Complex Neck Loading and Injury Tolerance in Lateral Bending With Head Rotation From Human Cadaver Tests

Advancements in automated vehicles may position the occupant in postures different from the current standard posture. It may affect human tolerance responses. The objective of this study was to determine the lateral bending tolerance of the head-cervical spine with initial head rotation posture using loads at the occipital condyles and lower neck and describe injuries. Using a custom loading device, head-cervical spine complexes from human cadavers were prepared with load cells at the ends. Lateral bending loads were applied to prerotated specimens at 1.5 m/s. At the occipital condyles, peak axial and antero-posterior and medial-lateral shear forces were: 316-954 N, 176-254 N, and 327-508 N, and coronal, sagittal, and axial moments were: 27-38 N·m, 21-38 N·m, and 9.7-19.8 N·m, respectively. At the lower neck, peak axial and shear forces were: 677-1004 N, 115-227 N, and 178-350 N, and coronal, sagittal, and axial moments were: 30-39 N·m, 7.6-21.3 N·m, and 5.7-13.4 N·m, respectively. Ipsilateral atlas lateral mass fractures occurred in four out of five specimens with varying joint diastasis and capsular ligament involvements. Acknowledging that the study used a small sample size, initial tolerances at the occipital condyles and lower neck were estimated using survival analysis. Injury patterns with posture variations are discussed.

Comparison of Load-Sharing Responses Between Graded Posterior Cervical Foraminotomy and Conventional Fusion Using Finite Element Modeling

Following the diagnosis of unilateral cervical radiculopathy and need for surgical intervention, anterior cervical diskectomy and fusion (conventional fusion) and posterior cervical foraminotomy are common options. Although patient outcomes may be similar between the two procedures, their biomechanical effects have not been fully compared using a head-to-head approach, particularly, in relation to the amount of facet resection and internal load-sharing between spinal segments and components. The objective of this investigation was to compare load-sharing between conventional fusion and graded foraminotomy facet resections under physiological loading. A validated finite element model of the cervical spinal column was used in the study. The intact spine was modified to simulate the two procedures at the C5-C6 spinal segment. Flexion, extension, and lateral bending loads were applied to the intact, graded foraminotomy, and conventional fusion spines. Load-sharing was determined using range of motion data at the C5-C6 and immediate adjacent segments, facet loads at the three segments, and disk pressures at the adjacent segments. Results were normalized with respect to the intact spine to compare surgical options. Conventional fusion leads to increased motion, pressure, and facet loads at adjacent segments. Foraminotomy leads to increased motion and anterior loading at the index level, and motions decrease at adjacent levels. In extension, the left facet load decreases after foraminotomy. Recognizing that foraminotomy is a motion preserving alternative to conventional fusion, this study highlights various intrinsic biomechanical factors and potential instability issues with more than one-half facet resection.

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.

A guide to finite element analysis models of the spine for clinicians

Finite element analysis (FEA) is a computer-based mathematical method commonly used in spine and orthopedic biomechanical research. Advances in computational power and engineering modeling and analysis software have enabled many recent technical applications of FEA. Through the use of FEA, a wide range of scenarios can be simulated, such as physiological processes, mechanisms of disease and injury, and the efficacy of surgical procedures. Such models have the potential to enhance clinical studies by allowing comparisons of surgical treatments that would be impractical to perform in human or animal studies, and by linking model results to treatment outcomes. While traditional ex vivo experiments are limited by variabilities in tissue, the complexity of test setup, cost, measurable biomechanical parameters, and the repeatability of experiments, FEA models can be used to measure a wide range of clinically relevant biomechanical parameters. Generic or patient-specific anatomical models can be modified to simulate different clinical and surgical conditions under simulated physiological conditions. Despite these capabilities, there is limited understanding of the clinical applicability and translational potential of FEA models. For spine surgeons, a comprehensive understanding of the key features, strengths, and limitations of FEA models of the spine and their ability to personalize treatment options and assist in clinical decision-making would significantly enhance the impact of FEA research. Furthermore, fostering collaborations between surgeons and engineers could augment the clinical use of these models. The purpose of this review was to highlight key features of FEA model building for clinicians. To illustrate these features, the authors present an example of the use of FEA models in comparing FDA-approved disc arthroplasty implants.

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.

Analysis of experimental injuries to obese occupants with different postures in frontal impact

The objective of the present study was to analyze injuries and their patterns to obese occupants in frontal impacts with upright and reclined postures using experimental data. Twelve obese post-mortem human subjects (PMHS) were positioned on a sled buck with seatback angles of 250 or 450 from the vertical, termed as upright and reclined postures. They were restrained with a seat belt and pretensioner. Frontal impact tests were conducted at 8.9 or 13.9 m/s, termed as low and high velocities. After the test, x-rays and CTs were taken, and an autopsy was conducted. The Maximum AIS (MAIS) and Injury Severity Score (ISS) were calculated, and injury patterns were analyzed. The mean age, stature, total body mass, and body mass indexes were 67 years, 112 kg, and 1.7 m, and 38 kg/m2. None of these parameters were statistically significantly different between any groups. The mean thickness of the soft tissues in the left anterior lateral, central, and right anterior lateral aspects were 44 mm, 24 mm, and 46 mm. In the low-velocity tests, the ISS data were 9, 18, and 9 for the upright, and 9, 9, and 4 for the reclined specimens, and in the high velocity tests, they were 29, 17, and 27 for the upright, and 27, 13, and 27 for the reclined postures. Other data are given in the paper. For both postures at the low velocity, injuries were concentrated at one body region, and the ISS data were in the mild category; in contrast, at the high velocity, other body regions also sustained injuries, and the ISS data were in the major trauma category. From MAIS perspectives, injuries to obese occupants did not change between postures and were independent of the energy input to the system. The association of chest with pelvis injuries in upright and reclined postures to obese occupants may have additional consequences following the initial injury to this group of our population.

Upper cervical spine bone mineral content variations in elderly females

The purpose of the study was to determine the bone mineral densities (BMDs) of the C1 and C2 vertebrae and discuss their implications for autonomous vehicle environments and vulnerable road users. Using quantitated computed tomography (QCT), the BMDs were obtained at eight regions for the C1 vertebra and seven regions for the C2 vertebra. The spine surgeon author outlined the boundaries of each region, and nine elderly female human cadaver specimens were used. The regions were based on potential stabilization locations for fracture fixation. In the C1 vertebra, the BMD was greatest at the anterior tubercle, followed by the posterior tubercle, the posterior arch, and the lateral and anterior lateral masses. In the C2 vertebra, the distal odontoid had the greatest BMD, followed by the spinous process, the C2-lateral mass, the odontoid-body interface, and the anterior inferior aspect of the body. Use of these data in female-specific finite element models may lead to a better understanding of load paths, injuries, mechanisms, and tolerance.

Regional brain strain dependance on direction of head rotation

Brain injuries in automated vehicles during crash events are likely to include mechanisms of head impact in non-standard positions and postures (i.e., occupants not facing forward in an upright position). Federal regulations currently focus on impact conditions in primary planes of motion, such as frontal or rear impacts (sagittal plane of motion) or side impact (coronal plane of motion) and do not account for out of position occupants or non-standard postures. The objective of the present study was to develop and use the anatomically accurate brain finite element model to parametrically determine the injury metrics under different vectors with head rotation. A custom developed brain finite element model with anatomical accuracy and several anatomical regions defined was used to evaluate whole-brain strain as well as regional brain strain. Cumulative Strain Damage Measure (CSDM) at a threshold of 20% strain and the 95th percentile of the maximum principal strain (MPS95) were calculated for the whole brain and each brain region under multiple rotational directions. The model was exposed to a sinusoidal angular acceleration pulse of 5000 rad per second squared (rad/s2-) over 12.5 ms. The same pulse was used in the primary axes of motion and (lateral bending, flexion, extension, axial rotation) and combined axes representing oblique flexion and oblique extension. Whole brain CSDM20 was highest for lateral bending. Whole brain MPS95 was highest for axial rotation. The rCSDM20 was more susceptible to impact direction, with several brain regions having substantial accumulation of strain for oblique flexion and lateral bending. Comparatively, rMPS95 was more consistent across all rotation directions. The present study quantified the regional brain strain response under multiple rotational vectors identifying a high amount of variability in the accumulation of strain (i.e., CSDM20) in the hypothalamus, hippocampus, and midbrain specifically. While there was a high amount of variability in the accumulation of strain for multiple regions, the maximum strain measured (i.e., MPS95) in the regions was more consistent.

