Sacralization is a congenital spinal variation in which the lowest lumbar vertebra, L5, partially or fully fuses with the sacrum, the triangular bone at the base of the spine. It falls under the umbrella of lumbosacral transitional vertebrae, and depending on the study, it shows up on imaging in roughly 15 to 25 percent of people. Many of them never know they have it. But for a meaningful subset, sacralization changes how the spine distributes mechanical stress, and that can lead to pain, disc problems, and diagnostic headaches that make treatment trickier than it should be.
What Sacralization Looks Like on Imaging
The fusion between L5 and the sacrum is not all-or-nothing. It exists on a spectrum. At the mild end, one or both of L5’s transverse processes (the bony wings that jut out from each side of the vertebra) are simply wider than normal. At the more dramatic end, L5 is completely incorporated into the sacrum, with no visible joint space remaining. In between, you can find partial bony bridges, full bony bridges on one side only, or a pseudo-joint (called a neoarticulation) where the enlarged transverse process presses against the sacral wing without true fusion.1Europe PMC / AJNR. Lumbosacral transitional vertebrae: classification, imaging findings, and clinical relevance
Clinicians typically use the Castellvi classification to describe what they see. Type I is an enlarged transverse process measuring at least 19 mm. Type II involves an actual joint between the transverse process and the sacrum. Type III is full bony fusion. Type IV is a mixed picture: one side has a joint, the other is fully fused. Each of these types can be unilateral (one side only) or bilateral. The distinction matters because unilateral forms create asymmetric forces across the spine, which has consequences for which structures wear out first.
How Common Is It
Prevalence estimates vary widely depending on the population studied and the imaging method used. A systematic review analyzing multiple studies placed the average prevalence of all lumbosacral transitional vertebrae at about 15 percent, with individual studies reporting anywhere from roughly 5 to 36 percent.2Brain and Spine. Prevalence, diagnosis, and clinical significance of lumbosacral transitional vertebrae: A systematic review and narrative analysis Studies that specifically look at sacralization (rather than lumbarization, its mirror image where S1 separates from the sacrum) tend to report rates in the range of 17 to 28 percent, depending on whether they are studying people with back pain or the general population.3PubMed Central. Is Sacralization Really a Cause of Low Back Pain?4PubMed Central. Prevalence of lumbosacral transitional vertebra in patients with chronic low back pain: a descriptive cross-sectional study
An interesting wrinkle: sacralization does tend to appear more often in people who present with low back pain, disc herniation, or scoliosis, but several studies comparing symptomatic and control groups have failed to find a statistically significant difference in sacralization rates alone.5PubMed Central. Is Sacralization Really a Cause of Low Back Pain? That does not mean sacralization is harmless. It means that whether sacralization causes symptoms depends heavily on which type you have, whether it is one-sided, and what is happening at the spinal levels nearby.
When Sacralization Causes Pain
When a sacralized L5 does produce symptoms, the resulting condition is called Bertolotti’s syndrome. The pain typically settles in the low back and can radiate into the buttock or leg. The mechanisms behind it are not a single story but several overlapping ones.
