In vivo, tissue-level, mechanical thresholds for axonal injury were determined by comparing morphological injury and electrophysiological impairment to estimated tissue strain in an in vivo model of axonal injury. Axonal injury was produced by dynamically stretching the right optic nerve of an adult male guinea pig to one of seven levels of ocular displacement (Nlevel = 10; Ntotal = 70). Morphological injury was detected with neurofilament immunohistochemical staining (NF68, SM132). Simultaneously, functional injury was determined by the magnitude of the latency shift of the N35 peak of the visual evoked potentials (VEPs) recorded before and after stretch. A companion set of in situ experiments (Nlevel = 5) was used to determine the empirical relationship between the applied ocular displacement and the magnitude of optic nerve stretch. Logistic regression analysis, combined with sensitivity and specificity measures and receiver operating characteristic (ROC) curves were used to predict strain thresholds for axonal injury. From this analysis, we determined three Lagrangian strain-based thresholds for morphological damage to white matter. The liberal threshold, intended to minimize the detection of false positives, was a strain of 0.34, and the conservative threshold strain that minimized the false negative rate was 0.14. The optimal threshold strain criterion that balanced the specificity and sensitivity measures was 0.21. Similar comparisons for electrophysiological impairment produced liberal, conservative, and optimal strain thresholds of 0.28, 0.13, and 0.18, respectively. With these threshold data, it is now possible to predict more accurately the conditions that cause axonal injury in human white matter.

Tissue-Level Thresholds for Axonal Damage in an Experimental Model of Central Nervous System White Matter Injury

This study is aimed to develop a next-generation, high quality, extensively validated finite element (FE) human head model for enhanced head injury prediction and prevention. The geometry of the model was based on CT and MRI scans of an adult male. A new feature-based multi-block technique was adopted to develop hexahedral brain meshes including the cerebrum, cerebellum, brainstem, corpus callosum, ventricles, and thalamus. Conventional meshing methods were used to create the bridging veins, cerebrospinal fluid (CSF), skull, facial bones, flesh, skin, and membranes - including falx, tentorium, pia, arachnoid, and dura. The head model has 270,552 elements in total. A total of 49 loading cases were selected from a range of experimental and real world head impacts to check the robustness of the model predictions based on responses including the brain pressure, relative skull-brain motion, intracranial strain, skull response, facial response, and bridging vein elongation. The brain pressure was validated against intracranial pressure data reported by Nahum et al. (1977) and Trosseille et al. (1992). The brain motion was validated against brain displacements under sagittal, coronal, and horizontal blunt impacts performed by Hardy et al. (2001, 2007). The facial bone responses were validated under nasal impact (Nyquist et al., 1986), zygoma and maxilla impact (Allsop et al., 1988). The skull bones were validated under frontal angled impact, vertical impact, and occipital impact (Yoganandan et al., 1995) and frontal horizontal impact (Hodgson et al., 1970). The FE head model was further used to study injury mechanisms and tolerances for brain contusion (Nahum et al., 1976), bridging vein rupture (Depreitere et al., 2006), and brain strains for real-world brain injury cases (Franklyn et al. 2005). Studies from 49 loading cases demonstrated that the FE head model had good biofidelity in predicting head responses under various impact scenarios. Furthermore, tissue-level injury tolerances were proposed. A maximum principal strain of 0.42% was adopted for skull cortical layer fracture and maximum principal stress of 20 MPa was used for skull diploe layer fracture. Additionally, a plastic strain threshold of 1.2% was used for facial bone fracture. Average of 17% of engineering tensile strain indicates bridging vein rupture. For brain contusion, 277 kPa of brain pressure was calculated from reconstruction of one contusion case. Lastly, the high strains predicted by the FE head model match the trend of brain injuries reported in four real-world cases. Language: en

Development of a finite element human head model partially validated with thirty five experimental cases.

This study was conducted to investigate differences in brain response due to frontal and lateral impacts based on a partially validated three-dimensional finite element model with all essential anatomical features of a human head. Identical impact and boundary conditions were used for both the frontal and lateral impact simulations. Intracranial pressure and localized shear stress distributions predicted from these impacts were analyzed. The model predicted higher positive pressures accompanied by a relatively large localized skull deformation at the impact site from a lateral impact when compared to a frontal impact. Lateral impact also induced higher localized shear stress in the core regions of the brain. Preliminary results of the simulation suggest that skull deformation and internal partitions may be responsible for the directional sensitivity of the head in terms of intracranial pressure and shear stress response. In previous experimental studies using subhuman primates, it was found that a lateral impact was more injurious than a frontal impact. In this study, shear stress in the brain predicted by the model was much higher in a lateral impact in comparison with a frontal impact of the same severity. If shear deformation is considered as an injury indicator for diffuse brain injuries, a higher shear stress due to a lateral impact indicate that the head would tend to have a decreased tolerance to shear deformation in lateral impact. More research is needed to further quantify the effect of the skull deformation and dural partitions on brain injury due to impacts from a variety of directions and at different locations.

Comparison of Brain Responses Between Frontal and Lateral Impacts by Finite Element Modeling

Rapid deformation of brain matter caused by skull acceleration is most likely the cause of concussion, as well as more severe traumatic brain injury (TBI). The inability to measure deformation directly has led to disagreement and confusion about the biomechanics of concussion and TBI. In the present study, brain deformation in human volunteers was measured directly during mild, but rapid, deceleration of the head (20–30 m/sec2 peak, ∼40 msec duration), using an imaging technique originally developed to measure cardiac deformation. Magnetic resonance image sequences with imposed "tag" lines were obtained at high frame rates by repeating the deceleration and acquiring a subset of image data each repetition. Displacements of points on tag lines were used to estimate the Lagrangian strain tensor field. Qualitative (visual) and quantitative (strain) results illustrate clearly the deformation of brain matter due to occipital deceleration. Strains of 0.02–0.05 were typical during these events (0.05 strain corres...

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Deformation of the human brain induced by mild acceleration.

Consequences of head size following trauma to the human head.

The dynamic response of the human head to side impact was studied by 2-dimensional finite element modeling. Three models were formulated in this study. Model I is an axisymmetric model. It simulated closed shell impact of the human head, and consisted of a single-layered spherical shell filled wiht an inviscid fluid. The other two models (Model II and III) are plane strain models of a coronal section of the human head. Model II approximated a 50th percentile male head by an outer layer to simulate cranial bone and an inviscid interior core to simulate the intracranial contents. The configuration of Model III is the same as Model II but more detailed anatomical features of the head interior were added, such as, cerebral spinal fluid (CSF); falx cerebri, dura, and tentorium. Linear elastic material properties were assigned to all three models. All three models were loaded by a triangular pulse with a peak pressure of 40 kPa, effectively producing a peak force of 1954 N (440 lb). The purpose of this study was to determine the effects of the membranes and that of the mechanical properties of the skull, brain, and membrane on the dynamic response of the brain during side impact, and to compare the pressure distributions from the plane strain model with the axisymmetric model. A parametric study was conducted on Model II to characterize fully its response to impact under various conditions.(ABSTRACT TRUNCATED AT 250 WORDS)

J. S. Ruan

Papers

Human head dynamic response to side impact by finite element modeling

Human Head Dynamic Response to Side Impact by Finite Element Modeling