Initial Evaluation

A thorough preoperative evaluation must be carried out before revision spine surgery is considered and requires a complete history and detailed general and neurological examination. Special attention is directed toward detailing the patient’s symptomatology before and after each previous spine operation. In particular, symptom-free periods or exacerbation of symptoms may indicate recurrent pathology such as disk herniation or failure of instrumentation. A lack of any symptom-free period may indicate residual or persistent pathology that was not fully addressed during the primary operation. Detailed records of each previous operation, including operative reports and preoperative and postoperative physical examinations, should be reviewed. Identification of the specific instrumentation construct used is essential to facilitate later removal if necessary.

Radiographic evaluation is made more difficult by previous spine operations. Plain anteroposterior (AP) and lateral radiographs provide the clinician with a great deal of anatomic information, as well as data on preexisting spinal instrumentation constructs. Dynamic radiographs such as flexion-extension films are very helpful for investigating the stability of the spinal column and for determining the integrity of instrumentation constructs and bone fusions. Standing 36-inch AP and lateral radiographs taken with both the hips and knees extended are useful in assessing the sagittal and coronal balance of the spinal column, especially in patients with spinal instability and deformity. Evaluation of bone anatomy and surgical instrumentation is best accomplished with computed tomography (CT) and plain radiography. The information provided by high-resolution, multiplanar CT is helpful when revising previous instrumentation constructs because it can serve as a surgical “road map.” This is particularly important during the revision of pedicle or lateral mass screws when planning new screw entry points and trajectories. High-resolution CT also aids in the assessment of spinal fusion, especially when combined with the information from dynamic radiographs; however, the “gold standard” for assessment of fusion remains open surgical exploration.^8,9 Magnetic resonance imaging (MRI) is the most useful study for evaluating the neural elements, ligamentous structures, and intervertebral disks. MRI with gadolinium enhancement is often used for the differentiation of recurrent herniated disk material from epidural scar tissue. Frequently, in patients with spinal instrumentation, imaging artifacts significantly limit detailed MRI evaluation of closely adjacent structures. Myelography, often accompanied by postmyelographic CT, is helpful in determining the presence of thecal sac or nerve root compression in cases in which MRI of the neural elements is limited by artifact.

The clinician must synthesize all available preoperative information and define the pathology that is amenable to surgical intervention. It is paramount to correlate the pathologic radiographic findings with the patient’s symptomatology and physical examination. Proper patient selection and good clinical outcomes rely on close correlation among symptoms, neurological findings, and surgically correctable pathology.

General Surgical Principles

Reoperations on the spine require a thorough understanding of general surgical principles, normal anatomy, and wound healing. If the reoperation is being carried out through the same approach, normal anatomic planes are disrupted and the surgeon may lose orientation because the expected anatomic landmarks are disturbed or have previously been excised. The surgical exposure should be extended beyond the margins of the previous operation and into normal areas in either a rostral-caudal or medial-lateral direction to allow the surgeon to work from the normal anatomy into the scarred and altered anatomy of the previous operative field. This technique is essential for maintaining proper orientation and allows the surgeon to dissect scar tissue from the bony and neural elements by using the adjacent normal anatomy as a point of reference. Use of visual augmentation techniques, such as the operating microscope or loupes, facilitates the surgeon’s visualization during surgical dissection and exposure. Sharp dissection, such as with an up-biting or forward-angle curet, greatly aids in dissection of scar tissue from both the neural and bony elements and reduces the risk of dural violation or injury to the neural elements during surgical exposure and decompression.

Attention to the integrity of superficial tissues is essential to decrease the risk for postoperative wound complications such as wound infection and dehiscence. Spine reoperations generally take longer than primary operations, and this is directly correlated with the risk for wound infection.⁷ Several techniques, such as frequent wound irrigation and hourly release of tissue retractors, help ensure adequate blood flow to tissues and lessen the risk for surgical infections. If the wound edges appear hypovascular or otherwise nonviable, they may be excised to allow primary closure of fresh wound edges.

