I thought I’d use this post to discuss the cervical spine, the cervical spine has some interesting points to consider when appreciating its clinical anatomy.
Primary and Secondary Curves
Next time you look at an image or model of the vertebral column, look closely at it’s lateral profile, the first thing you notice is that its not straight, rather its s-shaped. However, after birth the vertebral column is not s-shaped, rather its c-shaped. This c-shaped structure is known as the primary curvature. As we develop, two further curves develops, these are known as the secondary curvatures. The first of the two curvatures develops in the cervical region, this convex curve develops as the infant begins to develop the ability to lift its head. The second of the two curvatures develops in the lumbar region, this occurs at another important developmental milestone, standing up on our legs.
Let’s focus on the cervical spine, its composed of seven vertebrae (labelled C1 through to C7) and fibrocartilage discs (known as intervertebral discs) between each of the vertebrae, like all things there are exceptions, there are no discs between the skull (occipital bone) and the atlas (C1) and no disc between the atlas (C1) and the axis (C2). The discs contribute to almost a quarter of the height of the vertebral column and are thickest in the lumbar and cervical region, where they contribute to the greatest movement. Each disc is made of a tough outer fibrous ring known as the annulus fibrosus (AP) and a gelatinous core called the nucleus pulposus (NP). As we age the NP becomes increasingly desiccated, resulting in shrinkage of the disc and a loss of height with age.
Anterior to the vertebral bodies is the anterior longitudinal ligament, which will in the rostral end of the neck become the anterior atlanto-occipital membrane. The anterior longitudinal ligament is firmly attached to the anterior aspect of bodies and limits flexion of the spine. It is this ligament which is injured in hyper-extension injuries of the cervical spine. The posterior longitudinal ligament is loosely attached to the posterior aspect of the vertebral body, hence why intervertebral discs herniate in a posterior direction; it is also the ligament which is likely to be injured in hyper flexion of the neck. In the rostral end of the neck region the posterior longitudinal ligament becomes the tectorial membrane, see Figure 4C.
The seven cervical vertebrae can be divided into atypical and typical vertebrae. C1, C2 and C7 are atypical vertebrae and will be considered individually. C3,4,5 and C6 are typical vertebrae, please see Figure 1 below. These share the following features:
- Rectangular shaped body
- Triangular shaped vertebral canal
- Uncinate processes
- Bifid spinous processes
- Foramen transversarium in each transverse process (all cervical vertebrae)
- Superior and inferior articular facets
- Anterior and posterior tubercles on the transverse processes
Figure 1: Image showing the features of typical cervical vertebrae
Before we consider the atypical vertebrae, lets think about the joints which create the articulations between the vertebrae. There are 4 joints between each of the typical vertebrae:
- Uncovertebral joints:
- Uncinate process of vertebra below with uncus of vertebra above
- Exclusive to cervical vertebrae
- Cartilaginous (containing the intervertebral discs):
- Secondary or AKA. Fibrocartilaginous joint
- Joint consists of: Bone – fibrocartilage (disc) – bone
- Disc structure – NP surrounded by AP
- Synovial x 2 for each vertebra:
- Joint between superior and inferior facets
- Facets are found between pedicle and lamina
- Facets have articular cartilage.
- Articulation surrounded by synovial capsule
- Known as zygapophysial joints, AKA. facet joints
The Atypical Vertebrae
The C1 vertebra, also known as Atlas, is the most rostral vertebra and articulates with the occipital bone, forming the atlanto-occipital joint. The C1 vertebra is the first of the atypical vertebrae we will consider. The following features make the atlas atypical:
- No body
- Small anterior arch
- Large lateral masses
- Facets for the articulation of the dens
- No articular discs above or below
- No spinous processes
- Superior facets are elongated and concave
Figure 2: Case courtesy of OpenStax College, Radiopaedia.org, rID: 42770
The C2 vertebra, is known as the Axis, this vertebra does have a body, with the addition of a rostrally pointing projection known as the dens (AKA. odontoid peg). The dens used to be the body of the axis, but is now attached to the axis, providing the pivot for the atlantoaxial joint. The Axis does have a spinous process, which is usually not bifid, though in some cases may be bifid. Its other features include:
- Dens has an anterior facet for articulation with facet of anterior arch of Atlas
- Dens has a posterior facet for articulation with the transverse ligament
- Atlas rotates around the dens
Figure 3:Case courtesy of OpenStax College, Radiopaedia.org, rID: 42770
The C7 vertebra, is also known as Vertebral prominens and is the largest of the cervical vertebrae and resembles many features of the a thoracic vertebra, in particular its prominent spinous process, often easily palpable on examination of the back.
