Posted at 11:01 a.m. EDT Tuesday, April 10, 2001
Dale Earnhardt Accident
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See also: Accident Articles new 3/2/02
The purpose of this report is to explain the cause of death of Mr. Dale Earnhardt, with particular attention to the role of facial contact, inertial head loading (the whip mechanism), and impact near the top of the head.
Background
Mr. Earnhardt sustained a number of injuries as a result of his collision.
Mr. Earnhardt's fatal injury was the basilar skull fracture described in
the autopsy report. A basilar skull fracture is any fracture of the skull
in which the fracture originates in or propagates to the base (lower
portions) of the skull. There are several types of basilar skull fractures
including ring fractures, in which the fracture forms a complete or
incomplete ring around the foramen magnum (the hole through which the
spinal cord passes). Ring fractures are also a diverse group of fractures
and arise from many widely differing mechanisms. These include: impact to
the chin, jaw and face; impact to the head anteriorly, posteriorly or
laterally; impact near the top of the head; and inertial head loading, in
which the spine and neck muscles are called upon to stop the moving head.
In preparing this report, I considered and evaluated each of the possible
mechanisms.
The crash dynamics
Understanding the injury mechanism requires understanding and
reconstructing the crash. Mr. Earnhardt's car initially yawed
counterclockwise when viewed from above and traveled down (toward the
center of) the track. A right steer corrected this trajectory, however,
the vehicle yawed clockwise and began climbing up the track. As a result,
while most of the velocity of the car was directed along the track, a
significant component of velocity was directed toward the wall. Prior to
impacting the wall, Mr. Earnhardt's car contacted another vehicle, which
increased its clockwise yaw angle relative to its direction of travel. As
a result, the impact with the wall was equivalent to a passenger-side
angled barrier impact. As a result of the collision, Mr. Earnhardt's
velocity toward the wall was arrested and a component of his velocity
along the track was also arrested. This gave rise to a (change in
velocity) with a PDOF (direction), which is predominately frontal and from
the right. Because there was no significant additional rotation of the
vehicle after the crash, the (change in velocity) of the occupant is very
similar to the (change in velocity) of the vehicle. Mathematical analysis
and observation of the impact with the wall demonstrates that this was a
very severe crash. By contrast, many people have commented that the crash
did not "appear" to be severe. The reason for this common misconception is
that the frontal crash occurs in only one-tenth of a second. As such, the
crash is over in "a blink of an eye," and the (change in velocity) occurs
over a very short time, producing very large accelerations. Frontal
crashes which are angled to the right side of the vehicle are especially
dangerous for the head and the neck of the driver.
Occupant dynamics
In accordance with Newton's laws and as a result of the crash, Mr.
Earnhardt moved forward and slightly toward the right in the vehicle. This
motion was opposed by his restraint system, the steering wheel and the
other components of the vehicle interior with which he interacted (e.g.
the instrument panel with his knee, etc.). Initially, his head traveled
along this nearly straight line. As his chest and pelvis were stopped by
the restraint system, his head began to follow a circular arc forward and
down. Large inertial forces developed as the neck was stopping the forward
motion of both the head and the helmet. This is the basis of the whip
mechanism which occurs in right-side, angled, frontal collisions. In
crashes like Mr. Earnhardt's, these inertial forces alone can be large
enough to produce ring fractures of the skull base.
As the circular arcing motion continued, Mr. Earnhardt struck the steering wheel sub-mentally (on the underside of his chin). This caused significant deformation of the wheel rim and a spoke and resulted in the impression/abrasion described in the autopsy report. While an abrasion in and of itself might be associated with small or large forces, this particular abrasion was the result of a very significant impact involving large forces. Moreover, these forces were directed posterosuperiorly (upward and backward). These impact forces alone can be large enough to produce ring fractures of the skull base.
Noncontributing mechanisms
Other mechanisms of ring fractures include impacts to the top of the head
and impacts to the back of the head. Both of these have been suggested as
possible injury mechanisms in this case. The fracture pattern is not
consistent with either of these mechanisms, however. Impacts to the top of
the head produce ring fractures because the head is compressed between the
impact surface and the neck. These ring fractures commonly have associated
neck injuries and show evidence of compression in the fracture surfaces.
No neck injuries were present, and the fractures showed clear evidence of
tension. As such, the mechanism of an impact to the top of the head did
not cause Mr. Earnhardt's injury. Of note, I have not examined his helmet,
which would provide additional support for this conclusion. However, I do
not believe this is necessary in light of the other compelling findings.
