The Experimental Study Of Stress Distribution And Wave Propogation In Brain Tissue During Head Deceleration Injury

When the movement of head was suffocated due to the outside force, it could result in trauma at the local sites such as the scalp and the skull. The more serious case is that the movement of the brain tissue was lagging after that of skull. This could result in the coup site, contrecoup site and inner site of the brain tissue seriously injuried due to the mechanical effect of squeeze, fatigue, stretch, scrub and incision. This sort of injury is also called the deceleration injury and it often occurs in the traumatic brain injury (TBI) including traffic and falling injury with serious injury.Head decelerating impact injuries often occur in traffic accidents. It refers to the moving head impacts on an external object and slows down making the brain injuried. For example, when the moving vehicle impacts on something external to slow down then the people in the vehicle will impact on the objects in the vehicle leading to the decelerating impact injury; and in pedestrian-vehicle accidents, after the people is impacted by the vehicle and disengage from it he will then again impact on the ground resulting in the decelerating impact injury.In the biomechanical research of brain impacting injury, the results show that the inner stress response of the brain is closely associated with the brain injury. Therefore, it is necessary to further research the characteristics of stress distribution and the stress wave propogation in the brain. In this field, there are only a few researches in our country. The main reason is that it is a very difficult project to measure the stress distribution and the stress wave propogation of brain tissue in the skull using the experimental methods.In 1999, Yanping Jiang and Baosong Liu designed a planar photoelastic head model. Using the characteristics of the photoelastic material, the photoelastic stripes during the impact can be captured by the high-speed camera and then the stress distribution and the stress wave propogation in the brain can be obtained. This photoelastic head model is of important contribution to the research of traumatic brain injury. However, due to the limitations of the techniques and measuring methods at that time, the experiment did not make out the quantitative head impacting response characteristics.In 2004, Wayne State University and Ohio State University performed a very classical experiment to study the displacement of neutrally buoyant radio-opaque markers in cadaver brains during head acceleration. The markers were embedded in the brain tissue, then, a high-speed (250fps) bi-planar X-ray video camera was used to record the impacting process. By analyzing the video pictures, the“8”shape movement of the markers to the skull could be obtained. The research of the movements of brain tissue to the skull was the focus of the experiment but there was no pressure distribution or pressure wave propogation being detected in this experiment.Now, the only two high-speed X-ray video cameras in the world were in the Wayne State University. According to the researching results, actual experimental condition and the researching demands, this study prepares to set up a transparent physical brain model as well as its decelerating impact experimental platform. Then, a complete brain decelerating impact injurying experimental system is established. After this, the air bubbles are created in the brain tissue at the interested sites. The impacting process and the volume changes of the air bubbles are recorded and analyzed using the high-speed camera system and the three-dimension movement analyzing softwave. Finally, the stress distribution and the stress wave propogation of brain tissue are obtained. This experimental model provides both visuable and non-invasion methods to detect and analyze the stress distribution and the stress wave propogation of brain tissue. It is a non-contact and non-invasion measuring technique and is a new measuring concept. It is expected to disclosure the biomechanism of the brain decelerating impact using the experimental study and to provide the biomechanical basis for the defendence and diagnosis of the brain decelerating impact injury.The main studying methods and results are as follows:Ⅰ. A physical brain model with a toughened glass material is set up. This glass brain model is easy to obtain and convenient to operate. At the same time, the model is transparent enough to detect the inner markers with excellent effect. The clear changing process of the air bubbles’volume during the impacting process can be obtained using the high-speed camera system. So, this model is suit for qualitative analyzing of the biomechanics of the decelerating impact injury.A physical brain model based on the frame-turnover technique has been developed. PC colophony materail can provide a good property to endure impact. And the frame-turnover technique can provide a good comparability of geometry between the model and the real skull. So, this model is suit for quantitive analyzing of the biomechanics of the decelerating impact injury.Ⅱ. A brain decelerating impact injurying experimental platform is set up based on the toughened glass material physical brain model. This platform is suit for researching the low-grade brain decelerating impacting injury.A brain decelerating impact injurying experimental platform is set up based on the frame-turnover technique physical brain model. This platform is suit for researching the medium and severe brain decelerating impacting injury.Through the calculation of HIC, the degree of all kind brain injuries can be estimated quantitivly. Combining with the volume change of the bubbles in the brain and the stress change of the brain, the macroscopical lay and the microcosmic lay of the impact can be associated to research the biomechanics of the brain impacting injury.Ⅲ. Respectively based on the toughened glass material brain model, frame-turnover technique brain model and the macromolecular material brain model, the brain deceleration impact experiments have been executed on the platform. After the experiments, the stress distribution and the wave propogation of the brain tissue have been analyzed, while the results are as follows:①The negative pressure occurred at the contrecoup site, and the stress of the brain tissue during its deceleration impact showed itself a chart of grads.②A trough occurred in the stress wave of the brain tissue at the contrecoup site and a crest occurred in the stress wave of the brain tissue at the coup site. The occurrence of the trough was earlier than the occurrence of the crest. This property showed the propogation rule of the stress wave at site and between sites.③During the impact, the stress wave reflected on the inner wall of the skull. The reflected wave and the original wave met and overlaped to each other in the skull including the sites where the bubbles were created. ④The brain tissue at the contrecoup site was mainly suffered from the tensile stress and the brain tissue at the coup site was mainly suffered from the compressive stress and the brain tissue at the middle site was mainly suffered from the equivalent tensile or compressive stress. Because the tensile strength of the brain tissue was lower than the compressive strength of the brain tissue, under the state that the brain tissue at the contrecoup sites was mainly exposured to the tensile stress, and the brain tissue at this site was easier to destroy and injure than the brain tissue at the coup site. These loading properties of the brain tissue elementarily disclosured one of the biomechanical mechanisms of the contrecoup injuries which were often found in head deceleration injuries at traffic accidents.⑤At the contrecoup side, the nearer of the tissue to the impacting point the earlier of the trough of wave occured. At the coup side, the nearer the tissue to the impacting point, the earlier the crest of wave occured.⑥At the coup side, the brain tissue was exposured to the persistent positive pressure. At the contrecoup side, the brain tissue was exposured to the alternative positive and negative pressure during the impact. At the contrecoup side, the nearer the tissue to the middle site, the smaller the amplitude of the stress wave was, while the nearer the tissue to the middle site, the greater the frequency of the stress wave was. This increased frequency led to the increased opportunity of the brain tissue being injuried by the fatigue effect. This may be one of the biomechanical mechanisms of the contrecoup injuries which were often found clinically with the relative deep-seated tissue being injuried in head deceleration impact injuries.