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4 Injury Biomechanics Research and the Prevention of Impact Injury Improved protection against injury can be realized through a better understanding of the biomechanics of injury and disability. This chapter reviews the current state of knowledge on injury biomechanics and presents recommendations to advance the field by forming university centers for research, by training engineers and scientists in injury biomechanics, and by ensuring a long-term com- mitment of funds and leadership. By investing in the needed research, we can reduce injuries and preserve the well-being of countless people. STATE OF INJURY BlOMECHANICS RESEARCH Impact injury of the human body occurs by deformation of tissues beyond their failure limits, which results in damage of anatomic structures or alteration in function. Even if there is recovery from structural injury, normal physiologic function does not always return. For example, bony fractures can heal, but associated damage to the central nervous tissue might result in a permanent loss of motor and sensory function. Injury b~omechanics research uses the principles of mechanics to explore the mechanisms of physical and physiologic responses to mechanical forces. Injury is caused by penetrating or nonpenetrating blows to the body, and the energy delivered and the area of contact are important determinants of the results. Penetrating injuries are caused by high-speed projectiles, such as bullets, or by sharp objects moving at lower speeds, such as knives.) ~. Penetrating injury generally involves a concentration of mechanical energy in a small area of the body. Nonpenetrating injuries are caused by blunt 48

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49 ob jects that distr ibute energy over larger areas at a wide range of speeds. . 3 ~ a ~ ~ 7 7 ~ 9 3 Although in jury can occur by slow deformation of the body, such as in crushing, the predominant features of impact in jury are speed and violence, as in the rapid impact of the chest on the instrument panel of an automobile or a bullet's penetration into the chest cavity. Research in biomechanics involves a variety of disci- plines, including engineering, physiology, medicine, biology , and anatomy. Thus, there is no unified approach or single area of training, education, or exper fence . Research is often conducted by teams of engineering, medical, and other personnel, and the combining of such disciplines is an important element of a successful study. Detailed reviews of the literature on var ious aspects of in jury biomechanics are available,. 2 9 6 ~ O ~ ~ ~ ~ ~ 9 ~ 2 0 S but the research is so broad that no publication can cover the entire scope of this subject. MECHANISMS OF INJURY The severity of an impact depends not only on the ve- locity of the collision that produces it, but also on the shapes of the colliding objects and their rigidity. It can be reduced by energy-absorbing structures and padding material by allowing simultaneous deformation of the body and of the surface collided with. Il7 l.9 l'l i93 This extends the duration of impact and reduces the r isk of injury. Because of the inertial resistance of the body tis- sues and the elastic and viscous compliance of body struc- tures, force is developed on the body during impact. Force deforms and accelerates the body. 4-1) can be caused by: Injury (Figure Crushing deformation of the body, such as through chest compression, rib fracture, and sortie laceration. Impulsive impact, such as by violent sternal motion that deforms the heart beyond its viscous tolerance and causes contusion and rupture. Acceleration of the skeleton and tearing of internal organs, because of their inertia; for example, during head impact, the skull accelerates and the loosely attached brain lags,.. 'I 12] 129 \~l So injury is due in part to deformation of brain tissues beyond their limit of recovery.

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so ~1 Elastic At, ~ ~0 V iscous my, ~ \ ~ / I/ SN/` I nertial FIGURE 4-1 Three pr incipal mechanisms of impact in jury: left, compression of the body and injury when the crush exceeds elastic tolerances; center, high-speed impact with in jury when violence exceeds viscous tolerance; and right, body acceleration when internal organ motion lags the skeleton with injury due to organ inertia. A mechanism of injury involves the mechanical deforma- tions and physiologic responses that cause an anatomic lesion or functional change. Knowledge of injury mechan- isms is fundamental to the science of in jury biomechanics, because it points to the appropr late biomechanical mea- surements that characterize injuries. The human body has viscoelastic tissues that absorb energy and protect vital organs from the effects of impact. As long as the energy delivered to the tissue is below the limit of injury--whether it be the crush limit for for 137 20\ Me viscous limitless 106 203 or the acceleration limit ~ ~ 6 ~ 9 S--tbe energy will be absorbed without causing injury. The resistance of the human body to impact i'; responsible for survival of falls from extreme heightsS. ~ ~62 am -t'~~;*'-' me -~''~^ ". - - i. ~ Ace, is, - all. ~ motor-vehicle crashes. ~ ~ 6 ~ ~ 7 ~ a ~ Even though the body can survive great impact, the frequency and variety of impact incidents are so great that they constitute one of the leading causes of ser ious in jury , death , and disabil- ity in the United States. Ef fective in jury-prevention strategies must be based on knowledge of the mechanisms of injury and disability, as well of the body's biomech- anical responses and tolerances and techniques for

