Cervical Spine Anatomy
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An in depth understanding of cervical spine anatomy is essential to the diagnosis and management of cervical spine pathology. From the osseous anatomy down to the soft tissues structures that function to stabilize, maintain, and protect the spinal cord clinicians must be able to appreciate the biomechanics and the complex anatomy. For patients managed operatively, appropriate surgical planning and operative technique rely heavily upon a sound understanding of the intricate anatomy in this region. In this chapter we detail the cervical spine anatomy with particular emphasis on the osseous, muscular, ligamentous, and neurovascular tissues with the goal of providing clinicians a comprehensive review that they can depend on and refer to when treating patients with cervical spine disease.
KeywordsAnatomy Cervical spine Vertebrae Review
Subaxial Cervical Spine
The anterior aspect of the vertebra is a relatively cylindrical structure called the vertebral body. The body withstands the majority of the compressive loads placed on the vertebral column, and each is separated by an intervertebral disc that functions as a shock absorber. With descending levels down the spinal column, the height of the body slightly increases in height, with the occasional exception of C6, which can be shorter than C7. Each level is larger in the coronal plan than in the sagittal plane. The diameter of each body in the coronal plane is larger than that in the sagittal plane. The superior aspect is concave, and the inferior aspect is convex. Both the superior and inferior aspects contain a shell of cortical bone called the end plate, which eventually transitions into the fibrocartilaginous intervertebral disc. In the coronal plane, the lateral superior aspects of the body demonstrate a lip of bone that projects cranially is congruent with the inferior and lateral aspects of the adjacent cranial body and make up the uncovertebral joint, or joint of Luschka. The uncovertebral joint is important in resisting lateral translation of the vertebra and helps limit lateral bending. Intraoperatively, the uncovertebral joints are important anatomical landmarks that aid in identifying the lateral extent of the body and can act as a reference point for identifying the midline when placing implants during anterior-based procedures.
Projecting dorsolaterally each side of the body is the pedicle, which connects the body to the posterior arch. Unlike the pedicles of the thoracic and lumbar regions, those in the subaxial spine are located midway between inferior and superior end plates of the body on the coronal plane. Descending down the subaxial spine from C3 to C7, the pedicle height increases from an average 5.1 to 9.5 mm, and width increases from 3 to 7.5 mm. Average width and height increase from 5 to 7 mm, respectively (An et al. 1999; Ebraheim et al. 1997; Panjabi et al. 1991b). Additionally, the pedicle angle transitions from 45° to 30° as it descends down the subaxial spine.
Projecting laterally off of the posterior aspect of the body, anterior to the pedicle, are the bilateral transverse processes. Each transverse process contains both an anterior and posterior tubercle. The anterior tubercle is the origin of the longus colli cervicalis, anterior scalene, ventral intertransverse, and longus colli muscles. The posterior tubercle is the origin of the longissimus, levator scapulae, middle scalene, posterior scalene, splenius cervicalis, and iliocostalis muscles. The anterior tubercle of the C6 transverse process is also referred to as the carotid tubercle and is an important anatomical landmark, as it marks the transverse proves that separates the carotid artery from the vertebral artery. The transverse processes of C1 through C7 each contain a transverse foramen. The vertebral artery and vein travel through the transverse foramen of C1 through C6, and in the majority of cases, not through C7. The transverse foramen is bound by the lateral aspect of the pedicle, the posterior aspect of the anterior tubercle, and the anterior aspect of the posterior tubercle. Additionally, each transverse process contains a groove on its superior surface that runs posterior to the transverse foramen. This groove carries the exiting nerve at the corresponding level after it exits the neural foramen. For example, the groove on the C3 transverse process contains the exiting C3 nerve root.
