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, motor vehicles striking pedestrians, 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.
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 largely 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.29,30 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 fivefold 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.31
Understanding the changes in momentum, forces generated, and patterns of energy transfer between colliding vehicles is important. Yet, the behavior of the occupants of the passenger compartment in response to these 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.29,32
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 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 with a starburst pattern resulting 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 which may undergo 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.33 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 trunk causes a backward movement on the head causing hyperextension of the neck. Similarly, this injury pattern can also be seen in head-on collisions, where a sudden deceleration of the trunk with a continued forward movement of the head is followed by a backward rotation resulting from recoil.34,35
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 in that the level of a car’s bumper is at the height of the patient’s knee. This is the first contact point in this collision sequence, with the largest force being applied to the lower extremity. Those struck by a truck or other vehicle with a higher center of mass more frequently have serious injury to the chest and abdomen, since 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 trunk and head, which, by Newton’s first law, tend to stay at rest. 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, injury to the trunk such as rib fractures or rupture of the spleen, and injury to the brain.36,37
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.6 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% sustaining spinal fractures and 3.7% showing a neurologic deficit.38 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.39
Injury to the Head (Brain and Maxillofacial Injury)
The majority of closed-head injuries are caused by motor vehicle collisions, with an incidence of approximately 1.14 million cases each year in the United States.40,41 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.35,42,43 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.44 There are a multitude of mechanisms that occur under the broad heading of a traumatic brain injury. They are all 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 direct compressive strain associated with it. 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, will strike 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.45 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.46
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 phenomena. 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.47 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.48 An injury caused by shear strain is the 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 in motor vehicle trauma. The classic force vector that results in mid-face fractures is similar to that of a traumatic brain injury and occurs when the occupant of a motor vehicle 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.
The primary mechanism of blunt thoracic trauma 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 stress.
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 (ie, 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 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 a model of anterior blunt chest trauma using a pig has demonstrated that the heart can be compressed to half of its precrash diameter with a doubling of the pressure within the cardiac chambers.49 If 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.50 Mediastinal vascular injury and bronchial injury are examples of this mechanism. Transaction of the thoracic aorta is a classic deceleration injury mediated by shear forces. This injury can occur in frontal or lateral impacts51 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. 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 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 main stem bronchus (Fig. 1-7) and explains why the majority of blunt bronchial injuries occur within 2 cm of the carina.
Various mechanisms of injury to the thoracic aorta. 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.
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 main stem bronchus upon horizontal or vertical deceleration.
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. Yet, 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 that of 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.52 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.26
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 trauma occurs in approximately 3% of victims.53 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, while others believe that it is secondary to shear strains.54 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, which supports the shear force theory (Fig. 1-8).55 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.56 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. 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 intra-abdominal 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.
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.57 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.58 Once 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.
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.
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.34 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.26 Approximately 65% of injury is related to flexion–compression, about 30% to extension–compression, and 10% to extension–tension injuries.59 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 motor vehicle collisions 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.60 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.61
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.62 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.63 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.58 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.44 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.64
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.65 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.66,67