40 research outputs found
A quantitative analysis of the structure of human sternum
No full textAn extensive study of the human sternum has been carried out to obtain estimates of the omnidirectional path-length distributions and structural parameters for trabeculation and marrow spaces. Data for sternum samples have been collected, using an object plane scanning microscope. These data have been used to produce the omnidirectional path-length distributions and values of structural parameters for the whole sternum. For a typical adult man the mean trabecular and marrow space path lengths are 224 and 1364 µm, respectively. The percentage bone volume is 13.8 and the surface to volume ratio is 190 cm. Data on the structural variations within the whole sternum are presented. They show a percentage difference in bone volume between the manubrium and the body of sternum of about 36%
Multi-physics computational models of articular cartilage for estimation of its mechanical and physical properties
No full textRecent advances in the realm of computational modeling of complex multiphysics phenomena in articular cartilage enabled efficient and precise determination of articular cartilage properties. However, still accurate quantification of complicated indentation and diffusion processes tying closely with the inhomogeneity of articular cartilage remains challenging. In the present thesis accurate approaches are proposed to capture the mechanical and physical behavior of articular cartilage as faithfully as possible. Finite element models (FE-models) capable of detecting contact between indenter and cartilage surface are developed and applied to spherical indentation process. To predict mechanical and physical properties of cartilage artificial neural networks (ANN) were used and to guarantee the efficacy of the generated ANN they were trained using simulated noisy force-time data. The combination of FE-model and ANN trained with noisy data allowed obtaining cartilage properties robustly. FE-models taking the inhomogeneity of articular cartilage into account were developed and validated and applied to capture neutral (biphasicsolute model) and charged (multiphasic model) solute transfer across articular cartilage in a finite bath experimental setup. Those models could capture the behavior of solute diffusion across cartilage and provide diffusivities and fixed charge densities (FCD) of different cartilage zones. An algorithm consisting of inverse and forward ANNs was developed to obtain the diffusivities of cartilage layers which eliminates the need for computational expertise. The final goal of this algorithm is to introduce a methodology by which properties of cartilage can be determined without any need for computational expertise, which provides a promising opportunity to meet the needs for clinics when it comes to assess the healthiness of articular cartilage during osteoarthritis progression. Effects of bath osmolarity, concentration and charge of solute were investigated using a combination of micro-CT experiments and FE-models. The results suggested that solute charge unlike the osmoalrity and solute concentration had a profound effect on solute diffusion. Porosity and thickness of subchondral plate were identified as two primary factors affecting the diffusion of neutral solutes across subchondral plate. Using a developed multi-zone biphasic-solute model allowed obtaining the diffusivities of cartilage layers as well as subchondral plate. Using a multi-zone biphasic-solute model, we found that overlying bath size, bath stirring and thickness of the formed stagnant layer can substantially influence the diffusion across cartilage. This provides an opportunity to optimally design diffusion experiments
A Multi-Scale Approach to Implications of the Preferred Vertebral Trabecular Orientation on Spine Biomechanics
No full textKnowledge of the influence of loading directions on trabecular bone remodeling in spine is of significant value in understanding the development of spine deformities and vertebral bone quality across different scales. Information on the constitution of a preferred trabecular orientation and mechanical properties of trabecular bone are important indicators in this respect. The current thesis aimed at exploring these aspects across multiple length scales in the spine. The thesis is divided in two parts. The influence of loadings less dominant than compression, i.e. shear, on the constitution of a preferred trabecular orientation in the spine on the macro-tissue level (>10 mm) was investigated in the first part (Part I). This influence was related to mechanical characteristics of trabecular structures on the micro-tissue scale (1-10 mm) in the second part (Part II). In Part I, primary trabecular orientations (PTOsmacro) near the superior and inferior vertebral endplates of L1 and L5 of 6 human spine cadavers were determined on the macro level using micro computed tomography imaging (voxel size = 120 m3), by calculating the dominant fabric principal vector. Their relative deviations to the axial compression vectors in the spines, quantified by the normals to the endplate (NEs), were determined afterwards. The average deviation between the PTOmacro and NEs was 6.24⁰ (±4.34⁰). The PTOsmacro did not show a preference towards the anterior or posterior direction relative to the NE. From the deviations, it was concluded that trabecular bone in the spine predominantly adapts to compression loads. However, secondary loading directions, such as shear, are of additional influence. In Part II, 13 small cubes (6.0x6.0 mm) from the volumes of interest in Part I were analysed on the micro level with regard to elasticity. Components, component ratios and primary elastic orientations (PEOmicro) of elasticity tensors, computed by the simulation of mechanical tests in finite element (FE) models, were calculated. PTOs of the cubes (PTOsmicro) were compared to the PEOsmicro and related to the PTOsmacro and NEs (Part I) qualitatively. Elasticity tensor components were within a reasonable range (approximately 1-250 MPa, excluding outliers) and no material symmetry was found, i.e. the structures were mechanically anisotropic. PTOsmicro deviated 13.90⁰ (±8.04⁰) with respect to the PEOsmicro on average. 