Recurrent Lumbar Disk Herniation

Recurrent lumbar disk herniation is generally defined as ipsilateral or contralateral herniated disk material at the same level as the index operation that causes symptoms of radiculopathy after a postoperative symptom-free interval of at least 6 months.^1,10 Reherniation of a lumbar disk at the same level and site of a previous diskectomy causing recurrent radicular symptoms occurs in 3% to 19%, with higher rates being quoted in studies with longer follow-up.^11–16 This phenomenon is more common at the L4-5 level, perhaps because of its increased mobility relative to other spinal motion segments.¹⁷ Patients typically experience recurrent symptoms of radiculopathy after a postoperative symptom-free period of at least 6 months. Recurrent radiculopathy at the same level may also occur secondary to degenerative foraminal stenosis, a prolapsed annulus, or excessive epidural scar tissue formation.

Recurrent herniation may pose a diagnostic challenge because imaging studies will demonstrate significant postoperative changes in the epidural space at the operated level, such as epidural scar tissue, adhesions, and bony hypertrophy. Gadolinium-enhanced MRI is optimally used for the differentiation of postoperative changes from recurrent herniated disk material (Fig. 275-1).^18–20 Scar tissue generally exhibits a homogeneous pattern of enhancement, whereas disk material will usually demonstrate peripheral enhancement. The herniated disk fragment is typically continuous with the intervertebral parent disk unless sequestration of a free fragment has occurred. Neural elements are often displaced away from a herniated disk fragment, but thecal retraction toward epidural scar tissue can also be observed. Flexion-extension radiographs should be obtained to rule out dynamic instability and to evaluate the need for spinal fusion should revision surgery be necessary. If spinal fusion is being considered, standing 36-inch AP and lateral radiographs are used to assess the sagittal and coronal balance of the spinal column.

FIGURE 275-1 Sagittal (left) and axial (right) T1-weighted, gadolinium-enhanced magnetic resonance images of a 50-year-old woman in whom recurrent lower back pain and left S1 radiculopathy developed 20 months after undergoing left L5 hemilaminectomy and microdiskectomy. She initially experienced complete resolution of her symptoms for 19 months, followed by return of the radicular symptoms and the de novo development of a neurogenic bladder. Peripheral enhancement of the herniated disk fragment, continuity with the parent disk, and displacement of the thecal sac are demonstrated.

In most cases, the optimal treatment of recurrent lumbar disk herniation in the absence of a neurological deficit or spinal instability is similar to that for primary lumbar disk herniation. A treatment paradigm similar to that for de novo lumbar disk disease, including a trial of nonoperative therapy, is recommended before surgical therapy is considered. Failure of a comprehensive nonoperative treatment plan indicates the need for either repeated diskectomy or diskectomy plus spinal fusion if instability is present. In cases in which extensive destabilizing bony decompression is anticipated, spinal fusion is indicated.

Epidural scarring, which occurs after any procedure in the epidural space, may also cause recurrent radiculopathy in some patients.²¹ The degree of scarring has been correlated with the development of recurrent symptoms. Radiculopathy in this setting is thought to be secondary to nerve root tethering. The generally accepted teaching is that in the absence of a herniated disk, patients with epidural scar tissue and concurrent symptoms do not benefit from reoperation on a consistent basis whereas those with a recurrent herniated disk and concordant radiculopathy do benefit from reoperation.^22,23 Patients with degenerative foraminal stenosis or annular prolapse with neural compression may also benefit from revision surgery in selected cases.

The surgical technique for revision diskectomy entails exposure of the normal bony anatomy above and below the previous operative field. Sharp dissection under the operating microscope is used to identify the previous laminotomy site. Further sharp dissection is undertaken to dissect scar tissue cleanly from the bony margins. We prefer the use of up-biting curets for such dissection. The margins of the previous laminotomy are extended with a Kerrison rongeur to allow exposure of normal dura. Specific attention is devoted to preserving the integrity of the facet joint complexes laterally. Ligamentum and adherent epidural scarring are further exposed and excised by working from normal areas to those invested in scar tissue. Once the dura of the traversing and exiting nerve roots is identified, careful identification and dissection of the neural elements away from the scar tissue and recurrent disk fragment are then carried out. This technique effectively lessens the risk for dural laceration. All herniated disk material is resected with pituitary rongeurs. Significant epidural scar tissue is also resected with a sharp instrument such as a curet or scalpel while meticulously avoiding inadvertent violation of the dura. A sharp Kerrison rongeur may also be used to cut the scar tissue rather than using a pulling motion, which may place excessive traction on the neural elements. Should violation of the dura occur, it is preferably repaired primarily or, if not technically feasible, by using a dural graft combined with placement of a lumbar subarachnoid drain. Wide exposure permits exploration of the pedicles, both above and below the disk space, and the neural foramen for any residual herniated disk. Palpation of the paramedian disk space with an angled instrument such as a nerve hook is also critical to ensure complete removal of all herniated disk material and complete decompression of the neural elements. After the decompression is complete, the wound is closed in layers in the usual fashion.

