Special Feature
Motor Vehicle Crash Biomechanics: Interpreting the Polaroid
By Jeffrey W. Runge, MD, FACEP
Over the last decade, emergency medical services (EMS) personnel have become increasingly aware of the need to convey the circumstances of the motor vehicle crash to physicians caring for injured patients. First advocated by Hunt et al,1 the physician's index of suspicion increases when significant automobile damage is shown in a photograph. Many EMS services carry Polaroid cameras on their units with the notion that a picture is worth a thousand words. The premise behind this is very sound, in that it is as important to stratify the risk of significant injury based on the history as it is to stratify for risk of coronary artery disease in patients with chest pain. Unfortunately for the car crash victim, the relationship between automobile damage and risk of injury is not as widely known among physicians as are the epidemiologic risk factors for coronary artery disease. A few simple historical features can be used by physicians to discriminate between patients on backboards with cervical collars, even when all present similarly.
There are approximately 40,000 people killed on the roads of the United States every year, nearly equal to the death toll for the entire Vietnam War. Yet our culture continues to perpetuate the myth that motor vehicle crashes are accidents and that death by vehicle is somehow an act of God. Otherwise, the nation would make a top priority of the three factors that most affect motor vehicle crash death rates: seat belt use, speed, and alcohol use. Instead, 20-40% of people from state to state do not buckle their seat belts. Alcohol use is responsible for more than 15,000 fatalities per year and a far higher number of injuries. Nevertheless, due to the efforts of those who advocate stricter laws on drunk driving, the use of seat belts, and vehicle design enhancements, there was a 3.8% annual decline in fatalities between 1996 and 1997 in spite of a large increase in vehicle miles traveled over that same period.2
Understanding the Characteristics of Impact
The kinematics of crash injury are extremely complex. There are four questions that should be asked every time a patient presents with an injury from a motor vehicle crash to allow an estimate of the forces at play: What did the patient’s vehicle interact with (e.g., tree, guard rail, Yugo, or 18-wheeler)? What was the principal direction of impact — frontal, rear, near side, opposite side, or rollover? What was the estimated speed at impact (which is often difficult to precisely discern)? What protective devices were employed (air bag, 3-point seat belt, 2-point seat belt, or a child safety seat)?
About 40% of all fatalities are the result of single-vehicle crashes, one-half of which are rollovers. Of all crashes, fewer than 20% are single-vehicle, and only 6% are single-vehicle rollovers. Thus, single-vehicle crashes are more lethal, with rollovers being disproportionately high. Single-vehicle crashes more often involve occupants who are chemically impaired, traveling at high rates of speed, and unrestrained. Epidemiologists who study motor vehicle crash injury often use an index called "harm" that takes into account not only crash counts but also the economic impact of the crash and the resulting injuries. Although 21% of all crashes are rear, multi-vehicle crashes, they are responsible for only 3% of harm. Side-impact crashes, especially near-side impacts, produce disproportionately more harm.
Crash engineers refer to the severity of impact, related to the speed of impact, as sv. This is the change in velocity experienced by the vehicle and its occupants at impact until the velocity is zero. The average sv for a tow-away crash in the United States is around 19 mph. For all crashes, sv is around 5 mph, and for those where the police are involved, the average sv is around 10 mph. While more than one-half of tow-away crashes occur at v less than 19 mph, more than half of all harm is caused from crashes with sv = 16-30 mph, and 23% from crashes with sv > 30 mph. The clinical significance is that most of the injuries seen in a typical emergency department are caused by relatively low-velocity crashes and cause a small proportion of overall societal harm.
