What Is a Transitional Vertebrae and Does It Cause Pain?

A transitional vertebra is a spinal bone that sits at the junction between two regions of the spine and shares features of both, making it a kind of anatomical hybrid. The most common type occurs where the lumbar spine meets the sacrum, and these lumbosacral transitional vertebrae show up on imaging in roughly one in five to one in four people. Many of those people never have symptoms. But in a meaningful minority, the unusual anatomy does contribute to low back pain, hip pain, or nerve-related leg symptoms, a condition formally called Bertolotti’s syndrome.

What a Transitional Vertebra Actually Looks Like

The spine is organized into distinct regions: cervical (neck), thoracic (mid-back), lumbar (lower back), sacral, and coccygeal. Each region’s vertebrae have a characteristic shape suited to their mechanical role. A transitional vertebra is one that doesn’t fully commit to its assigned region. At the lumbosacral junction, this means the lowest lumbar vertebra may partially or fully fuse to the sacrum (called sacralization), or the top segment of the sacrum may separate and behave more like a free lumbar vertebra (called lumbarization).

These aren’t subtle variations. The morphology ranges from slightly enlarged transverse processes (the bony wings on either side of the vertebra) all the way to complete bony fusion with the sacrum.¹ In some cases, the enlargement is only on one side, creating an asymmetric arrangement that can torque the spine unevenly.

How They Form

Transitional vertebrae arise during embryonic development, when a molecular system of genes called HOX genes lays down the blueprint for which vertebrae belong to which spinal region. These genes are expressed in a precise sequence along the developing spine, and their overlapping activity domains tell each segment whether to become a thoracic vertebra with ribs, a lumbar vertebra without them, or a sacral vertebra that fuses into a single bone.² When this coding shifts slightly at a boundary zone, the vertebra at that junction receives mixed signals and develops features of both regions. Research mapping HOX gene expression in human embryonic spines has confirmed that the positional code is complex and involves at least 18 genes with position-specific patterns.³

Because this happens before birth, a transitional vertebra is present from day one. You don’t develop one from wear and tear or injury. Most people who have one discover it incidentally when they get imaging for something else.

How Common Are They

Prevalence estimates depend on the population studied and the imaging method used, but lumbosacral transitional vertebrae generally appear in somewhere between roughly 8% and 25% of people. A large study grading pelvic radiographs in over 4,600 people found 841 individuals with a transitional vertebra, with the mildest forms (enlarged transverse processes) and intermediate forms (a false joint between the transverse process and sacrum) each making up about 40% of cases, while complete bony fusion accounted for about 12%.⁴ A study in a southern European population found a prevalence of about 25%, with the majority falling into the intermediate categories.⁵

One reason estimates vary so widely is that the mildest forms, where the transverse process is simply a bit bigger than normal, shade into normal anatomy. Drawing the line between “unusually large” and “transitional” involves some judgment. And plain X-rays can misclassify the type of transitional vertebra more than a third of the time compared to CT imaging, which means some surveys over- or undercount depending on their imaging method.⁶

The Castellvi Classification

Radiologists grade lumbosacral transitional vertebrae using the Castellvi classification, which has four main types based on the size and behavior of the transverse processes. Type I means the transverse process is at least 19 mm wide but doesn’t form a joint or fuse with the sacrum. Type II means it forms a false joint (pseudoarticulation) with the sacrum. Type III means it has fused entirely to the sacrum with solid bone. Type IV describes an asymmetric combination, with fusion on one side and a pseudoarticulation on the other. Each type is further subdivided into “a” (one-sided) and “b” (both sides).⁴

This classification matters clinically because the more advanced types, particularly Types II through IV, are more closely associated with altered spinal mechanics and symptoms. Type I on its own is generally considered a normal variant that rarely causes trouble.

