Trauma, 7th Ed.

CHAPTER 1. Kinematics

John P. Hunt, Alan B. Marr, and Lance E. Stuke

Kin·e·mat·ics (kn-mtksn: The branch of mechanics that deals with pure motion without reference to the masses or forces involved in it. From Greek knma, knmat-, movement.1

As can be presumed from the derivation of the word kinematics, its essence revolves around motion. All injury is related to the interaction of the host and a moving object. That object may be commonplace and tangible, such as a moving vehicle or speeding bullet or more subtle as in the case of the moving particles and molecules involved in injury from heat, blasts, and ionizing radiation. Newtonian mechanics, the basic laws of physics, and the anatomic and material properties of the human body explain many of the injuries and injury patterns seen in blunt and penetrating trauma. Injury is related to the energy of the injuring element and the interaction between that element and the victim. Although most patients suffer a unique constellation of injuries with each incident, there are quite definable and understandable energy transfer patterns that result in certain predictable and specific injuries. Knowing the details of a traumatic event may aid the treating physician to further investigative efforts to uncover occult but predictable injuries.

This chapter has been organized in a stepwise fashion. First, the basic laws of physics and materials that dictate the interaction between the victim and the injuring element are reviewed. This is followed by a more detailed examination of penetrating and blunt trauma and a synopsis of mechanisms specific to organs and body regions. It is hoped that this will offer the reader a better understanding of specific injury patterns, how they occur, and which injuries may result.

BASIC PRINCIPLES

Image Newton’s Laws, Impulse, Momentum, Energy and Work, Elastic and Inelastic Collisions

Newton’s first law states that every object will remain at rest or in uniform motion in a straight line unless compelled to change its state by the action of an external force. This is the definition of inertia. Newton’s second law builds on the first and further defines a force (F) to be equal to the product of the mass (m) and acceleration (a).

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The application of a force does not occur instantaneously, but over time. If we multiply both sides of the above equation by time

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The product of force and time is known as impulse and multiplying acceleration by time yields velocity. Momentum (p) is defined to be the mass (m) of an object times its velocity (v).

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hence

impulse = change in momentum.

The important fact is that a force or impulse will cause a change in momentum and, likewise, a change in momentum will generate a force.2 This folds into Newton’s third law, which states that for every action or force there is an equal and opposite reaction.3 For instance, when two objects of equal velocity and mass strike each other, there velocities are reduced to zero (at the moment of impact). This change in velocity and, hence, momentum was caused by each object applying a force to the other. During impact the forces are equal and opposite. Recalling Newton’s second law, a force is associated with a change in momentum. In this system, the net force is zero and, therefore, the change in momentum is zero. This illustrates the law of conservation of momentum. The total momentum of a system will remain constant unless acted upon by an external force. The momentum of this two object system is the same after a collision as it was prior to impact.4

The next important basic principles are those of work and energy. Work (W) is defined as a force exerted over a distance and is frequently written as

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with image and image

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which after integration yields the familiar formula for kinetic energy: 1/2mv2

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Therefore, the work being done by a moving object, which interacts with a second object, equals the kinetic energy of the first object prior to doing work minus the kinetic energy after the interaction. In other words, the work done is equal to the change in kinetic energy of the first object.5 When this interaction sets the other body in motion, the second body now has kinetic energy of its own, equal to the work done. James Joule described the first law of thermodynamics in 1840, which simply states that energy can be neither created nor destroyed.6 Interactions in which both momentum and energy are conserved are termed elastic.

In trauma most collisions are inelastic. Inelastic collisions conserve momentum, but not kinetic energy. In these instances the kinetic energy “does work” in the deformation of materials even to the point where objects can conglomerate and form a single object. This is the hallmark of the inelastic collision. This energy transfer or work done is what is typically responsible for the injury sustained by the host.

Energy transfer and momentum conservation can be illustrated in the collision of two cars. Fig. 1-1(A) represents a head-on collision of two cars with equal mass and velocity and, thus, equal kinetic energy and momentum. The momentums are equal, but in opposite directions. Thus, the total momentum for the system is 0 prior to the crash and, by the law of conservation of momentum, must be 0 after the crash. Upon impact, both cars will come to rest. It is as if one of the cars struck an immovable wall. Recalling Newton’s second and third laws, this sudden change in momentum represents a force, which is equally applied to both cars. Because the final velocity is 0, the final kinetic energy is 0, meaning that all the kinetic energy has been converted to work that stops the other car and causes deformation such as breaking glass, bending metal, and causing physical intrusion into the passenger compartment. If the momentum of car A was greater than that of car B by having a greater mass or velocity, the resultant mass C will have momentum in the previous direction in which car A was traveling.

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FIGURE 1-1 Energy and momentum available in various motor vehicle crash scenarios. (A) Frontal collisions have the greatest change in momentum over the shortest amount of time and hence the highest forces generated. (B) T-bone collision. When cars A and B collide their resultant momentum directs them toward their final position C; the individual momentums in the x and y axis are dissipated over a greater time resulting in smaller forces then head-on collision. (C) Rear-end collision. Since these vehicles move in the same direction the change in momentum and forces generated are smaller.

In T-bone type crashes the directions of the momentum of cars A and B are perpendicular. Therefore, in the momentum axis of car A, car B has 0 momentum and, in the momentum axis of car B, car A has no momentum. The conglomerate C conserves momentum in both the A and B axes with the resultant direction as shown in Fig. 1-1(B). As a consequence, the changes in momentum and force generated are far less than that of a head-on collision. Also, C continues to have a velocity and, as such, kinetic energy. This means that some of the initial kinetic energy was not converted to work, and less damage to the automobiles will occur. In general, the closer to a head-on collision the greater the change in momentum and, thus, the greater the force generated.

In rear-end collisions the momentum of both cars is typically in the same direction, Fig. 1-1(C). Therefore, the changes in momentum and resultant forces generated are typically small as is the conversion of kinetic energy to work. These principles apply to all collisions whether they are a bullet penetrating a victim, a car hitting a pedestrian, or a driver impacting the windshield.

