There are more than 42,000 fatalities and 2.9 million people injured per year due to motor-vehicle accidents in the United States and an additional cost to society estimated at $230.6 billion per year, according to the National Highway Traffic Safety Administration (NHTSA 2005). Motor vehicle crashes remain a leading cause of death among the younger population between the ages of 4–34 and among the top ten causes of death for all age groups (NHTSA, 2006) and they deserve further study to prevent accidents and reduce their effects.
Side-impact crashes are the most harmful type of planar crashes. Although their frequency is about 28% of all crash types, they account for 30% of the serious injuries. One of the reasons for the higher injury potential of side-impact crashes is the reduced crush space between the passenger and the striking vehicle. Also, the fleet in the United States has shifted to a larger proportion of pickups and SUVs, whose size and weight make passenger cars more vulnerable than ever.
As will be discussed further in Chapter 3, blunt trauma aortic injuries are one of the leading causes of fatalities in side-impact crashes. The aorta is the main blood vessel of the human body and it supplies blood to all of the body’s vital organs. The blunt trauma that occurs in side-impacts can cause partial or total rupture of the aorta, resulting in excessive blood loss and, potentially, death.
Previous studies (Steps 2004) (Bertrand, et al., 2008) have established crash factors that could be used to predict aortic injury using real-world cases. These crash factors include age, restraints, delta-v, intrusion, crush, direction of force, and crash type. Other studies have attempted to establish the injury mechanisms for aortic injury, but to this date there is no general consensus on the evaluation criteria and the attempts to try to better understand these injury mechanisms are ongoing.
This study attempts to further investigate the proposed injury mechanisms for aortic injury such as Viscous Criterion, Chest Compression and the inertial effect of the heart in the thoracic cavity. Criteria that use Chest Compression and compression velocity have been researched by impacting the chest of cadavers with a cylindrical impactor (Hardy, et al., 2008). However, this type of testing is unable to evaluate how the inertial effect of the heart may contribute to loading the aorta. The reason was that the cadavers were not subjected to crash forces that simulated a side-impact. These studies demonstrated that the aorta is very weak in resisting tension loading that may be caused by the motion of the heart relative to the aortic arch. Other studies, with cadavers subjected to side-impact conditions, suggested that aortic injury was influenced by the magnitude of the upward acceleration acting parallel to the spine (Cavanaugh, et al., 2005). This type of acceleration would cause the heart to move upward and load the aorta in tension. One purpose of this study is to further evaluate the forces that act on the aorta, including those produced by the heart as a consequence of upward acceleration.
Several scenarios were modeled using LS-DYNA and MADYMO to reproduce currently available tests. These tests include the NCAP, NCAP Y-Damage and IIHS Side-impact test. The NCAP Y-Damage test was proposed by Steps as the test condition that most closely mimics the crash environment that produced the aortic injuries observed in low severity crashes (Steps, 2004). The NCAP and IIHS tests are routinely conducted to provide consumer information on crash safety. These scenarios were varied by adding airbags. The purpose of the air bag simulations was to determine the degree to which these safety systems reduced the risk of aortic injury. Sled tests were also modeled with and without a six inch pelvic offset in order to reproduce Cavanaugh’s cadaver sled tests (Cavanaugh, et al., 2005).
The modeling of these scenarios will be helpful to better understand the factors that contribute to the injury mechanism. Several injury parameters proposed by previous research studies (Cavanaugh, Koh, et al. 2005), such as Chest Compression, Viscous Criterion, Spinal Accelerations, etc. are analyzed. The effect of Spinal Acceleration is studied by adding a spring mass model within the Human Facet MADYMO Model, and exposing the resulting model to the selected crash environments. The inertia of the heart causing the aorta to stretch in the longitudinal direction is proposed as a possible injury mechanism.
Results conclude that the inertia effect is a possible factor in the injury mechanisms of aortic rupture. This stretching of the aorta as the result of inertia effect of the heart is present in the side-impact environments that were simulated. The aortic stretch is more severe in the higher severity cases and the Y-Damage pattern of the vehicle-to-vehicle simulations. It was also more severe in the pelvic offset sled tests, conforming to the previous cadaver research results from Cavanaugh.