Pedestrian lower limb injury criteria evaluation: a finite element approach

A detailed finite element model of a mid-sized male for the investigation of traffic pedestrian accidents

The pedestrian is one of the most vulnerable road users and comprises approximately 23% of the road crash-related fatalities in the world. To protect pedestrians during Car-to-Pedestrian Collisions (CPC), subsystem impact tests are used in regulations. These tests provide insight but cannot characterize the complex vehicle-pedestrian interaction. The main purpose of this study was to develop and validate a detailed pedestrian Finite Element (FE) model corresponding to a 50th percentile male to predict CPC induced injuries. The model geometry was reconstructed using a multi-modality protocol from medical images and exterior scan data corresponding to a mid-sized male volunteer. To investigate injury response, this model included internal organs, muscles and vessels. The lower extremity, shoulder and upper body of the model were validated against Post Mortem Human Surrogate (PMHS) test data in valgus bending, and lateral/anterior-lateral blunt impacts, respectively. The whole-body pedestrian model was validated in CPC simulations using a mid-sized sedan and simplified generic vehicles bucks and previously unpublished PMHS coronal knee angle data. In the component validations, the responses of the FE model were mostly within PMHS test corridors and in whole body validations the kinematic and injury responses predicted by the model showed similar trends to PMHS test data. Overall, the detailed model showed higher biofidelity, especially in the upper body regions, compared to a previously reported simplified pedestrian model, which recommends using it in future pedestrian automotive safety research.

Development and validation of an optimized finite element model of the human orbit

INTRODUCTION

The authors' main purpose was to develop a detailed finite element model (FEM) of the human orbit and to validate it by analyzing its behavior under the stress of blunt traumas.

MATERIALS AND METHODS

A pre-existing 3D FEM of a human head was modified and used in this study. Modifications took into account preliminary research carried out on PubMed database. Data from a CT scan of the head were computed with Mimics^® software to re-create the skull geometry. The mesh production, the model's properties and the simulations of blunt orbital traumas were conducted on Hyperworks^® software.

RESULTS

The resulting 3D FEM was composed of 640 000 elements and was used to perform blunt trauma simulations on an intact orbit. A total of 27 tests were simulated. Fifteen tests were realized with a metallic cylinder impactor; 12 tests simulated a hit by a closed fist. In all the tests conducted (27/27), the orbital floor was fractured. Fracture patterns were similar to those found in real clinical situations according to the buckling and hydraulic theories of orbital floor fractures.

DISCUSSION

The similitude between the fracture patterns produced on the model and those observed in vivo allows for a validation of the model. This model constitutes, at the authors knowledge, the most sophisticated one ever developed.

Analysis of the influence of passenger vehicles front-end design on pedestrian lower extremity injuries by means of the LLMS model

OBJECTIVE

This work aims at investigating the influence of some front-end design parameters of a passenger vehicle on the behavior and damage occurring in the human lower limbs when impacted in an accident.

METHODS

The analysis is carried out by means of finite element analysis using a generic car model for the vehicle and the lower limbs model for safety (LLMS) for the purpose of pedestrian safety. Considering the pedestrian standardized impact procedure (as in the 2003/12/EC Directive), a parametric analysis, through a design of experiments plan, was performed. Various material properties, bumper thickness, position of the higher and lower bumper beams, and position of pedestrian, were made variable in order to identify how they influence the injury occurrence. The injury prediction was evaluated from the knee lateral flexion, ligament elongation, and state of stress in the bone structure.

RESULTS

The results highlighted that the offset between the higher and lower bumper beams is the most influential parameter affecting the knee ligament response. The influence is smaller or absent considering the other responses and the other considered parameters. The stiffness characteristics of the bumper are, instead, more notable on the tibia. Even if an optimal value of the variables could not be identified trends were detected, with the potential of indicating strategies for improvement.

CONCLUSIONS

The behavior of a vehicle front end in the impact against a pedestrian can be improved optimizing its design. The work indicates potential strategies for improvement. In this work, each parameter was changed independently one at a time; in future works, the interaction between the design parameters could be also investigated. Moreover, a similar parametric analysis can be carried out using a standard mechanical legform model in order to understand potential diversities or correlations between standard tools and human models.

A Lower Limb-Pelvis Finite Element Model with 3D Active Muscles

A lower limb-pelvis finite element (FE) model with active three-dimensional (3D) muscles was developed in this study for biomechanical analysis of human body. The model geometry was mainly reconstructed from a male volunteer close to the anthropometry of a 50th percentile Chinese male. Tissue materials and structural features were established based on the literature and new implemented experimental tests. In particular, the muscle was modeled with a combination of truss and hexahedral elements to define its passive and active properties as well as to follow the detailed anatomy structure. Both passive and active properties of the model were validated against the experiments of Post-Mortem Human Surrogate (PMHS) and volunteers, respectively. The model was then used to simulate driver's emergency braking during frontal crashes and investigate Knee-Thigh-Hip (KTH) injury mechanisms and tolerances of the human body. A significant force and bending moment variance was noted for the driver's femur due to the effects of active muscle forces during emergency braking. In summary, the present lower limb-pelvis model can be applied in various research fields to support expensive and complex physical tests or corresponding device design.

