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The Normal Spinal Column: Thoracic, Lumbar, Sacral, and Coccygeal Segments


The spinal column consists of multiple segmented osseous vertebrae, the intervertebral discs interposed between these segments, and the ligaments and joints that bind the segments together. The spine has 32 to 35 vertebral segments, traditionally considered as 7 cervical, 12 thoracic, 5 lumbar, 5 sacral, and 3 to 5 coccygeal segments. The relative lengths of the cervical, thoracic, lumbar, and sacral spines are in proportion as 2:5:3:2. An introduction to the spinal column and a detailed anatomy of the cervical segment was presented in Chapter 3 ; in this chapter the thoracic, lumbar, sacral, and coccygeal segments of the spinal column are addressed and imaging of the bone marrow is discussed.

NORMAL ANATOMY

Segmental Osseous Morphology

Thoracic Vertebrae

The 12 rib-bearing thoracic vertebrae are similar to each other but show a progressive transition from more nearly cervical to more nearly lumbar shape over the length of the thorax ( Figs. 4-1 and 4-2 ). The vertebral bodies resemble cylinders with concave side walls and nearly equal anteroposterior and transverse dimensions. The T1 and T2 bodies show flat anterior faces. The T3 body is the smallest thoracic vertebral body and shows a convex anterior surface. From T4 downward the thoracic vertebral bodies become progressively larger to bear increasing weight. The two thoracic pedicles extend directly posteriorly from the vertebral body, nearly parallel to each other, rather than angling posterolaterally as do the cervical pedicles ( Figs. 4-3 and 4-4 ). The thoracic pedicles increase in size progressively from above downward. The articulations with the ribs extend onto the lateral surfaces of the pedicles. For that reason, the upper surfaces of thoracic pedicles (except T1) are straight or convex superiorly, with little to no superior vertebral notch. The inferior surfaces of the thoracic pedicles are markedly concave with deep inferior notches. As a consequence, the neural foramina lie predominantly behind the lower portion of the vertebral bodies. They are round, face directly laterally, and are nearly equal in size over the length of the thoracic spine.

FIGURE 4-1, Thoracic spine. Dried osseous vertebrae. A , Full anterior surface. B , Midportion of posterior surface. C , Corresponding lateral surface. Anteriorly these images demonstrate the changing sizes and relationships of the thoracic bodies and transverse processes (t). The costotubercular (costotransverse) facets (ct) for the rib articulations change orientation progressively over the length of the thoracic spine. At T1, the prominent costocapitular (costovertebral) facet ( black arrow in A ) forms a circular depression and raised edges that mold the lateral contour of the T1 body. Posteriorly, the broad laminae and long spinous processes (3-9) of the vertebrae overlap extensively. The tips of the spine are single, not bifid. Laterally, the vertebral bodies form a gentle kyphosis. The costocapitular (cc) demifacets for each articulating rib mold the adjoining inferior and superior margins of the vertebrae at each interspace. The lateral ends of the transverse processes (t) bear costotubercular (costotransverse) (ct) facets that face anterolaterally to articulate with the tubercles of the ribs. The zygapophyseal (facet) joints between the superior (S) and inferior (I) articular processes show steep, near-coronal angulation superiorly. These gradually become more shallow and complex inferiorly. The neural foramina lie between the deep inferior vertebral notch of the upper pedicle (p) and the shallow or flat superior vertebral notch of the lower pedicle. In the thoracic (and lumbar) regions, the nerve roots lie within the upper portion of the neural foramina, near to the superior pedicle, so they are named for the uppermost pedicle. White numbers 4 to 8 in C indicate the sites at which the nerve roots exit the spinal canal beneath the T4-T8 pedicles.

FIGURE 4-2, In-vivo 3D CT reformatted images from a 56-year-old man. Anterior ( A ), posterior ( B ), posteroinferior ( C ), posterolateral ( D ), lateral ( E ), and anterolateral ( F ) surfaces. G , Midsagittal CT reformatted image. H , 3D cut-plane removal of lateral bone from E reveals the surfaces of the contralateral side as viewed from the midsagittal plane.

