Download Intervertebral disc mechanics: a review

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Journal of the Royal Society of Medicine Volume 79 February 1986
Intervertebral disc mechanics: a review
J Nixon MA FRCS Nuffield Orthopaedic Centre, Headington, Oxford OX3 7LD
Introduction
Musculoskeletal problems account for up to 25% of
general practitioner consultations1'2. Out of every
100 patients on the general practitioner's list, 4 will
each year initiate a consultation for back pain3. Low
back pain is responsible for over thirty million working days lost in Britain per annum4. Between 21%5
and 84%6 of such 'lumbago' occurs without a
definite diagnosis, but it seems likely that degenerative disease of the spine underlies much of this
absenteeism. Some occupations are affected more
than others7. A study by the Royal College of Nursing8 reports the incidence of low back pain in nurses
to be as great if not greater than amongst manual
workers.
Following skeletal maturation the spine begins to
degenerate both biochemically and biomechanically, and this process continues throughout life at a
rate which may be genetically predetermined9.
Autoimmune mechanisms may also play a part in
this process0'11. Degeneration occurs most rapidly
at the L4/5 and L5/S1 levels. These are the levels subjected to the greatest mechanical stresses, so it is
reasonable to assume that mechanical forces play a
major role in disc degeneration. In this paper a
number of mechanical influences on the disc,
some physiological others pathological, will be
considered (Table 1).
Table 1. Intervertebral disc mechanics
(1) Spinal column flexibility
(2) Spinal column stability
(3) Intervertebral joint motion
(4) Loading mechanics
(5) Protective mechanisms
(6) Fatigue failure
(7) Acute mechanical failure
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The Royal
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Spinal column flexibility
The rigid elements of the spinal column consist of 24
presacral vertebrae which increase in size from
above downwards. Mechanically each vertebra can
be divided into four parts:
(1) The body which transmits most of the applied
load.
(2) Posterior facet joints which control and restrict
intervertebral joint motion and transmit a variable
load. Normal facets carry between 3% and 25% of
the applied load, whilst arthritic facet joints can
carry as much as 40%12.
(3) Pedicles and laminae which complete the spinal
canal.
(4) Transverse and spinous processes which provide
mechanically advantageous sites for muscle and
ligamentous attachments.
Flexibility of the spinal column is provided by the
disc which is composed of the central nucleus
pulposus and surrounding annulus fibrosus. Each
layer of the annulus has fibres directed at about 250
to 450 to the vertebral end-plate, and this angle
diminishes with age13. The direction of the fibres
alternates in each successive layer, providing flexibility whilst maintaining strength. If one layer of the
annulus is relatively slack during movement, its
adjacent layers will be taut.
Spinal column stability
The spinal column has both intrinsic and extrinsic
stability. Intrinsic stability results from the opposing forces of (a) ligaments restraining vertebral
motion, and (b) pressure within the nucleus pulposus
tending to push them apart. This latter is known as
the 'imbibition' pressure. Hydrophilic proteoglycans
of the nucleus pulposus imbibe and retain water
molecules to produce a turgid resilient nucleus. The
vertebrae are pushed apart with a force of about
100 kPa'4, which essentially pre-stresses the ligaments. Extrinsic stability results largely from trunk
musculature, and intra-abdominal pressure which is
in turn maintained by abdominal wall musculature.
With ageing the imbibition pressure falls. At birth
the nucleus contains about 88-90% water15, but by
70 years of age it is only 65 700/o water"6. These
values are similar for articular cartilage (approximately 75% by weight) where water content is
highest next to the articular surface"7. In a sense,
therefore, we do 'walk and move on water'. The
gradual reduction with age in the water content of
the nucleus occurs following proteoglycan breakdown, and associated ingrowth of collagenous
tissue. Although collagen is largely responsible for
the tensile strength of the disc, proteoglycan is the
major component resisting compression and providing resilience18.
