-afferent input stimuli? Here we provide an explanation for this phenomenon in the cat spinal cord, that we termed "ghost motor response". We recorded monosynaptic reflexes in the L7 ventral root, intracellular potentials in the motoneurons, and the associated muscular contractions elicited by stimulation of the lateral and medial gastrocnemius nerves. By stimulating with periodic pulses of sub-threshold intensities and distinct frequencies of 2 and 3 Hz the lateral and medial gastrocnemius nerves, respectively, we observed monosynaptic responses and phantom reflex muscle contractions occurring at the fundamental frequency (1 Hz), which was absent in the input stimuli. Thus we observed a reflex ghost motor response at a frequency not physically present in the inputs. We additionally studied the inharmonic case for sub-threshold stimuli and observed muscular contractions occurring at much lower frequencies, which were also conspicuously absent in the inputs. This is the first experimental evidence of a phantom reflex response in the nervous system. The observed behavior was modeled by numerical simulations of a pool of neurons subjected to two different input pulses.

Animals↗
PubMed2004

The development of a collaborative medical modelling service: organisational and technical considerations.

We describe the steps, problems, pitfalls and modifications in the development of a collaborative medical modelling service for a general hospital in the United Kingdom. We emphasise the value of having as much control as possible in the hands of clinicians so that the maximum relevant information can be obtained at minimum cost. Three-dimensional imaging, modelling and planning are now essential parts of any reconstructive surgery unit and must be adapted to make them as user friendly as possible for clinicians.

Computer-Aided Design↗
PubMed2006

Specimen-specific beam models for fast and accurate prediction of human trabecular bone mechanical properties.

Direct assessment of bone competence in vivo is not possible, hence, it is inevitable to predict it using appropriate simulation techniques. Although accurate estimates of bone competence can be obtained from micro-finite element models (muFE), it is at the expense of large computer efforts. In this study, we investigated the application of structural idealizations to represent individual trabeculae by single elements. The objective was to implement and validate this technique. We scanned 42 human vertebral bone samples (10 mm height, 8 mm diameter) with micro-computed tomography using a 20 microm resolution. After scanning, direct mechanical testing was performed. Topological classification and dilation-based algorithms were used to identify individual rods and plates. Two FE models were created for each specimen. In the first one, each rod-like trabecula was modeled with one thickness-matched beam; each plate-like trabecula was modeled with several beams. From a simulated compression test, assuming one isotropic tissue modulus for all elements, the apparent stiffness was calculated. After reducing the voxel size to 40 microm, a second FE model was created using a standard voxel conversion technique. Again, one tissue modulus was assumed for all elements in all models, and a compression test was simulated. Bone volume fraction ranged from 3.7% to 19.5%; Young's moduli from 43 MPa to 649 MPa. Both models predicted measured apparent moduli equally well (R2 = 0.85), and were in excellent agreement with each other (R2 = 0.97). Tissue modulus was estimated at 9.0 GPa and 10.7 GPa for the beam FE and voxel FE models, respectively. On average, the beam models were solved in 219 s, reducing CPU usage up to 1150-fold as compared to 40 microm voxel FE models. Relative to 20 microm voxel models 10,000-fold reductions can be expected. The presented beam FE model is an abstraction of the intricate real trabecular structure using simple cylindrical beam elements. Nevertheless, it enabled an accurate prediction of global mechanical properties of microstructural bone. The strong reduction in CPU time provides the means to increase throughput, to analyze multiple loading configuration and to increase sample size, without increasing computational costs. With upcoming in vivo high-resolution imaging systems, this model has the potential to become a standard for mechanical characterization of bone.

Biomechanical Phenomena↗
PubMed2003

Effectiveness of a dynamic breast examination training model to improve clinical breast examination (CBE) skills.

Despite the potential utility of clinical breast examination (CBE), doctors' palpation skills are often inadequate and difficult to train. CBE sensitivity ranges from 39-59%, in part because current training does not effectively teach tactile skills. To address CBE training limitations, we developed a breast examination training model with 15 dynamically controlled lumps, set to desired hardness within underlying rib and muscle structures, in a silicone breast. In an experiment of 48 medical students, training with the dynamic model increased lump detection by 1.35 lumps compared to 0.60 lumps for a traditional breast model (P=0.008), reduced false positives by -0.70 lumps compared to +0.42 lumps (P=0.0277), and demonstrated skill transfer with a 1.17 lump detection improvement on the traditional device compared to only a 0.17 lump detection improvement by traditional device trainees on the dynamic device (P<0.001). Findings demonstrate the advantage of the dynamic model over conventional models in training CBE tactile skills.

Adult↗
PubMed2006

Knee joint mechanics under quadriceps--hamstrings muscle forces are influenced by tibial restraint.