Pelvis-Sacrum-Lumbar Spine Injury Characteristics From Underbody Blast Loading

INTRODUCTION

Combat-related injuries from improvised explosive devices occur commonly to the lower extremity and spine. As the underbody blast impact loading traverses from the seat to pelvis to spine, energy transfer occurs through deformations of the combined pelvis-sacrum-lumbar spine complex, and the time factor plays a role in injury to any of these components. Previous studies have largely ignored the role of the time variable in injuries, injury mechanisms, and warfighter tolerance. The objective of this study is to relate the time or temporal factor using a multi-component, pelvis-sacrum-lumbar spinal column complex model.

MATERIALS AND METHODS

Intact pelvis-sacrum-spine specimens from pre-screened unembalmed human cadavers were prepared by fixing at the superior end of the lumbar spine, pelvis and abdominal contents were simulated, and a weight was added to the cranial end of the fixation to account for torso effective mass. Prepared specimens were placed on the platform of a custom vertical accelerator device and aligned in a seated soldier posture. An accelerometer was attached to the seat platen of the device to record the time duration to peak velocity. Radiographs and computed tomography images were used to document and associate injuries with time duration.

RESULTS

The mean age, stature, weight, body mass index, and bone density of 12 male specimens were as follows: 65 ± 11 years, 1.8 ± 0.01 m, 83 ± 13 kg, 27 ± 5.0 kg/m2, and 114 ± 21 mg/cc. They were equally divided into short, medium, and long time durations: 4.8 ± 0.5, 16.3 ± 7.3, and 34.5 ± 7.5 ms. Most severe injuries associated with the short time duration were to pelvis, although they were to spine for the long time duration.

CONCLUSIONS

With adequate time for the underbody blast loading to traverse the pelvis-sacrum-spine complex, distal structures are spared while proximal/spine structures sustain severe/unstable injuries. The time factor may have implications in seat and/or seat structure design in future military vehicles to advance warfighter safety.

Preliminary Data of Neck Muscle Morphology With Head-Supported Mass in Male and Female Volunteers

INTRODUCTION

This study quantified parameters related to muscle morphology using a group of upright seated female and male volunteers with a head-supported mass.

MATERIALS AND METHODS

Upright magnetic resonance images (MRIs) were obtained from 23 healthy volunteers after approval from the U.S. DoD. They were asymptomatic for neck pain, with no history of injury. The volunteers were scanned using an upright MRI scanner with a head-supported mass (army combat helmet). T1 and T2 sagittal and axial images were obtained. Measurements were performed by an engineer and a neurosurgeon. The cross-sectional areas of the sternocleidomastoid and multifidus muscles were measured at the inferior endplate in the sub-axial column, and the centroid angle and centroid radius were quantified. Differences in the morphology by gender and spinal level were analyzed using a repeated measures analysis of variance model, adjusted for multiple corrections.

RESULTS

For females and males, the cross-sectional area of the sternocleidomastoid muscle ranged from 2.3 to 3.6 cm2 and from 3.4 to 5.4 cm2, the centroid radius ranged from 4.1 to 5.1 cm and from 4.7 to 5.7 cm, and the centroid angle ranged from 75° to 131° and from 4.8° to 131.2°, respectively. For the multifidus muscle, the area ranged from 1.7 to 3.9 cm2 and from 2.4 to 4.2 cm2, the radius ranged from 3.1 to 3.4 cm and from 3.3 to 3.8 cm, the angle ranged from 15° to 24.4° and 16.2° to 24.4°, respectively. Results from all levels for both muscles and male and female spines are given.

CONCLUSIONS

The cross-sectional area, angulation, and centroid radii data for flexor and extensor muscles of the cervical spine serve as a dataset that may be used to better define morphologies in computational models and obtain segmental motions and loads under external mechanical forces. These data can be used in computational models for injury prevention, mitigation, and readiness.

Regional Strain Response of an Anatomically Accurate Human Finite Element Head Model Under Frontal Versus Lateral Loading

INTRODUCTION

Because brain regions are responsible for specific functions, regional damage may cause specific, predictable symptoms. However, the existing brain injury criteria focus on whole brain response. This study developed and validated a detailed human brain computational model with sufficient fidelity to include regional components and demonstrate its feasibility to obtain region-specific brain strains under selected loading.

METHODS

Model development used the Simulated Injury Monitor (SIMon) model as a baseline. Each SIMon solid element was split into 8, with each shell element split into 4. Anatomical regions were identified from FreeSurfer fsaverage neuroimaging template. Material properties were obtained from literature. The model was validated against experimental intracranial pressure, brain-skull displacement, and brain strain data. Model simulations used data from laboratory experiments with a rigid arm pendulum striking a helmeted head-neck system. Data from impact tests (6 m/s) at 2 helmet sites (front and left) were used.

RESULTS

Model validation showed good agreement with intracranial pressure response, fair to good agreement with brain-skull displacement, and good agreement for brain strain. CORrelation Analysis scores were between 0.72 and 0.93 for both maximum principal strain (MPS) and shear strain. For frontal impacts, regional MPS was between 0.14 and 0.36 (average of left and right hemispheres). For lateral impacts, MPS was between 0.20 and 0.48 (left hemisphere) and between 0.22 and 0.51 (right hemisphere). For frontal impacts, regional cumulative strain damage measure (CSDM20) was between 0.01 and 0.87. For lateral impacts, CSDM20 was between 0.36 and 0.99 (left hemisphere) and between 0.09 and 0.93 (right hemisphere).

CONCLUSIONS

Recognizing that neural functions are related to anatomical structures and most model-based injury metrics focus on whole brain response, this study developed an anatomically accurate human brain model to capture regional responses. Model validation was comparable with current models. The model provided sufficient anatomical detail to describe brain regional responses under different impact conditions.

A Novel Paradigm to Develop Regional Thoracoabdominal Criteria for Behind Armor Blunt Trauma Based on Original Data

INTRODUCTION

For behind armor blunt trauma (BABT), recent prominent BABT standards for chest plate define a maximum deformation distance of 44 mm in clay. It was developed for soft body armor applications with limited animal, gelatin, and clay tests. The legacy criterion does not account for differing regional thoracoabdominal tolerances to behind armor-induced injury. This study examines the rationale and approaches used in the legacy BABT clay criterion and presents a novel paradigm to develop thoracoabdominal regional injury risk curves.

MATERIALS AND METHODS

A review of the original military and law enforcement studies using animals, surrogates, and body armor materials was conducted, and a reanalysis of data was performed. A multiparameter model analysis describes survival-lethality responses using impactor/projectile (mass, diameter, and impact velocity) and specimen (weight and tissue thickness) variables. Binary regression risk curves with ±95% confidence intervals (CIs) and peak deformations from simulant tests are presented.

RESULTS

Injury risk curves from 74 goat thorax tests showed that peak deflections of 44.7 mm (±95% CI: 17.6 to 55.4 mm) and 49.9 mm (±95% CI: 24.7 to 60.4 mm) were associated with the 10% and 15% probability of lethal outcomes. 20% gelatin and Roma Plastilina #1 clay were stiffer than goat. The clay was stiffer than 20% gelatin. Penetration diameters showed greater variations (on a test-by-test basis, difference 36-53%) than penetration depths (0-12%) across a range of projectiles and velocities.

