Hierarchical process using Brier Score Metrics for lower leg injury risk curves in vertical impact

INTRODUCTION

Parametric survival models are used to develop injury risk curves (IRCs) from impact tests using postmortem human surrogates (PMHS). Through the consideration of different output variables, input parameters and censoring, different IRCs could be created. The purpose of this study was to demonstrate the feasibility of the Brier Score Metric (BSM) to determine the optimal IRCs and derive them from lower leg impact tests.

METHODS

Two series of tests of axial impacts to PMHS foot-ankle complex were used in the study. The first series used the metrics of force, time and rate, and covariates of age, posture, stature, device and presence of a boot. Also demonstrated were different censoring schemes: right and exact/uncensored (RC-UC) or right and uncensored/left (RC-UC-LC). The second series involved only one metric, force, and covariates age, sex and weight. It contained interval censored (IC) data demonstrating different censoring schemes: RC-IC-UC, RC-IC-LC and RC-IC-UC-LC.

RESULTS

For each test set combination, optimal IRCs were chosen based on metric-covariate combination that had the lowest BSM value. These optimal IRCs are shown along with 95% CIs and other measures of interval quality. Forces were greater for UC than LC data sets, at the same risk levels (10% used in North Atlantic Treaty Organisation (NATO)). All data and IRCs are presented.

CONCLUSIONS

This study demonstrates a novel approach to examining which metrics and covariates create the best parametric survival analysis-based IRCs to describe human tolerance, the first step in describing lower leg injury criteria under axial loading to the plantar surface of the foot.

Sensitivity of head and cervical spine injury measures to impact factors relevant to rollover crashes

OBJECTIVE

Serious head and cervical spine injuries have been shown to occur mostly independent of one another in pure rollover crashes. In an attempt to define a dynamic rollover crash test protocol that can replicate serious injuries to the head and cervical spine, it is important to understand the conditions that are likely to produce serious injuries to these 2 body regions. The objective of this research is to analyze the effect that impact factors relevant to a rollover crash have on the injury metrics of the head and cervical spine, with a specific interest in the differentiation between independent injuries and those that are predicted to occur concomitantly.

METHODS

A series of head impacts was simulated using a detailed finite element model of the human body, the Total HUman Model for Safety (THUMS), in which the impactor velocity, displacement, and direction were varied. The performance of the model was assessed against available experimental tests performed under comparable conditions. Indirect, kinematic-based, and direct, tissue-level, injury metrics were used to assess the likelihood of serious injuries to the head and cervical spine.

RESULTS

The performance of the THUMS head and spine in reconstructed experimental impacts compared well to reported values. All impact factors were significantly associated with injury measures for both the head and cervical spine. Increases in impact velocity and displacement resulted in increases in nearly all injury measures, whereas impactor orientation had opposite effects on brain and cervical spine injury metrics. The greatest cervical spine injury measures were recorded in an impact with a 15° anterior orientation. The greatest brain injury measures occurred when the impactor was at its maximum (45°) angle.

CONCLUSIONS

The overall kinetic and kinematic response of the THUMS head and cervical spine in reconstructed experiment conditions compare well with reported values, although the occurrence of fractures was overpredicted. The trends in predicted head and cervical spine injury measures were analyzed for 90 simulated impact conditions. Impactor orientation was the only factor that could potentially explain the isolated nature of serious head and spine injuries under rollover crash conditions. The opposing trends of injury measures for the brain and cervical spine indicate that it is unlikely to reproduce the injuries simultaneously in a dynamic rollover test.

Biomechanical responses due to discitis infection of a juvenile thoracolumbar spine using finite element modeling

Growth modulation changes occur in pediatric spines and lead to kyphotic deformity during discitis infection from mechanical forces. The present study was done to understand the consequences of discitis by simulating inflammatory puss at the T12/L1 disc space using a validated eight-year-old thoracolumbar spine finite element model. Changes in the biomechanical responses of the bone, disc and ligaments were determined under physiological compression and flexion loads in the intact and discitis models. During flexion, the angular-displacement increased by 3.33 times the intact spine and localized at the infected junction (IJ). The IJ became a virtual hinge. During compression loading, higher stresses occurred in the growth plate superior to the IJ. The components of the principal stresses in the growth plates at the T12/L1 junction indicated differential stresses. The strain increased by 143% during flexion loading in the posterior ligaments. The study indicates that the flexible pediatric spine increases the motion of the infected spine during physiological loadings. Understanding intrinsic responses around growth plates is important within the context of growth modulation in children. These results are clinically relevant as it might help surgeons to come up with better decisions while developing treatment protocols or performing surgeries.

Toward a more robust lower neck compressive injury tolerance-an approach combining multiple test methodologies

OBJECTIVE

The compressive tolerance of the cervical spine has traditionally been reported in terms of axial force at failure. Previous studies suggest that axial compressive force at failure is particularly sensitive to the alignment of the cervical vertebra and the end conditions of the test methodology used. The present study was designed to develop a methodology to combine the data of previous experiments into a diverse data set utilizing multiple test methods to allow for the evaluation of the robustness of current and proposed eccentricity based injury criteria.

METHODS

Data were combined from 2 studies composed of dynamic experiments including whole cervical spine and head kinematics that utilized different test methodologies with known end conditions, spinal posture, injury outcomes, and measured kinetics at the base of the neck. Loads were transformed to the center of the C7-T1 intervertebral disc and the eccentricity of the sagittal plane resultant force relative to the center of the disc was calculated. The correlation between sagittal plane resultant force and eccentricity at failure was evaluated and compared to the correlation between axial force and sagittal plane moment and axial force alone.

RESULTS

Accounting for the eccentricity of the failure loads decreased the scatter in the failure data when compared to the linear combination of axial force and sagittal plane moment and axial force alone. A correlation between axial load and sagittal plane flexion moment at failure (R² = 0.44) was identified. The sagittal plane extension moment at failure did not have an identified correlation with the compressive failure load for the tests evaluated in this data set (R² = 0.001). The coefficients of determination for the linear combinations of sagittal plane resultant force with anterior and posterior eccentricity are 0.56 and 0.29, respectively. These correlations are an improvement compared to the combination of axial force and sagittal plane moment.

CONCLUSIONS

Results using the outlined approach indicate that the combination of lower neck sagittal plane resultant force and the anterior-posterior eccentricity at which the load is applied generally correlate with the type of cervical damage identified. These results show promise at better defining the tolerance for compressive cervical fractures in male postmortem human subjects (PMHS) than axial force alone. The current analysis requires expansion to include more tolerance data so the robustness of the approach across various applied loading vectors and cervical postures can be evaluated.

Methodology to Quantify Human Cervical Spine Uncovertebral Joint Anatomy

The uncovertebral joints appear in the adult human cervical spinal column. While the descriptions of this structure have been reported, methods to quantify the dimensions of these joints are lacking. Therefore, in this study a preliminary attempt was made to develop a methodology to quantify the three-dimensional anatomical details of these joints in the adult human cervical spine using sequential cryomicrotome anatomic sections. Bilateral dorsal to ventral length, medial to lateral depth, and caudal to cranial height measurements were obtained from C2-T1 levels. The well developed larger joints were observed in the mid to lower cervical (C3-C7) regions and the smaller joints were noted in the most cranial and caudal (C2-C3, C7-T1) levels. Uncovertebral joints in the mid to lower cervical region extended further ventrally compared to the most cranial and caudal levels. The height of the uncovertebral joints was equal to the lateral height of the intervertebral discs throughout the extent of the joint. The mean overall medial to lateral depth of the joint was 3.8 mm (± 1.8). These quantitative three-dimensional descriptions assist in describing uncovertebral joints in stress analysis based finite element models to understand its effects on the cervical spine biomechanical behavior.

