Anatomy of the Spine
Description:
The
spinal column typically consist of 33 vertebrae. Seven cervical, 12 thoracic, and five lumbar vertebrae
compose the movable presacral spine; five fused segments form the sacrum
with four or five irregular ossicles caudal to the sacrum forming the
coccyx
- The
gross anatomic appearance is one of cervical lordosis, thoracic kyphosis,
and lumbar lordosis
- The
typical vertebrae consists of two major components: a roughly cylindrical
ventral mass of cancellous bone, the body and the dorsal vertebral arch
- The arch
is attached to the dorsolateral aspects of the body by two stout pillars,
the pedicles. These are united
dorsally by a pair of arched flat laminae that are surmounted in the
midline by a dorsal projection, the spinous process.
- The
pedicles, laminae, and dorsum of the body thus form the vertebral foramen,
a complete osseous ring that encloses the spinal cord.
- Near the
junction of the pedicles and the laminae are found the lateral transverse
processes and the superior and inferior articular processes. The transverse processes extend from
the sides of the vertebral arches.
The articular processes form the facet or zygapophyseal
joints.
- The pars
interarticularis is the portion of the lamina that lies between the
superior and inferior articular processes. It sees the dorso-ventral translational forces which has
been implicated in pars fractures.
- An
endplate composed of hyaline cartilage lies on both the superior and
inferior endplates of the vertebral bodies. Between them lie the intervertebral disc composed of a
circumferential annulus fibrosis and a central nucleus pulposus. The endplates are important in the
diffusion of nutrition and waste between the blood supply of the vertebral
bodies and the avascular discs.
Cervical
Spine:
- In the
cervical spine C1 and C2 have a distinct morphology compared to
C3-C7.
- C1, the
atlas, is a ring of bone that supports the base of the skull at the
atlanto-occipital articulation.
Half of the flexion and extension of the neck occur at this
articulation.
- C2,
known as the axis, provides a bearing surface on which the atlas may
rotate, but its most distinctive characteristic is the vertically
projecting odontoid process that serves as a pivotal restraint against
horizontal displacements of the atlas.
- Crucial
stabilizing ligaments maintain the relationships between the
occipito-atlanto-axial articulations.
Most notable is the transverse atlantal ligament which spans between
the atlantal condyles trapping the odontoid anteriorly against the
anterior arch of C1. This ligament
keeps C1 from sliding forward on C2 during flexion. While approximately half of the
rotation of the neck occurs at the C1-C2 articulation, it is limited by
the alar ligaments spanning from the odontoid to the anterior rim of the
foramen magnum.
- The
remainder of cervical vertebrae are similar in morphology as
described. The lateral edges of
their vertebral bodies turn sharply upward to form the uncinate
processes.
- This is
only present in the cervical spine.
The transverse processes in the cervical spine also contain
vertebral artery foramina which transmit the vertebral arteries within
them from C6 to C1, prior to the arteries penetrating the posterior
atlanto-occipital membrane and joining to become the basilar artery.
- From the
subaxial cervical spine inferiorly down to L5, the basic components of
vertebral structure are similar.
Ligaments of
the Spine:
- Supporting
ligaments give the column stability.
The anterior (ALL) and posterior longitudinal ligaments (PLL) run
from the foramen magnum to the sacrum.
- The ALL
largely inserts into the central aspect of the vertebral bodies.
- The PLL
primarily attaches to the superior and inferior edges of the vertebral
bodies and bowstrings across the central portion.
- The
ligamentum flavum is a very strong ligament that runs within the canal
attaching to the undersurface of the laminae. From segment to segment, the fibers of the ligamentum flavum
insert into the superior edge of the inferior lamina and then blend with
the undersurface of the superior lamina.
- An
interspinous ligament attaches the stalks of the spinous process while a
supraspinous ligament attaches to the tips of the spinous processes. Together, these two ligaments serve as a
tension band and act as a tether when the spine is flexed. These
ligaments
provide the spinal column with flexible stability.
Nerve
Anatomy:
- The
neurologic component of the spine is often misunderstood. The naming of the nerve roots in
relation to their exiting level changes from the cervical to the thoracic
spine.
- Because
there are seven cervical vertebrae and eight cervical nerves, the cervical
nerve roots exit above the pedicle of their corresponding level. That is, the C1 nerve root exits above
the C1 pedicle. At C7, the C7
nerve root exits above the C7 pedicle and the C8 nerve root exits below
the C7 pedicle.
- This
then sets up the pattern where the remainder of caudal nerve roots exit
below their corresponding pedicle.
It can, therefore, be expected that the L4 nerve root exits below
the L4 pedicle. The understanding
of this relationship is central to understanding the pathology of disc
herniation.