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Chapter 2 Applied Anatomy of the Spine 33
within the spinal canal have engendered much argument,
however, particularly concerning the segmental range of the
individual nerve ramications.
In illustrations based on dissections, Bogduk and colleagues44 and Parke52 agreed that each nerve supplies two
intervertebral discs via superiorly and inferiorly directed
branches—the inferiorly directed branch ramifying over the
dorsum of the disc at the level of entry and the longer, superiorly directed branch coursing along the edge of the posterior
longitudinal ligament to reach the disc of the next superior
level (Fig. 2.20). Dissections identify mainly the larger ramications. Smaller bers are usually localized with staining
techniques. Conventional methods of staining using silver or
lipotrophic stains have given controversial results, however,
because of a lack of specicity.
Groen and colleagues,53 using a highly specic acetylcholinesterase staining method on large cleared sections of
fetal human spines, resolved many conicts concerning the
1
2
3
4
5
12
11
10
FIG. 2.20 Schema of major intraspinal distribution of dorsal central
branches of segmental vertebromedullary arteries and distribution and
source of the sinuvertebral nerves. The pattern of the nerve shown entering
the superior foramen is derived from the data provided by Groen and
colleagues.53 Dotted lines show a composite of the variant ranges (arrows
indicate two or more segments) and ramications tabulated by these
authors. The nerve entering the inferior foramen shows the extent and
distribution described in previous reports. 1, Dorsal root ganglion. 2, Rami
communicantes. 3, Sinuvertebral nerve and its origin according to Groen
and colleagues. 4, Autonomic ganglion. 5, Nerve to anterior longitudinal
ligament. 6, Spinal nerve roots. 7, Sinuvertebral nerve arising from distal
pole of ganglion (thought to be its most common source before report
of Groen and colleagues). 8, Dorsal primary ramus of spinal nerve. 9, Ventral
primary ramus of spinal nerve. 10, Arteries entering basivertebral sinus
to supply cancellous bone. 11, Descending dorsal central branch of
vertebromedullary (spinal) artery. 12, Ventral branch of vertebromedullary
artery.
6
7
8
9
ramications of the nerves supplying spinal structures. ey
found that, in contrast to most previous reports, the human
sinuvertebral nerves were almost exclusively derivatives of
the rami communicantes close to their connections with the
spinal nerves. ese origins were fairly consistent throughout
the length of the thoracolumbar sympathetic trunk, but in the
cervical region they were also derived from the perivascular
plexus of the vertebral artery.
Five sinuvertebral nerves have been observed passing into
one intervertebral foramen. Typically, the group consists of
one thick nerve (perhaps the one seen in most conventional
dissections) and several ne bers. e thick, or predominant,
sinuvertebral nerve is oen absent, however, in the upper
cervical and sacral regions. e major sinuvertebral element
enters the foramen ventral to the spinal ganglion and gives o
some ne branches at this point. As the nerve enters the spinal
canal, the major branch usually divides into rami that course
in approximation to the distribution of the posterior central
branches of the segmental artery, with a long ascending
element and a shorter descending one. From these branches,
one to three coiled rami supply the ventral dura.
e acetylcholinesterase technique used by Groen and
colleagues53 made it possible to delineate details of the plexus
of the posterior longitudinal ligament. e work of these
authors supports the idea that the posterior longitudinal
ligament is highly innervated by an irregular plexiform distribution of bers that have a greater density in the ligament
expansions dorsal to the discs. ese authors were able to
note the primary direction, length, and “termination area”
of the branches of a single segmental sinuvertebral nerve.
ey classied the variations of individual nerves as follows:
(1) ascending one segment, (2) descending one segment, (3)
dichotomizing toward one segment caudal and one segment
cranial or horizontal, (4) ascending two or more segments,
and (5) descending two or more segments (see Fig. 2.20). e
existence of the latter two categories, although they are not
as common as the others, shows that the sinuvertebral nerve
can supply more than two adjacent segmental levels. A basis
for the poor pain localization of an oending disc may be
related to the generous distribution possible in the individual
sinuvertebral nerve. e large totomounts treated with acetylcholinesterase also showed that the patterns of sinuvertebral
nerve distribution to the posterior longitudinal ligament
did not display signicant regional variations apart from an
expected pronounced diminution in the plexus density in the
immovable lower sacral region.
e posterior longitudinal ligament is highly innervated
with complex encapsulated nerve endings and numerous lowmyelinated free nerve endings (Fig. 2.21). e lateral expansion of the posterior longitudinal ligament extends through
the intervertebral foramen covering all the dorsal and most of
the dorsolateral aspects of the disc. e elevation of this thin,
highly innervated strap of connective tissue may provide a
signicant component of the pain manifest in acute disc
protrusions.
e probable range of diverse functions of the sinuvertebral
nerve may be indicated by the analysis of its cross-sectional
composition. Stained preparations taken from a section near
SECTION
I
34 BASIC SCIENCE
AB
FIG. 2.21 Photomicrographs of nerve endings in posterior longitudinal ligament of a dog. (A) Section of
ligament dorsal to a lumbar intervertebral disc. The dark area is the central strap of the ligament, and the light
area is the thin lateral expansion over the dorsum of the disc. These ne nerve endings are characteristic of
those in known nociceptors. (B) Complex nerve ending from posterior longitudinal ligament. This type of
ending is believed to be a transducer of mechanical deformation for postural senses. (Methylene blue vital
tissue stain: [A], ×300; [B], ×500.)
the nerve origin show many small myelinated bers, although
some myelin sheaths are greater than 10 µm in diameter.54
Many of the smaller bers are postganglionic eerents from
the thoracolumbar autonomic ganglia that mediate the smooth
muscle control of the various vascular elements within the
spinal canal, and many of the larger bers are involved in
proprioceptive functions. Concerning the latter, Hirsch and
colleagues
46,55
found numerous complex encapsulated nerve
endings in the posterior longitudinal ligament (see Fig. 2.21B).
