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Chapter 2 Applied Anatomy of the Spine 43
e internal venous plexus is of more functional and anatomic interest. is plexus is essentially a series of irregular,
valveless epidural sinuses that extend from the coccyx to the
foramen magnum. Its channels are embedded in the epidural
fat and are supported by a network of collagenous bers,
but their walls are so thin that their extent or conguration
cannot be discerned by gross dissection. is latter property
may account for the fact that the epidural venous sinuses
have been periodically “rediscovered.” e epidural vertebral
veins were known to Vesalius and his contemporaries and
were described and illustrated in the rst part of the 19th
century by Breschet.81 Batson,82 Clemens,83 and others made
the functional and pathologic signicance of these vessels
apparent (Fig. 2.34).
e plexus does not entwine the dura in a completely haphazard fashion but is arranged in a series of cross-connected
expansions that produce anterior and posterior ladderlike
congurations up the vertebral canal. e main anterior
components of the epidural plexus consist of two continuous channels that course along the posterior surface of the
vertebral bodies just medial to the pedicles. ese channels
expand medially to create cross anastomoses over the central
dorsal area of each vertebral body and are thinnest where they
overlie the intervertebral discs. When injected with a contrast
medium, the main channels may appear as a segmental chain
of rhomboid beads. Chaynes and colleagues84 studied the
internal venous plexus using silicon injection techniques.
ey found that anterior longitudinal veins were located in
a “dehiscence” within the periosteum along the lateral aspect
of the spinal canal and that veins of each side communicated
with each other through a retrocorporeal vein. In the cervical
spine, the retrocorporeal vein was found deep to the posterior
longitudinal ligament, whereas it was supercial to the liga-
ment in the thoracic and lumbar regions.
Where the main anterior sinuses cross connect, they receive
the large unpaired basivertebral sinus that arises within the
dorsal central concavity of the spongiosa and drains the
intraosseous labyrinth of sinusoids. Regional visualization of
the epidural plexus can be accomplished by introducing a
radiopaque medium directly into the spongiosa or the cancellous bone of the spinous process (intraosseous venography).
e major external connections of the epidural plexus
consist of the veins that pass through the intervertebral
foramen and eventually empty into the segmentally available intercostal or lumbar veins (Fig. 2.35). Because these
sinuses are valveless, one cannot refer accurately to directions
of drainage and ow. e greatest functional signicance of
these vessels lies in their ability to pass blood in any direction according to the constantly shiing intraabdominal and
intrathoracic pressures. Breschet81 surmised that the epidural
plexus served as a collateral route for the valveless caval and
azygos systems. is ability has been shown by experimental
ligation of either the superior vena cava or the inferior vena
cava. In addition, the Queckenstedt maneuver, which tests
the patency of the spinal subarachnoid space by compressing
the jugular or intraabdominal veins, causes an increase in
cerebrospinal uid pressure through dural compression from
the expansion of the collaterally loaded epidural plexus.
e plexus is evidently capable of passing large quantities
of blood without developing varices. Clemens claimed that
this feature was due to the intricate network of collagenous
bers that supports the thin walls of the sinuses. Also, passive
SECTION
I
A
FIG. 2.34 (A) Posterior and (B) lateral illustrations of the spinal epidural venous plexus taken from hand-colored
copies of Breschet’s original work (ca. 1835). (Courtesy Scott Memorial Library, Jeerson Medical College.)
B
44 BASIC SCIENCE
21
1
6
2
3
9
8
3
7
FIG. 2.35 Schema showing venous relationships of a lumbar vertebra.
Engorgement and relative venous hypertension in the epidural vessels
exacerbate neuroischemic conditions in the lumbosacral roots. 1, Dorsal
external vertebral plexus. 2, Dorsal epidural plexus. 3, Ascending lumbar
veins. 4, Basivertebral vein. 5, Ventral external vertebral plexus. 6, Lumbar
segmental vein. 7, Muscular vein from posterior abdominal wall. 8,
Circumferential channels (sinuses) of epidural plexus. 9, ventral internal
vertebral plexus. 10, anterior and posterior longitudinal sinuses of internal
vertebral plexus.
10
4
5
congestion of the spinal cord is prevented by minute valves in
the radicular branches draining the spinal cord.83 is latter
fact is anatomically unique because valves exist nowhere else
in the venous channels associated with the central nervous
system. An ancillary function of the epidural plexus may be
to act in a mechanical capacity as a hydraulic shock-absorbing
sheath that helps buer the spinal cord during movements of
the vertebral column, similar to the epidural fat.
e vertebral sinuses are largest in the suboccipital and
upper cervical region. Here they also receive numerous nerve
endings from the sinuvertebral nerves and are associated with
glomerular arteriovenous anastomoses, which suggests a possible baroceptive function.85 e patency of these anastomoses
is most easily shown in the fetus, in which arterial injections
of a contrast medium may also ll the upper cervical epidural
sinuses. Similarly, the coccygeal bodies of the same specimen
pass the arterial injection directly into the epidural veins of
the lower sacral region.
e detrimental aspects of the vertebral epidural veins have
been well stated by Batson.82 Retrograde ow from venous
connections to the lower pelvic organs provides an obvious
route of metastasis for pelvic neoplasms to the spine itself and
to the regions of the trunk associated with valveless connections to the plexus. Batson82 claimed that direct metastatic
transfer can occur between the pelvic organs and the brain via
the vertebral epidural route.
Another extraspinal-intraspinal venous connection implicated in the transfer of pathologic processes involves the
pharyngovertebral veins.86 ese vessels constitute a system
that drains the superior posterolateral regions of the nasopharynx and coalesces into two to several veins that penetrate
1
2
23
3
4
5
6
7
8
10
9
11
T5
14
2
15
18
12
13
5
16
17
23
T11
22
24
21
20
19
FIG. 2.36 Composite schema of blood supply to spinal cord and nerve
roots showing two regions of the cord. Note the distinction between
medullary arteries and true radicular arteries and that the medullary arteries
usually run a course that is independent of the roots. 1, Dorsolateral
longitudinal artery. 2, Proximal radicular artery (of dorsal root). 3, Dorsal
medullary artery. 4, Dorsal root of thoracic spinal nerve. 5, Distal radicular
artery (of dorsal root). 6, Sinuvertebral nerve. 7, Dorsal ramus of spinal nerve.
