Pathology/pathogenesis of lumbar disc degeneration

INDEX
 BASIC DISC ANATOMY &
PHYSIOLOGY
 NORMAL DISC AGING
 DISC / VERTEBRAL COLUMN
PATHOLOGICAL CHANGES
IN DEGENERATIVE DISC
DISEASES
 THE VASCULAR BASIS FOR
DISC DEGENERATION
BASIC DISC ANATOMY & PHYSIOLOGY
The intervertebral discs may be thought of as soft tough pads that separate the bones
(vertebrae) of the spine from one another. Their basis function is three-fold: 1) they act as a
ligament by holding the vertebrae of the spine together, 2) they act as a shock absorber
which carries the downward weight of the body (axial load) while in an up right position,
and 3) they act as pivot point, which allows the spine to bend and twist.
www.yassermetwally.com
Professor Yasser Metwally

There are 23 discs in the human spine: 6 in the neck (cervical region), 12 in the middle
back (thoracic region), and 5 in the lower back (lumbar region). We shall focus on the
lumbar discs on this page.
Figure 1 Depicts a Front view
(AP) of the lumbar spine. Here
we can see how the discs (blue)
lie in between every vertebrae.
Spinal nerves (yellow) have
emerged from between every
two vertebrae and travel down
the lower limbs to innervate
(give life to) the skin and
muscle. Note how the sciatica
nerve is formed within the
pelvis by branches from the last
three lumbar spinal nerves. It is
this giant nerve that causes so
much trouble in many of us
chronic pain sufferers.
The disc is made up of three basic structures: the nucleus pulposus, the annulus fibrosus,
and the vertebral end-plates. Although their composition percentage differs, the latter
three structures are made of three basic components: proteoglycan (protein), collagen
(cartilage), and water. We will learn all about these structures below.

Figure 2. Shows a cut-away posterior view of
the lumbar spine. Now we can better visualize
how the sciatic nerve is formed and see just
how close the spinal nerve roots come to the
intervertebral disc. Any herniation of the
posterior disc may compress the spinal nerve
root and result in severe lower back pain and
lower limb pain (sciatica).
 Axial Disc Anatomy and physiology:
The Nucleus Pulposus is the water rich gelatinous center of the disc which is under very
high pressure when the human is up-right. It has two main functions, to bear or carry the
downward weight (axial load) of the body, and to act as a 'pivot point' from which all
movement of the lower trunk occurs. It's third function is to act as a ligament and bind the
vertebrae together.
The annulus Fibrosus is a much more fibrous structure that the nucleus pulposus. It has a
higher collagen content and lower water content. Its job is to 'corral' the pressurized
nucleus and keep it from exploding outward. It is made of 15 to 25 concentric sheets of
collagen, (a cartilage like substance) called the Lamellae. The lamellae are arranged in a
special configuration which makes them extremely strong and easily able to contain that
pressurized nucleus pulposus.
The spinal nerves roots and Spinal Nerves are extensions of the brain and spinal cord. Like
super highways the spinal nerves and nerve roots are constantly carrying electric messages
to and from the brain. The nerve roots exit the spine through bony holes called the
Intervertebral Foramen (IVF). As the nerve roots 'peal-off' from the cauda equina, one
sensory nerve root and one motor nerve root, and head for their respective IVF. Note
worthy is the fact that our two nerve root pass very close to the back of the disc. Once
within the IVF the two nerve roots merge into one spinal nerve. The spinal nerves are
called 'mixed nerves' for they contains both sensory nerve fiber (afferent) and motor nerve
fiber (efferent). After leaving the spine through the IVF, the nerve split into a posterior

division (Dorsal Ramus) and an anterior division (Ventral Ramus). The Dorsal Ramus
connect to the muscle and skin over the lower back and butt and the Facet Joint. The
Ventral Ramus combine in the pelvis and form the giant Sciatic Nerve and Lateral Femoral
Cutaneous Nerve which connect to all the skin and muscle of the lower limbs.
The ventral ramus has a 'recurrent branch' that connects to the back of the disc, as well as
the sympathetic nervous system (Grey Ramus Communicans); this special nerve is called
the Sinuvertebral nerve (SN) (see below). In the lumbar spine (below the L1 disc level)
there is no spinal cord. Instead the nerve roots hang in an enclosed sac which is called the
Thecal Sac. The thecal sac, which protects the dangling nerve roots, is made up of two
distinct but tightly bound layers called the dura mater and arachnoid mater. A clear fluid
called Cerebral Spinal Fluid is also found within the thecal sac. This fluid protects the
nerve roots and also supplies nutrients. Note how the nerve roots, that collectively are
called the Cauda Equina, are highly organized within the thecal sac.
Because of this arrangement, which always puts the lower level nerve root in front, it is
possible for a large compressive L4 disc herniation to irritate the S1 and/or L5 roots. This
may explain why disc herniations do not always match their dermatomal distribution - i.e.,
a disc herniation at L4 may clinically present as nerve root pain (radicular pain, sciatica)
and dysfunction in the S1 and/or L5 distributions! The Epidural Space is the space between
the bony neural canal and the thecal sac, or the space 'outside' of the thecal sac. Unlike my
drawing this space is filled with blood vessels and fat. Note worthy is the fact that this is the
region where 'epidural steroid injections' are placed.
The Facet Joints zygapophyseal joints) of the spine are where the vertebrae articulate
(join) with each other. Actually, the gap between the inferior and superior articular
processes is the true facet joint (white region). Collectively the inferior and superior
articular processes and the facet joint are called the zygapophyseal Joints or articular
pillars. These joints help carry the axial load of the body and limit the range of motion of
the spine. They also make up the back of the intervertebral foramen and may cause
stenosis if they hypertrophy in later life (lateral stenosis). The Ring Apophysis is the 'naked
bone' of the outer periphery of the vertebral bodies.
The very outer fibers of the disc (Sharpey's Fibers) anchor themselves into this region.
Bone spurs (Osteophytes) may arise from the ring apophysis as the result of the later stages
of Degenerative Disc Disease (DDD) and/or Osteoarthritis (Spondylosis). Specifically,
osteophytes arise from the prolonged 'pulling and tugging' of 'Sharpey's Fibers' at their
anchor points. The Posterior Longitudinal Ligament (PLL) is a strong ligamentous tissue
which courses down the anterior aspect of the vertebral canal and is attached to the outer
fibers of the annulus fibrosus. This highly innervated (supplied with pain carrying nerve
fiber) tissue is the last line-of-defense the posterior neural tissue has against the irritating
and inflammatory effects of nucleus pulposus.

