Strain rate dependent properties of younger human cervical spine ligaments

How Strong Is the Ligamentum Teres of the Hip? A Biomechanical Analysis

BACKGROUND

Intraarticular hip pain represents a substantial clinical challenge, with recent studies implicating lesions in the ligamentum teres as potential contributors. Even more so, damage to the ligamentum teres is particularly prevalent among young patients undergoing joint-preserving interventions. Although several studies have investigated the biomechanical attributes of the ligamentum teres, inconsistencies in reported findings and reliance on cadaveric or animal models have raised concerns regarding the extrapolation of results to clinical practice. Furthermore, there is a lack of research examining ligamentum teres biomechanics specifically within the relevant patient cohort-individuals who benefit from joint-preserving surgical interventions.

QUESTIONS/PURPOSES

We sought (1) to determine the biomechanical properties (ultimate load to failure, tensile strength, stiffness, and elastic modulus) of fresh-frozen ligaments from patients undergoing surgical hip dislocation, and (2) to identify patient-specific factors that are associated with them.

METHODS

This was an institutional review board-approved study on intraoperatively harvested ligamentum teres from 74 consecutive patients undergoing surgical hip dislocation for joint preservation (August 2021 to September 2022). After the exclusion of patients with previous surgery, posttraumatic deformities, avascular necrosis, slipped capital femoral epiphysis, and Perthes disease, 31 ligaments from 31 patients were analyzed. The mean age of the study group was 27 ± 8 years, and 61% (19) of participants were male. The main indication for surgery was femoroacetabular impingement. Standardized AP pelvic and axial radiographs and CT scans were performed in all patients for better radiological description of the population and to identify associated radiological factors. The ligament was thoroughly transected at its origin on the fossa acetabuli and at the insertion area on the fovea capitis and stored at -20°C until utilization. Specimens were mounted to a materials testing machine via custom clamps that minimized slippage and the likelihood of failure at the clamp. Force-displacement and stress-strain curves were generated. Ultimate failure load (N), tensile strength (MPa), stiffness (N/mm), and elastic modulus (MPa) were determined. Using a multivariate regression analysis and a subgroup analysis, we tested demographic, degenerative, and radiographic factors as potential associated factors.

RESULTS

The ligamentum teres demonstrated an ultimate load to failure of 126 ± 92 N, and the tensile strength was 1 ± 1 MPa. The ligaments exhibited a stiffness of 24 ± 15 N/mm and an elastic modulus of 7 ± 5 MPa. After controlling for potential confounding variables like age, fossa/fovea degeneration, and acetabular/femoral morphologies, we found that female sex was an independent factor for higher tensile strength, stiffness, and elastic modulus. Excessive femoral version was independently associated with lower load to failure (HR 122 [95% CI 47 to 197]) and stiffness (HR 15 [95% CI 2 to 27]). Damage to the acetabular fossa was associated with reduced load to failure (HR -93 [95% CI -159 to -27]).

CONCLUSION

Overall, the ligamentum teres is a relatively weak ligament. Sex, degeneration, and excessive femoral version are influencing factors on strength of the ligamentum teres. The ligamentum teres exhibits lower strength compared with other joint-stabilizing ligaments, which calls into question its overall contribution to hip stability.

CLINICAL RELEVANCE

Young patients undergoing hip-preserving surgery are the population at risk for ligamentum teres lesions. Baseline values for load to failure, tensile strength, elastic modulus, and stiffness are needed to better understand those lesions in this cohort of interest.

Biomechanical Analysis of Hybrid Artificial Discs or Zero-Profile Devices for Treating 1-Level Adjacent Segment Degeneration in ACDF Revision Surgery

OBJECTIVE

Cervical hybrid surgery optimizes the use of cervical disc arthroplasty (CDA) and zero-profile (ZOP) devices in anterior cervical discectomy and fusion (ACDF) but lacks uniform combination and biomechanical standards, especially in revision surgery (RS). This study aimed to investigate the biomechanical characteristics of adjacent segments of the different hybrid RS constructs in ACDF RS.

METHODS

An intact 3-dimensional finite element model generated a normal cervical spine (C2-T1). This model was modified to the primary C5-6 ACDF model. Three RS models were created to treat C4-5 adjacent segment degeneration through implanting cages plus plates (Cage-Cage), ZOP devices (ZOP-Cage), or Bryan discs (CDA-Cage). A 1.0-Nm moment was applied to the primary C5-6 ACDF model to generate total C2-T1 range of motions (ROMs). Subsequently, a displacement load was applied to all RS models to match the total C2-T1 ROMs of the primary ACDF model.

RESULTS

The ZOP-Cage model showed lower biomechanical responses including ROM, intradiscal pressure, maximum von Mises stress in discs, and facet joint force in adjacent segments compared to the Cage-Cage model. The CDA-Cage model exhibited the lowest biomechanical responses and ROM ratio at adjacent segments among all RS models, closely approached or lower than those in the primary ACDF model in most motion directions. Additionally, the maximum von Mises stress on the C3-4 and C6-7 discs increased in the Cage-Cage and ZOP-Cage models but decreased in the CDA-Cage model when compared to the primary ACDF model.

CONCLUSION

The CDA-Cage construct had the lowest biomechanical responses with minimal kinematic change of adjacent segments. ZOP-Cage is the next best choice, especially if CDA is not suitable. This study provides a biomechanical reference for clinical hybrid RS decision-making to reduce the risk of ASD recurrence.

Cervical Column and Cord and Column Responses in Whiplash With Stenosis: A Finite Element Modeling Study

Spine degeneration is a normal aging process. It may lead to stenotic spines that may have implications for pain and quality of life. The diagnosis is based on clinical symptomatology and imaging. Magnetic resonance images often reveal the nature and degree of stenosis of the spine. Stenosis is concerning to clinicians and patients because of the decreased space in the spinal canal and potential for elevated risk of cord and/or osteoligamentous spinal column injuries. Numerous finite element models of the cervical spine have been developed to study the biomechanics of the osteoligamentous column such as range of motion and vertebral stress; however, spinal cord modeling is often ignored. The objective of this study was to determine the external column and internal cord and disc responses of stenotic spines using finite element modeling. A validated model of the subaxial spinal column was used. The osteoligamentous column was modified to include the spinal cord. Mild, moderate, and severe degrees of stenosis commonly identified in civilian populations were simulated at C5-C6. The column-cord model was subjected to postero-anterior acceleration at T1. The range of motion, disc pressure, and cord stress-strain were obtained at the index and superior and inferior adjacent levels of the stenosis. The external metric representing the segmental motion was insensitive while the intrinsic disc and cord variables were more sensitive, and the index level was more affected by stenosis. These findings may influence surgical planning and patient education in personalized medicine.

Mechanical response of human thoracic spine ligaments under quasi-static loading: An experimental study

PURPOSE

This study aimed to investigate the geometrical and mechanical properties of human thoracic spine ligaments subjected to uniaxial quasi-static tensile test.

METHODS

Four human thoracic spines, obtained through a body donation program, were utilized for the study. The anterior longitudinal ligament (ALL), posterior longitudinal ligament (PLL), capsular ligament (CL), ligamenta flava (LF), and the interspinous ligament and supraspinous ligament complex (ISL + SSL), were investigated. The samples underwent specimen preparation, including dissection, cleaning, and reinforcement, before being immersed in epoxy resin. Uniaxial tensile tests were performed using a custom-designed mechanical testing machine equipped with an environmental chamber (T = 36.6 °C; humidity 95%). Then, the obtained tensile curves were averaged preserving the characteristic regions of typical ligaments response.

RESULTS

Geometrical and mechanical properties, such as initial length and width, failure load, and failure elongation, were measured. Analysis of variance (ANOVA) revealed significant differences among the ligaments for all investigated parameters. Pairwise comparisons using Tukey's post-hoc test indicated differences in initial length and width. ALL and PLL exhibited higher failure forces compared to CL and LF. ALL and ISL + SSL demonstrated biggest failure elongation. Comparisons with other studies showed variations in initial length, failure force, and failure elongation across different ligaments. The subsystem (Th1 - Th6 and Th7 - Th12) analysis revealed increases in initial length, width, failure force, and elongation for certain ligaments.

