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CONTENTS
Volume 1, Number 1, March 2014
 

Abstract
In this paper, a simple and accurate finite element model coupled to quasi-brittle damage law able to describe the multiple cracks initiation and their progressive propagation is developed in order to predict the complete force-displacement curve and the fracture pattern of human proximal femur under quasi-static load. The motivation of this work was to propose a simple and practical FE model with a good compromise between complexity and accuracy of the simulation considering a limited number of model parameters that can predict proximal femur fracture more accurately and physically than the fracture criteria based models. Different damage laws for cortical and trabecular bone are proposed based on experimental results to describe the inelastic damage accumulation under the excessive load. When the damage parameter reaches its critical value inside an element of the mesh, its stiffness matrix is set to zero leading to the redistribution of the stress state in the vicinity of the fractured zone (crack initiation). Once a crack is initiated, the propagation direction is simulated by the propagation of the broken elements of the mesh. To illustrate the potential of the proposed approach, the left femur of a male (age 61) previously investigated by Keyak and Falkinstein, 2003 (Model B: male, age 61) was simulated till complete fracture under one-legged stance quasi-static load. The proposed finite element model leads to more realistic and precise results concerning the shape of the force-displacement curve (yielding and fracturing) and the profile of the fractured edge.

Key Words
finite element; proximal femur fracture; crack propagation; fracture pattern; force-displacement curve

Address
Ridha Hambli: Prisme Institute - MMH 8, Rue Leonard de Vinci, 45072, Orleans cedex 2, France

Abstract
Computed tomography (CT) is being utilized in orthopaedics and related research to estimate bone strength. These applications benefit from calibration of Hounsfield units to mineral density typical of long bone, up to 1750mg/cm3. This study describes a method for establishing repeatable calibration of Hounsfield units to density, and determines the effects of imaging medium on calibration accuracy. Four hydroxyapatite standards were imaged in air on 7 occasions over 19 weeks using a helical multi-slice CT scanner. Each standard was scanned 5 times in different media: porcine soft tissue, water, and air. Calibrated densities were highly repeatable (CV<3.5%). No difference in density was observed between water and soft tissue conditions (p>0.08). This work provides a model for determining repeatable scanner-specific density calibration, demonstrates that the linear relationship between Hounsfield units and density extends to values typical of cortical bone, and supports the practice of imaging calibration standards in an environment similar to that of the target bone.

Key Words
computed tomography; reliability; cortical bone; trabecular bone; bone density

Address
Meghan Crookshank: Biomedical Engineering, University of Toronto, Toronto, ON M5B 1W8, Canada; Human Mobility Research Centre, Queen' s University & Kingston General Hospital, Kingston, ON K7L 3N6, Canada
Heidi-Lynn Ploeg: Human Mobility Research Centre, Queen' s University & Kingston General Hospital, Kingston, ON K7L 3N6, Canada; Mechanical Engineering, University of Wisconsin, Madison, WI 53706, USA
Randy Ellis: Human Mobility Research Centre, Queen' s University & Kingston General Hospital, Kingston, ON K7L 3N6, Canada;School of Computing, Queen' s University, Kingston, ON K7L 3N6, Canada
Norma J. MacIntyre: Human Mobility Research Centre, Queen' s University & Kingston General Hospital, Kingston, ON K7L 3N6, Canada;School of Rehabilitation Science, McMaster University, Hamilton, ON L8S 1C7, Canada

Abstract
This paper examines the blood chamber of a left ventricular assist device (LVAD) under static loading conditions and standard operating temperatures. The LVAD's walls are made of a temperature-sensitive polymer (ChronoFlex C 55D) and are covered with a titanium nitride (TiN) nano-coating (deposited by laser ablation) to improve their haemocompatibility. A loss of cohesion may be observed near the coating-substrate boundary. Therefore, a micro-scale stress-strain analysis of the multilayered blood chamber was conducted with FE (finite element) code. The multi-scale model included a macro-model of the LVAD's blood chamber and a micro-model of the TiN coating. The theories of non-linear elasticity and elasto-plasticity were applied. The formulated problems were solved with a finite element method. The micro-scale problem was solved for a representative volume element (RVE). This micro-model accounted for the residual stress, a material model of the TiN coating, the stress results under loading pressures, the thickness of the TiN coating and the wave parameters of the TiN surface. The numerical results (displacements and strains) were experimentally validated using digital image correlation (DIC) during static blood pressure deformations. The maximum strain and stress were determined at static pressure steps in a macro-scale FE simulation. The strain and stress were also computed at the same loading conditions in a micro-scale FE simulation.

