Traumatic head injuries represent a major global health burden, affecting up to 70 million people annually world-wide. To study head injury mechanisms and evaluate preventive measures, virtual, anatomically-detailed human surrogates, referred to as Human Body Models (HBMs), can be created using Finite Element (FE) modeling techniques. Such FE models can be used to computationally recreate real-world head traumas to study human response to impact and reveal injury mechanisms. However, since FE is an inherently heavy computational task, there are numerous modeling challenges associated with using FE analysis for this purpose: constitutive models need to be appointed to complex biological tissues, models need to be properly validated, the chosen approach should be feasible in terms of time, and so forth. This doctoral thesis aims to address a few of these difficulties.

This thesis is composed of four comprehensive studies, each related to the overall objective of developing new methodologies and models, and further developing existing ones, for in-depth FE reconstructions of real-world head trauma. To emphasize their applicability in head injury research, the four studies also feature in-depth reconstructions of real-world injurious events. In the first study, a male and female pedestrian HBM was developed based on an existing occupant HBM, along with an efficient framework for anthropometric personalization. In the second study, a framework for reconstructing head traumas of pedestrians and cyclists in real-world road traffic accidents was developed, validated and exemplified by reconstructing 20 real-world cases. In the third study, a material model for cranial bone was developed and validated, and used for predicting skull fractures in five fall accidents. Lastly, in the fourth study, the material model was applied to a subject-specific head model, used to conduct an in-depth reconstruction of a workplace fatality to assess the protective effect of construction helmets.

Together, these four studies highlight how in-depth FE reconstructions, involving geometrically personalized models of the human body, can provide head injury predictions with striking resemblance to real-world data. When conducted with care, such reconstructions can offer valuable insights into the complex dynamics of head trauma. They can be indispensable tools for evaluating injury prevention strategies, and can potentially be useful within the field of forensic medicine, as they may help open up for objectification of forensic evaluations.

Traumatiska huvudskador utgör en stor folkhälsoutmaning, med en årlig förekomst som uppskattas till uppemot 70 miljoner fall världen över. För att studera mekanismerna bakom huvudskador kan virtuella, anatomiskt detaljerade mänskliga surrogatmodeller, eller humanmodeller (eng: Human Body Model, HBM), skapas med hjälp av Finita Element (FE) metoden. Sådana FE-modeller kan användas för att rekonstruera huvudtrauman från verkliga olycksfall numeriskt, för att i sin tur synliggöra skademekanismer bakom skall- och hjärnskador. Det finns dessvärre många utmaningar med att använda FE-analys för detta ändamål: materialmodeller måste formuleras för komplexa biologiska vävnader, FE modeller bör valideras, tillvägagångssättet bör vara tidseffektivt och så vidare. Denna doktorsavhandling ämnar ta itu med några av dessa svårigheter.

Avhandlingen består av fyra delstudier, som alla förhåller sig till det övergripande målet att utveckla nya metoder och modeller, samt vidareutveckla  befintliga, för FE-rekonstruktioner av verkliga huvudtrauman. För att belysa deras tillämpning i huvudskadeforskning, behandlar de fyra studierna även rekonstruktioner av verkliga olycksfall. I den första studien utvecklades en manlig och kvinnlig fotgängar-HBM baserat på en befintlig passagerar-HBM, tillsammans med ett effektivt verktyg för att rätta till en HBMs antropometri. I den andra studien utvecklades en metodologi för att rekonstruera huvudtrauman i trafikolyckor (gångtrafikanter eller cyklister). Metodologin validerades genom att rekonstruera 20 verkliga olyckor. I den tredje studien utvecklades och validerades en materialmodell för mänskligt skallben, som senare användes för att prediktera skallfrakturer i fem verkliga fallolyckor. Materialmodellen applicerades på en individanpassad huvudmodell, som också användes i den fjärde studien, där en rekonstruktion av en arbetsplatsolycka genomfördes för att utvärdera skyddshjälmars effektivitet.

