Computer Simulation and Validation of the Archimedes Lever Hypothesis as a Mechanism for Aortic Isthmus Disruption in a Case of Lateral Impact Motor Vehicle Crash: A Crash Injury Research Engineering Network (CIREN) Study

OBJECTIVES:Can aortic isthmus disruption occurring in a lateral motor vehicle crash (LMVC) be explained by the Archimedes Lever Hypothesis, where the intrathoracic aorta, super-pressurized by the thoracic impact force, functions as a rigid lever system? The long arm of this lever system is the proximal aorta-aortic arch, the short arm is the aortic isthmus fixed distally at the descending aorta, and the fulcrum is at the great vessels, especially the left subclavian artery. METHODS:The theory was tested by a simulation technique using a computer-based finite element numerical model system. This simulation model included the dynamics of the crashed vehicles, the direction of force impact, and the structure of the thorax and intrathoracic viscera, including the entire intrathoracic aorta. The specific patient whose data were entered into the model was chosen from a study of 34 LMCV aortic injuries (AIs). The model was constrained by patient and vehicle data from this surviving case. RESULTS:Three sequential lateral thoracic levels impacted by the vehicle side structures were selected. At each level, the maximum mean intra-aortic pressure was 50 to 100 ms after impact, the structure dynamics of the actual crash and the resultant vehicle deformation were simulated; only when the lateral impact was induced in a transverse plane including the first 4 ribs at the level of the aortic arch/isthmus system, with intra-aortic pressures from 200 to 500 mm Hg, were AI-compatible stresses and deformations in the aortic wall achieved at the isthmus. CONCLUSIONS:In LMVC AI, the simulation suggests that the aorta functions as an Archimedes Lever System in which the magnified force mediated by the long lever arm produces sufficient strain on the short lever arm to rupture the aorta at the isthmus.