19 -Normal Anatomy and Congenital ANOMALIES of THE SPINE and SPINAL CORD .

It can be divided into anterior elements vertebral bodies and intervertebral disks, posterior elements pedicles, articular pillars, and facet joints, ligaments, soft tissues e.g., epidur

Normal Anatomy and Congenital Anomalies of the Spine and Spinal

Open Spinal Dysraphism

Occult Spinal Dysraphism

Anomalies of Abnormal Canalization and

Retrogres-sive Differentiation

Split Notochord Syndromes

Miscellaneous Malformations

Spine and spinal cord examinations comprise a

significant and important segment of clinical

neuroirnaging Familiarity with normal gross and

ra-diologic anatomy is a prerequisite to understanding the

broad spectrum of disorders that affect the spine and

spinal cord

In this chapter the normal gross and imaging anatomy

of the spine, spinal cord, and nerve roots, as well as their

congenital anomalies, are delineated Nonneoplastic

disorders, including trauma, infection, demyelinating,

vascular, and degenerative dis-

eases, are covered in Chapter 20 Tumors, cysts, and tumorlike masses are discussed in the concluding

chapter, Chapter 21

NORMAL ANATOMY Lumbosacral Spine

The lumbosacral spine has many components It can

be divided into anterior elements (vertebral bodies and intervertebral disks), posterior elements (pedicles, articular pillars, and facet joints), ligaments, soft tissues (e.g., epidural fat and venous plexuses), and neural tissue Neural tissue in this region includes the conus medullaris and cauda equina, lumbar roots and nerves, and the sacral plexus

Anterior elements

Vertebral bodies The lumbosacral spine normally

has five lumbar segments and the sacrum, which is composed of five fused segments Each lumbar seg-ment has a large, somewhat square-shaped vertebral body The superior and inferior end plates of the ver-tebral bodies are covered by a fenestrated cartilage to which the intervertebral disks attach (Figs 19-1 and 19-2).1

C H A P T E R

786 PART FIVE Spine and Spinal Cord

Fig 19-1 Anatomy of the lumbosacral spine in the axial plane A to C, Anatomic

draw-ings through the neural foramen (A), intervertebral disk (B), and pedicles (C)

Each vertebral body has an outer layer of dense,

compact cortical bone that surrounds an inner

med-ullary portion composed of bony trabeculae and

mar-row The two types of marrow, hematopoietically

ac-tive (red or cellular) and inacac-tive (yellow or fatty)

marrow, are easily distinguished on MR scans In

young children, marrow is typically cellular and

ap-pears isointense with paraspinous muscle on T1WI

(see Fig 19-15, B) In patients less than 2 years of

age, bone marrow and cartilage may show marked en-

hancement following contrast administration Mild marrow enhancement persists but gradually diminishes and disappears around age 7 years.2

From age 7 to adolescence there is also progressive conversion of red to yellow marrow.3 This replacement

of cellular marrow by fatty marrow results in high signal intensity on T1WI and relatively low signal intensity on standard T2-weighted spin-echo se-quences Inhomogeneous signal is common, and focal fat deposition is seen as localized zones of high

Fig 19-1, cont’d D, Axial cryomicrotome

section shows gross anatomy at the

intervertebral disc level E, Axial T1-weighted

MR scans show normal imaging anatomy of the

lumbosacral spine 1, Vertebral body 2,

Nucleus pulposus 3, Inner anular fibers of disk

4, Outer anular fibers of disk 5, Pedicles 6,

Lamina 7, Superior articular facet 8, Inferior

articular facet 9, Facet joint 10, Ligamentum

flavum 11, Epidural fat (curved arrow indicates

neural foramen) 12, Epidural venous plexus

13, Basivertebra venous plexus 14, Thecal sac

with roots of cauda equina 15, Exiting roots

16, Dorsal root ganglia 17, Extraforaminal

nerve 18, Transverse process 19, Pars

interarticularis 20, Spinous process (D,

Courtesy V.M Haughton.)

Chapter 19 Normal Anatomy and Congenital Anomalies of the Spine and Spinal Cord 787

signal intensity on T1WI (see Chapter 20).4 Marrow

in adolescents and adults normally does not enhance

following contrast administration.2

Intervertebral disks The intervertebral disks are

composed of a central gelatinous core (the nucleus

pulposus) surrounded by dense fibrocartilage and

fi-brous connective tissue (the anulus fibrosus) A

nor-mal lumbar intervertebral disk is slightly concave

posteriorly, except at L5-S1, where it appears

rounded

The intervertebral disks of infants are typically

high signal on T2-weighted scan except for a central

low signal area that represents the notochord

rem-nants (see Fig 19-19) Sharpey's fibers are seen at

the periphery as low signal intensity regions

Beginning in the second decade of life, a dark band

of compact fibrous tissue develops in the disk

centrum.5

Adult intervertebral disks are slightly hyperdense compared to adjacent muscle on NECT scans On MR scans, predominately fibrous compact tissue such as Sharpey's fibers and the outer anulus is low signal on both T1- and T2WI, whereas fibrocartilagenous tissue with mucoid matrix such as the nucleus pulposus, has high 5 signal intensity on T2WI (Figs 19-1, E; and 19-2, G).5 Age-related changes of disk dessication and degeneration begin in the midteens and continue

throughout life (see Chapter 20)