Image Penetrating Trauma and Ballistics

Although the above principles were elaborated in the setting of blunt trauma, they are equally applicable to penetrating trauma. The study of ballistics details the energy of projectiles as they leave the firearm and the energy transfer once the bullet strikes the victim. Theodore Kocher first proposed that the kinetic energy possessed by the bullet was dissipated in the four following ways: namely, heat, energy used to move tissue radially outward, energy used to form a primary path by direct crush of the tissue, and energy expended in deforming the projectile.7 Despite limited techniques for studying ballistics, Kocher was for the most part correct. Our more extensive knowledge of the behavior of projectiles in a host comes from the observed performance of bullets in gelatin, which has properties similar to that of muscle and is thought to reflect the way in which energy is transferred through tissue. From such experiments several characteristics of a projectile piercing tissue have been described. These include the following: (a) penetration (the distance the projectile passes through tissue is reflected in the distance from the cut edge of the gelatin block to where the projectile comes to rest); (b) fragmentation (the pattern is assessed by biplaner x-rays and the degree reflected in the difference of the weight of the prefired projectile minus the weight of the collected fragments); (c) permanent cavity (the tissue disintegrated by direct contact with the missile and preserved in the gelatin); and (d) temporary cavity (the amount of “stretch” caused by the passing projectile). This is reflected by the distance from the edge of the permanent cavity to the outer perimeter of the cracks within the gelatin.8

The performance of the bullet and the injury sustained is reliant upon velocity, construction of the bullet, and composition of the target.9 The energy and construction characteristics of the projectile will be discussed here while target properties will be reviewed in the section on biomaterials. The prominent 18th-century surgeon John Hunter stated, “If the velocity of the ball is small, then the mischief is less in all, there is not so great a chance of being compounded with fractures of bones etc.”10 This astute observation reflects the exponential importance of velocity in determining the amount of kinetic energy that a particular projectile is capable of transmitting to a given target (kinetic energy = 1/2mv2). As such, high-velocity missiles will generally cause more tissue destruction than their lower velocity counterparts. The velocities and kinetic energies11,12 of common handguns and rifles are listed in Table 1-1.

TABLE 1-1 Velocity and Kinetic Energy Characteristics of Various Guns

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The amount of energy imparted (or work) to the tissue by a projectile is equal to the kinetic energy of the missile as it enters the tissue minus the kinetic energy as it leaves the tissue. Bullets are extremely aerodynamic, causing little disturbance while passing through the air. To some extent, this is similar in tissue (i.e., if the projectile moves with the point forward and passes in and out of the tissue, only a small portion of its kinetic energy will be transferred to the target). The characteristics of damage created along the track of a bullet are divided into two components, the temporary and the permanent cavities. The temporary cavity is the momentary stretch or movement of tissue away from the path of the bullet. This could be construed as an area of blunt trauma surrounding the tract of the projectile. The temporary cavity increases in size with increasing velocity. The largest portion of the temporary cavity is on the surface where the velocity of the striking missile is the greatest.12 The concept of the temporary cavity has been used to advocate excessive tissue debridement in high-velocity wounds. In truth, postinjury observation of wound healing and animal experiments involving microscopic examination of tissue in the temporary cavity demonstrate that the momentary stretch produced does not usually cause cell death or tissue destruction.13 As such, debridement of high-velocity injuries should be confined to obviously devitalized tissue. Bullets can be constructed to alter their performance and increase the permanent cavity after they strike their target. This can be enhanced in four ways that all work by increasing the surface area of the projectile–tissue interface that facilitates the transfer of kinetic energy to the target. These include the following: (a) yaw, the deviation of the projectile in its longitudinal axis from the straight line of flight; (b) tumbling, the forward rotation around the center of mass; (c) deformation, a mushrooming of the projectile that increases the diameter of the projectile, usually by a factor of 2, increases the surface area, and, hence, the tissue contact area by four times; hollow point, soft nose, and dum–dum bullets all promote deformation; and (d) fragmentation, in which multiple projectiles can weaken the tissue in multiple places and enhance the damage rendered by cavitation. This usually occurs in high-velocity missiles. Nonfragmenting bullets will have a deeper penetration, whereas a fragmented projectile will not penetrate as deeply, but will affect a larger cross-sectional area.1416 If the bullet deforms, yaws, tumbles, or fragments, it will cause more tissue destruction. This occurs in deeper structures, not at the surface (Fig. 1-2). Wounds caused by knives are of very low energy and cause only a permanent cavity. With little energy transferred to the tissue, serious injury is caused by directly striking vital structures such as the heart, major vessels, lung, or abdominal organs.

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FIGURE 1-2 Yaw, tumble, deformation, and fragmentation. (A) Yaw describes deviation from flight path along the longitudinal axis. (B) Tumble is deviation in a “head over heels” manner. (C) Deformation occurs on impact and increases the actual surface area of the projectile. (D) Fragmentation involves the bullet scattering. All of these increase surface area of the projectile/tissue interface.

Image Blast Injury and Ionizing Radiation

The transfer of energy that results from explosions follows the previously stated rules of physics, but also has additional dimensions that deserve mention. The transmission of energy from an explosive blast is best understood in the context of wave mechanics. All conventional explosions have in common several characteristics in that they all involve a solid or liquid mixture that undergoes a rapid chemical reaction producing a gaseous by-product and a large amount of released energy. This release of energy pushes gaseous molecules from the explosion and within the atmosphere radially away from the explosion center producing a spherical wave of compressed gas, known as the blast wave, with increased density, pressure, and temperature when compared with the ambient air. The movement of these molecules creates what is known as a blast wind, and the compression of these molecules into a given space increases the density and pressure. This blast overpressure is defined as the wavefront pressure generated above ambient pressure. This peak overpressure is a function of the energy released from the blast and the distance from the point of detonation, and its decay is expressed as a scaling function17

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where W/W1 is a ratio of weights of a given explosive and D/D1 a ratio of distances from the epicenter. A compilation of experimental results showed that if a peak overpressure for one weight of explosive occurred at one distance, the same overpressure could be produced with a smaller weight of explosive at a shorter distance and for a larger weight of explosives at a longer distance (Fig. 1-3A). This relationship is known as the cube root rule or Hopkinson’s rule, and has been demonstrated to hold true for numerous modern-day explosive materials.18

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FIGURE 1-3 Physical characteristics of an explosive blast. (A) The Scaling Laws relate the overpressure at specific distances to the ratio of distances from the epicenter of a blast and the cube root of the ratios of corresponding weights of the charges. (B) The Pressure–time relationship at any given distance from the epicenter—the peak overpressure represents the passing wave front with a subsequent decrease in pressure until ambient pressure is reached. This is known as the positive phase. The passing wave will then cause a decrease in pressure below baseline resulting in a relative vacuum, or negative pressure phase.

At any given distance from the explosion there will be a distinct pressure–time curve with an abrupt increase in overpressure. Peak overpressure is dictated by the cube root rule and a decay in pressure that varies with the particular explosive compound and the time past the initial blast wavefront. As the wave moves past a given point, this positive pressure phase will be followed by a negative pressure phase19(Fig. 1-3B). Pressure is a force applied per unit area. When a force is applied over a given time, an impulse is present and has the ability to change momentum. This force when applied across a distance has the ability to transfer energy and do work.

