Having an extra vertebra is surprisingly common. Estimates vary widely depending on the study and the definition used, but somewhere between 4 and 30 percent of people have a lumbosacral transitional vertebra, where the lowest lumbar bone partially or fully merges with the sacrum, or where the sacrum sprouts a segment that behaves more like a lumbar bone. Some studies put the number even higher. Most people who have one never know it, because it rarely causes symptoms on its own.
What Counts as an “Extra” Vertebra
The standard human spine has 33 vertebrae: 7 cervical (neck), 12 thoracic (mid-back), 5 lumbar (lower back), 5 fused sacral, and 4 fused coccygeal. But that textbook number is an average, not a law. Variation happens most often at the borders between spinal regions, particularly between the lowest lumbar vertebra and the sacrum. The two main patterns are sacralization, where the fifth lumbar vertebra partially or fully fuses to the sacrum, and lumbarization, where the top segment of the sacrum separates and acts like an additional lumbar vertebra. Both are called lumbosacral transitional vertebrae, or LSTVs. Less commonly, a person can have a genuinely supernumerary vertebra inserted between normal segments, or have an unusual total count in the thoracic or cervical spine.
Because these variations sit on a spectrum from a slightly elongated transverse process to a fully fused extra segment, researchers sometimes disagree on where normal variation ends and a true “extra vertebra” begins. That disagreement is part of why prevalence estimates swing so widely from one study to the next.
Prevalence Numbers From the Research
A descriptive cross-sectional study of patients with chronic low back pain found that about 38 percent of those patients had an LSTV, compared with roughly 22 percent of control subjects without back pain. Sacralization was more common than lumbarization in both groups: about 28 percent versus 10 percent in the pain group, and about 17 percent versus 5 percent in the controls.1PubMed Central. Prevalence of lumbosacral transitional vertebra in patients with chronic low back pain: a descriptive cross-sectional study Another study screening eligible subjects found that about 36 percent were classified as having a transitional lumbosacral vertebra.2PubMed. The prevalence of transitional vertebrae in the lumbar spine A separate review put the general-population prevalence anywhere from 4 to 30 percent, highlighting how much the estimate depends on imaging methods and classification criteria.3PubMed Central. Bertolotti’s syndrome: an underdiagnosed cause for lower back pain
True numerical variants, where a person’s total vertebral count is genuinely different from 33, are less common but still not rare. A Chinese study of nearly 300 asymptomatic volunteers found that about 6 percent had only 11 thoracic vertebrae instead of 12, and about 6 percent had 6 lumbar vertebrae instead of 5.4PubMed Central. Rate of presence of 11 thoracic vertebrae and 6 lumbar vertebrae in asymptomatic Chinese adult volunteers A different analysis calculated a prevalence of about 0.8 percent for 6 lumbar vertebrae and about 1.8 percent for 4 lumbar vertebrae.5PubMed Central. Pelvic Incidence in Spines With 4 and 6 Lumbar Vertebrae The discrepancy between those two figures probably reflects population differences and how carefully the imaging was read. Overall, having a genuinely altered vertebral count is uncommon but far from unheard of.
Why the Spine Is Not as Fixed as We Think
Vertebral identity is set during embryonic development, when clusters of cells called somites form along the developing spine in a rhythmic, clock-like process. The total number of somites and the pace at which they form are defining features of a species.6PubMed Central. Cellular and molecular control of vertebrate somitogenesis Which somite becomes which type of vertebra is controlled by a family of genes called Hox genes, which switch on and off in overlapping patterns along the body axis. Research comparing Hox gene expression with vertebral shape has shown that vertebrae sharing the same Hox pattern cluster into the same morphological group, and that distinct shape changes between spinal regions coincide with shifts in which Hox genes are active.7PubMed Central. Correlation between Hox code and vertebral morphology in archosaurs Work on the developing human spine has identified at least 18 Hox genes with highly position-specific expression patterns.8Nature Communications. HOX gene expression in the developing human spine
When the boundaries between these Hox expression zones shift slightly, the result can be a vertebra with features of two adjacent regions. That is essentially what a transitional vertebra is: a bone that got mixed instructions. This is not a dramatic mutation. It is a small shift in gene-expression timing during development, which is why it happens so often and usually produces no symptoms at all.