8, Segmental artery. 9, Dorsal central artery. 10, Dorsal root ganglion. 11,
Anterior laminar artery. 12, Ventral ramus of spinal nerve. 13, Rami
communicantes. 14, Ventral root of spinal nerve. 15, Proximal radicular artery
of ventral root. 16, Periradicular theca of dura. 17, Dorsal meningeal branch
of vertebromedullary artery. 18, Dura. 19, Ventral meningeal plexus. 20, Great
ventral medullary artery (great “radicular” artery of Adamkiewicz). 21,
Anterior (ventral) spinal artery. 22, Vasa corona of spinal cord. 23, Spinal
nerve. 24, Ventral medullary artery of thoracic cord.
the anterior atlanto-occipital membrane to discharge into the
venous complex surrounding the median and lateral atlantoaxial joints. Because posterior pharyngeal infections have
been linked with the atlantoaxial rotatory subluxations characteristic of Grisel syndrome,87 it is believed that the pharyngovertebral veins are instrumental in transporting infectious
processes that may produce a hyperemic relaxation of the
atlantoaxial ligaments. e existence of this venous system
also explains the ease in transfer of superior pharyngeal metastatic processes to the upper cervical epidural veins.
Blood Supply of the Spinal Cord
roughout the length of the spinal cord, a system of three
longitudinal vessels receives blood from the irregularly located
medullary branches of the segmental spinal arteries and distributes it to the substance of the cord. is system consists of
the single median ventral anterior spinal artery and two
smaller dorsolateral spinal arteries (Fig. 2.36).
Chapter 2 Applied Anatomy of the Spine 45
Anterior Spinal Artery
Despite its great functional signicance, the anterior spinal
artery remains one of the more inaccurately described and
inadequately understood blood vessels. Derived from the
fusion of bilateral pairs of ascending and descending anastomotic branches of the original segmental arteries of the developing spinal cord,88 this median ventral pial vessel supplies
approximately 80% of the intrinsic spinal cord vasculature. It is
usually depicted in texts as a single continuous artery of nearly
uniform caliber that extends from the medulla oblongata to
the conus. e anterior spinal artery is actually a longitudinal
series of functionally independent vessels that may show wide
luminal variations and anatomic discontinuities.
Although the investigations of Crock and Yoshizawa75 have
tended to minimize the signicance of predominant regional
feeders, many functionally oriented reports have claimed that
the cord has three major arterial domains along its vertical
axis: (1) the cervicothoracic region (C1–T3), (2) the midthoracic region (T3–T8), and (3) the thoracolumbar (including sacral cord) region (T8-conus). e reports have also
claimed that these areas have little anastomotic exchange
between their junctions (Fig. 2.37).
Brewer and colleagues89 and Lazorthes and associates90
maintained that a series of human anterior spinal arteries
consistently show interruptions, or critically narrow zones,
VA
T3
88-90
in the mid-thoracic region, and these inuence the potential
collateral blood ow along the longitudinal axis of the cord.
It is not only the observed size of the vessel that is of physiologic signicance, however. e existence of a marked autoregulatory control of the intrinsic spinal cord blood ow has
been independently shown in many mammalian species.
32,91
Microscopic investigation92 of sections of the descending
and ascending contributions of the arteria medullaris magna
(artery of Adamkiewicz, also known as the arteria radicularis
magna) to the anterior spinal artery showed that these arteries,
in addition to their well-developed circumferential muscle of
the tunica media, also possess a layer of predominantly longitudinal intimal musculature. Located between the internal
elastic lamina and the endothelium, this layer ranges in thickness from one-h to one-half of the tunica media (Fig. 2.38).
In following a series of cranial to caudal sections of the
thoracolumbar anterior spinal artery, it was noted that the
intimal muscle layer did not extend into any of its branches.
At the mouth of the central (sulcal) arteries, which are the
largest anterior spinal artery derivatives, the intimal musculature stops abruptly, oen forming a liplike projection over
the opening of the branch vessel, but no intimal muscle bers
extend into the central arteries. A sphincter-like thickening
of the central artery tunica media, seen at the ostium of the
vessels, indicates that this muscle layer has a greater contractile
inuence at this point (see Fig. 2.38). e intimal musculature,
in addition to enhancing the luminal control of the anterior
spinal artery, also is involved in controlling the blood ow into
the central arteries. Where the intimal layer shows the liplike
projections, successive serial sections indicate that contraction
of the longitudinally disposed intimal muscle bers forms
an ellipsoidal buttonhole-shaped orice whose long axis is
parallel to that of the ber orientation. Such an arrangement
permits exquisite muscular control of the blood ow from the
anterior spinal artery to its central artery branches.
SECTION
I
T8
AMM
FIG. 2.37 Schema illustrating sources and relationships of medullary feeder
arteries to the spine and the spinal cord. Anterior spinal artery (ASA) is
shown to be formed by an anastomotic chain of ascending and descending
branches of medullary feeders. Cervicothoracic, mid-thoracic, and
thoracolumbar (includes sacral cord) regions are indicated, and their usual
boundaries at vertebral levels T3 and T8 are shown. Medullary feeders range
from 6 to 14, but the respective domains persist. Dotted line indicates
frequent position of a smaller accessory medullary feeder to the
thoracolumbar area. AMM, Arteria medullaris magna; VA, vertebral artery.
(From Parke WW, Whalen JL, Bunger PC, et al. Intimal musculature of the
lower anterior spinal artery. Spine. 1995;20:2074.)