CONCLUSIONS

While the original authors stressed limitations and the importance of additional tests for refining the 44 mm recommendation, they were not pursued. As live swine tests are effective in developing injury criteria and the responses of different areas of the thoracoabdominal regions are different because of anatomy, structure, and function, a new set of swine and human cadaver tests are necessary to develop scaling relationships. Live swine tests are needed to develop incapacitation/lethal injury risk functions; using scaling relationships, human injury criteria can be developed.

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.

Strain Response of an Anatomically Accurate Nonhuman Primate Finite Element Brain Model Under Sagittal Loading

INTRODUCTION

Prevention and treatment of traumatic brain injuries is critical to preserving soldier brain health. Laboratory studies are commonly used to reproduce injuries, understand injury mechanisms, and develop tolerance limits; however, this approach has limitations for studying brain injury, which requires a physiological response. The nonhuman primate (NHP) has been used as an effective model for investigating brain injury for many years. Prior research using the NHP provides a valuable resource to leverage using modern analysis and modeling techniques to improve our understanding of brain injury. The objectives of the present study are to develop an anatomically accurate finite element model of the NHP and determine regional brain responses using previously collected NHP data.

MATERIALS AND METHODS

The finite element model was developed using a neuroimaging-based anatomical atlas of the rhesus macaque that includes both cortical and subcortical structures. Head kinematic data from 10 sagittal NHP experiments, four +Gx (rearward) and six -Gx (frontal), were used to test model stability and obtain brain strain responses from multiple severities and vectors.

RESULTS

For +Gx tests, the whole-brain cumulative strain damage measure exceeding a strain threshold of 0.15 (CSDM15) ranged from 0.28 to 0.89, and 95th percentile of the whole-brain maximum principal strain (MPS95) ranged from 0.21 to 0.59. For -Gx tests, whole-brain CSDM15 ranged from 0.02 to 0.66, and whole-brain MPS95 ranged from 0.08 to 0.39.

CONCLUSIONS

Recognizing that NHPs are the closest surrogate to humans combined with the limitations of conducting brain injury research in the laboratory, a detailed anatomically accurate finite element model of an NHP was developed and exercised using previously collected data from the Naval Biodynamics Laboratory. The presently developed model can be used to conduct additional analyses to act as pilot data for the design of newer experiments with statistical power because of the sensitivity and resources needed to conduct experiments with NHPs.

Comparison of Axial Force Attenuation Characteristics in Two Different Lower Extremity Anthropomorphic Test Devices

INTRODUCTION

Any type of boot or footwear is designed to attenuate and distribute loading to the bottom of the foot. Anthropomorphic test device (ATDs) are used to assess potential countermeasures against these loads. The specific aims of this study were to compare and quantify force attenuation characteristics as a function of input energy for Hybrid-III and Mil-Lx ATD human surrogates.

MATERIALS AND METHODS

Two lower leg ATD surrogates (Mil-Lx and Hybrid-III) were tested to investigate the influence of a commercially available military boot on lower extremity force response and assess such differences against previously published postmortem human surrogate studies. The testing apparatus impacted the bottom of the foot using a rigid plate at velocities from 2 to 10 m/s. Tests were conducted on each ATD to obtain axial force response with and without boots as a function of input energy.

RESULTS

Peak forces ranged from 1 to 16.4 kN for the Hybrid-III, and 1 to 8.4 kN for the Mil-Lx for similar input conditions. The average force attenuation for the Hybrid-III at upper and lower load cells was 71% (59%-80%) and 70% (58%-78%). The average attenuation for the Mil-Lx at upper and lower load cells was 20% (13%-28%) and 37% (36%-37%), respectively. At the knee load cell, the attenuated peak loads ranged from 62% to 81% for the Hybrid-III and 16% to 30% for the Mil-Lx.

CONCLUSIONS

Force attenuation characteristics in the booted vs unbooted configuration of the Mil-Lx were significantly different than force attenuation characteristics of the H3 and may better represent in vivo forces during vertical impact injuries, such as IED blasts. Hence for military relevant applications where boots are used, the Mil-Lx may provide a more conservative evaluation of lower extremity protection systems.

Human pelvis injury risk curves from underbody blast impact

INTRODUCTION

Underbody blast loading can result in injuries to the pelvis and the lumbosacral spine. The purpose of this study was to determine human tolerance in this region based on survival analysis.

METHODS

Twenty-six unembalmed postmortem human surrogate lumbopelvic complexes were procured and pretest medical images were obtained. They were fixed in polymethylmethacrylate at the cranial end and a six-axis load cell was attached. The specimens were aligned in a seated soldier posture. Impacts were applied to the pelvis using a custom vertical accelerator. The experimental design consisted of non-injury and injury tests. Pretest and post-test X-rays and palpation were done following non-injury test, and after injury test medical imaging and gross dissections were done. Injuries were scored using the Abbreviated Injury Scale (AIS). Axial and resultant forces were used to develop human injury probability curves (HIPCs) at AIS 3+ and AIS 4 severities using survival analysis. Then ±95% CI was computed using the delta method, normalised CI size was obtained, and the quality of the injury risk curves was assigned adjectival ratings.

RESULTS

At the 50% probability level, the resultant and axial forces at the AIS 3+ level were 6.6 kN and 5.9 kN, and at the AIS 4 level these were 8.4 kN and 7.5 kN, respectively. Individual injury risk curves along with ±95% CIs are presented in the paper. Increased injury severity increased the HIPC metrics. Curve qualities were in the good and fair ranges for axial and shear forces at all probability levels and for both injury severities.

CONCLUSIONS

This is the first study to develop axial and resultant force-based HIPCs defining human tolerance to injuries to the pelvis from vertical impacts using parametric survival analysis. Data can be used to advance military safety under vertical loading to the seated pelvis.

Finite element modeling of the human cervical spinal cord and its applications: A systematic review

Background Context

Finite element modeling (FEM) is an established tool to analyze the biomechanics of complex systems. Advances in computational techniques have led to the increasing use of spinal cord FEMs to study cervical spinal cord pathology. There is considerable variability in the creation of cervical spinal cord FEMs and to date there has been no systematic review of the technique. The aim of this study was to review the uses, techniques, limitations, and applications of FEMs of the human cervical spinal cord.

Methods

A literature search was performed through PubMed and Scopus using the words finite element analysis, spinal cord, and biomechanics. Studies were selected based on the following inclusion criteria: (1) use of human spinal cord modeling at the cervical level; (2) model the cervical spinal cord with or without the osteoligamentous spine; and (3) the study should describe an application of the spinal cord FEM.

Results

Our search resulted in 369 total publications, 49 underwent reviews of the abstract and full text, and 23 were included in the study. Spinal cord FEMs are used to study spinal cord injury and trauma, pathologic processes, and spine surgery. Considerable variation exists in the derivation of spinal cord geometries, mathematical models, and material properties. Less than 50% of the FEMs incorporate the dura mater, cerebrospinal fluid, nerve roots, and denticulate ligaments. Von Mises stress, and strain of the spinal cord are the most common outputs studied. FEM offers the opportunity for dynamic simulation, but this has been used in only four studies.

Conclusions

Spinal cord FEM provides unique insight into the stress and strain of the cervical spinal cord in various pathological conditions and allows for the simulation of surgical procedures. Standardization of modeling parameters, anatomical structures and inclusion of patient-specific data are necessary to improve the clinical translation.

Effect of Cervical Stenosis and Rate of Impact on Risk of Spinal Cord Injury During Whiplash Injury

STUDY DESIGN

Finite Element Study.

OBJECTIVE

To determine the risk of spinal cord injury with pre-existing cervical stenosis during a whiplash injury.

SUMMARY OF BACKGROUND DATA

Patients with cervical spinal stenosis are often cautioned on the potential increased risk of spinal cord injury (SCI) from minor trauma such as rear impact whiplash injuries. However, there is no consensus on the degree of canal stenosis or the rate of impact that predisposes cervical SCI from minor trauma.

