Jiangyue Zhang

Physical properties of the human head: Mass, center of gravity and moment of inertia

This paper presents a synthesis of biomedical investigations of the human head with specific reference to certain aspects of physical properties and development of anthropometry data, leading to the advancement of dummies used in crashworthiness research. As a significant majority of the studies have been summarized as reports, an effort has been made to chronologically review the literature with the above objectives. The first part is devoted to early studies wherein the mass, center of gravity (CG), and moment of inertia (MOI) properties are obtained from human cadaver experiments. Unembalmed and preserved whole-body and isolated head and head-neck experiments are discussed. Acknowledging that the current version of the Hybrid III dummy is the most widely used anthropomorphic test device in motor vehicle crashworthiness research for frontal impact applications for over 30 years, bases for the mass and MOI-related data used in the dummy are discussed. Since the development and federalization of the dummy in the United States, description of methods used to arrive at these properties form a part of the manuscript. Studies subsequent to the development of this dummy including those from the US Military are also discussed. As the head and neck are coupled in any impact, and increasing improvements in technology such as advanced airbags, and pre-tensioners and load limiters in manual seatbelts affect the kinetics of the head-neck complex, the manuscript underscores the need to pursue studies to precisely determine all the physical properties of the head. Because the most critical parameters (locations of CG and occipital condyles (OC), mass, and MOI) have not been determined on a specimen-by-specimen basis in any single study, it is important to gather these data in future experiments. These critical data will be of value for improving occupant safety, designing advanced restraint systems, developing second generation dummies, and assessing the injury mitigating characteristics of modern vehicle components in all impact modalities.

Influence of angular acceleration–deceleration pulse shapes on regional brain strains

Recognizing the association of angular loading with brain injuries and inconsistency in previous studies in the application of the biphasic loads to animal, physical, and experimental models, the present study examined the role of the acceleration-deceleration pulse shapes on region-specific strains. An experimentally validated two-dimensional finite element model representing the adult male human head was used. The model simulated the skull and falx as a linear elastic material, cerebrospinal fluid as a hydrodynamic material, and cerebrum as a linear viscoelastic material. The angular loading matrix consisted coronal plane rotation about a center of rotation that was acceleration-only (4.5 ms duration, 7.8 krad/s/s peak), deceleration-only (20 ms, 1.4 krad/s/s peak), acceleration-deceleration, and deceleration-acceleration pulses. Both biphasic pulses had peaks separated by intervals ranging from 0 to 25 ms. Principal strains were determined at the corpus callosum, base of the postcentral sulcus, and cerebral cortex of the parietal lobe. The cerebrum was divided into 17 regions and peak values of average maximum principal strains were determined. In all simulations, the corpus callosum responded with the highest strains. Strains were the least under all simulations in the lower parietal lobes. In all regions peak strains were the same for both monophase pulses suggesting that the angular velocity may be a better metric than peak acceleration or deceleration. In contrast, for the biphasic pulse, peak strains were region- and pulse-shape specific. Peak values were lower in both biphasic pulses when there was no time separation between the pulses than the corresponding monophase pulse. Increasing separation time intervals increased strains, albeit non-uniformly. Acceleration followed by deceleration pulse produced greater strains in all regions than the other form of biphasic pulse. Thus, pulse shape appears to have an effect on regional strains in the brain.

Association of contact loading in diffuse axonal injuries from motor vehicle crashes

BACKGROUND Although studies have been conducted to analyze brain injuries from motor vehicle crashes, the association of head contact has not been fully established. This study examined the association in occupants sustaining diffuse axonal injuries (DAIs). METHODS The 1997 to 2006 motor vehicle Crash Injury Research Engineering Network database was used. All crash modes and all changes in velocity were included; ejections and rollovers were excluded; injuries to front and rear seat occupants with and without restraint use were considered. DAI were coded in the database using Abbreviated Injury Scale 1990. Loss of consciousness was included and head contact was based on medical- and crash-related data. RESULTS Sixty-seven occupants with varying ages were coded with DAI. Forty-one adult occupants (mean, 33 years of age, 171-cm tall, 71-kg weight; 30 drivers, 11 passengers) were analyzed. Mean change in velocity was 41.2 km/h and Glasgow Coma Scale score was 4. There were 33 lateral, 6 frontal, and 2 rear crashes with 32 survivors and 9 were fatalities. Two occupants in the same crash did not sustain DAI. Although skull fractures and scalp injuries occurred in some impacts, head contact was identified in all frontal, rear, and far side, and all but one nearside crashes. CONCLUSIONS Using a large sample size of occupants sustaining DAI in 1991 to 2006 model year vehicles, DAI occurred more frequently in side than frontal crashes, is most commonly associated with impact load transfer, and is not always accompanied by skull fractures. The association of head contact in >95% of cases underscores the importance of evaluating crash-related variables and medical information for trauma analysis. It would be prudent to include contact loading in addition to angular kinematics in the analysis and characterization of DAI.

