Surgical Anatomy of the Spine

Acknowledgments

We would like to express our gratitude to the body donors and their families, who, through their altruism, contributed to making this chapter possible.

We thank BodyParts3D© The Database Center for Life Science licensed under CC Attribution-Share Alike 2.1 Japan, for sharing their 3D structure database for anatomic concepts used in Figure 4.1A–D , Figure 4.2A–C , Figure 4.5A–D , and Figure 4.5F and H .

We also would like to acknowledge the National Library of Medicine’s Visible Human Program for sharing their publicly available image dataset for anatomic concepts used in Figures 4.4D, E, G, and H .

Finally, the authors are grateful for the contribution of Ankit Hirpara and Mohamed Yassin in this chapter.

This chapter includes an accompanying series of lecture presentations that have been prepared by the authors: , , , , , , , .

Key Concepts

The typical adult human spine has 7 cervical, 12 thoracic, 5 lumbar, 5 fused sacral, and 4 fused coccygeal segments that carry the spinal cord and its 31 paired nerve roots.
The vertebrae consist of the vertebral body and vertebral arch, and the morphology of vertebrae changes from cephalad to caudad. The junctional vertebrae have the characteristics of both the upper and lower regions.
Each level of the spine functions as a three-joint complex. There are two facet joints in the back and one large interbody joint in the front that compose each intervertebral segment.
The upper 26 segments of vertebrae, with the help of the intervertebral joints, ligaments, and surrounding muscles, form a chainlike structure with strong support and flexible movement.
The main functions of the spine are to (1) protect the spinal cord and thoracoabdominal organs in front of the spine, (2) provide structural support and balance to maintain an upright posture and transmit the load from the upper body to the pelvis to the lower extremities, and (3) enable flexible motion.

Overview of Spine Anatomy

Body Surface Projection and Palpable Landmarks of the Spine

Cervical Region

Posteriorly, the external occipital protuberance (inion) is in the center of the occipital squama. The spinous process of the axis is the first readily palpable bony structure in the posterior midline below the inion. The second prominent palpable structure is usually the spinous process of C7 (also known as the vertebra prominens ). In about 75% of the population, the C7 is the most prominent spinous process, whereas the spinous process of C6 or T1 is more evident in the other 10% and 15% of the population, respectively ( Fig. 4.1A ).

Figure 4.1, Body surface projection and palpable landmarks of the spine.

Laterally, the transverse process of the atlas may be palpable directly below and slightly anterior to the mastoid process (approximately at the midpoint between the mastoid process and the mandibular angle). The anterior tubercles of the transverse processes of C6 are especially large and are known as the carotid tubercles or Chassaignac tubercles (see Fig. 4.1A ), and they may be palpated at the level of the cricoid cartilage in the groove between the larynx and the sternocleidomastoid muscle. Care must be taken when palpating the carotid tubercles (and the other cervical transverse processes) because they are in proximity of the common carotid arteries, and they always should be palpated unilaterally.

Anteriorly, the superior border of the thyroid cartilage, which forms the laryngeal prominence (larger in the adult male) in the midline, may be used to identify the C4–C5 disk.

Thoracic Region

Posteriorly, the spinous process of T1 is usually the third most prominent (after C2 and C7) bony structure in the midline below the inion. The spinous process of T3 is located in the same horizontal plane as the root of the spine of the scapula. When patients are standing or sitting with their upper extremities resting along the sides of their trunk, the inferior scapular angle usually is at the horizontal level of the spinous process of T8. The costovertebral angle represents the level of the T12 vertebra and is also the boundary between the thoracic region and lumbar region (see Fig. 4.1B ).

Anteriorly, the jugular notch of the sternum (or suprasternal notch) is at the superior margin of the manubrium and corresponds with the horizontal plane of the T2–T3 disk. The sternal angle (of Louis) at the inferior margin of the manubrium is at the level of the T4–T5 disk. The xiphisternal junction, which corresponds with the body of T9, can be found near the inferior end of the sternum (see Fig. 4.1C ).