It is assumed that these may be associated with the larger
myelinated bers whose postganglionic axons enter the cord
to mediate postural reexes because similar bers in the cervical region of cats have been shown to be important in tonic
neck reexes.56 It seems, however, that the smaller bers
making up the bulk of the sinuvertebral nerve are aerents,
associated with simple, nonencapsulated, or “free” nerve
endings that are generally regarded as nociceptive (see Fig.
2.21A).
e fact that the sinuvertebral nerve carries pain bers has
been amply shown by clinical and laboratory experimentation.
Direct stimulation of tissues known to be served by the nerve
elicits back pain in humans. Pedersen and colleagues54 showed
that stimulation of these tissues in decerebrate cats resulted in
blood pressure and respiratory changes similar to those elicited by noxious stimuli to known pain receptors in other areas
of the body.
Disagreement exists over whether the anulus itself is innervated and, if so, how extensively. e classic work of Hirsch
and colleagues55 claimed that nerve endings are only in the
dorsal aspect of the most supercial layer of the anulus, and
these presumably are from branches of the same nerve bers
that innervate the overlying expansions of the posterior longitudinal ligament.
Pedersen and colleagues,54 Stilwell,57 and Parke52 have failed
to show nerve endings in the anulus. Because the connective
of nerve endings, Parke52 assumed that their disruption could
account for discogenic pain. Inappropriate methodology may
account for the failure to show intradiscal nerves. Malinsky,49
Bogduk and colleagues,
44,45
and Yoshizawa and colleagues58
published accounts showing nerve bers in the outer lamina
of the anulus. is work has now been supported by the
highly specic acetylcholinesterase method of Groen and
colleagues.
53
Most descriptions of the sinuvertebral nerve indicate that
the major meningeal bers to the spinal dura are distributed
to its ventral surface.59 e median dorsal dural surface has
been regarded as virtually free of nerve bers, a convenience
that permits its painless penetration during needle puncture.
Although Cyriax60 claimed that irritation of the ventral dura
during protrusion of the nucleus may contribute to discogenic
pain, a sucient distortion of the nerve bers on the movable
or unattached dura does not seem likely. e coiled conguration of these dural contributions of the sinuvertebral nerve,
noted by Groen and colleagues,61 may indicate a compensation
to permit a degree of dural movement without placing traction
on these nerves.
Parke and Watanabe62 observed that the ventral lower
lumbar dura is oen xed to the ventral canal surface by
numerous connective tissue bers, most rmly xed at the
margins of the lower lumbar discs. ese apparently acquired
adhesions are not to be confused with the ligaments of
Hofmann, which are normal straps of tissue connecting the
dura to the ventral canal surface that have been obliquely
positioned by the developmental cranial traction of the dura
and its contents. is observation has been supported by a
series of dissections by Blikra,63 who was seeking a rationale
for lower lumbar intradural disc protrusions. His analysis
showed that in some cases the dura may be suciently xed
to the ventral surface of the canal, particularly at the L4–L5
level, for protruding nucleus material to rupture the ventral
dura. Parke and Watanabe,62 by microscopic analysis of sections of the dura that had been forcibly freed from these
adhesions overlying the fourth or h lumbar disc, showed
disruption of the nerve bers bound in the adhesion. In the
numerous cases in which such adhesions are present, the
Chapter 2 Applied Anatomy of the Spine 35
forceful elevation of the dura by a disc protrusion may provide
an adjunctive source of the discogenic pain.
Spinal Motion Segment
e inclusion of all articular tissue, the overlying spinal
muscles, and the segmental contents of the vertebral canal and
intervertebral foramen into a single functional and anatomic
unit was rst suggested by Junghanns.
“motor” segment, this unit represents a useful concept that
stresses the developmental and topographic interdependence
between the brous structures that surround the intervertebral
foramen and the functioning of the structures that pass
through it. Although the 23 or 24 individual motion segments
must be considered in relation to the spinal column as a whole,
no congenital or acquired disorder of a single major component of a unit can exist without aecting rst the functions of
the other components of the same unit and then the functions
of other levels of the spine.
Although Junghanns64 dened the unit primarily in terms
of the movable structures making up the intervertebral articulations, a logical, if not necessary, extension of the motion
segment concept should include some aspect of the vertebral
elements. DePalma and Rothman66 included both adjacent
vertebrae in their illustration of the unit, but depiction of the
unit concept is improved by incorporating only the opposing
superior and inferior halves of each vertebra, eliminating
redundancy (see Fig. 2.16). In visualizing the motion segment
unit as a musculoskeletal complex surrounding a corresponding level of nervous structures, it must be realized that the
intervertebral disc and the facets are but two of the articulations involved. e interosseous brous connections that
include the interspinous, intertransverse, costovertebral, and
longitudinal ligaments and the ligamentum avum are varieties of syndesmoses.
