Spine anatomy

Osteology

The human spinal column is composed of seven cervical vertebrae, 12 thoracic vertebrae, five lumbar vertebrae, five sacral vertebrae and four coccygeal vertebrae. Of the 33 total vertebrae, the five sacral and the four coccygeal vertebrae are fused. The remaining 24 mobile vertebrae, with the exception of the C1-C2 level, are attached to the vertebrae directly adjacent by a series of ligaments, an interbody joint, and two facet joints. The facet joints are the only true di-arthrodial joints of the spine, being surrounded by a joint capsule, while the interbody joint contains the intervertebral disc that attaches the vertebral body endplates to levels cephalad and caudad. The intervertebral disc further serves as the center of the “functional spinal unit,” which is composed of the intervertebral disc and the vertebrae directly above and below.^1-4

In the non-pathologic state, the spinal column holds the head centered over the pelvis. When viewed in the coronal plane, the spine is straight from the top of the cervical spine down through the coccyx. In the sagittal plane, however, a physiologic amount of curvature can be seen within the different spinal regions: cervical, thoracic, lumbar, and sacral/coccygeal. The primary sagittal curvatures are those that exist in utero and consist of the thoracic and sacral/coccygeal kyphotic (apex posterior) curves. The typical range for thoracic kyphosis in the adult is 20-50º, with an average of 30º.^1,4

Secondary curves are formed after birth, once the spine is extended, to allow an upright posture.¹ The secondary curves are lordotic (apex anterior) and are found in the cervical and lumbar spine. The normal range of cervical lordosis is 16-22º in men and 15-25º in women.⁵ Lumbar lordosis ranges from 20-80º, with most of the curvature occurring between the L4 to S1 levels.³

Vertebrae

Each vertebra shares analogous structures with similar features. There are, however, important anatomic differences between the vertebrae that are specific to a particular area of the spine.

All vertebrae contain five centers of ossification: the centrum, which becomes the anterior vertebral body; two neural arches, which form the posterior vertebral body; pedicles, laminae, and spinous processes; and two costal arches, which form the transverse process or lateral mass on each side of the vertebrae.³

The size of the vertebrae increases from cephalad to caudad, with the cervical vertebrae being the smallest and the lumbar being the largest. The shape of the vertebrae also varies within the different regions:

In the lumbar spine, vertebrae are longer in the sagittal plane than the coronal plane.
The cervical vertebrae are wider in the coronal plane.
Within the thoracic spine, the upper vertebrae are shaped akin to the cervical vertebrae, while the lower thoracic levels are more similar to lumbar vertebrae.
The middle thoracic spine contains vertebrae that are nearly equal in width for both sagittal and coronal planes.
The middle thoracic spine is also the area where the spinal canal is the narrowest, from approximately T4 to T9.^1,6,7

Cervical vertebra

Thoracic vertebra

Lumbar vertebra

Vertebral Body

The largest and most anterior portion of the vertebrae is the vertebral body. With the exception of C1, all vertebral levels contain a vertebral body. Hard cortical bone surrounds the body, while the inside consists of softer cancellous bone.

The superior- and inferior-most aspects of the body are termed the vertebral endplates. These endplates are composed of thick cortical bone that articulates and provides nutrition to the intervertebral disc. The body is grossly cylindrical in shape, with some concavity at both the superior and inferior endplates, although there are several variations in body morphology within the different regions.

In the cervical spine, the superior endplate curves superiorly at the lateral borders bilaterally to form the uncinate processes. The inferior endplate in the cervical spine curves back into the body at the lateral border to form the unco-vertebral joint with the uncinate process from the level below. As the cervical vertebrae descend toward the thoracic spine, the caudad aspect of the body becomes larger than the cephalad aspect, with the width of the body being consistently larger than it is tall.⁸

The overall area of the thoracic vertebral body increases in a linear fashion from T1-T12. Additionally, the height of the body’s posterior margin is larger than the anterior margin and increases from cephalad to caudad, creating a slight wedge shape to the body and helping account for thoracic kyphosis.⁶

Similarly, in the lumbar spine, the body size continues to increase from cephalad to caudad. The lumbar bodies also have some asymmetry:

The upper lumbar levels have bodies that are taller on the dorsal sides.
The lower levels are taller on the ventral side.
L3 is nearly symmetric at the ventral and dorsal border.

These variations help account for physiologic lumbar lordosis.⁷

Another feature found in the thoracic spine is the costal facet which, in addition to the thoracic transverse process, serves as the point for rib articulation. The facet is located at the junction of the vertebral body and pedicle. At T11 and T12, each rib articulates with its respective vertebral body via the costal process; however, from T1 to T10, the ribs articulate with the vertebral body at their level and the body from the level above.⁶

Posterior Elements

The posterior elements of the spine, also termed the neural/vertebral arch, are composed of:

Pedicles
Transverse processes
Articular processes
Lamina
Spinous processes

The posterior elements lie directly dorsal to the vertebral body and are connected to the body by two pedicles. The pedicles originate from the posterior lateral margin on either side of the vertebral body and project away in a dorsolateral direction. At the dorsal limit of the pedicle is the pars interarticularis, which extends cephalad to become the superior articular process and caudad to become the inferior articular process. The transverse process projects laterally from the pars in the thoracic and lumbar spine.

