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Pathology of the Degenerative Cervical Spine
Wolfgang Rauschning, MD, PhD, Professor of Clinical Applied Anatomy, Department of Orthopaedic
Surgery, Academic University Hospital, S-75185 UPPSALA, Sweden
This pathoanatomical outline details some salient features of normal cervical spinal anatomy tht are are
essential for understanding the development of degenerative and posttraumatic derangements of the normal
cervical spine into spondylosis, segmental instability, uncovertebral osteophytosis and facet joint
osteoarthrosis. Roughly half of the rotatory movement of the entire cervical spine occurs in the atlantoaxial
motion segment which was formed by the assimilation of the bodies of C1 and C2. The atlantoaxial
articulation lacks congruity and stability and only the ligaments resist excessive motion. The alar ligaments
are strong as is the transverse ligament; the tectorial membrane and the apical ligament anchor the odontoid
process directly to the skull base. The tip of the odontoid process abuts the lower pons and the medulla
oblongata. The cisterna magna provides spatial reserve for a translatory posterior movement of the dens.
The vertebral column houses osseoligamentous spaces that accommodate, protect and transmit vital and
delicate neurovascular structures. Normally the anterior wall of the cervical vertebral canal is perfectly
straight. The uncinate processes project from the lateral borders of the vertebral bodies resist undue lateral
movement that would have deleterious effect on the vertebral artery which is tightly held in the
costotransverse foramina of C3 to C6 and the knee-shaped bony tunnel in the lateral mass of the axis. The
spinous processes are short and carry bifid tips. Whereas there is no supraspinous ligament sensu strictu, a
suspensory system comprising a midsagittal septum extends posteriorly to the strong elastic ligamentum
nuchae. On AP radiograms the articular pillars, comprised of the zygapophyseal joints project on each side
of the vertebral body column. The plane of the articular facets is offset from the coronal plane by 56-60
degrees. The joint capsules are thin and loose and meniscoid synovial folds project into the facet joints,
reflecting the high degree of mobility of these joints. The orientation of the facet joints and possibly those of
the uncinate processes, govern the pattern of segmental motion.
While the intrathecal neural elements are mobile, the thecal sac is segmentally attached to the vertebral
column by several important ligaments and membranes some of which are commonly known as the
Hoffmann ligaments. The thecal sac does not expand appreciably when intrathecal pressure raises because
the dura is not very elastic, but the thecal sac cannot resist compression. The thecal sac is wide and
surrounded by epidural fat and sinusoid epidural venous compartments. They are arbitrarily divided into
anterior and posterior internal veins. The vertebral canal contents could be conceived as a hydraulic system
in which the filling and emptying of the epidural veins and sinuses facilitate almost instantaneous variations
of the volume of the canals.
Mobility and the pattern of motion vary greatly between various regions of the spine. Every spinal motion
segment has a typical range and pattern of motion that is determined by the shape and three-dimensional
arrangement of bony and articular surfaces, intervening discs, ligaments and joint capsules. Mobility is also
determined and controlled by the insertion and origin of the muscles attaching to the spine, their mass and
fiber composition as well as their orientation in space. The most mobile portions of the spine are the entire
cervical spine and the mid- and lower lumbar spine.
In extension, there is segmental "shingling" caused by the retrolisthesis of the upper vertebra on the lower
vertebra due to the coupled motion induced by the obliquity of the facet joints. Instability at this level and/or
osteoarthrotic lipping of the joint surfaces may erode the vertebral artery or cause arterial thrombosis. The
size of the vertebral canal and its volume increases slightly in flexion and decreases in extension. The size
and volume of the intervertebral foramina (root canals) increases and decreases dramatically in flexion as
well as extension. These volume changes are most pronounced in the cervical spine.
