Comparison of biomechanical head-neck responses of hybrid III dummy and whole body cadaver during inverted drops.

The anthropometric hybrid III dummy test device is the most widely used physical model to assess the injury severity during automotive crashes. The dummy was designed to replicate the human neck in flexion and frontal impacts. Various investigators have compared the dummy neck with living human and cadaveric responses in frontal impacts. However, the comparison between the whole body human cadaver and dummy under an inverted drop is currently lacking. This study was designed to review the exiting data and obtain the comparative data between the human cadaver and the dummy. The vertical whole body cadaver drops from our laboratory and literature have been used. There exists a wide variation between the human and dummy neck responses. The dummy neck was approximately two to four times stiffer under axial compression in the quasi-static or in the dynamic mode. The present comparison will provide a basis to better design the anthropometric dummies in order to obtain an improved injury assessment during inverted drops.

An investigation of hybrid III and living human drop tests.

The effect of roof crush on restrained occupants has often been discussed without regard to the headroom available, effectiveness of belts, and location of roof crush. In this article, the question of the ability to protect a simply restrained occupant in an environment in which the roof does not crush is addressed. The subjects were inverted and dropped vertically in noncrushable production vehicle compartments and a specially designed drop fixture. Data collected includes head accelerations, vehicle accelerations, head displacements, belt angles, anchor point location, seat position, and belt tension for a variety of occupant sizes. To our knowledge, these are the first inverted living human vertical studies to be scientifically documented and reported. It was found that no head or neck injuries resulted from drops of up to 91 cm and velocities up to 4.2 m/sec for restrained occupants in the absence of roof crush.

Biomechanical analysis of injury criterion for child and adult dummies.

The development of human injury tolerance is difficult because of the physical differences between humans and animals, the available dummies, and tissue of the cadaver. Furthermore, human volunteer testing can clearly only be done at subinjurious levels. While considerable biomechanical injury evidence exists for the adult human based on cadaveric studies, little information is available for the pediatric population. However, some material is available from skull bone modulus studies and from the fetal tendon strength and early pediatric studies of the newborn. A review of living human, animal, and human cadaveric studies, which forms the basis for head-neck injury criterion are given. Examples of the use of the Hybrid III dummy for injury prediction such as in the Malibu rollover tests and air bag mechanisms show neck injury levels are considerably above the proposed Malibu 2000 N level.

Studies of vehicular padding materials.

The Federal Motor Vehicle Safety Standard 571.201 discusses occupant protection with interior impacts of vehicles. Rule making by the National Highway Traffic Safety Administration (NHTSA) has identified padding for potential injury reduction in vehicles. In these studies, head injury mitigation with padding on vehicular roll bars and brush bars was evaluated. Studies were conducted with free falling Hybrid 50% male head form drops on the forehead and side of the head and a 5% female head. Marked reductions in angular acceleration, as well as Head Injury Criterions (HIC), were observed when compared to unpadded roll bars and brush bars.

Finite-element models of the human head.

A review is presented of the existing finite-element (FE) models for the biomechanics of human head injury. Finite element analysis can be an important tool in describing the injury biomechanics of the human head. Complex geometric and material properties pose challenges to FE modelling. Various assumptions and simplifications are made in model development that require experimental validation. More recent models incorporate anatomic details with higher precision. The cervical vertebral column and spinal cord are included. Model results have been more qualitative than quantitative owing to the lack of adequate experimental validation. Advances include transient stress distribution in the brain tissue, frequency responses, effects of boundary conditions, pressure release mechanism of the foramen magnum and the spinal cord, verification of rotation and cavitation theories of brain injury, and protective effects of helmets. These theoretical results provide a basic understanding of the internal biomechanical responses of the head under various dynamic loading conditions. Basic experimental research is still needed to be determine more accurate material properties and injury tolerance criteria, so that FE models can fully exercise their analytical and predictive power for the study and prevention of human head injury.

Human head-neck biomechanics under axial tension.

