L6 Lumbar Vertebra Fused with Sacrum (Sacralization)
Clinical Significance and Management Approach
Sacralization of L6 (or L5 in typical anatomy) is a congenital anatomical variant that does not require surgical treatment unless it causes symptomatic biomechanical instability or pain refractory to conservative management. This transitional vertebra represents a structural adaptation rather than a pathological condition requiring intervention 1.
Understanding the Anatomical Variant
Sacralization occurs in approximately 4.8% of the population when the lowest lumbar vertebra becomes completely or partially fused with the sacrum 1.
This variant creates altered load-bearing patterns at the lumbosacral junction, making these regions more susceptible to degenerative changes above the transitional level 1.
The fused vertebra increases sacral height, width, and auricular surface area, representing a biomechanical compensation for reduced joint interfaces 1.
When Sacralization Becomes Symptomatic (Bertolotti's Syndrome)
Chronic low back pain associated with sacralized vertebrae is termed Bertolotti's syndrome, typically presenting as pain extending to the buttock region, often just above the ipsilateral sacroiliac joint 2.
The enlarged transverse process can form a pseudarthrosis with the sacral ala, creating a pain generator through abnormal motion at this junction 2.
In young patients with back pain, Bertolotti's syndrome should always be considered when radiographic investigation reveals anomalous enlargement of the transverse process 2.
Conservative Management (First-Line Approach)
A minimum 6-week comprehensive conservative treatment program is mandatory before considering any surgical intervention, including formal physical therapy, anti-inflammatory medications, and activity modification 3.
Targeted injections at the pseudarthrosis site may provide diagnostic and therapeutic benefit when the transitional segment is the confirmed pain generator 2.
Neuroleptic medications (gabapentin or pregabalin) may be beneficial if radicular symptoms are present 3.
Surgical Considerations (Only After Failed Conservative Management)
Indications for Surgical Intervention
Fusion is only indicated when there is documented instability at the level above the transitional vertebra (typically L4-5 in a sacralized L5, or L5-6 in a sacralized L6), combined with moderate-to-severe stenosis or spondylolisthesis 3, 4.
Isolated sacralization without instability or stenosis at adjacent levels does not meet criteria for surgical fusion 4.
The presence of the transitional vertebra itself is NOT an indication for surgery—only symptomatic pathology at adjacent mobile segments warrants intervention 3.
Critical Surgical Planning Considerations
Lateral transpsoas approaches (XLIF) at the L5-6 level in lumbarized sacra are challenging and often contraindicated due to altered psoas anatomy that resembles L5-S1 positioning in normal anatomy 5.
In patients with 6 lumbar vertebrae, only 20% could be safely treated using a lateral transpsoas approach at L5-6, with 80% requiring conversion to alternative approaches based on neuromonitoring feedback 5.
Preoperative axial MRI assessment is essential: a teardrop-shaped psoas detached from the lateral disc border indicates the level is unapproachable laterally, while a domed/helmet-shaped psoas attached to the disc suggests feasibility 5.
Advanced intraoperative neuromonitoring is mandatory when attempting lateral approaches at transitional levels to avoid neurological injury 5.
Biomechanical Pitfalls to Avoid
Extending fusion to the sacrum in patients with transitional anatomy requires meticulous attention to sagittal balance, as these patients have altered pelvic parameters and are at higher risk for sagittal decompensation 6.
Patients with high pelvic incidence (>60°) and preoperative sagittal imbalance are at significantly increased risk for postoperative sagittal decompensation when fusion extends to the sacrum 6.
Avoid distraction instrumentation when fusion must extend to the lower lumbar spine or sacrum, as this is the most frequently identified factor leading to flatback syndrome 7.
Pseudarthrosis at the lumbosacral junction occurs in patients with transitional anatomy, and 80% of these patients develop sagittal decompensation 6.
Specific Treatment Algorithm
Step 1: Confirm the Diagnosis
- Obtain standing AP and lateral radiographs to identify the transitional vertebra and count vertebral levels accurately 2.
- Perform flexion-extension radiographs to assess for instability at the mobile segment above the transitional level 3.
- MRI with axial cuts to evaluate for stenosis, disc degeneration, and psoas anatomy if surgery is being considered 5.
Step 2: Conservative Management (Minimum 6 Weeks)
- Structured physical therapy program focusing on core stabilization and postural correction 3.
- NSAIDs and activity modification 3.
- Consider diagnostic/therapeutic injection at the pseudarthrosis site if Bertolotti's syndrome is suspected 2.
Step 3: Surgical Decision-Making (Only if Conservative Management Fails)
- If no instability or stenosis at adjacent levels: Surgery is NOT indicated—continue conservative management 4.
- If instability or moderate-to-severe stenosis at the level above the transitional vertebra: Fusion may be appropriate at that specific level only 3, 4.
- Avoid prophylactic fusion of the transitional segment itself unless there is documented symptomatic pseudarthrosis refractory to injections 4.
Step 4: Surgical Approach Selection (If Surgery Indicated)
- For L4-5 pathology above a sacralized L5: Posterior approach (TLIF/PLIF) is preferred over lateral approach 5.
- For L5-6 pathology in a lumbarized sacrum: Posterior approach is strongly preferred; lateral approach has 80% conversion rate 5.
- If lateral approach is attempted: Mandatory preoperative axial MRI assessment of psoas morphology and intraoperative advanced neuromonitoring 5.
Common Clinical Pitfalls
Do not confuse the presence of a transitional vertebra with an indication for surgery—the vast majority of sacralized vertebrae are asymptomatic and require no intervention 1.
Do not perform fusion based solely on the transitional anatomy—fusion is only indicated for documented instability or stenosis at adjacent mobile segments 4.
Do not underestimate the technical difficulty of lateral approaches at transitional levels—have a backup posterior approach plan ready 5.
Do not extend fusion to the sacrum without careful assessment of sagittal balance and pelvic parameters—these patients are at high risk for sagittal decompensation 6.