Lumbosacral Region

From a posterior aspect, the spinous processes of L4 and L5 are shorter than the other lumbar spinous processes and are difficult to palpate. Historically, the L4 spinous process was considered to be in a horizontal plane with the superior margin of the iliac crests (the supracristal plane). However, more recent evidence using ultrasonography to localize intervertebral levels shows the supracristal plane to be at the level of L3–L4 in nearly 75% of normal volunteers. In the remainder of the healthy population, this plane is fairly evenly found at either L2–L3 or L4–L5 (see Fig. 4.1D ). The tips of the transverse processes of the lumbar vertebrae are located approximately 5 cm lateral to the midline and usually are not palpable.

The remnant of the spinous process of S2 is located at the extreme of the convexity of the sacral kyphosis and comprises the most prominent spinous tubercle on the sacrum. It is also in the same horizontal plane as the posterior superior iliac spines, which are readily palpable 3 to 4 cm lateral to the midline (see Fig. 4.1E ).

Anteriorly, the vertebral level of the umbilicus is typically at the level of L3, but this varies depending on body type and weight.

Vertebral Structures

The shape of the vertebrae changes from cephalad to caudad ( Fig. 4.2A–C ) . The junctional vertebrae have the characteristics of both the upper and lower adjacent regions. The outer aspect of each vertebra consists of dense, solid cortical bone. Inside each vertebra is cancellous bone, which is weaker than cortical bone and consists of loosely knit structures that resemble a honeycomb. Bone marrow, which forms red blood cells and certain types of white blood cells, is found within the cavities of cancellous bone. We will briefly introduce the various components of the vertebral structure in the following sections.

Figure 4.2, Sagittal and anterior view of the whole spine.

Vertebral Body

The vertebral body (VB), which approximates a cylinder, is the large anterior portion of a vertebra that acts to support the weight of the human frame. The cross-sectional area of the VB increases from top to bottom, and L5 is about three times as large as C3, ensuring that the force per unit area (pressure) is the same throughout the spine ( Fig. 4.3A–B ).

Figure 4.3, Morphology of the cervical, thoracic, and lumbar vertebra.

The superior and inferior surfaces of the VB begin as two pieces of complete hyaline cartilage before puberty. At the beginning of puberty, secondary ossification centers appear annularly around the cartilage, called epiphyseal rings. At around 25 years of age, the epiphyseal ring ossifies completely and fuses with the vertebral bone to form peripheral processes on the superior and inferior surfaces of the VB, which then give rise to the osseous end plate. The central part of the epiphyseal ring remains hyaline cartilage and lasts a lifetime. It adheres to the intervertebral disk (IVD), thereby forming the cartilaginous end plate. In abnormal circumstances, the VB may have two primary ossification centers on the left and right. If one of them is underdeveloped, it will form a hemivertebra, one of the causes of congenital scoliosis.

The upper and lower edges of the VB will sometimes have osteophyte formation, which is usually the product of vertebral functional compensation after spinal degeneration. The osteophyte can potentially compress against adjacent structures, including the nerve roots, spinal cord, vertebral artery (VA), or esophagus, leading to corresponding symptoms.

Vertebral Arch

The vertebral arch is the posterior part of a vertebra, and it is formed by the midline fusion of the left and right pedicles with the left and right vertebral laminae. The arch supports seven processes: four articular processes, two transverse processes, and one spinous process.

The pars, which is most prominent in the lumbar vertebrae, can be found at the junction of the pedicle and the lamina and in between the superior and inferior articular processes.

Pedicle

The pedicles create the narrow, anterior portions of the vertebral arch. They are short, thick, and rounded, and they attach to the posterior and lateral aspects of the VB (see Fig. 4.3A–B ). The anterior part of the pedicle connecting the VB is slightly wider and has more compact bone than the VB does. The posterior part of the pedicle attaches to the lamina, and the lateral and superior parts of the pedicle attach to the transverse process and the superior articular process. These junctions are areas of stress concentration, almost entirely composed of compact bone.

