Biomedical subjects

P Niederer

Publications and source records attributed to P Niederer.

At least 19 recordsLinked to original sources

PubMed2005

A finite element study relating to the rapid filling phase of the human ventricles.

During the rapid diastolic filling phase at rest, the ventricles of the human heart double approximately in volume. In order to investigate whether the ventricular filling pressures measured under physiological conditions can give rise to such an extensive augmentation in ventricular volumes, a finite element model of the human right and left ventricles has been developed, taking into account the nonlinear mechanical behavior and effective compressibility of the myocardial tissue. The results were compared with the filling phase of the human left ventricle as extrapolated from measurements documented in the literature. We arrived at the conclusion that the ventricular pressures measured during the rapid filling phase cannot be the sole cause of the rise of the observed ventricular volumes. We rather advocate the assumption that further dilating mechanisms might be part of ventricular activity thus heralding a multiple function of the ventricular muscle body. A further result indicates that under normal conditions the influence of the viscoelasticity of the tissue should not be disregarded in ventricular mechanics.

Biomechanical Phenomena↗

PubMed2005

Fast DRR generation for 2D/3D registration.

We present a simple and rapid method for generation of perspective digitally rendered radiographs (DRR) for 2D/3D registration based on splat rendering. Suppression of discretization artefacts by means of computation of Gaussian footprints--which is a considerable computational burden in classical splat rendering--is replaced by stochastic motion of either the voxels in the volume to be rendered, or by simulation of a X-ray tube focal spot of finite size. The result is a simple and fast perspective rendering algorithm using only a small subset of voxels. Our method generates slightly blurred DRRs suitable for registration purposes at framerates of approximately 10 Hz when rendering volume images with a size of 30 MB on a standard PC.

Algorithms↗

PubMed2005

Strain energy density as a rupture criterion for the kidney: impact tests on porcine organs, finite element simulation, and a baseline comparison between human and porcine tissues.

High-velocity (up to 25 m/s) impact tests were performed on pig kidneys to characterize failure behavior at deformation rates associated with traumatic injury. Cylindrical tissue samples (n = 45) and whole perfused organs (n = 34) were impacted using both falling weights and a high-velocity pneumatic projectile impactor. Impact energy was incrementally increased until visible rupture occurred. The strain energy density failure threshold fell between 25 and 60 kJ/m3 for excised porcine tissue samples, and between 15 and 30 kJ/m3 for whole, perfused organs. The relationship between localized failure in whole organ impacts and tissue level failure thresholds observed in cylindrical tissue samples was explored using a detailed finite element model of the human kidney. The model showed good correlation between experimentally observed injury patterns and predicted strain energy density distributions within the renal parenchyma. Finally, to facilitate interpretation of the porcine renal impact results with regard to human trauma, quasi-static compression test results of freshly excised human kidney cortex samples (n = 30) were compared against similar tests on pig kidneys. Human tissues failed at Lagrange strain levels similar to porcine tissue (63+/-6.3%), but at 52% lower Lagrange stress (116+/-28 kPa), and 35% lower strain energy density (17.1+/-4.4 kJ/m3). Thus conservative interpretation of porcine test results is recommended.

Animals↗

PubMed2005

Strain-rate dependent material properties of the porcine and human kidney capsule.

This study was performed to characterize the mechanical properties of the kidney capsular membrane at strain-rates associated with blunt abdominal trauma. Uniaxial quasi-static and dynamic tensile experiments were performed on fresh, unfrozen porcine and human renal capsules at deformation rates ranging from 0.0001 to 7 m/s (strain-rates of 0.005-250 s(-1)). Single stroke, dynamic tests were performed on samples of porcine renal capsule at strain-rates of 0.005 s(-1) (n = 33), 0.05 s(-1) (n = 17), 0.5 s(-1) (n = 38), 2 s(-1) (n = 10), 4 s(-1) (n = 10), 50 s(-1) (n = 21), 100 s(-1) (n = 18), 150 s(-1) (n = 17), 200 s(-1) (n = 10), and 250 s(-1) (n = 17). Due to limited availability of human tissues, only quasi-static tests were performed (0.005 s(-1), n = 25). Porcine renal capsule properties were found to match the material properties of human capsular tissue sufficiently well such that porcine tissue material can be used as a human test surrogate. The apparent elastic modulus and breaking stress of the porcine renal capsule were observed to increase significantly with increasing strain-rate (p < 0.01). Breaking strain was inversely related to strain-rate (p < 0.01). The effect of increasing strain-rate on material properties diminished appreciably at rates exceeding 150 s(-1). Empirically derived mathematical models of constitutive behavior were developed using a hyperelastic/viscoelastic Ogden formulation, as well as a Cowper-Symonds law material curve multiplication.