METHODS

A previously validated three-dimensional finite element model of the human head-neck complex with the spinal cord and activated cervical musculature was used. Rear impact acceleration was applied at 1.8 m/s and 2.6 m/s. Progressive spinal stenosis was simulated at the C5 to C6 segment, from 14 mm to 6 mm, at 2 mm intervals of ventral disk protrusion. Spinal cord von Mises stress and maximum principal strain were extracted and normalized with respect to the 14 mm spine at each cervical spine level from C2 to C7.

RESULTS

The mean segmental range of motion was 7.3 degrees at 1.8 m/s and 9.3 degrees at 2.6 m/s. Spinal cord stress above the threshold for SCI was noted at C5 to C6 for 6 mm stenosis at 1.8 m/s and 2.6 m/s. The segment (C6-C7) inferior to the level of maximum stenosis also showed increasing stress and strain with a higher rate of impact. For 8 mm stenosis, spinal cord stress exceeded SCI thresholds only at 2.6 m/s. Spinal cord strain above SCI thresholds were only noted in the 6 mm stenosis model at 2.6 m/s.

CONCLUSION

Increased spinal stenosis and rate of impact are associated with greater magnitude and spatial distribution of spinal cord stress and strain during a whiplash injury. Spinal canal stenosis of 6 mm was associated with consistent elevation of spinal cord stress and strain above SCI thresholds at 2.6 m/s.

Differences in spinal cord biomechanics after laminectomy, laminoplasty, and laminectomy with fusion for degenerative cervical myelopathy

OBJECTIVE

Spinal cord stress/strain during neck motion contributes to spinal cord dysfunction in degenerative cervical myelopathy (DCM), yet the effect of surgery on spinal cord biomechanics is unknown. It is expected that motion-preserving and fusion surgeries for DCM will have distinct effects on spinal cord biomechanics. The aim of this study was to compare changes in spinal cord biomechanics after laminectomy with fusion, laminectomy, and laminoplasty using a patient-specific finite element model (FEM) for DCM.

METHODS

A patient-specific FEM of the cervical spine and spinal cord was created using MRI from a subject with mild DCM. Multilevel laminectomy with fusion, laminectomy, and laminoplasty were simulated for DCM using the patient-specific FEM. Spinal cord von Mises stress and maximum principal strain during neck flexion-extension, lateral bending, and axial rotation were recorded. Segmental range of motion, intradiscal pressure, and capsular ligament strain were also measured. FEM outputs were calculated as a change with respect to the preoperative values and compared between the three models.

RESULTS

Across the surgical levels, spinal cord stress increased after laminectomy for neck flexion (+50%), neck extension (+37.8%), and axial rotation (+23%). Similarly, spinal cord strain increased in neck extension (+118.4%) and axial rotation (+75.1%) after laminectomy. Laminoplasty was associated with greater spinal cord stress in neck flexion (+57.4%) and increased strain in lateral bending (+56.7%) and axial rotation (+20.9%). Compared with laminectomy and laminoplasty, spinal cord biomechanics for laminectomy with fusion revealed significantly reduced median extension stress (13.7 kPa vs 9.7 kPa, p = 0.03), lateral bending strain (0.01 vs 0.007, p = 0.007), axial rotation stress (3.7 kPa vs 2.1 kPa, p = 0.04), and axial rotation strain (0.017 vs 0.009, p = 0.04).

CONCLUSIONS

Spinal cord strain decreased in neck flexion in all three models, yet spinal cord stress increased with neck flexion for laminectomy and laminoplasty. Changes in spinal cord biomechanics for laminoplasty parallel those for laminectomy with fusion except during neck flexion, lateral bending, and axial rotation. Compared with motion-preserving approaches such as laminectomy and laminoplasty, laminectomy with fusion was associated with the lowest spinal cord stress and strain in flexion-extension, lateral bending, and axial rotation of the neck.

Regional variations in C1-C2 bone density on quantitated computed tomography and clinical implications

Background

Our elderly population is growing and the number of spine fractures in the elderly is also growing. The elderly population in general may be considered as poor surgical candidates experience a high rate of fractures at C1 and C2 compared with the general population. Nonoperative management of upper cervical fractures is not benign as there is a high nonunion rate for both C1 and C2 fractures in the elderly, and orthosis compliance is often suboptimal, or complicated by skin breakdown. The optimal technique for upper cervical stabilization in the elderly may be different than in younger populations as the bone quality is inferior in the elderly. The objective of this basic science study is to determine whether the bone mineral density (BMD) of C1 and C2 vary by region, and if this is a gender difference in this elderly age group.

Methods

Twenty cadaveric spines from 45 to 83 years of age were used to obtain BMD using quantitated computed tomography (QCT). BMD was measured using a QCT. For C1, 8 regions were determined: anterior tubercle, bilateral anterior and medial lateral masses, bilateral posterior arches, and posterior tubercle. For C2, 7 regional BMDs were determined: top of odontoid, base of odontoid-body interface, mid body, bilateral lateral masses, anterior inferior body near the discs space, and the C2 spinous process.

Results

The BMD was greatest at the C1 anterior tubercle (564.4±175.8 mg/cm³) and C1 posterior ring (420.8±110.2 mg/cm³), and least at the anterior and medial lateral masses (262.8±59.5 mg/cm³, 316.9±72.6 mg/cm³). At C2 QCT BMD was greatest at the top of the dens (400.6±107.9 mg/cm³) decreasing down through the odontoid-C2 body junction (267.8±103.5 mg/cm³) and least in the mid C2 body 249.1±68.8 mg/cm³). The posterior arch of C1 and the spinous process of C2 had higher BMD's 420.8±110.2 mg/cm³ and 284.1±93.0 mg/cm³, respectively. A high correlation was observed between the BMD at the interface of the dens-vertebral body with the vertebral body with a Pearson correlation coefficient of 0.86. The BMD of the top of dens was significantly higher (p<.05) than all the regions in C2.

Conclusions

Regional and segmental BMD variations at C1 and C2 have clinical implications for surgical constructs in the elderly population. Given the higher BMDs of the C1 and C2 spinous process and posterior arches, consideration should be given to incorporate these areas using various C1-C2 wiring techniques. In the elderly, lateral masses particularly at C1 with lower BMD may result in potential screw loosening and nonunion in this age group. Old-school wiring techniques have a track record of efficacy and safety with less blood loss, reduced operative time, reduced X-ray exposure, and should be considered in the elderly as a primary stabilization technique or a belt-over suspenders approach based on regional variations in BMD in the elderly.

Temporal corridors of forces and moments, and injuries to pelvis-lumbar spine in vertical impact simulating underbody blast

Pelvis and lumbar spine fractures occur in falls, motor vehicle crashes, and military combat events. They are attributed to vertical impact from the pelvis to the spine. Although whole-body cadavers were exposed to this vector and injuries were reported, spinal loads were not determined. While previous studies determined injury metrics such as peak forces using isolated pelvis or spine models, they were not conducted using the combined pelvis-spine columns, thereby not accounting for the interaction between the two body regions. Earlier studies did not develop response corridors. The study objectives were to develop temporal corridors of loads at the pelvis and spine and assess clinical fracture patterns using a human cadaver model. Vertical impact loads were delivered at the pelvic end to twelve unembalmed intact pelvis-spine complexes, and pelvis forces and spinal loads (axial, shear and resultant and bending moments) were obtained. Injuries were classified using clinical assessments from post-test computed tomography scans. Spinal injuries were stable in eight and unstable in four specimens. Pelvis injuries included ring fractures in six and unilateral pelvis in three, sacrum fractures in ten, and two specimens did not sustain any injuries to the pelvis or sacrum complex. Data were grouped based on time to peak velocity, and ± one standard deviation corridors about the mean of the biomechanical metrics were developed. Time-history corridors of loads at the pelvis and spine, hitherto not reported in any study, are valuable to assess the biofidelity of anthropomorphic test devices and assist validating finite element models.

