Optical characterization of acceleration-induced strain fields in inhomogeneous brain slices.

The aim of this study was to measure high-resolution strain fields in planar sections of brain tissue during translational acceleration to obtain validation data for numerical simulations. Slices were made from fresh, porcine brain tissue, and contained both grey and white matter as well as the complex folding structure of the cortex. The brain slices were immersed in artificial cerebrospinal fluid (aCSF) and were encapsulated in a rigid cavity representing the actual shape of the skull. The rigid cavity sustained an acceleration of about 900m/s(2) to a velocity of 4m/s followed by a deceleration of more than 2000m/s(2). During the experiment, images were taken using a high-speed video camera and Von Mises strains were calculated using a digital image correlation technique. The acceleration of the sampleholder was determined using the same digital image correlation technique. A rotational motion of the brain slice relative to the sampleholder was observed, which may have been caused by a thicker posterior part of the slice. Local variations in the displacement field were found, which were related to the sulci and the grey and white matter composition of the slice. Furthermore, higher Von Mises strains were seen in the areas around the sulci.

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.

Characterisation of the mechanical behaviour of brain tissue in compression and shear.

No validated, generally accepted data set on the mechanical properties of brain tissue exists, not even for small strains. Most of the experimental and methodological issues have previously been addressed for linear shear loading. The objective of this work was to obtain a consistent data set for the mechanical response of brain tissue to either compression or shear. Results for these two deformation modes were obtained from the same samples to reduce the effect of inter-sample variation. Since compression tests are not very common, the influence of several experimental conditions for the compression measurements was analysed in detail. Results with and without initial contact of the sample with the loading plate were compared. The influence of a fluid layer surrounding the sample and the effect of friction were examined and were found to play an important role during compression measurements.To validate the non-linear viscoelastic constitutive model of brain tissue that was developed in Hrapko et al. (Biorheology 43 (2006), 623-636) and has shown to provide a good prediction of the shear response, the model has been implemented in the explicit Finite Element code MADYMO. The model predictions were compared to compression relaxation results up to 15% strain of porcine brain tissue samples. Model simulations with boundary conditions varying within the physical ranges of friction, initial contact and compression rate are used to interpret the compression results.

The influence of test conditions on characterization of the mechanical properties of brain tissue.

To understand brain injuries better, the mechanical properties of brain tissue have been studied for 50 years; however, no universally accepted data set exists. The variation in material properties reported may be caused by differences in testing methods and protocols used. An overview of studies on the mechanical properties of brain tissue is given, focusing on testing methods. Moreover, the influence of important test conditions, such as temperature, anisotropy, and precompression was experimentally determined for shear deformation. The results measured at room temperature show a stiffer response than those measured at body temperature. By applying the time-temperature superposition, a horizontal shift factor a(T)=8.5-11 was found, which is in agreement with the values found in literature. Anisotropy of samples from the corona radiata was investigated by measuring the shear resistance for different directions in the sagittal, the coronal, and the transverse plane. The results measured in the coronal and the transverse plane were 1.3 and 1.25 times stiffer than the results obtained from the sagittal plane. The variation caused by anisotropy within the same plane of individual samples was found to range from 25% to 54%. The effect of precompression on shear results was investigated and was found to stiffen the sample response. Combinations of these and other factors (postmortem time, donor age, donor type, etc.) lead to large differences among different studies, depending on the different test conditions.

The mechanical behaviour of brain tissue: large strain response and constitutive modelling.

The non-linear mechanical behaviour of porcine brain tissue in large shear deformations is determined. An improved method for rotational shear experiments is used, producing an approximately homogeneous strain field and leading to an enhanced accuracy. Results from oscillatory shear experiments with a strain amplitude of 0.01 and frequencies ranging from 0.04 to 16 Hz are given. The immediate loss of structural integrity, due to large deformations, influencing the mechanical behaviour of brain tissue, at the time scale of loading, is investigated. No significant immediate mechanical damage is observed for these shear deformations up to strains of 0.45. Moreover, the material behaviour during complex loading histories (loading-unloading) is investigated. Stress relaxation experiments for strains up to 0.2 and constant strain rate experiments for shear rates from 0.01 to 1 s(-1) and strains up to 0.15 are presented. A new differential viscoelastic model is used to describe the mechanical response of brain tissue. The model is formulated in terms of a large strain viscoelastic framework and considers non-linear viscous deformations in combination with non-linear elastic behaviour. This constitutive model is readily applicable in three-dimensional head models in order to predict the mechanical response of the intra-cranial contents due to an impact.

