Pedicle Subtraction Osteotomy

Introduction

Lumbar pedicle subtraction osteotomy (PSO) was first described by Thomasen in 1985 for the surgical treatment of 11 patients with severe disabling kyphosis secondary to ankylosing spondylitis. In this series, Thomasen performed posterior bony wedge resection (i.e., removal of the spinous process, neural arch, and posterior part of the index vertebral body) and subsequent osteotomy closure at the L2 vertebra to achieve the desired kyphosis correction. Since then, spine surgeons have used PSOs for deformity correction in pathologies other than ankylosing spondylitis such as congenital, posttraumatic, postinfectious, neoplastic, degenerative, idiopathic, or iatrogenic deformities. In this chapter, we review PSO in the context of revision lumbar spine surgery. We highlight iatrogenic flatback syndrome as a common cause of fixed spinal deformities that may require a lumbar PSO. We provide revision spine surgery case examples that illustrate lumbar PSO correction of iatrogenic flatback deformity resulting from pseudarthrosis with a loss of sagittal correction ( Fig. 18.1 ) and from prior distraction instrumentation ( Fig. 18.2 ).

Regardless of the specific etiology, rigid or fixed spinal deformities can be challenging to treat surgically and a PSO may be indicated. Briefly, a lumbar PSO involves performing a three-column osteotomy with transpedicular bony wedge resections extending from the posterior elements along both pedicles and into the vertebral body. The anterior vertebral cortex is left intact so that it can act as a hinge during subsequent closure of the wedge osteotomy defect. A considerable amount of surface area is available for osseous union after wedge closure brings the bony surfaces of the anterior, middle, and posterior columns in contact. Additionally, osteotomy wedge closure can shorten the posterior column without changing the length of the anterior column, which optimizes healing potential without stretching major abdominal vessels and viscera. After closure of the posterior wedge, two nerve roots exit on each side at the vertebral level of the osteotomy through the newly formed artificial neural foramina.

The traditional or classic PSO is categorized as a Schwab grade 3 osteotomy ( Table 18.1 ). A lumbar PSO allows for approximately 30 degrees of correction across the osteotomy level with a similar magnitude of corrective improvement in overall lumbar lordosis (LL); this can also result in up to 10 cm of posterior trunk translation (i.e., reduce global sagittal malalignment). Variations of the traditional PSO include the asymmetric PSO for biplanar correction (coronal as well as sagittal correction), as well as the extended PSO (Schwab grade 4 osteotomy) which allows more alignment correction by extending the wedge resection into the superior adjacent disc. Placement of an interbody cage at the osteotomy site may allow further correction when an extended PSO is performed. When performed appropriately, a lumbar PSO and its variants are powerful techniques that allow significant restoration of LL and global alignment in patients with severe fixed deformities. In addition, treatment of any potential underlying pseudarthrosis, which is often problematic during revision lumbar cases (e.g., iatrogenic flatback), can translate into improved pain and health-related quality of life (HRQL) scores in adequately aligned patients. Despite these positive outcomes, there are high complication risks associated with a PSO. Recent multicenter studies by the International Spine Study Group (ISSG) have advanced our understanding of the complication profile associated with this powerful but potentially morbid technique. After describing relevant patient indications, operative planning pearls, and the appropriate technique for a lumbar PSO, we conclude this chapter with a brief overview of these associated complications and emerging strategies to limit their occurrence.

Table 18.1

Schwab Classification of Spinal Osteotomies

Grade of Resection	Brief Description of Osteotomy
Grade 1: partial facet joint resection	Resection of inferior facet and joint capsule
Grade 2: complete facet joint resection	Resection of inferior and superior facets of an articulation
Grade 3: pedicle and partial body resection	Resection of posterior elements with pedicles Partial wedge resection of posterior vertebra
Grade 4: pedicle, partial body and disc resection	Resection of posterior elements with pedicles Wider wedge resection of posterior vertebra to remove end-plate and portion of adjacent disc
Grade 5: complete vertebra and discs resection	Complete resection of vertebral level and both adjacent discs
Grade 6: multiple adjacent vertebrae and discs resection	Resection of several adjacent vertebral levels

