## 6 Conclusions  
The limitation of in vivo studies on the human brain made the finite element modelling an important tool to study head injuries. The numerical modelling might provide additional data about brain injury propagation (occurrence of secondary brain trauma), and therefore have some input to medical practice in the treatment of certain trauma cases. It might point to potential lesions, which are not yet visible in radiological imaging, or not obvious on the initial examination. Compared with the adult FEHM, relatively few studies have been reported for modelling paediatric impact biomechanics. What is more, the degree of anatomical detail has been lagging behind that of the adult models. Scaling down an adult head to obtain a child head in testing should not be used and specific child geometry is needed to investigate child injury mechanisms. The aHEAD 2 yo model of a small child is very advanced in its structure and material compared to the models currently available in the literature. The authors believe this comprehensive work will push the state-of-the-art of paediatric head models. Although validation is limited due to the inherent difficulties of obtaining experimental data, a new high-quality model is presented, and the intracranial motion is assessed. These results will make it possible to further comparisons between future numerical paediatric head models. Thus, the authors believe that this model may serve as the benchmark model as all material data and the model structure have been revealed and referred. It should be mentioned that in the presented model, the space for cerebrospinal fluid was filled with the smoothed-particle hydrodynamics meshfree method. This approach allows cerebrospinal fluid to have complex dynamics, which was presented in the paper. On the other hand, modelling of local quasi-flow by implementing low shear-modulus by solid elements may inherit a significant, yet not intended, support for the brain within the skull. Nevertheless, the vast majority of FEHMs have set the cerebrospinal fluid as solid finite elements, which may not sufficiently reflect the biomechanics of the tissue. In addition, the aHEAD model consists only of hexahedral elements for 3D structures. However, in order for paediatric FEHMs to realise their full potential, extensive cadaveric or tissue tests need to be performed. Since this is unlikely to occur, mainly due to ethical responses, some new techniques and methods, such as the ones described in this study, must be developed to bridge the gaps in the state-of-the-art. The study showed that the use of the material model has a significant impact on the biomechanical response of the brain during impact. At the same time, it has been presented that the differentiation of white and grey matter may significantly affect brain displacement under the mechanical loading. It can be interpreted that the model divided into grey and white matter more accurately reflects the brain response in the real world compared to the unsegmented model. As a result, the biomechanics of destruction in the interface of grey/white matter can be better investigated. It should be noted that material models should be improved by conducting further experimental research on children's tissues. In particular, we still know very little about how the material characteristics change during brain development. During this period, the mechanical properties and tissue proportions including grey matter to white matter can change significantly. It is important to emphasize that currently,there is a lack of data for validation of children's head models. To date, the current validation of numerical models consists mainly of comparing acceleration and deflection force diagrams for fall and compression tests.