Spondylolisthesis Reduction and Fusion

Surgical Indications

Absolute Indications

• Progressive slip on serial radiographs greater than 10 percent in 6 months
• Neurologic deficit (L5 radiculopathy or cauda equina) not responding to conservative care
• Severe sagittal imbalance with compensatory lumbar hyperlordosis or knee flexion
• High-grade slip (Meyerding III-V) with intractable mechanical low-back pain

Relative Indications

• Symptomatic high-grade slip with slip angle greater than 30 degrees even without frank neurologic deficit
• Failed conservative management (greater than 6 months) with activity-limiting pain
• Patient preference for definitive correction of deformity and restoration of sagittal balance

Contraindications

Absolute:

• Active spinal infection or tumour
• Severe untreated osteoporosis (T-score less than -3.5) without augmentation plan
• Patient unable to tolerate major surgery or comply with postoperative restrictions

Relative:

• Low-grade slip (I-II) without neurologic deficit or instability — consider in-situ fusion or decompression alone
• Prior extensive abdominal surgery making anterior access difficult (though posterior approach avoids this)
• Smoker unwilling to quit — higher pseudarthrosis risk; consider staged or BMP-augmented procedure

Evidence for Reduction versus In-Situ Fusion

The Reduction Debate

• Reduction restores lumbosacral lordosis, improves sagittal balance, and unloads the L5-S1 disc, but carries a 5-15 percent risk of L5 nerve-root injury
• In-situ fusion avoids stretch injury but leaves residual kyphosis and may not halt progression in high-grade slips
• Modern series with neuromonitoring and wide decompression show permanent L5 palsy rates of 2-4 percent — acceptable when the alternative is untreated high-grade deformity

Key Evidence Summary

Key Evidence

Relevant Surgical Anatomy

Lumbosacral Junction and Spondylolisthesis Pathoanatomy

• The L5-S1 disc and facet joints bear the greatest shear forces in the lumbar spine
• In isthmic spondylolisthesis a pars interarticularis defect allows anterior translation of L5 on S1; the loose lamina and hypertrophic callus compress the L5 roots in the foramen
• Degenerative spondylolisthesis results from facet arthropathy and disc collapse without a pars defect; the L4-L5 level is most common and usually low-grade
• The L5 nerve root exits the L5-S1 foramen and courses over the sacral promontory; in high-grade slips the root is stretched and tented, already 1.5-2 cm longer than normal

Meyerding Grading and Slip Angle

• Measured on standing lateral radiograph: percentage of L5 body translation relative to S1
• Grade I less than 25 percent, II 25-50 percent, III 50-75 percent, IV 75-100 percent, V spondyloptosis
• Slip angle (lumbosacral angle) is the angle between the superior endplate of L5 and the posterior wall of S1; greater than 30 degrees predicts progression and favours reduction

Structures at Risk

• L5 nerve roots — most vulnerable during reduction; exit foramina and cross the sacral promontory
• Dura — adherent to the undersurface of the loose lamina at the pars defect
• L5 pedicles — small, sclerotic, or osteoporotic in chronic high-grade slips; limited screw purchase
• S1 endplate and promontory — may require resection to shorten the path of the L5 root after reduction
• Cauda equina — at risk during wide central decompression in high-grade slips with stenosis

Safe Intervals and Windows

• The Gill window is created by complete excision of the loose L5 lamina, exposing the L5-S1 disc and both L5 roots from the medial pedicle border to the sacral promontory
• The L5 root is mobilised from the pedicle to the foramen before reduction; any tethering tissue must be released
• The sacral promontory tip can be resected 5-8 mm to reduce tension on the L5 root after partial reduction

Positioning and Preparation

Patient position: Prone on Jackson table with hips extended to increase lumbar lordosis. Arms abducted less than 90 degrees. All pressure points padded. Foley catheter placed. Cell-saver and neuromonitoring (somatosensory and triggered EMG of L5 roots) mandatory.