FIGURE 4-3, Isolated dried T7 vertebra. Anterior ( A ), posterior ( B ), lateral ( C ), superior ( D ), and inferior ( E ) surfaces. The sides of the vertebral body are concave. At each level the widths of the vertebral bodies are nearly equal to the widths of the laminae. The pedicles arise at the lateral edges of the bodies and run nearly parallel. The transverse processes sweep posterolaterally. The superior articular processes (s, white arrow ) are much thinner than the inferior articular processes and point sharply superiorly. The inferior articular processes diverge inferolaterally, so the transverse distance between the superior articular processes is less than the transverse distance between the inferior articular processes. The laminae are roughened ( black arrows , B ) along their upper margin and upper posterior surface for the attachments of the paired ligamenta flava. The posterior surface of the vertebral body shows basivertebral venous foramina ( B , black arrowheads ). The superior and inferior vertebral end plates (sep; iep) show a perimeter of smooth cortical bone and a center of rougher more cancellous bone. In C , arrowheads indicate demifacets for costocapitular (costovertebral) joints and arrows indicate the costotubercular facet on the anterior face of the transverse process. In D and E , cc indicates costocapitular (costovertebral) facets. In this and other images, paired “drill holes” seen in the vertebral bodies indicate that the segmental vertebrae had been strung together to form a spinal column.

FIGURE 4-4, Anatomic and CT images of the thoracic spine. Cryomicrotome sections in the midsagittal ( A ), parasagittal ( B ), and axial ( C and D ) planes. Four specimens. E , Noncontrast in vivo CT similar to D . Midsagittal section shows the cortical bone and marrow of the vertebral bodies, the anterior ( white arrowheads ) and posterior ( black arrowheads ) longitudinal ligaments, ligamentum flavum (lf), steep angulation and marked overlap of the spines (S), and the interspinous muscles and ligaments. The gray matter and white matter of the spinal cord are well seen. The posterior subarachnoid space (SAS) appears large, because the spinal cord lies anteriorly along the thoracic kyphosis. Parasagittal section displays the cortical bone of the superior (S) and inferior (I) articular processes, the cartilage on the facet surfaces, and the oblique coronal inclination of the zygapophyseal joints. The neural foramina are defined superiorly by the deep inferior vertebral notch of the upper pedicle (p1), inferiorly by the shallow to flat superior vertebral notch of the lower pedicle (p2), anteriorly by the posterior surfaces of the upper vertebra and disc, and posteriorly by the ligamentum flavum and the superior articular process of the lower vertebra. The emerging nerve roots and dorsal root ganglia lie within the upper portion of the neural foramina, close to the upper pedicle. C , The axial section through the disc (D), pedicles (p), and facet joints (S/I) displays the round shape of the thoracic spinal canal, paired ligamenta flava (lf) just deep to the base of the spinous process (S), triangular packet of dorsal epidural fat (f), dura, subarachnoid space, and spinal cord. The round shape of the thoracic spinal cord, the “butterfly” of spinal gray matter, and the surrounding columns of white matter are clearly depicted. The ribs (r) flank the disc bilaterally. D , The axial section at the mid height of the vertebral body demonstrates the basivertebral channel (BV) leading to the anterior internal vertebral venous plexus posteriorly and the anterolaterally directed venous channels. The paired rib heads (c) articulate with the vertebral bodies at the costocapitular (cc) joints and with the tubercles of the ribs at the costotubercular joints. lf, ligamentum flavum. Noncontrast axial CT of a different patient shows the similar appearance of the midposterior channel for the basivertebral veins (BV), the anterolaterally directed venous channels within the vertebral body, and the articulations of the rib heads (c) with the vertebral body at the costocapitular (costovertebral) (cc) facets. The angular orientation of the anterior venous channels varies greatly from level to level and from person to person.

