Transitional Vertebral Anatomy

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The most common vertebral arrangement is of 24 presacral vertebrae, consisting of 7 cervical, 12 thoracic, and 5 lumbar vertebrae. The cervical spine has a fixed vertebral count of 7, however the lumbar and thoracic vertebrae counts can be variable. Transitional vertebrae occur when there is overlap of developing somites during cranial-caudal border shifting. The vertebrae affected have morphology of a combined nature of the two adjacent vertebral regions.

Some people with transitional anatomy have 23 or 25 presacral segments. The most common variations are the presence of 13 rib-bearing thoracic vertebrae with four lumbar-type vertebrae, and 12 rib-bearing thoracic vertebrae with six lumbar-type vertebrae.

Transitional anatomy can also occur at the atlanto-occipital junction (atlanto-occipital assimilation: complete or partial fusion of C1 and the occiput, occipital vertebra: an additional bone between C1 and the occiput), and at the cervicothoracic junction (cervical rib arising from C7).

Prevalence

• The lumbarisation of S1 occurs in around 2% of the population. The sacralisation of L5 occurs in around 17% of the population.
• Thoracolumbar transitional anatomy prevalence is unknown.
• Cervical ribs occur in around 0.2 to 0.5% of the population

Lumbosacral Transitional Anatomy

Numbering Technique

T2 Sagittal survey view of the entire spine done by a private Auckland radiology company showing 7 cervical, 12 thoracic, and 6 lumbar vertebrae.

It can be difficult to establish whether a LSTV is a lumbarised S1 or a sacralised L5, and there have been various techniques described, with the best technique being high quality imaging of the entire spine. MRI reports should state the method of numbering and how the LSTV was determined.

Survey sequences

In New Zealand, some private radiology providers will do a sagittal survey sequence of the entire spine (also called a sagittal localiser) as routine while others do not. You then count from the odontoid peg of C2 inferiorly, rather than counting cranially from L5. One can identify the L1 vertebral body, and subsequently determine the correct numeric assignment of the LSTV. Radiographs of the entire spine can fulfil the same purpose. If there is only a normal lumbar spine radiograph, then correct numeration may still be possible, but it can be tricky at the thoracolumbar junction to differentiate the hypoplastic rib from the transverse process. Thoracolumbar transitions can also make things difficult without a full spine view.

Using a sagittal localiser sequence assumes there are 7 cervical and 12 thoracic vertebra. The method does not account for thoracolumbar transitional anatomy or allow one to differentiate between dysplastic ribs and lumbar transverse processes. Adding a coronal localising sequence can sometimes increase the accuracy of locating the thoracolumbar junction.

Iliolumbar ligament

The Iliolumbar ligaments seen on an axial T2 weighted MRI sequence extending from the transverse process of L5 to the posteromedial iliac crest.

Locating the iliolumbar ligament is another technique described. This ligament is present at the L5 transverse process in 96.8% of people. On MRI it is a low signal intensity structure on T1 and T2 images and looks like a single or double band that extends from the transverse process of L5 to the posteromedial iliac crest. It serves to restrict flexion, extension, axial rotation, and lateral bending of L5 on S1. As with other landmarks, identification of the iliolumbar ligament is not sufficient to identify the L5 vertebral body, but it can be used to identify the lumbosacral junction as it identifies the lowest lumbar type vertebral segment.

Other anatomic markers

Using other markers such as the aortic bifurcation, right renal artery, and conus medullaris has also been described and are not satisfactory.

Other features

The lumbarised S1 may be squared and have facet joints and an intervertebral disc. The sacralised L5 may be wedged and have hypoplastic or absent jacet joints or intervertebral disc.

Castellvi classification

Type	Features
Type Ia	A unilateral TP height greater than or equal to 19 mm
Type Ib	Both processes heights greater than or equal to 19 mm
Type IIa	Presence of unilateral articulation between the TP and the sacrum
Type IIb	Presence of bilateral articulation between the TP and the sacrum
Type IIIa	Unilateral fusion of the TP and the sacrum
Type IIIb	Bilateral fusion of the TP and the sacrum
Type IV	Unilateral type II transition (articulation) with a type III (fusion) on the contralateral side

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Thoracolumbar transitional Anatomy

Throacolumbar transitional vertebrae (TLTV) may have hypoplastic ribs of <38mm in length, but this feature is not sufficient for detection.

Transitional vertebra (TV) characteristics.

TV Location	Naming	Frequency	Vertebral Segments	Anatomical Features
T12	T12TLTV	~2/3	24 (No additional)	Superior facets not directly posteriorly Intermediate mamillary bodies No prominent transverse process
L1	L1TLTV	~1/3	Mostly 24 Rarely 25 with 6 lumbar vertebra Rarely 25 with 13 thoracic vertebra and ribs	No costal facets for ribs. Transverse process aplasia/hypoplasia Superior facets asymmetrical and more posterior Remnant mamillary bodies

Other Articles

See Konin et al for an open access review on the lumbosacral transitional vertebral (LSTV) anatomy.