Method for Evaluating Cortical Bone Young's Modulus: Numerical Twin Reconstruction, Finite Element Calculation, and Microstructure Analysis.

The determination of bone mechanical properties remains crucial, especially to feed up numerical models. An original methodology of inverse analysis has been developed to determine the longitudinal elastic modulus of femoral cortical bone. The method is based on a numerical twin of a specific three-point bending test. It has been designed to be reproducible on each test result. In addition, the biofidelity of the geometric acquisition method has been quantified. As the assessment is performed at the scale of a bone shaft segment, the Young's modulus values obtained (between 9518.29 MPa and 14181.15 MPa) are considered average values for the whole tissue, highlighting some intersubject variability. The material microstructure has also been studied through histological analysis, and bone-to-bone comparisons highlighted discrepancies in quadrants microstructures. Furthermore, significant intrasubject variability exists since differences between the bone's medial-lateral and anterior-posterior quadrants have been observed. Thus, the study of microstructures can largely explain the differences between the elastic modulus values obtained. However, a more in-depth study of bone mineral density would also be necessary and would provide some additional information. This study is currently being setup, alongside an investigation of the local variations of the elastic modulus.

Mechanical effects of load speed on the human colon.

The aim of this study was to examine the mechanical behavior of the colon using tensile tests under different loading speeds. Specimens were taken from different locations of the colonic frame from refrigerated cadavers. The specimens were submitted to uniaxial tensile tests after preconditioning using a dynamic load (1 m/s), intermediate load (10 cm/s), and quasi-static load (1 cm/s). A total of 336 specimens taken from 28 colons were tested. The stress-strain analysis for longitudinal specimens indicated a Young's modulus of 3.17 ±â€¯2.05 MPa under dynamic loading (1 m/s), 1.74 ±â€¯1.15 MPa under intermediate loading (10 cm/s), and 1.76 ±â€¯1.21 MPa under quasi-static loading (1 cm/s) with p < 0.001. For the circumferential specimen, the stress-strain curves indicated a Young's modulus of 3.15 ±â€¯1.73 MPa under dynamic loading (1 m/s), 2.14 ±â€¯1.3 MPa under intermediate loading (10 cm/s), and 0.63 ±â€¯1.25 MPa under quasi-static loading (1 cm/s) with p < 0.001. The curves reveal two types of behaviors of the colon: fast break behavior at high speed traction (1 m/s) and a lower break behavior for lower speeds (10 cm/s and 1 cm/s). The circumferential orientation required greater levels of stress and strain to obtain lesions than the longitudinal orientation. The presence of taeniae coli changed the mechanical response during low-speed loading. Colonic mechanical behavior varies with loading speeds with two different types of mechanical behavior: more fragile behavior under dynamic load and more elastic behavior for quasi-static load.

Influence of gender, age, shelf-life, and conservation method on the biomechanical behavior of colon tissue under dynamic solicitation.

BACKGROUND: Data from biomechanical tissue sample studies of the human digestive tract are highly variable. The aim of this study was to investigate 4 factors which could modify the mechanical response of human colonic specimens placed under dynamic solicitation until tissue rupture: gender, age, shelf-life and conservation method. METHODS: We performed uniaxial dynamic tests of human colonic specimens. Specimens were taken according to three different protocols: refrigerated cadavers without embalming, embalmed cadavers and fresh colonic tissue. A total of 143 specimens were subjected to tensile tests, at a speed of 1 m s-1. FINDINGS: Young's modulus of the different conservation protocols are as follows: embalmed, 3.08 ±â€¯1.99; fresh, 2.97 ±â€¯2.59; and refrigerated 3.17 ±â€¯2.05. The type of conservation does not modify the stiffness of the tissue (p = 0.26) but does modify the stress necessary for rupture (p < 0.001) and the strain required to obtain lesions of the outer layer and the inner layer (p < 0.001 and p < 0.05, respectively). Gender is also a factor responsible for a change in the mechanical response of the colon. The age of the subjects and the shelf-life of the bodies did not represent factors influencing the mechanical behavior of the colon (p > 0.05). INTERPRETATION: The mechanical response of the colon tissue showed a biphasic injury process depending on gender and method of preservation. The age and shelf-life of anatomical subjects do not alter the mechanical response of the colon.

