Vincent J Devlin MD and Darren L Bergey MD

OSTEOLOGY

1. Describe the bony landmarks of the occiput.

The occiput forms the posterior osseous covering for the cerebellum. The foramen magnum is the opening through which the spinal cord joins the brainstem. The anterior border of the foramen magnum is termed the basion (clivus), and the posterior border is termed the opisthion. The inion or external occipital protuberance is the midline region of the occiput where bone is greatest in thickness. The superior and inferior nuchal lines extend laterally from the inion. The transverse sinus is located in close proximity to the inion (Fig. 1-1). The occipital area in the midline below the inion is the ideal location for screw insertion for occipitocervical fixation as it is the thickest portion of the occiput.

Figure 1-1. Posterior and lateral views of the occiput and cervical spine showing the basic bony anatomy. 1, Spinous process; 2, Lateral articular process or lateral mass; 3, Transverse process of C1; 4, Odontoid process of C2; 5, Foramen magnum; 6, Inferior nuchal line; 7, Inion; 8, Ligamentum nuchae; 9, Posterior arch of C1; 10, Spinous process of C2; 11, Lateral mass; 12, Supraspinous ligament;

13, Lateral articular process;

14, Uncinate process; 15, Anterior tubercle of transverse process;

16, Neural foramen; 17, Transverse foramen; 18, Carotid tubercle; 19, Intervertebral disc. (From An HS, Simpson JM. Surgery of the Cervical Spine. Baltimore: Williams & Wilkins; 1998, with permission.)

2. What is meant by typical and atypical cervical vertebrae?

C3, C4, C5, and C6 are defined as typical cervical vertebrae because they share common structural characteristics. In contrast, C1 (atlas), C2 (axis), and C7 (vertebra prominens) possess unique structural and functional features and are therefore termed atypical cervical vertebrae.

3. Describe a typical cervical vertebra.

The components of a typical cervical vertebra (C3-C6) include an anterior body and a posterior arch formed by lamina and pedicles. The lamina blend into the lateral mass, which comprises the bony region between the superior and inferior articular processes. The paired superior and inferior articular processes form the facet joint. The uncovertebral (neurocentral) joints are bony ridges that extend upward from the lateral margin of the superior surface of the vertebral body. The intervertebral foramina protect the exiting spinal nerves and are located behind the vertebral bodies between the pedicles of adjacent vertebra. The transverse processes of the lower cervical spine are directed anterolaterally and composed of an anterior costal element and a posterior transverse element. The transverse foramen, located at the base of the transverse process, permits passage of the vertebral artery. The spinous process originates in the midsagittal plane at the junction of the lamina and is bifid between C2 to C6 (Fig. 1-2).

Foramen transversarium

Lateral mass'

Neurocentral lips

Neurocentral lips

Foramen transversarium

Lateral mass'

Intervetebral foramen

Anterior tubercle

Posterior tubercle of transverse process

Bifid spinous process

Intervetebral foramen

Bifid spinous process

Anterior tubercle

Posterior tubercle of transverse process

Figure 1-2. Typical cervical vertebra (superior view). (From Raiszadeh K, Spivak JM. Spine. In: Spivak JM, DiCesare PE, Feldman DS, et al., editors. Orthopaedics: A Study Guide. New York: McGraw-Hill; 1999, p. 63-72, with permission.)

4. What are the distinguishing features of C1 (atlas)?

The ring-like atlas (C1) is unique because during development its body fuses with the axis (C2) to form the odontoid process. Thus, the atlas has no body. It is composed of two thick, load-bearing lateral masses, with concave superior and inferior articular facets. Connecting these facets are a relatively straight, short anterior arch and a longer, curved posterior arch. The anterior ring has an articular facet on its posterior aspect for articulation with the dens. The posterior ring has a grove on its posterior-superior surface for the vertebral artery. The weakest point of the ring is at the narrowed areas where the anterior and posterior arches connect to the lateral masses (location of a Jefferson fracture). The transverse process of the atlas has a single tubercle, which protrudes laterally and can be palpated in the space between the tip of the mastoid process and the ramus of the mandible (Fig. 1-3).

