Partial sacralization of L5 is a congenital spine variation where the lowest lumbar vertebra (L5) is partially fused or abnormally connected to the sacrum, the triangular bone at the base of your spine. Instead of being fully separate and mobile, the L5 vertebra develops an enlarged wing-like projection (called a transverse process) that forms a joint or partial bridge with the sacrum. This shows up in roughly 5 to 8 percent of the population on imaging, and many people never know they have it.
If you’re reading this, you likely saw the term on an X-ray or MRI report and want to know what it means for your back. The short answer: it’s a normal anatomical variant that causes no symptoms for most people. But in some cases, it can be a source of chronic low back pain.
How Partial Sacralization Differs From Complete
Sacralization exists on a spectrum. In complete sacralization, L5 fully fuses with the sacrum, essentially becoming part of it. The vertebra loses all independent movement at that level. In partial sacralization, the connection is incomplete. The transverse process of L5 is abnormally large (at least 19 mm wide) and forms a pseudo-joint, sometimes called a false joint, where it presses against or partially articulates with the top of the sacrum or the pelvis. This pseudo-joint isn’t a true joint with proper cartilage and lubrication. It’s an imperfect connection that can shift slightly with movement.
Partial sacralization can occur on one side (unilateral) or both sides (bilateral). The unilateral type tends to create more asymmetry in how the spine moves and bears weight, which is relevant to pain.
The Castellvi Classification
Radiologists use a grading system called the Castellvi classification to describe these transitional vertebrae. The types most relevant to partial sacralization are:
- Type I (Ia or Ib): The transverse process is enlarged but not yet forming a distinct joint with the sacrum. This can be on one side (Ia) or both (Ib).
- Type II (IIa or IIb): The enlarged transverse process has formed a pseudo-joint with the sacrum, creating a visible articulation on imaging. Again, one side (IIa) or both (IIb).
If your report mentions one of these types, it’s describing exactly how your L5 connects to the sacrum and how developed that connection is. Type II is what most clinicians consider true partial sacralization, since there’s an actual joint-like structure present.
Why It Sometimes Causes Pain
Most people with partial sacralization are completely asymptomatic. When it does cause pain, the condition is called Bertolotti syndrome, a term coined over a century ago. The pain typically centers in the lower back, often on the side of the abnormal connection, and it can radiate into the buttock or leg.
Pain can originate from several mechanisms. The pseudo-joint itself can become inflamed and arthritic over time, producing localized tenderness right at the site where the transverse process meets the sacrum. But the more common pain generator is actually the spinal level above. Because partial sacralization restricts normal movement at L5-S1, the segment above it (L4-L5) has to compensate by absorbing more motion and stress. This accelerated wear leads to disc degeneration, facet joint arthritis, and sometimes disc herniation at L4-L5. The narrowing of the nerve exit channels at that level can also compress nerve roots, causing leg symptoms like pain, numbness, or tingling.
An abnormal iliolumbar ligament, which connects the lumbar spine to the pelvis, may also play a role. When L5 is partially fused, this ligament can be stretched or positioned abnormally, contributing to pain.
Bertolotti syndrome tends to surface in younger adults, which is notable because most degenerative back pain appears later in life. A young person with persistent low back pain and no obvious cause on initial evaluation may have this overlooked variant driving their symptoms.
How It’s Diagnosed
Partial sacralization is visible on standard lumbar X-rays, but the best view is a Ferguson radiograph, which is an anteroposterior X-ray angled upward at 30 degrees. This angle gives a clearer look at the transverse processes and their relationship to the sacrum. On a lateral X-ray, a sacralized L5 often appears wedge-shaped compared to normal vertebrae.
One practical challenge with transitional vertebrae is that they can confuse vertebral counting. If a radiologist doesn’t recognize the transitional anatomy, they might mislabel which disc or nerve root is affected, potentially leading to treatment at the wrong spinal level. This is why whole-spine imaging or careful counting from the top of the spine down is sometimes used to confirm the exact anatomy before any procedure.
CT scans and MRI provide more detail when needed. MRI is particularly useful for evaluating disc health at L4-L5 and checking for nerve compression, while CT better shows the bony anatomy of the pseudo-joint itself.
Treatment When Symptoms Develop
For people whose partial sacralization is an incidental finding with no symptoms, no treatment is needed. It’s simply a variant of normal anatomy.
When Bertolotti syndrome is suspected, treatment starts conservatively. Physical therapy focuses on improving core stability, restoring range of motion, and reducing pain through targeted exercises and manual therapy. Motor control training helps retrain how the muscles around the lower spine coordinate movement, which can reduce stress on the compensating segments. A structured home exercise program is a central part of long-term management.
If physical therapy doesn’t provide enough relief, guided injections can serve a dual purpose. A steroid injection directly into the pseudo-joint can both confirm the diagnosis (if pain improves, the joint is likely the source) and provide therapeutic relief lasting weeks to months. For pain originating from the nerve roots or the level above, epidural steroid injections can be delivered near the affected nerve. The anatomy of a transitional vertebra can make these injections technically challenging, sometimes requiring the needle to be placed from the opposite side or through alternative approaches to navigate around the fused bone.
Surgical Options
Surgery is reserved for patients who have failed conservative treatment and whose pain has been clearly traced to the transitional anatomy, typically confirmed by a positive response to a diagnostic injection. Two main approaches exist. Resection involves surgically removing the enlarged transverse process to eliminate the pseudo-joint. This works best when the pseudo-joint itself is the pain source and the nearby discs are healthy. In one series of surgically treated patients, 10 out of 16 had improved low back pain after surgery, with 7 of those becoming completely pain-free. However, about 3 out of 8 resection patients in that group needed additional operations later, sometimes for disc problems that developed at the level above.
The second option is spinal fusion at the transitional level, which is more appropriate when disc degeneration is present below the transitional vertebra. The choice between resection and fusion depends on exactly where the pain originates and how much wear has already accumulated in the surrounding structures.
The L4-L5 Connection
One of the most clinically important things about partial sacralization is its effect on L4-L5. When L5 is tethered to the sacrum, the L4-L5 disc and facet joints experience forces similar to what happens at the level just above a surgical spinal fusion. This has led to a longstanding theory that sacralization accelerates degenerative spondylolisthesis (forward slipping of L4 on L5) and disc breakdown at that level. While some research supports this connection, at least one study found the influence of sacralization on L4-L5 degeneration may be less dramatic than previously assumed. Still, if you have partial sacralization and develop back or leg symptoms, your doctor will likely pay close attention to L4-L5 as a potential source.