Thoracic laminae are short, thick, and wide. From above downward they overlap each other like shingles of a roof. Thin superior articular processes arise from the junctions of the body with the pedicles, extend mostly superiorly, and display flat facets that face mostly posteriorly. The inferior articular processes project caudad from the inferior margins of the laminae and display facets that face predominantly anteriorly. The paired superior articular facets lie closer to the midline and to each other than do the paired inferior articular processes. Thoracic transverse processes arise from the pediculolaminar junctions and extend posterolaterally. The transverse processes are longest at T1 and decrease in length progressively down the spine, so the T12 transverse processes may be nearly absent ( Figs. 4-5 to 4-7 ).

FIGURE 4-5, In-vivo axial CT sections through T7 in a 27-year-old man. A to D , Serial axial images displayed from superior to inferior show the T6-T7 disc (2), T7 vertebral body (1), T7 pedicle (3), transverse process (4), base of the superior articular process (6), inferior articular process (7), neural foramen (8), transmitting the segmental T7 nerve roots, lamina (9), spinous process (10), spinal canal and cord (11), and costocapitular (costovertebral) joint (12). In A , the ribs are also seen to articulate with the transverse processes bilaterally at the costotubercular joints.

FIGURE 4-6, In-vivo axial T2W MR images through T7 in a 31-year-old woman. A to D , Serial axial MR images displayed from superior to inferior show the T7 vertebral body (1), T7-T8 disc (2), T7 pedicle (3), superior articular process of T7 (6), inferior articular process of T7 (7), laterally directed T7-T8 neural foramen (8), transmitting the segmental T7 nerve roots, lamina of T7 (9), spinous process of T7 (10), spinal cord (11), costal facet of the T7 rib (12), and flow voids from moving cerebrospinal fluid posterolateral to the cord (13).

FIGURE 4-7, A and B , In-vivo sagittal CT images in a 27-year-old man. C and D , T1W MR images in a 31-year-old woman. These images demonstrate the T7 vertebral body (1), T7-T8 intervertebral disc (2), T7 pedicle with deep inferior vertebral notch (3), superior articular process (6), inferior articular process (7), neural foramen containing the segmental T7 roots (8), low-hanging spinous process of T7 (10), and the spinal canal and cord (11). The upper surface of the pedicles is flat to convex superiorly, so there is little to no superior vertebral notch.

Lumbar Vertebrae

The five lumbar vertebrae are the largest of the spine ( Figs. 4-8 and 4-9 ). They increase in size from L1 to L3. L4 and L5 have variable sizes. The lumbar vertebral bodies are wider transversely than sagittally and show prominent concavity of their anterior surface. Short thick pedicles arise from the posterolateral surfaces of the vertebral bodies near to their superior margins. The superior vertebral notches are shallow. The inferior vertebral notches are deep. As a consequence, the neural foramina lie predominantly posterior to the upper vertebral body but do extend inferiorly behind the next lower vertebra ( Figs. 4-10 to 4-12 ).

FIGURE 4-8, Full lumbar spine. Isolated dried vertebrae. Anterior ( A ), posterior ( B ), and lateral ( C ) surfaces. From T12 to L3 the vertebral bodies increase in height, width, and span of the transverse processes. L4 and L5 morphology is variable. The orientation of the zygapophyseal (facet) joints modulates from above downward, appearing more nearly simple and coronal superiorly and more complex inferiorly. Prominent mammillary processes ( white and black arrows ) arise from the superior lateral aspects of the superior articular processes (s). The lumbar transverse processes (t) lie posterior to the neural foramina. Prominent accessory processes ( white arrowheads ) arise from the posteroinferior aspect of the base of each transverse process. The inferior articular processes (I) appear narrower and closer together in the upper lumbar region and then widen outward caudad. The spinous processes thicken and enlarge from T12 to L3 and angle inferiorly. At L4 and L5 the spinous processes usually become smaller and angle more directly posteriorly. The nerve roots emerge through the upper portion of the neural foramina, just beneath the upper pedicle, so they are named for the nearer pedicle ( white numbers in C ).

FIGURE 4-9, In-vivo 3D CT reformatted images in a 49-year-old man. Anterior ( A ), posterior ( B ), lateral ( C ), and posterolateral ( D ) surfaces. E , Midsagittal CT reformatted image. F , 3D cut-plane removal of lateral bone from C reveals the 3D surfaces of the contralateral side as viewed from the midsagittal plane.

