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Gilchrist et al • Anatomy of the Intervertebral Foramen For Evaluation Only. 372
Pain Physician, Volume 5, Number 4, pp 372-378
2002, American Society of Interventional Pain Physicians®
ISSN 1533-3159

Anatomical Review

Anatomy of the Intervertebral Foramen

Russel V. Gilchrist, DO*, Curtis W. Slipman, MD** and Sarjoo M. Bhagia, MD*

The intervertebral foramen serves as the doorway between that aids in preventing damage to this vital foraminal
the spinal canal and periphery. It lies between the pedicles structure. The presence of ligamentous structures within
of neighboring vertebrae at all levels in the spine. A number the foramen has been demonstrated by a number of recent
of categorization schemes have been attempted to describe studies. These ligaments serve a protective and
the boundaries of the intervertebral foramen. No uniform organizational role for the neurovascular structures of the
agreement has been made on which classification best foramen.
describes this area.
A thorough knowledge of the intervertebral foramen will
Studies of the nerve root canals have clearly noted allow the understanding of the pathological and
variations in the angle of take-off from the thecal sac, length degenerative changes that cause compression or injury to
of the nerve root, and placement of the dorsal root ganglion these foraminal structures.
from different lumbar levels. The nerve root canal receives
a dual blood supply from central and peripheral sources. Keywords: Intervertebral foramen, nerve root, canal,
The dorsal root ganglion also has a dual vascular supply ligaments

The intervertebral foramen transmits the spinal nerves, may serve a protective role in preventing injury to the
spinal arteries and veins, the recurrent meningeal nerves vasculature that pass through them.
and lymphatics (1). This foramen is unique in compari-
son to other foramens of the body due to its boundaries LIGAMENTS OF THE FORAMEN
consisting of two movable joints; the ventral interverte-
bral joint and the dorsal zygapophysial joint (2). The prox- Bourgery in 1832 was the first to report upon the presence
imity of these joints increased susceptibility of narrowing of ligaments passing across lumbar foramen (3). In the
from arthritic structural alterations. The foramen is es- 1940’s Larmon (4) and Magnuson (5) also noted foraminal
sentially a large osseous hole through which neurovascu- bands crossing the intervertebral foramen of the L5
lar structures pass. Within its boundaries is an intricate segment. Golub and Silverman (6) in 1969 were amongst
network of ligaments that divide the intervertebral fora- the first to report on the presence of ligamentous bands
men into multiple sub-compartments containing specific running across foramina of the lumbar spine at all lumbar
anatomic structures. It is conceivable these ligaments levels. They examined 10 cadaveric lumbar spines and
noted the inconsistent presence of band-like structures in
the foramen, most commonly occurring at the L1-2
From Penn Spine Research Group, University of Penn- foramen. They identified five major types of
sylvania Health System, Department of Rehabilitation transforaminal ligaments: superior corporotransverse,
Medicine, Philadelphia, Pennsylvania. *Dr. Gilchrist and inferior corporotransverse, superior transforaminal, mid-
Dr. Bhagia are fellows at the Penn Spine Center. **Dr. transforaminal, and inferior transforaminal. The superior
Slipman is the Director at Penn Spine Center, Associate corporotransverse ligament was the most frequently
Professor of the Department of Rehabilitation Medicine observed ligament. They postulated these bands were
and Chief of the Clinical Musculoskeletal Program. Ad- anomalous in origin and are a potential source of nerve
dress correspondence: Curtis W. Slipman, MD, 3400 root entrapment (6). Subsequent anatomic studies
Spruce Street, Philadelphia, PA 19104. E-mail: similarly documented the presence of foraminal ligaments
slipman@mail.med.upenn.edu (7-10). These studies indicated an increased presence of
ligaments in the fifth lumbar foramen. The results of these
Funding: No financial support was obtained in the prepa- studies created many discrepancies in the identification
ration of this manuscript. of these band-like structures as ligaments or fascial