64,65
Originally called the
Nutrition of the Intervertebral Disc
Most descriptive accounts of the intervertebral disc dismiss
the subject of its vascular nutrition with a brief mention of the
general agreement that the normal adult disc is avascular. e
demonstrable truth of this statement may give the impression
that the substance of the disc is inert biologically. Experimental evidence has indicated that the normal disc tissue is quite
vital and has a demonstrable rate of metabolic turnover.
contrast to the nonvascular cartilage in the diarthroses, the
cellular elements of the disc cannot receive the blood-borne
nutrients through the mediation of the synovial uid but must
rely on a diusional system with the vessels that lie adjacent
to the disc. Diusion is also the mechanism for removal of
products of metabolism from the disc, such as lactic acid.
e qualitative and quantitative aspects of the diusional
nutrition of the disc have been studied.
vascular plexus of the anulus and the vessels adjacent to the
hyaline cartilage of the bone-disc interface provide the two
sources for the diusion of metabolites into the disc. Although
68-72
e peripheral
67,68
In
69
the interface shows an average permeability of 40%, there is a
decreasing centrifugal gradient that starts with an 80% permeability at the center. Because diusion is the major mechanism
that carries small solutes through the disc matrix, the two
main parameters aecting this ow are the partition coecient,
which denes the equilibrium between the solutes within the
plasma and the solutes within the disc, and the diusion coef-
cient, which characterizes the solute mobility.
e partition coecient varies with the size and charge of
the solute particle. Small uncharged solutes show a nearequilibrium between their plasma and intradiscal concentrations, but because the disc matrix has a predominantly negative
charge, anionic solutes have a lower intradiscal concentration
in relation to the plasma, whereas the reverse is true for positively charged solutes, whose intradiscal concentration is
greater than that of the plasma. Because the range of these
eects depends on the concentration of the xed, negatively
charged, larger molecular aggregates (proteoglycans), the
partition coecient is regionally variable within the disc
matrix and especially pronounced in the inner annular lamellae and nucleus, where the concentration of proteoglycans is
the highest.
Solute mobility (the diusion coecient) within the disc is
slower than in the plasma because the presence of solids in the
form of collagen and proteoglycans impedes diusional progress. Without regard to charge, the diusion coecient
within the disc is 40% to 60% of free diusion within water,
and mobility is greatest in the inner anulus and nucleus where
the water concentrations are the highest.
Because of the regional dierentials in the densities of the
xed charges within the disc, the two vascular sources for disc
nutrition vary in their signicance in the supply of certain
solutes. With respect to the small uncharged particles, there is
little dierence in the transport potential of either the peripheral or the endplate vascular routes, but because of the greater
collective negative charge within the central substances of the
disc (from proteoglycans), the interface vasculature is a greater
source of cationic solutes, whereas the anions would gain
easier access through the peripheral vessels.
e eect of uid “pumping” under changes in the load
applied to the disc is minimal with respect to the transport of
small solutes because the matrix has a low hydraulic permeability relative to their higher rates of diusion. With regard
to the larger solutes, however, the pumping may have a more
substantial eect.
Metabolic turnover, as indicated by proteoglycan synthesis
in discs in dogs, is variable according to age within the range
of 2 to 3 years. It is roughly equivalent to that of articular
cartilage. e central disc tissues have a low oxygen tension
and a high concentration of lactic acid, indicating that the
inner disc cell respiration is primarily anaerobic. Because this
type of respiration is heavily dependent on glycolytic energy
requirements, the interface vasculature must deliver the
needed glucose to maintain the central disc cell viability.
Because this interface exchange is precariously dependent
on the integrity of the ne vasculature subjacent to the cartilaginous endplate, any change from the optimal state occasioned by age-dependent vagaries in the intrinsic vertebral
SECTION
I
36 BASIC SCIENCE
Lumb
PCB
vasculature may partly explain the marked predisposition to
degenerative changes characteristic of the aging disc. Calcication and sclerosis of the endplate decrease permeability and
contact area with the disc space, further contributing to disc
degeneration. MRI studies following the movement of contrast
medium into the disc have demonstrated a decreased ability
of a degenerated disc to accept solutes, both in speed and
quantity.
69
Blood Supply of the Vertebral Column
VA
&
DC
CC
IC
Cerv
Thor
e descriptions and terminology of the nutritional vessels
of the vertebrae vary considerably in anatomy texts. In general,
the texts illustrate and discuss the vascularity of a typical thoracic or lumbar vertebra, with a lack of agreement on such
basic issues as to whether the vertebral body does73 or does
not74 receive an anterior supply. In addition, discussions of
the vascularization of the atypical (craniocervical, cervical,
and sacral) vertebral regions are either supercial or entirely
lacking. Much of the information presented here is the result
of a de novo investigation by Parke and colleagues,77 and the
terminology ascribed to the vessels is derived from a selection
of what seem to be the most descriptive names previously used
in the literature.
