Pierre-Jean Arnoux

Finite element investigation of the loading rate effect on the spinal load-sharing changes under impact conditions

Sudden deceleration and frontal/rear impact configurations involve rapid movements that can cause spinal injuries. This study aimed to investigate the rotation rate effect on the L2-L3 motion segment load-sharing and to identify which spinal structure is at risk of failure and at what rotation velocity the failure may initiate? Five degrees of sagittal rotations at different rates were applied in a detailed finite-element model to analyze the responses of the soft tissues and the bony structures until possible fractures. The structural response was markedly different under the highest velocity that caused high peaks of stresses in the segment compared to the intermediate and low velocities. Under flexion, the stress was concentrated at the upper pedicle region of L2 and fractures were firstly initiated in this region and then in the lower endplate of L2. Under extension, maximum stress was located in the lower pedicle region of L2 and fractures started in the left facet joint, then they expanded in the lower endplate and in the pedicle region of L2. No rupture has resulted at the lower or intermediate velocities. The intradiscal pressure was higher under flexion and decreased when the endplate was fractured, while the contact forces were greater under extension and decreased when the facet surface was cracked. The highest ligaments stresses were obtained under flexion and did not reach the rupture values. The endplate, pedicle and facet surface represented the potential sites of bone fracture. Results showed that spinal injuries can result at sagittal rotation velocity exceeding 0.5 degrees /ms.

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.

Geometrical variations in white and gray matter affect the biomechanics of spinal cord injuries more than the arachnoid space

Traumatic spinal cord contusions lead to loss of quality of life, but their pathomechanisms are not fully understood. Previous studies have underlined the contribution of the cerebrospinal fluid in spinal cord protection. However, it remains unclear how important the contribution of the cerebrospinal fluid is relative to other factors such as the white/gray matter ratio. A finite element model of the spinal cord and surrounding morphologic features was used to investigate the spinal cord contusion mechanisms, considering subarachnoid space and white/gray matter ratio. Two vertebral segments (T6 and L1) were impacted transversely at 4.5 m s−1, which demonstrated three major results: While the presence of cerebrospinal fluid plays a significant contributory role in spinal cord protection (compression percentage decreased by up to 19%), the arachnoid space variation along the spine appears to have a limited (3% compression decrease) impact. Differences in the white and gray matter geometries from lumbar to thoracic spine levels decrease spinal cord compression by up to 14% at the thoracic level. Stress distribution in the sagittal spinal cord section was consistent with central cord syndrome, and local stress concentration on the anterior part of the spinal cord being highly reduced by the presence of cerebrospinal fluid. The use of a refined spinal cord finite element method showed that all the geometrical parameters are involved in the spinal cord contusion mechanisms. Hence, spinal cord injury criteria must be considered at each vertebral level.

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° 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.

Tonic finite element model of the lower limb

It is widely admitted that muscle bracing influences the result of an impact, facilitating fractures by enhancing load transmission and reducing energy dissipation. However, human numerical models used to identify injury mechanisms involved in car crashes hardly take into account this particular mechanical behavior of muscles. In this context, in this work we aim to develop a numerical model, including muscle architecture and bracing capability, focusing on lower limbs. The three-dimensional (3-D) geometry of the musculoskeletal system was extracted from MRI images, where muscular heads were separated into individual entities. Muscle mechanical behavior is based on a phenomenological approach, and depends on a reduced number of input parameters, i.e., the muscle optimal length and its corresponding maximal force. In terms of geometry, muscles are modeled with 3-D viscoelastic solids, guided in the direction of fibers with a set of contractile springs. Validation was first achieved on an isolated bundle and then by comparing emergency braking forces resulting from both numerical simulations and experimental tests on volunteers. Frontal impact simulation showed that the inclusion of muscle bracing in modeling dynamic impact situations can alter bone stresses to potentially injury-inducing levels.

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° 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.

Morphometrics of the entire human spinal cord and spinal canal measured from in vivo high-resolution anatomical magnetic resonance imaging.