Comparison of Two Intervertebral Disc Failure Models in a Numerical C4-C5 Trauma Model.

The intervertebral disc (IVD) is essential for the mobility and stability of the spine. During flexion-distraction injuries, which are frequent at the cervical spine level, the IVD is often disrupted. Finite element studies have been done to investigate injury mechanisms and patterns at the cervical spine. However, they rarely include IVD failure model. The aim of this paper was to implement and compare two types of IVD failure models and their impact on hyperflexion and hyperflexion-compression injuries simulations. The failure models were tested on a detailed C4-C5 finite elements model. The first failure model consisted in a maximal strain model applied to the elements of the annulus and nucleus. The second failure model consisted in the implementation of a rupture plane in the middle of the IVD with a tied interface created between the two sections. This interface is defined by threshold stress values of detachment in traction and shearing. The two failure models were tested in flexion only and in flexion-compression. The model without inclusion of an IVD failure model was also tested. Loads at failure and injury patterns were reported. Both failure models produce failure loads that were consistent with experimental data. Injury patterns observed were in agreement with experimental and numerical studies. However, in flexion-compression, the rupture plane model simulation reached important energy error due to high deformations in the IVD elements. Also, without inclusion of an IVD failure model, energy error forced the end of the simulation in flexion-compression. Therefore, inclusion of IVD failure model is important since it leads to realistic results, but the maximal strain failure model is recommended.

Head impact in a snowboarding accident.

To effectively prevent sport traumatic brain injury (TBI), means of protection need to be designed and tested in relation to the reality of head impact. This study quantifies head impacts during a typical snowboarding accident to evaluate helmet standards. A snowboarder numerical model was proposed, validated against experimental data, and used to quantify the influence of accident conditions (speed, snow stiffness, morphology, and position) on head impacts (locations, velocities, and accelerations) and injury risk during snowboarding backward falls. Three hundred twenty-four scenarios were simulated: 70% presented a high risk of mild TBI (head peak acceleration >80 g) and 15% presented a high risk of severe TBI (head injury criterion >1000). Snow stiffness, speed, and snowboarder morphology were the main factors influencing head impact metrics. Mean normal head impact speed (28 ± 6 km/h) was higher than equivalent impact speed used in American standard helmet test (ASTM F2040), and mean tangential impact speed, not included in standard tests, was 13.8 (±7 km/h). In 97% of simulated impacts, the peak head acceleration was below 300 g, which is the pass/fail criteria used in standard tests. Results suggest that initial speed, impacted surface, and pass/fail criteria used in helmet standard performance tests do not fully reflect magnitude and variability of snowboarding backward-fall impacts.

Biomechanical analysis of fracture risk associated with tibia deformity in children with osteogenesis imperfecta: a finite element analysis.

OBJECTIVES: Osteogenesis imperfecta (OI) frequently leads to long-bone bowing requiring a surgical intervention in severe cases to avoid subsequent fractures. However, there are no objective criteria to decide when to perform such intervention. The objective is to develop a finite element model to predict the risk of tibial fracture associated with tibia deformity in patients with OI. METHODS: A comprehensive FE model of the tibia was adapted to match bi-planar radiographs of a 7 year-old girl with OI. Ten additional models with different deformed geometries (from 2° to 24°) were created and the elasto-plastic mechanical properties were adapted to reflect OI conditions. Loads were obtained from mechanography of two-legged hopping. Two additional impact cases (lateral and torsion) were also simulated. Principal strain levels were used to define a risk criterion. RESULTS: Fracture risks for the two-legged hopping load case remained low and constant until tibia bowing reached 15° and 16° in sagittal and coronal planes respectively. Fracture risks for lateral and torsion impact were equivalent whatever the level of tibial bowing. CONCLUSIONS: The finite element model of OI tibia provides an objective means of assessing the necessity of surgical intervention for a given level of tibia bowing in OI-affected children.

