development and application of a complex numerical bone model
A. Geltmeier, C. Bludszuweit-Philipp, K. Luckow
ASD Advanced Simulation & Design GmbH, Rostock, Germany
12th workshop on "the Finite Element Method in biomedical engineering, biomechanics and related fields"
Ulm, germany, july 20-22 2005
In this study a complex numerical bone model which includes the orientation-dependent elasticity and strength material properties as well as the remodelling behaviour of bone was developed and its practical application was shown.
The first part of the project was an extensive literature research and the arrangement of the numerous anisotropic elasticity and strength material properties of different human bones in a data base.
Within the second part, a mathematical algorithm of bone remodelling procedure was implemented into the numerical analysis, validated and its application was shown. From the great variation of simple to very complicated theories an algorithm was choosen which is suited for implementation in a commercial FE-software and industrial applications. The chosen theory describes the strain induced internal bone adaptation. The implementation of this algorithm into the numerical analysis was realised by user subroutines within the commercial FE-Software package Marc / Mentat 2003. The algorithm was validated with a proximal femoral bone model under different load cases. The calculated density distribution of a typical daily load case was compared to an x-ray of a healthy femoral bone. A very good correspondence could be found. The application of the implemented algorithm was shown for a femoral bone model with an implanted hip stem under different contact conditions.
Summarising this study a complex numerical bone model was developed and validated which allows the consideration of anisotropic material properties as well as strain-induced remodelling behaviour. It represents a further important numerical tool for more realistic numerical investigations of bone structures itself and the interaction of bone and implants.
biomechanical optimisation of hip prostheses using patient specific data
A. Geltmeier1, C. Bludszuweit1, K. Krüger1, Silvia Raschke2
1ASD Advanced Simulation & Design GmbH, Rostock, Germany
2 BCIT - British Columbia Institute of Technology, Burnaby, Canada
9th workshop on "the Finite Element Method in biomedical engineering, biomechanics and related fields"
Ulm, germany, july 18-19 2002
In this study, a finite element model of femoral bone and hip prosthesis was developed which includes the patient specific gait analysis data as a more "dynamic" load case than one leg stance and stair climbing. This model together with a data base enables the optimisation of biomechanical parameters of the prostheses stem regarding minimisation of relative motion and bone strains.
The FE-model of femoral bone with and without prosthesis was developed using CT-data of cadaveric bones, which were used in strain gauge experiments. The material data for spongious and cortical bone were taken from the literatur and defined in terms of grey steps of the CT-slices.
Three load cases were investigated: one leg stance, stair climbing and one gait cycle (40 steps quasistatic). The loading during gait cycle was taken from gait analyses of different persons carried out in a gait analysis laboratory in Canada (BCIT). For all load cases, the influence of the biomechanical parameter femoral offset on relative motion of the prosthesis stem and the principal strains on the bone surface was investigated.
It could be shown that biomechanical parameters like the femoral offset have a considerable influence on long term standing of hip prostheses. This FE-model with the complex load cases enables the variation and optimisation of different design parameters and the prediction of the expected biomechanical durability. Together with the available patient specific data (age, weight, sizes, gait analysis data ...), an extensive data base was built, which allows a better preparation of the implantation and higher long-term success.
The Application of Computational Tools: Virtual Evaluation of Valve Design
P.V.Lawford1, D.R.Hose1, C.Bludszuweit2, L.A.Keggen1
1University of Sheffield, UK, 2ASD Advanced Simulation & Design GmbH, Rostock, Germany
Sixth Annual Hilton Head Workshop "Prosthetic Heart Valves: Past, Present and Future"
Hilton Head Island, South Carolina, USA
March 6-10 2002
The application of computational tools to heart valve design has been explored for two decades. In principle, gross fluid performance indices, such as pressure drop or effective orifice area, are readily computable for candidate designs using standard computational fluid dynamics tools. The solution of transient flow systems is now routine, and physical flow characteristics are typically determined from direct solution of the transient Navier Stokes equations. The recent development of fluid-solid interaction software has enabled the opening and closing characteristics of valves, and the influence on the flow fields, to be investigated. In parallel with these developments there has been significant progress in the use of transient particle tracking to monitor shear history data for particles in the flow. Although many challenges remain in the physics domain, the software is now sufficiently functional and mature to address meaningful problems in heart valve design.
This paper will present a solid-fluid interaction environment based on the external coupling of CFX to ANSYS, and will demonstrate applications in the cardiovascular sphere, including a study of the opening and stability characteristics of a mechanical valve under a prescribed pulsatile wave form.