Study Design. Measurements of cervical and thoracolumbar human spinal cord (SC) geometry based on in vivo magnetic resonance imaging and investigation of morphological “invariants.” Objective. The current work aims at providing morphological features of the complete in vivo human normal SC and at investigating possible “invariant” parameters that may serve as normative data for individualized study of SC injuries. Summary of Background Data. Few in vivo magnetic resonance image–based studies have described human SC morphology at the cervical level, and similar description of the entire SC only relies on postmortem studies, which may be prone to atrophy biases. Moreover, large interindividual variations currently limit the use of morphological metrics as reference for clinical applications or as modeling inputs. Methods. Absolute metrics of SC (transverse and anteroposterior diameters, width of anterior and posterior horns, cross-sectional SC area, and white matter percentage) were measured using semiautomatic segmentation of high resolution in vivo T2*-weighted transverse images acquired at 3 T, at each SC level, on healthy young (N = 15) and older (N = 8) volunteers. Robustness of measurements, effects of subject, age, or sex, as well as comparison with previously published postmortem data were investigated using statistical analyses (separate analysis of variance, Tukey-HSD, Bland-Altman). Normalized-to-C3 parameters were evaluated as invariants using a leave-one-out analysis. Spinal canal parameters were measured and occupation ratio border values were determined. Results. Metrics of SC morphology showed large intra- and interindividual variations, up to 30% and 13%, respectively, on average. Sex had no influence except on posterior horn width (P < 0.01). Age-related differences were observed for anteroposterior diameter and white matter percentage (P < 0.05) and all postmortem metrics were significantly lower than in vivo values (P < 0.001). In vivo normalized SC area and diameters seemed to be invariants (R2 > 0.74, root-mean-square deviation < 10%). Finally, minimal and maximal occupation ratio were 0.2 and 0.6, respectively. Conclusion. This study presented morphological characteristics of the complete in vivo human SC. Significant differences linked to age and postmortem state have been identified. Morphological “invariants” that could be used to calculate the normally expected morphology accurately, were also identified. These observations should benefit to biomechanical and SC pathology studies. Level of Evidence: N/A

Method to Geometrically Personalize a Detailed Finite-Element Model of the Spine

To date, developing geometrically personalized and detailed solid finite-element models (FEMs) of the spine remains a challenge, notably due to multiple articulations and complex geometries. To answer this problem, a methodology based on a free-form deformation technique (kriging) was developed to deform a detailed reference finite-element mesh of the spine (including discs and ligaments) to the patient-specific geometry of 10- and 82-year-old asymptomatic spines. Different kriging configurations were tested: with or without smoothing, and control points on or surrounding the entire mesh. Based on the results, it is recommended to use surrounding control points and smoothing. The mean node to surface distance between the deformed and target geometries was 0.3 ± 1.1 mm. Most elements met the mesh quality criteria (95%) after deformation, without interference at the articular facets. The methods novelty lies in the deformation of the entire spine at once, as opposed to deforming each vertebra separately, with surrounding control points and smoothing. This enables the transformation of reference vertebrae and soft tissues to obtain complete and personalized FEMs of the spine with minimal postprocessing to optimize the mesh.

Biomechanical analysis of pedicle screw pullout strength

Pedicle screws are widely used to treat severe cases of spinal pathologies and traumas. It is performed by inserting pedicle screws and connecting instrumentation rods in order to realign the vertebrae. In vitro experiments such as axial pullout tests provide insight into the biomechanics of screw–bone interactions, but show inherent limitations in terms of inter-individual variability (bone density, pedicle morphology, etc.) and reproducibility. The objective of this study was to develop a finite element model to simulate and biomechanically evaluate the pullout forces and stiffness of different pedicle screw designs and insertion techniques.

Evaluation of Knee Injury Threshold in Pedestrian-Car Crash Loading Using Numerical Approach

Abstract The present work evaluates the sensitivity of various loading parameters to the injury response of the human knee joint in pedestrian–car impact loading using a numerical model. The knee joint from a previously developed finite element lower limb model was validated against published postmortem human subject test results for stiffness and injury response. Previous studies have reported injury threshold for the knee joint based on validation with whole limb experimental tests in pure bending and shearing. The goal of the present study is to ascertain the validity of such injury thresholds when evaluated for isolated knee joint specimens, and furthermore, performing a sensitivity analysis by altering various loading parameters such as valgus bending rate, torsional degree of freedom and amount of lateral shear displacement. Findings of this study suggest that the threshold values for the cruciate ligaments are underestimated, attributed to the non-homogenous strain distribution of the ligaments which is not characterized in experimental results. Finally, the factors critical to injury of each major knee ligament have been described with the injury parameters recorded in each case. The reported injury parameters in the present study can be applied to modify the pre-existing injury thresholds incorporated in numerical models and mechanical surrogates, and design better countermeasures aimed at ameliorating pedestrian lower limb injuries.