Figure 3. The
Sinuvertebral
Nerve
 The Sinuvertebral Nerve
The Sinuvertebral Nerves (SN), is a mixed nerve as well. It carries both autonomic fiber
(sympathetic) and sensory (afferent) fiber. The sensory portion, which has the capability to
carry the feeling of PAIN, arises from the outer 1/3 of the posterior annulus fibrosus
(yellow balls, fig. 3) and Posterior Longitudinal Ligament. It then spilts and attaches to
both the dorsal ramus and the grey ramus communicans, although this nerves anatomy
and course seems to be quite anomalous. Of importance is the fact that if irritated, the
nerve ending within the disc have the potential to generate both back pain and/or lower
limb pain (discogenic pain).
o Discogenic Sciatica (referred discogenic pain)
It is believed that the sinuvertebral nerve-endings are 'sensitive' to the irritating effects of
degenerated nucleus pulposus, that may be leak into the outer region of the annulus from a
grade three annular tear. Amazingly, the sinuvertebral nerve also innervates (connects to)
the disc above and below! So, the sinuvertebral nerve of the L4 disc also innervates the L5
and L3 disc. This may help explain why a L4 disc herniation/annular tear may clinically
present with some signs of L5 and/or L3 involvement as well. It also carries autonomic
nerve fiber to the blood vessels (not shown in fig. 3) of the epidural space. Sympathetic
nerves control how the blood vessels function (vasomotor & vasosensory). Although rare,

injury to these sympathetic nerves may cause RSD symptoms in the patients lower limbs;
this usually would occur following surgery.
The exact pain-pathway (how pain travels from the disc to the spinal cord) of discogenic
pain is another fascinating and controversial subject. It seems that the sensory pathway
from the sinuvertebral nerves into the spinal cord, does not take the 'expected' route in
every patient. Some research has demonstrated that pain-signals travel from the disc, re-
enter the IVF (via sinuvertebral nerve) and Dorsal Root Ganglion at the same level. Other,
more recent research has indicated that pain-signals travel from the disc, through the
sinuvertebral nerve, through the Gray Rami Communicans, into the Sympathetic Trunk
(ST), up the sympathetic chain to the L2 vertebral level, through the gray rami
communicans, into the L2 dorsal rami, into the L2 IVF, and into the L2 Dorsal Root
Ganglion. The latter pain pathway is why some investigators believe that lower level disc
herniations may present as L2 dermatomal pain (groin region) in some patients!
Figure 4. A sagittal view (lateral view) of the 'motion
segment'; Two vertebrae which are 'sandwiching' the
intervertebral disc.
The disc (which is made of a annulus fibrosis and the nucleus pulposus) is made up of three
distinct areas: 1) The nucleus pulposus (green, fig 4), which is a water rich (due to
proteoglycan aggrecan & aggrecate molecules which trap and hold water within the disc)
gel in the center of the disc; 2) The annulus fibrosis (blue, fig 4), which is the fibrous outer
portions of the disc that is made up of type I collagen; and 3) The vertebral end-plates
(yellow, fig 4), which are cartilaginous plates that attach the discs to the vertebrae and
supply food (nutrients) to the inner 2/3 of the annulus and entire nucleus pulposus.
To further increase the strength of the annulus fibrosus, individual sheets of collagen are
layered throughout the annulus. There sheets of collagen are called lamellae (black curved
lines within blue, fig 4). The very outer lamellae (Sharpey's fibers), unlike the inner
lamellae, are anchored into the solid bony periphery (Ring Apophysis) of each vertebral
body. This is the region that 'osteophytes' or bone spurs typically like to form.

CT SCAN / MRI ANATOMY, CLINICAL CORRELATION
Figure 5. MR study of lumbar disc 5. Although this patient does have a moderate degree of
disc degeneration (black disc) and a small non-compressive 4 millimeter central disc
herniation, he had a very large 'central canal' which nicely demonstrates the axial MRI
anatomy. The nucleus pulposus is not visible on these images. One reason for this is
because the disc has too much desiccation to distinguish between the annulus and nucleus,
and the other reason is that this is a T1 Weighted image which will not differentiate
between the high water content of the nucleus and the more arid annulus; however, on a
normal, non-degenerated disc, you can easily see a nuclear region and an annular region on
the T2 Weighted image. The 'posterior neural structures' include the Traversing Nerve
Roots, the Thecal Sac (dura and arachnoid mater) and the exiting nerve roots that lie
within the IVF (pink zone) and not very visible on this particular image.