Odontoid process of C2

Anterior tubercle

Anterior arch

Transverse process

Foramen transversarium

Anterior tubercle

Anterior arch

Foramen transversarium

Posterior arch w

Posterior arch w

Transverse ligament

Figure 1 -3. Atlas (superior view). (From Raiszadeh K, Spivak JM. Spine. In: Spivak JM, DiCesare PE, Feldman DS, et al., editors. Orthopaedics: A Study Guide. New York: McGraw-Hill; 1999, p. 63-72, with permission.)

5. What are the distinguishing features of C2 (axis)?

The axis (C2) receives its name from its odontoid process (dens), which forms the axis of rotation for motion through the atlantoaxial joint (Fig. 1-4). The dens is a bony process extending cranial from the body of C2, formed from the embryologic body of the atlas (C1). The dens has an anterior hyaline articular surface for articulation with the anterior arch of C1 as well as a posterior articular surface for articulation with the transverse ligament. The C2 superior articular processes are located anterior and lateral to the spinal canal while the C2 inferior articular processes are located posterior and lateral to the spinal canal. The articular processes are connected by the pars interarticularis. Hyperflexion or hyperextension injuries may subject the axis to shear stresses, resulting in a fracture through the pars region (termed a hangman's fracture). The C2 pedicle is defined as that portion of the C2 vertebra connecting the dorsal elements with the vertebral body. This is a narrow area between the vertebral body and the pars articularis. The atlantodens interval is the space between the hyaline cartilage surfaces of the anterior tubercle of the atlas and the anterior dens. Normal adult and childhood measurements are 3 mm and 5 mm, respectively.

Odontoid process

Odontoid process

of C2-3 joint transversarium

Figure 1-4. Axis (lateral view). (From Raiszadeh K, Spivak JM. Spine. In: Spivak JM, DiCesare PE, Feldman DS, et al., editors. Orthopaedics: A Study Guide. New York: McGraw-Hill; 1999. p. 63-72, with permission.)

of C2-3 joint transversarium

Figure 1-4. Axis (lateral view). (From Raiszadeh K, Spivak JM. Spine. In: Spivak JM, DiCesare PE, Feldman DS, et al., editors. Orthopaedics: A Study Guide. New York: McGraw-Hill; 1999. p. 63-72, with permission.)

6. What are the distinguishing features of C7 (vertebra prominens)?

The unique anatomic features of the C7 vertebra reflect its location as the transitional vertebra at the cervicothoracic junction:

• Long non-bifid spinous process, which provides a useful landmark

• Its foramen transversarium usually contains vertebral veins but usually does not contain the vertebral artery, which generally enters the cervical spine at the C6 level

• The C7 transverse process is large in size and possesses only a posterior tubercle

• The C7 lateral mass is the thinnest lateral mass in the cervical spine

• The inferior articular process of C7 is oriented in a relatively perpendicular direction (like a thoracic facet joint) ARTICULATIONS, LIGAMENTS, AND DISCS

7. Describe how normal range of motion is distributed across the cervical region.

Facet joint orientation, bony architecture, intervertebral discs, uncovertebral joints, and ligaments all play a role in determining range of motion at various levels of the cervical spine. Approximately 50% of cervical flexion-extension occurs at the occiput-C1 level. Approximately 50% of cervical rotation occurs at the C1-C2 level. Lesser amounts of flexion-extension, rotation, and lateral bending occur segmentally between C2 and C7.

8. What are the key anatomic features of the atlantooccipital (O-C1) articulation?

The atlantooccipital joints are synovial joints comprised of the convex occipital condyles that articulate with the concave lateral masses of the atlas. Motion at the O-C1 segment is restricted primarily to flexion-extension due to bony and ligamentous constraints and absence of an intervertebral disc. The most important ligaments are the paired alar ligaments (extend from the tip of the dens to the medial aspect of each occipital condyle and restrict rotation of the occiput on the dens). The tectorial membrane is also important (continuous with the posterior longitudinal ligament and extends from the posterior body of C2 to the anterior foramen magnum and occiput). Less important ligaments include the anterior and posterior atlanto-occipital membrane, the O-C1 joint capsules, and the apical ligament (Fig. 1-5).