Lamina

The laminae are continuous with the pedicles. They are flattened from anterior to posterior and form the broad posterior portion of the vertebral arch. They curve posteromedially to unite with the spinous process, completing the vertebral foramen.

Spinous Process

The spinous process of each vertebra projects posteriorly and often inferiorly from the laminae, especially in the thoracic region. The size, shape, and direction of the spinous process vary greatly from one region of the vertebral column to the next. The spinous processes throughout the spine function as a series of levers both for muscles of posture and for muscles of active movement. Most of the muscles that attach to the spinous processes act to extend the vertebral column. Some muscles that attach to the spinous processes also rotate the vertebrae to which they are attached. In addition, the supraspinous ligaments and interspinous ligaments attach to the spinous process, and together with the posterior longitudinal ligaments (PLLs) and ligamentum flavum, help limit hyperflexion of the vertebral column.

Transverse Process

The transverse processes project laterally from the junction of the pedicle and lamina. Like the spinous processes, their exact direction varies considerably from one region of the spine to the next. The transverse processes of typical cervical vertebrae project obliquely anteriorly between the sagittal and coronal planes and are located anterior to the articular processes and lateral to their pedicles. A typical cervical transverse process is formed by the fusion of anterior and posterior parts, with the transverse foramen (anteromedial) and spinal nerve groove (posterolateral) between the two parts. In most cases, the bilateral vertebral arteries enter the transverse process of C6 and continue to ascend cephalad.

The thoracic transverse processes are different and project obliquely posteriorly and are located behind the articular processes, pedicles, and intervertebral foramina (IVF).

The lumbar transverse processes lie ventral to the lumbar articular processes and posterior to the pedicles and IVF. Multiple muscles and ligaments attach to the transverse processes of the lumbar spine and play an important role in the movement and stability of the spine. In some cases, L5 may have a larger transverse process, either articulated or fused with the sacral ala or iliac crest, which can lead to chronic, persistent lower back pain. This is known as Bertolotti syndrome and is a rare cause of low back pain in young patients.

Superior and Inferior Articular Processes

The superior articular processes (or zygapophyses) also arise from the pediculolaminar junction, while the left and right inferior articular processes project inferiorly from the pediculolaminar junction. The articular surface of the inferior articular process faces anteriorly, and the articular surface of the superior articular process faces posteriorly, although the precise direction varies from posteromedial in the cervical and lumbar regions to posterolateral in the thoracic region.

Vertebral Foramen

The vertebral foramen is the opening within each vertebra that is bounded by the VB, the left and right pedicles, the left and right laminae, and the spinous process. The size and shape of the vertebral foramina vary from one region of the spine to the next and even from one vertebra to the next. The spinal canal is the composite of all of the vertebral foramina.

Relationship Between Vertebrae

Three-joint Complex

Each level of the spine functions as a three-joint complex. Two facet joints in the back and a large interbody joint in the front comprise each intervertebral segment. The interbody joint is an amphiarthrosis joint that is connected by the IVD. The facet joints are a set of synovial plane joints with cartilage on the surface and synovium in the articular cavity. This tripod structure provides stability and support as well as the ability to move in all directions. The motions of these three joints are coupled, and abnormal motion of one joint will inevitably lead to abnormal motion of the other two joints.

Facet Joint

The articulating surface of each superior and inferior articular process is covered with a 1- to 2-mm-thick layer of hyaline cartilage. This hyaline-lined portion of a superior and inferior articular process is known as the articular facet. The junction found between the superior and inferior articular facets on one side of two adjacent vertebrae is known as a facet joint (also known as a zygapophysial joint or z-joint ) ( Fig. 4.4A–C ) . Mobility at the facet joints varies considerably between vertebral levels. The facet joint also helps form the posterior border of the intervertebral neural foramina.

