Sagittal Imbalance: Multiple Smith-Petersen Osteotomies Versus Pedicle Subtraction Osteotomies

Scoliosis is a complex three-dimensional rotational deformity affecting the spine in the coronal, sagittal, and axial planes. Treatment paradigms should address all three components of this disorder. Surgical treatment of spinal deformities may be accomplished through anterior, posterior, or combined approaches for correction. Fixed thoracic or lumbar deformity is a complex surgical problem that has been addressed by anterior release and posterior correction with fusion. An osteotomy carried through all three columns of the spine offers the advantage of avoiding anterior exposure. The posterior-only approach itself offers many options. Surgical correction through this approach includes various osteotomy techniques. The inferior facetectomy provides limited corrective ability, but the Smith-Petersen osteotomy (SPO), or a modification of this technique, provides comparatively more correction. Techniques with larger corrective potential include the pedicle subtraction osteotomy (PSO), the extended pedicle subtraction osteotomy, and the vertebral column resection (VCR).

In recent years, posterior-only approaches for treatment of deformity have provided the framework for surgical techniques for correction in the sagittal and coronal plane. Segmental fixation with pedicle screws and rods, and the use of osteotomies through a posterior approach, preclude the need for anterior releases or anterior corpectomies. Addressing deformity through a posterior-only approach avoids the morbidity of an anterior approach while obtaining correction equivalent to that achieved using a combined anterior-posterior approach. Before any osteotomy is attempted on the spine for correction of deformity, sufficient surgical experience and a thorough understanding of the anatomy of the spine and regional structures are imperative.

Osteotomies are performed to treat the following conditions: junctional kyphosis, ankylosing spondylitis, posttraumatic kyphosis, postlaminectomy kyphosis, idiopathic kyphoscoliosis, degenerative scoliosis, Scheuermann kyphosis, iatrogenic flat back, neuromuscular kyphosis, and congenital kyphosis. Patients are often classified as having either flexible or fixed deformity. Flexible sagittal deformity implies change in the sagittal vertical axis when gravity is removed as a variable (i.e., the patient is placed in a recumbent position). A flexible coronal deformity is not rigid, so that radiographs taken in the side-bending position demonstrate movement. Patients with a fixed sagittal deformity may have a subjective perception of imbalance. They may lean forward, have a stooped posture, develop early fatigue, experience intractable pain, and be unable to maintain a horizontal gaze. In cases of flexible sagittal imbalance or mild sagittal deformity, correction can be accomplished through positioning on the surgical table and segmental instrumentation. However, an osteotomy is often needed for correction in cases of rigid or fixed deformity. The goal is to restore sagittal balance so that the patient can stand without flexion at the hips or knees for compensation and to reduce pain with ambulation.

Case Presentation

Case 1

A 48-year-old woman developed progressive pain at the area of a previous lumbar fusion as well as worsening sagittal imbalance.

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PMH: Unremarkable
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PSH: The patient previously underwent an L3-S1 posterior lumbar interbody fusion at another institution. She also had facet arthropathy from T12 to L3.
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Exam: Neurologically normal except positive sagittal balance
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Imaging: Preoperative standing lateral 14 × 36 inch plain films demonstrating positive sagittal balance and lumbar hypolordosis

The patient underwent corrective surgery with removal of her segmental instrumentation from L3 to S1, exploration of her fusion from L3 to S1, placement of segmental instrumentation from T10 to S1, and performance of an L2 PSO for correction of kyphosis ( Figures 18-1 and 18-2 ).

Case 2

A 69-year-old man had progressive lower back pain and bilateral radiculopathy. He has been unable to stand fully erect or ambulate without severe back pain. He has failed conservative management. He also has noted weakness of 4 out of 5 strength in his left dorsiflexion.

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PMH: Unremarkable
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PSH: Unremarkable
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Exam: The patient also had a slight left foot drop.
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Imaging:

The patient underwent surgery that included placement of segmental instrumentation from T10 to S1/ilium, SPOs at T12-L1, L1-2, L2-3, L3-4, L4-5, and L5-6, and transforaminal lumbar interbody fusion at L6 to S1 for correction of kyphosis ( Figures 18-3 through 18-6 ).