In the cervical spine, the transverse process originates in a more ventral location and extends laterally from the junction between the vertebral body and the pedicle. Additionally, within the cervical region the pedicle and transverse process connect to contribute to the lateral mass and form a channel named the transverse foramen, which is where the vertebral artery is located as it ascends into the skull.

From the medial border of the pars, the lamina extend dorsally and medially to connect in the midline and complete the spinal canal. Finally, the spinous process extends dorsally from the junction of the two lamina at the midline of the posterior arch and is the most posterior portion of the spine.

Foramen

The bony architecture of the posterior elements forms two main foramen by which neurologic structures travel: the spinal canal and the intervertebral foramen.

The spinal canal is bordered anteriorly by the posterior vertebral body, medially and laterally by the pedicles, and posteriorly by the lamina.

Within the cervical spine, the spinal canal is largest at C1 and C2 and decreases at C3 to remain relatively constant until it reaches the thoracic spine.⁸
The thoracic spine has the narrowest canal, with the cross sectional area being the smallest between T3 and T8 (ranging between 189 and 199 mm²).⁶
In the lumbar spine the canal is largest at L1 and L5 and smaller between L2-L4.

According to the anatomic studies by Panjabi et al,⁸ the cross sectional area of the spinal column is largest at C2 averaging 374 mm².

The intervertebral or neural foramen creates the opening by which spinal nerves exit the spinal column to innervate the periphery.⁹ The foramen is bounded superiorly by the inferior border of the pedicle from the level above and inferiorly by the superior border of the pedicle from the level below. Dorsally, the foramen is bordered by superior and inferior articular processes of the facet joint and the ventral limit is the posterior edge of the vertebral body and intervertebral disc. The intervertebral foramen is oval shaped as the diameter is taller from superior to inferior than it is wide from anterior to posterior. Within the lumbar spine, the height of the foramen ranges from 12 to 19 mm.¹

Pedicles

The pedicles are cylindrically shaped channels of bone that connect the vertebral body to the posterior elements on both right and left sides. Hard cortical bone borders the outside of the pedicles, while their inner cores are filled with cancellous bone. The medial cortical wall of the pedicle is 2-3 times thicker than the lateral cortical wall.¹⁰

Over the past few decades, several quantitative anatomic studies have been performed to more accurately characterize the size, shape, and trajectory of the pedicles.^6,8,10-13 Values vary by a few millimeters or degrees depending on the particular study; however, the analyses have provided important anatomic information regarding these bony landmarks. Findings indicate that pedicles are:

Grossly oval in shape, being taller than they are wide
Larger at the cephalad aspect compared to the caudad aspect
Longer than they are tall, meaning that the length from anterior to posterior is greater than the length from superior to inferior
Projecting in a medial direction when the spine is viewed from posterior to anterior

Cervical Pedicles

Panjabi et al⁸ found that in the cervical spine, the C2 pedicle was largest and the C3 pedicle was smallest, with pedicles sizes increasing from C3 to C7. Cervical pedicles originate halfway between the superior and inferior endplates of the vertebral body.

In the upper cervical spine, the pedicles are sloped superiorly in the sagittal plane.
In the mid-cervical spine, the pedicles are parallel to the horizontal plane.
In the lower cervical spine, the pedicles are sloped downward.^8,12,14

The C2 pedicle also has the highest degree of medial angulation heading from posterior to anterior, and the C7 pedicle has the least. The range of angulation was found to be between 29 and 42º.⁸

In 2004, Bozbuga¹⁵ published the dimensions of the cervical pedicles to guide safe placement of pedicle screws, adding to pre-existing literature on cervical pedicle sizes.^8,12,16

The length of the pedicles were found to be between 5.3 and 6.7 mm, increasing from C2 to C7.
The width of the pedicles ranged from 4.4 to 4.9 mm.
Pedicle height was between 6.9 and 7.2 mm.

An important and often confusing anatomic region is the distinction between the C2 pedicle and the pars interarticularis. To clarify the difference, Ebraheim et al¹⁷ defined the C2 pedicle as the area inferior to the superior articular facet and anterior medial to the transverse foramen, and the C2 pars as the narrowest area between the superior and inferior articular processes.

