Some patients arrive in my office with years of low back pain, a stack of MRIs, and a sentence they've heard from multiple doctors: "You have an extra bone down there, but it's just an incidental finding." Sometimes that's true — many people carry this anatomy without any symptoms. But sometimes it isn't, and the pain has a name: Bertolotti syndrome. This post is about the anatomy, the pain it can cause, and how to tell the difference.
What "transitional anatomy" means
At the base of the spine, the last lumbar vertebra (L5) meets the sacrum (S1). In most people the boundary is clean: five lumbar vertebrae, then the sacrum. In a lumbosacral transitional vertebra — LSTV for short — that boundary blurs. The lowest lumbar vertebra takes on sacral characteristics (sacralization of L5), or the top of the sacrum takes on lumbar characteristics (lumbarization of S1). The hallmark is an enlarged transverse process — the bony wing off the side of the vertebra — that reaches toward the sacrum or pelvis. It can form a false joint (a pseudoarticulation) with the sacral ala, or fuse to it solidly. Transitional vertebrae are common on imaging; studies report them in a substantial minority of spines scanned for any reason. Most are silent. Bertolotti syndrome is the name for when this anatomy is actually producing pain.
The Castellvi types, in plain language
Spine surgeons classify transitional vertebrae with the Castellvi system, proposed in 1984, based on what the enlarged transverse process is doing. Type I is simply an enlarged, dysplastic transverse process — bigger than normal but not touching anything. Type II is the important one for pain: the enlarged process forms a pseudoarticulation, a false joint, with the sacrum — and Type II is the pattern most associated with Bertolotti syndrome, because a false joint can move, wear, and become arthritic like any joint. Type III is complete bony fusion of the process to the sacrum — solid, no motion. Type IV is mixed: a false joint on one side and fusion on the other. The asymmetry of one-sided (unilateral) patterns matters, because unbalanced anatomy loads the spine unevenly.
What Bertolotti syndrome actually is
Bertolotti syndrome, described in 1917, is chronic low back pain attributed to a lumbosacral transitional vertebra. The pain typically sits low and off to one side — over the buttock, the sacroiliac region, sometimes the groin or hip — and it is frequently mistaken for SI joint pain, which is one reason the diagnosis gets missed. The pain can come from several sources: arthritis in the false joint itself, overload of the facet joint on the opposite side (in unilateral patterns), accelerated wear of the disc just above the transitional level, or — less commonly — pinching of the L5 nerve root where the enlarged process crowds the space it exits through. It tends to show up in younger adults, often aggravated by activity and prolonged standing.
Why it's missed
Three reasons. First, transitional anatomy is common enough that radiologists routinely — and usually correctly — call it incidental, because most of the time it is. Second, the standard lumbar X-ray series doesn't include the Ferguson view, the 30-degree angled AP projection that best profiles the lumbosacral junction; without it, the pseudoarticulation hides behind overlapping bone. Third, the pain pattern mimics SI joint dysfunction so closely that the workup stops there. The result: a patient with real, anatomically explainable pain gets told nothing is wrong, sometimes for years. The distinction that matters is not whether the transitional vertebra exists — it's whether it's the pain generator.
How it's diagnosed
Diagnosis is a process of confirmation, not just observation. It starts with the right imaging: dedicated lumbosacral views including the Ferguson projection, CT for the bony detail (CT shows the pseudoarticulation and arthritic changes best), and MRI to assess the discs and nerves around it. SPECT/CT can show increased bone activity at a stressed false joint. But the confirmatory step is a diagnostic injection — numbing medication placed precisely into the pseudoarticulation under fluoroscopic guidance. If the pain drops substantially while the anesthetic is working, the false joint is the source. That single test separates Bertolotti syndrome from the far larger group of people who have transitional anatomy and pain from something else entirely.
What can be done about it
As with most spine problems, the ladder starts conservatively: anti-inflammatory medication, targeted physical therapy, and activity modification carry many patients. The next rung is therapeutic injection into the pseudoarticulation — the same injection used for diagnosis, now with steroid added, which can give meaningful relief. Radiofrequency ablation around the transitional segment is another option reported to help select patients. Surgery is reserved for those who fail these measures, and it is targeted: resection (removal) of the anomalous transverse process to eliminate the painful false joint, decompression if a nerve is entrapped, or fusion in select cases where instability demands it. The operation matches the pain source — which is why the diagnostic injection matters so much.
Why this is a second-opinion problem
Bertolotti syndrome lives in a diagnostic blind spot: too specific for a routine workup, too often dismissed as incidental. I see patients who were told their imaging was "normal" despite a visible transitional vertebra, and patients whose SI joint was treated repeatedly while the real generator sat one level up. A second opinion with attention to the lumbosacral junction — the right views, the right questions about where the pain actually sits — can reframe years of unexplained back pain. If you've been told your transitional anatomy means nothing while your pain says otherwise, that disagreement deserves a closer look.
Frequently asked questions
What is a lumbosacral transitional vertebra?
A lumbosacral transitional vertebra is a congenital variation where the boundary between the lumbar spine and sacrum blurs — the lowest lumbar vertebra takes on sacral features (sacralization) or the top sacral segment takes on lumbar features (lumbarization). The hallmark is an enlarged transverse process that may form a false joint with, or fuse to, the sacrum. It is common on imaging and usually causes no symptoms.
What is Bertolotti syndrome?
Bertolotti syndrome is chronic low back pain attributed to a lumbosacral transitional vertebra. The pain typically sits low and to one side — over the buttock or sacroiliac region, sometimes the groin or hip — and can come from arthritis in the false joint, overload of nearby structures, or nerve irritation. It is the painful subset of transitional anatomy, not the anatomy itself.
How is Bertolotti syndrome diagnosed?
Diagnosis combines dedicated imaging — lumbosacral X-rays including the Ferguson view, CT for bony detail, MRI for discs and nerves — with a confirmatory diagnostic injection: numbing medication placed into the pseudoarticulation under fluoroscopic guidance. If pain drops while the anesthetic works, the false joint is the source.
Is a transitional vertebra always the cause of back pain?
No — and this is the critical distinction. Transitional vertebrae are common and most are silent. Bertolotti syndrome is only diagnosed when the transitional anatomy is proven to be the pain generator, typically through a diagnostic injection. Imaging findings alone are never enough.
What are the treatment options for Bertolotti syndrome?
Treatment starts conservatively with anti-inflammatory medication and physical therapy. The next steps are therapeutic injections into the pseudoarticulation and, in select patients, radiofrequency ablation. Surgery — usually resection of the anomalous transverse process, sometimes decompression or fusion — is reserved for those who fail conservative and injection-based care.
I was told my transitional vertebra is 'just incidental.' Should I get a second opinion?
If your pain pattern fits — low, one-sided back or buttock pain, worse with activity — and no one has evaluated the transitional segment as a pain source with dedicated views or a diagnostic injection, a second opinion is reasonable. Being common on imaging doesn't automatically make it irrelevant to your pain.