Transitional vertebrae (TV) are a common congenital variation where a vertebral segment develops features characteristic of two adjacent spinal regions. This anomaly occurs during fetal development when the vertebral column undergoes segmentation. While the spine is typically organized into distinct sections—seven cervical, twelve thoracic, and five lumbar vertebrae—a TV is a segment that deviates from this standard morphology. Although present from birth and often asymptomatic, a TV can sometimes contribute to chronic back pain.
Defining Transitional Vertebrae
A transitional vertebra is defined by its hybrid nature, essentially bridging the gap between two spinal regions. This congenital variation results from an alteration in the body plan during early embryogenesis. Instead of a clear distinction between the end of one vertebral section and the start of the next, one vertebra partially adopts the form of its neighbor. Prevalence ranges widely, from 4% to 36%, due to differences in definition and imaging criteria across studies. The two main forms are “sacralization” and “lumbarization,” involving the lowest lumbar or uppermost sacral segment taking on characteristics of the other.
Primary Locations and Anatomical Classifications
The most frequently encountered type of transitional vertebra is the Lumbosacral Transitional Vertebra (LSTV), which occurs at the junction where the lumbar spine meets the sacrum. In sacralization, the fifth lumbar vertebra (L5) partially or completely fuses with the sacrum (S1). In lumbarization, the first sacral segment (S1) separates and appears more like an additional lumbar vertebra. Other less common transitional segments can occur, such as at the cervicothoracic junction, where the seventh cervical vertebra (C7) might exhibit features of a thoracic vertebra, sometimes developing an extra rib-like structure.
Due to the clinical importance of LSTV, a standardized system, known as the Castellvi classification, is used to categorize the degree and pattern of the variation.
- Type I involves an enlarged and misshapen transverse process of the vertebra, measuring at least 19 millimeters, but without articulation or fusion to the sacrum.
- Type II is characterized by an incomplete fusion, where the enlarged transverse process forms a “pseudoarticulation,” or false joint, with the sacrum.
- Type III represents a complete bony fusion between the enlarged transverse process and the sacrum.
- Type IV features a pseudoarticulation (Type II) on one side and a complete bony fusion (Type III) on the opposite side.
Both Type II and Type III can be unilateral (affecting one side) or bilateral (affecting both sides), designated by ‘a’ and ‘b’ subtypes, respectively. This classification helps medical professionals standardize communication regarding the specific anatomical structure of the LSTV.
Clinical Relevance and Associated Symptoms
While many people with LSTV remain completely asymptomatic, the presence of this anatomical variation can lead to altered spinal biomechanics, which may cause pain over time. The most well-known clinical entity associated with a painful LSTV is Bertolotti’s Syndrome, first described in 1917. This diagnosis is applied only when the transitional vertebra is identified as the definitive source of a patient’s chronic low back pain.
The mechanism for pain often relates to the loss of normal motion at the L5/S1 segment, which forces the adjacent spinal levels to compensate. This compensatory movement can lead to premature degeneration of the intervertebral disc immediately above the LSTV, most commonly the L4-L5 disc. The reduced mobility at the transitional segment also places increased mechanical stress on the facet joints and surrounding soft tissues.
In cases of a pseudoarticulation (Castellvi Type II or IV), the anomalous joint between the vertebra and the sacrum can become inflamed and arthritic due to repetitive friction. This localized irritation is a common source of chronic low back pain, often felt on one side or radiating into the hip and buttock area. Nerve root compression can also occur when the enlarged transverse process narrows the space through which the spinal nerves exit, leading to symptoms resembling sciatica.
Diagnosis and Treatment Approaches
The identification of a transitional vertebra is typically made incidentally when a patient undergoes imaging for non-specific lower back pain. Standard X-rays, particularly those focused on the lumbosacral region, are sufficient to show the anomalous bone structure and allow for Castellvi classification. Magnetic resonance imaging (MRI) or computed tomography (CT) scans may also be used to evaluate associated soft tissue structures, such as the adjacent discs, nerves, and the anomalous joint itself.
For a symptomatic patient, the initial approach to treatment is conservative, focusing on reducing pain and improving function. Physical therapy is a primary tool, aimed at strengthening the core and surrounding muscles to stabilize the spine and manage altered biomechanics. Pharmacological management typically involves non-steroidal anti-inflammatory drugs (NSAIDs) to decrease local inflammation and pain.
If conservative measures prove insufficient, targeted interventional procedures may be employed to confirm and treat the pain source. A diagnostic injection of a local anesthetic and steroid directly into the pseudoarticulation can offer temporary relief and confirm that the transitional joint is the pain generator. Surgical intervention is reserved for a small subset of patients whose pain is definitively localized to the LSTV and who have failed all conservative treatments. These procedures may involve resection of the enlarged transverse process or spinal fusion to stabilize the segment.