A major challenge in the application of these software tools is the building of the bridge from computable physical system characteristics to physiological processes such as haemolysis and thrombosis. The haemolytic potential of a candidate valve design might be assessed by integration of damage parameters over particle paths in the flow. Example computations based on cfd and mri data will be presented. The latter part of the blood coagulation cascade can be represented by the clotting of enzyme-activated milk. Results will be presented for in vitro tests studying the clotting of milk on test objects and on valve prostheses. The paper will illustrate the application of a computational model of thrombosis based on the changing of viscous fluid properties as a function of the local flow field. A simulation of the clotting process on a test object will be demonstrated and compared with in vitro data.
Towards the virtual artificial organ
Bludszuweit C., Rosenow S.-E., ASD Advanced Simulation & Design GmbH, Germany
CFX-Update, No. 15, Spring 1998
Artificial organ engineers are generally faced with complex and demanding design challenges. Devices must both fulfil organ-specific functions such as physical/chemical transport and exchange processes and still be bio-compatible. Blood in direct contact with a mechanical system cannot be altered beyond a physiologically repairable degree.
ASD Advanced Simulation & Design GmbH has developed a powerful method for designing and optimising such devices with its "Virtual Artificial Organ" concept. In this technique, prototypes are developed using modern simulation tools such as CFD. Functionality and haemocompatibility can be predicted in advance, thus reducing costs as well as time invested in intensive animal and in-vitro testing. As such, development time is shortened considerably.
ASD has applied this concept using CFX-TASCflow at organ-specific level with a centrifugal blood pump, an oxygenator and a haemodialyser, as these are the most significant organ assist and replacement devices for heart, lung and kidney.
Centrifugal blood pumps have a particularly high potential for causing mechanical blood damage. The numerical simulation performed therefore sought to maximise pump performance while minimising blood trauma. A flow analysis of the entire stator/rotor configuration in a centrifugal blood pump yielded 3D distributions of velocity, static pressure, turbulence parameters and molecular as well as Reynolds stresses. Particle streak lines with representative stress values and transit times were calculated to quantify the mechanical loading of blood elements on their passage through the pump and the most recent mechanical blood damage models were incorporated to help damage assessment. It was thus possible to study geometrical performance parameters and damage characteristics effectively.
Oxygenators and dialysers carry out mass exchange by using large artificial surface areas (hollow fibres) which are a potential source for bio-incompatibility. High mass transfer and transport in future generations must, hence, be achieved with a much reduced fibre area. ASD has numerically predicted exchange-unit 3D flow processes and optimised the results. Analysis of global flow characteristics such as blood velocity distribution within the entire fibre bundle has been combined with a break-down of highly complex local external fibre flow to enable optimisation of the design and operation of these devices.
A deeper understanding of physical/chemical processes and their complex interactions in artificial organs can be expected as the virtual organ concept is further developed.
Evaluation and optimisation of artificial organs by computational fluid dynamics
Bludszuweit C., ASD Advanced Simulation & Design GmbH, Germany
1997 ASME Fluids Engineering Division Summer Meeting FEDSM'97
June 22-26 1997
Artificial organs are required to satisfy high functionality and bio-compatibility demands for successful use. Efforts in the development of a modern generation of artificial organs are supported by the application of advanced Computational Fluid Dynamics (CFD) methods with the study presented.
Based on a detailed specification of design criteria and the incorporation of a novel model for mechanical blood damage prediction, a numerical design evaluation was performed for three characteristic artificial organs.
Flow conditions within a centrifugal blood pump, an oxygenator and a haemodialyzer were analysed for performance characteristics and blood trauma potential. Parameter variations were simulated and conclusions for latent design improvements drawn.
The results revealed that powerful simulation tools can be used to accompany the artificial-organ design optimisation process effectively.
Three-dimensional numerical prediction of stress loading of blood particles in a centrifugal pump
Bludszuweit C., Gaylor J.D.S.
Artificial Organs, 19(7):590-596, Blackwell Science Inc., Boston
© 1995 International Society for Artificial Organs
The successful use of centrifugal pumps as temporary cardiac assistance devices strongly depends on their degree of blood trauma. The mechanical stress loading experienced by cellular components on their passage through the pump is a major cause of blood trauma. Prediction of the mechanical stresses will assist optimization of pump design to minimize haemolyses and platelet activation. As a theoretical approach to this task, the determination of the complete three-dimensional (3D) flow field including all regions of high shear stresses is therefore required. A computational fluid dynamics (CFD) software package, Tascflow, was used to model flow within a commercially available pump, the Aries Medical Isoflow Pump. This pump was selected in order to demonstrate the ability of the of the CFD software to handle complex impeller geometries. A turbulence model was included, and the Newtonian as well as the Reynolds stress tensor calculated for each nodal point.
