Poroelastic analysis of bone tissue differentiation by using the boundary element method

Fracture healing is initiated and tightly regulated mainly by growth factors and by mechanical environment around the callus site. Biomechanics of fracture healing have been previously studied. Most computational models are based on finite elements and some of them study the level of strain or stress in the different tissues. These strain/stress fields are the main mechanical stimuli affecting cell differentiation and ossification pathway. In this work, we incorporated that hypothesis into a poroelastic axi-symmetric boundary element callus model, where the pore pressure was included as a part of the stimuli function. This analysis allowed us to extend the observations made by other authors and a new poroelastic correlation between mechanical conditions and local tissue formation is proposed. This work shows the capability of the boundary element method to characterize the tissue phenotypes during a progressive healing process. The results were in good agreement with those reported in previous works.     

A Systematic Study of Restorative Crown-Materials Combinations for Dental Implants: Characterization of Mechanical Properties under Dynamic Loads

This study aimed to find the optimum mechanical characteristics of the restorative materials for the manufacture of implant crowns subjected to impact loading when different combinations of materials are used for the inner and outer crown. Several combinations of external–internal crown restorative materials were analyzed. The dynamic stresses at eight different zones of a dental implant subjected to an impact load and the influence of several mechanical properties, such as the Young’s modulus, Poisson’s ratio, density, and initial velocity, were analyzed and compared. A detailed 3D model was created, including the crown, the retention screw, the implant, and a mandible section. The model was then built by importing the 3D geometries from CAD software. The whole 3D model was carefully created in order to guarantee a finite element mesh that produced results adjusted to physical reality. Then, we conducted a numerical simulation using the finite element method (FEM). The results of the FEM analysis allowed for evaluating the effect that different combinations of restorative materials and mechanical properties had on the stress distribution in various regions of the implant. The choice of restorative material is a factor to be considered in order to preserve the integrity of osseointegration. Restorative materials transfer more or less stress to the dental implant and surrounding bone, depending on their stiffness. Therefore, an inadequate Young’s modulus of the rehabilitation material can affect the survival of the implant over time. Eight interactive graphics were provided on a web-based surface platform to help clinical dentists, researchers, and manufacturers to select the best restorative materials combination for the crown.     

The p-adaptive boundary integral equation method

This work is concerned with the development and application of the p-adaptive boundary integral equation method (BIEM) in practical elastostatics engineering situations. Some basic concepts inherent to the p-adaptive technique are summarized and discussed. A pseudocode which illustrates the way for generating the p-adaptive system of equations in microcomputers is also provided.

Two numerical examples, which show the accuracy of the method discussed herein are included.     

Hierarchical statistical shape models of multiobject anatomical structures: application to brain MRI

The accurate segmentation of subcortical brain structures in magnetic resonance (MR) images is of crucial importance in the interdisciplinary field of medical imaging. Although statistical approaches such as active shape models (ASMs) have proven to be particularly useful in the modeling of multiobject shapes, they are inefficient when facing challenging problems. Based on the wavelet transform, the fully generic multiresolution framework presented in this paper allows us to decompose the interobject relationships into different levels of detail. The aim of this hierarchical decomposition is twofold: to efficiently characterize the relationships between objects and their particular localities. Experiments performed on an eight-object structure defined in axial cross sectional MR brain images show that the new hierarchical segmentation significantly improves the accuracy of the segmentation, and while it exhibits a remarkable robustness with respect to the size of the training set.     

Full Multiresolution Active Shape Models

The incorporation of a multiresolution image approach is one of the most popular variants of Active Shape Models (ASMs), providing a more robust algorithm and minimizing its initialization dependency. Using the wavelet transform, the present paper extends the multiresolution analysis to the shape space, developing a novel multiresolution shape framework, capable of being incorporated into most of ASM variants. The tests performed with two different types of images, face images (AR database) and chest radiographs (JSRT database), demonstrate how this new generation of algorithms significantly reduce the computational cost, more than halving it, while maintaining the same levels of accuracy.     

Double Raman Amplified Bus Networks for Wavelength-Division Multiplexing of Fiber-Optic Sensors

In this paper, three different double Raman fiber bus networks are compared and demonstrated experimentally for the first time as a means of gathering information from wavelength- division-multiplexed optical sensors: the double-bus scheme, the improved double-bus configuration, and the hybrid topology. We report how these structures reduce the received amplified- spontaneous-scattering noise generated. This low-noise configuration yields signal-to-noise ratios over 43 dB and increases the number of sensors that could be multiplexed in a single structure. Furthermore, the last one enables the reutilization of the gratings' wavelengths.     