A significant majority of cervical spine biomechanics studies has applied the external loading in the form of compressive force vectors. In contrast, there is a paucity of data on the tensile loading of the neck structure. These data are important as the human neck not only resists compression but also has to withstand distraction due to factors such as the anatomical characteristics and loading asymmetry. Furthermore, evidence exists implicating tensile stresses to be a mechanism of cervical spinal cord injury. Recent advancements in vehicular restraint systems such as air bags may induce tension to the neck in adverse circumstances. Consequently, this study was designed to develop experimental methodologies to determine the biomechanics of the human cervical spinal structures under distractive forces. A part-to-whole approach was used in the study. Four experimental models from 15 unembalmed human cadavers were used to demonstrate the feasibility of the methodology. Structures included isolated cervical spinal cords, intervertebral disc units, skull to T3 preparations, and intact unembalmed human cadavers. Axial tensile forces were applied, and the failure load and distraction were recorded. Stiffness and energy absorbing characteristics were computed. Maximum forces for the spinal cord specimens were the lowest (278 N +/- 90). The forces increased for the intervertebral disc (569 N +/- 54). skull to T3 (1555 N +/- 459), and intact human cadaver (3373 N +/- 464) preparations, indicating the load-carrying capacities when additional components are included to the experimental model. The experimental methodologies outlined in the present study provide a basis for further investigation into the mechanism of injury and the clinical applicability of biomechanical parameters.

Cervical vertebral strain measurements under axial and eccentric loading.

The mid to lower cervical spine is a common site for compression related injury. In the present study, we determined the patterns of localized strain distribution in the anterior aspect of the vertebral body and in the lateral masses of lower cervical three-segment units. Miniature strain gages were mounted to human cadaveric vertebrae. Each preparation was line-loaded using a knife-edge oriented in the coronal plane that was moved incrementally from anterior to posterior to induce compression-flexion or compression-extension loading. Uniform compressive loading and failure runs were also conducted. Failure tests indicated strain shifting to "restabilize" the preparation after failure of a component. Under these various compressive loading vectors, the location which resulted in the least amount of deformation for a given force application (i.e., stiffest axis) was quantified to be in the region between 0.5- 1.0 cm anterior to the posterior longitudinal ligament. The location in which line-loading produced no rotation (i.e., balance point) was in this region; it was also close to where the vertebral body strains change from compressive to tensile. Strain values from line loading in this region produced similar strains as recorded under uniform compressive loading, and this was also the region of minimum strain. The region of minimum strain was also more pronounced under higher magnitudes of loading, suggesting that as the maximum load carrying capacity is reached the stiffest axis becomes more well defined.

Biomechanics of skull fracture.

This study was conducted to determine the biomechanics of the human head under quasistatic and dynamic loads. Twelve unembalmed intact human cadaver heads were tested to failure using an electrohydraulic testing device. Quasistatic loading was done at a rate of 2.5 mm/s. Impact loading tests were conducted at a rate of 7.1 to 8.0 m/s. Vertex, parietal, temporal, frontal, and occipital regions were selected as the loading sites. Pathological alterations were determined by pretest and posttest radiography, close-up computed tomography (CT) images, macroscopic evaluation, and defleshing techniques. Biomechanical force-deflection response, stiffness, and energy-absorbing characteristics were obtained. Results indicated the skull to have nonlinear structural response. The failure loads, deflections, stiffness, and energies ranged from 4.5 to 14.1 kN, 3.4 to 16.6 mm, 467 to 5867 N/mm, and 14.1 to 68.5 J, respectively. The overall mean values of these parameters for quasistatic and dynamic loads were 6.4 kN (+/- 1.1), 12.0 mm (+/- 1.6), 812 N/mm (+/- 139), 33.5 J (+/- 8.5), and 11.9 kN (+/-0.9), 5.8 mm (+/- 1.0), 4023 N/mm (+/- 541), 28.0 J (+/- 5.1), respectively. It should be emphasized that these values do not account for the individual variations in the anatomical locations on the cranium of the specimens. While the X-rays and CT scans identified the fracture, the precise direction and location of the impact on the skull were not apparent in these images. Fracture widths were consistently wider at sites remote from the loading region. Consequently, based on retrospective images, it may not be appropriate to extrapolate the anatomical region that sustained the impact forces. The quantified biomechanical response parameters will assist in the development and validation of finite element models of head injury.

