Expanded polystyrene (EPS) is a cellular material widely used in energy-absorbing systems due to its ability to dissipate kinetic energy through compression. EPS can be recycled, offering a promising avenue for sustainable material use. This study aims to characterize the mechanical response of recycled EPS under dynamic loading conditions. Both external factors (temperature, impact velocity) and internal factors (density, recycling ratio) were investigated, and the behavior of recycled EPS was compared to that of classical material. The results show that recycled EPS exhibits no significant sensitivity to temperature within the range of -20 degrees C to 18 degrees C. However, its mechanical response is strongly influenced by impact velocity and foam density. No statistical difference was observed between recycled and classical EPS at a density of 60 kg/m3. In contrast, at 80 kg/m3, recycling induced a marked shift in the stress-strain curves, indicating that higher densities amplify the effects of the recycling process on mechanical performance.

Introduction:Proximal junctional kyphosis/failure (PJK/PJF) remains a frequent and severe complication following adult spinal deformity (ASD) surgery. While alignment risk factors are known, the specific mechanical influence of anterior malalignment and pelvic retroversion across different fusion levels remains poorly understood. Research question:How do postoperative anterior malalignment and pelvic retroversion influence PJK/PJF risk and biomechanical forces at the proximal junction based on upper instrumented vertebra (UIV) selection? Material and methods:We retrospectively analyzed 351 ASD patients fused to the pelvis, stratified by UIV: lower thoracic (LT, T9-T11; n = 206) or upper lumbar (UL, T12-L2; n = 145). Radiographic spinopelvic alignment was evaluated. Additionally, a validated finite element model (FEM) of T10-pelvis and L2-pelvis constructs simulated progressive anterior offsets and pelvic retroversion to quantify UIV endplate compressive and shear forces. Results:PJK/PJF incidence was comparable between groups (LT: 22.4%, UL: 22.2%). In both cohorts, PJK patients exhibited greater 6-week postoperative global sagittal malalignment and pelvic retroversion. LT failures were driven by higher SVA and thoracic kyphosis, whereas UL failures associated with increased segmental T10-L2 kyphosis. FEM showed LT constructs experienced predominant compressive forces scaling with anterior offset, while UL constructs experienced predominant posterior shear forces. Pelvic retroversion offered negligible mitigation against compression and limited shear reduction. Discussion and conclusion:UIV selection dictates the biomechanical failure mechanism, not the overall PJK/PJF risk. LT instrumentation exposes the proximal junction to compression, whereas the UL spine is susceptible to shear-driven failure. Pelvic retroversion cannot compensate for residual anterior malalignment. Therefore, UIV choice must account for regional alignment and predictable force vectors.

OBJECTIVE:The objective of this study was to evaluate the impact of four-rod (4R) constructs and interbody cages (IBCs) on pseudarthrosis and rod breakage (PA/RB) in patients with adult spinal deformity (ASD) who had undergone surgery with pelvic fixation and Schwab grade 2 osteotomies, using a combined finite element model (FEM) and clinical data analysis. METHODS:A validated FEM simulated Schwab grade 2 osteotomies at the L4-5 level in two-rod and 4R configurations, with or without IBCs at L4-5 and L5-S1. Rod strain and range of motion were calculated under a 7.5-Nm moment. Clinical analysis was conducted on ASD patients with pelvic fixation and Schwab grade 2 osteotomies and ≥ 2 years of follow-up. Patients were classified into 2 groups depending on the presence or absence of PA/RB. Demographic, surgical, radiographic, and patient-reported outcome measure (PROM) data were compared. RESULTS:The FEM analysis revealed maximal rod strain of 399 MPa at the osteotomy site in flexion. The 4R constructs and IBCs reduced strain to 114 MPa at L4-5 and 80 MPa at L5-S1. Among the 213 patients included in the study, PA/RB occurred in 61 (28.6%). Multivariate analysis revealed the use of 4R constructs (OR 0.331, 95% CI 0.16-0.71, p = 0.004) and IBCs (OR 0.46, 95% CI 0.23-0.94, p = 0.033) as protective factors. Patients with PA/RB experienced more unplanned reinterventions, worse scores on PROMs, and greater loss of sagittal alignment at 2 years postoperatively. CONCLUSIONS:Constructs with 4Rs and IBCs in ASD surgeries with pelvic fixation and Schwab grade 2 osteotomies significantly reduced rod strain and decreased the risk of PA/RB, leading to better scores on PROMs and decreasing unplanned reinterventions and loss of alignment.

