h1 class=”calibre8″>1 Anatomy of the Cervical Spine

Ilyas Aleem, Samuel Rosenbaum, and Bradford L. Currier

Keywords: cervical anatomy, cervicothoracic junction, foramen magnum, atlas, axis, subaxial spine

1.1 Craniovertebral Junction

The craniovertebral junction (CVJ) is highly complex and is composed of the foramen magnum, occiput, atlas, axis, and their associated ligamentous, muscular, and neurovascular structures. This together represents the transition between the brain and cervical spine, and provides an extremely complex unit of stability and flexibility not found in any other region of the spine. ¹

1.1.1 Osteology of the Craniovertebral Junction

Foramen Magnum and Occipital Condyle

The foramen magnum is an oval shaped opening in the occipital bone that allows for the transition between the cranium and the spinal column (▶ Fig. 1.1). The opisthion and basion are the midpoints on the posterior and anterior margins of the foramen magnum, respectively. The average sagittal diameter of the foramen magnum measures 34.7 ± 2.5 mm. ² The foramen magnum is flanked anterolaterally by the occipital condyles, which form bilateral inferior convexities allowing articulation with the atlas. ³ The lateral-faced occipital condyles articulate with the superomedially facing concave articular surfaces on the superior aspect of the atlas lateral masses, permitting flexion and extension of the cranium on the anterior half of the foramen magnum. ^3,4 In articulating with the trapezoidal lateral mass of the atlas below, the condylar external surface is convex downward, facing outward and sloping cephalocaudal in both sagittal and coronal views. ⁵ Mean intercondylar distance is 29.4 mm (range: 26.2–37.0 mm). ⁵

The Atlas (C1)

The atlas (C1) is an atypical vertebra without a body or spinous process, resulting in a ring-like appearance (▶ Fig. 1.2). ⁶ It comprises a short anterior arch and longer posterior arch connected by two dense, cortical lateral masses on either side. Instead of a body, it has an anterior tubercle that serves as an attachment for the longus colli muscle. The posterior tubercle serves as an attachment point for the rectus capitis posterior minor muscle and the suboccipital membrane. The obliquus capitis superior and inferior muscles originate from the transverse process of C1 and insert into the base of the occipital bone and into the spinous process of C2, respectively. The transverse process contains the foramen transversarium, through which the vertebral artery passes. The lateral masses are located at the junction of the anterior and posterior arch providing a secure anchor for placement of lateral mass screws. The concave superior facet of the lateral mass articulates above with the occipital condyle, and the flatter inferior facet of the lateral mass articulates below with C2. Just posterior to the lateral mass is the groove for the vertebral artery. When exposing the posterior arch of C1, it is recommended that exposure not go beyond 1 to 1.5 cm from midline to avoid injury to the vertebral artery. The atlanto-occipital articulation primarily permits extension and flexion, as well as lateral flexion. ⁷

C1 lateral mass screw insertion places the vertebral and carotid arteries, hypoglossal nerve, and C2 nerve roots at risk. ⁸ The depth of bicortical screw insertion is approximately 19.3 mm in the axial plane and 20.9 mm in the sagittal plane. ^8,9,10 The ideal start point for a C1 lateral mass screw is at the junction of the medial edge of the posterior arch attaching to the lateral mass with 10 to 15 degree angulation upward and 5 to 10 degrees medially. ^11,12 The height for lateral mass screw placement, measured as the cephalad–caudad distance from the bottom of the inferior facet to the top of the posterior arch at the vertebral artery groove, measures 9.0 mm with a minimum height of 4.7 mm. In a computed tomography (CT) study of 50 head and neck scans, Currier et al found the mean distance from the internal carotid artery (ICA) to the anterior aspect of the C1 lateral mass to be 2.9 mm (range: 0–7.2 mm) (▶ Fig. 1.3). ¹³ Furthermore, the proximity of the ICA to C1 posed a high risk of arterial injury from a drill bit or screw in 12% of cases. An alternative entrance point that avoids the troublesome bleeding often encountered when dissecting in the region of the lateral mass is on the inferior third of the posterior arch through the pedicle analog. However, prior to the use of this entry point, the preoperative imaging must be carefully evaluated to ensure that the bone caudal to the groove of the vertebral artery is sufficient to allow screw placement. ¹⁴

The Axis (C2)

The axis (C2) was so named because it functions as a pivot for the atlas, allowing the head to rotate. Like the atlas, the axis is also unique due to the odontoid process or dens, which is a bony process that protrudes upward from the body of C2 (▶ Fig. 1.2). It is generally 1.0 to 1.5 cm long and 1 cm wide, inclining posteriorly up to 30 degrees relative to the body of C2. ¹⁵ The ventral surface of the odontoid articulates with the posterior aspect of the anterior arch of C1. The transverse ligament traverses from one side of the C1 ring to the other side in the transverse groove on the dorsal surface of the dens, providing stability to the dens. Fifty percent of cervical spine rotation occurs at the atlantoaxial joint.

