A Study Of Brain Mechanics Response And Injury Mechanisms Under Impact Loading

Traumatic brain injury(TBI)is a major cause of disability and death all around the world.TBI,also known as intracranial injury,caused by the application of external mechanical forces to the head,may result in temporary or permanent cognitive impairment and brain damage.The TBI research can reveal not only the pathological and physiological mechanism of TBI,but also the variety of physical processes involved contributes to TBI to provide explanations for the cause of brain damage.The current TBI research has become an interdisciplinary field involving both medical and mechanical studies.The investigation of TBI is the combination of experimental research and numerical simulation.The numerical simulation has attracted increasing attention and gradually become a successful tool for TBI research due to its flexibility and effectiveness,particularly for human brain modeling.The numerical simulation research can provide comprehensive information about the prediction,mechanism,and protection of TBI without the special requirements of the experimental samples and equipment.In this dissertation,TBI and its complication injury on the optic nerve were investigated using numerical simulation technology.The dissertation is mainly composed of the following aspects:In Chapter 1,the anatomical structure of the human head was firstly introduced,and the current epidemiological research of TBI is summarized.Subsequently,the current research status of TBI and complicating optic nerve injury was reviewed.The advantages and difficulties of current TBI experimental and numerical research were analyzed.The key problems in TBI research were also elaborated.In Chapter 2,the mechanical behavior of the cranial pia mater was characterized.The limited in vivo data of mechanical property of human cranial pia make it difficult to theoretically modeling TBI.Therefore,in vitro tensile and stress-relaxation experiments of ovine cranial pia mater specimens were conducted at eight strain rates to characterize its material property.The cranial pia mater exhibited a rate-dependent hyper-viscoelastic property in the tensile and stress-relaxation experiments.The tensile and stress-relaxation experimental data were fitted by an Ogden hyper-viscoelastic model with a strain rate function to describe the mechanical behavior of the cranial pia mater.In Chapter 3,we used reverse engineering technology to construct a complete finite element(FE)model of the human head based on medical images.Firstly,the computed tomography(CT)and magnetic resonance imaging(MRI)scans were used to build the geometric models of the human head tissues.The geometric models were then meshed and assembled after manual correction and inspection of the models.The material properties of different anatomical tissues were from published literatures to ensure that the verified material properties have been employed in our head model.Finally,the biofidelity of the head FE model was evaluated by the brain deformation data in vivo obtained from a magnetic resonance elastography(MRE)experiment.In Chapter 4,the mechanical response of brain under vibration loading was simulated.The shear wave propagation in the brain was firstly studied based on the head model.We identified the mechanical role of the falx and tentorium membranes while the head is affected by harmonic loading.In addition,the translational and rotational vibration simulations of the head model were performed to reveal the injury mechanism of the brain.The simulation results mimic the most severe vibration circumstance of the brain tissues.Finally,the frequency response analysis of the brain under small amplitude(<1.1 mm)was performed to reveal the injury frequency range and area of human brain.As a result,the shear wave gradually attenuates during its propagation from outside to inside of the brain.However,the wave refection from the falx and tentorium modulates the brain deformation field,which is one of the leading reasons of brain injury.The shear effect at the interface of skull and brain is another main cause of brain injury under vibration loading.Futhermore,our model predictes large strains that could potentially induce brain injuries for frequencies of brain vibration higher than 10,000 Hz.In Chapter 5,we developed a head model with a biofidelic orbit to advance the mechanistic understanding of traumatic optic neuropathy.Impact simulations were conducted using two types of impactors:a metal cylinder and a soccer ball.The simulated results from both impactors suggest that forehead impacts were those to which the optic nerve is most vulnerable.The mode and location of optic nerve injury was significantly different between the impacting conditions.Simulated results using a relatively rigid impactor(metal cylinder)suggest that the deformation of the skull at the optic canal was the primary mode of injury.On the other hand,simulated results using a relatively compliant impactor(soccer ball)suggest that primary mode of injury came from the brain tugging upon the optic nerve(from where it was affixed to the intracranial end of the optic canal)during coup and countercoup motion of the brain.