Center of mass and anatomical coordinate system definition for sheep head kinematics, with application to ovine models of traumatic brain injury

Pathological outcomes of traumatic brain injury (TBI), including diffuse axonal injury, are influenced by the direction, magnitude, and duration of head acceleration during the injury exposure. Ovine models have been used to study injury mechanics and pathological outcomes of TBI. To accurately describe the kinematics of the head during an injury exposure, and better facilitate comparison with human head kinematics, anatomical coordinate systems (ACS) with an origin at the head or brain center of mass (CoM), and axes that align with the ovine Frankfort plane equivalent, are required. The aim of this study was to determine the mass properties of the sheep head and brain, and define an ACS(virtual) for the head and brain, using anatomical landmarks on the skull with the aforementioned origins and orientation. Three-dimensional models of 10 merino sheep heads were constructed from computed tomography images, and the coordinates of the head and brain CoMs, relative to a previously reported sheep head coordinate system (ACS(physical)), were determined using the Hounsfield unit-mass density relationship. The ACS(physical) origin was 34.8 +/- 3.1 mm posterosuperior of the head CoM and 43.7 +/- 1.7 anteroinferior of the brain CoM. Prominent internal anatomical landmarks were then used to define a new ACS (ACS(virtual)) with axes aligned with the Frankfort plane equivalent and an origin 10.4 +/- 3.2 mm from the head CoM. The CoM and ACS(virtual) defined in this study will increase the potential for comparison of head kinematics between ovine models and humans, in the context of TBI.