Lumbosacral Transitional Vertebrae Detected on Routine Sacroiliac Joint Radiographs in a Rheumatology Cohort: Comparison of Castellvi and Jenkins Systems

The Mystery of the "Extra" Bone

Imagine your spine is a magnificent, flexible tower built to keep you upright, allowing you to dance, run, and reach for the stars. But sometimes, nature decides to add a little extra flair to the construction. At the very bottom of this tower, where the flexible spine meets the sturdy pelvic bowl, there is a spot that can get a bit confused. This is the lumbosacral junction. Sometimes, the last bone of the spine (the L5) doesn't quite know if it wants to be part of the spine or part of the pelvis. It might grow a little too big, or it might try to fuse with the pelvis, creating a "transitional vertebra." Think of it like a doorframe that is slightly too wide, or a bridge that accidentally touches both banks of a river at the same time.

When this happens, it can cause a condition known as Bertolotti syndrome, which is a fancy name for lower back pain caused by this weird bone setup. Doctors have been trying to figure out how often this happens and how to spot it. For a long time, they used a specific rulebook called the "Castellvi classification" to sort these bones into different types, kind of like sorting LEGO bricks by shape. But recently, a new rulebook called the "Jenkins classification" was proposed. This new system uses a ruler to measure the distance between the bone and the pelvis, flagging even connections that are quite far apart as "transitional." The big question for scientists was: How many people actually have these extra bones? And does the new ruler find way more of them than the old rulebook?

The Great Bone Hunt in the Rheumatology Clinic

In this study, a team of researchers from Poland decided to play detective. They went on a treasure hunt through the medical records of 817 patients who had visited a rheumatology clinic (a place for treating joint and muscle diseases) between 2022 and 2024. These patients had X-rays taken of their sacroiliac joints—the joints connecting the spine to the pelvis—usually to check for inflammation or arthritis. The researchers didn't just look at the joints; they zoomed in on the very bottom of the spine to see if they could spot any of those "confused" transitional vertebrae.

They used two different maps to guide their search: the old, well-known Castellvi map and the new, distance-measuring Jenkins map. Here is what they found, and it was a bit of a shocker.

When they used the Castellvi map, they found these special bones in 248 out of the 817 patients. That's about 30.35% of the group. It's a significant chunk, but not a majority. The most common type they saw was a "Type IA," where the bone was just a bit enlarged but didn't quite touch the pelvis.

But then, they switched to the Jenkins map. Suddenly, the numbers exploded. Using this new system, they found these bones in 735 out of 817 patients. That is a whopping 89.96% of everyone they looked at! Almost everyone had a "Jenkins-positive" bone. The researchers noticed something very interesting about this new map: it seemed to be very sensitive to age. In people under 20, only 68% had a Jenkins-positive bone. But as people got older, the number climbed steadily, reaching 100% for everyone aged 70 and older. It's as if the new ruler starts to see "connections" everywhere as people get older, perhaps because the bones change shape over time.

The Missing Report and the Pain Puzzle

One of the most surprising discoveries in this paper was what the doctors didn't write down. Even though the X-rays were right there, clear as day, none of the original radiological reports mentioned these transitional bones. The radiologists were so focused on checking the joints for arthritis that they completely missed the "extra" bones at the bottom of the spine. This suggests that in everyday practice, these bones are being overlooked, even when the pictures are right in front of the doctors' eyes.

The team also tried to solve a bigger mystery: Does having these bones actually cause the lower back pain that brought these patients to the clinic? They looked at whether patients with these bones were in more pain than those without. The answer was a bit complicated. They found that having a rheumatologic disease (like ankylosing spondylitis, a type of inflammatory arthritis) was strongly linked to back pain. In fact, patients with ankylosing spondylitis were much more likely to be in pain than those without.

However, when they looked specifically at the transitional bones, the story was less clear. The study did not find a strong link between having these bones and having back pain in this specific group of patients. The pain seemed to be driven mostly by the inflammatory diseases they already had, rather than the extra bones. The researchers suggest that because so many people in this group had inflammatory diseases, it was hard to tell if the bones were the real culprit. They also warned that the Jenkins system might be "over-enthusiastic," flagging bones as problematic even when they might just be normal variations or changes due to aging. The new system uses a broad threshold (measuring a 10mm distance) that might be too permissive, counting bones as "transitional" even when the connection is quite loose or distant.

What This Means for You

So, what's the takeaway from this big bone hunt? First, the researchers suggest that the standard X-rays taken in rheumatology clinics are actually good enough to spot these bones if someone just takes a closer look. The problem isn't the picture; it's that the doctors often forget to look at the bottom of the spine. If they started checking for these "transitional" bones systematically, they might catch more cases of Bertolotti syndrome.

Second, the study highlights a debate between the two rulebooks. The old Castellvi system found about 30% of people with these bones, while the new Jenkins system found nearly 90%. The researchers suspect the Jenkins system might be too sensitive, especially for older people, because it counts tiny, harmless gaps as "transitional."

Finally, the paper reminds us that having a weird bone doesn't automatically mean you have a painful back. In this group of patients, the pain was mostly coming from their rheumatologic diseases. The study doesn't prove that these bones cause pain, but it does suggest that we need to be smarter about how we look at them. If you have chronic back pain and the usual treatments aren't working, it might be worth asking a specialist to take a second, closer look at that bottom part of your spine, just to make sure there isn't a hidden "extra" bone causing the trouble.