Spinal Instability and Spinal Fusion Surgery 

Updated: Feb 26, 2018
Author: Peyman Pakzaban, MD; Chief Editor: Brian H Kopell, MD 

Overview

Background

In recent decades, increased understanding of spinal biomechanics, proliferation of sophisticated spinal instrumentation devices, advances in bone fusion techniques, refinement of anterior approaches to the spine, and development of microsurgical and minimally invasive methods have made it possible to stabilize every segment of the spine successfully, regardless of the offending pathology. Accordingly, use of spinal fusion and instrumentation has increased. The question facing the modern spine surgeon is not so much how to stabilize the spine but when to do so.

As defined by White and Panjabi,[1] spinal stability is the ability of the spine under physiologic loads to limit patterns of displacement so as not to damage or irritate the spinal cord and nerve roots and, in addition, so as to prevent incapacitating deformity or pain due to structural changes; instability (acute or chronic) refers to excessive displacement of the spine that would result in neurologic deficit, deformity, or pain. (See Definition of Spinal Instability.)

Strictly defined, spinal fusion is an operation designed to treat spinal instability. In practice, however, this definition is not particularly useful, in that it fails to establish the indications for spinal fusion. The problem is threefold, as follows:

  • The current definitions of spinal instability are not uniformly accepted and applied
  • It is difficult to measure instability in individual clinical circumstances
  • Class I and II scientific evidence regarding spinal fusion is relatively scarce

In this setting, clinical practice is guided by an understanding of the principles of spinal biomechanics (see Pathophysiology) and knowledge of the generally accepted indications, contraindications, and controversies regarding fusion surgery (see Treatment).

It should be noted that whereas the term fusion, as used in this article and in spine literature to refer to the concept of internal stabilization of the spine, generally refers to fusion with instrumentation (instrumented fusion), such stabilization has also, albeit with decreasing frequency, been accomplished by means of bone grafting alone.

Degenerative spine disease without instability

A great deal of controversy remains regarding the application of fusion surgery in the treatment of degenerative spine disease without overt instability. In the future, these controversies will be addressed by a two-pronged approach. First, rigorous randomized controlled trials are needed to better assess the efficacy of existing methods of fusion. Second, novel treatment strategies are needed to replace fusion surgery.

Disk arthroplasty and posterior dynamic stabilization devices are two strategies that are under investigation. Some brands of artificial disk (see the image below) for treatment of symptomatic lumbar degenerative disk disease have been approved by the US Food and Drug Administration (FDA). Short-term studies revealed equivalent results for disk arthroplasty and lumbar fusion.[2]

Artificial lumbar disk. Image courtesy of Synthes, Artificial lumbar disk. Image courtesy of Synthes, Inc.

A prospective, randomized, controlled multicenter study designed to show the "noninferiority" of cervical total disk replacement (TDR) revealed that this technology was at least equivalent to anterior cervical diskectomy and fusion with regard to outcome at 24 months.[3]  Although most primary outcome measures (eg, pain scores and neurologic success) were equivalent in the two groups, the disk replacement group showed a lower requirement for analgesics and lower reoperation rate than the fusion group at 24 months.

Although these results show promise for total disk replacement, it should be noted that this study was limited to patients with single-level disk disease with radiculopathy and and that the results therefore cannot be generalized to patients with multilevel disk herniations, spondylosis, spondylolisthesis, and degenerative disk disease. In addition, long-term follow-up studies are needed to determine whether these benefits last, whether motion preservation with artificial disks persists over the long term, and whether the frequency of transition-level syndrome is decreased.

Posterior dynamic stabilization devices come in several varieties. The most promising of these are pedicle screw-based system, where the screws are linked by flexible members instead of rigid rods. The theoretical goal is to limit movement to a zone where neutral or near-neutral loading of spine occurs, or conversely prevent movement into a zone where abnormal loading occurs. Again, the clinical trials that have been conducted to date have produced clinical outcomes comparable with fusion.[4]

The challenges facing artificial disks and posterior dynamic stabilization devices are twofold. First, they must improve upon lumbar fusion outcomes. Second, these mechanical devices must continue to function indefinitely, as opposed to current spine implants, which are shielded from biomechanical stress once bony fusion is achieved.

In the long-term future, biologic rather than mechanical treatment strategies directed at repairing and maintaining the degenerated spine elements are more likely to provide a satisfactory solution to the problem of degenerative spine disease.

Anatomy

Regional variations in vertebral anatomy affect the incidence and consequences of spinal instability in different parts of the spine and dictate the surgical means by which the spine can be stabilized.

Vertebral body size increases as one descends the spine, accompanied by a corresponding increase in vertebral axial load-bearing capacity. The greater cancellous-to-cortical bone ratio in the vertebral body as compared with the posterior vertebral elements makes the body more susceptible to neoplastic and infectious diseases, and its relation to the instantaneous axis of rotation (IAR) makes it more susceptible to compressive injuries. The relative preponderance of these disorders anterior to the spinal cord makes their surgical management more challenging, often necessitating an anterior surgical approach to the spine.

On the other hand, the large surface area and volume of the vertebral body make it an excellent target for insertion of screw/plate systems, which can be used to stabilize every segment of the subaxial spine.

