Development of a finite element human head model partially validated with thirty five experimental cases.

This study is aimed to develop a high quality, extensively validated finite element (FE) human head model for enhanced head injury prediction and prevention. The geometry of the model was based on computed tomography (CT) and magnetic resonance imaging scans of an adult male who has the average height and weight of an American. A feature-based multiblock technique was adopted to develop hexahedral brain meshes including the cerebrum, cerebellum, brainstem, corpus callosum, ventricles, and thalamus. Conventional meshing methods were used to create the bridging veins, cerebrospinal fluid, skull, facial bones, flesh, skin, and membranes-including falx, tentorium, pia, arachnoid, and dura. The head model has 270,552 elements in total. Thirty five loading cases were selected from a range of experimental head impacts to check the robustness of the model predictions based on responses including the brain pressure, relative skull-brain motion, skull response, and facial response. The brain pressure was validated against intracranial pressure data reported by Nahum et al. (1977, "Intracranial Pressure Dynamics During Head Impact," Proc. 21st Stapp Car Crash Conference, SAE Technical Paper No. 770922) and Trosseille et al. (1992, "Development of a F.E.M. of the Human Head According to a Specific Test Protocol," Proc. 36th Stapp Car Crash Conference, SAE Technical Paper No. 922527). The brain motion was validated against brain displacements under sagittal, coronal, and horizontal blunt impacts performed by Hardy et al. (2001, "Investigation of Head Injury Mechanisms Using Neutral Density Technology and High-Speed Biplanar X-Ray," Stapp Car Crash Journal, 45, pp. 337-368; and 2007, "A Study of the Response of the Human Cadaver Head to Impact," Stapp Car Crash Journal, 51, pp. 17-80). The facial bone responses were validated under nasal impact (Nyquist et al. 1986, "Facial Impact Tolerance and Response," Proc. 30th Stapp Car Crash Conference, SAE Technical Paper No. 861896), zygoma and maxilla impact (Allsop et al. 1988, "Facial Impact Response - A Comparison of the Hybrid III Dummy and Human Cadaver," Proc. 32nd Stapp Car Crash Conference, SAE Technical Paper No. 881719)]. The skull bones were validated under frontal angled impact, vertical impact, and occipital impact (Yoganandan et al. 1995, "Biomechanics of Skull Fracture," J Neurotrauma, 12(4), pp. 659-668) and frontal horizontal impact (Hodgson et al. 1970, "Fracture Behavior of the Skull Frontal Bone Against Cylindrical Surfaces," 14th Stapp Car Crash Conference, SAE International, Warrendale, PA). The FE head model was further used to study injury mechanisms and tolerances for brain contusion (Nahum et al. 1976, "An Experimental Model for Closed Head Impact Injury," 20th Stapp Car Crash Conference, SAE International, Warrendale, PA). Studies from 35 loading cases demonstrated that the FE head model could predict head responses which were comparable to experimental measurements in terms of pattern, peak values, or time histories. Furthermore, tissue-level injury tolerances were proposed. A maximum principal strain of 0.42% was adopted for skull cortical layer fracture and maximum principal stress of 20 MPa was used for skull diploë layer fracture. Additionally, a plastic strain threshold of 1.2% was used for facial bone fracture. For brain contusion, 277 kPa of brain pressure was calculated from reconstruction of one contusion case.

Why is CA3 more vulnerable than CA1 in experimental models of controlled cortical impact-induced brain injury?

One interesting finding of controlled cortical impact (CCI) experiments is that the CA3 region of the hippocampus, which is positioned further from the impact than the CA1 region, is reported as being more injured. The current literature has suggested a positive correlation between brain tissue stretch and neuronal cell loss. However, it is counterintuitive to assume that CA3 is stretched more during CCI injury. Recent mechanical studies of the brain have reported on a level of spatial heterogeneity not previously appreciated-the finding that CA1 was significantly stiffer than all other regions tested and that CA3 was one of the most compliant. We hypothesized that mechanical heterogeneity of anatomical structures could underlie the proposed heterogeneous mechanical response and hence the pattern of cell death. As such, we developed a three-dimensional finite element (FE) rat brain model representing detailed hippocampal structures and simulated various CCI experiments. Four groups of material properties based on recent experiments were tested. In group 1, hyperelastic material properties were assigned to various hippocampal structures, with CA3 more compliant than CA1. In group 2, linear viscoelastic material properties were assigned to hippocampal structures, with CA3 more compliant than CA1. In group 3, the hippocampus was represented by homogenous linear viscoelastic material properties. In group 4, a homogeneous nonlinear hippocampus was adopted. Simulation results demonstrated that for CCI with a 5-mm diameter, flat shape impactor, CA3 experienced increased tensile strains over a larger area and to a greater magnitude than did CA1 for group 1, which best explained why CA3 is more sensitive to CCI injury. However, for groups 2-4, the total volume with high strain (>30%) in CA3 was smaller than that in CA1. The FE rat brain model, with detailed hippocampal structures presented here, will help to engineer desired experimental neurotrauma models by virtually characterizing brain biomechanics before testing.