CONCLUSIONS

Variations of both the geometric and mechanical properties of the ligaments were noticed, highlighting their unique characteristics and response to tensile force. Presented results extend very limited experimental data base of thoracic spine ligaments existing in the literature. The obtained geometrical and mechanical properties can help in the development of more precise human body models (HBMs).

Quantifying the Importance of Active Muscle Repositioning a Finite Element Neck Model in Flexion Using Kinematic, Kinetic, and Tissue-Level Responses

PURPOSE

Non-neutral neck positions are important initial conditions in impact scenarios, associated with a higher incidence of injury. Repositioning in finite element (FE) neck models is often achieved by applying external boundary conditions (BCs) to the head while constraining the first thoracic vertebra (T1). However, in vivo, neck muscles contract to achieve a desired head and neck position generating initial loads and deformations in the tissues. In the present study, a new muscle-based repositioning method was compared to traditional applied BCs using a contemporary FE neck model for forward head flexion of 30°.

METHODS

For the BC method, an external moment (2.6 Nm) was applied to the head with T1 fixed, while for the muscle-based method, the flexors and extensors were co-contracted under gravity loading to achieve the target flexion.

RESULTS

The kinematic response from muscle contraction was within 10% of the in vivo experimental data, while the BC method differed by 18%. The intervertebral disc forces from muscle contraction were agreeable with the literature (167 N compression, 12 N shear), while the BC methodology underpredicted the disc forces owing to the lack of spine compression. Correspondingly, the strains in the annulus fibrosus increased by an average of 60% across all levels due to muscle contraction compared to BC method.

CONCLUSION

The muscle repositioning method enhanced the kinetic response and subsequently led to differences in tissue-level responses compared to the conventional BC method. The improved kinematics and kinetics quantify the importance of repositioning FE neck models using active muscles to achieve non-neutral neck positions.

Determinants of spinal cord stress and strain in degenerative cervical myelopathy: a patient-specific finite element study

Degenerative cervical myelopathy (DCM) is the commonest cause of spinal cord dysfunction in older adults and is characterized by chronic cervical spinal cord compression. Spinal cord stress and strain during neck motion are also known contributors to the pathophysiology of DCM, yet these factors are not routinely assessed for surgical planning. The aim of this study was to measure spinal cord stress/strain in DCM using patient-specific 3D finite element models (FEMs) and determine whether spinal cord compression is the primary determinant of spinal cord stress/strain. Three-dimensional patient-specific FEMs were created for six DCM patients (mild [n = 2], moderate [n = 2] and severe [n = 2]). Flexion and extension of the cervical spine were simulated with a pure moment load of 2 Nm. Segmental spinal cord von Mises stress and maximum principal strain were measured. Measures of spinal cord compression and segmental range of motion (ROM) were included in a regression analysis to determine associations with spinal cord stress and strain. Segmental ROM in flexion-extension and axial rotation was independently associated with spinal cord stress (p < 0.001) and strain (p < 0.001), respectively. This relationship was not seen for lateral bending. Segmental ROM had a stronger association with spinal stress and strain as compared to spinal cord compression. Compared to the severity of spinal cord compression, segmental ROM is a stronger determinant spinal cord stress and strain. Surgical procedures that address segmental ROM in addition to cord compression may best optimize spinal cord biomechanics in DCM.

Strain-rate-dependent material properties of human lung parenchymal tissue using inverse finite element approach

Automobile crashes and blunt trauma often lead to life-threatening thoracic injuries, especially to the lung tissues. These injuries can be simulated using finite element-based human body models that need dynamic material properties of lung tissue. The strain-rate-dependent material parameters of human parenchymal tissues were determined in this study using uniaxial quasi-static (1 mm/s) and dynamic (1.6, 3, and 5 m/s) compression tests. A bilinear material model was used to capture the nonlinear behavior of the lung tissue, which was implemented using a user-defined material in LS-DYNA. Inverse mapping using genetic algorithm-based optimization of all experimental data with the corresponding FE models yielded a set of strain-rate-dependent material parameters. The bilinear material parameters are obtained for the strain rates of 0.1, 100, 300, and 500 s-1. The estimated elastic modulus increased from 43 to 153 kPa, while the toe strain reduced from 0.39 to 0.29 when the strain rate was increased from 0.1 to 500 s-1. The optimized bilinear material properties of parenchymal tissue exhibit a piecewise linear relationship with the strain rate.

Subaxial Cervical Spine Motion With Different Sizes of Head-supported Mass Under Accelerative Forces

INTRODUCTION

The evolution of military helmet devices has increased the amount of head-supported mass (HSM) worn by warfighters. HSM has important implications for spine biomechanics, and yet, there is a paucity of studies that investigated the effects of differing HSM and accelerative profiles on spine biomechanics. The aim of this study is to investigate the segmental motions in the subaxial cervical spine with different sizes of HSM under Gx accelerative loading.

METHODS

A three-dimensional finite element model of the male head-neck spinal column was used. Three different size military helmets were modeled and incorporated into head-neck model. The models were exercised under Gx accelerative loading by inputting low and high pulses to the cervical vertebra used in the experimental studies. Segmental motions were obtained and normalized with respect to the non-HSM case to quantify the effect of HSM.

RESULTS

Segmental motions increased with an increase in velocity at all segments of the spine. Increasing helmet size resulted in larger motion increases. Angulations ranged from 0.9° to 9.3° at 1.8 m/s and from 1.3° to 10.3° at 2.6 m/s without a helmet. Helmet increased motion between 5% to 74% at 1.8 m/s. At 2.6 m/s, the helmet increased segmental motion anywhere from 10% to 105% in the subaxial cervical spine. The greatest motion was seen at the C5-C6 level, followed by the C6-C7 level.

CONCLUSIONS

The subaxial cervical spine experiences motion increases at all levels at both velocity profiles with increasing HSM. Larger helmet and greater impact velocity increased motion at all levels, with C5-C6 demonstrating the largest range of motion. HSM should be minimized to reduce the risk of cervical spine injury to the warfighter.

Overstrain on the longitudinal band of the cruciform ligament during flexion in the setting of sandwich deformity at the craniovertebral junction: a finite element analysis

BACKGROUND CONTEXT

In the setting of "sandwich deformity" (concomitant C1 occipitalization and C2-3 nonsegmentation), the C1-2 joint becomes the only mobile joint in the craniovertebral junction. Atlantoaxial dislocation develops earlier with severer symptoms in sandwich deformity, which has been hypothesized to be due to the repetitive excessive tension in the ligaments between C1 and C2.

PURPOSE

To elucidate whether and how the major ligaments of the C1-2 joint are affected in sandwich deformity, and to find out the ligament most responsible for the earlier development and severer symptoms of atlantoaxial dislocation in sandwich deformity.

STUDY DESIGN

A finite element (FE) analysis study.

METHODS

A three-dimensional FE model from occiput to C5 was established using anatomical data from a thin-slice CT scan of a healthy volunteer. Sandwich deformity was simulated by eliminating any C0-1 and C2-3 segmental motion respectively. Flexion torque was applied, and the range of motion of each segment and the tension sustained by the major ligaments of C1-2 (including the transverse and longitudinal bands of the cruciform ligament, the alar ligaments, and the apical ligament) were analyzed.

RESULTS

Tension sustained by the longitudinal band of the cruciform ligament and the apical ligament during flexion is significantly larger in the FE model of sandwich deformity. In contrast, tension in the other ligaments is not significantly changed in the sandwich deformity model compared with the normal model.

CONCLUSIONS

Considering the importance of the longitudinal band of the cruciform ligament to the stability of the C1-2 joint, our findings implicate that the early onset, severe dislocation, and unique clinical manifestations of atlantoaxial dislocation in patients with sandwich deformity are mainly due to the enlarged force loaded on the longitudinal band of the cruciform ligament.

CLINICAL SIGNIFICANCE

The enlarged force loaded on the longitudinal band of the cruciform ligament can add to its laxity and thus reducing its ability to restrict the cranial migration of the odontoid process. This is in accordance with our clinical experience that dislocation of the atlantoaxial joint in patients with sandwich deformity is mainly craniocaudal, which means severer cranial neuropathy, Chiari deformity, and syringomyelia, and more difficult surgical treatment.