Posterior elements The pedicles and neural arch

form the posterior part of the vertebral column The neural arch is composed of the articular pillars and facet (zygoapophyseal) joints, the laminae, and the spinous processes

Pedicles The pedicles are thick, bony pillars that

Fig 19-2 Anatomy of the lumbosacral spine in the sagittal plane is depicted A and B,

Anatomic drawings show structures in the midline (A) and in the neural foramen (B)

C to E, Cryornicrotome section shows anatomy in the midline (C) and neural foramen (D) Close-up view (E) of the neural foramen (C to E, Courtesy V.M Haughton.)

mostly consist of dense cortical bone They project

posterolaterally from the vertebral bodies, connecting

them with the neural arch and forming the spinal canal

(Fig 19-1, C)

Articular pillars The articular pillars consist of the

pars interarticularis and the superior and inferior articular

facets The pars interarticularis is a bony plate that extends

posteriorly from the pedicle and gives rise to the superior

and inferior articular facets

Facet joints The facet joints are diarthrodial

synovial-lined joints that connect the posterosuperior articular process of a lower vertebra with the poster-oinferior articular process of the vertebra above (Figs 19-1, B and D; and 19-2, B and D).6 A tough, fibrous capsule is present along the posterolateral aspect of each facet joint There is no fibrous capsule on the ventral aspect of the joint; here, the ligamentum flavum and synovial membrane are the only barriers between the facet joint space and the spinal canal.7 The synovial membrane is intimately bound to the

Chapter 19 Normal Anatomy and Congenital Anomalies of the Spine and Spinal Cord 789

T2-weighted (G) scans demonstrate normal midline anatomy H, Sagittal

T1-weighted scan through the neural foramina shows the relationship of the soft tissues to the surrounding bone and intervertebral disk 1, Vertebral body 2, Intervertebral disk (nucleus pulposus) 3, Anterior longitudinal ligament 4, Posterior longitudinal ligament 5, Basivertebral venous plexus 6, Epidural fat 7, Epidural veins 8, Spinous processes 9, Interspinous ligament 10, Ligamentum flavum 11, Pedicle 12, Neural foramen with epidural fat and veins 13, Dorsal root ganglion

14, Superior articular facet 15, Inferior articular facet 16, Intranuclear cleft 17, Inner anular fibers of disk 18, Outer anular fibers of disk 19, Cauda equina 20, Conus medullaris 21, Pars interarticularis 22, S1 root 23, Sharpey fibers 24, Facet joint

fat in the posteromedial and anterior recesses of the

joint space.6 Synovium and joint space extend a

vari-able distance along the articular processes and under

the capsule The facet joint capsules are richly

inner-vated by sensory fibers that arise from medial

branches of the posterior spinal nerve rami.6

In the upper lumber spine the articular pillars and

facet joints are oriented nearly in the parasagittal

plane, whereas they are positioned more obliquely in

the lower lumbar region.1,8 On axial imaging studies

the facet joint has a mushroom-shaped appearance; the

superior articular facet forms the "cap" and the

inferior articular facet and spinal lamina form the

"stem" (Fig 19-1, E) On sagittal MR scans the pars

interarticularis lies between the more pointed superior

articular facet above and the somewhat

rounded-appearing inferior articular facet below (Fig 19-2, H)

Laminae and spinous processes The laminae are

comparatively flat bony plates that extend posteriorly from the articular pillars and join together at the midline where they form the root of the spinous process The spinous processes extend posteriorly and inferiorly from the neural arch (Fig 19-2, A)

Ligaments and soft tissues In the lumbosacral spine

the ligaments, epidural fat, and the epidural venous plexuses form prominent extradural soft tissues that surround the thecal sac and exiting nerve roots

Ligaments The anterior (ALL) and posterior (PLL)

longitudinal ligaments are thick, dense fibrous bands that extend along the anterior and posterior surface of each vertebral body from the skull base to the sacrum (Fig 19-2).9 They connect the vertebral bodies and are attached

to the intervertebral disks

The ALL extends from the basiocciput to S1 It is identified on sagittal T1-weighted MR scans as a very