Nuclear blast waves have a similar pressure–time relationship, but their positive phase may last several seconds as compared to the milliseconds of conventional munitions.20 The energy released from a nuclear explosion is on the order of thousands of times greater than conventional explosives with a corresponding increase in overpressure. The energy available is dictated by Einstein’s mass–energy equivalency formula:21

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A large portion of this energy is released in the form of kinetic energy that dictates the wave characteristics of the blast. Also, there is the production of high-energy subatomic particles, such as gamma radiation, which has the ability to cause destruction at the cellular level. Where h = Planck’s constant, the energy of these particles is directly related to their frequency (v).22

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They can be released at the time of the blast, but also for a period of time after the explosion as unstable products of a nuclear reaction undergo radioactive decay. Therefore, a nuclear explosion has the ability to transfer energy to a victim and do damage long after the initial blast.

PROPERTIES OF BIOMATERIALS

Image Stress, Strain, Elasticity, and Young’s modulus

When a force is applied to a particular material, it is typically referred to as a stress, which is a load or force per unit area. This stress will cause deformation of a given material. Strain is the distance of the deformation caused by the stress, divided by the length of the material to which the stress was applied.23

Strain can be tensile, shear, compressive, or overpressure (a relative of compressive strain) (Fig. 1-4). Tensile strain of a particular structure or organ occurs as opposing forces are applied to the same region. The forces are opposite and concentrated upon a particular point. This essentially interrupts the integrity of the structure by pulling it apart. Shear strain occurs as opposing forces are applied to a particular structure, but at different points within that structure. This can be caused by an application of opposing external forces or can arise from a relative differential in the change of momentum within a single structure or between structures that are attached to one another.24

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FIGURE 1-4 Biomechanical mechanisms of injury. (A) Tensile strain—Opposite forces stretching along the same axis. (B) Shear strain—Opposite forces compress or stretch in opposite direction but not along the same axis. (C) Compressive strain—Stress applied to a structure usually causing simple deformation. (D) Overpressure—A compressive force increases the pressure within the viscus passing the “breaking point” of the wall.

Compressive strain is the direct deformation that occurs as a result of impact. The energy involved with a particular force does work on the structure causing a crushing-type injury resulting in deformation and interruption of the structural integrity of the injured organ. Overpressure is a type of compressive strain that is applied to a gas- or fluid-filled cavity. The energy applied to a gas- or fluid-filled viscus can deform that structure and cause a decrease in the volume of the structure. Following Boyle’s law:

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The product of the pressure and the volume prior to an applied force must be equal to the product afterward.4 Therefore, a decrease in its original volume will increase the pressure inside that viscus. If the rise in pressure, which is a force, overcomes the tensile strength of the viscus, it will rupture.25

When stress is plotted on the same graph as strain, there are several clear and distinct aspects to the curve. The elastic modulus is that part of the curve in which the force does not cause permanent deformation, and a material is said to be more elastic if it restores itself more precisely to its original configuration.26 The portion of the curve beyond this is called the plastic modulus and denotes when an applied stress will cause permanent deformation.27 The tensile, compressive, or shear strength is the level of stress at which a fracture or tearing occurs.28 This is also known as the “failure point.” The area under the curve is the amount of energy that was applied to achieve the given stress and strain (Fig. 1-5).29

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FIGURE 1-5 The concept of stress, strain, elastic modulus, plastic modulus, tensile strength and energy as demonstrated by a tensile stress applied to a given structure. The tensile strain is the change in length under a stress divided by the original length. This concept is applicable to compressive and shear strain. In the stress/strain relationship the elastic modulus is the portion of the curve where permanent deformation does not occur as opposed to the plastic modulus where it does fracture or tearing occurs at the tensile strength. The energy applied is the area under the curve.

How well tissue tolerates a specific insult varies with the type of force applied and the tissue in question. In blunt and penetrating trauma, the higher the density of a particular tissue, the less elastic it is and the more energy is transferred to it in a collision. Lung is air-filled and extremely elastic. In lower velocity blunt trauma, energy tends to be dissipated across the lung easily, while in penetrating trauma the actual destruction of the permanent cavity and stretch caused by the temporary cavity are better tolerated because of the elasticity of the lung. In contrast, solid organs such as spleen, liver, or bone tend to absorb energy and will have greater tissue destruction as a consequence.30In blast injury it is the air-filled structures of the lung and bowel that tend to be injured because of their ability to transmit the blast wave and cause localized pressure increases that overcome the structural failure point of the organ.20

BLUNT TRAUMA MECHANISMS AND PATTERNS OF INJURY

The transfer of energy and application of forces in blunt trauma is often much more complex than that of penetrating trauma. The most frequent mechanisms of blunt trauma include motor vehicle crashes, auto–pedestrian crashes, and falls from a significant height. In these instances there are typically varying energies and forces in both the victim and the striking object. Other variables that complicate care include the larger surface area over which the energy is dispersed as compared to penetrating trauma and the multiple areas of contact that can disperse energy to different regions of the victim’s body. The interactions and directions of these lines of force and energy dispersion are often instrumental in causing specific kinds of injury.

Image Motor Vehicle Crashes

Although there are frequently confusing vectors for energy transfer and force in a victim of a motor vehicle crash, mortality is directly related to the total amount of energy and force available. Mortality from motor vehicle crashes is accounted for in large part by head-on collisions with mortality rates up to 60%. Side impact collisions (20–35%) and rollovers (8–15%) have progressively lower mortality rates with rear-end collisions (3–5%) having the lowest.3132 Rollover crashes have a lower than expected mortality because the momentum is dissipated, and forces generated and projected to the passenger compartment are in a random pattern that frequently involves many different parts of the car. Although there are certain forces and patterns of energy exchange that occur in a motor vehicle crash, the vehicle itself does offer some degree of protection from the direct force generated by a collision. Patients who are ejected from their vehicle have the velocity of the vehicle as they are ejected and a significant momentum. They typically strike a relatively immobile object or the ground and undergo serious loads. Trauma victims who were ejected from the vehicle were four times more likely to require admission to an intensive care unit, had a 5-fold increase in the average Injury Severity Score, were three times more likely to sustain a significant injury to the brain, and were five times more likely to expire secondary to their injuries in one study.33

