Anatomy of the spine

Introduction

A thorough understanding of spinal anatomy requires comprehension of the intimate relationships that exist between neurovasculature, osseous components, and soft tissue throughout the spine. A heterogeneous structure, the spine is a unique structure as segmental anatomy varies extensively throughout its 33 levels. Subtle differences in structure throughout the length of the spine must be fully understood to treat pathology effectively at any given level.

As the spine extends caudally, 31 pairs of spinal nerve roots emanate from the spinal cord, beginning cranial to the first cervical vertebrae and ending with a pair of nerves leaving the coccyx. While both spinal column and cord project from the foramen magnum of the skull, the spinal cord generally tapers as the conus medullaris at the level of the L1–L2 intervertebral disc in adults (range T12-L3); while the osseous spine continues to project caudally culminating in the coccyx. Nerve roots projecting from the conus medullaris form the cauda equina, a collection of axonal structures traveling caudally within the spinal canal to traverse vertebral foramina at each respective lumbar level.

Each vertebra, with the exception of the sacrum, coccyx and first two cervical vertebrae, follows a basic structure [ , ]. As is true with all biology, structure defines function, and the spine is no exception. Spinal anatomy will be illustrated with an emphasis on relevant structures in this chapter, highlighting osseous structures and their articulations, neuroanatomy, ligaments, muscles, relevant vasculature, and surrounding soft tissue.

Osseous spine

Superiorly, the spinal column articulates with the occiput and terminates caudally as the coccyx near the anus. Each vertebra (fused coccyx considered as a whole) consists of a central canal and contributes to bilateral intervertebral (IV) foramina, allowing for the spinal cord and its projections to travel from the brain to target destinations in a coordinated fashion. Although symmetric under normal circumstances in the coronal plane, the spinal column, depicted in Fig. 1.1 , is not rigid or perfectly linear in the sagittal plane but curved. Normal curvature includes cervical lordosis, thoracic kyphosis, lumbar lordosis, and sacral kyphosis (see Chapter 16 ) [ ]. Although debated, mean cervical lordosis has been reported as approximately 40 degrees, with most lordosis occurring at the C1-2 level [ ]. Thoracic kyphosis ranges from 20 degrees to 50 degrees, and lumbar lordosis ranges from 40 degrees to 60 degrees. As illustrated in Fig. 1.1 , the sagittal axis runs from the odontoid process of C2 through the C7-T1 intervertebral disc (IVD) (see Chapter 6 ), anterior to the thoracic spine, through the T12-L1 IV disc, posterior to the lumbar spine, through the lumbosacral joint, and anterior to the rest of the column starting at S2 [ ].

The basic vertebral structure consists of an anterior body and posterior arch with a central canal as displayed in Fig. 1.2 . Vertebral bodies increase in size caudally. Arches consist of lateral pedicles, the strongest components of vertebrae, and posterior laminae. At the interface of the pedicle and lamina, or isthmus, lie the superior and inferior articular processes forming the facet or zygapophyseal joint, and the laterally projecting transverse process. Together with the vertebral body and pedicle, the facet joint (see Chapter 14 ) forms the IV foramen through which the nerve roots exit. Finally, the spinous process projects posteriorly from the fused midline between laminae, increasing in size from the cervical to lumbar spine [ ]. Between the vertebrae are the IVDs, representing 25% of total spinal height, but gradually declining characteristically with aging (see Chapter 6, Chapter 7 ) [ ]. In regards to classification and stability in the face of fracture, the spine is divided into three columns [ ]. The anterior column consists of the anterior longitudinal ligament (ALL) and anterior two-thirds of the vertebral body/IVD, the middle column the posterior longitudinal ligament (PLL) and posterior one-third of the vertebral body/IVD, and the posterior column the pedicles, lamina, spinous and articular processes, and ligaments (see Chapter 12 ).

