Front Impact Crashes

Front impact crashes are the type of crash that most people think of when talking about motor vehicle accidents. Textbooks tend to concentrate on this type of accident, because it is easy to explain and understand. When two vehicles crash head-on, or a vehicle crashes into a fixed object, unless the driver and passengers are restrained, they will continue their forward movement, even though the car has stopped. If unrestrained, the driver's knees will impact the instrument panel; the chest the steering wheel; and the head the windshield, sun visor region above the windshield, or the frame (generally in this order). The same pattern of injuries would be true for unrestrained passengers, except they would impact the dashboard rather than the steering wheel. If the front impact is off-center, the driver or passenger might impact the A pillar with their head.

If the driver is restrained by belts, but without an airbag, the knees still impact the instrument panel but the head flexes forward, with the chin impacting either the sternum or, in severe collisions, the steering wheel. Unrestrained individuals in the back seat will hit the back of the front seat, the passengers in the front seat come up against the windshield or the sun visor area.

Objects protruding from the instrument panel, such as levers or knobs, can produce patterned abrasions on the victims.

If the drivers and passengers have restraint devices and if the passenger compartment retains its integrity, then the occupants of the vehicle should survive without any significant injury. The intrusion of part of the vehicle or another object into the passenger compartment may be transitory, with the portion of car or the object springing back. Thus, it might not at first be obvious that there has been violation of the integrity of the passenger compartment.

If the head of the driver or front seat passenger impacts the windshield, there will be abrasions and superficial cuts of the forehead, nose, and face, with the injuries having a vertical orientation (Figure 9.1)Thin slivers of windshield glass might be embedded in the wounds or be found loose on the clothing. The glass cuts, per se, are not serious, thanks to the construction of the windshield. Windshields are designed to prevent serious cuts and people going through them. The latter phenomenon, however, can still occur if the windshield pops out of the frame. Present-day windshields consist of thin outer and inner layers of glass with a thick core of plastic. Impaction of the head against the windshield with great force can cause the glass to shatter, but the plastic will only bulge.

Blunt force impact on the windshield, while not causing serious incised wounds, can, with enough force, produce fairly severe soft tissue injuries. There can be partial avulsion of the skin with the avulsed skin anchored superiorly (Figure 9.2). These wounds, because of their location, often bleed very heavily, appearing very dramatic and life threatening. This has caused emergency room physicians to concentrate all their attention on these seemingly severe, but actually non-life-threatening head injuries, and to neglect chest and abdominal injuries that eventually caused death (Figure 9.3). In addition to the external injuries, impaction of the head with the frame of the car above the windshield can cause basilar skull fractures, closed head injury, and fractures of the neck. Basilar fractures tend to run along the length of the petrous ridges passing through the sella turcica ("hinge fractures"). Less common are ring fractures and multiple fracture lines of the base of the skull.

In neck injuries, the most common fatal injuries are upper cervical fractures or dislocation at the atlanto-occipital junction (Figure 9.4). This

Windshield Fracture Due Passenger
Figure 9.1 (A and B) Abrasions and superficial cuts of the forehead caused by impacting the windshield.
Skin Avulsion
Figure 9.2 Avulsion of skin caused by impact with windshield.

can result in either complete transection or crushing of the cord. In other instances, the cord is violently pulled down, with partial or complete avulsion of the brain stem, ventrally, at the ponto-medullary junction.

The chest of the driver can impact the steering wheel; the chest of the passenger, the dashboard. Evidence of injury from such an impact varies from imprinted abrasions/contusions of the wheel or instrument panel to complete absence of any evidence of external injuries (Figure 9.5). The following internal injuries are fairly typical, depending on the amount of force and the age of the victim:

• Transverse fracture of the sternum (usually at the third intercostal space)

• Bilateral rib fractures

• Impaling injuries of the lung caused by fractured ribs

• Contusions, internal lacerations, and rupture of the pulmonary parenchyma

• Rupture of the heart

• Transection of the aorta

• Lacerations of the liver and spleen

Pontomedullary Lacerations
Figure 9.3 (A) A 24-year-old male with severe laceration of forehead caused by impact with windshield. Treated for non-existent brain injuries. (B) Cause of death: a 1-cm laceration of right atrium with hemopericardium (250 ml).
Atlanto Occipitale Dislocation
Figure 9.4 Fracture-dislocation of neck at atlanto-occipital joint.