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.

Morphometry of lumbar muscles in the seated posture with weight-bearing MR scans

Conventional imaging studies of human spine are done in a supine posture in which the axial loading of the spine is not considered. Upright images better reveal the interrelationships between the various internal structures of the spine. The objective of the current study is to determine the cross-sectional areas, radii, and angulations of the psoas, erector spinae, and multifidus muscles of the lumbar spine in the sitting posture. Ten young (mean age 31 ± 4.8 years) asymptomatic female subjects were enrolled. They were seated in an erect posture and weight-bearing T1 and T2 MRIs were obtained. Cross-sectional areas, radii, and angulations of the muscles were measured from L1-L5. Two observers repeated all the measurements for all parameters, and reliability was determined using the inter- and intra-class coefficients. The Pearson product moment correlation was used for association between levels, while level differences were used using a linear regression model. The cross-sectional areas of the psoas and multifidus muscles increased from L1 to L5 (1.9 ± 1.1 to 12.1 ± 2.5 cm2 and 1.8 ± 0.3 to 5.7 ± 1.4 cm2). The cross-sectional area of the erector spinae was greatest at the midlevel (13.9 ± 2.2 cm2) and it decreased in both directions. For the angle, the range for psoas muscles was 75-105°, erector spinae were 39-46° and multifidus was 11-19°. Correlations magnitudes were inconsistent between levels and muscle types. These quantitated data improve our understanding of the geometrical properties in the sitting posture. The weight-bearing MRI-quantified morphometrics of human lumbar spine muscles from this study can be used in biomechanical models for predicting loads on spinal joints under physiological and traumatic situations.

An investigation of elderly occupant injury risks based on anthropometric changes compared to young counterparts

OBJECTIVE

The objective of the study was to investigate the difference between elderly and young occupant injury risks using human body finite element modeling in frontal impacts.

METHODS

Two elderly male occupant models (representative age 70-80 years) were developed using the Global Human Body Consortium (GHBMC) 50th percentile as the baseline model. In the first elderly model (EM-1), material property changes were incorporated, and in the second elderly model (EM-2), material and anthropometric changes were incorporated. Material properties were based on literature. The baseline model was morphed to elderly anthropometry for EM-2. The three models were simulated in a frontal crash vehicle environment at 56 km/h. Responses from the two elderly and baseline models were compared with cadaver experimental data in thoracic, abdominal, and frontal impacts. Correlation and analysis scores were used for correlation with experimental data. The probabilities of head, neck, and thoracic injuries were assessed.

RESULTS

The elderly models showed a good correlation with experimental responses. The elderly EM-1 had higher risk of head and brain injuries compared to the elderly EM-2 and baseline GHBMC models. The elderly EM-2 demonstrated higher risk of neck, chest, and abdominal injuries than the elderly EM-1 and baseline models.

CONCLUSIONS

The study investigated injury risks of two elderly occupants and compared to a young occupant in frontal crashes. The change in the material properties alone (EM-1) suggested that elderly occupants may be vulnerable to a greater risk of head and thoracic injuries, whereas change in both anthropometric and material properties (EM-2) suggested that elderly occupants may be vulnerable to a greater risk of thoracic and neck injuries. The second elderly model results were in better agreement with field injury data from the literature; thus, both anthropometric and material properties should be considered when assessing the injury risks of elderly occupants. The elderly models developed in this study can be used to simulate different impact conditions and determine injury risks for this group of our population.

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.

Obese Occupant Response in Reclined and Upright Seated Postures in Frontal Impacts

The American population is getting heavier and automated vehicles will accommodate unconventional postures. While studies replicating mid-size and upright fore-aft seated occupants are numerous, experiments with post-mortem human subjects (PMHS) with obese and reclined occupants are sparse. The objective of this study was to compare the kinematics of the head-neck, torso and pelvis, and document injuries and injury patterns in frontal impacts. Six PMHS with a mean body mass index of 38.2 ± 5.3 kg/m2 were equally divided between upright and reclined groups (seatback: 23°, 45°), restrained by a three-point integrated belt, positioned on a semi-rigid seat, and exposed to low and moderate velocities (15, 32 km/h). Data included belt loads, spinal accelerations, kinematics, and injuries from x-rays, computed tomography, and necropsy. At 15 km/h speed, no significant difference in the occupant kinematics and evidence of orthopedic failure was observed. At 32 km/h speed, the primary difference between the cohorts was significantly larger Z displacements in the reclined occupant at the head (190 ± 32 mm, vs. 105 ± 33 mm p < 0.05) and femur (52 ± 18 mm vs. 30 ± 10 mm, p < 0.05). All the moderate-speed tests produced at least one thorax injury. Rib fractures were scattered around the circumference of the rib-cage in the upright, while they were primarily concentrated on the anterior aspect of the rib-cage in two reclined specimens. Although MAIS was the same in both groups, the reclined specimens had more bi-cortical rib fractures, suggesting the potential for pneumothorax. While not statistical, these results suggest enhanced injuries with reclined obese occupants. These results could serve as a data set for validating the response of restrained obese anthropometric test device (ATDs) and computational human body models.

Response of Thoraco-Abdominal Tissue in High-Rate Compression

Body armor protects the human from penetrating injuries. However, in the process of defeating a projectile, the back face of the armor can deform into the wearer at extremely high rates. This deformation can cause a variety of soft and hard tissue injuries. Finite element modeling represents one of the best tools to predict injuries from this high-rate compression mechanism. However, the validity of a model is reliant on accurate material properties for biological tissues. In this study, we measured the stress-strain response of thoraco-abdominal tissue during high-rate compression (1000 and 1900 s-1) using a split Hopkinson pressure bar. Using this method, high-rate material properties of porcine adipose, heart, spleen, and stomach tissue were characterized. At a strain rate of 1000 s-1, adipose (E=4.7MPa) was the most compliant, followed by spleen (E=9.6MPa), and then heart (E=13.6MPa) tissue. At a strain rate of 1900 s-1, adipose (E=7.3MPa) was most compliant, followed by spleen (E=10.7MPa), heart (E=14.1MPa), and stomach (E=32.6MPa) tissue. Only adipose tissue demonstrated a consistent rate dependence across these rates, with a stiffer response at 1900 s-1. However, comparison of these tissues to previously published quasi-static and intermediate dynamic experiments revealed a strong rate dependence with increasing stress response from quasi-static to dynamic to high strain rates. Together, these findings can be used to develop finite element models for high-rate compression injuries.

Comparison of small female occupant model responses with experimental data in a reclined posture

Objective: The objective of the current study was to compare the GHBMC female model responses with in-house sled test data for three small female post mortem human surrogates (PMHS) at 32 km/h and a seatback recline angle of 45 degrees. The kinematics and the seatbelt forces were used to compare the female PMHS and model responses. The study aimed to identify updates that may be needed to the model.Methods: In-house experimental sled test kinematic and seatbelt response data for the small females were obtained. The 5^th female GHBMC was simulated with the same boundary conditions as in the experiments. In addition, using the PMHS computed tomography (CT) and test environment scans, the female model geometry was updated to a subject-specific model for one of the specimens, and the models were simulated to obtain 5^th female and subject-specific model responses. The kinematic response and the seatbelt forces for the two models were compared with the average of the three experimental data.Results: The head, T8 and L4 excursions, head and pelvis accelerations and seatbelt forces for the two female models were compared with the experimental data. The model responses were in agreement with the PMHS; however, the subject-specific model showed a closer agreement with the kinematic response. The subject-specific model did not submarine as in the experiments, whereas the 5^th female model submarined. However, the subject-specific model showed 20% higher seatbelt forces than the PMHS.Conclusion: This study showed that anthropometric differences may significantly alter occupant kinematics in reclined posture and need to be incorporated to investigate kinematics and injury mechanisms. The next step of the study involves incorporating age-specific material changes and investigating the subject-specific injury mechanisms. The results will be useful to develop countermeasures for autonomous vehicles.