Figure 4.4, Overview of the osseous and ligamentous anatomy of the spine.

The biomechanical function of each pair of facet joints is to guide and limit the motion between vertebrae. The angle between the C2–C3 intervertebral facet plane and the horizontal plane is about 45 degrees, and the facet plane gradually tends to be more horizontal in lower cervical vertebrae. The capsule of the cervical facet joint is wide and loose, leading to more motion and greater likelihood of dislocation rather than fracture when subjected to trauma. The angle between the thoracic facet plane and the horizontal plane is about 60 to 80 degrees, and the extension and rotation of the thoracic facet joint is limited by the rib cage and spinous processes. The plane of the lumbar facet joint is angled approximately perpendicular to the horizontal plane and about 45 degrees to the sagittal plane; the capsule is tight with limited motion. The lumbar facet joint allows flexion, extension, and lateral flexion but has limited rotation. When subjected to trauma, the facet joints or pars are more prone to fracture than dislocation.

The superior articular processes of sacrum generally face posteriorly and slightly medially. However, the plane in which these processes lie varies considerably. Their orientation ranges from nearly a coronal plane to almost a sagittal one. These processes also frequently are asymmetric in orientation, with one process more coronally oriented and the other more sagittally oriented. Such asymmetry is known as tropism and usually can be detected on standard anterior-posterior x-ray films. The superior articular processes possess articular facets on their posterior surfaces that articulate with the inferior articular facets of the L5 vertebra. The facet joints formed by these articulations are more planar than those between two adjacent lumbar vertebrae, and they usually are much more coronally oriented than the lumbar facet joints. However, because of the wide variation of the plane in which the superior articular processes lie, the orientation of the lumbosacral facet joints also varies in corresponding fashion.

Intervertebral Disk

There are 32 IVDs, which account for one-third of the spine’s height. On one hand, IVDs act as structures to hold the vertebrae together, provide stability, and absorb shock. On the other hand, the limited motion of each IVD is compounded, which makes the spine as a whole have unique multidirectional motion. The morphologic characteristic of the IVD is one of the factors that contribute the physiologic curvature of the spine. Also, degeneration of the IVD is the primary cause of many degenerative spine diseases. It is important to note that there is no vascular distribution inside the IVD. Nutritional support mainly comes from diffusion exchange through the cartilaginous end plate. Therefore, disk degeneration can sometimes be attributed to sclerosis of the end plate, which consequently affects the nutritional supply of the IVD.

The components of each disk (the nucleus pulposus, its surrounding annulus fibrosus, and the superior and inferior cartilaginous end plates) have different compositions of proteoglycan, collagen, and water. These components have correspondingly different function, making the disk a very specialized component of the spine (see Fig. 4.4D ). The nucleus pulposus is a jellylike substance with hydraulic viscoelastic properties, located slightly posterior to the center of the IVD. Essentially, the nucleus pulposus resists and redistributes pressure from the spinal column. The annulus fibrosus, which is stiffer and tougher, is composed of collagenous lamellae that are arranged around the nucleus pulposus like the plies of a tire. Its main function is to withstand tension, which comes from the horizontal expansion of the nucleus pulposus being compressed, the torsional stress of the spine column, or the tension of the lateral flexion and extension (see Fig. 4.4E ).

The outermost layer of the annulus fibrosus fuses with the periosteum of the VB and the anterior longitudinal ligament (ALL) and PLL. In the anterior one-third of the IVD, the fibrous bundles are strongest. In the dorsal part of the nucleus pulposus, the lamellar layers are thin. Therefore, when subjected to trauma, the weaker posterior region of the disk is more likely to rupture, especially in the lateral posterior region without the reinforcement of the central bundle of PLL.