2
ASA LUM
1
5
4
7
10 mm
FIG. 2.38 High-power cross section of thoracolumbar anterior spinal artery
(ASA) wall at junction with one side of a central artery. The intimal
musculature (1) may extend as a liplike projection (6) over the central artery
orice. This muscle layer stops at this point and does not extend into
branch vessels. A sphincter-like enlargement of the conventional circular
muscle of the central artery (7) is indicated. Endothelium (3) and internal
elastic lamina (4), tunica media (2), and adventitia-pia (5) are labeled.
ART, artery; LUM, lumbar. (From Parke WW, Whalen JL, Bunger PC, et al:
Intimal musculature of the lower anterior spinal artery. Spine 1995;20:2075.)
6
Central
ART
LUM
3
46 BASIC SCIENCE
a ASA
d ASA
AMM
FIG. 2.39 Schema derived from sections of arteria medullaris magna
(AMM)–anterior spinal artery (ASA) junction to show distribution of intimal
musculature (solid black) in this region. Intimal cushions are shown guarding
the orice of the ascending ASA (a ASA) and a typical distribution is found
in the arch of the descending ASA (d ASA). (From Parke WW, Whalen JL,
Bunger PC, et al. Intimal musculature of the lower anterior spinal artery.
Spine. 1995;20:2076.)
In addition to the fairly uniform layer of the intimal musculature throughout the walls of the examined sections of
the thoracolumbar anterior spinal artery, serial sections cut
through the arch-shaped junction of the arteria medullaris
magna and the descending anterior spinal artery branches
show that this intimal layer, in most cases, is organized into
prominent intimal cushions. ese muscular thickenings are
erratically distributed along the lumen of the hairpin-shaped
arterial arch and the initial segment of the ascending branch of
the anterior spinal artery (Figs. 2.39 and 2.40). is latter location is of considerable interest because its prominent cushions,
with reinforced thickenings of the underlying tunica media,
could exert considerable inuence over the quantity of blood
ow between the thoracolumbar and mid-thoracic vascular
domains. is intimal control system, when coupled with
the intramedullary arteriovenous anastomoses (described in
a subsequent section on intrinsic vascularity), provides an
anatomic basis for the dramatic range of spinal cord blood
ow autoregulation. e presence of the intimal cushions
explains the oen-noted failure of the arteria medullaris
magna to supply adequately the mid-thoracic cord region
above the arteria medullaris magna–anterior spinal artery
junction during aortic cross clamping.
e ventral position of the anterior spinal artery and its
nutritional importance may have consequence in spinal stenosis. Particularly in the lower cervical region, its compression
by dorsal osteophytes and cartilaginous protrusions related to
cervical disc degeneration may lead to the neurologically
disastrous anterior spinal artery syndrome.93 e medullary
feeder arteries that supply the anterior spinal artery may arise
from any spinal segmental artery. Studies by Dommissee94
showed, however, that there are statistical preferences for
certain segmental levels. ere are usually three anterior
5
2
4
FIG. 2.40 Sagittal section through junction of arteria medullaris magna
(AMM) arch and ascending anterior spinal artery (aASA) showing the intimal
cushions guarding the aASA orice (1). These may be reinforced by
underlying enhancement of the circular bers of the tunica media (2).
Endothelium (3) and elastic lamina (4) are indicated. The longitudinal
disposition of the intimal muscle bers is apparent, particularly in the
intimal cushion on the right side. The contraction of these muscular systems
would dramatically alter the radius of the aASA lumen. 5, Adventitia-pia.
(From Parke WW, Whalen JL, Bunger PC, et al. Intimal musculature of the
lower anterior spinal artery. Spine. 1995;20:2076.)
4
3
aASA
1
1
AMM
10mm
4
2
medullary arteries for the cervical region, one or two for the
thoracic region, and a conspicuous medullary vessel (the
arteria medullaris magna) for the lumbosacral cord region.
e levels of origin for all these vessels center around certain
“average” locations in each region. e anterior spinal artery
is usually of greatest caliber in the lumbosacral part of the
cord, where it supplies the considerable tissue mass of the
proximal cauda equina in addition to the lumbosacral cord
intumescence.
e dorsolateral spinal arteries arise from the posterior
inferior cerebellar vessels and are of lesser caliber and nutritional signicance. ey also are less likely to be longitudinally
continuous and oen present a more plexiform distribution
over the dorsum of the cord. ey have a greater frequency of
smaller medullary sources.
e larger intradural spinal arteries are unusual in that,
similar to the cerebral arteries, they have no signicant vasa
vasorum. In all other regions of the body, a vessel with an
external diameter approaching 1 mm shows a ne vascular
plexus (vasa vasorum) on its external surface that supplies
nutrients to its outer layers of tissue. Because the cerebral
and spinal vessels are bathed in the nutrient-rich cerebrospinal uid, their external layers presumably derive metabolic
exchange from this source.
Chapter 2 Applied Anatomy of the Spine 47
Lateral Spinal Arteries of the Cervical Cord
e highest three to four segments of the cervical spinal cord
receive blood from a unique pair of vessels, the lateral spinal
arteries. Although, ontogenetically, these seem to be the most
rostral expressions of the dorsolateral spinal arteries, they have
a more extensive distribution and are without equivalents in
other levels of the cord. ey usually arise from the intradural
parts of the vertebral arteries near the origins of the posterior
inferior cerebellar arteries, or they may arise from the proximal sections of the posterior inferior cerebellar arteries
themselves. eir typical course carries them anterior to the
posterior roots of the cervical spinal nerves C1 to C4, dorsal
to the denticulate ligaments, and parallel to the spinal components of the 11th cranial nerve. eir general distribution is
to the dorsolateral and ventrolateral cord regions caudad to
the olives.