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51 assessing the injury-prevention benefits of safety technology. Deformation of tissues beyond a recoverable limit is the most common origin of in jury. s g 6 5 ~ 2 ~ From an engineering point of view, deformation of a tissue or structure is measured according to change in shape (such as the change in length divided by the in itial length ) or strain. 6 2 The two main types of strain (Figure 4-2) that can damage tissue are tensile strain and shear strain; a third type is compressive strain, which is responsible for crushing injuries. Stretching of an artery increases its length and increases strain. If the strain is too great, the tissue will break . There are many ways to stretch tissue and thus produce tensile strain. For example, the motion of the heart during TENSILE STRAIN SHEAR STRAIN - - .%, If: cV~O Hemorrhage ~50:__ ( ~ ~Adventitial Seal fit Pseudo~aneurism (pulsating hematorna) lit' do, -or ~` ado' i"\ (A Hemorrhage A) Adventitial Seal Pseudo~neurism (pulsating hematomaJ FIGURE 4-2 Stretch of vessel can tear tissue (partial tear shown) with loss or containment of blood. Opposing forces across vessel can cause shear injury (complete tear shown) with or without loss of blood.

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S2 chest compression stretches the aorta near points of attachment; the stretch is primarily along the axis of the vessel and generally leads to a transverse laceration when the recoverable limit of tissue strain is exceeded.~9 An increase in vascular pressure dilates blood vessels and produces tensile strain in the tissue; in this case, the strain is in both axial and transverse directions (i.e., it is biaxial), and pressures beyond the recoverable limit cause a bursting of the tissue. Axial impact on the femur causes an increase in its curvature, tensile strain on its enter for surface, and compressive strain on its posterior surface. Midshaft failure of the femur occurs when the tensile strain exceeds the recoverable limit. s 2 2 o ~ This mechanism is common in the r ibs, where compression of the chest causes tensile strain on the outer surfaces. Bending failure of bone and failure of vessel walls are two common examples of tensile failure due to tissue stretch. Shear strain occurs when forces oppose each other across a tissue (see Figure 4-2 ) . The movement of tissue in opposite directions separates it when the recoverable limit is exceeded. For example, the dif ferential movement of the brain with respect to the accelerated skull during head impact causes a combination of shear and tensile strain at the interface between brain and skull. ~3 Damage occurs when the strain exceeds the resistance of the tissue. Differential motion of lobes of the liver can shear and lacerate hepatic vessels when the strain exceeds the recoverable limit.~` to 6 Stretch and shear of tissue are the primary mechanisms of laceration, fracture, rupture, and avulsion in the human body. The strain mechanisms commonly occur when an organ moves relative to its attachment during deformation of the body. This strain explains major vessel ruptures in the liver during deformation of the abdomen and laceration of the aorta due to compression of the chest. Deformation or strain is also a principal factor in contusion injury (Figure 4-3). In this case, the surface of the tissue is not damaged, but the deformations stretch and shear internal vessels and increase intraluminal pres- sures, which can damage vessels and initiate hemorrhage. The rate of loading, or strain rate, is important in the production of injury. Biologic tissues are visco- elastic, and their response and tolerance depend on both strain and strain rate. 6 ~ 6 S 2 For example, compact bone fails at a lower strain applied at higher rates, even though the load it carries at failure is higher.

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53 ), -rl, If' Spasm -I (O)J ~ ~ \: ~ j Local Thrombosis \ 57 True Aneurism FIGURE 4-3 Vessels beneath sk in can be torn by stretch or shear with laceration of skin, resulting in bruise, or contusion. The rate of loading is critical in soft-tissue injury, particularly when the viscous tolerance of the tissue is exceeded.~ 6 2 0 3 This tolerance is proportional to the product of loading rate and amount of compression. The faster a tissue is loaded, the lower is its tolerance to compression. The viscous in jury mechanism is important in cases of high-speed impact, particularly when rupture and contusion occur. RESEARCH NEEDS Injury Mechanisms Gaining knowledge of the mechanisms of injury is the first step in injury biomechanics research. It permits an understanding of the deformations associated with gross anatomic lesions or damage to biologic tissues that result in functional change. The study of injury meabanisms makes use of clinical and accident data and data from experiments with human cadavers and anesthetized animal`;. Mechanisms of many of the important in jury- related anatomic lesions have been studied. For example, the mechanisms of hear t and great-vessel rupture have been discussed extensively; ~ ~ ~ ~ the sequence of events that follow blunt impact and lead to rupture and laceration has been descr ibed ; and f racture of the femur and dislocation of the knee are generally well under- stood,~39 2 o 2 although much less is known about injuries of the other long bones and many joints.

