Vertebrae and Joints
The seven cervical vertebrae are
relatively small and enclose a wide vertebral canal with adequate space for the
cervical part of the spinal cord. Each transverse process is perforated by a
foramen transversarium transmitting the vertebral vessels. The spinous
processes all give attachment to a strong midline elastic ligament, the
ligamentum nuchae.
Four of the cervical vertebrae
(numbers 3–6) have a typical appearance whereas the first, second and seventh
are modified. The typical vertebrae (Fig. 8.11)
possess short bifid spines and their transverse processes have anterior and
posterior tubercles. Often the upper and lower surfaces of the vertebral bodies
are not flat but curve upwards at their lateral edges. The facets on the
superior articular processes face obliquely backwards and upwards and therefore
rotation and lateral flexion always occur together.
The first cervical vertebra, the atlas
(Fig. 8.12), has anterior and posterior arches,
relatively large transverse processes and two lateral masses. The atlas has no
body and its spinous process is represented by a tubercle. On the superior
surface of each lateral mass is a concave facet, which articulates with the
convex occipital condyle of the skull. The atlantooccipital joints permit
flexion and extension (nodding movements).
The second cervical vertebra, the axis
(Fig. 8.13), possesses some of the features of a typical cervical vertebra but
it has a unique vertical projection, the dens (odontoid process). This projects
superiorly from the upper surface of its body and represents the body of the
atlas. The dens articulates by a synovial joint with a facet on the posterior
surface of the anterior arch of the atlas, where it is retained by the alar,
apical and transverse ligaments (Figs 8.15 & 8.16). The planes of the
lateral atlantoaxial joints and the pivot joint of the dens (Fig. 8.14) allow
rotation of the head as in looking from side to side.
The seventh cervical vertebra (Fig. 8.17) possesses a long, non-bifid spine, which
provides the inferior attachment for the ligamentum nuchae. The spinous process
is easily palpable and hence, the vertebra is called the vertebra prominens.
The foramina transversaria of this vertebra are traversed by the vertebral
veins but not by the arteries. The costal element of the seventh vertebra,
represented by the anterior tubercle and the bar of bone in front of the
transverse foramen, may form a cervical rib. The subclavian artery and the
first thoracic nerve root may be stretched and distorted as they pass over
(superior to) a cervical rib, leading to arterial damage and pain referred
along the medial side of the upper limb.
The joints of the whole cervical
column allow movements of extension, rotation and lateral flexion. These
movements are brought about by the prevertebral and postvertebral muscles (Fig.
8.18), assisted by sternocleidomastoid and trapezius. The prevertebral muscles
comprise the scalene group (p. 331) and the longus colli group. The latter
passes from the base of the skull down the anterior surface of the vertebral
column into the thorax. The prevertebral muscles are much smaller than the
postvertebral group which has an antigravity action in keeping the head
upright.
Arthritis involving joints of the
cervical spine is often associated with the formation
of bony outgrowths (osteophytes), which may compress the nerve roots that
contribute to the brachial plexus (p. 80). Injuries to the cervical column,
particularly involving fracture or dislocation of vertebrae, may result in spinal cord injury leading to
quadriplegia or death. The atlantoaxial joint is particularly liable to
disruption in hyperextension injuries.
The bodies of the 12 thoracic
vertebrae increase in size from above downwards. The bodies bear characteristic
costal facets (Fig. 8.19), which form synovial joints with the heads of the ribs. Typically a vertebral
body possesses one pair of facets (superior and inferior costal facets) on each
side adjacent to the attachment of the pedicle. The upper facet receives the
rib whose number corresponds to the vertebra, while the lower facet articulates
with the rib below. However the tenth, eleventh and twelfth vertebrae possess
facets on each side, which are for articulation with their own ribs. The
vertebral canal is smaller than in any other region.
The transverse processes project
laterally and backwards and typically each bears near its tip a facet for the
tubercle of the corresponding rib. The spinous processes are long and slope
steeply downwards. The plane of the joints between the facets on the articular
processes is almost vertical and permits rotation. However, all movements in
the thoracic region are restricted by the rib cage.
The upper four lumbar vertebrae are
very similar. The vertebral foramina are moderate in size (Fig.
8.20) but the bodies are
comparatively large, with concave sides.
The transverse processes taper and are directed laterally and slightly
backwards. The spinous processes are deep and rectangular. Facets on the
superior articular processes face medially and ‘grasp’ the laterally directed
inferior facets of the vertebra above, permitting wide ranges of flexion, extension
and lateral flexion but severely restricting rotation.
The fifth lumbar vertebra has shorter
transverse processes and a less angular spinous process. Its inferior articular
facets are widely separated and face anteriorly. They articulate with the
sacrum (Fig. 8.21) and prevent forward displacement of the vertebra. A fracture or
developmental defect between the superior and inferior articular processes of
the fifth lumbar vertebra will allow its body to slip anteriorly, a condition
called spondylolisthesis, which may stretch or compress
the cauda equina (p. 411). One or both transverse processes may be fused with
the upper part of the sacrum (sacralization of the fifth lumbar vertebra),
which can cause difficulty in the interpretation of radiographs.