When an Extra or Transitional Vertebra Causes Pain
The vast majority of people with an LSTV go their whole lives without knowing they have one. But a subset develops chronic low back pain attributable to the anomaly, a condition called Bertolotti’s syndrome. The pain typically centers around the spot where the transitional vertebra articulates or fuses with the sacrum, and it can mimic other causes of lower back pain like disc herniation or sacroiliac joint dysfunction.9PubMed Central. A Narrative Review of Bertolotti’s Syndrome: Etiology, Classification, Diagnosis, and Treatment
Because Bertolotti’s syndrome is often underdiagnosed, its true prevalence is unknown. The problem is that LSTVs are frequently found on imaging of people with back pain, but they are also frequently found on imaging of people without it. The association between the anatomical variant and actual pain is genuine but muddied by how common the variant is in the first place.3PubMed Central. Bertolotti’s syndrome: an underdiagnosed cause for lower back pain Doctors face a chicken-and-egg problem: if a person has back pain and an LSTV shows up on an MRI, is the variant the cause of the pain, or just a bystander?
There is some biomechanical evidence that the variant can genuinely alter spinal mechanics. Finite element modeling has shown that people with fewer presacral vertebrae (say, 23 instead of the standard 24 or 25) can experience concentrated stress in the upper lumbar discs and greater mobility at the transitional segment, which could contribute to disc degeneration and instability over time.10PubMed Central. Biomechanical influence of numerical variants of lumbosacral transitional vertebra with Castellvi type I on adjacent discs and facet joints based on 3D finite element analysis So while the extra or altered vertebra itself is not necessarily painful, the way it changes the mechanical behavior of the segments above it might be.
Extra Segments in the Neck and Thorax
The lower back is where extra and transitional vertebrae show up most frequently, but variations happen elsewhere too. One of the better-known examples is the cervical rib, an extra rib that develops from the seventh cervical vertebra rather than from a thoracic vertebra. Cervical ribs occur in a small percentage of the population and are usually discovered incidentally. When they do cause problems, the result is thoracic outlet syndrome, a neurovascular condition where the extra rib compresses nearby nerves or blood vessels.11PubMed Central. Thoracic outlet syndrome: a rare case with bilateral cervical ribs and bilateral anterior scalene hypertrophy
Patients who develop thoracic outlet syndrome because of a cervical rib tend to experience it at a younger age than patients who develop the syndrome from other causes like muscle hypertrophy. In one comparative study, the cervical-rib group presented at about 25 years of age versus about 37 in the non-rib group, and was more likely to have bilateral symptoms.12Egyptian Journal of Neurosurgery. Clinical characteristics and surgical outcomes in thoracic outlet syndrome: a comparative study of cases with and without cervical rib Still, most cervical ribs never cause symptoms and are found only when imaging is done for unrelated reasons.
Supernumerary ribs can also appear in the lumbar region, where vertebrae normally do not carry ribs at all. These lumbar ribs are considered rare and are typically discovered incidentally during routine X-rays or skeletal examination.13PubMed Central. Supernumerary lumbar ribs: a rare occurrence on an adult African male skeleton Unlike cervical ribs, lumbar ribs seldom cause any clinical problems.