Postoperative Spinal Instability and Deformity

Postoperative spinal instability can arise as a consequence of surgical intervention, the primary spinal pathology, or a combination of both. Such instability may lead to the development of either a mobile or rigid deformity. Iatrogenic spinal instability, both with and without deformity, is more common in the cervical than in the lumbar and thoracic spine as a result of the relatively increased mobility of the cervical motion segments. Iatrogenic spinal destabilization is more commonly encountered after dorsal surgery because ventral decompressive procedures generally lead to spinal destabilization and are often supplemented with interbody strut grafting, with or without instrumentation, during the primary operation. The most common types of postoperative spinal deformity are cervical postlaminectomy kyphosis and lumbar postlaminectomy instability leading to focal kyphosis or spondylolisthesis.

Iatrogenic disruption of the posterior tension band, paraspinal muscles, and facet joint complexes may result in the development of spinal instability, with or without subsequent deformity. Additionally, abnormalities of the ventral spinal column may lead to or hasten the development of spinal instability after dorsal surgery (Fig. 275-2). Other factors, including disruption of more than half of the medial facets or young age, also increase the risk for iatrogenic spinal destabilization, especially in the cervical spine.²⁴

FIGURE 275-2 Upper left, Sagittal computed tomography (CT) reconstruction in a 12-year-old boy who underwent C3, C4, and C5 laminectomies for resection of extradural Ewing’s sarcoma 14 months before evaluation for new complaints of axial neck pain and an abnormal posture. He had a 29-degree kyphotic deformity measured from the inferior end plate of C2 to the superior end plate of C6 and centered at the C4 level. C3-4 anterolisthesis and lytic erosion of the C3 vertebral body are also demonstrated. Upper right, Sagittal magnetic resonance image demonstrating infiltration of the C3 vertebral body by tumor. A lateral cervical spine radiograph (lower left) and a sagittal reformatted CT scan (lower right) were obtained after C3 corpectomy, C4-5 and C5-6 diskectomy, internal reduction of the kyphotic deformity, C2-6 anterior screw-plate stabilization, and supplemental C2-6 posterior screw-rod stabilization. A sagittal plane correction of 17 degrees was achieved.

In addition to symptoms of gross spinal deformity, such as the presence of an abnormal posture or an inability to raise the head, patients may have symptoms caused by impingement of neural elements, such as myelopathy or radiculopathy. Dynamic segmental instability may be associated with the development of axial mechanical pain.

Clinical evaluation includes MRI or CT myelography to look for compression of neural elements. Static and dynamic radiographs are examined to demonstrate the degree of deformity and detail any specific areas of dynamic spinal instability or fixed deformity. Standing 36-inch AP and lateral radiographs are critical in determining the total sagittal and coronal balance of the spinal column, which must be taken into account during preoperative planning for a deformity correction procedure. High-resolution CT is often helpful in demonstrating the bony anatomy, as well as the integrity of any preexisting spinal instrumentation.

Neurological injury is one of the most serious complications that may occur during surgery for correction of deformity. Many surgeons elect to perform intraoperative neurophysiologic monitoring (IOM) in the form of somatosensory evoked potentials, motor evoked potentials, and electromyography to minimize this risk.^25–29 Although several studies have demonstrated that IOM may be beneficial in predicting and possibly minimizing the occurrence of postoperative neurological deficits, the importance of proper surgical technique and close attention to the integrity of the neural elements can be neither overemphasized nor replaced. Despite the increasing use of IOM in many centers, its routine use has yet to become universally accepted. This may, in part, be due to concerns regarding the time added to the procedure, cost, availability, and the potential for generating misleading information and unnecessary measures taken to address any false-positive alerts generated by IOM.^30–32

In general, for most cases of revision surgery involving correction of deformity, we do not routinely perform IOM but instead rely on direct decompression and visualization of the neural elements. When performing surgery for correction of postoperative deformity, we first place the patient in a relatively neutral position before performing a full decompression. Only after full decompression of the neural elements has been completed is any attempt made to reduce the deformity. In so doing, we are able to directly visualize the neural elements during reduction of the deformity and visually confirm the absence of any neural compression or tethering. We have found this practice to be a reliable and reproducible method of minimizing neurological injury during revision surgery involving spinal deformity without the need for the routine use of IOM.