Thoracic and Lumbar Pedicles

The thoracic and lumbar pedicles originate on the superior portion of the posterior vertebral body, close to the superior endplate, thus differing from the cervical pedicles that more central location in the body. These pedicles are angled in the cephalad direction from T1 through L3, and the L5 pedicle is angled slightly caudad.¹¹

Thoracic Pedicles
The height of the thoracic pedicle increases from T1 to T12, with the range reported between 9 and 16 mm.⁶ In contrast to height, pedicle width is narrowest in the middle thoracic spine between T3-T10. Pedicle width decreases from T1 to T4 and then has a mild increase until T12,¹⁸ with values ranging from 4 to 8 mm depending on the level.¹⁹ Cinotti et al¹⁹ found that nearly half of measured pedicles between T4 and T8 were less than 5 mm in width, highlighting the potential challenge of placing thoracic pedicle screws.

Similar to width, the cross sectional area of thoracic pedicles decreases from T1 to T3, remains relatively constant in the middle region, and then markedly increases from T8-T12.⁶ The medial angulation of thoracic pedicles is highest at T1, with a mean value of 27º, and decreases in the caudad direction to T12, with a mean value of 10º.⁶

Lumbar Pedicles
The pedicles of the lumbar spine are the largest in the body. Lumbar pedicle height ranges from 14 to 20 mm, with L3 being the shortest and L5 the tallest. Pedicle width increases in the caudad direction. L1 and L2 have similar widths and values, then gradually increase at L5, which has a width reported at 19 mm.²⁰

Likewise, cross sectional pedicle area is comparable between L1 and L2 and increases caudally towards L5, reaching a mean value of 144 mm2.²⁰ In contrast to the thoracic spine, the medial angulation of the lumbar pedicle increases from cephalad to caudad. L1 pedicles angle medially between 10º and 15º, while L5 pedicles angle medially between 25º and 30º.^11,20

As demonstrated, much attention has been paid to the morphometry of the pedicle. This is secondary to its importance during spinal instrumentation procedures, and the fact that an intricate knowledge of pedicle anatomy is critical for accurate pedicle screw placement.

Furthermore, understanding of the relationship between the neurologic structures and the pedicles is essential for safe pedicle screw insertion. The nerve roots lie closest to the medial and inferior pedicles walls in the thoracic and lumbar spine, and the thecal sac is closest to the medial wall. Secondary to this anatomic feature, pedicle screws that breach either of these walls are at highest risk of producing a neurologic deficit.

In the lumbar spine, the distance from the lateral border of the dura to the medial pedicle wall is approximately 1.5 mm, and although small, is greater than the distance between the pedicle and dura in the thoracic spine, which is occasionally too small to measure.^21,22 The distance from the superior pedicle wall to the nerve roots exiting through the neural foramen above is 1.9 to 3.9 mm in the thoracic and 4.1 to 5.5 mm in the lumbar spine.^21-23 The distance between the inferior pedicle wall and nerve root exiting the foramen below is smaller, and ranges between 1.7 to 2.8 mm in the thoracic and 1.3 to 1.5 mm in the lumbar.^21-23

Facet Joint

The superior articular facet from the level below and the inferior articular facet from the level above articulate to form the facet, or zygapophyseal, joint. The facet joint represents the only true diarthrodial joint in the spine, having two surfaces covered in articular cartilage and surrounded by a joint capsule filled with synovial fluid. Neural innervation of the joints is supplied by branches from the posterior primary rami.

The exact orientation of the facets varies depending on the specific region of the spinal column. In the cervical spine, the articular processes arise from the dorsal portion of the lateral mass, and project off at 45º angles.⁴ The superior articular process angles in a ventral direction, and the inferior process angles in a dorsal direction. The superior process, therefore, makes up the ventral half of the joint, and inferior process makes up the dorsal half.

The articular surfaces of the facet joints in the cervical spine are aligned close to parallel to the coronal plane. This joint orientation holds true in the upper and middle thoracic spine, with the exception that the articular processes in these regions do not arise from the lateral mass and are oriented at less of an oblique angle. The mean angle of the thoracic superior facet to the sagittal plane is between 74-88° and the mean angle of the inferior facet to the sagittal plane is 74-104°.²⁴

The facet joints in the thoracolumbar spine begin to change their orientation and become less perpendicular to the mid-sagittal plane. In this region, the superior articular facet composes the more lateral half of the joint and the inferior facet the more medial half.

The change in orientation continues into the lumbar spine where the articular surfaces are aligned close to parallel to the sagittal plane, with the superior process lateral and the inferior process medial. A unique feature most prominent on the lumbar superior articular facet is the mammillary process, which is located on the superior and lateral aspect of the facet and serves as an attachment for paraspinal muscles.¹

Transverse and Spinous Processes

The transverse process projects laterally from both right and left sides of each vertebra. In the cervical spine, the transverse process has a more ventral location, originating directly from the vertebral body. Here it serves as the lateral border of the transverse foramen that creates a passageway for the vertebral artery as it travels towards the brain.