The Surgical Counting Problem
One of the most consequential real-world implications of having an extra or transitional vertebra is the risk it creates during spine surgery. Surgeons identify the correct operative level by counting vertebrae from anatomical landmarks, usually the last rib-bearing vertebra or the sacrum. When a patient has an atypical number of vertebrae, the count can be off by one, and the surgeon may operate on the wrong level. A multicenter study of wrong-level spine surgeries found that anatomical variations were among the identified risk factors.14PubMed. Wrong-level spine surgery: A multicenter retrospective study
A case report described how unusual segmentation in a patient’s spine contributed to wrong-level surgery, and recommended that surgeons designate pathological levels by their relationship to physical landmarks rather than by label names like “T7-8.” The reasoning is simple: if a vertebra that is normally T12 turns out to be an extra segment, calling the operative target “T7-8” based on a count from the top leads to the wrong disc space. Counting from the last rib-bearing vertebra downward is more reliable in these situations.15PubMed Central. Unusual spine anatomy contributing to wrong level spine surgery: a case report and recommendations for decreasing the risk of preventable ‘never events’
Standard lumbar MRIs, which image only the lower back, are particularly unreliable at catching these variants. One study found that using a standard lumbar MRI to identify transitional vertebrae by the last costal-bearing vertebra had a sensitivity of only about 74 percent, partly because roughly 30 percent of sacralization cases in their sample had the last rib at a non-standard level.16PubMed Central. Role of Anatomical Landmarks in Identifying Normal and Transitional Vertebra in Lumbar Spine Magnetic Resonance Imaging Whole-spine imaging is the only reliable way to identify both numerical variants and the true anatomical nature of transitional vertebrae.17PubMed Central. Incidence of numerical variants and transitional lumbosacral vertebrae on whole-spine MRI That is an important consideration if you are being evaluated for spinal surgery and have any suspicion of anatomical variation.
Vertebral Anomalies as Part of Broader Conditions
Most extra or transitional vertebrae are isolated findings, but vertebral anomalies can also appear as part of congenital syndromes. The most well-known is VACTERL association, a pattern of birth defects affecting the vertebrae, anus, heart, trachea, esophagus, kidneys, and limbs. Vertebral anomalies are one of the most common features, reported in roughly 60 to 95 percent of all VACTERL patients.18PubMed Central. The genetic landscape and clinical implications of vertebral anomalies in VACTERL association The vertebral findings in these patients can include hemivertebrae, block vertebrae, and supernumerary or absent segments. Unlike the transitional vertebrae described above, which are typically harmless variants discovered by accident, vertebral anomalies in VACTERL tend to be structurally more dramatic and are identified in infancy alongside the syndrome’s other features.
The distinction matters because finding one transitional vertebra on an adult’s X-ray is almost never a reason to suspect a congenital syndrome. It is simply part of the broad normal variation in human anatomy. Syndromic vertebral anomalies are identified early, present with multiple other organ defects, and are managed from childhood.
Why Mammals Have Such Stable Vertebral Counts
Given how often transitional and extra vertebrae appear in humans, it is worth asking why the standard count is so consistent across mammals in the first place. Nearly all mammals, from giraffes to mice, have exactly seven cervical vertebrae. Research has argued that this constancy is the result of strong selective pressure against changes in vertebral number in fast-running and agile species. Species that rely on efficient galloping locomotion appear to be strongly penalized for deviations in trunk vertebral count, while slower or more sedentary mammals tolerate more variation.19PubMed Central. Fast running restricts evolutionary change of the vertebral column in mammals
Humans, as upright bipeds who no longer depend on a gallop, may sit in a kind of evolutionary gray zone: the developmental program that specifies vertebral number is ancient and well-conserved, but the selective pressure against small deviations is weaker than it is in a cheetah or a horse. That could help explain why transitional vertebrae are so common in people without causing major functional problems. The machinery that builds the spine is precise but not perfect, and in a species where a one-segment shift at the lumbosacral junction rarely matters for survival, natural selection has little reason to clamp down on it.
Identification in Forensic Science
Extra vertebrae have a practical significance beyond the clinic. In forensic anthropology, correctly counting and classifying vertebrae is part of the process of identifying skeletal remains and determining whether a set of bones belongs to one individual or multiple people. A documented autopsy case involved human skeletal remains with an extra vertebra at the thoracolumbar boundary, sitting between T12 and L1 and displaying features intermediate between the two. The authors emphasized that forensic scientists should be more aware of these numerical variations, because mistaking an extra segment for a bone from a second individual could lead to incorrect conclusions about the number of people represented in a set of remains.20Legal Medicine. An autopsy case of human skeletal remains with a numerical variation in thoraco-lumbar vertebrae
This concern may sound niche, but in mass-disaster or archaeological contexts, vertebral counts are a basic step in sorting commingled remains. If examiners assume every person has exactly 24 presacral vertebrae and find 25 bones in a set, they might wrongly infer contamination from a second person. Awareness of normal human variation is, in these settings, a practical necessity rather than an academic curiosity.