372 Pain Physician Vol. 5, No. 4, 2002

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Copyright(C) by Foxit Software Company,2005-2007
Gilchrist et al • Anatomy of the Intervertebral Foramen For Evaluation Only. 373

condensations of overlying muscles. The last intraforaminal ligament noted was a strong,
transversely oriented band originating from the anterior
Golub and Silverman (6) hypothesized that the ligaments upper portion of the superior articular facet and attaching
were condensations in the fascia of overlying muscles to the postero-lateral surface of the vertebra above. The
adjacent to the lumbar foramen. They described the exiting spinal nerve was noted to lye directly overtop this
obliquely running bands as superior and inferior ligament in all specimens (1).
corporotransverse ligaments and transversely directed
bands as transforaminal ligaments. Macnab (7) in 1977 The external ligaments all had a common attachment to
categorized the bands as corporotransverse ligaments. the root of the transverse process. From this position the
Bachop and others (8-10) identified the ligaments in a bands ran in a superior, inferior, and transverse direction.
directional capacity only, primarily consisting of All bands were seen to insert into the vertebral bodies at
superopostero-laterally running bands. the same level and the level below. These three external
ligaments have also been called the superior, middle, and
A more recent anatomic study of twelve lumbar spines inferior corporotransverse ligaments. The position of these
investigated the presence of ligamentous structures in the ligaments creates multiple sub-compartments just external
foraminal canals of the first four lumbar levels (1). In to the foramen. A large central compartment was seen
contrast to earlier works, this study documented ligaments encasing the exiting ventral rami. Anterior and superior
at all foraminal levels. No muscular attachments to the to this central compartment are two smaller openings
ligaments were noted. The ligaments were much better through which the spinal artery, recurrent meningeal nerve
defined in the upper lumbar levels than lower levels. In and a small branch of the segmental artery travel. Inferior
fact, the upper lumbar foraminal ligaments had a more to the ventral rami foramen are typically two or more small
thick, rounded appearance in comparison to the lower compartments through which veins were seen traversing.
lumbar ligaments. In the posterior aspect of the external foramen exist
superior and inferior compartments. The superior
The ligaments were also found to exist in three different compartment contained the medial division of posterior
zones of the lumbar foramen; internal, intraforaminal, and primary ramus and branches of the lumbar artery and vein.
external zones. The internal ligaments were commonly The inferior tunnel transmitted the lateral division of the
found in the inferior aspect of the medial portion of the posterior ramus and branches of the segmental artery and
foramen. They were broad ligaments attaching to the veins (1).
posterolateral aspect of the intervertebral disc and anterior
surface of the superior articular facet. These attachments Kuofi et al (1) were able to establish the consistent
gave the ligaments an obliquely running course inferiorly presence of ligaments within the immediate region of the
and posterior. In its course the internal ligament bridges lumbar intervertebral foramen. Based on topographical
across the top of the superior vertebral notch, thereby mapping of the ligaments it was possible to conclude, in
converting it into a sub-compartment in the lower contradiction to earlier reports, that these ligaments are
foraminal canal. Veins were commonly noted to be running not responsible for entrapment of the spinal nerve resulting
through this sub-compartment (1). in radicular pain symptoms (6). In fact, the orientation of
the ligaments precludes compression of the nerve root
The intraforaminal ligaments ran in three typical during dynamic alteration of the foramen. The presence
distributions. The first type traveled from the root of the of thick bands forming the compartments through which
pedicle to the inferior border the same vertebral body. The the vascular supply travels also denotes a protective role
recurrent meningeal nerve and a branch of the spinal artery for these ligaments (1).
were observed within the compartment formed by this
ligament. A second distribution was the attachment to the BOUNDARIES OF THE NEURAL FORAMEN
angle between the posterior end of the pedicle and the
root of the transverse process extending to the postero- The boundaries of the lumbar foramen contain not only
lateral surface of the same vertebral body. These osseous structures, but also have ligamentous structures
attachments create an antero-superior compartment that aide in defining its borders. These foramens are also
through which a large branch of the segmental artery was unique because two joints form part of their boundaries.
observed to travel in all specimens examined (1). This arrangement allows the foramen to dynamically

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Gilchrist et al • Anatomy of the Intervertebral Foramen For Evaluation Only. 374

