American Football On-Field Head Impact Kinematics: Influence of Acceleration Signal Characteristics on Peak Maximal Principal Strain.

Recorded head kinematics from head-impact measurement devices (HIMd) are pivotal for evaluating brain stress and strain through head finite element models (hFEM). The variability in kinematic recording windows across HIMd presents challenges as they yield inconsistent hFEM responses. Despite establishing an ideal recording window for maximum principal strain (MPS) in brain tissue, uncertainties persist about the impact characteristics influencing vulnerability when this window is shortened. This study aimed to scrutinize factors within impact kinematics affecting the reliability of different recording windows on whole-brain peak MPS using a validated hFEM. Utilizing 53 on-field head impacts recorded via an instrumented mouthguard during a Canadian varsity football game, 10 recording windows were investigated with varying pre- and post-impact-trigger durations. Tukey pair-wise comparisons revealed no statistically significant differences in MPS responses for the different recording windows. However, specific impacts showed marked variability up to 40%. It was found, through correlation analyses, that impacts with lower peak linear acceleration exhibited greater response variability across different pre-trigger durations. Signal shape, analyzed through spectral analysis, influenced the time required for MPS development, resulting in specific impacts requiring a prolonged post-trigger duration. This study adds to the existing consensus on standardizing HIMd acquisition time windows and sheds light on impact characteristics leading to peak MPS variation across different head impact kinematic recording windows. Considering impact characteristics in research assessments is crucial, as certain impacts, affected by recording duration, may lead to significant errors in peak MPS responses during cumulative longitudinal exposure assessments.

Automatic detection of spinal injuries under dynamic compressive loading using high-speed cine-radiography.

Every year, new cases of individuals suffering from traumatic spinal injuries are detected. Advances in numerical models have allowed for the understanding of the damage caused by trauma and its impact on the patient's life. However, the kinematics and dynamics of vertebral fracture formation from its point of origin to the speed of propulsion of the fragments remain unknown. This is mainly due to the lack of data that essentially includes high-speed videos, load and displacement measurements during experimental tests reproducing spinal traumatic loading conditions. This lack of data can be addressed by the analysis of X-Ray images of animal specimens acquired during the traumatic spinal injury formation process. Thus, the purpose of this study was to develop an approach to automatically detect and track in vitro vertebral fractures using high-speed cine-radiography imaging. Four segments of porcine thoracolumbar vertebrae were dynamically compressed using a servo-hydraulic test bench. The compression process was filmed with a custom high-speed cine-radiography device, and the imaging parameters were optimized based on the physical properties of vertebrae. This paper demonstrates the feasibility of using high-speed cine-radiography imaging in this way, combined with an image processing pipeline to allow automatic documentation of the fracture's appearance and its evolution in the vertebra over time.Clinical Relevance- The proposed method will provide helpful information for proper handling of traumatic spinal injuries.

Contribution of impactor misalignment to the neurofunctional variability in porcine spinal cord contusion models.

Traumatic spinal cord lesions studies are often carried out with animal models or numerical simulations. Unfortunately, animal models usually present a high variability in severity and type of neurofunctional impairments following impact surgery. We postulate that the variability of outcomes is strongly dependent on the positioning and alignment of the impactor during the contusion. A finite elements model of the spinal cord, predicting the action potential (AP) conduction alteration, was proposed and used to perform nine numerical simulations of a 50 g weight dropped from 200 mm on the exposed spinal cord in its spinal canal. Simulations followed a 32 factorial design with impactor eccentricity and spinal cord tilt angle as factors on two outcomes: injured spinal cord area (AP < 10 % of its baseline, 1h post-injury), and asymmetry of injury (ratio of right/left injured area of both half spinal cord). Eccentricity contributed highly and significantly on both outcomes, but not tilt angle. Damaged axons were found in conscious motor, sensory, and unconscious proprioception tracts. Variability in impactor alignment beyond ±6.2 % of the spinal canal width affects neurofunctional outcomes, and careful assessment of the impactor course is therefore key when producing spinal cord injury by contusion.Clinical Relevance- A precision value is proposed to mitigate the contribution of impactor misalignment to neurofunctional variability in animal models, allowing the reduction of animal used in research. The proposed method of action potential conduction assessment could easily be implanted in human numerical models for the cross-study of patient's cases.

Gait analysis of a post induced traumatic spinal cord injury porcine model.