Because of the elastic nature of the sternum or ribs in young individuals, there can be extensive thoracic injuries without fracture. Serious chest injuries from impaction against the steering wheel became less frequent with the introduction of energy-absorbing compressible steering wheel columns in the late 1960s. At the time, this was estimated to have reduced overall driver fatalities in frontal crashes 12%, with serious injury (including fatalities) reduced by 38%.4

One of the most common fatal thoracic injuries is transsection of the aorta (Figures 5.2 and 5.3). Typically, this occurs immediately distal to the origin of the left subclavian artery. Occasionally, one will see at autopsy individuals who have survived motor vehicle accidents in the past and died for other reasons. On opening the aorta, faint, horizontally oriented, linear scars on the intimal surface of the aorta distal to the left subclavian artery can be seen. These represent incomplete lacerations of the intima that subsequently healed. While the etiology of thoracic aortic lacerations has classically been ascribed to rapid deceleration, more recent work suggests that it is caused by chest compression.5

Injury to the heart is less common than aortic injuries. The most common injuries are myocardial contusion, laceration of the pericardial

Chest Bruise Steering Wheel
Figure 9.5 Imprint of steering wheel on chest.

sac, rupture of the right atrium, rupture of the right ventricle anteriorly at the interventricular septum, rupture of the left atrium, and laceration of the interatrial septum (Figure 9.6). Occasionally, individuals die from intraparenchymal pulmonary hemorrhage caused by massive pulmonary contusions.

In addition to the chest injuries, there can be lacerations of the liver and spleen. The injuries of the liver range from superficial capsular lacerations to bursting rupture. With massive blunt trauma, there might be rupture of the left hemi-diaphragm. With both splenic and hepatic injuries, the extent of the injuries may not be appreciated initially, when there may only be production of subcapsular hematomas. The individual will be examined and thought to have only minor injuries. A peritoneal tap will reveal no blood. The patient will die hours, if not days, later when the subcapsular hepatic or splenic hematoma ruptures. The authors have seen deaths in individuals where the physicians were so concentrated on other injuries that they ignored the possibility of abdominal injuries. In one case, the patient was given an initial peritoneal tap with no blood found. Although the patient subsequently complained of abdominal pain and had a drop in hematocrit, the physicians were so concerned with the other injuries that they lost their patient to the liver injury.

Occasionally, there will be a motor vehicle accident in which the driver impacts the steering wheel and in which no anatomical cause of death

Steering Wheel Chest Injuries
Figure 9.6 Rupture of heart caused by chest's impacting steering wheel.

presents after a complete autopsy and toxicological screen. There may be soft tissue trauma to the chest and a fractured sternum or ribs, but insufficient injuries to explain death. Such deaths are caused by fatal cardiac arrhythmia secondary to a cardiac contusion. Examination of the heart might fail to reveal any evidence of impact because of the suddenness of the death. Some individuals do not develop cardiac arrhythmias until hospitalized. In these individuals, the presence of a cardiac contusion is confirmed by enzymatic tests and EKGs. The patients usually recover, though occasionally, they will die from an arrhythmia. Rarely, the authors have seen traumatic dissecting aneurysms of the left anterior descending coronary artery caused by blunt trauma to the chest.

Before death can be ascribed to cardiac contusion, positional or traumatic asphyxia must first be ruled out. A careful dissection of the neck, both anteriorly and posteriorly, in the upper cervical region must also be performed. The authors have seen a number of deaths caused by posterior fracture or dislocation of the upper cervical region, while no hemorrhage was seen anteriorly or in the subarachnoid space of the brain stem. This injury would have been missed if the posterior aspect of the cervical spine had not been explored. These injuries are caused by hyperflexion of the head.

