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Файл:Ординатура / Хирургия / Библиотека им академика М.И. Перельмана / Книга_6044_Библиотеки_им_академика_М_И_Перельмана.pdf
29
4.3 Whiplash Injury Mechanism
In a rear-end collision, the chest is pushed forwards by the seatback; the burden on
the neck results from the strength of inertia that works through the center of gravity
of the head in a front-back direction; in this case, the neck injury mechanism is one
of hyperextension and tension (“traction” in mechanical terminology) of the cervical rachis (Fig. 4.2 ). It should be pointed out that not all hyperextension/tension
injury mechanisms are caused by whiplash.
Chronic radicular symptoms have been documented in whiplash patients, potentially caused by cervical neural tissue compression during an automobile rear crash.
Simulated rear crashes with whiplash protection system (WHIPS) and active head
restraint (AHR) have been compared to those obtained with no head restraint (NHR)
with the aim to determine how whiplash may induce neural space narrowing of the
lower cervical spine [ 6 , 7 ]. Average peak canal and foramen narrowing could not be
statistically differentiated between WHIPS, AHR, and NHR. While lower cervical
spine cord compression during a rear crash is unlikely in those with normal canal diameters, it has been demonstrated [ 8 ] that foraminal kinematics is suffi cient to compress
spinal ganglia and nerve roots. Disc strains are highest in the C4-C5-C6 segments.
4.4 Technical and Structural Limits
The technical norms now in force in Europe prescribe that the seatback must be
able to withstand the application of a static moment of 53 (m daN). Even though
from a theoretical point of view it is not possible to defi ne a dynamic stress that
Fig. 4.2 Injury mechanism
in whiplash
4 Kinematics and Dynamics of the Vehicle/Seat/Occupant System
30
is equivalent to the static one, on the basis of experimental data and for normal
seats with 50 % male anthropometric characteristics, we can consider that such a
burden corresponds to the capacity to withstand the chest’s inertia load in a rearend collision between vehicles of equal mass with a closure speed of about
35 km/h.
Ignoring the retaining action of the headrest, we can deduce that speed variations of the vehicle higher than 18 km/h the seatback can rotate backwards absorbing part of the chest kinetic energy; these phenomena in fact limit both the angle of
rotation of the head and the value of the extension moment.
The technical norms assure that the headrest withstands additional loadings
compared to the seatback.
We can therefore conclude that the crash that must be considered is a
rear-end one at AV < 18 km/h with a longitudinal movement of the passenger
compartment.
Review of whiplash injury mechanisms and effects of anti-whiplash systems,
including active head restraint (AHR) and Whiplash Protection System (WHIPS),
investigated whether seat design and biomechanical knowledge of proposed whiplash injury mechanisms translates to understanding outcomes of rear crash occupants. In fact, in attempt to reduce whiplash injuries, some newer automobiles
incorporate anti-whiplash systems such as AHR or WHIPS. During a rear crash,
mechanically based systems activate by occupant momentum pressing into the
seatback, whereas electronically based systems activate using crash sensors and an
electronic control unit linked to the head restraint.
Biomechanical studies of simulated rear crashes have been performed using
human volunteers, mathematical models, crash dummies, whole cadavers, and
hybrid cadaveric/surrogate models. They indicated that AHR and WHIPS reduce
the potential for some whiplash injuries but did not completely eliminate the risk of
injury.
Epidemiological outcomes indicate reduced whiplash injury claims or subjective
complaints of crash-related neck pain between 43 and 75 % due to AHR and
between 21 and 49 % due to WHIPS as compared to conventional seats and head
restraints [ 9 ], concluding that energy-absorbing seats aim to reduce occupant loads
and accelerations, whereas AHRs aim to provide early head support to minimize
head and neck motions. Continued objective biomechanical and epidemiological
studies of anti-whiplash systems together with industry, governmental, and clinical
initiatives will ultimately lead to reduced whiplash injuries through improved prevention strategies [ 10 ].
Some differences have been observed [ 11 ] regarding gender-based whiplash
effects in the dynamic response for the females with respects to the males in volunteer experiments. In fact the peak head acceleration in the posterior-anterior direction was higher and occurred earlier for the females than for the males. These
experiments could be used in developing and evaluating a female dummy model for
rear-impact safety assessment in order to design gender-specifi c protective
systems.