Porcine model constitutes a potential translational model to study traumatic spinal cord injuries (TSCI) considering its recent use in numerous studies. Recovery of the animal is currently monitored through a qualitative evaluation of the gait. Adding a quantitative evaluation might help to better assess the functional recovery of the animal. In this study, a new controlled method involving the use of an electro-magnetic actuator was used on a pig to induce a TSCI. Chronic monitoring was done using a quantitative analysis of the gait. Results show both, the injury of the pig and its functional recovery. This large animal model will help to provide a better understanding of injury and recovery mechanisms and thus could constitute a strong preclinical model for future therapeutic studies.Clinical Relevance- Methodology and results from this study would provide a better insight on the functional recovery after traumatic spinal cord injuries.

In vitro assessment of the role of the nucleus pulposus in the mechanism of vertebral body fracture under dynamic compressive loading using high-speed cineradiography.

Traumatic Spinal Cord Injuries (TSCI) have a disastrous effect on the physical and mental health of both the patients and their relatives. Around 15 % of these injuries are caused by burst fractures, a sub-type of compressive fractures of the vertebral body. The transient dynamics of these fracture have been studied through in vitro experiments coupled with numerical simulations, but no direct observation have ever been made of their genesis and evolution and the behaviour of the nucleus pulposus under compressive loading has only been hypothesized. The purpose of this study was to evaluate the interactions between the vertebral body and the nucleus pulposus under dynamic compressive loading using high-speed cineradiography. A radiopaque agent was injected into the nuclei pulposi of 4 young porcine thoraco-lumbar and lumbar cadaveric segments, and a dynamic compressive load was applied to them using a servo-hydraulic bench-test. The compression process was filmed with a custom high-speed fluoroscope. The nucleus pulposus loaded the vertebral endplate up to 14,142 ± 486 N, before fracturing it and diffusing into the vertebral body. Then, internal pressure seemingly built up until an outward projection of the nucleus pulposus, at an antero-posterior velocity up to 2.9 m.s-1, or until retroprojection of bony fragments into the spinal canal. These results directly corroborate the hypotheses previously made by other studies and stress the unprecedented advantages of using high-speed cineradiography for the study of complex fractures genesis and evolution.Clinical Relevance- Methodology and results from this study would provide an unprecedented insight on the genesis and transient evolution of complex spinal fractures.

Ultrashort laser induced spatial redistribution of silver species and nano-patterning of etching selectivity in silver-containing glasses.

Femtosecond laser-induced spatial redistribution of silver species (ions, clusters, and hole centers) in a silver-containing phosphate glass is investigated by correlative means of near-field scanning optical microscopy (NSOM) images, numerical simulations, chemical micro-probe analysis, and nanoscale spatial profiles after soft etching. In particular, we found that the chemical etching selectivity for nanoscale patterning is strongly dependent upon the irradiation of femtosecond laser due to the spatial redistribution of silver species within the affected area. These results strongly indicate that controlling the distribution of silver species by femtosecond laser irradiation may open new routes for surface nanoscale chemical and/or spatial patterning for the fabrication of 2D surface photonic crystals.

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.

A novel spinal cord surrogate for the study of compressive traumatic spinal cord injuries.

Although in vitro studies are frequent for the study of traumatic spine and spinal cord injuries, few include the spinal cord due to its prompt post-mortem decay. Several materials have been proposed to mimic the spinal cord behaviour, but none matched its mechanical properties under transverse compression, which is vital for the study of burst fractures and other injury mechanisms leading to spinal cord compression. In this study, a new material named Soma Foama 15 (Reynolds Advanced Material, USA) was used to manufacture three spinal cord surrogates at 3 mixing ratios of elastomer to catalyst (1:1, 2:1 and 3:1) and tested at three different strain rates (0.5, 5 and 50 .s-1). The mixing ratio 3:1 presents a mechanical behaviour comparable to that of the porcine spinal cord at each of these strain rates, making the surrogate a valid substitute up to 75 % of transverse compression.

Laser writing of nonlinear optical properties in silver-doped phosphate glass.