Understanding the changes in momentum, forces generated, and patterns of energy transfer between colliding vehicles is important. For example, the principal direction of force in a head-on collision is affected by the degree of overlap of the vehicles.34 Yet, the behavior of the occupants of the passenger compartment in response to this is what helps identify specific patterns of injury. In frontal collisions the front of the vehicle decelerates as unrestrained front-seat passengers continue to move forward in keeping with Newton’s first law. Lower extremity loads, particularly those to the feet and knees, occur early in the crash sequence and are caused by the floorboard and dashboard that are still moving forward. Therefore, relative contact velocity and change in momentum are still low. Contact of the chest and head with the steering column and windshield occurs later in the crash sequence; therefore, contact velocities and deceleration, change in momentum, and contact force are higher.31,35

Types of injuries are dependent on the path the patient takes. The patient may slide down and under the steering wheel and dashboard. This may result in the knee first impacting the dashboard causing a posterior dislocation and subsequent injury to the popliteal artery. The next point of impact is the upper abdomen or chest. Compression and continued movement of solid organs results in lacerations to the liver or spleen. Compression of the chest can result in rib fractures, cardiac contusion, or a pneumothorax from the lung being popped like a paper bag. Finally, the sudden stop can cause shear forces on the proximal descending thoracic aorta resulting in a partial- or full-thickness tear. The other common path is for the occupant to launch up and over the steering wheel. The head then becomes the lead point and strikes the windshield resulting in a starburst pattern on the windshield. The brain can sustain direct contusion or can bounce within the skull causing brain shearing and a contrecoup injury. Once the head stops, forces are transferred to the neck that may sustain hyperflexion, hyperextension, or compression injuries, depending on the angle of impact. Once the head and neck stop, the chest and abdomen strike the steering wheel with similar injuries to the down and under path.

Lateral collisions, specifically those that occur on the side of a seated passenger, can be devastating because of the small space between the striking car and the passenger. Therefore, resistance to slow momentum of the striking car prior to contact with the passenger is limited. If the side of the car provides minimal resistance the passenger can be exposed to the entire momentum change of the striking car. These loads are usually applied to the lateral chest, abdomen, and pelvis and, as such, injuries to the abdomen and thorax are more frequent in lateral collisions than in frontal collisions.35 Injuries to the chest include rib fractures, flail chest, and pulmonary contusion. Lateral compression often causes injuries to the liver, spleen, and kidneys, as well. Finally, the femoral head can be driven through the acetabulum.

Rear-end collisions are classically associated with cervical whiplash-type injury and are a good example of Newton’s first law at work. When the victim’s car is struck from behind, the body, buttressed by the seat, undergoes a forward acceleration and change in momentum that the head does not. The inertia of the head tends to hold it in a resting position. The forward pull of the victim’s trunk causes a backward movement on the head leading to hyperextension of the neck. Similarly, this injury pattern can also be seen in head-on collisions where a sudden deceleration of the victim’s trunk with a continued forward movement of the head is followed by a backward rotation resulting from recoil.36,37

Image Pedestrian Injuries

Pedestrian injuries frequently follow a well-described pattern of injury depending on the size of the vehicle and the victim. Nearly 80% of adults struck by a car will have injuries to the lower extremities. This is intuitively obvious as the level of a car’s bumper is at the height of the patient’s knee and this is the first contact point in this collision sequence. A victim struck by a truck or other vehicle with a higher center of mass will more frequently have serious injuries to the chest and abdomen because the initial force is applied to those regions. In the car–pedestrian interaction, the force applied to the knee region causes an acceleration of the lower portion of the body that is not shared by the victim’s trunk and head, which tend to stay at rest, by Newton’s first law. As the lower extremities are pushed forward they will act as a fulcrum bringing the trunk and head forcefully down on the hood of the car applying a secondary force to those regions, respectively. The typical injury pattern in this scenario is a tibia and fibula fracture or dislocation of the knee joint, injury to the trunk such as rib fractures or rupture of the spleen, and injury to the brain.38,39

Image Falls

Falls from height can result in a large amount of force transmitted to the victim. The energy absorbed by the victim at impact will be the kinetic energy at landing. This is related to the height from which the victim fell. The basic physics formula describing the conservation of energy in a falling body states that the product of mass, gravitational acceleration, and height, the potential energy prior to the fall, equals the kinetic energy as the object strikes the ground. With mass and gravitational acceleration being a constant for the falling body, velocity, and, therefore, momentum and kinetic energy are directly related to height.4 The greater the change in momentum upon impact the larger the load or force applied to the victim. Injury patterns will vary depending upon which portion of the victim strikes the ground first and, hence, how the load is distributed.

The typical patient with injuries sustained in a free fall has a mean fall height of just under 20 ft. One prospective study of injury patterns summarized the effects of falls from heights ranging between 5 and 70 ft. Fractures accounted for 76.2% of all injuries, with 19–22% of victims sustaining spinal fractures and 3.7% developing a neurological deficit.40 Nearly 6% of patients had intra-abdominal injuries, with the majority requiring operative management for injury to a solid organ. Bowel and bladder perforation were observed in less than 1% of injuries.41

ANATOMIC CONSIDERATIONS

Image Injury to the Head (Brain and Maxillofacial Injury)

The majority of closed-head injuries are caused by motor vehicle collisions (MVCs), with an incidence of approximately 1.14 million cases each year in the United States.42,43 The severity of traumatic brain injury represents the single most important factor contributing to death and disability after trauma and may contribute independently to mortality when coexistent with extracranial injury.37,44,45 Our knowledge of the biomechanics of injury to the brain comes from a combination of experiments conducted with porcine head models, biplaner high-speed x-ray systems, and computer-driven finite element models.46

There are a multitude of mechanisms that occur under the broad heading of traumatic brain injury. All are a consequence of loads applied to the head resulting in differing deceleration forces between components of the brain. Brain contusion can result from impact and the associated direct compressive strain. The indirect component of injury to the brain on the side opposite to that of impact is known as the contrecoup injury. This occurs because the brain is only loosely connected to the surrounding cranium. As a result, after a load is applied to the head causing a compressive strain at the point of impact and setting the skull in motion along the line of force, the motion of the brain lags behind the skull. As the skull comes to rest, or even recoils, the brain, still moving along the line of the initial load, strikes the calvarium on the opposite side and another compressive strain is generated. The existence of the coup–contrecoup injury mechanism is supported by clinical observation and has been confirmed by a three-dimensional finite element head model and pressure-testing data in cadavers.47 It is even suspected that this forward acceleration of the brain relative to the skull may set up a tensile strain in the bridging veins causing their laceration and formation of a subdural hematoma.48

Injury to the superficial regions of the brain is explained by these linear principles; however, injury to the deep structures of the brain, such as diffuse axonal injury (DAI), is more complicated. Several authors have tried to explain DAI as a result of shear strain between different parts of the brain, but there is also another model known as the stereotactic phenomenon. This model relies more on wave propagation and utilizes the concavity of the skull as a “collector,” which focuses multiple wave fronts to a focal point deep within the brain, causing disruption of tissue even in the face of minimal injury at the surface of the brain.49 This “wave propagation” through deeper structures within the brain, such as the reticular-activating system, with subsequent disruption of their structural integrity is thought to account for a loss of consciousness, the most frequent serious sign after blunt trauma to the brain.50 Current research characterizes DAI as a progressive process induced by the forces of injury, gradually evolving from focal axonal alteration to eventual disconnection. Traumatically induced focal axolemmal permeability leads to local influx of Ca2+ causing the release of proteases that digest the “membrane skeleton.” This ultimately leads to local axonal failure and disconnection.51 An injury caused by shear strain is a laceration or contusion of the brainstem. This is explained by opposing forces applied to the brain and the spinal cord perpendicular to their line of orientation, with the spinal cord and brainstem being relatively fixed in relation to the mobile brain.

