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

Abstract

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.

Geometric and Inertial Properties of the Pig Head and Brain in an Anatomical Coordinate System

“…The larger scatter in the head CoM location likely reflects greater variability in the amount and distribution of soft tissue external to the skull, related to scanning position, total body weight, and animal-specific anthropometry. Similar to the findings in sheep [ 31 ], the head and brain CoMs in the pig were not coincident and were separated by 49.3 ± 4.8 mm. Given the distance between the pig head and brain CoMs, reporting kinematics at their respective locations may be prudent rather than assuming an interchangeable or coincident CoM as previously applied in non-human primate TBI research [ 1 ].…”

“…Using the mass and volume of each voxel, the total mass and volume of the head and brain was calculated, respectively. Using the mass and CT coordinates of each voxel [ 31 ], the CoM of the head and brain was calculated in the CT coordinate system using Equation 1 where are the CoM coordinates, m i is the mass of each voxel, and x i , y i , z i are the coordinates of each voxel. The coordinates of the AOJ was also defined by the midpoint of the most caudal aspect of the right and left occipital condyles (as defined for the head-neck cutting plane).…”

“…Therefore, translation of kinematics data from porcine studies can be aided by defining the location of the pig head and brain CoMs, and the AOJ. Additionally, since injury severity can be dependent on the impact direction relative to anatomical features (e.g., orientation of spine axis to the brain) [ 8 , 31 ], developing an anatomically-relevant head coordinate system for the quadrupedal pig can be used to translate kinematics data to bipedal human models. If defined by palpable bony landmarks, such an anatomical coordinate system (ACS) could be particularly valuable in injury biomechanics studies as it eliminates the need for 3D medical imaging and analysis, and allows real-time location of the head in the laboratory coordinate system.…”

“…Additionally, the human head ACS is typically defined relative to the Frankfort plane which is nominally oriented horizontally [ 34 ] in the standard forward-gaze reference position, and is approximately perpendicular to the spine axis in the bipedal human [ 39 ]. The head and brain CoMs, and an associated ACS, have also been defined for adult sheep [ 31 ], but similar work has not been reported for the pig head and brain. The ACS defined by Sharkey et al [ 31 ] using internal skull landmarks on CTs of sheep heads, corresponded to an equivalent forward-gaze head posture in this quadruped.…”

“…The head and brain CoMs, and an associated ACS, have also been defined for adult sheep [ 31 ], but similar work has not been reported for the pig head and brain. The ACS defined by Sharkey et al [ 31 ] using internal skull landmarks on CTs of sheep heads, corresponded to an equivalent forward-gaze head posture in this quadruped. An ACS for the pig head, with similitude to the human ACS, will improve the translational application of head kinematics data in biomechanics models using pigs.…”