proximal-cadaveric-femur-preparation-for-fracture-strength-testing

Proximal Cadaveric Femur Preparation for Fracture Strength Testing and Quantitative CT-based Finite Element Analysis

Cadaveric fracture testing is routinely used to understand factors that affect proximal femur strength.  Because ex vivo biological tissues are prone to ...

method-instrumented-fixture-for-femoral-fracture-testing-sideways

Method and Instrumented Fixture for Femoral Fracture Testing in a Sideways Fall-on-the-Hip Position

Mechanical testing of femora brings valuable insights into understanding the contribution of clinically-measureable variables such as bone mineral density ...

a-method-to-estimate-cadaveric-femur-cortical-strains-during-fracture

A Method to Estimate Cadaveric Femur Cortical Strains During Fracture Testing Using Digital Image Correlation

This protocol describes the method using digital image correlation to estimate cortical strain from high speed video images of the cadaveric femoral ...

Mayo Clinic

The results of a computational study of a helmeted human head are presented in this paper. The focus of the work is to study the effects of helmet pad materials on the level of acceleration, inflicted pressure and shear stress in a human brain model subjected to a ballistic impact. Four different closed cell foam materials, made of expanded polystyrene and expanded polypropylene, are examined for the padding material. It is assumed that bullets cannot penetrate the helmet shell. Finite element modelling of the helmet, padding system, head and head components is used for this dynamic nonlinear analysis. Appropriate contacts and conditions are applied between the different components of the head, as well as between the head and the pads, and the pads and the helmet. Based on the results of simulations in this work, it is concluded that the stiffness of the foam has a prominent role in reducing the level of the transferred load to the brain. A pad that is less stiff is more efficient in absorbing the impact energy and reducing the sudden acceleration of the head and consequently lowers the brain injury level. Using the pad with the least stiffness, the influence of the angle of impacts as well as the locations of the ballistic strike is studied.

A computational study of influence of helmet padding materials on the human brain under ballistic impacts.

In this study, the optimal viscoelastic material parameters of axon and extracellular matrix (ECM) in porcine brain white matter were identified using a genetic algorithm (GA) optimization procedure. The procedure was combined with micromechanical finite element analysis (FEA) of brain tissue and experimental stress relaxation tests on brainstem specimens to find the optimal material coefficients of axon and ECM. The stress relaxation tests were performed in tension on 10 brainstem specimens at 3% strain level. The axonal volume fraction in brainstem was measured from the Scanning Electron Microscopy images of the brain tissue. A square periodic volume element was selected to represent the microscale homogenized brainstem tissue. Periodic boundary conditions were applied on the square volume element to mimics the repetitive nature of the volume element. Linear viscoelastic material properties were assumed for the brain tissue constituents under small deformation. The constitutive behavior was expressed in terms of Prony series. The GA procedure searched for the optimal material parameters by fitting the time-dependent tissue stresses of brain tissue FEA to the stresses of relaxation tests under the same loading conditions. The optimization procedure converged after 60 iterations. The initial elastic modulus of axon was found to be 12.86kPa, three times larger than that of ECM. The long-term elastic modulus of axon was 3.7kPa, while for ECM this value was 1.03kPa. The concordance correlation coefficient between FEA estimated elastic modulus of brainstem tissue using the optimal material properties and the experimental elastic modulus of brainstem specimens was 0.952, showing a strong agreement. The optimal material properties of brain tissue constituents can find applications in micromechanical analysis of brain tissue to gain insight into diffuse axonal injures (DAIs).

A micromechanical procedure for viscoelastic characterization of the axons and ECM of the brainstem.

A parametric study was conducted to delineate the efficacy of personal protective equipment (PPE), such as ballistic faceshields and advanced combat helmets, in the case of a blast. The propagations of blast waves and their interactions with an unprotected head, a helmeted one, and a fully protected finite element head model (FEHM) were modeled. The biomechanical parameters of the brain were recorded when the FEHM was exposed to shockwaves from the front, back, top, and bottom. The directional dependent tissue response of the brain and the variable efficiency of PPE with respect to the blast orientation were two major results of this study.

Examination of the protective roles of helmet/faceshield and directionality for human head under blast waves.

Although a large number of studies have addressed the age-related changes in bone mineral density (BMD), there is a paucity of data for the assessment of femoral strength loss with age in both genders. We determined the variation of strength with age in femurs of women and men by mechanical tests on a cohort of 100 cadaveric femurs. In addition, the age-related neck BMD loss in our cadaveric cohort was found to be similar with BMD loss of four published population-based studies. Given the strong correlation found in our cadaveric study between BMD and femoral strength, we also estimated the femoral strength of the four populations based on their reported neck BMDs. Our study showed that men's femurs in our cadaveric cohort were stronger than women's femurs by about 800 N at the same BMD level, and by 1750 N at the same age. The strength differences were not explained satisfactorily by the size difference between men's and women's bones. Similar to the findings of clinical studies, the BMD values of men at all ages were larger than that of women. The age-related loss rates in BMD and strength were not statistically different between the two genders of our cadaveric cohort. After normalization, strength decreased more than 40% faster than BMD. On average, men reached a certain BMD value about 16 years later than women, and for strength about 23 years later, which may explain the higher rate of hip fracture in postmenopausal women. In patient population cohorts men reached a similar BMD value about 16 to 25 years later than women, whereas for estimated strength, sometimes more than 40 years later. © 2015 American Society for Bone and Mineral Research.