Thoracic deformation and velocity analysis in frontal impact.

The objective of the present study was to measure dynamic chest deformations and compute chest velocity and viscous criterion during real world frontal impacts conducted on a horizontal sled. Four unembalmed human cadavers were restrained using a three-point belt restraint in the driver seat of a sled buck. Two chest bands (each with a 24 gauge capability) were placed on the thorax to record the temporal deformation patterns during impact. All tests were conducted at a velocity of approximately 50 kph. Biomechanical data were gathered digitally at a sampling rate of 12,500 Hz. Multiple rib fractures were identified in all specimens at autopsy. Analysis of approximately 800 temporal deformation contours of the thorax demonstrated regional differences. The overall mean maximum normalized chest deflections, maximum chest compression velocities, and peak viscous response variables ranged from 0.15 to 0.51, 1.79 to 4.87 m/s, and 0.15 to 1.95 m/s, respectively. These findings clearly illustrate the potential use of the chest band output to correlate injury with biomechanical variables and establish thoracic impact tolerance.

Continuous motion analysis of the head-neck complex under impact.

The objective of the present study was to analyze the localized kinematic biodynamics of the human head-neck complex under impact loading. Unembalmed human cadaveric head-neck complexes were subjected to axial compressive forces delivered using an electrohydraulic testing device. The head-neck complex was aligned along the stiffest-axis; musculature was simulated using preloaded springs and cables; and retroreflective targets were inserted into the vertebral body, the facet joint articulation, and the spinous process at every level of the cervical column. At dynamic loading rates (1.8-5.1 m/min), mid to lower cervical spine injuries consistently occurred in these preparations. Continuous motion analysis of the components (vertebral body, intervertebral disk, facet joint, and the spinous process) at all levels of the cervical spine showed the temporal order of the transfer of the external load. Injuries documented by computed tomography and cryomicrotomy techniques correlated with the kinematics of the structure. The application of dynamic loading to the head-neck complex coupled with high-speed, continuous-motion analysis of the intervertebral components of the entire cervical column makes possible the definition of the temporal kinematic mechanics that are fundamental to the understanding of the biodynamics of cervical spine trauma. Using these procedures, we have correlated the kinematics with the onset and pattern of neck injury secondary to impact forces.

Kinematics of the lumbar spine following pedicle screw plate fixation.

This investigation was conducted to determine the kinematic response of the lumbar spine instrumented with transpedicular screws and plates. Seven unembalmed human cadaveric lumbar spines were used. Retroreflective targets were inserted into the bony landmarks of each vertebral body, facet column, and spinous process. The specimen was quasistatically loaded until failure (initial cycle) using an electrohydraulic testing device at a rate of 2.5 mm/sec. After radiography, the specimen was again loaded (injury cycle) to the failure compression determined in the previous cycle. Transpedicular screws then were inserted bilaterally at one level proximal and distal to injury. The stabilized cycle of loading was conducted using the procedure adopted in the injury cycle. Comparative analysis of the localized kinematic data between the stabilized and injured columns indicated a reduction in motion between fixated levels, increasing the rigidity of the column. At levels proximal and distal to fixation, however, motion increased, indicating added flexibility. These alterations in the motion, observed during single-cycle loading, may be further accentuated in vivo, leading to hypermobility and degeneration of the spine.

Energy absorption characteristics of football helmets under low and high rates of loading.

The purpose of this study was to examine the force-deformation characteristics of football helmets subjected to compressive loading on the crown surface. Tests were conducted at quasi-static and dynamic rates of loading. Energies were computed from the force-deformation data. The padding systems represented by the helmets differed in their ability to absorb energy under varying loading rates. Helmets using pneumatic or combination pneumatic-foam padding systems were the most successful while suspension helmets were able to absorb the least amount of energy. The evaluation of energy absorption characteristics is an alternative method of describing the effectiveness of football helmets in preventing head injury.