The mucopolysaccharide-protein complexes of
young discs are large molecules composed primarily
of chondrotin sulphate A and C. These macromolecules are strongly hydrophilic and increase disc
viscosity'9. During the early 20s they begin to break
down into smaller molecules, possibly under the
influence of changes in oxygen tension20 and blood
supply. Mucopolysaccharide production is switched
to chondroitin sulphate B and keratosulphate,
smaller molecules which bind less water. The
annulus fibrosus does not lose water with age21, but
changes in the glycosaminoglycan ground substance
'lubrication' between adjacent lamellae may generate shear. stresses with movement22.
The imbibition pressure of the nucleus is largely
responsible for the variation in a person's height
from morning to evening. Up to the age of 10 a child
Journal of the Royal-Society of Medicine Volume 79 February 1986
is 2% shorter at the end of the day than at the beginning. Young adults are on average 1.1 cm taller in
the morning, whilst a 70-year old individual shows
little change. A more bizarre example is an increase
in height of over 5 cm of an astronaut on return to
earth through imbibition pressure unopposed by
gravity.
As imbibition pressure falls with age, the nucleus
pulposus becomes less turgid, more fibrotic, and less
efficient at redistributing vertical compressive loads
centrifugally to the inner layers of the annulus. A
larger proportion of the load is therefore taken
directly by the annulus itself. Bulging of the annulus
occurs with loading" , and this may compress adjacent nerve roots or cauda equina23. Loss of disc
height with advancing age results in relative lengthening of the ligaments, with consequent reduction
of intrinsic spinal stability. This occurs at a time
when the tone of trunk and abdominal musculature
is reduced, compromising extrinsic stability also.
which provides extrinsic stability and enables the
spine to support tremendous loads. Vector diagram
analysis at the lumbosacral junction of a person
bending over to pick up a weight indicates that
forces in the order of ten times the lifted load are
generated. This implies that lifting a load of only
20 kg would be sufficient to fracture the vertebral
end-plate30, since cadaver studies have shown endplate failure to occur under loads as small as
10 kN31 32. The normal disc is much stronger than
the vertebral end-plate of the cadaver spine on
vertical loading33. Mechanisms must exist, therefore, to protect the vertebral end-plate from
tremendous loads transmitted by the disc. A number
of protective mechanisms have been postulated.
Theories of protective mechanisms
Hydrostatic properties of the disc
The hydrostatic model of the disc was first postulated by Schmorl34'35, and demonstrated in vivo by
Nachemson36, both in normal and slightly degenerate discs. Pressure within the nucleus pulposus
under loading conditions is redistributed both
radially to the inner layers of the annulus fibrosus as
well as vertically to the vertebral end-plates. This
effectively converts compressive loading of the
nucleus to tension within the annulus, helping, to
unload adjacent vertebrae. Pressure is always present within the disc, even at rest37, so the annulus is
effectively pre-stressed.
Intervertebral joint motion
The intervertebral joint (one disc and two facet
joints) is a type of universal joint with six degrees
of freedom24: three of translation (sideways,
backwards-and-forwards, up-and-down) and three of
rotation (side-to-side, bending forwards and backwards, and longitudinal rotation). In practice, however, the joint does not perform simple movements in
isolation, but always couples one translational with
one rotational movement. For example, as the spine
axially rotates there is always lateral flexion and
vice versa. Three-dimensional radiographic studies
of spinal movement25 have shown that relatively
little rotation occurs in the lumbar spine due to facet
joint orientation. In the thoracic spine the centre of
rotation lies within the nucleus, so the disc is subjected to rotational forces26, whereas in the lumbar
spine this axis lies posterior to the disc, subjecting
the disc to translational shear forces. Shear and
rotational stresses are probably the most significant
ones contributing to degeneration.
Facet joint tropism, a rotational malalignment,
displaces the axis of rotation of the intervertebral
segment away from the midline, thus increasing the
translational and rotational stresses imposed upon
the opposite facet joint and contralateral side of the
disc. This probably accelerates their degeneration,
and it has been noted that the incidence of facet
joint degeneration and annular rupture is increased
on the side opposite the blocked facet joint27.
Stress analysis of the intervertebral joint by
Farfan et al.28 indicated that the greatest stresses
are generated during rotation when only half of the
oblique fibres of the annulus are under tension, and
the others are relaxed. At this time, the nucleus
pulposus is contained by only half of the annular
fibres. Many patients with acute disc prolapse do
report a twisting of the trunk in addition to forward
bend as a precipitating factor.