Femoral Strength Changes Faster With Age Than BMD in Both Women and Men: A Biomechanical Study.

Three different human head models in a free space are exposed to blast waves coming from four different directions. The four head-neck-body models composed of model a, with the neck free in space; model b, with neck fixed at the bottom; and model c, with the neck attached to the body. The results show that the effect of the body can be ignored for the first milliseconds of the head-blast wave interactions. Also one can see that although most biomechanical responses of the brain have similar patterns in all models, the shear stresses are heavily increased after a few milliseconds in model b in which the head motion is obstructed by the fixed-neck boundary conditions. The free-floating head model results are closer to the attached-body model.

Computational biomechanics of human brain with and without the inclusion of the body under different blast orientation.

Head protective tools such as helmets and faceshields can induce a localized high pressure region on the skull because of the underwash of the blast waves. Whether this underwash overpressure can affect the brain tissue response is still unknown. Accordingly, a computational approach was taken to confirm the incidence of underwash with regards to blast direction, as well as examine the influence of this effect on the mechanical responses of the brain. The variation of intracranial pressure (ICP) as one of the major injury predictors, as well as the maximum shear stress were mainly addressed in this study. Using a nonlinear finite element (FE) approach, generation and interaction of blast waves with the unprotected, helmeted, and fully protected (helmet and faceshield protected) FE head models were modeled using a multi-material arbitrary Lagrangian-Eulerian (ALE) method and a fluid-structure interaction (FSI) coupling algorithm. The underwash incidence overpressure was found to greatly change with the blast direction. Moreover, while underwash induced ICP (U-ICP) did not exceed the peak ICP of the unprotected head, it was comparable and even more than the peak ICP imposed on the protected heads by the primary shockwaves (Coup-ICP). It was concluded that while both helmet and faceshield protected the head against blast waves, the underwash overpressure affected the brain tissue response and altered the dynamic load experienced by the brain as it led to increased ICP levels at the countercoup site, imparted elevated skull flexure, and induced high negative pressure regions. Copyright © 2016 John Wiley & Sons, Ltd. Copyright © 2016 John Wiley & Sons, Ltd.

Evaluation of brain tissue responses because of the underwash overpressure of helmet and faceshield under blast loading.

Underwash occurs as the incoming shockwaves enter the helmet subspace and develop a high pressure region at the opposite side of the head. The mechanism leading to the underwash is yet not well understood. To investigate this effect, the turbulent, supersonic flow of compressible air approaching the head-helmet assembly from different directions was studied through computational fluid dynamics simulations. The effects of different incident overpressures and helmet gap size on the underwash incidence were further evaluated. The backflow-induced pressure from the air traveling outside of the helmet on the outflow from the helmet, as well as the momentum change in the backside curve of the helmet were postulated as the main reasons for this effect. Side shockwaves predicted the highest underwash overpressures. The increase rate of the underwash reduced with increasing the incident shockwave intensity.

Article Total : 3	Year
Publication title	2017
Publication title	2017
Publication title	2017

Article	Year
A computational study of influence of helmet padding materials on the human brain under ballistic impacts. Computer methods in biomechanics and biomedical engineering\| PubMed ID: 23281845	2014
A micromechanical procedure for viscoelastic characterization of the axons and ECM of the brainstem. Journal of the mechanical behavior of biomedical materials\| PubMed ID: 24361933	2014
Examination of the protective roles of helmet/faceshield and directionality for human head under blast waves. Computer methods in biomechanics and biomedical engineering\| PubMed ID: 25413615	2015
Femoral Strength Changes Faster With Age Than BMD in Both Women and Men: A Biomechanical Study. Journal of bone and mineral research : the official journal of the American Society for Bone and Mineral Research\| PubMed ID: 26096829	2015
Computational biomechanics of human brain with and without the inclusion of the body under different blast orientation. Computer methods in biomechanics and biomedical engineering\| PubMed ID: 26442577	2016
Evaluation of brain tissue responses because of the underwash overpressure of helmet and faceshield under blast loading. International journal for numerical methods in biomedical engineering\| PubMed ID: 26968860	2017
CFD modeling of the underwash effect of military helmets as a possible mechanism for blast-induced traumatic brain injury. Computer methods in biomechanics and biomedical engineering\| PubMed ID: 27269066	2017

Asghar Rezaei

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