2002 - University of Washington Bone and Joint Sources

etween the head (at the center of
gravity) and the sternum was reduced
from about 48 mm to 20 mm.
Measurements indicated lower peak
accelerations and less variation in
subject responses with the modified
head restraint.
Computer modeling
MADYMO (Mathematical
Dynamic Modeling, TNO Automotive,
Delft Netherlands) is used in the
automotive industry for design of safety
devices. It simulated the motion of the
occupant modeled as rigid bodies (the
bones) connected by joints, with
properties well defined by cadaveric
testing. The seat was modeled as a finite
element object based on mechanical
testing of the seat frame and cushion.
Using this approach allowed the
internal forces in the cervical spine to
be defined since the model includes
individual vertebrae and their
connecting soft tissues. Responses at
impact speeds much higher than
tolerable by volunteers were studied.
Also, various properties of the seat were
altered and the response of the
occupant determined.
The response of the model in terms
of acceleration of the head and chest
fell well within the responses measured
in the volunteer studies described
above. As shown in Figure 3, with a
head restraint that reaches the top of
the occupant’s head and an initial gap
of about 4 inches, the major force acting
is shear, that is a horizontal force across
the spinal vertebrae. With the head
initially upright and momentarily
stationary, while the torso is thrust
forward by its contact with the seatback,
an anterior shear or horizontal force
acts on the cervical spine. Later, as the
torso is stopped by the shoulder strap
and the head is thrust forward by its
contact with the head restraint, a
secondary shear force develops.
Reducing the head to restraint distance
did not significantly alter these forces.
It did prevent extension of the cervical
spine and head but instead created
greater flexion during the forward
rebound of the head. Making the seat
and head restraint softer decreased the
magnitude of the primary shear force
and reduced the head rebound from the
restraint by absorbing energy, therefore
decreasing the secondary shear force.
Cadaveric Studies
In order to gain insight into the
detailed deformations of cervical spine
tissues, a series of 11 cadaveric cervicothoracic
specimens with an
instrumented artificial skull was tested.
Each specimen was mounted onto a
platform, replacing the seat, on the
same cart used for human volunteer
testing. The same head restraint was
used and the impactor was adjusted so
that the acceleration at vertebra T1
(about top of sternum level) was similar
to that recorded by volunteers. High
speed video was used to film the
positions of markers on the lateral
masses and the vertebral bodies and
measurements were made of the
contact pressures in the facet joints, and
around the nerve roots. The effect of
the modified head restraint was
studied.
Figure 4 showed formation of a
partial “S” curvature, specifically
extension of the lower cervical spine
which seemed to be most prominent
when the head motion was stopped by
the head restraint. Following rebound
from the head restraint the spine was
subjected to both anterior shear and
tensile forces as the head moved
forward of the lower spine.
Reproducible pressure pulses occurred
around the nerve roots, highest at levels
C7-T1, and were maximum after peak
acceleration of the chest but before peak
acceleration of the head occurred.
Pressure measurements in the facets
showed pinching (a high pressure point
in the joint instead of a uniform
contact), in about 70% of tests, with
C4-5 and C5-6 having the greatest
likelihood of pinching. The modified
head restraint reduced head
acceleration and resulting spinal shear
forces by about 30%.
CONCLUSION
In a rear-end vehicle impact, even
when the occupant’s head is positioned
close to the head restraint, significant
forces can be developed in the cervical
spine with resulting intervertebral
displacements and tissue pressures and
deformations. These forces are a
primary anterior shear, due to the torso
being thrust under the stationary head
by its contact with the seat, and a
secondary force from the head being
thrust forward by its contact with the
head restraint as the torso motion stops.
The basic method for reducing spinal
forces and intersegmental
displacements is to soften the torso-toseat
and head-to-restraint contact, and
importantly, use the head restraint to
absorb the energy of these impacts.
RECOMMENDED READING
Insurance Institute for Highway Safety
“Whiplash Injuries”, Status Report,
30:no 8, pp1-12, Sept 16, 1995.
Tencer AF, Mirza SK, Bensel K, The
response of human volunteers to rearend
impacts, The effect of head
restraint properties, Spine, 26: Nov,
2001.
Tencer AF, Mirza SK, Bensel KD,
Internal loads in the cervical spine
during motor vehicle rear-end impacts:
The effect of acceleration and head-tohead
restraint proximity, Spine, 27: Jan,
2001.
Tencer AF, Mirza SK, Cummings, P, Do
whiplash victims with neck pain differ
from those with neck pain and other
symptoms Proceedings of the
Association for the Advancement of
Automotive Medicine, Sept, 2001.
Ono, K, Kaneoka K, Wittek, A, Kajzer J,
Cervical injury mechanism based on
the analysis of human cervical vertebral
motion and head-neck-torso
kinematics during low speed rear
impacts, Transactions of the Society of
Automotive Engineers, 973340, 1997.
Yoganandan, N, Pintar FA, Cusick, J,
Biomechanical analyses of whiplash
injuries using experimental models,
Proceedings of the World Congress on
Whiplash and Associated Disorders,
Vancouver, BC, Feb, 1999, pp 325-344,
1999.
Deng B, Begeman PC, Yang KH,
Tashman S, King AI, Kinematics of
human cadaver cervical spine during
low speed rear-end impacts, STAPP Car
Crash Journal, 44:171-188, 2000.
24 2002 ORTHOPAEDIC RESEARCH REPORT