change its configuration according to movements of the the lumbar spine. An average measurement of 7mm was
trunk. Under normal conditions, these dynamic changes reported from the front to the back of the foramen.
are easily tolerated by the neurovascular structures that Measurement of the foraminal canal by three-dimensional
run through them without any compromise of the computed tomography (3DCT) has been attempted.
neurovascular components. The boundaries of the Unfortunately, comparative studies using cadaveric
intervertebral foramen have not been well defined. specimens have shown the 3DCT to be an unreliable study
Different authors have chosen separate and distinct to quantify the dimensions of the lumbar foramen (17).
classifications to describe the foramen (11-13). Crock Magnetic resonance imaging has also been performed in
(11) in 1981 has described the intervertebral foramen as a healthy subjects to measure normal values for the height
single sagittal slice through the narrowest portion of the of the intervertebral foramen (18). In one study, twenty
nerve root canal. Lee et al (12) in 1988 divided the male volunteers with no history of back pain or
foramen into three zones: lateral recess zone, midzone, radiculopathy underwent magnetic resonance imaging
and the exit zone. We have chosen a more generalized (MRI). The height of the intervertebral foramen was
anatomic approach here to allow for a comprehensive measured between the inferior margin of the pedicle of
description of the foraminal area. the upper vertebra and the superior margin of the pedicle
of the lower pedicle in two hundred and thirty-three levels,
When looking outward through the intervertebral foramen L1-L2 to L4-L5. The mean heights of the foramen were
from the spinal canal the foramen takes on the appearance reported as follows: L1-L2, 17.1+/- 2mm; L2-L3, 18.4+/
of an oval, round, or inverted teardrop-shaped window - 1.7mm; L3-L4, 18.1+/- 1.5mm; and L4-L5, 17.1 +/-
(14). The roof of the intervertebral foramen is the inferior 3.6mm (18). No comparison was made in that study to
aspect of the vertebral notch of the pedicle of the superior cadaveric or radiographic measurements. However, prior
vertebra (1), the ligamentum flavum at its outer free edge cadaveric studies have noted the lumbar foramens to have
(11), and posteriorly lays the pars interarticularis and the pedicle-to-pedicle heights varying from 11-19 mm (15,
zygapophysial joint. The floor of the nerve root canal is 16). The mean widths and heights of the pedicles of the
the superior vertebral notch of the pedicle of the inferior L1 through L5 vertebra were also measured. The mean
vertebra (1), postero-inferior margin of the superior pedicle widths were seen to increase from the L2 to the
vertebral body (11), the intervertebral disc, and the L5 level. The pedicle heights appeared to remain relatively
postero-superior margin of the inferior vertebral body. unchanged from level to level (18).
Multiple structures are involved in bounding the anterior
aspect of the foramen. They include the posterior aspect THE NERVE ROOT CANAL
of the adjacent vertebral bodies, the intervertebral disc,
lateral expansion of the posterior longitudinal ligament, The true anatomic nerve root canal initially arises from
and the anterior longitudinal venous sinus. Posteriorly, the lateral aspect of the dural sac and travels through the
the foramen is bounded by the superior and inferior neural lumbar foramen. At each level, anywhere from
articular process of the facet joint at the same level as the two to six anterior and posterior roots converge in the
foramen, and the lateral prolongation of the ligamentum thecal sac to form anterior and posterior roots (19).
flavum. The medial canal border contains the dural sleeve. Extensions of the dural sheath encase all nerve roots as
The lateral boundary is a fascial sheet and overlying psoas they depart from the thecal sac. In the lumbar spine the
muscle (1). A distal and proximal oval perforation is seen nerve roots regularly exit the thecal sac approximately one
in the fascia. The distal perforation houses the nerve root, segmental level above their respective foraminal canal
and the smaller proximal perforation regularly have blood (20). They take an oblique course downwards and laterally
vessels traversing through them (1). The height of the toward the intervertebral foramen. This oblique angle has
foramen is dependent upon the vertical height of the modest differences based upon the lumbar level in
corresponding intervertebral disc. With aging there is a question. In the upper lumbar nerve roots their orientation
natural tendency toward disc degeneration and loss of disc is more at a right angle to the dural sac than the distal
height. This decrease in disc height has direct anatomic nerve roots (21). This right angle makes the intraspinal
consequences to the area of the foramen and resultant portions of the upper nerve roots very short. In fact, in
availability of space for neurovascular structures to pass. the upper lumbar area the thecal sac lies against the medial
Direct cadaveric measurements of lumbar foraminal wall of the pedicles, therefore the nerve roots exit
heights have varied from 11-19 mm (15, 16). Magnusson immediately into the intervertebral foramen (11). Distal
(16) also reported on foraminal width measurements of to the L3 vertebral body level the dural sac is seen to taper

