The Biomechanics of Impact Injury

This book deals with the subject of impact forces acting on the human body and the injuries resulting therefrom. The motivation for doing research to uncover the effects of impact on biological systems is to lower the rate of carnage on US highways and bi-ways that have become unacceptably high. The surprising fact is that the USA has lost over 3.6 million lives due to traffic crashes since 1899. This number is larger than that of the lives lost in all the wars it has been involved in since 1775. In 1966, the National Research Council published a report entitled Accidental Death and Disability: The Neglected Disease of Modern Society principally to deal with the issue of the rapidly rising fatality rate from automotive crashes. It rose from just over 36,000 in 1960 to almost 51,000 in 1966. The National Highway Traffic Safety Bureau was established in 1966 to set safety standards for motor vehicles sold in the USA. In 1983, Congress authorized the US Department of Transportation to initiate a study by the National Academy of Sciences (NAS) by convening a panel of experts to determine what is known about injury and what research is needed to prevent or ameliorate it, including the role the federal government should play to increase the knowledge of injury. A NAS report, entitled Injury in America: A Continuing Public Health Problem, was published in 1985, and the Centers for Disease Control and Prevention (CDC) was commissioned to form the Center for Injury Prevention and Control to assist the Department of Transportation in enabling injury research in the USA. Automotive safety was high on the list of priorities. At the same time, the automotive industry was keenly aware of the problem but was resistant to federal intervention which can result in regulations that add to the cost of building a car. For the rest of the twentieth century, industry opposition gradually subsided, and the larger automotive companies became substantive sponsors of automotive safety research at many US universities and laboratories. As a result, injury research accelerated through government and industry funding, and the driving public was the principal beneficiary of this joint effort. The fatality rate in 2013 was 32,719.

Brain injury is a major public health problem and is commonly seen in falls and automotive crashes as well as in other environments, such as contact sports, military action, and assaults. Although the brain is protected by the skull, it can be injured in relatively low-speed impacts, such as in American football. Statistics on traumatic brain injury (TBI) provided by the National Center for Injury Prevention and Control reveal that, in 2010, there were over 50,000 deaths due to TBI and that TBI was diagnosed in more than 280,000 hospitalizations and in 2.2 million emergency room visits. Falls are the leading cause of TBI, especially among the youngest and oldest age groups, while motor vehicle crashes were the third leading cause of TBI (14 %). The various causes are shown in Fig. 2.1.

In retrospect, the experimental research carried out in head injury had the biomechanical objectives that were outlined in Chap. 1 (Sect. 1.​6) although they were not clearly explained until much later. The research began with the work of Gurdjian and associates in the mid-1950s followed by the work of Ommaya and associates. The purpose of their work was to try to understand the mechanisms of brain injury. Suffering from the lack of what we now call modern technology, the researchers used head acceleration and intracranial pressure as possible parameters that might be able to explain how the brain is injured. These were the only measurable parameters available at the time, and they were used to try to explain how brain injury occurs. Out of that research came the two competing theories of brain injury—the linear and angular acceleration mechanisms.

The purpose of modeling head impact is to try to understand the effect of a blow to the brain. Thus, it is essential that the brain be modeled in as much detail as possible. Then, of course, it will be necessary to assess injury to the brain by computing its response. Based on what we know about brain injury, we hypothesize that strain in the axons is a likely cause of diffuse axonal injury (DAI) and intracranial pressure wave propagation can be a second parameter of interest. Because of the complexity of the geometry of the head and brain, the many different types of tissues involved, and the lack of data on their material properties under high strain rate conditions, the modeling task is far from being simple. In the pre-finite element era, simplifying assumptions were made to facilitate the formulation of equations that describe the impact event. For example, the first known model of head impact was proposed by Anzelius (1943) who assumed the head to be a rigid sphere and the brain to be a liquid. He solved the governing equations in closed form, and his model predicted coup and contrecoup pressures at the site of impact and at a site diametrically opposite to the site of impact, respectively.

