Numerical Investigation of Spinal Cord Injury After Flexion-Distraction Injuries at the Cervical Spine.

Flexion-distraction injuries frequently cause traumatic cervical spinal cord injury (SCI). Post-traumatic instability can cause aggravation of the secondary SCI during patient care. However, there is little information on how the pattern of disco-ligamentous injury affects the SCI severity and mechanism. This study objective was to analyze how posterior disco-ligamentous injuries affect spinal cord compression and stress and strain patterns in the spinal cord during post-traumatic flexion and extension. A cervical spine finite element model including the spinal cord was used and different combinations of partial or complete intervertebral disc (IVD) rupture and disruption of various posterior ligaments were modeled at C4-C5, C5-C6, or C6-C7. In flexion, complete IVD rupture combined with posterior ligamentous complex rupture was the most severe injury leading to the highest von Mises stress (47-66 kPa), principal strains p1 (0.32-0.41 in white matter) and p3 (-0.78 to -0.96 in white matter) in the spinal cord and the highest spinal cord compression (35-48%). The main post-trauma SCI mechanism was identified as the compression of the anterior white matter at the injured level combined with distraction of the posterior spinal cord during flexion. There was also a concentration of the maximum stresses in the gray matter during post-traumatic flexion. Finally, in extension, the injuries tested had little impact on the spinal cord. The capsular ligament was the most important structure to protect the spinal cord. Its status should be carefully examined during the patient's management.

Simulation of the objective occlusion effect induced by bone-conducted stimulation using a three-dimensional finite-element model of a human head.

The occlusion effect (OE) refers to the phenomenon that more bone-conducted physiological sounds are transmitted into the earcanal when it is blocked and may cause discomfort on users of hearing protection devices. Models have been proposed to study the OE as they can help understand the physical mechanisms and can be used to evaluate the individual contribution on the OE of the factors that may affect it (i.e., occlusion device, ear anatomy, and stimulation). The existing finite element models developed to study the OE are limited by their truncated ear geometries. In order to progress in the understanding of the OE, the goal of this paper is to develop a finite element model of an entire head to predict the sound pressure field in its earcanals, open or occluded by earplugs. The model is evaluated by comparing the computed input mechanical impedances and OEs in various configurations with literature data. It is able to reproduce common behavior of the OE reported in the literature. In addition, the model is used to assess the effects on the simulated OEs of several parameters, including the modeling of the external air, the boundary condition at the head base and the material properties.

A Sensitive and Fast Fiber Bragg Grating-Based Investigation of the Biomechanical Dynamics of In Vitro Spinal Cord Injuries.

To better understand the real-time biomechanics of soft tissues under sudden mechanical loads such as traumatic spinal cord injury (SCI), it is important to improve in vitro models. During a traumatic SCI, the spinal cord suffers high-velocity compression. The evaluation of spinal canal occlusion with a sensor is required in order to investigate the degree of spinal compression and the fast biomechanical processes involved. Unfortunately, available techniques suffer with drawbacks such as the inability to measure transverse compression and impractically large response times. In this work, an optical pressure sensing scheme based on a fiber Bragg grating and a narrow-band filter was designed to detect and demonstrate the transverse compression inside a spinal cord surrogate in real-time. The response time of the proposed scheme was 20 microseconds; a five orders of magnitude enhancement over comparable schemes that depend on costly and slower optical spectral analyzers. We further showed that this improvement in speed comes with a negligible loss in sensitivity. This study is another step towards better understanding the complex biomechanics involved during a traumatic SCI, using a method capable of probing the related internal strains with high-spatiotemporal resolution.

Biomechanical evaluation of bovine stifles stabilized with an innovative braided superelastic nitinol prosthesis after transection of the cranial cruciate ligament.