FIGURE 4-10, A to C , Isolated dried L3 vertebra: anterior, superior, and inferior surfaces. D to F , L3-L4 articulations: posterior, lateral, and posterolateral surfaces. The sides of the vertebral body are deeply concave. The pedicles (p) are robust. The superior articular processes (S) are broad with prominent mammillary processes ( white arrows ) protruding posterolaterally from their surfaces. The laminae are short and squat. The inner aspect of the laminae shows a sharp demarcation ( black arrows in C ) between the smooth upper lamina that is free of ligamentum flavum and the roughened lower lamina ( asterisks , C ) to which the ligamentum flavum attaches. The inferior articular processes (I) are relatively narrow, so the transverse distance between the superior articular processes is wider than the transverse distance between the inferior articular processes. This is the reverse of their relationship in the thoracic spine. The facet surfaces are cupped, so the lateral portions of the facet joints are nearly sagittal and the medial portions nearly coronal in orientation. The bases of the transverse processes (t) show accessory processes ( black and white arrowheads in D , E , and F ). i, pars interarticularis. The inferior margin of the spinous process (s) shows a sharply demarcated depression ( asterisk , E ) for the interspinous muscles and ligaments.

FIGURE 4-11, Sagittal cryomicrotome sections of the lumbar spine. A , Midsagittal section at L1-L2 displays the anterior longitudinal ligament ( white arrowheads ) and posterior longitudinal ligament (PLL) ( black arrowheads ), the dense white cortical bone at the edges of the vertebral body, mottled red and yellow marrow within the cancellous bone, the basivertebral channel (v) draining into the anterior internal vertebral venous plexus ventral to the PLL along the mid posterior vertebral body, lipofucsin aging pigment within the intervertebral discs, the dura, dorsal epidural fat (f) and ligamentum flavum (lf) along the posterior wall of the spinal canal, the spines (S), and the interspinous muscles and ligaments. The tip of the spinal cord (conus medullaris) lies at L1-L2, surrounded by the roots of the cauda equina. B , Parasagittal section from L1 to uppermost S1 demonstrates uniform color and height of young adult discs, homogeneous red marrow within the vertebral bodies, keyhole shape of the lumbar neural foramina, the nerve roots and dorsal root ganglia within the upper portions of the neural foramina, the angulation of the zygapophyseal (facet) joints between the superior (S) and inferior (I) articular processes of adjoining vertebrae, and the bright yellow ligamenta flava that extend from the inferior half of the anterior surface of the upper vertebra to insert into the lamina and articular processes of the next-lower vertebra. The ligamentum flavum forms the anterior surface of the lumbar facet joints.

FIGURE 4-12, Axial cryomicrotome sections of the lumbar spine. Upper lumbar vertebrae ( A ) show short robust, nearly parallel pedicles, while lower lumbar vertebrae ( B ) show thicker pedicles that diverge posterolaterally. The midline basivertebral venous channel (BV) leads into the prominent venous plexus of the ventral epidural space (vv). The thick dural sac contains the subarachnoid fluid and roots of the cauda equina. C , Section through the intervertebral disc, neural foramina, and facet joints displays the superior (S) and inferior (I) articular processes of the cup-shaped facet joint and the spinous process (s). As they sweep across the back of the spinal canal, the paired ligamenta flava form the posterior wall of the spinal canal, the anterior walls of the synovial joints, and the posterior walls of the neural foramina. The midline triangle of dorsal epidural fat (f) fits snuggly between the posteromedial margins of the ligamenta flava. D , Diagrammatic representation of the facet joints in C demonstrates one method to measure the articular angles and assess articular tropism. Here the right facet measures 23.7 degrees to the midline, whereas the left measures 23.0 degrees. An angular difference of less than 7 degrees is considered to be within normal limits.

Lumbar laminae are short, broad, and overlap less than do thoracic laminae. The superior articular processes have concave articular facets that face posteromedially ( Figs. 4-13 to 4-15 ). They exhibit prominent mammillary processes on their posterior border. The inferior articular processes have convex articular facets that face anterolaterally. At L1 to L3 the transverse width between superior articular processes is greater than the transverse width between inferior articular processes. At L4 and L5, these widths are nearly equal.