Although these vessels were observed in the later 19th
century, they were usually regarded as variants, and their
functional signicance was not appreciated. Lasjaunias and
colleagues95 compiled an extensive report on the variations
and selective angiography of these important vessels.
Intrinsic Vascularity of the Spinal Cord
e tissues of the spinal cord are supplied by two systems of
vessels that enter its substance. e rst is a centripetal
arrangement of arteries that supplies the supercial tracts of
the ventral and lateral funiculi, all of the dorsal funiculus, and
the extremities of the dorsal horns. ey are radially penetrating branches of the vasa corona and the dorsolateral spinal
arteries, which serve only a little more than one-fourth of the
cord. e greater part of the cord and almost all of its gray
matter is supplied by a second centrifugal system of vessels
derived from the sulcal (or central) arteries.96 ese arteries
are a repetitive series of branches derived from the dorsal
aspect of the anterior spinal artery that penetrate the depths
of the anterior median ssure. In the mid-sagittal plane, they
form a close palisade of vessels that occur with a frequency of
3 to 8 arteries per 1 cm in the cervical region and 2 to 6 per
1 cm in the thoracic cord; they are densest in the lumbar
region, where they number 5 to 12 per 1 cm of the anterior
spinal artery. e average diameters of the sulcal arteries are
greater in the cervical (0.21 mm) and lumbosacral (0.23 mm)
regions than in the thoracic cord (0.14 mm).
As these vessels approach the anterior commissure, most
turn to either the right or the le and supply only the corresponding side of the cord.
14,73,98,99
is unilateral proclivity
reects their origins in the early embryonic stages when the
anterior spinal arteries rst condensed from a primitive plexus
as a symmetrical pair of longitudinal vessels, each supplying
its respective half of the cord. In subsequent development,
these two vessels fused in the midline to form the denitive
single median anterior spinal artery, but their sulcal branches
retained their original unilateral anities. Bilateral distributions occur in 9%, 7%, and 14% of the cervical, thoracic, and
lumbar vessels.
97,100
97
Although the sulcal arteries may give infrequent branches
to the septomarginal white bers as they extend into the
median anterior ssure, their major distribution is derived
aer they enter the substance of the cord, just ventral to the
anterior white commissure. Here the individual right and le
arteries subdivide into dorsal and ventral branches. A group of
ventral branches supplies the ventral horns and, through more
radial extensions, provides vessels to Clarke column and the
deeper bers of the anterior and lateral funiculi. e smaller,
more dorsal group of branches supplies the gray commissure
and the ventral one-half to two-thirds of the dorsal horns. A
few second-order or third-order branches form anastomotic
arcades with their counterparts of adjacent sulcal artery
territories. All these vessels provide the ner arterioles that
eventually lead to the spinal capillary beds.
e greater metabolic requirements of the spinal gray
matter, in contrast to the funicular tissue, are dramatically
reected in their relative capillary densities. Quantication
of the microvascularity in the spinal cord has shown that the
capillary density of the gray matter is four to ve times as
great as the white matter.
101
e capillary distribution within
the gray matter is not homogeneous, however, and varies with
the regional concentrations of the nuclei. e nuclei of the
dorsal horn are fairly uniformly distributed. e ventral horn
shows segmental nuclear clusters, which display distinct nerve
cell groups.
As noted by Feeney and Watterson,
102
the capillary densities
of the white and gray matter of the central nervous system are
established at a level that is minimally requisite for the metabolic needs of the given tissue. is situation is in contrast to
most other body tissues, which have a capillary “reserve” and
normally function with only part of their capillary channels
open, varying their intrinsic vascular resistance by dilation
of the accessory channels. Nevertheless, despite the lack of
this method of control, the spinal cord exhibits a remarkable
range of blood ow autoregulation.
1,32,103
e intrinsic cord
vasculature maintains a constant blood ow throughout a
wide range of systemic blood pressure alterations, although
each animal species has a denite upper and lower limit to
the systemic blood pressure at which the regulation decompensates. Because transection of the upper cervical cord has
no eect on this autoregulatory capacity, it may be assumed
that this reex is local and independent of autonomic nerve
control.
Numerous third-order branches of the sulcal arteries communicate directly with veins through convoluted anastomoses.
ese vascular structures are located primarily in the area that
divides the ventral two-thirds of the dorsal horn from the
dorsal one-third and in the more central regions of the ventral
horn. ey show a paucity of contractile elements and instead
exhibit an “epithelioid” type of media that seems capable of
swelling and diminishing its thickness. Because this action
could rapidly control the caliber of the anastomotic lumina in
immediate response to local metabolic changes, these anastomotic convolutions may be the site of the reex adjustment in
the ow resistance of the spinal cord vasculature.
104
Perhaps the most essential part of knowledge of the vascular supply of the spinal cord is awareness of the ranges of
SECTION
I
48 BASIC SCIENCE
individual variability. e numerous successful surgical cases
in which the arteria medullaris magna has been inadvertently
interrupted without producing a disastrous spinal cord
ischemia give the impression that an adequate collateral vascularity may protect the cord in most individuals when a
single major artery is compromised. In procedures involving
the interruption of blood ow in numerous consecutive segmental branches of the aorta, such as aortic cross clamping for
abdominal vascular surgery, the maintenance of adequate
spinal cord blood ow, particularly in the thoracic area, seems
to depend more on the regional competence of the anterior
spinal artery than on the number of collateral sources to the
cord. Spinal cord injury aer cross clamping without adjunctive vascular support has been reported to range from 15% to
25%, depending on the series of cases reviewed.
105,106
Proximalto-distal aortic shunting may alleviate the undesirable hypertension in the aortic distribution proximal to the rst clamp
and the hypotension in the segments distal to the second
clamp. e work of Molina and colleagues
105
on dogs indicated,
however, that the shunt capacity should provide more than
60% of the baseline descending aortic ow and have a diameter
greater than one-half of the descending aorta to be eective.