The vertebral canal normally has a reserve capacity since the spinal cord is surrounded (and protected) by
two hydraulic spaces: 1. the subarachnoid cerebrospinal fluid compartment and 2. the peridural venous
sinusoids. The midsagittal diameter of the cervical spinal canal is 17 to 18 mm between C3 and C7, as
compared to the normal average diameter of the spinal cord of 10 mm (range 8.5 to 12 mm). If the reserve
space is limited or the spinal nerve roots and/or the dural sac may already be compressed. Dynamic
changes in the nerve root canal are less marked than in the central spinal canal. They are caused by bulging
or increased bulging of the postero-lateral region of the disc into the subarticular portion of the nerve root
canal and buckling of the anterior capsule of the facet joint covered by the ligamentum flavum as a result of
a downward sliding of the inferior articular process with respect to the superior during extension of the spine.
Rotational forces affect the subarticular portion of the nerve root canal. Torsion of a vertebra results in a
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posterior displacement of the lateral portion of the vertebra, on this side the root canal decreases in width
and this may be responsible for compression of the nerve root.
From piercing the thecal sac to the tubercles at the tips of the transverse processes the roots take a two
centimeter long course through the subaxial cervical root canals (neuroforamina). Anatomical relationships
are: the upper border of the pedicle and the composite transverse process inferiorly, the upper articular
process posteriorly, the uncinate process and vertebral artery anteriorly. The root sleeve, ganglion and
nerve cannot yield superiorly or posteriorly because they are recessed in a deep furrow at the anterior
aspect of the superior articular process. Bordered and held anteriorly by the vertebral artery. The
postganglionic nerves eventually exit through the musculotendinous slits between the insertions of the
scalenus muscle bundles into the anterior and posterior tubercles of the composite transverse process.
In the subaxial cervical spine traumatic disc injuries were the most common injuries, ranging from discrete
annular tears to frank protrusions or herniations. Detachment of the annulus from the apophyseal ring (“rim
lesions”) was common. In young subjects we found avulsions of the discs from the vertebral bodies, similar
to epiphysiolysis-type lesions, these cartilaginous endplate avulsions were partial or complete, in extreme
cases they were associated with segmental gaping. At such levels frank avulsion of the nerve rootlets as
they emerge from the spinal cord has been observed. Traumatic lesions of the facet joint included capsular
ruptures, torn meniscoid synovial folds, hemarthrosis, small facet fractures and avulsions of the ligamentum
flavum capsules.
In a large number of traumatized specimens a consistent pattern of disc injury was observed. In presence of
a complete segmental disruption, such as in fracture dislocations and bilateral facet dislocations, the
periosteum and PLL is avulsed from the (anteriorly dislodged) upper vertebra. These lesions are consistently
combined with extensive posterior annulus rupture and, in severe cases, compete ruptures and avulsions of
large disc fragments. Large lumps of disc material have been found entrapped behind the vertebra in the
triangular pocket-shaped space under the avulsed periosteum-PLL membrane. There are relatively few
clinical reports on spinal cord compression by non-recognized extruded discs and neurological impairment
after posterior reduction of fracture dislocations. MR has been helpful in demonstrating such extruded disc
fragments. The reserve capacity of the cervical vertebral canal (see above) may explain the discrepancy
between the high incidence of traumatic disc extrusions in specimens and clinical manifestations of such
disc ruptures.
The large hematomas observed deep in the cervical soft tissues, surrounding the nerves and blood vessels,
may lead to restricted neurovascular element mobility by adhesions and strangulation by scar tissue. In
previously traumatized cervical spines severe reactions of the vertebral endplates were observed, ranging
from healed endplate depression fractures with subchondral endplate sclerosis and endplate irregularity, to
persistent Schmorl's nodes that were filled with blood-tinted disc material. In odontoid pseudarthrosis the
development of pannus-type reactive soft tissue masses that compress the brain stem was observed in
several specimens. In still other cases persistent instability had caused the formation of osteophytes at the
insertion of ligaments and the outermost annulus fibrosus ("enthesopathy" = ligament insertion site
ossification).
