Lumbar Pedicle Subtraction/Extension Osteotomy

Lumbar pedicle subtraction osteotomy (PSO) is an effective tool for correction of fixed sagittal deformity. As such, it is used primarily in the treatment of degenerative and iatrogenic kyphoscoliosis. This procedure entails shortening of the posterior column through removal of the posterior elements, both pedicles, and a wedge-shaped segment of the vertebral body with subsequent closure of the osteotomy defect. An extended PSO involves removal of the superjacent end plate and disk in addition to the standard midvertebral wedge resection. PSO is typically performed below the conus to reduce the risk of neurologic injury and maximize sagittal plane correction. In the lumbar spine, its primary effect is to restore lordosis or reverse pathological kyphosis.

In the posterior correction of sagittal malalignment, less severe (sagittal vertical axis [SVA] < 10 cm), smooth, and flexible (unfused) deformities can often be managed with multiple Smith–Petersen osteotomy (SPO); however, more severe (SVA > 10 cm), sharp angular, and/or fixed (fused) deformities may require a PSO, which can provide up to 30 degrees (or more with an extended PSO) of segmental correction in the sagittal plane and can be performed asymmetrically to address concomitant coronal plane deformity when required.

Despite advances in surgical techniques, anesthetic management, and neurophysiological monitoring, lumbar PSO carries substantial risk. In particular, the risk of neurologic injury, significant blood loss, and pseudarthrosis necessitate caution in selecting and performing this procedure.

46.2 Patient Selection

Positive sagittal malalignment frequently causes progressive pain, disability, and a reduced health-related quality of life. Restoration of global alignment is associated with improvement in these measures. Pedicle subtraction osteotomy is typically indicated in the setting of severe, fixed deformity when realignment requires significant correction in the sagittal plane. It may also be used in the setting of combined coronal and sagittal plane deformity. As a result of the associated risks and technical demands of the procedure, it is generally reserved for patients who require an additional 30 degrees or more of lumbar lordosis and those with an SVA > 10 cm that cannot be corrected by less aggressive techniques. Commonly, these patients have had a prior fusion procedure or have degenerative or inflammatory disease that has caused spontaneous fusion.

The clinical conditions where PSO can be an effective procedure include patients with flat-back deformity after Harrington rod instrumentation and fusion for idiopathic scoliosis, iatrogenic loss of lumbar lordosis during the treatment of degenerative conditions, ankylosing spondylitis, progressive adult lumbar idiopathic scoliosis, degenerative scoliosis, postinfectious deformity, and progressive kyphosis after a fracture. It may also be beneficial in a select group of patients who have had previous anterior surgery to avoid the need for a revision anterior approach with its attendant increased morbidity.

Physical examination will often reveal a forward stoop with pelvic retroversion, which results in flattening of the buttocks and hip and knee flexion. Patients typically complain of mechanical pain that is far worse when they attempt to stand or walk and relieved, at least partially, by rest. When considering lumbar PSO, a thorough medical evaluation is required as this procedure’s associated morbidity may preclude its use in some elderly patients or in those with significant medical comorbidities.

Full-length posteroanterior and lateral radiographs are essential for evaluating patients with spinal deformity. Overall sagittal balance is measured by the SVA, a measure of the distance from a plumb line dropped from the C7 centroid to the posterior superior corner of S1. The pelvic incidence (PI) determines the amount of lumbar lordosis (LL) required for harmonious spino-pelvic alignment. The pelvic tilt (PT) is a measure of pelvic retroversion and should be factored into surgical planning to avoid under-correction when it is high (> 20 degrees). These measures are necessary to quantify the severity of deformity and determine the need for PSO and the overall surgical strategy including approach, fusion levels, other osteotomies, and interbody grafts.

Computed tomography (CT) myelogram is a useful and often necessary adjunct to radiographs. This modality defines the bony anatomy of the spine, including the presence of fused segments that require an osteotomy to mobilize and effect correction. It also defines any areas of stenosis that should be addressed during surgery. Magnetic resonance imaging (MRI) of the spine provides excellent detail of the spinal cord, cauda equina and nerve roots to further aid in surgical planning. The severity and flexibility of the deformity are the key radiographic features in appropriately selecting patients for lumbar PSO. It is also important to account for any reduction in the magnitude of sagittal malalignment on supine full-length lateral radiographs or CT scout images, as this may change the decision as to whether a PSO is necessary or whether one or more SPOs may suffice.

