Lumbosacral transitional vertebra (Bertolotti)

What it means

The lowest movable bone of the lower back is normally called L5, sitting just above the sacrum, the triangular bone that connects the spine to the pelvis. In a lumbosacral transitional vertebra, that boundary is blurred: the transverse process — a wing-like projection off the side of the vertebra — is enlarged on one or both sides and either touches, forms a false joint with (pseudarthrosis), or is fully fused to the sacrum or the nearby pelvic bone (the ilium). This is sometimes called Bertolotti syndrome when it's linked to pain, after the physician who first described it.

Why it appears on a CT or MRI report

Radiologists identify it by counting vertebrae and noting the shape and contact points of the lowest lumbar segment. The report typically describes which side is enlarged, whether the connection to the sacrum or ilium is a joint-like articulation or a solid bony fusion, and uses a grading system (commonly Castellvi type I through IV) to describe how extensively the two bones are joined. This detail matters because it can also change how spine levels are counted and labelled on the rest of the report.

What it usually means

This is a congenital anatomic variant — something a person is born with — rather than a disease or an injury, and it's seen in roughly one in ten to one in five people, most of whom never have a problem from it. When it does cause symptoms, the mechanism is mechanical: the extra joint or fusion point alters normal movement and load-sharing at the very bottom of the spine, which can lead to extra stress at that level and, over time, one-sided low back pain, sometimes radiating into the buttock or hip. Not everyone with this variant on their scan has pain from it — many people with the same finding are entirely symptom-free, so the anatomy alone isn't a diagnosis of pain.

When to follow up

If this variant is found incidentally and you have no matching symptoms, it typically needs no treatment or follow-up at all. If you do have persistent low back pain on the same side as the finding, mention it to your doctor — treatment usually starts conservatively with physical therapy, targeted injections, or activity modification, and is escalated only if those don't help. Bring up any new numbness, tingling, or leg weakness with your doctor as well, since those warrant their own evaluation.

A plain-language way to picture it

Picture a set of stacking chairs where the bottom chair's legs are a little wider and touch the floor stand slightly differently from all the chairs above it. The stack still works and stands fine, but that bottom connection point takes on load in a slightly different way than the rest. For most people, that's simply how their particular set of chairs was built — sturdy and unremarkable. For a smaller number, that different connection point becomes the spot that eventually aches from the years of slightly uneven loading.