The C2 pedicle averages approximately 8.7 mm high with a mean width of 5.8 mm and an overall transverse angle of 43.2 degrees which makes it a robust option for C2 pedicle screw placement. ^16,17 The anatomic median angle of the pedicle is 10.4 degrees and the angle of declination is 28.4 degrees. The safe site of screw entry is the superior and medial third of the posterior surface of the C2 pedicle. The safe trajectory for a C2 pedicle screw is 40 degrees medial and 20 degrees superior. ¹⁸ In a radiographic analysis of C2 pedicle screws, Chin et al found the mean distance along the laminar surface between the isthmus and starting point to be 8.1 mm, and the mean distance from the superior border of the lamina to the starting point to be 5.7 mm. ¹⁹ The transverse processes of the axis are small lateral projections demarcating the lateral margin of the foramen transversarium, in which the vertebral artery courses upward before deviating medially over the C1 superior sulcus (▶ Fig. 1.4).

Translaminar axis screws are a robust option when fixation through the C2 pedicle or pars is problematic from either an anatomic or technical perspective. ^20,21,22 Saetia and Phankhongsab described transverse diameters of the C2 lamina, C2 laminar length, and spinolaminar angles in 200 adult C2 CT scans. ²³ They found the mean inner and outer transverse diameter of the C2 lamina to be 4.2 and 6.6 mm, respectively. The mean C2 laminar length was 37.3 mm and the mean spinolaminar angle was 56.4 degrees. ²³ In another study of 420 adult C2 specimens, Cassinelli et al found 71% of specimens had a laminar thickness of ≥ 5 mm, and 93% of specimens had a laminar thickness of ≥ 4 mm. ²⁴

1.1.2 Ligamentous Anatomy of the Craniovertebral Junction

The ligamentous components of the CVJ can be classified as either extrinsic or intrinsic. ^6,25 The extrinsic ligaments include the fibroelastic membranes, the ligamentum flavum, and the ligamentum nuchae. The ligamentum nuchae extends from the occipital protuberance to the posterior aspects of the atlas and upper cervical spinous processes. The intrinsic ligaments are composed of the tectorial membrane, accessory atlantoaxial, cruciate, and odontoid ligaments, and the anterior atlantooccipital membranes (▶ Fig. 1.5). All of the ligaments that make up the intrinsic layer are located anterior to the dura. ²⁵

Cruciform Ligaments

The cruciform ligaments, as their name suggests, are composed of transverse and vertical components crossing behind the dens. The transverse atlantal ligament attaches to a small bony tubercle on the medial side of the lateral masses of the atlas on either side, arching across the ring of the atlas behind the dens. As it crosses behind the dens in the transverse groove, small bands are directed upward and downward, toward the clivus and C2 body, respectively. ⁶ The transverse atlantal ligament is the strongest ligament of the CVJ and is the predominant stabilizer of the atlas. ²⁶ The transverse ligament serves as an anteroposterior stabilizer holding the odontoid in its vertical position, thus allowing stable rotation of the head. ^3,25,26 Laxity or injury to the transverse ligament can lead to atlantoaxial instability. ²⁷

Alar Ligaments

Paired alar ligaments originate on the upper portion of the posterior surface of the dens and travel laterally to insert on the C1 lateral masses (atlantoalar band) and occiput (occipitoalar band). ¹⁵ The alar ligaments play an important role in the stabilization of the head during movement and are the primary restraint to axial rotation. ²⁵ If the transverse ligament ruptures, the alar ligaments become responsible for preventing atlantoaxial subluxation. Damage to the alar ligaments results in further instability with axial rotation.

Tectorial Membrane

The tectorial membrane is a cephalad extension of the posterior longitudinal ligament (PLL), attaching to the body of C2 caudally and basilar groove of the occiput cranially. The tectorial membrane is firmly adherent at its cranial and caudal regions. It serves as a significant secondary stabilizer in the prevention of ventral compression of the thecal sac by the odontoid. ^6,25 The tectorial membrane is composed of a lateral part (also referred to as the accessory atlantoaxial ligament) joining the atlanto-occipital capsular ligaments (Arnold ligaments) and a central part that merges with the dura mater. ²⁵ Superior and inferior crura arise from the transverse ligament as it crosses the dens, attaching to the anterior foramen magnum and body of the dens, respectively. ³

Capsular Ligaments and Atlanto-occipital Membranes

Capsular joint ligaments connect the occiput to the C2 complex. Capsular ligaments extend across the convex-concave articulations, ensuring stable movement through a wide range of motion. The atlanto-occipital articulation is reinforced by the cephalic extension of the anterior longitudinal ligament (ALL) and ligamentum flavum, respectively termed the anterior and posterior atlanto-occipital membranes at this level. The anterior and posterior atlanto-occipital membranes extend from the foramen magnum to the anterior and posterior arch of the atlas, respectively. They contribute very little to CVJ stability. ^3,25

Accessory Atlantoaxial Ligament and Apical Ligament

The accessory atlantoaxial ligament and apical ligament offer little to no contribution to CVJ stabilization. ²⁵ The accessory atlantoaxial ligament connects the lateral mass of the atlas to the body of the axis and extends to the occipital bone. The apical ligament, also known as the suspensory ligament or odontoid ligament, extends from the tip of the dens to the anterior border of the foramen magnum. It lies between the anterior atlanto-occipital membrane and the cruciform ligament, in the triangular space created by the paired alar ligaments inside of which is connective tissue, fat, and a small venous plexus. ^6,25