Tillsammans belyser dessa fyra studier hur FE-rekonstruktioner, som involverar individanpassade biomekaniska FE-modeller, kan förutsäga huvudskador med slående likhet med verkliga data. När rekonstruktioner genomförs noggrant kan de hjälpa till att åskådliggöra den komplexa dynamiken bakom skall- och hjärnskador. De kan vara oumbärliga verktyg för att utvärdera skadeförebyggande åtgärder och undersöka orsakssamband inom rättsmedicinska sammanhang.

To combat the global fatality rates among vulnerable road users (VRUs), prioritizing research on head injury mechanisms and human tolerance levels in vehicle-to-VRU traffic collisions is imperative. A foundational step for VRU injury prevention is often to create virtual reconstructions of real-world collisions. Thus, efficient and trustworthy reconstruction tools are needed to make use of recent advances in accident data collection routines and Finite Element (FE) human body modeling. In this study, a comprehensive and streamlined reconstruction methodology, starting from a video-recorded accident, has been developed. The workflow, that includes state-of-the-art tools for personalization of human body models (HBMs) and vehicles, was evaluated and demonstrated through 20 real-world VRU collision cases. The FE models successfully replicated the vehicle damage that was observed in on-scene photographs of the post-impact vehicle, as well as impact kinematics captured in dash cam or surveillance recordings. The findings highlight how video evidence can considerably narrow down the number of plausible impact scenarios and raise the credibility of virtual reconstructions of real-world VRU collision events. More importantly, this study demonstrates how, with an efficient and systematic methodology, FE might be feasible also for large-scale VRU accident datasets.

Developing vehicle finite element (FE) models that match real accident-involved vehicles is challenging. This is related to the intricate variety of geometric features and components. The current study proposes a novel method to efficiently and accurately generate case-specific buck models for car-to-pedestrian simulations. To achieve this, we implemented the vehicle side-view images to detect the horizontal position and roundness of two wheels to rectify distortions and deviations and then extracted the mid-section profiles for comparative calculations against baseline vehicle models to obtain the transformation matrices. Based on the generic buck model which consists of six key components and corresponding matrices, the case-specific buck model was generated semi-automatically based on the transformation metrics. Utilizing this image-based method, a total of 12 vehicle models representing four vehicle categories including family car (FCR), Roadster (RDS), small Sport Utility Vehicle (SUV), and large SUV were generated for car-to-pedestrian collision FE simulations in this study. The pedestrian head trajectories, total contact forces, head injury criterion (HIC), and brain injury criterion (BrIC) were analyzed comparatively. We found that, even within the same vehicle category and initial conditions, the variation in wrap around distance (WAD) spans 84–165 mm, in HIC ranges from 98 to 336, and in BrIC fluctuates between 1.25 and 1.46. These findings highlight the significant influence of vehicle frontal shape and underscore the necessity of using case-specific vehicle models in crash simulations. The proposed method provides a new approach for further vehicle structure optimization aiming at reducing pedestrian head injury and increasing traffic safety.

ObjectivesVulnerable road users are globally overrepresented as victims of road traffic injuries. Developing biofidelic male and female pedestrian human body models (HBMs) that represent diverse anthropometries is essential to enhance road safety and propose intervention strategies.MethodsIn this study, 50th percentile male and female pedestrians of the SAFER HBM were developed via a newly developed image registration-based mesh morphing framework. The performance of the HBMs was evaluated by means of a set of cadaver experiments, involving subjects struck laterally by a generic sedan buck.ResultsIn simulated whole-body pedestrian collisions, the personalized HBMs effectively replicate trajectories of the head and lower body regions, as well as head kinematics, in lateral impacts. The results also demonstrate the personalization framework's capacity to generate personalized HBMs with reliable mesh quality, ensuring robust simulations.ConclusionsThe presented pedestrian HBMs and personalization framework provide robust means to reconstruct and evaluate head impacts in pedestrian-to-vehicle collisions thoroughly and accurately.