75-77
Despite the fact that regional variations may at rst seem
to thwart the perception of a common pattern of vertebral
vascularization, the homologous origin of all vertebral elements nevertheless provides a certain constancy. From a
segmental artery, or its regional equivalent, each vertebra
receives several sets of nutritional vessels, which consist of
anterior central, posterior central, prelaminar, and postlaminar branches. e rst and last of these are derived
from vessels external to the vertebral column, whereas the
posterior central and prelaminar branches are derived from
spinal branches that enter the intervertebral foramina and
supply the neural, meningeal, and epidural tissues as well. In
the mid-spinal region, the internal arteries (i.e., the posterior
central and prelaminar branches) provide the greater part of
the blood supply to the body and vertebral arch, but reciprocal arrangements may occur, particularly in the cervical
region.
is general pattern of the vasculature is best shown in the
area between the second thoracic and h lumbar vertebrae,
where the segments are associated with paired arteries that
arise directly from the aorta (Fig. 2.22). Typically, each segmental artery leaves the posterior surface of the aorta and
follows a dorsolateral course around the middle of the vertebral
body. Near the transverse processes, it divides into a lateral
(intercostal or lumbar) and a dorsal branch. e dorsal branch
runs lateral to the intervertebral foramen and the articular
processes as it continues backward between the transverse
processes eventually to reach the spinal muscles. Because the
segmental artery is closely applied to the anterolateral surface
of the body, its rst spinal derivatives are two or more anterior
central branches that directly penetrate the cortical bone of
the body and that may be traced radiologically into the spongiosa (Figs. 2.23 and 2.24). e same region of the segmental
LUA
IS
&
MS
Sacr
AB
FIG. 2.22 (A) Anteroposterior and (B) lateral radiographs of spine of an
8-month fetus injected with nely divided barium sulfate. Traditional
regional subdivisions of the spine are indicated on the left, and regional
arteries that provide the segmental branches to the individual vertebrae are
shown on the right. The upper cervical region is supplied by vertebral and
deep cervical arteries (VA & DC), the lower cervical and upper two thoracic
segments are supplied by the costocervical trunk (CC), and the remaining
thoracic vertebrae receive intercostal vessels (IC). The lumbar arteries (LUA)
supply their regional vertebrae, and the sacral segments are provided with
branches from lateral sacral (LS) and middle sacral (MS) arteries.
DB
T6
IA
FIG. 2.23 Ventral radiograph of section through T6 of a specimen from a
6-year-old child injected with barium sulfate. The intercostal arteries (IA)
give rise to dorsal branches (DB) that provide spinal branches to the
vertebral canal and posterior branches to the arch and dorsal musculature.
The posterior central branches (PCB) are well shown as they send vessels
into the vertebral body. Fine anterior central and anterior laminar and
posterior laminar vessels can be seen. Note the neurocentral synchondrosis.
artery also supplies longitudinal arteries to the anterior longitudinal ligament (Fig. 2.25).
Aer the segmental artery divides into its dorsal and
lateral branches, the dorsal component passes lateral to the
intervertebral foramen, where it gives o the spinal branch
that provides the major vascularity to the bone and contents
of the vertebral canal. is branch may enter the foramen as a
PLB
PCB
ACB
LA
ALB
Chapter 2 Applied Anatomy of the Spine 37
SECTION
T10
LB
NB
L2
FIG. 2.24 Vertical radiograph of section through lumbar vertebra of a
6-year-old child. The vascularity of the lumbar vertebra may be regarded as
the archetypal pattern from which other regions evolved variations. The
segmental lumbar artery (LA) gives rise to numerous anterior central
branches that penetrate the cortical bone of the body. The spinal branch
(SB) sends prominent posterior central branches to the dorsum of the body,
whereas the dorsal branch (DB) supplies the anterior (ALB) and posterior
(PLB) laminar branches. Neural branches (NB) follow the nerve roots to the
cord. In this section, the arteria radicularis magna is seen as a neural branch
on the right side. ACB, anterior central branches; LB, lumbar branches; PCB,
posterior central branches.
T10
PCB
NB
L2
FIG. 2.25 Lateral view of lumbar vertebra shown in Fig. 2.24. Longitudinal
anastomoses of posterior central branches (PCB) can be appreciated, and
the disposition of neural branches (NB) is claried. The lumbar arteries also
supply small longitudinal branches to the anterior longitudinal ligament.
DB
SB
single vessel, or it may arise from the dorsal segmental branch
as numerous independent rami. In either case, it ultimately
divides into a triad of posterior central, prelaminar, and
intermediate neural branches. e posterior central branch
passes over the dorsolateral surface of the intervertebral disc
and divides into a caudal and a cranial branch, which supply
the two adjacent vertebral bodies.
PCB
ARM
L2
FIG. 2.26 Anteroposterior arteriogram of lower thoracic and upper lumbar
vertebrae in a 6-year-old child. The interlocking anastomotic pattern formed
by the posterior central branches (PCB) and the manner in which four
branches converge over the center of the dorsum of the body of each
vertebra are well shown. The arteria radicularis magna (ARM), which forms a
major contribution to the anterior spinal artery of the cord, can be seen
arising at L2.
Coursing in the same plane as the posterior longitudinal
ligament, these branches vascularize the ligament and the
related dura before entering the large concavity in the central
dorsal surface of the vertebral body. e dorsum of each
vertebral body is supplied by four arteries derived from two
intervertebral levels. As these vessels tend to converge toward
the dorsal central concavity, where they are cross-connected
with their bilateral counterparts, their connections with other
vertebral levels give the appearance of a series of rhomboid
anastomotic loops (Fig. 2.26) that illustrate the extent of collateral supply to a single vertebra.
e prelaminar branch of the spinal artery follows the
inner surface of the vertebral arch, giving ne penetrating
nutrient branches to the laminae and ligamenta ava, while
also supplying the regional epidural and dorsal tissue. e
neural branches that enter the intervertebral foramen with the
above-described vessels supply the pia-arachnoid complex
and the spinal cord itself. In the fetus and the adult, the neural
or radicular branches are not segmentally uniform in their size
or occurrence. Although all spinal nerves receive ne twigs to
their ganglia and roots, the major contributions to the cord
are found at irregular intervals. Several larger radicular arteries may be discerned in the cervical and upper thoracic
regions, but the largest, the arteria radicularis magna (artery
of Adamkiewicz76), is an asymmetrical contribution from one
of the upper lumbar, or lower thoracic, segmental arteries. It
travels obliquely upward with a ventral spinal root to join the
anterior spinal artery in the region of the conus medullaris.
