Development and Validation of a Generic Finite Element Ribcage to be used for Strain-based Fracture Prediction

Finite element human body models, comprising detailed anatomical descriptions, can complementanthropomorphic test devices (ATDs) in the development of new restraint systems. Human body models (HBMs)can evaluate injury on tissue level, i.e. rib strain can be used to evaluate the risk of rib fracture, although theHBM must accurately predict the rib strain distribution to be effective. Current HBMs are not validated for ribstrain, and it remains unknown if any represent an average-shaped ribcage. Thus, a new generic ribcage wascreated, representing an average male, based on a combination of averaged geometrical and material data fromin-vivo and in-vitro datasets. The ribcage was incorporated into the THUMS AM50 Version 3, resulting in theSAFER HBM Version 9. Validation of ribcage kinetic, kinematics and strain distribution was carried out at threelevels of complexity: anterior-posterior rib bending tests; rigid impactor table-top test; and a 40 km/h frontalsled test. The rib strains in the single rib load case were predicted within \ub1 one standard deviation for 91% of themeasuring points. The biofidelity for the rib strains in the table-top and sled test load cases was deemed ‘fair’using CORA analysis. This study is an important step in the development and validation process of strain-basedrib fracture criteria for HBMs

Generic finite element models of human ribs, developed and validated for stiffness and strain prediction - To be used in rib fracture risk evaluation for the human population in vehicle crashes

To enable analysis of the risk of occupants sustaining rib fractures in a crash, generic finite element models of human ribs, one through twelve, were developed. The generic ribs representing an average sized male, were created based on data from several sources and publications. The generic ribs were validated for stiffness and strain predictions in anterior-posterior bending. Essentially, both predicted rib stiffness and rib strain, measured at six locations, were within one standard deviation of the average result in the physical tests. These generic finite elements ribs are suitable for strain-based rib fracture risk predictions, when loaded in anterior-posterior bending. To ensure that human variability is accounted for in future studies, a rib parametric study was conducted. This study shows that the rib cross-sectional height, i.e., the smallest of the cross-sectional dimensions, accounted for most of the strain variance during anterior-posterior loading of the ribs. Therefore, for future rib fracture risk predictions with morphed models of the human thorax, it is important to accurately address rib cross-sectional height

Predicting pelvis geometry using a morphometric model with overall anthropometric variables

Pelvic fractures have been identified as the second most common AIS2+ injury in motor vehicle crashes, with the highest early mortality rate compared to other orthopaedic injuries. Further, the risk is associated with occupant sex, age, stature and body mass index (BMI). In this study, clinical pelvic CT scans from 132 adults (75 females, 57 males) were extracted from a patient database. The population shape variance in pelvis bone geometry was studied by Sparse Principal Component Analysis (SPCA) and a morphometric model was developed by multi- variate linear regression using overall anthropometric variables (sex, age, stature, BMI). In the analysis, SPCA identified 15 principal components (PCs) describing 83.6% of the shape variations. Eight of these were signifi- cantly captured (α < 0.05) by the morphometric model, which predicted 29% of the total variance in pelvis geometry. The overall anthropometric variables were significantly related to geometrical features primarily in the inferior-anterior regions while being unable to significantly capture local sacrum features, shape and position of ASIS and lateral tilt of the iliac wings. In conclusion, a new detailed morphometric model of the pelvis bone demonstrated that overall anthropometric variables account for only 29% of the variance in pelvis geometry. Furthermore, variations in the superior-anterior region of the pelvis, with which the lap belt is intended to interact, were not captured. Depending on the scenario, shape variations not captured by overall anthropometry could have important implications for injury prediction in traffic safety analysis

Influences of human thorax variability on population rib fracture risk prediction using human body models

Rib fractures remain a common injury for vehicle occupants in crashes. The risk of a human sustaining rib fractures from thorax loading is highly variable, potentially due to a variability in individual factors such as material properties and geometry of the ribs and ribcage. Human body models (HBMs) with a detailed ribcage can be used as occupant substitutes to aid in the prediction of rib injury risk at the tissue level in crash analysis. To improve this capability, model parametrization can be used to represent human variability in simulation studies. The aim of this study was to identify the variations in the physical properties of the human thorax that have the most influence on rib fracture risk for the population of vehicle occupants. A total of 15 different geometrical and material factors, sourced from published literature, were varied in a parametrized SAFER HBM. Parametric sensitivity analyses were conducted for two crash configurations, frontal and near-side impacts. The results show that variability in rib cortical bone thickness, rib cortical bone material properties, and rib cross-sectional width had the greatest influence on the risk for an occupant to sustain two or more fractured ribs in both impacts. Therefore, it is recommended that these three parameters be included in rib fracture risk analysis with HBMs for the population of vehicle occupants