First, when L5 is locked to the sacrum, the mobile segment above it (L4-L5, or what effectively becomes the last free joint in the lumbar spine) takes on extra mechanical load. Studies have found that disc herniations, disc degeneration, and facet joint wear all occur more frequently at this level compared to people without sacralization.6Orthopedic Reviews. A Comprehensive Update of the Treatment and Management of Bertolotti’s Syndrome: A Best Practices Review In adolescents with sacralization, disc herniations at L4-L5 were found to be far more common than at L5-S1, essentially reversing the usual pattern seen in adults without the anomaly.7PubMed. Lumbosacral Transitional Vertebra: Possible Role in the Pathogenesis of Adolescent Lumbar Disc Herniation
Second, when sacralization is unilateral, the asymmetry puts the opposite-side facet joint under unusual stress. Pain from the contralateral facet is a well-recognized pattern in Bertolotti’s syndrome.8Europe PMC / AJNR. Lumbosacral transitional vertebrae: classification, imaging findings, and clinical relevance
Third, the pseudo-joint itself, the neoarticulation between the transverse process and the sacrum, can become a source of pain. It behaves like a joint that was never supposed to exist: it has no proper cartilage surface, can develop its own version of arthritis, and can generate local inflammation.9PubMed. Extraforaminal entrapment of the fifth lumbar spinal nerve by nearthrosis in patients with lumbosacral transitional vertebrae
Effects on Disc Aging
Sacralization does not just shift where disc problems happen. It changes how discs age. Research on young men found that having a transitional vertebra was linked to faster degeneration of the disc above the transition, the one that inherits the extra workload. At the same time, the disc below the transition (between the fused segment and the sacrum) actually appeared to be protected, showing less degeneration than the same disc in people with normal anatomy.10Spine. Lumbosacral Transitional Vertebra: Relation to Disc Degeneration and Low Back Pain This makes intuitive sense: a disc that sits between two fused bones barely moves, so it barely wears. The disc above it, though, absorbs the motion that was supposed to be shared across two levels.
The clinical finding that disc herniation at the level above the transition occurs at a younger average age (around 35, compared to 59 in people without the anomaly) underscores just how much the altered biomechanics can accelerate wear and tear.11Orthopedic Reviews. A Comprehensive Update of the Treatment and Management of Bertolotti’s Syndrome: A Best Practices Review
Nerve Root Trapping
A less obvious complication involves the L5 nerve root. In sacralization, the bony overgrowth or pseudo-joint tissue around the enlarged transverse process can gradually narrow the space where the L5 nerve exits the spine. Over time, the new bone and synovial-like tissue that forms around the neoarticulation can physically pinch the nerve.12PubMed. Extraforaminal entrapment of the fifth lumbar spinal nerve by nearthrosis in patients with lumbosacral transitional vertebrae This tends to happen outside the main spinal canal, in the extraforaminal zone, which is a location that standard MRI sequences sometimes miss.13PubMed Central. An Unusual Case Report of Bertolotti’s Syndrome: Extraforaminal Stenosis and L5 Unilateral Root Compression (Castellvi Type III an LSTV)
Patients with this type of nerve entrapment often describe pain and numbness in the L5 distribution: the outer calf, top of the foot, and big toe. Because the compression site is outside the normal canal, a standard discectomy would miss it entirely. Recognizing that the anatomy is abnormal is the first step toward getting the right treatment.
The Counting Problem in Diagnosis and Surgery
Sacralization creates a surprisingly dangerous diagnostic pitfall: miscounting vertebral levels. When L5 is fused to the sacrum, it can look like part of the sacrum on imaging, making what is actually L4 appear to be L5. A surgeon who operates on “L5-S1” based on a standard lumbar MRI might actually be operating one level too high.
This is not a theoretical concern. A study examining patients with lumbosacral transitional vertebrae found that vertebral counts from outpatient assessments were wrong in over half of cases. In about 15 percent of cases, recognizing the true count required changing the surgical plan entirely.14The Tohoku Journal of Experimental Medicine. Lumbosacral Transitional Vertebrae Cause Spinal Level Misconception in Surgeries for Degenerative Lumbar Spine Disorders Whole-spine imaging that includes the cervical region, where you can reliably count down from C2, is the standard recommendation to avoid wrong-level surgery.
MRI alone, without a full-spine view, has limitations for identifying transitional vertebrae. Using the last rib-bearing vertebra as a landmark on lumbar MRI achieved a sensitivity of about 74 percent in one study, meaning roughly one in four transitional vertebrae went undetected.15PubMed Central. Role of Anatomical Landmarks in Identifying Normal and Transitional Vertebra in Lumbar Spine Magnetic Resonance Imaging That 26-percent miss rate is a significant gap in a pre-surgical setting.
