Page 354 - Kinesiology of the musculoskeletal system foundations for physical rehabilitation

330 Section III Axial Skeleton
rectly, the apophyseal joints. A dehydrated and thinned disc
places disproportionately greater compressive loads on the
apophyseal joints. For this reason, some authorities claim that
a degenerated disc leads to subsequent arthritis (or arthrosis)
of the apophyseal joints.97 Some authors, however, argue the
opposite cause-and-effect relationship—that the loss of joint
space within a degenerated apophyseal joints favors disc
degeneration.61 Both arguments are indeed valid.
The intervertebral discs are very important stabilizers of
the spine. This stabilizing function is primarily a result of the
structural configuration of the collagen fibers within the
annulus fibrosus. As shown in Figure 9-34, most fibers are
oriented in a rather precise geometric pattern. In the lumbar
region, collagen rings are oriented, on average, about 65
degrees from the vertical, with fibers of adjacent layers travel-
ing in opposite directions.23,122 This structural arrangement
offers significant resistance against intervertebral distraction
(vertical separation), shear (sliding), and torsion (twisting).85
If the embedded collagen fibers ran nearly vertically, the disc
would most effectively resist distraction forces, but not sliding
or torsion. In contrast, if all fibers ran nearly parallel to the
top of the vertebral body, the disc would most effectively
resist shear and torsion, but not distraction. The 65-degree
angle likely represents a geometric compromise that permits
tensile forces to be applied primarily against the most natural
movements of the lumbar spine. Distraction forces are an
inherent component of flexion, extension, and lateral flexion,
occurring as one vertebral body tips slightly and thus sepa-
rates from its neighbor. Shear and torsion forces are produced
during virtually all movements of the vertebral column.
Because of the alternating layering of the annulus, only col-
lagen fibers oriented in the direction of the slide or twist
become taut; fibers in every other layer slacken.
In contrast to the lumbar region, the annulus fibrosus in
the cervical region does not have complete concentric rings
that surround the nucleus.130 When a cervical disc is viewed
from above, the annulus has a near-crescent shape, thick
along the anterior rim and progressively tapering to a very
thin layer at the disc’s lateral margins. Little or no annular
fibers exist at the region of the uncovertebral joints. A small
fissure (or cleft) typically extends horizontally inward from
each uncovertebral joint, coursing to the deeper regions of
the disc.130 Although the function of the fissures is uncertain,
they likely increase the freedom of movement within the
cervical region. The posterior annulus is separate from the
anterior and lateral regions; it is thin and oriented vertically,
parallel with the adjacent posterior longitudinal ligament.130
Vertebral Endplates
The vertebral endplates in the adult are relatively thin cartilagi- nucleus responds by deforming radially and outwardly against
nous caps of connective tissue that cover most of the superior
and inferior surfaces of the vertebral bodies (see Figure 9-33).
At birth the endplates are very thick, accounting for about
50% of the height of each intervertebral space. During child-
hood the endplates function as growth plates for the verte-
brae; in the adult the endplates recede and occupy only about
5% of the height of each intervertebral space.169
The surface of the vertebral endplate that faces the disc is
composed primarily of fibrocartilage, which binds directly and
strongly to the collagen within the annulus fibrosus (Figure
9-35). This fibrocartilaginous bond forms the primary adhe-
sion between consecutive vertebrae. In contrast, the surface of
Nucleus pulposus Vertebral endplate
Annulus
fibrosus
FIGURE 9-35. A vertical slice through the interbody joint shows the
relative position of the vertebral endplates. (Modified from Bogduk
N: Clinical anatomy of the lumbar spine, ed 4, New York, 2005,
Churchill Livingstone.)
the endplate that faces the vertebral body is composed primar-
ily of calcified cartilage that is weakly affixed to the bone. This
endplate-bone interface is often described as the “weak link”
within the interbody joint, often the first component of the
interbody joint to fracture under high or repetitive compres-
sive loading.136 A perforated or fractured endplate can allow
the proteoglycan gel to leak from the nucleus pulposus, causing
structural disruption of the disc.77,162 Such disruption has been
shown to cause spinal instability.227
Only the outer, more peripheral rings of the annulus fibro-
sus contain blood vessels. For this reason, most of the disc
has an inherently limited healing capacity. Essential nutrients,
such as glucose and oxygen, must diffuse a great distance to
reach the deeper cells that sustain the disc’s low but essential
metabolism. The source of these nutrients is in the blood
vessels located in the more superficial annulus and, more
substantially, blood stored in the adjacent vertebral bodies.77
Most of these nutrients must diffuse across the vertebral
endplate and through the disc’s extracellular matrix, eventually
reaching the cells residing deep in the disc.62,165 These cells
must receive nourishment to manufacture extracellular pro-
teoglycans of the essential quantity and quality. Aged discs,
for example, typically show reduced permeability and increased
calcification of the vertebral endplates, which, in turn, reduce
the flow of nutrients and oxygen into the disc.25,169 This age-
related process can inhibit cellular metabolism and synthesis
of proteoglycans. Less proteoglycan content reduces the ability
of the nucleus to attract and retain water, thereby limiting its
ability to effectively absorb and transfer loads.91
Intervertebral Disc as a Hydrostatic Pressure Distributor
The vertebral column is the primary support structure for the
trunk and upper body. While one stands upright for example,
approximately 80% of the load supported by two adjoining
lumbar vertebrae is carried through the interbody joint; the
remaining 20% is carried by posterior elements, such as
apophyseal joints and laminae.5
The intervertebral discs are uniquely designed as shock
absorbers to protect the bone from excessive pressure that
might result from body weight or strong muscle contraction.
Compression forces push the endplates inward and toward
the nucleus pulposus. Being filled mostly with water and
therefore essentially incompressible, the young and healthy
the annulus fibrosus (Figure 9-36, A). Radial deformation is