DEFINITION

Smith-Petersen osteotomy (SPO) is a chevron resection of the posterior elements that shorten the posterior column and lengthen the anterior column upon closure (FIG 1A). The chevron osteotomy is called a Smith-Petersen osteotomy if performed through a prior fusion or a Ponté osteotomy if done through a nonfused spinal segment.

Pedicle subtraction osteotomy (PSO) is a posterior-based osteotomy that requires resection of the posterior elements, pedicles, and decancellation of the vertebral body in a V-shaped fashion through the transpedicular corridor (FIG 1B). The osteotomy hinges on the anterior column with closure of the middle and posterior columns creating a large cancellous bone footprint for fusion.

 

 

ANATOMY

 

A thorough understanding of spinal anatomy including spinal cord, nerve root, and vertebral segments is needed to safely perform these procedures. For an SPO, understanding the relationship of the interspinous ligaments, ligamentum flavum, facet joints, nerve roots, and spinal cord is important to resect enough posterior elements to allow osteotomy closure without posterior impingement. In a PSO, it is important to understand these same relationships, but in addition, the relationship of the exiting and traversing nerve roots to the corresponding pedicle is necessary to allow safe osteotomy closure.

 

SPO involves creating a chevron trough in the posterior elements by resecting the posterior elements through the facet joints and pars intra-articularis and posterior ligaments (supraspinous, intraspinous, and ligamentum flavum). A mobile disc space allows for closure of the middle and posterior columns and spontaneous opening of the anterior column.

 

 

 

 

FIG 1 • A. Area of bone resection for an SPO. B. Area of bone resection for a PSO.

 

 

A PSO requires a wide laminectomy from the pedicle above to pedicle below the osteotomy level, resection of

the bilateral pedicles at the PSO level, and vertebral body decancellation to the anterior vertebral body in a wedge shape.

 

PATHOGENESIS

 

Sagittal imbalance is classified into type I and type II.

 

 

Type I sagittal imbalance is when there is a region of the spine that is fused in a hypolordotic or kyphotic position, but overall sagittal balance is satisfactory (sagittal C7 plumb falling through the L5-S1 disc space or slightly behind it on a standing long cassette lateral radiograph) as the patient is able to compensate through nonfused segments.

 

A type II imbalance is one in which the patient cannot compensate due to adjacent level degeneration resulting in a positive sagittal imbalance (patient leans forward in the sagittal plane).

 

Type I patients often maintain their balance by hyperextending through mobile lumbar segments below the kyphotic segment. In type II imbalance, vertebral segments above or below the kyphotic area are substantially degenerated or fused and, therein, the spine is unable to hyperextend and maintain balance (FIG 2).

 

Kyphosis can be smooth and span several segments, such as in Scheuermann kyphosis (FIG 3), or sharp and angular, over one or two segments, such as in congenital or posttraumatic kyphosis.

 

 

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FIG 2 • A,B. Patient (45-year-old woman) with eight prior spinal fusions presenting with fixed sagittal and coronal imbalances. An asymmetric L2 PSO was performed for spinal realignment. At 6-year follow-up, a solid fusion was achieved with improvement in radiographic and clinical appearance.

 

 

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FIG 3 • A,B. Patient (31-year-old woman) with three prior posterior spinal fusions at outside institution for treatment of Scheuermann kyphosis presented with worsened thoracic kyphosis, multiple pseudarthrosis, and sagittal imbalance. A revision T3-L2 posterior spinal fusion with SPOs T5-T12 and anterior spinal fusion was performed for spinal realignment. At 5-year follow-up, a solid AP fusion was achieved with improvement in radiographic and clinical appearance.

 

 

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These osteotomies are most often used for the correction of sagittal imbalance or kyphosis. SPOs are most often used to correct sagittal imbalance between 5 and 10 cm or smooth gradual kyphosis, whereas a PSO is used to treat sagittal imbalance greater than 10 cm or sharp, angular kyphosis within the lumbar spine. An asymmetric PSO can be done for a deformity that has both a coronal imbalance and sagittal imbalance together. A vertebral column resection (VCR) can be used to treat sharp, angular kyphosis within the thoracic or thoracolumbar spine.

