Last Updated on August 24, 2025
A lumbosacral transitional vertebra (LSTV) refers to a congenital segmentation anomaly at the junction between the lumbar spine and the sacrum. It represents a developmental spectrum in which the morphology of the terminal lumbar vertebra (usually L5) or the uppermost sacral segment (S1) is altered. These changes range from:
- Sacralization: Complete or partial fusion of L5 to the sacrum, reducing the lumbar vertebrae count to four.
- Lumbarization: Partial or complete separation of S1 from the sacral mass, resulting in six lumbar-type vertebrae.
- Intermediate forms: Numerous partial fusions or pseudoarticulations collectively classified as transitional states.
These anomalies are clinically important because they can affect spinal biomechanics, complicate imaging interpretation, and occasionally contribute to low back pain (Bertolotti syndrome).
Epidemiology and Genetics
Lumbosacral transitional vertebrae are relatively common, with reported prevalence ranging from 4% to 35.9%, and a mean prevalence of about 12%. While the anomaly itself is frequent, only a subset of individuals develop symptoms attributable to it; most cases are incidental findings.
When unilateral, the anomaly occurs more often on the left side, though the reason remains unclear. Sacralization is more frequently reported in males, whereas lumbarization occurs more often in females.
A 2025 prognostic study confirmed that patients with LSTV have higher odds of advanced lumbar disc degeneration, especially in females, independent of Castellvi subtype [1]. The presence of LSTV is also associated with a greater likelihood of a thoracolumbar transitional vertebra, suggesting a systemic segmentation tendency.
Developmental segmentation is regulated by HOX10 and HOX11 gene clusters. Aberrations in these genes are implicated in transitional morphology. Recent literature suggests that genetic predisposition may influence whether the sacrum incorporates L5 or “releases” S1, possibly mediated by load-bearing cues during early development.
Types of Lumbosacral Transitional Vertebra
Castellvi Classification
Accurate classification of LSTV is essential for radiological interpretation, communication among clinicians, and surgical planning. Two widely used systems are the Castellvi classification and the Mahato morphological classification, each addressing different aspects of the anomaly. [2]
Castellvi Classification (Radiographic Focus)
Developed for plain radiographs, Castellvi’s system is based on the morphology of the L5 transverse processes and their articulation or fusion with the sacrum:
- Type I: Dysplastic transverse process(es) ≥19 mm craniocaudally
- Ia: Unilateral
- Ib: Bilateral
- Type II: Incomplete lumbarization/sacralization with pseudoarticulation
- IIa: Unilateral diarthrodial joint
- IIb: Bilateral
- Type III: Complete osseous fusion
- IIIa: Unilateral
- IIIb: Bilateral
- Type IV: Mixed – Type II on one side, Type III on the other
It is simple and widely used, but does not account for changes in the neural arch or auricular surface.
Mahato Classification (Morphological Continuum)
Mahato proposed a more detailed system integrating the morphology of transverse processes, neural arch, and auricular surface into a biomechanical continuum.
- Type I: Dysplastic L5 transverse process
- Ia: unilateral TP is shorter than 19mm
- Ia F i/c – Presence of rudimentary TP ipsilateral/contralateral
- Ib: Bilateral TPs> 19mm
- Ib F – Presence of bilateral rudimentary facets
- Ia: unilateral TP is shorter than 19mm
- Type II: Accessory articulations
- IIa – Unilateral L5-S1 accessory articulation
- IIa F i/c- Presence of i/c rudimentary facet
- IIb- Bilateral L5-S1 accessory articulations
- IIb F- Presence of bilateral rudimentary facet
- IIa – Unilateral L5-S1 accessory articulation
- Type III: Sacralization
- Type III a: Unilateral L5-S1 sacralization
- Type III b: Unilateral complete sacralization with contralateral L5-S1 pseudoarthrosis
- Type III C: Bilateral complete L5-S1 sacralization
- F is added when a rudimentary facet is present ipsilateral/contralateral to sacralization or bilaterally
- Type IV: Lumbarization
- Type IV A: Incomplete/partial lumbarization of S1 as an accessory S1-2 articulation
- Type IV B: Unilateral complete separation of S1 from sacral mass
- Type IV: Bilateral S1-2 accessory articulation
- Type IV D: Complete sacralization with residual four-segment sacrum
- F is added to denote the presence of a rudimentary facet
In modern practice, Castellvi is often used for radiographic description, while Mahato provides greater surgical and biomechanical insight.