Postlaminectomy Cervical Kyphosis

Postoperative deformity of the cervical spine most commonly occurs in the form of kyphosis or anterior subluxation. Accordingly, postlaminectomy cervical kyphosis is a well-established complication of cervical laminectomy.^33–38 Cervical kyphosis in this regard refers to a loss of the normal lordotic cervical curvature of at least 5 degrees per motion segment (C2-7); however, overt reversal of the natural lordotic curvature to the point of global cervical kyphosis is not uncommonly encountered in this population. Cervical laminectomy involves resection of the spinous processes, laminae, and interspinous and supraspinous ligaments. The muscular attachments of the paraspinal muscles are disrupted and the erector spinae muscles are weakened as a result of partial denervation.³⁹ The disrupted dorsal elements and weakened posterior tension band favor the development of cervical kyphosis because the natural balance between the extension and flexion moments of the cervical spine is shifted toward the latter.^40,41 In the pediatric population, this imbalance often leads to the development of anteriorly wedged vertebral bodies because the growth of immature bone is strongly influenced by the asymmetry of the altered flexion and extension moments.⁴² Resection of more than 50% of the cervical facet joint complexes is also a strong risk factor for the development of cervical kyphosis and anterolisthesis.^24,43,44

The true incidence of postlaminectomy cervical kyphosis has not been fully established, in part because of conflicting definitions of deformity, lack of the long-term follow-up required to detect the development of slowly progressive kyphosis in the adult population, and inclusion of only symptomatic patients in some studies. However, the risk for postlaminectomy cervical deformity in the adult population is reported to be between 5% and 50%.^{35,43,45–47} An incidence as high as 75% has been reported in patients who have undergone extensive (>50%) bilateral facet resection, thus highlighting the significant stabilizing force of the facet joint complexes.^40,43 The exact proportion of patients in whom subsequent clinically symptomatic instability develops is not well established, but the rate may be as low as 5%.^33,34

Although the precise incidence of postlaminectomy kyphosis is not known, several risk factors have been identified, including young age (<25 years), preoperative evidence of cervical instability or deformity, a neutral cervical spine, laminectomy performed for degenerative spondylosis, coexisting vertebral fracture or ligamentous injury at the levels to be operated, coexistent structural abnormalities of the ventral elements, the extent of laminectomy (partial resection of the facet joint complexes), and the number of segments (more than four) treated.^{36,40,46,48,49} Given the relative frequency of this complication, many surgeons elect to perform laminaplasty or prophylactically perform a dorsal instrumented fusion in patients with one of the aforementioned risk factors.^37,42,48

Indications for reoperation include compression of the neural elements leading to myelopathy or radiculopathy, progressive or severe spinal deformity, or symptoms of axial mechanical pain. A low-grade kyphosis without associated symptoms of neural compression does not often produce axial pain and may be observed clinically and radiographically. In these patients, either the development of radiculopathy or myelopathy, progression of the deformity, or the development of axial pain may warrant corrective surgery.

Techniques for the correction of cervical postlaminectomy kyphosis include a dorsal approach, ventral approach, or a combined dorsal and ventral approach.38–40,50–53 Preoperative traction may be used in patients with a kyphotic deformity that does not fully reduce with simple neck extension but is not clearly a rigid deformity.^38,39,54 The choice of surgical approach is dependent on several factors, including the degree of kyphotic angulation, the structural integrity of the anterior and posterior elements, the presence and location of compression of neural elements, and the presence of either a fixed, incompletely reducible or rigid deformity.³⁹ In general, either a ventrally or dorsally performed revision surgery for cervical postlaminectomy kyphosis requires instrumentation and fusion.

Ventral Approach

The ventral surgical approach is chosen when the kyphotic deformity is either rigid or minimally reducible with neck extension and preoperative traction.^38,39 In addition, the ventral approach is used when the ventral bony elements are structurally compromised or are of insufficient height to maintain satisfactory alignment and a long-segment interbody fusion is required (see Fig. 275-2). Ventral decompression may be performed in patients with significant neural compression by ventral bony elements, prolapsed intervertebral disks, and osteophytic ridges. In those with high-degree kyphotic deformity, the spinal cord may be draped over the ventral bony elements (Figs. 275-3 and 275-4). In such cases, ventral decompression is required before any procedure for correction of the kyphotic deformity is performed because the application of corrective distraction forces will increase ventral compression of the spinal cord and may lead to spinal cord injury. The ventral approach affords the additional opportunity to perform ventral release of the longus colli muscles, the anterior longitudinal ligament, and the anterior annulus, thereby facilitating further anterior reduction of the kyphotic deformity.