Thoracic injury metrics with side air bag: stationary and dynamic occupants

OBJECTIVE

Injury risk from side air bag deployment has been assessed using stationary out-of-position occupant test protocols. However, stationary conditions may not always represent real-world environments. Therefore, the objective of the present study was to evaluate the effects of torso side air bag deployment on close-proximity occupants, comparing a stationary test protocol with dynamic sled conditions.

METHODS

Chest compression and viscous metrics were quantified from sled tests utilizing postmortem human specimens (PMHS) and computational simulations with 3 boundary conditions: rigid wall, ideal air bag interaction, and close-proximity air bag deployment. PMHS metrics were quantified from chestband contour reconstructions. The parametric effect of DeltaV on close-proximity occupants was examined with the computational model.

RESULTS

PMHS injuries suggested that close-proximity occupants may sustain visceral trauma, which was not observed in occupants subjected to rigid wall or ideal air bag boundary conditions. Peak injury metrics were also elevated with close-proximity occupants relative to other boundary conditions. The computational model indicated decreasing influence of air bag on compression metrics with increasing DeltaV. Air bag influence on viscous metric was greatest with close-proximity occupants at DeltaV = 7.0 m/s, at which the response magnitude was greater than linear summation of metrics resulting from rigid impact and stationary close-proximity interaction.

CONCLUSIONS

These results suggest that stationary close-proximity occupants may not represent the only scenario of side air bag deployment harmful to the thoraco-abdominal region. The sensitivity of the viscous metric and implications for visceral trauma are also discussed.

ES2 neck injury assessment reference values for lateral loading in side facing seats

Injury assessment reference values (IARV) predicting neck injuries are currently not available for side facing seated aircraft passengers in crash conditions. The aircraft impact scenario results in inertial loading of the head and neck, a condition known to be inherently different from common automotive side impact conditions as crash pulse and seating configurations are different. The objective of this study is to develop these IARV for the European Side Impact Dummy-2 (ES-2) previously selected by the US-FAA as the most suitable ATD for evaluating side facing aircraft seats. The development of the IARV is an extended analysis of previously published PMHS neck loads by identifying the most likely injury scenarios, comparing head-neck kinematics and neck loads of the ES2 versus PMHS, and development of injury risk curves for the ES2. The ES2 showed a similar kinematic response as the PMHS, particularly during the loading phase. The ES2 exhibited a stiffer response than the PMHS in the thoracic region, resulting in a faster rebound and smaller excursions in the vertical direction. Neck loads were consistent with results from previous authors and served as the basis for the ES2 neck injury risk curve developed here. Regression analysis of the previously published PMHS neck loads indicated that the tension force at the occipital condyles was the only neck load component with a significant correlation (Pearson r2 = 0.9158) to AIS3+ classified injuries. Tension force in the ES2 upper neck showed a weaker but still significant correlation with injury severity (r2 = 0.72) and is proposed to be used as an IARV with a tolerance of 2094 N for 50% AIS3+ risk. Although the prime focus of this study is on loading conditions typical in an aircraft crash environment, it is expected that the proposed IARV's can be used as an extension of typical automotive conditions, particularly for military vehicles and public transport applications where side facing upright seating configurations are more common.

Lightweight low-profile nine-accelerometer package to obtain head angular accelerations in short-duration impacts

Despite recognizing the importance of angular acceleration in brain injury, computations using data from experimental studies with biological models such as human cadavers have met with varying degrees of success. In this study, a lightweight and a low-profile version of the nine-accelerometer system was developed for applications in head injury evaluations and impact biomechanics tests. The triangular pyramidal nine-accelerometer package (PNAP) is precision-machined out of standard aluminum, is lightweight (65 g), and has a low profile (82 mm base width, 35 mm vertex height). The PNAP assures accurate orthogonal characteristics because all nine accelerometers are pre-aligned and attached before mounting on a human cadaver preparation. The feasibility of using the PNAP in human cadaver head studies is demonstrated by subjecting a specimen to an impact velocity of 8.1 m/s and the resultant angular acceleration peaked at 17 krad/s2. The accuracy and the high fidelity of the PNAP device at high and low angular acceleration levels were demonstrated by comparing the PNAP-derived angular acceleration data with separate tests using the internal nine-accelerometer head of the Hybrid III anthropomorphic test device. Mounting of the PNAP on a biological specimen such as a human cadaver head should yield very accurate angular acceleration data.

Validation of a head-neck computer model for whiplash simulation

A head-neck computer model was comprehensively validated over a range of rear-impact velocities using experiments conducted by the same group of authors in the same laboratory. Validations were based on mean +/- 1 standard deviation response curves, i.e. corridors. Global head-neck angle, segmental angle and local facet joint regional kinematic responses from the model fell within experimental corridors. This was true for all impact velocities (1.3, 1.8 and 2.6 m s(-1)). The non-physiological S-curvature lasted approximately 100 ms. The present, comprehensively validated model can be used to conduct parametric studies and investigate the effects of factors such as active sequential and parallel muscle contractions, thoracic ramping and local tissue strain responses, as a function of cervical level, joint region and impact velocity in whiplash injury assessment.

Biomechanics of abdominal injuries

Although considerable efforts have been advanced to investigate the biomechanical aspects of abdominal injuries, reviews have been very limited. The purpose of this article is to present a comprehensive review of the topic. Traumatic abdominal injuries occur due to penetrating or blunt loading. However, the present review is focused on blunt trauma. Because of the complexity of the abdomen, biomechanically relevant anatomical characteristics of the various abdominal organs are presented. The proposed mechanism of injury for these organs and methods for abdominal injury quantification are described. This is followed by a detailed analysis of the biomechanical literature with particular emphasis on experiments aimed to duplicate real world injuries and attempt to quantify trauma in terms of parameters such as force, deflection, viscous criteria, pressure criteria, and correlation of these variables with the severity of abdominal injury. Experimental studies include tests using primates, pigs, rats, beagles, and human cadavers. The effects of velocity, compression, padding, and impactor characteristics on tolerance; effects of pressurization and postmortem characteristics on abdominal injury; deduction of abdominal response corridors; and force-deflection responses (of the different abdominal regions and organs) are discussed. Output of initial research is presented on the development of a device to record the biomechanical parameters in an anthropomorphic test dummy during impact. Based on these studies and the current need for abdominal protection, recommendations are given for further research.

Whiplash injury determination with conventional spine imaging and cryomicrotomy

STUDY DESIGN

Soft tissue-related injuries to the cervical spine structures were produced by use of intact entire human cadavers undergoing rear-end impacts. Radiography, computed tomography, and cryomicrotomy techniques were used to evaluate the injury.

OBJECTIVES

To replicate soft tissue injuries resulting from single input of whiplash acceleration to whole human cadavers simulating vehicular rear impacts, and to assess the ability of different modes of imaging to visualize soft tissue cervical lesions.

SUMMARY OF BACKGROUND DATA

Whiplash-associated disorders such as headache and neck pain are implicated with soft tissue abnormalities to structures of the cervical spine. To the authors' best knowledge, no previous studies have been conducted to determine whether single cycle whiplash acceleration input to intact entire human cadavers can result in these soft tissue alterations. There is also a scarcity of data on the efficacy of radiography and computed tomography in assessing these injuries.

METHODS

Four intact entire human cadavers underwent single whiplash acceleration (3.3 g or 4.5 g) loading by use of a whole-body sled. Pretest and posttest radiographs, computed tomography images, and sequential anatomic sections using a cryomicrotome were obtained to determine the extent of trauma to the cervical spine structures.