Of particular signicance was the study by Svensson and
colleagues
107
on the blood ow in the baboon spinal cord and
its implications in aortic cross clamping. is animal was
chosen because its spinal vascularity is similar to humans in
that its anterior spinal artery is a continuous vessel without
the occasional interruptions noted in some quadrupeds. is
study indicated that in baboons, as in humans, the caliber of
the anterior spinal artery is oen critically narrowed where the
thoracic anterior spinal artery joins the lumbar segment of this
vessel at their common junction with the arteria medullaris
magna. e functional implication is that the shunting of the
cross-clamped aorta may help maintain an adequate ow in
the lumbosacral sections of the cord but is of little help to the
supply of the lower sections of the thoracic cord, owing to the
marked discrepancy that usually exists between the anterior
spinal artery diameters above and below the junction of the
arteria medullaris magna.
In accordance with the hemodynamic principles of Poiseuille’s equation, the resistance to blood ow upward from
the arteria medullaris magna junction was more than 50 times
greater than the ow resistance downward into the lumbosacral anterior spinal artery in the baboon. Because a series of
direct measurements showed that this discrepancy in the
anterior spinal artery diameters was even greater in humans,
Svensson and colleagues
107
concluded that even the lowest
segments of the thoracic cord were dependent on a blood ow
from the superior end of the thoracic anterior spinal artery
despite the shunting.
Intrinsic Venous Drainage of the Spinal Cord
Compared with the arterial anatomy, the structural and functional aspects of the venous drainage of the spinal cord have
been relatively neglected. In contrast to other organ systems
in which the equivalent orders of veins and arteries tend to
course in a common vascular bundle, the veins of the central
nervous system are generally less numerous than the arteries, they are larger than their corresponding eerent vessels,
the larger branches may not show a pattern concurrent with
the arterial distribution, and they are not accompanied by
lymphatics.
e internal substance of the dorsal half of the cord drains
by a centrifugal arrangement of intrinsic vessels that are tributaries, by way of a venous vasa corona, to a large median dorsal
longitudinal spinal vein; the ventral half sends tributaries to
sulcal veins that empty into a large median ventral longitudinal
vein that runs parallel to the anterior spinal artery. Both of
these longitudinal vessels are circumferentially connected by
a prominent venous vasa corona. is entire system drains
into the epidural venous plexus by medullary (previously
called radicular) veins that are as infrequent in their distribution as the medullary arteries.
108
e proximal sections of the
spinal nerve roots drain centripetally into the vasa corona and
longitudinal veins of the cord and then to the epidural system
via the medullary veins.
Vascularization of the Spinal Nerve Roots
Although it has been generally recognized that much of the
pain consequent to degenerative changes in the spinal motion
segment is associated with compression or tension on the
spinal nerve roots, the mechanisms that initiate the actual
nerve discharge have remained obscure. Because experimental
studies on peripheral nerves and observations on numerous
cases of neurogenic claudication have suggested that much of
the pain may have a neuroischemic basis, investigations were
undertaken to determine the nature of the intrinsic vascularity
of the spinal nerve root and its response to localized compression or tension. e nerve roots had long been regarded as
part of the peripheral nervous system and were viewed as
histologically and vascularly similar to peripheral nerves.
Consequently, research on the latter was oen uncritically
extrapolated to apply to the nerve roots.
e very long roots of the lumbosacral spinal nerves
seemed to be particularly vulnerable because their vascularity
was initially believed to be supplied only from their distal ends
without the access to the frequent collateral support that is
characteristic of peripheral nerves. Because the nerve root
fasciculi do not have a strong connective tissue support, it also
seemed that the ne vascularity they possessed would be at
risk from the repeated tension and relaxation resulting from
the exion and extension of the spine. Parke and colleagues
and Parke and Watanabe
110
showed by vascular injection that
the roots receive their arterial supply from both ends (Fig.
2.41; see Fig. 2.36), however, a fact physiologically conrmed
by Yamamoto.
111
e existence of many redundant coils along the branches
of the true radicular arteries ameliorates the stresses that
would result from the interfascicular movements that accompany the repeated stretch and relaxation. A signicant nding
was the occurrence of numerous, relatively large arteriovenous
anastomoses throughout the length of the root (Fig. 2.42).
109
Chapter 2 Applied Anatomy of the Spine 49
SECTION
T10
L4
FIG. 2.41 Schema indicating directions of normal blood ow in cauda
equina. The anterior spinal artery of the lumbosacral part of the cord is
supplied by medullary arteries and supplies 75% at the cord substance and
upper parts of the cauda equina via the proximal radicular arteries. This
accounts for enlargement of the anterior spinal artery in the lumbosacral
region.
ese vascular cross connections apparently allow blood ow
to be maintained in sections of the root above and below a
point of compression. Of particular signicance to root nutrition is the work of Rydevik and colleagues
111a
who, using
isotopically labeled methylglucose, showed that approximately
50% of the root nutrition is derived from the ambient cerebrospinal uid; this necessitates a gauzelike architecture of the
radicular pia-arachnoid sheath (Fig. 2.43; see Fig. 2.42B).
A study by Watanabe and Parke
112,113
of chronically compressed roots indicated that the compressed segment is most
likely metabolically deprived. It has been suggested that
radicular pain is related to root ischemia because a reduction
of oxygen intake in patients with neurogenic claudication
exacerbates the symptoms.
114
e arterial side of the vasa
radiculorum seems to be well compensated, however, and
maintains a continuity despite severe chronic compression.
Further study has indicated that the venous side of the
radiculomedullary circulation is more vulnerable.
113
Because
the roots are part of the central nervous system, the relationships of the arteries to the veins resemble those of the brain
more than those of peripheral nerves. e radicular veins do
not follow the arterial pattern. ey are fewer in number and
run a separate and usually deeper (more central) course. Being
thin-walled, they are more liable to the spatial restrictions
imposed by degenerative changes in the dimensions of the
spinal canal and intervertebral foramina and show complete
interruption in the chronically compressed root. e metabolically deprived, or inamed, nerve root becomes hypersensitive to any mechanical deformation, and any additional
insult to such a nerve may initiate ectopic impulses that
produce pain.