 

NATURAL HISTORY

 

The natural history of the diseases/conditions leading to sagittal imbalance and kyphosis are variable, and a complete workup is necessary before recommending an osteotomy as a corrective operation.

 

Deformities that progress become rigid and, uncompensated, may present with intolerable pain, decreased ability to perform activities of daily living, or myelopathy and nerve root impingement.

 

PATIENT HISTORY AND PHYSICAL FINDINGS

 

The thorough history should include an understanding of the patient's main reason(s) for seeking treatment, for example, progressive deformity, pain, loss of function, and neurologic deterioration.

 

The history should include a careful assessment of current pain medication usage as preoperative narcotic usage may complicate the perioperative care. Additionally, any medications that may confer a risk of increased bleeding (eg, acetylsalicylic acid) should be noted, and the patient is cautioned to stop them prior to surgery.

 

Patients should be questioned on their use of nicotinecontaining products, particularly cigarettes, as the risk of perioperative complications and pseudarthrosis is increased in these patients and may be a relative contraindication to these procedures.

 

Those patients with diabetes mellitus must have well-controlled blood glucose levels before and after surgery, as uncontrolled blood glucose levels are associated with increased risk of perioperative infection.

 

A patient's nutritional status should be assessed and optimized prior to surgery. In addition, a bone density test should be performed to assess for osteoporosis, and appropriate treatment of these deficiencies or referral for their treatment should be initiated.

 

Patients with respiratory disease may require consultation with a pulmonologist or assessment of lung function by pulmonary function tests. Cardiac history should be assessed with the assistance of a cardiologist. Often, coordination with the patient's primary care physician is necessary to get the patient ready for these surgeries.

 

The overall coronal and sagittal plane balance should be observed with the patient standing upright.

 

The deformity should be assessed for its flexibility by placing the patient prone and supine on the examination table. Several minutes of supine positioning will allow one to assess the flexibility of a kyphotic deformity.

 

A detailed neurologic examination assessing sensation, strength, reflexes, and pathologic reflexes is necessary. A complete neurologic examination should assess for signs of myelopathy (gait disturbance such as a wide-based gait, imbalance) or nerve root palsies (foot drop). In addition, assessment of hip and knee contractures is required as these conditions may make osteotomy correction and postoperative recovery more difficult.

 

IMAGING AND OTHER DIAGNOSTIC STUDIES

 

Radiographic assessment includes a series of standing fulllength 36-inch radiographs in the anteroposterior (AP) and lateral planes, left and right side bending radiographs if coronal deformity is present, and full-length supine or prone radiographs to assess spontaneous deformity correction.

 

Hyperextension radiographs (bolster placed at apex of kyphosis) and hyperflexion radiographs (bolster at apex of lordosis) help assess sagittal plane rigidity.

 

For sagittal plane deformity, comparison of standing AP and lateral radiographs to prone and/or supine fulcrum hyperextension long-cassette radiographs will help assess deformity flexibility.

 

Computed tomography (CT) scan is often obtained to assess prior fusion masses, bone quality, relevant bone anatomy at proposed osteotomy site, and bone anomalies (small pedicles) that may preclude safe fixation point placement. A CT myelogram may help assess areas of stenosis.

 

Magnetic resonance imaging (MRI) is often obtained to evaluate the spinal cord and nerve roots in addition to assessing for neural axis anomalies.

 

If SPOs are planned, assessment for mobile disc spaces is paramount as this is a requirement for this osteotomy.

 

DIFFERENTIAL DIAGNOSIS

Smooth global kyphosis (Scheuermann kyphosis) Sharp angular kyphosis (posttraumatic)

Sagittal imbalance (types I and II) (flat back syndrome, postlaminectomy kyphosis)

 

 

NONOPERATIVE MANAGEMENT

 

Patients with static deformities and only mild pain or physical impairment should be managed with a trial of nonoperative therapy.