Figure 1-5. Ligamentous and bony anatomy of the upper cervical region. 1, Anterior tubercle; 2, Superior articular facet; 3, Vertebral artery; 4, Anterior longitudinal ligament; 5, Anterior atlas-axis membrane; 6, Anterior arch of atlas; 7, Apical ligament; 8, Vertical cruciform ligament; 9, Anterior atlas-occipital membrane; 10, Attachment of tectorial membrane; 11, Anterior edge of foramen magnum; 12, Tectorial membrane; 13, Vertebral artery; 14, Atlas; 15, Transverse ligament;

16, Origin of tectorial membrane;

17, Posterior longitudinal ligament;

18, Spinous process (axis); 19, Atlas; 20, Transverse ligament; 21, Dens (odontoid process); 22, Alar ligament; 23, Deep tectorial membrane. (From An HS, Simpson JM. Surgery of the Cervical Spine. Baltimore: Williams & Wilkins; 1998, with permission.)

9. What are the key anatomic features of the atlantoaxial (C1-C2) articulation?

The atlantoaxial articulation is composed of three synovial joints—paired lateral mass articulations and a central articulation between the dens and the anterior C1 arch and transverse ligament (see Fig. 1-5). The primary motion at the atlantoaxial joint is rotation with approximately 50% of rotation of the cervical spine occurring at the C1-C2 joints. The approximation of the odontoid against the anterior arch of C1 resists translation of C1 relative to C2.

The transverse atlantal ligament, the major stabilizer at the C1-C2 level, attaches to the medial aspect of the lateral masses of the atlas (see Fig. 1-5). This ligament has a wide middle portion where it articulates with the posterior surface of the dens. Superior and inferior longitudinal fasciculi extend to insert on the anterior foramen magnum and the posterior body of the axis respectively. These structures are collectively named the cruciform ligament. This ligament holds the dens firmly against the anterior arch of the atlas. Other important ligaments attaching to C2 include:

• Anterior atlantoaxial ligament—continuous with the anterior longitudinal ligament in the lower cervical spine

• Posterior atlantoaxial ligament—continuous with the ligamentum flavum in the subaxial spine

• Apical ligament—extends from the tip of the dens to the foramen magnum

• Alar ligaments—extend from the lateral dens and attach to the medial border of the occipital condyles

10. Name the arrangement of ligaments at the craniovertebral junction as the spine is sectioned in an anterior to posterior direction.

1. Anterior atlantooccipital membrane (continuous with anterior longitudinal ligament)

2. Apical ligament (extends from tip of the dens to anterior edge of foramen magnum)

3. Alar ligaments (extend from the tip of the dens to the medial aspect of each occipital condyle)

4. Cruciform ligament

5. Tectorial membrane (continuous with the posterior longitudinal ligament)

6. Posterior atlantooccipital membrane (continuous with the ligamentum flavum)

11. Describe the ligament anatomy of the subaxial spine.

The major ligaments of the subaxial cervical spine are:

• Anterior longitudinal ligament (ALL)—this strong ligament extends from the body of the axis to the sacrum binding the anterior aspect of the vertebral bodies and intervertebral discs together. It resists hyperextension of the spine and gives stability to the anterior aspect of the disc space. It is continuous with anterior atlanto-occipital membrane.

• Posterior longitudinal ligament (PLL)—this is a weaker ligament, which extends from the axis to the sacrum. It is thicker and wider in the cervical spine than in the thoracolumbar segments. It serves to protect from hyperflexion injury and reinforces the intervertebral discs from herniation. It is continuous with tectorial membrane.

• Ligamentum flavum—this structure may be considered to be a segmental ligament which attaches to adjacent lamina. This structure attaches to the ventral aspect of the superior lamina and the dorsal aspect of the inferior lamina. Laterally, the ligamentum flavum is in continuity with the facet capsules.

• Interspinous and supraspinous ligaments—these ligaments lie between or dorsal to the spinous processes, respectively. The supraspinous ligament is in continuity with the ligamentum nuchae, which runs from C7 to the occiput and acts as a posterior tension band to maintain an upright neck posture.