Aspects of seat modelling for seating comfort analysis.

The development of more comfortable seats is an important issue in the automotive industry. However, the development of new car seats is very time consuming and costly since it is typically based on experimental evaluation using prototypes. Computer models of the human-seat interaction could accelerate this process. The objective of this paper is to establish a protocol for the development of seat models using numerically efficient simulation techniques. The methodology is based on multi-body techniques: arbitrary surfaces, providing an accurate surface description, are attached to rigid bodies. The bodies are connected by kinematic joints, representing the seat back recliner and head restraint joint. Properties of the seat foam and frame have been lumped together. Further, experiments have been defined to characterise the mechanical properties required for the seat model for comfort applications. The protocol has been exemplified using a standard car seat. The seat model has been validated based on experiments with rigid loading devices with human-like shapes in terms of force-deflection characteristics. The response of the seat model agrees well with the experimental results. Therefore the presented method can be a useful tool in the seat development process, especially in early stages of the design process.

A finite element model of the human buttocks for prediction of seat pressure distributions.

Seating comfort is becoming increasingly important for the automotive industry. Car manufacturers use seating comfort to distinguish their products from those of competitors. However, the development and design of a new, more comfortable seat is time consuming and costly. The introduction of computer models of human and seat will accelerate this process. The contact interaction between human and seat is an important factor in the comfort sensation of subjects. This paper presents a finite element (FE) model of the human buttocks, able to predict the pressure distribution between human and seating surface by its detailed and realistic geometric description. A validation study based on volunteer experiments shows reasonable correlation in pressure distributions between the buttocks model and the volunteers. Both for simulations on a rigid and a soft cushion, the model predicts realistic seat pressure distributions. A parameter study shows that a pressure distribution at the interface between human and seat strongly depends on variations in human flesh and seat cushion properties.

Estimation of spinal loading in vertical vibrations by numerical simulation.

OBJECTIVE: This paper describes the prediction of spinal forces in car occupants during vertical vibrations using a numerical multi-body occupant model. BACKGROUND: An increasing part of the population is exposed to whole body vibrations in vehicles. In literature, vertical vibrations and low back pain are often related to each other. The cause of these low back pains is not well understood. A numerical human model, predicting intervertebral forces, can help to understand the mechanics of the human spine during vertical vibrations. METHODS: Numerical human and seat models have been used. Human model responses have been validated for vertical vibrations (rigid and standard car seat condition): simulated and experimental seat-to-human frequency response functions have been compared. The spinal shear and compressive forces have been investigated with the model. RESULTS: The human model seat-to-pelvis and seat-to-T1 frequency response functions in the rigid seat condition and all seat-to-human frequency response functions in the standard car seat condition approach the experimental results reasonably. The lumbar and the lower thoracic spine are subjected to the largest shear and compressive forces. CONCLUSIONS: The human model responses correlate reasonable with the volunteer responses. The predicted spinal forces could be used as a basis for derivation of hypothetical mechanisms and better understanding of low back pain disorders. RELEVANCE: In order to solve the problem of whole body vibration related injuries, knowledge about the interaction between human spinal vertebrae in vertical vibrations is required. This interaction cannot be measured in volunteer experiments. This paper describes the application of a numerical human model for prediction of spinal forces, that could be used as a basis for derivation of hypotheses regarding low back pain disorders.

Comparison of the Rear Impact Biofidelity of BioRID II and RID2.

Researchers worldwide try to define a unique test procedure for the assessment of whiplash protection of seats and restraint systems in low speed rear-end impact. Apart from valid injury criteria and uniform crash conditions, there is no clear answer to the question, which dummy to use. There are two impact dummies currently available, which have been designed for rear-end impact testing: BioRID and RID2. Both dummies have been evaluated in several test programs, however, both dummies have never been compared with each other in the test conditions, which form the basis of their design. BioRID was based on and validated against volunteer tests performed by Davidsson and Ono, while RID2 was designed with and validated against PMHS tests done by Bertholon and compared to volunteer tests reported by Van den Kroonenberg. This paper compares the responses of both rear impact dummies and the Hybrid III for the test conditions mentioned above. The setup of Davidsson used a rigid seat with flexible back and head restraint panels, while the setups from Ono and Bertholon used a rigid seat without a head restraint, in spite of being not representative for real car seats. This configuration creates a well defined test environment which will not affect nor obscure the dummy response Results of the performance of both rear impact dummies and the Hybrid III in comparison to the human responses will be presented in this paper. The results show that both rear impact dummies are capable of simulating rear impact responses, especially the head-neck kinematics. A difference in load pattern was found, which could be relevant when injury criteria will be based on neck forces and/or torques. Moreover, the dummies show a different interaction with the seat back, illustrated by the differences in T1 kinematics: BioRID shows larger T1 rotation and more ramping up than RID2, while spine straightening is comparable for both dummies. The current study showed good scores for both dummies in the setup on which they are based. The biofidelity score of BioRID is slightly better than for RID2, while the performance of the Hybrid III is relatively poor. However, repeatability, reproducibility and handling are not part of the evaluation, even though they are important for the practical use of the dummies.