Surgical Options

Preoperative workup is important for all patients undergoing osteotomies. Patients may be evaluated preoperatively by an internist, family physician, and/or a cardiologist depending on medical comorbid conditions. Patients often undergo computed tomography (CT) of the spine covering the regions to be included in the intended fusion to evaluate for bone integrity. Thin-cut CT scans with sagittal reconstructions are valuable in accurately identifying ankylosed regions of the spine, both in the anterior and middle columns as well as along the facet joints. Standing long-cassette anteroposterior (AP) and lateral radiographs are obtained for all patients. Also, initial workup includes assessment of the flexibility of the spine. Spinal rigidity is assessed based on standing and supine radiographs. For sagittal deformity, this is accomplished by obtaining long-cassette AP and lateral radiographs with the patient standing, supine, prone, or supported by a bolster. For coronal deformity, lateral side-bending radiographs are also obtained. If the deformity is fixed and no correction is observed with positional change, or if it is partially fixed with some degree of correction occurring through adjacent mobile segments, then an osteotomy may be appropriate.

Patient risk and intraoperative morbidity, including blood loss from surgery, are weighed when deciding what corrective osteotomy will be advantageous for a patient with coronal and/or sagittal imbalance. The larger corrective osteotomies, although more powerful, are associated with increased technical demands, longer operative time, greater blood loss, and procedure-related morbidity. The VCR, in particular, carries the greatest risk to the patient in terms of possible neurologic injury, operative time, and potential morbidity, but achieves the most correction in the coronal and sagittal planes. This chapter focuses on the SPO and the PSO.

Fundamental Technique

Smith-Petersen Osteotomies

In 1945, Smith-Petersen and colleagues first described the technique of posterior-element osteotomy and posterior compression. It was principally used for treating ankylosing spondylitis. This technique also was used to treat flexion deformities in individuals with rheumatoid arthritis and ankylosed or autofused spines. They resected the spinous processes, removed the edges of the laminae, resected the ligamentum flavum, and performed oblique osteotomies with an osteotome forward and upward through part of the inferior and superior articular processes (the synostosed articular processes). Correction through rupture of the anterior tension band results in profound anterior lengthening. The middle column or posterior part of the annulus serves as the pivot or fulcrum, and because the wedge is closed posteriorly, the disk space opens anteriorly. The SPO involves removal of the posterior ligaments (supraspinous, intraspinous, and ligamentum flavum) and resection of the facets at one or more levels to produce a posterior release, which aids in coronal correction and sagittal plane realignment. The osteotomies either hinge or slightly shorten the middle column. Also, producing an anterior gap may make arthrodesis less reliable. Rupture of the anterior longitudinal ligament carries the risk of injury to vascular structures anterior to the spine. The lengthening of the anterior column can be associated with vascular and neurologic complications. This may be especially true in the elderly or in patients with ankylosing spondylitis, who may have significantly calcified vessels, including the aorta and the vena cava. Modifications have been suggested to avoid such complications. Law described a modification of the corrective lumbar osteotomy for ankylosing spondylitis, but found it unnecessary to divide the anterior longitudinal ligament.

Hehne and colleagues also described a multisegmented osteotomy that achieved lordosis through resection of a portion of the posterior elements at each level, producing about 10 degrees per segment. The lordosis correction was described as “harmonious.” In addition, Ponte and associates further elaborated on the use of wide segmental osteotomies and posterior compression along unfused regions of the spine in patients with Scheuermann kyphosis. Ponte advocated a posterior-only procedure consisting of posterior column shortening via multiple wide segmental osteotomies and posterior compression with instrumentation and fusion. This modification of the SPO did not require anterior osteoclasis. They were developed for a gentle correction at multiple levels using the same technique. This method gave a more overall correction from the closing of the dorsal osteotomy without fracturing the anterior column.

Although the name Smith-Petersen osteotomy seems to encompass the original osteotomy technique involving posterior column resection and anterior osteoclasis and also the modification, the technique most commonly used today (modification) is referred to in this chapter. Multiple SPOs have been useful for treating fixed sagittal imbalance in conditions like ankylosing spondylitis and iatrogenic sagittal imbalance. Compression of an SPO after transpedicular fixation can correct kyphosis. An adequate intervertebral disk height and a mobile disk space anteriorly add to the corrective potential.