Figure 6. Demonstrates another axial view of a normal health L4 disc. Now we can see a
distinct nuclear region and annular region. Note the Facet Joints (pink star on one of them)
are imaged 'white' as well. Again this indicates a high fluid content; in this case it's the
synovial fluid of the facet joint that is cause the increased signal.
Figure 7. CT myelography of the lumbar region. BLUE: This is the 'Central Region' and is
located directly behind the disc and encompasses the anterior aspect of the thecal sac. Since

the PLL (posterior longitudinal ligament) is at its thickest in this region, the disc usually
herniates slightly to the left or right of this central zone. PINK: This is the 'Paracentral
Region' or 'Lateral Recess' and is located just outside of the Central Region. Because the
PLL is not as thick in this region, disc herniations are frequently found here; in fact, this is
the number one region for disc herniations to occur in. The Traversing Nerve Roots, which
are the neural structures found in this zone, are frequently contacted, deviated and
compressed in this zone. GREEN: This is the 'Intraforaminal Zone', also known as the
'Subarticular Zone', and is located within the intervertebral foramen (IVF). It is fairly rare
for a disc to herniate into this region or beyond; in fact, only 5% to 10% of all disc
herniation occur here or farther out. When herniations do occur in this zone, they are often
very troublesome for the patient. This is because a super-delicate neural structure called
the 'Dorsal Root Ganglion' (DRG) lives in this zone. Any compression of the DRG can
result in severe pain, sciatica ( radiculopathy) and nerve cell body (neuron) damage.
YELLOW: This is the 'extraforaminal zone' and, as the name implies, is just outside
(lateral to) of the IVF. Again, it is very rare for a disc to herniate into this region, but when
it does happen, it is often very troublesome for the patient and surgeon. A herniation in this
zone may also irritate the 'Sympathetic Nervous System' and cause RSD like symptoms in
the lower limb.

Figure 8. CT myelography.
Normal structure at L5 vertebra
DISC PHYSIOLOGY
The normal human intervertebral disc, which is considered the largest avascular structure
in the human body, is made up of two main components, proteoglycan and collagen (type I
and type II). The annulus is mostly made of collagen, which is a tough fibrous tissue similar
to the cartilage that is found in the knee, and the nucleus is made mostly of proteoglycan.
Proteoglycans, which are produced by disc cells that resemble chondrocytes, are extremely
important for disc function and are what 'trap' and hold water molecules (H20) within the
tissue of the disc. In fact both the disc and annulus are comprised mainly of water, i.e., the
nucleus is 80% water, and the annulus is 65% water. Proteoglycans are the building blocks
of the aggrecan molecule which is the true 'water trap' of the disc. Aggrecans combine
within the disc on strands of hyaluronan acid to form huge structures called 'Aggregates'.
These super water-filled proteoglycan aggregates are what give the healthy young disc its
amazing strengths and pliability, in fact a well hydrated disc is often even stronger than the
bony vertebral body.

Figure 9. Here we have the healthy disc
of a teenager (cadaver). The water
content is extremely high as you can even
see by the 'glistening' appearance of the
nucleus (which is the gray center of the
white disc).
 Disc Function
In order for a disc to function properly, it must have high water content; this is especially
true of the nucleus. A well hydrated (with water) disc is both strong and pliable. The
nucleus pulposus needs to be strong and well hydrated to do its job (axial load), for it is this
structure that supports or carries the lion's share of the axial load (downward weight of
body) of the body. With an undamaged annulus, strongly corralling a fully hydrated
nucleus, the disc can easily support even the heaviest of bodies! As the disc dehydrates
(loses water) the disc loose ability to support the axial load of the body (loses hydrostatic
pressure); this causes a weight bearing shift' from the nucleus, outward, onto the annulus,
outer vertebral body, and zygapophyseal joints (facets). Now, we have an 'over-load' on the
annulus (which may trigger other destructive biochemical reactions) which, if severe
and/or is imposed upon a genetically inferior annulus, will result in pathological
degenerative disc disease (spondylosis).
Hydration also is important with respect to disc nutrition. As we have already mentioned,
nutrients (which all living tissue needs in order to survive) must diffuse (soak) through the
discal tissue in order to reach the hungry disc cells. This diffusion process is much faster
and easier if the diffusing tissue has a high water content. We may use 'swimming' as an
analogy: It's easier to swim through the water, than through the sands of a desert. The
sands of the desert would be a dehydrated disc, and the water would be a hydrated disc. So,
water and disc hydration are one of the key factors for a normally functioning spine and
well fed disc.

 Proteoglycans, which are produced by disc cells that resemble chondrocytes, are
extremely important for disc function and are what 'trap' and hold water molecules (H20)
within the tissue of the disc. In fact both the disc and annulus are comprised mainly of
water, i.e., the nucleus is 80% water, and the annulus is 65% water. Proteoglycans are the
building blocks of the aggrecan molecule which is the true 'water trap' of the disc.
Aggrecans combine within the disc on strands of hyaluronan acid to form huge structures
called 'Aggregates'. These super water-filled proteoglycan aggregates are what give the
healthy young disc its amazing strengths and pliability.
 Water is held within the disc by tiny sponge-like molecules called proteoglycan
aggrecans. These 'super sponges' have an amazing ability to attract and hold water
molecules, and can in fact hold over 500 times their own weight in water; this gives the
non-dehydrated disc the tremendous 'hydrostatic pressure' which is needed to bear the
axial load of the body.
o Disc hydration
Water is held within the disc by tiny sponge-like molecules called proteoglycan aggrecans.
These 'super sponges' have an amazing ability to attract and hold water molecules, and can
in fact hold over 500 times their own weight in water; this gives the non-dehydrated disc
the tremendous 'hydrostatic pressure' which is needed to bear the axial load of the body.
Amazingly, the aggrecans water absorption is so powerful that over night (non-axial
loading) the height of the disc and the body will actually measurably increase due to the
discs engorgement with water. This phenomenon is called 'Diurnal Change' and is only
present in non-degenerated discs.
Disc cells, particularly the chondrocyte-like cells of the nucleus and inner annulus,
manufacture proteoglycan aggrecan molecules. Like little factories, they create, replace
and rebuild aggrecan molecules. As long as the disc cells have food (glucose), building
material (amino acids) and oxygen all is well in disc-land. It is also important for them to
have a non-acidic working environment, which is taken care of, since wastes are diffused
out of the disc through the same way nutrients diffuse in. In the living disc up to 100
aggrecans combine on a long piece of 'hyaluronan acid' to form giant proteoglycan
aggregate molecules. It's these aggregates that are found within the disc in the real world.
The nucleus pulposus has a unique ability to bind water. Initially the water binding capacity of the nucleus was
attributed to fluid exchange governed by osmotic pressure. The cartilaginous end-plates were thought to act as a
semipermeable membranes with the nucleus drawing water from the vertebral bodies. It is presently thought that
the hydrodynamics of the disc depend upon the nucleus possessing the properties of a gel rather than upon
osmosis. this gel contains some cartilaginous cells, fibroblasts, collagen framework and the ground substance which
is mostly mucopolysaccharides with varying amount of salt and water. The high imbibition pressure is produced
by the mucopolysaccharides contained within the gel which can bind almost nine times its volume of water. The
water content of the nucleus is not chemically bonded and can be expressed from the nucleus by prolonged
mechanical pressure.