A biomechanical impact test system for head and facial injury assessment and model development.

A biomechanical test system has been developed and validated to conduct controlled uniaxial impact experiments of head and facial trauma. The design reduces off-axis accelerations which are not in the direction of impact and allows accurate positioning of test specimens. Impact forces, displacement histories, impulses at impact and spectral responses are compared to free-fall test results at contact velocities representative of facial injuries (2.5, 3.1 and 3.8 m s-1). Models based on the experimental results are developed to reveal stiffness and inertial properties of impact for use in the design of biomechanically protective steering wheels, air bags and other potential impact structures. The results indicate that the system provides a flexible yet controllable method for positioning and testing impact structures reliably.

Biomechanical properties of human lumbar spine ligaments.

Biomechanical properties of the six major lumbar spine ligaments were determined from 38 fresh human cadaveric subjects for direct incorporation into mathematical and finite element models. Anterior and posterior longitudinal ligaments, joint capsules, ligamentum flavum, interspinous, and supraspinous ligaments were evaluated. Using the results from in situ isolation tests, individual force-deflection responses from 132 samples were transformed with a normalization procedure into mean force-deflection properties to describe the nonlinear characteristics. Ligament responses based on the mechanical characteristics as well as anatomical considerations, were grouped into T12-L2, L2-L4, and L4-S1 levels maintaining individuality and nonlinearity. A total of 18 data curves are presented. Geometrical measurements of original length and cross-sectional area for these six major ligaments were determined using cryomicrotomy techniques. Derived parameters including failure stress and strain were computed using the strength and geometry information. These properties for the lumbar spinal ligaments which are based on identical definitions used in mechanical testing and geometrical assay will permit more realistic and consistent inputs for analytical models.

Strength and kinematic response of dynamic cervical spine injuries.

This study was conducted to evaluate the biodynamic strength and localized kinematic response of the human cervical spine under axial loading applied to the head. Intact ligamentous fresh human cadaveric head-neck complexes were subjected to dynamic compressive forces with a custom-designed electrohydraulic testing device at varying rates. The structure included the effects of anterior and posterior cervical spine muscles with a system of pulleys, dead weights, and spring tension. Localized kinematic data were obtained from retroreflective targets placed on the bony landmarks of the specimen at every level of the spinal column. Input forces, accelerations, displacement, and output generalized force histories were recorded as a function of time with a digital data acquisition system at dynamic sampling rates in excess of 8,000 Hz. High-speed photography at 1,000-1,200 frames/sec also was used. Pathologic alterations to the head-neck complex were evaluated with conventional radiography, computed tomography, and cryomicrotomy. In all specimens, cervical spine injuries occurred as a result of impact. Compressive forces recorded at the distal end of the preparation indicated large-duration, short-magnitude pulses in contrast to short-duration, high-amplitude input waveforms at the head, suggesting decoupling characteristics of the head-neck system. Cervical vertebral body accelerations were consistently smaller than the accelerations recorded on the head. Kinematic data demonstrated temporal deformation characteristics as well as a plausible sequence of spinal deformations leading to injury, which were correlated with the pathoanatomic alterations documented with the post-test computed tomographic and sequential cryomicrotome sections.

Traumatic facial injuries with steering wheel loading.

This study was conducted to evaluate the biomechanics of facial fractures caused by steering wheel loading. Twelve intact fresh human cadaver heads were impacted onto standard or energy-absorbing steering wheels with a custom-designed and validated vertical-drop apparatus. Either zygoma was impacted once at a velocity of 2.0-6.9 m/s. The specimens were oriented to permit a direct comparison between pretest and posttest radiography, and two-dimensional and three-dimensional CT images. Bone mineral content was determined, and biomechanical forces, accelerations, and deformations were recorded. More severe fractures were associated with higher forces on the zygoma. With increasing velocities, fractures initiated at the zygomatic region propagated to other unilateral regions such as the mandible and orbit or to the contralateral side. Less facial trauma was observed with energy-absorbing steering wheels compared with standard wheels at similar impact velocities. Bone mineral content did not correlate well with specimen age or with fracture severity. Clinically significant fractures were identifiable on 3-D CT images. The flexibility of 3-D CT in evaluating the spatial extent of facial abnormalities in different orientations may have significant impact in planning surgical procedures.

