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What is a Transitional Segment?
Picture this. You’re looking at your low back on the screen and I’m pointing to the very bottom of it. “See that?” I say. “That’s a transitional segment.” If you’ve ever heard those words and wondered what they actually mean, you’re in good company. It’s one of the most common variations in spinal anatomy, and for most people it never causes a problem. Here’s what it is, how common it really is, and when it matters.
What Is a Transitional Segment?
During early development, the spine starts out as a series of segments that eventually form and harden into bone. Most of us end up with five lumbar vertebrae and a sacrum made of five fused segments. But in about 12% of people, the blueprint changes at the junction of L5 and S1. The lowest lumbar vertebra develops an enlarged transverse process that reaches toward the sacrum, and sometimes it forms a joint with it, or even fuses to it. We call that sacralization. Less commonly, the first segment of the sacrum, S1, ends up looking more like an extra lumbar vertebra. We call that lumbarization. Either way, that border segment never fully commits to being lumbar or sacral. So it’s transitional.
Think of the spine like a game of Jenga. Normally each block is separate, and the whole tower depends on that. Now imagine that the second to last block and the block below it are actually one single fused piece. The tower still stands and still works, but the mechanics around that spot are different. That’s a transitional segment. An architectural variation, not a disease.
This X ray shows a normal low back. You can count five distinct lumbar vertebrae, identified by the pairs of small white dots (the pedicles) on either side, sitting above the large triangular sacrum. The fifth vertebra, L5, has two small transverse processes projecting out to the sides. In a transitional segment, that tidy picture changes. The transverse process of L5 is broad, it reaches down toward the sacrum, and an extra joint line often forms between them. That extra joint is called a pseudoarticulation, and it’s the key to understanding when a transitional segment matters.
How Common Are They?
About 1 in 8 people has one. Estimates range from 4% to 35.9% of the general population, with most reviews landing right around 12% (Jancuska et al., 2015).
It’s more common in men than women. One large imaging study found it in 28.1% of men compared to 11.1% of women.
It shows up more often in chronic low back pain patients. A 2023 study of 385 people with chronic pain found a transitional segment in about 10% of them.
About 13% of people who have one never have any symptoms. For many, it’s just an incidental finding on imaging done for something else.
It tends to run in families. Variations in the HOX10 and HOX11 genes, which help set the number of lumbar and sacral segments, appear to play a role.
Not All Transitional Segments Are the Same
Radiologists sort transitional segments into four types using something called the Castellvi classification. The type depends on how far the transition goes.
Type I. An enlarged transverse process, at least 19 mm wide, that does not touch the sacrum. One side is Ia, both sides is Ib.
Type II. The enlarged process forms a joint with the sacrum. That’s the pseudoarticulation, or false joint. One side is IIa, both sides is IIb.
Type III. Complete bony fusion of the transverse process to the sacrum. Full sacralization or lumbarization.
Type IV. A combination, with Type II on one side and Type III on the other.
Types I and II are the most common, each accounting for about 40% of cases. Type III makes up about 12%, and Type IV is the rarest at about 5%. The one to remember is Type II, the false joint. A joint that was never meant to exist can develop arthritis over decades of use, and that’s the type most often linked to pain.
Why Does It Matter?
A transitional segment changes the way forces travel through your low back. Research points to four main mechanisms:
Stiffness at the transition. The extra joint or fusion limits normal motion at L5 and S1.
Extra load on the level above. L4 and L5 pick up the slack and carry more than their usual share. That’s why disc problems are more common above a transitional segment and less common below it.
Arthritis in the false joint. The pseudoarticulation can wear down over time, and that can be a real source of pain.
Narrowing of the nerve openings. Altered mechanics and facet wear can pinch or irritate nerve roots.
None of this means pain is inevitable. Most people with a transitional segment never have a problem from it. But it does explain why the finding is worth taking seriously when symptoms are present.
When a Transitional Segment Causes Pain
When a transitional segment is confirmed as the actual source of low back pain, it has a name: Bertolotti syndrome. It’s named after Mario Bertolotti, the Italian radiologist who first described the link back in 1917. It’s one of the most commonly missed causes of chronic low back pain.
It’s found in about 4% to 8% of people with low back pain, and it’s frequently overlooked.
It tends to show up in the 30s to 50s. The average patient is between 30 and 50, with one group of 268 patients averaging 47.7 years.
Diagnosis is often slow. One review found it took an average of 41 months from the start of symptoms to diagnosis, partly because it mimics muscle strain, SI joint problems, facet arthritis, and disc issues.
It matters in the operating room too. The altered anatomy can lead to wrong level surgery if the segments are miscounted, which is why anyone with a transitional segment heading into spine surgery needs that anatomy on the radar.
Here’s what a false joint looks like. Notice how broad the transverse process of L5 is and how it reaches down into the sacrum. You can see the extra joint line that formed between them. That’s the hallmark of a Type II transitional segment.
What Can You Do About It?
The good news is that most people with a transitional segment never need treatment, let alone surgery. When it does contribute to pain, the approach starts conservative and only escalates if needed.
Start with conservative care. Activity modification, medication for pain and inflammation, and chiropractic care focused on mobility and strength. Physical therapy aimed at lumbosacral mobility is the standard starting point.
Consider targeted injections. An injection into the false joint can help confirm it as the pain source and can provide relief in some cases. One honest caveat from the research: steroid injections tend to work less well in Bertolotti syndrome than in straightforward disc herniations, so expectations should be realistic.
Radiofrequency ablation is an option in select cases. Case reports show it can help when the false joint is the culprit.
Surgery is reserved for people who fail conservative care. The two main options are removing the enlarged transverse process or fusing the segment. Long term results favor fusion: at 12 months or more, pain improved in 78% of the fusion group versus 28% of the resection group in one study.
The Bottom Line
A transitional segment is a common congenital variation. You’re born with it, it doesn’t progress, and it doesn’t mean your spine is broken. It means your spine was built a little differently, and for most people that difference never causes a problem. But when low back pain hangs around, especially in your 30s to 50s, a transitional segment deserves to be part of the conversation. When it really is the source, it’s identifiable and treatable, and the sooner it’s recognized, the sooner the right care can start. So if you’ve been told you have one, the question isn’t whether you have it. The question is whether it’s actually the source of your pain. That’s exactly what a thorough evaluation is for.
Sources
Jancuska J, et al. "A Review of Symptomatic Lumbosacral Transitional Vertebrae: Bertolotti’s Syndrome." International Journal of Spine Surgery, 2015
"An Update on the Prevalence and Management of Bertolotti’s Syndrome." Frontiers in Surgery, 2024
"Prevalence, Patterns, Functional Disability of Bertolotti Syndrome Among Patients with Low Back Pain" (Mulago National Referral Hospital), 2023. Full text
"Lumbosacral Transitional Vertebrae: Association with Low Back Pain." Radiology, 2012