Angular acceleration was implicated as a cause of brain injury, beginning with the theory by Holbourn (1943) and the extensive research conducted by Ommaya and Hirsch (1971), Ommaya et al. (1967), and Gennarelli et al. (1982). The concept of angular acceleration was proposed by Sir Isaac Newton in the 1680s (Newton’s Principia) where he laid down the laws of motion. However, Newton did not say how this quantity could be measured. In 2-D, the measurement is accomplished by using a pair of linear accelerometers placed a known distance apart and facing the same direction (Mertz 1967). For 3-D motion, many schemes have been proposed (see, e.g., Kane 1968). It turns out that all of the schemes can potentially yield unreliable results even though the equations used are sophisticated. In this chapter, the traditional method is first described and is shown to be numerically unstable. Then a different scheme is introduced to show that it is numerically stable but requires more sensors.

In this chapter, three real-world problems will be discussed to show how head injury modeling can be helpful in providing information on human tolerance to head impact and the impact parameters that are good predictors of brain injury. The first deals with the problem of estimating human tolerance to mild concussion as experienced by athletes who play American football. It was of interest to professional football in the USA, and the study reported below was supported in part by the National Football League (NFL) which is a nonprofit trade association made up of professional football teams around the country. The second problem is to simulate a well-documented automotive crash at an intersection, including the injuries sustained by one of the drivers. The third problem is the simulation of the crash of a racecar and the response of the brain to the crash.

The three major functions of the neck are to support the head, to allow it move three-dimensionally, and to conduct nerve signals to and from the brain via the spinal cord. Many muscles in the neck provide the flexibility for head motion, while a bony vertebral column protects the delicate tissues of the spinal cord. This protection, however, is not adequate for high-speed crashes, and a variety of neck injuries occur when the head is impacted directly or inertially. In order to attain a better understanding of the injury mechanisms involved, a brief review of spinal anatomy is needed. This review covers the cervical spine as well as the thoracolumbar spine to avoid repetition in subsequent chapters. It also stresses certain anatomical features that are normally glossed over in anatomical texts.

Up until the advent of active safety systems, rearend collisions are a common occurrence, especially on busy urban roads where drivers are distracted, going too fast, and in a hurry. The most common scenario is the impact of a car stopped at a red light or on the roadway by the car behind it. The impacted vehicle is accelerated forward and the seatbacks push the torso of the occupants forward as well. Without a headrest in contact with the head, the head is left behind until shear forces are developed at each cervical vertebral level to bring the head forward along with the torso. This delay results in hyperextension of the head and neck or in whiplash. Many whiplash victims complain of neck pain, some for a few days or weeks, while others develop chronic pain syndromes that are difficult to treat. The problem is aggravated by our legal system which allows plaintiffs to sue for damages without having to pay their attorney in advance. Safety engineers are thus faced with a challenge to prevent this injury. Prior to the availability of an active pre-collision braking and warning system, the only recourse was to install headrests to prevent hyperextension. The federal government required these headrests to be installed in passenger cars in 1969, but complaints of neck pain did not abate. This called for research into the causes of neck pain due to whiplash because it became obvious that it is difficult to prevent an injury if the cause is not well understood. Additionally, without knowing the cause, it is also difficult to treat whiplash-related neck pain. Usually, there is little that can be seen from CT or MRI scans to indicate the source of the pain.

Impact injuries to the thoracolumbar spine are rare in automotive crashes. They take the form of vertebral body wedge fractures and, at times, burst fractures, particularly among the elderly. The cause is not vertical acceleration of the vehicle but is instead the straightening effect of the thoracic spine when the shoulder belt restraint is used. However, impact injuries due to vertical acceleration do occur in other environments, especially in the military environment. One of the first military problems is that of seat ejection – the emergency exit of a pilot from a disabled military jet aircraft. Some pilots sustain anterior wedge fractures of the thoracolumbar spine due to the 20 g acceleration of the seat. The injury was first recognized by the Luftwaffe or the German air force during World War II and was studied intensely in Britain and the USA for several decades after the war. The current problem is injury to the spine, pelvis, and lower extremities sustained by mounted soldiers whose vehicle they are riding in encounters an improvised explosive device. In this book, the seat ejection problem will be addressed, but blast-related injuries will not. Civilian injuries to the thoracolumbar spine due to falls also produce similar injuries. Falling from a height and landing on one’s buttocks generate wedge-type vertebral injuries. Fracture-dislocations can occur in more severe impacts. Such injuries are catastrophic because they can result in damage to the spinal cord and paralysis from the waist down. Ejection from a moving automobile or rollover of a vehicle can also cause these injuries.