Uncovertebral Joint

The lateral margin of the superior surface of each subaxial cervical VB extends cranially as a bony process called the uncinate process. These processes articulate with a reciprocal convex area (uncus) on the inferolateral aspect of the upper VB. This articulation is named the uncovertebral joint or Luschka joint and is only located in the cervical region of the vertebral column between C3 and C7 (see Fig. 4.4F ). Functionally, the uncovertebral joint limits the lateral displacement of the IVD. The uncinate process, unique to the cervical spine, constitutes the anteromedial boundary of the cervical intervertebral foramen, and its hyperostosis may compress the spinal nerve root or the VA.

Spinal Canal

The spinal canal is the collection of all of the vertebral foramina. It is enclosed by the vertebral bodies, disks, and PLL from the front, by the laminae and the ligamenta flava from the back, and by the pedicles and IVF from lateral aspects.

The vertebral canal follows the normal contour of the curves of the spine. The vertebral foramina are relatively large and triangular in the cervical and lumbar regions, where there is considerable spinal movement. The vertebral canal in the thoracic region is smaller and almost circular in configuration.

The spinal canal is an osseous and fibrous passage that contains the spinal cord, nerve roots, cauda equina, CSF, dura mater, vessels, venous plexus, and adipose tissue that fills the epidural space (see Fig. 4.4G ). Despite the spinal canal having room for the neural elements, osseous or fibrous structural abnormalities can lead to spinal canal stenosis, which can cause neural compression and clinical manifestations. Sometimes the dura mater and periosteum are tightly adherent at the compression site of spinal canal stenosis, increasing the difficulty of the operation.

The volume of spinal canal also varies with body position. During extension, the volume of cervical spinal canal decreases, and the ligamentum flavum folds toward the spinal canal. If there is preexisting spinal stenosis, the spinal cord can be more compressed. This should be considered when positioning the patient for surgery because an overextended neck can make the spinal cord vulnerable to injury.

Intervertebral Foramina

Bilateral IVF are located between all of the adjacent vertebrae from C2 to the sacrum. The sacrum also has a series of paired dorsal and ventral foramina. The IVF lie posterior to the vertebral bodies and between the inferior and superior pedicle notches of adjacent vertebrae. The width of the pedicles in the horizontal plane gives depth to these openings, actually making them neural canals rather than foramina, but the name IVF remains. In addition to the pedicle above (roof) and the pedicle below (floor), the neural foramen or IVF is also formed by (1) the IVD and vertebral bodies above and below (anterior wall), (2) the ligamentum flavum, and (3) the facet joint (posterior wall) (see Fig. 4.4G–J ).

The dimension of the IVF varies by region. They are smallest in the cervical region, and generally there is a gradual increase in IVF dimensions to the L4 vertebra. In addition, because the IVF is formed by two moveable vertebrae and joints between these two vertebrae (the IVD and facet joints), the superior-inferior dimension of the IVFs can change more than 44% during flexion and extension, becoming larger in spinal flexion and smaller in spinal extension.

A number of structures pass through the IVF, including: (1) the spinal nerve (union of dorsal and ventral roots) and the dural root sleeve, (2) the spinal branch of the segmental artery, (3) communicating (intervertebral) veins between the internal and external vertebral venous plexuses, (4) the meningeal branches of the spinal nerves (also known as recurrent meningeal nerves, sinuvertebral nerves, or recurrent nerves of Luschka ), (5) lymphatic channel(s), and (6) transforaminal ligaments and adipose tissue that surround all of the aforementioned structures.

The dorsal and ventral roots unite to form the spinal nerve in the region of the IVF, and the spinal nerve is surrounded by the dural root sleeve. The recurrent meningeal nerves are a number of small nerves that originate from the most proximal portion of the ventral ramus. These nerves reenter the IVF and provide sensory innervation (including nociception) to the annulus fibrosus of the IVD, the spinal dura mater, and the ligaments and periosteum of the spinal canal. The nucleus pulposus of the IVD has no nociception innervation.

