Download The Intervertebral Disk

Chapter 20
The Intervertebral Disk
Overview
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The IVD forms a symphysis or
amphiarthrosis between two adjacent
vertebrae
Represents the largest avascular structure in
the body
In the human spinal column, the combined
heights of the IVD accounts for
approximately 20-33% of the total length of
the spinal column
Overview
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Intervertebral discs are able to distribute
compressive stress evenly between adjacent
vertebrae because the NP and inner AF act
like a pressurized fluid, in which the
pressure does not vary with location or
direction
The biomechanical studies of the IVD seem
to indicate that the disc acts to provide
flexibility at low loads, and stability at high
loads
Anatomy
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Lumbar Disk
– The lumbar disc is approximately cylindrical, its
shape being determined by the integrity of the
annulus fibrosis (AF)
– The AF consists of approximately 10-12 (often as
many as 15-25) concentric sheets of
predominantly type I collagen tissue bound
together by proteoglycan gel
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The number of annular layers decreases with age, but
there is a gradual thickening of the remaining layers
The fibers of each successive sheet or lamella maintain
the same inclination of 65º but in the opposite
direction to the preceding lamella, resulting in every
second sheet having the same orientation
Anatomy
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Lumbar Disk
– Although the posterior aspect of the IVD is
thinner, the collagen is more tightly packed than
it is anteriorly
– Consequently, the posterior part of the annulus
will have thinner but stronger fibers, and it is
capable of withstanding tension applied to this
area during flexion activities and postures which
occur more frequently than with extension
– However, due to the predominance of flexion
activities in life, fatigue damage may occur in the
posterior aspect of the disc, making it a common
site of injury
Anatomy
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Lumbar Disk
– With the exception of early youth, there
is no clear boundary between the NP and
AF, and it resembles a transitional zone
– The biomechanical make up of the NP is
similar to that of the AF, except that the
NP contains mostly type II collagen, as
opposed to type I
Anatomy
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Lumbar Disk
– Each vertebral end plate consists of a layer of
hyaline and fibrocartilage about 0.6 to 1
millimeter thick, which covers the top or bottom
aspects of the disc, and separates the disc from
the adjacent vertebral body
– The two end plates of each disc, therefore, cover
the NP in its entirety, but fail to cover the entire
extent of the AF
Anatomy
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Lumbar Disk
– The outer half of the IVD, the posterior
longitudinal ligament, and the dura are
innervated by the sinuvertebral nerve,
which is considered to arise from the
ventral ramus and the sympathetic trunk
Biomechanics
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Lumbar Disk
– Although the lumbar IVD appears destined for
tissue regression and destruction, it remains
unclear why similar age-related changes remain
asymptomatic in one individual, yet may cause
severe low back pain in others
– The basic changes that influence the responses
of the disc to aging appear to be biochemical,
and may concern the collagen content levels in
the NP.
Pathology
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Lumbar disk
– Three main types of lumbar disc herniation are
recognized:
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Contained herniation (protrusion)
– With this type, the nuclear material bulges outwards
through the tear to strain, but not escape from, the
outer AF and/or the posterior longitudinal ligament
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Extrusion (prolapse)
– The nuclear material remains attached to the disc, but
escapes the AF and/or the posterior longitudinal
ligament to bulge posterior-laterally into the
intervertebral canal
Pathology
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Lumbar disk
– Sequestration
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The migrating nuclear material escapes
contact with the disc entirely, and becomes a
free fragment in the intervertebral canal
Pathology
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Nerve Compression
– Mechanical compression of the nerve root alone
does not explain sciatic pain and radiculopathy
– Recent models of lumbar radiculopathy suggest
that the underlying mechanisms are probably
due, in part, to a local chemical irritant such as
proteoglycans released from a disc creating an
inflammatory reaction, an autoimmune reaction
from exposure to disc tissues, or an increased
concentration of lactic acid, and/or a lower pH
around the nerve roots
Pathology
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Specific Lumbar Disc Lesions
– At the L 1 and L 2 levels, the nerves exit
the intervertebral foramen above the disc.