Box 1 Factors necessary for chondrocytes-like disc cells to manufacture proteoglycan
aggrecan molecules
 Glucose
 Building material (amino acid)
 Oxygen
 Non-acidic environment
 Disc Nutrition
The intervertebral disc is the largest avascular structure in the
human body. The reason for this is because it has no direct
blood supply like most other body tissue. Nutrients for the disc
are found within tiny capillary beds (black arrows, fig. 6) that
are located in the subchondral bone, just above the vertebral
end-plates . This subchondral vascular network 'feeds' the disc
cells of the nucleus and inner annulus through the diffusion
process. Figure (fig. 10) shows the 'disc feeding vascular
system' . Note that the outer annulus has its own blood supply
that is embedded within the very outer annulus. For the outer
annulus this is a much more efficient system and nutrients
don't have to diffuse very far to find their disc cells. The 'more direct' blood supply of the
outer annulus is why tears of the outer 1/3 of the annulus will heal with the passage of time,
which unfortunately is not true for tears of the rest of the disc.
Research has indicated that disc tears will not heal in the inner zones of the disc - probably
because of the avascular nature of the inner two thirds of the disc. Note the nutrients (pink
balls, fig. 10)) diffuse directly into the tissue of the outer annulus, where as the nucleus and
inner annulus has a much longer diffusion route that is might be blocked by the vertebral
end-plates if calcified. Note how the nutrients (pink balls) are released from the blood
vessels (red, fig. 10) in the subchondral bone just under the vertebral end-plates. These
nutrients must 'diffuse' or soak their way through the vertebral end-plates and into the
disc. This 'diffusion method' is how the cells of the disc get the nutrients oxygen, glucose,
and amino acids which are required for normal disc function and repair. This poor
blood/nutrient supply system to the disc is one of the main reasons that the disc ages and
degenerates so early in life.
Nutrients for the disc are
found within tiny capillary
beds that are located in the
subchondral bone, just
above the vertebral end-
plates. This subchondral
vascular network 'feeds' the
disc cells of the nucleus and
inner annulus through the
diffusion process.

Figure 10. Disc anatomy, disc
vascular system and disc nutrition
The 'diffusion feeding process' is enhanced somewhat by a phenomena called 'Diurnal
Change'. Our discs have the ability to expand and compress over the course of a day. As we
start the day our discs, like squeezing out a sponge, will compress and dehydrate because of
the gravity and physical activity which place axial loads upon the discs. In fact a healthy
disc will shrink down some 20%, which in turn decreases our height by 15 to 25mm. As we
sleep and decompress our spines, our discs swell with water plus nutrients and expand
back to their fully hydrated state. This tide-like movement of fluids in and out of the disc
will help with the movement of nutrients into the avascular center of the disc.
 Advanced Anatomy & Physiology
o The Nucleus Pulposus
The nucleus pulposus is a hydrated gelatinous structure located in the center of each
intervertebral disc that has the consistency of toothpaste. Its main make-up is water (80%).
Its solid/dry component make-up are proteoglycan (65%), type II collagen fiber (17%) and
a small amount of elastin fibers. Collectively the proteoglycans and the collagen are called
the 'nuclear matrix'. The cells of the disc, which produce the water holding proteoglycan
molecules are very similar to chondrocytes seen in articular cartilage and are also held
within the matrix.

Proteoglycans are found in several structural forms within the disc but the most important
'arrangement' is called a proteoglycan aggrecan. These aggrecans main function is to trap
and hold water, which is what gives the nucleus its strength and resiliency. Like a 'super
sponge', aggrecans can trap and hold over 500 times their weight in water!
The nucleus has two functions. The first is to bear most of the tremendous axial load
coming from the weight of the body above and second to 'stand-up' the lamellae of the
annulus - so that the annulus can reach its full weight baring potential. In order for proper
weight bearing the nucleus and the annulus must work hand in hand.
o The annulus Fibrosis
The annulus is the outer portion of the disc that surrounds the nucleus. It is made up of 15
to 25 collagen sheets which are called the 'lamellae'. The lamellae are 'glued' together with
a proteoglycans. These sheets encircle the disc and, in concert with the nucleus, give the
disc tremendous axial load strength.
Figure 11. A, The annulus Fibrosis. B, The 'Nucleus Pulposus' (pink), which is a water-rich
gel-like mass of proteoglycan material, has the duty to support the tremendous 'Axial-
Load' (weight) of the body. This nucleus is 'corralled' by the stronger 'annulus Fibrosus'.
The annulus is made out of concentric rings of a cartilage-like material called 'lamellae'. It
is this specially arranged collagen that gives the annulus the tremendous strength needed to
hold that nucleus in place.