Degeneration of the "disc joint" encompasses reactions of the intervening disc, the endplates and especially
reactive degenerative changes of the rim of the vertebral bodies at which the outermost annulus fibrosus
inserts into the assimilated ring apophyses. Early degenerative changes include loss of discal turgor
(possibly instability) and progressive reduction of disc height. Due to the obliquity of the articular facets, any
decrease of segmental height causes a slight retrolisthesis of the suprajacent vertebra. In later stages
osteophytic ridging or lipping, probably prompted by reactions of the periosteum to undue traction forces
exerted by the inserting annulus fibrosus (enthesopathy, see above), encroaches on the anterior epidural
space. The retrolisthesis of the vertebra above the level of the disc degeneration further encroaches on the
canal dimensions and cause venous stasis and congestion, compromise of the segmental arterial blood
supply and, eventually, compression of the spinal cord and anterior spinal artery.
Once the normal kinematics of the disc is compromised by repetitive or subclinical traumatic changes, the
vertebral endplates may yield to excessive pressure from the disc, ranging from small infractions of the
endplate to large defects and Schmorl's nodes. Thickening of the dura, in extreme cases coalescing of the
dura with the PLL and the vertebral body periosteum, may completely obliterate the anterior epidural
vascular space, thereby impeding epidural venous circulation. Ossification of the posterior longitudinal
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ligament has been identified as a major cause of cervical spinal myelopathy, not only in Japanese, but also
in Western populations.
The uncinate processes project superiorly from the lateral-posterior circumference of the upper endplates. At
C3-C4 they are sagittally oriented, towards the lower cervical spine they migrate more posteriorly, in the
upper thoracic spine they emerge from the posterolateral corner of the vertebrae. The so-called vonLuschka pseudojoints between the uncinate process and a beveled notch at the inferior-lateral ridge of the
vertebra above ("echancrure"). These clefts develop during childhood and increase in width during
adolescence. The onset of cervical disc degeneration most frequently starts with a progressive delamination
and fissuring at the site of the uncovertebral clefts; later in the process of internal disc disruption there is a
progression of fissuring toward the center of the disc.
This "centripetal" fissuring eventually encompasses the entire disc. At the end-stage of disc degeneration
the entire disc has resorbed and the endplates are remodelled and resemble ball-and-socket joints. Penning
has introduced the term "saddle joints" to typify this late stage of spondylosis. Concurrently with the endplate
reactions, sclerosis and uncovertebral osteophytosis ensues. The uncovertebral spondylophytes
(spondylophytes are osteophytes emanating from a spondylos = vertebra) project posterolaterally and
compress and dislodge the vertebral artery (see below: "vertebral artery") and also encroach on the
foramina (root canals). Not infrequently, the uncus spondylophytes swerve markedly posteriorly. Since the
cervical roots follow the upper surface of the pedicle below, they not infrequently become circumferentially
encased in a shell of bone. These curved spondylophytes are common in the lower cervical spine and are
best seen on oblique radiographs.
Whereas degenerative changes in the lumbar spine cause compromise of the roots and segmental blood
vessels by soft tissue compromise (bulging disc anteriorly and infolding ligamentum flavum and joint capsule
posteriorly), the cervical roots are almost entirely bordered by bone (uncinate process anteriorly, pedicle
inferiorly, upper articular process notch posteriorly). Due to the marked obliquity of the zygapophyseal joints,
a loss of disc height inevitably entails a retrolisthesis of the upper vertebra on the lower vertebra ("frontback-stenosis") that compresses the "root" between the lower endplate and the upper articular process.
While structural compression due to osteophytosis per se may cause chronic impairment of nerve root
function and segmental radicular circulation, extension or, even worse, sudden hyperextension with or
without rotation, may severely jeopardize the radicular structures. These cannot yield to the sharp lowerendplate spondylophytes because they are recessed in furrows in the upper articular process, and because
uncinate process spondylophytes may partially or completely encase the "root", thereby rendering it less
mobile. Since the pattern of combined structural-degenerative and functional neurovascular element
compression in the root canals is highly complex and multifactorial, it is essential to accurately determine the
nature and cause of the compression.