Expanded polystyrene (EPS) is commonly used in protective systems due to its high capability to absorb energy. Thus, understanding the dynamic properties is essential to predict performance of such materials. This paper presents dynamic compressive experiments conducted on EPS at various densities. The influence of the temperature, the impact velocity and the density on the material's behavior are presented. In addition, various stress-strain curve parameters are extracted and analyzed. The results show that EPS behavior mainly depends on its density and the impact velocity. However, it is independent of the temperature on the studied range. Based on the experimental data, mathematical models are developed that can be used to predict the material's behavior in different configurations or to incorporate EPS properties into numerical simulations. All proposed models align with values coming from previous studies.

Individuals exposed to the propagation of shock waves generated by the detonation of explosive charges may suffer Traumatic Brain Injury. The mechanism of cranial deflection is one of many hypotheses that could explain the observed brain damage. To investigate this physical phenomenon in a reproducible manner, a new simplified cranial substitute was designed with a mechanical response close to that of a human skull when subjected to this type of loading. As a first step, a Finite Element Model was employed to dimension the new substitute. The objective was indeed to obtain a vibratory behavior close to that of a dry human skull over a wide range of frequencies up to 10 kHz. As a second step, the Finite Element Model was used together with Experimental Modal Analyses to identify the vibration modes of the substitute. A shaker excited the structure via a metal rod, while a laser vibrometer recorded the induced vibrations at defined measurement points. The results showed that despite differences in material properties and geometry, the newly developed substitute has 10/13 natural frequencies in common with those of dry human skulls. When filled with a simulant of cerebral matter, it could therefore be used in future studies as an approximation to assess the mechanical response of a simplified skull substitute to a blast threat.

Model-based brain injury criteria can present higher potential to predict injury than global head kinematic parameters. Numerical head injury prediction tools are time consuming and require Finite Element (FE) skilled users. To address these difficulties, a deep learning technique was applied to an existing previously developed brain FE model for which an injury risk curve has been proposed in terms of maximum Von Mises stress to predict moderate diffuse axonal injury. A total of 4492 experimental head impacts coming from experimental helmet testing were considered as input for the analysis. Each input was expressed in terms of three linear accelerations and three angular velocities versus time, when the target metric was the time history Von Mises Stress (vms) curve computed within the brain via the FE analysis. The architecture used for the Deep Learning (DL) model is the U-Net and four models based on it were evaluated. The dataset was split into three datasets dedicated for learning and testing. The quality of the DL models were assessed via the Maximum Absolute Error between FE and DL models computed brain maximum vms. Further, a regression analysis of brain response with both methods was conducted. The results demonstrated that deep learning methods can be applied in the context of brain response estimation when helmet assessment is considered without any FE computation, only by considering the 6 D head kinematic vs time demonstrating that the deep learning approach should be further developed for research and industrial applications.

Although blast-induced Traumatic Brain Injury (bTBI) has become a signature wound of conflict, its cause is not yet fully understood. Regarding primary blast injuries, i.e., those caused by the propagation of shock waves in the body, four direct and two indirect injury mechanisms have been mainly proposed in the literature. Since numerous authors have exposed instrumented animals, Post-Mortem Human Subjects (PMHS), and head substitutes to blast conditions, the aim of this review is to classify them in terms of threat, instrumentation, and investigated mechanisms. In the first part, data are collected from 6 studies on PMHS, 1 on primates, 11 on rodents, and 6 on swine for comparison purposes. Peak amplitudes of reflected pressures, intracranial pressures and cranial strains are extracted and analyzed to establish trends. Despite the small number of comparable studies, several similarities can be highlighted. Indeed, the analyses revealed a dose-response effect for most measurements. The results also depend on the orientation of the subject (forward, backward, and sideways) for the PMHS, primates, and swine. The second goal of this review is to evaluate the behavior of substitutes developed to replace PMHS experiments. Shell strains and internal pressures are thus collected on 19 geometric and anthropomorphic substitutes to assess whether they faithfully represent a human head. The results showed that these substitutes are for the most part not properly designed and therefore cannot yet reliably replace PMHS experimental data.