A straightforward method of measuring MPRT using LC test cells

Abstract— A simple method based on integration over time to measure motion‐picture response time (MPRT) has been developed. Liquid‐crystal response time (LCRT) alone cannot express the motion blur perceived by the observer; one must also take into account the transition between intermediate gray levels and the sample‐and‐hold effect. The method shows similar results to other previously reported methods based on temporal integration, while being simpler and more straightforward. Indeed, just monopixel test cells are required for measuring MPRT. This method can be used for comparison between different materials, alignments surfaces, or other manufacturing details with no need of fabricating the whole LCD structure. Nevertheless, the method could also be used for characterization of commercial displays with major changes. A comparison of MPRT values for three different liquid‐crystal materials is presented in this work. The behavior of the MPRT parameter in the case of an ideal liquid‐crystal material with an LCRT equal to zero has also been studied. The results obtained for this material have been used as a reference to establish comparisons with real materials.     

Identification of the dynamical properties of structures using free vibration data and distributed genetic algorithms

The dynamical properties of structures, such as natural frequencies, damping ratios and mode shapes, can be obtained by several identification methods. Some are based on the direct signal processing in a time domain; others transform response data to the frequency domain. The development of these techniques is useful in the production of more accurate structural models; they can be also used to test the level of damage in structures (or verify their strength to support new load actions) by using experimental data. There are situations where frequency domain algorithms and conventional system identification techniques fail, do not allow adequate solution of the identification problems or become trapped in a local optimum. It is in these cases where evolutionary optimization techniques are important tools for evaluating the dynamical properties of structural systems in practical applications. This article presents a methodology to determine the dynamic properties of structures knowing their response in terms of displacement, velocities or accelerations in the time domain when they are subjected to a free vibration excitation. In order to do that, a specialized evolutionary algorithm capable of adapting its parameters to the different types of registers obtained from the dynamic time response of a structure is implemented in a robust way, making this approach useful in practical situations. A distributed real genetic algorithm (DRGA) based on an island model of different subpopulations is used to adjust a simulated response signal of a building to the real response signal. Initially, computer-generated synthetic response signals are used but, in future, the approach will be validated with signals obtained from free vibration experimental tests and will be extended to other types of dynamical excitation signals. Finally, the method will be tested with data obtained from earthquake events.     

Analysis of 3D transient blood flow passing through an artificial aortic valve by Lattice-Boltzmann methods

The development of flow instabilities due to high Reynolds number flow in artificial heart valve geometries inducing high strain rates and stresses often leads to hemolysis and related highly undesired effects. Geometric and functional optimization of artificial heart valves is therefore mandatory. In addition to experimental work in this field it is meanwhile possible to obtain increasing insight into flow dynamics by computer simulation of refined model problems. After giving an introductory overview we report the results of the simulation of three-dimensional transient physiological flows in fixed geometries similar to a CarboMedics bileaflet heart valve at different opening angles. The visualization of emerging complicated flow patterns gives detailed information about the transient history of the systems dynamical stability. Stress analysis indicates temporal shear stress peaks even far away from walls. The mathematical approach used is the Lattice Boltzmann method. We obtained reasonable results for velocity and shear stress fields. The code is implemented on parallel hardware in order to decrease computation time. Finally, we discuss problems, shortcomings and possible extensions of our approach.     

A bi-cubic transformation for the numerical evaluation of the Cauchy principal value integrals in boundary methods

The numerical strategies employed in the evaluation of singular integrals existing in the Cauchy principal value (CPV) sense are, undoubtedly, one of the key aspects which remarkably affect the performance and accuracy of the boundary element method (BEM). Thus, a new procedure, based upon a bi-cubic co-ordinate transformation and oriented towards the numerical evaluation of both the CPV integrals and some others which contain different types of singularity is developed. Both the ideas and some details involved in the proposed formulae are presented, obtaining rather simple and-attractive expressions for the numerical quadrature which are also easily embodied into existing BEM codes. Some illustrative examples which assess the stability and accuracy of the new formulae are included.