Radicular contributions to the dorsal spinal plexus may usually
be distinguished by their more tortuous course (see Figs. 2.25
and 2.26).
I
38 BASIC SCIENCE
Aer the dorsal branch of the segmental artery has provided the vessels to the intervertebral foramen, it passes
between the transverse processes, where it gives o a ne spray
of articular branches to the joint capsule of the articular
processes. Immediately distal to this point, it divides into
dorsal and medial branches; the larger, dorsal branch ramies
in the greater muscle mass of the erector spinae, whereas the
medial branch follows the external contours of the lamina and
the spinous process. is postlaminar artery supplies the
musculature immediately overlying the lamina and sends ne
nutrient branches into the bone. e largest of these branches
penetrates the lamina through a nutrient foramen located just
dorsomedial to the articular capsule.
Regional Variations in Spinal Vasculature
Only vertebrae that are related to the aorta have access to
direct segmental branches. e cervical, upper thoracic, and
sacral regions have dierent patterns in their segmental supply
that aect to various extents the arrangements of the ner
vessels. In an arteriogram of the entire fetal spine (see Fig.
2.22), it can be seen that the greater part of the cervical region
is supplied by the vertebral arteries and the deep cervical
arteries. An intermediate area that usually includes the lower
two cervical and upper two thoracic vertebrae is supplied by
costocervical branches of the subclavian artery that are of
variable pattern and oen bilaterally dissimilar. From T2 to
L3, the typical segmental arrangement prevails, but in the
sacral area lateral sacral branches of the hypogastric artery and
middle sacral branches assume the function of supporting the
nutritional vasculature to the vertebral elements.
Cervical Region
C1
A
1
10
C2
C3
11
1
6
2
3
4
5
6
7
8
9
10
3
5
e general patterns of the arterial supply with respect to the
typical cervical vertebrae are schematically represented in
Figs. 2.27A and 2.28.77 e vertebral arteries represent a lateral
longitudinal fusion of the original segmental vessels and
provide a ventrally coursing anterior central artery and a
medially directed posterior central artery to each subaxial
vertebral element. e anterior spinal plexus is best developed
in the cervical region, where it exhibits a rectangular mesh of
vessels in which the transverse members (anterior central
arteries) run along the upper ventral edges of their respective
intervertebral discs. e conspicuousness of this plexus reects
the fact that it also serves the cervical prevertebral musculature. e thyrocervical and costocervical trunks assist in the
lower cervical region, and the upper cervical part of the plexus
receives contributions from the ascending pharyngeal arteries
(Fig. 2.29).
Atlantoaxial Complex
With their complex phyletic and developmental history, the
components of the atlantoaxial articulation display the most
atypical vascular pattern of all the vertebrae. Although the
odontoid process represents the denitive centrum of the rst
11
B
FIG. 2.27 (A) Schema of arterial supply to bodies of the upper cervical
vertebrae and the odontoid process. Numerical designations apply to the
same structures in (B). 1, Hypoglossal canal passing meningeal artery.
2, Occipital artery. 3, Apical arcade of odontoid process. 4, Ascending
pharyngeal artery giving collateral branch beneath anterior arch of atlas.
5, Posterior ascending artery. 6, Anterior ascending artery. 7, Precentral and
postcentral arteries to typical cervical vertebral body. 8, Anterior spinal
plexus. 9, Medullary branch of vertebral artery; radicular, prelaminar, and
meningeal branches are also found at each level. 10, Collateral to ascending
pharyngeal artery passing rostral to anterior arch of atlas. 11, Left vertebral
artery.
cervical vertebra, it develops and remains as a projecting
process of the axis that is almost completely isolated from the
rest of the atlas by synovial joint cavities. Its xed position
relative to the rotation of the atlas and the adjacent sections
of the vertebral arteries prevents formation of major vascularization by direct branches at its corresponding segmental level.
Chapter 2 Applied Anatomy of the Spine 39
PLB
SB
C4
ACB
FIG. 2.28 Vertical radiograph of section through fourth cervical vertebra of
a 6-year-old child, showing vascularity. The deep cervical artery (DC)
provides the posterior laminar branches (PLB). Vertebral arteries show
numerous anastomoses with other cervical arteries and send spinal
branches (SB) that form posterior central branches (PCB) of the body and
anterior lamina branches of the arch. Anterior central branches (ACB) may
arise independently from the vertebral arteries (VA).
T1
FIG. 2.29 Arteriogram of cervical and upper thoracic regions of the
6-year-old spine seen in Figs. 2.23 and 2.24. The vertebral artery ( VA) and
deep cervical branch (DC) of the costocervical trunk (CC) supply segmental
branches to each vertebra. The costocervical artery also typically supplies T1
and T2, but in this case T2 receives a high intercostal (IC) branch on the left
side.