Although the biomechanics community has now accepted the concept of facet loading, the idea took a long time to take hold. Even in the 1980s, it was necessary to continue to prove conclusively that facet loads are real. To that end, El-Bohy et al. (1989) obtained contact pressure data from the tip of an inferior lumbar facet to show that it did indeed bottom out on the lamina below in order to transmit spinal load. Other topics covered in this chapter are spinal models simulating seat ejection, a model simulating the ditching of an aircraft at sea, and a brief overview of finite element models of the spine simulating impact.

The thorax occupies the upper part of the torso and contains the lung and heart that are enclosed by a rib cage. Of course, the lung, heart, and the great vessels are vital organs that need to be protected from external forces but the enclosure also needs to be expandable to assist in the respiratory function. The rib cage is capable of expanding the thorax and can provide some protection to the thoracic organs. However, for high speed impacts, the ribs are vulnerable to fracture. Although multiple rib fractures are serious injuries, the fracture of a rib or two is relatively minor. But, when cadavers are used to assess thoracic injury, rib fracture is the only measure because injuries to the heart and lung are generally not assessable in dead tissue. Because of the variability in human tolerance, the number of rib fractures and the number of fractured ribs cannot be correlated to the severity of injuries to the thoracic organs.

The abdominal organs are located under the dome-shaped diaphragm and most of the organs have no skeletal protection. The organs directly under the diaphragm have minimal protection from the lower ribs which have cartilaginous connections to the sternum and are not very strong. However, these organs are vulnerable to injury in an automotive crash and a delayed diagnosis of severe trauma can be fatal. Thus, tolerance of the abdomen to blunt trauma is a concern for automotive safety engineers. The topic is covered quite completely by Rouhana (1993) in a book chapter that details almost all aspects of the biomechanics of abdominal injury. It is recommended reading for anyone interested in the details of abdominal injury due to blunt impact.

Pelvis is the Latin word for basin. It holds the organs of the lower abdomen and is anatomically part of the abdomen but the skeletal pelvis has a load bearing function because it transmits the weight of the head and torso to the lower extremities via the sacrum which is firmly attached to the pelvis. In this chapter, we will study the biomechanics of pelvic response to impact and the injuries that result from pelvic impact. The pelvis also plays a crucial role in restraining automotive occupants during a crash because the lapbelt is designed to hold the torso to the seat so that it can ride down with the car and prevent severe impacts of the head and the torso against parts of the vehicle.

This chapter deals with the biomechanics of impact injuries sustained by the upper and lower legs, or, in anatomical terms, the thigh and the leg. Injuries to the foot will be discussed in Chap. 15. In both frontal and lateral impacts, the lower extremities are at risk of being injured when they come into contact with the dash or the car door. The mechanisms of injury are explored and the types of injuries are discussed.

This chapter deals with the biomechanics of impact injuries of the foot. In current vehicles, foot injuries are usually the result of footwell intrusion caused by an offset frontal impact. The force applied to the plantar (bottom) surface of the foot can result in injury to both the midfoot and the hindfoot. In the early days of aviation, the brake pedal in open cockpit single seaters was a round bar and there are anecdotal records of pilots breaking their feet (midfoot) while doing a crash landing. In the nineteenth century, French surgeon, Jacques Lisfranc de St. Martin (1790–1847) treated injuries to the midfoot of Napoleon’s cavalrymen when their foot got caught in the stirrups after they fell from their horse. This serious foot injury is now named after Lisfranc.