12. Describe the articulations between vertebrae in the subaxial cervical spine (C3-C7).

The anatomy of the lower cervical spine (C3-C7) can be described in terms of a functional spinal unit consisting of two adjacent vertebrae, an intervertebral disc, and related ligaments and joint capsules. The anterior elements include the vertebral body and intervening intervertebral disc. Paired lateral columns consist of pedicles, lateral masses, and facet joints. The posterior structures include the laminae, spinous processes, and posterior ligamentous complex. Various theories have conceptualized the functional anatomy of the cervical spine in terms of a columnar structure. (Fig. 1-6).

Posterior ligamentous complex

• Facet capsules

• Interspinous ligaments

Anterior ligamentous complex

• Anterior longitudinal ligament — • Annulus fibrosus

Middle ligamentous complex

• Posterior longitudinal ligament Annulus fibrosus

Anterior ligamentous complex

• Anterior longitudinal ligament — • Annulus fibrosus

Anterior column I Middle column ^Posterior column

Figure 1-6. Components of the three columns of the cervical spine. (From Stauffer ES, MacMillan M. Fractures and dislocations of the cervical spine. In: Rockwood CA, Green DP, Bucholz RW, et al., editors. Fractures in Adults, vol. 2. 4th ed. Philadelphia: Lippincott-Raven; 1996, p. 1473-1628, with permission.)

13. What are the unique features of the subaxial cervical facet joints?

At each cervical level (C3-C7) there are paired superior and inferior articular processes. The superior articular process is positioned anterior and inferior to the inferior articular process of the adjacent cranial vertebra. These articulations are covered with hyaline cartilage and form synovial zygapophyseal (facet or Z) joints. The orientation of the facet joints is a major factor in the range of motion of the cervical spine. The typical cervical facet joints are oriented 45° in the sagittal plane and 0° in the coronal plane. These are the most horizontally oriented regional facet joints in the spinal column. Laxity of the joint capsule permits sliding motion to occur and explains why unilateral or bilateral dislocation without fracture may occur. The orientation of these facets allows flexion and extension, lateral bending, and rotation of the lower cervical spine. Flexion and extension are greatest at the C5-C6 and C6-C7 levels. This has been postulated to be responsible for the relatively high incidence of degenerative changes noted at these two cervical levels.

14. What are the uncovertebral joints (joints of Luschka)?

When viewed anteriorly, the lateral margin of the superior surface of each subaxial cervical vertebral body extends cranially as a bony process called the uncinate process. These processes articulate with a reciprocal convex area on the inferolateral aspect of the next cranial vertebral body. This articulation is named the uncovertebral joint or neurocentral joint of Luschka. It is believed to form as a degenerative cleft in the lateral part of the annulus fibrosus. The uncinate process, unique to the cervical spine, serves as a "rail" to limit lateral translation or bending and as a guiding mechanism for flexion and extension.

15. What are the components of the intervertebral disc?

Each intervertebral disc is composed of a central gel-like nucleus pulposus surrounded by a peripheral fibrocartilaginous annulus fibrosus. The endplates of the vertebral bodies are lined with hyaline cartilage and bind the disc to the vertebral body. The annulus fibrosus (predominantly type 1 collagen) attaches to the cartilaginous endplates via collagen fibers, which run obliquely at a 30° angle to the surface of the vertebral body and in a direction opposite to the annular fibers of the adjacent layer. The nucleus pulposus is composed primarily of glycosaminoglycans and type 2 collagen, which have the capacity to bind large amounts of water. In a normal healthy disc, loads acting on the disc are transferred to the annulus by swelling pressure (intradiscal pressure) generated by the nucleus. With aging, biochemical changes occur which limit the ability of the nucleus pulposus to bind water. Dehydration of the nucleus and increased loading of the annulus occurs. Fissuring and disruption of the annulus develops and migration of nuclear material through the annulus may occur.

NEURAL ANATOMY

16. Describe the cross-sectional anatomy of the spinal cord and the location and function of the major spinal cord tracts.