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Gilchrist et al • Anatomy of the Intervertebral Foramen 375

progressively. The distal nerve roots are seen to exit from reaches the foraminal outlet it curves around antero-
the thecal sac at more oblique angles after the L3 level. laterally the base of the subjacent pedicle and transverse
Bose and Balasubramaniam (22), in 1984, demonstrated process. Around this exit zone of the foramen the spinal
a gradual decrease in the angle of inclination through the nerve divides into primary anterior and posterior rami.
L1-2 through L5-S1 levels. This finding was later disputed Just outside the foramen the primary rami run between
by Cohen et al (23) who noted no change in the angle of the deep layers of the psoas muscle and the vertebral
inclination from the exiting nerve roots in the L1 through column (19). Within the psoas muscle the lumbar nerves
L5 nerve roots, but did note a significant drop-off in the coalesce into trunks that run down vertically along the
angle of inclination at the S1 level. A more recent surface of the junctional area between the body and the
morphometric study using MRI to analyze angle of pedicle of the lumbar spine (29).
inclination from exiting nerve roots noted the L1 nerve to
have a greater angle of departure from the thecal sac than The nerve roots have two areas of fixation to surrounding
all other lumbar nerve roots. In addition, the S1 root had structures. The first area of fixation occurs at the neck of
a significantly smaller take-off angle than the other lumbar the nerve root sheath as it exits the dural sac. The fibrous
roots (18). All previous studies have demonstrated that attachments were located both ventrally and dorsally on
the intra spinal course of the lumbosacral nerve roots is the neck of the nerve root sheath, with both bands attaching
successively longer for each caudal level encountered (18, to the periosteum of the subjacent pedicle. The second
22, 23). Epidural fat surrounds each nerve root throughout area of fixation occurred at the lateral aspect of the
their course to the intervertebral foramen (11). Just prior foramen. These fibrous expansions are attached to both
to its entrance to the neural foramen the lumbar roots fit pedicles superiorly and inferiorly to the nerve root (30).
into an osseous groove at the medial base of the pedicle Avulsions are commonly noted to occur at these two areas
(19). This groove may be more pronounced at the level of fixation. The average rupture force in one study for
of the fifth lumbar vertebral foramen secondary to a more roots of the L1-L4 level was 7.25 kg, 13 kg for the L5,
trefoil shape of the spinal canal (24). The term lateral and 11.5 for the S1 nerve root (30).
recess has been used to describe this well-defined area.
Verbiest (25) in 1954 was first to note that narrowing of VASCULAR SUPPLY OF THE NERVE ROOT
this groove, or lateral recess stenosis, could cause radicular AND DRG
leg pain in patients. As the nerve root slides under the
medial edge of the pedicle it takes an inferior and oblique The vascular supply to the lumbar nerve roots will be briefly
direction away from the pedicle (19). At this point the discussed here. For a more detailed description the reader
nerve roots are located within the neural foramen, and they is referred to a prior publication by these authors (31). A
commonly combine to form the spinal nerve. Just prior to more detailed look at the vascular supply of the dorsal root
the formation of the spinal nerve a small enlargement of ganglion (DRG) will be presented here. Blood supply to
the dorsal root is noted. This enlargement is called the the lumbar spinal nerve roots occurs proximally from
dorsal root ganglion (DRG), which contains the cell bodies branches of the longitudinal vessels of the conus
of sensory neurons. The DRG location in perspective to medullaris. These vessels only travel a few centimeters
the foramen can be quite variable. However, there are along the rootlets before terminating (32). The posterior
some general trends that are consistently reproduced in (or dorsal) nerve roots receive their vascular supply via
anatomical studies. The majority of DRG’s in the lumbar the dorsolateral longitudinal spinal arterial system. This
levels are located within the anatomic boundaries of the system is an extension of the vasa corona that forms a
intervertebral foramen (18, 22, 23). Most commonly, the plexiform, interrupted network of vessels that is always in
position of DRG within the foramen is located directly close proximity to the posterior rootlets. The ventral roots
beneath the foramen (22). Only at the S1 level is this rule each receive a direct branch from the nearest vasa corona
not applicable. Studies have reported that the S1 DRG (32).
exist within the spinal canal approximately 80% of the
time (18, 26). This intraspinal placement places the S1 The remaining proximal portions of the nerve roots receive
DRG at increased risk of injury from disc herniations or blood supply via the dorsal and ventral proximal radicular
degenerative changes of the L5-S1 intervertebral disc (18). arteries (33). These blood vessels are derived from the
In the foramen, nerve roots typically occupy approximately dorsal longitudinal spinal artery and accessory
30% of the available foraminal area (27), but numbers as anterolateral artery, respectively. The proximal radicular
high as 50% have been reported (28). As the spinal nerve