Development and Evaluation of a New Rear-Impact Crash Dummy: The RID 2.

Low severity neck injuries due to vehicle accidents are a serious problem in our society. In 1997 the European Whiplash project started with the aim to develop passive safety methodologies to reduce the frequency of neck injuries in rear-end impacts. This project has resulted, among others, in a rear impact crash dummy, the so-called RID2. The objective of this paper is present the design of this dummy and to present its performance in comparison with human volunteer and post mortem human subject (PMHS) tests. Also a comparison is made with the Hybrid III dummy in similar test conditions. In the comparison with human volunteers in a real car seat, both the RID2 and the Hybrid III showed realistic kinematics. Lower neck rotation as well as the typical S-shape in the neck were found in the RID2, but not in the Hybrid III dummy. Ramping up was not found in the Hybrid III, while the RID2 did show limited ramping up. The upper neck forces measured in both dummies were reasonably good in the regular car seat, but upper neck torques were not well predicted in either dummy. Compared to post mortem human subjects placed on a rigid seat without a head restraint, the Hybrid III was found to be less biofidelic than the RID2, as the kinematics of the human subjects were better approximated by the RID2 than by the Hybrid III, which was mainly attributed to the stiff spine and neck of the Hybrid III.

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.

The large shear strain dynamic behaviour of in-vitro porcine brain tissue and a silicone gel model material.

The large strain dynamic behaviour of brain tissue and silicone gel, a brain substitute material used in mechanical head models, was compared. The non-linear shear strain behaviour was characterised using stress relaxation experiments. Brain tissue showed significant shear softening for strains above 1% (approximately 30% softening for shear strains up to 20%) while the time relaxation behaviour was nearly strain independent. Silicone gel behaved as a linear viscoelastic solid for all strains tested (up to 50%) and frequencies up to 461 Hz. As a result, the large strain time dependent behaviour of both materials could be derived for frequencies up to 1000 Hz from small strain oscillatory experiments and application of Time Temperature Superpositioning. It was concluded that silicone gel material parameters are in the same range as those of brain tissue. Nevertheless the brain tissue response will not be captured exactly due to increased viscous damping at high frequencies and the absence of shear softening in the silicone gel. For trend studies and benchmarking of numerical models the gel can be a good model material.

[Stress and kinematic analysis of the head and neck in frontal collision. A comparison of voluntary probands and postmortem human test cadavers]. / Belastung und Kinematik des Kopf/Nackensystems bei der Frontalkollision. Ein Vergleich zwischen Freiwilligen und PMTO.

Investigations comparing dynamics and kinematics in volunteers and postmortem human subjects (PMHS) under the same collision conditions are rare, although such results are of elementary importance in order to estimate the suitability of PMHS as injury indicators in crash tests. Under this aspect, a report on 12 experimental frontal collisions has been made where PMHS were loaded and restrained by a 80 mm broad double shoulder lap-step belt. The impact velocity amounted 60 km/h, the mean vehicle decelerations were at 11 g and 15 g. In the Naval Biodynamics Laboratory in New Orleans, the comparing collective of 9 volunteers in the age range of 18 to 22 years has been investigated by a mean deceleration of 11 g at otherwise comparable conditions. Optical targets have been fixed at the 1st thoracic vertebra, the occiput and the top of the head in the PMHS, the collision phase has been documented by a laterally to the sled mounted high-speed camera with a frame rate of 1000 p/s. The accelerations have been measured tri-axial at the top of the head, at the clivus and the 1st thoracic vertebra. For the simulation of the muscle tonus of the volunteers, formaldehyde has been injected into the neck muscles. In the PMHS as well as in the volunteers no thoracic- or abdominal injuries occurred during the above mentioned loads. However, the spinal column of the PMHS remained uninjured only in 3 cases, in 7 cases hemorrhages in the intervertebral disc, lacerations of the ligamentum flavum or hemorrhages in the vertebral joints were observed.