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54 Cerv ical spine in jury has been emphas ized, because of the ser ious functional consequences associated with spinal cord damage . 3 ~ Neck in juries occur in rollover automo- bile crashes, shallow-water diving, and contact sports. Although the literature and hypotheses on fracture- d islocation of the neck are extensive, 8. 94 iso ~ 7 2 little of the underlying sequence of injury events has been verified. The mechanisms of functional damage to the central nervous system (CNS) are less well estab- lished, primarily because the problem has not been studied from the point of view of impact deformation and functional loss. Although isolated tissue preparations have provided information on the underlying mechanisms of functional change, the problem is complicated by the abundance of physiologic responses to injury. But CNS injury is paramount, because it so often causes functional disability 99 ~ 3 ~ ~ S 7 ~ 9 2 We need to study the sequence of events and biomechanics of impact injury to the CNS. Many of the mechanisms responsible for functional change are speculative and generally unknown, but they deserve a balance of biomechanical and physiologic research. Measurement of Biomechanical Responses The first step in understanding biomechanical responses is to measure changes in shape of the body, organ, or tissue caused by impact. ~ ~ ~ 2 ~ ~ 9 ~ The elastic and viscous resistance of biologic tissue to impact deformation and the inertial resistance of the body to motion must be characterized. Measurement of biomechanical responses is used to analyze the injury process. Studies of volunteers, limited by rigid regulations and guidelines, can be conducted with impacts below the pain threshold. Several military research facilities use military volunteer subjects, but the United States has only 8 few research facilities for civilian impact experiments. Although noninjurious responses can be measured in volunteer experiments, the basic study of impact responses must use surrogate humans. The primary research tools to evaluate injurious bio- mechanical responses are human cadavers and anesthetized animals that are exposed to impact and detailed response measurement. Cadavers are suitable research models that simulate gross geometric and material properties of living humans, and they are often used to study kinematics, such as the motion of a body during deceleration, or the

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55 mechanical response of a body segment, such as deformation of the chest. Animal research is vital in obtaining information on physiologic responses tr iggered by in jury; there is no other way to simulate pathophysiologic responses or itical to developing information on disabling brain and spinal cord injuries and life-threatening arrhythmia or shock. Mathematical models9' i 8 8 ~9 and anthropomorphic dummiesS 7 ~ 2 ~ ~ 2 ~ are used extensively as predictive tools, particularly in connection with body k inematics and acceleration during impact. However, simulations are only as accurate as the biomechanical information used in their formulation. Mathematical models generally lack accuracy for interactive forces developed during contact. In many instances, the lack of basic research data on biomechanical responses has retarded the development of more sophisticated models or dummies. Tolerance The threshold of injury is a degree of deformation, energy absorption, or biomechanical response beyond which damage occurs to the tissue or structure. Damage in this case can be a gross anatomic lesion or an injury that results in a permanent alteration of function. The threshold is not fixed, but is a function of a variety of characteristics, including the type of tissue and the type of test subject. Our current knowledge concerning human impact tolerances is sparse, and experimentally derived data on women, children, and other segments of the population are highly limited. Determination of human tolerances to impact is compli- cated by many factors, including the magnitude, direction, distribution, duration, and pulse shape of the force of the impact; the body orientation; tightness and configura- tion of restraint; and structure of the striking object. Biologic factors may also influence human tolerance, including sex, age, physical and mental condition, and body size.70 ~ i94 Individual variability must be considered, because tolerance under identical test conditions can vary in the same person, as well as from one person to another. Furthermore, although data are available on impact forces in some body orientations, such as forward or rearward, less is known about the effects of lateral or multiple-direction impact.