 

This includes a directed physical therapy program, to include cardiovascular conditioning, postural training, and abdominal strengthening.

 

For those patients with moderate to severe pain, a referral to a pain specialist, most notably for those patients with complaints of pain not consistent with their presenting pathology or other signs of nonorganic causes of pain.

 

Epidural and transforaminal steroid injections offer a less invasive, potentially diagnostic and/or therapeutic intervention for patients with nerve root compression.

 

SURGICAL MANAGEMENT

 

 

SPO is often used to treat smooth, gradual kyphotic deformities or positive sagittal imbalance of 5 to 10 cm. Usually, multiple SPOs are performed through the apex of the deformity.

 

It is important to perform a wide facetectomy as posterior compression closes down the neural foramina and may

 

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cause nerve root impingement. In general, the degree of kyphotic correction with a single SPO is 10 degrees per level or 1 degree per millimeter of bone resected.

 

A PSO is most often performed for sharp angular kyphosis, gradual kyphosis that lacks mobile disc spaces, or positive sagittal imbalance greater than 10 cm. In general, the degree of kyphotic correction with a lumbar PSO is approximately 30 to 40 degrees.

 

Preoperative Planning

 

A multidisciplinary team approach is often necessary in the treatment of patients with complex deformity that require multilevel SPOs or a PSO.

 

Preoperative assessment of the patient's cardiovascular, pulmonary, nutritional, hematologic, and metabolic systems is required to maximize the patient's preoperative reserve.

 

 

SPOs can be performed within any region of the spine, most often in the thoracic or lumbar spine.

 

PSOs are most often performed in the lumbar spine, usually at L2 or L3. A PSO in the lower lumbar spine, L4 or L5, reduces the number of available distal fixation points.

 

Positioning

 

An Orthopedic Systems Incorporated (OSI) Jackson frame with six pads is our preferred operative table when performing corrective deformity surgery. The pads are strategically placed to allow the abdomen to rest free, reducing intra-abdominal pressure and intraoperative bleeding. In addition, the axilla should be free to help reduce the risk of brachial plexus injury.

 

We prefer to place a halo or Gardner-Wells tongs with 5 to 15 pounds of traction that allows for rigid positioning of the skull and allows the face and eyes to remain free during surgery.

 

Arms are placed in a 90/90 position on padded arm boards with no pressure on the axillae and elbow padding to decrease the risk of brachial plexopathy or ulnar nerve neuropathy.

 

The hips are gently extended, and the knees are slightly flexed. For PSO correction, the hips can be extended further to help close down the osteotomy.

 

Spinal cord monitoring leads are placed to monitor the sensory and motor function of the lower extremities.

 

Approach

 

The standard posterior subperiosteal approach is used from the transverse processes of the most superior instrumented level to the most distal vertebra or ilium that are to be fused.

 

The approach can be done in stages, lumbar followed by thoracic or vice versa, to help reduce blood loss if adequate surgical help is not available. In conjunction with the anesthesiologist, hypotensive anesthesia is used to help reduce blood loss.

 

 

In addition, the use of antifibrinolytic medications can assist in helping reduce blood loss during these procedures.

 

TECHNIQUES

• Smith-Petersen Osteotomy

Identify the pedicles at all levels where SPOs are planned by placing pedicle screws.

Alternatively, SPOs can be done prior to placement of pedicle screws. For large deformities or abnormal pedicle anatomy, performing an SPO first can help identify the medial and superior borders of the pedicle to assist in locating of the starting point for pedicle screw placement.

Remove the interspinous ligaments down to level of ligament flavum and identify the median raphe. Ensure adequate space between ligament flavum and dura with a Woodson elevator and use Kerrison punches to resect a V of bone that starts centrally and works out laterally through the facet joints and pars (see FIG 1A).

Closure of the osteotomy is through a combination of compression and cantilever forces (TECH FIG 1).

In treatment of a smooth gradual kyphosis, bilateral rods are contoured to the desired sagittal plane profile and secured into the cephalad pedicles.

Gradual cantilever (downward) force is applied, and the rods are sequentially captured within the caudally located pedicle screws.

