The spine consists of a series of vertebral segments. The spinal cord itself has neurological segmental levels marked by the spinal roots. A spinal root is named after the vertebra it exits from. When traced back to cord spinal cord segmental levels do not necessarily correspond to the bony vertebral level.
Let us have a look at vertebrrae and nerve root numbers
- Cervical – 7 vertebrae and 8 cervical roots. This discrepancy is because there ar total 8 nerve roots exiting from the cervical vertebrae
- Thoracic 12 vertebrae and 12 roots
- Lumbar – 5 vertebrae and 5 nerve roots
- Sacral – 5 vertebrae and 5 nerve roots
The spinal cord segments are not necessarily situated at the same vertebral levels. This has been caused by difference in vertebral height and height of spinal segment. Thus while first cervical spinal segment is within C1 vertebra, the T12 cord comes to lie at the T8 vertebra.
Similarly the entire lumbar spinal segment lies between T9 and T11 vertebrae and the the sacral cord is between the T12 to L2 vertebrae.
Course of Spinal Roots
C1 spinal root exit the spinal column at the atlanto-occiput junction. C2 root exits at the atlanto-axis. The C3 roots exit between C2 and C3. The C8 root exits between C7 and T1.
The first thoracic root or T1 exits the spinal cord between T1 and T2 vertebral bodies. The T12 root exits the spinal cord between T1 and L1. The L1 root exits the spinal cord between L1 and L2 bodies. The L5 root exits the cord between L1 and S1 bodies.
Relation of Spinal and Vertebral Segments
- First two cervical cord segments roughly match the first two cervical vertebral levels.
- C3 – C8 segments of the spinal cords are situated between C3 through C7 bony vertebral levels.
- Likewise, in the thoracic spinal cord, the first two thoracic cord segments roughly match first two thoracic vertebral levels.
- However, T3 through T12 cord segments are situated between T3 to T8. The lumbar cord segments are situated at the T9 through T11 levels while the sacral segments are situated from T12 to L1. The tip of the spinal cord or conus is situated at L2 vertebral level. Below L2, there is only spinal roots, called the cauda equina.
A rough calculation can be done as follow to obtain a relation between vertebral and segmental level.
- From C2-C6 vertebra add 1 to obtain the spinal segment level
- From T1-T6 vertebra add 2 to obtain the spinal segment level
- From T7-T9 vertebrae add 3 to obtain the spinal segment level
- T10-T12 vertebrae have whole of lumbar segments
- L1 vertebra has sacral & coccygeal segments
- L2 onwards is cauda equina
This has an important beearing in marking the level of injury and correlating with the anatomy. For example if an injury occurs at T10-11 vertebral level, it is going to damage her lower thoracic as well lumber spinal segments.
This also helps to pin point the level of compression.
For all practical purposes whenever lesion in the spine is examined, it is always good to mention injured vertebral and spinal levels sparately.
If they roughly match, the level has been ascertained. if they grossly mismatch somthing might have been missed. There might be another lesion or cord edema.
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