6
5
4
5
5
3
A
1
2
3
7
6
4
B
FIG. 2.42 (A) Low-power (×20) transillumination photomicrograph of
midsection from part of L4 nerve root treated with hydrogen peroxide after
vascular injection with latex–India ink but before clearing in a solution of
tributyl-tricresyl phosphates. The peroxidases within the residual blood
elements inated the radicular veins (4) to provide a temporary contrast
medium. Note the frequency of the large arteriovenous anastomoses
(5) that permitted the latex–India ink to enter the veins. (B) Compilation
showing structure of a typical lumbosacral nerve root derived from data
obtained by injection studies and scanning electron microscopy (see
Fig. 2.38). The gauzelike pia-arachnoid membranes permit the cerebrospinal
uid to percolate into nerve tissues and assist metabolic support. Numbers
in (A) and (B) are common to equivalent structures. 1, Fascicular pia.
2, Interfascicular and intrafascicular arteries showing compensating coils to
allow interfascicular movement. 3, Longitudinal radicular artery. 4, Large
radicular vein (does not course with arteries). 5, Arteriovenous anastomosis.
6, Collateral radicular artery. 7, Gauzelike pia-arachnoid that permits
percolation of cerebrospinal uid to assist in metabolic support.
6
4
FIG. 2.43 Scanning electron photomicrograph of section of proximal part
of L5 ventral nerve root. The gauzelike pia-arachnoid sheath is very evident.
The numbers correspond to the structures labeled in Fig. 2.42.
5
3
1
7
I
50 BASIC SCIENCE
Impedance of the radiculomedullary venous return can
occur without topographically related venous constriction.
e exacerbation of neurogenic pain in cases in which spinal
stenosis has been associated with venous hypertension has
been recorded by clinical investigators. LaBan
and Wesolowski
116
noted that patients with diminished right-
115
and LaBan
sided heart compliance and spinal stenosis may eventually
exhibit neurogenic pain even in static or recumbent situations.
ey attributed this phenomenon to an increased external
pressure on the already sensitized roots by the engorgement
of the epidural venous sinuses (see Fig. 2.35), but the venous
hypertension alone may be sucient to impede the venous
return from an already compromised radicular circulation.
Madsen and Heros
117
showed that “arterialization” of spinal
veins by abnormal arteriovenous shunts in the region of the
conus medullaris exacerbates the neurogenic pain in patients
with spinal stenosis. eir hypothesis suggested that a variable combination of increased mechanical constriction by
dilated epidural veins and the direct increased resistance to
the radicular circulation by the venous hypertension could
contribute to the elicitation of pain. Aboulker and col-
118
leagues
also concluded that epidural venous hypertension
alone may produce radicular symptoms or cord symptoms or
both without adjunctive stenotic compression.
If the intrinsic circulation of the nerve root is impeded in
either its arterial input or its venous outow, the net eect
seems to be the same: a neuroischemia of the compressed root
segment that may enhance the generation of ectopic nerve
impulses. A phenomenon that could be related to radicular
venous stasis is the swelling of the disc-distorted nerve root
that Takata and colleagues
119
showed in CT myelograms. is
phenomenon is dicult to explain because extravasated uids
in the root tissues should have free access to the surrounding cerebrospinal uid. Nevertheless, the uid balance of the
root tissues seems to be altered, particularly in the segment
proximal to the level of the oending disc. e intricacies of
the hemodynamic relationships responsible for this change
remain unknown.
e role of the ubiquitous arteriovenous anastomosis in
autoregulation of the intrinsic radicular vasculature also oers
a fertile eld for clinical investigations. Because these vascular
shunts are mostly without contractile elements but seem instead
to control their lumina by the thickening response of an epithelioid endothelium, they probably react to chemical changes
in the blood within their lumina and can oer an immediate
local reex to alterations in the nerve root metabolism.
Functional Anatomy of the Spine
e biomechanics of the spine is a very complex and extensive
subject. A comprehensive discussion is beyond the scope of
this chapter, so the reader is directed to the work of White and
Panjabi,
this eld. Because an appreciation of the essential functional
relationships of the spinal components does enhance an
understanding of their anatomy, however, a brief overview
follows.
120
which is generally regarded as the major book in
e spine is capable of ventroexion, extension, lateral
exion, and rotation. is remarkable universal mobility may
seem at odds with the fact that its most essential function is
to provide a rm support for the trunk and appendages. e
apparent contradiction may be resolved when one realizes that
the total ranges of motion are the result of a summation of
limited movements permitted between the individual vertebrae and that the total length of the spine changes very little
during its movements. e role of the musculature in the
performance of the supportive functions cannot be minimized,
as the disastrous scolioses that result from their unilateral loss
in a few motor segment units may attest.
e degree and combination of the individual types of
motion described earlier vary considerably in the dierent
vertebral regions. Although all subaxial-presacral vertebrae
are united in a tripod arrangement consisting of the intervertebral disc and the two zygapophyseal articulations, the relative size and shape of the former and the articular planes of
the latter determine the range and types of motion that an
individual set of intervertebral articulations contributes to the
total mobility of the spine. In general, exion is the most
pronounced movement of the vertebral column as a whole. It
requires an anterior compression of the intervertebral disc and
a gliding separation of the articular facets, in which the inferior set of an individual vertebra tends to move upward and
forward over the opposing superior set of the adjacent inferior
vertebra. e movement is checked mainly by the posterior
ligaments and epaxial muscles.
Extension tends to be a more limited motion, producing
posterior compression of the disc, with the inferior articular
process gliding posteriorly and downward over the superior
set below. It is checked by the anterior longitudinal ligament
and all ventral muscles that directly or indirectly ex the spine.