INVESTIGATION ON THE KNEE STIFFNESS UNDER THE LATERAL IMPACT FOR THE DESIGN OF PEDESTRIAN LEGFORM IMPACTOR

The objective of the present study is to investigate the stiffness of the knee joint, in order to provide a basis for developing a biofidelic pedestrian legform impactor. A biofidelic lower limb model was employed to replicate structural responses of the human knee joint by finite element simulations. In the simulation, a single displacement was imposed on the thigh or leg, and constrained six freedoms of the other part. As a result, nonlinear load-displacement data were approximated by a linear regression to determine the stiffness. Considering knee joint kinematics under lateral car-pedestrian impacts, a stiffness matrix was established including lateral bending, lateral shearing and torsion effects that significantly influence pedestrian lower limb injuries. Then, this stiffness matrix was applied in a developed legform model. The structural responses of the legform were obtained by the impact with a family car model. Finally, the legform biofidelity was evaluated by comparing the global kinematics of the pedestrian lower limb.

Coupling lateral bending and shearing mechanisms to define knee injury criteria for pedestrian safety

OBJECTIVE

In car-pedestrian accidents, lateral bending and shearing kinematics have been identified as principal injury mechanisms causing permanent disabilities and impairments to the knee joint. Regarding the combined lateral bending and shearing contributions of knee joint kinematics, developing a coupled knee injury criterion is necessary for improving vehicle countermeasures to mitigate pedestrian knee injuries.

METHODS

The advantages of both experimental tests and finite element (FE) simulations were combined to determine the reliable injury tolerances of the knee joint. First, 7 isolated lower limb tests from postmortem human subjects (PMHS) were reported, with dynamic loading at a velocity of 20 km/h. With the intention of replicating relevant injury mechanisms of vehicle-pedestrian impacts, the experimental tests were categorized into 3 groups by the impact locations on the tibia: the distal end to prioritize pure bending, the middle diaphysis to have combined bending and shearing effects, and the proximal end to acquire pure shearing. Then, the corresponding FE model was employed to provide an additional way to determine exact injury occurrences and develop a robust knee injury criterion by the variation in both the lateral bending and shearing contributions through a sensitivity analysis of impact locations.

RESULTS

Considering the experimental test results and the subsequent sensitivity analysis of FE simulations, both the tolerances and patterns of knee joint injuries were determined to be influenced by impact locations due to various combined contributions of lateral bending and shearing. Both medial collateral ligament and cruciate ligament failures were noted as the onsets of knee injuries, namely, initial injuries. Finally, a new injury criterion categorized by initial injury patterns of knee joint was proposed by coupling lateral bending and shearing levels.

CONCLUSIONS

The developed injury criterion correlated the combined joint kinematics to initial knee injuries based on subsegment tests and FE simulations conducted with a biofidelic lower limb model. This provides a valuable way of predicting the risk of knee injury associated with vehicle-pedestrian crashes and thereby represents a further step to promote the design of vehicle countermeasures for pedestrian safety.

Injury tolerance of tibia for the car-pedestrian impact

Lower limbs are normally the first contacted body region during car-pedestrian accidents, and easily suffer serious injuries. The previous tibia bending tolerances for pedestrian safety were mainly developed from three-point bending tests on tibia mid-shaft. The tibia tolerances of other locations are still not investigated enough. In addition, tibia loading condition under the car-pedestrian impact should be explored to compare with the three-point bending. This work aims to investigate the injury tolerance of tibia fracture with combined experimental data and numerical simulation. Eleven new reported quasi-static bending tests of tibia mid-shaft, and additional eleven dynamic mid-shaft bending test results in the previous literature were used to define injury risk functions. Furthermore, to investigate the influence of tibia locations on bending tolerance, finite element simulations with lower limb model were implemented according to three-point bending and pedestrian impact conditions. The regressive curve of tibia bending tolerance was obtained from the simulations on the different impact locations, and indicated that tibia fracture tolerance could vary largely due to the impact locations for the car-pedestrian crash.