Biomechanical and Structural Changes
The sacrum transmits axial load to the sacroiliac joints. Anomalies at the lumbosacral junction alter these load-transfer dynamics.
During early development, the sacrum may incorporate L5 if the sacral surface area is insufficient to handle the load (sacralization), or release S1 if the load-bearing capacity is robust (lumbarization).
Transitional states thus reflect both genetic programming and mechanical adaptation.
Classic alterations include:
Sacralization: All dimensions, including pedicle height, sagittal and transverse dimensions, and sagittal angulation, are reduced, and the downward slope is increased.
The height of the pars interarticularis and the widths of the laminae are significantly smaller in the terminal lumbar segment of sacralized specimens. This predisposes spondylolysis and spondylolisthesis at the lumbosacral junction.\
Lumbarization: Lumbarization of S1 results in a shorter distance between the facet and sacral promontory, more obtuse pedicles in the sagittal plane, and less steep in front.
Facet asymmetry is Common and contributes to uneven load sharing.
Disc height is reduced below the transitional level in Castellvi II–IV. It may mimic degenerative collapse.
The iliolumbar ligaments may be hypoplastic above the transitional level, contributing to instability. Bony articulation or fusion between the transverse process and sacrum may function as a compensatory stabilizer. It could be responsible for vertebral segment instability and could subsequently lead to early disc degeneration.
Motion at L5–S1 is typically reduced in sacralized segments, with compensatory hypermobility and torque at adjacent levels. In unilateral LSTV, asymmetric loading leads to pelvic obliquity (lateral tipping of the iliac crest) and scoliosis convex toward the fused side.
Asymmetry can cause early degenerative changes and influence disc degeneration.Neural compression by new bone formation below an LSTV may occur.
Shift in Landmarks due to Lumbosacral Transitional Vertebra
Conus medullaris is significantly higher in the presence of a sacralized L5 and significantly lower in the presence of a lumbarized S1. This might be the reason for the neurological discrepancies observed among neurologic injuries at the thoracolumbar junction.
Non-spinal landmarks like the aortic bifurcation, IVC confluence, right renal artery, celiac trunk, and superior mesenteric artery root are located 1-3 levels more caudal than normal in the case of lumbarization, and 1-3 levels more cephalic than normal in the case of sacralization.
Nonspinal landmarks have a variable location and also change with age.
Due to altered vertebral, neural, and possible vascular anatomy, modification of surgical approaches may be necessary.
Dynamic radiographic studies in 2025 confirmed markedly reduced L5–S1 motion in Castellvi II–IV cases, with compensatory motion distributed across several levels[1]. A 2025 multicenter study showed that spinopelvic parameters should be measured relative to L5 (not S1) in high-grade sacralization to avoid errors. [3]
Imaging of Lumbosacral Transitional Vertebra
Accurate imaging evaluation is essential to avoid wrong-level interventions.
X-rays
Standard AP radiographs demonstrate 76%-84% accuracy for lumbosacral transitional vertebra detection.
- Standard AP films detect most cases (up to 85%)
- The 30° angled AP radiograph (Ferguson radiograph) serves as the reference standard method to detect LSTV.
- Lateral films show vertebral body “squaring” and reduced transitional disc height.
- Sclerotic changes and osteophytes at pseudoarticulations suggest symptomatic joints.
CT:
- Provides superior bony detail and classification.
- 3D reconstructions help assess neural arch and auricular surface changes.
MRI:
- Crucial for disc and neural assessment.
- Exaggerated lumbar lordotic curvature and a lack of sharp angulations at the lumbosacral junction on mid-sagittal MRI suggest a lumbosacral transitional vertebra.