Maxillofacial injuries are associated with injuries to the head and brain in terms of mechanism and are a common presentation after motor vehicle crashes. The classic force vector that results in mid-face fractures is similar to that of traumatic brain injury and occurs when a motor vehicle occupant impacts the steering wheel, dashboard, or windshield. Nearly all of these subtypes of injury are secondary to compressive strain. This mechanism is associated with the greatest morbidity for the driver and front-seat passenger, while the forces are attenuated for the back-seat passenger impacting the more compliant front seat.

Image Thoracic Injury

The primary mechanism of blunt trauma to the chest wall involves inward displacement of the body wall with impact. Musculoskeletal injury in the chest is dependent upon both the magnitude and rate of the deformation of the chest wall and is usually secondary to compressive strain from the applied load. Patterns of injury for the internal organs of the thorax frequently reflect the interactions between organs that are fixed and those that are relatively mobile and compressible. This arrangement allows for differentials in momentum between adjacent structures that lead to compressive, tensile, and shear stresses.

The sternum is deformed and rib cage compressed with a blunt force to the chest. Depending on the force and rate of impact in a collision, ribs may fracture from compressive strain applied to their outer surface and consequent tensile strain on the inner aspects of the rib. Indirect fractures may occur due to stress concentration at the lateral and posterolateral angles of the rib. Furthermore, stress waves may propagate deeper into the chest resulting in small, rapid distortions or shear forces in an organ with significant pressure differential across its parenchymal surface (i.e., the air and tissue interface of the lung). This is thought to be the mechanism causing a pulmonary contusion.

Blunt intrusion into the hemithorax and a pliable lung could also result in overpressure and cause a pneumothorax. A direct load applied to the chest compresses the lung and increases the pressure within this air-filled structure beyond the failure point of the alveoli and visceral pleura. This overpressure mechanism may also be seen with fluid (blood) instead of air in a blunt cardiac rupture. High-speed cine-radiography in an anterior blunt chest trauma model in the pig has demonstrated that the heart can be compressed to half of its pre-crash diameter with a doubling of the pressure within the cardiac chambers.52If the failure point is reached, rupture occurs with disastrous results.

There are several examples of indirect injury secondary to asynchronous motion of adjacent, connected structures and development of shear stress at sites of attachment.53 Mediastinal vascular injury and bronchial injury are examples of this mechanism. Rupture or transection of the descending thoracic aorta is a classic deceleration injury mediated by shear forces. This injury can occur in frontal or lateral impacts and occurs because of the continued motion of the mobile and compressible heart in relation to an aorta that is tethered to more fixed structures.54 In frontal and lateral impacts the heart moves in a horizontal motion relative to an aorta that is fixed to the spinal column by ligamentous attachments. This causes a shear force applied at the level of the ligamentum arteriosum. When the stress is applied in a vertical direction, such as a fall from a height in which the victim lands on the lower extremities, the relative discrepancy in momentum is in that plane and a tensile strain is generated at the root of the ascending thoracic aorta (Fig. 1-6). Injury to a major bronchus is another example of this mechanism. The relatively pliable and mobile lung generates a differential in momentum in a horizontal or vertical plane depending on the applied load as compared to the tethered trachea and carina. This creates a shear force at the level of the mainstem bronchus and explains why the majority of blunt bronchial injuries occur within 2 cm of the carina (Fig. 1-7).

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FIGURE 1-6 Various mechanisms of injury for thoracic aorta injury. In a horizontal deceleration the heart and arch move horizontally away from the descending aorta causing shear strain and tearing at the ligamentum arteriosum. A vertical deceleration causes caudad movement of the heart, causing a strain at the root of the ascending aorta.

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FIGURE 1-7 Mechanisms of injury for bronchial injury. The carina is tethered to the mediastinum and spinal complex while the lungs are extremely mobile, setting up shear strain in the mainstem bronchus upon horizontal or vertical deceleration.

Image Abdominal Injury

Abdominal organs are more vulnerable than those of the thorax because of the lack of protection by the sternum and ribs. A number of different mechanisms account for the spectrum of injury observed in blunt trauma to the abdomen. With regard to the solid abdominal organs, a direct compressive force with parenchymal destruction probably accounts for most observed injuries to the liver, spleen, and kidney. Shear strain can also contribute to laceration of these organs. As with the previous description of strain forces, a point of attachment is required to exacerbate a differential in movement. This can occur at the splenic hilum resulting in vascular disruption at the pedicle or at the ligamentous attachments to the kidney and diaphragm. Shear forces in the liver revolve upon the attachments of the falciform ligament anteriorly and the hepatic veins posteriorly, explaining injuries to the parenchyma in these areas. Another significant injury related to this mechanism is injury to the renal artery. The renal artery is attached proximally to the abdominal aorta, which is fairly immobile secondary to its attachments to the spinal column, and distally to the kidney, which has more mobility. A discrepancy in momentum between the two will exact a shear stain on the renal artery resulting in disruption.55 This same relation to the spinal column occurs with the pancreas (Fig. 1-8). The relatively immobile spine and freely mobile pancreatic tail predispose to a differential in momentum between the two in a deceleration situation leading to fracture in the neck or body of the pancreas. The biomechanics of such injuries suggest that the body’s tolerance to such forces decreases with a higher speed of impact, resulting in an injury of greater magnitude from a higher velocity collision.29

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FIGURE 1-8 Points of shear strain in blunt abdominal trauma. All of these points occur where a relatively fixed structure is adjacent to a mobile structure.