Animals↗

PubMed2005

[Supernormal sight and high-resolution retinal imaging: a review].

BACKGROUND: An analysis of the wavefront in human eyes has yielded new findings with respect to the cones and other elements of the retina. But the correction of higher-order wavefront aberrations in photorefractive processes is still associated with basic difficulties. MATERIALS AND METHODS: The world literature relating to wavefront corrections in experimental research and the correction of photorefractive aberrations in the human eye is surveyed. RESULTS AND CONCLUSIONS: The correction of higher-order aberrations has permitted a vast improvement in the resolution of the cone-pattern picture existing in vivo. As a consequence, new findings with regard to colour physiology have been forthcoming. On the other hand, the application of wavefront corrections in photorefractive procedures has been only partially successful.

Color Perception↗

PubMed2005

Some aspects of wave aberrations of the human eye and supervision: a review.

Supervision is defined by a visual acuity of 20/10 or 20/8 and may be attained by custom-correcting the aberrations of higher order of the human eye. Higher order aberrations are those aberrations which are left in the eye after having corrected lower order aberrations, i.e., defocus (myopia, hypermetropia) regular astigmatism, and which can be corrected by ordinary spectacle lenses or contact lenses. Higher order aberrations are found to a higher or lesser degree in normal or pathological human eyes and in eyes having undergone conventional corneal surgery. According to custom keratorefractive surgery limits, given by the neural visual apparatus and the receptor mosaic, supervision (i.e., 20/10 or even 20/8) may be attained. A number of dedicated sensors have been developed in recent years that are able to detect and measure aberrations of the wave front which is a sensitive procedure for the determination and surgical control of the optical quality of the eye. Not every custom keratorefractive procedure results in supervision, however. This is because not every "normal" eye is able to reach such limits because of its basic design (anatomy or function) and also because keratorefractive procedures neglect the plastic behaviour of the cornea. The plasticity of the central neural system may furthermore interact with corrected or non-corrected visual function.

Algorithms↗

PubMed2004

The forces generated within the musculature of the left ventricular wall.

OBJECTIVES: To test the hypothesis that two populations of myocardial fibres-fibres aligned parallel to the surfaces of the wall and an additional population of fibres that extend obliquely through the wall-when working in concert produce a dualistic, self stabilising arrangement. METHODS: Assessment of tensile forces in the walls of seven porcine hearts by using needle probes. Ventricular diameter was measured with microsonometry and the intracavitary pressure through a fluid filled catheter. Positive inotropism was induced by dopamine, and negative inotropism by thiopental. The preload was raised by volume load and lowered by withdrawal of blood. Afterload was increased by inflation of a balloon in the aortic root. The anatomical orientation of the fibres was established subsequently in histological sections. RESULTS: The forces in the fibres parallel to the surface decreased 20-35% during systolic shrinkage of the ventricle, during negative inotropism, and during ventricular unloading. They increased 10-30% on positive inotropic stimulation and with augmentation in preload and afterload. The forces in the oblique transmural fibres increased 8-65% during systole, on positive inotropic medication, with an increase in afterload and during ventricular shrinkage, and decreased 36% on negative inotropic medication. There was a delay of up to 147 ms in the drop in activity during relaxation in the oblique transmural fibres. CONCLUSION: Although the two populations of myocardial fibres are densely interwoven, it is possible to distinguish their functions with force probes. The delayed drop in force during relaxation in obliquely oriented fibres indicates that they are hindered in their shortening to an extent that parallels any increase in mural thickness. The transmural fibres, therefore, contribute to stiffening of the ventricular wall and hence to confining ventricular compliance.

Animals↗

PubMed2004

A finite element model for the simulation of hydrometra.

