74 Dance Anatomy and Kinesiology
distinctive shape is termed the vertebra prominens
(L. prominent). The paired transverse processes
project posterolaterally from the junction of the
pedicles and the laminae. The spinous and transverse
processes form important sites for the attachment of
ligaments and muscles of the vertebral column, and
they function to increase the mechanical advantage
of these structures by allowing for an attachment
farther away from the axis of rotation. The paired
superior and inferior articular processes are located
at the junctions of the pedicles and laminae. As their
name suggests, these processes form joints with the
vertebrae immediately above and below.Joint Structure and Movements of the Vertebral Column
The vertebrae are generally joined to adjacent ver-
tebrae both at the vertebral bodies and at the verte-
bral arches. These joints are collectively termed the
intervertebral joints. There are also some additional
specialized joints that occur in specific regions of
the vertebral column and serve to help connect the
skull, ribs, and pelvis to the spine.Joints Between the Vertebral Bodies
The bodies of adjacent vertebrae from the second cer-
vical vertebra to the first sacral vertebra are connected
by cartilaginous joints, and the interposed cartilage
is termed the intervertebral disc. Each intervertebral
disc consists of an outer ring, termed the annulus
fibrosus, and an inner gelatinous mass, termed the
nucleus pulposus as shown in figure 3.3. The annulus
(L. annulus, ring) fibrosus is composed of concentric
sheets or lamellae of fibrocartilage. The fibers run in
approximately the same direction in a given band but
in the opposite direction in any two adjacent bands
as shown in figure 3.3A. This structural arrangement
provides strength to the disc to help it withstand forces
and limit excessive motion in many directions. The
nucleus pulposus (L. the inside of a thing + fleshy)
is a deformable gel-like core that is about 80% water
in a healthy disc (Deckey and Weidenbaum, 1997)
and allows for rocking and rotating motion between
adjacent vertebrae, as well as essential absorption of
compression forces for the vertebrae.
During weight bearing, the nucleus pulposus
is compressed and exerts a large centrifugal force
on the fibers of the annulus as seen in figure 3.3B.
The nucleus pulposus contains charged molecules
(proteoglycans) that tend to pull water into the
disc—important to counter the tendency for water tobe pushed out of the disc by the compression associ-
ated with weight bearing. However, with repetitive
loading, a small amount of water is lost from the
disc such that the spine undergoes up to an 0.8-inch
(2-centimeter) loss in height during the day (Hall,
1999), which is restored when pressures are relieved
such as during sleep and recumbency. Conversely,
when compression is decreased due to the loss of
gravity during space flight, astronauts can undergo
a temporary increase in the height of the spine of
approximately 2 inches (5 centimeters).
Each end of the disc is centrally closed by a thin
cartilaginous plate (vertebral endplate) that is firmly
adhered to the body of the adjacent vertebrae (figure
3.3). The inner zone of the annulus fibers is attached
to this endplate, while the outer peripheral zone
attaches directly into the bony tissue of the vertebral
body (epiphyseal ring). When the spine is subjected
to very large compression forces, these vertebral
endplates can sometimes fracture (Panjabi, Tech,
and White III, 1980).
The intervertebral disc constitutes 20% to 33% of
the total height of the vertebral column (White III
and Panjabi, 1978), with disc thickness varying in dif-FIGURE 3.3 The intervertebral disc. (A) Transverse sec-
tion, (B) sagittal section.