Final
Report
to the
Air
Force
Office
of Scientific
Research
Hygroviscoelasticity of
the
Human
Intervertebral Disc
by
W.
G.
Knauss
California Institute of
Technology
Pasadena,
California
91125
July
1980
Graduate Aeronautical Laboratories
-
Report
GALCIT
SM
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LONDON-A
tiny
mrts.ihr
leaves
a
fot.
to
be
disired.;ii
a
midnight
trysting
Got,
two
secret
lovers
have
discov-
':
ere&
-
.'
wedg&
into's
two-&at&,
'anear-nake'd
rnan,ivas
sud-
denly
immobilized
by
a
slipped
disc;
trapping
his,woma?
companion
beneath
him,
according to
a
doctor
writing
ina
rnedcal
jowal,
here.
'
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2-2
5
7
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,
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The
desperate
woman
ked
to
summon
he$
by
honking
the
horn
with
her
foot.
A
doctor,
ambulanceman,
firemen
And
a
group
of
interested passers-by quickly surrounded
the
couple's
car
in
Regents
Park.
,
:
a
-:
-
.z
A-
-=-
,.
:
-.
A
"me
lady found
herself
trapped beneath
200
pounds
of
pain-racked; immobile
...
man,:'
@d
Dr.
Brian
Richards
of
KmL::
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4
+
,
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--;
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2
7
.
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"To
free
the
couple,
fi5men
hadto
cut
away
th6
,c&
frame,"
he
add&.-
:
-
:+
-
.
,
'-
,.-
-.
;
. .
...
.
---..--
.
The
distraught
woman,
he~ded
out
of
thk
cargnd
ih
a
SUMMARY
In order
to gain
an improved understanding
of the behav-
ior of human
intervertebral
disc
material under
various
kinds
of loads
the viscoelastic properties
of small
specimens
excised
from
human
L4-L5
discs
were examined. Excisions
were
made
from
donated spine segments procured
a few hours after death
and then
frozen.
Material examined
was in the form
of single
lamellar specimens
as well
as specimens containing
several
lamellae.
Tensile relaxation
tests
were
performed
on single
lamellae
prepared
such
that
the collagen fibers
were
a) aligned
with
the tension
axis,
b)
normal
to the tension
axis
and c) at an
angle
of about
30'
with
that axis.
The multi-lamellar speci-
mens were
excised from
the disc such
that
one set produced
the
tensile
axis
to run parallel
to the disc
circumference
(sur-
rounding
the spinal
axis)
while
another
set caused the
tensile
axis
to run parallel
to the spinal
axis.
It was found
early
in the study
that the
water content
of the disc
material has
a profound
effect
on its
mechanical
response.
Consequently
the diffusion
and swelling
character-
istics
of the material
in different
water
environments
were
studied.
Primarily
air of
differing relative
humidity
and
various concentrated solutions
of
NaCl
were
used
to provide
for different
water
concentrations
in the material.
For the relaxation
studies
the same environments
were
used.
This
allowed achieving
water
concentrations ranging
from
virtually
dry to in-vivo
conditions.
The main
findings
of this
work
are:
1) Water affects
the relaxation
time
in a sensitive
way,
A
few percent change
in water content
can change
the re-
laxation time
by an order
of magnitude
or more.
This
fact
is important
when
one is concerned
with
laboratory
test-
ing without
being
able
to control
the water
content
at
all times.
2)
Diffusion
is a surprisingly slow process
taking
place
over several
hours
(2-3)
in specimens only
thick. The
amount
of water
take-up
is controlled
by the environment
(distilled
water
destroys
samples
into fissures)
and
by
constraints offered
by the fiber structure
and the
stresses
it engenders
(see
point
5)
below).
3) The stiffness
of the
disc
material
is on the order
of
3
2
to 10
x
lov6
N/m.
relaxation
occuring,
on the whole
rather
slowly,
i.e.,
on the order
of 10% per decade.
The relax-
ation
modulus
at right
angles
to the collagen fibers
is
about
1/3
that of
the modulus along
the fiber direction.
4)
The relaxation
modulus
of multi-layer
specimens
cut from
the disc
in
a
circumferential and
a spine-axial
direction
are very
nearly
the same;
the modulus
for the circumfer-
ential
material
is slightly
higher.
The consequence
of
this
would
be that although one
would
expect
a
markedly
anisotropic behavior
of the disc
material
on the basis of
its fiber and lamellar
construction,
the disc
material
does
not deviate
very
strongly
from
isotropic
response.
(Isotropy
as-referred
to a plane
that is tangent
to the
disc perimetric surface.
)
Stress affects
the equilibrium concentration
of water
in
the material.
The implication
-
and test
result
-
is that
once
the stress has changed
the equilibrium
of water
con-
tent
with
the environment
is disturbed
and the concentra-
tion
changes. That
change,
in turn,
causes
a change
in
relaxation
behavior such
that
tensile stresses cause
an
acceleration
of relaxation
or creep
(sdftening),
while
com-
pression
would cause
the opposite
affect
(hardening).
The
material responds
thus
to stress
as a system
that
is open
to the environment.
1. PUBLICATIONS
AND REPORTS
1. Panagiotacopulos,
N. D., Bloch,
R.,
Knauss,
W.
G.,
Harvey,
P., and Patzakis,
M.; On the Mechanical
Proper-
ties
of the Human Intervertebral
Disc;
California
In-
stitute
of Technology,
Pasadena,
CA 91125;
GALCIT
SM
78-13;
AFOSR-TR-0054;
1978.
2. Panagiotacopulos,
N.
D.,
Knauss,
W.
G., and Bloch,
R.;
On the Mechanical Properties
of
Human
Intervertebral
Disc
Material;
Biorheology
Vol.
16, 317-330;
1979.
3.
Knauss,
W.
G., Kenner,
V.
H.;
A Technique
to Measure
Poisson
Contraction in Small Biological
Specimens;
California
Institute
of Technology, Pasadena,
CA 91125;
GALCIT
SM 80-2;
1980.
4. Knauss,
W.
G.; Hygroviscoelastic
Behavior
of Human
Inter-
vertebral Disc
Material;
California Institute
of Technol-
ogy,
Pasadena,
CA 91125;
GALCIT
SM 80-14;
1980.
(in prep
aration)
.