Mechanical properties of animal ligaments: a review and comparative study for the identification of the most suitable human ligament surrogates

The interest in the properties of animal soft tissues is often related to the desire to find an animal model to replace human counterparts due to the unsteady availability of human tissues for experimental purposes. Once the most appropriate animal model is identified, it is possible to carry out ex-vivo and in-vivo studies for the repair of ligamentous tissues and performance testing of replacement and support healing devices. This work aims to present a systematic review of the mechanical properties of ligaments reported in the scientific literature by considering different anatomical regions in humans and several animal species. This study was conducted according to the Preferred Reporting Items for Systematic Reviews and Meta-Analyses (PRISMA) method. Moreover, considering the lack of a standard protocol for preconditioning of tissues, this aspect is also addressed. Ninety-six studies were selected for the systematic review and analysed. The mechanical properties of different animal species are reported and summarised in tables. Only results from studies reporting the strain rate parameter were considered for comparison with human ligaments, as they were deemed more reliable. Elastic modulus, ultimate tensile stress, and ultimate strain properties are graphically reported identifying the range of values for each animal species and to facilitate comparison between values reported in the scientific literature in animal and human ligaments. Useful similarities between the mechanical properties of swine, cow, and rat and human ligaments have been found.

A comparison of intervertebral ligament properties utilized in a thoracic spine functional unit through kinematic evaluation

Ligament properties in the literature are variable, yet scarce, but needed to calibrate computational models for spine clinical research applications. A comparison of ligament stiffness properties and their effect on the kinematic behavior of a thoracic functional spinal unit (FSU) is examined in this paper. Six unique ligament property sets were utilized within a volumetric T7-T8 finite element (FE) model developed using computer-aided design (CAD) spinal geometry. A 7.5 Nm moment was applied along three anatomical planes both with and without costovertebral (CV) joints present. Range of Motion (RoM) was assessed for each property set and compared to published experimental data. Intact and serial ligament removal procedures were implemented in accordance with experimental protocol. The variance in both kinematic behavior and comparability with experimental data among property sets emphasizes the role nonlinear characterization plays in determining proper kinematic behavior in spinal FE models. Additionally, a decrease in RoM variation among property sets was exhibited when the model setup incorporated the CV joint. With proper assessment of the source and size of each ligament, the material properties considered here could be expanded and justified for implementation into thoracic spine clinical studies.

Cervical facet capsular ligament mechanics: Estimations based on subject-specific anatomy and kinematics

Background

To understand the facet capsular ligament's (FCL) role in cervical spine mechanics, the interactions between the FCL and other spinal components must be examined. One approach is to develop a subject-specific finite element (FE) model of the lower cervical spine, simulating the motion segments and their components' behaviors under physiological loading conditions. This approach can be particularly attractive when a patient's anatomical and kinematic data are available.

Methods

We developed and demonstrated methodology to create 3D subject-specific models of the lower cervical spine, with a focus on facet capsular ligament biomechanics. Displacement-controlled boundary conditions were applied to the vertebrae using kinematics extracted from biplane videoradiography during planar head motions, including axial rotation, lateral bending, and flexion-extension. The FCL geometries were generated by fitting a surface over the estimated ligament-bone attachment regions. The fiber structure and material characteristics of the ligament tissue were extracted from available human cervical FCL data. The method was demonstrated by application to the cervical geometry and kinematics of a healthy 23-year-old female subject.

Results

FCL strain within the resulting subject-specific model were subsequently compared to models with generic: (1) geometry, (2) kinematics, and (3) material properties to assess the effect of model specificity. Asymmetry in both the kinematics and the anatomy led to asymmetry in strain fields, highlighting the importance of patient-specific models. We also found that the calculated strain field was largely independent of constitutive model and driven by vertebrae morphology and motion, but the stress field showed more constitutive-equation-dependence, as would be expected given the highly constrained motion of cervical FCLs.

Conclusions

The current study provides a methodology to create a subject-specific model of the cervical spine that can be used to investigate various clinical questions by coupling experimental kinematics with multiscale computational models.

Development and validation of lumbar spine finite element model

The functional biomechanics of the lumbar spine have been better understood by finite element method (FEM) simulations. However, there are still areas where the behavior of soft tissues can be better modeled or described in a different way. The purpose of this research is to develop and validate a lumbar spine section intended for biomechanical research. A FE model of the 50th percentile adult male (AM) Total Human Model for Safety (THUMS) v6.1 was used to implement the modifications. The main modifications were to apply orthotropic material properties and nonlinear stress-strain behavior for ligaments, hyperelastic material properties for annulus fibrosus and nucleus pulposus, and the specific content of collagenous fibers in the annulus fibrosus ground substance. Additionally, a separation of the nucleus pulposus from surrounding bones and tissues was implemented. The FE model was subjected to different loading modes, in which intervertebral rotations and disc pressures were calculated. Loading modes contained different forces and moments acting on the lumbar section: axial forces (compression and tension), shear forces, pure moments, and combined loading modes of axial forces and pure moments. The obtained ranges of motion from the modified numerical model agreed with experimental data for all loading modes. Moreover, intradiscal pressure validation for the modified model presented a good agreement with the data available from the literature. This study demonstrated the modifications of the THUMS v6.1 model and validated the obtained numerical results with existing literature in the sub-injurious range. By applying the proposed changes, it is possible to better model the behavior of the human lumbar section under various loads and moments.

On implementation of fibrous connective tissues' damage in Abaqus software

Connective fibrous tissues, such as tendons and ligaments, in humans and animals exhibit hyperelastic behaviour. The constitution of the material of these tissues is anisotropic due to the presence of the collagen fibres, where one family of fibres is the typical case. Traumatic events and/or aging may sometimes lead to the damage of the tissue. The study of motion of affected joints or limbs is usually not permitted in vivo. This is where finite element method (FEM) becomes useful as a premise for general analysis, surgical planning or designing of implants and medical treatment. One of the most often used FEM commercial programs is the field of the biomechanics is Abaqus. The present study discusses the potential of user subroutine UANISOHYPER_INV in this code to analyse response of transversely isotropic tissue with damage in quasi-static range. This subroutine requires providing the material energy function and its derivatives only. The stress tensor and constitutive matrix are computed by the software automatically. To the best of the Authors' knowledge this procedure provides the easiest way to simulate the anisotropic hyperelastic material behaviour in Abaqus. In this study its usage is extended onto the damage response simulation. The verification of the approach and its validation against experimental data indicates its efficiency.

UBC Neck C4-C5: An Anatomically and Biomechanically Accurate Surrogate C4-C5 Functional Spinal Unit

Millions of people worldwide suffer from spinal cord injuries (SCIs) and traumatic brain injuries (TBIs) annually. Safety devices meant to protect against SCIs and TBIs, such as helmets, airbags, seat belts, and compliant floors are often evaluated with the use of anthropometric test devices (ATD s); however, there are currently no neck surrogates appropriate for the multiplane loading that often occurs in real-world scenarios leading to injury. As such, our objective in this study was to design and create an anatomically correct functional spinal unit (FSU) that produces a repeatable and biofidelic response to lateral bending, axial rotation, and quasistatic flexion-extension motion. This is a critical step in developing a biofidelic omnidirectional surrogate that can be used in future evaluations of safety devices in transportation, occupational, and sports settings. To create a biofidelic C4-C5 FSU, anatomically accurate C4 and C5 vertebrae were designed and manufactured using a 3D printer using geometry derived from the CT scans of a healthy 31-year-old male. Potential intervertebral disc and ligament surrogate materials were tested in compression and tension, respectively, to select representative materials for the surrogate intervertebral disc and cervical ligaments. The C4-C5 FSU was assembled and tested repeatedly in quasistatic flexion-extension, axial rotation, and lateral bending. Kinematic results were captured and compared to previously published cadaver data. The surrogate disc showed excellent Biofidelity (ISO/TR 9790) in compression, and the surrogate ligaments were within 25 N/mm of linear cadaveric stiffness ranges. The assembled FSU named UBC Neck C4-C5 showed good biofidelity under quasistatic axial rotation, lateral bending, flexion-extension, and coupled motion (ISO/TR 9790). However, the instantaneous centre of rotation was not similar to ex vivo or in vivo published studies. The UBC Neck C4-C5 FSU resulted in good biofidelity ratings and will inform future construction of a full surrogate neck to be used in the testing of head and neck safety equipment.

