The human spinal column is typically composed of 33 vertebrae divided into five regions: cervical, thoracic, lumbar, sacral, and coccygeal. This complex structure provides support, allows movement, and protects the spinal cord. While the standard count is common, variations in the number and form of these bones are frequent congenital anomalies. The phenomenon described as having an “extra vertebrae” is a congenital variation where the boundary between the lumbar and sacral regions is altered. This anatomical variation represents a structural difference in the lower back, though it does not always lead to symptoms.
The Medical Terminology
The specific medical term for the congenital variation often referred to as an “extra vertebrae” in the low back is Lumbosacral Transitional Vertebra (LSTV). This condition is not strictly about having an additional bone, but rather a transformation or blending of the final lumbar segment (L5) and the first sacral segment (S1). The LSTV typically occurs in two main forms: sacralization and lumbarization.
Sacralization is the more common form, where the lowest lumbar vertebra, L5, takes on characteristics of the sacrum, often partially or completely fusing to it. Conversely, lumbarization occurs when the uppermost sacral segment, S1, separates from the rest of the sacrum, acquiring the features of a lumbar vertebra and making the lumbar spine appear to have six segments instead of five. This variation is present in up to 30% of the general population.
To standardize the identification of these structural differences, the Castellvi classification system is used. It categorizes LSTV into four types based on the morphology and degree of connection between the enlarged transverse process and the sacrum or ilium.
- Type I involves a broadly enlarged transverse process without articulation or fusion.
- Type II features a pseudoarticulation, or a “false joint,” between the enlarged process and the sacrum.
- Type III is defined by complete bony fusion.
- Type IV is a mixed type, showing a pseudoarticulation on one side and complete fusion on the other.
This classification helps assess the potential for biomechanical stress and pain.
How Extra Vertebrae Develop
Lumbosacral transitional vertebrae arise from an error in the segmentation process during embryonic development. The spine forms from somites, blocks of mesoderm that differentiate into vertebrae, ribs, and skeletal muscle. The identity of each vertebral segment (cervical, thoracic, lumbar, or sacral) is controlled by Homeobox (Hox) genes.
These Hox genes are expressed in specific patterns along the embryo’s axis, providing a blueprint for segment identity. A subtle alteration, or “homeotic transformation,” in the Hox gene expression boundary can cause one segment to assume the identity of its neighbor. For example, a shift in the expression pattern defining the lumbar region can result in the lowest lumbar segment developing sacral characteristics, leading to sacralization.
Recognizable Symptoms and Associated Conditions
While the majority of individuals with an LSTV remain completely asymptomatic throughout their lives, this anatomical variant can be the source of chronic lower back pain in a subset of patients. When the LSTV is definitively identified as the cause of a patient’s pain, the clinical diagnosis is referred to as Bertolotti’s Syndrome. This syndrome, first described in 1917, is most frequently associated with Castellvi Types II and IV, where an anomalous articulation or pseudoarthrosis is present.
The pseudoarthrosis, or false joint, between the enlarged transverse process and the sacrum or ilium is a common source of mechanical irritation and localized pain. This abnormal joint lacks the smooth cartilage of a true joint, leading to bone-on-bone contact, inflammation, and eventual arthritic changes. This localized discomfort is often unilateral, corresponding to the side of the abnormal articulation.
LSTV significantly alters the normal biomechanics of the lumbosacral region. The transitional segment is often stable or fused, restricting movement at the L5-S1 level. To compensate, the spinal segment immediately above the LSTV—the L4-L5 disc—experiences increased stress and hypermobility. This accelerated wear often results in disc degeneration, herniation, and nerve root irritation, which can lead to radiating leg pain (radiculopathy).
Diagnosis and Management
Imaging Modalities
The presence of a lumbosacral transitional vertebra is most commonly established through standard X-rays of the lumbosacral spine. A specialized view, such as the Ferguson projection, is often used to clearly visualize the bony structure and its articulation with the sacrum. This imaging allows a physician to apply the Castellvi classification to determine the specific type of LSTV present.
If a patient presents with symptoms, other imaging modalities assess soft tissues and nerve structures. Magnetic Resonance Imaging (MRI) evaluates the health of the intervertebral discs and checks for nerve root compression or inflammation caused by the enlarged transverse process. Computed Tomography (CT) scans provide greater detail of the bony fusion or pseudoarticulation.
Conservative and Interventional Treatment
The management of symptomatic LSTV, or Bertolotti’s Syndrome, typically begins with conservative treatments. Physical therapy is a primary approach, focusing on exercises to strengthen the core muscles and stabilize the spine to mitigate biomechanical stress. Anti-inflammatory medications, such as non-steroidal anti-inflammatory drugs (NSAIDs), are used to reduce pain and inflammation associated with the pseudoarthrosis.
When conservative measures fail, localized injections are often the next step. A diagnostic and therapeutic injection of a local anesthetic and corticosteroids can be administered directly into the pseudoarticulation joint to confirm it is the source of the pain and reduce inflammation. Surgical intervention is generally considered a last resort for patients who do not respond to conservative care. Options include partial resection of the enlarged transverse process, which removes the source of mechanical irritation, or spinal fusion to permanently stabilize the segment.