Validation of a clinical finite element model of the human lumbosacral spine

Very few finite element models on the lumbosacral spine have been reported because of its unique biomechanical characteristics. In addition, most of these lumbosacral spine models have been only validated with rotation at single moment values, ignoring the inherent nonlinear nature of the moment–rotation response of the spine. Because a majority of lumbar spine surgeries are performed between L4 and S1 levels, and the confidence in the stress analysis output depends on the model validation, the objective of the present study was to develop a unique finite element model of the lumbosacral junction. The clinically applicable model was validated throughout the entire nonlinear range. It was developed using computed tomography scans, subjected to flexion and extension, and left and right lateral bending loads, and quantitatively validated with cumulative variance analyses. Validation results for each loading mode and for each motion segment (L4-L5, L5-S1) and bisegment (L4-S1) are presented in the paper.

Effects of tissue preservation temperature on high strain-rate material properties of brain

Postmortem preservation conditions may be one of factors contributing to wide material property variations in brain tissues in literature. The objective of present study was to determine the effects of preservation temperatures on high strain-rate material properties of brain tissues using the split Hopkinson pressure bar (SHPB). Porcine brains were harvested immediately after sacrifice, sliced into 2 mm thickness, preserved in ice cold (group A, 10 samples) and 37°C (group B, 9 samples) saline solution and warmed to 37°C just prior to the test. A SHPB with tube aluminum transmission bar and semi-conductor strain gauges were used to enhance transmitted wave signals. Data were gathered using a digital acquisition system and processed to obtain stress-strain curves. All tests were conducted within 4 h postmortem. The mean strain-rate was 2487±72 s(-1). A repeated measures model with specimen-level random effects was used to analyze log transformed stress-strain responses through the entire loading range. The mean stress-strain curves with ±95% confidence bands demonstrated typical power relationships with the power value of 2.4519 (standard error, 0.0436) for group A and 2.2657 (standard error, 0.0443) for group B, indicating that responses for the two groups are significantly different. Stresses and tangent moduli rose with increasing strain levels in both groups. These findings indicate that storage temperatures affected brain tissue material properties and preserving tissues at 37°C produced a stiffer response at high strain-rates. Therefore, it is necessary to incorporate material properties obtained from appropriately preserved tissues to accurately predict the responses of brain using stress analyses models, such as finite element simulations.

Force and Acceleration Corridors from Lateral Head Impact

This study was conducted to provide force and acceleration corridors at different velocities describing the dynamic biomechanics of the lateral region of the human head. Temporo-parietal impact tests were conducted using specimens from ten unembalmed post-mortem human subjects. The specimens were isolated at the occipital condyle level, and pre-test x-ray and computed tomography images were obtained. They were prepared with multiple triaxial accelerometers and subjected to increasing velocities (up to 7.7 m/s) using free-fall techniques by impacting onto a force plate from which forces were recorded. A 40-durometer padding (50-mm thickness) material covering the force plate served as the impacting boundary condition. Computed tomography images obtained following the final impact test were used to identify pathology. Four specimens sustained skull fractures. Peak force, displacement, acceleration, energy, and head injury criterion variables were used to describe the dynamic biomechanics. Force and acceleration responses obtained from this experimental study along with other data will be of value in validating finite element models. The study underscored the need to enhance the sample size to derive probability-based human tolerance to side impacts.