Pneumatic properties ofabdominal cavity
Cross-sections through the trunk and CT scans
demonstrate that the abdominal cavity lies laterally
as well as anterior to the spinal column. It was first
postulated by Bartelink38 that this cavity may,
under loaded conditions, act as a semi-rigid cylinder
capable of unloading to some extent the spinal
column. To test this hypothesis, subjects swallowed
tubes into the oesophagus and stomach and electromyographic measurements of various paraspinal
muscles were taken39. The subject then lifted
weights from 50 lb to 200 lb (22.7-90.9 kg), and as
muscle activity increased, so did intra-abdominal
pressure and to a lesser extent intrathoracic
pressure. The same occurred on static loading of the
spine when the subject pulled on a strain ring. On
lifting 91 kg (200 lb), the increased intra-abdominal
pressure reduced the calculated forces at the
lumbosacral junction from 939 kg (2071 lb) to 673 kg
(1483 lb), a drop of 30%/0.
Eie40 tested Nordic ski-jumpers who swallowed
intra-abdominal tubes linked to tape recorders with
a retarder in the pocket to measure the gravitational
force on landing. He found the landing force to be
10 G, which would theoretically have generated
420 kg of compression at the lumbosacral junction,
but this was reduced to 310 kg by intra-abdominal
pressure elevation since the jumper held his breath
at the moment of impact.
Loading mechanics
The ligamentous cadaver spine devoid of muscle
control can support a compressive weight of only
20 N (4 lb or approximately 2 kg) before collapse of
the column occurs29. It is the trunk musculature
Dorsolumbarfascia
Selective layers of the anterior abdominal wall arise
posteriorly from the dorsolumbar and iliolumbar
fascia and maintain its shape and length during lifting. Gluteus maximus and the ham-strings also
generate forces in this fascia during lifting. The
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102
Journal of the Royal Society of Medicine Volume 79 February 1986
posterior layer of fascia converges upon the tips of
the spinous processes posterior to the erector
spinalis muscle"1. Its action through a long lever
arm is therefore to extend the spine and possibly to
reduce the force required in the erector spinae muscles during lifting.
Blood diversion
During lifting, blood is diverted from the inferior
vena cava via Batson's plexus to the epidural veins.
From here it flows freely into the spongiosa of the
vertebral bodies to increase the intraosseous venous
pressure. This may strengthen the vertebral body
against compressive loads, but the hypothesis
awaits experimental confirmation.
Weight-lifters attempting to break their record
wear a broad belt pulled tightly around the waist
which increases abdominal'pressure. It seems likely,
however, that the 'turgor hypothesis' would be more
effective in conditions of acute compression, such as
ski-jumpers landing or aircraft pilots ejecting whilst
performing a Valsalva manoeuvre42.
Measurement of intradiscalpressure
Intradiscal pressures have been measured in
normal subjects (medical students) by inserting a
needle attached to a pressure transducer posterolaterally36. The pressure was increased during
sitting, and especially when slumped forwards. This
may help to explain a patient's increased back pain
and sciatica in these positions. Lifting a- heavy
object with the knees bent and trunk straight generates a lower pressure than lifting whilst bending at
the waist42. The distance between the centre of
gravity of the lifted load and the body is of most
significance in reducing intradiscal pressure. Intraabdominal pressure is conversely much higher when
lifting properly. This type of pressure measurement
has been employed to aid the design of car seats in
Sweden.
Some physiotherapy exercises recommended for
patients with low back pain were found to generate
enormous intradiscal pressures,' particularly sit-up
exercises. Isometric exercises produced least elevation of intradiscal pressure'3. Pressures were
measured in 2 patients following spinal fusion and
were found to be reduced during lifting.
Fatigue failure
On histological examination of cadaver discs of
people who had not had significant back pain,
Farfan25 and Hirsch44" demonstrated distortion of
the lamellae of the annulus fibrosus particularly
posteriorly, and in 10-15% radial fissures were seen
posterolaterally. Recently nerve endings have been
visualized in the annulus fibrosus, and it may be that
fissures or the products of degeneration associated
with them are a source of pain45. Fissures appear to
be more a fatigue phenomenon -than the result of
acute trauma'6. Although theoretical stress
analysis would suggest that the proximity and orientation of the facet joints does to some extent protect
the posterior annulus during axial rotation, such
protection is not afforded during spinal flexion when
the annulus is highly stressed posteriorly.