Once the rod is captured within the pedicle screws, sequential compression through the pedicle screws toward the deformity apex can be added to close down the SPOs.

 

Compressive forces reduce spinal kyphosis.

 

 

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TECH FIG 1 • A,B. Gradual, smooth thoracic kyphosis with multilevel pedicle screw placement. (continued)

 

 

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TECH FIG 1 • (continued) C,D. Multiple thoracic SPOs performed through the apex of the smooth kyphosis; an ideal situation for sagittal correction with multiple SPOs. (continued)

 

 

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TECH FIG 1 • (continued) E,F. Cantilever and compression forces through bilateral rods allow for gradual, controlled correction of the smooth kyphosis by closing down of the SPOs.

• Pedicle Subtraction Osteotomy

   

  Resect all the posterior elements around the pedicles with a combination of Leksell rongeurs, high-speed burr, and Kerrison punches.

   

  The pedicles are surrounded medially, laterally, superiorly, and inferiorly (TECH FIG 2A).

   

  Decancellate the pedicles and the vertebral body (TECH FIG 2B).

   

  Thin the posterior vertebral body wall with a curette until it is wafer thin. Greenstick the posterior vertebral cortex with a Woodson elevator or reverse-angled curette (TECH FIG 2C).

   

  Resect the lateral vertebral cortex with a Leksell rongeur bilaterally (TECH FIG 2D).

   

  To close the osteotomy, apply gentle downward pressure on the two segments along with compression through the pedicle screws and rods to approximate the two osteotomy edges (TECH FIG 2E).

   

  Cantilevering through the rods can also assist in osteotomy closure.

   

  Sometimes, placing more pillows/pads underneath the patient's hips and legs can extend the pelvis/hips and help with osteotomy closure.

   

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  A third rod technique can also be used. This relies on establishing midline fixation points, often through a prior fusion mass, above and below the PSO site. A rod within these fixation points can then be used to use sequential compression to bring the osteotomy to closure.

   

  We do not routinely use special osteotomy tables; however, these do exist that can be used to assist in gradual osteotomy closure.

   

  Inspect the osteotomy site to make sure that there is adequate decompression without dorsal impingement of the thecal sac or nerve roots after the osteotomy is closed (TECH FIG 2F,G).

   

   

   

  TECH FIG 2 • A. All posterior elements around the pedicles are resected. B. The pedicles and vertebral bodies are decancellated. C. The posterior vertebral cortex is greensticked with a Woodson elevator or reverse-angled curette. D. The lateral vertebral cortex is resected with Luxel bilaterally. (continued)

   

   

  An asymmetric PSO requires resection of more bone on the convex side of the deformity when creating the wedge resection so that during closure, the convex side is closed more than the concave side, allowing rebalancing of the patient in the coronal plane and sagittal plane.

   

  Not only is the posterior column resection bigger on the convexity, but the middle and anterior columns have to be resected more generously on the convexity as well.

   

  This involves turning the corner on the vertebral body and what amounts to resecting two-thirds of the

  convexity of the vertebral body both laterally and anteriorly, which otherwise would not be necessary with a standard PSO.

   

   

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  TECH FIG 2 • (continued) E. The osteotomy is closed with compression, cantilever, and extension of the chest and lower extremities. F,G. Intraoperative photographs showing the surgeon assessing for adequate decompression posteriorly at the PSO site prior to and after PSO closure to help prevent iatrogenic impingement of neural elements with PSO closure.

   

   

  PEARLS AND PITFALLS
  		Smith-Petersen osteotomy

  • Fixation points can be placed before or after the osteotomies are performed.

  • For large deformities or abnormal pedicle anatomy, performing an SPO first, prior to pedicle screw placement, can help in identifying the medial and superior borders

   

  of the pedicle to assist in locating the starting point.

  • Undercut the lamina as much as possible to remove all ligamentum flavum.

  • If possible, limit the amount of forces placed on the pedicle screws and apply forces more through the posterior elements.

     

    Pedicle subtraction osteotomy

• Resect a symmetric wedge of bone within the vertebral body to minimize the potential for coronal decompensation with osteotomy closure.