FIGURE 4-13, In-vivo axial CT sections through L3 in a 27-year-old man. A to D , Serial axial images displayed from superior to inferior show the L3 vertebral body (1) with basivertebral channel in the midposterior surface, L3-L4 intervertebral disc (2), L3 pedicle (3), transverse process (4), superior articular process (6), inferior articular process (7), L3-L4 neural foramen containing the segmental L3 roots (8), L3 lamina (9), L3 spinous process (10), and spinal canal containing the cauda equina (11).

FIGURE 4-14, In-vivo axial T2W MR sections through L3 in an 18-year-old woman. A to D , Serial axial images displayed from superior to inferior show the L3 vertebral body (1), L3-L4 intervertebral disc (2), L3 pedicle (3), transverse process (4), superior articular process (6), inferior articular process (7), L3-L4 neural foramen containing the segmental L3 roots (8), L3 lamina (9), L3 spinous process (10), roots of cauda equina within the spinal canal (14), and the pair of L4 nerve roots moving caudally and laterally toward the next-lower L4-L5 neural foramen ( black arrowhead , D ).

FIGURE 4-15, In-vivo sagittal sections of the L2-L4 spine centered on L3. A to C , Reformatted sagittal CT sections in a 27-year-old man. D to F , T1W MR images in an 18-year-old woman. These images display the L3 vertebral body (1), L3-L4 disc (2), L3 pedicle (3), superior articular process (6), inferior articular process (7), L3-L4 neural foramen (8) containing the segmental L3 roots ( white arrowheads ), and L3 spinous process (10). The ligamentum flavum ( white arrows ) forms the posterior wall of the neural foramina. The dorsal epidural fat appears segmentally as interrupted plates.

The lumbar transverse processes are relatively thin and increase in length from L1 to L3. The L4 transverse process remains thin and is slightly shorter than the L3 transverse process. The L5 transverse process is thicker, more robust, and variably shorter or longer than those above. It typically angles posterosuperiorly and exhibits a blunt tip. At its root, the posteroinferior surface of each lumbar transverse process displays a small accessory process. Along its anterior surface, each lumbar transverse process displays a vertical ridge to which the anterior layer of the thoracolumbar fascia attaches. At its tip, each transverse process gives attachment to the middle layer of thoracolumbar fascia. The spinous processes of L1-L4 are nearly horizontal with thick posteroinferior margins. The spinous process of L5 is small with a rounded downturned apex. The tips of the lumbar spinous processes give attachment to the posterior layer of thoracolumbar fascia.

Sacrum

The sacrum is the name given to the triangular bony mass made by fusion of the five sacral vertebrae ( Figs. 4-16 and 4-17 ). It has a superior base that articulates with L5, an inferior apex that articulates with the coccyx, a concave ventral (deep) surface that forms the posterior superior wall of the pelvis, and a convex dorsal (superficial) surface that is palpable beneath the skin. The dorsal surface displays a series of tubercles. These tubercles mark the borders of the multiple spinal elements that fused into the single dorsal bone plate. On each side, lateral surfaces articulate with the iliac bones.

