Importance of Physical Properties of the Human Head on Head-Neck Injury Metrics

Objectives: To demonstrate the importance of using specimen-specific head physical properties in head-neck dynamics. Methods: Eight postmortem human subjects were subjected to side impact. A 9-axis accelerometer package was used to obtain head translational accelerations. After test, the head was isolated at the skull base, circumference, breadth, and length were obtained, and mass, center of gravity, and occipital condylar locations and moments of inertia were determined. Using specimen-specific and gathered accelerations, 3-dimensional head center of gravity accelerations and forces and moments at the occipital condyles were computed. Head physical properties were also extracted from regression equations using external dimensions of each subject. Using these properties and gathered kinematics, above-described accelerations and forces and moments were computed and compared with specimen-specific results. Results: Head masses predicted by stature and total body mass were more in close agreement with specimen-specific data than head masses predicted by head circumference or head circumference and head length. The center of gravity to the occipital condyle vector was shorter in the literature-based dataset than the actual specimen-specific vector. Differences in moments of inertias between predicted and specimen-specific data ranged from -15 to 59 percent. Variations in peak antero-posterior shear, lateral shear, and axial force ranged from -12 to 46 percent, -21 to 78 percent, and -17 to 50 percent. Differences in peak lateral moment, sagittal moment, and axial torque ranged from -45 to 78 percent, -86 to 327 percent, and -96 to 112 percent. These were normalized using specimen-specific data. Conclusions: Considerable variations in physical properties and injury metrics between data obtained from literature-based regression equations and actual data for each specimen suggest the critical importance of specimen-specific data to accurately describe the biodynamic response and establish tolerance criteria. Because neck dynamics control head kinematics (and vice versa), these results emphasize the need to determine physical properties of each specimen following impact tests.