DC
PCB
VA
VA
DC
CC
IC
One might assume that the nutrition of the dens would
easily be accomplished by interosseous vessels derived from
the spongiosa within the supporting body of the axis. It is
axiomatic, however, that the vascular patterns of bones were
developmentally established to supply the original ossication
centers within the nonvascular cartilage matrices, and despite
the eventual obliteration of the separating cartilage, the original
patterns of vascularity generally prevail throughout life. e
transient cartilaginous plate, which represents an incipient
intervertebral disc between the atlas and axis, does not calcify
until the latter half of the rst decade and eectively prevents
the development of any signicant vascular communication
between the axis centrum and the odontoid process. Occasionally, noncalcied remnants of this plate may persist in
adults; although there may be a stable union between the two
elements, a radiolucent area may suggest a fracture nonunion
or a “false” os odontoideum.
In light of the foregoing facts, it was not unexpected that
the investigations of Schi and Parke78 revealed that the
odontoid process was supplied primarily by pairs of anterior
and posterior central branches that coursed upward from the
surfaces of the body of the axis and were derived from the
vertebral arteries at the level of the foramen of the third cervical nerve. e posterior ascending arteries are the larger
members of these two sets of vessels and usually arise independently from the posteromedial sides of their respective
vertebral arteries. e individual artery enters the vertebral
canal through the foramen between the second and third
vertebrae and trifurcates on the dorsum of the axis body. e
typical posterior central perforators course medially passing
deep to the posterior longitudinal ligament (called the tecto-
rial membrane in the craniocervical region) to penetrate into
the spongiosa of the axis. A small descending branch anastomoses distally with vessels of the next lower segment.
e major part of the posterior ascending artery crosses the
dorsal surface of the transverse ligament of the atlas about
1.5 mm lateral to the neck of the odontoid process (see Fig.
2.27). Dorsal to the alar ligament, it sends an anterior anasto-
motic branch over the cranial edge of this ligament to form
collateral connections with the anterior ascending artery. e
posterior ascending artery continues on a medial course to
meet its opposite counterpart and forms the apical arcade that
arches over the apex of the odontoid process.
e smaller anterior ascending arteries arise from the
anteromedial aspect of the vertebral arteries and pass to the
ventral surface of the axis body. Fine medial branches send
perforators into the substance of the vertebral body and meet
in a median anastomosis typical of the anterior central
branches of the lower cervical region. e rostral continuance
of the anterior ascending arteries brings them dorsal to the
anterior arch of the atlas. Here each artery sends numerous
ne perforators into the anterolateral surfaces of the neck of
the odontoid process and terminates in a spray of vessels that
supply the synovial capsule of the median atlantoaxial joint.
Fine branches from the anterior and posterior ascending
arteries also assist in the nutrition of the syndesmotic relations
of the atlantoaxial and craniovertebral articulations. e main
blood supply to the atlanto-occipital joint is provided by a
complex of vessels derived from the vertebral and occipital
arteries.
Collateral vessels pass over and under the anterior arch of
the atlas to anastomose with the apical arcade and ascending
arteries.79 ese are derived from some component of the
external carotid system. ese vessels are branches of the
ascending pharyngeal artery, which has a nearly ubiquitous
distribution in the upper pharyngeal region and sends a
branch along the inner aspect of the carotid sheath that, on
SECTION
I
40 BASIC SCIENCE
reaching the base of the skull, becomes recurrent and descends
deep to the prevertebral fascia to supply the upper prevertebral
cervical muscles and anastomose with the anterior spinal
plexus. Numerous small-bore vessels that descend from the
rim of the foramen magnum to anastomose with the apical
arcade are derivatives of a meningeal branch of the occipital
artery that enters the skull through the hypoglossal canal (see
Fig. 2.27). Its descending branches supply the periforaminal
dura, the tectorial membrane and alar and apical ligaments,
and the ne anastomoses to the arcade.
Sacroiliolumbar Arterial System
From the second thoracic vertebra to the fourth lumbar vertebra, the spine and its regionally related structures are supplied by pairs of segmental arteries that are direct branches of
the aorta. Because the aorta terminates in a bifurcation ventral
to the fourth lumbar vertebral body, the vertebrae and the
associated tissues caudad to this point rely on an arterial
complex derived mostly from the internal iliac (hypogastric)
arteries. is “sacroiliolumbar system” consists of contributions from the fourth lumbar artery, the iliolumbar artery, and
the middle and lateral sacral arteries.
With the increasing use of percutaneous approaches to the
lower lumbar discs, this infra-aortic system of vessels has
assumed some surgical signicance, particularly because, in
contrast to the conventional segmental supply to the more
superior vertebrae, its major components are longitudinally
related to the dorsolateral surfaces of the discs most frequently
involved in these procedures.
Fourth Lumbar Arteries
e peculiarities of the sacroiliolumbar system of arteries may
best be understood if compared with the pattern of distribution of the typical aortic segmental branches. e ramications
of the fourth lumbar arteries were selected for this purpose
because they not only exemplify the conventional segmental
distribution, but oen are involved in the nutrition of the next
lower segments by variable contributions to the iliolumbar
vessels. ese vessels oen may be twice the caliber of their
more cephalad homologues because of a greater muscular and
intersegmental distribution.
As depicted in Figs. 2.30 and 2.31, the distribution of the
major ramications is similar to that of the thoracic segmental
vessels, with the exception of additional branches that supply
the psoas and quadratus lumborum muscles. e lateral
muscular branch (equivalent of the thoracic intercostals) may
be quite large at the fourth lumbar level, where, in contrast to
the other lumbar laterals, it passes anterior, rather than posterior, to the quadratus lumborum. It then continues to supply
the lower posterolateral abdominal wall as it courses superior
to the crest of the ilium. As can be seen in Fig. 2.30, it may be
equivalent in size to the iliac branch of the iliolumbar artery.
