The Virginia Tech – Wake Forest University Center for Injury Biomechanics (CIB) has completed 10 years of research in the fields of automobile safety, military restraints, and sports biomechanics. The CIB has grown to include 74 researchers lead by 10 faculty and 48 staff and student researchers. These researchers examine human tolerance across a range of dynamic loading environments for all body regions. The CIB is the largest university based injury biomechanics research group in the world. It includes five separate laboratory facilities with over 40,000 sq.ft. of dedicated research space. The faculty and staff have published nearly 300 journal and refereed conference papers in 10 general research areas. The objective of this paper is to summarize briefly the CIB research capabilities and accomplishments.

Virginia Tech-Wake Forest University Center for Injury Biomechanics 10 Year History

The research presented in this thesis investigates eye injuries caused by blunt impacts and blast overpressure. This research represents part of an ongoing investigation to accurately quantify and predict eye injuries and injury mechanisms for various loading schemes. It has been shown that blunt trauma can cause severe eye injuries but it remains undecided whether blast overpressure alone can cause eye injury. Presented herein are four experimental studies that quantify eye injuries and implement a technique for predicting injury risk. Isolated porcine or human eyes were subjected to various loading conditions consisting of blunt projectiles, water streams, remote control helicopter blades, and blast overpressure. All eyes were prepared in a similar manner that required the insertion of a miniature pressure sensor into the globe through the optic nerve. This sensor measured intraocular pressure throughout each event. Using previously published injury risk curves, this intraocular pressure data was used to predict the injury risk for four eye injuries: hyphema, lens damage, retinal damage, and globe rupture. Injuries sustained were quantified upon direct inspection of the globe following testing. No serious eye injuries were observed for any of the tests and all tests resulted in low predicted injury risks consistent with the lack of observed injury. The research presented in this thesis provides a robust low injury level dataset for eye injuries. This data could be useful for designing and validating computational models and anthropomorphic test device eyes, and serves as a basis for future work with more dangerous projectiles and higher pressure levels.