OBJECTIVE: To determine the stability bovine stifles stabilized with nylon or nitinol superelastic prostheses after transection of the cranial cruciate ligament (CCL). STUDY DESIGN: Ex vivo study. SAMPLE POPULATION: Stifles (n = 15) harvested from adult bovine cadavers. METHODS: The stifles were randomly assigned pairwise to a ligament reconstruction technique (n = 5): (1) and (2) Hamilton's technique using a prosthesis made of 24 nitinol strands (0.39 mm) braided at 40°or single 600-lb test nylon implant, and (3) nitinol prosthesis placed in femoral and tibial bone tunnels (bone-to-bone). Craniocaudal tibial translation at ±2000 N was applied to the tibia, and mediolateral angular displacement via measured under torsional tibial loading at ±60 Nm on three occasions: intact CCL, transected, and stabilized. Outcomes were evaluated with a mixed effect linear model for repeated measures. RESULTS: Bone-to-bone using nitinol was the only repair that decreased tibial translation after CCL transection (p = .001) with a 23% change magnitude compared with intact CCL. Hamilton was the only stabilization reestablishing angular displacement, similar to intact CCL (p = .109 and .134 for nitinol and nylon). Bone-to-bone nitinol stabilization decreased angular displacement after CCL-transection with an 8% change magnitude (p = .040) without returning to normal values. CONCLUSION: CCL replacement with nylon did restore joint stability. Nitinol prostheses passed through single femoral and tibial bone tunnels (bone-to-bone) were the only techniques reducing tibial translation. CLINICAL SIGNIFICANCE/IMPACT: Bone-to-bone stabilization with a nitinol prosthesis may be considered as an alternative to nylon for CCL replacement in cattle. These results provide evidence to justify clinical evaluation in cattle undergoing CCL replacement.

Traumatic Spinal Cord Injuries with Fractures in a Québec Level I Trauma Center.

BACKGROUND: Traumatic spinal cord injuries (TSCI) have devastating consequences on patients' quality of life. More specifically, TSCI with spinal fractures (TSCIF) have the most severe neurological impairment, although limited data are available. This study aimed at providing data and analyzing TSCIF in a level I trauma center in the province of Québec, Canada. METHODS: Two hundred eighty-two TSCIF were reviewed. Spinal injuries and neurological impairment were assessed with AO classification and AIS, respectively. Variables included age, sex, cause, location, mechanism of injury (MOI), and severity of TSCIF. Chi-squared Pearson determined significant associations (p < 0.05). RESULTS: Male-to-female ratio was 3.21:1. Patients were 42.5 ± 18.7 years. The leading causes of TSCIF were high-energy falls (28.4%), cars (26.2%) and vehicle without restraint system (motorcycle, all-terrain vehicle, snowmobile, and bicycle) (21.3%). Vehicle collisions, pooling cars and unrestrained vehicles, mostly affected the 20-49-year population (62.2%). The main MOI was distraction in males (47.9%), and axial compression in females (44.8%). There were significant associations between causes and injured spinal level, as well as between MOI and injured spinal level, sex, and TSCIF severity. Most patients involved in unrestrained vehicle accidents sustained a thoracolumbar spine distraction with complete motor deficit. A severe neurologic deficit affected most patients following car accidents that caused cervical spine distraction or axial torsion. CONCLUSIONS: In Québec, most TSCIF caused by vehicle collisions affect a young population and have severe neurological impairments. Future efforts should focus on better understanding accidents involving the unrestrained vehicle category to further improve preventive measures.