Does It Cause Pain

The honest answer is: sometimes. Plenty of people walk around with a transitional vertebra and never know it. But the association with low back pain is real and has been demonstrated across multiple studies. A cross-sectional study found that people with chronic low back pain had a significantly higher rate of transitional vertebrae (about 38%) compared to a control group (about 22%).⁷ In the southern European study, people with transitional vertebrae were significantly more likely to report low back pain and score higher on disability questionnaires.⁵ An analysis of nearly 600 patients found a modest but statistically significant association between transitional vertebrae and low back pain, with an odds ratio of about 1.5.⁸

When a transitional vertebra does cause chronic low back pain, the condition is called Bertolotti’s syndrome. It is diagnosed in a relatively small fraction of back pain patients, estimated at roughly 4 to 8%.⁹ The pain most commonly centers around the sacroiliac joint, groin, or hip, and it may or may not involve radiating leg symptoms.¹⁰

Why the Abnormal Anatomy Creates Problems

The pain pathways in Bertolotti’s syndrome involve several mechanisms, and more than one can be active in the same person.

The most studied mechanism is accelerated disc degeneration at the level just above the transitional vertebra. A combined clinical and cadaveric study found that lumbar discs immediately above a transitional vertebra were significantly more degenerated than discs at other levels, while the disc between the transitional vertebra and the sacrum was actually better preserved. The same researchers found that the iliolumbar ligament at the level above the transitional vertebra was thinner and weaker, which likely allows more motion at that segment and drives earlier disc breakdown.¹¹ Finite element modeling has confirmed this pattern: when the number of freely moving lumbar segments is reduced by a transitional vertebra, the segments above it experience higher stress and greater range of motion, increasing the risk of disc problems over time.¹²

A second mechanism involves the false joint itself. When a transverse process forms a pseudoarticulation with the sacrum (Castellvi Type II), that abnormal joint can become a pain generator through the same kind of wear-and-tear arthritis that affects any other joint. Similarly, when a new joint-like structure (neoarticulation) forms between the transverse process and the iliac bone of the pelvis, it can become a chronic source of pain that proves difficult to treat.¹³

A third mechanism is direct nerve compression. The L5 nerve root exits the spine in close proximity to an enlarged or fused transverse process, and bone overgrowth in that region can squeeze the nerve. One study found that the L5 nerve root was compressed by abnormal bone at the false joint in 64% of symptomatic patients with transitional vertebrae.¹⁴ A separate imaging study found evidence of nerve compression below the transitional vertebra in about 13% of patients with a pseudoarticulation, with symptoms attributable to that compression in a subset of those cases.¹⁵

Why Diagnosis Can Be Tricky

Bertolotti’s syndrome is widely considered underdiagnosed, partly because the differential for low back pain is enormous. Clinicians have to distinguish transitional vertebra pain from disc herniation, facet joint arthritis, sacroiliac dysfunction, spondylolisthesis, muscle strain, and other causes.¹⁶ Making matters worse, several of these conditions can coexist with a transitional vertebra, so seeing one on imaging doesn’t automatically mean it’s the pain source.

The standard approach for pinning down the transitional vertebra as the culprit involves a combination of imaging, clinical history, and a diagnostic injection. A local anesthetic block is placed at the pseudoarticulation or the contact point between the transverse process and sacrum. If the injection temporarily eliminates or dramatically reduces the pain, it strongly implicates the transitional anatomy as the source.¹⁷

The Wrong-Level Problem

One of the most serious practical consequences of a transitional vertebra has nothing to do with pain itself: it can cause surgeons to operate on the wrong spinal level. When someone has an extra lumbar vertebra (lumbarization) or one fewer (sacralization), the standard method of counting vertebrae from the bottom up can misidentify which disc is L4-L5 and which is L5-S1. A study of patients with transitional vertebrae undergoing treatment for lumbar radiculopathy found that determination and treatment were performed at the wrong anatomical level in about 17% of cases, and in nearly half the cases, the correct level couldn’t be determined with certainty from the available records.¹⁸

A systematic review of surgical cases involving transitional vertebrae found that wrong-level surgery occurred in about 1.4% of patients, a rate that is small in percentage terms but significant given the consequences.¹⁹ If you know you have a transitional vertebra and are facing any spinal procedure, it’s worth making sure your surgical team is aware and has used whole-spine or cross-reference imaging to confirm vertebral numbering.