The behaviour of the human uterus under an internal (intracavital) pressure of 150 mm Hg (20 kPa) was modelled. The application of such an intracavital or intrauterine pressure corresponds to the procedure which is performed at the beginning of hysteroscopy (hydrometra). Homogenous, isotropic material laws were implemented in a three dimensional, finite element model. The volume of the distended uterine cavity was calculated with different parameters obtained from in vivo aspiration experiments on human uteri as well as from ex vivo tensile tests on rabbit uteri for comparison purposes. The calculated results were in general agreement with in vivo measurements of hydrometra performed at the University Hospital of Zurich.

Animals↗

PubMed2003

A new subdural probe for combined intracranial pressure (ICP) and cerebral blood flow (CBF) monitoring.

We report the development of a new subdural probe for combined intracranial pressure (ICP) and cerebral blood flow (CBF) monitoring with near infrared spectroscopy (NIRS) and indocyanine green (ICG) dye dilution. For NIRS a conventional subdural ICP monitoring probe was supplied with two fiber bundles and 90-degree prisms. Injections of 25 mg ICG were performed. Regional values for the mean transit time of ICG (rmtt(ICG)), cerebral blood flow (rCBF) and cerebral blood volume (rCBV) were calculated. With prototypes of the probe in two patients with intracerebral haemorrhage 18 comparative measurements obtained simultaneously with conventional NIRS (optodes placed on the skin) and the subdural NIRS probe were performed. The new subdural NIRS probe allows combined monitoring of ICP and cerebral hemodynamics in the brain directly, without the influence of extracerebral tissue.

Blood Flow Velocity↗

PubMed2003

A finite element analysis for the prediction of load-induced fluid flow and mechanochemical transduction in bone.

Interstitial fluid flow through the lacunocanalicular cavities of mechanically loaded bone provides the biophysical basis for a number of postulates regarding mechanotransduction in bone. Recently, the existence of load-induced fluid flow and its influence on molecular transport through bone has been confirmed using tracer methods to visualize fluid flow induced by in vivo four-point-bending of rat tibiae. In this paper, we present a theoretical two-stage approach for the calculation of load-induced flow fields and for the evaluation of their influence on molecular transport in bone loaded in four-point bending, analogous to the aforementioned experimental model. In the first stage, the fluid velocities are calculated using a three-dimensional, poroelastic finite element model. In the second stage, mass transport analysis, this calculated fluid flow serves as a forced convection flow and its contribution to the total transport potential is determined. Based on this combined approach, the overall tracer concentration in the loaded bone is significantly higher than that in the unloaded bone. Furthermore, augmentation of mass transport through convective flow is more pronounced in the tension band of the tissue, as compared to the compression band. In general, augmentation of tracer concentration via convective mechanisms is most pronounced in areas corresponding to lowest fluid velocities, which is indicative of fluid flow direction and areas of increased "dwell time" or accumulation during the loading cycle. This theoretical model, in combination with the corresponding experimental model, provides unique insight into the role of mechanical loads in modulating local flow distributions and concentration gradients within bone tissue.

Animals↗

PubMed2003

Pressure aberrations inside the spinal canal during rear-end impact.

Minor soft tissue injuries of the cervical spine increasingly pose problems in public health. Such injuries are conveyed particularly often in rear-end automobile collisions at low impact speeds and it has been established that they may be associated with long-term impairment. As a possible cause for this type of injury it has been hypothesized that pressure pulses induced in cervical fluid compartments during the impact could damage the membrane of spinal nerve cells. To date, animal as well as cadaver experiments performed support this hypothesis. A theoretical analysis has been undertaken in order to investigate the pressure and flow pulse emerging in a cervical fluid compartment under conditions representing rear-end impacts with a Dv of 15 km/h. Using the finite element (FE) method, a three-dimensional model of the cervical spine was developed. The model consists of eight vertebrae (C1-T1), the intervertebral discs, the intervertebral joints, all the major ligaments, most of the neck muscles and the head. Additionally, a typical venous blood vessel was included. To determine the pressure behaviour inside the blood vessel, fluid-structure interaction was taken into account. For the time interval including the development of the S-shape, the pressure pulses were calculated and found to be in qualitative agreement with the reported measurements. The shear stresses acting on the vessel wall can be determined from the associated flow pulses. An extrapolation of the results into the interstitial space where nerve cells are located at this stage does not allow assessment of whether a damage threshold may be reached.