Numerical Reproducibility of Human Body Model Crash Simulations

The numerical reproducibility of a Finite Element (FE) Human Body Model (HBM) was evaluated by quantifying the variation in model predictions for diverse computer systems at different sites and settings. Repeated simulations, with varying number of Central Processing Unit (CPU) cores and model decomposition, of four high severity load cases – a full frontal, near-side frontal oblique and side impact with a full set of driver restraints, as well as a full frontal with a seat belt only restraint – was carried out on five computer systems. HBM responses were found to vary randomly with the Number of CPU cores (NCPU), but not due to different hardware or message parsing interface software at each computer system used. Implemented HBM updates reduced the variation in the near-side frontal oblique load case. When the NCPU used was fixed, identical results were obtained from all computer systems. This means the variation of HBM responses is due to the model decomposition. It is possible to quantify the numerical reproducibility of an FE HBM by repeated simulations, varying the NCPU and analyzing the coefficient of variation of the responses

Evaluation of a diverse population of morphed human body models for prediction of vehicle occupant crash kinematics

Morphing can be used to alter human body models (HBMs) to represent a diverse population of occupants in car crashes. The mid-sized male SAFER HBM v9 was parametrically morphed to match 22 Post Mortem Human Subjects, loaded in different configurations. Kinetics and kinematics were compared for the morphed and baseline HBMs. In frontal impacts, the morphed HBMs correlated closer with the kinematics of obese subjects, but lower to small females. In lateral impacts HBM responses were too stiff. This study outlines a necessary evaluation of all HBMs that should be morphed to represent the diverse population in vehicle safety evaluations

A numerical study on the safety belt-to-pelvis interaction

The slide of the lap belt over the iliac crest of the pelvis during vehicle frontal crashes can substantially increase the risk of some occupant injuries. A multitude of factors, related to occupants or the design of belt, are associated with this phenomenon. This study investigates safety belt-to-pelvis interaction and identifies the most influential parameters. It also explores how initial lap belt position influences the interaction between lap belt and pelvis. A finite element model of the interaction between lap belt with pelvis through a soft tissue part was created. Belt angle, belt force, belt loading rate and belt-to-body friction as belt design parameters, and pelvis angle, constitute parameters of soft tissue, and soft tissue-to-pelvis friction as occupant parameters were inspected. For the soft tissue part, subcutaneous adipose tissue with different thicknesses was created and the effect initial lap belt position may have on lap belt-to-pelvis interaction was investigated. The influential parameters have been identified as: the belt angle and belt force as belt design parameters and the pelvis angle and compressibility of soft tissue as occupant parameters. The risk for the slide of lap belt over the iliac crest of the pelvis was predicted higher as the initial lap belt positions goes superior to the pelvis. Of different submarining parameters, the lap belt angle represents the most influential one. The lap belt-to-pelvis interaction is influenced by the thickness of subcutaneous adipose tissue between lap belt and pelvis indicating a higher risk for obese occupants

Rib Cortical Bone Fracture Risk as a Function of Age and Rib Strain: Updated Injury Prediction Using Finite Element Human Body Models

To evaluate vehicle occupant injury risk, finite element human body models (HBMs) can be used in vehicle crash simulations. HBMs can predict tissue loading levels, and the risk for fracture can be estimated based on a tissue-based risk curve. A probabilistic framework utilizing an age-adjusted rib strain-based risk function was proposed in 2012. However, the risk function was based on tests from only twelve human subjects. Further, the age adjustment was based on previous literature postulating a 5.1% decrease in failure strain for femur bone material per decade of aging. The primary aim of this study was to develop a new strain-based rib fracture risk function using material test data spanning a wide range of ages. A second aim was to update the probabilistic framework with the new risk function and compare the probabilistic risk predictions from HBM simulations to both previous HBM probabilistic risk predictions and to approximate real-world rib fracture outcomes. Tensile test data of human rib cortical bone from 58 individuals spanning 17–99 years of ages was used. Survival analysis with accelerated failure time was used to model the failure strain and age-dependent decrease for the tissue-based risk function. Stochastic HBM simulations with varied impact conditions and restraint system settings were performed and probabilistic rib fracture risks were calculated. In the resulting fracture risk function, sex was not a significant covariate—but a stronger age-dependent decrease than previously assumed for human rib cortical bone was evident, corresponding to a 12% decrease in failure strain per decade of aging. The main effect of this difference is a lowered risk prediction for younger individuals than that predicted in previous risk functions. For the stochastic analysis, the previous risk curve overestimated the approximate real-world rib fracture risk for 30-year-old occupants; the new risk function reduces the overestimation. Moreover, the new function can be used as a direct replacement of the previous one within the 2012 probabilistic framework