Treatment Options
Most people with sacralization who have no symptoms need no treatment at all. The anomaly is an incidental finding on imaging in the majority of cases. For those who do develop Bertolotti’s syndrome, treatment follows a stepwise approach.
Conservative management comes first: physical therapy, anti-inflammatory medications, and activity modification. If the neoarticulation is identified as the pain generator, a diagnostic injection of local anesthetic into the pseudo-joint can confirm the source. This can be combined with a corticosteroid injection for longer-lasting relief, though the evidence on duration is not encouraging. In reported cases, steroid injections into the neoarticulation provided relief lasting anywhere from a few weeks to about three months, with symptoms returning after that.16PubMed Central. Bertolotti’s syndrome: A commonly missed cause of back pain in young patients17Korean Journal of Pain. Bertolotti Syndrome: A Diagnostic and Management Dilemma for Pain Physicians
Radiofrequency ablation, which uses heat to destroy the small nerve fibers carrying pain signals from the pseudo-joint, has been tried in some cases. Case reports describe the procedure providing temporary relief, but the evidence base remains thin, mostly individual case reports rather than controlled trials.18Pain Medicine. Radiofrequency Sensory Ablation as a Treatment for Symptomatic Unilateral Lumbosacral Junction Pseudarticulation (Bertolotti’s Syndrome): A Case Report
When conservative approaches fail, surgery becomes an option. Two main surgical strategies exist: resecting the enlarged transverse process to eliminate the pseudo-joint, or fusing the transitional segment to the sacrum to stabilize it. A follow-up study of 16 surgically treated patients found that about two-thirds improved, with seven achieving complete pain relief. Both resection and fusion produced similar outcomes, though the average post-surgical disability level corresponded to a moderate result, not a full cure.19PubMed. Surgical treatment of Bertolotti’s syndrome. Follow-up of 16 patients The evidence base for surgery is small, and there are no large randomized trials comparing surgical versus non-surgical management for Bertolotti’s syndrome specifically.
Why the Spine Develops This Way
Sacralization is not caused by an injury, poor posture, or anything that happens after birth. It is determined during embryonic development, when a family of genes called Hox genes lays out the body plan along the head-to-tail axis. These genes tell each developing vertebra what type to become: cervical, thoracic, lumbar, sacral, or coccygeal. Research in mice has shown that Hox group 11 genes are specifically responsible for telling vertebrae to take on sacral identity, and this instruction has to be delivered at a precise stage of development, before the cells have formed the somites that will become actual vertebral segments.20Genes & Development. Hox genes specify vertebral types in the presomitic mesoderm
When the boundary between “lumbar” and “sacral” programming shifts slightly upward, L5 receives sacral instructions instead of lumbar ones, and you get sacralization. When it shifts slightly downward, S1 receives lumbar instructions instead, and you get lumbarization. Studies of skeletal variation have found that the genetic influences behind sacralization and lumbarization are not simply mirror images of each other; the sets of co-occurring vertebral changes differ between the two conditions, implying partly different genetic pathways.21PubMed. Lumbarization and sacralization: Domains of their co-occurrence with other costal-vertebral transformations are not identical Hox genes are deeply conserved across vertebrates, and extensive work in mice and chick embryos has established that they are the master regulators of what each vertebra looks like.22PubMed. Hox genes and vertebrate axial pattern
The Evolutionary Angle
The lumbosacral boundary is not just variable between individuals. It has shifted across evolutionary time. Humans and our closest living relatives, the African great apes, share an unusually short trunk compared to most mammals, a defining feature of the ape lineage.23PubMed. Vertebral numbers and human evolution But the path to the modern human vertebral formula, with five lumbar vertebrae fusing into a broad sacrum, involved independent episodes of lumbar reduction in different lineages. Early human ancestors likely had six lumbar vertebrae, and the shortening to five happened after our lineage split from chimpanzees and bonobos.24PubMed. The vertebral formula of the last common ancestor of African apes and humans
Selection for upright walking appears to have narrowed the range of vertebral variation in humans relative to other apes.25PubMed. Homeotic change in segment identity derives the human vertebral formula from a chimpanzee-like one Bipedalism demands a specific geometry: the lumbar spine needs to curve inward (lordosis) to stack the torso over the pelvis, and the sacrum needs to be broad and angled to transmit body weight to the hip joints. Having too many or too few free lumbar segments could compromise that geometry. Sacralization, in this light, is a remnant of the natural developmental variability that evolution has been working to constrain but has not entirely eliminated.