FIGURE 275-3 Sagittal T1-weighted magnetic resonance image of an 11-year-old girl with neurofibromatosis type 2 and a recurrent intramedullary ependymoma. Four years ago she had undergone laminectomy at C6-7, followed by suboccipital craniectomy and C1-5 laminectomy 4 months earlier. She complained of mechanical neck pain and an inability to fully raise her head, which developed over a period of 3 months. A cervical kyphotic deformity (50 degrees) involving C3-6 and severe ventral spinal cord compression are demonstrated.

FIGURE 275-4 Flexion (upper left) and extension (upper right) lateral cervical spine radiographs of the patient in Figure 275-3 demonstrating a 48-degree kyphotic deformity measured from C2 to C7 and centered on the C5 vertebral body. The deformity partially reduces to 13 degrees of kyphosis in extension. Lower left, Lateral cervical spine radiograph after 25 lb of Gardner-Wells traction, which provided an additional 4 degrees of correction of the deformity. Lower right, Postoperative lateral cervical spine radiograph 1 year after anterior diskectomies (C3-6), internal reduction, and interbody fusion. A supplemental occipital-T2 posterior fusion with screw-rod stabilization was also performed during the same procedure. An additional 11 degrees of correction of the deformity was achieved and resulted in 2 degrees of lordosis (C2-7).

Gardner-Wells traction is applied and a shoulder roll is placed beneath the scapulae to encourage cervical lordosis. Intraoperative lateral fluoroscopy is valuable in assessing alignment of the cervical spine and centering the incision above the levels to be operated on and during the placement of instrumentation. Either a transverse skin incision or, alternatively, an oblique incision along the anterior border of the sternocleidomastoid muscle (when exposure of three or more spinal segments is required) is performed. The usual method of anterior cervical spinal exposure as described by Cloward is then carried out.⁵⁵ Caspar distraction posts are placed parallel to the vertebral body end plates, which will lie in a convergent fashion in patients with cervical kyphosis. Distraction is applied along the Caspar posts to place the posts in a parallel orientation. This effectively extends the cervical spine and reduces the kyphotic deformity; however, this maneuver must be used judiciously in patients with suboptimal bone mineral density to avoid vertebral body fracture. Additional distraction and reduction of the deformity are achieved by increasing axial traction via Gardner-Wells tongs by approximately 5 lb per cervical level. Further reduction of the kyphotic deformity is accomplished after performing anterior release of the anterior longitudinal ligament and the outer annulus.

Anterior decompression is performed, as necessary, depending on the degree of kyphotic angulation, the presence of compression of neural elements by ventral structures, and the structural integrity of the ventral bony elements. With regard to correction of deformity, we preferably perform multiple diskectomies rather than corpectomy, when possible, because we are able to obtain improved correction of a kyphotic deformity with the use of multiple lordotic interbody grafts rather than larger strut grafts. Single or multilevel discectomy is performed in the usual manner, depending on the number of segments involved in the kyphotic deformity. Should significant ventral bony compression of the neural elements or evidence of vertebral body collapse exist on preoperative studies, single or multilevel corpectomies are performed at the involved levels. This may be combined with additional discectomies above and below the corpectomy segment or segments as needed, depending on the number of segments with kyphosis. Some authors have advocated preservation of an intermediate point of fixation rather than performing multiple adjacent corpectomies for ventral correction of cervical kyphosis.⁵³ This technique provides three- or four-point bending forces, which enhances the stability of the construct in addition to allowing the surgeon to “deliver” the intermediate point of fixation to a contoured implant for reduction of the deformity. Once complete anterior decompression has been achieved, manual intraoperative neck extension may be used to further increase cervical lordosis.

