ANATOMY
OF SUBAXIAL CERVICAL SPINE
The subaxial cervical spine consists of five cervical vertebrae. These vertebrae begin with C3 and end at C7. The overall balance of the cervical spine is slightly lordotic, which contributes to normal global sagittal alignment with the head appropriately aligned over the pelvis. This transition begins at the cervicothoracic junction, where the normal kyphosis of the thoracic spine gives way to the lordotic cervical spine. The cervical vertebrae have a common fundamental design but are unique from all other vertebral types owing to the presence of their transverse foramen and uncovertebral joints.
The subaxial cervical vertebral
bodies are morphologically unique and smaller than those of the other
movable vertebrae and increase in size caudally. The superior surfaces of the
vertebral bodies are concave from side to side and slightly convex from front
to back. The inferior surfaces are slightly curved. The lateral edges of the
superior body are slightly raised, and the lower surfaces are beveled with
small clefts. These clefts, which seem to articulate with the slightly raised
lateral edges of the inferior vertebral body, are known as “uncovertebral
joints,” although their actual function is unclear and they do not seem to be
true joints. Surgically, they provide a marker for the lateral extent of
decompression of the spinal cord and nerve roots during ventral surgery.
The spinal canal in
the subaxial region is comparatively large to accommodate the cervical
enlargement of the spinal cord; it
is bound by the bodies, pedicles, and laminae of the vertebrae. The pedicles
project posterolaterally from the bodies and are grooved by superior and
inferior vertebral notches, almost equal in depth, which form the
intervertebral foramina by connecting with similar notches on adjacent vertebrae.
The medially directed laminae are thin and relatively long and fuse
posteriorly to form short, bifid spinous processes (C3 to C6).
Projecting laterally from the junction of the pedicles and laminae are
articular pillars supporting superior and inferior articular facets.
Each transverse
process is pierced by a foramen, through which the vertebral artery
passes. Foramina are bound by narrow bony bars ending in anterior and posterior
tubercles; these are interconnected lateral to the foramen by the so-called costotransverse
bar. Only the medial part of the posterior bar represents the true transverse process;
the anterior and costotransverse bars and the lateral portion of the posterior
bar constitute the costal element. Abnormally, these elements, especially in C7
and C6, or both, develop to form cervical ribs. The upper surfaces of the
costotransverse bars are grooved and lodge the anterior primary rami of the
spinal nerves. The anterior tubercles of C6 are large and are termed the carotid
tubercles, because the common carotid arteries lie just anteriorly and can
be com- pressed against them. The dorsal facet joints formed from the inferior
and superior articulating processes of adjacent vertebrae form the dorsum, or
roof, of the neural foramina
through which the spinal nerves exit the spinal column. Clinically, the foramen
is very important because it is a common site of nerve root compression as
people age. The surgically relevant borders of the foramen are the disc
ventrally, the lateral dura medially, the inferior articular process dorsally,
and the pedicle inferiorly.
The seventh cervical
vertebra (C7) is called the vertebra prominens, because its spinous
process is long and “proud” and ends in a tubercle that is easily palpable at
the base of the neck.
LIGAMENTS AND
MUSCULATURE
The splenius muscle
serves as a strap, covering and holding in the deeper muscles of the back of
the neck (see Plate 1-9). It takes origin
from the ligamentum nuchae and the
spinous processes of C7 to T6. The muscle may be divided into two parts the splenius
capitis muscle, which inserts on the mastoid process and the lateral third
of the superior nuchal line of the skull, and the splenius cervicis muscle, which
terminates in the posterior tubercles of the first two or three cervical
vertebrae. The cervicis portion is the outer and lower portion of the splenius
muscle, and its inserting bundles curve deeply along its lateral margin.
The splenius muscle
draws the head and neck backward and rotates the face toward the side of the
muscle that is acting. Both sides contracting together extend the head and
neck. The muscle is innervated by the lateral branches of the dorsal rami of
the second to fifth or sixth cervical nerves. It lies directly under the
trapezius and is covered by the nuchal fascia; its mastoid insertion is deep to
that of the sternocleidomastoid, and it overlies the erector spinae and the semispinalis. The longissimus
cervicis muscle arises medial to the upper end of the longissimus thoracis,
from the transverse processes of about the upper four to six thoracic
vertebrae. Its slips of insertion end in the transverse processes of C2 to
C6. The longissimus capitis muscle connects the articular processes of the
lower four cervical vertebrae with the posterior margin of the mastoid process.
The spinalis cervicis
muscle is frequently absent or poorly developed. When completely represented,
it arises from the ligamentum nuchae and from the spinous processes of C7 and, perhaps, the upper thoracic vertebrae. Its
insertion may reach the spinous processes of the axis and sometimes extends to
the C3 and C4 vertebrae. The spinalis capitis muscle is not a separate muscle
but is blended laterally with the semi-spinalis capitis.
The ligamentum
nuchae is a fibroelastic membrane stretching from the external occipital
protuberance and crest to the posterior tubercle of the atlas and the spinous
processes of all the other cervical vertebrae. It provides areas for muscular
attachments and forms a midline septum between the posterior cervical muscles.
Previously thought to be of minimal biomechanical significance, the ligamentum
nuchae now seems to play a role in the preservation of range of motion in
humans. The ligamenta flava contain a high proportion of yellow elastic
fibers and connect the laminae of adjacent vertebrae. As in the remainder of the spine, the anterior
longitudinal ligament and posterior longitudinal ligament border the
anterior and posterior components of the spinal canal, respectively.
NERVES
The cervical spinal
nerves are similar in form and function to the nerves found in the other areas
of the spine. A dorsal and ventral ramus combines to form the distal nerve,
which then branches to provide sensory and motor function to its appropriate
dermatome and myotome. What differs importantly is the numbering of spinal
nerves. There are eight cervical nerves with only seven cervical vertebrae.
This occurs because the first through seventh cervical nerves exist above the level of the
corresponding vertebral body. As a result, the eighth cervical nerve exits
below the C7 vertebra.
VASCULATURE
The vertebral artery
enters the spine through the trans- verse foramen of C6 in approximately 90% of
people. On the right, it originates from the subclavian artery, and on the left
it comes from the aortic arch; and the arteries course upward into the
craniovertebral junction. It
is divided into four segments (V1 to V4). The first (extraosseous) segment
originates from its respective parent artery and ends at the transverse foramen
of C6. The V2 (foraminal) segment consists of the vertebral artery as it lies
within the transverse foramina from C6 to the atlas. The V3 (extraspinal)
segment originates at the foramen of C1 and terminates as the vertebral artery
pierces the dura at the level of the foramen magnum. The V4 (intradural)
segment comprises the remainder of the vertebral artery until the two arteries
unite in the midline of the brainstem at the junction of the pons and midbrain
and create the basilar artery. In humans, one vertebral artery is almost always
dominant, with 75% of individuals possessing a dominant left vertebral artery.
The blood supply to the musculature
and bones of the cervical spine is supplied through a series of innominate
small vessels that originate from the subclavian artery, including the anterior
spinal artery and posterior spinal artery.
It is critical that
surgeons understand the potential for anomalous positions of the vertebral
artery. The artery will enter into the foramen transversarium at levels other
that C6 in approximately 10% of people. This has implications for anterior
surgical approaches to the cervical spine. The vertebral artery may also course
through the lateral aspect of the vertebral body. This occurs in approximately
2.7% of people. It is critical that surgeons evaluate for these anomalies with
a thorough preoperative review of advanced imaging (i.e., MRI or CT).