Mechanisms of cervical spine injury and coupling response with initial head rotated posture - implications for AIS coding

Objective: This objective of the present study is to describe the responses of the human head-cervical spine in terms of injuries, injury mechanisms, injury scoring, and quantify multiplanar loads.Methods: Pretest radiographs of pre-screened five human cadaver head-neck complexes were obtained. Cranium contents and sectioned the structure rostral to skull base. The caudal end was embedded, and cervical-thoracic disc was unconstrained condition. The loading was applied as a torque about the occipital condyle joint. The head and T1 were angulated 30 degrees and 25 degrees. Peak forces and moments at the occipital condyles were recorded using a six-axis load cell. After testing, x-rays and CT images were obtained. Injuries were scored using the Abbreviated Injury Scale, AIS 2015 version.Results: The mean age, stature, total body mass, body mass index of the five subjects were as follows: 63 years, 1.7 m, 78.0 kg, and 28.1 kg/m². The mean peak axial force and coronal, sagittal, and axial bending moments were: 754 N, and 36.8 Nm, 14.8 Nm, and 9.5 Nm. All but one specimen sustained injury. Injuries were scored at the AIS 2 level. Two specimens sustained left anterior inferior lateral mass fractures of the atlas. While the transverse atlantal ligament was intact, some capsular ligament involvement was observed. In the other two specimens, although the same injury was noted, joint diastasis of the atlas-axis joint was identified.Conclusions: Using a PMHS model, the present study described the biomechanics of the initially head rotated head-neck complex under lateral bending in terms of injuries, injury mechanisms, quantification of the multiplanar loads at the occipital condyles, and underscored potential injury scoring issues for occupant protection. The issue of diastasis is not addressed in the AIS 2015 version. While this may not always result in immediate instability and require surgical intervention, it may be necessary to revisit this issue. Upper cervical fractures with diastasis and or transverse atlantal ligament involvement may be potential injury scoring factors for AIS consideration.

Loading rate effect on tradeoff of fractures from pelvis to lumbar spine under axial impact loading

Objectives: The transmission of impact loading from the seat-to-pelvis-to-lumbar spine in a seated occupant in automotive and military events is a mechanism for fractures to these body regions. While postmortem human subject (PMHS) studies have replicated fractures to the pelvis or lumbar spine using isolated/component models, the role of the time factor that manifests as a loading rate issue on injuries has not been fully investigated in literature. The objective of this study was to explore the hypothesis that short duration pulses fracture the pelvis while longer pulses fracture the spine, and intermediate pulses involve both components.Methods: Unembalmed PMHS thoracolumbar spine-pelvis specimens were fixed at the superior end, and a six-axis load cell was attached. The specimens were mounted on a vertical accelerator, and noninjury and injury tests were conducted by applying short, medium, or long pulses with 5, 15, or 35 ms durations, respectively. Peak axial, shear and resultant forces were obtained. Injuries were documented using posttest x-ray and computed tomography images and scaled using the AIS (2015).Results: The mean age, stature, weight, body mass index, and BMD of twelve specimens were 64.8 ± 11.4 years, 1.8 ± 0.01 m, 83 ± 13 kg, 26.7 ± 5.0 kg/m², and 114.5 ± 21.3 mg/cc, respectively. For the short, long, and medium duration pulses, the mean resultant forces were 5.6 ± 0.9 kN, 5.9 ± 0.94 kN, and 5.4 ± 1.8 kN, and time durations were 4.8 ± 0.5 ms, 16.3 ± 7.3 ms, and 34.5 ± 7.5 ms, respectively. For the short pulse, pelvis injuries were more severe in 3 out 4 specimens, for the medium pulse, they were distributed between the pelvis and spine, and for the long pulse, spine injuries were more severe in 3 out of 4 specimens.Conclusions: While acknowledging the limitations of the sample size, the results of this study support the hypothesis of the time variable in the tradeoff between pelvis and spine injuries with pulse duration. The tradeoff pattern is attributed to mass recruitment: short pulse biases injuries to pelvis while limiting spinal injuries, and the opposite is true for the longer pulse, thus supporting the hypothesis. It is important to account for the time variable in injury analysis.

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.

Quantifying the Effect of Pelvis Fracture on Lumbar Spine Compression during High-rate Vertical Loading

Fracture to the lumbo-pelvis region is prevalent in warfighters seated in military vehicles exposed to under-body blast (UBB). Previous high-rate vertical loading experimentation using whole body post-mortem human surrogates (PMHS) indicated that pelvis fracture tends to occur earlier in events and under higher magnitude seat input conditions compared to lumbar spine fracture. The current study hypothesizes that fracture of the pelvis under high-rate vertical loading reduces load transfer to the lumbar spine, thus reducing the potential for spine fracture. PMHS lumbo-pelvis components (L4-pelvis) were tested under high-rate vertical loading and force and acceleration metrics were measured both inferior-to and superior-to the specimen. The ratio of inferior-tosuperior responses was significantly reduced by unstable pelvis fracture for all metrics and a trend of reduced ratio was observed with increased pelvis AIS severity. This study has established that pelvis fracture reduces compression forces at the lumbar spine during high-rate vertical loading, thus reducing the potential for fracture to the lumbar spine. Therefore, pelvis injury potential should be considered when implementing lumbar injury criteria specific to UBB.

Importance of neural foraminal narrowing in lumbar spine fractures of low AIS severity

OBJECTIVE

In recent years, based on injuries predicted using machine learning, there have been efforts to reduce imaging performed on trauma patients. While useful, such efforts do not incorporate results from studies investigating the pathophysiology of traumatic events. The objective of this study was to identify potentially symptomatic vertebral foramen narrowing in the presence of minor to moderate (AIS ≤ 2 levels of severity) thoracolumbar fractures sustained in motor vehicle crashes (MVCs).

METHODS

Hospital records and images of patients admitted to a Level One trauma center between the years 2014 and 2018 with the diagnosis of thoracolumbar fracture were reviewed. Spinal injuries were scored using the AIS v2015. In addition, the geometry of the neural foramina, particularly the height of the foramina and intervertebral disk at the posterior region, were measured using reconstructed sagittal computed tomography (CT) images. The criteria for foraminal narrowing were associated with <15 mm for the foraminal height and <4 mm for the height of the posterior disk.

RESULTS

24 patients with MVCs associated thoracolumbar fractures, who met both the clinical and imaging criteria for radiculopathy and foraminal narrowing without spinal cord injury, were considered for the present clinical study. 54% of the total lumbar fracture cases reported were rated as AIS 2 injuries. AIS ≥ 3 cases reported 50% narrowing of foramen, which was expected. However, it was surprising to note that the AIS 2 cases also sustained foraminal stenosis, narrowing ranging from 13% to 20%.

CONCLUSIONS

Low severity (AIS ≤ 2) injuries were often found to be associated with foraminal narrowing leading to clinical complaints. While the present clinical study cannot determine if narrowing existed prior to the trauma, they were certainly asymptomatic prior to the trauma. The present findings emphasize the need for detailed imaging in all instances of thoracolumbar trauma, as clinically significant nerve compression may occur even with modest vertebral body injury.

Application of complex neck loads to human spine at the occipital condyle joint: Implications for nonstandard postures for automated vehicles

OBJECTIVE

The automotive industry's shift toward automated vehicles allows the occupants to assume postures different from the standard upright seated position. Injury criteria assessments are needed under these nonstandard postures to advance safety. The objective of this study is to develop a new device that can position the human cadaver head-neck structures in different nonstandard pre-postures using custom devices and apply external loading anticipated in modern and future automotive and military scenarios.