Post-mortem computed tomography (PMCT) enables the creation of subject-specific 3D head models suitable for quantitative analysis such as finite element analysis (FEA). FEA of proposed traumatic events is an objective and repeatable numerical method for assessing whether an event could cause a skull fracture such as seen at autopsy. FEA of blunt force skull fracture in adults with subject-specific 3D models in forensic pathology remains uninvestigated. This study aimed to assess the feasibility of FEA for skull fracture analysis in routine forensic pathology. Five cases with blunt force skull fracture and sufficient information on the kinematics of the traumatic event to enable numerical reconstruction were chosen. Subject-specific finite element (FE) head models were constructed by mesh morphing based on PMCT 3D models and A Detailed and Personalizable Head Model with Axons for Injury Prediction (ADAPT) FE model. Morphing was successful in maintaining subject-specific 3D geometry and quality of the FE mesh in all cases. In three cases, the simulated fracture patterns were comparable in location and pattern to the fractures seen at autopsy/PMCT. In one case, the simulated fracture was in the parietal bone whereas the fracture seen at autopsy/PMCT was in the occipital bone. In another case, the simulated fracture was a spider-web fracture in the frontal bone, whereas a much smaller fracture was seen at autopsy/PMCT; however, the fracture in the early time steps of the simulation was comparable to autopsy/PMCT. FEA might be feasible in forensic pathology in cases with a single blunt force impact and well-described event circumstances.

Finite element human body models (HBMs) are becoming increasingly important numerical tools for traffic safety. Developing a validated and reliable HBM from the start requires integrated efforts and continues to be a challenging task. Mesh morphing is an efficient technique to generate personalized HBMs accounting for individual anatomy once a baseline model has been developed. This study presents a new image registration-based mesh morphing method to generate personalized HBMs. The method is demonstrated by morphing four baseline HBMs (SAFER, THUMS, and VIVA+ in both seated and standing postures) into ten subjects with varying heights, body mass indices (BMIs), and sex. The resulting personalized HBMs show comparable element quality to the baseline models. This method enables the comparison of HBMs by morphing them into the same subject, eliminating geometric differences. The method also shows superior geometry correction capabilities, which facilitates converting a seated HBM to a standing one, combined with additional positioning tools. Furthermore, this method can be extended to personalize other models, and the feasibility of morphing vehicle models has been illustrated. In conclusion, this new image registration-based mesh morphing method allows rapid and robust personalization of HBMs, facilitating personalized simulations.

Traumatic head injuries remain a leading cause of death and disability worldwide. Although skull fractures are one of the most common head injuries, the fundamental mechanics of cranial bone and its impact tolerance are still uncertain. In the present study, a strain-rate-dependent material model for cranial bone has been proposed and implemented in subject-specific Finite Element (FE) head models in order to predict skull fractures in five real-world fall accidents. The subject-specific head models were developed following an established image-registration-based personalization pipeline. Head impact boundary conditions were derived from accident reconstructions using personalized human body models. The simulated fracture lines were compared to those visible in post-mortem CT scans of each subject. In result, the FE models did predict the actual occurrence and extent of skull fractures in all cases. In at least four out of five cases, predicted fracture patterns were comparable to ones from CT scans and autopsy reports. The tensile material model, which was tuned to represent rate-dependent tensile data of cortical skull bone from literature, was able to capture observed linear fractures in blunt indentation loading of a skullcap specimen. The FE model showed to be sensitive to modeling parameters, in particular to the constitutive parameters of the cortical tables. Nevertheless, this study provides a currently lacking strain-rate dependent material model of cranial bone that has the capacity to accurately predict linear fracture patterns. For the first time, a procedure to reconstruct occurrences of skull fractures using computational engineering techniques, capturing the all-in-all fracture initiation, propagation and final pattern, is presented.