Des lésions traumatiques de la moelle épinière associées à des fractures dans le cadre d'un centre de traumatologie de niveau 1 du Québec Contexte: Les lésions traumatiques de la moelle épinière (LTME) ont des conséquences catastrophiques sur la qualité de vie des patients qui en sont victimes. De façon plus particulière, il faut savoir que les LTME associées à des fractures vertébrales sont celles qui entraînent, bien que les données à ce sujet soient limitées, les déficiences neurologiques les plus graves. Cette étude vise à collecter des données et à analyser les LTME associées à des fractures vertébrales dans un centre de traumatologie de niveau 1 situé au Québec (Canada). Méthodes: Au total, nous avons examiné 282 cas de LTME associés à des fractures vertébrales. Pour ce faire, nous avons évalué ces fractures au moyen de la classification Müller AO ; quant au niveau de déficience neurologique, nous l'avons évalué au moyen de l'échelle ASIA. Parmi les variables incluses dans cette étude, mentionnons l'âge, le sexe, la cause, l'endroit de l'incident, le mécanisme de blessure (mechanism of injury) ainsi que la gravité des LMTE associées à des fractures vertébrales. Enfin, c'est au moyen du test du X2 de Pearson qu'on a pu déterminer des associations statistiques valables (p < 0,05). Résultats: Le rapport hommes/femmes était de 3,2 :1. En moyenne, les patients étaient âgés de 42,5 ans ± 18,7 ans. Les principales causes de LMTE associées à des fractures vertébrales se sont révélées être des chutes à haut transfert d'énergie (28,4 %), des accidents de la route impliquant des automobiles (26,2 %) et des accidents impliquant des moyens de transport (motocyclettes, VTT, motoneiges et vélos) dépourvus d'un dispositif de retenue (21,3 %). Tant les collisions à bord d'une automobile que celles impliquant un moyen de transport sans dispositif de retenue ont surtout affecté la population des 20 à 49 ans (62,2 %). Chez les hommes, le principal mécanisme de blessure était la distraction de la colonne (47,9%) alors que chez la femme, c'était la compression axiale (44,8%). Des associations significatives sont apparues entre les causes énumérées ci-dessus et la gravité des blessures à la colonne vertébrale de même qu'entre le mécanisme de blessure et la gravité des blessures à la colonne vertébrale, le sexe des patients et la gravité des LMTE associées à des fractures vertébrales. La plupart des patients victimes d'un accident sur un véhicule sans dispositif de retenue ont subi une distraction thoraco-lombaire de la colonne vertébrale jumelée à un déficit moteur complet. Enfin, un déficit neurologique marqué a affecté la plupart des patients victimes d'un accident de la route ayant subi une distraction cervicale et une torsion axiale. Conclusions: Au Québec, la plupart des LMTE associées à des fractures vertébrales et causées par des accidents de la route affectent une population plus jeune et entraînent de graves déficits neurologiques. À l'avenir, on devrait tenter de mieux comprendre les accidents impliquant des moyens de transport dépourvus de dispositif de retenue afin d'améliorer davantage les mesures préventives.

Use of magnetic resonance image registration to estimate displacement in the human earcanal due to the insertion of in-ear devices.

In-ear devices are used in a wide range of applications for which the device's usability and/or efficiency is strongly related to comfort aspects that are influenced by the mechanical interaction between the device and the walls of the earcanal. Although the displacement of the earcanal walls due to the insertion of the device is an important characteristic of this interaction, existing studies on this subject are very limited. This paper proposes a method to estimate this displacement in vivo using a registration technique on magnetic resonance images. The amplitude, the location and the direction of the earcanal wall displacement are computed for four types of earplugs used by one participant. These displacements give indications on how each earplug deforms the earcanal for one specific earcanal geometry and one specific earplug insertion. Although the displacement due to a specific earplug family cannot be generalized using the results of this paper, the latter help to understand where, how much, and how each studied earplug deforms the earcanal of the participant. This method is revealed as a promising tool to investigate further acoustical and physical comfort aspects of in-ear devices.

Short-term changes in the physiology of the primary motor cortex following head impact exposure during a Canadian football game.

OBJECTIVE: This study investigated the association between head impact exposure (HIE) during varsity Canadian football games and short-term changes in cortical excitability of the primary motor cortex (M1) using transcranial magnetic stimulation (TMS). METHODS: Twenty-nine university-level male athletes wore instrumented mouth guards during a football game to measure HIE. TMS measurements were conducted 24 hours before and 1-2 hours after the game. Twenty control football athletes were submitted to a noncontact training session and underwent identical TMS assessments. Between-group changes in short-interval intracortical inhibition (SICI) ratios over time were conducted using two-way ANOVAs. The relationship between HIE (i.e., number, magnitude, and cumulative forces of impacts) and SICI (secondary outcome) was also investigated using Pearson correlations. RESULTS: Relative to controls, the group of athletes who had played a full-contact football game exhibited a significant intracortical disinhibition (p = 0.028) on the SICI 3-msec protocol (i.e., short interstimulus interval of 3 msec) within hours following the game. Moreover, exposure to ≥ 40g hits positively correlated with SICI disinhibition (p < 0.05). CONCLUSIONS: Athletes exposed to subconcussive hits associated with Canadian football exhibit abnormal M1 corticomotor inhibition function, particularly when the recorded impact magnitude was ≥ 40g. Given the deleterious effects of decreased inhibition on motor control and balance, systematically tracking head impact forces at each game and practice with contacts could prove useful for injury prevention in contact sports.

