In a patient with a mild grade‑1 L4‑over‑L5 anterolisthesis and dynamic instability, will the fusion typically cover L3‑S1 or just the L4‑L5 segment (with possible inclusion of S1)?

Fusion Extent for L4-L5 Anterolisthesis with Dynamic Instability

For a patient with mild grade-1 L4-over-L5 anterolisthesis and dynamic instability, fusion should typically cover only the L4-L5 segment (or L4-S1 if L5-S1 is also symptomatic and unstable), not extending routinely to L3-S1.

Rationale for Limited Fusion Construct

The key principle is to fuse only the symptomatic and unstable segments while preserving motion at adjacent levels. In your scenario with isolated L4-L5 pathology:

Primary Fusion Strategy

• L4-L5 fusion alone is appropriate when instability and symptoms are confined to this level 1
• The 2014 neurosurgical guidelines demonstrate that fusion technique selection (posterolateral fusion vs. interbody approaches) matters less than addressing the specific unstable segment 1
• Recent evidence from 2026 confirms that selective decompression without extending to S1 yields comparable clinical outcomes when L5-S1 lacks instability (>3mm translation or >10° angulation on flexion-extension films) 2

When to Extend to S1

Extension to the sacrum (L4-S1) is warranted only if:

• L5-S1 demonstrates radiographic instability (>3mm translation or >10° angulation on dynamic films) 2
• L5-S1 is symptomatic with significant disc degeneration requiring treatment 3
• Preoperative planning identifies L5-S1 pathology requiring fusion 4

Critical pitfall to avoid: Do not reflexively extend to S1 "for stability" when L5-S1 is asymptomatic and stable. This increases operative time (148 vs. 205 minutes), blood loss (377 vs. 653 mL), and exposes patients to S1 screw loosening complications (23.3% incidence) without clinical benefit 2.

Why Not L3-S1?

There is no indication to extend proximally to L3 in your described scenario:

• L3-L4 is not involved in the pathology
• Extending fusion increases adjacent segment stress without addressing the primary problem
• Biomechanical studies show that extending fusion to the sacrum already increases L3-L4 motion by 15% in flexion-extension 5
• Unnecessary multilevel fusion increases morbidity, costs, and adjacent segment degeneration risk

Surgical Decision Algorithm

1. Confirm instability at L4-L5 with flexion-extension radiographs (>3mm translation or >10° angulation)
2. Assess L5-S1 stability with dynamic imaging
   • If stable and asymptomatic → L4-L5 fusion only
   • If unstable or symptomatic → L4-S1 fusion
3. Verify L3-L4 is asymptomatic clinically and radiographically
   • No indication to include L3 unless separate pathology exists

Technical Considerations

The guideline evidence shows that instrumented fusion with pedicle screws provides superior outcomes compared to noninstrumented approaches for spondylolisthesis with instability 1. Choice between posterolateral fusion (PLF) and interbody techniques (PLIF/TLIF) shows similar functional outcomes, though interbody approaches may offer better lordosis restoration at L5-S1 specifically 3.

Bottom line: Your fusion should cover L4-L5 (or L4-S1 if L5-S1 is also unstable/symptomatic), not L3-S1. The question appears to contain a typo—extending three levels from L3-S1 would be excessive for isolated L4-L5 pathology.