In the thoracic and lumbar spine the transverse process originates at the junction of the pars and pedicle. Within the thoracic spine, the transverse process generally decreases in size from T1 to T12,⁶ whereas the overall size of the transverse processes in the lumbar spine tends to increase from L1 to L5, becoming shorter but stouter in the lower lumbar region.²⁰

The spinous process begins at the midline junction of the left and right lamina and projects dorsally, creating a point for ligament and muscle attachments. The cervical spine’s spinous processes are small and angle in a caudad direction.

Within the cervical spine, C2 and C7 are the largest spinous processes and serve as important anatomic landmarks, while C2 through C6 are typically bifid.^1,4,25 In the thoracic spine, the spinous processes are long, slender, and angled caudally. The spinous processes of the upper and lower thoracic spine angle caudally at approximately a 40º angle, while the middle thoracic spinous processes have a steeper angle of 60º and completely overlap the level below.¹ The lumbar spine has thick and broad spinous processes that angle dorsally as in the cervical and thoracic spine, but are larger.

Pars Interarticularis

The area that connects the superior and inferior articular processes is termed the pars interarticularis. In the cervical spine, the area is more commonly referred to as the lateral mass. It contributes to the dorsal border of the intervertebral foramen and, in the thoracic and lumbar spines, is the location where the transverse process originates. The area is subjected to high stresses during adjacent segment movement and has important clinical implications. In a C2 Hangman’s type fracture, the pars is the portion of the bone that fractures, and in isthmic spondylolithesis, the pars is the area where the lytic defect is located.

Laminae

The right and left laminae connect in the midline, form the dorsal roof of the spinal canal, and serve as a location for muscle and ligament attachment.

In the cervical and thoracic spine, there is some degree of laminar overlap between the level above and the level below, creating a “shingle” effect and a narrow interlaminar space. In contrast, the laminae of the lumbar spine do not overlap, resulting in a wider interlaminar space in the lumbar levels.

Laminae are tallest in the lower thoracic spine at around 20-25 mm, and are shortest in the mid-cervical spine at approximately 10 mm.²⁶ The lamina at the L5 level is markedly shorter in comparison to the other lumbar laminae. The laminae in the cervical spine and lumbar spine are the longest, ranging between 11 and 16 mm, while the laminae of the mid-thoracic spine are the shortest, measuring approximately 6 mm.²⁶ The short laminae of the thoracic spine correspond to the narrow spinal canal at the mid-thoracic levels. Finally, cervical laminae are the thinnest, while the proximal thoracic spine contains the thickest laminae.

Atlas and Axis Anatomy

The atlas (C1) and axis (C2) have anatomic features that are unique and not found within the vertebrae of lower levels. The spinal canal is large in the upper cervical spine and the dimensions of the canal formed by the C1 ring have been reported to be 32 mm from anterior to posterior and 31 mm from medial to lateral.²⁷

The atlas is the only level that does not have a vertebral body. It has anterior and posterior rings that are connected by a large lateral mass on either side, with the anterior ring being slightly shorter than the posterior ring. The rings are round and contain thick cortices that are thickest in the anterior ring where it articulates with the dens.

The posterior ring contains grooves on either side of its superior surface to accommodate the vertebral artery, which travels over the ring to ascend intracranially. During a posterior approach to the atlas, dissection is limited to 12 mm lateral of the midline to avoid injury to the vessel.²⁸

At C1, the superior and inferior facets are oriented with their surfaces close to the horizontal. The superior facets are located on the top of the lateral mass and have some internal rotation, allowing articulation with the occipital condyles. Likewise, the inferior facets are on the bottom of the lateral mass to articulate with the superior facet of C2. The size of the C1 lateral mass has been reported to be 13 mm from anterior to posterior, 4 mm from medial to lateral, and 5 mm from cephalad to caudad.²⁹

The axis supports C1 to permit rotational movement at the atlantoaxial joint. Located on the lateral mass, the superior articular facet of C2 faces superiorly with internal rotation to articulate with the inferior facet of C1. The inferior articular facet of C2 transitions from the articular processes of the upper cervical spine to become similar to the facets in the sub-axial cervical spine.

Projecting superiorly from the anterior vertebral body of C2 is the odontoid process, or dens. The dens articulates with the posterior aspect of C1’s anterior ring, where it is surrounded by a ligamentous complex that adds stability to the articulation. It is approximately 15 mm in height, widest at the superior half and narrowest at its junction with the C2 body.³⁰ The dens is instrumental to the stability and the axial rotation of the C1-C2 joint.¹

Intervertebral Discs and Spinal Ligaments

Discs

The 23 intervertebral discs are located between the vertebral bodies of each mobile spinal segment, with the exception of the C1-C2 level. In the typical adult spine, the most caudal disc is located between L5 and S1. Twenty-five percent of the spinal column’s height comes from the intervertebral discs, with the biggest contribution coming from the lumbar discs, which are larger than the cervical and thoracic discs.¹ The discs of the cervical and thoracic spine are symmetrical in height; however, the lumbar discs are taller dorsally, helping to account for the physiologic lordosis found in the lumbar spine.⁴

Intervertebral discs are connected to the vertebral bodies through the cartilage layer on the vertebral endplate, termed the lamina cribosa.⁴ This layer functions to allow diffusion of nutrients from the each vertebral body into the intervertebral discs, which is critical for the health of the largely avascular discs.

