L5 sacralization is a congenital variation of the spine where the fifth lumbar vertebra (L5) partially or completely fuses with the sacrum, the triangular bone at the base of the spine. This anomaly is present from birth and alters the normal anatomy of the lumbosacral junction. For many individuals, this spinal variation causes no symptoms and is often discovered incidentally during imaging for unrelated conditions. The seriousness of L5 sacralization depends on the potential biomechanical complications it introduces, which may lead to chronic back pain in a subset of people.
Understanding the L5 Sacralization Anomaly
L5 sacralization is one form of lumbosacral transitional vertebra (LSTV), representing a structural difference in the junction between the lower back and the pelvis. The prevalence of this variation is relatively common, estimated to occur in approximately 7% to 30% of the general population. This condition is not an injury or a disease that develops over time, but rather a difference in skeletal development during the embryonic stage.
The anomaly involves the transverse processes of the L5 vertebra, which are the small bony projections extending sideways. Instead of remaining free, these processes enlarge and connect to the sacrum or the ilium (pelvic bone). This connection can be unilateral, occurring on only one side, or bilateral, involving both sides of the L5 vertebra.
Clinicians use the Castellvi classification system to categorize the structural degree of this fusion. The type of sacralization does not automatically predict whether a person will experience pain.
- Type I describes an enlarged, non-fused transverse process measuring at least 19 millimeters wide.
- Type II involves an incomplete fusion, where the enlarged process forms a pseudoarthrosis, or a “false joint,” with the sacrum.
- Type III is defined by a complete bony fusion between the L5 transverse process and the sacrum.
- Type IV presents with a pseudoarthrosis on one side and a complete bony fusion on the opposite side.
Addressing the Seriousness: When Sacralization Causes Symptoms
The seriousness of L5 sacralization, sometimes termed Bertolotti’s Syndrome when symptomatic, stems from the altered mechanics of the lower spine. By partially or completely fusing the L5 segment, the condition reduces motion at the lumbosacral junction. This shifts mechanical stress to the spinal segment immediately above the fusion, typically the L4-L5 disc and facet joints.
Studies show that sacralization can lead to an increase in shear stress at the L4-L5 disc, predisposing it to earlier degeneration. This accelerated wear can result in disc herniation or facet joint arthritis at the L4-L5 level, which then becomes the primary source of pain. The symptoms often manifest as chronic low back pain.
The enlarged L5 transverse process itself can also become a direct source of pain. In Castellvi Type II and Type IV variations, the pseudoarthrosis (false joint) is a region of tissue that can develop arthritic changes. This false joint may become inflamed and painful due to movement and friction, often localized to the buttock or low back.
The bony anomaly can also directly irritate or compress the L5 spinal nerve root as it exits the spine. The enlarged transverse process can narrow the space through which the nerve root travels, leading to radicular symptoms such as sciatica. While most people with L5 sacralization remain asymptomatic, those who develop symptoms often experience chronic pain that is challenging to manage due to the underlying structural change.
Diagnosis and Treatment Pathways
The initial diagnosis of L5 sacralization is typically made through X-rays, which clearly visualize the bony connection between the L5 vertebra and the sacrum. While finding sacralization is straightforward, establishing that it is the definitive source of a patient’s back pain requires a thorough clinical assessment. Clinicians must rule out other common causes of low back pain, as the spinal variation may be an incidental finding unrelated to the patient’s discomfort.
Advanced imaging, such as Magnetic Resonance Imaging (MRI), helps evaluate the surrounding soft tissues, including the intervertebral discs and nerve roots. This imaging identifies secondary problems like L4-L5 disc degeneration or nerve root compression.
Targeted diagnostic injections, where a local anesthetic is injected directly into the pseudoarthrosis or the joint above, can confirm if the transitional segment is the pain generator. If the pain is temporarily relieved by the injection, it supports the diagnosis of symptomatic sacralization.
Treatment for symptomatic L5 sacralization is conservative and focuses on alleviating pain and improving function. Physical therapy is a primary management strategy, concentrating on strengthening the core and stabilizing muscles to better support the altered spinal biomechanics. Non-steroidal anti-inflammatory drugs (NSAIDs) may be used to manage pain and reduce inflammation, particularly when the pseudoarthrosis is inflamed.
If conservative measures fail to provide sufficient relief, targeted injections of a corticosteroid and anesthetic mixture may be administered directly into the painful pseudoarthrosis. Surgical intervention is considered a last resort and is reserved for a small number of patients whose pain is severe, chronic, and unresponsive to conservative management. When surgery is pursued, it may involve the minimally invasive resection, or removal, of the painful pseudoarthrosis to eliminate the source of joint friction and irritation.