• Ensure that the ventral dura is free from the posterior vertebral cortex and that the posterior vertebral cortex is adequately thin to allow for controlled implosion of the bone into the osteotomy site.

• Attempting to greenstick the posterior vertebral cortex that is too thick may require too much force and increases the risk of a ventral dural tear.

• Remember, as the osteotomy is closed, the contour in the rods will have to change. As more closure is achieved, more lordosis is needed in the rods.

• If possible, limit the amount of forces placed on the pedicle screws and apply forces more through the posterior elements.

• With pedicle subtraction procedures, there is some risk of dural buckling and the posterior elements impinging on the dura. Our preference is to enlarge the field centrally to observe dural buckling and to “feel” the dorsal canal with nerve hooks/Woodson elevators. Watch carefully for vertebral subluxation at the osteotomy site.

• Neuromonitoring is followed for up to 1 hour after final osteotomy compression. A formal wakeup test is often done after the osteotomy closure, as neuromonitoring is at times unable to detect nerve root injury. A formal wake-up test is performed prior to leaving the operating room.

 

 

 

POSTOPERATIVE CARE

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Patients are often sent to the intensive care unit for close monitoring (for 24 to 48 hours as needed) and then transitioned to the hospital ward.

 

Patients are mobilized on postoperative day 1.

 

 

Drains are retained until recorded output is less than 30 mL per 8-hour shift. Diet is advanced slowly, with the return of bowel sounds.

 

Deep vein thrombosis prophylaxis is provided with sequential compressive devices and thromboembolic deterrent hose.

 

Avoid flexion and axial loading of the spine for at least 4 months postoperatively.

 

No cast or brace is necessary.

OUTCOMES

Studies have shown improvement (20% to 30%) in Scoliosis Research Society (SRS)-30 and Oswestry Disability Index scores in most patients at 2- and 5-year follow-up.1,2,3

Three SPOs accomplish approximately what is accomplished with one pedicle subtraction procedure.

 

The blood loss is greater with a pedicle subtraction procedure.1,5

 

 

 

COMPLICATIONS

Substantial complications associated with PSOs include neurologic deficit, substantial blood loss, and adding on to the sagittal deformity if the entire thoracic and lumbar spine is not fused.2

The neurologic risk with performing a pedicle subtraction procedure in the lumbar spine exceeds the risk associated with three SPOs. The complications associated with the procedure in older patients are more

substantial.4

A review of 108 PSOs revealed an intraoperative and postoperative neurologic deficit rate of 11.1%, with 2.8% of deficits being permanent.4

Multiple SPOs can accomplish substantial correction of major fixed sagittal imbalance. However, there is a risk of pitching the patient to the concavity because of the fact that the osteotomies are often performed through areas of residual scoliosis and the SPO shortens the concavity/posterior elements and lengthens

the convexity/anterior disc spaces.1

 

 

REFERENCES

1. Booth KC, Bridwell KH, Lenke LG, et al. Complications and predictive factors for the successful treatment of flatback deformity (fixed sagittal imbalance). Spine 1999;24(16):1712-1720.

    

    

2. Bridwell KH, Lewis S, Edwards C, et al. Complications and outcomes of pedicle subtraction osteotomies for fixed sagittal imbalance. Spine 2003;28(18):2093-2101.

    

    

3. Bridwell KH, Lewis SJ, Rinella A, et al. Pedicle subtraction osteotomy for the treatment of fixed sagittal imbalance. Surgical technique. J Bone Joint Surg Am 2004;86A(suppl 1):44-50.

    

    

4. Buchowski JM, Bridwell KH, Lenke LG, et al. Neurological complications of lumbar pedicle subtraction osteotomy: a 10-year assessment. Spine 2007;32(20):2245-2252.

    

    

5. Cho K, Bridwell KH, Lenke LG, et al. Comparison of Smith-Petersen versus pedicle subtraction osteotomy for the correction of fixed sagittal imbalance. Spine 2005;30(18):2030-2037.