FIGURE 4-16, Sacrum and lumbosacral articulation. Anterior ( A ) and posterior ( B ) surfaces of L5 plus sacrum. The sacrum is roughly triangular. The transverse processes of S1 form the widest portion of the sacral alae (wings). On the anterior surface, ridges (and occasional gaps) mark the sites of the embryonic interspaces. The upper four sacral nerve roots S1 to S4 emerge through four paired sacral foramina. The S5 root emerges just inferior to S5 (right). If the transverse process for coccygeal 1 fuses to the sacrum (left) then the S5 root emerges via its own “fifth sacral” foramen. On the posterior surface, the rudimentary spinous processes fuse into a median sacral crest ( black arrowheads , B ) with variably distinct spinous tubercles. At S3 or S4 the spinal canal opens onto the dorsal surface at the sacral hiatus ( large vertical black arrow , B ). The articular processes fuse into paired intermediate sacral crests with variably distinct articular tubercles. The transverse processes (t) fuse into the lateral sacral crest (t on sacrum) with variably distinct transverse tubercles. In this specimen, the laminae of S1 did not fuse, leaving an S1 spina bifida occulta. Far laterally, the posterior surface is roughened ( asterisk ) for attachment of the multiple sacroiliac ligaments. Two small vertical black arrows in B mark the basivertebral venous channels in the midposterior surface of L5. C , Lateral surface of L4, L5, and S1 (second specimen). The sacrum articulates with the ilium laterally via the ear-shaped auricular facet (Au). The roughened surface for tendinous attachments lies posterior to this facet. White arrowheads indicate spinous tubercles of S1and S2. Black arrow in C shows the articular tubercle of S1-S2. D , Top surface of S1. The sacral promontory ( white arrowhead ) is the most anterior portion of the upper sacrum. The sacral alae (A) evolve from the transverse processes: the true transverse processes (diapophyses) posterolaterally and the pleurapophyses (costal element) anterolaterally. The laminae ( white arrows ) remain unfused in the midline. sep, Superior end plate.

FIGURE 4-17, 3D CT of the sacrum faithfully demonstrates all of the anatomic features of the sacrum, as labeled on Figure 4-16 .

Base

The base of the sacrum is formed by the wide upper surface of the S1 vertebral body ( Fig. 4-18 ). The prominent anterior lip of S1 forms the most anterior portion of the sacrum, designated the sacral promontory (see Fig. 4-16D ). The S1 transverse processes merge with broad bone masses that arise from the body, pedicle, and superior articular process on each side. These project posterolaterally to form the upper portion of each sacral ala. The pedicles are short and diverge posterolaterally to form the narrow sacral canal. The oblique laminae converge posteromedially to form the roof of the spinal canal in the midline. The superior articular processes of S1 project upward to articulate with L5 and display cortical thickenings analogous to the lumbar mammillary processes.

FIGURE 4-18, Axial cryomicrotome section through the anterior portion of L5-S1 disc (D) and the sacral alae (SA) displays the laminae (la) and inferior articular processes (I) of L5 forming the facet joint with S1, and the paired ligamenta flava (yellow ligaments) (lf). The sacral spinal canal is triangular. The S1 (1) root sleeves bud off the thecal sac anterolaterally. The anterior internal venous plexus (v, v) lies ventral to the superficial layer of the posterior longitudinal ligament.

Apex

The inferior surface of S5 has an oval facet that articulates with the coccyx.

Pelvic Surface

The pelvic surface is concave ventrally, although the anterior surface of the S2 body often forms a prominent forward bulge ( Figs. 4-19 and 4-20 ). Medially, the superior and inferior margins of the original sacral vertebral bodies form prominent transverse ridges that mark the old interspaces. Four pairs of ventral sacral foramina extend from the intervertebral foramina to the ventral surface to give exit to the ventral rami of the sacral roots. The laterally directed bony bars between the foramina are formed by costal elements (rib analogs) fused to the vertebrae. Lateral to the foramina, these costal elements fuse with each other and with the true transverse processes behind them to form the lateral portion of the sacral triangle. Superiorly, this surface enlarges into the sacral alae. Inferiorly, it curves gently medially to the inferolateral angle, then angles more sharply medially to the apex.

FIGURE 4-19, In-vivo CT sections through the sacrum in a 27-year-old man. A to D , Axial images. E , Reformatted midsagittal image. F , Reformatted coronal image. These studies display the sacral promontory ( black arrowhead ) and the five sacral segments from S1 downward, the neural foramina (1, 2, 3) for the sacral nerve roots, the sacral alae (SA), the sacroiliac joints formed between the sacral alae and the ilia (IL), and a portion of the coccygeal cornua (Co) that articulates with S5. Inferiorly the sacral spinal canal opens dorsally to form the sacral hiatus.