Projecting from both the superior and inferior aspects of the lateral mass is an articular process that is congruent with the adjacent articular process of the neighboring vertebra, which together comprises the facet joint. The superior articular process of the vertebra articulates with the inferior articular process of the cephalad vertebra, and the inferior facet of the vertebra articulates with the facet of the superior articular process of the caudal vertebra. The facet joint is a diarthrodial joint with a relatively lax capsule that allows for appropriate motion to occur. In the sagittal plane, these joints are oriented obliquely from anterior-superior to posterior-inferior at approximately 45° (Fletcher et al. 1990). This orientation differs from the relatively vertically oriented facet joints in the coronal plane of the lumbar spine, and in conjunction with the relatively lax capsule, allows for a broad range of motion in flexion, extension, lateral bending, and rotation (Bland 1987). Just as in other diarthrodial joints throughout the body, the facet joint is susceptible to degenerative changes such as joint swelling, cartilage thinning, and osteophyte formation. Given its close proximity to the neural foramina and spinal canal, these changes can have significant clinical implications.
The cervical spinal canal is bordered ventrally by the posterior aspect of the vertebral body, ventrolaterally by the pedicles transverse near the location of the neural foramina, laterally by the lateral masses, and posteriorly by the lamina and spinous process. The lateral diameter of the spinal canal is larger than the anterior-posterior (AP) diameter at all levels of the subaxial spine. The AP diameter is approximately 17 mm at the C3 level and decreases to 15 mm at C7, which has the lowest cross-sectional area.
Located dorsolateral to the pedicle is a cylindrical piece of bone termed the lateral mass. The lateral mass is analogous to the pars interarticularis of the thoracic and lumbar spinal regions, as it is the structure that connects the superior and inferior articular surfaces that make up the cephalad and caudal facet joints. It is directly dorsal to the midportion of the transverse process, and as such it is in very close proximity to the exiting nerve root. It has a bony projection dorsomedially, which becomes confluent with the lamina. The lateral masses of the subaxial cervical spine have an average depth and width of approximately 13 mm and 12 mm, respectively, and slightly decrease at each descending level to C7 where it is thinnest (Mohamed et al. 2012). Given the relatively small pedicle size in this region of the spine, screw fixation within the lateral mass is sometimes a desired treatment option, and thus a proper understanding of the lateral mass size is crucial in avoiding complications.
Lamina and Spinous Process
The dorsomedial projection from the lateral masses is termed the lamina. As they continue posteriorly bilaterally, they merge to form the posterior-most bony prominence called the spinous process. The C2 through C6 vertebra are normally bifid, but the C7 spinous process is not. The lamina and the spinous process make up the posterior aspect of the spinal canal. An important anatomic landmark is the junction of the lamina and spinous process posteriorly.
The subaxial cervical spine contains an array of ligaments. The ligaments contribute significantly to the stability and alignment of the bony structures in the region and allow for motion in various planes while restricting extremes of motion that could compromise proper anatomic alignment and integrity of the local structures.
The anterior and posterior aspects of the vertebral bodies and intervertebral discs are bound by both the anterior and posterior longitudinal ligaments. The anterior longitudinal ligament (ALL) is composed of longitudinal fibers that run in a cranial and caudal direction spanning the base of the skull to the sacrum. The ALL attaches to the anterior surfaces of the vertebral bodies and intervertebral discs and acts as a restraint to hyperextension of the mobile segments of the vertebral column. The ALL is narrow and thick over the concave surface of the vertebral bodies but becomes more wide and thin over the discs. The posterior longitudinal ligament (PLL) also spans the length of the vertebral column, fanning out to form the tectorial membrane at its most cranial aspect, and attaches to the sacrum caudally. Just like the ALL, the PLL is more narrow over the bodies and wide over the discs (Parke and Sherk 1989) (Fig. 1).
The ligamentum nuchae is composed of the interspinous and supraspinous ligaments. The interspinous ligament is a relatively thin structure that connects the spinous processes of adjacent vertebra. It runs obliquely from the anteroinferior aspect of the cephalad spinous to the posterosuperior aspect of the caudal spinous process. It is bound by the ligamentum flavum anteriorly and the supraspinous ligament posteriorly. The supraspinous ligament connects the posterior tips of the spinous processes along the length of the vertebral column. However, in the subaxial spine, these two ligaments are less distinct as individual structures until the level of C7 but rather form a complex of thick ligamentous elastic tissue that is referred to the ligamentum nuchae. The ligamentum nuchae runs from the inion of the occiput to the spinous process of C7 and acts as an attachment point for the nuchal musculature in the region.