790 PART FIVE Spine and Spinal Cord

low signal line that is in direct contact with and follows

the ventral surface of the vertebral bodies and disks

(Fig 19-2, A) The PLL is a thinner band that extends

from C1 to the first sacral vertebra.1 In contrast to the

ALL, the PLL does not adhere to the vertebral body.9

The PLL has a more narrow central segment that

widens laterally at the intervertebral disks and attaches

firmly to the anulus fibrosus, reinforcing the midline

and paramedian zones of the disk.1

On midline sagittal MR scans, the PLL is seen as a

continuous low signal band that is molded to the

pos-terior disk surface but spans the vertebral body

con-cavities like a bowstring (Fig 19-2, G) Epidural fat and

veins are interposed between the PLL and the vertebral

body

The ligamentum flavum (LF) arises from the anterior

aspect of the lower margin of one lamina and inserts on

the posterior surface of the lamina below.1 The

appearance of the LF on sagittal MR scans varies with

its distance from the midline.10 It is thinnest at the

midline where it is seen as an oblique, linear band of

low signal that attaches to the superior border of one

spinous process and the inferior surface of the next (Fig

19-2, F) On parasagittal scans the LF appears as an

inhomogeneous triangle with a narrow base inferiorly

and a broader base at its caudal end near the lamina.10

At the neural foramen it is seen as a curvilinear, low

signal structure covering the anterior surface of the

facet joint (Fig 19-2, H)

On axial CT and MR studies the LF is seen as a

V-shaped structure that covers the facet joint anteriorly

and is sometimes filled with fat posteriorly (Fig 19-1,

E) On NECT scans the LF is similar in attenuation to

muscle; signal on MR is variable because the LF

undergoes age-related degenerative change and can

calcify or become infiltrated with fat (see subsequent

discussion)

Small ligaments, the corporotransverse and

trans-foraminal ligaments, are often found in the neural

fo-ramina These fibrous bands originate from the

inter-vertebral disk and attach to the pedicle, superior

ar-ticular process, or ligamentum flavum They reduce the

potential space available for nerve roots that traverse

the neural foramen.11

Epidural fat and veins Extradural fat surrounds the

lumbosacral thecal sac and root sleeves The epidural

fat contains numerous small veins that connect to each

other in the midline between the PLL and posterior

vertebral body to form the epidural venous plexus.9

Basivertebral veins traverse the lumbar vertebral bodies

and emerge near the midline to drain into this plexus

(Figs 19-1, C; and 19-2, A).1

The lumbar epidural venous plexus is seen as thin,

linear, low signal foci on T1- and T2-weighted MR

scans (Fig 19-2, F) Enhancement following contrast

administration is variable but can sometimes be intense

Nerves and meninges

Conus medullaris and cauda equina The distal Spinal

cord terminates in a slight, diamond-shaped enlargement: the conus medullaris The conus tip is normally at about the Ll-L2 level The lower spinal nerve roots exit the conus medullaris and pass inferiorly within the thecal sac, forming the cauda equina, or "horse's tail" (Fig 19-3, A) Using heavily T2-weighted spin-echo sequences (Figs 19-2, G; and 19-3, G), MR "myelography" provides detailed definition of the thecal margins, nerve roots, and root sheaths that approaches conventional water-soluble

lumbar myelograms and CT-myelography (Fig 19-3, E and F) 12 On axial section, the roots of the filum terminale typically he in a symmetric, crescent-shaped pattern with the lower sacral roots positioned dorsally and the lumbar

roots positioned more anterolaterally (Fig 19-3, F and

G).13

Lumbar nerves and neural foramina Between L1 and

L5, the nerve roots exit the spinal canal at about a 45 degree angle The nerve root axillae are lateral outpouchings of dura and arachnoid that surround the exiting roots (Fig 19-3, E) The motor roots lie ventral to the sensory roots from the thecal sac exit to the dorsal root ganglia.14 The dorsal root ganglia normally vary considerably in size, and range from 6 mm at L1 to 15

mm at S2.14 The pedicles form the superior and inferior borders of the neural foramen; the articular facet and ligamentum flavum form its posterior border (see Fig 19-2, B) The anterior border is comprised of the vertebral body superiorly and the intervertebral disk and PLL inferiorly.15,16

The normal lumbar neural foramen is widest in its superior aspect and narrows inferiorly Each lumbar nerve root exits the spinal canal through the superior part of the foramen, above the level of the intervertebral disk In 90%

of cases, the dorsal root ganglion is directly inferior to the pedicle.14 On sagittal MR scans the fat-filled foramen looks like the head and beak of a bird, with the dorsal root

ganglion forming its eye (see Fig 19-2, H)