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Gilchrist et al • Anatomy of the Intervertebral Foramen 376

arteries enter the nerve root and follow the length of the photomicrographs of the intraganglionic vasculature.
nerve distally to anastomose with the distal radicular artery Their findings were later supported by Day (37) in 1964.
(33). Their entrance into the proximal nerve roots occurs However, Day was only able to publish photographs of a
slightly distal to the roots exit from the spinal cord. This limited number of lumbosacral DRG’s. In 1973, Somogyi
delay in contact is most likely due to proximal regions of et al (38) also demonstrated similar drawings to Bergmann
the nerve root already receiving vascular supply from the and Alexander of the T6 and L4 DRG’s.
dorsal and anterior longitudinal vessels. As the proximal
radicular artery enters the nerve root it follows along with More recently, Parke and Whalen (39) in 2002 examined
one of the main fascicular bundles. A number of collateral the arterial supply of three human perinatal cadavers, one
branches occur directly off of the main radicular artery. adult rabbit, and venous injections of vertebral segmental
These smaller branches tend to form parallel courses along tissues of two adult human anatomic cadavers. After
other nerve root fascicles (32). Precapillary branching injection of India Ink into the perinatal cadavers,
from these long running parallel vessels give supply to transilluminated microphotographs were taken of the
the subdivisions of fiber bundles not directly overlying DRGs. These photographs supported previous findings
the radicular arteries. These branches are unique in that of vascular supply throughout all levels tested (36-38).
they are coiled shaped. Coiling of these precapillary
vessels has been noted to endow these vessels with a The vascular supply to the DRG consists of a two primary
resistance to compression during flexion and extension plexuses, one superficial and one deep into the substance
moments of the spine, thereby preventing ischemia to nerve of the DRG. The two plexuses are connected via fine,
root fascicles (32). centripetally running, anastomotic channels. Both internal
and peripheral plexuses of arteries are derived from distal
The distal radicular artery branches from the lumbar artery and proximal polar arteries. Epidural branches of the
at the level of the intervertebral foramen. It then divides intersegmental vertebral arteries feed these polar arteries.
into two branches, one entering each dorsal and ventral The intersegmental radicular arteries are branches from
root. At this point it travels both proximal and distally the larger and epi-spinal segmental arteries distally, and
along the length of the nerve roots giving it blood supply from the anterior and posterior spinal arteries centrally.
(34). As the distal radicular artery travels proximally it This basic vascular pattern is seen throughout all vertebral
forms numerous arteriovenous anastomoses with levels, and also supported by previous works (36-39).
neighboring veins (35). Nutrient arteries have also been noted to directly branch
off the spinal segmental artery and give blood supply to
There are two venous systems involved in drainage of the the DRG (40). In comparison to the nerve root, the DRG
lumbar nerve roots, which are divided into proximal and has more abundant intrinsic vascular supply. A reason
distal radicular venous systems. The distal radicular veins for this increased vascular supply is most likely due to
drain into the lumbar vein at the level of the intervertebral cellular elements that exist within the DRG (40). A dense
foramen. The proximal radicular veins drain into the spinal network of continuous and fenestrated capillaries is seen
cord venous plexuses (34). They have been documented surrounding the parenchymal cells of the DRG (40). A
to return via the vasa corona and then pass proximally in superficial periganglionic venous plexus is noted
the anterior and posterior longitudinal veins of the cord surrounding the DRG. Multiple anastomotic channels are
(34). It has been demonstrated that the veins are similar to seen connecting the parenchyma of the DRG to the
nerve root arteries in they have a variable location and periganglionic plexus (39).
number in the nerve root (33). The major veins of the
nerve root demonstrate a more similar morphology and The importance of the DRG is evidenced by the intricate
arrangement to those of the central nervous system. Their arterial supply that surrounds and invests this structure.
walls are comparatively thin and lack a tunica media. In In addition, blood flow volume to the DRG has been
contrast to peripheral nerves, nerve roots tend to have measured. Hachiya et al (41, 42) in 1989-1990 examined
fewer numbers of veins and tend toward a more spiraling the blood flow volume of the DRG in the dog. In their
course through the deeper portions of the nerve root (32). studies an electrochemically generated hydrogen washout
The vascular supply of the DRG was first investigated by method was used with a tissue blood flow meter. They
Bergmann and Alexander (36) in 1941. They used pen found the absolute blood flow volume of the dorsal nerve
and ink drawings to depict the macro-vascular flow to the root to be an average of 26.9 ml/min/100g, whereas the
DRG. Microscopic cross-sections were illustrated by blood flow of the DRG of the dog was an average of 56.1

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Gilchrist et al • Anatomy of the Intervertebral Foramen 377

ml/min/100g. These studies demonstrated about twice as

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