The two main structural regions of the intervertebral disc are the peripheral annulus fibrosis and the central nucleus pulposus.

The annulus fibrosis is composed of concentric layers of fibrous collagen tissue that are oriented at 30º from the long axis of the spine.⁴ Fiber orientation alternates between the layers, improving the stability of the annulus to resist torsional and tension stresses that are placed on the spine.
Moving centrally, the annulus merges through an indistinct transition zone to become the nucleus pulposus in the center of the disc.³¹ The nucleus is a remnant of the embryonic notochord and functions to distribute compressive forces. The exact location of the nucleus is eccentric, as it is positioned slightly dorsal to the center of the intervertebral disc.

Cervical vertebra showing annulus fibrosis and nucleus pulposus in the intervertebral disc

Ligaments

The spinal ligaments play a critical role in resisting excessive motion between the mobile segments of the spine through static restraint.

Anterior Longitudinal Ligament
The anterior longitudinal ligament (ALL) is located on the anterior vertebral body and intervertebral disc. The ligament is continuous and runs from the occiput to the sacrum. It overlays approximately one third of the vertebral body, is widest at the center of the body, and attaches most firmly at the vertebral endplates.^1,4 In comparison to its attachments to the body, the ALL is loosely connected to the intervertbral disc and is narrower in this location.

On closer inspection of the ligament fibers, three layers can be appreciated: superficial, intermediate and deep. The superficial layer has fibers that span approximately four or five spinal levels; the intermediate fibers span two to three spinal levels; and the deep layer has fibers that span only one spinal level.

Posterior Longitudinal Ligament
Traveling along the posterior vertebral bodies is the posterior longitudinal ligament (PLL). The PLL begins at C2, names the tectorial membrane at this level, and extends caudad to the sacrum. Similar to the ALL, the PLL has superficial fibers that span five spinal levels, an intermediate layer that spans two to three spinal segments, and deep fibers that span one level.

Despite this, there are several anatomic differences between the ALL and PLL. The PLL is thinner than the ALL. It is widest over the intervertebral disc space and thinnest over the body. Furthermore, it is firmly attached to the intervertebral disc and only loosely attached to the body, which is the opposite of the ALL.^1,4 In the cervical spine, the deep layers of the PLL and ALL travel around the vertebral body, forming a continuous ligament layer.³²

Ligamentum Flavum
The ligamentum flavum, or yellow ligament, forms the dorsal ligamentous roof of the spinal canal. Each level has two ligaments, one on either side, composed of highly elastic fibers that meet and occasionally fuse in the midline. The ligament originates halfway up the ventral border of the lamina from the level above and travels inferiorly to insert on the superior edge of the lamina from the level below.

The ligamentum flavum is composed of two layers: superficial (dorsal) and deep (ventral). The superficial layer attaches to the cephalad level on the base of the inferior spinous process, the inferior edge of the lamina, and the inferior ventral cortex of the lamina; the deep layer attaches to a ridge approximately halfway up the ventral cortex of the lamina. On the caudal lamina, the superficial layer attaches to the dorsal superior surface and the deep layer attaches to the ventral superior surface of the lamina.[33] The ligaments cover the interlaminar space from the midline, where they join, laterally to the facet joints.

Other Ligaments and Structures
The supraspinous and interspinous ligaments are midline ligaments that connect the spinous processes and are composed of highly elastic tissue.

The interspinous ligament travels from the inferior border of the spinous process from the level above to the superior border of the process from the level below.
The supraspinous ligament runs along the dorsal surface of the spinous processes. It begins at C7 and travels down to the sacrum.
Superior to C7, the supraspinous ligament transitions to its homologue in the cervical spine, named the ligamentum nuchae, which continues in a superior direction and connects to the external occipital protuberance.

The ligamentous structures in the upper cervical spine have many similar features to those in the subaxial cervical spine, although with some important distinctions. The PLL becomes the tectorial membrane as it extends superiorly from the C2 body, covering the odontoid process and anterior ring to attach onto the foramen magnum. The posterior atlanto-occipital membrane is the continuation of the ligamentum flavum, while the anterior atlanto-occipital membrane is a continuation of the ALL.

In addition to the homologous ligaments, a highly intricate ligament complex connects the dens, anterior ring, and occiput, providing critical stability to the articulation. The transverse ligament attaches to tubercles on either side of the anterior ring and runs directly posterior to the dens. It has a length of approximately 21 mm and width estimated between 2 and 3 mm.^34,35 The integrity of the transverse ligament is the major factor that determines stability in C1 burst-type fractures.