The posterior portion of the annulus if further strengthened by the 'posterior longitudinal
ligament'. This structure is the final barrier between the disc and the delicate spinal cord,
and nerve roots.
The biochemical make up of the annulus is similar to that of the nucleus with different
proportions. The annulus is 65% water, with the collagen, both type I and II making up
55% of the dry weight, and proteoglycans (mostly the larger aggregate type - 60%) making
up 20% of the dry weight. 10% of the annulus also contain 'elastic fiber' that are seen near
where the annulus attaches into the vertebral end-plate.
The lamellae are made up of both Type I (very strong type) and Type II collagen fiber. The
very outer lamellae are almost all Type I. As we move inward toward the nucleus the more
Type II is seen and less Type I. The very inner layers are very hard to distinguish from the
nucleus. There is not a clear boundary between the nucleus and the annulus.
A simply amazing fact about the lamellae design is that the collagen fibers that make-up
each lamellae run parallel at a 65 degree angle to the sagittal plane. Even more amazing is
the fact that the each lamellae are flipped so that the 65 degree angle alternates between
every lamellae, one to the right then one to the left. This design greatly increases the shear
strength of the annulus and makes it stronger for cracks to develop through the layers of
the annulus.
The function of the annulus is to help the nucleus support the axial weight from the body.
The annulus does need some help from the nucleus in order to achieve its strongest
configuration. It relies on the nucleus to push it outward which keeps the lamellae from
collapsing inward. The nucleus must keep a very high hydrostatic pressure to achieve this.
Box 2. Function of the proteoglycan
1. Proteoglycan traps water. Water is held within the disc by tiny sponge-like
molecules called proteoglycan aggrecans. These 'super sponges' have an amazing
ability to attract and hold water molecules, and can in fact hold over 500 times their
own weight in water; this gives the non-dehydrated disc the tremendous 'hydrostatic
pressure' which is needed to bear the axial load of the body.
2. The lamellae of the annulus fibrosus are glued together by proteoglycan
o The Vertebral End-Plates
Both the top and the bottom of each vertebrae (spinal bones) are capped with a thin ¾
millimeter cartilaginous pad called the 'Vertebral End-Plate'. Despite their name, these
end-plates are not attached to the subchondral bone of the vertebrae but are instead
strongly interwoven into the annulus of the disc. It is for this reason, as well as strong
morphological similarities, that the vertebral end-plates are considered part of the disc and
not part of the vertebral body.

Figure 12. The Vertebral End-Plates
The biochemical morphology of the end-plates is extremely similar to that of the disc:
Water, proteoglycans, collagen and cartilage cells (chondrocytes). The concentration
scheme of these components also mirrors that of the disc: The center of the end-plate is
mostly water and proteoglycan. As we move outward toward the periphery, more and
more collagen is seen with less and less proteoglycans. This similar biochemical makeup
and distribution scheme helps the diffusion of nutrients between the subchondral bone of
the vertebra and the depths of the disc.
The very outer rim of the vertebrae is not covered by the end-plate, which leaves a ring of
exposed bone on the periphery of the top and bottom of each vertebra. This exposed
peripheral area is called the 'Ring Apophysis' and is often a site for the development of
spur formation associated with the degeneration process.
BIOMECHANICS OF DISC DEGENERATION
In the lumbar region, the compressing forces occur primarily in a vertical axis. Both the nucleus
and annulus act to absorb these forces and redistribute them evenly in all direction. The nucleus
bears all the vertical load and the annulus bears only the tangential load. The liquid property of
the nucleus enables it to alter its shape freely under pressure. This shape altering property of the
nucleus enable it to withstand stresses varying in duration and magnitude, transmitting some
radially to the annulus and the rest over the entire cartilaginous end- plate.
With degeneration of the disc, the gel inhibition pressure of the nucleus is impaired, marked
change occurs in the transmission of forces occurring along the vertical axis of the spine. The
desiccated nucleus is unable to redistribute much of the vertical load radically, causing the
annulus and the facet joints to sustain much of this load.

NORMAL DISC AGING
Unlike other tissues of the body, the intervertebral disc under goes an early and often
severe form of aging and degeneration . In most humans, this aging/degeneration process is
slow and steady, but in some the process rapidly accelerates and may lead to catastrophic
failure of the disc; which in turn may lead to chronic pain and disability. This 'accelerated'
form of aging/degeneration may be called Degenerative Disc Disease (DDD), although the
term is commonly and erroneously used to describe any form of disc degeneration.
Figure 13. The figures to the left
show the difference between the
normal discs and degenerative disc
disease. Note the difference
between the 'normal discs' (white
structures) and the 'degenerated
discs' (right). This is an example of
moderate to severe degenerative
disc disease. The discs have both
collapsed and severely dehydrated.
They have also an ugly brown
color from the glycation process.
Research has strongly linked degenerative disc disease to back pain, and sciatica, although
not in every case, for it is well known that degenerative disc disease, disc protrusion, and
stenosis do occur in completely asymptomatic people, but for about 10% of the population,
degenerative disc disease will result in permanent chronic pain and disability. Technically
it's not the actual process of degenerative disc disease that results in pain; it's the evil 'end-
phases' of the disease that have the potential to generate back pain. These end-phases
include annular tears (Internal Disc Disruption or IDD); disc protrusions; nerve in-growth
; and the ultimate end-phase, stenosis.
The most common and striking feature of disc aging and degeneration is the loss of the
proteoglycan molecule from the nucleus of the disc. Other findings of aging include a
progressive dehydration, a progressive thickening (via cross-linking, glycation and CML
formation), brown pigmentation formation and increased 'brittleness' of the tissues of the
disc.
 The three main factors of disc aging:
There are three main factors that are involved in the aging process of the disc and both of
these factors are amplified because of the already poor vascular supply of the disc: 1)
Idiopathic blood vessel/nutrient loss and dehydration, 2.) Non-Enzymatic Glycation,
Glycosylation, 3) Free radical oxidation