Its position superior to the crest indicates that it is more likely
to be encountered by percutaneous instrumentation than the
latter vessel.
79
1
18
2
17
16
15
14
13
12
11
FIG. 2.30 Distribution and major variations of sacroiliolumbar system of
arteries that supply the vertebrae and their associated structures inferior to
the fourth lumbar vertebra. These patterns of the vessels were derived from
radiographs of perinatal specimens and dissections of adults and drawn
against a tracing of the lumbosacral region taken from a left anterior
oblique radiograph of a man. The aorta lies to the left of center as it
approaches the bifurcation ventral to the fourth lumbar vertebra. This
schema shows the more frequent arrangement of the sacroiliolumbar
system on the right side of the illustration, where the iliolumbar vessel
(7) has a single origin from the dorsum of the posterior division of the
(removed) internal iliac artery. The left side shows the common variation
where the iliac artery and the lumbar artery (14) are derived separately.
The middle sacral artery (16) is in its typical position, and the anastomotic
contribution from the fourth lumbar artery (4) shows its most frequent form.
1, Aorta. 2, Musculocutaneous branch of third lumbar artery. 3, Muscular
branch to posterior abdominal wall. 4, Anastomotic contribution of fourth
lumbar artery to sacroiliolumbar system. 5, Lumbar branch of iliolumbar
artery. 6, Iliac branch of iliolumbar artery. 7, Iliolumbar artery. 8, Left lateral
sacral artery. 9, Posterior division of internal iliac artery. 10, Superior and
inferior gluteal arteries. 11, External iliac artery. 12, Anterior (visceral) division
of internal iliac artery. 13, Internal iliac artery. 14, Variant origin of lumbar
branch of iliolumbar artery from lateral sacral artery. 15, Common iliac artery.
16, Middle sacral artery. 17, Left fourth lumbar segmental artery. 18, Left
second lumbar segmental artery.
10
9
3
4
5
6
7
8
e dorsal musculocutaneous branch of the fourth lumbar
artery is equivalent in distribution to other thoracolumbar
segmental arteries. It usually has a medial branch that supplies
the external aspects of the facet joints and neural arch components and the transversospinal group of muscles and a
lateral branch to the transversocostal group of the erector
spinae. e vertebromedullary (spinal) branches of the fourth
lumbar artery are also similar to those of other segmental
arteries (see Fig. 2.24). ey are a group of vessels of variable
caliber that may generally be sorted into three divisions: (1)
the ventral periosteal and osseous branches that supply the
posterior longitudinal ligament, the periosteum, and the
Chapter 2 Applied Anatomy of the Spine 41
ASA
L2
L5
DRA
FIG. 2.31 Anteroposterior radiograph of spine from a perinatal cadaver
injected with barium sulfate. The aorta and common iliac vessels have been
removed before radiography. This specimen shows considerable variation
between the two sides of the sacroiliolumbar system. On the right, a small
lumbar branch and a descending branch from the fourth lumbar artery
(4LA) enter the L5-S1 intervertebral foramen. On the left, there is no lumbar
branch, and a descending branch of the L4 artery supplies all of the vessels
to the L5-S1 foramen. The middle sacral artery is also absent, and other
branches of the system supply its domain. The radicular branches of the
vertebromedullary vessels supply the distal radicular arteries (DRA) and
reveal the positions of the lower ends of the lumbosacral nerve roots.
AMM, Arteria medullaris magna; ASA, anterior spinal artery; IIA, internal iliac
artery; ILA, iliolumbar artery.
AMM
4LA
ILA
IIA
A major peculiarity of the fourth lumbar artery is its proclivity toward providing a relatively large, caudally directed
intersegmental branch that arises near the level of the intervertebral foramen and becomes reciprocally involved with the
lumbar branch of the iliolumbar artery. When this latter vessel
is small or absent, the descending branch of the fourth lumbar
artery may be suciently large to provide the predominant
nutritional system to two vertebral segments caudad to its
origin (see Figs. 2.30 and 2.31).
Iliolumbar Artery
As opposed to the mostly visceral distribution of the anterior
division of the internal iliac (hypogastric) artery, the posterior
division is essentially a somatic artery giving rise to gluteal,
iliolumbar, and lateral sacral branches. e iliolumbar artery
most frequently is the rst branch of this dorsal division. It is
directed dorsosuperiorly, passing close to the ventrolateral
surface of the rst sacral vertebral segment. It courses superiorly, dorsal to the obturator nerve and ventral to the lumbosacral trunk. Lateral to the inferior margin of the L5-S1 disc,
the iliolumbar artery usually divides into a laterally directed
iliac artery and an ascending lumbar artery. e rst of these
crosses the sacroiliac joint to reach the iliac fossa of the pelvis,
where it courses inferior to the iliac crest and usually deep to
the muscle to provide muscular branches to the iliac muscle
and articular twigs to the acetabulum and eventually anastomoses with the deep circumex branch of the femoral artery.
e lumbar artery ascends posterolateral to the L5-S1 disc,
still between the obturator nerve and the lumbosacral trunk,
to provide the vertebromedullary vessels to the L5-S1 intervertebral foramen (Fig. 2.32; also see Figs. 2.30 and 2.31). In
most cases, a branch of this vessel continues rostrally to
anastomose with the descending branch of the fourth lumbar
artery. e lumbar branch of the iliolumbar artery provides
regional branches to the psoas and quadratus lumborum
muscles.