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:: S6 i Thus, it is not possible to state the human tolerance to impact definitively without knowing specific condi- tions, because tolerance depends on so many conditions. In fact, there is likely to be a distr ibution of toler- ances for a given population and a given impact. Falls provide a means of estimating human tolerances to impact, and particularly to extreme impact beyond which volunteers may not be subjected.S. '8 I.2 l8. Data from falls have limited value, however, because responses are not measured during the event, and informa- tion on the impact dynamics must be reconstructed after the fact. Other estimates of impact tolerance are obtained from clinical studies of impact injury and from reconstruction of automotive crashes and aircraf t crashes. i , 6 ~ 7 9 In these cases, although the resulting injuries are known, impact conditions can only be estimated. Assessment of Safety Technology A research goal is to develop a test tool, such as a dummy, and a test method, such as a crash simulation, to study the effectiveness of safety technology. ~ 2 ~ ~ ~ 9 If a test is sufficiently representative of a range of exposures in which injury might occur, the test tool and method can be used to a';sess the r isk of in jury and disability. Automotive crashes are violent events of short dura- tion, so they are typically studied in the laboratory with dummies. The purpose of such studies is to evaluate methods for reducing the overall risk of impact injury to havens, but dummies provide only the most basic informa- tion, which usually cannot be correlated accurately with human r esponse and injury. For anatomic injur ies , the predictive capacity of dummy tests is marginal. More important, current dummies cannot be used to evaluate functional changes that result in severe cognitive dysfunction, in quadriplegic, or in fatal ventricular fibr illation . Advances have recently been made in the mathematical simulation of crashes, - ~ and this simulation can be used to ';tudy the effects of a wide range of design changes and improvements. Thus, current Cools and techniques constitute only a first step in the development of adequate evaluative procedures based on understanding of trauma mechanisms, biomechanical responses, and tolerance of humans .

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57 Other Factors in Injury Many factors influence the occurrence, severity, and outcome of impact. Hypertension and arteriosclerosis tend to increase the severity of cardiovascular injury and might be important in injuries among the older population. Osteoporosis, a disease of the skeleton that reduces the impact resistance of bone, s 2 S 9 6 S is a factor in skeletal injury, particularly in compression fractures of vertebrae and fractures of the femoral neck and ribs. This disease is more likely to affect older women, reinforcing the belief that age, sex, and well- being influence injury. Chronic use of alcoholic beverages interferes with normal body repair processes and is important in injury causation. It is now becoming evident that use of alcoholic beverages predisposes to more severe and extensive injury than would be experienced by nondrinkers, given impact of the same severity S6 IS 10? Alcoholic beverage use that produces even moderate or low blood alcohol concentration can signif icantly increase the fatality rate associated with cardiac injury and the debilitating effects associated with CNS damage. Physiologic experiments have demonstrated a higher fatality rate associated with a combination of acute alcoholic-beverage use and blunt thoracic impact than that associated with impact alc~ne.~' A. The combination of alcohol and impact impairs cardiac performance, and death is caused by electromechanical dissociation, or decoupling, of the excitation- contraction process. Because of the high incidence of alcoholic beverage use by seriously and fatally injured road-accident victims (33 percent of seriously injured and 50 percent of fatally in jured), the reduced tolerance associated with moderate blood alcohol concentration might be one of the most important factors influencing injury, but little is known about the mechanism for this . CONCLUS IONS The current state of knowledge of injury mechanisms, the understanding of impact responses and tolerances, and the availability of useful technology for injury evalua- tion were analyzed by the Committee on Trauma Research and are summarized in Table 4-1. There is a reasonable understanding of mechanisms of anatomic damage, but the