Cervical spine

The first two vertebrae of the cervical spine have distinctive characteristics and are named the atlas and axis, respectively. These vertebrae function to connect the lower cervical spine to the occiput of the skull and allow for neck rotation and flexion/extension. The atlas (C1), shown in Fig. 1.3 , lacks a body but instead possesses an anterior and posterior arch that connects two lateral masses. The lateral masses possess superior (concave) and inferior articular surfaces, allowing for interaction with the occipital condyles and axis (C2), respectively. The anterior arch has an anterior tubercle and a posterior facet that articulates with the dens or odontoid process of the axis. Lateral to each lateral mass is a transverse process with a transverse foramen. The cephalically traveling vertebral artery exits the transverse foramen of C1 and wraps along the anterior portion of the superior surface of the posterior arch in its respective groove before turning upward to enter the foramen magnum [ , ]. On the anterior and posterior edges of the posterior arch, the medial edges of the vertebral artery groove measure 10 and 18 mm from the posterior midline on average, respectively, with minimum distances of 8 and 12 mm limiting lateral dissection [ ]. The axis ( Fig. 1.4 ) more closely resembles the other cervical vertebrae but has an odontoid process projecting superiorly from the anterior body to articulate with the posterior aspect of the anterior arch of the atlas. The odontoid process measures 15 mm in height on average [ ], and is held against the atlas by the transverse atlantal ligament (TAL) [ ]. The pedicle of the axis is larger than the other cervical vertebrae and lays posteromedial to the transverse foramen, bounded medially by the superior articular facet [ ]. Axis variants include a persistent ossiculum terminale, resembling a Type I dens fracture, as well as an os odontoideum that may be situated near the basion of the foramen magnum or above the base of the dens resembling a Type II dens fracture [ ].

The lower cervical vertebrae (C3-7) resemble each other in structure ( Fig. 1.5 ) and usually have bifid spinous processes with the exception of C7, which has the largest process and is referred to as the vertebra prominens [ ].

In this region, inferior processes are dorsal to the articulating superior processes of caudal vertebrae [ ]. Like the atlas and axis, C3-6 possess transverse foramina for the vertebral arteries, which are lateral to the vertebral body by a mean of 2 mm if viewed anteriorly [ ]. Viewed posteriorly, the transverse foramen lies anteromedial (C3-5) and directly anterior (C6) to the posterior midpoint of the lateral mass. As such, lateral screw placement obviates injury to the vertebral artery if placed perpendicular or 10 degrees lateral to the posterior midpoint of the lateral body at C3-5 and C6, respectively. Additionally, screw length is significant, as the average distance from the posterior midpoint of the lateral body to the transverse foramen at C3-6 has been reported as low as 9 mm [ ]. C3-7 are unique in that they have uncinate processes projecting superiorly from the lateral edges of the vertebral bodies that articulate with their adjacent, superior vertebra [ ].

Thoracic spine

The thoracic spine is unique as it articulates with the ribs through the costovertebral joints. Vertebral bodies have costal facets on their lateral edges that articulate with ribs at their head ( Fig. 1.6 ). The upper thoracic vertebrae have a superior and inferior costal facet, with the superior facet articulating with the same numbered rib and inferior facet with that below. The lower thoracic vertebrae possess a single costal facet oriented laterally to articulate with the associated rib. Additionally, T1-10 possess transverse costal facets on the superior surface of the lateral aspect of each transverse process that articulate with the tubercle off the neck of their respective ribs [ , ]. The articulation between rib and vertebra provides additional stability and contributes to stiffness in the thoracic column. The facet joints between the thoracic vertebrae are oriented in a more coronal plane, providing stability in flexion. The spinal canal is round and provides less free space for the spinal cord than the neighboring portions of the column [ ].

Lumbar spine

The vertebral bodies of the lumbar spine are large, heavy, and kidney shaped ( Fig. 1.7 ). Canal stenosis is more likely at upper lumbar levels as the canal becomes progressively triangular with increasing epidural space caudally (see Chapter 13 ). Facet joints are oriented in the sagittal plane to allow for flexion/extension, with the superior articular facets lateral to inferior articular facets (see Chapter 14 ). Mammillary processes project off the posterior aspect of each superior articular facet. The pedicles are short with a medial inclination and the laminae oriented vertically in the sagittal plane. At the lateral borders of the spinal canal are the lateral recesses, permitting passage of nerve roots in an obliquely downward manner [ , , ]. A height of at least 5 mm is normal for the lateral recess, with anything less contributing to stenotic symptoms [ ]. The IV foramen measures roughly 20 mm in mean height [ ], with measurements less than 15 mm associated with nerve root compression 80% of the time [ ]. Sacralization of L5 is a congenital anomaly, fusing to the ilium and/or sacrum, and may become symptomatic with age.

