The Intervertebral Disk

1
Chapter 20

• The Intervertebral Disk

2
Overview

• 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

3
Overview

• 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

4
Anatomy

• 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
• 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

5
Anatomy

• 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

6
Anatomy

• 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

7
Anatomy

• 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

8
Anatomy

• 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

9
Biomechanics

• 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.

10
Pathology

• Lumbar disk
• Three main types of lumbar disc herniation are
  recognized
• 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
• 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

11
Pathology

• Lumbar disk
• Sequestration
• The migrating nuclear material escapes contact
  with the disc entirely, and becomes a free
  fragment in the intervertebral canal

12
Pathology

• 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

13
Pathology

• 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

14
Pathology

• 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

15
Pathology

• 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

16
Pathology

• 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

17
Pathology

• 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

18
Pathology

• 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

19
Pathology

• 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

20
Pathology

• Schmorl's node
• A herniation of disc substance through the
  cartilaginous vertebral end plate of the IVD into
  the body of the adjacent vertebra

21
Examination

• 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

22
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 (25) is
  greatest in the cervical spine, and the
  intervertebral discs make up approximately 25 of
  the superior-to-inferior height of the cervical
  spine

23
Anatomy

• 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

24
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

25
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

26
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

27
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

28
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

29
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

30
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

31
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

32
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

33
Anatomy

• Thoracic disk
• 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 15,
  compared to 25 in the cervical spine, and 13 in
  the lumbar spine, making it the smallest ratio in
  the spine, and affording the least amount of
  motion

34
Anatomy

• Thoracic disk
• 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

35
Anatomy

• 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

36
Pathology

• Thoracic disk
• 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)

37
Examination

• Thoracic disk
• The clinical manifestations of thoracic disk
  herniation are extremely variable and vague.
  This often results in long delays between
  presentation and diagnosis