by John H. Siegel

John H. Siegel, MD, professor of cell biology and molecular medicine, and professor of surgery, UMDNJ-New Jersey Medical School

Motor vehicle crashes are the third most common cause of mortality in the world. In the last year for which full data are available (2002), there were 42,815 fatalities and more than 5 million injuries related to MVCs in the U.S. and more than 356,000 individuals suffered incapacitating injuries requiring hospitalization. While exact figures are difficult to derive, it has been reliably estimated that the present annual cost of motor vehicle crashes is $230 billion, or approximately 2.6 times the cost of the present year's appropriation for the current expenditure in Iraq. This MVC cost represents 2.3% of the gross national product of the U.S. Each critically injured survivor costs an average of $1.1 million. Serious MVC related injuries and deaths are disproportionately represented among younger adults, although the incidence in the elderly population is rising. The magnitude of medical costs, the degree of disability and the impact on social services have raised the appreciation of this type of injury to the status of a major public health issue.

Enlarge the image Figure 1: Location of sites of aortic rupture. P--proximal aorta, A--aortic arch, I- aortic isthmus, D--descending aorta. ME--patient dead at scene of accident, HD--death after hospital admission usually due to associated injuries, S--hospital survivor. 75% of all aortic ruptures occur at the aortic isthmus.

The incidence and consequences of this component of trauma have been quantified by the General Estimates System of the National Highway Traffic Safety Administration (NHTSA) since 1988. However, NHTSA's sampling system provides only a statistical estimate of the magnitude of this problem. Therefore, I was awarded a grant by the agency in 1988, when I was the Deputy Director of the Maryland Institute for Emergency Medical Services Systems, to study the causes, mechanisms and medical consequences of severe post-traumatic injuries resulting from motor vehicle crashes. This program was the initial nucleus of what is now the NHTSA sponsored Crash Injury Research Engineering Network (CIREN). When I came to New Jersey Medical School (NJMS) in 1991 as Wesley J. Howe Professor of Trauma Surgery and Chairman of the then Department of Anatomy, Cell Biology and Injury Sciences, I brought the NHTSA supported CIREN program with me. There are now 10 collaborating CIREN centers in nine states and the District of Columbia investigating various problems related to the mechanisms of MVC induced injury and the effectiveness of various new safety devices and motor vehicle construction standards.

Enlarge the image Figure 2: Vehicle of driver who sustained a fatal aortic rupture after a side-impact motor vehicle crash. The maximum crush on the side-door was 70 centimeters. There was no side airbag.

The NJMS CIREN center has been active for the past 13 years. During this period it has carried out detailed examinations of MVC patient injuries and in each case related the occurrence and mechanism of these injuries to the findings obtained from a detailed study of the actual motor vehicle damage and crash site. The NJMS CIREN center investigations have resulted in published studies demonstrating the differing patterns of injuries incurred by drivers and passengers in cars struck by automobiles versus those in cars impacted by the larger and heavier SUVs, small vans and pickup trucks. The severity and anatomic pattern of patient injuries were shown to be a function of the direction of the crash, the differences in the structure and mass of the vehicles, the forces involved, and the use or absence of use of airbags and/or seat-belts. Additional studies by the NJMS CIREN group have demonstrated the protective effect of airbags in reducing the functional severity of brain injuries incurred in frontal MVCs, and the nature of the impact energy/crash deceleration velocity relationship on the production of spine fractures and spinal cord injuries. Most recently, the NJMS CIREN center has been working to elucidate the impact energy/crash deceleration relationships responsible for the production of traumatic ruptures of the thoracic aorta, an injury that currently has a nearly 65% instantaneous fatality rate induced at the moment of crash and only a 15-20% total survival rate even if the patient reaches the hospital alive. As shown in Figure 1, the vast majority of thoracic aortic ruptures, and nearly all of the potential survivors' aortic lacerations, occur in the short free segment of the descending portion of the aorta, known as the isthmus. Moreover, this anatomic region is the most vulnerable to disruption, regardless of whether the MVC is the result of a frontal or a lateral crash.