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Compression can lead to narrowing of the neural foramina, which necessitates a preceding wide facetectomy to prevent nerve root impingement.

However, correction may be challenging if there is a fully ossified anterior longitudinal ligament. PSOs have also been described for the treatment of these conditions.

Indications for SPO include ample intervertebral disk height anteriorly and laterally, a mobile disk without ankylosis, and a smooth curve. The SPO requires disk space flexibility to allow for coronal and/or sagittal correction. A long, rounded, smooth kyphosis is often an ideal target for multiple SPOs. The classic indication for an SPO is a long, gradual, rounded kyphosis as in Scheuermann kyphosis. The surgeon may consider another type of osteotomy if there is a thick bridging anterior osteophyte. Although an anterior gap may be present after an SPO, there is no need for an anterior bone graft. Coronal and sagittal imbalance may be addressed with an SPO; it affords gradual correction of kyphotic or scoliotic curves. The degree of kyphotic correction afforded by an SPO has been reported to be in the range of 9.3 to 10.7 degrees per level, with approximately 1 degree per millimeter of bone resected. Technically, these osteotomies can be used at multiple levels within a fusion construct and offer powerful correction globally across a kyphotic segment. However, focal regions of kyphosis may be amenable to use of other osteotomies for correction. If there is a fine bony ridge anteriorly, it can potentially be broken with closed reduction. After closure of an SPO, there is bone on bone of the posterior column centrally and laterally.

At the time of the surgery, patients are appropriately padded and positioned prone on a Jackson table. This permits the patient’s abdomen to float freely and allows gravity to assist in pulling the lumbar spine back into lordosis. Lordosis can be achieved by using an open Jackson table so that the abdomen hangs free. This is particularly helpful for nonrigid or flexible sagittal imbalance. Placing additional height on the chest pad helps to achieve further lumbar lordosis on the table. Intraoperatively, you obtain lordosis based on your correction technique. Neurophysiologic monitoring with measurement of somatosensory evoked potentials and transcranial motor evoked potentials is often used for SPOs (and routinely for PSOs). Normotension is always rigorously maintained throughout surgery to ensure adequate spinal cord perfusion. Patients typically have an arterial line and a central venous line in place to allow for blood pressure monitoring and volume resuscitation during surgery. The coagulation profile is closely monitored intraoperatively by the anesthesiologist. The incision is made and subperiosteal dissection is performed to expose the posterior spinal elements. If needed, existing segmental instrumentation is removed, and the old fusion mass is explored if present. Pedicle screws are placed at all levels involved in the fusion. For completion of an SPO, the lamina and facet joints are removed completely using an osteotome, Kerrison rongeur, or high-speed drill in an oblique manner at the desired level. A V-shaped gutter is created. ³The width of the gutter should be between 10 and 15 mm. Correction is performed by compression of the posterior elements until closure. Compression should be performed gradually over multiple segments at the same time to redistribute the corrective forces over a large area of the spinal column. Rods are set and decortication is performed before wound closure. A cross-table scoliosis radiograph is taken at the end of the procedure to ensure that adequate correction has been obtained.

Pedicle Subtraction Osteotomy

In 1963, Scudese and Calabro described a vertebral wedge osteotomy for the correction of lumbar kyphosis in a patient with ankylosing spondylitis. They removed the back part of the upper surface of the body of a lumbar vertebra and wedged the disk space and body with posterior narrowing. This led the way to the technique known today as the pedicle subtraction osteotomy. The PSO was first described by Thomasen in 1985. This osteotomy has also been referred to as a transpedicular wedge procedure, closing wedge osteotomy, and eggshell osteotomy. The technique involves removal of the posterior ligaments and facets—as in an SPO—followed by bilateral pediculectomy and decancellation of a wedge of the vertebral body via a transpedicular corridor. A resection that includes the disk space above the decancellated segment is described as an extended PSO. Closure of this osteotomy occurs in wedge fashion, which brings about kyphosis correction through posterior shortening. If performed in an asymmetric fashion, the osteotomy can also lead to significant coronal correction. This closure also creates a large contact area of cancellous bone, which is beneficial for fusion of the vertebral body. The technique also creates a superforamen. Segmental correction by a PSO depends on the level at which the osteotomy is performed.

For a PSO, pedicle screws are placed above and below the level of the osteotomy.

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