The cervical spine contains six intervertebral fibrocartilaginous discs which separate the vertebral bodies (Fig. 1). There is no disc between the occiput and the atlas or between the atlas and the axis. The first disc is located between C2 and the C3 body. The junction of the disc with the adjacent bodies is lined by a cartilaginous layer termed the end plates. The disc itself is composed of two primary components – the nucleus pulposus and the annulus fibrosus. The nucleus pulposus is the centrally located portion of the disc that is the remnant of the primitive notochord and is comprised with primarily type II collagen, proteoglycans, and water. This makeup of the nucleus pulposus results in a gelatinous type substance that allows for force dissipation to the annulus fibrosis and both end plates when compression is applied to the vertebral column. The annulus fibrosus is the component of the disc that surrounds the nucleus pulposus circumferentially and composed of type I collagen, proteoglycans, and water. It is characterized by multiple circumferential layers of fibers that run in an oblique pattern from the cephalad to caudal vertebral bodies. The annulus fibrosus has a high tensile strength that contributes to the stability within a pair of vertebra, which is assisted in the lateral direction by the uncovertebral joint. As the aging process progresses, the margin between the nucleus pulposus and annulus fibrosus becomes more difficult to distinguish (Bland and Boushey 1990). In the coronal view, the superior aspect of the disc is concave, and the inferior aspect is convex as to contour its respective adjacent end plates. The height of the intervertebral disc is slightly larger anteriorly than posteriorly, which contributes to the lordotic curvature of the cervical spine.
The next layer is the pre-tracheal layer, which houses many structures and likewise is referred to by multiple names including the middle cervical fascia or the visceral layer. This multifaceted aponeurosis envelops the infrahyoid muscles as well at the omohyoid muscles which lie just superficial to the visceral space. This space is residence to important soft tissue structures such as the thyroid gland, larynx, trachea, and esophagus and deep to this layer run the thyroid vessels. Its superior attachments are the hyoid and thyroid cartilage and inferiorly to the clavicles and sternum. The carotid sheath makes up its lateral margin (Fig. 6).
The prevertebral layer is a thick fascial plane that surrounds the vertebral column and its muscles. This layer includes the longus and the scalene muscles. The longus colli is a notable structure that aids in establishing midline during an anterior cervical approach. Identifying this structure also helps protect the cervical portion of the sympathetic chain during anterior dissection by retracting laterally. The alar layer is also included as part of the prevertebral layer and encloses the carotid sheath, which houses the vagus nerve, carotid artery, and internal jugular vein (Fig. 6).
The dorsal muscle groups provide tension to the vertebrae to keep them in an upright position and deliver balance as well. These muscles are innervated by the dorsal rami. The erector muscles take advantage of the tension band principle to provide sagittal support and symmetric balance in an effort to preserve lordosis of the cervical region. Loss of strength, often attributed to pain, can lead to progressive loss of lordosis and a relative kyphotic deformity. In the coronal plane, the lateral tension bands provide support. Imbalance in any of these planes can lead to deformity seen in abnormal cervical spine curves. All the muscles in the dorsal compartment spread out into three layers discussed below.
The transversospinalis group makes up the deepest layer and lies along the spinous process and lamina of the cervical spine. They consist of the multifidus and rotator muscles. They are innervated by the dorsal rami of the spinal nerves of the cervical spine (Fig. 10).
