Lumbarization of S1: Definition and Management
Lumbarization of S1 is a congenital spinal anomaly where the first sacral vertebra (S1) separates from the sacrum and takes on lumbar characteristics, creating a functional sixth lumbar vertebra—this is typically an incidental radiographic finding that requires no treatment unless symptomatic.
What is Lumbarization?
Lumbarization represents a lumbosacral transitional vertebra (LSTV) where S1 fails to fuse with S2, occurring in approximately 1.8-2.1% of the population 1. This anatomical variant can present as:
- Partial lumbarization: Incomplete separation of S1-S2 bodies and neural arches
- Complete lumbarization: Full separation of S1 from S2, creating a distinct vertebral segment with well-developed S1-S2 disc space 2, 1
Key Anatomical Features
When lumbarization is present, specific morphological changes occur 1, 3:
- Auricular surfaces positioned lower (at S2-S3 level) compared to normal sacra
- Smaller overall sacral dimensions in the remaining S2-S5 segments
- Accessory articulations may develop between the transverse processes of the separated S1 and the S2 ala (present in 4 of 6 cases in one series)
- Attenuated laminae with occasional spina bifida
- Altered pedicle anatomy: S1 pedicles are more obtuse with the sagittal plane and less steep anteriorly 4
Radiographic Identification
Critical imaging pitfall: Lumbarization can be misinterpreted as normal lumbar anatomy if only lumbar MRI is obtained 2. Always obtain whole-spine radiography when LSTV is suspected.
Rapid identification methods 3:
- High iliac crest line (ICL): When the ICL is positioned higher than the inferior edges of the transverse processes of the vertebra above the last presacral vertebra (sensitivity: 81.1% for lumbarization)
- Anterior superior angle (ASA) of last presacral vertebra: <85.7° (vs. 90.6° in normal anatomy)
- Posterior angle (PA): <19.7° (vs. 36.9° in normal anatomy)
- Combined criteria: When ICL is high AND PA <20°, 95.8% have lumbarization 3
Management Approach
Asymptomatic Lumbarization
No treatment is required for incidentally discovered lumbarization in asymptomatic patients. This is a normal anatomical variant that does not inherently cause pathology.
Symptomatic Presentations
When lumbarization becomes clinically relevant, management depends on the specific pathology:
1. Acute Trauma
For isolated fractures of lumbarized S1 (e.g., spinous process fractures from direct trauma) 5:
- Conservative management is first-line: analgesics, local heat, physical therapy
- Most patients return to normal activities within 3 weeks
- Surgery is rarely indicated for isolated fractures
2. Disc Herniations
Calcified disc herniations can develop at both L5-S1 and the lumbarized S1-S2 levels 2:
- Percutaneous endoscopic interlaminar discectomy (PEID) is effective for decompression at both levels
- Can be performed in a single surgical session
- Excellent neurological recovery reported (JOA score improvement from 12 to 26 at 6 months)
3. Degenerative Spondylolisthesis
Rare but documented at the S1-S2 level with lumbarization 6:
- Surgical decompression and stabilization at affected levels (L5, S1, S2)
- Autologous bone graft placement
- Clinical improvement typically achieved by 1 year
4. Chronic Low Back Pain
If chronic low back pain develops and is refractory to conservative treatment, follow established guidelines for degenerative disc disease 7:
Conservative treatment first (minimum trial):
- Physical therapy
- Other nonoperative measures for adequate duration
If conservative treatment fails and pain is attributed to 1-2 level degenerative disc disease:
- Lumbar fusion is recommended (Grade B evidence) 7
- Alternative: Comprehensive rehabilitation program incorporating cognitive behavioral therapy (equivalent outcomes to fusion) 7
Important caveat: The guidelines note that determining the actual pain source in chronic low back pain remains challenging, and MRI findings lack specificity 7. Discography has insufficient evidence as an independent predictor of fusion success 8.
Surgical Considerations
When surgery is required for lumbarized anatomy 4, 9:
- Pedicle morphology differs from normal S1: more obtuse sagittal angulation, less steep anterior slope
- Preoperative mapping essential to avoid inadvertent surgical injuries
- Spinopelvic parameters: Use L5 incidence or L4-sciatic notch angle formulas when S1 endplate is altered 9
- Screw placement requires adjustment for altered pedicle dimensions and angulation
Clinical Pitfalls to Avoid
- Wrong-level surgery: Lumbarization is a main cause of wrong-level procedures—always count from C2 or obtain whole-spine imaging 3
- Missed dual pathology: Check both L5-S1 AND the lumbarized S1-S2 disc spaces for herniation 2
- Inadequate imaging: Lumbar-only MRI may miss the transitional anatomy—obtain AP whole-spine radiographs
- Assuming normal pedicle anatomy: Altered dimensions require preoperative CT measurement for safe instrumentation 4