Perforation of a hollow viscus in blunt abdominal trauma occurs in approximately 3% of victims.56 The exact cause is a matter of debate. Some believe that it is related to compressive forces, which cause an effective “blowout” through generation of significant overpressure, whereas others believe that it is secondary to shear strains. Both explanations are plausible, and clinical observations have supported the respective conclusions. Most injuries to the small bowel occur within 30 cm of the ligament of Treitz or the ileocecal valve, supporting the shear force theory57 (Fig. 1-8). Yet, injuries do occur away from these points of fixation. Also, experiments have documented that a “pseudo-obstruction” or temporarily closed loop under a load can develop bursting pressures as described by the overpressure theory.58Clinically, this is confirmed by the largest percentage of small intestinal injuries being of the “blowout” variety.59 Most likely, both proposed mechanisms are applicable in individual instances. The most common example of the pseudo-obstruction type is blunt rupture of the duodenum, where the pylorus and its retroperitoneal location can prevent adequate escape of gas and resultant high pressures that overcome wall strength.

Another important example of overpressure is rupture of the diaphragm. The peritoneal cavity is also subject to Boyle’s law, which states that volume of a gas is inversely proportional to pressure. A large blunt force, such as that related to impact with the steering wheel, applied to the anterior abdominal wall will cause a temporary deformation and decrease in the volume of the peritoneal cavity. This will subsequently raise intraabdominal pressure. The weakest point of the cavity is the diaphragm with the left side being the preferred route of pressure release as the liver absorbs pressure and protects the right hemidiaphragm. The relative deformability of the lung on the other side of the diaphragm facilitates this.

Image Musculoskeletal Injury

By far, the most common type of blunt injury in industrialized nations is to the musculoskeletal system. The ratio of orthopedic operations to general surgical, thoracic, and neurosurgical operations is nearly 5:1. As stated earlier, seatbelts and air bags have significantly decreased the incidence of major intracranial and abdominal injuries; however, they have not decreased the incidence of musculoskeletal trauma. Although these are not usually fatal injuries, they often require operative repair and rehabilitation and can leave a significant proportion of patients with permanent disability.60 With the advent of seatbelt laws, improved restraint systems, and air bags in motor vehicles, the incidence of lower extremity trauma, in particular, has increased. It is thought that these patients in the past may have suffered fatal injuries to the brain or torso and, therefore, their associated fractures of the femur, tibia, and fibula were not included in the overall list of injuries.

The type and extent of injury is determined by the momentum and kinetic energy associated with impact, underlying tissue characteristics, and angle of stress of the extremity. High-energy injuries can involve extensive loss of soft tissue, associated neurovascular compromise, and highly comminuted fracture patterns. Low-energy injuries are often associated with crush or avulsion of soft tissue in association with simple fractures. Injuries to soft tissue are usually secondary to compressive strain with crush injury as an example. Tensile and shear strain mechanisms, however, are present with degloving and avulsion injuries, respectively.

Most of that written about musculoskeletal injury involves fractures of long bones. Although each fracture is probably a consequence of multiple stresses and strains, there are four basic biomechanisms (Fig 1-9). In a lateral load applied to the mid shaft of a long bone, bowing will occur and compressive strain occurs in the cortex of the bone where the load is applied. The cortex on the opposite side of the bone will undergo tensile strain as the bone bows away from the load. Initially, small fractures will occur in the cortex undergoing tensile strain because bone is weaker under tension than it is under compression.61Once the failure point is reached on the far side from the load, the compressive strain increases markedly and the failure point for the side near the applied load is reached, also, resulting in a complete fracture. This mechanism can be seen in passengers in lateral collisions, pedestrians struck by a passenger car in the tibia and fibula region, or in the upper extremities from direct applied force in victims of assault with a blunt instrument.

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FIGURE 1-9 Fracture mechanics. A lateral load causing “bowing” will create tensile strain in the cortex opposite the force and compressive strain in the adjacent cortex. If a longitudinal stress caused “bowing” a similar strain pattern occurs. If no bowing occurs the strain is all compressive. A torsion load will cause a spiral fracture.

When a longitudinal load is placed on a long bone, bowing can also occur, and the compressive and tensile strain patterns will be similar to that previously described. If bowing does not occur, then only a compressive strain is seen and a compression fracture can occur. In the case of the femur this usually occurs distally with the shaft being driven into the condyles. These mechanisms can be seen in falls from a height, but are more frequently seen in head-on collisions resulting in fractures of the femur or tibia. In these cases deceleration occurs and the driver’s or passenger’s feet receive a load from the floorboard or the knee receives a load from the dashboard upon deceleration. This causes a longitudinal force to be applied to the tibia or femur, respectively. A torsional load will cause the bone to fracture in a spiral pattern.

Image Injury to the Spine and Whiplash

Injury to the vertebral column and spinal cord can be devastating and is frequently the result of a complex combination of specific anatomic features and transmitted forces. These can cause a wide variety of injury patterns distributed through the different portions of the vertebral column. Deceleration forces in motor vehicle crashes, such as impact with the windshield, steering assembly, and instrument panel, inertial differences in the head and torso, or ejection are responsible for both flexion and hyperextension injuries. Although the biomechanics of transmission of force can be readily demonstrated for the vertebral column’s individual components (disks, vertebrae, etc.), a model demonstrating injury patterns in the intact spinal unit is lacking.36 The cervical spine is most frequently injured in motor vehicle crashes, due to its relatively unprotected position compared to the thoracic and lumbar regions. Injuries are related to flexion, extension, or lateral rotation, along with tension or compression forces generated during impact of the head. The direction and degree of loading with impact account for the different injury patterns in trauma to the cervical spine.29 Approximately 65% of injury is related to flexion–compression, about 30% to extension–compression, and 10% to extension–tension injuries.62 Fracture dislocations of the vertebrae are related to flexion and extension mechanisms, whereas fractures of the facets are related to lateral-bending mechanisms. In contrast to trauma to the cervical spine, injury to the thoracic or lumbar spine is more likely related to compressive mechanisms. The rib cage and sternum likely provide stabilizing forces in motor vehicle crashes and lessen the risk of injury in these regions.

Whiplash refers to a pattern of injury seen often in MVCs with a rear-end impact. The injury is usually a musculoligamentous sprain, but may be combined with injury to cervical nerve roots or the spinal cord. Patients typically experience neck pain and muscle spasm, although an additional spectrum of symptoms has been described.63 The etiology of whiplash probably relates to acceleration and extension injury, with some rotational component in non–rear-impact crashes. Factors related to poor recovery following whiplash injury are a combination of sociodemographic, physical, and psychological, and include female gender, low level of education, high initial neck pain, more severe disability, increased levels of somatization, and sleep difficulties.64

Image Kinematics in Prevention

The ideas of William Haddon have become the cornerstone of injury prevention, and approximately a third of his strategies involve altering the interaction of the host and the environment.65 Understanding forces and patterns of energy transfer have allowed the development of devices to reduce injury. Most of this understanding has been applied to the field of automotive safety.

