Lumbarization or sacralization-?. Concept of lumbosacral transitional vertebra by MRI
Lumbosacral transitional vertebrae (LSTV) occur when the last lumbar vertebra shows elongation of its transverse process, with varying degrees of fusion to the first sacral segment. The term “LSTV” is used to avoid having to decide whether such a vertebra is a “sacralized L5″ or a “lumbarized S1.” Although LSTV can be sensitively identified on sagittal lumbar spine MRI, based on abnormal morphology of the lumbosacral junction no standard method is established for their numbering. Techniques that have been used include the addition of cervicothoracic localizer scans and determining lumbar levels by identification of the right renal artery(in relation to the L1-2 disk space ). These techniques are prone to substantial error.Both anatomic and MRI studies have indicated that the iliolumbar ligament (ILL) arises almost exclusively from the L5 transverse process However, these studies are limited by relatively small numbers .if ILLs can be reliably identified on routine axial MR images through thelumbosacral junction and if they arise solely from L5, then their level of origin can serve as a marker of lumbar levels, which may enable the confident numbering of LSTV.
The ILLsappear as either a single or a double hypointense band arising from the transverse process and extending to the posteromedial aspect of the iliac crest. If an LSTV is present, it is numbered as a transitional L5 or transitional S1 segment based on the following method. When the ILL arises at
the level above the LSTV, the vertebral level of origin of the ILL was labeled L5 and, consequently, the LSTV was numbered S1 . If the ILL is not visualized, the LSTV is numbered L5 This numbering system was based on the hypothesis that in the presence of healthy lumbosacral segmentation, the ILL always arises from L5
and is therefore a marker of the L5 level.
Using identification of the iliolumbar ligaments as a marker of the L5 vertebral level, we can number LSTV as L5 transitions and S1 transitions. The iliolumbar ligament is readilyidentifiable on axial lumbar spine MRI and always arises from L5.We suggest that its position can be used to confidently assign lumbar levels in patientswith LSTV.