Numerical Accident Reconstructions A Biomechanical Tool to Understand and Prevent Head Injuries MADELEN FAHLSTEDT Doctoral Thesis KTH Royal Institute of Technology School of Technology and Health Neuronic Engineering SE- 141 52 Huddinge, Sweden TRITA-STH Report 2015:4 ISSN 1653-3836 ISRN/KTH/STH/2015:4-SE ISBN 978-91-7595-512-4 Academic dissertation which with permission from Kungliga Tekniska Högskolan (Royal Institute of Technology) in Stockholm is presented for public review for passing the doctoral examination on Friday, May 8th 2015, at 9:00 in lecture hall 3-221, Alfred Nobels Allé 10, Huddinge, Sweden. © Madelen Fahlstedt, 2015 Abstract Traumatic brain injuries (TBIs) are a major health and socioeconomic problem throughout the world, with an estimated 10 million deaths and instances of hospitalization annually. Numerical methods such as finite element (FE) methods can be used to study head injuries and optimize the protection, which can lead to a decrease in the number of injuries. The FE head models were initially evaluated for biofidelity by comparing with donated corpses experiments. However, there are some limitations in experiments of corpses, including material degradation after death. One feasible alternative to evaluating head models with living human tissue is to use reconstruction of real accidents. However, the process of accident reconstruction entails some uncertainties since it is not a controlled experiment. Therefore, a deeper understanding of the accident reconstruction process is needed in order to be able to improve the FE human models. Thus, the aim of this thesis was to evaluate and further develop more advanced strategies for accident reconstructions involving head injuries. A FE head model was used to study head injuries in accidents. Existing bicycle accident data was used, as were hypothetical accident situations for cyclists and pedestrians. A FE bicycle helmet model having different designs was developed to study the protective effect. An objective method was developed based on the Overlap Index (OI) and Location Index (LI) to facilitate the comparison of FE model responses with injuries visible in medical images. Three bicycle accident reconstructions were performed and the proposed method evaluated. The method showed to have potential to be an objective method to compare FE model response with medical images and could be a step towards improving the evaluation of results from injury reconstructions. The simulations demonstrated the protective effect of a bicycle helmet. A decrease was seen in the injurious effect on both the brain tissue and the skull. However, the results also showed that the brain tissue strain could be further decreased by modifying the helmet design. Two different numerical pedestrian models were compared to evaluate whether the more time- efficient rigid body model could be used, instead of a FE pedestrian model, to roughly determine the initial conditions as an accident reconstruction involves some uncertainties. The difference, in terms of the head impact location, rotation and velocity, attributable to the two models was in the same range as differences due to uncertainties in some of the initial parameters, such as vehicle impact velocity. Keywords Head injuries; Accident reconstruction; Finite element analysis; Injury prevention; Helmet; Cyclist; Pedestrian; Epidemiology i ii Sammanfattning Hjärnskador är ett stort hälso- och socialekonomiskt problem i världen. Det uppskattas att 10 miljoner människor drabbas av dödliga hjärnskador eller en hjärnskada som kräver inläggning på sjukhus varje år. Numeriska metoder, såsom finita element (FE) metoden kan användas för att studera huvudskador och optimera skyddssystem för att i sin tur minska antalet skador. FE- huvudmodeller har initialt utvärderats mot försök på donerade kroppar. Men det finns begränsningar med donerade kroppar, som t.ex. att det sker en nedbrytning av död vävnad. För att utvärdera modellerna mot levande vävnad kan rekonstruktioner av verkliga olyckor användas. Men olycksrekonstruktionsprocessen innehåller också några osäkerhetsfaktorer. Därför behövs en ökad förståelse av dessa osäkerheter i olycksrekonstruktionsprocessen för att kunna utveckla dagens humana FE-modeller. Så syftet med denna avhandling var att utvärdera och utveckla mer avancerade strategier för olycksrekonstruktioner av huvudskador. En FE-modell av huvudet användes för att studera huvudskador i olyckor. Existerande olycksdata av cykelolyckor användes samt hypotetiska olyckssituationer för fotgängare och cyklister. Dessutom skapades en FE modell av en hjälm med olika design för att studera hjälmens skyddande effekt. En objektiv metod baserad på ”Overlap Index” (mått på överlapp mellan modellsvar och medicinska bilder) och ”Location Index” (mått på skillnad i lokaliseringen av skadan) har utvecklats. Metoden utvärderades för tre cykelolycksrekonstruktioner. Metoden visade på potential att vara en objektiv metod för att jämföra resultat från FE-modeller med medicinska bilder och kan ses som ett steg närmare en objektiv utvärdering av skaderekonstruktioner. Simuleringarna