Transitional vertebra at any junction is characterized by features retained from two adjacent regions in the vertebral column. For example in the lumbosacral transitional vertebra, there is an L5 vertebra can become like S1 vertebra or S1 can leave the sacrum and become L6.
As we know from our knowledge of the anatomy of the spine, the anatomy of vertebrae varies according to the region and function.
Thus cervical vertebrae have different anatomy than thoracic and lumbar and sacral. The size shape and different features are different. Most of the typical vertebrae behave like that.
The issue of transitional vertebra arises when there is a change of region. These regions are from skull to cervical vertebrae, cervical to thoracic, thoracic to lumbar and lumbar to sacrum.
Out of these lumbosacral transitional vertebrae is most common.
Thus the transitional vertebra is found in cervicothoracic, thoracolumbar, or lumbosacral junction.
The transitional vertebra has indeterminate characteristic and features of vertebrae from adjacent vertebral segments.
This transitional vertebra causes a change in anatomy and biomechanics. These may cause symptoms and can lead to confusion when describing findings [the difficulty in labeling the correct number of the vertebra].
Different Types of Transitional Vertebra
Following are the different transitional vertebrae
Atlanto-occipital Junction
Transitional vertebra here can present as
Atlanto-occipital assimilation
Complete or partial fusion of C1 and the occiput
Occipital vertebra
Here there is an additional bone present between C1 and the occiput
Cervicothoracic junction
Cervical rib arising from C7 is an example of transition. Normally, the ribs begin from T1 vertebra and arising from C7 indicates transition.
Thoracolumbar junction
Here, there may be a lumbar rib or 13th rib arising from T13 vertebrae or L1 vertebra( the label varies according to the method of numbering chosen.
Other examples of transition are
- Aplasia or hypoplasia of the transverse process
- Irregular orientation on the superior articular process
- Atypical mammillary bodies [Mamillary body is a small apophysis or tubercle on the dorsal margin of the superior articular process of each of the lumbar vertebrae and often of T12 too].
Lumbosacral junction
Lumbosacral transitional vertebra
It is the most common transitional vertebra and most researched also. Its spectrum ranges from complete lumbarization to sacralization.
In lumbarization, there are six lumbar vertebrae and four sacral because first sacral vertebrae have left the sacrum during development and became like the lumbar vertebra.
Sacralization is the opposite of the above. The last lumbar vertebra gets fused with the sacrum. Now sacrum has six vertebrae and lumbar spine has only 4.
Transitional lumbosacral is also thought to be the cause of back pain. Bertolotti syndrome is symptomatic transitional lumbosacral vertebra.
In presence of LSTV, the numbering of vertebrae has to be done carefully to ascertain correct levels and confirmed using more than one methods.
Detection
These vertebrae are best identified on MRI on sagittal T1W and T2W images. For correct identification, the entire spine is imaged.
Clinical Significance
Transitional vertebrae could be found on images incidentally or could present as a set of symptoms as defined by the area where they are present.
It is important to mention that just the presence of transitional vertebra doe does not mean that, it is the originator of the symptoms. Therefore clinical relevance and correlation of the findings are very necessary.
The presence of transitional vertebra could confuse the numbering of the vertebral levels. Therefore, it is important to ascertain the correct levels by using more than one method to count the vertebrae.
In some case, the whole spine may be needed to be imaged to ascertain the correct level.