Sacral plexus The sacral plexus is formed by the

ventral rami of the L4-L5 and S1-S4 nerves (Fig 19-3, A) Medial to the psoas muscle, the L4-L5 nerves join to form the lumbosacral trunk After they exit the spine, the S1-S4 nerves converge in front of the piriformis muscle and join with the lumbosacral trunk to form the sacral plexus The sciatic nerve (L4-S3) is the continuation of the sacral plexus The sciatic nerve leaves the pelvis through the greater sciatic foramen to enter the thigh.17

Fig 19-3 Anatomy of the conus medullaris, cauda

equina, and exiting nerve roots A and B, Anatomic drawings with coronal (A) and axial (B) views C and

D, Cryornicrotome sections show gross anatomy of the

distal cord and filum terminale in sagittal section (C)

Axial section (D) illustrates the cauda equina 1,

Thoracic cord with central gray matter 2, Conus medullaris 3, Subarachnoid space 4, Anterior roots

5, Posterior roots 6, Cauda equina 7, Sacral plexus 8, Sciatic nerve 9, Pedicles 10, Basivertebral vein 11, Exiting roots 12, Dorsal- root ganglion 13, Central

gray matter 14, Posterior longitudinal ligament (C and D, Courtesy V.M Haughton.)

Chapter 19 Normal Anatomy and Congenital Anomalies of the Spine and Spinal Cord 791

Continued.

Multimodality imaging studies show

the conus me dullaris and filum

terminale Water soluble myelogram,

AP view (E) Axia CT scan (F) with

intrathecal contrast Axial

T2-weighted MR scan (G) through

cauda equina Compare with F H

and I Axial T2-weighted MR scans

through conus medullaris with “MR

myelogram” effect Compare with

(J), an axial post myelograrn CT scan

of the conus medul laris 1, Thoracic

cord with central gray matter 2,

Conus medullaris 3, Sub arachnoid

space 4, Anterior roots 5, Posterior

roots 6, Cauda equina 7, Sa cral

plexus 8, Sciatic nerve 9, Pedicles

10, Basivertebral vein 11, Exiting

roots 12, Dorsal root ganglion 13,

Centra gray matter 14, Posterior

longitudina ligament

Chapter 19 Normal Anatomy and Congenital Anomalies of the Spine and Spinal Cord 793

Thoracic Spine Anterior elements

Vertebral bodies The dorsally convex thoracic spine

consists of twelve vertebrae that gradually increase in size from rostral to caudal (Fig 19-4, A) The weight-bearing vertebral bodies are slightly wedge-shaped from front to back and appear somewhat cone- or triangular-shaped in axial section.18

Interverteral disks The height of the thoracic

in-tervertebral disks is less than either the cervical or lumbar counterparts, but the anulus fibrosus is thicker here

Posterior elements

Pedicles and laminae The pedicles project posteriorly

from the superior aspects of each vertebral body The laminae are broad, short, and overlap each other like the tiles on a roof (Fig 19-4, B).18 The laminae fuse in the midline to form the dorsal canal wall and give origin to the spinous processes The thoracic spinous processes are long and gracile, extending posteriorly and inferiorly from the spinal canal (Fig 19-4, D)

Articular pillars and joints Articular processes arise

from the superior and inferior aspects of the laminae and form the facet joints In the thoracic spine, most facet joints lie in the coronal plane Transverse processes project laterally from the articular pillars between the superior and inferior articu-

Fig 19-3, cont'd J, Axial postmyelogram CT scan

of the conus medullaris

Fig 19-4 Anatomy of the thoracic spine and spinal cord A to C, Anatomic drawings

with sagittal midline view (A), sagittal view through the neural foramen (B), and axial view (C) 1, Spinal cord with central gray matter 2, Conus medullaris 3, Spinous pro-

cess 4, Ligamentum flavum 5, Dura 6, Cauda equina 7, Subarachnoid space 8, Rib

9, Facet joints 10, Basivertebral venous plexus 11, Superior articular facets 12, Inferior articular facets 13, Lamina 14, Posterior longitudinal ligament 15, Dentate ligaments 16, Epidural fat 17, Epidural veins 18, Nerve root 19, Costovertebral joint 20, Pedicle 21 Neural foramen

Fig 19-4, cont'd D, Sagittal midline cryornicrotome

shows the thoracic spine and intervertebral disks in a

young child E to G, Imaging anatomy of the thoracic

spinal cord is shown on T2-weighted MR scans Sagittal

scans through the midline (E) and neural foramina (F)

are shown Axial view (G) shows the rib articulations 1,

Spinal cord with central gray matter 2, Conus

medullaris 3, Spinous process 4, Ligamentum flavum

5, Dura 6, Cauda equina 7, Subarachnoid space 8, Rib

9, Facet joints 10, Basivertebral venous plexus 11,

Superior articular facets 12, Inferior articular facets 13,

Lamina 14, Posterior longitudinal ligament 15, Dentate

ligaments 16, Epidural fat 17, Epidural veins 18,

Nerve root 19, Costovertebral joint 20, Pedicle 21,

Neural foramen (D, Courtesy V.M Haughton.)