RESULTS

Routine radiography identified the least number of lesions (one lesion in two specimens). Although computed tomography was more effective (three lesions in two specimens), trauma was not readily apparent to all soft tissues of the cervical spine. Cryomicrotome sections identified structural alterations in all four specimens to lower cervical spine components that included stretch and tear of the ligamentum flavum, anulus disruption, anterior longitudinal ligament rupture, and zygopophysial joint compromise with tear of the capsular ligaments.

CONCLUSIONS

These results clearly indicate that a single application of whiplash acceleration pulse can induce soft tissue-related and ligament-related alterations to cervical spine structures. The pathologic changes identified in this study support previous observations from human volunteers observations with regard to the location of whiplash injury and may assist in the explanation of pain arising from this injury. Although computed tomography is a better imaging modality than radiography, subtle but clinically relevant injuries may be left undiagnosed with this technique. The cryomicrotome technique offers a unique procedure to understand and compare soft tissue-related injuries to the cervical anatomy caused by whiplash loading. Recognition of these injuries may advance the general knowledge of the whiplash disorder.

Mechanisms and factors involved in hip injuries during frontal crashes

This study was conducted to collect data and gain insights relative to the mechanisms and factors involved in hip injuries during frontal crashes and to study the tolerance of hip injuries from this type of loading. Unembalmed human cadavers were seated on a standard automotive seat (reinforced) and subjected to knee impact test to each lower extremity. Varying combinations of flexion and adduction/abduction were used for initial alignment conditions and pre-positioning. Accelerometers were fixed to the iliac wings and twelfth thoracic vertebral spinous process. A 23.4-kg padded pendulum impacted the knee at velocities ranging from 4.3 to 7.6 m/s. The impacting direction was along the anteroposterior axis, i.e., the global X-axis, in the body-fixed coordinate system. A load cell on the front of the pendulum recorded the impact force. Peak impact forces ranged from 2,450 to 10,950 N. The rate of loading ranged from 123 to 7,664 N/msec. The impulse values ranged from 12.4 to 31.9 Nsec. Injuries were not apparent in three tests. Eight tests resulted in trauma. Fractures involving the pelvis including the acetabulum and proximal femur occurred in five out of the eight tests, and distal femoral bone fracture occurred in one test. These results underscore the importance of leg pre-positioning and the orientation of the impacting axis to produce specific types of trauma to the pelvic region of the lower extremity.

Experimental trauma to the malar eminence: fracture biomechanics and injury patterns

OBJECTIVES

To document patterns of facial fractures after trauma to the malar eminence and to elucidate biomechanical factors relevant to the injury patterns.

STUDY DESIGN AND SETTING

Studies were conducted on 14 cadaver heads. Study variables included impact velocity, contact area, impact force, and zygomatic skin thickness. Bony fractures and clinical injury patterns were documented. A fracture severity rating scale was devised and statistically correlated to the study variables using regression ANOVA analysis.

RESULTS

A broad spectrum of facial fracture patterns was found. Skin thickness and surface area did not correlate with fracture severity (P = 0.67, P = 0.83, respectively). Impact force demonstrated a trend toward significance (P = 0.14). Velocity was most correlative with fracture severity (P = 0.07). A critical threshold velocity (3.5 m/s) was found to correlate with the most severe fracture patterns.

CONCLUSIONS

A broad spectrum of facial fracture patterns was demonstrated after experimental trauma to the malar eminence. Contact surface area and zygomatic skin thickness were not found to be significant factors in fracture severity. Velocity, rather than impact force, was most correlative with fracture severity. The most severe fracture patterns were elicited by velocities above 3.5 m/s.

Contribution of disc degeneration to osteophyte formation in the cervical spine: a biomechanical investigation

Cervical spine disorders such as spondylotic radiculopathy and myelopathy are often related to osteophyte formation. Bone remodeling experimental-analytical studies have correlated biomechanical responses such as stress and strain energy density to the formation of bony outgrowth. Using these responses of the spinal components, the present study was conducted to investigate the basis for the occurrence of disc-related pathological conditions. An anatomically accurate and validated intact finite element model of the C4-C5-C6 cervical spine was used to simulate progressive disc degeneration at the C5-C6 level. Slight degeneration included an alteration of material properties of the nucleus pulposus representing the dehydration process. Moderate degeneration included an alteration of fiber content and material properties of the anulus fibrosus representing the disintegrated nature of the anulus in addition to dehydrated nucleus. Severe degeneration included decrease in the intervertebral disc height with dehydrated nucleus and disintegrated anulus. The intact and three degenerated models were exercised under compression, and the overall force-displacement response, local segmental stiffness, anulus fiber strain, disc bulge, anulus stress, load shared by the disc and facet joints, pressure in the disc, facet and uncovertebral joints, and strain energy density and stress in the vertebral cortex were determined. The overall stiffness (C4-C6) increased with the severity of degeneration. The segmental stiffness at the degenerated level (C5-C6) increased with the severity of degeneration. Intervertebral disc bulge and anulus stress and strain decreased at the degenerated level. The strain energy density and stress in vertebral cortex increased adjacent to the degenerated disc. Specifically, the anterior region of the cortex responded with a higher increase in these responses. The increased strain energy density and stress in the vertebral cortex over time may induce the remodeling process according to Wolff's law, leading to the formation of osteophytes.

Biomechanics of calcaneal fractures: a model for the motor vehicle

Changes in legislation, availability of passive or active restraint systems, or both, together with increased public awareness for safety and the need for use of restraint, have shifted the spectrum of trauma in motor vehicle crashes from the head and torso to other regions. Lower extremity trauma in motor vehicle crashes continues to be a significant problem. The objective of this study was to investigate the biomechanics of the human foot and ankle complex under impact loading and replicate calcaneal fractures routinely seen in motor vehicle crashes. Twenty-two unembalmed cadaver lower extremity specimens were subjected to dynamic loads using a minisled pendulum device. Input and output forces and results of pathologic analysis were obtained using load cell data, radiographs obtained before and after testing, and gross dissection. The intraarticular fracture patterns produced were similar to those seen clinically and described in the literature. Maximum forces ranged from 3.6 to 11.4 kN for the fracture, and 0.5 to 7.3 kN for the nonfracture groups. Logistic regression analysis revealed a 50% probability of calcaneal fracture at 5.5 kN and a 25% probability at 4.0 kN. These studies will lead to an understanding of the tolerance of the lower extremity in sustaining calcaneal fractures under impact. Implications of the work are in the design of crash test dummies, data acquisition, and modifications in motor vehicle design and safety.

Whiplash syndrome: kinematic factors influencing pain patterns

STUDY DESIGN

The overall, local, and segmental kinematic responses of intact human cadaver head-neck complexes undergoing an inertia-type rear-end impact were quantified. High-speed, high-resolution digital video data of individual facet joint motions during the event were statistically evaluated.

OBJECTIVES

To deduce the potential for various vertebral column components to be exposed to adverse strains that could result in their participation as pain generators, and to evaluate the abnormal motions that occur during this traumatic event.

SUMMARY OF BACKGROUND DATA

The vertebral column is known to incur a nonphysiologic curvature during the application of an inertial-type rear-end impact. No previous studies, however, have quantified the local component motions (facet joint compression and sliding) that occur as a result of rear-impact loading.

METHODS

Intact human cadaver head-neck complexes underwent inertia-type rear-end impact with predominant moments in the sagittal plane. High-resolution digital video was used to track the motions of individual facet joints during the event. Localized angular motion changes at each vertebral segment were analyzed to quantify the abnormal curvature changes. Facet joint motions were analyzed statistically to obtain differences between anterior and posterior strains.

RESULTS

The spine initially assumed an S-curve, with the upper spinal levels in flexion and the lower spinal levels in extension. The upper C-spine flexion occurred early in the event (approximately 60 ms) during the time the head maintained its static inertia. The lower cervical spine facet joints demonstrated statistically greater compressive motions in the dorsal aspect than in the ventral aspect, whereas the sliding anteroposterior motions were the same.