Mechanical differences of anterior and posterior spinal nerve roots revealed by tensile testing.

Cervical spondylotic myelopathy can damage the nerves and structures of the subarachnoidal canal. The spinal cord is attached to the subarachnoidal canal via structures such as nerve roots (NR), which play an important role in its positioning and can be affected by cervical spondylotic myelopathy Understanding the tensile mechanical properties of nerve roots is therefore crucial. A total of 37 swine nerve samples (15 bundles, 12 posterior roots, and 10 anterior roots) were mechanically tested within 12 h of sacrifice by a tensile test on a Mach 1 system (Biomomentum, Montreal, Canada) equipped with a 17 N load cell. Bi-linear piecewise fitting was performed to determine the elastic modulus, maximal strain and stress at failure, as well as the coefficients of the 1st- and 3rd-order Ogden models. Additionally, a sensitivity analysis on the Ogden coefficients was performed. The elastic modulus for the bundles, posterior roots, and anterior roots were 3.7 ± 3.1 MPa, 0.23 ± 0.18 MPa, and 0.31 ± 0.27 MPa, respectively. Significant differences (P < 0.05) were found between the bundles and the anterior/posterior roots. Coefficients for the 1st-, 2nd- and 3rd-order Ogden models were provided for each type of sample. Nerve roots have different properties depending on their types. The 1st-order Ogden model may be used to model the mechanical properties of NR using constitutive laws.

Cervical spine injury response to direct rear head impact.

BACKGROUND: Direct rear head impact can occur during falls, road accidents, or sports accidents. They induce anterior shear, flexion and compression loads suspected to cause flexion-distraction injuries at the cervical spine. However, post-mortem human subject experiments mostly focus on sled impacts and not direct head impacts. METHODS: Six male cadavers were subjected to a direct rear head impact of 3.5 to 5.5 m/s with a 40 kg impactor. The subjects were equipped with accelerometers at the forehead, mouth and sternum. High-speed cameras and stereography were used to track head displacements. Head range of motion in flexion-extension was measured before and after impact for four cadavers. The injuries were assessed from CT scan images and dissection. FINDINGS: Maximum head rotation was between 43 degrees and 78 degrees, maximum cranial-caudal displacement between -12 mm and - 196 mm, and antero-posterior displacement between 90 mm and 139 mm during the impact. Four subjects had flexion-distraction injuries. Anterior vertebral osteophyte identification showed that fractures occurred at adjacent levels of osteophytic bridges. The other two subjects had no anterior osteophytes and suffered from C2 fracture, and one subject also had a C1-C2 subluxation. C6-C7 was the most frequently injured spinal level. INTERPRETATION: Anterior vertebral osteophytes appear to influence the type and position of injuries. Osteophytes would seem to provide stability in flexion for the osteoarthritic cervical spine, but to also lead to stress concentration in levels adjacent to the osteophytes. Clinical management of patients presenting with osteophytes fracture should include neck immobilization and careful follow-up to ensure bone healing.

Experimental Bi-axial tensile tests of spinal meningeal tissues and constitutive models comparison.