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61 mechanisms of functional changes are less well understood. Very little is known regarding the mechanisms of brain and spinal cord damage. These tissues control life processes and cannot be adequately protected if the mechanisms of functional in jury to them are not better understood. A better understanding of impairment of cardiac function due to thoracic impact is also needed. The understanding of impact responses and tolerances is not as advanced as that of mechanisms of anatomic injury, because of the lack of experimental models for research to measure biomechanical responses and tolerances of humans. The use of human cadavers has allowed the assessment of the response of the ribcage to impact and the response of the femur to axial loading at the knee,~S ]03 200 but most recent research has emphasized the biomechanical responses and tolerances of the chest and extremities. Earlier cadaver experiments involving head impact provided the basis for the current understand- ing of the heads impact response and tolerance . ~ 2 ~ ~ 2 9 However, the current techniques are not sufficient to assess the risk of severe and moderate in jury to the brain with confidence. The measurement of head-impact responses and tolerances is an urgent sub ject for research and must use a physiologic model for study. The frequency of head impact is high, and the consequences of brain damage are severe.'s ' t ~ 2 ~ ~ ~ O ~ ~ ~ ~ S 7 Of similar importance is development of response and tolerance data on the spinal cord. The seriousness of quadriplegic is obvious, and useful measures of the r isk of damage to the central nervous system are needed. l.7 Although hypotheses; are available for the mechanism of cervical spine fracture-dislocation, there is a paucity of experimental verification and a lack of correlation of fracture-dialocation with spinal cord damage. Information on the impact responses of the abdomen and external tissues is also sparse. Test dummies are the primary tools for predicting injury, but only a few measures of potential injury are assessed with current techniques during impact tests. The most common ones are the acceleration responses of the head and chest and the measurement of force applied to the femur.5' i21 12. The measurement of femoral forces is well accepted and used, but the assessment of femoral injury is not based on the underlying mechanism of injury, which is bending and not axial compression. The criterion for evaluating the risk of head injury has been well publicized, but it has limited exper isaental

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62 verification and has not been correlated with the risk of brain damage or facial injury. Limited experimental criteria are available for evaluating neck in jury, but they do not assess functional changes associated with the risk of quadriplegic. Clearly, the two most.important body regions, the head and neck, are inadequately evaluated with the current testing technology. but they are regions that suffer the most harm in motor-vehicle crashes.29 RECO - ENDATIONS 1. A multidisciplinary approach to injury bio- mechanics research should be coordinated to include: Injury investigation, in jury-mechanism study, biomedical research on response and tolerance, study of pathophysiologic response to impact, and research on disabling injuries, particularly to the central nervous system. Support for the training of scientists and engineers in injury biomechanics, to overcome a serious shortage of such workers. . Support and incentives for established investigators on university faculties to develop curricula in the mechanics and physiology of in jury . . Development and nurturing of a core of leaders by establishing university centers for scientific study of injury and disability. Adequate support for long-term and applied research should be emphasized and an effort made to curb the rapid loss of trained researchers primarily to service as expert witnesses in lawsuits. Maintenance of a balanced extramural research program managed by engineers, physiologists, and physicians. Foster ing of cooperation and exchange of information with existing federal organizations. . Promotion of the development of information on important problems in trauma, independent of the regulatory function of the government. 2. High priority should be given to research that can provide a clearer understanding of injury mechanisms. The crucial subjects of research are as follows:

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63 . The relative contr ibution of linear and angular acceleration of the head to deformation and in jury of brain and spinal cord . Ver if ication of proposed mechan isms of in jury to the neck--head motion can impose a variety of combined axial, bending, and shear loads that r esult in many k inds of in jury. . Verification of mechanisms of in jury to the thoracic and abdominal viscera; these mechanisms vary from unknown to slightly understood. Musculoskeletal in jury that leads to functional joint disability. 3. Research in mechanical responses requires sustained support. The responses of most body regions to mechanical loading have not been measured adequately. The critical regions are the following: . Central nervous system, where the accurate measurement of linear and angular acceleration is needed for use in b~omechanics exper iments . Thoracic viscera, including motion of internal organs and vessels that leads to injury. Abdominal viscera. . knee . Joints, with the possible exception of the Muscles and peripheral nerves. 4. High priority should be given to obtaining and defining limits of human tolerance to injury, particularly with regard to the following general subjects : . Segments of the population on which data are extremely limited, including children, women, and the aged. Both whole-body and regional tolerances, to provide an improved basis for design of less hazardous products and environments. The effect of variables that influence and modify tolerance, such as substance abuse and energy-attenuating and restraint systems. Physiologic tolerances, particularly in the central nervous system. Long-term effects of rapid deceleration on the body, particularly the brain, spinal cord, and joints.

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64 Survival of extreme impact, to provide a better basis for understanding the limits of tolerance. 5. Improvement in injury-assessment technology is needed. Although some tools and techniques are available for this purpose, they are inadequate, because of the wide diversity of potential injury and disability. Work in this field should include: Development of means of assessing the important debilitating injuries and causes of fatality. Improvement of anthropomorphic dummies. Development of computer models that can be used to predict injury and response in complex crash conditions.