To assess the fatigue effect of flexion on the loaded
spine simulating heavy manual labour such as
digging, a cadaver spine was cyclically loaded in a
jig within the physiological range of flexion. A compressive force of between 1500 and 6000 Newtons,
depending upon age and sex, was applied 40 times
per minute to simulate the fastest a British workman could possibly work, and this was continued for
a maximum of four hours giving 9600 loading
cycles'6. Some vertebral bodies failed through
fatigue fracture within 4 hours of 'testing. Following
this, dissection of the discs revealed that in over 50%
the lamellae were closely packed anteriorly, widely
spaced laterally, and tightly curved posterolaterally. Posterolateral radiate fissures were found
in 70% of all discs, and in 100% of L4-5 and L5-S1
discs from 30-50-year-olds. These occurred most
noticeably where the annulus was most stretched by
an inbuilt eccentric setting of the flexion apparatus.
The degree of distortion of the lamellae is
determined by the centrifugal force of the nucleus
against the inner layers of the annulus, posterior
displacement of the nucleus during flexion, and
vulnerability of the annulus posterolaterally to
'creep' or stretching. It may be that when radial
fissures become confluent, a pathway is provided
by which nuclear material can escape, usually
posterolaterally.
Clearly, in vivo the long-term effects of fatigue are
modified by the cellular repair mechanisms. Chondrocytes of a mature disc can manufacture collagen
and large proteoglycan molecules. The turnover of
proteoglycans is 500 days in dogs, and collagen production is probably even slower47, such that the
repair processes will have only minimal effect over
days or weeks, but may be substantial over a period
of months or years.
Acute mechanical failure
The mechanism underlying acute extrusion of the
nucleus pulposus must be clarified. As stated, pure
compression of the intervertebral joint leads invariably to vertebral body failure, even with a fatigue
pattern of loading31. Flexion just beyond the physiological limit results in damage to the ligaments of
the neural arch but not the disc48. Torsion damages
the articular facets and, if carried beyond the
physiological range, produces circumferential tears
of the annulus fibrosus49. However, when the intervertebral joint is flexed a few degrees beyond the
normal range and then compressed with about 8000
Newtons, comparable to the compressive forces
generated within the discs of a healthy young man
straining to lift a heavy weight, acute disc prolapse
has been produced in cadaver spines in 26 out of 61
joints'9. Adams and Hutton'9 found that the intervertebral joint could fail in one offour ways:
(1) Nuclear extrusion, with nuclear material
appearing on a posterolateral edge of the vertebral
body or within the canal.
(2) Annular protrusion, with an annular ring disruption occurring at a point of bulging and the
nucleus displaced posterolaterally behind the bulge.
(3) Compression fracture - anterior crushing of the
vertebral body and occasional end-plate fracture,
with nuclear material sometimes entering the vertebral body.
(4) Hyperflexion fracture - even before compressive
loads were applied, the posterior rim of the vertebral
body could become detached and the anterior rim of
Journal ofthe Royal Society of Medicine Volume 79 February 1986
the body fracture. Sufficient flexion had to occur to
render the annulus vulnerable and this is achieved
by overstretching the interspinous ligaments. It
was reported many years ago that the spinous and
interspinous ligaments were frequently found to
be ruptured in patients undergoing disc surgery the so-called 'sprung back' described by Philip
Newman50.
Across the age range, 26 out- of 61 discs tested
failed by nuclear extrusion, but on looking at the
IA-6 and Lb-Si-discs of cadaver specimens from
30-50-year-old patients, the rate of prolapse rose to
14 out of the 17 discs tested. It is possible that there
is a greater degree of pre-existing degeneration at
these two levels resulting from their increased range
of flexion. Alternatively, these levels may be more
susceptible to prolapse because of morphological or
biochemical variations.
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(Accepted 6 August 1985)