Enlarge the image Figure 3: Autopsy specimen of aorta from driver killed at scene of crash in vehicle shown in Figure 2.

To study the force/velocity mechanisms involved, we have investigated the crashes of survivors, as well as scene and hospital fatalities, with the help of the regional medical examiner. In each case, the location and magnitude of the impact energy and crash deceleration are computed from engineering examination of the patient's vehicle. An example of a vehicle in which a fatal aortic rupture occurred is shown in Figure 2. These data are then correlated with the contact points within the passenger compartment of the vehicle that produced the patient's injuries. The injury data, therapies and outcomes are obtained from the sanitized patient record after obtaining patient or next of kin permission, or from similarly sanitized medical examiner records. To make this CIREN study possible, the absolute confidentiality of all patient related data is protected by a special Certificate of Confidentiality issued by the Secretary of Health and Human Services under a specific Congressional law.

In the vehicle shown earlier, the crash victim's resulting aortic laceration occurred at the isthmus just below the origin of the subclavian artery, as shown in this autopsy specimen, Figure 3. In this case, the change in velocity (decleration) on impact was 34 miles per hour. The crash delivered an impact energy of 163,692 joules to the vehicle. This impact was transmitted to the patient's left lateral chest, including the first five ribs, which define the anatomic location of the aortic arch and the upper and lower limits of the aortic isthmus. Similar detailed examination of clinical, radiographic and autopsy data have demonstrated that in the vast majority of cases, a critical level of this impact energy/crash deceleration relationship, focused on the patient's thorax over this specific defined anatomic level, was associated with the production of an isthmus related aortic disruption. From this type of study, the NJMS CIREN effort has produced a new theory of the mechanism of MVC induced aortic rupture, which is currently being evaluated using computer simulation technology in conjunction with the biomedical engineering group at Wayne State University. This simulation is shown in Figure 4, where the area marked in red shows the area of maximum strain to correspond to the aortic isthmus where the patient's actual aortic tear occurred. Simulations such as this based on real patient and vehicle data may allow us to develop new safety or improved devices to reduce the incidence of this highly fatal disease.

Enlarge the image Figure 4: Finite element model of aorta used to simulate a side impact crash in a complex model that included the automobile as well as the patient's entire thoracic anatomy, including the chest wall, ribs, lung, heart and aorta. Only the aorta is shown for simplicity. The intensity of the color (red) shows the point of maximum strain after an upper thorax impact by the intruded side door structures and B-pillar which separates the front and rear doors.

The research investigations at individual CIREN centers such as NJMS and the collective work of the entire CIREN center group have assisted NHTSA to develop an awareness of the nature of the differing pattern of organ and extremity injuries resulting from different types of MVCs. These studies have also had an important role in assisting the NHTSA to evaluate the effectiveness of the various new safety devices and motor vehicle construction standards in reducing the incidence and severity of motor vehicle induced injuries.

John H. Siegel, MD, FACS, FCCM, graduated from Cornell University and received his medical degree from Johns Hopkins University School of Medicine. Dr. Siegel trained in surgery at Yale, worked as an investigator at the National Heart Institute and completed his residency in general surgery at the University of Michigan. His academic career has included appointments at the Albert Einstein College of Medicine and the State University of New York at Buffalo. Dr. Siegel was Deputy Director of the Maryland Institute for Emergency Medical Services Systems and simultaneously professor of surgery at both the University of Maryland and Johns Hopkins University School of Medicine. He came to NJMS in 1991 as the Wesley J. Howe professor of trauma surgery and also as chair of the then Department of Anatomy, Cell Biology and Injury Sciences. Dr. Siegel retired from these full-time positions in 2001 to devote himself to his ongoing research studies on MVC injuries through a grant from the National Highway Traffic Safety Administration, and teaching and writing. His more than 360 publications have included such topics as the cardiovascular and metabolic response to shock and sepsis. Dr. Siegel is currently professor of cell biology and molecular medicine and professor of surgery at NJMS. §