10 out of 13 PEOsmicro had similar anterior or posterior tendencies as the PTOsmacro with respect to the NEs. 11 out of 13 PTOsmicro had similar anterior or posterior tendencies as PTOsmacro with respect to the NEs. Elastic properties of typical trabecular structures in the vertebral bodies were successfully determined. Due to a relatively low resolution, PEOsmicro deviated strongly with the PTOsmicro. Such deviations could function as indicators for bone quality in skeletal disease diagnostics using low resolution imaging. PTOsmicro and PEOsmicro agreed relatively well to the PTOsmacro on the macro-tissue level, in terms of anteriorly or posterior tendencies relative to axial loading in the spine. This outcome shows promise for multi-scalar biomechanical analysis of trabecular bone.A Multi-Scale Approach to Implications of the Preferred Vertebral Trabecular Orientation on Spine BiomechanicsBiomedical Engineering | Tissue Biomechanics and Implant
The development of a Statistical Shape- and Finite Element-Model of the human pelvis to analyse the performance of current acetabular implants and validate the concept of a new deformable design
No full textBackground: Patients who suffer from acetabular bone defects are often subjected to a decreased mobility and a loss of independence. To treat these bone defects, standard hemispherical acetabular implants offer in most cases the solution. However, when these defects are large, a custom triflange acetabular implant is needed. Although this type of implant performs well in general, it also has some drawbacks. These include higher costs and a longer design- and development cycle, since every implant is custom-made to the patient. In addition, it is expected that the triflange implant causes more stress-shielding (loss in bone-mineral-density (BMD) due to insufficient loads) than the standard implant. However, the evidence for this last statement is quite weak, since studies on both cups use different parameters,materials and environments. Objective: The objective of this study is to determine if the custom triflange cup causes indeed more stress-shielding than the standard hemispherical cup. Furthermore, the goal is to find a solution for this and the other above-mentioned drawbacks of the custom triflange cup. Therefore, the goal is to study the potential of a deformable acetabular cup, which in theory should solve these issues of the triflange cup. Methods: First a statistical shape model (SSM) of the human pelvis was developed to find patterns in defects of the pelvis, which could eliminate the need for customization. Next, a Finite Element (FE) Model with a bone-remodelling algorithm was developed to determine the difference in stress-shielding between the standard and the triflange cup. Furthermore, a FE model of a deformable implant is developed which is pressed into multiple defected pelvises. Finally, a machine-algoirthm is trained to predict the optimal deformable cup parameters, based on the type of defects. Results: Several modes of deformation were found with the SSM, which were utilised in creating damaged pelvises for the Finite Element Model. Furthermore, it was found that the custom triflange cup decreases the BMD of the pelvis by 28.6% compared to pelvis without cup. With a deformable implant, this decrease can be reduced to 7.1%, which is in the similair range as the standard hemispherical cup. Finally, it was found that the machine-learning algorithm can successfully predict the optimal cup parameters, based on the type and size of defects.ProsperosMechanical Engineerin
The effects of cutouts on output, mean energy and percentage depth dose of 12 and 14 MeV electrons
No full textOptimization of a stiffness-graded Fracture Fixation plate
No full textIt has been observed that fractured bones which are stabilized with a titanium fracture fixation plate, once healed, often refracture at an edge of the plate. This is believed to be caused by stress concentrations in the bone that take place at the location of the edges of the plate. Various implant parameters are known to affect these concentrations of stress, but we further hypothesized that the material of the plate has the most significant influence in reducing these concentrations, here referred to as ‘peak stresses’. Moreover, it was reasoned, based on relevant literature, that minimizing the peak stresses was not the only criteria that should be considered for the design of an optimal implant, the effect of the Interfragmentary strain on the healing outcome in the early stages after implantation is also crucial for the success of the implant. Otherwise the bone will never heal in the first place.Thus, by assuming that different regions of the plate have a different influence in the peak stresses, it was suggested that a stiffness graded plate could minimize the peak