The transforaminal ligaments are a group of accessory ligaments within the IVF, and they are not always present. When they are present, these ligaments are much more common in the lower thoracic and lumbar regions than in the cervical region. The transforaminal ligament historically had been implicated as an anomalous structure and a cause of both low back pain and nerve root compression. However, the transforaminal ligament is now considered to be a normal anatomic structure with the responsibility of holding other structures (e.g., spinal nerves, vessels) in their proper position within the IVF and protecting them.

Spinal Cord

The spinal cord is 40 to 50 cm long and stretches from the foramen magnum to the first or second lumbar vertebral segment. It is covered by three meninges just like the brain: the pia, the arachnoid, and the dura, which is the outermost layer. The spinal cord is connected to the dura via lateral denticulate ligaments that emerge from the pia. Nerve rootlets protrude from the dorsolateral and ventrolateral aspect of the spinal cord and join distally to form 31 pairs of nerves. Similar to the regional categorization of the spine, the spinal cord is also divided into the cervical, thoracic, lumbar, and sacral regions. Visually the cord is enlarged in size from C3 to T1 and from L1 to S2, known as the cervical enlargement and lumbar enlargement, respectively.

There are 8 cervical nerves, 12 thoracic nerves, 5 lumbar nerves, 5 sacral nerves, and 1 coccygeal nerve. The IVF are openings between vertebral segments that allow the dorsal and ventral nerve roots to leave the spinal column and connect to other parts of the body. Because of particularities of embryonic development, all spinal nerves except those associated with the cervical region exit via foramina below their associated vertebral segment.

Cross-sections of the spinal cord reveal peripheral white matter, inner gray matter, and a central canal lined with ependymal cells, all surrounded by CSF. Lower levels of the cord have a higher ratio of gray matter to white matter relative to higher levels because lower levels have fewer ascending and descending nerve fibers. The gray matter consists mainly of cell bodies of neurons and glia and is divided into the dorsal horn, intermediate column, lateral horn, and ventral horn column. The dorsal horn is found at all levels and is involved in somatosensorial and signal transmission to the midbrain and diencephalon. The intermediate column and lateral horn involve autonomic neurons that innervate organs in the pelvic region. Lastly, the ventral horn involves motor neurons that communicate with skeletal muscle.

Ascending tracts derived from the spinal cord’s white matter arise from the neurons in the dorsal root ganglion and transmit sensory information. The dorsal funiculus in the dorsal column, composed of the ascending gracile and cuneate fasciculi, transmits information related to discrimination of simultaneously applied pressure, vibration, position, and conscious proprioception stimuli. In the lateral column, the neospinothalamic tract carries pain, temperature, and crude touch information. Dorsal and ventral spinocerebellar tracts transmit information from unconscious proprioception of the lower extremities to the cerebellum.

The conus medullaris is the tapered, lower end of the spinal cord. A bundle of spinal nerves and spinal nerve rootlets, which is called cauda equina, arise from the lumbar enlargement and the conus medullaris of the spinal cord. The cauda equina is composed of the second through fifth lumbar nerve pairs, the first through fifth sacral nerve pairs, and the coccygeal nerve, innervating the pelvic organs and lower extremities. In addition, the cauda equina provides sensory innervation of the perineum and, partially, parasympathetic innervation of the bladder. The filum terminale, which is a delicate strand of non-neurogenic leptomeningeal tissue, arises from the end of the conus medullaris and attaches distally to the dorsal aspect of the first coccygeal VB. It can stabilize and tether the lower end of the spinal cord.

Arterial Supply of the Spinal Cord

The arterial supply of the spinal cord originates from the vertebral arteries and segmental arteries. The thoracic spinal cord between T4 and T9 is poorly vascularized. This region is termed the critical vascular zone of the spinal cord and corresponds to the narrowest region of the spinal canal.

