A homeomorphic finite-element model of impact head injury
Description
The mechanics of head injury has been of special interest in biomechanics research. Since experimentation is costly and often difficult to perform, mathematical and computer models which simulate head injury are primary research tools. Previous investigators have formulated finite element models of the human brain and of the human skull. A finite element model of the human head and neck, as a system, is presented which incorporates the brain and skull models of previous investigators and a model of the cervical spine into a single model. The 'exact' geometry of the cerebrospinal fluid (CSF) space is included, and morphological continuity of the CSF space and brain material in the region of the head/spine connection is maintained in order that the proper 'flow' of material between the head and spinal cavity may take place during a simulated head impact or whiplash. The 3-dimensional geometry of the brain, spinal cord, CSF space, cervical vertebrae, and the intervertebral disks is defined by means of eight-node, isoparametric brick elements. The jaw and the thick portion of the skull in the floor of the cranium are also described by brick elements, but the frontal and parietal bones of the skull and the superior portions of the occipital and temporal bones of the skull are represented by thin shell elements. The falx cerebri and the tentorium cerebelli are partitioned into plane stress membrane elements. Midsagittal symmetry was assumed in the construction of the model Accelerations usually exceed 100 g's during a head impact. Hence, inertial forces will cause large changes in the pressure and shear stress gradients within the central nervous system (CNS) during the contact phase of the impact. The model was therefore made inertially correct, at least to a reasonable approximation, by optimally locating point masses at the surface nodes of the skull in order that the total mass, center of mass, and mass and product moments of inertia of the head/neck system would agree with experimentally determined values in the literature. The point masses represent the additional mass of the system contributed by the hair, facial and neck muscles, and other soft tissue of the head and neck, and therefore care was taken so that the point mass distribution would be anatomically correct The material properties assigned to the brain and spinal cord elements of the model were the standard literature values of E = 66.7 kPa (9.68 psi) and a Poisson's ratio of (nu) = .499, which expresses the essential incompressibility of the brain material. The boundary conditions consist of fixing the spine at the T1 level in order to simulate the effect of the large torso mass on the head/neck system. A half-sine wave contact force of 4 ms duration with a peak load of 6000 N (1349 lb) was applied to the occiput of the skull over an area of 26.7 cm('2) (4.14 in('2)), and the model was exercised using the linear finite element program EASE2 with the split energy option The results gave peak pressures in the coup region of the brain of approximately 93.5 kPa (13.6 psi) at 1.6 ms; the peak negative pressure in the contre-coup region was -100.1 kPa (-14.5 psi) at 1.6 ms. These values agree with the experimental data reported in the literature if they are scaled relative to the magnitude of the contact force used in the experiment (impact study)