The spinal cord starts from the foramen magnum and ends at a level between the twelvth thoracic vertebra and the first lumbar vertebra. Segments of the spinal cord aren't generally called depending on relevant vertebral bodies. It was known that the spinal cord neural segments were located at one vertebral body level above their corresponding relevant vertebrae in the cervical region and at two vertebral body levels above in the thoracic region5)
. Nevertheless, it was difficult to find anatomical studies on the relationship between vertebral bodies and the neural segments of the spinal cord from which the relavent rootlets originated. While such morphologic knowledge helps in the understanding of spinal cord trauma, myelopathy due to pathological lesions of the spinal cord, and radicular pains; it was extremely difficult to standardize the anatomy of the spinal cord or nerve roots because of the individual variations in size and shape4,8)
. Considering the fundermental data presented above, safe and sufficient decompression can be achieved when surgical treaments are performed on the thoracic or cervical vertebrae and the spinal cord. It would also be helpful in determinig the cause of unexpencted disorders or surgical complications.
The normal relationships between the segments of the cervical or thoracic spinal cord and the intervetebral discs have already been mentioned in several anatomical textbooks. However, these were not clearly defined and classifed. There were even several different criteria used for the such relationships. Each segment of the spinal cord was defined and demarcated as either; 1) the segment in between the most superior rootlet of a nerve root to the most superior rootlet of the adjacent nerve root, 2) the middle portion of two adjacent nerve roots, or 3) the part in which a pair of nerve roots were attached to. This study used the first definition. Kubo6)
reported that, with some exceptions, the seventh cervical nerve root generally started at the level of the fifth and sixth cervical intervertebral disc (C5-C6) and the eighth cervical nerve root originated at the level of the sixth and seventh cervical intervertebral disc (C6-C7) in eighty percent of subjects. Kim and Shin5)
described that the seventh cervical nerve root was mostly located at the level of the fifth and sixth cervical intervertebral disc (C5-C6), the fifth cervical nerve root was at the level of the fourth cervical vertebra (C4), and the sixth cervical nerve root was at the level of the fifth cervical vertebra (C5). Tanaka et al.10)
reported that the starting point of the fifth cervical nerve root was at the level of the fourth cervical vertebra (C4), the sixth cervical nerve root was at the level between the fourth and fifth cervical vertebrae, and the seventh cervical nerve root was at the level between the fifth and sixth cervical vertebrae. While there were some differences between the reported studies, the results led to very similar conclusions.
Based on the previous studies, a formula for the relationship between the neural segments of the spinal cord and vertebral bodies can be derived. The spinal cord neural segements of the posterior root entry zones for the cervical nerve roots down to the first thoracic nerve root were located at the level one vertebral body segment immediately superior to their corresponding relevent vertebrae of exit. Seventy-five percent (75%) of the specimens examined showed that the spinal cord neural segment for the posterior root entry zone of the second thoracic nerve root was located one and a half vertebral body level above its corresponding relevent vertebrae of exit, which is between the seventh cervical vertebra and the first thoracic vertebra. Otherwise, it was located at the level of the seventh cervical vertebra or the first thoracic vertebra. Caudad of the third thoracic nerve root, the locations of the spinal cord neural segments for the posterior root entry zone were at the level 2 vertebral segments superior to their corresponding relevant vertebra of exit. Following the above rule, the eighth cervical spinal cord neural segment would have its posterior root entry zone located at the level of the seventh cervical vertebra. However, our cadeveric studies showed that it was located at the level of the sixth cervical vertebra. This may be due to the non-existence of an eighth cervical vertebra and the relative thickening of the spinal cord at the fourth, fifth, and sixth cervical vertebral levels. It was generally known and accepted that the cervical spinal cord neural segments were located at a level one vertebral segment above and the neural segments in the thoracic spinal cord were located at a level two vertebral segments above their corresponding relevant vertebra. This study showed that the generally accepted "one vertebral segment above" or "two vertebral segments above" rule is actually demarcated at the second thoracic nerve root. This result may be clinically interpreted as it could efffect the next inferior nerve root when C3-C4 intervertebral disc was posteriorly bulged and two nerval segments were related. In case where the thoracic spine is involved, three spinal cord neural segments could be affected.
