A lumbosacral transitional vertebra (LSTV) is a common congenital variation of the spine occurring where the lumbar spine meets the sacrum. This anomaly, present from birth, involves the lowest lumbar vertebra (typically L5) and the uppermost segment of the sacrum (S1). The LSTV exhibits characteristics of both regions, often involving an atypical bony connection or fusion between these segments. This structural difference can potentially impact spinal mechanics and overall health.
Defining Lumbosacral Transitional Vertebrae
The standard human spine contains five distinct lumbar vertebrae (L1 through L5) resting upon the sacrum, which is formed by five fused segments. An LSTV represents a structural deviation from this standard anatomy, affecting the articulation between L5 and S1. This variation manifests in two primary ways: sacralization or lumbarization. Sacralization occurs when L5 partially or completely fuses with the sacrum, making it appear more like a sacral segment.
Lumbarization is the less common variation, where the first sacral segment (S1) is not fused to the rest of the sacrum, causing the lumbar spine to appear to have six vertebrae instead of five. In both types, the transverse processes of the transitional vertebra are often enlarged, creating an atypical joint or a complete bony bridge with the sacrum or the ilium. This anatomical difference is common, with prevalence estimates ranging between 4% and 30% of the general population. Despite this high prevalence, the majority of people with an LSTV remain completely without symptoms.
Understanding the Classification System
The Castellvi Classification system is widely used to standardize the description of LSTV based on its radiographic appearance. This system divides the anomaly into four types, distinguishing between incomplete and complete fusion, and unilateral versus bilateral involvement. The classification relies on the morphology of the enlarged transverse processes of the L5 vertebra and their connection to the sacrum. Type I involves a dysplastic, enlarged transverse process (at least 19 millimeters wide) that does not form a joint or fusion with the sacrum.
Type I is subdivided into Ia (unilateral enlargement) and Ib (bilateral enlargement). Type II describes an incomplete sacralization characterized by a pseudoarticulation, or false joint, between the enlarged transverse process and the sacrum. This Type II pseudoarticulation is a functional joint prone to movement-related pain. Type III is defined by complete osseous fusion of the transverse process to the sacrum, forming a solid bony union. Type IV describes a mixed presentation, featuring a Type II pseudoarticulation on one side and a Type III complete fusion on the contralateral side.
Symptoms and Associated Pain
When an LSTV becomes symptomatic, the condition is known as Bertolotti’s Syndrome, characterized by chronic low back pain directly attributed to the anomaly. This syndrome is often seen in younger adults because the atypical anatomy disrupts normal biomechanics and weight distribution across the pelvis. The stiff or fused segment (especially Types II, III, and IV) prevents normal motion at the L5-S1 junction. This loss of motion forces the segment immediately above (L4-L5) to compensate with increased movement and stress, a phenomenon known as hypermobility. This compensatory mechanism can accelerate disc degeneration and arthritic changes in the facet joints at the L4-L5 level, leading to pain.
Pain can also originate directly from the anomalous joint itself, particularly in Type II pseudoarticulations, which may develop arthritis and inflammation due to mechanical grinding. Patients typically report localized lower back pain that may radiate into the buttock, hip, or groin area. This pain is sometimes confused with sacroiliac joint pain. In some cases, the enlarged transverse process or altered alignment can compress surrounding nerve roots, leading to radiating leg pain or radiculopathy.
Identification and Management
Identifying an LSTV typically begins with imaging studies of the lower back. The initial diagnostic tool is usually a standard X-ray, often including the Ferguson radiograph, which is angled to provide a clear image of the lumbosacral junction. This plain film effectively visualizes the bony structure and allows for classification using the Castellvi system. Advanced imaging, such as MRI or CT scans, may be used to evaluate soft tissue structures like nerves, discs, and the integrity of the anomalous joint. For patients experiencing pain, a diagnostic injection of local anesthetic and corticosteroid into the pseudoarticulation is often performed to confirm the LSTV as the source of symptoms.
The management of symptomatic LSTV (Bertolotti’s Syndrome) is predominantly conservative. Initial treatment involves physical therapy to strengthen core muscles and improve biomechanics, combined with nonsteroidal anti-inflammatory drugs (NSAIDs) to manage inflammation. Local corticosteroid injections into the painful pseudoarticulation can provide temporary or long-term relief. Surgical intervention is rare and reserved for patients whose severe pain is confirmed to originate from the LSTV and fails to improve after extensive conservative treatment. Surgery typically involves resection of the enlarged transverse process and pseudoarticulation to eliminate the source of mechanical irritation.