Injury Biomechanics of the Human Eye During Blunt and Blast Loading

The current study focuses on the development of a finite element model of the hip capsule ligament to represent the joint between the femur and the pelvis. The finite element model of the hip capsule is represented as comprising of eight sectors, consisting of the important ligaments in those regions of the capsule. The geometric parameters for the capsule are determined from available literature. A soft tissue material model is used to represent the nonlinear behavior of the capsule ligaments. Available hip distraction test data is used to validate the finite element model. A parametric study is conducted with the isolated capsule sector finite element model to understand the effects of different material parameters on the tensile load-deflection response of the capsule. The material properties for each sector of the hip capsule are systematically estimated to match the corresponding values from isolated capsule sector tensile tests reported in the literature. A verification exercise for the sector hip capsule material properties is carried out by considering a setup comprising an integrated pelvis-hipcapsule-femur finite element model and comparing with data available from hip distraction tests in the literature. The stiffness of the hip capsule finite element model is found to lie within the reported test corridor from hip distraction tests. The developed hip capsule finite element model will be of use in examining the stresses and strains in the joint ligaments. INTRODUCTION The pelvis is one of several human body regions that are susceptible to injury in the event of an automobile impact [Teresinski, 2001, Aekbote et al., 2003, Kuppa and Fessahaie, 2003, Rupp and Schneider, 2004, Sochor and Rupp, 2005]. The simulation and prediction of such injuries requires the use of detailed computer models. One of the challenges of developing such computer models is in the accurate representation of various human body joints in these models. Several contemporary computer models (e.g. Anthropometric Test Device (ATD) Models, ESI's H-Model [Choi et al., 1999], the GM-UVA Human Body Model [Deng, 2007]), intended to simulate the human body in an automobile impact scenario, represent the connection between the pelvis and the femur head by a simplified rigid kinematic joint, with appropriate mechanical properties (joint stiffness in various directions). Such a representation limits injury prediction in the pelvis region on account of the requirement of typical finite element software like LS-DYNA [LS-Dyna, 2006] for the connecting bodies (ilium and femoral head) to be rigid. A natural progression towards accurate representation of the hip capsule is through 1-D elements. Vezin et al. [Vezin and Verriest, 2005] idealized the hip capsular ligament as a set of 1-D elements in the HUMOS model. While this constituted a step forward from the idealized rigid kinematic joint, this model could not be used to perform detailed stress analysis in the capsule region, or to predict phenomena like capsule laxity, ligament tear, etc. [Teresinski, 2001, Philippon and Schenker, 2005]. Stewart and co-workers [Stewart et al., 2004] developed a model of the hip capsular ligament with solid elements and experimentally obtained tissue material properties for simulating total hip arthroplasty (THA) applications. One advantage of representing the hip capsule in the model as soft tissue is the more accurate capture of the load transfer mechanism between the pelvis and the lower extremity in comparison to rigid kinematic joints or 1-D beam representations. This could turn out to be significant for a dynamic load case like pedestrian impact. The hip joint is essentially a ball and socket joint wherein the ligaments and the contact interface between the acetabulum and the femur head constrain the femur head (ball) from slipping out of the acetabulum (socket). The hip capsule is an important part of the hip joint that allows for joint articulation and provides stability. The hip capsule is strengthened by three important ligaments, namely the iliofemoral ligament, the ischiofemoral ligament and the pubo-capsular ligament [Gray and Carter, 1995]. The elastic behavior of the ligament soft tissue allows for flexion, abduction and internal rotation of the lower extremities about the hip joint. Several studies [Wingstrand and Wingstrand, 1997, Scifert et al., 1999] have been carried out to understand hip joint biomechanics, but these have been focused more on sport related injuries and total hip arthroplasty (THA). To date, the availability of hip capsule response data in a dynamic impact scenario is very limited. The availability of relevant material data comprises one of the most significant prerequisites for building accurate finite element (FE models). Until recently, not much data has been published regarding mechanical properties of the hip capsule. Hewitt et al., 2000 performed the first set of tests to understand the material behavior of the important ligaments in the hip capsule. They carried out tensile tests on cadaveric ligament specimens to failure. Several important properties like stress and strain at failure, toe-region and linear region elastic modulus were documented. They also reported the variation of cross -sectional area of the ligaments along the fiber direction (from the acetabulum side to the femoral side). Their study highlighted the mechanical heterogeneity of the hip capsule. It was observed in their tests that the iliofemoral ligament (anterior region of the capsule) was stiffer than the ischiofemoral ligament (posterior region of capsule). However these tests were limited only to key capsule ligaments like the ischiofemoral, iliofemoral and pubofemoral ligaments. This study was also performed at low rates to avoid visco-elastic effects. Stewart et al., 2002 performed more detailed tests on the whole hip capsule instead of a few important hip capsule ligaments. They reported regional material and geometrical properties of the whole human hip capsule ligament. Distraction tests were performed on cadaveric specimens to calculate the stiffness of the whole hip capsule ligament. After each test the whole capsule was cut into several sectors and subjected to tensile load until failure to calculate the stiffness of each sector of the capsule. The sectors were carefully cut so as to not cut across ligament fibers. This study reported several mechanical properties of the capsule like stiffness and failure load of each capsule sector, typical load-deflection curves for the whole capsule and individual sectors for distraction and tensile tests respectively. They also performed tests at various distraction rates but concluded that strain rate effects were insignificant for their load case. However, other studies [Kemper et al., 2007] have reported strain rate effects in testing the iliofemoral and ischiofemoral ligaments. Stewart et al., 2004 used some of their own test data [Stewart et al., 2002] to build finite element models to simulate total hip dislocation. They reported significant improvement in the FE model joint behavior. The focus of the current work is to develop a hip capsule ligament model similar to the one developed in [Stewart et al., 2004], but for eventual application to occupant and pedestrian impact scenarios. The current study focuses on the development of a finite element model of the hip capsule ligaments for a human body model representative of a 50th percentile male. This model is further used to connect the femur to the pelvis. A recently developed material model [LS-DYNA, 2006] for soft tissue, which takes into account viscoelastic effects as well as tissue fiber direction, is used for the ligament model. As a first step, a validated FE model was developed to simulate the individual capsule sector tensile test. Parametric studies were conducted using this model to identify the most important tissue material parameters which influence the capsule stiffness. Subsequently, the material parameters of each sector were estimated. The second step involved the development of a FE model to simulate a hip distraction test. The sectoral material properties obtained in the first step were used to develop a whole hip capsule model to simulate hip distraction test. These FE results verified the estimated material properties of individual capsule sector in the previous step. The integrated hip capsule FE model was further used to conduct a brief parametric study by varying capsule material model parameters. This study provides a glimpse of the influence of material model parameters on overall load-deflection behavior of the hip capsule. METHODS Hip capsule geometry The hip capsule is a dense fibrous tissue with the shape of a cylindrical sleeve. It spans the circumference of the acetabulum rim along the medial end. The other end of the capsule surrounds the neck of the femur. It is attached anteriorly, to the intertrochanteric line, posteriorly to the lower part of the neck closer to lesser trochanter, superiorly and inferiorly to the base of the femur neck [Gray and Carter, 1995]. The thickness of the tissue varies both along the circumference and length of the capsule. The fiber directions vary in different parts of the capsule. There are three dominant dense regions which are identified as the iliofemoral ligament, the ischiofemoral ligament and the pubofemoral ligament. It is very difficult to identify the distinct boundaries of these ligaments and therefore it is better to model the ligaments as a single unified capsule model. The iliofemoral ligament part of the capsule is the thickest and has its fibers run longitudinally. The anterior region is the thinnest and weakest part of the capsule. Stewart et al., 2002 have reported circumferential variation of capsule geometric parameters (length, thickness). They have dissected each of ten cadaveric hip joint specimens into eight sectors and reported the range of these geometr

Development of human hip capsule finite element model

The object of this study was to develop a quasi-linear viscoelastic model for the iliofemoral and ischiofemoral hip ligaments. In order to accomplish this, a total of 56 axial tension tests were performed on 8 bone-ligament-bone specimens prepared from 4 fresh frozen male cadavers. Each specimen went through a battery of 7 tests including a series of step-and-hold tests and load-and-unload ramp tests. The bone-ligament-bone specimens were situated so that the load from a servo-hydraulic Material Testing System would be applied on the long axis of each ligament. The reduced relaxation data was fit to a two exponential damping function while the instantaneous elastic response was fit to a power-law function. These two constituents were then combined to create a single constitutive equation for each ligament. The quasi-linear viscoelastic model presented in this study can be used to improve the biofidelity of computational models of the human hip.

Quasi-linear viscoelastic characterization of human hip ligaments

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Quasi-linear viscoelastic characterization of human hip ligaments