After decompression, lordotically shaped interbody strut grafts are applied. The interbody grafts should be placed in a ventral position, flush with the anterior margin of the vertebral bodies, to more effectively counteract the flexion moment in the sagittal plane because they will lie in a position as far ventral to the internal axis of rotation (IAR) as possible. This provides the interbody graft with a mechanical advantage to resist the sagittal flexion moment because the force required to resist the flexion moment decreases in a linear manner with increasing distance from the IAR.⁵⁶ The Caspar distracting pins are then removed and the axial traction is relieved. This maneuver effectively applies an axial load on the interbody strut grafts and thereby enhances fusion and, in effect, offloads the screw-plate system. When performing deformity correction procedures it is preferable to place instrumentation and achieve immediate internal stabilization.^50,52,57,58 An appropriately contoured screw-plate construct is then inserted under intraoperative fluoroscopic guidance. If an intermediate point of fixation has been preserved, it may be delivered to the plate system during screw tightening to further enhance lordosis as it is brought in toward the cervical plate. Use of a dynamic anterior screw-plate system, which permits axial subsidence while maintaining lordosis, both lessens the incidence of instrumentation failure and relieves mechanical stress at the screw-bone interface.^59–61 The incision is closed in layers and a rigid cervical collar is applied for at least 12 weeks.

If a combined dorsal-ventral procedure has been planned, the patient may be placed in the prone position and dorsal decompression and fusion performed in the same setting or in a staged manner. In general, we prefer to supplement ventral surgery involving corpectomies at two or more levels with a dorsal lateral mass–pedicle screw instrumentation and fusion procedure.^40,57,58,62 In cases in which significant posterior spinal canal compromise exists, dorsal decompression can be performed before dorsal fixation, as outlined earlier.

Segmental Instability after Posterior Lumbar Decompression

Dorsal decompression of the lumbar spinal canal is a well-established treatment of degenerative lumbar stenosis.^63–67 In some patients who undergo dorsal decompression of the lumbar spine, segmental instability will develop as a consequence of both surgery and, to a lesser degree, structural compromise of the ventral elements. Iatrogenic disruption of the posterior tension band, paraspinal muscles, and facet joint complexes may lead to the development of segmental instability, with or without subsequent spinal deformity.⁶⁸ Additionally, coexisting abnormalities of the ventral spinal elements, which often arise from degenerative disease, may also contribute to the development of spinal instability after dorsal surgery.

Several risk factors for the development of spinal instability after posterior lumbar decompression have been established, including preoperative radiologic evidence of dynamic instability or anterior spondylolisthesis, preserved disk space height, sagittally oriented facet joints, multilevel decompression, and the extent of facet joint complex or pars resection.^44,69–72 It is also important to note that glacial spinal instability may develop in a minority of patients who undergo only partial laminectomy and diskectomy, particularly if more than half to a third of the facet joint complex has been disrupted or if overaggressive resection of the pars interarticularis has been performed.⁴⁴ This may especially hold true when performing multilevel lumbar laminectomies because the distance between the pars interarticularis progressively decreases at the more cranial vertebral levels. Overaggressive pars resection is more likely to occur at the cranial levels, particularly L3 and above, if the decompression follows a rectangular rather than a trapezoidal shape. Injury to the facet joint from direct capsular cauterization may also contribute to facet joint denervation and glacial destabilization. Nevertheless, the presence one or more of the aforementioned risk factors should encourage the surgeon to consider the inclusion of an instrumented fusion in addition to a dorsal decompressive procedure. Furthermore, inclusion of an instrumented fusion will allow substantially more aggressive decompression of the neural elements because resection of the facet joints and pars interarticularis can be carried out. Such destabilizing maneuvers are frequently required in patients with high-grade canal and foraminal stenosis.

The clinical evaluation includes a detailed history and physical and neurological examination, as outlined earlier. Imaging studies such as static and dynamic radiographs are critical in detailing both the degree and the specific location of the segmental instability and spinal deformity. Standing 36-inch AP and lateral radiographs are obtained to assess the sagittal and coronal balance of the spinal column. MRI is an important adjunctive modality for investigating the neural elements, ligamentous structures, and intervertebral disks. Most importantly, MRI is useful in defining any areas of compression of the neural elements. Information obtained from electromyography may also be helpful in determining nerve root dysfunction, which may arise as a result of foraminal stenosis. Comprehensive synthesis of the information obtained from the history and neurological examination, imaging studies, and records of previous operations is required to formulate a detailed treatment plan.

Segmental instability, which is defined as pathologic rotational or translational motion of the intervertebral motion segment, often leads to the development of spinal deformities such as spondylolisthesis, scoliosis, subluxation, or lateral listhesis. Although these deformities may be apparent on static radiographs, evidence of excessive translation or rotation of the vertebral motion segment on dynamic radiographs is required for the identification of segmental instability rather than a fixed deformity.

