Transitional Vertebra¶

Definition¶

A transitional vertebra (lumbosacral transitional vertebra, LSTV) is a congenital variant in which the lowest lumbar vertebra has characteristics of the sacrum (sacralization of L5) or the highest sacral segment has characteristics of a lumbar vertebra (lumbarization of S1). LSTVs are common, present in approximately 15–30% of the population.

Classification — Castellvi¶

• Type I — Dysplastic (enlarged) transverse process (>19 mm). Unilateral (Ia) or bilateral (Ib).
• Type II — Pseudoarticulation between the enlarged transverse process and the sacral ala, forming a diarthrodial joint. Unilateral (IIa) or bilateral (IIb).
• Type III — Complete bony fusion of the transverse process to the sacral ala. Unilateral (IIIa) or bilateral (IIIb).
• Type IV — Type II on one side and Type III on the other (asymmetric).

Clinical Significance¶

Bertolotti Syndrome¶

Low back pain attributed to a transitional vertebra — the pseudoarticulation (Type II) or the contralateral facet joint and disc can be a pain generator.

Numbering Errors¶

The most important clinical implication of LSTVs is vertebral numbering confusion — if a radiologist and surgeon count vertebral levels differently, the wrong disc may be operated on. Whole-spine or cervicothoracic scout images help establish accurate numbering.

Imaging Findings¶

Radiography¶

• AP view — Enlarged, "winged" transverse process at L5 (or lumbarized S1) that may articulate with or fuse to the sacral ala
• Unilateral or bilateral

CT¶

• Best demonstrates the bony morphology — pseudoarticulation, fusion, or dysplastic transverse process
• Helps with accurate vertebral numbering

MRI¶

• The disc above a sacralized L5 is typically the last mobile disc and may show premature degeneration
• The "last normal disc" sign — the lowest disc with a normal nucleus pulposus signal helps identify the transitional level
• The most accurate method of numbering the vertebrae is to count down the vertebrae from C2 from the localizer sequences of the entire spine
• The iliolumbar ligament usually arises from the transverse process of L5 and can also be used to base numbering

Key Points¶

• Present in 15–30% of the population — common variant
• Castellvi classification based on transverse process morphology (I–IV)
• Biggest clinical risk is wrong-level surgery from vertebral numbering errors
• Whole-spine or cervicothoracic localizer ensures accurate numbering
• Bertolotti syndrome — back pain from the pseudoarticulation or adjacent segment degeneration

References¶

1. Castellvi AE, Goldstein LA, Chan DP. Lumbosacral transitional vertebrae and their relationship with lumbar extradural defects. Spine (Phila Pa 1976). 1984;9(5):493–495. doi:10.1097/00007632-198407000-00014. PMID: 6495013.
2. Konin GP, Walz DM. Lumbosacral transitional vertebrae: classification, imaging findings, and clinical relevance. AJNR Am J Neuroradiol. 2010;31(10):1778–1786. doi:10.3174/ajnr.A2036. PMID: 20203111.
3. Nardo L, Alizai H, Virayavanich W, et al. Lumbosacral transitional vertebrae: association with low back pain. Radiology. 2012;265(2):497–503. doi:10.1148/radiol.12112747. PMID: 22952380.
4. Hughes RJ, Saifuddin A. Imaging of lumbosacral transitional vertebrae. Clin Radiol. 2004;59(11):984–991. doi:10.1016/j.crad.2004.02.019. PMID: 15488846.
5. Miller AE, Zhang A. Bertolotti Syndrome. In: StatPearls [Internet]. Treasure Island (FL): StatPearls Publishing; 2023 [updated 2023 Jul 25]. Available from: https://www.ncbi.nlm.nih.gov/books/NBK585032/. PMID: 36251807.
6. Gaillard F, et al. Lumbosacral transitional vertebra. Radiopaedia.org. Available from: https://radiopaedia.org/articles/lumbosacral-transitional-vertebra.
7. Gaillard F, et al. Castellvi classification of lumbosacral transitional vertebrae. Radiopaedia.org. Available from: https://radiopaedia.org/articles/castellvi-classification-of-lumbosacral-transitional-vertebrae.

Related Articles¶

• Vertebral Column Overview
• Lumbar Spine