There are two alar ligaments, one on either side, that originate on the superior aspect of the dens and travel superiorly to attach on the medial border of right and left occipital condyles, respectively. These ligament are between 10 and 13 mm in length and serve to limit the axial rotation of the atlanto-occipital joint.³⁶

Musculature

The muscles along the posterior spinal column are divided into superficial, middle, and deep layers and vary anatomically according to location (cervical, thoracic, or lumbar spine).

Lumbar and Thoracic Spine
The superficial layer of the lumbar spine is composed of the latissimus dorsi. This muscle originates on the posterior iliac crest, the lumbodorsal fascia, and the lower ribs. It inserts on the medial border of the humerus and is innervated by the thoracodorsal nerve.

In the thoracic spine, the trapezius represents the superficial layer and is innervated by the spinal accessory nerve. The trapezius originates from the supraspinous ligament in the thoracic spine and the ligamentum nuchae in the cervical spine. It inserts via a broad attachment onto the clavicle, acromion, and the spine of the scapula.

The rhomboids and serratus posterior superior make up the intermediate layer in the thoracic spine, while the serratus posterior inferior represents the intermediate layer in the lumbar spine.

The rhomboids are innervated by the dorsal scapular nerve, originate on the transverse processes in the lower cervical and upper thoracic spine, and have an insertion on the medial border of the scapula.
The serratus posterior superior of the thoracic spine originates from the thoracic spinous processes and inserts onto the ribs. It is innervated by the ventral rami of spinal nerves.^1,25
The serratus posterior inferior of the lumbar spine originates on lumbar spinous processes to insert on the ribs. This muscle is also innervated by the ventral rami of spinal nerves.^{1, 25}

The deep muscle layer in the thoracic and lumbar spine share similar features and are composed of the paraspinal muscles groups. These muscle groups run longitudinally along both right and left sides of the dorsal spinal column.

In the lumbar spine, the paraspinal muscles are arranged in the sagittal plane and are named from lateral to medial: iliocostalis, longissimus, and spinalis. This lumbar muscle groups is also referred to as the sacrospinalis or erector spinae muscles. They originate as a broad tendon from the dorsal aspect of the sacrum and are covered by the thick lumbodorsal fascia in the lumbar and thoracolumbar spine. The lumbodorsal fascial layer originates from the supraspinous ligament and iliac spines to travel laterally and transition into the transversus abdominus fascia.

Similar to the lumbar spine, the iliocostalis, longissimus, and spinalis travel along the dorsal spine in the thoracic region. In addition to these muscles, the most midline portion of the thoracic paraspinal muscles is made up of three muscles, arranged in the coronal plane, and named the semispinalis, multifidus, and rotatores.²⁵

The semispinalis is the most superficial and spans five vertebral levels.
The multifidus is the intermediate layer and spans three levels.
The rotatores are closest to the bone, spanning one level.¹

The thoracic and lumbar paraspinal muscles receive a segmental vascular supply from segmental arteries that originate directly from the left and right side of the thoracic and lumbar aorta. Neural innervation to the paraspinal muscles is also received in a segmental fashion via the posterior primary rami that run alongside the segmental vessels.

Cervical Spine
Similar to the thoracic spine, the superficial muscle layer of the cervical spine consists of the trapezius, which originates off the spinous processes of the cervical vertebrae. The middle and deep muscle layers of the cervical spine are different than those of the thoracic and lumbar.

In the cervical spine, the middle layer is filled by the splenius capitus, which originates from the spinous processes of the upper thoracic and lower cervical vertebrae, and inserts onto the occiput. Lateral to the splenius capitus is the levator scapulae.

A paraspinal muscle mass makes up the deep layer in the cervical spine and is further divided into superficial, middle, and deep layers. The superficial paraspinal muscle is the semispinalis capitis, which originates from the cervical vertebrae transverse processes and inserts onto the occiput. The middle paraspinal muscle layer is filled by the semispinalis cervicis and the deep layer by the multifidus. Lateral to the deep paraspinal muscles are the longissimus capitis and longissimus cervicus; anterior to these are the posterior, middle, and anterior scalenes.^25,37

Between the axis (C2) and superior nuchal line on the occiput is the suboccipital, or posterior, triangle of the neck. The muscles that fill this region are responsible for fine control of head rotation through regulation of atlas and axis movements. The suboccipital triangle is innervated by the greater occipital and subocciptal nerves and lies deep to the superficial and intermediate muscle layers in the lower cervical spine.