CONCLUSIONS

The nonphysiologic kinematic responses during a whiplash impact may induce stresses in certain upper cervical neural structures or lower facet joints, resulting in possible compromise sufficient to elicit either neuropathic or nociceptive pain. These dynamic alterations of the upper level (occiput to C2) could impart potentially adverse forces to related neural structures, with subsequent development of a neuropathic pain process. The pinching of the lower facet joints may lead to potential for local tissue injury and nociceptive pain.

Biomechanics of the cervical spine Part 3: minor injuries

Minor injuries of the cervical spine are essentially defined as injuries that do not involve a fracture. Archetypical of minor cervical injury is the whiplash injury. Among other reasons, neck pain after whiplash has been controversial because critics do not credit that an injury to the neck can occur in a whiplash accident. In pursuit of the injury mechanism, bioengineers have used mathematical modelling, cadaver studies, and human volunteers to study the kinematics of the neck under the conditions of whiplash. Particularly illuminating have been cinephotographic and cineradiographic studies of cadavers and of normal volunteers. They demonstrate that externally, the head and neck do not exceed normal physiological limits. However, the cervical spine undergoes a sigmoid deformation very early after impact. During this deformation, lower cervical segments undergo posterior rotation around an abnormally high axis of rotation, resulting in abnormal separation of the anterior elements of the cervical spine, and impaction of the zygapophysial joints. The demonstration of a mechanism for injury of the zygapophysial joints complements postmortem studies that reveal lesions in these joints, and clinical studies that have demonstrated that zygapophysial joint pain is the single most common basis for chronic neck pain after injury.

Biomechanical modeling of penetrating traumatic head injuries: a finite element approach

Due to advances in emergency medical care and modern techniques, treatment of gunshot wounds to the brain have improved and saved many lives. These advances were largely achieved using retrospective analysis of patients with recommendations for treatment. Biomechanical quantification of intracranial deformation/stress distribution associated with the type of weapon (e.g., projectile geometry) will advance clinical understanding of the mechanics of penetrating trauma. The present study was designed to delineate the biomechanical behavior of the human head under penetrating impact of two different projectile geometry using a nonlinear, three-dimensional finite element model. The human head model included the skull and brain. The qualitative comparison of the model output with each type of projectile during various time steps indicated that the deformation/stress progressed as the projectile penetrated the tissues. There is also a distinct difference in the patterns of displacement for each type of projectile. This observation matches our previous study using a physical gelatin model of delineate the penetrating wound profiles for different projectile types. The present study is a first step in the study of biomechanical modeling of penetrating traumatic brain injuries.

Biomechanics of the cervical spine Part 2. Cervical spine soft tissue responses and biomechanical modeling

OBJECTIVE

The responses and contributions of the soft tissue structures of the human neck are described with a focus on mathematical modeling. Spinal ligaments, intervertebral discs, zygapophysial joints, and uncovertebral joints of the cervical spine are included. Finite element modeling approaches have been emphasized. Representative data relevant to the development and execution of the model are discussed. A brief description is given on the functional mechanical role of the soft tissue components. Geometrical characteristics such as length and cross-sectional areas, and material properties such as force-displacement and stress-strain responses, are described for all components. Modeling approaches are discussed for each soft tissue structure. The final discussion emphasizes the normal and abnormal (e.g., degenerative joint disease, iatrogenic alteration, trauma) behaviors of the cervical spine with a focus on all these soft tissue responses. A brief description is provided on the modeling of the developmental biomechanics of the pediatric spine with a focus on soft tissues. Relevance. Experimentally validated models based on accurate geometry, material property, boundary, and loading conditions are useful to delineate the clinical biomechanics of the spine. Both external and internal responses of the various spinal components, a data set not obtainable directly from experiments, can be determined using computational models. Since soft tissues control the complex structural response, an accurate simulation of their anatomic, functional, and biomechanical characteristics is necessary to understand the behavior of the cervical spine under normal and abnormal conditions such as facetectomy, discectomy, laminectomy, and fusion.

Pediatric neck injury scale factors and tolerance

Although significant research efforts have been made to determine the tolerance for the adult neck, relatively little research has been conducted to derive the pediatric neck injury parameters. The existing approach to determine injury for the one, three and six year old pediatric populations is based on extrapolations from the adult male and calcaneal tendon tensile data. This study addresses the scale factors for pediatric age groups using data obtained from spinal components and neck geometry. The analysis included the determination of scale factors under extension, tension, compression and flexion loading modes as a function of age. The variations in biomechanical properties of each spinal component were determined from human cadaver studies. Active spinal components were identified under each loading mode and relationships were established for each component to obtain material-based scale factors. The scale factors and resulting injury tolerance values based on spine component material properties are more appropriate than values extrapolated from the calcaneal tendon.

Development of extension kinematic corridors to validate a head/neck finite element model

The objective of the current study was the development of experimental response corridors for the purpose of validating a finite element head-neck model in simulated vehicular rear impact. Six intact human head-neck cadaver complexes were used to understand and quantify the kinematics of the cervical spine secondary to low-speed rear impact. The first and second thoracic vertebrae were mounted in a fixative and attached to a minisled/pendulum apparatus. The specimens experienced live different input velocities applied to the first thoracic vertebral, created t),y the pendulum. The response of the specimen was digitally imaged at 1000 Hz from the right lateral side. Relative angles between vertebrae were analyzed in the sagittal plane at 100 ms after impact of the pendulum. Results correlated well with published physiologic range of motion data and dynamic full-body cadaver real impact experiments. Data obtained from this study will be used to validate the macroscopic motions of a finite element model, which will be used to understand the injury mechanisms involved in low-speed vehicular rear impacts.

Geometric and mechanical properties of human cervical spine ligaments

This study characterized the geometry and mechanical properties of the cervical ligaments from C2-T1 levels. The lengths and cross-sectional areas of the anterior longitudinal ligament, posterior longitudinal ligament, joint capsules, ligamentum flavum, and interspinous ligament were determined from eight human cadavers using cryomicrotomy images. The geometry was defined based on spinal anatomy and its potential use in complex mathematical models. The biomechanical force-deflection, stiffness, energy, stress, and strain data were obtained from 25 cadavers using in situ axial tensile tests. Data were grouped into middle (C2-C5) and lower (C5-T1) cervical levels. Both the geometric length and area of cross section, and the biomechanical properties including the stiffness, stress, strain, energy, and Young's modulus, were presented for each of the five ligaments. In both groups, joint capsules and ligamentum flavum exhibited the highest cross-sectional area (p < 0.005), while the longitudinal ligaments had the highest length measurements. Although not reaching statistical significance, for all ligaments, cross-sectional areas were higher in the C5-T1 than in the C2-C5 group; and lengths were higher in the C2-C5 than in the C5-T1 group with the exception of the flavum (Table 1 in the main text). Force-deflection characteristics (plots) are provided for all ligaments in both groups. Failure strains were higher for the ligaments of the posterior (interspinous ligament, joint capsules, and ligamentum flavum) than the anterior complex (anterior and posterior longitudinal ligaments) in both groups. In contrast, the failure stress and Young's modulus were higher for the anterior and posterior longitudinal ligaments compared to the ligaments of the posterior complex in the two groups. However, similar tendencies in the structural responses (stiffness, energy) were not found in both groups. Researchers attempting to incorporate these data into stress-analysis models can choose the specific parameter(s) based on the complexity of the model used to study the biomechanical behavior of the human cervical spine.