Patient Selection and Operative Indications

In general, a lumbar PSO is indicated for revision spine surgery if significant correction is needed for a severe fixed spinal deformity. Patients with severe fixed spinal deformities causing positive sagittal imbalance (i.e., pitched forward with head anterior to pelvis) may present with early fatigue and disability from difficulty maintaining an erect posture and horizontal gaze. Even mildly positive sagittal imbalance can be detrimental to HRQL, and the severity of clinical symptoms increases linearly with progressive sagittal imbalance. The debilitating impact on health from adult spinal deformity can exceed the disability of more recognized chronic diseases. For example, patients with lumbar scoliosis combined with severe positive sagittal imbalance demonstrated worse HRQL scores than patients with limited use of their arms and legs. In addition to an observable “forward pitch” from positive sagittal imbalance, clinicians should be also be aware of potential compensatory mechanisms that these patients often use to partially mitigate global imbalance (e.g., knee flexion, hip extension, pelvic retroversion, increased cervical lordosis). Compensatory pelvic retroversion for sagittal spinal malalignment is measured radiographically as pelvic tilt (PT), and high values of this variable correlate with worse HRQL. Finally, patients with severe fixed deformities may also complain of intractable back pain exacerbated by ambulation or upright posture, and partly relieved with rest or recumbency.

For select patients with rigid deformities at locations where previous anterior spinal surgery was performed, a PSO remains an attractive treatment option if a high magnitude of correction is needed. Many studies have assessed lumbar PSOs with focus on sagittal plane correction, and have suggested the following potential indications: fixed deformities with sharp or angular kyphosis and sagittal vertical axis (SVA) greater than 8 cm, high spinopelvic mismatch requiring more than 30 degrees restoration of LL but insufficient anterior curve flexibility for effective multilevel posterior column osteotomies (PCO; Schwab grade 2), and severe deformities of high enough magnitude which exceed the corrective capacity of multilevel PCOs. Potential PSO patients may have a history of prior fusion surgery, degenerative or inflammatory disease resulting in spontaneous fusion with sharp angular kyphosis, ankylosing spondylitis with thoracolumbar kyphosis, posttraumatic vertebral fractures, and/or progressive untreated idiopathic scoliosis. Because this review series focuses on revision lumbar spine surgery, we highlight iatrogenic flatback syndrome as a common cause of fixed deformity that may warrant a lumbar PSO. Significant contributing factors to iatrogenic flatback deformity have been identified and include distraction instrumentation in the lower lumbar spine or sacrum (e.g., Harrington distraction instrumentation), pseudarthrosis resulting in loss of sagittal correction, improper patient positioning causing reduced LL during transpedicular instrumentation, and segmental distraction for “foraminal enlargement” or during interbody device or graft placement. We provide revision case examples illustrating PSO correction of iatrogenic flatback deformity (see Figs. 18.1 and 18.2 ). Of note, in addition to being indicated for patients with fixed positive sagittal malalignments, a PSO may also be indicated for patients with a pelvic incidence (PI) to LL mismatch who remain globally sagittally aligned. In a study from Smith et al. and the ISSG, this form of compensated flatback syndrome was shown to be a significant source of pain and disability and demonstrated similar HRQL improvement after surgical correction compared with deformity correction surgery on patients with positive sagittal malalignment.

The magnitude of correction necessary for achieving optimal outcomes varies with each deformity patient, and factors such as preoperative health status, medical comorbidities, and baseline global or regional alignment are important surgical considerations. Surgeons should also consider preoperative segmental curve flexibility and the compensatory ability of postoperative adjacent unfused segments. Curve flexibility can influence surgical planning when considering the approach (anterior, posterior, lateral), number of spinal levels to fuse, and the type of corrective osteotomy. Deformities with more than approximately 30% correction on dynamic radiographs are generally considered more flexible and less likely to require a three-column osteotomy such as a PSO. Curves with less than 30% correction on dynamic imaging, however, may generally be considered fixed or rigid and are more likely to require a three-column osteotomy.

Operative Planning

There are many factors to consider when planning a PSO for revision lumbar spine surgery. First, the amount of necessary correction should be considered. This includes both focal correction at the osteotomy wedge and the ensuing regional and global alignment changes that it can produce. A subset of patients with fixed sagittal deformity may have positive sagittal imbalance indicated by increased SVA (horizontal offset from the center of the C7 vertebral plumb line to the posterior-superior border of the S1 vertebra on standing 36-inch long-cassette radiograph). Although definitive ranges for normal versus pathological SVA are debated, the Scoliosis Research Society and Schwab et al. classified SVA as follows: normal SVA (<4 cm), moderately increased SVA (4–9.5 cm), and severe positive SVA (>9.5 cm). Studies have demonstrated positive SVA and progressive sagittal imbalance correlate with worse pain and disability. Therefore restoration of more normal sagittal balance is critically important in reconstructive spinal surgery. Like SVA, T1-spinopelvic inclination (T1-SPI, the angle between the vertical plumb line and the line drawn from the vertebral body center of T1 and the center of the femoral heads) is another global spinopelvic parameter that refers to truncal inclination, but has been relatively underused despite having strong correlation to HRQL outcome measures.