Also, the laminae and spinous processes may sharply limit
extension. Lateral exion is accompanied by some degree of
rotation. It involves a rocking of the bodies on their discs, with
a sliding separation of the diarthroses on the convex side and
an overriding of the diarthroses related to the concavity. e
rotational component brings the anterior surface of the bodies
toward the convexity of the exure and the spinous processes
toward its concavity. is phenomenon is well illustrated in a
dried preparation of a scoliotic spine. Lateral exion is checked
by the intertransverse ligaments and the extensions of the ribs
or their costal homologues.
Pure rotation is directly proportional to the relative thickness of the intervertebral disc and is mainly limited by the
geometry of the planes of the diarthrodial surfaces. Although
the architecture of the disc permits limited rotation between
the bodies, it also serves to check this movement by its resistance to compression. e consecutive layers of the anulus
brosus have their bers arranged in an alternating helical
fashion, and rotation in either direction can be accompanied
only by increasing the angularity of the opposing bers to the
horizontal, which requires compression of the disc.
e entire vertebral column rotates approximately 90
degrees to either side of the sagittal plane, but most of this
traversion is accomplished in the cervical and thoracic sections. It exes nearly the same amount, using primarily the
Chapter 2 Applied Anatomy of the Spine 51
cervical and thoracic regions. Approximately 90 degrees of
extension is permitted by the cervical and lumbar regions,
whereas lateral exion with rotation is allowed to 60 degrees
to both sides, again primarily by the cervical and lumbar areas.
Specic Regional Considerations
e atlanto-occipital joints mostly permit exion and extension with a limited lateral action, all being checked by the
suboccipital musculature and the atlanto-occipital ligaments.
e atlantoaxial articulations allow only rotation, the pivoted
joint being stabilized and checked by the alar ligaments
and the ligaments forming the capsules of the atlantoaxial
diarthroses.
One-half of the rotational mobility of the entire cervical
region takes place between the atlas and the axis, and the
remainder is distributed among the joints of the subaxial
vertebrae. e atlanto-occipital joint accounts for approximately half of the cervical exion. e remaining 50% is not
evenly distributed among the cervical vertebrae but is greater
in the upper section.
e subaxial part of the cervical region shows the ranges
of motion that are the most free of all the presacral vertebrae.
e discs are quite thick in relation to the heights of the vertebral bodies and contribute about one-fourth of the height of
this part of the column. In addition, a sagittal section shows
the middle part of the cervical disc to be lenticular, so that the
anteroinferior lips of the bodies are more capable of sliding
slightly forward and overriding one another. e range of
spinal exion is greatest in the cervical region, and although
the posterior nuchal ligaments and muscles may tend to resist
this motion, it is ultimately checked by the chin coming to rest
on the chest.
e cervical spine is normally carried in a moderately
extended position and shows a median variation of 91 degrees
between extension and exion. Extension is checked by the
anterior longitudinal ligament and the combined resistances
of the anterior cervical musculature, fascia, and visceral
structures, all three of which may be traumatized in hyperextension injuries.
Cervical lateral exion is quite limited by the articular
pillars and the intertransverse ligaments, and most lateral
motion involves considerable rotation. e nearly horizontal
position of the planes of the cervical articular facets provides
good supportive strength to the articular pillars but increases
the lateral rigidity, so that hyperextension injuries may be
more disastrous if the head is rotated at the time of impact
from the rear.
e mobility of the thoracic region is also not uniform
throughout its length. Although the upper segments resemble
the cervical vertebrae with respect to the size of the bodies and
the discs, the ribs attached to the sternum greatly impair the
ranges of motion. e circumferential arc of the plane of the
articular facets shows that rotation is the movement least
restricted by these structures.
Flexion and extension become freer in the lower thoracic
region, where the discs and vertebral bodies progressively
increase in size and the more mobile and less restrictive they
become. e last few thoracic vertebrae are transitional with
respect to the surfaces of the articular facets. ese begin to
turn more toward the sagittal plane and tend to limit rotation
and permit greater extension.
e articulations of the lumbar region permit ventroex-
ion, lateral exion, and extension, but the facets of the synovial
joints lie in a ventromedial to dorsolateral plane that virtually
locks them against rotation. is lumbar nonrotatory rigidity
is a feature shared with most mammals and achieves its greatest manifestation in certain quadrupeds in which the inferior
articulation ts like a cylindric tenon into the semicircular
mortise of the corresponding superior process of the vertebra
below. It provides a gliding action that permits the neural
arches to separate or approximate each other only during
extension and exion. e morphology of the joints can be
well appreciated in an appropriate cut of loin chop or T-bone
steak.
e synovial articulations at the lumbosacral junctions are
unique. In contrast to the more superior lumbar joints, the
facets of the inferior articulating processes of the h lumbar
vertebra face forward and slightly downward, to engage the
reciprocally corresponding articular processes of the sacrum.
Because of the position of these joint surfaces, a certain
amount of rotation should be possible between the h
lumbar segment and the sacrum, but the presence of the
strong iliolumbar ligaments quite likely restricts much motion
of this type.
e most essential function of the synovial lumbosacral
articulations involves their role as buttresses against the
forward and downward displacement of the h lumbar
vertebra in relation to the sacrum. When one considers that
each region of the spine has its own characteristic curvature,
the tracing of the vertical line indicating the center of gravity
shows that it intersects the column through the bodies of
the transitional vertebrae. e normal cervical lordosis places
most of the cervical vertebrae anterior to the center of gravity,
and the compensating thoracic kyphosis places the thoracic
vertebrae posterior to the center of gravity. e lumbar lordosis brings the middle lumbar vertebrae anterior to the line.
e transitional vertebrae between each region intersect the
center of gravity and seem to be the most unstable regions
of the spine; this is emphasized by the fact that disc problems and fractures most frequently occur in the transitional
vertebrae.