METHODS

An isolated head to T1 human cadaver specimen was attached to a load cell at T1. The load cell was fixed to the top of a six-degree-of-freedom custom spinal positioning device to orient the specimen such that the occipital condyle joint was in line with the torque axis of a custom angular displacement test device. The angular device converted the linear motion of a vertically oriented electro-hydraulic piston to a torque about the occipital condyle joint of the specimen. The head was pre-rotated in the axial plane, approximately 20 degrees to the left, while maintaining the coronal alignment of the lower cervical spine. Targets were secured at the head and spine (details in the body of the manuscript), and their three-dimensional positions were measured using a seven-camera optical motion capture system. Right and then left lateral bending tests were conducted. Occipital condyle joint loads were determined from the superior load cell, and the stiffness difference between the left and right lateral bending was determined.

RESULTS

The peak coronal bending moments were 27.1 Nm and 47.6 Nm for the right and left lateral bending tests. At the time of the peak x-moment, the y moments were 1.6 and 9.1 Nm, and the z moments were 3.1 and 4.8 Nm. The head angle with respect to T1 at the time of peak x-moments was 28.1 and 27.7 deg about x, 11.0 and 11.7 deg about y, and 33.9 and 21.8 deg about z axes for the right and lateral bending tests. C1 left lateral mass fractured following the left lateral bending test.

CONCLUSIONS

The stiffness of the spine increased by approximately three times due to asymmetries in posture and loading. The present system of custom spinal positioning and angular displacement test devices and loading methodologies can be used in conjunction with a conventional piston testing apparatus to conduct additional experiments to delineate the injury patterns and mechanisms and develop injury criteria applicable to modern and future vehicle environments.

Belt-induced abdominal injuries in recent frontal impact CIREN cases

OBJECTIVE

The objective is to report sex-related variation in 3-point belt-related abdominal injuries in Crash Injury Research Engineering Network (CIREN) cases.

METHODS

A query of CIREN cases was made for those with the highest ranked Collision Deformation Classification (CDC) to the front plane, a principal direction of force (PDOF) ±20° from 0°, and Abbreviated Injury Scale (AIS) 2+ abdomen injuries attributed to the seat belt. Patterns of injury were categorized as above the crest of the ilium, injuries below the crest of the ilium, and injuries above and below the ilium. This was done in the context of autonomous vehicle occupant kinematics testing results. Twelve 5th and 95th percentile 3-point belt-restrained postmortem human subjects were subjects; test speeds and recline angles varied. Abdomen injuries were anticipated; none were observed.

RESULTS

Thirty-five occupants with belt-related abdominal injuries were identified. Seventeen case occupants sustained an injury only within the pelvic contents: 5 women and 12 men. Nine of the 17 were at or above the 81st percentile for height, 13 were between the 62nd and 80th percentile for height, and 4 were less than the 50th percentile for height.

CONCLUSIONS

The stature component of the body mass index (BMI) appears to be a plausible candidate for an independent variable that is a contributing factor explaining the incidence of pelvic contents injuries when a 3-point belt-restrained occupant is involved in a frontal impact.

Deflection-based parametric survival analysis side impact chest injury risk curves AIS 2015

OBJECTIVES

The objective of this study was to reanalyze lateral postmortem human surrogate (PMHS) sled test chestband data to construct updated lateral thoracic injury risk curves (IRCs) using survival analysis.

METHODS

Chestband and injury data were gathered from 16 previously conducted PMHS sled tests. Briefly, 2 chestbands were wrapped around the thorax's circumference at the levels of ribs 4 and 8. Tests were conducted at 6.7 m/s on a rigid and padded load wall fixed to the top of a rebound sled. The injuries were reclassified using the Abbreviated Injury Scale (AIS) 2015 coding scheme. Chestband signals were combined with pretest specimen measurements to calculate the chest deflection contour time history. Deflections were determined using updated processing techniques calculating the change in length of every point on the contour from the impacted side using the thorax's midpoint as the origin. Four candidate metrics were selected: the deflection from rib 4, the deflection from rib 8, the greater of the deflections from ribs 4 and 8, and the average of the deflections from ribs 4 and 8. AIS 3+ IRCs were developed considering outcomes of AIS ≥3 injuries. All injury data were uncensored, and noninjury data were right-censored. Three specimen mass-based IRCs were determined using the IRC with the lowest Brier score metric (BSM): The first corresponded to the 5th percentile female mass (49 kg), the second to the 50th percentile male mass (77 kg), and the third to the average mass of the PMHS ensemble (65 kg).

RESULTS

Sixteen PMHS were used in the current study. Six specimens were right-censored, and 10 were uncensored. The average metric had the lowest BSM, and mass was a significant covariate with 50% risk of AIS3+ injury at 72mm of chest deflection. The 50% risk deflection magnitudes for the 5th percentile female (49 kg), 50th percentile male (77 kg), and PMHS ensemble (PMHS-E) (65 kg) were 59, 81, and 71 mm. IRCs for the 4 metrics and the 3 occupant masses are given.

CONCLUSIONS

IRCs were developed using survival analysis, and the average of the peak deflections was found to best represent the thoracic chest deflection response. Mass-based side impact IRCs were calculated for occupants representing the WorldSID 5th percentile female and 50th percentile male anthropomorphic test device.

Normalized vertebral-level specific range of motion corridors for female spines in rear impact

OBJECTIVE

It is well known that the biomechanical responses of female and male spines are different in rear impacts. Female-specific finite element models are being developed as improvements over generic models. Such advancements need female-specific segmental responses for validation. The objectives of the study were to develop vertebral level-specific range of motion corridors from female human cadaver head-neck complexes exposed to rear impact loading.

METHODS

Previously conducted experiments from five human cadaver head-neck complexes were used in this analysis-based study. Briefly, the female head-neck complexes were isolated at the second thoracic vertebral level from the whole body such that the skin and the surrounding tissues of the osteoligamentous complex were intact. The distal end was fixed to the platform of a min-sled testing device. The anterior angulation of T1 was at 25 degrees with respect to the horizontal axis to simulate the normal driver posture. The occipital condyles were directly superior to the T1 body, and the Frankfort plane was horizontal. Rear impact loading were applied at a velocity of 2.6 m/s. The range of motion was defined as the inter-segmental angle at each level of the subaxial spinal column, and it was obtained by tracking the motion of the retroreflective targets that were secured on vertebral bodies and lateral masses of C2 through C7 vertebrae. Data were normalized with respect to the fifth percentile female total body mass, and corridors were developed using the equal stress equal velocity approach and expressed as mean ± 1 standard deviation corridors for each segment.

RESULTS

The segmental motions of the subaxial cervical spinal column were such that the upper regions responded with flexion while the lower regions responded with extension during the initial accelerative loading phase of the impact, resulting in a non-physiological curvature. During the later phase, all segments were in extension. individual corridors are presented as temporal responses in the body of the manuscript. A comparison of the mean temporal responses at each segment are presented to depict the angulation motion differences within the spinal column.

CONCLUSIONS

The present corridors are unique to the female spines. Because female spines have significantly (p < 0.05) different biomechanical responses when compared to male spines, local anatomical differences exist between male and female spines, and field data and clinical studies show female bias to whiplash associated disorders under the rear impact of loading, the present set of corridors serve as a fundamental dataset for the validation of female-specific finite element models. Current computational models can also use these corridors for improved validation to add confidence in their outputs.

Sagittal plane moment responses of the THOR-05F anthropomorphic test device

OBJECTIVE

Anthropomorphic test devices (ATD) are used in crashworthiness studies to advance safety in automotive, military, aviation, and other environments. The Test Device for Human Occupant Restraint (THOR) is an advancement over the widely used Hybrid III ATD. The female version THOR-05F is different from the male as it is not a scaled-down version of the male, and it is based on the recognition that the cervical spines (necks) of females have a different response than males. The objective of this study is to evaluate its response at dynamic rates of loading and compare it with previous postmortem human surrogate (PMHS) responses under sagittal plane bending.