Introduction This study aims at identifying mechanical characteristics under bi-axial loading conditions of extracted swine pia mater (PM) and dura and arachnoid complex (DAC). Methods 59 porcine spinal samples have been tested on a bi-axial experimental device with a pre-load of 0.01 N and a displacement rate of 0.05 mm·s-1. Post-processing analysis included an elastic modulus, as well as constitutive model identification for Ogden model, reduced Gasser Ogden Holzapfel (GOH) model, anisotropic GOH model, transverse isotropic and anisotropic Gasser models as well as a Mooney-Rivlin model including fiber strengthening for PM. Additionally, micro-structure of the tissue was investigated using a bi-photon microscopy. Results Linear elastic moduli of 108 ± 40 MPa were found for DAC longitudinal direction, 53 ± 32 MPa for DAC circumferential direction, with a significant difference between directions (p < 0.001). PM presented significantly higher longitudinal than circumferential elastic moduli (26 ± 13 MPa vs 13 ± 9 MPa, p < 0.001). Transversely isotropic and anisotropic Gasser models were the most suited models for DAC (r2  =  0.99 and RMSE:0.4 and 0.3 MPa) and PM (r2 = 1 and RMSE:0.06 and 0.07 MPa) modelling. Conclusion This work provides reference values for further quasi-static bi-axial studies, and is the first for PM. Collagen structures observed by two photon microscopy confirmed the use of anisotropic Gasser model for PM and the existence of fenestration. The results from anisotropic Gasser model analysis depicted the best fit to experimental data as per this protocol. Further investigations are required to allow the use of meningeal tissue mechanical behaviour in finite element modelling with respect to physiological applications. STATEMENT OF SIGNIFICANCE: This study is the first to present biaxial tensile test of pia mater as well as constitutive model comparisons for dura and arachnoid complex tissue based on such tests. Collagen structures observed by semi-quantitative analysis of two photon microscopy confirmed the use of anisotropic Gasser model for pia mater and existence of fenestration. While clear identification of fibre population was not possible in DAC, results from anisotropic Gasser model depicted better fitting on experimental data as per this protocol. Bi-axial mechanical testing allows quasi-static characterization under conditions closer to the physiological context and the results presented could be used for further simulations of physiology. Indeed, the inclusion of meningeal tissue in finite element models will allow more accurate and reliable numerical simulations.

Calibration of hyperelastic material properties of the human lumbar intervertebral disc under fast dynamic compressive loads.

Under fast dynamic loading conditions (e.g. high-energy impact), the load rate dependency of the intervertebral disc (IVD) material properties may play a crucial role in the biomechanics of spinal trauma. However, most finite element models (FEM) of dynamic spinal trauma uses material properties derived from quasi-static experiments, thus neglecting this load rate dependency. The aim of this study was to identify hyperelastic material properties that ensure a more biofidelic simulation of the IVD under a fast dynamic compressive load. A hyperelastic material law based on a first-order Mooney-Rivlin formulation was implemented in a detailed FEM of a L2-L3 functional spinal unit (FSU) to represent the mechanical behavior of the IVD. Bony structures were modeled using an elasto-plastic Johnson-Cook material law that simulates bone fracture while ligaments were governed by a viscoelastic material law. To mimic experimental studies performed in fast dynamic compression, a compressive loading velocity of 1 m/s was applied to the superior half of L2, while the inferior half of L3 was fixed. An exploratory technique was used to simulate dynamic compression of the FSU using 34 sets of hyperelastic material constants randomly selected using an optimal Latin hypercube algorithm and a set of material constants derived from quasi-static experiments. Selection or rejection of the sets of material constants was based on compressive stiffness and failure parameters criteria measured experimentally. The two simulations performed with calibrated hyperelastic constants resulted in nonlinear load-displacement curves with compressive stiffness (7335 and 7079 N/mm), load (12,488 and 12,473 N), displacement (1.95 and 2.09 mm) and energy at failure (13.5 and 14.7 J) in agreement with experimental results (6551 ± 2017 N/mm, 12,411 ± 829 N, 2.1 ± 0.2 mm and 13.0 ± 1.5 J respectively). The fracture pattern and location also agreed with experimental results. The simulation performed with constants derived from quasi-static experiments showed a failure energy (13.2 J) and a fracture pattern and location in agreement with experimental results, but a compressive stiffness (1580 N/mm), a failure load (5976 N) and a displacement to failure (4.8 mm) outside the experimental corridors. The proposed method offers an innovative way to calibrate the hyperelastic material properties of the IVD and to offer a more realistic simulation of the FSU in fast dynamic compression.

Tensile mechanical properties of the cervical, thoracic and lumbar porcine spinal meninges.