1.2 Subaxial Cervical Spine Anatomy

1.2.1 Osteology

The C3 to C6 vertebrae are considered typical vertebrae while C7 (along with C1 and C2) is considered atypical as it has several unique characteristics leading to the spine’s transition at the cervicothoracic junction. Each vertebra in the subaxial spine consists of a body anteriorly which connects to two lateral masses through posterolateral bony projections, the pedicles and transverse processes which form the anterolateral and posteromedial walls of the transverse foramen. The lateral masses contain the superior and inferior articular facets and connect to the spinous process posteromedially through the lamina. Together, the lamina and the pedicles form the vertebral arch (▶ Fig. 1.6).

Vertebral Body

The vertebral body consists of a thin cortical shell filled with softer cancellous bone. The superior and inferior endplates of the vertebral body are typically saddle shaped with the superior endplate convex in the sagittal plane and concave in the coronal plane while the inferior endplate is concave in the sagittal plane and convex in the coronal plane. The anterior lip inferiorly will occasionally overlap the superior endplate at the next level. ²⁸ The endplates are elliptical with their widths greater than depths. The widths and depths of both the superior and inferior endplates increase from C3 to C7. The depth ranges from 15 mm at C3 to 18.1 mm at C7 and the width ranges from 15.8 to 23.4 mm. Posterior vertebral body heights range from 10.9 to 15.14 mm and remain relatively constant from C3 to C7, whereas anterior vertebral body height increases from 14.37 mm at C3 to 14.97 mm at C7. ^29,30

Uncinate Process

The uncinate process (also termed uncal process or uncus) is an articular projection at the posterolateral aspect of the superior endplate unique to the cervical spine. It forms the uncovertebral joint or joint of Luschka with the inferior beveled aspect of the vertebra above. It generally is positioned more posteriorly and increases in height and width from C3 to C7. It functions as a “guide rail” for anterior–posterior motion during flexion and extension, assists with coupling of rotation and lateral bending, contributes to stability to the spinal column in the medial–lateral plane, and protects the neural foramen from herniated disc material. ³¹ The uncinate process helps define the outer limits of anterior decompression and serves as a key guide to avoid injury to the vertebral artery that lies in the foramen transversarium just lateral to it. The width of the uncinate process (and therefore distance to the foramen) ranges from 3.5 to 6 mm. ^{31,32,33,34,35}

Transverse Process

The transverse processes in the subaxial spine are unique when compared to the thoracic and lumbar spine. They project posterolaterally off the vertebral body and contain the foramen transversarium or transverse foramen, which transmits the vertebral artery (usually entering at C6) and the venous plexus at C7. The transverse process begins at the anterior root that extends medially from the vertebral body to the anterior tubercle, from which the anterior scalene, longus capitis, longus colli, and ventral intertransverse muscles originate. The anterior tubercle of C6 is larger and termed the carotid tubercle or Chassaignac tubercle. The costotransverse bar connects the anterior tubercle to the posterior tubercle and has a concave groove superiorly for the exiting nerve root. The posterior tubercle serves as the origin of the splenius cervicis, longissimus, levator scapulae, middle scalene, posterior scalene, and iliocostalis muscles. ^36,37

Pedicles

The pedicles are cylindrical bony corridors that connect the vertebral body posterolaterally to the lateral masses. They are formed by a cortical shell with a soft cancellous interior. A vertebral notch is located on the superior and inferior aspects of each pedicle to contribute to the neural foramen. Morphometric and cadaveric measurements of pedicles are numerous. ^{38,39,40,41,42,43,44,45,46} Mean pedicle lengths range from 5.2 mm at C3 to 5.7 mm at C7 while mean pedicle axis length from the posterior pedicle cortex to the anterior body cortex is relatively similar and ranges from 32.3 mm at C7 to 34.2 mm at C6 (C3–C5 lying somewhere in between). Mean pedicle width increases from 5.2 mm at C3 to 6.6 mm at C7 and mean pedicle height is quite similar from C3 to C7 ranging from 6.7 mm at C6 to 7.0 mm at C4. The mean medial pedicle transverse angles range between 47 and 49 degrees at C3 to C5, 44.2 degrees at C6, and 38.7 degrees at C7. The mean pedicle sagittal angle in reference to the horizontal decreases considerably from 14.2 degrees at C3 to −3.27 degrees at C6 and −1.9 degrees at C7. ⁴²

From an anterior cervical approach, the pedicle, which is the anatomic landmark defining the boundaries of the foramen, is hidden from view intraoperatively. This can potentially lead to incomplete surgical decompression of the foramen. In a radiographic CT analysis of 100 patients, we recently showed that the posterior endplate valley (PEV), defined as the posterior margin of the caudal vertebral body of the segment to be decompressed, is an accurate surgical landmark that is consistently at most 1 mm from the superior aspect of the cervical pedicle in the subaxial spine (▶ Fig. 1.7). ⁴⁷

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