A cross-sectional view of the spinal cord demonstrates a central butterfly-shaped area of gray matter and peripheral white matter (Fig. 1-7). The central gray matter contains the neural cell bodies. The peripheral white matter contains the axon tracts. Tracts are named with their point of origin first. Ascending (afferent tracts) carry impulses toward the brain, whereas the descending (efferent tracts) carry nerve signals away from the brain. The axon tracts may receive and transmit signals to the same side of the body (uncrossed tracts) or may transmit or receive signals from the opposite side (crossed tracts). The major spinal tracts important to the clinician include:

• Corticospinal tracts: The lateral corticospinal tract (pyramidal tract) is a descending tract located in lateral portion of the cord that transmits ipsilateral motor function. The tract is anatomically organized with efferent motor axons to the cervical area located medially and sacral efferent axons located laterally. The anterior corticospinal tract is a crossed tract, which facilitates skilled movements.

• Spinothalamic tracts: Ascending tracts located in the anterior and lateral portion of the cord that transmit sensations of pain and temperature. Light touch sensation is carried primarily in the ventral spinothalamic tract. These tracts cross shortly after entering the spinal cord and therefore transmit sensations from the contralateral side of the body.

• Dorsal column tracts: Ascending tracts that convey proprioception, vibration, and discriminative touch sensation from the ipsilateral side of the body.

Ascending (sensory) tracts

Descending (motor) tracts

Posterior

Dorsal column

Posterior

Lateral spinothalamic tract

Lateral corticospinal tract

Anterior tract

Lateral corticospinal tract

Figure 1-7. Cross-sectional anatomy of the spinal cord. (From Raiszadeh K, Spivak JM. Spine. In: Spivak JM, DiCesare PE, Feldman DS, et al., editors. Orthopaedics: A Study Guide. New York: McGraw-Hill; 1999, p. 63-72, with permission.)

Lateral spinothalamic tract

Anterior spinothalamic tract

Anterior corticospinal

Anterior tract

17. How many spinal nerves exit from the spinal cord?

The spinal nerves exit from the spinal cord in pairs. There are 31 pairs of spinal nerves: 8 cervical, 12 thoracic, 5 lumbar, 5 sacral, and 1 coccygeal nerve root pairs.

18. What structures contribute to the formation of a spinal nerve? What are the branches of a spinal nerve?

Each spinal nerve (Fig. 1-8) is composed of both sensory and motor fibers. The collection of sensory fibers is termed the dorsal root. The cell bodies for these sensory fibers are located in the dorsal root ganglion. The collection of motor fibers is termed the ventral or anterior root. A typical spinal nerve is formed by the union of the dorsal and ventral roots, which occurs just distal to the dorsal root ganglion. The spinal nerve becomes covered by a common dural sheath and gives off the following branches:

• Dorsal ramus: Provides sensation to the medial two-thirds of the back, the facet joint capsules, and the posterior ligaments. The dorsal ramus also innervates the deep spinal musculature.

• Ventral ramus: Supplies all other skin and muscles of the body. In the cervical and lumbar regions the ventral rami form plexuses (cervical plexus, brachial plexus, lumbar plexus, lumbosacral plexus). In the thoracic levels ventral rami form the intercostal nerve.

• Recurrent meningeal branch (sinuvertebral nerve): Innervates the periosteum of the posterior aspect of the vertebral body, basivertebral and epidural veins, epidural adipose tissue, posterior annulus and posterior longitudinal ligament, and anterior aspect of the dural sac.

Figure 1-8. Components of a spinal nerve. 1, Spinal ganglion; 2, Dentate ligament; 3, Pia mater; 4, Dorsal root of spinal nerve; 5, Dura mater; 6, Subdural space; 7, Periosteum; 8, Epidural space; 9, Arachnoid membrane; 1o, Subarachnoid space; 11, Dorsal ramus; 12, Spinal nerve;

13, Ventral ramus of spinal nerve;

14, Ramus communicans; 15, Periosteum; 16, Medulla spinalis; 17, Dura mater; 18, Ventral root of spinal nerve. (From An HS, Simpson JM. Surgery of the Cervical Spine. Baltimore: Williams & Wilkins; 1998, with permission.)