Spinal Cord Boundary Conditions Affect Brain Tissue Strains in Impact Simulations

Brain and spinal cord injuries have devastating consequences on quality of life but are challenging to assess experimentally due to the traumatic nature of such injuries. Finite element human body models (HBM) have been developed to investigate injury but are limited by a lack of biofidelic spinal cord implementation. In many HBM, brain models terminate with a fixed boundary condition at the brain stem. The goals of this study were to implement a comprehensive representation of the spinal cord into a contemporary head and neck HBM, and quantify the effect of the spinal cord on brain deformation during simulated impacts. Spinal cord tissue geometries were developed, based on 3D medical imaging and literature data, meshed, and implemented into the GHBMC 50th percentile male model. The model was evaluated in frontal, lateral, rear, and oblique impact conditions, and the resulting maximum principal strains in the brain tissue were compared, with and without the spinal cord. A new cumulative strain curve metric was proposed to quantify brain strain distribution. Presence of the spinal cord increased brain tissue strains in all simulated cases, owing to a more compliant boundary condition, highlighting the importance of the spinal cord to assess brain response during impact.

Spinal Cord Stress After Anterior Cervical Diskectomy and Fusion: Results from a Patient-Specific Finite Element Model

Degenerative cervical myelopathy (DCM) is the commonest cause of cervical spinal cord dysfunction in older adults and is characterized by spinal cord compression and stress during neck motion. Although surgical decompression eliminates static spinal cord compression, cord stress resulting from flexion-extension motion of the spinal column has not been determined for single and multi-level surgical interventions. The effect of surgery on spinal cord stress is expected to change with the number of surgical levels as well as patient-specific anatomy. Using a MRI-derived patient-specific finite element model, we simulated 1-, 2- and 3-level anterior cervical diskectomy and fusion (ACDF) surgery for DCM. A substantial decrease in spinal cord stress at the level of spinal cord decompression was noted in all simulations. This was associated with a considerable increase in spinal cord stress rostral to the surgical level, and the magnitude of stress was higher in multi-level surgery. Increased spinal cord stress at the rostral adjacent segment correlated with increased segmental range of motion (r = 0.69, p = 0.002) and disk pressure (r = 0.57, p = 0.05). Together, these results indicate that ACDF for DCM is associated with adverse spinal cord stress patterns adjacent to the fusion construct, and further research is needed to determine if the altered stress is associated with clinical outcomes after surgery for DCM.

Gender Differences in Cervical Spine Motions and Loads With Head Supported Mass Using Finite Element Models

While many studies have been conducted to delineate the role of gender in rear impact via experiments, clinical investigations, modeling, and epidemiological research, the effect of the added head mass on segmental motions has received less attention. The objective of the study is to determine the role of the head supported mass on the segmental motions and loads on the cervical spinal column from rear impact loading. The study used finite element modeling. The model was subjected to mesh convergence studies. It was validated with human cadaver experimental data by applying the rear impact acceleration pulse to the base of the spine. At all levels of the subaxial spinal column, a comparison was made between male and female spines and with and without the use of an army combat helmet. For this purpose, segmental motions, forces, and bending moments were used as biomechanical parameters. Results showed that female spines responded with increased motions than males, and the presence of a helmet increased motions and loads in males and female spines at all levels. Numerical data are given. Head supported mass affects spine responses at all levels. The present computational modeling study, from one geometry for the male spine and one geometry for the female spine (limitations are addressed in the paper), provided insights into the mechanisms of the internal load transfer with the presence of head supported mass, prevalent in certain civilian occupations and active-duty Service members in the military.

Evaluation of muscle strain injury severity in active human body models

Even though significant efforts in the field of injury detection with finite element active human body models (FE AHBMs) have been made, injuries of the muscle-tendon unit (MTU) have not yet been taken into consideration. Therefore, the goal of this study was to define a muscle strain injury criterion (MSIC) to evaluate the damage sustained by the musculature during muscle driven movement scenarios. The MSIC was derived from biomechanical tests found in the literature and the proposed threshold values were substantiated through a comparison to an estimate of the ultimate tensile strength of human skeletal muscle and the forces acting on the biceps femoris long head muscle during one sprinting gait cycle. The application of the MSIC to state-of-the-art FE AHBMs was demonstrated by evaluating the strain injury severity of selected neck muscles of a full-body AHBM during two seat rotation load cases. The results of the MSIC substantiation suggest that all three injury threshold values proposed in this work fall in a plausible corridor of forces acting on the MTU. The combined results of the AHBM simulations indicate that neither of the two examined seat rotations are likely to cause strain injury to the neck muscles and that the proposed MSIC can easily be applied to current AHBMs without further modification of the model architecture or the muscle parameters. The MSIC was also used to formulate a hypothesis on the aetiology of muscle strain injuries, through which it was demonstrated that material inhomogeneities in the MTU might be the cause for strain injuries sustained during otherwise physiological movements. This work is a first step in the direction of the definition of a wholistic injury criterion for the human skeletal muscle fibre.

Biomechanical Analysis of 3-Level Anterior Cervical Discectomy and Fusion Under Physiologic Loads Using a Finite Element Model

Objective

Pseudarthrosis and adjacent segment degeneration (ASD) are 2 common complications after multilevel anterior cervical discectomy and fusion (ACDF). We aim to identify the potential biomechanical factors contributing to pseudarthrosis and ASD following 3-level ACDF using a cervical spine finite element model (FEM).

Methods

A validated cervical spine FEM from C2 to C7 was used to study the biomechanical factors in cervical spine intervention. The FEM model was used to simulate a 3-level ACDF with intervertebral spacers and anterior cervical plating with screw fixation from C4 to C7. The model was then constrained at the inferior nodes of the T1 vertebra, and physiological loads were applied at the top vertebra. The pure moment load of 2 Nm was applied in flexion, extension, and lateral bending. A follower axial force of 75 N was applied to reproduce the weight of the cranium and muscle force, was applied using standard procedures. The motion-controlled hybrid protocol was utilized to comprehend the adjustments in the spinal biomechanics.

Results

Our cervical spine FEM demonstrated that the cranial adjacent level (C3–4) had significantly more increase in range of motion (ROM) (+90.38%) compared to the caudal adjacent level at C7–T1 (+70.18%) after C4–7 ACDF, indicating that the cranial adjacent level has more compensatory increase in ROM than the caudal adjacent level, potentially predisposing it to earlier ASD. Within the C4–7 ACDF construct, the C6–7 level had the least robust fixation during fixation compared to C4–5 and C5–6, as reflected by the smallest reduction in ROM compared to intact spine (-71.30% vs. -76.36% and -77.05%, respectively), which potentially predisposes the C6–7 level to higher risk of pseudarthrosis.

Conclusion

Biomechanical analysis of C4–7 ACDF construct using a validated cervical spine FEM indicated that the C3–4 has more compensatory increase in ROM compared to C7–T1, and C6–7 has the least robust fixation under physiological loads. These findings can help spine surgeons to predicate the areas with higher risks of pseudarthrosis and ASD, and thus developing corresponding strategies to mitigate these risks and provide appropriate preoperative counseling to patients.