lar facets The tip of each transverse process from T1

to T10 bears an oval costal facet Costotransverse

joints are formed by the articulation of the rib

tuber-cles and tips of the transverse processes.19

Ribs Ribs articulate with the thoracic vertebrae at

two sites Rib heads articulate with the vertebrae at

the disk (Fig 19-4, C and G), and the rib tubercle

joins with the transverse process at the

costotransverse articulation (see previous

discussion).19 At all levels except T1, TH, and T12,

demifacets above and below the disk articulate with

the rib head to form the costovertebral joint, which is

a true synovial joint The rib heads are therefore

helpful landmarks in identifying the intervertebral

disk during axial imaging.19

Ligaments The anterior longitudinal ligament is

thicker in the thoracic region than in the cervical or lumbar spine.18 It is also more prominent opposite the vertebral bodies than the disks The posterior lon-gitudinal ligament is also thicker in the thoracic spine Other ligaments such as the ligamentum fla-vum and the interspinous ligaments are not signifi-cantly different from their configuration at other spi-nal segments.19

Nerves A number of rootlets emerge from the

thoracic spinal cord and merge to form two roots: a large dorsal sensory root and a smaller ventral motor

root (see Figs 19-3, H and 19-4, C) These descend a

Chapter 19 Normal Anatomy and Congenital Anomalies of the Spine and Spinal Cord 795

Fig 19-5 Axial anatomy of the cervical spine and spinal cord A, Anatomic drawing depicting

the pedicles and lateral recesses B and C, Axial cryomicrotome sections through the C6-C7

interspace and the low vertebral body of C7 are shown 1, Vertebral body 2, Intervertebral disk

3, Uncinate processes 4, Neural foramen 5, Anterior roots 6, Posterior roots 7, Ganglion 8,

Cervical spinal cord 9, Ventral median fissure 10, Central gray matter 11, Subarachnoid space

12, Dura 13, Vertebral artery in foramen transversarium 14, Transverse process 15, Superior

articular facet 16, Inferior articular facet 17, Facet joint 18, Pedicle 19, Lamina 20, Spinous

process 21, Ligamentum flavum 22, Epidural fat 23, Epidural veins 24, Root sleeve (B and

C, Courtesy V.M Haughton.) Continued

variable distance within the subarachnoid space to

exit through the neural foramina.18

Cervical Spine

The upper two cervical vertebrae differ in size and

configuration from the lower five segments.20 The

anatomy and pathology of the craniovertebral

junc-tion are discussed in Chapter 12

C1, the atlas, is a bony ring with ellipsoid, superior

articular surfaces that combine with the occipital

condyles to form the atlantooccipital joint The

infe-rior facets are round or oblong and articulate with the

superior facets of C2 to form the atlantoaxial joints

The second cervical vertebra, the axis, is notable cause of the dens (odontoid process), a cone-shaped bony prominence that extends superiorly from the C2 body nearly to the clivus.18 The dens articulates an-terosuperiorly with the anterior arch of C1

be-C3-C7 are functionally and anatomically quite ilar and are therefore discussed together

sim-Anterior elements

Vertebral bodies and uncovertebral joints (Figs

19-5 and 19-6) The C3-C7 vertebral bodies are somewhat box-shaped and gradually increase in size from C3 to C7 (Fig 19-6, A) Each has superior pro-

796 PART FIVE Spine and Spinal Cord

Fig 19-5, cont’d D to G, Multimodality imaging studies depict axial anatomy Axial CT scans (D

and E) with intrathecal contrast are shown at the level of the uncovertebral joints and neural foramina (D) and the pedicles (E) F and G, Axial T2-weighted MR scans depict normal cervical

spinal cord and soft tissue anatomy Prominent areas of high velocity signal loss from pulsatile CSF flow are present 1, Vertebral body 2, Intervertebral disk 3, Uncinate processes 4, Neural foramen

5, Anterior roots 6, Posterior roots 7, Ganglion 8, Cervical spinal cord 9, Ventral median fissure

10, Central gray matter 11, Subarachnoid space 12, Dura 13, Vertebral artery in foramen transversarium 14, Transverse process 15, Superior articular facet 16, Inferior articular facet 17, Facet joint 18, Pedicle 19, Lamina 20, Spinous process 21, Ligamentum flavum 22, Epidural fat