BACKGROUND: To investigate the role of quadriceps and hamstrings muscle recruitments on knee joint mechanics, measurement studies constrain the tibial anterior-posterior translation at a point away from the joint. This generates a restraining force perpendicular to the tibia thus introducing an artefact shear force that likely alters joint mechanics and forces in cruciate ligaments. METHODS: A 3D nonlinear finite element model of the entire knee joint, including tibiofemoral and patellofemoral joints, was used to investigate joint mechanics in flexion (0 degrees -90 degrees ) under isolated and combined hamstrings and quadriceps activation. The effect of tibial restraint at two locations on results was studied and compared with the reference boundary condition of tibia constrained by pure moments. FINDINGS: Tibial restraint by a force rather than a pure moment substantially influences the joint response. For identical forces, hamstrings have much greater moment generating capacity at larger flexion angles while quadriceps are more effective at smaller angles. INTERPRETATION: Tibial constraint by a restraining force rather than a pure moment causes an artefact force on the joint that vary with muscle forces and restraining location. These artefact shear forces, especially when placed closer to the joint, considerably reduce forces in cruciate ligaments; in anterior cruciate ligament at near full extension and in posterior cruciate ligament at larger flexion angles. The beneficial effects of muscle co-contraction in reducing anterior cruciate ligament forces at near full extension and in posterior cruciate ligament forces at near 90 degrees , however, disappear as the restraint on the tibia approaches the joint. The artefact forces could distort results and their interpretations.

Biomechanical Phenomena↗
PubMed2006

Measuring knee joint laxity: a review of applicable models and the need for new approaches to minimize variability.

Knee joint laxity can result from soft tissue injury, such as a ligament tear, or from genetic factors such as joint hypermobility syndrome and various forms of Ehlers-Danlos Syndrome. The location of a subject's passive knee laxity along a continuous spectrum is dependent on the mechanical properties of the existing structures, and the increased motion that often follows joint injury. At a threshold along the spectrum, a patient will be at risk for joint instability and further injury to joint structures. Links between instability and laxity may be better understood if laxity can be reliably and accurately quantified. Current measures of laxity have not been compared to a 'gold standard' in all cases, and when they have, were found to overestimate the laxity values. This is attributed to soft tissue deformation. Consequently, a noninvasive measure of laxity with improved accuracy and repeatability would be useful clinically and in the research sector. In this review, current clinical measures of laxity are critiqued, criteria for a measure of laxity are identified, and three theoretical models of knee laxity are outlined. These include contact, lumped parameter, and finite element models, with emphasis on applicability, strengths, and limitations of each. The long term goal is to develop a model and method able to differentiate subjects along a spectrum of laxity, and understand the functional implications of altered joint integrity. This would allow careful scrutiny of clinical interventions aimed at improving joint health and provide a valuable research tool to study joint injury, healing, and degeneration.

Anterior Cruciate Ligament↗
PubMed2006

Model-based estimation of muscle forces exerted during movements.

Estimation of individual muscle forces during human movement can provide insight into neural control and tissue loading and can thus contribute to improved diagnosis and management of both neurological and orthopaedic conditions. Direct measurement of muscle forces is generally not feasible in a clinical setting, and non-invasive methods based on musculoskeletal modeling should therefore be considered. The current state of the art in clinical movement analysis is that resultant joint torques can be reliably estimated from motion data and external forces (inverse dynamic analysis). Static optimization methods to transform joint torques into estimates of individual muscle forces using musculoskeletal models, have been known for several decades. To date however, none of these methods have been successfully translated into clinical practice. The main obstacles are the lack of studies reporting successful validation of muscle force estimates, and the lack of user-friendly and efficient computer software. Recent advances in forward dynamics methods have opened up new opportunities. Forward dynamic optimization can be performed such that solutions are less dependent on measured kinematics and ground reaction forces, and are consistent with additional knowledge, such as the force-length-velocity-activation relationships of the muscles, and with observed electromyography signals during movement. We conclude that clinical applications of current research should be encouraged, supported by further development of computational tools and research into new algorithms for muscle force estimation and their validation.

Algorithms↗
PubMed2005

A computationally efficient method for accurately solving the EEG forward problem in a finely discretized head model.

OBJECTIVE: Solution of the forward problem using realistic head models is necessary for accurate EEG source analysis. Realistic models are usually derived from volumetric magnetic resonance images that provide a voxel resolution of about 1 mm3. Electrical models could, therefore contain, for a normal adult head, over 4 million elements. Solution of the forward problem using models of this magnitude has so far been impractical due to issues of computation time and memory. METHODS: A preconditioner is proposed for the conjugate-gradient method that enables the forward problem to be solved using head models of this magnitude. It is applied to the system matrix constructed from the head anatomy using finite differences. The preconditioner is not computed explicitly and so is very efficient in terms of memory utilization. RESULTS: Using a spherical head model discretized into over 4 million volumes, we have been able to obtain accurate forward solutions in about 60 min on a 1 GHz Pentium III. L2 accuracy of the solutions was better than 2%. CONCLUSIONS: Accurate solution of the forward problem in EEG in a finely discretized head model is practical in terms of computation time and memory. SIGNIFICANCE: The results represent an important step in head modeling for EEG source analysis.

Algorithms↗
PubMed2006

Efficiency differences in computational simulations of the total cavo-pulmonary circulation with and without compliant vessel walls.