The first set of design features revolve around the concept of decreasing the force transmitted to the passenger compartment. This includes the “crumple zone,” which allows the front and rear ends of a car to collapse upon impact. The change in momentum the passenger compartment undergoes in a collision will, therefore, occur over a longer period. Going back to the impulse and momentum relation, this means less force will be transmitted to the passenger compartment. In terms of energy, work is done in the crumple zone and energy is expended before reaching the passenger compartment.66 The second design feature directs the engine and transmission downward and not into the passenger compartment decreasing intrusion into the passenger compartment.

Passenger restraint systems, which include safety harnesses and child car seats, keep the passengers’ velocity equal to that of the car and prevent the passengers from generating a differential in momentum and striking the interior of the car. Also, they more evenly distribute loads applied to the victim across a greater surface area thus decreasing stress.

Even with restraint systems the occupants of a car can develop relative momentum and kinetic energy during a crash. This energy and momentum can be dissipated by air bags, which convert it into the work of compressing the gas within the device. The helmets used by cyclists and bicyclists work on a similar principle in that a compliant helmet absorbs some of the energy of impact, which is therefore not transmitted to the brain. Many studies have demonstrated the benefits of using seatbelts and air bags with mortality reductions ranging from 41 to 72% for seatbelts, 63% for air bags, 80% for both, and 69% for child safety seats.61 Seatbelts and air bags have also significantly reduced the incidence of injuries to the cervical spine, brain, and maxillofacial region by keeping the forward momentum of the passenger to a minimum and preventing the head from striking the windshield.46 Also worth mentioning is the headrest that has decreased whiplash-type injury by 70% by preventing a difference in momentum between the head and body and hyperextension of the neck in rear-end collisions.67

Despite their effectiveness, air bags can be responsible for injury in motor vehicle crashes. Approximately 100 air bag–related deaths were confirmed by National Highway Traffic Safety Administration (NHTSA) over a 5-year period, many associated with improper restraint of small adults or children in front-seat locations. Additionally, a spectrum of minor injuries such as corneal abrasions and facial lacerations have been seen in low-speed impacts. Injuries can occur from the use of safety belts, as well. Lap seatbelts can cause compressive injuries such as rupture of the bowel, pelvic fractures, and mesenteric tears and avulsions. They can also act as a fulcrum for the upper portion of the trunk and be associated with hyperflexion injuries such as compression fractures of the lumbar spine. As a consequence, newer automobiles are required to have the more extensive and protective lap and shoulder harness style belts. Even still, shoulder harnesses can cause intimal tears or thrombosis of the great vessels of the neck and thorax and fracture and dislocation of the cervical spine in instances of submarining, where the victim slides down under the restraint system.68 Even when a shoulder harness system works as intended, clavicular and rib fractures or perforations of hollow viscera in the abdomen secondary to a compressive-type mechanism can occur.6970

SPECIAL CONSIDERATIONS

Image Pediatrics

Differences have been noted between adults and children in both patterns of injury and physiological responses to injury. In one analysis of adults and children sustaining comparable degrees of injury from blunt trauma, significant differences were noted in the incidence of thoracic, spinal, and pelvic injuries in children. Although the overall incidence of injury to the brain is higher in blunt pediatric trauma, thoracic and pelvic injuries occur less frequently.71

Overall mortality is generally higher for adults than it is for children sustaining comparable degrees of injury. When assessed by mechanism, however, mortality is slightly higher for children in motor vehicle crashes.72

The most significant difference between adults and children is in the compliance of the bony structures. This difference is seen commonly in the resilience of the chest wall. The incidence of rib fractures, flail chest, hemo-pneumothorax, and injury to the thoracic aorta in children is significantly less than that in adults, though the incidence of pulmonary contusion is higher. Because of this resilience, the chest wall can absorb a greater impact in children while demonstrating less external sign of injury. In children the index of suspicion for a pulmonary contusion, in the absence of rib fractures, must be higher than in an adult.

Injury to the spinal cord is rare in children, representing only 1–2% of all pediatric trauma. The cervical spine is injured in the majority of cases (60–80%), compared to 30–40% in adult injury. The immature spinal column has incomplete ossification, a unique vertebral configuration, and ligamentous laxity, which accounts for this difference in pattern of injury. The proportionally larger head and less developed neck musculature of younger children (<10 years old) account for more torque and acceleration stress in the higher cervical spine during injury, as well. Young children have high rates of dislocations and spinal cord injury without radiographic abnormality (SCIWORA), and these are more likely to be seen at the upper cervical levels. As older children have a low fulcrum of cervical motion (C5–C6) and more ossification and maturity of the vertebral bodies and interspinous ligaments, they have a high incidence of fractures in the lower cervical spine.73 SCIWORA is associated with 15–25% of all injuries to the cervical spine in pediatrics and represents a transient vertebral displacement and realignment during injury, resulting in damage to the spinal cord without injury to the vertebral column.

Childhood obesity is recognized as a leading public health issue in the United States. Childhood obesity is defined as an age and sex-specific body mass index at the 95th percentile or higher. Based on this definition, 14–17% of all children were obese from 1999 to 2004.69 When compared to their nonobese counterparts, obese children between the ages of 2 and 5 who are injured in a MVC are at an increased risk for major injuries to the brain and chest. Obese children above the age of 5 involved in an MVC are at an increased risk for major thoracic and lower extremity injuries in comparison to nonobese children of the same age and sex.74

Nothing has reduced the incidence of injury to children and infants more than the mandatory use of safety belts and restraints. The problem still to be faced is the different contours and shapes with infant restraints. Also, there has been increased interest in the issue of pediatric restraint systems because of a number of injuries related to air bags. It is recommended that restrained infants and children not be placed in a front seat and that all children under age 12 ride in the rear seat. Injuries related to air bags have ranged from minor orthopaedic trauma to fatal injury to the brain.75

Unfortunately, child abuse is a reality in the pediatric population and must be considered when evaluating a child who has been injured in less than clear circumstances or has multiple injuries of varying ages. Although injury to soft tissue is the most common presentation, fractures follow as a close second. There is a high rate of spiral fractures of the humerus and femur secondary to a torsional force, applied by an adult grabbing the child’s extremity in a twisting motion. Injury to the brain is the third most common injury, with skull fractures thought to be secondary to direct blows to the child’s head or the dropping and throwing of the child. Intracranial hemorrhage has been noted in the “shaken impact syndrome” and is thought to result from significant acceleration and deceleration forces followed by direct force transfer with impact. Subdural and subarachnoid hemorrhages can often result, as blood vessels between the brain and skull are ruptured. Retinal hemorrhage may also be identified in this pattern of injury and occurs in approximately 3% of cases. Impact injury to the abdomen is common in child abuse and can result in injury to solid organs (liver, spleen, or kidney), duodenal hematoma (sometimes with delayed symptoms of intestinal obstruction), pancreatitis, injury to the colon or rectum, or mesenteric bleeding. In addition, falls from even very small heights may cause severe intracranial hemorrhage in the infant or child.76