Many patients who harbor radiographic evidence of iatrogenic segmental instability remain asymptomatic. However, spinal canal and foraminal stenosis will develop in a proportion of these patients as a consequence of the segmental instability and spinal deformity. The resultant spinal canal and nerve root compression may lead to symptoms of neurogenic claudication and radiculopathy, respectively. Furthermore, pathologic motion of unstable spinal segments is frequently associated with axial mechanical lower back pain.

The initial treatment of lumbar spinal instability after lumbar decompression includes a comprehensive trial of nonoperative therapy. If nonoperative therapy fails to control the symptoms or if symptoms of neural compression are present, revision surgery is undertaken. The goals of revision surgery must be carefully and specifically defined. The surgical approach is influenced by the presence of compression of neural elements, the type and severity of the spinal deformity, and the need for grafts or instrumentation to address the deformity. In our experience, patients with lumbar instability and evidence of neural compression will attain maximal benefit from dorsal decompression and instrumented posterior lumbar interbody fusion (Fig. 275-5). The goals of this procedure are to provide decompression of the neural elements, reduction of the deformity, restoration of sagittal alignment, and 360-degree stabilization through one approach. Alternatively, if there is no evidence of neural compression, either an anterior, posterior, or combined stabilization and fusion procedure can be performed. Because many of these patients harbor superimposed progressive multilevel degenerative spinal disease, additional procedures to address the degenerative spine disease at adjacent levels may also be performed as indicated during the same procedure. Accordingly, decompression plus instrumented fusion is often performed at adjacent levels not included in the previous operation.

FIGURE 275-5 A 46-year-old man sustained an L2 burst fracture 31 years previously and was treated by decompressive laminectomy of L2 and L3. Two years later he underwent noninstrumented posterolateral fusion of L1-2. He came to our institution with a 2-year history of intractable lower back pain and an externally visible thoracic spinal deformity. Left, Lateral 36-inch standing radiograph demonstrating an L2 fracture and a 49-degree kyphotic deformity measured from the top of T12 to the bottom of L4. There is a marked ventral compression deformity of L2. The patient is noted to have six lumbar-type vertebrae. Right, Postoperative lateral 36-inch standing radiograph after pedicle subtraction osteotomy at L2, internal reduction, T12-L4 posterior fusion, and screw-rod stabilization. A 50-degree correction of the deformity was achieved.

The surgical technique of posterior lumbar interbody fusion for postlaminectomy lumbar instability consists of exposure of the normal anatomy not included in the previous operation, decompression of the neural elements by further resection of bone and epidural scar tissue, posterior interbody fusion, and instrumented lateral fusion. Under general anesthesia, the patient is placed on a radiolucent Jackson table in the prone position. It is essential that the hips be extended fully to allow the lumbar spine to achieve maximal lordosis. Any hip flexion can be associated with relative kyphotic angulation of the lumbar spine and may predispose to the development of flat back syndrome.⁷³ A radiolucent table is used to aid in intraoperative fluoroscopic imaging in both the coronal and sagittal planes.

A standard midline approach is performed. The operative exposure includes the normal bony anatomy above and below or lateral to the previous operative field. Care is taken to avoid cauterization of the facet joint capsules of the adjacent levels that will not be included in the fusion procedure. The lateral extent of the exposure includes the transverse processes at each level and the sacral alae for fusions that extend to the sacrum. In patients with evidence of neural compression, sharp dissection is undertaken to cleanly dissect scar tissue from the bony margins of the previous laminectomy. We prefer the use of up-biting curets for this dissection. The margins of the previous laminectomy are extended with a Kerrison rongeur to allow exposure of normal dura. The previous bone decompression is extended by bilaterally removing the remaining laminae, pars interarticularis, and inferior facets at the levels to be operated. After bone decompression, careful identification and dissection of the neural elements away from the scar tissue are then carried out. This maneuver effectively lessens the risk for dural laceration. Significant epidural scar tissue is also resected with sharp instruments such as a curet or scalpel or a sharp Kerrison rongeur as described earlier. After decompression, the thecal sac and the exiting and traversing nerve roots are widely exposed. Pedicle screws are then inserted under fluoroscopic guidance.

