Assessment of Macropus giganteus as a Biomechanical Model of the Pediatric Thorax

A leading cause of pediatric injury is motor vehicle crashes, in particular injuries to the head which is highly dictated by the thoracic response with the restraint. Due to the dearth of pediatric data available (specifically no existing pediatric thoracic response data in high speed frontal impacts) and the biofidelic uncertainty in current pediatric models, this thesis identifies and assesses an animal model as a surrogate for a six-year-old pediatric thoracic model. The animal model was chosen for its availability and geometric, length, mass and modulus similitude to a human six-year-old. The similitude is important in order to minimize or eliminate the need for scaling. Additionally, the presence of a clavicle was considered necessary for representing belt loading on the human chest. The eastern grey kangaroo satisfied many similitude characteristics and was chosen as the animal model to assess thoracic force-deformation response using three experimental studies: CPR loading, blunt hub loading, and an accelerated sled environment. After juvenile kangaroo carcasses were obtained, CPR and blunt hub loading tests were performed replicating the test conditions of previous pediatric experiments under these loading environments. Finally, kangaroo carcasses were subjected to belted simulated frontal crashes at low (9 ± 1 km/h) and high (39 ± 1 km/h) speeds.

In the CPR tests, the kangaroo thorax did not fully recover after each loading cycle which was not as prominent in pediatric data; however, they had a comparable thoracic stiffness response to pediatric subjects. In the blunt hub tests, the juvenile kangaroos did not have a large inertial spike in the thoracic force-deformation curve as observed in pediatric subjects. The lack of a sharp inertial force resulted in a third less work done on the chest deformation during loading and unloading of the kangaroo impact compared to pediatric subjects, and consequently more kinetic energy transferred to the kangaroo torso. The thoracic anatomical structures engaged by the loading geometries of the CPR and hub tests differ from a shoulder belt, so the kangaroos were tested in a belted sled accelerated environment. The sled tests showed that positioning the shoulder belt on the kangaroo torso was challenging due to the narrow shoulders and length of the torso. The overall kinematics of the kangaroo sled tests showed large amounts of lumbar lordosis due to the cranial positioning of the belt and more caudal center of gravity, which was probably larger than what a human child would exhibit.

In conclusion, although the eastern grey kangaroo has a similar size and growth development to human children and some anatomical similarities, the thoracic kinematics exhibited by the kangaroos in the sled tests were considered to be unrepresentative of the human child to such an extent that the force-deflection response of the kangaroo would be unlikely to reflect that of the child. This thesis justifies the importance of geometric similitude and mass distribution in the development of pediatric biomechanical models and its effects on thoracic structural behavior when loaded in an inertial environment.