Lumbosacral transitional vertebra

Last revised by Dr Sonam Vadera on 04 Oct 2021

Lumbosacral transitional vertebrae (LSTV) are a relatively common variant and can be seen in ~25% (range 15-35%) of the general population 1-3. Non-recognition of this variant and/or poor description in the report can lead to operations or procedures performed at the wrong level. 

Depending on the number of thoracic vertebrae, lumbar vertebrae, and sacral segments, they can be thought of as a lumbarized S1 segment or sacralized L5 segment. There can be a varying degree of transition, from partial to complete fusion. 

  • assimilation of S1 to lumbar spine
  • less common than sacralization, occurring in ~2% of the population 2
  • presence of six rib-free lumbar-type vertebrae, which may have the following features:
    • squaring of highest sacral (transitional) vertebral body
    • facet joints (even rudimentary) and intervertebral disc at S1-S2
  • assimilation of L5 to the sacrum
  • more common than lumbarization, occurring in ~17% of the population 2
  • presence of four rib-free lumbar type vertebrae, which may have the following features:
    • wedging of the lowest lumbar (transitional) vertebral body
    • broadened transverse processes, potentially with pseudoarticulation or fusion with one or both sacral ala
    • hypoplastic or absent facet joints or intervertebral disc at L5-S1

The Castellvi classification is commonly used to classify transitional anatomy. 

Ferguson projection (AP lumbar spine radiograph, 30° cranial tilt) and lateral projection have been traditionally used as the best method for identification of lumbosacral transitional vertebra, although this has been largely replaced by cross-sectional imaging.

The most accurate method of enumerating the vertebrae requires taking localizer sequences of the entire spine to count down from C2. However, these are often not part of lumbar spine MRI protocols.

Numbering can also be ascertained on axial MRI images (see case 9) at the sacral level using nerve morphology given that the L5 nerve characteristically does not split proximally and has twice the caliber of the L4 peroneal branch at this level 5. The level is determined as follows:

  • 4 lumbar segments (23 presacral vertebrae with completely sacralized L5): a bundle of several splitting nerves at the lateral sacrum represents the L4 nerve
  • 5 lumbar segments (24 presacral vertebrae with normal anatomy, partial L5 sacralization, or partial S1 lumbarization): a thin nerve joining a thicker nerve at the lateral sacrum represents the peroneal branch of L4 and the L5 nerve
  • 6 lumbar segments (25 presacral vertebrae, completely lumbarized S1): two nerves of similar caliber at the lateral sacrum represent the L5 and S1 nerves

The iliolumbar ligament is an alternative landmark on which to base numbering (see case 4) as it usually arises from the transverse process of L5 1. Although the ligament seems to always arise from the last lumbar vertebra, the accuracy of this method has been questioned in cases of lumbosacral transitional anatomy 5,6.

Despite classification systems, nothing replaces an explicit description of what nomenclature has been used and assumptions made in cases where the anatomy is non-standard. For example, one may have total spine imaging or correlation with prior imaging to allow numbering from C1/2. Alternatively, one may number presuming the last rib-bearing vertebra is T12, presuming the last fully formed intervertebral disc space is L5-S1, or using the L5 nerve morphology or iliolumbar ligament localization methods described above.

A common but not universal nomenclature designates 7 cervical and 12 thoracic vertebrae, even in cases where there are 13 rib-bearing vertebrae (considered cervical ribs at C7 or lumbar ribs of L1) rather than designating a T13 vertebra. After T12, the vertebrae are considered lumbar-type and the 24th vertebra is designated L5 and the 25th vertebra is designated S1, even in cases of lumbosacral transitional anatomy (considered partially or completely sacralized L5 or lumbarized S1) rather than omitting L5 or designating an L6 vertebra 5.

  • an association with low back pain (Bertolotti syndrome) remains controversial 3,4
  • increased disc degeneration to the level above the lumbosacral transitional vertebra 4

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Cases and figures

  • Case 1: with unilateral incomplete sacralization of L5
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  • Case 2: lumbarization of S1
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  • Case 3: sacralization of L5
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  • Case 4: iliolumbar ligaments on CT
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  • Case 5: six lumbar-type vertebrae
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  • Case 6: sacralization of L5
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  • Case 7: hemilumbarisation of S1
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  • Case 8: 11 ribs pairs and L5 sacralization
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  • Case 9: left S1 partial lumbarization
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