A finite element study on the effects of follower load on the continuous biomechanical responses of subaxial cervical spine

In spine biomechanics, follower loads are used to mimic the in vivo muscle forces acting on a human spine. However, the effects of the follower load on the continuous biomechanical responses of the subaxial cervical spines (C2-T1) have not been systematically clarified. This study aims at investigating the follower load effects on the continuous biomechanical responses of C2-T1. A nonlinear finite element model is reconstructed and validated for C2-T1. Six levels follower loads are considered along the follower load path that is optimized through a novel range of motion-based method. A moment up to 2 Nm is subsequently superimposed to produce motions in three anatomical planes. The continuous biomechanical responses, including the range of motion, facet joint force, intradiscal pressure and flexibility are evaluated for each motion segment. In the sagittal plane, the change of the overall range of motion arising from the follower loads is less than 6%. In the other two anatomical planes, both the magnitude and shape of the rotation-moment curves change with follower loads. At the neutral position, over 50% decrease in flexibility occurs as the follower load increases from zero to 250 N. In all three anatomical planes, over 50% and 30% decreases in flexibility occur in the first 0.5 Nm for small (≤100 N) and large (≥150 N) follower loads, respectively. Moreover, follower loads tend to increase both the facet joint forces and the intradiscal pressures. The shape of the intradiscal pressure-moment curves changes from nonlinear to roughly linear with increased follower load, especially in the coronal and transverse planes. The results obtained in this work provide a comprehensive understanding on the effects of follower load on the continuous biomechanical responses of the C2-T1.

Arc-Length Re-Parametrization and Signal Registration to Determine a Characteristic Average and Statistical Response Corridors of Biomechanical Data

A characteristic average and biofidelity response corridors are commonly used to represent the average behaviour and variability of biomechanical signal data for analysis and comparison to surrogates such as anthropometric test devices and computational models. However, existing methods for computing the characteristic average and corresponding response corridors of experimental data are often customized to specific types or shapes of signal and therefore limited in general applicability. In addition, simple methods such as point-wise averaging can distort or misrepresent important features if signals are not well aligned and highly correlated. In this study, an improved method of computing the characteristic average and response corridors of a set of experimental signals is presented based on arc-length re-parameterization and signal registration. The proposed arc-length corridor method was applied to three literature datasets demonstrating a range of characteristics common to biomechanical data, such as monotonic increasing force-displacement responses with variability, oscillatory acceleration-time signals, and hysteretic load-unload data. The proposed method addresses two challenges in assessing experimental data: arc-length re-parameterization enables the assessment of complex-shaped signals, including hysteretic load-unload data, while signal registration aligned signal features such as peaks and valleys to prevent distortion when determining the characteristic average response. The arc-length corridor method was shown to compute the characteristic average and response corridors for a wide range of biomechanical data, while providing a consistent statistical framework to characterize variability in the data. The arc-length corridor method is provided to the community in the freely available and open-source software package, ARCGen.

Characterizing the Mechanical and Viscoelastic Response of the Porcine Facet Joint Capsule Ligament in Response to a Simulated Impact

The facet capsule ligament (FCL) is a structure in the lumbar spine that constrains motions of the vertebrae. Subfailure loads can produce micro-damage resulting in increased laxity, decreased stiffness, and altered viscoelastic responses. Therefore, the purpose of this investigation was to determine the mechanical and viscoelastic properties of the FCL under various magnitudes of strain from control samples and samples that had been through an impact protocol. Two hundred FCL tissue samples were tested (20 Control & 180 Impacted). Impacted FCL tissue samples were obtained from functional spinal units that had been exposed to one of nine sub-failure impact conditions. All specimens underwent to following loading protocol: preconditioning with 5 cycles of 5% strain, followed by a 30 second rest period, 5 cycles of 10% strain and 1 cycle of 10% strain with a hold duration at 10% strain for 240 seconds (4 minutes). The same protocol followed for 30% and 50% strain. Measures of stiffness, hysteresis and force-relaxation were computed. No significant differences in stiffness were observed for impacted specimens in comparison to control. Impacted specimens from the 8g Flexed and 11g Flexed and Neutral conditions exhibited greater hysteresis during the cyclic-30% and cyclic-50% portion of the protocol in comparison to controls. In addition, specimens from the 8g and 11g Flexed conditions resulted in greater stress decay for the 50%-hold conditions. Results from this study demonstrate viscoelastic changes in FCL samples exposed to moderate and highspeed single impacts in a flexed posture.

FEBio finite element model of a pediatric cervical spine

OBJECTIVE

The underlying biomechanical differences between the pediatric and adult cervical spine are incompletely understood. Computational spine modeling can address that knowledge gap. Using a computational method known as finite element modeling, the authors describe the creation and evaluation of a complete pediatric cervical spine model.

METHODS

Using a thin-slice CT scan of the cervical spine from a 5-year-old boy, a 3D model was created for finite element analysis. The material properties and boundary and loading conditions were created and model analysis performed using open-source software. Because the precise material properties of the pediatric cervical spine are not known, a published parametric approach of scaling adult properties by 50%, 25%, and 10% was used. Each scaled finite element model (FEM) underwent two types of simulations for pediatric cadaver testing (axial tension and cardinal ranges of motion [ROMs]) to assess axial stiffness, ROM, and facet joint force (FJF). The authors evaluated the axial stiffness and flexion-extension ROM predicted by the model using previously published experimental measurements obtained from pediatric cadaveric tissues.

RESULTS

In the axial tension simulation, the model with 50% adult ligamentous and annulus material properties predicted an axial stiffness of 49 N/mm, which corresponded with previously published data from similarly aged cadavers (46.1 ± 9.6 N/mm). In the flexion-extension simulation, the same 50% model predicted an ROM that was within the range of the similarly aged cohort of cadavers. The subaxial FJFs predicted by the model in extension, lateral bending, and axial rotation were in the range of 1-4 N and, as expected, tended to increase as the ligament and disc material properties decreased.

CONCLUSIONS

A pediatric cervical spine FEM was created that accurately predicts axial tension and flexion-extension ROM when ligamentous and annulus material properties are reduced to 50% of published adult properties. This model shows promise for use in surgical simulation procedures and as a normal comparison for disease-specific FEMs.

Potential effect of pre-activated muscles under a far-side lateral impact

OBJECTIVE

The goal of this study is to evaluate the potential effect of muscle pre-activation under a lateral impact scenario, in this case focusing on a far-side impact, using an Active Human Body Model.

METHODS

In total fourteen simulations were run, out of these, twelve were computed with an Active Human Body Model and two with a passive one. The models were subjected to a far-side impact scenario reaching up to 14 g's. Two different pre-crash scenarios were analyzed with the Active Human Body Model: (1) constant velocity, and (2) braking deceleration. During the pre-crash phase a lambda control based on the muscle length computed the muscle activation. Since there is no available data concerning the neuromuscular strategy of the occupants subjected to high accelerations, six different control strategies were analyzed during the in-crash phase. Besides, rib fracture and brain injury risk were analyzed, since they are the two most relevant body regions in this simplified far-side crash scenario.

RESULTS

The pre-activation of the muscles showed an effect on both the occupant kinematics and estimated injury risks. Depending on the considered muscle strategy, the head lateral excursion varied up to 75 mm, specifically for the scenario with constant velocity. Moreover, the rib fracture probability and the brain injury indicator revealed higher injury risks for the passive Human Body Model. When applying the constant velocity during pre-crash, the fracture probability for two or more ribs ranged from 9.91 to 46.06% for the Active Human Body Model, whereas it reached 84.3% for the passive model. The brain injury indicator was reduced by about 10% when using the active model compared to the passive one.

CONCLUSIONS

The numerical results show that the pre-activation of the muscles affects the kinematic and injury outcomes in car crashes. In this study, six muscular control strategies have been proposed. The two muscular controls that may be most realistic are: constant activation after the in-crash phase starts, by trying to hold the position prior to the crash, or no stimulation, by not responding to the upcoming in-crash event.

Biomechanical effects of uncinate process excision in cervical disc arthroplasty

BACKGROUND

Studies on the role of uncinate process have been limited to responses of the intact spine and patient's outcomes, and procedures to perform the excision. The aim of this study was to determine the role of uncinate process on the biomechanical response at the index and adjacent levels in three artificial discs used in cervical disc arthroplasty.

METHODS

A validated finite element model of cervical spine was used. Flexion, extension, and lateral moments and follower load were applied to Bryan, Mobi-C, and Prestige LP artificial discs at C5-C6 level with and without uncinate process. Ranges of motion at index level and adjacent caudal and cranial segments, intradiscal pressures at adjacent segments, and facet loads at index level and adjacent segments were obtained. Data were normalized with respect to the preservation of uncinate process.