Figure 14. Here we have early disc aging. The tissue of the disc has turned brown,
secondary to the glycation process, and the disc has become much 'drier. The disc seen in
adolescents, for comparison sake, shows us what the disc of a teenage looks like. Note the
well demarcated, wet looking, nucleus (gray center) and no ugly brown tissue
1) Idiopathic blood vessel/nutrient loss and dehydration:
 Short term result
For unknown reasons the nucleus of the disc losses much of its vital blood supply during
the first decade of life (6). Without sufficient nutrients (which are contained in the blood)
the cells of the disc begin to die and the disc (especially the nucleus) becomes depleted of
water. The drop in water/proteoglycan content is one or the classic signs of disc aging.
Because of this dehydration of the nucleus, there is ultimately a 'weight-bearing shift' that
occurs from the nucleus onto the outer annulus, ring apophysis, and the zygapophyseal
joints. This increase stress upon the preceding posterior structures may lead to further
more severe forms of aging, i.e., degenerative disc disease.
 Long result
Under the physiology section of the 'Disc Anatomy', we have learned how important disc
nutrition is in maintaining a normally functioning disc. As long as the cells of the disc
receive an adequate nutrient supply (which is obtain from the diffusion of oxygen, glucose,
and amino acids from capillary beds just above the end-plates, into the disc), they will
manufacture the proteoglycan molecule, which combines within the disc to form the larger
aggrecan and aggregate molecules. It is these aggrecan molecules that trap and hold water
within the disc. A fully hydrated disc will have a very high hydrostatic pressure (osmotic
pressure) which makes the nucleus pulposus (which is 80% water in a normal disc)
incredibly strong and able support the lion's share of the axial load from the body. The
nutrients to inner annulus and nucleus have a long way to 'diffuse' in order to reach all the
disc cells. (pink balls, fig 6)
Without an adequate supply of nutrients, the cells of the disc will die. If the cells of the disc
failed to get proper nutrients - such as oxygen, or glucose - or if the pH level of the disc rose
(because waste is not being diffused out of the disc), disc cells would die and stop producing

the vital proteoglycan molecule; without proteoglycans, the disc losses its water content
(dehydrates) and losses its hydrostatic pressure (osmotic pressure). This lost proteoglycan
content is the most striking feature of disc aging and degeneration. By adulthood over 50%
of the cells of the disc are dead.
So what's killing the disc cells and resulting in this
loss of proteoglycan content?. It seems that the human
disc becomes 'nutritionally compromised' from the
moment we begin to stand and walk. An idiopathic
"obliteration" of portions of the nutrient-providing
capillary beds, which lie just above the vertebral end-
plates. (These capillary beds are the only source of nutrients for the cells of the inner
annulus and nucleus) is frequently observed very early in life, amazingly, this 'auto-
destruction' begins within the first two years of life, and worsens over the next 8 years and
between the ages of 3 and 10 there is "a dramatic decrease of physiologic vessels in the end-
plate and an abundance of areas with obliterated vessels. and a substantial increase in
(disc) cell death." These findings suggest that the initial causation of disc aging and
degeneration is 'nutritional compromise', secondary to an idiopathic loss of the discal blood
supply above the vertebral end-plates.
Other factors affecting disc nutrition via diffusion rates of nutrients through the vertebral
end-plates include end-plate calcification, the effects of changes in blood flow patterns
secondary to arterial stenosis, smoking, diabetes, and exposure to vibration.
Box 3. Pathophysiological / biochemical steps of disc degeneration
 Obliteration of portions of the nutrient-providing
capillary beds, which lie just above the vertebral end-
plates.
 Death of the proteoglycan forming disc cells
 Progressive loss of the water trapping capacity of the
nucleus
 Loss of disc hydration
 Disc degeneration
 The Vicious Cycle of Disc Aging
This progressive loss of proteoglycan and dehydration begins to 'snowball' out of control.
Not only because of the progressive loss of nutrients, but also because of the fact that
decreased hydrostatic pressure also slows the production of proteoglycan by the disc cell.
Here's what this vicious cycle looks like.
As the nutrient supply within the disc drops (because of blood vessel obliteration and later
end-plate mineralization), the disc cells start to die. Because there are fewer available disc
cells around to make proteoglycan, there is a drop in the amount of circulating
An idiopathic "obliteration" of
portions of the nutrient-providing
capillary beds, which lie just above
the vertebral end-plates is frequently
observed very early in life.