46.3 Preoperative Preparation

As outlined herein, radiographic evaluation determines both the need for PSO and the degree of correction required. In general, a PSO performed lower in the lumbar spine results in greater overall correction of positive sagittal malalignment and a larger improvement in pelvic retroversion. Preferably, the level is below the conus medullaris to reduce the risk of spinal cord injury. This permits sufficient retraction of the thecal sac to safely perform the osteotomy. The level chosen is most frequently L2, L3, or L4 since these most closely approximate the apex of lumbar lordosis in a sagittally balanced spine: the L3–4 disk space. Performing the PSO at L2, L3, or L4 also allows for multiple points of fixation below the osteotomy (a minimum of four fixation points inferior to the osteotomy is ideal, for example, bilateral pedicles of L4 and L5 for an L3 PSO). Placing the apex of the resected wedge more anteriorly or widening the posterior aspect of the wedge increases the amount of correction achieved during closure of the osteotomy.

Neurophysiological monitoring is essential for performing deformity correction because of the risk of traction on nerve roots or the spinal cord. Use of multiple modalities maximizes sensitivity and specificity. The risk of neurologic injury is highest during closure of the osteotomy. Therefore, this stage should be performed in a stepwise fashion with frequent monitoring checks.

46.4 Operative Procedure

While the patient is being positioned on the Jackson table (OSI, Union City, California), all pressure points should be well padded. The neurophysiological monitoring we use includes somatosensory evoked potential, transcranial motor evoked potential, and free-running electromyography. The PSO procedure can be associated with substantial intraoperative blood loss. The use of antifibrinolytics such as tranexamic acid can reduce blood loss and associated complications.

The shoulders and elbows should not be extended greater than 90 degrees when placing the patient in a prone position, with both arms on the arm rest by the patient’s head. The hip pads should be placed as low as possible while still padding the anterior–superior iliac spine to maximize extension through the lumbar spine and can be lowered further during closure of the osteotomy.

The first step involves meticulous exposure of the bony anatomy. A minimum of two levels above and two levels below where the osteotomy is to be performed is exposed, after which pedicle screw fixation points are placed throughout the indicated fusion levels with the exception of the PSO level itself. The goal should be a “harmonious” placement of pedicle screws to facilitate the application of a contoured rod. Careful attention should be placed in localizing the pedicle, especially in patients with associated coronal plane deformity and those with fusion mass obscuring the normal anatomic landmarks. Intraoperative fluoroscopy or image guidance may be used to place the pedicle screws.

A wide laminectomy is performed at the level of the osteotomy. Partial laminectomies are also performed at the levels above and below the planned osteotomy level. Bilateral facetectomies above and below the pedicle are then performed, thus exposing the nerve roots running inferiorly and superiorly. The transverse process is disconnected from the lateral aspect of the pedicle with a narrow Leksell rongeur (V. Mueller, Germany). In essence, complete posterior bony removal is performed from the level of the pedicle above to the pedicle below the level of the osteotomy.

After isolation of the pedicles, the Cobb elevator (Codman, Raynham, Massachusetts) is used in the subperiosteal plane to gently reflect the psoas muscle and soft tissue off the lateral aspect of the pedicle and vertebral body, attempting to avoid injuring the segmental vessels or the exiting nerve roots. The medial pedicle wall is delineated, and the thecal sac is protected with a nerve root retractor. Gentle traction may be applied with the retractor on the thecal sac if the level of the osteotomy is below the conus. The nerve root above the pedicle is also protected with a retractor or a Penfield no. 4.

The residual pedicle stump is removed with a Leksell rongeur (V. Mueller, Germany) until it is flush with the vertebral body ( ▶ Fig. 46.1). Pedicle decancellation is performed through the residual pedicle stump with straight and curved osteotomes. Bony removal is extended in a wedge-shaped fashion into the vertebral body with the apex at the anterior cortex ( ▶ Fig. 46.2). The lateral bony wall is removed next with an osteotome or Leksell rongeur without violating the anterior cortical wall ( ▶ Fig. 46.3). Alternatively, the pedicle can be removed by performing three successive osteotomies: (1) a right-angle osteotome is used with the corner placed at the inferomedial aspect of the pedicle, (2) a straight osteotome placed above the pedicle, and (3) a straight osteotome medial to the pedicle. The first two osteotomies are positioned such that they meet at the anterior aspect of the vertebral body, thus initiating the wedge to be removed with the pedicle.

Fig. 46.1 Pedicle resection at the lumbar spine level, with a nerve root retractor protecting the thecal sac. Note the complete pars to pars bony resection before pedicle removal.

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