Key Words
 finite element method (FEM); representative volume element (RVE); polymer; nano-coating; titanium nitride (TiN); digital image correlation (DIC); left ventricular assist device (LVAD)

Address
Magdalena Kopernik and Andrzej Milenin: AGH University of Science and Technology, Kraków, Poland

Abstract
Acellular intra-myocardial biomaterial injections have been shown to be therapeutically beneficial in inhibiting ventricular remodelling of myocardial infarction (MI). Based on a biventricular canine cardiac geometry, various finite element models were developed that comprised an ischemic (II) or scarred infarct (SDI) in left ventricular (LV) antero-apical region, without and with intra-myocardial biomaterial injectate in layered (L) and bulk (B) distribution. Changes in myocardial properties and LV geometry were implemented corresponding to infarct stage (tissue softening vs. stiffening, infarct thinning, and cavity dilation) and injectate (infarct thickening). The layered and bulk injectate increased ejection fraction of the infarcted LV by 77% (II+L) and 25% (II+B) at the ischemic stage and by 61% (SDI+L) and 63% (SDI+B) at the remodelling stage. The injectates decreased the mean end-systolic myofibre stress in the infarct by 99% (II+L), 97% (II+B), 70% (SDI+L) and 36% (SDI+B). The bulk injectate was slightly more effective in improving LV function at the remodelling stage whereas the layered injectate was superior in functional improvement at ischemic stage and in reduction of wall stress at ischemic and remodelling stage. These findings may stimulate and guide further research towards tailoring acellular biomaterial injectate therapies for MI.

Key Words
myocardial infarct; therapeutic injection; cardiac remodelling; finite element method

Address
Jeroen Kortsmit, Neil H. Davies, Renee Miller and Peter Zilla: Cardiovascular Research Unit, Chris Barnard Division of Cardiothoracic Surgery, University of Cape Town, South Africa
Thomas Franz: Cardiovascular Research Unit, Chris Barnard Division of Cardiothoracic Surgery, University of Cape Town, South Africa; Center for Research in Computational and Applied Mechanics, University of Cape Town, Rondebosch, South Africa; Research Office, University of Cape Town, Mowbray, South Africa; Center for High Performance Computing, Rosebank, South Africa

Abstract
The purpose of the study was to investigate the kinematics between the double collar-tie and double underhook Thai Boxing clinching positions. Ten amateur mixed martial arts athletes executed six knee strikes for both clinching positions with their dominant limb directed towards a target. A standard two-dimensional video motion analysis was conducted, and the results showed a statistical significant difference at the hip joint angle and the angular acceleration of the knee and ankle. Within both clinching positions, there was a statistically significant correlation between the hip and knee joint angles, hip and knee angular velocities, and hip angular acceleration. Between both clinching positions, there was a statistically significant correlation at the knee joint angle, knee angular velocity, and hip angular acceleration. This study demonstrates the importance of the hip and knee joint movements in both clinching positions, which implies the applications of strength training and flexibility at these joints for sports performance and injury prevention. It is suggested that future studies analyzing the non-dominant leg are warranted to fully understand the Thai Boxing clinch.

Key Words
boxing; clinch; kinematics; lower extremity; martial arts

Address
William Trial and Tom Wu: Department of Movement Arts, Health Promotion, and Leisure Studies, Bridgewater State University, Bridgewater, USA

Abstract
Bone properties are one of the key components when constructing models that can simulate the mechanical behavior of a mandible. Due to the complexity of the structure, the tooth, ligaments, different bones etc., some simplifications are often considered and bone properties are one of them. The objective of this study is to understand if a simplification of the problem is possible and assess its influence on mandible behavior. A cadaveric toothless mandible was used to build three computational models from CT scan information: a full cortical bone model; a cortical and cancellous bone model, and a model where the Young

Key Words
biomechanics; finite element method; modeling and simulation; dental mechanics; bone biomechanics

Address
António Ramos and Hugo Marques: TEMA, Department of Mechanical Engineering, University of Aveiro, Portugal
Michel Mesnard: University of Bordeaux, I2M, CNRS UMR 5295, Talence, France


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