Biomechanics of human occupants in simulated rear crashes: documentation of neck injuries and comparison of injury criteria

The objective of this study was to subject small female and large male cadavers to simulated rear impact, document soft-tissue injuries to the neck, determine the kinematics, forces and moments at the occipital condyles, and evaluate neck injury risks using peak force, peak tension and normalized tension-extension criteria. Five unembalmed intact human cadavers (four small females and one large male) were prepared using accelerometers and targets at the head, T1, iliac crest, and sacrum. The specimens were placed on a custom-designed seat without head restraint and subjected to rear impact using sled equipment. High-speed cameras were used for kinematic coverage. After the test, x-rays were obtained, computed tomography scans were taken, and anatomical sections were obtained using a cryomicrotome. Two female specimens were tested at 4.3 m/s (mean) and the other two were tested at 6.8 m/s (mean), and one large male specimen was subjected to 6.6 m/s velocity. One female specimen tested at 4.1 m/s did not sustain injury. All others produced injuries to soft tissue and joint-related structures that included tearing of the anterior longitudinal ligament, rupture of the ligamentum flavum, hematoma at the upper facet joint, anterior disc disruption at the lower spine, and facet joint capsule tear. Compressive forces (100 to 254 N) developed within 60 ms after impact. Tensile forces were higher (369 to 904) and developed later (149 to 211 ms). While peak shear forces (268 to 397 at 4.3 m/s and 257 to 525 N at 6.8 m/s) did not depend on velocity, peak tensile forces (369 to 391 N at 4.3 m/s and 672 to 904 N at 6.8 m/s) seemed to correlate with velocity. Peak extension moments ranged from 22.0 to 33.5 Nm at low velocity and 32.7 to 46.6 Nm at high velocity. All these biomechanical data attained their peaks in the extension phase (with very few exceptions), which ranged from 179 to 216 ms. The neck injury criterion, NIC, exceeded the suggested limit of 15 m(2)/s(2) in all specimens. Axial force and bending moment data were used to evaluate various neck injury criteria (N(ij), N(TE), peak tension and peak extension). The risk for AIS >/= 3 injury for the combined tension-extension criteria was 30 percent in one female specimen tested at 6.8 m/s. For the other specimens the risk of AIS >/= 3 injury was less than five percent using all criteria.

Child neck strength characteristics using an animal model

The purpose of this study was to determine neck strength characteristics of children indirectly using scaling relationships through a caprine animal model. Because of the necessity to evaluate airbag designs for injury risk to the out-of-position child, a strong foundation of experimental data is needed to obtain appropriate tolerance values. Cadaver caprine cervical spines of different ages were tested mechanically in nondestructive bending and destructive tensile modes. Injuries induced in destructive testing such as endplate failure and ligament tears were consistent with clinical observations. Specimens demonstrated statistically significant increased strength characteristics with age. Scaling relationships were developed with respect to the adult specimens. For the tensile failure load the scaling percentages were 78%, 38%, 20%, and 12% for the twelve-, six-, three-, and one-year-old, respectively. The tensile stiffness parameter yielded slightly higher scale factors of 85%, 54%, 23%, and 17%, whereas the bending stiffness parameter resulted in 62%, 57%, 15%, and 11% values for twelve-, six-, three-, and one-yearold age groups, respectively. The caprine model therefore, suggests that tolerance values for the oneand three-year-old may be lower than the values obtained using current scaling techniques.

Head-neck finite element model for motor vehicle inertial impact: material sensitivity analysis

The aim of this study was to conduct a material sensitivity analysis using a head-neck finite element model (FEM). The model included the skull, C1-T1 vertebrae, intervertebral discs, facet joints, and biomechanically relevant ligaments. Poisson's ratio and elastic modulus of the head-neck components were varied. The loading condition included the impact load applied to the first thoracic vertebra. Commercially available software (LS-DYNA) was used for the analysis. Head angle versus time, head center of gravity trajectory, and head center of gravity angular acceleration responses were computed. In general, the variation of elastic modulus had a higher effect on the response compared to variation of Poisson's ratio. As the elastic modulus was increased, the head angle and angular acceleration increased. The present findings form a first step in the study of computational biomechanics of vehicular-related trauma.

Finite element model of human cervical spinal column

The purpose of this study was to develop a detailed anatomically accurate finite element model (FEM) of the whole cervical column (C2-T1) using sagittal and coronal computed tomography (CT) scan images and cryomicrotome anatomical sections. The bony vertebrae were defined using CT scan images. The geometrical details of intervertebral discs, uncovertebral joints and ligaments were obtained from cryomicrotome sections. The following steps were used to develop the FEM: wire mesh, surface model and solid model. The wire mesh of each vertebra and disc was generated from the CT and cryomicrotome sections, respectively. The surface model was developed by connecting the wire mesh lines. The solid model was obtained by filling the volume enclosed by the surface model. Finally, the FEM was constructed by discretizing the solid model using the mapped-mesh option. Appropriate finite element types were assigned to each component. For example, the isoparametric eight-noded solid elements were used to define the cancellous bone of a vertebra. The material properties reported in literature were adopted in the model. Commercially available software (IDEAS and ABAQUS) was used to develop the FEM.

Experimental production of extra- and intra-articular fractures of the os calcis

Although studies have been conducted in the past to duplicate traumatic fractures of the os calcis, biomechanical force data as a function of extra- and intra-articular fractures are not available. Consequently, in this study, a dynamic single impact model was used to provide such information. Using intact human cadaver lower extremities, impact loading was applied to the plantar surface of the foot using a mini-sled pendulum equipment. The proximal tibia was fixed in polymethylmethacrylate. Following impact, pathology to the os calcis was classified into intact (no injury; 14 cases), and extra-articular (6 cases) and intra-articular (6 cases) fractures. Peak dynamic forces were used to conduct statistical analysis. Mean forces for the intact and (both) fracture groups were 4144 N (standard error, SE: 689) and 7802 N (SE: 597). Mean forces for the extra- and intra-articular fracture groups were 7445 N (SE: 711) and 8159 N (SE: 1006). The peak force influenced injury outcome (ANOVA, p<0.005). Differences in the forces were found between intact and injured specimens (p<0.01); intact specimens and specimens with extra-articular pathology (p<0.001); intact specimens and specimens with intra-articular pathology (p<0.005). The present experimental protocol, which successfully reproduced clinically relevant os calcis pathology, can be extended to accommodate other variables such as the simulation of Achilles tendon force, the inclusion of other angles of force application, and the application of the impact force to limited regions of the plantar force of the foot in order to study other injury mechanisms.

Biomechanical study of pediatric human cervical spine: a finite element approach

Although considerable effort has been made to understand the biomechanical behavior of the adult cervical spine, relatively little information is available on the response of the pediatric cervical spine to external forces. Since significant anatomical differences exist between the adult and pediatric cervical spines, distinct biomechanical responses are expected. The present study quantified the biomechanical responses of human pediatric spines by incorporating their unique developmental anatomical features. One-, three-, and six-year-old cervical spines were simulated using the finite element modeling technique, and their responses computed and compared with the adult spine response. The effects of pure overall structural scaling of the adult spine, local component developmental anatomy variations that occur to the actual pediatric spines, and structural scaling combined with local component anatomy variations on the responses of the pediatric spines were studied. Age- and component-related developmental anatomical features included variations in the ossification centers, cartilages, growth plates, vertebral centrum, facet joints, and annular fibers and nucleus pulposus of the intervertebral discs. The flexibility responses of the models were determined under pure compression, pure flexion, pure extension, and varying degrees of combined compression-flexion and compression-extension. The pediatric spine responses obtained with the pure overall (only geometric) scaling of the adult spine indicated that the flexibilities consistently increase in a uniform manner from six- to one-year-old spines under all loading cases. In contrast, incorporation of local anatomic changes specific to the pediatric spines of the three age groups (maintaining the same adult size) not only resulted in considerable increases in flexibilities, but the responses also varied as a function of the age of the pediatric spine and type of external loading. When the geometric scaling effects were added to these spines, the increases in flexibilities were slightly higher; however, the pattern of the responses remained the same as found in the previous approach. These results indicate that inclusion of developmental anatomical changes characteristic of the pediatric spines has more of a predominant effect on biomechanical responses than extrapolating responses of the adult spine based on pure overall geometric scaling.