Introduction Soccer is the most popular sport in the world. This contact sport carries the risk of exposure to repeated head impacts in the form of subconcussions, defined as minimal brain injuries following head impact, with no symptom of concussion. While it has been suggested that exposure to repetitive subconcussive events can result in long-term neurophysiological modifications, and the later development of chronic traumatic encephalopathy, the consequences of these repeated impacts remain controversial and largely unexplored in the context of soccer players. Methods and analysis This is a prospective, single-centre, exposure/non-exposure, transverse study assessing the MRI and neuropsychological abnormalities in professional retired soccer players exposed to subconcussive impacts, compared with high-level athletes not exposed to head impacts. The primary outcome corresponds to the results of MRI by advanced MRI techniques (diffusion tensor, cerebral perfusion, functional MRI, cerebral volumetry and cortical thickness, spectroscopy, susceptibility imaging). Secondary outcomes are the results of the neuropsychological tests: number of errors and time to complete tests. We hypothesise that repeated subconcussive impacts could lead to morphological lesions and impact on soccer players’ cognitive skills in the long term. Ethics and dissemination Ethics approval has been obtained and the study was approved by the Comité de Protection des Personnes (CPP) No 2021-A01169-32. Study findings will be disseminated by publication in a high-impact international journal. Results will be presented at national and international imaging meetings. Trial registration number NCT04903015.

To compare instrumentation configurations consisting of bilateral single or double rods and additional interbody cages (IBCs) at different levels in terms of Range of Motion (ROM) and distribution of von Mises stress in rods. A previously validated L1-pelvis finite element model was used and instrumented with configurations consisting of single or double bilateral rods and IBCs at multiple levels. Pure moments of 7.5 N.m were applied to L1 in main directions in addition to a follower load of 280 N. Global, segmental ROM and distribution of von Mises stress in rods were studied. All configurations reduced segmental and global ROM from 50 to 100% compared to the intact spine. Addition of IBCs slightly increased ROM at levels adjacent to the IBC placement. The simple rod configuration presented the highest von Mises stress (457 MPa) in principal rods at L5-S1 in flexion. Doubling rods and IBC placement reduced this value and shifted the location of maximum von Mises stress to other regions. Among studied configurations, double rods with IBCs at all levels (L2-S1) showed the lowest ROM. Maximal von Mises stresses in secondary rods were lower in comparison to main rods. Double rods and IBCs reduced global and segmental ROM as well as von Mises stress in rods. The results suggest a possible benefit in using both strategies to minimize pseudarthrosis and instrumentation failure. However, increased ROM in adjacent levels and the shift of maximal von Mises stress to adjacent areas might cause complications elsewhere.

Surgical corrections of degenerative lumbar scoliosis and sagittal malalignment are associated with significant complications, such as rod fractures and pseudarthrosis, particularly in the lumbosacral junction. Finite element studies can provide relevant insights to improve performance of spinal implants. The aim of the present study was to present the development of non-instrumented and instrumented Finite Element Models (FEMs) of the lumbopelvic spine and to compare numerical results with experimental data available in the literature. The lumbo-pelvic spine FEM was based on a CT-scan from an asymptomatic volunteer representing the 50th percentile male. In a first step a calibration of mechanical properties was performed in order to obtain a quantitative agreement between numerical results and experimental data for defect stages of spinal segments. Then, FEM results were compared in terms of range of motion and strains in rods to in-vitro experimental data from the literature for flexible non-instrumented and instrumented lumbar spines. Numerical results from the calibration process were consistent with experimental data, especially in flexion. A positive agreement was obtained between FEM and experimental results for the lumbar and sacroiliac segments. Instrumented FEMs predicted the same trends as experimental in-vitro studies. The instrumentation configuration consisting of double rods and an interbody cage at L5-S1 maximally reduced range of motion and strains in main rods and thus had the lowest risk of pseudarthrosis and rod fracture. The developed FEMs were found to be consistent with published experimental results; therefore they can be used for further post-operative complication investigations.