In a side impact, the struck vehicle is at a disadvantage in terms of occupant safety because of the proximity of the side structures (e.g., the side door) to the occupant compared to the space available to the occupant in a frontal impact. The seat belt system is also not effective in preventing injury from a side impact. As a result, before side impact airbags were available, the fatality rate was high even though the speed of impact of the striking vehicle is low. In fact, in the 1990s, the annual fatality rate for side impact was close to 10,000 before FMVSS 214 for side impact was implemented, as shown in Fig. 16.1. However, after the standard came into full effect in 1997, the rate showed no substantive drop. The total fatality rate was 42,013 in 1997 and it dropped from a high of 43,510 in 2005 to 32,575 in 2014. That is, even with the introduction of active safety into our vehicles, side impact fatalities remain unchanged and is becoming a larger part of the fatality problem. The reasons for this anomaly are discussed in this chapter.

Road users who are not protected by the vehicular structure of an automobile are particularly vulnerable in a crash situation. These road users include pedestrians, bicyclists, motor cyclists, and people riding in non-motorized vehicles, such as horse-drawn carriages and the like. By far, the largest group of vulnerable road users is the pedestrian who travels in close proximity to automobiles and, in fact, often cross the roadways and are at high risk of being impacted by these vehicles. The most effective way of protecting the pedestrian is to separate vehicular and pedestrian traffic, especially in busy urban areas, by the use of overpasses and underpasses at intersections. These are costly solutions and are not available at most intersections, even in highly developed countries of North America and Europe. Strict enforcement of traffic regulations regarding pedestrians in intersections is helpful but many are still injured or killed.

There is an advertising poster put out in 1940 by the now defunct Packer Motors that suggested an unusual way for the right front passenger to protect him/herself before an impending crash. The ad suggested that the passenger in the “dead man’s seat” curl up in the footwell to ride out the crash. This is possible for a small person in a large car but it is not a practical suggestion because by the time the passenger manages to get into the footwell, the crash would have occurred already. The more practical form of protection is the use of automotive restraint systems. There are two forms of safety restraints. Seatbelts constitute the active restraint system which requires the occupant to actively participate in its use. There are forms of automatic seatbelts but so far their use has been limited. The most popular form of passive restraint is the airbag which is deployed at the time of the crash, hence the name passive restraint. Both systems afford good protection for the occupant but when used together, they are very effective in mitigating injuries and preventing fatalities. The biomechanics behind the use of these restraint systems is the subject of this chapter.

Sports-related injuries are rarely fatal but are very common. They are more common in contact sports, such as American football, but are also seen in non-contact sports, such as basketball and baseball. The knee is the most commonly injured body region in sports because the joint is not well protected by bony structures and is heavily used. However, catastrophic injuries can occur due to impacts to the head, neck, and chest. Examples of such injuries include fatal heart injuries in baseball, head injuries in football (soccer), baseball and basketball and knee injuries in jogging and tennis. The topics covered in this chapter are mild traumatic brain injury in American football, catastrophic neck injuries due to crown impacts, cardiac injuries due to sternal impacts, and knee injuries due to a lateral impact.

The human race has been living with accidental injury and death since its beginning. Injury prevention was a personal issue that was learned through personal experience or that of elders. Even in the modern-day medical school, injury prevention is not only not taught but also not considered preventable as other diseases. The high fatality rate due to automotive collisions in the 1960s prompted congressional action which finally brought injury prevention to the attention of the public and attracted researchers in the field of epidemiology to look for preventative measures. However, researchers in impact biomechanics had been aware of the need for prevention some two decades earlier. It turns out that preventative methods using behavioral control are not as effective as those using environmental control. In automotive safety. Behavioral control means driving at a safe speed, responding to changing road conditions, and not drinking and driving. Automotive environmental control includes airbags, padded interiors, headrests, and crushable body structure. Biomechanics played and continues to play a large role in the design of many components of environmental safety control because the underlying causes of injury must be properly understood and the design should not have injurious side effects. The padding used to protect the knees of front seat occupants is a prime example. Its stiffness and thickness protect the knees but do not cause femoral fractures.