Upper Neck Forces and Moments and Cranial Angular Accelerations in Lateral Impact

Biomechanical studies using postmortem human subjects (PMHS) in lateral impact have focused primarily on chest and pelvis injuries, mechanisms, tolerances, and comparison with side impact dummies. A paucity of data exists on the head–neck junction, i.e., forces and moments, and cranial angular accelerations. The objective of this study was to determine lateral impact-induced three-dimensional temporal forces and moments at the head–neck junction and cranial linear and angular accelerations from sled tests using PMHS and compare with responses obtained from an anthropomorphic test device (dummy) designed for lateral impact. Following initial evaluations, PMHS were seated on a sled, restrained using belts, and lateral acceleration was applied. Specimens were instrumented with a pyramid-shaped nine-accelerometer package to record cranial accelerations. A sled accelerometer was used to record the input acceleration. Radiographs and computed tomography scans were obtained to identify pathology. A similar testing protocol was adopted for dummy tests. Results indicated that profiles of forces and moments at the head–neck junction and cranial accelerations were similar between the two models. However, peak forces and moments at the head–neck junction were lower in the dummy than PMHS. Peak cranial linear and angular accelerations were also lower in the dummy than in the PMHS. Fractures to the head–neck complex were not identified in PMHS tests. Peak cranial angular accelerations were suggestive of mild traumatic brain injury with potential for loss of consciousness. Findings from this study with a limited dataset are valuable in establishing response corridors for side impacts and evaluating side impact dummies used in crashworthiness and safety-engineering studies.

Methodology to determine skull bone and brain responses from ballistic helmet-to-head contact loading using experiments and finite element analysis

The objective of the study was to obtain helmet-to-head contact forces from experiments, use a human head finite element model to determine regional responses, and compare outputs to skull fracture and brain injury thresholds. Tests were conducted using two types of helmets (A and B) fitted to a head-form. Seven load cells were used on the head-form back face to measure helmet-to-head contact forces. Projectiles were fired in frontal, left, right, and rear directions. Three tests were conducted with each helmet in each direction. Individual and summated force- and impulse-histories were obtained. Force-histories were inputted to the human head-helmet finite element model. Pulse durations were approximately 4 ms. One-third force and impulse were from the central load cell. 0.2% strain and 40 MPa stress limits were not exceeded for helmet-A. For helmet-B, strains exceeded in left, right, and rear; pressures exceeded in bilateral directions; volume of elements exceeding 0.2% strains correlated with the central load cell forces. For helmet-A, volumes exceeding brain pressure threshold were: 5-93%. All elements crossed the pressure limit for helmet-B. For both helmets, no brain elements exceeded peak principal strain limit. These findings advance our understanding of skull and brain biomechanics from helmet-head contact forces.

Role of Falx on Brain Stress-Strain Responses

The objective of this chapter is to provide a review of the role of falx cerebri on brain mechanics, specifically stress and strain responses due to dynamic loading. Because stress-strain responses are inherently intrinsic, the review is focused on physical and computational models using the finite element method. In order to maintain the focus, although experimental animal models are used as validations tools for ensuring the confidence in the finite element or physical model output, discussions from biological tests are not a subject matter. While finite element modeling of the human head has been a subject matter f investigation for decades, a review of literature provides very few analyses regarding the role of falx on the internal stress-strain responses of the brain. As described, physical and finite element models have shown that the falx cerebri, present in the human head, affects the intrinsic response of the brain under contact- and inertia-induced dynamic loads. Physical models using a brain substitute have also shown a similar response. Regional stresses and strains from these models are discussed. The chapter concludes with some recommendations for further studies.

Biomechanical Aspects of Blunt and Penentrating Head Injuries

The objective of this presentation is to discuss certain biomechanical aspects of head injuries due to blunt and penetrating impacts. Emphasis is given to fundamental data leading to injury criteria used in the United States (US) regulations for motor vehicle safety. Full-scale and component tests done under US Federal Motor Vehicle Safety Standards (FMVSS) are described. In addition, results providing occupant safety and vehicle crashworthiness information to the consumer from frontal and lateral impact crash tests are discussed with an emphasis on head injury assessment and mitigation. In the area of penetrating impact, newer experimental techniques are described for a better understanding of head injury secondary to penetrating impacts, with specific reference to the civilian population.