Posture and muscular behaviour in emergency braking: an experimental approach

In the field of numerical crash simulations in road safety research, there is a need to accurately define the initial conditions of a frontal impact for the car occupant. In particular, human models used to simulate such impacts barely take into account muscular contracting effects. This study aims to quantify drivers' behaviour in terms of posture and muscular activity just before a frontal impact. Experiments on volunteers were performed in order to define these conditions, both on a driving simulator and on a real moving car. Brake pedal loads, lower limbs kinematics and muscle activation were recorded. Coupling instantaneous data from both experimental protocols (simulator versus Real car), a standard emergency braking configuration could be defined as (1) joint flexion angles of 96 degrees, 56 degrees and 13 degrees for the right hip, knee and ankle respectively; (2) a maximum brake pedal load of 780N; (3) a muscular activation of 55% for the anterior thigh, 26% for the posterior thigh, 18% for the anterior leg and 43% for the posterior leg. The first application of this research is the implementation of muscle tone in human models designed to evaluate new safety systems.

A new approach to multibody model development: pedestrian lower extremity

OBJECTIVE

The goal of this study was to develop a mathematical model of the 50th percentile male lower extremity capable of predicting injury risk and simulating the kinetic and kinematic response of the pedestrian lower extremity under vehicle impact loading.

METHODS

The hip-to-foot multibody model was developed for the MADYMO software platform using exterior and interior geometry and inertial properties from a detailed finite element model (FEM) of the human lower extremity and stiffness and failure tolerance data from the literature. The leg and thigh models' structural and contact parameters were simultaneously optimized to validate model response in simulations replicating previous dynamic bending experiments. The aggregate model's full-scale kinematic response was verified by comparing 3-D local (knee bending angles) and global (linear accelerations and velocities) frame leg and thigh kinematics from vehicle impact simulations with data generated from seven vehicle-pedestrian (PMHS) impact experiments.

RESULTS

By optimizing contact and structural response variables, the applied moment vs. deflection response of the leg and thigh showed excellent correlation with the experimental corridor averages in component-level bending simulations. The full-scale kinematic response of the 50th percentile male model showed good correlation with the PMHS response data in both the rate of valgus knee bending (approximately 3 degress/ms) and in the timing and magnitude of the peak thigh and leg accelerations (250 g and 400 g). Additionally, as a result of vehicle interaction, both the model and the experiments showed that the thigh and leg are initially accelerated upward (100 g) and downward (100 g), respectively, and then downward (60 g) and upward (100 g), respectively. The model also predicted a valgus knee injury and a tibia fracture similar to those seen in the PMHS.

CONCLUSIONS

The use of a facet surface model of the lower extremity skin and simultaneous optimization of the model's structural response and contact parameters resulted in a model capable of accurately predicting the detailed kinematic response of the lower extremity under vehicle impact loading at 40 km/h. The model can be scaled to represent varying pedestrian anthropometries and can assess the risks associated with sustaining the most common pedestrian injuries. As a vehicle design tool, the model can be used to optimize front-end designs, or it can be used in combination with a detailed FEM to reduce the vast design space prior to FE simulations. Additionally, the model can be used as a tool to study pedestrian impact kinematics, real-world case reconstructions, or particular vehicle countermeasures.

Injury Tolerance and Moment Response of the Knee Joint to Combined Valgus Bending and Shear Loading

Valgus bending and shearing of the knee have been identified as primary mechanisms of injuries in a lateral loading environment applicable to pedestrian-car collisions. Previous studies have reported on the structural response of the knee joint to pure valgus bending and lateral shearing, as well as the estimated injury thresholds for the knee bending angle and shear displacement based on experimental tests. However, epidemiological studies indicate that most knee injuries are due to the combined effects of bending and shear loading. Therefore, characterization of knee stiffness for combined loading and the associated injury tolerances is necessary for developing vehicle countermeasures to mitigate pedestrian injuries. Isolated knee joint specimens (n=40) from postmortem human subjects were tested in valgus bending at a loading rate representative of a pedestrian-car impact. The effect of lateral shear force combined with the bending moment on the stiffness response and the injury tolerances of the knee was concurrently evaluated. In addition to the knee moment-angle response, the bending angle and shear displacement corresponding to the first instance of primary ligament failure were determined in each test. The failure displacements were subsequently used to estimate an injury threshold function based on a simplified analytical model of the knee. The validity of the determined injury threshold function was subsequently verified using a finite element model. Post-test necropsy of the knees indicated medial collateral ligament injury consistent with the clinical injuries observed in pedestrian victims. The moment-angle response in valgus bending was determined at quasistatic and dynamic loading rates and compared to previously published test data. The peak bending moment values scaled to an average adult male showed no significant change with variation in the superimposed shear load. An injury threshold function for the knee in terms of bending angle and shear displacement was determined by performing regression analysis on the experimental data. The threshold values of the bending angle (16.2deg) and shear displacement (25.2mm) estimated from the injury threshold function were in agreement with previously published knee injury threshold data. The continuous knee injury function expressed in terms of bending angle and shear displacement enabled injury prediction for combined loading conditions such as those observed in pedestrian-car collisions.