Can the L5‑S1 level remain unfused after performing a decompression and fusion at L4‑5 if the L5‑S1 segment is structurally stable?

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Last updated: October 13, 2026 • View editorial policy

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Can L5-S1 Remain Unfused After L4-5 Fusion with Decompression?

Yes, L5-S1 can remain unfused after performing decompression and fusion at L4-5, provided the L5-S1 segment demonstrates no preoperative or intraoperative instability. The evidence consistently supports that fusion should be reserved for segments with documented instability, spondylolisthesis, or deformity—not performed prophylactically 1.

Key Decision Framework

When L5-S1 Should NOT Be Fused:

  • No radiographic instability (defined as <3mm translation or <10° angulation on flexion-extension films) 2
  • No spondylolisthesis at L5-S1 1
  • No deformity requiring correction 1
  • Structurally intact facets and posterior elements after decompression 1

When L5-S1 MUST Be Fused:

  • Preoperative instability (>5mm translation on dynamic radiographs) 1
  • Intraoperative iatrogenic instability from extensive facet resection 1
  • Coexisting spondylolisthesis at L5-S1 1
  • Significant deformity involving L5-S1 3

Evidence Supporting Selective Approach

The 2014 Journal of Neurosurgery guidelines explicitly state that fusion is appropriate only when there is preoperative radiographic or intraoperative evidence of instability 1. Multiple systematic reviews concluded that in the absence of deformity or instability, lumbar fusion was not associated with improved outcomes compared with decompression alone 1.

Clinical Outcomes Data:

Recent high-quality evidence from 2026 demonstrates that selective L5-S1 decompression without fusion in patients undergoing multi-level fusion above achieved:

  • Comparable clinical outcomes at 6 and 24 months (ODI and VAS scores) 2
  • Significantly reduced operative time (148 vs 205 minutes, p<0.001) 2
  • Lower blood loss (377 vs 653 mL, p<0.001) 2
  • Avoided S1 screw complications (23.3% loosening rate when S1 was instrumented) 2

Adjacent Segment Disease Risk:

The data strongly favors NOT extending fusion to L5-S1 prophylactically:

  • Caudal (L5-S1) ASD incidence is significantly lower than rostral ASD after L4-5 fusion (2.3% vs 18.3%, p<0.001) 4
  • Pre-existing L5-S1 degeneration does NOT affect outcomes after isolated L4-5 fusion over 9+ years follow-up 5
  • Only 1.7% required subsequent L5-S1 surgery in long-term follow-up 4
  • No symptomatic L5-S1 instability developed in 181 patients with floating L5-S1 segments at 5.1 years 6

Critical Pitfalls to Avoid

Do NOT Fuse L5-S1 Based On:

  • Mild disc degeneration alone on MRI—this does not predict symptomatic ASD 5, 6
  • Prophylactic reasoning—the evidence shows this increases complications without benefit 4
  • Concern about "leaving pathology behind"—asymptomatic radiographic changes are common and well-tolerated 6

DO Assess Carefully:

  • Flexion-extension radiographs to document stability (>3mm translation or >10° angulation = instability) 2
  • Intraoperative stability after decompression—if >50% facet resection required, consider fusion 1
  • Sagittal alignment—significant L5-S1 disc collapse or kyphosis may warrant inclusion 3

Biomechanical Considerations

When L4-5 is fused, reciprocal compensatory changes occur at L5-S1 (decreased segmental lordosis) 7, but this is:

  • Physiologically adaptive, not pathologic 7
  • Does not translate to clinical symptoms in stable segments 5, 6
  • Part of normal load redistribution after fusion 8

The L5-S1 segment has inherent biomechanical stability from:

  • Iliolumbar ligaments
  • Large facet surface area
  • Sacral inclination
  • Making it more resistant to instability than upper lumbar segments 4

Practical Algorithm

  1. Preoperative assessment: Obtain flexion-extension films of L5-S1

    • If >3mm translation or >10° angulation → Include in fusion
    • If stable → Plan decompression only

  2. Intraoperative decision: After L5-S1 decompression

    • If >50% bilateral facet resection required → Add fusion
    • If facets preserved → Leave unfused

  3. Document stability: Test under fluoroscopy if uncertain

    • Excessive motion → Fuse
    • Normal motion → Decompress only

The weight of evidence strongly supports that extending fusion to a stable L5-S1 segment increases surgical morbidity (longer operative time, more blood loss, S1 screw complications) without improving clinical outcomes or preventing future surgery 2, 4. The guideline consensus is clear: fusion should be reserved for documented instability, not performed prophylactically 1.

References

7
Research

Association of operative-level segmental lordosis correction with reciprocal changes in sagittal and spinopelvic parameters after single-level lateral lumbar interbody fusion at L4-5.

European spine journal : official publication of the European Spine Society, the European Spinal Deformity Society, and the European Section of the Cervical Spine Research Society, 2026

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This information is intended for healthcare professionals. Any medical decision-making should rely on clinical judgment and independently verified information. The content provided herein does not replace professional discretion and should be considered supplementary to established clinical guidelines. Healthcare providers should verify all information against primary literature and current practice standards before application in patient care. Dr.Oracle assumes no liability for clinical decisions based on this content.