A novel aspect was the assignment of scalar stress values streak lines representing particle paths through the pump. Scalar stress values were obtained by formulating a theory that enabled the comparison of a three-dimensional state of stress with a uniaxial stress as applied in all mechanical blood damage tests. Stress loading time functions for fluid particles passing inlet, impeller and outlet domains were obtained. These showed that particles undergo complex, irregularly fluctuating stress loading. Future blood damage theories would have to consider an unsteady stress loading regime that realistically reflects the flow conditions occurring within the pump. Validation of the pump modelling was demonstrated with pressure head discrepancies predicted to within 15% of measured values.
Model for general mechanical blood damage prediction
Bludszuweit C., Gaylor J.D.S.
Artificial Organs, 19(7):583-589, Blackwell Science Inc., Boston
© 1995 International Society for Artificial Organs
Knowledge of the correlation between mechanical loading of formed blood elements and the amount of their destruction is important for the prediction of blood trauma in artificial circulatory devices as well as in natural circulation. A haemodynamic assessment and optimization of artificial organs to minimize trauma could be undertaken in the design phase given a comprehensive mechanical blood damage model.
A theory to determine blood trauma theoretically as a combination of a mechanical loading analysis and a phenomenological blood damage resistance hypothesis was presented. The arbitrary stress-time function of blood particles predicted by flow analysis were reduced to a set of simple time function for which the damage behaviour may, in principle, be obtained from mechanical blood damage tests.
A classification of those stress functions into damaging and non-damaging parts was followed by an overall trauma prediction taking account of cumulative effects by means of a damage accumulation hypothesis. Theoretical determination of blood destruction caused by mechanical stresses in a centrifugal pump is one possible application of the proposed theory.
The strategy of haemolysis prediction was demonstrated for the Aries Medical Isoflow Pump. Irregular stress-time loading functions of particles passing the pump domain obtained by three-dimensional numerical flow simulations were reduced and classified into harmonic components. To relate these functions to their haemolytic response can only be done in a qualitative manner since blood damage behaviour under transient stress loading has not been sufficiently investigated. Accurate prediction of blood trauma using the proposed theory will require detailed study of the influence of frequency and amplitude of harmonic stress loading on blood elements formed.
A theoretical approach to the prediction of haemolysis in centrifugal blood pumps
Bludszuweit C.
Ph.D. Thesis, University of Strathclyde, Glasgow, Scotland, U.K., 1994
The successful use of centrifugal pumps as temporary cardiac assistance devices strongly depends on the extent to which they damage blood. The development of a theoretical pump evaluation model was performed in this study to facilitate effective pump optimisation. The optimisation process sought to maximise flow performance and minimise blood trauma which is primarily caused by hydrodynamic stresses. A general mechanical blood damage theory was developed comprising a combination of information about mechanical blood loading with the knowledge of its resistance properties. In this theory arbitrary loading-time functions were reduced to simple loading functions for which the damage behaviour is known. A linear damage accumulation theory contributed towards determining partial damage and the correlation in the overall damage process.
The application of this novel blood damage prediction theory was demonstrated for haemolysis prediction in a centrifugal blood pump. Particle loading-time functions were determined via a 3-dimensional numerical flow analysis of the entire pump domain by means of assigning scalar stress values to particle streak lines. Scalar stress values were obtained by a theory comparing a six-component stress tensor with uniaxial stresses as applied in blood damage tests. It was shown that particles undergo a complex, irregularly fluctuating stress loading and that turbulent stresses and flow conditions in the outlet domain are the most critical factors. Haemolysis tests using a oscillating capillary tube arrangement were performed to investigate blood damage resistance properties under cyclic stress loading in hitherto unexplored amplitude and frequency ranges. A non-linear damage curve for the stress amplitude-cycle number was derived which indicated the existence of a red blood-cell endurance strength.
Detailed information about the mechanical loading of blood within a centrifugal pump was obtained for the first time and linked to its traumatic effect. It offers the possibility of effective, multi-parameter optimisation of blood pumps in the design phase.
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