A transitional vertebra is a congenital spinal variation where a single vertebral segment exhibits anatomical characteristics belonging to two different regions of the spine. This anomaly results from a developmental irregularity and is present from birth. The most common location for this structural change is the junction between the lower back and the pelvis, known as the lumbosacral region. While this structural difference alters the typical number and shape of the bones in a spinal area, it is only considered a clinical condition if it causes symptoms. Prevalence in the general population ranges widely from 4% to 35%.
Defining the Anatomical Variation
The majority of transitional vertebrae occur at the lumbosacral junction, termed a Lumbosacral Transitional Vertebra (LSTV). A typical spine has five lumbar vertebrae (L1-L5) and a sacrum composed of five fused segments (S1-S5). An LSTV disrupts this pattern by exhibiting features of both the fifth lumbar vertebra and the first sacral segment.
This variation manifests in two forms: sacralization or lumbarization. Sacralization occurs when the lowest lumbar vertebra (L5) partially or completely fuses with the sacrum, reducing the number of mobile lumbar segments to four. Lumbarization is the process where the uppermost sacral segment (S1) fails to fully fuse with the remainder of the sacrum, causing it to appear and function like an extra, sixth lumbar vertebra.
The defining physical feature of an LSTV is an enlarged transverse process, usually on L5. This projection may be elongated, form a pseudoarticulation (false joint) with the sacrum or ilium, or be completely fused to the sacrum, creating a solid bony bridge. The degree of fusion or articulation is classified using the Castellvi system, which categorizes anomalies based on laterality and whether bony fusion or pseudoarticulation is present.
Embryological Basis of Formation
The existence of a transitional vertebra is rooted in embryonic development during the formation of the spine. Vertebrae arise from somites, blocks of tissue that determine the body’s segmented pattern. The identity of each vertebra (cervical, thoracic, lumbar, or sacral) is governed by regulatory genes known as Hox genes.
Hox genes are expressed along the body’s axis, with specific combinations defining the boundaries between spinal regions. A transitional vertebra results from a slight shift in these established Hox gene expression boundaries during segmentation. This shift causes a segment to be patterned with the identity of the adjacent region, creating a blended form.
For example, a shift in the boundary where sacral identity is established can result in the S1 segment adopting features of a lumbar vertebra, leading to lumbarization. The timing of this developmental event occurs early in gestation, making the resulting LSTV a fixed congenital characteristic of the individual’s skeletal anatomy.
Associated Symptoms and Clinical Impact
While a significant portion of individuals with a transitional vertebra remain entirely without symptoms, the presence of an LSTV can lead to a specific type of chronic low back pain known as Bertolotti’s Syndrome. The underlying cause is a disruption of normal lower spine biomechanics. The bony articulation or fusion restricts natural movement at the L5-S1 level, which is typically the most mobile segment.
This restriction forces the segments immediately above it, primarily the L4-L5 disc and facet joints, to compensate. The resulting hypermobility at L4-L5 subjects this segment to excessive torque and mechanical stress. This abnormal loading often accelerates degenerative changes, leading to premature disc degeneration or arthrosis in the facet joints above the LSTV.
The pain associated with Bertolotti’s Syndrome is chronic and localized to the lower back, sometimes radiating into the buttock or hip. Pain can arise directly from the anomalous pseudoarticulation, which may become inflamed and arthritic due to repetitive movement. This localized pain is often exacerbated by activities involving arching the back or prolonged standing.
Furthermore, the enlarged transverse process can impinge on the adjacent nerve root, most commonly the L5 root. This mechanical compression causes symptoms of sciatica, characterized by pain, numbness, or tingling traveling down the leg.
Diagnosis and Treatment Approaches
The initial step in diagnosing a transitional vertebra involves a thorough clinical history and physical examination to correlate a patient’s symptoms with the anatomical finding. Imaging studies are essential for confirming the presence and specific type of LSTV. Plain X-rays, often utilizing a specific angled view called the Ferguson projection, are the primary tool for identifying bony characteristics, such as the enlarged transverse process and the degree of articulation or fusion.
A Computed Tomography (CT) scan provides a detailed assessment of the bony anatomy, aiding in precise classification. Magnetic Resonance Imaging (MRI) evaluates soft tissues, particularly intervertebral discs and nerve roots, checking for associated degeneration or compression. The definitive diagnosis of symptomatic Bertolotti’s Syndrome requires demonstrating that the LSTV is the source of pain, typically confirmed when a targeted anesthetic and steroid injection at the anomalous joint provides significant, temporary relief.
Management for a symptomatic transitional vertebra is conservative. Initial treatment focuses on reducing inflammation and strengthening supportive musculature. This includes non-steroidal anti-inflammatory drugs (NSAIDs) and structured physical therapy aimed at improving core stability and correcting biomechanical imbalances.
If conservative measures fail, localized injections of corticosteroids and anesthetics into the pseudoarticulation or adjacent nerve root sheath are considered. Surgical intervention is reserved for patients with persistent, debilitating pain unresponsive to conservative treatments. Surgical options include resection of the enlarged transverse process and its pseudoarticulation or, less commonly, spinal fusion to stabilize the hypermobile segment above the LSTV.