Accidents, Traffic↗

PubMed2003

Comparing a simplified FEM approach with the mass-spring model for surgery simulation.

Virtual reality based surgical simulators offer a very elegant approach to enhancing traditional training in endoscopic surgery. In this context a realistic soft tissue model is of central importance. The most accurate procedures for modeling elastic deformations of tissue use the Finite Element Method (FEM) to solve the governing mechanical equations. An alternative are mass-spring models which are a crude approximation of the real physical behavior. The main reason given when using the mass-spring approach is the computational complexity of FEM. In this study we show that an optimized linear FEM model requires computation time similar to the mass-spring approach, while giving better results.

Computer Simulation↗

PubMed2002

New methods for monitoring cerebral oxygenation and hemodynamics in patients with subarachnoid hemorrhage.

Radiographic cerebral vasospasm (CVS) after subarachnoid hemorrhage (SAH) do not reflect cerebral hemodynamics and oxygenation and may occur in the absence of clinical deficit and vice-versa. This report is to describe preliminary findings in further development of a non invasive method to estimate regional cerebral oxygenation and perfusion. Measurements were performed with a technique combining near infrared spectroscopy (NIRS) and indocyaningreen (ICG) dye dilution. Successful data analysis has been performed based on the decomposition in pulsatile and non-pulsatile components of NIRS absorption data collected before and during the passage of ICG through the vascular bed under the NIRS-detector. First measurements in patients with CVS suggest that the technique could become a powerful tool in the detection and treatment of CVS. This non invasive technique can be done at the bedside, it seems to be safe, easy to perform and less time-consuming compared to conventional techniques. The influence of extracerebral bone and surface tissue on cerebral NIRS signal has not been clarified yet. Therefore a new subdural NIRS probe has been developed, which gives the opportunity to measure directly the concentration of the chromophores in the brain without the influence of extracerebral contamination. In future comparative measurements with conventional NIRS probes on the scalp will allow to quantify and eliminate extracerebral contamination from the NIRS signal.

Adult↗

PubMed2001

Investigation of the morphology of the lacunocanalicular system of cortical bone using atomic force microscopy.

Mechanical loading has been implicated as a powerful driving mechanism for interstitial fluid flow through bone. However, little information is available with regard to the morphology of bone fluid spaces, e.g., the canalicular wall, which would be expected to dictate the type of flow regime developing in the lacunocanalicular system under mechanical loads. The purpose of this study was to examine the fine structure of the lacunocanalicular system in cortical bone using atomic force microscopy (AFM), resin casting methods, and selective etching of the specimen surface. A resin-cast replica of the canalicular wall was produced and surface morphology and dimensions were observed using AFM in tapping mode. Material contrast was obtained using surface potential measurements. A striped pattern perpendicular to the canaliculus long axis with a periodicity of 125 nm dominated the structure of the canalicular wall; it is likely that this was caused by the imprint of collagen fibrils arranged in parallel, lining the canaliculus wall. The largest dimension measured for canalicular diameter was on the order of 500 nm. The regular dips and ridges caused by the collagen that lines the wall are a source of roughness which may influence shear stresses imparted by the fluid on the cell surface as well as mixing of solutes within the lacunocanalicular system. In addition, the lacunocanalicular wall lining is likely to affect physicochemical interactions between the fluid and bone matrix. This has important implications for modeling and understanding the microfluid mechanics and rheology of the fluid-filled lacunocanalicular network.

Animals↗

PubMed2000

A finite difference model of load-induced fluid displacements within bone under mechanical loading.