The rectus capitus posterior major and rectus capitus posterior minor originate from the spinous process of the axis and the posterior arch of the atlas and insert onto the superior nuchal line.
Spanning the distance between the transverse processes of the axis and atlas is the obliqus capitus inferior.
The obliqus capitus superior orginates on the transverse process of the atlas and inserts on the superior nuchal line.^25,38

In addition to the posterior muscle groups of the cervical spine, the anterior musculature bears clinical significance, as the anterior surgical approach is commonly used to address cervical pathology.^39,40 The platysma is a thin superficial muscle that is covered by the superficial cervical fascia and is the first muscle that is encountered during anterior approaches. The sternocleidomastoid (SCM) lies just deep to the platysma and is the largest muscle in the anterior neck. An important anatomic landmark during surgical approaches, the SCM originates from the clavicle and sternum and inserts onto the mastoid process. Innervation to the SCM is supplied by the spinal accessory nerve.

Deep to the SCM and lying along the anterior vertebrae are the longus coli and longus capitis muscles. The longus coli runs along the anterolateral surface of the C3-T1 vertebral bodies, while the longus capitus is lateral, originating on the transverse processes of C3-C6 and inserting onto the occiput.

Spinal Vasculature

Spinal Column

The spinal column receives a robust vascular supply that is richest in the cervical and lumbar regions, corresponding to the enlargements in the spinal cord at these levels.⁴¹ Blood to the spine is received in a segmental fashion from both right and left sides of the body. Because of the bilateral supply, and the fact that each segment receives blood from the level above and below, there are several areas of collateral and redundant flow within the spine. The area of poorest blood supply is within the middle thoracic spine, and this area is sometimes referred to as a watershed zone.⁴¹

The vertebral arteries are the primary vessels that supply blood to the cervical spine. They are the first arteries to branch from the subclavian artery, traveling in a superior direction to enter the transverse foramen at the C6 level. Of note, while the vertebral arteries typically enter the transverse foramen at C6, they can enter at different levels (C4, C5, or C7), variations that have been reported to occur between in 5% and 7% of cases.^42,43

Once they enter the transverse foramen, the vertebral arteries ascend through the foramen at each successive level to finally exit through the transverse foramen of C1. After exiting, they curve in a posteromedial direction to cross directly over the posterior arch of the atlas and pierce the posterior atlanto-occipital membrane, where they then turn superiorly and enter the foramen magnum. At each level, the vertebral arteries gives off right and left segmental branches that further divide into anterior and posterior central branches to provide blood to the spinal cord and the bony column.⁴⁴

The lower cervical spine and two upper-most levels of the thoracic spine are supplied by the costocervical trunk. This vessel is the second branch from the subclavian artery. It further divides to give off the deep cervical artery to transfuse the lower cervical spine and the most superior intercostal artery to transfuse the upper thoracic segments.⁴⁵

The segmental arteries on the right and left side that branch directly from the thoracic and abdominal aorta are the primary blood supply to the spinal column from T2-L5.^41,46,47 The segmental arteries derive from the lateral aspect of the aorta and course in a posterior direction along the midpoint of the vertebral body. After passing posterior to the vertebral body, the segmental arteries divide into dorsal and lateral branches. The lateral branch travels to give blood to the paraspinal muscles, while the dorsal branch supplies the majority of the vertebrae and spinal cord.

After dividing, the dorsal branch continues in a posterior direction, traveling past the intervertebral foramen, lateral to the pars interarticularis and inferior to the transverse process. Once it passes posterior to the pars, the dorsal branch sub-divides to supply the deep paraspinal muscles and the posterior vertebral arch. When the dorsal branch passes the intervertebral foramen it gives off the spinal branch which enters the foramen to supply the contents of the spinal canal. As the spinal branch enters the foramen, it divides at a distribution point into three branches named the posterior central, pre-laminar, and intermediate neural branches.^1,2

The sacroiliaclumbar vascular system, which is a complex set of vessels containing multiple anastomoses and areas of collateral flow, supply the sacrum and lumbar spine below L4. The majority of blood in this system comes from the internal iliac artery; the most significant vessels are named the iliolumbar, fourth lumbar, and middle sacral arteries.^41,45*

The iliolumbar artery branches directly from the internal iliac artery and travels superiorly to the L5-S1 disc space, where it branches into the lateral iliac artery and an ascending lumbar artery.
The fourth lumbar artery branches from the aorta, as a segmental artery that gives blood to that level, before it heads inferiorly and anastomoses with the iliolumbar artery.
The middle sacral artery, the last branch from the aorta and derived at the bifurcation, travels inferiorly along the ventral surface of L4, L5, and sacrum to give off segmental branches at each level.