FIGURE 4-20, In-vivo oblique coronal T1W MR sections ( A to F ) through the sacrum in a 62-year-old man. This plane demonstrates the relationships among the sacral segments S1-S5, the sacral alae (SA), the ilia (IL), the sacroiliac joints, the sacral foramina, and the emerging nerve roots.

Dorsal Surface (From Medial to Lateral)

In the midline, the spinous processes of S1 to S3 (or S4) fuse into a median sacral crest that retains three to four spinous tubercles at the tips of the spines (see Fig. 4-16B ). Just inferior to the lowest tubercle, the laminae of S5 (or S4 and S5) remain unfused, leaving an arched sacral hiatus that opens into the vertebral canal. Lateral to the median sacral crest, the sacral laminae and articular processes fuse to form the dorsal wall of the sacrum. The lateral edges of these exhibit four pairs of articular tubercles, collectively designated the intermediate sacral crests (articular crests). Just lateral to these, four pairs of dorsal sacral foramina extend from the intervertebral foramina to the dorsal surface to give exit to the dorsal rami of the sacral roots S1 to S4. Each foramen lies immediately superolateral to each articular tubercle. Caudad, the inferior articular processes of S5 morph into prominent paired ridges—the sacral cornua—that flank the sacral hiatus. The sacral cornua articulate with the paired coccygeal cornua of Co1 inferior to them. The S5 roots emerge just medial to the sacral cornua and groove the lateral surface of S5. Farther laterally, the transverse processes of S1 to S5 fuse together lateral to the dorsal sacral foramina to form the lateral sacral crest. The apices of these transverse processes form a vertical line of small transverse tubercles.

Lateral Surface

The lateral surface of the sacrum is formed by the fused costal elements (anteriorly) and the fused transverse processes (posteriorly) (see Fig. 4-16C ). Anteriorly and superiorly the fused costal processes form an auricular facet that articulates with the ilia. The upper portion of the facet is formed by the costal elements of S1. The inferior portion is formed by the costal elements of S2 and part of S3. Behind the auricular facet the lateral surface is roughened for attachment of ligaments. Inferior to S3 the lateral surface of sacrum does not articulate with other bones and rapidly becomes reduced in size.

Lumbosacral Vertebral Specialization

Transitional lumbosacral vertebrae typically occur in 5% to 7% of the population, although numbers as high as 21% have been reported. These transitional vertebrae may be classified by the Castellvi criteria ( Table 4-1 ; Fig. 4-21 ). Overall, sacralization of L5 occurs about as often as lumbarization of S1. Among types II-IV, subtypes IIA and IIIB are most common and IIIA least common.

TABLE 4-1
Classification of Lumbosacral Transitional Vertebrae
From Castellvi AE, Goldstein LA, Chan DP. Lumbosacral transitional vertebrae and their relationship with lumbar extradural defects. Spine 1984; 9:493-495.
Type Description
I Dysplastic transverse process of the vertebra with width >19 mm
IA Unilateral
IB Bilateral
II Incomplete lumbarization or sacralization. Enlarged transverse process that has unilateral or bilateral pseudoarthrosis with the adjacent sacral ala.
IIA Unilateral
IIB Bilateral
III Complete lumbarization or sacralization. Enlarged transverse process that has unilateral or bilateral complete fusion with the adjacent sacral ala.
IIIA Unilateral
IIIB Bilateral
IV Mixed type with type IIA on one side and type IIIA on the other

FIGURE 4-21, Transitional lumbosacral vertebra: anterior (pelvic) ( A ) surface and dorsal ( B ) surface. C , 3D surface reformatted image of this dried specimen. The S1 vertebra is partially lumbarized with a lumbariform right transverse process (t) and sacriform left transverse process that is incompletely united ( arrows , A ) with the sacral ala formed by S2. Note that the fused sacral bones enclose only three neural foramina, indicating that the fused sacrum includes only S2-S5. B , The left S1 inferior articular process ( upper asterisk ) is hypoplastic and articulates with an anomalous S2 superior articular process. The spinous processes of L5 (5) and S1 (1) are free, while those of S2 (2) and S3 (3) have fused into the median sacral crest. The sacral hiatus ( black arrowheads ) opens dorsally ( vertical black arrow ) at S4. The articular tubercles ( oblique black arrows ) mark the intermediate sacral crest (synonym: articular crest). The foramina for the dorsal sacral roots S2-S4 lie immediately lateral to these. Further laterally, the transverse tubercles of S2-S5 (t, t, t) mark the lateral sacral crest. The sacriform left transverse process of S1 ( upper t) aligns with these. Further laterally, the roughened sacral surface ( lower two asterisks ) gives attachment to numerous tendons. In this case, the transverse processes of the first coccygeal segment did not fuse to the sacrum, so the S5 roots ( white 5) exit lateral to S5 with no osseous neural foramen (see Fig. 4-16 ).