Patterns of abdominal injuries in frontal and side impacts

Public awareness for safety and vehicle improvements has contributed to significant reduction in injuries secondary to motor vehicle crashes. The spectrum of trauma has shifted from one region of the body to another with varying consequences. For example, airbags have minimized head and neck injuries for adults while emphasizing the lower regions of the human body. Studies have concentrated on the changing patterns of these injuries in frontal impacts. However, there is almost a paucity of data with regard to the characterization of abdominal injuries. Consequently, this study was conducted to determine the patterns of abdominal injuries in frontal and side impacts with an emphasis on more recent crashes. In particular, the frequency and severity of trauma were investigated with a focus on the various abdominal organs (e.g., spleen and liver). Results indicate that side crashes contribute to a large percentage of injuries to the abdomen. The liver and spleen organs are most vulnerable; therefore, it may be beneficial to apply concerted efforts to focus on injury biomechanics research and prioritization activities in these areas of the abdomen. These data may be of benefit to develop anthropomorphic dummies with improved biofidelity.

Morphology of young and old cervical spine intervertebral disc tissues

The intervertebral disc of the cervical spine undergoes degenerative changes during the aging process. Although many studies have reported the qualitative changes in the disc, methodology to quantify these changes is lacking. The present study was designed to quantify the geometrical variations of the nucleus pulposus, annulus fibrosus and uncovertebral joints. The age groups of specimens were classified as juvenile, adult and aged. Fresh intervertebral discs with adjacent vertebral bodies of the lower cervical spine of primates were isolated. The specimens were sectioned sequentially in a coronal plane. Sections were stained with Hematoxylin and Eosin, Verhoeff's, Safranin O, and Trichrome methods to distinguish the nucleus, annulus and uncovertebral joints. Histological images were examined using light microscopy and processed using a computer imaging program to trace the boundaries of the disc components. Dorsal-to-ventral depth and medial-to-lateral width of the nucleus pulposus, and its relative location to the annulus pulposus were also obtained. In the juvenile and adult discs, the nucleus appeared as a light opaque region with scattered notochordal cells with a clear distinction from the annulus region. In contrast, in the aged discs, the nucleus appeared as a dense region of amorphous, irregular collagen materials with less distinction from the annulus region. With the progression of aging, the dorsal-to-ventral depth of the nucleus decreased considerably compared to medial-to-lateral width. The uncovertebral joints were clear in the adult discs. The joints were less distinct in the aged discs and their size decreased. Quantification of three-dimensional geometrical variations will assist in better defining the disc tissue in the mathematical models.

Airbag effectiveness on brain trauma in frontal crashes

The purpose of this study was to evaluate the effectiveness of frontal restraint systems in reducing the potential for head injuries, specifically brain injuries and skull fractures. The US DOT NASS database files from 1991-1998 were evaluated for drivers and right front seat occupants in frontal crashes. Of the total driver and right front seat occupants in this data set, 3.83% sustained a brain injury without skull fracture, 0.05% sustained a skull fracture without a brain injury, and 0.16% sustained both brain injury and skull fracture. The incidence of head injury was lowest among occupants who were restrained by belt alone (2.76%) and by both airbag and belt systems (3.51%). The unrestrained population had a 10.39% incidence of at least one type of head injury. In general, for maximum AIS > or = 2 head injuries, airbag effectiveness was greatest between 16-45 kph crash delta V. For the more severe maximum AIS > or = 3 head injuries, the airbag restraint had its greatest effect up to 35 kph. It can be concluded that brain injury in frontal crashes is substantially reduced with the presence of a restraint system and the use of both airbag and belt restraint offers the greatest protection across all delta V categories. Restraint system effectiveness for the non-head-injured occupant is variable but, generally, the belted occupant sustained the lowest percentage of injuries. Skull fractures in frontal impact were relatively rare and the incidence appeared to be unaffected by the presence of a restraint system.

Finite element modeling of the cervical spine: role of intervertebral disc under axial and eccentric loads

An anatomically accurate, three-dimensional, nonlinear finite element model of the human cervical spine was developed using computed tomography images and cryomicrotome sections. The detailed model included the cortical bone, cancellous core, endplate, lamina, pedicle, transverse processes and spinous processes of the vertebrae; the annulus fibrosus and nucleus pulposus of the intervertebral discs; the uncovertebral joints; the articular cartilage, the synovial fluid and synovial membrane of the facet joints; and the anterior and posterior longitudinal ligaments, interspinous ligaments, capsular ligaments and ligamentum flavum. The finite element model was validated with experimental results: force-displacement and localized strain responses of the vertebral body and lateral masses under pure compression, and varying eccentric anterior-compression and posterior-compression loading modes. This experimentally validated finite element model was used to study the biomechanics of the cervical spine intervertebral disc by quantifying the internal axial and shear forces resisted by the ventral, middle, and dorsal regions of the disc under the above axial and eccentric loading modes. Results indicated that higher axial forces (compared to shear forces) were transmitted through different regions of the disc under all loading modes. While the ventral region of the disc resisted higher variations in axial force, the dorsal region transmitted higher shear forces under all loading modes. These findings may offer an insight to better understand the biomechanical role of the human cervical spine intervertebral disc.

Biomechanical effect of anterior cervical spine fusion on adjacent segments

The biomechanical effects of superior (C4-C5) and inferior (C5-C6) level fusions with different graft materials on the adjacent unaltered components were quantified using an anatomically accurate and experimentally validated C4-C5-C6 finite element model. Smith-Robinson and Bailey-Badgley fusion procedures were analyzed with five different types of inter-body fusion materials with varying stiffnesses. Intact and surgically altered finite element models were subjected to physiologic compression, flexion, extension and lateral bending. The external axial and angular stiffness, and the internal unaltered intervertebral disc (C5-C6 for the superior and C4-C5 for inferior fusion) and C5 vertebral body stresses were determined. The superior level fusion resulted in the highest increase in external response in lateral bending for all implant materials in both surgical procedures. In contrast, the inferior level fusion produced a higher increase in the C4-C5 disc and C5 vertebral body stresses in compression than the superior level fusion in both surgical procedures. The increased internal stress responses reflecting the changes in the load-sharing following inferior level fusion may explain clinical observations such as enhanced degeneration subsequent to surgery. Because of the inclusion of three levels in the present multi-segment finite element model, it was possible to determine these responses in the unaltered adjacent components of the cervical spine.

Evaluation of cervical laminectomy and laminoplasty. A longitudinal study in the goat model

STUDY DESIGN

An evaluation of the longitudinal radiologic changes up to 6 months induced by multilevel laminectomy and laminoplasty and the biomechanical responses in the goat model, complemented by biomechanical studies of intact specimens.

OBJECTIVES

To determine the long-term radiographic differences and biomechanical responses of laminectomy and laminoplasty in an in vivo animal model.

SUMMARY OF BACKGROUND DATA

Previous clinical and laboratory studies have indicated that multilevel laminectomy can cause increased flexibility in the cervical spinal column. Although the potential for laminoplasty to resolve these changes has been suggested, other evaluations have not supported this contention. Clarification of this controversy with long-term in vivo studies has not been performed.