Sacrum and coccyx

The sacrum ( Fig. 1.8 ) and coccyx are fused segments of five and four bones, respectively. The sacrum functions to transmit body weight from the spine to the pelvis. Kyphosis of the sacrum is approximately 25 degrees, with the apex at S3. L5 sits on the sacral promontory and articulates with S1 at its superior articular facet, located just lateral to the sacral canal and facing posteromedially. At the lateral border of the sacrum are the posterior and larger anterior foramina, which allow for the passage of nerve roots. The sacral canal narrows caudally and ends as the sacral hiatus at S4 or more commonly S5 [ , , ]. Lumbarization, or an unfused S1, may be a symptomatic congenital anomaly.

The lateral sacral mass, identified superiorly as the ala, is composed of the fusion of transverse processes and costal elements and forms the lateral border of the sacrum as well as the sacroiliac joints [ , , ]. The ala expands laterally and is sloped anteroinferiorly at an angle of ~37 degrees caudal to the coronal plane between its superior edge and that of S1 [ ]. Ventrally, transverse ridges mark the fusion of vertebral bodies. Dorsally, spinous processes, fused articular processes, and fused transverse processes comprise the medial, intermediate, and lateral crests, respectively. At S5, the intermediate crest projects inferiorly as the sacral cornua and articulates with the coccygeal cornua [ ].

The coccyx is composed of rudimentary vertebrae. The coccygeal nerve courses under the transverse process of the first coccygeal vertebra after exiting the sacral hiatus. Although roles for movement of the coccyx have been postulated, the coccyx mainly functions as an attachment site for the levator ani, coccygeus and gluteus maximus muscles, as well as the anococcygeal ligament and occasionally filum terminale externum [ ]. Not all vertebrae of the coccyx are fused.

Joints, discs, and ligaments

Intervertebral articulation is made possible by the interaction of many diverse soft tissue structures. The IVDs are avascular, and function to absorb or redistribute energy while facilitating flexibility (see Chapter 6 ). Their structure is composed of an outer annulus fibrosis (AF) surrounding an inner nucleus pulposus (NP). The AF is itself composed of an outer and inner layer, with dense type I collagen on the outside and fibrocartilage and loose type II collagen on the inside. Fibers are oriented in an oblique manner and function to resist tensile loads [ ]. Posteriorly, fibers are oriented relatively vertically and the AF is thinner, responsible for predilection of posterior disc herniation (see Chapter 8 ) [ ]. The NP, composed of proteoglycan, type II collagen and mostly water, resists compressive loads (see Chapters 2 and 3 ) [ ]. Its herniation through the AF may result in nerve root compression, ~95% of which occurs at the L4-5 or L5-S1 IV discs in individuals aged 25–55 years [ ]. The NP may herniate generally through the anterosuperior endplate of its adjacent vertebra creating a limbus vertebra, resembling fracture (see Chapter 10 ).

The outer edges of the AF are consistent with the ALL and PLL. The ALL runs from the skull to the sacrum and resists hyperextension, attaching to the anterior surface of vertebral bodies and IV discs. The ALL thins laterally and over IV discs [ , ]. The PLL also runs from the skull to the sacrum but limits hyperflexion and posterior protrusion of the IV disc, attaching to the posterior surface of vertebral bodies and IVDs. In the cervical spine, the PLL consists of two layers: the deep or ventral layer connects to the AF and extends laterally from the IV foramen to fuse with the lateral margin of the ALL, and the superficial or dorsal layer envelops the dura mater, nerve roots, and vertebral arteries within the spinal canal [ ]. The PLL is broad in the upper cervical spine and over IVDs in the thoracic and lumbar regions [ ].