proteoglycan aggrecan molecules. This decrease in the aggrecan molecule, (which is what
holds water within the disc) results in both dehydration, and a decrease in hydrostatic
pressure within the nucleus. The loss of hydrostatic pressure has two negative effects on the
disc: a) it will cause a further decrease in the amount of circulating proteoglycan aggrecan
molecules, for we know that disc cells need a constant hydrostatic pressure level of 3 atm to
function normally. Any increase or decrease in hydrostatic pressure caused a reduction
proteoglycan production, which in turn decreases hydrostatic pressure even more - hence
the vicious cycle. b) Now, these biochemical changes begin to change the biomechanics of
the disc: With the decrease of hydrostatic pressure the nucleus, like a deflating beach-ball,
can no longer carry the full axial-load (weight) of the body. A 'shift' in the axial-load
distribution begins to occur, with the periphery of the disc (outer annulus, ring apophysis,
and zygapophyseal joints) taking on more and more of the load and stress. Experimentally,
the annulus of a degenerated disc shows a very high 'stress-load' on the annulus and not
the nucleus. We will later learn that this 'load-shift' can be greatly accelerated if the
volume of the nucleus is increased by trauma-induced structural damage to either the end-
plate (compression fracture) and/or tearing of the inner annulus.
2.) Non-Enzymatic Glycation: Glycosylation
Glycation (Glycosylation , or non-enzymatic glycation) is a biochemical reaction which
occurs when reduced sugars (like glucose) come in contact with proteins (like disc collagen)
in an avascular environment. The more avascular the tissue, the more severe this reaction
occurs. Since the disc is the largest avascular tissue in the body, the glycation process
thrives within its substance and results in a slow but steady transformation of disc collagen
into a thicker and more brittle substance. Specifically, this reaction occurs between the
protein molecules within the collagen, and free floating glucose (reduced sugar). This
reaction is called 'posttranslational protein modification' or Glycation. Here's how it
works: In the absents of oxygen, reduced sugars start to 'rub against' (bind) the proteins
within the collagen. The proteins are transformed into what is called an 'Advanced
Glycation End-Product' or AGE. These converted discal collagen strands (AGEs) become
much more brittle and also much more 'sticky', i.e., they love to combine with their
glycated neighbors in a process called 'cross-linking'. This 'cross-linking' phenomenon
makes the disc thicker, more fibrous and more susceptible to the development of
degenerative disc disease. It also stains the discal tissue a distinct shade of brown as noted
in figure 15,16
3) Free radical oxidation
Finally, the unstable AGEs molecules are oxidize by free radicals into a much more stable
structure called a CML (N- Carboxymethyl -lysine). CML formation has been found to be
an excellent indication of discal aging

Figure 15. Dessication of the nucleus pulposus associated with multiple annular tears (eg,
radial, circumferential). Notice that the disc is bulging without actual herniation. Because
of the existing disc degeneration, it is possible to differentiate between the nucleus pulposus
and annulus fibrosus. 1. annulus fibrosis 2. Circumferential annular tears 3- Posterior
longitudinal ligament 4. The desiccated nucleus pulposus
Table 1. Factors implicated in disc aging and disc degeneration
Factor Comment
Nutritional compromise will
deprive the disc cells of the
capability to form
proteoglycan. Without
proteoglycans aggrecan &
aggrecate, the water trapping
function of the nucleus is lost,
the disc losses its water content
(dehydrates) and losses its
hydrostatic pressure (osmotic
pressure)
 Idiopathic loss of the discal blood supply above the
vertebral end-plates
 End-plate calcification
 Vascular risk factors resulting in arteriolosclerosis of tiny
capillary beds that are in the subchondral bone, just above
the vertebral end-plates (subchondral vascular network)
Non-Enzymatic Glycation
Specifically, this reaction occurs between the protein molecules
within the collagen of the annulus, and free floating glucose
(reduced sugar) in an avascular medium. This reaction is called
'posttranslational protein modification or Glycation. The proteins
are transformed into what is called an 'Advanced Glycation End-
Product' or unstable AGE.
Free radical oxidation
The unstable AGEs molecules is oxidize by free radical into a
much more stable structure called a CML (N- Carboxymethyl -
lysine). CML formation has been found to be an excellent
indication of discal aging

Figure 16. Dessication of the nucleus pulposus
associated with multiple annular tears (eg,
radial, circumferential). Notice that the disc is
bulging without actual herniation. Because of the
existing disc degeneration, it is possible to
differentiate between the nucleus pulposus and
annulus fibrosus. Notice the existence of
articular facet pathology and tegmental flavum
hypertrophy.
1. annulus fibrosis
2. Circumferential annular tears with infiltration
by nuclear material
3,4 . The desiccated nucleus pulposus
DISC / VERTEBRAL COLUMN PATHOLOGICAL CHANGES IN DEGENERATIVE
DISC DISEASES
Degenerative disc disease commonly induces pathological changes in both the
intervertebral discs and the vertebral column, collectively these are called spondylosis.
Table 2
Table 2. Disc/ vertebral column pathological changes in degenerative disc disease
Pathology Comment
Disc pathology Annular bulging, annular tears, nucleus pulposus herniation, vacuum disc,
canal stenosis
Pars
interarticularis
defect
Stress fractures of one or both sides of the neural arch through the
relatively weak pars.
Facet
pathology
Facet Joint disease can be defined as osteophytosis and hypertrophy of the
articular process.. etc
MRI bone
marrow signal
changes
MRI bone marrow signal changes in the vertebral endplates of the
degenerated discs

Figure 17. A,
Normal spine,
B,
spondylosis
 Role of end plate degeneration in intradiscal changes among patients with low back
pain
The end plate is the main route of diffusion. The nutrition provided by arterial flow is
delivered through the end plate (Roberts et al. 1989, Moore et al. 1992). A comparison
between end plate degeneration in MRI and intradiscal changes revealed in discography
showed end plate degeneration to be associated more clearly with long-term disc damage,
i.e. degeneration, than acute disc damage, i.e. annular rupture. The primary nutritional
pathway into the avascular disc is assumed to be from the subchondral vessels via the end
plate (Roberts et al. 1989, Moore et al. 1992). If this route were disturbed because of a
calcified endplate or occlusion of the subchondral vessels, internal disc disruption could
follow (Roberts et al. 1989). End plate degeneration has been found to occur parallel to
other disc degeneration events (Horner & Urban 2001). Histologic studies have revealed
coincidence of microfractures in the end plate and intradiscal tears and clefts (Moon et al.
2000, Boos et al. 2002). It is therefore assumed that the degenerative process affecting the
end plate, the NP and the AF is a temporal sequence of nutritional deficiency (Boos et al.
2002).
No differences were found between the end plate degeneration categories and pain
provocation, and the results were hence opposite to the previous findings (Weishaupt et al.
2001), which showed end plate changes to be associated with low back pain. It is yet
possible that the irritation of a ruptured disc by contrast medium is such a dominant factor
in producing pain that minor variables, such as possible end plate irritation, will be
masked, which might explain why no evident correlation between pain provocation and
end plate changes was found. On the other hand, we only evaluated intradiscal pain, and
the possible end plate-originated symptoms may not be discogenic. Yet, the results showed
that a significant number of positive pain provocations were associated with no end plate
degeneration, which also indicates the negative role of end-plate degeneration in producing
low back pain. Findings of concordant pain during discography in discs with annular
ruptures support the results of previous studies showing an association between intradiscal
ruptures and low back pain (Vanharanta et al. 1988, Moneta et al. 1994).