METHODS

The head/neck assembly was separated from the thorax, and a lower neck plate was attached to the head/neck assembly to mount the preparation to the frame of an electro-hydraulic testing device. A custom upper neck interface plate was attached to a novel angular displacement test device that converted the linear motion of the vertical electrohydraulic piston to moment loading at the occipital condyle joint. The neck was preconditioned by applying a sinusoidal 10-degree flexion-extension cycle for 90 s and then three repeat dynamic tests at a target rate of 90 Nm/s. Flexion and extension tests were performed with and without the front and rear neck cables of the THOR-05F neck. Targets were fixed to the upper neck adapter plate, occipital condyle joint, mid-spine aluminum puck, and lower neck adapter plate. The targets' three-dimensional positions were measured using a seven-camera optical motion capture system. Upper neck load cell and occipital condyle potentiometer data were sampled at 20 kHz, and loading rates were determined by calculating the sagittal moment slope between 15% and 85% of the signal.

RESULTS

The mean occipital condyle angle versus sagittal moment response from the 12 tests (three tests each with and without cables and under flexion and extension) are given in the body of the manuscript. With and without cables, the loading rates for flexion tests were 89.3 ± 0.5 Nm/s and 86.3 ± 0.4 Nm/s, and for extension tests they were 90.8 ± 1.2 Nm/s and 88.0 ± 1.5 Nm/s. The average peak sagittal moments were 34.2 ± 0.3 Nm and 30.3 ± 0.2 Nm for flexion and 50.6 ± 0.3 Nm and 47.0 ± 0.3 Nm for extension tests. The mean peak occipital condyle angles were 23.5 ± 0.2 deg and 25.3 ± 0.1 deg for flexion and 22.7 ± 0.2 deg and 25.8 ± 0.1 deg for extension.

CONCLUSION

Using the angular motion as a basis and comparing it with the previously conducted PMHS tests, the THOR-05F neck has approximately twice the stiffness of the human under sagittal plane bending.

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.

A novel posture control device to induce high-rate complex loads for spine biomechanical studies

Modern environmental scenarios such as autonomous vehicles, aircrafts, and military vehicles position the human body in a nonstandard posture and induce multiplanar loads; however, current spine alignment methods and loading are based on sagittal and planar loads. The objective of this study is to develop a posture control device and demonstrate its ability to induce multiplanar loads to the human cadaver spinal columns. The inferior end of the device was designed to allow a full six degree-of-freedom control for positioning the specimen via a coupled x-y cross table, vertical lift platform, and triaxial rotation mechanism. The superior end of the device was designed such that the cranial fixation of the specimen could be attached to the piston of the electrohydraulic testing apparatus directly or via a rotary disc through a slider-crank mechanism. The former attachment induces complex forces and moments, while the latter induces controlled moments with minimal forces. The usability of the posture control device was demonstrated by conducting experiments with a thoracolumbar spinal column for combined forces and moments, and with a head-neck column for complex moments, and in both cases, the uniaxial travel of the piston was at a dynamic rate. The posture control device can be used to study the biomechanics of the spine under complex loads and with different postures and develop injury criteria for different field environments.

Biomechanical Study of Cervical Disc Arthroplasty Devices Using Finite Element Modeling

Many artificial discs for have been introduced to overcome the disadvantages of conventional anterior discectomy and fusion. The purpose of this study was to evaluate the performance of different U.S. Food and Drug Administration (FDA)-approved cervical disc arthroplasty (CDA) on the range of motion (ROM), intradiscal pressure, and facet force variables under physiological loading. A validated three-dimensional finite element model of the human intact cervical spine (C2-T1) was used. The intact spine was modified to simulate CDAs at C5-C6. Hybrid loading with a follower load of 75 N and moments under flexion, extension, and lateral bending of 2 N·m each were applied to intact and CDA spines. From this work, it was found that at the index level, all CDAs except the Bryan disc increased ROM, and at the adjacent levels, motion decreased in all modes. The largest increase occurred under the lateral bending mode. The Bryan disc had compensatory motion increases at the adjacent levels. Intradiscal pressure reduced at the adjacent levels with Mobi-C and Secure-C. Facet force increased at the index level in all CDAs, with the highest force with the Mobi-C. The force generally decreased at the adjacent levels, except for the Bryan disc and Prestige LP in lateral bending. This study demonstrates the influence of different CDA designs on the anterior and posterior loading patterns at the index and adjacent levels with head supported mass type loadings. The study validates key clinical observations: CDA procedure is contraindicated in cases of facet arthroplasty and may be protective against adjacent segment degeneration.

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.

Upright Magnetic Resonance Imaging Study of Cervical Flexor/Extensor Musculature and Cervical Lordosis in Females After Helmet Wear

INTRODUCTION

Addition of head-supported mass imparts greater demand on the human neck to maintain functionality. The same head-supported mass induces greater demand on the female spine than the male spine because female necks are comparatively slender. Prevalence of neck pain is greater in military than civilian population because of the head-borne mass (among other factors). The goal of this study is to determine quantifiable parameters related to muscle geometry using female human volunteers and upright magnetic resonance imaging.

MATERIALS AND METHODS

Young healthy subjects were consented. Demographics and head-neck anthropometry were recorded. For all the 7 subjects, the T1- and T2-weighted magnetic resonance imaging in the neutral sitting position was obtained immediately following donning and after 4 hours of continuous wear of standard issued military helmet, while seated in the same posture for 4 hours. Cross-sectional areas of sternocleidomastoid and multifidus muscles from C2-C7, overall and segmental Cobb angles (C2-T1), and centroid and radius of each muscle were calculated. Data were compared with determine differences with the continuous helmet wear.

RESULTS

There were level specific changes in morphological parameters for each of the muscles. Significant difference (P < 0.05) in cross-sectional areas was noted at C2-3 level for sternocleidomastoid and at C3-4 and C5-6 levels for multifidus. For centroid angles, significant difference (P < 0.05) was observed at C2-3 and C5-6 levels for sternocleidomastoid and at C3-4 level for multifidus. There was no significant difference (P > 0.05) in muscle centroid radii between the pre- and posttest conditions.

CONCLUSIONS

Alterations in muscle geometries were muscle specific and level specific: sternocleidomastoid was significant at the upper level, whereas multifidus was significant at the mid-lower cervical spine segments. The insignificant difference in the Cobb angles was attributed to length of time of continuous helmet wear attributed and sample size. Helmet wear can lead to morphometric alterations in cervical flexor/extensor musculature in females.

Neck Vertebral Level-specific Forces and Moments Under G-x Accelerative Loading

INTRODUCTION

It is important to determine the local forces and moments across the entire cervical spine as dysfunctions such as spondylosis and acceleration-induced injuries are focused on specific levels/segments. The aims of the study were to determine the axial and shear forces and moments at each level under G-x accelerative loading for female and male spines.

METHODS

A three-dimensional finite element model of the male head-cervical spinal column was developed. G-x impact acceleration was applied using experimental data from whole body human cadaver tests. It was validated with experimental head kinematics. The model was converted to a female model, and the same input was applied. Segmental axial and shear forces and moments were obtained at all levels from C2 to T1 in male and female spines.

RESULTS

The time of occurrence of peak axial forces in male and female spines ranged from 37 to 41 ms and 31 to 35 ms. The peak times for the shear forces in male and female spines ranged from 65 to 86 ms and 58 to 78 ms. The peak times for the bending moment ranged from 79 to 91 ms for male and 75 to 83 ms for female spines. Other data are given.

CONCLUSIONS

All metrics reached their peaks earlier in female than male spines, representing a quicker loading in the female spine. Peak magnitudes were also lower in the female spines. Moments and axial forces varied differently compared to the shear forces in the female spine, suggesting that intersegmental loads vary nonuniformly. Effects of head inertia contributed to the greatest increase in axial force under this impact acceleration vector. Because female spines have a lower biomechanical tolerance to injury, female spines may be more vulnerable to injury under this load vector.