23, Epidural veins 24, Root sleeve

jections, the uncinate processes, that indent the pos-

terolateral margin of the intervertebral disk and ver

tebral body above, forming the uncovertebral joints

(Fig 19-5, A).21 Some uncovertebral joints are filled

with loose connective tissue; others are lined with

synovium.21

Intervertebral disks In the cervical spine, the

in-tervertebral disks are kidney bean-shaped structures

that are normally somewhat thicker anteriorly than

posteriorly (Figs 19-5, A and F) These disks have a

central amorphous nucleus pulposus and a denser

peripheral fibrocartilaginous anulus fibrosus.20

Transverse processes and foramina transversaria

The transverse processes project anterolaterally from

the vertebral bodies The anterior and posteriorof the transverse processes are connected by a thin bony bar, the costotransverse bar The canal that is thus created

is the transverse foramen The foramina transversaria

contain the vertebral arteries and veins (Fig 19-5, B

and E)

Posterior elements

Pedicles The pedicles are short, cylindric

struc-tures that project posteriorly and slightly laterally from the vertebral bodies, connecting them to the ar-ticular pillars (Fig 19-6, B and D)

Articular pillars and facet joints The cervical

ar-ticular pillars are rhomboid-shaped bony projections

Fig 19-6 Sagittal anatomy of the cervical spine and spinal cord A and B, Anatomic

drawings through the midline (A) and neural foramina (B) C and D, Cryomicrotome

sections with midline anatomy (C) and close-up view of the neural foramen (D) E,

Mid-line sagittal T2-weighted MR scan shows the cervical spine and spinal cord F, More

lateral scan shows the neural foramina and exiting roots 1, Dens with odontoid process 2,

C1 3, Vertebral body 4, Intervertebral disk 5, Dura 6, Clivus 7, Anterior longitudinal

ligament 8, Posterior longitudinal ligament 9, Cervicomedullary junction 10, Cervical

spinal cord with central gray matter 11, Subarachnoid space 12, Ligamentum nuchae 13,

Spinous process 14, Interspinous ligament 15, Superior articular facet 16, Inferior

articular facet 17, Facet joint 18, Pedicle 19, Neural foramen 20, Epidural veins and fat

21, Anterior (ventral) roots, 22, Posterior roots and dorsal root ganglia 23, Ligamentum

flavum (C and D, Courtesy V.M Haughton.)

Chapter 19 Normal Anatomy and Congenital Anomalies of the Spine and Spinal Cord

797

798 PART FIVE Spine and Spinal Cord

that arise at the junction between the pedicle and lamina

The facet joints are formed by the superior and inferior

articular facets of adjacent vertebrae (Fig 19-6, B and

D)

In the sagittal plane, the facet joints angle obliquely

downward (Fig 19-6, B and D) On axial section, they are

oriented perpendicular to the vertebral body, with the

superior articular processes positioned anterior to the

inferior ones (Fig 19-5, B to E) Together the superior

and inferior articular facets look like two slightly

flattened half-moon-shaped structures with an interposed

joint space (Fig 19-5, E) The facet joints are true

synovial joints with a fibrous capsule The anterior aspect

of the ligamentum flavum covers the joints

Laminae and spinous processes The cervical laminae

are thin bony plates that project dorsally and are fused in

the midline, covering the spinal canal (Fig 19-5, B and

C) The spinous processes project posteroinferiorly from

the spinolaminar junction (Fig 19-5, E) The spinous

processes are often bifid C7 has the longest spinous

process

The spinal canal on axial views is roughly shaped like

an equilateral triangle (Fig 19-5) Its anteroposterior

diameter varies in size from a normal lower limit of 12

mm in the lower canal to 15 to 16 mm at C1 and C2.21

Neural foramina and nerves

Neural foramina The cervical neural foramina are

formed by the vertebral bodies anteriorly, the pedicles

above and below, and the articular pillars and

lig-amentum flavum posteriorly (Fig 19-6, B and D)

Nerves The cervical nerve roots extend slightly

in-feriorly and anterolaterally from the cord at about a 45

degree angle Cervical nerve roots are located within the

root sheath in the inferior half of the neural foramen; the

upper half of the cervical neural foramen contains fat and

small veins (Fig 19-6, D).22 The dorsal roots lie above

and behind the ventral nerve roots (Fig 19-6, B and C)

The dorsal root ganglion lies outside the neural foramen

between the vertebral artery anteriorly and the superior

articular facet posteriorly.22

Ligaments and soft tissues

Ligaments As in the thoracic and lumbar spine, the

anterior and posterior longitudinal ligaments connect the

cervical vertebrae Fibers from the PLL diverge from the

midline at each disk level, merging with the anulus

fibrosus and attaching to the adjacent vertebral end

plates 21 The PLL extends cephalad to merge into the

tectorial membrane and dura mater (Fig 19-6, C) Just

behind the dens, the inferior and superior cruciate

Other cervical ligaments are similar to their thoracic and lumbar counterparts The interspinous ligament extends between spinous processes (Fig 19-6, C and E) The ligamentum flavum is continuous along the posterior cervical spinal canal, attaching to the laminae and covering the facet joint capsules (see Fig 19-5).21