Implementation and Calibration of Active Reflexive Cervical Muscles on Female Head-Neck Model

Problem outlineViVA OpenHBM is an open source human body model that represents the 50th percentile female population for assessing whiplash protection systems in car. ViVA OpenHBM was developed with intention to fill the gap of current available HBMs which excluded the average female size although injury statistics since 1960s have shown that females have three times higher risk to sustain whiplash injury compared to males. In this study, the current model is being enhanced by implementing active muscles as previous studies have shown that cervical muscles could alter the head and neck kinematicsof the occupant during low-speed rear- crashes.Study objectivesThe first goal of this study was to implement a Proportional Integral Derivative (PID) feedback control mechanism adding to the Finite Element models of cervical muscles. The second goal was to calibrate the PID control gains by conducting an optimization-based parameter identification with publishedvolunteer data and to analyze the effects of three calibration objectives to the head and cervical kinematics of the model.MethodologyThe VIVA OpenHBM head-neck model, previously validated to PMHS data, was used. To represent the 34 cervical muscles, 129 beam elements with Hill-type material models were implemented. A closedloop control strategy was applied to activate these muscles mimicking the human body’s vestibularsystem. Calibration studies of head and cervical spine kinematics were conducted by comparing the model against published-volunteer responses to identify reasonable gain values for the controller. Three different calibrations were conducted with three different objectives: head kinematics in linear and angular direction, head and cervical spine kinematics in angular direction, and head and cervical spine kinematics in linear and angular direction.Results and ConclusionThe simulation results show that the reflexive feedback control was numerically stable and able to control model muscle’s activation. Gain values of the implemented muscle control strategies were able to be identified from calibration simulations. Muscle activation changed the head kinematics by reducingpeak linear and angular displacements, as compared to the model without muscle activation. The agreement of specific kinematic variables such as head kinematic and cervical spine angular displacement was dependent on the controller calibration objectives. Best agreement of head\ua0 kinematics was observed in the model that calibrated against only volunteer head kinematics. However, in the vertical and angular direction there was a discrepancy of head response caused by anteriorposterior buckling of the cervical spine. In the model that was calibrated against head and cervical spine in angular direction, less contraction of cervical muscles was observed. As the result, good agreement was obtained in the cervical spine angular kinematics but not in the head kinematics. The best agreement was obtained by the model that calibrated against both linear and angular displacement of volunteer head and cervical spine kinematics although reduced the agreement of head kinematics compared to the model that was calibrated against only volunteer head kinematics. This was because of different calibration objectives that opposing each other

Multi-Scale Validation of a Rib Fracture Prediction Method for Human Body Models

A multi-scale validation of the capability of the SAFER human body model (v9) to predict the risk for an occupant to sustain two or more rib fractures in vehicle crashes was carried out. The rib fracture risk was evaluated by means of a probabilistic rib fracture prediction method. A variety of loading conditions was evaluated, from published lab tests with post mortem human subjects (PMHS) to detailed accident reconstructions and population-based reconstructions. The PMHS load cases were table-top, impactor and sled tests. The detailed accident reconstructions included 20 occupants involved in real-world crashes. For the population-based reconstructions more than 100 simulations with a generic vehicle interior model were carried out. Parameters regarding both the generic model and the occupant were varied in the population-based simulations. The predicted risk for an occupant to sustain two or more rib fractures was evaluated for the PMHS sled reconstructions as well as for the detailed and population-based reconstructions. The predicted 2 or more rib fracture risk was compared to the actual number of fractured ribs sustained by the PMHS and the occupants. Generally, two or more fractured ribs observed in the PMHS tests, the vehicle crashes and NASS data were successfully predicted with the model