Regarding the impact to the back of the head, both the fracture pattern
and the associated injuries are not consistent with the type of ring
fracture which Mr. Earnhardt suffered. Specifically, the anterior
diastasis (opening at the front) with comparative posterior fracture
stability, and the absence of externally mediated posterior bruising, show
that an impact to the back of the head was not the cause Mr. Earnhardt's
ring fracture.
Helmet effects and restraint effects
It has been suggested that a full face helmet might have been an important
aid in preventing the injuries in this crash. This is incorrect. If Mr.
Earnhardt had worn a full face helmet, he would still have experienced the
same tragic outcome. There are several reasons for this. Addition of a
full face helmet does not (affect) the inertial (whip) mechanism. In the
chin impact mechanism, the impact was submental and as such directed
forces posterosuperiorly (upward and backward). Addition of the full face
helmet would not significantly alter how that force was transmitted
through the maxilla and mandibular condyles (face and jaw). Moreover, it
would not have significantly changed the ride-down distance and therefore
the deceleration of the head during the phase in which the steering wheel
impact and deformation occurred. In contrast, a full face shield provides
benefit in protecting the jaw and face from direct trauma. It also can
work in conjunction with other systems to control the head. Thus, while
not ameliorating Mr. Earnhardt's injury, a full face shield is of
potential benefit to other drivers.
Review of the vehicle photographs shows that the left (outboard) lap belt webbing is separated and appears torn. For the purposes of this analysis, I am assuming that the belt was torn as a result of crash forces as opposed to being cut following the crash (i.e. assuming a worst-case scenario). Physical evidence clearly shows that the restraint system, including the left outboard lap belt, carried large forces during the crash sequence. This evidence includes the rib and sternal fractures as well as the seat-belt abrasions, including an abrasion over the left iliac crest and left lower quadrant. The fact that Mr. Earnhardt's head rotated and was struck from below also shows that his upper torso was restrained. This upper torso restraint is also supported by the absence of steering wheel deformation characteristic of chest impact, and by the absence of steering wheel mediate chest trauma. As such, the restraint system functioned to slow Mr. Earnhardt's body. This includes the outboard lap belt for some significant portion of the crash. If the outboard lap belt had remained intact throughout the crash, Mr. Earnhardt's head would still likely have experienced similar inertial forces and similar contact forces with the steering wheel. As such, the restraint failure does not appear to have played a role in Mr. Earnhardt's fatal injury. That said, an important part of occupant protection, including preventing head injury, is having an appropriately tuned restraint system that gives both occupant ride-down and strongly couples the occupant to the vehicle during the crash-pulse. As such, while lap belt failure did not contribute to this injury, the restraint system should be appropriately studied as part of an ongoing safety effort.
Relative importance of the mechanisms
Understanding the relative contributions of the inertial mechanism and the
submental impact mechanism to the creation of Mr. Earnhardt's ring
fracture is challenging. Both forms of loading can cause the injury.
Moreover, the chin impact occurred during the inertial loading, so the two
mechanisms occurred concurrently. Under many conditions, contact of the
face with the wheel actually serves a protective role for the head and
neck.
This protection occurs because the steering wheel deforms, absorbs energy over time and helps the neck stop the head. Working in conjunction with facial protection, this method of protection can be very effective. Unfortunately, that was not the case in Mr. Earnhardt's crash because the impact came to the underside of his chin. The features revealed at autopsy suggest that the chin impact did cause, or at the very least enhanced, his injury. That said, absent the chill impact, crashes of this kind call give rise to forces which are more than large enough to produce this injury, as revealed by study of similar crash tests using instrumented crash test devices (dummies). In other words, if Mr. Earnhardt did not hit his chin, he still could have suffered the same fatal injury in this crash.
Solutions
Better understanding of the relative contributions of these two mechanisms
to Mr. Earnhardt's fatal injuries could be achieved through extensive
biomechanical testing. Such testing would need to be repeated until very
accurate occupant kinematics were recreated. Such efforts are not
required, however, as the reduction in injury risk from either the
inertial or chin impact mechanisms occurs through improved control of the
head during a crash. Achieving good control of the head in a crash is the
result of design efforts which include seat and vehicle geometry, belt
design and head-helmet-suspension-retention system use. Many people have
asked whether the use of a HANS device would have prevented this injury.
Such a device offers the potential to help control the head and prevent
injuries, as do the other elements of the design. Without detailed sled
and crash test data, however, I cannot evaluate its role in this
particular crash. Clearly, further study is merited of methods to control
the head while still allowing adequate driver mobility for the unique
demands of NASCAR racing.
Barry Myers, M.D., Ph.D Duke University
Associate Professor, Department of Biomedical Engineering
Associate Professor, Division of Orthopaedic Surgery
Assistant Professor, Department of Biological Anthropology and Anatomy
Timeline Accident Photo Sequence
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