Epidural fat and veins Compared to the lumbosacral

region, cervical epidural fat is sparse, whereas the epidural veins are larger The anterior epidural space contains a prominent venous sinusoidal plexus (Fig 19-6, A and C) This plexus consists of longitudinal vascular channels that are located in the anterolateral recesses of the epidural space and connected to each other via a network of retrocorporeal veins.23 The epidural venous plexus communicates anteriorly with the basivertebral venous system It also forms a venous plexus in each neural foramen that extends through the foramen to surround the vertebral arteries (Fig 19-6, B and D).23

The cervical anterior epidural venous plexus (CAEVP) is visualized on 90% of contrast-enhanced MR scans and is particularly prominent at the C1-C3 level, whereas only 20% of CAEVPs are visualized at C6-C7.24

Meninges and Spinal Cord Meninges

Dura and subdural space The spinal dura is dense

fibrous tube that encloses the leptomeninges, cerebrospinal fluid, spinal cord, and proximal nerve roots The dura is continuous cephalad with the inner layer of the cranial

pachymeninges (see Chapter 12) The spinal dura extends

inferiorly to the second sacral segment, below which it blends into the solid filum terminale externum and attaches

to the coccyx.1 The spinal subdural space is normally very small

Arachnoid and subarachnoid space The arachnoid is

loosely attached to the inner aspect of the dura The subarachnoid space lies under the leptomeninges and contains cerebrospinal fluid, spinal cord, conus medullaris, filum terminale internum, and nerve roots, The cervical subarachnoid space is widest at the craniovertebral junction and gradually tapers from the foramen magnum to C2 The subarachnoid space from C3 to C7 ranges from 10 to 15 mm

in anteroposterior diameter.21 The spinal subarachnoid space is continuous cephalad with the intracranial CSF cisterns

The thoracic subarachnoid space is relatively constant, typically measuring 12 to 13 nun in sagittal diameter Thin septae extend from the posterior surface of the thoracic cord

to the arachnoid The most prominent and constant of these

is the midline septum posticum.18 Other delicate ligaments, the dentate ligaments, extend laterally from the cord to the

Epidural veins, venous plexus

Dorsal root ganglia

Marrow, intervertebral disks (older children, adults)

Congenital Malformations of the Spine and

Neural tissue exposed

Examples

Myelocele (neural placode flush with surface) Myelomeningocele (protruding placode)

tionally divide the thoracic subarachnoid space into

compartments that intercommunicate but may differ in

CSF flow rates This sometimes results in prominent

flow-related artifacts that can mimic vascular

malformations on MR imaging (see Fig 20-15)

The lumbar subarachnoid space is larger and more

variable, ranging from 15 to 20 mm in sagittal diameter.1

All of the spinal meninges have a blood supply with a

fenestrated capillary endothelium, although their

extravascular space is relatively small This limits the

amount of contrast pooling, and thus the spinal meningeal

enhancement normally seen on postcontrast T1-weighted

MR scans is relatively modest (see box, above).25

Spinal cord

Gross configuration The cervical spinal cord is

somewhat elliptic in cross section, whereas the thoracic

cord appears more round The conus medullaris has a

diamond-shaped enlargement before it terminates in the

cauda equina The normal conus in adults ends above

L2-L3, typically at the L1-L2 level This so-called "adult"

position is attained during the first few months of life and

varies little thereafter.26

Cord surface topography is exquisitely delineated on

axial T2-weighted MR scans or CT myelograms A

prominent cleft, the ventral median fissure, is seen in the

midline anteriorly (see Fig 19-5, D); a more shallow

dorsal median sulcus is present posteriorly The

dorsolateral sulci lie adjacent to the dorsal nerve roots,

and the dorsal intermediate sulci separate the gracile and

cuneate fasciculi.27

Internal anatomy Cross sections of the spinal cord

delineate the centrally placed gray matter and the

surrounding white matter The central gray matter has a

characteristic butterfly or H-shaped configuration that is

formed by the dorsal and ventral horns

(see Fig 19-3, H).28 These extend throughout the tire length of the-.spinal cord Smaller lateral horns are present from T1 to the conus medullaris The cen-tral gray matter volume appears relatively increased in the cervical and lumbar enlargements.29

en-The spinal cord white matter is divided into three funiculi on each side.29 The anterior funiculi he be-tween the ventral median fissure medially and the exit zone for the ventral nerve rootlets The lateral funiculi lie between the dorsal and ventral spinal nerve roots The posterior funiculi are the white matter between the dorsal median sulcus and the dorsolateral fasciculus and dorsal horns on each side.27