Spinal Cord

Arterial System
While the spinal column receives the majority of blood via segmental vessels that travel horizontally to supply respective levels, the spinal cord itself is supplied by vessels that run longitudinally down the length of the cord. The three main vessels in this system are the anterior spinal artery and the left and right dorsal lateral, or posterior, spinal arteries.^46-48 The anterior spinal artery is the largest and is responsible for giving approximately 80% of the blood to the cord, while the dorsal lateral arteries make smaller contributions.⁴⁶

These arteries begin in the upper cervical spine and are initially derived from branches of the vertebral arteries. Additionally, the upper cervical spine receives blood from lateral spinal arteries that branch from the vertebral artery close to the origination of the inferior cerebellar artery.⁴⁹ As the anterior and posterior spinal arteries descend inferiorly along the spine, the segmental vessels at each level feed into the spinal canal and form radicular arteries to supply the vessels.

In the thoracic and thoracolumbar regions, the spinal arteries are divided into cervicothoracic, midthoracic, and thoracolumbar portions based on which segmental arteries are providing blood to the area.¹ The artery of Adamkiewicz, or arteria radicularis magna, is the largest radicular artery and feeds into the anterior spinal artery. The artery is typically formed on the left side of the body, being derived from a lower thoracic or upper lumbar segmental artery between T9 and T11.^41,50 As the segmental artery enters the spinal canal and becomes the radicular artery, it takes a characteristic “hairpin” turn to head superiorly and join the anterior spinal artery around the level of the conus medullaris, at approximately T4-T6.⁵⁰

Venous System
The venous system draining the spine is highly variable and contains valveless veins, allowing blood to flow in either direction pending the pressure gradient. In general, it can be grouped into an intra-vertebral system consisting of the veins inside the spinal canal, and an extra-vertebral system consisting of the veins external to the vertebrae.

The intra-vertebral system surrounds the dura within epidural fat along the entire length of the spinal column. It is highly irregular, receives drainage from the spinal canal contents, and subsequently feeds into the extra-vertebral system.
The extra-vertebral system is more predictable, tends to follow the arterial system, and feeds into the azygous vein and vena cava.

Batson’s plexus is a named system, described in 1940, used to detail the pattern of venous drainage from the spinal column and its relation to the spread of metastases. It includes veins contained both intra- and extra-durally and has three components: extra-vertebral venous plexus, extra-dural venous plexus, and veins of the bony spinal column.⁵¹

Spinal Cord and Nerves

A detailed discussion on the intra-dural neural anatomy is beyond the scope of this review. The following will focus on the gross anatomy of the cord and nerves, and their relation to the bony spinal column.

The spinal cord and nerve roots are part of the central nervous system (CNS) and, being so, are surrounded by meninges composed of the pia mater, arachnoid mater, and dura mater. Cerebral spinal fluid (CSF) is contained in the interval between the pia and arachnoid mater, termed the subarachnoid space. Inside the bony spinal column, the cord is suspended by the dentate ligaments.

Spinal nerves are formed by the junction of ventral and dorsal nerve roots that are derived from their respective sides of the spinal cord. The formation of the spinal nerve marks the boundary of the CNS and peripheral nervous system (PNS) that is no longer surrounded by meninges.

The spinal nerves extend from the spinal cord and exit through the intervertebral foramen on the right and left sides of the spinal column. There is no C1 spinal nerve; therefore, the nerve that exits the foramen at C1-C2 is the C2 spinal nerve. In total, there are eight cervical, 12 thoracic, five lumbar, five sacral, and one coccygeal spinal nerves.

The spinal nerves of the cervical spine are named for the pedicle they exit superior to in the intervertebral foramen (ie, the C5 nerve exits through the C4-C5 foramen). This pattern changes at the transition from the cervical to thoracic spine because there are eight cervical nerves and only seven cervical vertebrae. Therefore, the C8 nerve exits through the C7-T1 foramen and cephalad to this point, the spinal nerve exits the foramen below its corresponding pedicle (ie, the T5 nerve exits through the T5-T6 intervertebral foramen).

When the cervical spine is viewed in the coronal plane, the spinal nerves roots extend from the cord at angles that are closed to perpendicular. At T1, the nerve root comes off the cord at an approximate angle of 120º, and from this point, the cord-to-nerve angles decrease in the caudad direction. The T12 nerve branches from the cord at an angle of approximately 57º, and the lumbar spinal nerves are oriented approximately 37º to the cord.^21,22

The adult spinal cord averages between 40 and 45 cm in length and fills two-thirds of the bony spinal column.⁴ The cord ends at the L3 body in the immature spine and at the inferior border of the L1 body in the mature spine, secondary to the different amount of growth between the bone and neural structures.

The cord is largest in the cervical region between C5-T1 and the lumbar region between L1-S2 to correspond with the brachial and lumbar nerve plexuses. The cord terminates as the conus medullaris, and this region contains the portion of the cord that supplies that sacral spinal nerves. The filum terminale extends from the conus medullaris and is a fibrous band of tissue that attaches to the most distal end of the thecal sac. &The lower four lumbar nerve roots and sacral nerve roots travel inferiorly to the conus prior to exiting the spinal column, a region termed the cauda equina.

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