Analysis of the mechanism of lateral impact aortic isthmus disruption in real-life motor vehicle crashes using a computer-based finite element numeric model: with simulation of prevention strategies.

INTRODUCTION

Despite advances in the surgical therapy of aortic injury (AI) using endovascular prostheses, more than 60% of motor vehicle crash (MVC) induced AIs die at the scene. In 80 cases of MVC AI, both change in velocity on impact (Delta V) and impact energy (IE) were correlated with autopsy or surgical findings. Of the 34 AIs due to lateral impact MVCs (LMVC), 91% had an aortic isthmus laceration. Computer simulation is used to study the cause of LMVC AI.

METHODS

To delineate AI mechanism, 10 real life LMVCs (8 left, 2 right) were simulated using a computer-based finite element numerical model. Each began with the initial vehicle impact with another vehicle or fixed object, followed by the vehicle's compartment structures' impact with the patient's chest wall, causing a rise in intra-aortic pressure and the resulting location and pattern of aortic wall stresses and strains. In the real LMVCs, the Delta V ranged from 27.5 to 62 kph with impact energies of from 46,051 to 313,502 joules. In both real-life and the model, the main cause of the chest wall impact was intrusion of the car's B-pillar. Dynamic simulations delineate increased stress and strains at the aortic Isthmus. In some LMVCs, the B-pillar intrusion was also seen to impact the head in the AI cases.

RESULTS

In the simulations, aortic pressure rose from 100 mm Hg precrash to as high as 1,322 mm Hg. Both the maximum aortic longitudinal tensile strain and the von Mises Stress were proportional to the maximum force impacted on the chest wall. Aortic isthmus maximum stresses ranged from 1.1 Mega Pascal (MPa) to 3.2 MPa, with longitudinal tensile strains ranging from 8.2% to 48.5%. The simulation dynamics demonstrated that the proximal pressurized turgid aorta initially moves toward the LMVC impact. As a result, the ascending aorta and aortic arch (proximal ascending aorta) rotate about the fulcrum of the great vessels, so that this aortic unit, acting as the long-arm of an Archimedes lever system, exerts the maximum stress and strain at the aortic isthmus or short-arm, where the real-life aortic rupture occurs.

CONCLUSION

Simulation supports the lever hypothesis that the force on the short-arm aortic isthmus is proportionally greater than at the long-arm proximal aorta. Simulation also suggests improved vehicle construction techniques, which increase the strength and resistance to deformation of the B-pillar and vehicle side structure plus a B-pillar airbag will limit the intrusion forces causing LMVC AIs and reduce the incidence of associated head injuries.