Treatment Options

Most people with a symptomatic transitional vertebra start with conservative treatment: anti-inflammatory medications, physical therapy, and activity modification. For many, these measures are sufficient. When they aren’t, steroid injections at the pseudoarticulation or neoarticulation can provide temporary relief, sometimes lasting up to two years.¹³ Radiofrequency ablation, which uses heat to deaden the nerve fibers carrying pain signals from the abnormal joint, is another option that has shown partial to significant improvement in small case series.²⁰

When conservative approaches fail, surgery becomes a consideration. Two main procedures exist: resection of the enlarged transverse process (cutting away the part that contacts the sacrum or ilium) and fusion of the transitional segment. Both approaches provide good short-term pain relief, with fusion and resection performing similarly in the first six months. However, longer-term results diverge. A comparative study found that about 78% of fusion patients reported sustained pain improvement beyond 12 months, compared to only 28% of resection patients.²¹

The choice isn’t straightforward, though. Cadaveric biomechanical testing has shown that fusion increases motion at the adjacent segment above, which could set up future problems, while resection does not.²² The evidence base for both procedures remains relatively small, and the decision often depends on whether the spine above the transitional level is already showing instability.

Transitional Vertebrae at Other Spinal Levels

Lumbosacral transitional vertebrae get the most attention, but the same kind of identity confusion can happen at the thoracolumbar junction, where T12 meets L1. A thoracolumbar transitional vertebra typically involves an abnormal rib at T12, either absent, hypoplastic, or unusually short. A CT-based study found these in about 11% of patients, with absent ribs being the most common presentation.²³ Other research has placed the prevalence around 16% in both men and women.²⁴

Having a thoracolumbar transitional vertebra is strongly associated with also having a lumbosacral one, and with having an abnormal total rib count. One study using whole-spine CT found an odds ratio above 7 linking the two types of transitional vertebrae, and an odds ratio above 117 linking thoracolumbar transitional vertebrae to abnormal rib counts.²⁵ The clinical significance of thoracolumbar variants is less clear cut. One large study found no significant correlation between these spinal anomalies and clinical symptoms on their own, though women with an abnormal total vertebral count did show a higher rate of degenerative spondylolisthesis.²⁴

Genetics and Heritability

The HOX gene story explains the developmental mechanism, but what about family patterns? Studying heritability of transitional vertebrae in humans is difficult because the trait is usually discovered incidentally and most family members don’t get spinal imaging. The best data on inheritance comes from veterinary research, particularly in dogs, where standardized hip and spine screening programs generate large datasets with known pedigrees.

A study in German shepherd dogs estimated a moderate heritability of about 0.27, meaning that roughly a quarter of the variation in whether an animal develops a transitional vertebra can be attributed to genetic factors.²⁶ Research across 14 dog breeds in Norway found that prevalence differed significantly by breed, further supporting a heritable component.²⁷ Whether the same degree of heritability applies in humans is unknown, but given that the underlying HOX-gene patterning system is conserved across mammals, a genetic contribution is widely assumed. No human genome-wide association study has yet identified specific risk variants, which leaves this as one of the more conspicuous gaps in the research.

Activity, Athletes, and When Age Matters

A common worry for younger people diagnosed with a transitional vertebra is whether they need to limit physical activity. The evidence is somewhat reassuring on this front. The study that found a significant overall association between transitional vertebrae and low back pain also tested whether physical activity level modified that relationship and found that it did not. Active and inactive people with transitional vertebrae had similar odds of pain.⁸ What did matter was whether there was concurrent disc disease at the level above, which was strongly associated with pain regardless of activity.

Age matters in a different way. About 18.5% of patients diagnosed with Bertolotti’s syndrome are under 30, which is younger than the typical age for degenerative back pain.⁹ In a young person with persistent low back pain and no obvious disc or muscle explanation, a transitional vertebra is worth investigating. A number of these patients cycle through years of generic “mechanical back pain” diagnoses before the transitional anatomy is recognized as relevant.

Transitional Vertebrae and Evolutionary Variation

The fact that humans exhibit so much variation at the lumbosacral boundary fits into a broader evolutionary picture. Research on vertebral numbers across primates has shown that humans and great apes share a characteristically short trunk compared to most mammals, a trait that defines the ape lineage. Debates about whether early human ancestors had more or fewer lumbar vertebrae than modern humans turn in part on how you define a lumbar vertebra, since transitional forms blur the boundary.²⁸ The frequency of transitional vertebrae in living humans may reflect the evolutionary instability of this boundary, a zone where slight shifts in developmental patterning are tolerated without major functional cost, at least most of the time.

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