Lumbarization is a congenital spinal variation where the first sacral vertebra (S1) partially or fully separates from the rest of the sacrum and takes on the characteristics of a lumbar vertebra. This effectively gives the person six lumbar vertebrae instead of the typical five. It occurs in roughly 3.4% to 7.2% of the general population, and many people never know they have it because it often causes no symptoms at all.
How Lumbarization Changes Spinal Anatomy
The sacrum is normally a single fused bone made up of five vertebral segments. In lumbarization, the top segment (S1) fails to fully fuse with the segments below it during development. Instead, it behaves more like a free-standing lumbar vertebra. The changes can be subtle or dramatic: a lumbarized S1 takes on a squared shape instead of the typical triangular sacral profile, develops functional facet joints between itself and S2 (where solid bone fusion would normally exist), and forms a full-sized disc below it rather than the tiny vestigial disc usually found between S1 and S2.
These changes can be partial, affecting just one side of the vertebra, or complete on both sides. Some people have only a slight loosening of the S1 segment, while others have an S1 that is virtually indistinguishable from a lumbar vertebra on imaging.
Lumbarization vs. Sacralization
Lumbarization and sacralization are opposite versions of the same phenomenon, both falling under the umbrella of “lumbosacral transitional vertebrae.” In sacralization, the lowest lumbar vertebra (L5) fuses partially or completely into the sacrum, reducing the number of mobile lumbar segments. The L5 vertebra becomes wedge-shaped and its transverse processes enlarge to anchor against the sacrum or pelvis. In lumbarization, the reverse happens: S1 breaks away from the sacrum, gaining mobility and lumbar-like features.
The practical difference matters. Sacralization reduces spinal mobility at the base, while lumbarization adds an extra mobile segment. Each creates distinct stress patterns on surrounding discs and joints. Notably, sacralization tends to be more common in males, while lumbarization is more common in females.
When Lumbarization Causes Pain
Most people with lumbarization are asymptomatic. When the variation does produce pain, the condition is called Bertolotti syndrome, a term that applies to low back pain caused by any lumbosacral transitional vertebra. Pain from Bertolotti syndrome can show up in several ways, and physical exams are often frustratingly non-specific: tenderness in the lower back and limited range of motion, with no obvious pointer to the underlying cause.
The pain itself can mimic several other conditions. Contact between bones at a “false joint” (pseudoarticulation) where the transitional vertebra meets the sacrum can produce sacroiliac-type pain, hip pain, groin pain, or even pain that radiates down the leg in a pattern that looks like a pinched nerve root. This mimicry is one reason lumbarization often goes undiagnosed for years, especially in younger patients whose back pain gets attributed to muscle strain or poor posture.
Biomechanical Effects on Neighboring Segments
The extra mobile segment created by lumbarization shifts mechanical stress to the vertebrae above and below it. The level immediately above the transitional vertebra experiences hypermobility and abnormal rotational forces. Over time, this predisposes that disc and its facet joints to early degeneration. If the lumbarization is one-sided (affecting only the left or right transverse process), the asymmetry compounds the problem: the facet joint on the normal side bears uneven loads, accelerating wear on that side specifically.
The sacroiliac joint on the affected side also gets involved. The enlarged transverse process of the transitional vertebra increases the contact area with the pelvis, leading to greater loading, more joint irritation, and increased one-sided muscle tension. Below the transitional vertebra, the story is different. Restricted movement there actually protects those lower segments from degeneration, but at the cost of altered facet joint shape and orientation.
Nerve Function Can Shift
One of the more surprising effects of lumbarization is that it can rewire which nerve root does what. In patients with a lumbarized S1, research has shown that the S1 nerve root takes over the job normally performed by the L5 nerve root. Compression of the S1 root in these patients produces the pattern of muscle weakness and sensory changes you would normally expect from L5 compression in someone with standard anatomy.
This has real consequences for diagnosis and treatment. A surgeon relying on standard anatomical maps to identify a compressed nerve could target the wrong level if they don’t account for the transitional vertebra. The condition known as “Far-Out Syndrome” can also occur, where a lumbar nerve gets pinched between the transverse process of L5 and the top of the sacrum, causing pain that radiates into the leg.
How It’s Identified on Imaging
Lumbarization is usually discovered incidentally on an X-ray, CT scan, or MRI ordered for back pain. The key features radiologists look for include the squared shape of the S1 vertebra, the presence of a full-sized disc between S1 and S2, and facet joints at a level where fusion would normally exist.
Counting vertebrae sounds simple but can actually be tricky. Because lumbarization adds an extra segment that looks lumbar, it’s easy to miscount and mislabel vertebral levels. Newer identification methods use specific angle measurements on CT scans and the position of the iliac crest (the top of the hip bone) relative to the spine on full-length images. When the iliac crest sits higher than expected and certain vertebral angles are unusually small, there’s a strong likelihood that lumbarization is present. In one study, 95.8% of patients who showed both a high iliac crest line and a small posterior angle on imaging turned out to have lumbarization. When these findings appear on standard lumbar imaging, a full-spine X-ray is recommended to confirm the diagnosis by counting from the top of the spine down.
Impact on Disc Health Over Time
The segment just above a transitional vertebra bears the brunt of the altered mechanics, and disc degeneration at that level is a well-documented long-term risk. Research on the parallel condition of sacralization (which creates similar stress concentration patterns) found that 62.2% of patients with the transitional vertebra had severe disc degeneration at the level above, compared to 41.5% of patients without the variation. Vertebral slippage at that level was also significantly worse, and the degree of slip correlated strongly with the severity of disc breakdown.
While this data comes from sacralization studies, the underlying principle applies to lumbarization as well: when one spinal segment is abnormally fixed or abnormally mobile, the segments next to it compensate, and that compensation accelerates wear. For people with lumbarization who develop progressive back pain, this pattern of adjacent-segment degeneration is often the underlying driver.
Managing Symptoms
Because most people with lumbarization never develop symptoms, no treatment is needed unless pain arises. When it does, the approach typically starts conservatively. Physical therapy focused on core stability, pelvic alignment, and reducing asymmetric muscle tension can address much of the biomechanical imbalance. Targeted injections into the pseudoarticulation (the false joint between the transverse process and sacrum) can both confirm the diagnosis and provide temporary relief.
For people whose pain persists despite conservative measures, surgical options exist but are considered a last resort. The specific procedure depends on where the pain originates: the pseudoarticulation itself, a degenerated disc above the transitional vertebra, or a compressed nerve root. Accurate identification of the transitional vertebra’s level is critical before any procedure, since mislabeling the vertebrae can lead to surgery at the wrong spinal segment.