Image Pregnancy

Injury to pregnant women in motor vehicle crashes is estimated to account for 1500 to 5000 fetal deaths each year. There has been little investigation into specific forces and the kinematics of injury in pregnancy. Several studies have demonstrated that the most common cause of fetal demise in motor vehicle crashes with a viable mother is placental abruption. The biomechanics of this injury involves generation of tensile and shear forces, with the circumferential forces in the uterine wall inducing a shear strain across the placental surface, resulting in placental strain and subsequent abruption. Shorter women have a higher incidence of fetal demise with automotive crashes because of their close proximity to the steering wheel. As in other populations, restraints have been demonstrated to increase survival in both mother and fetus.77

Image Geriatrics

Trauma remains a disease of the young, though there is a significant incidence of morbidity and mortality in the elderly population. Death from trauma represents the fifth leading cause of mortality in persons over 65 years of age. The most common mechanisms of injury in the elderly are falls, fires, and vehicular trauma.

When patients with similar injury levels are compared with respect to age and mortality, the incidence of fatality in older persons is 5- to 10-fold higher than it is in the younger population. It is not the severity of injury that is crucial, but rather the incidence of comorbid factors in this population, especially cardiac and vascular disease. Most likely it is the patient’s inability to demonstrate a cardiovascular reserve that is a contributing factor to their subsequent increased morbidity and mortality. The Injury Severity Score and other predictors of outcome do not hold up in the geriatric or pediatric populations. Another significant finding in this population is that most (as many as 88%) of these patients never return to their previous level of independence.78

The incidence of falls in the geriatric population is high, with an annual incidence of approximately 30% in those over 65, and approximately 50% in those over 80 years of age. Falls account for approximately half the cases of geriatric trauma. Most falls in the elderly occur from standing with mortality secondary to the comorbid factors mentioned earlier.79 The propensity for fracture is also increased secondary to a loss of bone density with aging, with hip fractures being one of the most common injuries.

BLAST INJURIES

Blast injuries are among the most dramatic and devastating wounds encountered by the trauma community. The National Counterterrorism Center documented approximately 11,800 terrorist attacks in 2008, resulting in over 54,000 deaths and injuries.80 Although the number of terrorist incidents decreased from the previous year, overall fatalities had increased.8081 The vast majority of these attacks occurred in the Middle and Far East, but the United States was not immune from blast incidents. The Bureau of Alcohol, Tobacco, and Firearms noted an average of 182 annual injuries and 23 annual deaths from explosive incidents in the United States from the period of 2004 to 2006.82

Blast injuries are broadly categorized as primary, secondary, tertiary, quaternary, and quinary, based on a taxonomy of explosive injuries published by the Department of Defense in 2006.83 The trauma practitioner should be familiar with each of these patterns of injury and be able to predict associated injuries from each category (Table 1-2).

TABLE 1-2 Department of Defense Classification of Blast Injuries from Explosive Devices

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Primary blast injuries occur when the blast overpressure transmits forces directly onto a person, causing tissue damage. The air-filled organs are the most likely affected by a primary blast injury and include the tympanic membrane, lungs, and gastrointestinal tract.84 Primary blast injuries are less common in open-space explosions but are increased in situations where the explosion occurs within a confined space, which allows the blast wave to reflect off of fixed structures.85 Rupture of the tympanic membrane is the most common manifestation of primary blast injury, occurring in up to one half of patients injured in an explosion.84 Some have considered an intact tympanic membrane to be a strong negative predictor of severe blast injury, although this has proven not to be the case.8687 The orientation of the patient to the blast wave (perpendicular vs. parallel), the presence or absence of cerumen in the ear canal, and whether the patient was wearing hearing protection at the time of the blast will all work to alter the true impact of the blast on the tympanic membrane.84 Therefore, an intact tympanic membrane does not rule out blast injury.

The most common fatal injury among blast victims is to the lung, often referred to as “blast lung injury.” The blast wave causes tissue disruption at the capillary–alveolar interface, resulting in pulmonary edema, pneumothorax, parenchymal hemorrhage, and, occasionally, air embolus from alveolovenous fistulas.84 Clinical diagnosis of blast lung injury is dependent on the presence of the triad of hypoxia, respiratory distress, and bilateral or central infiltrates on a chest radiograph.88 The infiltrates are usually present on admission and can worsen with aggressive fluid resuscitation. These central infiltrates are also referred to as “butterfly” or “batwing” infiltrates and are pathognomonic for blast lung injury, in contrast to the peripheral infiltrates commonly seen with pulmonary contusions from blunt injury. Management of the ventilated patient with blast lung injury includes avoidance of positive pressure ventilation, minimization of positive-end expiratory pressure (PEEP), and judicious fluid resuscitation.84Fluid management in these patients will often be challenging due to associated injuries from secondary and tertiary blast effects, which often require greater amounts of intravenous fluid for adequate resuscitation.

Secondary blast injuries are created by debris from the explosive device itself or from surrounding environmental particles. Many devices contain additional munitions consisting of nails, pellets, ball bearings, and scrap metal designed to increase the lethality of the explosion. Fragments from the surrounding environment, including glass and small rocks, can become secondary missiles, as well. Secondary blast injuries are more common than primary blast injuries as the debris and added fragments travel over a much greater distance than does the shock wave from the primary blast.89 Lacerations, penetrating injury, and significant soft tissue defects are the most common injuries seen from secondary blast injuries.

Tertiary blast injuries are caused by the body being physically thrown a distance or from a solid object falling onto a person as a result of the explosion. Most tertiary injuries are from a blunt mechanism, and crush injuries or traumatic amputations are not uncommon. Quarternary and quinary blast injuries have only recently been defined. They are miscellaneous blast injuries caused directly by the explosion but often due to other mechanisms, such as burns, inhalation injuries, and radiation effects.

Children injured by explosions suffer a different injury pattern as compared to adults.90 Children are more likely to sustain life-threatening injuries and traumatic brain injury. They are less likely to have an extremity injury or significant open wounds. The adolescent injury pattern resembles that of the adult, although they are more likely to have fewer internal injuries, more contusions, and have a higher risk of requiring surgical intervention for mild or moderate wounds when compared to adults.

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