Anterior Spinal Artery

At the level of the foramen magnum, before the VA converges to the basilar artery, one branch from each side converges downward to form the anterior spinal artery (ASA), which then courses along the anterior median sulcus of the spinal cord and terminates with the filum terminale (see Fig. 4.4K ). The ASA is reinforced by several anterior segmental medullary arteries, the largest of which is called the artery of Adamkiewicz. This artery can have a variable origin and side but typically arises from the left posterior intercostal artery between T9 and L2. Damage to the artery of Adamkiewicz can theoretically cause paralysis from spinal cord infarction because it is the dominant supplier to the lumbosacral cord segments. The ASA in the midthoracic portion of the cord (T4 to T9) often receives only a single contribution from a small artery located at about T7, most often on the left. The ASA has its smallest diameter in this region, and it is sometimes—but not usually—discontinuous with the vessel in more rostral or caudal regions. The ASA mainly supplies the anterior two-thirds of the spinal cord. ^,

Posterior Spinal Artery

The posterior spinal arteries appear in pairs and arise from the VA or posterior inferior cerebellar artery (see Fig. 4.4L ). They descend along the posterolateral spinal cord and are reinforced by several posterior segmental medullary arteries along the way. Posterior spinal arteries supply the posterior third of the spinal cord.

Radicular Artery

The anterior and posterior radicular arteries run along with the anterior and posterior roots of the spinal nerves and provide blood supply to nerve roots. Radicular arteries can sometimes be replaced functionally by segmental medullary arteries. However, unlike segmental medullary arteries, radicular arteries do not join the anterior or posterior spinal arteries.

Muscles Associated With the Spine

Muscles associated with the spine can be divided into the back muscles group and anterolateral muscles group.

Dorsal Musculature

The muscles in this group are divided into superficial and deep layers.

Superficial Muscles

These muscles originate from spinous processes and terminate at the bones of upper extremities, the superior portion of the humerus, or the ribs. Their main functions are to maintain the movement of the upper limbs and ribs, many of which act on the spine. The trapezius muscle is a large muscle that is located at the neck and upper back; the latissimus dorsi is located at the lower back ( Fig. 4.5A ). Their role in the spine is to pull the spine backward. Under the trapezius muscle and the latissimus dorsi, the levator scapulae muscle can be found in the cervical region, the rhomboid muscles and serratus posterior superior muscles in the thoracic region, and the serratus posterior inferior muscles in the lumbar region (see Fig. 4.5B ).

Deep Muscles

Deep muscles of the back are well developed and extend longitudinally from the sacrum to the base of the skull. They are associated with the posture and movements of the vertebral column. Again, the deep back muscles can be divided into three layers.

First Layer

The first-layer muscles include the splenius capitis and splenius cervicis, which expand from the ligamentum nuchae and upper thoracic spinous processes to the occipital and cervical transverse processes. They play roles in extension, lateral flexion of the neck, and rotation of the head (see Fig. 4.5C ).

Second Layer

The second-layer muscles include the iliocostalis, longissimus, and spinalis. Together these muscles form a column known as the erector spinae, which acts unilaterally to bend the vertebral column laterally and bilaterally to extend the vertebral column and head. All have a common tendinous origin, which arises from lumbar and lower thoracic vertebrae, sacrum, the posterior aspect of the iliac crest, and sacroiliac and supraspinous ligaments (see Fig. 4.5C ).

The iliocostalis muscle, which is located laterally within the erector spinae, arises from the common tendinous origin and attaches to the costal angle of the ribs and the cervical transverse processes. It is associated with the ribs and can be divided into three parts: lumborum, thoracis, and cervicis. The longissimus muscle is situated between the iliocostalis and spinalis, expanding from the common tendinous origin to the lower ribs, C2 to T12 transverse processes, and the mastoid process of the skull. It is the largest of the three columns and can be divided into three parts: thoracic, cervicis, and capitis. The spinalis muscle is located medially within the erector spinae, expanding from common tendinous origin to the spinous processes of C2 and T1–T8, and the occipital bone. It is the smallest of the three muscle columns and can be divided into the thoracic, cervicis, and capitis.

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