The formation of both local second- and third-harmonic generations (SHG and THG) induced by a train of femtosecond laser pulses in silver-doped phosphate glasses is addressed. Based on modeling calculations, including various diffusion and kinetic processes, THG is shown to result from the formation of silver clusters. The latter organize into a ring-shape structure, leading to the emergence of a static electric field. By breaking the glass centro-symmetry, this field gives rise to a local effective second-order susceptibility, inducing SHG. Both theoretically predicted SHG and THG evolutions with respect to the number of pulses in the train are in good agreement with experimental observations. In particular, the observed reaching of a maximum in the nonlinear optical responses after a few thousands of pulses is explained by the competition of various physical processes. A cooling of the glass is shown to improve the process efficiency of the laser writing of second-order nonlinearity.

Biomechanical testing of a hybrid locking plate fixation of equine sesamoid osteotomies.

OBJECTIVE: To compare the biomechanical properties of a hybrid locking compression plate (LCP) construct with the compression screw technique as a treatment for transverse mid-body proximal sesamoid bone fractures. METHODS: Ten paired forelimbs from abattoir horses were used. The medial proximal sesamoid bone of each limb was osteotomized transversely and randomly assigned, to either repair with a two-hole 3.5 mm LCP or a 4.5 mm cortical screw placed in lag fashion. Each limb was tested biomechanically by axial loading in single cycle until failure. The point of failure was evaluated from the load-displacement curves. Then a gross evaluation and radiographs were performed to identify the mode of failure. RESULTS: The loads to failure of limbs repaired with the hybrid LCP construct (4968 N ± 2167) and the limbs repaired with the screw technique (3009 N ± 1091) were significantly different (p <0.01). The most common mode of failure was through a comminuted fracture of the apical fragment of the proximal sesamoid bone. CLINICAL SIGNIFICANCE: The LCP technique has potential to achieve a better fracture stability and healing when applied to mid-body fractures of the proximal sesamoid bone. Further testing, particularly fatigue resistance is required to corroborate its potential as a treatment option for mid-body fractures of the proximal sesamoid bone.

Biomechanical evaluation of pedicle screw loosening mechanism using synthetic bone surrogate of various densities.

Pedicle screw fixation is a well-established procedure for various spinal disorders. However, pedicle screws failures are still reported. Therefore, there is a need for a greater understanding of the pedicle screw failure mechanism. This experimental study investigates the biomechanical stability of pedicle screws using a synthetic bone surrogate with a special focus on the screw loosening mechanism. Pedicle screws have been inserted in thirty six polyurethane foam blocks of three different densities. In half of the specimens from each density group, pedicle screws were submitted to cyclic bending (toggling) before pullout. The rest of specimens were solely loaded in axial pullout. The peak pullout force and stiffness were determined from load-displacement curve of each specimen. Statistical analyses were performed to investigate on the effect of toggling and bone surrogate density on the pedicle screw's pullout force. The results suggest that the pullout force and stiffness were significantly affected by toggling and density. Higher pullout forces resulted from higher grades of density. The proposed method allowed investigating the pedicle screw loosening mechanism. However, conducing further experimental tests on animal or cadaveric vertebrae are needed to confirm these findings.

Estimation of pedicle screw fixation strength from probe indentation force and screw insertion torque: a biomechanical study on bone surrogates of various densities.

Posterior pedicle screw fixation is commonly used for patients with spinal disorders. However, failure of fixation is reported in many cases and surgeons have only little information. The objective of this study was to assess the correlation between the probe indentation force, screw insertion torque and the pullout force using bone surrogates of different densities. The indentation force and insertion torque were measured using a custom made test bench during screw insertion into polyurethane foam blocks. The two variables were significantly correlated to pullout force and to density. A high correlation was also found between indentation force and the peak insertion torque. The proposed methods for measuring indentation force and screw insertion torque were reproducible. This study suggests that the peak screw insertion torque and the indentation force can predict the screw fixation strength in synthetic bone models. Additional tests should be performed on animal and human specimens to confirm and to translate these findings to clinical applications.

Two-photon excited fluorescence in the LYB:Eu monoclinic crystal: towards a new scheme of single-beam dual-voxel direct laser writing in crystals.

We report on two-photon excited fluorescence in the oriented Eu(3+)doped LYB monoclinic crystal under femtosecond laser tight focusing. Due to spatial walk-off, the two polarization modes of the incident femtosecond beam simultaneously provide the independent excitation of two distinct focuses, leading to a single-beam dual-voxel nonlinear excitation of fluorescence below material modification threshold. These observations emphasize on the anisotropy of both two-photon absorption as well as fluorescence emission. They demonstrate the localized control of the nonlinear energy deposit, thanks to the adjustment of both the input power and polarization, by properly balancing the injected energy in each voxel. Such approach should be considered for future direct laser writing of waveguides in propagation directions out of the dielectric axes, so as to optimally cope with the highly probable anisotropy of laser-induced material modification thresholds in these crystals. These results open new ways for further potential developments in direct laser writing as the simultaneous inscription of double-line structures for original waveguides processes.