Load-induced fluid flow in the lacunocanalicular network, induced by the mechanical loading of bone, is believed to play an important role in bone modelling, remodelling and adaptation processes. There are strong indications that this fluid flow is responsible for the mechanotransduction from external mechanical loads to the cells responsible for bone apposition or removal. Since direct flow measurements (especially in compact bone, in vivo and in situ) are not yet possible, theoretical modelling offers an alternative approach to determine the fluid flow velocities, displacements and effects of interstitial fluid flow. In this model, the fluid displacements in a middiaphyseal slab of a rat tibia under a cyclic four-point-bending load were calculated by applying Biot's theory of poroelasticity. The resulting differential equations were solved numerically for the fluid displacement vectors using the finite difference method. Thereby, the cross section located in the middle between the two inner points of force application was chosen for examination, such that the problem, although formulated in three dimensions, reduced itself to an essentially planar form. The maximal fluid displacements for the vector components in the cross sectional plane were found in the proximity of the neutral axis of bending. The direction of the displacement vectors was from the lateral aspect, which was in compression in the examined loading situation, towards the medial aspect in tension. In a parameter study it was found that the fluid displacement pattern and the distribution of fluid displacements remained constant for all the examined parameters, while the magnitude was influenced by the model parameters Young's modulus, Poisson's ratio and porosity. This study represents a further step in the examination of load-induced fluid displacements in loaded bone using theoretical models, aiming to understand the relationship between mechanical loading and bone modelling, remodelling and functional adaptation.

Adaptation, Physiological↗

PubMed2000

[Protein-condensation diseases--molecular basis of cataract formation].

There are reasons to classify a number of apparently disparate diseases as "condensation" (or molecular aggregation) diseases. Examples of such condensation diseases include the late phase of diabetes mellitus, Alzheimer's disease and others. With an expanding knowledge, the list of these diseases is likely to increase. We shall describe the underlying common mechanisms, the aim being to find anticataractogenic drugs based on this insight. The common, most important denominator of various clinically differing condensation diseases derives from the interaction of the macromolecules which is in part attractive and in part repulsive. Aggregation resp. clumping of the macromolecules of the crystalline lens, the reasons for light scattering, may be prevented by introducing a number of molecules of various designs into the original macromolecular complex which reduce the tendency of aggregation. Cataract inhibitors of this category may be regarded as effective if they are able to increase the time constant of the normal aging process (i.e. the increment of scatter) by about 20%.

Age Factors↗

PubMed2000

In vivo demonstration of load-induced fluid flow in the rat tibia and its potential implications for processes associated with functional adaptation.

Load-induced extravascular fluid flow has been postulated to play a role in mechanotransduction of physiological loads at the cellular level. Furthermore, the displaced fluid serves as a carrier for metabolites, nutrients, mineral precursors and osteotropic agents important for cellular activity. We hypothesise that load-induced fluid flow enhances the transport of these key substances, thus helping to regulate cellular activity associated with processes of functional adaptation and remodelling. To test this hypothesis, molecular tracer methods developed previously by our group were applied in vivo to observe and quantify the effects of load-induced fluid flow under four-point-bending loads. Preterminal tracer transport studies were carried out on 24 skeletally mature Sprague Dawley rats. Mechanical loading enhanced the transport of both small- and larger-molecular-mass tracers within the bony tissue of the tibial mid-diaphysis. Mechanical loading showed a highly significant effect on the number of periosteocytic spaces exhibiting tracer within the cross section of each bone. For all loading rates studied, the concentration of Procion Red tracer was consistently higher in the tibia subjected to pure bending loads than in the unloaded, contralateral tibia. Furthermore, the enhancement of transport was highly site-specific. In bones subjected to pure bending loads, a greater number of periosteocytic spaces exhibited the presence of tracer in the tension band of the cross section than in the compression band; this may reflect the higher strains induced in the tension band compared with the compression band within the mid-diaphysis of the rat tibia. Regardless of loading mode, the mean difference between the loaded side and the unloaded contralateral control side decreased with increasing loading frequency. Whether this reflects the length of exposure to the tracer or specific frequency effects cannot be determined by this set of experiments. These in vivo experimental results corroborate those of previous ex vivo and in vitro studies. Strain-related differences in tracer distribution provide support for the hypothesis that load-induced fluid flow plays a regulatory role in processes associated with functional adaptation.

Adaptation, Physiological↗

PubMed2000

Mechanical modeling of soft biological tissues for application in virtual reality based laparoscopy simulators.

For application in a Virtual Reality (VR) based laparoscopic surgery simulator, computationally efficient algorithms for the description of the mechanical behavior of soft tissue have been developed. The explicit Finite Element Method has turned out to be a robust method for this purpose provided that absolute strain formulations are applied. Furthermore, a VR model of a uterus and its adnexe has been generated and simulation results are presented.

Adnexa Uteri↗