P.L. Ardoino and F. Ioppolo
31
4.5 Phases of the Collision
A stereotypical kinematic and neuromuscular response has been observed in human
subjects exposed to rear-end impacts [ 6 ]. Combined with various models of injury,
these response data have been used to develop anti-whiplash seats that prevent
whiplash injury in many, but not all, individuals exposed to a rear-end crash.
Understanding of the occupant kinematics and neuromuscular responses, combined
with data from various seat-related interventions, have shown that differential
motion between the superior and inferior ends of the cervical spine is responsible
for many whiplash injuries. The number of whiplash injuries not prevented by current anti-whiplash seats suggests that further work remains, possibly related to
designing seats that respond dynamically to the occupant and collision properties.
Neck muscles alter the head and neck kinematics during the interval in which injury
likely occurs, even in initially relaxed occupants. It remains unclear whether muscle
activation mitigates or exacerbates whiplash injury. If muscle activation mitigates
injury, then advance warning could be used to help occupant tense their muscles before
impact. Alternatively, if muscle activation exacerbates whiplash injury, then a loud
preimpact sound that uncouples the startle and postural components of the muscle
response could reduce peak muscle activation during a whiplash exposure [ 12 , 13 ].
Generally speaking, rear-end collision is characterized by three different phases [ 14 ]:
• First phase: crash between the vehicles until the struck one assumes the speed
variation (AV) due to the impact conditions.
• Second phase: interaction between the seat and its occupant.
• Third phase: loadings on the occupant; the three phases surely are linked and
temporarily, partially overlapping.
4.5.1 First Phase
Regardless of the initial speed of the two vehicles, the struck one in this phase
assumes a speed variation that is a function of the masses of the vehicles, their closure speed, and the spring restitution coeffi cient; the analytic expression of the
speed variation of the vehicle that is hit is the following:
ΔV
MM
MM
VVA=
AB
AB
AB1e
×
+
()
+
()
-
where:
A = struck vehicle
B = striking vehicle
V = speed at the impact
∆ V = speed variation
M = mass
e = spring restitution coeffi cient
( V A − V B) = closure speed
4 Kinematics and Dynamics of the Vehicle/Seat/Occupant System
32
Higher or lower stiffness of the vehicles brings a different degree of deformation
and therefore infl uences the duration of the collision and the values of average
acceleration, but does not modify the extent of the variation in speed.
4.5.2 Second Phase
The variations in motion of the hit vehicle are also those of the fastening of the
seat to the body shell; the seat, mostly the seatback, is the element through
which the occupant adapts its motion situation to the fi nal one of the hit
vehicle.
In a rear impact, the seatback is the actual restraint system of the occupant, just
as the seatbelt is in a frontal impact.
It is important to point out that the fact of wearing the seat belt does not infl uence
the kinematics and the dynamics of the occupant during a lengthwise rear-end collision; the only function of the seat belt in this particular case is to hold the occupant
once the crash is over.
Recently Viano et al. [ 15 ] analyzed matched rear sled tests with all belts to seat
(ABTS) and conventional seats from the same vehicle model to determine differences in BioRID IIg dummy responses. The BioRID IIg rear-impact dummy was
placed on ABTS or conventional seats and subjected to 10 m/h rear sled tests
using the Insurance Institute for Highway Safety (IIHS) whiplash assessment protocol. Measurements in the dummy included head and pelvis triaxial accelerations, T1 and L1 biaxial accelerations, and upper and lower neck triaxial forces
and moments. High-speed video captured the dummy and seat kinematics during
seat loading and rebound into the lap-shoulder belts. Four vehicles were used with
conventional and ABTS seats in the same model. They were the 2007–2008
Chrysler Sebring, 2006 Ford F-150, 2005–2007 Saab 9–3, and 2006–2007 BMW
3 series.
Authors noticed that the upper neck tension was 44 % higher and the lower neck
extension moment was 102 % higher and that the Saab 9–3 responses were lower
than the 3 other vehicles for both the conventional and ABTS seats. There was less
rearward shear and extension of the neck in the Saab seats. In conclusion, the tests
show that ABTS seats involved signifi cantly higher neck tensions, rearward shear
forces, and extension moments than matched conventional seats. Overall, ABTS
seats applied more load on the head and spine, had less control of neck kinematics,
and had higher risks for whiplash and more severe injury than conventional seats in
the same vehicle model.