The incidence of sacral spina bifida has been reported to range from 1.2% to 50%. Fidas and associates found a 23% incidence of spina bifida occulta in 2707 Swedish adults. Spina bifida occulta was twice as common in men as women. Most commonly, it affected S1 alone or S1 plus S2. Isolated defects at L5 or S2 were rare ( Table 4-2 ).

TABLE 4-2
Incidence and Combinations of Spina Bifida Occulta in 2707 Adults
From Fidas A, MacDonald HL, Elton RA, et al: Prevalence and patterns of spina bifida occulta in 2707 normal adults. Clin Radiol 1987;38:537.
Type Males (%) Females (%)
None 70.4 83
L5 only 0.1 0.1
S1 only 17.8 11.5
S2 only 1.0 0.7
L5 + S1 0.8 0.4
L5 + S2 0.1 0.0
S1 + S2 8.5 3.7
All three 1.1 0.6
Any form 29.6 17

Coccyx

The coccyx is a small, triangular bone formed by the fusion of three to five rudimentary coccygeal vertebrae ( Fig. 4-22 ). It may show incomplete fusion and irregular asymmetric shape. In analogy with the sacrum, the upper surface of the coccyx (Co1) is designated the coccygeal base. The caudal portion of the lowest coccygeal segment is designated the coccygeal apex. The base has an oval facet that articulates with the apex of the sacrum. The coccyx angles forward at the sacrococcygeal joint, so the pelvic surface of the coccyx inclines superiorly and ventrally. The pedicles and superior articular processes of Co1 morph into coccygeal cornua that articulate with the sacral cornua of S5. On one or both sides, rudimentary transverse processes of Co1 may project superolaterally to articulate or fuse with the inferolateral angle(s) of the lateral sacral surfaces to form (variably) complete S5 neural foramina. Inferior to Co1, Co2 to Co4 decrease rapidly in size and may resemble fused bony nodules.

FIGURE 4-22, Coccyx. Anterior ( A ) and posterior ( B ) surfaces. The first coccygeal segment (Co1) has coccygeal cornua (s) that articulate with S5. The transverse processes (t) of Co1 may articulate with the sacrum at the inferolateral angles to create osseous S5 neural foramina. The caudal coccygeal segments (c) often manifest as unequal and asymmetric nubbins.

Ligaments

Ligamentum Flavum

The posterior edges of the paired ligamenta flava may fuse completely or leave dorsal midline gaps (see detailed review in Chapter 3 , The Normal Spinal Column: Overview and Cervical Spine). In the low thoracic and lumbosacral regions, the paired ligamenta flava appear to be either completely fused or completely separated, without the intermediate partial fusions seen in the cervical and upper thoracic regions. Within the lower thoracic spine, the incidence of midline gaps is very low from T6-7 to T8-9, increases caudally to a peak of 28% to 35% at T10-11 and T11-12, and decreases again further inferiorly. Within the lumbar spine, the incidence of midline gaps is generally lower than in the low thoracic region, but shows a peak incidence at L1-2. Specifically, the incidences of midline gaps in the ligamentum flavum are T6-7: 4.4%; T7-8: 2.1%; T8-9: 4.4%; T9-10: 17.9%; T10-11: 35.2%; T11-12: 28.5%; T12-L1: 15.8; L1-2: 22.2%; L2-3: 11.4%; L3-4: 11.1%; L4-5: 9.3%; and L5-S1: 0%.

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