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cancellous bone of the vertebral body; (2) the radiculomedullary division that provides the irregularly located
medullary arteries of the cord and the constant distal radicular
arteries to all the roots; and (3) the dorsal division that supplies ne articular branches to the deep aspects of the facet
joints and the periosteum of the deep surfaces of the laminae
and their associated ligaments. e rst two divisions usually
originate from a common branch of the segmental artery and
enter the intervertebral foramen just rostral to their respective
vertebral pedicle and ventral to the dorsal root ganglion,
whereas the dorsal division arises from the musculocutaneous
branch of the segmental artery and enters the foramen dorsal
to the nerve components. All the vertebromedullary branches
may provide ne branches to the spinal dura.
e aortic segmental arteries course around their respective vertebral body at its narrowest circumference and are
positioned almost equidistant between the adjacent discs.
ese parts of the arterial distribution are relatively safe from
instrumentation properly positioned to enter the discs.
Sacral Arteries
Lateral Sacral Arteries
Lateral sacral arteries usually form the second branch of the
dorsal division of the internal iliac arteries and course down
the pars lateralis on each side of the sacrum. Opposite the
sacral foramina, they give o medial branches that dorsally
enter the foramina. Aer providing the typical vertebromedullary derivatives, their dorsal muscular branches exit through
the dorsal sacral foramina to supply the sacral origins of the
erector spinae muscles.
Middle Sacral Artery
e middle sacral artery is an unpaired vessel that is the last
branch of the aorta, usually derived from its dorsal median
surface just above the carina of the bifurcation (Fig. 2.33; also
see Fig. 2.30). It descends down the ventral surface of the
anterior longitudinal ligament over the fourth and h lumbar
42 BASIC SCIENCE
MSAISA HA
L5
MSA
HA
S1
LS
PA
FIG. 2.32 Anteroposterior arteriogram of sacral region in a 7-year-old child.
The lateral sacral arteries (LS) can be seen coming from the hypogastric
vessels (HA). The middle sacral artery (MSA) is atypical in this specimen
because it stops at S1. Just anterior to the coccyx, the coccygeal bodies (CB)
are indicated as small knots of arteriovenous anastomoses. Pudendal
arteries (PA) are well injected.
SG
CB
bodies and down the ventral sacrum to terminate at the
sacrococcygeal junction in a vascular glomus (sacrococcygeal
body) in tail-less mammals or continues ventral to the coccygeal (caudal) vertebrae in tailed mammals as the caudal
artery. In humans, this is a variable vessel, being totally absent
in some cases or replaced by a branch of one of the lateral
sacral arteries. Where it is a signicant component of the
sacroiliolumbar system, its rst lateral branches on the ventral
surface of the h lumbar body may entirely replace this segment’s contributions from the iliolumbar or fourth lumbar
vessels and provide its osseous, muscular, and vertebromedullary requirements.
Where it is conspicuously present in the sacral region, the
middle sacral artery may also contribute a vertebromedullary
branch to each anterior sacral foramen. When it is absent,
these ventral sacral territories are provided with segmental
medial branches from the lateral sacral arteries.
Functional Signicance
e sacroiliolumbar system, despite its complexity and seemingly endless combinations of reciprocal substitutions, supplies
the lower lumbosacral elements of the spine and the inferior
half of the lumbosacral spinal nerve roots (cauda equina) and
the back musculature inferior to the L4 level. It is also a major
contributor to the vasa nervorum of the lumbosacral plexus.
e distal radicular arteries dene the positions of the lumbosacral roots (see Fig. 2.25). Although signicant medullary
branches to the spinal cord are seldom found below L4, they
do occur, and from the preceding descriptions it is obvious
why the ligation of both internal iliac arteries during radical
cystoprostatectomy can result in spinal cord ischemia.
80
S2
S3
FIG. 2.33 Radiograph of horizontal section through sacroiliac joint. The
natural curvature of the sacrum provided oblique sections through
segments 2, 3, and 4. The hypogastric artery (HA) gives o the lateral sacral
artery (LSA) that sends anastomotic branches to join the middle sacral
artery (MSA); from these, the sacral segments receive the penetrating
anterior central branches. The dorsal branches pass into the anterior sacral
foramina to provide posterior, central, neural, and prelaminar branches. The
dorsal branches leave through the posterior sacral foramina to supply the
muscles and posterior laminar branches.
S4
Venous System of the Vertebral Column
An external plexus and an internal plexus of veins are associated with the vertebral column. e distribution of the two
systems roughly coincides with the areas served by the external
and internal arterial supplies. e external venous plexus also
consists of an anterior and a posterior set of veins. e small
anterior external plexus is coextensive with the anterior central
arteries and receives tributaries that perforate the anterior and
lateral sides of the vertebral body.
e more extensive posterior external veins drain the
regions supplied by posterior (muscular and postlaminar)
branches of the segmental artery. e posterior external veins
form an essentially paired system, which lies in the two vertebrocostal grooves, but has cross anastomoses between the
spinous processes. It is a valveless venous complex that receives
the draining segmental tributaries of the internal veins through
the intervertebral foramina and communicates ultimately
with the lumbar and intercostal tributaries of the caval and
azygos system. e posterior external plexus becomes most
extensive in the posterior nuchal region, where it receives the
intraspinous tributaries via the vertebral veins and drains into
the deep cervical and jugular veins.