Because the sacrovertebral angle produces the most abrupt
change of direction in the column, and the center of gravity,
which passes through the h lumbar body, falls anterior to
the sacrum, there is a marked tendency for the thick, wedgeshaped h lumbar disc to give way to the shearing vector that
the lumbosacral angularity produces. e resulting condition,
spondylolisthesis, most frequently reveals a deciency in the
laminae (spondylolysis) that fails to anchor the h vertebral
body to the sacrum and allows its forward displacement.
ere has been considerable discussion as to whether spondylolysis is congenital or acquired, but the spondylolisthesis
seldom occurs without the laminar deciencies as a preceding
condition.
SECTION
I
52 BASIC SCIENCE
Biomechanics of the Intervertebral Disc
It is axiomatic in mechanical engineering that a well-designed
machine automatically reveals its function through the analysis of its structure. ere are few instances in biologic circumstances in which this statement is more applicable than in the
case of the intervertebral disc. Even when the disc is simply
divided with a knife and examined grossly, it is apparent that
one is dealing with an organ that is remarkably constructed
simultaneously to alleviate shock and transmit forces from
every conceivable combination of vectors. is appreciation
of the functional competency of the disc increases as its
structure is analyzed at the ner levels of organization.
e internal composition of the disc has evolved to withstand great stresses through the liquid and elastic properties
of nucleus and anulus acting in combination. e nucleus is
distorted by compression forces, but being liquid it is in itself
incompressible. It serves to receive primarily vertical forces
from the vertebral bodies and redistribute them radially in a
horizontal plane. It is the distortion of the anulus by the
internal pressure of the nucleus that gives the disc its compressibility, and its resilience makes possible the recovery from
pressure.
Were the nucleus pulposus simply a cavity lled with water,
it would momentarily act in the same capacity, but the ability
to maintain the appropriate quantity of uid during the continual compression and recovery cycle would be lacking. is
ability to absorb and retain relatively large amounts of water
is the unique property of the living tissue of the nucleus.
essential compound involved in this process is a proteinpolysaccharide gel, which through a high imbibition pressure
binds nearly nine times its volume of water. It is apparent that
the hydrophilia is not a form of biochemical bonding because
a quantity of water can be expressed from the nucleus by
prolonged mechanical pressure. is accounts for the diurnal
decrease in the total length of the spine and its recovery in the
supine position at night.
e anulus must receive the ultimate eects of most forces
transmitted from one vertebral body to another. Because the
major loading of the intervertebral disc is in the form of vertical compression, it may seem paradoxical that the anulus is
best constructed to resist tension, but the nucleus transforms
the vertical thrust into a radial pressure that is resisted by the
tensile properties of the lamellae. Although the basic plan of
alternating bands of bers is one of the obvious sources of the
tensile strength of the anulus, this arrangement is not uniform
with respect to the directions of the bers or the degrees of
resistance and resilience encountered throughout the anulus.
e bers generally become longer, and the angle of their
spiral course becomes more horizontal near the circumference
of the disc because it is here that the shearing stresses of
vertebral torsions would be most eective. Experimental
analysis has also shown that various parts of the anulus do not
respond equally to the same degree of tension, and the discrepancies were related to the plane of section and the location
of the sample.
122
e anulus proved to have the greatest
resistance and the greatest recovery in horizontal sections of
121
e
the peripheral lamellae, whereas vertical and more medial
sections were more distensible.
Because the spine acts as a exible boom to the guidewire
actions of the erector spinae muscles, it is essentially the
fulcrum of a lever system of the rst class, in which the loading
has a considerable mechanical advantage. Pure vector analysis
has indicated that a theoretical pressure of approximately
three-fourths of a ton could be applied to a disc when 100 lb
is lied by the hands,21 but this is considerably in excess of the
actual pressures achieved. Increased intrathoracic and intraabdominal pressures alleviate much of the fulcrum compression of the discs by eectively counteracting the load of the
anterior lever arm.
e actual pressure variations occurring with postural
changes have been recorded by inserting transducers into
the third lumbar disc.
123,124
is procedure indicated that the
internal disc pressure increases from approximately 100 kg
in a standing position with the spine erect to 150 kg when
the trunk is bent forward and to 220 kg when a 70-kg man
lis a 50-kg weight. It was particularly revealing that the
pressure showed a considerable increase when the equivalent
maneuvers were repeated in a sitting position, and the weight
liing ultimately created a pressure of 300 kg on the third
lumbar disc.
e disc is also “preloaded.” e inherent tensions of the
intervertebral ligaments and the anulus exert a pressure of
about 15 kg because this weight is required to restore the
original thickness of the disc aer the ligaments have been
divided.
110
From a comparative standpoint, this preloading
probably oers increased stability to the spine as a functional
exible rod. One is almost induced unconsciously to use teleologic thinking in terms of the vertical thrust resistance when
regarding the structure of the disc. In perspective, however,
the intervertebral disc shows a consistent morphology in all
mammals, yet humans are the only species that truly stand
erect. Although analysis of muscular action would most likely
show that all mammalian discs must dissipate and transfer
axial thrusts, the preloading would enhance the “beam
strength” that is obviously necessary in the vertebral column
of quadrupeds.
Acknowledgments
e authors acknowledge Wesley W. Parke, PhD (deceased),
an original author of this chapter in previous editions who was
responsible for much of the critical information included in
this work.
e vascular studies presented in this chapter were
supported by National Institutes of Health research grant
HL-14035.
KEY REFERENCES
1. Bajwa NS, Toy JO, Ahn NU. L5 pedicle length is increased in
subjects with spondylolysis: an anatomic study of 1072
cadavers. Clin Orthop Relat Res. 2012;470(11):3202-3206.
2.
Goldstein RY, Sunde CD, Assaad P, et al. Location of the
vertebral artery at C1 in children: how far out laterally can one
safely dissect? J Bone Joint Surg Am. 2014;96(18):1552-1556.