Higher-order Kerr improve quantitative modeling of laser filamentation.

We test numerical filamentation models against experimental data about the peak intensity and electron density in laser filaments. We show that the consideration of the higher-order Kerr effect improves the quantitative agreement without the need of adjustable parameters.

Comparison of the influences of structural characteristics on bulk mechanical behaviour: experimental study using a bone surrogate.

An experimental study was conducted to classify the influence of trabecular architecture and cortical shell thickness on the mechanical properties using a bone surrogate. Thirty-six rectangular prisms and 18 vertebral-shaped specimens were fabricated with fused deposition modelling (FDM) as a bone surrogate with controlled structural characteristics (cortical wall thickness, strut spacing, strut angle and strut orientation). The apparent density of the FDM specimens was evaluated using quantitative computed tomography (QCT) imaging and related to the apparent elastic modulus measured with compression testing. The effects of the structural parameters on the apparent elastic modulus were analysed using analysis of variance (ANOVA). The results obtained corroborate that the structural parameters have a significant effect on the apparent mechanical properties of the bulk material. The cortical shell thickness was found to have more influence than trabecular architecture. Therefore, accurate modelling of the cortical shell thickness should be considered more important than trabecular architecture in development of bone finite element models and bone surrogates.

Field measurements suggest the mechanism of laser-assisted water condensation.

Because of the potential impact on agriculture and other key human activities, efforts have been dedicated to the local control of precipitation. The most common approach consists of dispersing small particles of dry ice, silver iodide, or other salts in the atmosphere. Here we show, using field experiments conducted under various atmospheric conditions, that laser filaments can induce water condensation and fast droplet growth up to several µm in diameter in the atmosphere as soon as the relative humidity exceeds 70%. We propose that this effect relies mainly on photochemical formation of p.p.m.-range concentrations of hygroscopic HNO(3), allowing efficient binary HNO(3)-H(2)O condensation in the laser filaments. Thermodynamic, as well as kinetic, numerical modelling based on this scenario semiquantitatively reproduces the experimental results, suggesting that particle stabilization by HNO(3) has a substantial role in the laser-induced condensation.

Strength evaluation of a variable diameter acetabular trial implant under realistic loading conditions.

A variable diameter acetabular trial implant (VDATI) was designed to reduce the costs related to cleaning, sterilisation and storage of surgical instruments used for total hip arthroplasty. The purpose of this study was to evaluate the mechanical strength of a functional prototype of the VDATI. Experimental testing was performed to identify if the VDATI can resist loading conditions similar to the ones occurring during the surgical procedure and to validate a finite element model (FEM) of the VDATI. The results highlighted the potential of the current concept of the VDATI and demonstrated the relevance to continue its development.

Finite element model of a greater trochanteric reattachment system.

Detachment of greater trochanter (GT) is generally associated with hip arthroplasty complications and needs for repositioning and fixation. A new GT reattachment system (Y3) was proposed to reduce GT displacements in anterior-posterior direction to decrease non-union issues. The goal of this study is to develop and validate a FEM of the Y3 GTR system. FEM validation suggests a good concordance between numerical and experimental GT displacements. Sensitivity study show that the transition between proximal and distal branches of Y3 design is the most influent part on all GT displacements. The anterior branch affects more anterior-posterior displacements and rotation while the posterior branch affects more proximal displacements and rotation. This study provides an improved understanding of the influence of Y3 geometry on GT displacements.

Finite element analysis of an acetabular trial implant.

A variable diameter trial liner (VDTL) was designed to reduce the costs related to cleaning, sterilisation and storage of surgical instruments used for total hip arthroplasty. The purpose of this study was to develop a finite element model to evaluate the mechanical behavior of the VDTL before manufacturing of a functional prototype. The finite element analysis consists to identify the maximum stresses applied on the VDTL and compare these values to the yield strength of stainless steel 17-4 (540 MPa) to asses if the VDTL will resist to loading conditions similar to the ones occurring during the surgical procedure. The results demonstrated the need to improve the mechanical strength of the current concept of the VDTL. These results will serve as indications for upcoming design refinements.