Finite element analysis of head-neck kinematics under simulated rear impact at different accelerations

Rear-Impact Neck Whiplash: Role of Head Inertial Properties and Spine Morphological Variations on Segmental Rotations

Whiplash injuries continue to be a concern in low-speed rear impact. This study was designed to investigate the role of variations in spine morphology and head inertia properties on cervical spine segmental rotation in rear-impact whiplash loading. Vertebral morphology is rarely considered as an input parameter in spine finite element (FE) models. A methodology toward considering morphological variations as input parameters and identifying the influential variations is presented in this paper. A cervical spine FE model, with its morphology parametrized using mesh morphing, was used to study the influence of disk height, anteroposterior vertebral depth, and segmental size, as well as variations in head mass, moment of inertia, and center of mass locations. The influence of these variations on the characteristic S-curve formation in whiplash response was evaluated using the peak C2–C3 flexion marking the maximum S-curve formation and time taken for the formation of maximum S-curve. The peak C2–C3 flexion in the S-curve formation was most influenced by disk height and vertebral depth, followed by anteroposterior head center of mass location. The time to maximum S-curve was most influenced by the anteroposterior location of head center of mass. The influence of gender-dependent variations, such as the vertebral depth, suggests that they contribute to the greater segmental rotations observed in females resulting in different S-curve formation from men. These results suggest that both spine morphology and head inertia properties should be considered to describe rear-impact responses.

Simulated effects of head movement on contact pressures between headforms and N95 filtering facepiece respirators-part 1: headform model and validation

In a respirator fit test, a subject is required to perform a series of exercises that include moving the head up and down and rotating the head left and right. These head movements could affect respirator sealing properties during the fit test and consequently affect fit factors. In a model-based system, it is desirable to have similar capability to predict newly designed respirators. In our previous work, finite element modeling (FEM)-based contact simulation between a headform and a filtering facepiece respirator was carried out. However, the headform was assumed to be static or fixed. This paper presents the first part of a series study on the effect of headform movement on contact pressures-a new headform with the capability to move down (flexion), up (extension), and rotate left and right-and validation. The newly developed headforms were validated for movement by comparing the simulated cervical vertebrae rotation angles with experimental results from the literature.

Quantifying cervical-spine curvature using Bézier splines

Knowledge of the distributions of cervical-spine curvature is needed for computational studies of cervical-spine injury in motor-vehicle crashes. Many methods of specifying spinal curvature have been proposed, but they often involve qualitative assessment or a large number of parameters. The objective of this study was to develop a quantitative method of characterizing cervical-spine curvature using a small number of parameters. 180 sagittal X-rays of subjects seated in automotive posture with their necks in neutral, flexed, and extended postures were collected in the early 1970s. Subjects were selected to represent a range of statures and ages for each gender. X-rays were reanalyzed using advanced technology and statistical methods. Coordinates of the posterior margins of the vertebral bodies and dens were digitized. Bézier splines were fit through the coordinates of these points. The interior control points that define the spline curvature were parameterized as a vector angle and length. By defining the length as a function of the angle, cervical-spine curvature was defined with just two parameters: superior and inferior Bézier angles. A classification scheme was derived to sort each curvature by magnitude and type of curvature (lordosis versus S-shaped versus kyphosis; inferior or superior location). Cervical-spine curvature in an automotive seated posture varies with gender and age but not stature. Average values of superior and inferior Bézier angles for cervical spines in flexion, neutral, and extension automotive postures are presented for each gender and age group. Use of Bézier splines fit through posterior margins offers a quantitative method of characterizing cervical-spine curvature using two parameters: superior and inferior Bézier angles.

The effects of ligamentous injury in the human lower cervical spine

Damage is often sustained by the anterior longitudinal ligament (ALL) and ligamentum flavum (LF) in the cervical spine subsequent to whiplash or other cervical trauma. These ligaments afford substantial cervical stability when healthy, but the ability of the ALL and LF to stabilize the spine when injured is not as conclusively studied. In order to address this issue, the current study excised ALL and LF tissues from cadaveric spines and experimentally simulated whiplash-type damage to the isolated ligaments. Stiffnesses and toe region lengths were measured for both the uninjured and damaged states. These ligamentous mechanical properties were then inputted into a previously-validated finite element (FE) model of the cervical spine and the kinematic effects of various clinically relevant combinations of ligamentous injury were predicted. The data indicated three and five-fold increases in toe region length for the LF and ALL injury variants, respectively. These toe length distensions resulted in FE predictions of supra-physiologic ranges of motion, and these motions were comparable to spines with no ligamentous support. Finally, a set of cadaveric cervical spine ligament-sectioning experiments confirmed the FE predictions and supported the finding that partial injury to the relevant ligaments produces equivalent cervical kinematic signatures to spines that have completely compromised ALL and LF tissues.