Two characteristics of the seat infl uence the stress transfer from the vehicle to the
occupant: (1) the seat foam which cushions the stresses, fi lters, and further offsets
the small transversal and rotational movements of the vehicle; (2) the elasticity of
the seatback structure which obviously acts only in situations which do not exceed
the structural limits indicated before.
The relative head/headrest position plays a basic role in the extensional traction
movement of the neck and therefore in limiting the whiplash injuries.
P.L. Ardoino and F. Ioppolo
33
4.5.3 Third Phase
The human body is an articulated system of body segments which are submitted to
external loads, applied mainly through the seat, and they interact among themselves.
In order to analyze the stresses on the single-body segments, an experimental rearend collision test has been carried out on a crash simulator with the previously
described conditions (Fig. 4.3 ).
A 17 km/h variation in speed has been applied to the passenger compartment
with an acceleration curve corresponding to a rear-end collision with a rigid moving
barrier and, therefore, a particularly severe one (Fig. 4.4 ) segments.
Fig. 4.3 Rear-impact
simulation. Test confi guration
*
*
*
*
*
*
*
*
*
*
80
60
40
20
Accelaeration (g)
Velocity (km/n)
0
−20
−40
−60
0 40 80 120 160 200
Time (ms)
240 260 320 360 400
80
180
120
80
Displacement (cm)
40
0
Acceleration
Velocity
Displacement
60
40
20
0
Fig. 4.4 Time histories of the passenger compartment
4 Kinematics and Dynamics of the Vehicle/Seat/Occupant System
34
A Hybrid III dummy has been used in the front passenger position, provided with
the necessary instruments in order to obtain (Fig. 4.5 ): (1) the pelvis, chest, and
head accelerations; (2) the axial loads (compression and traction), the moments
(fl exion and extension), and the shear force on the neck; (3) the rotation of the head
with regard to the basis of the neck.
Figure 4.6 shows the time histories (acceleration, velocity, displacement) of the
occupant’s pelvis, chest, and head in the direction of the applied forces that, in this
particular case, were horizontal-longitudinal.
Head accel.
3 chs
Head rotation
2 chs
Chest accel.
3 chs
Pelvis accel.
3 chs
Neck load
3 chs
Fig. 4.5 Hybrid III instrumentation
P.L. Ardoino and F. Ioppolo
35
It is necessary to point out that the head’s speed and displacement values above
130 ms do not correspond to the actual values of the speed and displacement components along the X -axis, due to the rotation of the head.
It is therefore evident that the stresses on the three-body segments have different
values and are transmitted in different times.
80
60
40
Acceleration (g)Acceleration (g)Acceleration (g)
Velocity (km/h) Velocity (km/h) Velocity (km/h)
20
0
80
BPX
TOPX
TPX
60
40
20
0
160
120
80
40
0
160
120
80
40
0
160
120
80
Displacement (cm)
Displacement (cm) Displacement (cm)
40
0
80
60
40
20
0
80
60
40
20
0
80
60
40
20
0
−20
−40
−60
0 40 80 120 160 200
Time (ms)
240 280 320 360 400
80
60
40
20
0
Fig. 4.6 Pelvis, chest, head time histories
4 Kinematics and Dynamics of the Vehicle/Seat/Occupant System
36
The pelvis compared to the chest is subject to a stress that is shorter lasting, with
an average acceleration value which is higher, but it has a variation in speed which
does not exceed that of the vehicle (17 km/h); this means that, at pelvis level, the
seat and the seatback cushion the crash and do not elastically return energy to the
pelvis itself.
The duration of the stress on the chest is longer due to the spring restitution of
the seatback which, with a coeffi cient of about a 0.3, brings the chest’s variation in
speed to about 23 km/h.
Regarding the head, though we do not have the exact value of the variation in
speed along the horizontal-longitudinal axis, we have noticed a higher stress degree
in terms of both mean acceleration and variation in speed.
The head’s increase in speed variation compared to that of the chest is mainly
due to the neck’s spring restitution.