FINDINGS

Uncinate process removal increased motions up to 27% at index and decreased up to 10% at adjacent levels, decreased disc pressures up to 14% at adjacent segments, decreased facet loads at adjacent segments up to 14%, while at index level, change in loads depended on mode and arthroplasty, with Mobi-C responding with up to 51% increase and Bryan disc up to 11% decrease, while Prestige LP increased loads by 17% in extension and decreased by 9%% in lateral bending.

INTERPRETATION

As surgical selection is based on morphology and surgeon's experience, the present computational findings provide quantitative information for an optimal choice of the device and procedure, while further studies (in vitro/clinical) would be required.

An Electromyographically Driven Cervical Spine Model in OpenSim

Relatively few biomechanical models exist aimed at quantifying the mechanical risk factors associated with neck pain. In addition, there is a need to validate spinal-rhythm techniques for inverse dynamics spine models. Therefore, the present investigation was 3-fold: (1) the development of a cervical spine model in OpenSim, (2) a test of a novel spinal-rhythm technique based on minimizing the potential energy in the passive tissues, and (3) comparison of an electromyographically driven approach to estimating compression and shear to other cervical spine models. The authors developed ligament force-deflection and intervertebral joint moment-angle curves from published data. The 218 Hill-type muscle elements, representing 58 muscles, were included and their passive forces validated against in vivo data. Our novel spinal-rhythm technique, based on minimizing the potential energy in the passive tissues, disproportionately assigned motion to the upper cervical spine that was not physiological. Finally, using kinematics and electromyography collected from 8 healthy male volunteers, the authors calculated the compression at C7-T1 as a function of the head-trunk Euler angles. Differences from other models varied from 25.5 to 368.1 N. These differences in forces may result in differences in model geometry, passive components, number of degrees of freedom, or objective functions.

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's 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 different flexion-distraction disco-ligamentous injuries affect the SCI mechanisms 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 most extreme von Mises stress (47 to 66 kPa), principal strains p1 (0.32 to 0.41 in white matter) and p3 (-0.78 to -0.96 in white matter) in the spinal cord and to the most important spinal cord compression (35 to 48 %). The main post-trauma SCI mechanism was identified as 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 after injury. Finally, in extension, the injuries tested had little impact on the spinal cord. The capsular ligament was the most important structure in protecting the spinal cord. Its status should be carefully examined during patient's management.

Importance of the cervical capsular joint cartilage geometry on head and facet joint kinematics assessed in a Finite element neck model

Finite element human neck models (NMs) aim to predict neck response and injury at the tissue level; however, contemporary models are most often assessed using global response such as head kinematics. Additionally, many NMs are developed from subject-specific imaging with limited soft tissue resolution in small structures such as the facet joints in the neck. Such details may be critical to enable NMs to predict tissue-level response. In the present study, the capsular joint cartilage (CJC) geometry in a contemporary NM was enhanced (M50-CJC) based on literature data. The M50-CJC was validated at the segment and full neck levels and assessed using relative facet joint kinematics (FJK), capsular ligament (CL) and intervertebral disc (IVD) strains, a relative vertebral rotation assessment (IV-NIC) and head kinematics in frontal and rear impact. The validation ratings at the segment level increased from 0.60 to 0.64, with improvements for modes of deformation associated with the facet joints, while no difference was noted at the head kinematic level. The improved CJC led to increased FJK rotation (188%) and IVD strain (152.2%,) attributed to the reduced facet joint gap. Further enhancements of the capsular joint representation or a link between the FJK and CL injury risk are recommended. Enhancements at the tissue level demonstrated a large effect on the IVD strain, but were not apparent in global metrics such as head kinematics. This study demonstrated that a biofidelic and detailed geometrical representation of the CJC contributes significantly to the predicted joint response, which is critical to investigate neck injury risk at the tissue level.

Comparative Finite Element Modeling Study of Anterior Cervical Arthrodesis Versus Cervical Arthroplasty With Bryan Disc or Prodisc C

INTRODUCTION

Cervical disc arthroplasty (CDA), a motion-preserving alternative to anterior cervical discectomy and fusion (ACDF), is used in military patients for the treatment of disorders such as spondylosis. Since 2007, the FDA has approved eight artificial discs. The objective of this study is to compare the biomechanics after ACDF and CDA with two FDA-approved devices of differing designs under head and head supported mass loadings.

MATERIALS AND METHODS

A previously validated osteoligamentous C2-T1 finite element model was used to simulate ACDF and two types of CDA (Bryan and Prodisc C) at the C5-C6 level. The hybrid loading protocol associated with in vivo head and head supported mass was used to apply flexion and extension loading. First, intact spine was subjected to 2 Nm of flexion extension and the range of motion (ROM) was measured. Next, for each surgical option, flexion-extension moments duplicating the same ROM as the intact spine were determined. Under these surgery-specific moments, ROM and facet force were obtained at the index level, and ROM, facet force, and intradiscal pressure at the rostral and caudal adjacent levels.

RESULTS

ACDF led to increased motion, force and pressures at the adjacent levels. Prodisc C led to increased motion and facet force at the index level, and decreased motion, facet force, and intradiscal pressure at both adjacent levels. Bryan produced less dramatic biomechanical alterations compared with ACDF and Prodisc C. Numerical results are given in the article.

CONCLUSIONS

Recognizing that ROM is a clinical measure of spine stability/performance, CDA demonstrates a more physiological biomechanical response than ACDF, although the exact pattern depends on the implant design. Anterior and posterior column load-sharing patterns were different between the two implants and may affect implant selection based on the anatomical and pathological state at the index and adjacent levels.

The importance of intervertebral disc material model on the prediction of mechanical function of the cervical spine

BACKGROUND

Linear elastic, hyperelastic, and multiphasic material constitutive models are frequently used for spinal intervertebral disc simulations. While the characteristics of each model are known, their effect on spine mechanical response requires a careful investigation. The use of advanced material models may not be applicable when material constants are not available, model convergence is unlikely, and computational time is a concern. On the other hand, poor estimations of tissue's mechanical response are likely if the spine model is oversimplified. In this study, discrepancies in load response introduced by material models will be investigated.

METHODS

Three fiber-reinforced C2-C3 disc models were developed with linear elastic, hyperelastic, and biphasic behaviors. Three different loading modes were investigated: compression, flexion and extension in quasi-static and dynamic conditions. The deformed disc height, disc fluid pressure, range of motion, and stresses were compared.

RESULTS

Results indicated that the intervertebral disc material model has a strong effect on load-sharing and disc height change when compression and flexion were applied. The predicted mechanical response of three models under extension had less discrepancy than its counterparts under flexion and compression. The fluid-solid interaction showed more relevance in dynamic than quasi-static loading conditions. The fiber-reinforced linear elastic and hyperelastic material models underestimated the load-sharing of the intervertebral disc annular collagen fibers.

CONCLUSION

This study confirmed the central role of the disc fluid pressure in spinal load-sharing and highlighted loading conditions where linear elastic and hyperelastic models predicted energy distribution different than that of the biphasic model.

Development of a detailed human neck finite element model and injury risk curves under lateral impact

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.

Influence of cervical spine sagittal alignment on range of motion after corpectomy: a finite element study

BACKGROUND

Sagittal alignment of the cervical spine might influence the development of radiological adjacent segment pathology (RASP) after central corpectomy (CC). Range of motion (ROM) of the adjacent segments is closely linked to the development of RASP.

METHODS

To investigate the ROM of the adjacent segments after CC, we developed a C2-T1 finite element (FE) model. The model with a lordotic sagittal alignment served as the baseline model. Models with straight and kyphotic alignment were generated using mesh morphing methods. Single-level corpectomy at C5 was done on these models. Segmental ROMs of intact and corpectomized spines were compared for physiologic flexion-extension loads.

RESULTS

The flexion ROM decreased by an average of 13% with the change in sagittal alignment from lordosis to kyphosis; however, a consistent decrease was not observed in extension. After CC, the ROM increased by an average of 95% and 31% in the superior and inferior adjacent segments. With kyphotic change in the sagittal alignment, the postoperative increase in flexion ROM exhibited a decreasing trend, while this was not seen in extension.

CONCLUSIONS

Kyphotic changes of the intact spine resulted in segmental stiffening, and after corpectomy, it resulted in inconsistent variations of segmental extension ROMs.

