A Thesis entitled The Effect of Head Restraint Material Properties, Initial Backset, and T1 Acceleration Magnitude on the Risk of Whiplash Injury: A Finite Element Study by Dhanvin Sunil Desai Submitted to the Graduate Faculty as partial fulfillment of the requirements for the Master of Science Degree in Bioengineering _________________________________________ Dr. Vijay K. Goel, Committee Chair _________________________________________ Dr. Anand K.Agarwal, Committee Member _________________________________________ Dr. Scott C. Molitor, Committee Member _________________________________________ Dr. Patricia R. Komuniechi, Dean College of Graduate Studies The University of Toledo August 2013 Copyright 2013, Dhanvin Sunil Desai This document is copyrighted material. Under copyright law, no parts of this document may be reproduced without the expressed permission of the author. An Abstract of The Effect of Head Restraint Material Properties, Initial Backset, and T1 Acceleration Magnitude on the Risk of Whiplash Injury: A Finite Element Study by Dhanvin Sunil Desai Submitted to the Graduate Faculty as partial fulfillment of the requirements for the Master of Science Degree in Bioengineering The University of Toledo August 2013 Whiplash sprains and/or strains occur in 28-53% of motor vehicle collision victims, making it the most common type of injury in these collisions. The costs annually in the United States of whiplash injury are approximately between 4.5 to 8 billion dollars. Whiplash can have long-term symptoms which can lead to chronic pain. Theories have linked the risk of whiplash injury to facet joints, ligaments, intervertebral discs, vertebral arteries, dorsal root ganglia, and neck muscles. Head restraints were invented in the 1960s to reduce spinal motion by limiting relative motion between the head and thorax. The effectiveness of headrests in reducing injury has been limited to only 13 to 18% reduction in neck injury claims. It was reported that 85% of all whiplash injuries occur during rear- end impacts. A detailed cervical model was created using a female CT scan. The scan was imported into Mimics. The model was meshed in IA-FEMesh and then imported into ABAQUS. The headrest model was created in SolidWorks and meshed in 3-Matic. All of the material properties were derived from literature. The bony structures were modeled as linear elastic material models and the discs and ligaments were modeled as non-linear models. This thesis aims to provide a detailed cervical spine finite element model and iii studies the effect of headrest material properties, initial headrest distance, and T1 acceleration magnitudes on the risk of whiplash injury. The initial goal was to validate the model under static and dynamic loading. Static validation was done in flexion/extension, lateral bending, and axial rotation and comparing the data with cadaver means and standard deviations. Dynamic loading was performed by providing an input acceleration pulse to T1 and comparing the segmental motion with the cadaver data corridors. A sled dummy test performed at The University of Toledo was used as input variables to the finite element model. An HIII 50th percentile dummy was used for the test. The Y and Z chest accelerations were used as input accelerations to the finite element model. An actuator arm was used to provide the force used to accelerate the sled. The dummy was seated on a standard seat, which was bolted to the sled. The sled was placed on a wheel system that sat on the sled frame. A brake system was used to provide stopping force. Four polyurethane material properties were derived from literature and were used as variables for the headrest material properties. The headrest was modeled as a hyperfoam material model in ABAQUS. The initial headrest distance was also varied from 0 mm, 5 mm, 10 mm and 20 mm. The final part was to vary the input T1 acceleration magnitude from 7.5 G, 9.8 G, 13.8 G, and 15 G. The risk of whiplash injury was quantified by measure lower cervical intra-discal pressures, lower cervical peak facet stresses, lower cervical ALL ligament strains, ALAR ligament strains, and lower cervical facet capsular ligament strains. These variables have been defined in literature as the iv most common sites for cervical spine injury during whiplash. The risk of head trauma was also quantified by measuring the peak head stress during impact. Results showed a stiffer headrest material decreased the risk of cervical soft tissue injury while increasing the risk of head trauma. As the initial backset distance increased, the risk of cervical soft tissue injury increased and the risk of head trauma increased. As the T1 acceleration magnitude increased, the risk of cervical soft tissue injury increased and the risk of head trauma increased. It can be concluded that these three factors play a role in the risk of whiplash injury. It can also be concluded that a less stiff material should not be used if the goal is to reduce cervical soft tissue injury. The ideal way to reduce the risk of injury would be to make sure the initial headrest distance away from the head is as minimal as possible. v I would like to dedicate this work to my family and friends. Thank you. vi Acknowledgements I would like to acknowledge my advisor Dr. Vijay Goel. His knowledge, experience and guidance have allowed me to reach this point. I would also like to thank my committee members, Dr. Anand Agarwal and Dr. Scott Molitor for their support and mentorship. I would also like to thank Dr. Deniz Erbulut for his assistance with this project. I would like to thank the University of Toledo, Department of Bioengineering, and Koc University for funding this project. I would like to thank all of my lab members, specifically Vivek Palepu for his continued support throughout this project. I would also like to thank Aakash Agarwal, Vikas Kaul and Ata Kiapour for their continued assistance on this project. vii Table of Contents Abstract .............................................................................................................................. iii Acknowledgements ........................................................................................................... vii Table of Contents ............................................................................................................. viii List of Tables ................................................................................................................... xi List of Figures ................................................................................................................... xii List of Abbreviations .........................................................................................................xx List of Symbols ................................................................................................................ xxi 1 Introduction ..........................................................................................................1 1.1 Overview ..........................................................................................................1 2 Literature Review.....................................................................................................6 2.1 Overview ..........................................................................................................6 2.2 Individual Structures ..........................................................................................7 2.2.1 Body ....................................................................................................7 2.2.2 Pedicles ...............................................................................................7 2.2.3 Processes .............................................................................................7 2.2.4 The Laminae .......................................................................................8 2.3 Anatomy of the Cervical Spine ..........................................................................8 2.3.1 Common Sections (C3-C6) .................................................................8 viii 2.3.2 Vertebral Section – C1 ......................................................................10 2.3.3 Vertebral Section – C2 ......................................................................12 2.3.4 Vertebral Section – C7 ......................................................................14 2.4 Current Head-Restraint Effectiveness ..............................................................14 2.5 Head-Restraint Safety Standards .....................................................................15 2.6 Active Head-Restraint Systems .......................................................................16 2.7 Whiplash Injury ...............................................................................................17 2.8 Types of Whiplash Injury ................................................................................18 2.9 Whiplash Statistics and Cost ............................................................................19 2.10 Computer Modeling of Whiplash ..................................................................20 2.10.1 Multi-Body Modeling .....................................................................21 2.10.2 Finite Element Modeling ................................................................24 2.11 Human Modeling and Testing .......................................................................24 2.12 In Vivo Testing ..............................................................................................25