The Fontan operation is a palliative surgical procedure performed on children born with congenital defects of the heart that have yielded only a single functioning ventricle. The total cavo-pulmonary connection (TCPC) is the most popular variant of the Fontan procedure. The objective of the study was to quantify and compare the efficiency of numerical models of the TCPC with rigid versus elastic vessel wall models. The pressure drop and power loss through both type TCPC models was measured. Significant differences in efficiencies exist between rigid versus elastic numerical models. We have shown incorporating elasticity into numerical models of the total cavo-pulmonary connection is important when determining circuit efficiencies.

Blood Flow Velocity↗
PubMed2006

FEM simulation of the eye structure with bio-heat analysis.

Computer simulation on medical sciences has gain increasing popularity as computational technology advances. Successful thermal modeling of the human eye will assist in enabling early detections of eye abnormalities such as inflammatory. However, validity of every computer simulated results must be benchmarked with experimental measurement and this can be a daunting task especially in biomedical fields where experimental data is not in abundance. This paper presents a 2D finite element (FE) human eye model developed to simulate its thermal steady state conditions based on the properties and parameters reported in the open literatures. The results are verified with experimental and computational results obtained by previous studies on human as well as animal eyes. Results show discrepancy of only 0.33% when compared to images from infrared (IR) screening and a difference of only 0.127% compared to another finite element model. The sensitivity analysis also provides good agreement with results by previous studies. This promising simulation allows new possibility in computational methods for eye health care.

Artificial Intelligence↗
PubMed2006

Viscoelastic studies of human subscapularis tendon: relaxation test and a Wiechert model.

Numerical techniques such as the finite element method employ the material constitutive laws for their analysis. With regards to finite element analysis involving viscoelastic solids, the Generalized Standard Linear Solid (Wiechert) model has been a popular choice among available constitutive laws. Although numerous models have been developed to specifically describe the viscoelastic behavior of tendons and ligaments, most of them have not been implemented in commercial finite element packages. This paper describes a stress relaxation test on the human subscapularis tendon, and then presents an approach for obtaining constitutive parameters of a Wiechert model for the human subscapularis tendon using experimental data from the aforementioned relaxation test. The approach is general and thus, can be applied to other tendons and ligaments, as well as any linear viscoelastic solid materials. The Wiechert model is required if finite element analysis using the commercial finite element package ANSYS is to be performed for a biomechanic structure composed of tendons and/or ligaments.

Algorithms↗
PubMed2004

The effect of inlet velocity profile on the bifurcation COPD airway flow.

The effect of inlet velocity profile on the flow features in obstructed airways is investigated in this study. In reality, the inlet velocity distributions on such models, which are extracted from medial branches of natural human lung, should be neither uniform, nor symmetric parabolic, but skewed-parabolic due to having been skewed by the upper carina ridges. Four different three-dimensional three-generation models based on the 23 generations model of Weibel have been considered, respectively. The fully three-dimensional incompressible laminar Navier-Stokes equations and continuity equation have been solved using CFD solver on unstructured tetrahedral meshes. To reduce the complexity of the simulations, only one Reynolds number of 900 was used in this calculation. Four types of inlet boundary conditions, namely uniform, parabolic, positive-skewed parabolic (skewed to the positive x-direction), and negative-skewed parabolic, were imposed on the obstructed airway models, which were considered to be obstructed at either the second generation or the third generation airways, respectively. The results show that the inlet velocity profile has significant influence on the flow patterns, mass distributions, and pressure drops in either the symmetric model, or the three obstructed models. The three generation airways may not be enough to study the bifurcation flow in chronic obstructive pulmonary disease (COPD) airways, and a four-generation or more airway model is necessary to get better predictive results.

Computer Simulation↗
PubMed2005

Understanding velocity of sound in trabecular bone via computer simulations.

Osteoporosis is a condition characterized by low bone mass and micro-architectural deterioration leading to non-traumatic fractures of the skeleton. It is a potentially debilitating condition especially for senior citizens. There is growing interest for quantitative ultrasound to measure bone mineral density. However, understanding of ultrasound-bone interaction is limited. Simulating ultrasound propagation of bone can help us better understand ultrasound-bone interaction and provide insight into bone architecture. In this study a mathematical model for the propagation of sound in bone is presented. Results demonstrate the suitability of the proposed modeling approach and the model's capability to reproduce conditions in the lab.

Animals↗
PubMed2006

Ocular surface temperature: a 3D FEM prediction using bioheat equation.

Computational and mathematical human eye models from previous studies which were constructed in two-dimensions (2D) did not give a precise representation of the actual human eye. This work is an extension from an earlier published work on the 2D model. In this paper, a 3D FEM model of the human eye is simulated for the steady state temperature distribution during normal condition and during electromagnetic (EM) wave radiation. Results show a discrepancy of 0.49% for a normal condition as opposed to 1.9% of a 2D model when compared to experimental results from open literatures. Investigations on the EM wave radiations found an average power absorption density of 15,151 and 22,145 Wm(-3) for the 750 and 1500 MHz radiation, respectively. A peak temperature of 38.18( composite function)C was predicted for the 750 MHz radiation while 41.19( composite function)C was computed for the 1500 MHz radiation. These temperatures are in reasonable agreement with the simulated results computed by another report in the past.

Body Temperature↗