The outer circumferential layer is the white matter and is composed of myelinated axons. In a similar manner to the gray matter, it is separated into posterior, lateral, and anterior columns. The lateral column houses the lateral corticospinal tract which provides efferent motor innervation control ipsilateral extremity motion. Also within the lateral column is the lateral spinothalamic tract which is sensory pathway that transmits contralateral pain and temperature. This tract decussates to the other side of the spinal cord at the anterior white commissure, usually 1–2 spinal nerve segments above the entry point. The posterior column, composed of the fasciculus gracilis and cuneatus, is the structure responsible for ascending sensory signals transmitting proprioception, vibration, and fine touch. Sensory information from this pathway is also from the contralateral extremity although its crossover is much higher, located in the brain stem. The anterior column houses both sensory and motor systems, as well as the anterior spinothalamic tract which is responsible for crude touch.
Meninges and Dura
In the cervical spine, there are eight rootlets that exit the spinal cord that unite and form the dorsal and ventral roots. These form nerve roots at each corresponding level and pass through the dura to the intervertebral foramina. In the cervical spine, the nerve roots pass above the corresponding pedicle, except for the C8 nerve root which travels underneath the C7 pedicle. These nerves also leave the spinal cord at an angle that approximates a right angle and explains why foraminal and central herniations will affect the same nerve root. In the foramen, the nerve root takes up one-third of the space, medially it is located at the caudal portion of the superior articular process and as it travels laterally adopts a more inferior position above the pedicle (Daniels et al. 1986). When the neck is extended, the foramen size decreases in overall volume, and the nerve takes up a more superior position within the foramen; when flexed, the foramen size increases, and the nerve root assumes a position in the caudal half of the foramen (Rauschning 1991). The remaining space is filled with fat, which provides cushion to the nerve (Flannigan et al. 1987).
Spinal Cord Blood Supply
The vertebral arteries are the primary blood supply to the cervical spine which branch of the subclavian arteries and ultimately form the basilar artery. In general, each vertebral artery enters the transverse foramen at C6 and courses rostrally until C1 (Rickenbacher et al. 1982). It is important to note, during an anterior approach that the vertebral artery is located in the middle one-third of the vertebral body, just lateral to the uncinated process. At the atlas, the vertebral arteries curve around and enter the foramen magnum to unite with the contralateral artery to become the basilar artery. Throughout their course, they give off feeding branches to the spinal cord known as the anterior and posterior spinal arteries. The anterior spinal artery supplies the anterior two-thirds of the spinal cord, while the posterior spinal artery assumes the remaining one-third.
Venous outflow of the spinal cord consists of three anterior and three posterior veins. The most prominent are the anterior venous structures and are located medial to the pedicles. The posterior venous plexus surrounds the spinal cord.
Important Ventral Structures
Superior Laryngeal Nerve
A branch off the vagus nerve, the superior laryngeal nerve runs medial to the carotid sheath and bifurcates at the level of the hyoid to provide motor function the inferior pharyngeal constrictors. It also has sensory branch that provides sensation to the base of the tongue and the larynx. Injury to this nerve can be manifested with poor gag reflex and voice control especially with high pitches. Loss of the gag reflex can be most debilitating as these patients are often at increased risk for aspiration.
Inferior (Recurrent) Laryngeal Nerve
The inferior laryngeal nerve, commonly known as the recurrent laryngeal nerve, has a U-shaped course in the thorax, specifically in the tracheoesophageal groove. As it pierces the inferior pharyngeal constrictor, it provides motor function to the intrinsic laryngeal muscles. Its course in the neck is not symmetric. On the left, it loops under the aortic arch, and on the right, it loops under the right subclavian artery.
The hypoglossal can be located in the carotid triangle, deep to the belly of the digastric muscle, and as previously discussed, in between the carotid artery and internal jugular vein. Before heading toward the oral cavity to innervate the tongue, it gives off a branch to innervate the strap muscles, which is termed the ansa cervicalis.
The sympathetic chain resides in the prevertebral fascia, just ventral to the longus colli muscles. It surrounds the vertebral artery during its ascension toward the cranial vault. Injury to this structure during an anterior approach can cause ipsilateral Horner syndrome, which is characterized by ptosis, miosis, and anhidrosis.
Surgical Anatomy: The Cervical Triangles
Ventral (4 Types)
Dorsal (2 Types)
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