BACKGROUND: The spinal meninges play a mechanical protective role for the spinal cord. Better knowledge of the mechanical behavior of these tissues wrapping the cord is required to accurately model the stress and strain fields of the spinal cord during physiological or traumatic motions. Then, the mechanical properties of meninges along the spinal canal are not well documented. The aim of this study was to quantify the elastic meningeal mechanical properties along the porcine spinal cord in both the longitudinal direction and in the circumferential directions for the dura-arachnoid maters complex (DAC) and solely in the longitudinal direction for the pia mater. This analysis was completed in providing a range of isotropic hyperelastic coefficients to take into account the toe region. METHODS: Six complete spines (C0 - L5) were harvested from pigs (2-3 months) weighing 43±13 kg. The mechanical tests were performed within 12 h post mortem. A preload of 0.5 N was applied to the pia mater and of 2 N to the DAC samples, followed by 30 preconditioning cycles. Specimens were then loaded to failure at the same strain rate 0.2 mm/s (approximately 0.02/s, traction velocity/length of the sample) up to 12 mm of displacement. RESULTS: The following mean values were proposed for the elastic moduli of the spinal meninges. Longitudinal DAC elastic moduli: 22.4 MPa in cervical, 38.1 MPa in thoracic and 36.6 MPa in lumbar spinal levels; circumferential DAC elastic moduli: 20.6 MPa in cervical, 21.2 MPa in thoracic and 12.2 MPa in lumbar spinal levels; and longitudinal pia mater elastic moduli: 18.4 MPa in cervical, 17.2 MPa in thoracic and 19.6 MPa in lumbar spinal levels. DISCUSSION: The variety of mechanical properties of the spinal meninges suggests that it cannot be regarded as a homogenous structure along the whole length of the spinal cord.

Biomechanical analysis of pedicle screw placement: a feasibility study.

In spinal instrumentation surgery, the optimal placement of pedicle screws that takes into account the cortical/cancellous bone quality, geometry and property distribution, and screw design is still undetermined despite several in vitro experiments. The objective of this study was to evaluate the feasibility of using a detailed finite element model (FEM) of an instrumented vertebra to simulate screw axial pull-out and to analyze the bone-screw mechanical interaction. The FEM was built using CT-scan images of the L3 vertebra (0.6mm thick contiguous slices) of a 50th percentile human male volunteer, in order to virtually implant a fully customizable pedicle screw in a straight-forward position. The 753,000 elements model takes into account local cortical bone thickness and integrates advanced material behavior (elasto-plastic) laws that simulate bone failure. Screw axial pull-out was simulated and compared to in vitro experimental data, and the stress distribution at the screw thread-bone interface was analyzed. The simulated screw pull-out force (non-linear response with a failure at 640N) was within the range of experimental data (500-660N). Von Mises stresses in the bony structures were concentrated around the root of each internal thread, with the maximum stress located near the first proximal thread, in the cortical bone of the posterior wall of the pars. This study shows the feasibility and relevance of using a detailed FEM to simulate screw pull-out and to analyze the bone-screw mechanical interaction.

Effect of experimental, morphological and mechanical factors on the murine spinal cord subjected to transverse contusion: A finite element study.

Finite element models combined with animal experimental models of spinal cord injury provides the opportunity for investigating the effects of the injury mechanism on the neural tissue deformation and the resulting tissue damage. Thus, we developed a finite element model of the mouse cervical spinal cord in order to investigate the effect of morphological, experimental and mechanical factors on the spinal cord mechanical behavior subjected to transverse contusion. The overall mechanical behavior of the model was validated with experimental data of unilateral cervical contusion in mice. The effects of the spinal cord material properties, diameter and curvature, and of the impactor position and inclination on the strain distribution were investigated in 8 spinal cord anatomical regions of interest for 98 configurations of the model. Pareto analysis revealed that the material properties had a significant effect (p<0.01) for all regions of interest of the spinal cord and was the most influential factor for 7 out of 8 regions. This highlighted the need for comprehensive mechanical characterization of the gray and white matter in order to develop effective models capable of predicting tissue deformation during spinal cord injuries.

Dynamics of spinal cord compression with different patterns of thoracolumbar burst fractures: Numerical simulations using finite element modelling.