METHODS

Ten adult goats were divided into two groups, one undergoing C3-C5 laminectomy and the other open-door laminoplasty. Lateral cervical spine radiographs were obtained at 4-week intervals for a 6-month period. After the goats were killed, biomechanical testing was performed using pure moment loading on the surgically treated specimens and on three intact (without surgery) cervical spinal columns.

RESULTS

In the laminectomy preparations, the cervical curvature index was noted to decrease by 59% at 16 weeks (P < 0.028) and by 70% at 24 weeks (P < 0.002), whereas the decrease in laminoplasty was not significantly different. Biomechanical testing indicated a significantly increased sagittal-plane slack motion in the laminectomy group (55 degrees) compared with that in intact specimens (39 degrees), but no significant difference between the laminoplasty and intact groups with respect to this motion. Laminectomy was found to be significantly stiffer (36%) in flexion than in extension, whereas the contrary was true for laminoplasty (37%).

CONCLUSIONS

Radiographic and biomechanical results in the goat model suggest that laminoplasty is superior to laminectomy in maintaining cervical alignment and preventing postoperative spinal deformities.

Finite element analysis of the cervical spine: a material property sensitivity study

OBJECTIVE

The study determined the effect of variations in the material properties of the cervical spinal components on the output of the finite element analysis (external and internal responses of the cervical spine) under physiologic load vectors.

DESIGN

A three-dimensional (3D) anatomically accurate finite element model comprising of the C4-C5-C6 cervical spine unit including the three vertebrae, two interconnecting intervertebral discs, and the anterior and posterior ligament complex is used.

BACKGROUND

The effect of material property variations of spinal components on the human lumbar spine biomechanics is extensively studied. However, a similar investigation of the cervical spine is lacking.

METHODS

Parametric studies on the variations in the material properties of all the cervical spine components including the cortical shell, cancellous core, endplates, intervertebral disc, posterior elements and ligaments were conducted by exercising the 3D finite element model under flexion, extension, lateral bending and axial torsion loading modes. Low, basic and high material property cases for each of the six components under all the four physiologic loading modes were considered in the finite element analysis. A total of 432 results were evaluated to analyze the external angular rotation, and the internal stresses in the middle vertebral body, the superior and inferior endplates and the two intervertebral discs.

RESULTS

Variations in the material properties of the different cervical spinal components produced dissimilar changes in the external and internal responses. Variations in the material properties of the cancellous core, cortical shell, endplates and posterior element structures representing the hard tissues did not affect the external angular motion, and the internal stresses of the inferior and superior intervertebral discs under all four loading modes. In contrast, variations in the material properties of the intervertebral disc and ligament structures representing the soft tissues significantly altered the angular motion, and the stresses in the inferior and superior intervertebral discs of the cervical spine.

CONCLUSION

The material properties of the soft tissue structures have a preponderant effect on the external and internal responses of the cervical spine compared with the changes in the material properties of the hard tissue structures.

RELEVANCE

Bone remodeling (e.g., osteophyte) secondary to degeneration of the human cervical joints may be explained by a change in the material property of the soft tissues, coupled with an increase in stress (due to these material property variations) in the spinal components. Consequently, to accurately predict the biomedical effects of cervical spine degeneration, it is critical to accurately determine the material property of these components.

Static and dynamic bending responses of the human cervical spine

The quasi-static and dynamic bending responses of the human mid-lower cervical spine were determined using cadaver intervertebral joints fixed at the base to a six-axis load cell. Flexion bending moment was applied to the superior end of the specimen using an electrohydraulic piston. Each specimen was tested under three cycles of quasi-static load-unload and one high-speed dynamic load. A total of five specimens were included in this study. The maximum intervertebral rotation ranged from 11.0 to 15.4 deg for quasi-static tests and from 22.9 to 34.4 deg for dynamic tests. The resulting peak moments at the center of the intervertebral joint ranged from 3.8 to 6.9 Nm for quasi-static tests and from 14.0 to 31.8 Nm for dynamic tests. The quasi-static stiffness ranged from 0.80 to 1.35 Nm/deg with a mean of 1.03 Nm/deg (+/- 0.11 Nm/deg). The dynamic stiffness ranged from 1.08 to 2.00 Nm/deg with a mean of 1.50 Nm/deg (+/- 0.17 Nm/deg). The differences between the two stiffnesses were statistically significant (p < 0.01). Exponential functions were derived to describe the quasi-static and dynamic moment-rotation responses. These results provide input data for lumped-parameter models and validation data for finite element models to better investigate the biomechanics of the human cervical spine.

Effect of age and loading rate on human cervical spine injury threshold

STUDY DESIGN

Statistical analysis of human cadaver cervical spine compression experiments.

OBJECTIVES

To quantify the cervical spine compressive injury threshold as a function of the person's age, gender, and external loading rate.

SUMMARY OF BACKGROUND DATA

Results of epidemiologic studies have indicated that most survivors of cervical spinal cord injury have spinal column fractures and dislocations that result from a compression or compression-flexion force vector. Cervical spinal column injury thresholds are dependent on many factors. Delineation of the injury thresholds according to age, gender, and loading rate is necessary to improve clinical assessments and prevention strategies.

METHODS

Twenty-five human cadaver head-neck compression tests were included in the analysis. Two statistical models were used to quantify the effects of age, gender, and loading rate on the force required to induce failure in the cervical spine. A multiple linear regression model provided a direct equation that quantified the effects of the variables, and a proportional hazards model was used to quantify probability of injury with each factor.

RESULTS

The regression model had a correlation coefficient of 0.87. There was an interactive effect between age and loading rate: Increasing age reduced the effect of loading rate and at approximately 82 years, loading rate had no effect. Men were consistently 600 N stronger than women. The 50% probability of failure for a 50-year-old man at a 4.5-m/sec loading rate was approximately 3.9 kN. Differences in probability curves followed the same trends as seen in the regression model.

CONCLUSIONS

The effects of age on cervical spine injury threshold are coupled with the rate of loading experienced through the external force vector that causes the trauma. Assessment of injury mechanisms and thresholds should be based on the person's age, gender, and loading rate to determine treatment and prevent injuries.

Modular data acquisition system updated using LabWindows/CVI Graphical User Interface

The Biomechanics Laboratory of the Neuroscience Department of the Medical College of Wisconsin is currently engaged in research involving trauma biomechanics. For some experiments, 24 channels of analog data must be sampled at 10,000 Hertz. The Modular Data Acquisition System (MDAS) is able to acquire up to 60 channels of analog data at sampling intervals as low as 6 microseconds. This excellent hardware system has only menu-driven utility software that is no longer supported and required updating to a Graphical User Interface (GUI). Using National Instruments LabWindows/CVI software a GUI was developed. The GUI consists of 9 Graphical User Interface panels controlled by a 3000 line C program "MDAS3SAM". "MDAS3SAM" controls a driver program "MDAS_SAM" which communicates with the MDAS unit via a National Instruments GPIB interface. The 9 GUI panels allow the user to configure the MDAS system (selecting channels, sampling interval, triggering levels etc.), start sampling the data, writing the data to hard disk, graphing the data and printing the graphs. The new system allows the user to quickly reconfigure the MDAS unit and obtain accurate results.

Biomechanics of penetrating trauma

It is well known that injuries and deaths due to penetrating projectiles have become a national and an international epidemic in Western society. The application of biomedical engineering to solve day-to-day problems has produced considerable advances in safety and mitigation/prevention of trauma. The study of penetrating trauma has been largely in the military domain where war-time specific applications were advanced with the use of high-velocity weapons. With the velocity and weapon caliber in the civilian population at half or less compared with the military counterpart, wound ballistics is a largely different problem in today's trauma centers. The principal goal of the study of penetrating injuries in the civilian population is secondary prevention and optimized emergency care after occurrence. A thorough understanding of the dynamic biomechanics of penetrating injuries quantifies missile type, caliber, and velocity to hard and soft tissue damage. Such information leads to a comprehensive assessment of the acute and long-term treatment of patients with penetrating injuries. A review of the relevant military research applied to the civilian domain and presentation of new technology in the biomechanical study of these injuries offer foundation to this field. Relevant issues addressed in this review article include introduction of the military literature, the need for secondary prevention, environmental factors including projectile velocity and design, experimental studies with biological tissues and physical models, and mathematical simulations and analyses. Areas of advancement are identified that enables the pursuit of biomechanics research in order to arrive at better secondary prevention strategies.

