Instrumentation, Model, and Prototype for Generating Mechanics Needed to Produce Diffuse Brain Injury in the Rat

Due to the frequency of diffuse brain injuries across the population and cost of treatment, there is great interest in the mechanical parameters involved in provoking the injury. Rotationally induced diffuse brain injury has been studied since the early seventies on a spectrum of animal surrogates from primates to rats. Among the devices designed to produce brain injury in the rat, none have induced rotational diffuse brain injury from an impact event along with developing descriptive mechanics of the event. Since diffuse brain injury is most commonly caused from an impact event, it is critical to use an impact as the loading medium for rotating the head. The purpose of this study was to develop the injury mechanism to produce biomechanical parameters equivalent to those needed to induce diffuse brain injury from rotational loading caused by impact along with the instrumentation to monitor these parameters. Such a mechanism consisted of a spring-loaded impactor ejection system, impact helmet and interface characteristics. A mathematical model was developed to describe the impact event using a lumped parameter Maxwell model. A prototype was created from design specifications developed with the use of anthropometric scaling ratios and previous research on various animal surrogates. Experimental test drops were conducted using a clay model of equivalent head weight and dimensions to the rat. Five drops with an accelerometer on the helmet, five drops with high-speed video, ten drops with a force transducer on the site of impact, five drops measuring relative motion of a rat head with respect to the helmet, and five drops measuring relative motion of a rat head with respect to the helmet with a mouth bar to stabilize the bead were conducted to verify the mathematical model. Results indicated the injury mechanism exceeded the minimum threshold for 50% probability of diffuse concussion and will produce an abbreviated injury score of 2, indicating classical concussion. The results also indicated mass moment of inertia and damping coefficient are sensitive parameters that may alter the response of the helmet significantly. Relative motion of the bead with respect to the helmet indicated 91.3% of the helmet angular acceleration was seen by the skull. Insertion of a horizontal metal bar across the mouth of the specimen caused a significant increase in the angular acceleration of the skull. There was a 1 ms average delay between the response of the helmet and response of the head