Figure 1-8. Components of a spinal nerve. 1, Spinal ganglion; 2, Dentate ligament; 3, Pia mater; 4, Dorsal root of spinal nerve; 5, Dura mater; 6, Subdural space; 7, Periosteum; 8, Epidural space; 9, Arachnoid membrane; 1o, Subarachnoid space; 11, Dorsal ramus; 12, Spinal nerve;

13, Ventral ramus of spinal nerve;

14, Ramus communicans; 15, Periosteum; 16, Medulla spinalis; 17, Dura mater; 18, Ventral root of spinal nerve. (From An HS, Simpson JM. Surgery of the Cervical Spine. Baltimore: Williams & Wilkins; 1998, with permission.)

19. Describe the relationship of the exiting spinal nerve to the numbered vertebral segment for each spinal region.

In the cervical region there are eight cervical nerve roots and only seven cervical vertebra. The first seven cervical nerve roots exit the spinal canal above their numbered vertebra. For example, the C1 root exits the spinal column between the occiput and the atlas (C1). The C5 nerve root passes above the pedicle of the C5 vertebra and occupies the intervertebral foramen between C4 and C5. The C8 nerve root is atypical because it does not have a corresponding vertebral element and exits below the C7 pedicle and occupies the intervertebral foramen between C7 and T1. In the thoracic and lumbar spine, the nerve roots exit the spinal canal by passing below the pedicle of their named vertebra. The T12 nerve passes below the T12 pedicle and exits the neural foramen between T12 and L1. The L4 nerve root passes beneath the L4 pedicle and exits the neural foramen between L4 and L5.

20. How does the course of the recurrent laryngeal nerve differ from left to right?

The recurrent laryngeal nerve originates from the vagus nerve and enters the tracheoesophageal groove. On the right side, it passes around the subclavian artery; on the left side, it passes under the aortic arch. Anterior surgical exposure of the lower cervical spine must be carefully performed in the interval between the tracheoesophageal sheath and carotid sheath to avoid injury to this nerve. The right recurrent laryngeal nerve is at greater risk of injury than the left nerve during surgical exposure because it reaches the tracheoesophageal groove at a higher cervical level and has a less predictable course

VASCULAR STRUCTURES OF THE CERVICAL REGION

21. Describe the course of vertebral artery.

The vertebral artery (Fig. 1 -9) is the first branch off the subclavian artery and provides the major blood supply to the cervical spinal cord, nerve roots, and vertebrae. It can be divided into four segments. During its first segment, the vertebral artery passes from the subclavian artery anterior to C7 to enter the C6 transverse foramen. In the second segment, it continues from the C6 transverse foramina along its course through the cephalad transverse foramina to the level of the atlas. During its course it lies lateral to the vertebral body and in front of the lateral mass. During its upward course between C6 and C2, the vertebral artery gradually shifts to an anterior and medial position, thereby placing the artery at greater risk of injury during anterior decompressive procedures at the upper cervical levels. In its third segment, the artery exits C1 and curves around the C1 lateral mass, running medially along the cranial surface of the posterior arch of C1 in its sulcus, before passing through the atlantooccipital membrane and entering the foramen magnum. The artery stays at least 12 mm lateral from midline of C1, making this a safe zone for dissection. In its fourth segment, the vertebral artery joins the contralateral vertebral artery to form the basilar artery.

Vertebral artery

Vertebral artery

Figure 1-9. Vertebral artery anatomy. (From Emery SE, Boden SD. Surgery of the Cervical Spine. Philadelphia: Saunders; 2003, p. 6.)

Posterior atlantooccipital membrane

First spinal nerve (C1)

Vertebral artery

Posterior atlantooccipital membrane

First spinal nerve (C1)

Ligamentum flavum

Figure 1-9. Vertebral artery anatomy. (From Emery SE, Boden SD. Surgery of the Cervical Spine. Philadelphia: Saunders; 2003, p. 6.)