The rate of tendon failure in a collagen fibre recruitment-based model

Accurate characterization of the mechanical response of collagenous tissues is critical for investigations into mechanisms of soft tissue injury. These tissues are inherently viscoelastic, exhibiting strain-rate dependent stiffnesses, creep, and stress-relaxation. The strain-rate features of the failure portion of the stress-strain curve are less well developed. Collagen-distribution based models are improving and capable of reproducing the non-linear aspects of the elastic response of soft tissues, but still require parameterization of failure regions. Therefore, the purpose of this investigation, was to determine whether the parameters characterizing the rate of damage accumulation in a collagen-distribution model are proportional to strain rate. Fifty rat tail tendons were subjected to one of five different strain rates (0.01, 0.05, 0.1, 0.15, 0.20 s^-1) until failure in an uni-axial strain test. To test the hypothesis that the parameters associated with damage rate are proportional to strain rate, a collagen distribution model was employed with the parameters describing the rate of fibre damage being obtained by least-squares and regressed against the strain rate. The breaking function was found to be proportional to strain rate, with a proportionality constant of 60.7 s^-1. Properties characterizing the failure portion of the stress-strain curves for rat tail tendons are also reported. The Young's Modulus did not vary with strain rate and was found to be 103.3 ± 49.5 MPa. Similarly, failure stresses and strains did not vary across the strain rates tested, and were 15.6 ± 6.1 MPa and 32.2 ± 9.1%, respectively.

A detailed finite element model of a mid-sized male for the investigation of traffic pedestrian accidents

The pedestrian is one of the most vulnerable road users and comprises approximately 23% of the road crash-related fatalities in the world. To protect pedestrians during Car-to-Pedestrian Collisions (CPC), subsystem impact tests are used in regulations. These tests provide insight but cannot characterize the complex vehicle-pedestrian interaction. The main purpose of this study was to develop and validate a detailed pedestrian Finite Element (FE) model corresponding to a 50th percentile male to predict CPC induced injuries. The model geometry was reconstructed using a multi-modality protocol from medical images and exterior scan data corresponding to a mid-sized male volunteer. To investigate injury response, this model included internal organs, muscles and vessels. The lower extremity, shoulder and upper body of the model were validated against Post Mortem Human Surrogate (PMHS) test data in valgus bending, and lateral/anterior-lateral blunt impacts, respectively. The whole-body pedestrian model was validated in CPC simulations using a mid-sized sedan and simplified generic vehicles bucks and previously unpublished PMHS coronal knee angle data. In the component validations, the responses of the FE model were mostly within PMHS test corridors and in whole body validations the kinematic and injury responses predicted by the model showed similar trends to PMHS test data. Overall, the detailed model showed higher biofidelity, especially in the upper body regions, compared to a previously reported simplified pedestrian model, which recommends using it in future pedestrian automotive safety research.

Febio finite element models of the human cervical spine

Finite element (FE) analysis has proven to be useful when studying the biomechanics of the cervical spine. Although many FE studies of the cervical spine have been published, they typically develop their models using commercial software, making the sharing of models between researchers difficult. They also often model only one part of the cervical spine. The goal of this study was to develop and evaluate three FE models of the adult cervical spine using open-source software and to freely provide these models to the scientific community. The models were created from computed tomography scans of 26-, 59-, and 64-year old female subjects. These models were evaluated against previously published experimental and FE data. Despite the fact that all three models were assigned identical material properties and boundary conditions, there was notable variation in their biomechanical behavior. It was therefore apparent that these differences were the result of morphological differences between the models.

Application of Simulation Methods in Cervical Spine Dynamics

Neck injury is one of the most frequent spine injuries due to the complex structure of the cervical spine. The high incidence of neck injuries in collision accidents can bring a heavy economic burden to the society. Therefore, knowing the potential mechanisms of cervical spine injury and dysfunction is significant for improving its prevention and treatment. The research on cervical spine dynamics mainly concerns the fields of automobile safety, aeronautics, and astronautics. Numerical simulation methods are beneficial to better understand the stresses and strains developed in soft tissues with investigators and have been roundly used in cervical biomechanics. In this article, the simulation methods for the development and application of cervical spine dynamic problems in the recent years have been reviewed. The study focused mainly on multibody and finite element models. The structure, material properties, and application fields, especially the whiplash injury, were analyzed in detail. It has been shown that simulation methods have made remarkable progress in the research of cervical dynamic injury mechanisms, and some suggestions on the research of cervical dynamics in the future have been proposed.

Sex-Specific Intubation Biomechanics: Intubation Forces Are Greater in Male Than in Female Patients, Independent of Body Weight

Background

Studies of head, neck, and cervical spine morphology and tissue material properties indicate that cervical spine biomechanics differ between adult males and females. These differences result in sex-specific cervical spine kinematics and injury patterns in response to standardized loading conditions. Because direct laryngoscopy and endotracheal intubation require the application of a load to the cervical spine, intubation biomechanics should be sex-specific. The aim of this study was to determine if intubation forces during direct laryngoscopy differ between male and female patients and, if so, is the difference independent of body weight.

Methods

We pooled original data from three previously published adult clinical intubation studies that used methodologically reliable intubation force measurements (measured total laryngoscope force applied to the tongue, and force values were insensitive to or accounted for other laryngoscope blade forces). All patients had undergone direct laryngoscopy and orotracheal intubation with a Macintosh 3 blade under general anesthesia. Patient data included sex, age, height, weight, and maximum intubation force. Least squares multivariable linear regression was performed between the dependent variable (maximum intubation force) and two independent variables (patient sex and patient weight). A third term was added for the interaction between patient sex and weight.

Results

Among all patients (males n=42, females n=59), the median intubation force was 42.2 N (25^th to 75^th percentiles: 31.5 to 57.4 N). While controlling for patient body weight, intubation force differed between the sexes; P=0.011, with greater intubation force in male patients. While controlling for patient sex, there was a positive association between patient body weight and intubation force; P=0.009. In addition, there was a significant interaction between patient sex and weight; P=0.002, with intubation force in male patients having greater dependence on body weight. The difference in intubation force between male and female patients who had the same body weight exceeded 5 N when body weight exceeded 75 kg, and intubation force differences between male and female patients increased as patient body weight increased. Additional analyses using robust regression and using body mass index instead of weight provided comparable results.

Conclusion

In adult patients, the biomechanics of direct laryngoscopy and intubation are sex-specific. Our findings support the need to control for patient sex and weight in future clinical and laboratory studies of the human cervical spine and head and neck biomechanics.

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.

Analysis of the mechanical response of damaged human cervical spine ligaments

BACKGROUND

Cervical spine ligaments that protect the spinal cord and stabilize the spine are frequently injured in motor vehicle collisions and other traumatic situations. These injuries are usually incomplete, and often difficult to notice. The focus of the presented study is placed on analysis of the effect of subfailure load on the mechanical response of the three main cervical spine ligaments: the anterior and the posterior longitudinal ligament and the ligamentum flavum.

METHODS

A total of 115 samples of human cadaveric ligaments removed within 24-48 h after death have been tested. Uniaxial tension tests along the fiber direction were performed in physiological conditions on a custom designed test equipment. The ligaments were loaded into an expected damage zone at two different subfailure values (based on previously reported reference group of 46 samples), and then reloaded to failure.

FINDINGS

The main effect of a high subfailure load has proven to be the toe elongation change. The toe elongation increase is affected by the subfailure load value. While anterior and posterior longitudinal ligament showed similar changes, the smallest subfailure effect was found in ligamentum flavum.

INTERPRETATIONS

The normal physiological region of the cervical spine ligaments mechanical response is modified by a high subfailure load. The observed ligament injury significantly compromises ligament ability to give tensile support within physiological spinal motion.

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.

