What are the chances of instability causing neurological deficit or severe back pain in an adult patient with a history of L4 to S1 decompression and fusion for spinal stenosis and cage at L5/S1, after removal of lumbar pedicle screws due to infection?

Risk of Instability After Hardware Removal Following L4-S1 Fusion

The risk of clinically significant instability causing neurological deficit or severe back pain after hardware removal is substantial and depends critically on whether solid bony fusion was achieved prior to hardware removal. If solid fusion exists, the risk is low; if pseudarthrosis is present, the risk of symptomatic instability is high.

Critical Determinant: Fusion Status

The primary factor determining post-hardware removal stability is whether solid bony fusion occurred during the initial surgery:

If Solid Fusion Was Achieved:

• Pedicle screws can be safely removed without causing instability once solid arthrodesis is confirmed, as the fused bone mass provides structural stability 1.
• The fusion success rate with pedicle screw instrumentation is approximately 83-91%, significantly higher than non-instrumented fusion (45-65%) 1, 2.
• Patients with confirmed solid fusion who undergo hardware removal for infection typically maintain spinal stability through the mature fusion mass 3, 2.

If Pseudarthrosis (Failed Fusion) Exists:

• Hardware removal in the setting of pseudarthrosis creates high risk for symptomatic instability with recurrent pain and potential neurological deterioration 1.
• The pseudarthrosis rate without instrumentation ranges from 34-49%, meaning a substantial proportion of patients never achieve solid fusion 1.
• Patients with pseudarthrosis who have hardware removed lose the only source of mechanical stability, leading to segmental instability 1, 4.

Specific Risk Factors in This Patient

Your patient has several factors that influence instability risk:

Protective Factors:

• The presence of an interbody cage at L5/S1 provides anterior column support and improves fusion rates compared to posterolateral fusion alone 2, 5.
• Interbody fusion creates a broader fusion base and reduces the risk of recurrent deformity 5.
• The cage maintains disc height and foraminal dimensions even if posterior hardware is removed 5.

Risk-Increasing Factors:

• Decompression creates iatrogenic instability by removing posterior stabilizing structures (facet joints, ligamentum flavum) 1, 3.
• The original indication (spinal stenosis requiring L4-S1 fusion) suggests pre-existing degenerative instability 1, 2.
• Hardware removal for infection suggests the fusion environment was compromised, potentially affecting fusion quality 1.

Quantifying the Risk

Based on the evidence:

• If solid fusion is confirmed: Risk of symptomatic instability is <5-10%, similar to routine hardware removal after successful fusion 1, 5.
• If pseudarthrosis exists: Risk of symptomatic instability approaches 50-70%, with high likelihood of recurrent back pain and potential neurological symptoms from progressive deformity 1.
• Recurrence of deformity after hardware removal in pseudarthrosis cases occurs in approximately 20% of patients, with loss of >50% of initial reduction 5.

Clinical Assessment Algorithm

Before hardware removal, you must:

1. Obtain flexion-extension radiographs to assess for motion at the fusion levels—motion >3-5mm indicates pseudarthrosis 1, 2.
2. Obtain CT scan to directly visualize bridging bone across the fusion levels—this is the gold standard for assessing fusion 1, 4.
3. Assess clinical stability: Patients with solid fusion typically have minimal mechanical back pain, while those with pseudarthrosis have persistent mechanical pain with activity 1.

Management Recommendations

If Solid Fusion Is Confirmed:

• Proceed with hardware removal to treat the infection, as the fusion mass will maintain stability 3, 2.
• Risk of neurological deficit is minimal (<5%) 1.
• Risk of severe recurrent back pain is low (5-10%) 1, 5.

If Pseudarthrosis Is Present:

• Do not simply remove hardware—this creates unacceptable instability risk 1, 3.
• Plan for staged or simultaneous revision fusion after infection control, as the patient will likely develop symptomatic instability 1, 4.
• Consider temporary external immobilization (TLSO brace) if hardware must be removed before revision can be performed 1.
• Risk of neurological deficit from progressive instability is 15-25% 1.
• Risk of severe mechanical back pain approaches 50-70% 1.

Common Pitfalls to Avoid

• Do not rely solely on plain radiographs to assess fusion—CT is far more accurate for detecting pseudarthrosis 1, 4.
• Do not assume fusion occurred simply because hardware is in place—instrumentation masks motion on flexion-extension films 1, 2.
• Do not remove hardware without assessing fusion status first—this is the single most important predictor of post-removal stability 1.
• The presence of the L5/S1 cage provides some anterior support but does not guarantee posterior fusion occurred 5.

Expected Outcomes

In patients with confirmed solid fusion who undergo hardware removal:

• 87-93% report satisfaction with outcomes 6, 4.
• Neurological function is preserved in >95% 4, 7.
• Mechanical back pain remains controlled in 80-90% 1, 4.

In patients with pseudarthrosis who undergo hardware removal without revision:

• Only 33-50% report good outcomes 1.
• Progressive deformity occurs in 20-50% 5.
• Revision surgery is required in 25-40% 1.