Understanding Energy Transfer During a Crash
Whereas most injury biomechanics discussions begin with the formula e = mv2, the energy forces during a motor vehicle crash are better understood by considering stopping distance and stopping time as a determinant of G-forces delivered to the body. In order to calculate or estimate G-forces, one must have some knowledge of the sv and the stopping distance (the distance traveled from the beginning of impact until the velocity reaches zero). The formula is:
sv (ft/sec)2
G = stopping distance (ft) ´ 30
For a healthy adult younger than 60 years of age, average maximum human tolerance to quick-pulse velocity change is approximately 30 G-forces. Using this relationship, it is apparent that in order to reduce G-forces, one may reduce the sv, as in the enforcement of speed laws or effective braking. Equally effective are methods to increase the duration of the impact and increase the distance the body travels throughout the duration of the crash, as in the depth of an air bag, the deformation of the vehicle, padded surfaces within vehicles, and energy-absorbing barriers in the environment. Even small increases in stopping distance decrease the G-forces passed to the body by a factor of 30. It is also easy to understand why near-side impact crashes are more dangerous. Near-side impact crashes allow far less stopping distance than do frontal or rear crashes, and the chances for passenger department incursion are much higher.
The Four Collisions in a Crash and "Ride Down"
Understanding the four collisions during crash makes it apparent why seat belts work, and why the lack of a seat restraint invariably leads to increased tissue damage. The first collision occurs when the vehicle strikes another object. The second collision occurs when the occupant contacts fixed objects in the vehicle (e.g., the seat belt in the case of a restrained person, or the steering wheel, windshield, roof rail, or other objects when occupants are unrestrained). The third collision occurs when the occupants’ organs continue to move relative to the body surface, and are damaged either through contact with fixed tissue such as the skull, or by tethering as in the duodenum or aortic arch. A fourth collision may occur if there are loose objects in the vehicle, such as backseat passengers who are unrestrained, or other loose objects, making contact with the occupant.
Under conditions ideal for injury prevention, the first collision occurs in a vehicle that has a large capacity for deformation, creating a longer stopping distance and absorption of a larger amount of energy. The second collision would ideally occur with a seat belt early in the impact phase allowing the occupant to "ride down" the crash, decelerating over the entire duration of the crash. This sustained duration reduces the damage from the third collision, as the person’s organs experience less deformation, as opposed to a quick-pulse deceleration. Further stopping distance can be added to the restrained occupant by a fully deployed air bag. An unrestrained occupant keeps moving during the first part of the impact and stops suddenly on contact with a vehicle surface. In cars equipped with air bags, unrestrained occupants may contact the air bag prior to or during deployment, which may add to the deleterious effects of the second collision as the speed and energy of the air bag deployment is added to the sv.
Part of the history that should be obtained from the patient or EMS personnel is the type of vehicles involved and the surface contacted. Clearly, a three-cable guard rail in the middle of an interstate, designed to give 12 to 15 feet of stopping distance at 60 mph, or a water-filled barrier at an exit ramp, afford greater stopping distance and time duration than does a large tree. Moreover, a modern vehicle with crumple zones and a collapsible steering column will afford greater stopping distance than a 1957 solid chrome collector’s special. Where multi-vehicle crashes occur, size matters. Occupants of large vehicles that hit small vehicles fare better than do occupants of small vehicles interacting with large vehicles. Thus, an estimation of energy forces using the available parameters of mass and velocity and stopping distance should be ascertained when taking the history.
While biomechanics is not a simple subject that can be summarized in a page or two, these basic concepts can help the emergency physician treat motor vehicle crash patients less blindly. An estimation of velocity at impact, principal direction of impact, seat belt use and air bag presence, and external vehicle and environmental interactions, when taken together, serve as a useful guide to the likelihood of severe injury. Risk stratification and a more studious approach to test ordering is good for patients and results in a better use of resources.
References
1. Hunt RC, et al. Comparison of motor vehicle damage documentation in EMS run reports compared with photo documentation. Ann Emerg Med 1993;22:651-656.
2. U.S. Department of Transportation. National Highway Traffic Safety Administration. "Traffic Safety Facts." http://www.nhtsa.dot.gov.
3. Peterson TD, et al. Motor vehicle safety: Current concepts and challenges for emergency physicians. Ann Emerg Med 1999;34:384-393.
10. Which of the following is not one of the four collisions in a crash?
a. Vehicle strikes object.
b. Occupant strikes fixed interior object.
c. Organs move within occupant till stopped.
d. Cell-cell deceleration.
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