Most descriptions of the intervertebral disc (henceforth, disc) relate to the lumbar spine and the rest of the discs are assumed to be similar. Nothing can be further from the truth. The intervertebral discs of the cervical spine are quite dissimilar to those in the lumbar spine. The cervical spine has the uncovertebral joints that support the disc in the lateral and posterolateral regions and that has an affect on the anatomy of the its disc. There is also a concavity of the endplates that affects the structure.
The function of the cervical disc is much different than that of the lumbar spine. The lumbar spine is involved in weight-bearing and its robust structure reflects this. The cervical spine is primarily involved in mobility and also has close involvement with the upper extremities. This is reflected in the unique structure of the cervical disc.
|Vertical view of a cervical disc
NP = nucleus pulposus
AF = anulus fibrosus
UP = uncinate processes
ALL = anterior longitudinal ligament
PLL = posterior longitudinal ligament
The anulus fibrosus of the cervical disc does not have the relatively consistent cross-sectional thickness of the lumbar spine. The anterior aspect of the anulus fibrosus has been found to be thick, about 30% thicker than the posterior anulus. (Mercer & Bogduk 1999, Tonelli et al 2005), but becomes thinner laterally towards the uncinate processes. (Mercer & Bogduk 1999) There is a thin collagenous layer that separates the outer layer of the anterior anulus from the anterior longitudinal ligament. The fibers of this layer are oriented inferiorly and laterally and is anchored to the margin of the vertebral bodies. (Mercer & Bogduk 1999) As in the rest of the spine, the layers of the anterior anulus have oblique and opposite orientations between laminae. Towards the midline at the anterior aspect, the fibers of the laminae from each side do not pass as separate laminae but interweave. (Mercer & Bogduk 1999) In the outer laminae, the anular fibers attach to the opposing vertebral bodies throughout the span of the anterior laminae. In the deeper layers, the anular fibers attach primarily near the mid-line. This gives the anterior anulus a crescent shape when viewed from above rather than an even thickness of layers throughout. (Mercer & Bogduk 1999)
At the anterior margin of the uncovertebral region, the disc is very thin with only a single lamina that is oriented obliquely. (Mercer & Bogduk 1999) Along the uncovertebral region, there is a layer of thin periosteal tissue. (Mercer & Bogduk 1999)
The posterior aspect of the anulus is thin and the fibers are longitudinally oriented, rather than having an oblique orientation. (Mercer & Bogduk 1999) The posterior anulus fibrosus of the cervical disc has often been described as an extension of the posterior longitudinal ligament and in some cases is barely present. (Tonelli et al 2005) The posterior anulus is 1 mm or less thick and covers the posteriormost aspect of nucleus or fibrocartilage core. (Mercer & Bogduk 1999)
The nucleus pulposus appears to be proportional larger in the cervical spine than in the lumbar spine. The cervical nucleus contains a high and constant amount of collagen throughout life. In comparison, the collagen content of the lumbar nucleus begins to increase significantly after age 10 years. (Scott, et al 1994) The nucleus of an infant has proteoglycan and fibrous and notochordal tissue. (Bland & Boushey 1990, Oda et al 1988) By the teen years, there is more fibrocartilage and the notochordal cells diminish until they disappear in the 20s. In the adult, the nucleus is largely fibrous tissue and has been called “dry.” (Bland & Boushey 1990, Oda, et al 1988) In the mature disc, there is chondrocytic activity in the nucleus adjacent to the endplates as well as fibers extending from the endplate into the nucleus. (Oda et al 1988)
In its lateral aspect, there are clefts that extend from the uncovertebral region. In younger spines, the clefts do not penetrate very deep into the nucleus pulposus. In older spines, the clefts can completely transect the posterior two-thirds of the disc. (Bland & Boushey 1990, Cramer 2005, Mercer & Bogduk 1999) This can leave an island of nuclear material between the cleft and the posterior anulus. (Mercer & Bogduk 1999) The opposing surfaces of the cleft contain anular-like tissue, fibrocartilage, hyaline cartilage, tissues similar to tendons, and calcified regions. (Bland & Boushey 1990) It is uncertain whether the clefts are caused by axial rotation of the cervical spine.. The clefts do allow the vertebral body to make a slight lateral shift during axial rotation of the cervical spine and increase the rotation. (Mercer & Bogduk 1999)
The vertebral endplates were described in June 2006 issue. In the cervical spine, the endplates appear to begin to show areas of calcification in the third decade. The calcified areas are infiltrated by blood vessels. (Oda et al 1988) In the immature cartilaginous endplate, there is extensive vascular supply. These have largely disappeared and are replaced by cartilaginous tissue in the second decade. In the adult, the blood vessels again reappear in calcified regions of the endplates. (Oda et al 1988) If the calcification becomes extensive, that can interfere with the nutrient transport system and may result in disc degeneration.