From L 2 downward, the nerves leave the
dura slightly more proximally than the
foramen through which they pass, and at
a decreasing angle of obliquity, and an
increasing length within the spinal canal
Pathology
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High Lumbar Disc Lesions
– The high lumbar radiculopathy does not
typically radiate pain down the back of
the leg, but instead causes an insidious
onset of pain in the groin or anterior
thigh, which is often relieved in a flexed
position and worsens with standing
– The superficial cremasteric reflex is also
invariably present
Pathology
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Third Lumbar Nerve Root
– The L 3 nerve root travels behind the
inferior aspect of the vertebral body and
the L 3 disc
– Clinical findings may include:
Symptoms in the mid lumbar, upper buttock,
whole anterior thigh and knee, medial knee,
and just above the ankle
 Slight weakness of iliopsoas, grosser loss of
quadriceps
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Pathology
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Fourth Lumbar Nerve Root
– About 40% of IVD impairments affect this
level, about an equal amount as those
that affect the L 5 root.
– Clinical findings may include:
Symptoms located in the lumbar area or iliac
crest, inner buttock, outer thigh and leg, and
over the foot to the great toe
 Weak dorsi-flexion
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Pathology
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Fifth lumbar nerve root
– Frequently compressed by the L 4-5 disc
as well as the L5-S1 disc
– Clinical findings may include:
Pain in the sacroiliac area, lower buttock,
lateral thigh and leg, inner 3 toes and medial
sole of the foot
 Weakness of peroneal muscles, extensor
hallucis and hip abductor muscles
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Pathology
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1st, 2nd and 3rd sacral nerve roots
– Can be compressed by a fifth lumbar disc
protrusion
– The clinical findings with a lesion at the
S1 level may include:
Pain in the low back to buttocks to sole of
foot and heel
 Weakness of the calf muscles, peronei, and
hamstrings
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Pathology
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4th sacral nerve root
– A lesion of this nerve root is always a
concern as a permanent palsy may lead
to incontinence and impotence
– Clinical findings may include:
Pain in the lower sacral, peroneal and genital
areas
 Saddle paresthesia
 Bladder, bowel and/or genital dysfunction
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Pathology
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Schmorl's node
– A herniation of disc substance through
the cartilaginous vertebral end plate of
the IVD into the body of the adjacent
vertebra
Examination
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The conventional physical examination for a
suspected disc herniation consists of tests
for strength and range of motion, reflex,
and sensory testing, and dural mobility tests
such as the SLR test
It must be remembered that no single test
in the physical examination has a high
diagnostic accuracy alone for disc herniation
Anatomy
 Cervical Disk
– In the cervical spine, there are five discs, with
the first disc located between C 2 and C 3
– The cervical discs are named after the vertebra
above (the C 4 disc lies between C 4 and C 5)
– The IVD height to body height ratio (2:5) is
greatest in the cervical spine, and the
intervertebral discs make up approximately 25%
of the superior-to-inferior height of the cervical
spine
Anatomy
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Cervical Disk
– Anteriorly, the cervical AF consists of
interwoven, alar fibers, whereas posteriorly,
the AF lacks any oblique fibers, and consists
exclusively of vertically orientated fibers
– In no region of the cervical AF, do successive
lamellae exhibit alternating orientations
– Protection against disc herniation is afforded
by the uncovertebral joints
Anatomy
 Cervical Disk
– As in the lumbar spine, the cervical IVD
functions as a closed but dynamic system,
distributing the changes in pressure equally to
all components of the container, i.e., the end
plates and the AF, and across the surface of the
vertebral body
Anatomy
 Cervical Disk
– It has been observed that in the first and second decades of life,
before complete ossification occurs, lateral tears occur in the
annulus fibrosus, most probably induced by motion of the cervical
spine in the bipedal posture
– The tears in the lateral part of the disc tend to enlarge toward the
medial aspect of the intervertebral disc