Liver injuries in frontal crash situations a coupled numerical - experimental approach.

From clinical knowledge, it has been established that hepatic traumas frequently lead to lethal injuries. In frontal or lateral crash situations, these injuries can be induced by pure deceleration effects or blunt trauma due to belt or steering wheel impact. Concerning the liver under frontal decelerations, how could one investigate organ behaviour leading to the injury mechanisms? This work couples experimental organ decelerations measurements (with 19 tests on cadaver trunks) and finite element simulation, provides a first analysis of the liver behaviour within the abdomen. It shows the influence of the liver attachment system that leads to liver trauma and also torsion effects between the two lobes of the liver. Injury mechanisms were evaluated through the four phases of the liver kinematics under frontal impact: (1) postero-anterior translation, (2) compression and sagittal rotation, (3) rotation in the transverse plane and (4) relaxation.

[Pregnant woman and road safety: a numerical approach. Application to a restrained third trimester pregnant woman in frontal impact]. / Femme enceinte et accidentologie routière: intérêt de l'approche numérique. Application à un choc frontal au troisième trimestre de la grossesse avec analyse de l'effet de la ceinture de sécurité

OBJECTIVES: The goal of our work is the development of a numerical model of pregnant woman in driving position. We present an application to the study of injury mechanisms during a frontal car crash for a seat belt restrained pregnant woman in driving position. MATERIALS AND METHODS: We integrated a digital representation of a pregnant uterus, foetus and placenta in a previous existing numerical model of non pregnant Human body in driving position, the Humos model. The realization of a numerical simulation of a frontal car crash enabled us to analyze the part played by the safety belt in the organic traumatisms. RESULTS: Three phases were highlighted. The first phase consists of a translation forwards of the pregnant uterus during the impact. The second phase is a rotation forwards in the sagittal plan of the pregnant uterus with for axis of rotation the posterior wall of the pubis. The third phase is a vertical adjustment coupled to a translation of the uterus towards the back. This translation leads the uterus to impact the spine. CONCLUSION: The development of a pregnant numerical model in the field of accidentology allows the analysis of organic traumatisms. That makes it possible to study the role played by the existing safety systems. This model might make it possible to develop safety systems specific to the pregnant woman.

Pedestrian lower limb injury criteria evaluation: a finite element approach.

OBJECTIVE: In pedestrian traumas, lower limb injuries occur under lateral shearing and bending at the knee joint level. One way to improve injury mechanisms description and consequently knee joint safety is to evaluate the ultimate shearing and bending levels at which ligaments start being injured. METHODS: As such data cannot easily and accurately be recorded clinically or during experiments, we show in this article how numerical simulation can be used to estimate such thresholds. This work was performed with the Lower Limb Model for Safety (LLMS) in pure lateral bending and shearing conditions, with an extended range of impact velocities. RESULTS: One result concerns the ultimate knee lateral bending angle and shearing displacement measurements for potential failure of ligaments (posterior cruciate, medial collateral, anterior cruciates and tibial collateral). They were evaluated to be close to 16 degrees and 15 mm, respectively. CONCLUSION: The lower leg model used in this study is an advanced FE model of the lower limb, validated under various situations. Its accurate anatomical description allows a wide range of applications. According to the validity domain of the model, it offered a valuable tool for the numerical evaluation of potential injuries and the definition of injury risk criterion for knee joint.

Using a finite element model to evaluate human injuries application to the HUMOS model in whiplash situation.