The anterior longitudinal ligament in the cervical spine consists of four layers. The most superficial layer has longitudinally-oriented fibers attaches to the central two-thirds of the anterior vertebral bodies and the anterior tubercle of the atlas. (Mercer & Bogduk 1999) The next layer also has longitudinally-oriented fibers but are shorter than the most superficial layer. They extend from adjacent vertebral bodies and cover the anterior disc. (Mercer & Bogduk 1999) The third layer is longitudinally-oriented but are very short. They cover one disc and attached to the anterior vertebral bodies near its margin, cranial and caudal to the disc. (Mercer & Bogduk 1999) The deepest layer is oriented in an alar pattern. It attaches to the superior vertebral body to the disc and extend inferiorly and laterally over the disc to attached to the vertebral body inferior to the disc and extends laterally to the uncinate processes. (Mercer & Bogduk 1999)
The posterior longitudinal ligament in the cervical spine is much thicker than that in the lumbar and thoracic spine. (Bland & Boushey 1990) It has three layers. The most superficial layer has fibers that attach centrally to the posterior vertebral body. (Mercer & Bogduk 1999) The central fibers are oriented longitudinally and travel multi-segmentally. There are fibers from this layer that are laterally-oriented and pass over a disc and attach at the base of the pedicle, one or two segments caudally. (Mercer & Bogduk 1999) The middle layer is longitudinally-oriented and centrally located and pass over one disc. it attached to the adjacent posterior vertebral bodies. (Mercer & Bogduk 1999) The deep layer travels over one disc from the inferior aspect of the superior posterior vertebral body, travel inferiorly and laterally to the subadjacent posterior vertebral body to the posterior aspect of the uncinate processes. (Mercer & Bogduk 1999) These laterally projecting fibers do the work of the anulus which is lacking in the posterolateral aspect. (Mercer & Bogduk 1999)
I did a write-up on the uncovertebral joints of von Luschka in the June 2003 issue. These joints are only found in bipedal animals. (Gatterman & Panzer 1990) They are formed by lateral, superiorly directed extension of the lateral and posterolateral margins of the superior aspect of the cervical vertebral body called the uncinate process or uncus and a beveled margin on the inferior surface of the vertebral body above. It is bounded by a capsule formed by ligaments on the lateral aspect and the anulus on the medial aspect. With maturity, there is a tendency for the uncinate processes to enlarge and flatten. (Bland & Boushey 1990)
Tonelli et al and others opine that anterior divergence of the plane lines (“wedging”) of the opposing endplates in the cervical spine are normal. In my opinion, and in the opinion of Gonstead chiropractors, it is not, unless there is a distortion of the endplate or other structural changes that would allow it.
One wonders if the histological structure of a mature disc that appears to have a normal disc space and alignment on x-ray and full range of motion is similar to those found in these studies. Are those in the studies pathological? Has it become “dry” and fibrous or does it still have vestiges of the gelatinous nucleus of the immature spine. Does it have extensive cleft formation from the uncinate processes? If it has become fibrotic, how is the disc space maintained and why doesn’t it appear to show radiographic signs of degeneration.
The cervical intervertebral disc has not been as extensively studied as has the lumbar discs. This has led to incorrect assumptions on the structure of the cervical discs.
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