A probe for measuring current density during magnetic stimulation.

Time-varying magnetic fields induce currents in conductive media, and when the induced current is large enough in excitable tissue, stimulation occurs. This phenomenon has been applied to the human brain and peripheral nerves for diagnostic evaluation of the neural system. One important aspect that is presently unknown is the current level necessary in tissue for stimulation induced by magnetic fields. This study presents a method of measuring the induced current density from pulsed magnetic fields in vitro and in vivo. The current-density probe was inserted into three concentrations of saline and into the brains of ten anesthetized cats. Two stimulation systems with coils 9 cm and 5 cm in diameter were used. The two systems provided sinusoidal and pulsatile coil currents. Measurements made in saline were compared with those calculated theoretically for a semi-infinite medium. The measured values were within 5% of the calculated values. Measurements made in the cat brain showed a 67% decrease compared with the theoretic model. This variance is attributed to the finite bounds of the skull. The results indicate that direct measurement of current density is possible. Subsequent measurements will aid in the design of improved magnetic stimulation systems.

Strength and motion analysis of the human head-neck complex.

This study was conducted to correlate the pathology of the experimentally tested human cervical spine with biomechanical strength information and localized temporal movements of the various spinal components. Eight fresh human cadaveric head-neck complexes were subjected to compressive forces at a quasistatic rate of 2.5 mm/s until failure. Biomechanical force and deflection data were collected. Localized kinematic data as a function of time were obtained from retroreflective targets placed in the anterior and posterior regions of the vertebral body, facet column, and spinous process at every level of the cervical spine. The specimens were radiographed prior to, during, and following failure; they were then deep frozen at the level of failure to preserve the localized tissue deformations. Specimens underwent computed tomography scanning and sequential sectioning using a cryomicrotome. The failure forces and compressions ranged from 1.3 to 3.6 kN and 0.9 to 3.7 cm. Stiffness and energy-absorbing characteristics ranged from 96.1 to 220.5 kN/m and 12.2 to 53.6 J, respectively. Varying localized temporal motions among spinal components were found to exist at all levels of the head-neck complex. With increasing compressive loads, the specimen components reorient as demonstrated by kinematic changes in the spinal elements; failure was imminent when the structure no longer resisted any further increase in external load. The study demonstrated that an evaluation of the human head-neck complex in a relaxed state, as in clinical observations on posttraumatic radiographs, is often different from that documented immediately following the traumatic insult; this underscores the importance of conducting controlled in vitro investigations to determine the injury biomechanics of the human cervical spine.

Biomechanics of lumbar pedicle screw/plate fixation in trauma.

This investigation was conducted to determine alterations in the biomechanical strength and stiffness characteristics of the lumbar spine fixated with Steffee instrumentation. Comparative studies of these parameters were conducted using seven lumbar columns from fresh human cadavers. Three runs were conducted on each T12-L5 column: control, injured, and fixated. The specimens were loaded under the compression-flexion mode until failure (control run) and then reloaded (injury run) to the failure deformation determined in the control run. Screw/plates were then inserted one level proximal and distal to injury, and the specimens were reloaded (fixation run). Radiographs were taken before and after each trial. Data on deformation and force histories were gathered. The load-deflection response of the injured and fixated specimens were bimodal with two representative stiffnesses. Control failure loads and stiffnesses were higher than those for the injured (P less than 0.001) or fixated (P less than 0.01) spine. Initial stiffness was significantly higher for the fixated than for injured columns (P less than 0.001), but the final stiffnesses were similar. The increase in the initial stiffness in the fixated specimen compared to the injured specimen indicates the strength added to the posterior region of the spine. The relatively smaller alteration in the final stiffness between the fixated and the injured columns, corresponding to the load shared by the anterior column, may suggest that, above a critical strain level, the anterior column absorbs a higher portion of the external load and posterior fixation may be inadequate as sole treatment in trauma.