stresses while still allowing for an acceptable interfragmentary strain at the early stages of healing, through an optimization. In order to prove or disprove all the hypotheses mentioned above, a Finite Element model of a fracture fixation construct was developed. For the selection of many of the modelling assumptions, a literature study was carried out. For the selection of the contact properties to be used, a graphical comparison study was done. For choosing an appropriate mesh, a mesh study was implemented. Using this Finite Element model, it was possible firstly to show that while the material properties of the plate do in fact appear to be very influential in reducing the peak stresses, the thickness seems to be even more influential. For the latter a parametric study was carried out using the Taguchi method. Secondly, it was shown that different regions of the fracture fixation plate do have a different influence in the peak stresses of the bone. The outer most sections of the plate seem to always be the most influential. Lastly, optimisations were carried out in three different ways and although they all reduced the peak stresses, the method thought to be the simplest, yielded the most useful results. This consisted of dividing the plate only into three sections and assigning one material to the outer sections and another to the inner section.Mechanical Engineering | BioMechanical Desig
Determine the Dose Distribution Using Ultrasound Parameters in MAGIC-f Polymer Gels
No full textIn this study, using methacrylic and ascorbic acid in gelatin initiated by copper (MAGIC-f) polymer gel after megavoltage energy exposure, the sensitivity of the ultrasound velocity and attenuation coefficient dose-dependent parameters was evaluated. The MAGIC-f polymer gel was irradiated under 1.25 MeV cobalt-60, ranging from 0 to 60 Gy in 2-Gy steps, and received dose uniformity and accuracy of ±2%. After calibration of the ultrasonic systems with a frequency of 500 kHz, the parameters of ultrasound velocity and attenuation coefficient of the irradiated gel samples were measured. According to the dose–response curve, the ability of ultrasonic parameters was evaluated in dose rate readings. Based on a 4-order polynomial curve, fitted on the dose–response parameters of ultrasound velocity and attenuation coefficient and observed at 24 hours after irradiation, ultrasonic parameters had more sensitivity. The sensitivity of the dose–velocity and dose-attenuation coefficient curves was observed as 50 m/s/Gy and 0.06 dB/MHz/Gy over the linear range of 4 to 44 Gy, respectively. The ultrasonic parameters at 5°C, 15°C, and 25°C on the gel dosimeter after 0 to 60 Gy irradiation showed that readings at 25°C have higher sensitivity compared to 15°C and 5°C. Maximum sensitivity time and temperature readings of the MAGIC-f ultrasonic parameters were concluded 24 hours after irradiation and at a temperature of 25°C
Production and dosimetry of simultaneous therapeutic photons and electrons beam by linear accelerator: A Monte Carlo study
No full textEnhancing the effect of 4 MeV electron beam using gold nanoparticles in breast cancer cells
No full textElectron Beam Dosimetry in Heterogeneous Phantoms Using a MAGIC Normoxic Polymer Gel
No full textIntroduction: Nowadays radiosensitive polymer gels are used as a reliable dosimetry tool for verification of 3D dose distributions. Special characteristics of these dosimeters have made them useful for verification of complex dose distributions in clinical situations. The aim of this work was to evaluate the capability of a normoxic polymer gel to determine electron dose distributions in different slab phantoms in presence of small heterogeneities. Materials and Methods: Different cylindrical phantoms consisting gel were used under slab phantoms during each irradiation. MR images of irradiated gel phantoms were obtained to determine their R2 relaxation maps. 1D and 2D lateral dose profiles were acquired at depths of 1 cm for an 8 MeV beam and 1 and 4 cm for the 15 MeV energy, and then compared with the lateral dose profiles measured using a diode detector. In addition, 3D dose distributions around these heterogeneities for the same energies and depths were measured using a gel dosimeter. Results: Dose resolution for MR gel images at the range of 0-10 Gy was less than 1.55 Gy. Mean dose difference and distance to agreement (DTA) for dose profiles were 2.6% and 2.2 mm, respectively. The results of the MAGIC-type polymer gel for bone heterogeneity at 8 MeV showed a reduction in dose of approximately 50%, and 30% and 10% at depths 1 and 4 cm at 15 MeV. However, for air heterogeneity increases in dose of approximately 50% at depth 1 cm under the heterogeneity at 8 MeV and 20% and 45% respectively at 15 MeV were observed. Discussion and Conclusion: Generally, electron beam distributions are significantly altered in the presence of tissue inhomogeneities such as bone and air cavities, this being related to mass stopping and mass scattering powers of heterogeneous materials. At the same time, hot and cold scatter lobes under heterogeneity regions due to scatter edge effects were also seen. However, these effects (increased dose, reduced dose, hot and cold spots) at deeper depths, are compensated with the contributions of scattered electrons. Our study showed that normoxic polymer gels are reliable detectors for determination of electron dose distributions due to their characteristics such as tissue equivalence, energy independence, and 2D and 3D dose visualization capabilities