Sacral and coccygeal vertebrae
The sacrum is a triangular bone formed
by the fusion of five vertebrae (Figs 8.21 & 8.22). The upper surface of the sacrum resembles that
of a lumbar vertebra and carries the lumbosacral disc. Below the apex of the
sacrum lies the coccyx (Fig. 8.23), which may be a small single bone or up to
four rudimentary vertebrae. The coccyx and the sacrum usually articu- late via
a small intervertebral disc, although they may be fused. The sacrum slopes
backwards and downwards and is concave anteriorly. The bone in the female has
relatively small joint surfaces and larger alae, while in the male the larger
sacral promontory often creates a heart-shaped pelvic inlet (p. 214). The fused
pedicles and laminae enclose the sacral canal, triangular in cross-section,
which opens posteroinfe- riorly at the V-shaped sacral hiatus. The canal
contains the lower part of the cauda equina, comprising the roots of the sacral
and coccygeal nerves. The anterior rami of the upper four sacral nerves pass
into the pelvis via the anterior sacral foramina and contribute
to the sacral
plexus. The posterior rami
traverse the posterior sacral foramina (Fig. 8.22). Lateral to the foramina are
the lateral masses, each of which bears an auricular surface for articulation
with the ilium (Fig. 8.23). Anaesthetic may be injected through the sacral hiatus and the caudal
canal into the epidural space to anaesthetize the cauda equina.
The sacroiliac joint is synovial but
allows very little movement because of the irregularity of the articulating
surfaces and the thick posterior interosseous ligament. Each joint is further
supported by the anterior and posterior sacroiliac ligaments and the
iliolumbar, sacrospinous and sacrotuberous ligaments. Body weight, acting downwards
through the lumbosacral disc, tends to rotate the lower part of the sacrum
backwards, a movement prevented by the sacrospinous and sacrotuberous ligaments
(Fig. 8.25).
The plane synovial joints between the
facets of adjacent superior and inferior articular processes are called
zygapophysial or facet joints. The joints in the different regions of the
vertebral column allow different movements, determined by the orientations of
the articular processes.
Intervertebral discs connect adjacent
vertebral bodies (Fig. 8.24) and act as fibrocartilaginous joints along the
whole length of the vertebral column. Like the vertebral bodies, the discs
gradually increase in size from above downwards, the largest being the
lumbosacral disc between the fifth lumbar vertebra and the sacrum (Fig. 8.25).
The discs contribute about one-fifth of the length of the vertebral column.
Each disc consists of a laminated anulus fibrosus surrounding a gelatinous
nucleus pulposus (Fig. 8.28). The nucleus pulposus lies closer to the posterior
surface of the disc and thus is more liable to posterior herniation when the
disc is damaged. This herniation, often called a slipped disc, may occur near
the midline and compress the spinal cord or cauda equina. Posterolateral
herniation may compress nerves near the intervertebral foramen (p. 396) and
cause muscle weakness and referred pain. Usually, the herniation affects nerve
roots passing through the intervertebral foramen below the affected disc. In
the cervical region, herniation most commonly occurs between vertebrae C6–C7,
affecting nerve C7, and between vertebrae C7–T1, affecting nerve C8.
Compression of nerve C7 may produce pain in the dermatome C7 (p. 74) and
weakness of extension of the elbow and wrist joints. Compression of nerve C8
may produce pain in the dermatome C8 (p. 74) and weakness of finger movements.
In the lumbar spine, herniation most commonly occurs between vertebrae L4–L5,
affecting nerve L5, and between vertebrae L5–S1, affecting nerve S1.
Compression of nerve L5 may produce pain in the L5 dermatome (p. 258) and weakness
of ankle dorsiflexion and extension of the great toe. Compression of nerve S1
may produce pain in the S1 dermatome (p. 258) and weakness of plantar flexion.
Pain referred from the back into the lower limb is often called sciatica.
The intervertebral discs are
reinforced by posterior and anterior longitudinal ligaments (Figs 8.26 &
8.27). These ligaments attach to vertebral bodies and intervertebral discs and
anchor inferiorly to the sacrum and superiorly to the cervical vertebrae or
skull. Whiplash injuries involving excessive extension-flexion are caused by
rear-end car crashes. There may be damage to the joints and ligaments of the
cervical spine, including the anterior longitudinal ligament, resulting in
cervical pain and restricted movement.
Other ligaments interconnect the
laminae, spinous processes and transverse processes of adjacent vertebrae.
Ligamenta flava interconnect the laminae within the vertebral canal. The high
content of elastic tissue gives these ligaments their yellow appearance and
they assist return of the vertebral column to the erect position following
flexion. Supraspinous and interspinous ligaments connect adjacent spinous
processes of thoracic and lumbar vertebrae. It is through these ligaments that
a needle is inserted to withdraw cerebrospinal fluid during lumbar puncture.
The supraspinous and interspinous ligaments are replaced in the cervical region
by the ligamentum nuchae, which attaches to the skull at the external occipital
protuberance and crest and to the spinous processes of all the cervical
vertebrae. Intertransverse ligaments connect the transverse processes of
adjacent vertebrae. The lumbosacral joint is reinforced by the iliolumbar
ligament, which attaches the transverse process of the fifth lumbar vertebra to
the iliac crest (Fig. 8.28).
