Physical Modeling of Brain and Head Kinematics

Abstract: The development of head injury criteria and countermeasures for head protection requires understanding of how different factors affect brain and head kinematics. In this thesis, 2D physical models of the human head were developed and used to evaluate how various factors affect brain kinematics during angular head kinematics. Furthermore, an experimental model of frontal head impact into padded vehicle upper interior structures consisting of a Hybrid III head-neck structure mounted on a mini-sled platform was used to evaluate how peak responses of linear head kinematics correlate with peak responses of angular head kinematics.

Sagittal plane physical models were used to investigate effects of the lateral ventricles and irregular skull base during sagittal plane angular head dynamics. Coronal plane angular head dynamics were simulated in a coronal plane physical model based on the hypothesis that acute subdural hematoma (ASDH) is related to cerebral vertex displacement and diffuse axonal injury (DAI) to local Green-Lagrange strain.

Silicone gel simulated the brain and was separated from the surrounding skull vessel by paraffin, which provided a slip interface between the gel and vessel. The physical models were exposed to a biphasic time history of centroidal rotation characterized by a rapid acceleration phase followed by free rotation and a more gradual deceleration phase. Rigid body displacement, shear and principal strains were determined from high speed video recorded trajectories of grid markers in the surrogate brain.

The results from the sagittal plane physical models indicate that the inability of the intraventricular cerebrospinal fluid to transfer shear and tensile forces involves strain relief in cerebral structures located inferior and superior to the lateral ventricles. Consequently, the lateral ventricles seem to provide protection for important structures against strain related injury. Nonetheless, the ventricular boundaries seem to be subjected to greater strain intensity as compared to adjacent tissue. Apart from influencing kinematics throughout the cerebrum, the irregular skull base seems to protect nerves and vessels passing through the cranial floor by limiting the mobility of the frontal and temporal lobes. Realistic representations of the lateral ventricles and skull base are thus necessary in head injury modeling and for an understanding of brain injury mechanisms.

The results from the coronal plane physical model indicate that ASDH caused by bridging vein rupture is more likely to occur on the contralateral than on the ipsilateral side of the falx and that the presence of sulci in the brain increases the risk of ASDH. The strain in the fiber direction of the corpus callosum is predicted to be close to the direction of minimum principal strain, indicating a degree of natural protection of the axons. The data support the use of the peak change in angular velocity as a descriptor of the risk of DAI but not of ASDH.

Using the experimental model of frontal head impact into padded vehicle upper interior structures, sagittal plane linear and angular head accelerations were measured in frontal head impacts into foam samples of various stiffness and density at three different impact speeds. Provided that the foam samples do not bottom out, peak angular acceleration and peak change of angular velocity increase approximately linearly with increasing peak resultant linear acceleration and HIC₃₆. The results indicate that the padding that produces the lowest peak angular acceleration and peak change in angular velocity without causing high peak forces is the one that produces the lowest possible HIC₃₆ without bottoming out in the Free Motion Headform (FMH) test specified in Federal Motor Vehicle Safety Standard (FMVSS) No. 201.