Concussion, in spite of being a mild traumatic brain injury, involves serious long term consequences and can adversely affect the life of an individual, their family and the wider society. Since, diffuse axonal injury (DAI) is known to be one of the most frequent pathological features of traumatic brain injury (TBI), knowledge of the mechanical threshold for concussion in terms of axonal strain can help in developing better brain injury prediction tools in the context of head protection system optimization and the management of sport related concussions. This paper presents development, validation and utilization of an anisotropic viscous hyperelastic finite element rat brain model for investigation of the mechanical threshold for concussion in terms of axonal strain. For the investigation, twenty-six well documented cases of experimental concussion were simulated. A thorough statistical analysis of global kinematic parameters (maximum rotational acceleration and duration) and intra-cerebral parameters (maximum axonal strain, maximum strain energy, maximum von Mises stress, maximum von Mises strain, maximum shear stress, maximum shear strain, maximum principal stress, maximum principal strain, minimum pressure and maximum pressure) revealed that intra-cerebral parameters are better suited for the prediction of concussion than the global kinematic parameters. The estimated tolerance level for a 50% risk of concussion was found to be 8.97% of maximum axonal strain. The results are promising and hence, this study is not only a key step towards better understanding of concussion, but it also contributes towards concussion related investigations.

Traumatic brain injury is the leading cause of death and permanent impairment in accidents. In both the severe and mild TBI, diffuse axonal injury (DAI) is the most common pathology. Computation of axon elongation by using finite element head model in numerical simulation can enlighten the DAI mechanism and helps to establish advanced tissue level head injury criteria. The main objective of this research is to propose a brain injury criterion based on multiscale computation of axonal elongation under real-world head trauma.

Advanced neck finite element modeling and development of neck injury criteria are important for the design of optimal neck protection systems in automotive and other environments. They are also important in virtual tests. The objectives of the present study were to develop a detailed finite element model (FEM) of the human neck and couple it to the existing head model, validate the model with kinematic data from legacy human volunteer and human cadaver impact datasets, and derive lateral impact neck injury risk curves using survival analysis from the upper and lower neck forces and moments. The detailed model represented the anatomy of a young adult mid-size male. It included all the cervical and first thoracic vertebrae, intervening discs, upper and lower spinal ligaments, bilateral facet joints, and passive musculature. Material properties were obtained from literature. Frontal, oblique, and lateral impacts to the distal end of the model was applied based on human volunteer and human cadaver experimental data. Corridor and cross-correlation methods were used for validation. The CORrelation and Analysis (CORA) score was used for objective assessments. Forces and moments were obtained at the occipital condyles (OC) and T1, and parametric survival analysis was used to derive injury risk curves to define human neck injury tolerance to lateral impact. The Brier Score Metric (BSM) was used to determine the hierarchical sequence among the injury metrics. The CORA scores for the lateral, frontal, and oblique impact loading conditions were 0.80, 0.91, and 0.87, respectively, for human volunteer data, and the mean score was 0.7 for human cadaver lateral impacts. Injury risk curves along with ±95% confidence intervals are given for all the four biomechanical metrics. The OC shear force was the optimal metric based on the BSM. A force of 1.5 kN was associated with the 50% probability level of AIS3+ neck injury. As a first step, the presented risk curves serve as human tolerance criteria under lateral impact, hitherto not available in published literatures, and they can be used in virtual testing and advancing restraint systems for improving human safety.

Multy-body simulation is applied to compute the motorcyclist's kinematic in case of virtual motorcycle versus car accident. Results show that for over 50% of the simulations, the impact velocities of the whole body exceed 8 m/s when impacting the vehicle and 4 m/s at the time of road impact. The head impacts the vehicle most often with a speed exceeding 7.5 m/s in the fronto-lateral impact configuration, when thorax, abdomen and pelvis are the most exposed segments for the other impact configurations, with impact velocities beeing in the range of 10 to 15 m/s. Changes in BMI leads to minor influence on the impact velocities of the different body, especially for the first impact. As expected, body impact speed increases with the speed of the motorcycle, for both, the first and the second impacts. These results will contribute to an improvement in the evaluation and optimization of motorcyclist's protection garnement.