In any automobile death in which no cause of death is found at autopsy, the back of the neck should be explored.

In elderly individuals, death can occur with injuries that a younger person might survive. Thus, rib fractures, minor cardiac contusions, pulmonary contusions, and some minor intrathoracic or intraabdominal injury that younger people will survive might cause death in an elderly individual with an unstable cardiac status.

If the knees impact the dashboard, there may be fractures of the patella or the distal femur, as the patella is driven as a wedge between the malleoli. There also can be dislocation at the hip joint or a fracture of the femur at its neck. In one case the authors saw, a woman in her early fifties was a passenger in the front seat of a car. She impacted her knees against the dashboard in a minor crash. She was seen in the emergency room of a prestigious trauma center where she complained of leg pains and had obvious bruising of the knees. The knees were X-rayed, but no fractures were seen. The woman said that she could not walk, so she was given crutches and sent home. A day and a half later, she was found dead in bed. At autopsy, a fracture of the neck of the right femur was found, with massive bleeding into the musculature and soft tissue of the thigh. Death was caused by exsanguination. The doctors in the emergency room, while dutifully X-raying the knees, had not examined the rest of the femur and had completely missed the fracture that produced death.

If the integrity of the passenger compartment is violated by the engine's being driven backward, the injuries produced can be the massive crushing injuries previously noted, plus injuries of the pelvis. Occasionally, the compartment may be violated, the injuries produced minor, yet death occurs. Death in these cases is usually caused by traumatic asphyxia with compression of the chest by the intruding portion of the car so that respiration is impossible.

In unrestrained individuals, the usual sequence and pattern of injuries is knee-femur-hip-chest-head. In restrained (belted) individuals, the nature of the injuries is dependent on the severity (force) of the crash. Knee contact usually still occurs. In moderate-speed collisions (30 mph and above), the forward movement of the head is such that it might impact the steering wheel. In all collisions, the individual is propelled toward the point of impact. This is true in angled front impact collisions. Thus, in an impact to the left front of the vehicle, the head of the driver moves front-left and might impact the A pillar while the passenger might impact the rearview mirror.6

If the driver and passenger are restrained by seat (lap and shoulder) belts and there is no intrusion into the passenger compartment, the probability of survival is great. In addition to preventing, or at least minimizing, occupant-to-interior impacts, seat belts virtually eliminate the risk of ejection, even in rollover crashes. Use of seat belts reduces the risk of fatalities to frontseat occupants by 45%.7

Seat belts and airbags, while effective in decreasing the incidence of death and injury, can, themselves, produce injuries, even death. Lap belts can produce tears of the mesentery and omentum and occasionally laceration of the bowel. Shoulder belt use may be reflected by a linear abrasion running downward and medially on the left side of the neck of the driver or the right side of the neck of the front passenger (Figure 9.7A). A poorly defined area of abrasion and contusion indicating the distribution of the belt might be seen on the skin of the lower abdomen (Figure 9.7B).

In addition to the aforementioned injuries, one also sees dicing injuries, which are superficial cuts of the skin caused by the fragments of glass produced when the side and back windows of a car shatter (Figure 9.8). Glass used in these windows is tempered glass, which is designed to shatter into little glass cubes on violent impact. This is to prevent the individuals in the car from incurring serious cuts from slivers of glass. The marks produced by these little cubes of glass tend to be linear, right angled, and very superficial. They are not life threatening. Drivers typically have them on the left side of the face and forehead and the left arm; passengers on the right side of the face and forehead and right arm.

In head-on crashes, the floorboards can be driven upward and inward, twisting the foot on the ankle and causing a fracture. In other instances, if the seat goes forward, the foot can be trapped beneath the seat, breaking the ankle.

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