THE VASCULAR BASIS FOR DISC DEGENERATION
 Arterial findings, diffusion and disc degeneration among people without low back
pain
Atherosclerotic changes in the lumbar arteries have been shown to be associated with
lumbar disc degeneration (Kauppila et al. 1997). There is moderate variation in the
findings on lumbar artery status between the disc levels. It has been demonstrated that the
correlation of atherosclerotic changes with disc degeneration is stronger at the upper
compared to the lower levels of the lumbar spine (Kauppila 1994)
The well-being of a disc is greatly dependent on its cells (Maroudas 1988, Boos et al. 2002).
The cell density of a disc is controlled by nutritional factors (Stairmand et al. 1991). During
the degenerative process, considerable alterations occur in the structure and biochemistry
of the intervertebral disc.
 Arterial findings, disc degeneration and low back pain
The major findings in the literature concerning the role of blood flow and disc
degeneration can be summarised as follows.
In cadaveric studies, atherosclerotic manifestations in the abdominal aorta and the lumbar
arteries correlated with low back pain (Kauppila et al. 1997) and disc degeneration
(Kauppila et al. 1994).
In a cross-sectional cohort study, calcification of the abdominal aorta was found to
correlate with disc degeneration (Kauppila et al. 1997). In some follow-up studies, vascular
disease and LBP have been found to correlate (Penttinen 1994, Kauppila et al. 1997), and
one post-mortem study also showed this association (Kauppila & Tallroth 1993).
In epidemiological studies, low back pain and arterial disease have been found to have
some common risk factors (Frymoyer & Gordon 1989, Battie et al. 1991). The nutrient
supply to the nucleus cells can be disturbed at several points. Factors that affect the blood
supply to the vertebral body such as vascular risk factors, the metabolic syndrome (type 2
diabetes, inulin resistance, hypertension, increased blood viscosity and atherosclerosis
...etc) can all result in disturbances of the nutrient supply to the nucleus cells and
subsequent disc degeneration. Histologic and experimental studies have revealed the
association between nutritional factors and disc degeneration (Horner & Urban 2001, Boos
et al. 2002). It has been shown anatomically that vascular buds penetrate the cartilaginous
end plates on the vertebral side, but rarely communicate with the disc substance itself
(Moore et al. 1992), and that the nutrition of discs occurs mainly via diffusion from the
blood vessels into the surrounding structures (Holm et al. 1981, Horner H.A. & Urban J.
2001). Thus, changes in the end plate and the vessels may cause a reduction in this
pathway.

It has been found in an experimental study that the reduction in nutrition may cause
intradiscal tears and reduce the regenerative ability of cells in discs. The mechanical stress
is also more likely to cause tears in conditions of reduced nutrition(Boos et al. 2002). These
remaining tears could be a source of intradiscal pain and thus explain the difference in the
degree of disc degeneration and lumbar artery occlusion between people with and without
low back pain.
Vascular changes causing insufficient blood supply in the abdominal aorta and the lumbar
arteries are more common in the LBP population. Atheromatous lesions in the abdominal
aorta most commonly occur around the aortic bifurcation, where the orifices of the lumbar
arteries are also located (Rogoff S.M. & Lipchik E.O. 1970, Ross R 1988). Patients with
chronic low back pain were more likely to have atheromatous plaques in the abdominal
aorta than patients without back pain, as analysed by CT, and this association was most
distinct in the youngest patients. Also, when sciatica patients were compared to an
asymptomatic population, intervertebral disc degeneration and narrowing of the lumbar
arteries were significantly more pronounced in the sciatic population at almost every disc
level. In general, it can be concluded that the present study showed multiple evidence of an
association between vascular changes and low back pain and disc degeneration.
CONCLUSION
1. The metabolic syndrome (truncal obesity, type 2 diabetes, inulin resistance,
hypertension, increased blood viscosity and atherosclerosis ...etc) is a risk factor for
disc degeneration and back pain. Vascular risk factors can result in disturbances of
the nutrient supply to the nucleus cells and subsequent disc degeneration. Disc
degeneration is a vascular process rather than a mechanical one.
2. Cardiovascular/cerebrovascular risk factors are significantly and independently
associated with symptomatic lumbar disc herniation. These findings provide further
confirmation that atherosclerosis may be involved in spinal disc degeneration.
Modification of risk factors, particularly smoking, may also prove to be beneficial.
3. Stenosis of lumbar arteries is significantly associated with decreased diffusion of
lumbar discs and may thus play an important role as a mechanism of disc
degeneration.
4. End plate degeneration is associated with disc degeneration, but not with disc
rupture. Nor is end plate degeneration associated with discogenic pain caused by CT
discography.
5. Atheromatous lesions in the abdominal aorta are more common and more extensive
in patients with chronic low back pain than in asymptomatic people. There was no
association between atheromatous lesions in the abdominal aorta and disc
degeneration among chronic low back pain patients.
6. Stenosis of lumbar arteries is associated with disc degeneration among sciatica
patients, and the grade of occlusion of lumbar arteries and the severity of disc
degeneration are significantly higher among sciatica patients compared to
asymptomatic subjects.
7. Stenosis of lumbar arteries was associated with back pain symptoms and physical
activity among sciatica patients in three-year follow-up.

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Pathology/pathogenesis of lumbar disc degeneration

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