STUDY DESIGN: In the field of numerical simulation, the finite element method provides a virtual tool to study human tolerance and postulate on potential trauma under crash situations, particularly in case of whiplash trauma. OBJECTIVES: To show how medical and biomechanical interpretations of numerical simulation can be used to postulate on human injuries during crash situations. This methodology was applied to whiplash trauma analysis. A detailed analysis of kinematics of joints, stress level in hard tissues, and strain level in soft tissues was used to postulate on chronology and patterns of injury. Data were compared with published biomechanical and clinical studies of whiplash. SUMMARY OF BACKGROUND DATA: Although many in vitro and in vivo studies have been conducted to investigate whiplash cervical injury, and despite the number of finite element models developed to simulate the biomechanical behavior of the cervical spine, to date, there are only limited finite element models reported in the literature on the biomechanical response of the whole cervical spine in these respects. METHODS: A complete finite element model of the human body (HUMOS) build in a sitting position in a car environment was created to investigate injury mechanisms and to provide data for automotive safety improvements. It includes approximately 50,000 elements, including descriptions of all bones, ligaments, tendons, skin, muscles, and internal organs. A 15-g whiplash injury was simulated with the HUMOS model. The model predicted cervical motion segment kinematics, deformations of disks and ligaments, and stresses in bone. Model output was then compared with experimental and clinical whiplash literature. RESULTS: In term of kinematics during the chronology of whiplash, two injury phases were identified: the first was hyperextension of the lower cervical spine (C6-C7 and C5-C6) and mild flexion of the upper cervical spine(C0-C4). The amount of upper cervical flexion was 15 degrees from C0 to C4. The second phase was hyperextension of the entire cervical spine. Potential patterns of ligamentous injuries were observed; the anterior longitudinal ligament experienced the most strain (30%) at the lower cervical spine at the time of lower cervical extension and the interspinous ligament experienced the most strain (60%) at the time of upper cervical flexion. Von Mises stresses in bone do not exceed 15 Mpa, which is largely under injury levels reported in the literature. CONCLUSIONS.: This study reports a methodology to describe and postulate on human injuries based on finite element model analysis. The output of the HUMOS model in the context of whiplash shows a strong correlation with clinical and experimental reported data. HUMOS shows promise for the modeling of other types of trauma as well.

A human model for road safety: from geometrical acquisition to model validation with radioss.

In order to investigate injury mechanisms, and to provide directions for road safety system improvements, the HUMOS project has lead to the development of a 3D finite element model of the human body in driving position. The model geometry was obtained from a 50th percentile adult male. It includes the description of all compact and trabecular bones, ligaments, tendons, skin, muscles and internal organs. Material properties were based on literature data and specific experiments performed for the project. The validation of the HUMOS model was first achieved on isolated segments and then on the whole model in both frontal and lateral impact situations. HUMOS responses were in good agreement with the experimental data used in the model validation and offers now a wide range of applications from crash simulation, optimization of safety systems, to biomedical and ergonomics.

A visco-hyperelastic model with damage for the knee ligaments under dynamic constraints.

The aim of this study was to identify the behaviour laws governing the knee ligaments, accounting for the damage incurred by the structure under dynamic constraints. The model is developed using a thermodynamic formulation based on the coupling between a viscoelastic model and a damage model. Identification is carried out using the results of dynamic traction tests performed on a bone ligament/bone complex to which traction velocities of around 1.98 m/s were applied. The results show the ability of the model to account for the brittle and ductile failure processes occurring in the cruciate and lateral ligaments, respectively.

Lower Limb: Advanced FE Model and New Experimental Data.

The Lower Limb Model for Safety (LLMS) is a finite element model of the lower limb developed mainly for safety applications. It is based on a detailed description of the lower limb anatomy derived from CT and MRI scans collected on a subject close to a 50th percentile male. The main anatomical structures from ankle to hip (excluding the hip) were all modeled with deformable elements. The modeling of the foot and ankle region was based on a previous model Beillas et al. (1999) that has been modified. The global validation of the LLMS focused on the response of the isolated lower leg to axial loading, the response of the isolated knee to frontal and lateral impact, and the interaction of the whole model with a Hybrid III model in a sled environment, for a total of nine different set-ups. In order to better characterize the axial behavior of the lower leg, experiments conducted on cadaveric tibia and foot were reanalyzed and experimental corridors were proposed. Future work will include additional validation of the model using global data, joint kinematics data, and deformation data at the local level.