This study did not look into the effect of aging on the morphometric relationship between the cervico-thoracic cord segments and the vertebral body. Payne and Spillane9)
reported that the nerve roots were pointing in the inferior direction due to the different growth rates of the spinal cord and vertebral bodies. They also mentioned that the height of both intervertebral discs and vertebral bodies decreased with age. Considering small amount of the decrease and effects on overall height of whole spine, such decline of the height made the spinal cord lower comparing with vertebral bodies and the angle of the nerve roots could be reduced while Hwang et al.3)
decribed that the angle was 60-68 degree. Spinal nerve roots are consisted of anterior horn beginning from the spinal cord and posterior horn going to the spinal cord. Kubo et al.6)
discussed that the anterior root exit zone may not be anatomically defined but it could be defined as an oval shape from the most superior anterior rootlet to the most inferior anterior rootlet. Also, they studied anatomical difference of posterior root exit zone that posterior rootlet disposed in lines along the posterolateral furrow. Generally, the posterior rootlets were thicker than the anterior rootlets in the cervical spine. The mean vertical length of the anterior root exit zone was 10-15 mm and an anterior nerve root consisted of 17-25 rootlets. The vertical length of the posterior root exit zone was 6-14 mm and the number of rootlets was 7-12. It has been mentioned that there was a difference in the number of rootlets between the anterior and the posterior nerve roots even though they were on the same spinal cord neural segment. Clark2)
described variations in the number of rootlets composing the posterior root exit zone but it was mostly 7-12.
This study was limited to the investigation of the posterior side of the spinal cord only. It showed similar results that vertical length of posterior root exit zone was 10-12 mm. The numer of rootlets (7) found in the current investigation was different from the referred studies. The reason for this difference may be because a microscope was used in the previous studies and this study relied on macrography. It is important, from a clinical standpoint, to know that the rootlets of the posterior root exit zone progressively shifted more medially as one follows the spinal cord inferiorly from the cervical spine region to the thoracic spine region. It was known that lesions on the spinal cord cause spinal cord diseases and leisons outside the spinal cord that press the nerve root, cause radicular pain. Even though this study was limited to the posterior spinal cord, the study was able to show that both myelopathy and radicular pain can result from a lesion on the spinal cord. Moreover, Kubo et al.6)
and Tanaka et al.10)
mentioned that on the anterior side of the spinal cord, the left and right anterior root exit zones progressivley moved medially as it went caudally and their distance was 1-3 mm. This meant that a medially herniated intervertebral disc can cause nerve root compression and lead to radicular pain even without intervertebral foraminal stenosis.
This study is in agreement with the results of previous research regarding the vertical length of the posterior root exit zone being shorter and the number of rootlets being less in the thoracic spinal cord than in the cervical spinal cord. The reason for a greater number of cervical rootlets than the number of thoracic rootlets may be related to the functions required of the nerves. Since the upper limbs need to perform much more functions than others, it is understandable that the upper spinal cord has to have more rootlets. Also, it can be related to width of the spinal cord that Chang et al.1)
reported that the spinal cord was widest at the level of the sixth-seventh cervical spine.
It was observed that the rootlets were anastomosed before entering the intervertebral foramen. Most rootlets were anastomosed between the 5th and 6th nerve roots, between the 6th and 7th nerve roots, and between the 7th and 8th nerve roots. However, there were big individual differences. This result was the same with previous studies6,7,10)
. Since this anastomosis may cause confusion regarding the lesion's location, pre-operative neurological tests should be performed and it should be clearly confirmed through radiological images.