Spinal Alignment and Pelvic Shape
Beyond the local effects on discs and nerves, sacralization can alter the global geometry of the spine. When L5 is incorporated into the sacrum, it effectively raises the base of the mobile spine. Research has found that individuals with high-grade sacralization show a different distribution of lumbar lordosis, with the apex and inflection point of the lumbar curve sitting higher than normal.26Spine. The Implications of Sacralized Transitional Vertebra on Spinal Alignment For a spine surgeon planning a fusion or deformity correction, ignoring this shift could lead to a construct that doesn’t match the patient’s natural balance.
Sacralization also alters the sacrum itself. In women, a transitional sacrum can change the dimensions of the dorsal pelvis, the bony ring the baby passes through during delivery. A study examining female sacra with transitional anatomy found structural alterations that could change the available dorsal pelvic space and potentially affect the biomechanics of labor.27PubMed Central. Transitional Female Sacrum: Dimensions, Alterations in Dorsal Pelvic Structure, and Potential Obstetric Implications This is still a theoretical concern based on anatomical measurements, not clinical delivery outcomes, but it illustrates how sacralization reaches further than the spine itself.
Sacralization in Forensic and Military Settings
Sacralization has practical value in one unexpected field: forensic identification. Because it is a congenital, stable skeletal feature, it can serve as an identifying marker when comparing unknown remains to pre-existing medical imaging of missing persons. Forensic anthropologists treat sacralization of L5 as one of several “epigenetic skeletal traits” useful for narrowing or confirming identity.28Forensic Anthropology. Epigenetic Skeletal Traits in Forensic Identification If someone had an abdominal CT scan years before disappearing, and that scan shows a sacralized L5, matching that feature to recovered remains adds a piece to the identification puzzle.
In military aviation, the stakes are different but equally concrete. The vibration and G-forces that fighter pilots endure put enormous stress on the lumbar spine, and a spine with abnormal biomechanics from a transitional vertebra may be more vulnerable to injury under those conditions. A study of Indian military aviation candidates found that nearly two-thirds of those with lumbosacral transitional vertebrae were deemed unfit for flying duties under existing medical standards.29Indian Journal of Aerospace Medicine. Lumbosacral transitional vertebra in military aviation candidates: A cross-sectional study Whether such blanket disqualification policies are too aggressive is debatable, given that many people with sacralization never develop symptoms, but the military tends to err on the side of caution with spinal anomalies in high-performance aviation.
Living with a Sacralized Vertebra
If you have been told you have sacralization on an X-ray or MRI, the most useful thing to understand is that having it does not mean you will have problems. A large share of people with this anatomy go through life without symptoms. The features that push sacralization toward becoming clinically relevant are unilateral fusion (which creates asymmetric stress), the presence of a pseudo-joint (which can become its own pain source), and the cumulative effect of altered mechanics on the disc and facet joints at the level above.
If you do develop low back pain and have known sacralization, it is worth making sure your treating physician is aware of it. The diagnosis influences which level a surgeon would target, which injection site a pain specialist would choose, and which MRI sequences a radiologist should order. Bringing any prior imaging that shows your full spine can help prevent the miscounting errors that are surprisingly common in this population. In a condition where the most dangerous complication may be operating on the wrong level, the patient’s own awareness of their anatomy is a genuine safety net.