Rear-Impact Neck Whiplash: Role of Head Inertial Properties and Spine Morphological Variations on Segmental Rotations

Whiplash injuries continue to be a concern in low-speed rear impact. This study was designed to investigate the role of variations in spine morphology and head inertia properties on cervical spine segmental rotation in rear-impact whiplash loading. Vertebral morphology is rarely considered as an input parameter in spine finite element (FE) models. A methodology toward considering morphological variations as input parameters and identifying the influential variations is presented in this paper. A cervical spine FE model, with its morphology parametrized using mesh morphing, was used to study the influence of disk height, anteroposterior vertebral depth, and segmental size, as well as variations in head mass, moment of inertia, and center of mass locations. The influence of these variations on the characteristic S-curve formation in whiplash response was evaluated using the peak C2–C3 flexion marking the maximum S-curve formation and time taken for the formation of maximum S-curve. The peak C2–C3 flexion in the S-curve formation was most influenced by disk height and vertebral depth, followed by anteroposterior head center of mass location. The time to maximum S-curve was most influenced by the anteroposterior location of head center of mass. The influence of gender-dependent variations, such as the vertebral depth, suggests that they contribute to the greater segmental rotations observed in females resulting in different S-curve formation from men. These results suggest that both spine morphology and head inertia properties should be considered to describe rear-impact responses.

Validated thoracic vertebrae and costovertebral joints increase biofidelity of a human body model in hub impacts

Objective: A recent emphasis on nontraditional seating and omnidirectional impact directions has motivated the need for deformable representation of the thoracic spine (T-spine) in human body models. The goal of this study was to develop and validate a deformable T-spine for the Global Human Body Models Consortium (GHBMC) M50-O (average male occupant) human model and to demonstrate improved biofidelity.Methods: Eleven functional spinal units (FSUs) were developed with deformable vertebrae (cortical and trabecular), spinal and costovertebral ligaments, and intervertebral discs. Material properties for all parts were obtained from the literature.FSUs were subjected to quasistatic loads per Panjabi et al. (1976) in 6 degrees of freedom. Stiffness values were calculated for each moment (Nm/°) and translational force (N/µm). Updated costovertebral (CV) joints of ribs 2, 6, and 10 were subjected to moments along 3 axes per Duprey et al. (2010). The response was optimized by maximum force and laxity in the ligaments. In both cases, updated models were compared to the baseline approach, which employed rigid bodies and joint-like behavior. The deformable T-spine and CV joints were integrated into the full M50-O model Ver. 5.0β and 2 full-body cases were run: (1) a rear pendulum impact per Forman et al. (2015) at speeds up to 5.5 m/s. and (2) a lateral shoulder impact per Koh (2005) at 4.5 m/s. Quantitative evaluation protocols were used to evaluate the time history response vs. experimental data, with an average correlation and analysis (CORA) score of 0.76.Results: All FSU responses showed reduced stiffness vs. baseline. Tension, extension, torsion, and lateral bending became more compliant than experimental data. Like the experimental results, no trend was observed for joint response by level. CV joints showed good biofidelity. The response at ribs 2, 6, and 10 generally followed the experimental data.Conclusions: Deformable T-spine and CV joint validation has not been previously published and yielded high biofidelity in rear impact and notable improvement in lateral impact at the full body level. Future work will focus on localized T-spine injury criteria made possible by the introduction of this fully deformable representation of the anatomy.

Cervical spine morphology and ligament property variations: A finite element study of their influence on sagittal bending characteristics

Cervical spine finite element models reported in biomechanical literature usually represent a static morphology. Not considering morphology as a model parameter limits the predictive capabilities for applications in personalized medicine, a growing trend in modern clinical practice. The objective of the study was to investigate the influence of variations in spinal morphology on the flexion-extension responses, utilizing mesh-morphing-based parametrization and metamodel-based sensitivity analysis. A C5-C6 segment was used as the baseline model. Variations of intervertebral disc height, facet joint slope, facet joint articular processes height, vertebral body anterior-posterior depth, and segment size were parametrized. In addition, material property variations of ligaments were considered for sensitivity analysis. The influence of these variations on vertebral rotation and forces in the ligaments were analyzed. The disc height, segmental size, and body depth were found to be the most influential (in the cited order) morphology variations; while among the ligament material property variations, capsular ligament and ligamentum flavum influenced vertebral rotation the most. Changes in disc height influenced forces in the posterior ligaments, indicating that changes in the anterior load-bearing column of the spine could have consequences on the posterior column. A method to identify influential morphology variations is presented in this work, which will help automation efforts in modeling to focus on variations that matter. This study underscores the importance of incorporating influential morphology parameters, easily obtained through computed tomography/magnetic resonance images, to better predict subject-specific biomechanical responses for applications in personalized medicine.

Objective Evaluation of Whole Body Kinematics in a Simulated, Restrained Frontal Impact

The use of human body models as an additional data point in the evaluation of human-machine interaction requires quantitative validation. In this study a validation of the Global Human Body Models Consortium (GHBMC) average male occupant model (M50-O v. 4.5) in a restrained frontal sled test environment is presented. For vehicle passengers, frontal crash remains the most common mode, and the most common source of fatalities. A total of 55-time history traces of reaction loads and kinematics from the model were evaluated against corresponding PMHS data (n = 5). Further, the model's sensitivity to the belt path was studied by replicating two documented PMHS cases with prominent lateral and medial belt paths respectively. Results were quantitatively evaluated using open source CORA software. A tradeoff was observed; better correlation scores were achieved on gross measures (e.g. reaction loads), whereas better corridor scores were achieved on localized measures (rib deflections), indicating that subject specificity may dominate the comparison at localized anatomical regions. On an overall basis, the CORA scores were 0.68, 0.66 and 0.60 for force, body kinematics and chest wall kinematics. Belt force responses received the highest grouped CORA score of 0.85. Head and sternum kinematics earning a 0.8 and 0.7 score respectively. The model demonstrated high sensitivity to belt path, resulting in a 20-point increase in CORA score when the belt was routed closer to analogous location of data collection. The human model demonstrated overall reasonable biofidelity and sensitivity to countermeasures in frontal crash kinematics.

The denticulate ligament - Tensile characterisation and finite element micro-scale model of the structure stabilising spinal cord

BACKGROUND

Damage to the spinal cord is one of the most debilitating pathologies with considerable health, economic and social impact. Improved prevention, treatment and rehabilitation after spinal cord injury (SCI) requires the complex biomechanics of the spinal cord with all its structural elements and the injury mechanism to be understood. This comprehensive understanding will also allow development of models and tools enabling better diagnosis, surgical treatment with increased safety and efficacy and possible development of regenerative therapies. The denticulate ligaments play an important role in stabilising spinal cord within the spinal canal. They participate in spinal cord movements and play a role in determining the stress distribution during physiological but also traumatic loading. We present detailed tensile characterisation of the denticulate ligaments and a Finite Element micro-scale model of the ligament relating its structure with the distribution of stress under physiological loading.

METHOD

Denticulate ligaments were dissected from cervical spinal levels from 6 porcine cervical specimens with fragments of the pia and dura mater and characterised in terms of their geometry and response to uniaxial tensile loading. The stress-strain characteristics were recorded until rupture of the ligament, ultimate parameters and Young's moduli were determined. The parametric micro-structural Finite Element model was constructed based on literature microscope and histological images of a denticulate ligament as a phenomenological representation of the complex microstructure of a soft tissue. The model was validated against the experimental data.

RESULTS

Stress-strain characteristics obtained in tensile test were typical for a soft tissue behaviour. No statistically relevant differences in ultimate strength, strain and Young's moduli were observed between the ligaments harvested from different vertebral levels. Average ultimate tensile stress was 1.26 ± 0.20 MPa at strain 0.51 ± 0.00, rupturing force (1.01 ± 0.21 N) was in agreement with results obtained previously. The Finite Element model accurately predicted the extension-load behaviour of the denticulate ligament in elastic regime. The micro-scale structural representation enabled capturing deformation modes representative of the experimentally observed behaviour.

CONCLUSIONS

The presented stress-strain characteristics of the denticulate ligaments add valuable data to the understanding of the biomechanics of the spinal cord and enable development of more accurate models. The developed micro-scale model was capable of capturing biomechanical response of collagenous tissue under tensile loading, it can be applied for the prediction of other soft tissues behaviours. The denticulate ligament model should be included into future spinal cord models to fully represent the complex system's biomechanics and enable development of surgical aid tools to improve patient outcomes and future regenerative therapies.