 The development of such tears through both sides may result in a
complete transverse splitting of the disc
 Such a process can be observed in the second and third decades of
life in the lower cervical spine when the intervertebral disc is split in the
middle into equal halves
 With this aging process, the NP rapidly undergoes fibrosis such that, by
the third decade, there is barely any nuclear material distinguishable
Pathology
 Cervical disk
– When considering cervical IVD, it is clear that
the pathology affecting the cervical IVD is
different from that affecting the lumbar disc
Pathology
 Cervical disk
– Almost everyone older than 40 years of age has
evidence of cervical disc degeneration
– According to Töndury and Theiler, the NP
usually dries out in the fourth and fifth decades
of life and acute extrusion is not expected then
Pathology
 Cervical disk
– Anteriorly, compression of the nerve roots is
likely caused by protruding discs and
osteophytes of the uncovertebral region,
whereas the superior articular process, the
ligamentum flavum, and the periradicular fibrous
tissues often affect the nerve posteriorly
Pathology
 Cervical disk
– Considering the structure of the cervical AF, the
possibilities that emerge for mechanisms of
discogenic pain are strain or tears of the
anterior AF, particularly after hyperextension
trauma, and strain of the alar portions of the
posterior longitudinal ligament when stretched
by a bulging disc
Pathology
 Cervical disk
– In the lumbar disc, a prolapse is common. In the
cervical spine, a straightforward prolapse is uncommon,
and a cervical disc herniation should not be considered
as a miniature version of lumbar disc herniation
– Acute disc herniations may result in compression of
nerve roots. Cervical discs may become painful as part
of the degenerative cascade, from repetitive
microtrauma, or from an excessive single load
Pathology
 Cervical disk
– The most common level of cervical nerve root
involvement has been reported at the seventh
(C7, 60%) and sixth (C6, 25%), followed by the
C4-C5 disc
Examination
 As with the lumbar spine, the conventional
physical examination for a suspected
cervical disk herniation consists of tests for
strength and range of motion, reflex, and
sensory testing, and dural mobility tests
Anatomy
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Thoracic disk
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Thoracic disks have been poorly researched. They are
narrower and flatter than those in the cervical and lumbar
spine, and contribute approximately one-sixth of the length
of the thoracic column
Disk size in the thoracic region gradually increases from
superior to inferior
The disk height to body height ratio is 1:5, compared to 2:5
in the cervical spine, and 1:3 in the lumbar spine, making it
the smallest ratio in the spine, and affording the least amount
of motion
Anatomy
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Thoracic disk
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In the thoracic spine, the segmental nerve roots are
situated mainly behind the inferior-posterior aspect
of the upper vertebral body rather than behind the
disk, which reduces the possibility of root
compression in impairments of the thoracic disk
Anatomy
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Thoracic disk
In contrast to the cervical and lumbar regions, where
the spinal canal is triangular/oval in cross section
and offers a large lateral excursion to the nerve
roots, the mid thoracic spinal canal is small and
circular, becoming triangular at the upper and lower
levels
 At the levels of T 4 through to T 9 the canal is at its
narrowest
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Pathology
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Thoracic disk
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Herniated disks have been found at every level of the
thoracic spine, although they are more common in the lower
thoracic spine
The intra-spinal course of the upper thoracic nerve root is
almost horizontal (as in the cervical spine). Therefore, the
nerve can only be compressed by its corresponding disk.
More inferiorly in the spine though, the course of the nerve
root becomes more oblique, and the lowest thoracic nerve
roots can be compressed by disk impairments of two
consecutive levels (T 12 root by 11th or 12th disk)
Examination
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Thoracic disk
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The clinical manifestations of thoracic disk
herniation are extremely variable and vague. This
often results in long delays between presentation and
diagnosis