- Indicative angles that suggest a lumbosacral transitional vertebra.
- Angle >39.8°, between by a line parallel to the superior surface of the sacrum and a line perpendicular to the axis of the scan table on mid-sagittal T2-weighted MRI
- Angle >35.9° between a line parallel to the superior endplate of the L3 vertebra and a line parallel to the superior surface of the sacrum, suggests a lumbosacral transitional vertebra.
- Paraspinal structures in positions outside their frequent location may signify the presence of a transitional vertebra.
Correct Numbering of the Vertebrae on Imaging
It is important to ensure the correct numbering of vertebrae to avoid intervention or surgery at the incorrect level.
A lumbosacral transitional vertebra can make the correct numbering of lumbar and sacral vertebrae difficult on imaging.
X-rays
On a sagittal radiograph, the last vertebra with a rectangular shape is generally considered to be L5, and then the vertebral bodies are numbered from the bottom to the top. In the presence of an LSTV, the rectangular-shaped last vertebra can be L4 or L6.
This is confirmed by counting from above, T12 downwards. The vertebra with the last rib is deemed as T12 [ But that can be fallacious, as many persons have a last rib absent]
In such cases/doubt, whole spine images, including the cervical spine, can be used for confirmation, starting from C2 downward.
MRI
Numbering is done from C2 on whole-spine MR images, is remains the best option. In a child, sagittal images of the sacrum and coccygeal bone on T2-weighted can help. The counting is done up from S5 and S1 is determined correctly.
Determination of S1 enables detection of the L5 and, in turn, all other vertebrae.
Whole-spine sagittal T2 MRI or stitched scout views remain best for numbering, counting from C2 down.
Iliolumbar ligament identification on MRI aids numbering. [4]
Clinical Significance of LSTV
Low back pain in the presence of an LSTV was originally noted by Mario Bertolotti in 1917 and termed Bertolotti’s Syndrome.
The link between LSTV and low back pain remains debated. Bertolotti syndrome refers to symptomatic cases described initially in 1917.
Pain sources
- Early disc degeneration or herniation above the transitional level.
- Pseudoarticulation arthrosis (especially Castellvi II).
- Facet arthropathy, foraminal stenosis.
- Extraforaminal nerve entrapment between TP and sacral ala (Far-Out Syndrome).
Clinical presentation
Localized low back pain, buttock pain, or radicular symptoms; unilateral cases may have pelvic obliquity and scoliosis convex toward the fused side.
Diagnosis
- Correlation of symptoms with imaging is essential. Despite the presence of LSTV and back pain, over-attribution should be avoided.
- Diagnostic injections with local anesthetic corticosteroid into the pseudoarticulation or facet confirm pain source.
Management
- Conservative therapy: NSAIDs, physiotherapy.
- Interventions: Image-guided pseudoarticulation injections; radiofrequency ablation (RFA) has shown benefit in refractory cases (2021 onward).
- Surgery: Processectomy (pseudoarticulation resection) in confirmed symptomatic cases, fusion when instability/degeneration is present.
Lumbosacral transitional vertebrae are common in patients undergoing surgery for the last mobile disc herniation. In axial spondyloarthritis, prevalence is high but does not worsen disease activity or treatment response; it can complicate imaging interpretation.
References
- Kuoppala, K., et al. (2025). Lumbosacral transitional vertebrae and lumbar disc degeneration: a prognostic analysis. Archives of Orthopaedic and Trauma Surgery, 145, 1503–1514. [PubMed]
- Konin GP, Walz DM. Lumbosacral transitional vertebrae: classification, imaging findings, and clinical relevance. AJNR American journal of neuroradiology. 2010;31(10):1778–86.
- Khalifé, M., et al. (2025). The implications of sacralized transitional vertebrae on spinopelvic measurements: a multicenter stereoradiographic analysis. Spine, 50(8), 1234–1242. [PubMed]
- Hughes RJ, Saifuddin A. Numbering of lumbosacral transitional vertebrae on MRI: role of the iliolumbar ligaments. AJR American journal of roentgenology. 2006;187(1): W59–65. [PubMed]