Atlanto-occipital and atlantoaxial joints
Figure 4: Comparison between the atlanto-occipital and atlantoaxial joints
Now lets focus on the occipitoatlantal joints, this joint forms the articulation between the occipital bone of the skull and the Atlas. The joint permits flexion and extension of the head upon the neck, just as when you nod your head. It does not allow rotation (turning your head around), this occurs at the atlantoaxial joint. The occiptoatlantal joints are two (one on each side) synovial condyloid joints; between the superior articular facets of the Atlas and the occipital condyles of the occipital bone. Important ligaments related to this joint include the following (see figure 4A & B):
- Anterior atlanto-occipital membrane:
- Attaches anterior arch of the atlas to the anterior margin of the foramen magnum
- Posterior atlanto-occipital membrane:
- Attaches the posterior arch of the atlas to the posterior margin of the foramen
The atlantoaxial joint is a synovial pivot between the dens of the axis and a ring created by the anterior arch of the Atlas and the transverse ligament. The joint also consists of articulation between the articular facets of both the Atlas and the Axis. These joints are each encased in a capsule. The important ligaments related to the atlantoaxial joint are:
- Apical ligament
- Attaches the top of the dens with the anterior margin of the foramen magnum
- Cruciate ligament:
- As the name suggests, the ligament is cross shaped, the horizontal component is known as the transverse ligament and is particularly important
Important anatomical relationships of the vertebral column, are the relationships of the spinal nerves and the spinal cord to each vertebra. The spinal cord is located in the vertebral canal and each spinal nerve exits the intervertebral foramen. The intervertebral foramen is actually a foramen created by the space between the pedicles of adjacent vertebrae, see Figure 5.
Radiculopathy and myelopathy
Posterior to the intervertebral foramen are the facets joints. Therefore diseases affecting synovial joints can cause deformation of these joints (thickening of the capsule, bony spur formation), encroaching on the intervertebral foramen, causing compression of spinal nerves, this is known as a radiculopathy. Anterior to the exit of spinal nerve are the vertebral arteries.
Figure 5: Case courtesy of OpenStax College, Radiopaedia.org, rID: 42770
As discussed earlier, herniation of the disc usually occurs in a posterior direction, due to the posterior longitudinal ligament not being as firmly attached as the anterior longitudinal ligament. Herniation of the disc in a posterior and lateral direction may compress a spinal nerve (radiculopathy), or if the herniation is posterior and central, this may cause compression of the spinal cord itself, known as a myelopathy.
When assessing the potential effects of trauma to the neck, alignment of the vertebral bodies and assessment of the dens are crucial. Therefore it is important to ask for lateral, AP and peg (dens/odontoid peg) views. The radiographs must also include the T1 vertebra. The most common injuries of the cervical spine are in the C5 to C7 region and at atlantoaxial joint.
Alignment is assessed by looking at 4 almost (slightly lordotic – convex anterior) parallel lines, see Figure 6. These are the anterior vertebral and posterior vertebral lines, the posterior margin of the spinal canal (spinolaminal line) and the tips of the spinal processes (spinous process line).
The peg is held in place by the apical, alar and cruciate ligaments. Damage to these ligaments may be indicated by unequal distances between the peg and the adjacent lateral masses on peg view radiographs, no greater than 2mm (on a peg view film). The distances may appear unequal due to slight rotation of the neck, so how do you tell the difference between rotation and traumatic displacement? In rotation the lateral margins of C1 and C2 should still be inline, see Figure 7A. Compare Figure 7A with 7B, showing a Jefferson fracture (bilateral breaks in both anterior and posterior arches – 4 fractures), one can clearly see the misalignment of the lateral masses of C1 and C2.
Figure 7A: Normal PEG view. Case courtesy of Andrew Murphy, Radiopaedia.org, rID: 48418
Figure 7B: Case courtesy of Dr Andrew Dixon, Radiopaedia.org, rID: 9601
The distance between the peg and the anterior arch (as seen on the lateral view) must not be greater than 3mm in an adult. On lateral views alignments of vertebral bodies should be checked as indicated in Figure 6.
On the AP view (figure 8), check that the spinous process are in a straight line and equidistant apart, if not this may indicate displacement of an adjacent vertebra or severe neck flexion.
The shadow of the prevertebral soft tissues can also be assessed from a lateral projection, however this is beyond the scope of this article.
Another condition where the anatomy of the atlantoaxial joint is important, is Rheumatoid arthritis. Remember RA is a disease process affecting the synovium, the atlantoaxial joint is a synovial joint; here the synovium thickens to form a thick pannus which disrupts the transverse ligament, potentially causing the dens to slip backwards (subluxation) and compressing the spinal cord.
In approximately 0.5% of the population, an additional rib or pair or ribs may be present, each rib may not be complete. These are cervical ribs, which arise from the C7 vertebra, they are clinically relevant as in some patients they might cause compression of the subclavian artery, vein or brachial plexus (often inappropriately known as thoracic outlet syndrome). Looking at at an AP view, count downwards from C2 and you are looking for a rib attached the transverse process of the C7 vertebra.
Figure 8: Case courtesy of Dr M Osama Yonso, Radiopaedia.org, rID: 17212