BIOMECHANICAL CHARACTERIZATION OF BICYCLE ACCIDENTS RELATED HEAD INJURIES

Cycling in Belgium represents one of the main transportation means to work and school, and is a popular recreational activity. However, road traffic is inherently unsafe. Traffic safety was not considered as a relevant goal when initially designing traffic infrastructure, and although efforts are being made to accommodate the wide range of road users, road accidents can still result from errors or unpredictable actions. The kinetic forces resulting from the differences in mass and velocity of crash opponents largely dictates the severity of the outcomes. Especially severe crash outcomes are expected for vulnerable road users such as pedestrians and cyclists who lack by far the same level of protection offered to cars and other vehicle occupants. Moreover, single bicycle crashes are also a source of injuries through falls and collisions with obstacles, which can result in serious traumas, especially for elderly population of cyclists and those not wearing a helmet. In view of all these observations, head injury represents a real economic, social and health burden on society. Along the years a lot of research has been employed in order to improve our understanding of head injury mechanism, to develop appropriate lesion-specific tolerance criteria, and to improve protective and preventive measures. Despite all efforts, the field of head injury biomechanics still raises many unanswered questions and debates. For almost 13 years, the multidisciplinary Bicycle Helmet research group concentrated its research activity efforts on the head injury biomechanics field, with the main goal of providing lesion-specific head injury criteria and developing a new helmet prototype that offers a better head protection in a bicycle accident. The goals of the present dissertation are situated within this framework. In view of the formulated goals, this dissertation is structured as follows: Chapter 1 describes the context and the aim of the global research project, the position of the current research in the context of the global project and the specific hypotheses and research questions of the current dissertation. In Chapter 2, the validity of an energy criterion for skull fractures is investigated. Despite all progress made in the field of skull fracture biomechanics, the mechanism of skull fracture is far from being fully understood. The investigation comprises a series of experimental head impacts in various anatomical locations, using a double pendulum set-up. The methodology allowed for the calculation of the biomechanical force-deflection response and energy-absorbing characteristics of the skull. The resulting data indicated the skull to have nonlinear structural response suggesting the existence of an energy failure level in the range of 5-15 J for temporal dynamic loading conditions. It is likely that energy criteria impact location dependent have the potential to accurately predict skull fracture by encompassing impact as well as structural characteristics. In Chapter 3, the aim is to assess brain-skull relative motion in quasistatic circumstances and to correlate cortical regions with high motion amplitudes with predilection sites for cerebral contusions. Brain-skull relative motion plays a primordial role in the etiology of devastating head injuries. The investigation included thirty healthy volunteers scanned using a clinical 3 Tesla MR scanner in four different head positions. Through image processing and 3D models registration, pairwise comparisons were performed to calculate the brain shift between sagittal and coronal head positional change. Next, local brain deformation was evaluated by comparison between cortical and ventricular amplitudes. Finally, the influence of age, gender, and skull geometry on the cortical and ventricular motion was investigated. The results described complex brain shift patterns, with high regional and inter-individual variations. Regions with maximum motion amplitudes were identified at the inferolateral aspects of the frontal and temporal lobes, congruent with predilection sites for contusions. The results of this project contribute to a better understanding of the frontotemporal contusion mechanopathogenesis and are useful for the optimization of finite element head models and neurosurgical navigation procedures. Chapter 4 focuses on the separate description of the various brain motion components building further on the data generated in the MRI experiment described in chapter 3. The rigid body motion of the brain was extracted by means of a rigid-body algorithm employed using SPM 8.0. The net brain displacement reported in the current study did not vary significantly from 0, the maximum translational displacement being as high as 1.3mm and maximum rotation angle being 0.85. Larger brain motion amplitudes in the sagittal direction resulted when gravitational force had a similar orientation. Due to skull symmetry in the coronal plane and the falx as a fibrous interhemisferic partition not interfering with the resulting motion in the sagittal direction, the out of plain motion of the brain was less pronounced after a prone-supine head positional change. In contrast, under the gravitational force acting in lateral direction the brain not only translates along the gravitational axis, but also significantly rotates in the sagittal and axial plane The observations made in this investigation study contribute to a better understanding of the brain s dynamic behaviour and of the relation between brain motion and specific brain injury mechanopathogenesis. New insights on the boundary conditions in the human head are extremely helpful for improvement and validation of finite element models.Chapter 5 presents an investigation on the bridging veins-superior sagittal sinus failure mechanism under axial stretch in relation with ASDH mechanopathogenesis. In this study, bridging vein-superior sagittal sinus (BV-SSS) units were axially stretched until failure for strain rates ranging from 2.66s-1 to 185.61s-1, in order to investigate any strain rate dependency in their mechanical behaviour. The results show that up to 200s-1, the effect of the strain rate on the veins mechanical behaviour is outweighed by the large morphological intra- and inter- individual variations. The veins dimensions had the strongest influence on the BV mechanical behaviour and on the failure mechanism. The study brings important contribution to ASDH research, emphasising the importance of considering the BV-SSS complex as a whole when trying to describe the ASDH mechanopathology. The research on head injury biomechanics described in this dissertation has helped to fine tune and to define better lesion-specific tolerance criteria. Nevertheless it contributes to the development of more detailed and more accurate finite element models in which extrapolation is possible. Hence, a final aspect discussed in this dissertation and included in the appendix and relates to the investigation on the mechanical characterisation of the helmets by using the finite element method as a first step in the helmet optimisation and improvement process. The relation between different parameters (materials properties, vents number and localization, etc.) and the risk of head injury is evaluated. Different finite elements models were built to compare the mechanical behaviour of the different types of cyclist helmets during the impact using MSC Marc Mentat. The linear acceleration and the energy absorption were evaluated for a better understanding of the mechanism through which the helmet protects the head during an impact. The finite element study showed that all materials studied will protect the head by reducing the linear acceleration by more than 80% and stress values into the head by more than 65%. The anisotropic foam studied seems to offer the best protection against linear acceleration, stress and deformation, showing promising results for the use of anisotropic foams in the process of improving the head protection offered by a headgear.Valhelm" FWO Vlaanderen Project Levenslijn G.0001.08.nrpages: 209status: publishe

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