Anaesthesia: General endotracheal with total intravenous anaesthesia to allow neuromonitoring. Arterial line for blood-pressure control. Preoperative antibiotics (cefazolin or vancomycin if MRSA risk). Tranexamic acid 1 g IV at induction and 1 g at closure.

Consent discussion: Risk of L5 nerve injury (5-15 percent transient, 2-4 percent permanent foot drop), dural tear (5-8 percent), pseudarthrosis (5-12 percent with interbody), hardware failure, adjacent-segment disease (10-20 percent at 10 years), infection, and possible need for staged or revision surgery.

Posterior Approach and Gill Decompression

Step 1: Exposure

Midline incision from L4 to S2. Subperiosteal dissection to expose L4-L5 facets, L5 transverse processes, and sacral ala. Identify the pars defect at L5 and the loose lamina. Expose the L5-S1 disc space laterally.

Step 2: Gill Procedure — Complete Laminectomy

Remove the loose L5 lamina en bloc from the pars defect to the spinous process. Release dural adhesions first with a Penfield dissector. Identify both L5 roots at the medial border of the L5 pedicles. Undercut the superior S1 facet and resect any hypertrophic tissue compressing the L5 root in the foramen. Continue decompression until the L5 root is freely mobile from the pedicle to the sacral promontory.

Step 3: Pedicle Screw Placement

Place pedicle screws at L4 (or L5 if adequate purchase), L5, and S1 bilaterally. Use fluoroscopy or navigation. In osteoporotic bone consider cement augmentation. Aim for bicortical S1 purchase when possible.

Step 4: Disc Space Preparation and Interbody Placement

After wide decompression, resect the L5-S1 disc. Prepare endplates to bleeding bone. Insert one or two lordotic structural cages (TLIF or PLIF technique) packed with autograft. The cage restores disc height and provides anterior column support.

Step 5: Gradual Reduction

Connect rods to the L5 and S1 screws. Use reduction screws or persuaders to gradually translate L5 posteriorly in 2-3 mm increments. Pause 5 minutes between increments. Monitor triggered EMG continuously. If amplitude drops greater than 50 percent, release correction immediately. Aim for 50-75 percent reduction in high-grade slips; complete anatomic reduction is rarely necessary and increases nerve risk.

Step 6: Final Fixation and Compression

After acceptable reduction, lock the rods. Compress the screws to restore lumbosacral lordosis. Confirm no residual foraminal stenosis with a probe. Irrigate copiously. Place posterolateral autograft over the decorticated transverse processes and sacral ala.

Step 7: Closure

Layered closure with absorbable sutures. Drain if extensive dissection. Sterile dressing. Log-roll precautions.

Postoperative Protocol

Immediate (Day 0-3)

• Log-roll only; no bending, lifting, twisting
• Mobilise with physiotherapist on day 1-2 with brace if surgeon preference
• Drain removal when output less than 50 mL/24 h
• Multimodal analgesia; avoid high-dose opioids

Early Phase (Week 1-6)

• Gradual increase in walking distance
• No lifting greater than 5 kg
• Wound review at 2 weeks; sutures or staples removed
• Begin gentle core activation exercises at 4 weeks

Intermediate Phase (Week 6-12)

• Progressive strengthening and proprioception
• Transition out of brace if used
• Return to light work at 8-12 weeks if fusion progressing

Late Phase (Month 3-12)

• Full activities as tolerated once radiographic fusion confirmed (usually 6-9 months)
• Sport-specific rehabilitation after 6 months
• Annual radiographic surveillance for 5 years to monitor adjacent segments

Rehabilitation Milestones

• Day 1-2: Sit to stand, short walks
• Week 6: Independent ADLs, 30-minute walks
• Month 3: Return to light duty or office work
• Month 6: Fusion mass usually consolidating; begin impact activities
• Month 12: Full unrestricted activity if solid fusion

Follow-up Imaging

• Standing radiographs at 6 weeks, 3 months, 6 months, 12 months, then annually
• CT at 6-9 months if clinical concern for pseudarthrosis or to confirm fusion before high-demand activities