BACKGROUND: In thoracolumbar burst fractures, spinal cord primary injury involves a direct impact and energy transfer from bone fragments to the spinal cord. Unfortunately, imaging studies performed after the injury only depict the residual bone fragments position and pattern of spinal cord compression, with little insight on the dynamics involved during traumas. Knowledge of underlying mechanisms could be helpful in determining the severity of the primary injury, hence the extent of spinal cord damage and associated potential for recovery. Finite element models are often used to study dynamic processes, but have never been used specifically to simulate different severities of thoracolumbar burst fractures. METHODS: Previously developed thoracolumbar spine and spinal cord finite element models were used and further validated, and representative vertebral fragments were modelled. A full factorial design was used to investigate the effects of comminution of the superior fragment, presence of an inferior fragment, fragments rotation and velocity, on maximum Von Mises stress and strain, maximum major strain, and pressure in the spinal cord. FINDINGS: Fragment velocity clearly was the most influential factor. Fragments rotation and presence of an inferior fragment increased pressure, but rotation decreased both strains outputs. Although significant for both strains outputs, comminution of the superior fragment isn't estimated to influence outputs. INTERPRETATION: This study is the first, to the authors' knowledge, to examine a detailed spinal cord model impacted in situ by fragments from burst fractures. This numeric model could be used in the future to comprehensively link traumatic events or imaging study characteristics to known spinal cord injuries severity and potential for recovery.

Numerical investigation of the relative effect of disc bulging and ligamentum flavum hypertrophy on the mechanism of central cord syndrome.

BACKGROUND: The pathogenesis of the central cord syndrome is still unclear. While there is a consensus on hyperextension as the main traumatic mechanism leading to this condition, there is yet to be consensus in studies regarding the pathological features of the spine (intervertebral disc bulging or ligamentum flavum hypertrophy) that could contribute to clinical manifestations. METHODS: A comprehensive finite element model of the cervical spine segment and spinal cord was used to simulate high-speed hyperextension. Four stenotic cases were modelled to study the effect of ligamentum flavum hypertrophy and intervertebral disc bulging on the von Mises stress and strain. FINDINGS: During hyperextension, the downward displacement of the ligamentum flavum and a reduction of the spinal canal diameter (up to 17%) led to a dynamic compression of the cord. Ligamentum flavum hypertrophy was associated with stress and strain (peak of 0.011 Mpa and 0.24, respectively) in the lateral corticospinal tracts, which is consistent with the histologic pattern of the central cord syndrome. Linear intervertebral disc bulging alone led to a higher stress in the anterior and posterior funiculi (peak 0.029 Mpa). Combined with hypertrophic ligamentum flavum, it further increased the stress and strain in the corticospinal tracts and in the posterior horn (peak of 0.023 Mpa and 0.35, respectively). INTERPRETATION: The stenotic typology and geometry greatly influence stress and strain distribution resulting from hyperextension. Ligamentum flavum hypertrophy is a main feature leading to central cord syndrome.

Contribution of injured posterior ligamentous complex and intervertebral disc on post-traumatic instability at the cervical spine.

Posterior ligamentous complex (PLC) and intervertebral disc (IVD) injuries are common cervical spine flexion-distraction injuries, but the residual stability following their disruption is misknown. The objective of this study was to evaluate the effect of PLC and IVD disruption on post-traumatic cervical spine stability under low flexion moment (2 Nm) using a finite element (FE) model of C2-T1. The PLC was removed first and a progressive disc rupture (one third, two thirds and complete rupture) was modeled to simulate IVD disruption at C2-C3, C4-C5 and C6-C7. At each step, a non-traumatic flexion moment was applied and the change in stability was evaluated. PLC removal had little impact at C2-C3 but increased local range of motion (ROM) at the injured level by 77.2% and 190.7% at C4-C5 and C6-C7, respectively. Complete IVD rupture had the largest impact on C2-C3, increasing C2-C3 ROM by 181% and creating a large antero-posterior displacement of the C2-C3 segment. The FE analysis showed PLC and disc injuries create spinal instability. However, the PLC played a bigger role in the stability of the middle and lower cervical spine while the IVD was more important at the upper cervical spine. Stabilization appears important when managing patients with soft tissue injuries.

Effect of impact velocity and ligament mechanical properties on lumbar spine injuries in posterior-anterior impact loading conditions: a finite element study.