After pedicle screw placement, a diskectomy is then performed. Care is taken to preserve the ventral and lateral margins of the annulus to allow the graft material, both structural and morselized, to be contained within the interbody space. Once this is completed, the interbody graft is placed. We prefer the use of two carbon fiber cages packed with morselized graft material, although other types of structural interbody graft can be used as well. The carbon fiber cages that we use have a lordotic sagittal contour with the ventral surface being 2 mm larger than the dorsal surface. In patients with significant loss of lumbar lordosis or a positive sagittal balance, correction of the deformity can be achieved with the use of lordotically shaped interbody grafts.^74,75 In patients with spondylolisthesis, a cage of shorter depth is used. For higher grades of spondylolisthesis, the interbody graft is placed after at least partial reduction of the deformity has been accomplished. We perform interbody graft placement via the transforaminal approach. By resecting the medial aspects of the subjacent superior facets, manipulation of the thecal sac and nerve roots is kept to a minimum during placement of the interbody graft.⁷⁶

A generous amount of morselized autograft is placed over the decorticated transverse processes. After bilateral placement of the lateral fusion bone, the screws are connected to each other with either plates or rods. The plates or rods are contoured in the sagittal plane to achieve lumbar lordosis. If restoration of lordosis is not fully attained after placement of the lordotic cages, the rods may be overbent to further enhance the lordotic curvature of the spine. In this maneuver, distraction devices are used to deliver the pedicle screws to the overly contoured rods or plates, with the amount of lordosis being increased to match that of the contoured rods or plates. If necessary, compression of the screws posteriorly can be performed to improve the extent of lordosis that can be achieved. However, such compression should be used only in conjunction with ventral interbody support. In cases of spondylolisthesis, significant reduction of the deformity is often achieved after connecting the pedicle screws to the contoured rods or plates. Two large-bore drains are placed and standard wound closure is then performed in layers.

Pseudarthrosis

Pseudarthrosis is a commonly encountered complication of both instrumented and noninstrumented spinal fusion surgery.^77–86 Pseudarthrosis is defined as the presence of symptomatic bony nonunion more than 1 year after fusion surgery.⁵⁶ It may lead to motion of the operated spinal motion segment with correlative signs and symptoms of spinal instability. The term nonunion is defined as permanent failure of bone growth across a fracture, regardless of the presence or absence of symptoms and signs of instability. Spinal nonunion can result in a variety of clinical manifestations ranging from an asymptomatic radiographic finding to persistent mechanical or radicular pain or, in its most severe form, to catastrophic construct failure with resultant deformity and neurological injury. Although pseudarthrosis is one of the leading complications of cervical, thoracic, and lumbar arthrodesis surgery, there is a wide range of its reported incidence in the literature.^{9,78,79,81–83,85–91} This variability is due to diverse surgical and patient factors and the varying methods used to assess bony fusion. The incidence of pseudarthrosis in spinal fusion surgery, in general, has been reported to be approximately 15%⁹¹; however, rates approaching 50% have also been reported.^85,86,90 This figure varies greatly according to the indications for and the type of surgery, the number of levels fused, the surgical technique, the use of different graft materials or fixation devices, patient demographics, and the presence of systemic factors that could adversely affect bone fusion.

Modern spinal instrumentation systems allow the surgeon to apply sustained multiplanar corrective forces to the spine to establish or maintain proper alignment. Long-term structural stability is conferred only by solid fusion of the bony elements so that they can endure biomechanical stresses during the patient’s lifetime. Spinal instrumentation systems provide immediate segmental immobilization of the bony elements; however, the primary goal of instrumented fixation in patients with a normal life expectancy is to achieve the structural stability necessary to allow bone fusion to occur. The widespread availability and technologic advances in spinal instrumentation systems have led to the increased use of internal fixation devices. Concurrent advances in general medicine and anesthesia techniques have allowed surgeons to address complex spinal diseases in patients with increasingly serious medical comorbidities and advanced age, both of which are well-established factors that adversely affect the ability to achieve solid bone fusion. Thus, pseudarthrosis remains a leading complication of spine surgery as surgical boundaries continue to expand.

Importantly, the surgeon can minimize the risk for nonunion by directing attention to optimization of medical comorbid conditions, eliminating risk factors known to adversely affect bone healing, and using specific surgical techniques that promote bone growth across the operated motion segments and enhance the likelihood of achieving a solid spinal fusion. These same general principles apply to both primary arthrodesis and surgery undertaken to address a failed fusion.