22. Describe the blood supply to the spinal cord.

The anterior median spinal artery and the two posterior spinal arteries supply the spinal cord. The anterior spinal artery supplies 85% of the blood supply to the cord throughout its length. Radicular or segmental arteries feed these arteries. In the cervical spine, the majority of radicular arteries arise from the vertebral artery. These arteries enter the spinal canal through the intervertebral foramina and divide into anterior and posterior radicular arteries. The most consistent radicular artery in the cervical spine is located at the C5-C6 level. On average, there are 8 radicular feeders to the anterior spinal artery and 12 to the posterior spinal arteries throughout the length of the spinal cord. The basilar artery also anastomoses with the anterior spinal artery, variably supplying the cord to the fourth cervical level.

FASCIA AND MUSCULATURE OF THE CERVICAL SPINE

23. What are the fascial layers of the anterior neck?

The fascial layers of the neck consist of a superficial layer and a deep layer. The superficial layer of the cervical fascia surrounds the platysma muscle. The deep cervical fascia consists of three layers:

1. Superficial layer: Surrounds the sternocleidomastoid and trapezius muscles.

2. Middle layer: Consists of the pretracheal fascia, which surrounds the strap muscles, trachea, esophagus, and thyroid gland. This layer is continuous with the lateral margin of the carotid sheath.

3. Deep layer: Consists of the prevertebral fascia, which surrounds the posterior paracervical and anterior prevertebral

24. Describe the muscular triangles of the neck.

The anterior aspect of the neck is divided by the sternocleidomastoid into an anterior and posterior triangle. The posterior triangle borders are the trapezius, sternocleidomastoid, and middle third of the clavicle. The inferior belly of the omohyoid further divides this space into subclavian (lower) and occipital (upper) triangles. The anterior triangle is bounded by the sternocleidomastoid, the anterior median line of the neck, and lower border of the mandible. It is further subdivided into the submandibular, carotid, and muscular triangles. The posterior belly of the digastric separates the carotid from the submandibular triangles. The superior belly of the omohyoid separates the carotid from the muscular triangles (Fig. 1 -10). The standard anterior approach to the midcervical spine is done through the muscular triangle.

25. Name the muscles most commonly encountered during anterior and posterior cervical spine procedures.

• Anterior muscles, platysma, sternocleidomastoid, strap muscles of the larynx, omohyoid, longus colli

• Posterior muscles, superficial layer—trapezius; middle layer—splenius capitis, splenius cervicis; deep layer— semispinalis capitis, longissimus capitis; muscles of the suboccipital triangle—rectus capitis posterior major and minor, obliquus capitis superior and inferior musculature.

Figure 1-10. Muscular triangles of the neck. (From Raiszadeh K, Spivak JM. Spine. In: Spivak JM, DiCesare PE, Feldman DS, et al., editors. Orthopaedics: A Study Guide. New York: McGraw-Hill; 1999, p. 63-72, with permission.)

Sternocleidomastoid

Figure 1-10. Muscular triangles of the neck. (From Raiszadeh K, Spivak JM. Spine. In: Spivak JM, DiCesare PE, Feldman DS, et al., editors. Orthopaedics: A Study Guide. New York: McGraw-Hill; 1999, p. 63-72, with permission.)

Sternocleidomastoid

Key Points

1. Appreciation of the distinguishing features of typical (C3-C6) and atypical (C1, C2, C7) vertebrae is important for understanding cervical spinal anatomy.

2. There are eight pairs of cervical nerve roots but only seven cervical vertebra.

Websites

1. Spinal cord, topographical and functional anatomy: http://emedicine.medscape.com/article/1148570-overview

2. See cervical spine anatomy: http://www.orthogate.org/patient-education/cervical-spine/cervical-spine-anatomy.html

3. See spine anatomy index section: http://www.spineuniverse.com/displayarticle.php/article1297.html

BiBLiOGRAPHY

1. Aebi M, Arlet V, Webb JK. AO Spine Manual. New York: Thieme; 2007.

2. An HS, Simpson JM. Surgery of the Cervical Spine. Baltimore: William & Wilkins; 1998.

3. Clark CR. The Cervical Spine. 4th ed. Philadelphia: Lippincott; 2005.

4. Emery SE, Boden SD. Surgery of the Cervical Spine. Philadelphia: Saunders; 2003.

5. Kim Dh, Henn JS, Vaccaro aR, et al., editors. Surgical Anatomy and Techniques to the Spine. Philadelphia: Saunders; 2006.

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