In addition to SVA and T1-SPI, other studies have emphasized the importance of pelvic parameters, including PI and PT, in the assessment of sagittal spinal alignment. Abnormal pelvic retroversion (increased PT) may help mitigate positive sagittal malalignment, but can adversely affect ambulation and increase energy utilization, which may negatively impact HRQL. Based on LL and these spinopelvic parameters, Schwab et al. provided evidence to suggest potential threshold values for predicting severe disability (Oswestry Disability Index [ODI] >40) that could help guide clinical assessments for therapeutic decision-making. Determined threshold values for severe disability included: SVA more than 4.7 cm, PT more than 22 degrees, and PI-LL mismatch greater than 11 degrees. Therefore, when planning a PSO for revision lumbar spine surgery, achieving more harmony among spinopelvic parameters with low SVA and PT is a critical goal. Reasonable postoperative target alignment includes the following: SVA greater than 5 cm, PT greater than 25 degrees, and LL proportional to the PI (e.g., PI-LL mismatch <10 degrees). However, it is important to recognize that these realignment target values should be adjusted for age, with older patients having less stringent alignment targets.

Mathematical Calculations for Pedicle Subtraction Osteotomy Correction and Postoperative Sagittal Vertical Axis

Preoperative surgical planning is important to determine whether a PSO can effectively restore appropriate sagittal spinal alignment. When performing a PSO, placing the apex of the resected wedge more anteriorly and/or widening the posterior aspect of the wedge increases the amount of correction achieved during closure of the osteotomy. Based on proposed target alignment goals for spinopelvic mismatch (from Schwab et al. ), a simple method for preoperative planning is to estimate the amount of LL restoration needed and then perform the PSO accordingly. This method is limited because it does not account for global alignment changes (e.g., postoperative SVA), which can lead to an undercorrection and poorer clinical outcomes. To address this problem, various studies have proposed mathematical calculations to predict global alignment changes after a lumbar PSO. Some of these mathematical formulas used the tangent function to calculate the size of the desired wedge osteotomy resection. However, these trigonometric formulas were limited by a lack of input variables assessing the pelvis and unfused spinal segments (e.g., reciprocal alignment changes in the thoracic spine). To improve postoperative SVA prediction accuracy, the “Lafage formulas” were devised to include dynamic changes that occur in the pelvis and unfused spinal segments after a lumbar PSO. In another study that supported inclusion of pelvic variables when planning a PSO, Smith et al. compared available mathematical formulas and concluded that the formulas by Lafage et al. demonstrated the greatest accuracy predicting postoperative SVA after PSO adult reconstructive surgery.

Level of Pedicle Subtractive Osteotomy and Segmental Correction

An important decision when planning a PSO for revision lumbar spine surgery is choosing the appropriate osteotomy level. This decision can be complex and may be influenced by the experience and preferences of the surgeon. Many surgeons prefer to perform the PSO in regions distal to the conus medullaris (to reduce risk of neurological injury) and in sites where the spine has been previously fused. Accordingly, the lumbar PSO procedure has been commonly described for the L3 and L4 vertebral levels, with mean LL and SVA correction of approximately 30 degrees and 10 cm, respectively. The anatomical features and location of the L3 vertebra can facilitate performing a PSO with less technical difficulty in comparison to more caudal regions in the lumbar spine. Of note, nonphysiological corrections focused too cephalad in the lumbar spine can be associated with higher complication risks. Therefore we highlight that appropriate distribution of LL restoration via “harmonious” correction is critical and has been well described by Roussouly. For example, a patient with a high PI may have a more cephalad apex of LL, prompting the surgeon to possibly choose this level (or adjacent level) for the PSO. In many cases, the apex of kyphotic deformity should orient the surgeon to choose the appropriate PSO level; however, careful planning is still recommended to prevent nonphysiological, nonharmonious LL restoration.

More recently, some surgeons have reported results from a single-level L5 PSO. For example, Alzakri et al. demonstrated improvement in mean LL from 22.5 degrees (range: 8 to 33 degrees) corrected to 58.5 degrees (range: 40 to 79 degrees), and improvement in mean SVA of 13.7 cm (range: 3.5 20 cm) corrected to 4.6 cm. The authors highlighted the technically challenging lateral dissection along the L5 body because of its pedicle orientation, the shallow vertebral body height, and significant vascular environment. Although more technically demanding, this caudal level of PSO may allow for more PT correction. Of note, a recent study by Lafage et al. demonstrated that a more caudal PSO level was correlated with a greater reduction in PT. However, no correlation was demonstrated between the PSO level and the SVA correction.