In foraminotomy it is especially important to precisely determine the location of the offending lesions before
decompressive surgical procedures such as uncoforaminotomy, spondylophyte resection and any type of
stabilizing fusions are performed. The term "root" is simplistic because it connotes a single structure. The
root complex really is a tubular structure of considerable length that comprises a variety of anatomically and
neurophysiologically different structures: medially the root sleeve that envelopes the (still intrathecal)
cervical rootlets and roots. More laterally the root sheath becomes contiguous with the richly vascularized
dorsal root ganglion ("segmental mini-brains"), while the ventral (motor) root still runs separately from the
ganglion, typically recessed into the anterior surface of the ganglion and encased by its capsule. More
laterally still the motor root merges with the postganglionic cervical spinal nerve. Especially in the lower
cervical spine the root elements, in particular the ganglia, are large since they constitute brachial plexus
elements. Severe spondylosis in elderly may be completely asymptomatic. It therefore is crucial to
anatomically pinpoint any offending lesion in symptomatic patients. Only when there is an unequivocal
correlation between the patient's symptom and signs and the diagnostic imaging studies will a highly
targeted decompression be successful.
Any loss of disc height inevitably entails a subluxation of the oblique articular facets of the cervical
zygapophyseal joints. While degenerative changes of the facet are common, they appear to contribute to a
lesser extent to compression of the spinal cord and the radicular structures. Hypertrophy and reactive
remodeling of the articular processes causes the joints to grow longer anteromedially and posterolaterally,
rendering the joints an abnormal reciprocal relationship of the facets as a result of vertebral slipping. It
cannot be excluded that the process of remodeling represents a compensation mechanism aimed at limiting
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vertebral slipping. Degenerative changes of the superior articular processes are responsible for narrowing of
the root canals, but it should be noted that the tip of the upper articular process and their osteophytes is
located high above the root bundle which lies on top of the pedicle below (see above: uncovertebral
osteophytosis). Narrowing may be slight and not cause compression of the nerve roots or may produce
stenosis of varying entity of the root canals. When hypertrophy and outgrowth of the medial border of the
facets are severe, the latter encroach on the central spinal canal and may compress, even markedly, the
dural sac.
The hypertrophied inferior articular processes narrow the posterior portion of the central spinal canal which
is further constricted to a significant extent by vertebral slipping. In fact due too the integrity of the posterior
vertebral arch, the dural sac follows the slipped vertebra and distally to this undergo a sort of guilloutinement
between the intervertebral disc and the posterior superior endplate of the vertebra below, on one side and
the superior articular processes of this vertebra and the posterior arch of the slipped vertebra as well as the
ligamenta flava on the other side. The pathological effects of degenerative changes of the articular
processes and olisthesis are strictly related to the original dimensions of the vertebral canal. If the latter is
wide and vertebral slipping of slight entity, compression of the dural tube may not occur. When the spinal
canal is of medium size, the dural sac is usually encroached upon at the level of its lateral portions, which
are in direct contact with the medial border of the facet joints. I the dimensions of the spinal canal are at the
lower limits of normal or below these, marked compression of the dural sac and the emerging nerve roots
occurs even in the presence of mild degenerative changes of the articular processes or moderate vertebral
slipping.
The vertebral artery is firmly held in the chain of transverse foramina between C3 and C6 and in the
coronally oriented and laterally verted knee-shaped osseous tunnel in C2. The thick nerves of the brachial
plexus that swerve around the lateral aspect of the artery at mid-level of each vertebral body further restrain
the artery. Uncovertebral osteophytes that project laterally inevitably indent the artery, causing it to take a
markedly undulating cause in advanced stages of uncovertebral spondylophytosis. Such multisegmental
vertebral artery compression is consistently demonstrated on coronal views of vertebral artery angiograms.
In addition to the sharp sclerotic osteophytes, laterally projecting recesses of the widened and flaccid
uncinate joint "capsules" may further accentuate compression of the artery. These capsular recesses are
best seen on coronal MR images and discograms. Undue movements in spondylotic cervical spines may
cause attrition of the vertebral arteries, endothelium lesions, microfractures of atheromatous plaques and
eventually vertebral artery thrombosis. Not surprisingly, a high incidence of vascular complications has been
reported following manipulative treatment of cervical spines.
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