Traumatic events may lead to lumbar spine injuries ranging from low severity bony fracture to complex fracture dislocation. Injury pathomechanisms as well as the influence of loading rate and ligament mechanical properties were not yet fully elucidated. The objective was to quantify the influence of impact velocity and ligament properties variability on the lumbar spine response in traumatic flexion-shear conditions. An L1-L3 finite element spinal segment was submitted to a posterior-anterior impact at three velocities (2.7, 5, or 10 m/s) and for 27 sets of ligament properties. Spinal injury pathomechanism varied according to the impact velocities: initial osseous compression in the anterior column for low and medium velocities versus distraction in the posterior column for high velocity. Impact at 2.7 and 5 m/s lead to higher extent of bony injury, i.e., volume of ruptured bone, compared to the impact at 10 m/s (1140, 1094, and 718 mm3 respectively), lower L2 anterior displacement (2.09, 5.36, and 7.72 mm respectively), and lower facet fracture occurrence. Ligament properties had no effect on bony injury initiation but influenced the presence of facet fracture. These results improve the understanding of lumbar injury pathomechanisms and provide additional knowledge of lumbar injury load thresholds that could be used for injury prevention. Graphical abstract Stress distribution analysis at the injury initiation and final injury pattern identification for a lumbar segment submitted to a traumatic posterior-anterior impact.

High-speed video analysis improves the accuracy of spinal cord compression measurement in a mouse contusion model.

BACKGROUND: Animal models of spinal cord injuries aim to utilize controlled and reproducible conditions. However, a literature review reveals that mouse contusion studies using equivalent protocols may show large disparities in the observed impact force vs. cord compression relationship. The overall purpose of this study was to investigate possible sources of bias in these measurements. The specific objective was to improve spinal cord compression measurements using a video-based setup to detect the impactor-spinal cord time-to-contact. NEW METHOD: A force-controlled 30kDyn unilateral contusion at C4 vertebral level was performed in six mice with the Infinite Horizon impactor (IH). High-speed video was used to determine the time-to-contact between the impactor tip and the spinal cord and to compute the related displacement of the tip into the tissue: the spinal cord compression and the compression ratio. RESULTS & COMPARISON WITH EXISTING METHOD(S): Delayed time-to-contact detection with the IH device led to an underestimation of the cord compression. Compression values indicated by the IH were 64% lower than those based on video analysis (0.33mm vs. 0.88mm). Consequently, the mean compression ratio derived from the device was underestimated when compared to the value derived from video analysis (22% vs. 61%). CONCLUSIONS: Default time-to-contact detection from the IH led to significant errors in spinal cord compression assessment. Accordingly, this may explain some of the reported data discrepancies in the literature. The proposed setup could be implemented by users of contusion devices to improve the quantative description of the primary injury inflicted to the spinal cord.

Quasi-static tensile properties of the Cranial Cruciate Ligament (CrCL) in adult cattle: towards the design of a prosthetic CrCL.

Mechanical properties of the Cranial Cruciate Ligament (CrCL) in adult cattle are not well documented and protocols used in the literature focus on testing a full femur-CrCL-tibia complex rather than an isolated CrCL. The aim of this study was to assess a wider range of tensile properties of the CrCL along its anatomic axis with experimental measurements of the global elongation, displacement and strain fields, in order to provide guidelines for the design of CrCL prosthetic surrogates. Fourteen bovine CrCL were harvested from seven mature cows (5.1 ± 1.3 years) weighing 631 ± 90kg. The mean CrCL length was 41.4 ± 1.5mm and its mean cross-section was 103.9 ± 23.8mm2. Pre-conditioning was achieved with 30 cycles of loading from 30 to 200N at a strain rate of 0.02s-1. Specimens were then loaded to failure at the same strain rate. The following results were obtained: the mean ultimate tensile load (UTL) 4372 ± 1485N and the median [quartiles] maximal global elongation 19.3 [17.8; 21.4] %. At first physical signs of tearing, the mean load was 3315 ± 1336N and mean elongation 13.5 ± 4.9%. The mean absorbed energy at failure was 5.23 ± 2.08 MJ.mm-3 and the mean stiffness at various levels of elongation was: 220 ± 195N.%-1 (5%), 285 ± 162N.%-1 (10%), 239 ± 200N.%-1 (15%), 146 ± 59N.%-1 (20%), 153 ± 136N.%-1 (25%). None of these properties were related to the bovine weight, age and side of the body (p > 0.05). An ideal prosthetic surrogate should then follow these sets of properties and the experimental data suggest that the in-vivo maximal elongation is below 13.5%.