Cervical spine vertebral and facet joint kinematics under whiplash

Whiplash injuries sustained during a rear-end automobile collision have significant societal impact. The scientific literature on whiplash loading is both diverse and confusing. Definitive studies are lacking to describe the local mechanisms of injury that induce either acute or chronic pain symptoms. A methodology has been presented to quantify the kinematics of the cervical spine components by inducing controlled whiplash-type forces to intact human head-neck complexes. The localized facet joint kinematics and the overall segmental motions of the cervical spine are presented. It is anticipated that the use of this methodology will assist in a better delineation of the localized mechanisms of injury leading to whiplash pain.

Finite element modeling approaches of human cervical spine facet joint capsule

The human cervical spine facet joint capsule was modeled using four nonlinear finite element approaches: slideline, contact surface, hyperelastic, and fluid models. Slideline elements and contact surface definitions were used in the first two models to simulate the synovial fluid between the articulating cartilages. Incompressible solid elements approximated the synovial fluid in the hyperelastic model. Hydrostatic fluid elements idealized the synovial fluid in the fluid model. The finite element analysis incorporated geometric, material and contact nonlinearities. All models were subjected to compression, flexion, extension, and lateral bending. The fluid model idealization better approximates the actual facet joint anatomy and its behavior than the gap assumption in the slideline and contact surface models, and the solid element simulation in the hyperelastic model.

Biomechanics of human thoracic ribs

Considerable advances have been made to determine the failure biomechanical properties of the human thoracic spinal column and its components. Except for a few fundamental studies, there is a paucity of such data for the costovertebral elements. The present study was designed to determine the biomechanics of the human thoracic spine ribs from a large population. Seventh and eighth ribs bilaterally were tested from 30 human cadavers using the principles of three-point bending techniques to failure. Biomechanical test parameters included the cross-sectional area (core, marrow, and total), moment of inertia, failure load, deflection, and the Young's elastic modulus. The strength-related results indicated no specific bias with respect to anatomical level and hemisphere (right or left), although the geometry-related variables demonstrated statistically significant differences (p < 0.05) between the seventh and the eighth ribs. This study offers basic biomechanical information on the ultimate failure and geometric characteristics of the human thoracic spine ribs.

Finite element analysis of anterior cervical spine interbody fusion

The present study investigated the external and the internal biomechanical responses of anterior cervical discectomy coupled with fusion. Five different types of interbody fusion materials were used: titanium core, titanium cage, tricortical iliac crest, tantalum core, and tantalum cage. Two different types of surgical procedures were analyzed: Smith-Robinson and Bailey-Badgley. A validated three-dimensional anatomically accurate finite element model of the human cervical spine was used in the study. The finite element model was exercised in compression, flexion, extension, and lateral bending for the intact case and for the two surgical procedures with five implant materials. The external response in terms of the stiffness and angular rotation, and the internal response in terms of the disc and the vertebral stresses were determined. The Smith-Robinson technique resulted in the highest increase in external response under all modes of loading for all implant materials. In contrast, the Bailey-Badgley technique produced a higher increase in the disc and the vertebral body stresses than the Smith-Robinson technique. As experimental human cadaver tests can only determine the external response of the non-fused spine simulating immediate post-operative structure, the present finite element studies assist in the understanding of biomechanics of interbody fusion by delineating the changes in the extrinsic and intrinsic characteristics of the cervical spine components due to surgery.

Axial impact biomechanics of the human foot-ankle complex

Recent epidemiological, clinical, and biomechanical studies have implicated axial impact to the plantar surface of the foot to be a cause of lower extremity trauma in vehicular crashes. The present study was conducted to evaluate the biomechanics of the human foot-ankle complex under axial impact. Nine tests were conducted on human cadaver below knee-foot-ankle complexes. All specimens were oriented in a consistent anatomical position on a mini-sled and the impact load was delivered using a pendulum. Specimens underwent radiography and gross dissection following the test. The pathology included intra-articular fractures of the calcaneus and/or the distal tibia complex with extensions into the anatomic joints. Impactor load cell forces consistently exceeded the tibial loads for all tests. The mean dynamic forces at the plantar surface of the foot were 7.7 kN (SD = 4.3) and 15.1 kN (SD = 2.7) for the nonfracture and fracture tests, respectively. In contrast, the mean dynamic forces at the proximal tibial end of the preparation were 5.2 kN (SD = 3.1) in the nonfracture group, and 10.2 kN (SD = 1.5) in the fracture group. The foot and tibial end forces were statistically significantly different between these two groups (p < 0.01). The present investigation provides fundamental data to the understanding of the biomechanics of human foot-ankle trauma. Quantifying the effects of other factors such as gender and bone quality on the injury thresholds is necessary to understand foot-ankle tolerance fully.

Inertial loading of the human cervical spine

While the majority of experimental cervical spine biomechanics research has been conducted using slowly applied forces and/or moments, or dynamically applied forces with contact, little research has been performed to delineate the biomechanics of the human neck under inertial "noncontact" type forces. This study was designed to develop a comprehensive methodology to induce these loads. A minisled pendulum experimental setup was designed to test specimens (such as human cadaver neck) at subfailure or failure levels under different loading modalities including flexion, extension, and lateral bending. The system allows acceleration/deceleration input with varying wave form shapes. The test setup dynamically records the input and output strength information such as forces, accelerations, moments, and angular velocities; it also has the flexibility to obtain the temporal overall and local kinematic data of the cervical spine components at every vertebral level. These data will permit a complete biomechanical structural analysis. In this paper, the feasibility of the methodology is demonstrated by subjecting a human cadaver head-neck complex with intact musculature and skin under inertial flexion and extension whiplash loading at two velocities.

Wire fixation techniques of the cervical facets

STUDY DESIGN

The changes in the biomechanical responses of the cervical spine altered by multilevel laminectomy to various facet wiring techniques were evaluated.

OBJECTIVE

To determine the effectiveness of various proposed techniques of cervical facet wiring used to offer rigid internal fixation after multilevel laminectomy.

METHODS

Eight human cadaveric spine segments from C2-11 underwent combined flexion-compression loading. After testing intact and three-level laminectomy (C4-C6) preparations, two techniques of facet wiring fixation were evaluated in an identical manner. Force, displacement, and kinematics data at every level of the column were obtained.

RESULTS

The mean stiffness of the intact column was significantly greater than the mean stiffness for laminectomized specimens. Individual facet wiring to the bone graft and through the spinous process below the laminectomy failed to restore stiffness to the laminectomized preparations, whereas the Luque rectangle method restored the stiffness to that found in the intact column. The increases in segmental and overall sagittal rotations resulting from multilevel laminectomy were not decreased significantly by the individual facet wiring technique, but the Luque rectangle technique demonstrated a reduction of sagittal rotations compared with laminectomy without fixation.

CONCLUSIONS

The significant increases in total column flexibility and segmental flexural rotations after multilevel laminectomy were not corrected by techniques that depend on individual facet wires secured to an overlying strut, including wiring to the inferior intact segment. Crosslinking of the facet wire fixation above and below the laminectomized segments, as exemplified by the Luque rectangle technique, restored column stiffness and reduced segmental sagittal rotations.