med och utan hjälm visade på en skyddande effekt av hjälmen. Både påfrestningarna på hjärnvävnaden och skallbenet sjönk när en hjälm inkluderades i simuleringarna. Men resultaten visade också att påfrestningarna på hjärnvävnaden kan vara ännu mindre med andra hjälmdesigner. Två numeriska fotgängarmodeller jämfördes för att undersöka om den mer tidseffektiva stelkroppsmodellen kunde användas, istället för en FE-modell, för att grovt bestämma initialvillkoren för den mer tidskrävande och detaljerad FE-modell eftersom en olycksrekonstruktion innehåller flera osäkerheter. Resultatet visade att skillnaden i huvudets islagspunkt på fordonet, rotation och islagshastighet mellan modellerna var i samma storleksordning som andra initiala parametrar, som fordonshastighet. Nyckelord Huvudskador; Olycksrekonstruktion; Finita element analys; Skadeprevention; Hjälm; Cyklist; Fotgängare; Epidemiologi iii iv Dissertation List of Appended Papers Paper A Pedersen, K., Fahlstedt, M., Jacobsson, A., Kleiven, S., von Holst, H., A National Survey of Traumatic Brain Injuries Admitted to Hospital in Sweden from 1987 to 2010, Accepted for publication in Neuroepidemiology Paper B Fahlstedt, M., Depreitere, D., Halldin, P., vander Sloten, J., Kleiven, S., Correlation between Injury Pattern and Finite Element Analysis in Biomechanical Reconstructions of Traumatic Brain Injuries, Published in Journal of Biomechanics Paper C Fahlstedt, M., Halldin, P., Kleiven, S., The Protective Effect of Bicycle Helmets, Manuscript Paper D Fahlstedt, M., Halldin, P., Kleiven, S, Importance of the Bicycle Helmet Design and Material for the Outcome in Bicycle Accidents, in the Proceedings of the International Cycling Safety Conference 2014, 18-19 November 2014, Gothenburg, Sweden Paper E Fahlstedt, M., Halldin, P., Kleiven S., Comparison of MADYMO and Finite Element Human Body Models in Pedestrian Accidents with the Focus on Head Kinematics, Submitted to Traffic Injury Prevention Reprints were made with permission from the respective publishers. v Division of Work Paper A The study design was done by HvH. A.J generated the data from National Patient Register. M.F and HvH analyzed the data and wrote the manuscript. Paper B The study design was done by M.F and S.K. B.D and JvS contributed with the information concerning the accidents. M.F performed the simulations and was responsible for the analysis of the results and wrote the manuscript under supervision of P.H and S.K. All authors critically reviewed the manuscript. Paper C The study design was done by M.F, P.H and S.K. The helmet modeling and evaluation was performed by M.F and P.H. M.F performed the simulations, analyzed the results and wrote the manuscript under supervision of P.H and S.K. Paper D The study design was done by M.F and P.H. M.F performed the simulations, analyzed the results and wrote the manuscript under supervision of P.H and S.K. Paper E M.F. designed the study and performed the simulations. M.F analyzed the results and wrote the manuscript under supervision of P.H and S.K. vi Other Scientific Contributions Not Included in This Thesis Journal Article Hedman, L., Fahlstedt, M., Schlickum, M., Möller, H., von Holst, H., Felländer-Tsai, L., 2014. A Pilot Evaluation of an Educational Program that Offers Visualizations of Cervical Spine Injuries: Medical Students’ Self-efficacy Increases by Training. Informatics for Health and Social Care 39, 33- 46. Hedman, L., Fahlstedt, M., Schlickum, M., Möller, H., Halldin, P., von Holst, H., Felländer-Tsai, L., 2012. Training Diagnosis and Treatment of Cervical Spine Trauma Using a New Educational Program for Visualization through Imaging and Simulation (VIS): A First Evaluation by Medical Students. Studies in Health Technology and Informatics 173, 171-174. Courteille, O., Ho J., Fahlstedt, M., Fors, U., Felländer-Tsai, L., Hedman, L., Möller, H., 2013. Face Validity of VIS-Ed: A Visualization Program for Teaching Medical Students and Residents the Biomechanics of Cervical Spine Trauma, Studies in Health Technology and Informatics 184, 96-102. Conference Proceedings Fahlstedt, M., Baeck, K., Halldin, P., Vander Sloten, J., Goffin, J., Depreitere, B., Kleiven, S., 2012. Influence of Impact Velocity and Angle in a Detailed Reconstruction of a Bicycle Accident. International Research Council on Biomechanics of Injury (IRCOBI) Conference, Dublin, Ireland, pp. 787-799. Alvarez, V.S., Fahlstedt, M., Halldin, P., Kleiven S. Importance of Neck Muscle Tonus in Head Kinematics during Pedestrian Accidents. (2013). International Research Council on Biomechanics of Injury (IRCOBI) Conference, Gothenburg, Sweden, pp. 747-761 Conference Abstracts Fahlstedt, M., Halldin, P., 2010. Development of a Ligament Rupture Model Used in a Finite Element Model of the Neck, World Congress of Biomechanics, Singapore, p.305 Fahlstedt, M., Halldin, P., Ho, J., Kleiven, S., 2011. Visualisering av huvud- och nacktrauma med finita element metoden, Riksstämman, Stockholm, Sweden Fahlstedt, M., Halldin, P., Kleiven, S., 2015. Dagens cykelhjälmars skyddande effekt och dess förbättringspotential. Transportforum, Linköping, Sweden vii viii