It should be noted that all the recorded acceleration values are at least one order
of magnitude below the human tolerance levels universally accepted in the biomechanic fi eld for these body segments. The fact, which is also emphasized by the
accident analysis, that in rear-end collisions of this severity no pelvis, chest or head
injuries occur, does not alter the signifi cance of the dynamic and kinematic trends
of the system which are the input conditions for the analysis of stresses on the neck.
The overlapping of the pelvis and chest speed curves (Fig. 4.7 ) shows how, in the
time interval between 80 and 200 ms, the variation in speed between chest and pelvis passes from about –l0 to +7 km/h.
The same behavior has been noticed between head and chest (Fig. 4.8 ). Regarding
stresses on the neck (moments and axial loads) (Fig. 4.9 ), we noticed a time corre-
spondence with the acceleration of the head; the same can be said for the rotation of
the head with regard to the basis of the neck. The head acceleration and speed
80
60
40
Chest
Pelvis
0
40 80
120
160 200
Times (ms)
240 280 320 360
400
20
Velocity (km/h)
0
−20
Fig. 4.7 Pelvis, chest velocity vs time
P.L. Ardoino and F. Ioppolo
37
curves (Fig. 4.6 ) show, at about 80 ms, a small backward movement of the head, due
to the lever effect of the pelvis, chest, and seatback, to which a small bending
moment value corresponds (Fig. 4.9 ).
There is a question whether the standing or seated pelvis should be used in
Hybrid III dummy evaluations of seats and belt restraint systems in severe rear
impacts. Viano and Parenteu performed [ 12 ] sled tests in a belted standing and
seated Hybrid III dummy. The head, chest, and pelvis were instrumented with triaxial accelerometers and the upper and lower neck, thoracic spine, and lumbar spine
had transducers measuring triaxial loads and moments. Belt loads were measured.
In 40 km/h sled tests, the dummy motion and excursion were essentially similar
with the standing and seated pelvis. The similarities included the lap belt interaction
with the pelvis and the leg movement upward fl exing the hip joint. Overall, similar
biomechanic and kinematic responses were found, including the pelvic acceleration, spinal forces, and moments. For the lower speed tests at 10, 16, and 24 km/h,
the motion sequence was also similar with the two different pelvises, including the
upward movement of the legs as the seat was loaded and rebound kinematics. The
biomechanical responses were similar. The seated pelvis involves only a small portion of the upper leg molded into the vinyl skin of the pelvis and does not limit leg
rotation at the hip joint. Furthermore, lap belt loads were minimal during the rearward movement of the dummy. In conclusion, tests showed no signifi cant difference
in occupant kinematics or biomechanical responses between the standing and seated
pelvis in rear sled tests.
By examining Figs. 4.6 and 4.9 , the following can be noted:
• The chest starts its forward movement, pushed by the seatback, at about 50 ms.
• The rotation of the head starts at about 90 ms.
• The head’s center of gravity starts to shift forwards at about 120 ms.
80
60
40
Head
0
40 80
120
160 200
Times (ms)
240 280 320 360
400
20
Velocity (km/h)
0
−
20
Fig. 4.8 Chest, head velocity vs time
4 Kinematics and Dynamics of the Vehicle/Seat/Occupant System
38
• The maximum stresses on head and neck occur at about 150 ms. Based on what
was mentioned above, it is clear that the neck – the link between chest and head
– is stressed in different ways during the crash (Fig. 4.10 ):
– Between 50 and 90 ms, the chest moves forwards and the head stays in its rest
position; the neck is essentially subject to small shear strains.
– Between 90 and 120 ms, the forward movement of the chest continues; the
head rotates due to the fact that the neck is subject to the application of an
extension moment.
40
20
Head – resultant acceleration in the vertical longitudinal plane
Neck – bending moment in the verticl longitudinal plane
Flexion
Extension
Times (ms)
Neck – axial load
Tension
Head/chest – rotation in the vertical longitudinal plane
Acceleration (g)Moment (N.m.)Load (DAN)Rotation (GR)
0
40
20
0
100
0
20
0
−20
−40
−60
Fig. 4.9 Head and neck loadings
P.L. Ardoino and F. Ioppolo