The dynamic response of the human thoracic spine is not well understood in the field of injury biomechanics, largely because of experimental challenges in isolating the properties of the upper thoracic spine segment from the rest of the thorax. Research suggests that increased flexibility in the anthropomorphic test device (ATD) posterior thorax improves the biofidelity of head kinematics, and this effect is especially important in child ATDs for evaluating booster seat performance.
The objectives of this study were to (a) characterize the dynamic response of the intact human upper thoracic spine while considering the effects of pectoral girdle restraint configuration, speed, and anthropometry, (b) quantify the link between upper thoracic system dynamics and whole-body thoracic spine kinematics in crash simulation (sled) testing, and (c) develop a methodology to estimate large child UTS-PG properties using anthropometry, kinematics, and adult UTS-PG material properties.
A novel approach, Isolated Segment Manipulation (ISM), was introduced to quantify the intact upper thoracic spine – pectoral girdle (UTS-PG) dynamic response of nine adult post-mortem human subjects (PMHS). The ISM dynamic properties were confirmed to be applicable in more realistic crash conditions by applying them in a sled model with an input acceleration applied directly to the mid-thoracic spine. Thoracic spine displacements were measured in twelve HYGE sled tests conducted on three of the nine PMHS at various speeds (3.8 – 7.0 m/s) associated with spine velocities observed in typical belted sled tests. Using two different models, it was determined that the dynamic properties from ISM testing could be used to accurately predict T3 spine displacements for multiple sizes of PMHS and various combinations of restraint and speed. Head, shoulder, and spine kinematics were calculated through three-dimensional kinematic measurement, and T3 displacement vs. T6 force relationships were presented as preliminary ATD response targets.
Anatomic and kinematic statistical analyses were then completed to aide in translating the adult UTS-PG data to the child population. Structural anatomy measurements were taken from radiology data of both adult PMHS and pediatric patients, and statistically significant age-dependent measures were identified for scaling purposes. Head displacements of both child and adult occupants from sled evaluations in the literature were statistically analyzed and supported qualitatively by crash data occupant available space (OAS) calculations, and it was determined that the mean 10YO displacement, when normalized by stature, is 46% greater than that of an adult. A distributed parameter analysis was employed to estimate the elastic modulus of the adult UTS-PG to be 7.5 – 16.5 MPa using anthropometric, ISM, and sled kinematic data from this study. Extension of this methodology to the large child using age-dependent scale factors from this study and the literature resulted in normalized mode shape differences between large child and adult that were consistent with kinematic differences from experimental literature.
Using the techniques, findings, and tools from this study, it is believed that biofidelity response corridors and a test method can be developed for the upper thoracic region of large child ATDs used to evaluate booster seat designs.