Reciprocal Changes in Unfused Spine

As previously discussed in the section on mathematical PSO calculations and the Lafage formulas, another significant contributor to be considered during surgical planning are the expected reciprocal changes in the alignment of unfused spinal segments. These reciprocal alignment changes are compensatory effects of reconstructive spinal surgery, and are more likely when major deformity correction spans fewer segmental levels (e.g., single-level lumbar PSO). Lafage et al. demonstrated the negative impact of increased reciprocal thoracic kyphosis on postoperative SVA after lumbar PSO and noted that these are not simply caused by junctional failures.

Choosing Proximal and Distal Fixation Levels for Lumbar Pedicle Subtractive Osteotomy

If there is a coexisting flexible thoracolumbar kyphosis, the instrumentation can be extended proximally in the thoracic spine with the possible use of multilevel PCOs, if needed. However, in the setting of coexisting rigid thoracolumbar kyphosis, an additional PSO can usually be performed at the apex of the cervical or thoracic kyphosis. Regarding distal fixation, a sufficient instrumentation construct to biomechanically support the extensive focal correction created by a lumbar PSO should ideally have a minimum of four points inferior to the PSO level (e.g., bilateral pedicles of L4 and L5 after an L3 PSO). One of the important factors that guides the overall success of the surgery is whether to extend the fusion across the lumbosacral junction. The status of the lower lumbar spine discs and magnitude of the lumbosacral fractional curve impacts this decision-making process. Of note, many patients undergoing revision lumbar spine surgery likely have concomitant advanced lumbar degeneration and a high risk of lumbosacral breakdown. Therefore extension of instrumentation to the pelvis is generally recommended. In our experience, we typically extend instrumentation to the pelvis with bilateral iliac screw fixation after performing a lumbar PSO.

Surgical Technique

Performing a lumbar PSO is challenging and requires meticulous attention to detail and technique. We summarize the critical steps for this procedure in Table 18.2 . First, the patient is positioned prone on an open-frame Jackson table with arms outstretched approximately 90 degrees. Given the potential for long operative duration, the surgeon must ensure sufficient padding of all contact points and bony prominences to avoid complications such as pressure ulcers or compression neuropathies (e.g., ulnar neuropathy). The abdomen should hang freely within the confines of the open-frame Jackson table. Avoiding abdominal compression can reduce epidural venous pressure, which helps reduce surgical blood loss. The patient should be positioned in slight reverse Trendelenburg (approximately 15 degrees) to reduce intraocular pressure and limit the risk of postoperative vision loss. The chest pad and hip/thigh pad should be positioned such that optimal correction of lordosis can be achieved when compressing across the PSO. When performing a PSO at lower levels such as L5, the hip pads may be placed further distal leaving the anterior superior iliac spine free to allow pelvic anteversion. This may help correct potential sacropelvic pathology often associated when performing a PSO at these caudal levels. Neuromonitoring leads may be placed to monitor motor-evoked potentials, somatosensory-evoked potentials, and electromyography. Intraoperative fluoroscopy can be used to identify the appropriate spinal levels when planning the incision and should remain in the room for subsequent localization after subperiosteal exposure. Of note, patients may have significant improvement in LL after positioning, which is important to consider when planning surgical correction.

Table 18.2

Summary of the Critical Steps for Single-Level Lumbar Pedicle Subtraction Osteotomy

Steps to Perform Lumbar Pedicle Subtraction Osteotomy

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Meticulous exposure of the posterior elements of the spine with care to maintain hemostasis.
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Transpedicular instrumentation at least two levels above and below the index osteotomy level, with likely pelvic extension for increased biomechanical support.
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Perform Smith-Petersen osteotomies above and below the osteotomy level.
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Begin the pedicle subtraction osteotomy by first resecting the posterior elements bilaterally, including the lamina, facets, and pars interarticularis, thereby exposing the pedicles.
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Use a drill and rongeur to decancellate and resect the pedicle bilaterally to the level of the vertebral body while protecting the thecal sac and nerve root.
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Use an osteotome or high-speed drill to create a wedge-shaped, triangular resection; do not violate the anterior cortex of the vertebral body; temporary rods can be placed.
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Enlarge the vertebral wedge resection using a rongeur or osteotome thereby extending the resection to involve the lateral walls of the vertebral body and remove any bone fragments.
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Remove the posterior cortical wall ventral to the dura by using a curette to collapse it into the decancellated cavity.
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Compress across the pedicle screws at the involved level to close the wedge.
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Osteotomy. A cantilever maneuver can be used to achieve a complete, symmetric closure (temporary rods previously placed can help with this step, followed by permanent rod placement).
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Confirm no nerve root or thecal sac compression and observe for neuromonitoring changes.
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Perform posterolateral decortication and arthrodesis with application of harvested bone graft.