Decisions and Expectations with Osteotomy Surgery for Fixed Sagittal Imbalance Syndromes: Smith-Petersen vs Pedicle Subtraction

Problems with Fixed Kyphosis:• Cosmesis awful.
• Function poor.
• Fatigue pain from neck, hip extensors.
• Effect on levels above/below - predisposes to early degeneration.

Diagnosis/Etiology:
• Post-traumatic - fusing the thoracolumbar or lumbar spine in kyphosis.
• Post fusion.

-Scoliosis - don't shorten the anterior column (Zielke, Dwyer) or lengthen the posterior column (HR, other).

-Degenerative - don't fuse segmental kyphosis without correction; will produce “fall-off” above.

-Post-traumatic - don't fuse the spine in kyphosis without correction.

-Long fusion if pullout of posterior segmental spinal instrumentation (PSSI) or displacement of structural grafts anteriorly.

-Settling of long fusion done without structural grafting or 4-point fixation of sacrum/pelvis. “Settling” = loss of anterior column height and subsequent kyphosis.

-LDK (Lumbar Degenerative Kyphosis).

• Systemic.

- JRA.
- Ankylosing spondylitis and related disorders.
- Osteoporosis.

Indications for Osteotomy:
• Fixed kyphosis associated with pain/pseudarthrosis,
• Definition of balance/out of balance; see definition below.
• Emotionally stable patient with family support

Spectrum of Deformity:
• Type I (compensated) - patient segmentally flat but globally in balance. Able to compensate by hyperextending segments above/below. Discs below are not severely degenerated but appear triangular on a standing x-ray (abnormal) and trapezoidal on a supine x-ray (normal). Triangular = markedly convergent posteriorly or anterior height by > 5 mm -- posterior height.

• Type II (decompensated) - patient segmentally and globally flat and out of balance. C7 in front of L5-S 1. Patient unable to compensate by hyperextending segments below.

• Get long cassette lateral x-ray with the patient’s hips and knees extended and his or her hands just in front of the waist. Drop a plumb from C7 to produce the sagittal vertical axis (SVA). SVA anterior to L5/Sl disc = positive sagittal balance. SVA posterior to L5/Sl disc = negative sagittal balance. SVA through the L5/S1 disc = neutral sagittal balance. Physiologic = neutral or slightly negative SVA.

Smith-Petersen Osteotomies:

• Shortens posterior column.

• Hinges on middle column.

• Lengthens anterior column.

• May require structural grafting of anterior column.

• If there is significant residual deformity/rotation/ scoliosis, it may shorten the concavity, lengthen the convexity, and displace the patient towards the concavity.

Indications for Multiple Smith-Petersen Osteotomies Without Anterior Surgery:
• Fusing short of sacrum (usually type I deformity).
• Young patient.
• Excellent bone stock.Mild/moderate correction at several levels.
• Fat discs anteriorly.
• Physiologic sagittal restoration achievable.
• Excellent segmental fixation achievable.

Indications for Anterior Releases/Morselized Graft Then Multiple Smith-Petersen Osteotomies.    No Structural Grafting Needed.
• Narrow or ossified discs need anterior release.

• If pseudos exist, an anterior procedure is advisable to 8 surface area for fusion.

• Weight-bearing line behind the osteotomies post-operatively if so, may not need structural grafting if there are not big gaps anteriorly.

Indications for Posterior Smith-Petersen Osteotomies with Structural Grafting Anteriorly [Cages, Fresh Frozen Femoral Rings, Tricortical Iliac Graft (Auto or Allo)]:
• “Big” gaps (> 10 mm) anteriorly after osteotomies.
• Incomplete sagittal restoration.
• If extending fusion to sacrum, structurally graft at least 4-5 and 5-1, too.

Transpedicular Three-Column (Pedicle/Vertebral Body Subtraction) Osteotomy:
• Anticipate 30E correction at one level.
• One approach (posterior).
• No gap anteriorly.
• The anterior column is not opened up as in a Smith-Petersen osteotomy.

All 3 columns should be bone on bone at the completion.
• Do it at L1 or below.

• Blood loss variable, but usually more than with Smith-Petersen osteotomy because of work on the floor of the canal (epidural veins) + the vertebral body (very vascular).

• Do at apex if possible - OK to do even if a rotational deformity exists there.

• Shortens posterior and middle column; hinges on anterior column.

• Should open up/enlarge the canal centrally (as one would for a Simmons cervical-thoracic osteotomy) if not being done through a previous fusion or if any hint of subluxation.

Step-by-Step Technique of PSO:

Step 1. The extent of the osteotomy indicated with shade.

Step 2. The posterior columns resected but the pedicles preserved. The pedicles surrounded medially/laterally/ superiorly/inferiorly.

Step 3. A cavity created under the pedicles.

Step 4. The pedicles resected and the posterior vertebral body wall resected. Thin the posterior vertebral body wall with a curette until it is wafer thin. Then resect the lateral portions with a Kerrison or Leksell. Greenstick the posterior cortex centrally with a Woodson or reverse angled curette.

 Step 5. The lateral vertebral body wall resected at the level of the pedicles.

Step 6.  The osteotomy closed*

*If the canal is at all stenotic centrally, then a wide posterior column decompression should be performed centrally, the osteotomy should be open centrally and only closed laterally along the foramen bilaterally.

Step 7. The spine reconstructed.**

**Potential fixation points include screws and hooks. “Closing” forces are cantilever and compression.

Calculations:
• Need 30E more lumbar lordosis than thoracic kyphosis, i.e. match 30E lumbar lordosis with OE thoracic kyphosis, 70E lumbar lordosis (TI2-S1) with 40E thoracic kyphosis (T3-T12).

• Assume 1E correction per mm bone resected with SPO (10-15 mm per level).

• Assume 30E-35E correction per PSO.

• Assume 30E-35E of lordization/correction is needed to posteriorly displace the plumb/SVA from C7 12-15 cm.

Complications of Osteotomies:
• Wound - may be significant if there have been multiple previous surgeries. Do TPN!

• CSF leak - always a risk, but rarely “a problem” if the osteotomies are closed “tightly.”

• Blood loss - may want to “stage” procedures 5 days apart.  Stand/walk patient between. Use catheter and hyperal if not able to do in one day.

• Loss of fixation - rare.

• Non-union - rare at osteotomy level(s) if enough correction and “stability” is achieved. More likely in segments being added to the fusion. Those distal lumbar segments being added should be structurally grafted!

• Slow recovery by the patient in some cases due to magnitude of the surgery.

• Neurologic: SSEP, MEP, multiple wake-up tests advised.

• Coronal decompensation - more likely with SPO than PSO - especially if done at apex with residual rotation.

Results of Osteotomies:
• Fusion rate: high if osteotomy closed posteriorly, stable fixation, and sagittal restoration.

• Patient satisfaction: high if sagittal restoration complete without “complications” and patient comorbidities are low. (Booth, Bridwell et al, SRS 1998, AAOS 1999, Spine 1999.)

• Patient satisfaction lower if:

-Wound infection.
-Coronal imbalance.
-Poor family/social support.

Patient motivation is important.

Surgeon should prepare the patient and patient family for “the worst.”

Bibliography:

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