CONGENITAL MALFORMATIONS OF THE SPINE AND SPINAL CORD

An exhaustive description of the numerous genital malformations that affect the spine and cord is beyond the scope of this text We discuss the most important entities in two broad categories of congen-ital malformations, open spinal dysraphism and occult spinal dysraphism Two other groups o lesions that are also occult dysraphic disorders are the abnormalities

con-of canalization and retrograde differentiation and the split notochord syndromes We will discuss each of these categories separately, then close our consideration of congenital malformations by sum-marizing a few miscellaneous but important anoma-lies

Open Spinal Dysraphism

The general term "spinal dysraphism" refers to those spinal anomalies that have incomplete midline closure of mesenchymal, osseous, and neural tissue

(see box, above).30

In open spina bifida, also called spina bifida aperta

or spina bifida cystica, there is a dysraphic spine with posterior protrusion of spinal contents through

800 PART FIVE Spine and Spinal Cord

Fig 19-7 Anatomic drawing depicts myelocele (A), myelomeningocele (B),

lipomyelomeningocele (C), and intradural lipoma (D) The neural placode is shown

in red (A to C) The lipoma and subcutaneous fat are shown in yellow (A to D) The

CSF space is shown in gray, and the dura is indicated by the heavy black line

(arrowheads) The pia-arachnoid is shown by the thin black lines (arrows) (Adapted

from Barkovich AJ: Pediatric Neuroimaging, New York, Raven Press, 1990.)

the dorsal bony defect In this section we discuss the two

forms of open spinal dysraphism, myelocele and

myelomeningocele

Myelocele Myelocele is a neural tube closure disorder

similar in embryogenesis to myelomeningocele (see

subsequent discussion) A midline plaque of neural tissue

(the neural placode) is flush with the surface laterally

The placode is not covered with skin and is thus open to

the air (Fig 19-7, A) The dura is also deficient

posteriorly, whereas the pia and arachnoid line the ventral

surface of the neural placode and dura The arachnoid sac

thus formed is continuous with the lumbar subarachnoid

space.30

Myelomeningocele

Etiology and pathology Myelomeningocele (MM) and

myelocele both result from failure of the embryonic

neural folds to flex and fuse into a tube (see box) 31

Instead, they persist as a flat plate of urneurulated tissue called the neural placode The superficial ectoderm does not undergo disjunction from the neural ectoderm and

remains in lateral position (see Chapter 1)

Bone, cartilage, muscle, and ligament also develop in abnormal position ventrolateral to the neural tissue and remain bifid and everted.32,33 A midline defect is present, and the everted, elevated neural plate and meninges are continuous laterally with the subcutaneous tissues (Fig 19-7, B) The spinal cord is thus tethered and relatively immobile The dorsa roots arise from the anterior surface of the neural plate lateral to the ventral roots Both cross the CSF-filled sac to exit the neural foramina (Fig 19-8; see Fig 19-7, B).33

Incidence, age, and gender Myelomeningocele

anencephaly, and cephalocele are all considered neural tube defects Together, their incidence in the

Neural tube closure defect

Pathology

Dysraphic spine with dorsal protrusion of meninges,

CSF, neural tissue; not covered with skin

Location

Nearly always lumbar

Imaging

Intrauterine ultrasound discloses widely open neural

arch with flared laminae; meningocele sac; signs of

Chiari II malformation ("lemon" and "banana" signs;

hydrocephalus and callosal dysgenesis common)

MR, CT nearly always postoperative, show repair;

tethered cord often persists

Fig 19-8 Gross autopsy specimen demonstrates the

pathologic findings in myelomeningocele The spinal

cord (large arrow) is tethered into a CSF-filled dural

sac that protrudes dorsally through a widely dysraphic lumbosacral spine Note the nerve roots

(small arrows) that course anteriorly across the sac

from the neural placode (curved arrows) (Courtesy Royal College of Surgeons of England, Slide Atlas of

Pathology, Nervous System Gower Medical Publishing.)

Fig 19-9 Obstetric ultrasound findings of Chiari II malformation and

myelomeningo-cele are illustrated on these prenatal scans A, Transverse view through the fetal head

shows a small posterior fossa with compressed cerebellum around the midbrain

form-ing the "banana sign" (small arrows) Note bifrontal concavities, the so-called "lemon

sign" (white arrows) B, Coronal view through the lower fetal spine shows widened

lower lumbar and sacral canal with flared posterior elements (arrows) (Courtesy C

Sistrom.)

Continued.