Fusako Sato

Effects of whole spine alignment patterns on neck responses in rear end impact

ABSTRACT Objective: The aim of this study was to investigate the whole spine alignment in automotive seated postures for both genders and the effects of the spinal alignment patterns on cervical vertebral motion in rear impact using a human finite element (FE) model. Methods: Image data for 8 female and 7 male subjects in a seated posture acquired by an upright open magnetic resonance imaging (MRI) system were utilized. Spinal alignment was determined from the centers of the vertebrae and average spinal alignment patterns for both genders were estimated by multidimensional scaling (MDS). An occupant FE model of female average size (162 cm, 62 kg; the AF 50 size model) was developed by scaling THUMS AF 05. The average spinal alignment pattern for females was implemented in the model, and model validation was made with respect to female volunteer sled test data from rear end impacts. Thereafter, the average spinal alignment pattern for males and representative spinal alignments for all subjects were implemented in the validated female model, and additional FE simulations of the sled test were conducted to investigate effects of spinal alignment patterns on cervical vertebral motion. Results: The estimated average spinal alignment pattern was slight kyphotic, or almost straight cervical and less-kyphotic thoracic spine for the females and lordotic cervical and more pronounced kyphotic thoracic spine for the males. The AF 50 size model with the female average spinal alignment exhibited spine straightening from upper thoracic vertebra level and showed larger intervertebral angular displacements in the cervical spine than the one with the male average spinal alignment. Conclusions: The cervical spine alignment is continuous with the thoracic spine, and a trend of the relationship between cervical spine and thoracic spinal alignment was shown in this study. Simulation results suggested that variations in thoracic spinal alignment had a potential impact on cervical spine motion as well as cervical spinal alignment in rear end impact condition.

A female head–neck model for rear impact simulations

Several mathematical cervical models of the 50th percentile male have been developed and used for impact biomechanics research. However, for the 50th percentile female no similar modelling efforts have been made, despite females being subject to a higher risk of soft tissue neck injuries. This is a limitation for the development of automotive protective systems addressing Whiplash Associated Disorders (WADs), most commonly caused in rear impacts, as the risk for females sustaining WAD symptoms is double that of males. In this study, a finite element head and neck model of a 50th percentile female was validated in rear impacts. A previously validated ligamentous cervical spine model was complemented with a rigid body head, soft tissues and muscles. In both physiological flexion-extension motions and simulated rear impacts, the kinematic response at segment level was comparable to that of human subjects. Evaluation of ligament stress levels in simulations with varied initial cervical curvature revealed that if an individual assumes a more lordotic posture than the neutral, a higher risk of WAD might occur in rear impact. The female head and neck model, together with a kinematical whole body model which is under development, addresses a need for tools for assessment of automotive protection systems for the group which is at the highest risk to sustain WAD.

Age-Dependent Factors Affecting Thoracic Response: A Finite Element Study Focused on Japanese Elderly Occupants

Objectives: The ultimate goal of this research is to reduce thoracic injuries due to traffic crashes, especially in the elderly. The specific objective is to develop and validate a full-body finite element model under 2 distinct settings that account for factors relevant for thoracic fragility of elderly: one setting representative of an average size male and one representative of an average size Japanese elderly male. Methods: A new thorax finite element model was developed from medical images of a 71-year-old average Japanese male elderly size (161cm, 60 kg) postmortem human subject (PMHS). The model was validated at component and assembled levels against original series of published test data obtained from the same elderly specimen. The model was completed with extremities and head of a model previously developed. The rib cage and the thoracic flesh materials were assigned age-dependent properties and the model geometry was scaled up to simulate a 50th percentile male. Thereafter, the model was validated against existing biomechanical data for younger and elderly subjects, including hub-to-thorax impacts and frontal impact sled PMHS test data. Finally, a parametric study was conducted with the new models to understand the effect of size and aging factors on thoracic response and risk of rib fractures. Results: The model behaved in agreement with tabletop test experiments in intact, denuded, and eviscerated tissue conditions. In frontal impact sled conditions, the model showed good 3-dimensional head and spine kinematics, as well as rib cage multipoint deflections. When properties representative of an aging person were simulated, both the rib cage deformation and the predicted number of rib fractures increased. The effects of age factors such as rib cortical thickness, mechanical properties, and failure thresholds on the model responses were consistent with the literature. Aged and thereby softened flesh reduced load transfer between ribs; the coupling of the rib cage was reduced. Aged costal cartilage increased the severity of the diagonal belt loading sustained by the lower loaded rib cage. Conclusions: When age-specific parameters were implemented in a finite element (FE) model of the thorax, the rib cage kinematics and thorax injury risk increased. When the effect of size was isolated, 2 factors, in addition to rib material properties, were found to be important: flesh and costal cartilage properties. These 2 were identified to affect rib cage deformation mechanisms and may potentially increase the risk of rib fractures.

Analysis of Intervertebral Strain Response during Rear Impact Using Head-Neck Finite Element Model

Minor neck injuries in rear collision accidents have become a huge problem in many countries. Therefore, it is urgent to develop a suitable criterion for assessing neck injury risk. In this study, a detailed head-neck finite element (FE) model was developed. Skull and vertebrae models were created based on CT images of a typical Japanese male. Models of intervertebral discs, ligaments and muscles were also created according to literatures. Furthermore, material properties were taken from the published data. In order to evaluate intervertebral soft tissue strain due to translational rotational coupled motion of vertebrae, a 2D strain analysis method was also proposed. The method was applied to cervical vertebral motion data obtained from previous rear impact tests using human volunteers and from test reconstruction using the head-neck model. A potential correlation between intervertebral strain and neck injury was clarified from the comparison between the intervertebral strain level and existence of neck discomforts. The model’s response is also in good agreement with the volunteers’ response, indicating that the head-neck model is suitable for minor neck injury analysis and that it is possible to analyze the intervertebral strain with a head-neck model.