The IVD and ligaments contributing to spinal stability are demonstrated in Figs. 1.9–1.12 . The ligamenta flava (LF) span the spinal column but are interrupted and not a single, continuous structure. LF attach to the anterior laminae of superior vertebrae and posterior laminae of inferior vertebrae [ ]. Laterally, the LF fuses with the capsule of the facet joint. It is composed of strong, yellow elastic fibers oriented vertically that thicken caudally. Over time, elasticity is lost and LF hypertrophy, leading to stenosis [ ]. The supraspinous ligament is a strong ligament that runs the length of the nonfused spinal column, connecting the tips of spinous processes. It may be variably absent in the lower lumbar spine. Above C7, it is referred to as the ligamentum nuchae, connecting the occipital protuberance to the posterior arch of C1 and spinous processes of C2-6. The ligamentum nuchae is fibroelastic and serves as the attachment site of cervical back muscles [ , ]. Interspinous ligaments are weak and thicken caudally, attaching from the posterosuperior border of the inferior spinous process to the anteroinferior border of the superior spinous process in an oblique orientation [ ]. The intertransverse ligaments limit lateral flexion and connect adjacent transverse processes. They are most pronounced in the lumbar region and lie directly above the lumbar nerve roots lateral to the IV foramina [ ].

Articulations between adjacent, distinct regions of the spinal column represent junctions of morphologically diverse vertebrae, posing challenges to maintaining proper alignment and function. The occipital condyle and superior facet of the atlas form the occipitoatlantal joint ( Fig. 1.10 ), which allows for 25 degrees of flexion//extension. The anterior atlantooccipital ligament is a continuation of the ALL, the posterior atlantooccipital ligament is homologous to LF, and the tectorial membrane is a continuation of the PLL. However, the tectorial membrane functions differently than the PLL: it limits extension and is the primary stabilizer of the occipitoatlantal joint. Caudally, the atlantoaxial joint ( Fig. 1.10 ) consists of the two facet joints and the articulation of the dens (C2) with the anterior arch of C1, as previously described. These articulations supply 50% of cervical rotation, allowing 40 degrees rotation per side, as well as 20 degrees of flexion/extension. The TAL, the primary stabilizer, spans across the anterior arch of the atlas and holds the posterior dens in place at a normal atlas dens interval of ≤3 mm in flexion. The superior and inferior longitudinal ligaments attach the dens to the anterior foramen magnum and body of axis, respectively, and function as secondary stabilizers of the cruciate ligament in conjunction with TAL. The alar ligament functions to limit lateral bending and rotation, and attaches the dens to the occipital condyle; injury to this ligament may result in instability. The apical ligament attaches the dens to the anterior foramen magnum anterior to the superior longitudinal ligament, and the accessory ligament attaches the axis body to the occipital condyle. Both are secondary stabilizers [ ].

To aid stability at the cervicothoracic junction, C7 is in some ways more similar to T1 and T2 compared to the other cervical vertebrae. C7 has a larger body and transverse and spinous processes, and thinner spinal canal and lateral mass [ ]. In the thoracic spine the radiate, intraarticular, costotransverse, superior costotransverse, and lateral costotransverse ligaments ( Fig. 1.11 ) all stabilize thoracic vertebrae with the same numbered rib, while the superior costotransverse ligament attaches the transverse process of a vertebra to the inferior rib [ ]. The iliolumbar ligament attaches the transverse process of L5 to the ala and ilium and inhibits the anterior subluxation of L5 over the sacrum [ ]. Ligaments contributing to the sacroiliac articulation are pictured in Fig. 1.12 .

Muscles and fasciae

Spinal muscles are divided into three groups: anterior, lateral, and posterior (see Chapter 15 ). Anterior and lateral muscles ( Fig. 1.13 ) mostly function in flexion and rotation and are innervated by cranial nerves (CN) or the ventral rami of spinal nerves. Muscles of the posterior back mostly function in extension, rotation, and lateral flexion and are considered in multiple layers: superficial ( Figs. 1.14 and 1.15 ), intermediate ( Fig. 1.16 ), and deep ( Figs. 1.16 and 1.17 ). The origin, insertion, function, and innervation of the anterior, lateral, and posterior muscles of the spine are summarized in Tables 1.1–1.2 , respectively. The erector spinae muscles are the main extensors of the back and include the iliocostalis, longissimus, and spinalis. These three muscles are themselves divided into three parts: cervicis, thoracis, and either capitis or lumborum. The anterior neck possesses several fascia layers to protect the vital structures it contains; this compartmentalizes the neck, aiding in dissection [ , , ]. Functions are summarized in Tables 1.1 and 1.2 [ , ]. To support compartmentalization, the sternocleidomastoid (SCM) is considered to divide the neck into anterior and posterior triangles when viewed anteriorly.