Facet joints have a transverse orientation in the cervical spine and gradually acquire a more sagittal orientation throughout the thoracic and upper lumbar spine. They then become more coronally oriented as one descends the lumbar spine. The transverse orientation of the facet joints and the loose facet capsules in the cervical spine allow relatively free movement of the neck in all three planes and do not protect the cervical spine against flexion injuries.

In the thoracolumbar junction, the sagittal orientation of the facet joints and the strong capsular ligaments permit greater movement in the sagittal plane than in other directions. This facet orientation and the transitional location of the thoracolumbar spine between the ribcage-stabilized thoracic spine and the more robust lumbar spine make the thoracolumbar junction more susceptible to flexion injuries.

The more coronal orientation of the L5-S1 facet joints as compared with the L4-5 facets accounts for the lower incidence of degenerative spondylolisthesis at L5-S1, in spite of the biomechanically disadvantaged angle of the lumbosacral junction. In contrast, isthmic spondylolisthesis, where the presence of spondylolysis bypasses the resistance of facet joints against translation, is more frequent at L5-S1.

The spinal canal is narrowest in the thoracic spine. On the other hand, the thoracic spine is stabilized by the ribcage, making it relatively immune to degenerative instability and increasing its resistance to traumatic instability. Consequently, if the force vector is great enough to overcome the stability of thoracic spine and produce a fracture-dislocation, the likelihood and severity of spinal cord injury would be greater in this area than elsewhere in the spine.

The pedicles in the cervical spine are quite narrow, short, acutely oriented, and juxtaposed to the transverse foramina (of the vertebral artery); accordingly, they are relatively undesirable for screw insertion. In contrast, the large size, strength, and favorable cylindrical anatomy of the pedicles in the lumbar spine makes them ideal for screw insertion. The pedicle screws at different segments are then linked by rods to stabilize the spine.

The pedicles acquire a relatively sagittal orientation in the thoracic and upper lumbar spine but then point inward again as one approaches the sacrum, a fact that must be taken into account when pedicle screws are to be inserted. In the thoracic spine, the pedicles have a narrow transverse diameter, exhibit a slight downward angle, and are located next to the narrow thoracic spinal canal.

Because of these anatomic considerations, wires and hooks have been used more frequently than screws to anchor rods against the thoracic spine, necessitating long instrumentation constructs to stabilize a short segment of instability ("rod long, fuse short"). Increasingly, screws are used in the thoracic spine to create shorter and stronger instrumentation constructs. In this setting, it is imperative to select screws of appropriate diameter on the basis of preoperative computed tomography (CT) and to avoid breach of the medial pedicle wall, erring toward the laterally located and protective costovertebral articulation, if necessary.

On the other hand, the relatively generous sagittal diameter of thoracic pedicles and the smaller size and lesser functional importance of thoracic nerve roots make screw misdirection in the sagittal plane less costly in the thoracic spine than it would be in the lumbar spine.

Cervical vertebrae have anatomic structures not found elsewhere in the spine: the lateral masses. Juxtaposed between the pedicles and the lamina and delimited by the articular surfaces of the adjacent facet joints, the paired lateral masses are satisfactory targets for screw insertion. Lateral mass screws at adjacent segments are linked by plates or rods to stabilize the cervical spine.

Laminae, spinous processes, and transverse processes can be used as anchor points for wires and hooks connected to rods to form three-point-bending instrumentation constructs. Alternatively, these structures can be wired to each other at different segments to produce tension band constructs. In general, these types of constructs provide less stiffness than screw/rod or screw/plate systems.

Comparison of vertebral anatomy in cervical, thora Comparison of vertebral anatomy in cervical, thoracic, and lumbar spine. Note variation in anatomy and size of pedicles.

Pathophysiology

The pathophysiology of spinal instability is variable and dependent on the etiology of instability. However, an understanding of certain biomechanical principles can guide the surgeon in diagnosing spinal instability and selecting the appropriate treatment method.

The three-column concept of the spine, as defined by Denis, is widely used as the conceptual framework for diagnosing acute overt spinal instability.[5] Although originally devised on the basis of a retrospective review of traumatic injuries to the thoracic and lumbar spine, it is now also applied to the subaxial (below C2) cervical spine and to nontraumatic instability. In this system, the columns are defined as follows:

  • The anterior column consists of the anterior vertebral body (usually the anterior two thirds), the anterior annulus, and the anterior longitudinal ligament
  • The middle column comprises the posterior wall of the vertebral body, the posterior annulus, and the posterior longitudinal ligament
  • The posterior column refers to the posterior ligamentous complex that connects adjacent neural arches, consisting of facet capsules, ligamentum flavum, interspinous ligament, and supraspinous ligament

Failure of two or more columns generally results in instability.

In this context, a simple compression wedge fracture occurs as a consequence of failure of the anterior column with preservation of the middle column (stable). On the other hand, a burst fracture is due to compression failure of both the anterior and the middle column (usually unstable), often resulting in bone retropulsion into spinal canal. A seatbelt-type injury is attributed to distraction failure of the posterior and middle columns with hinging of an intact anterior column (unstable). Fracture-dislocations involve failure of all three columns and are considered highly unstable.

The IAR is the axis about which a vertebral segment would rotate when exposed to an asymmetric application of force. Although in theory, there are three axes of rotation—corresponding to rotation in the sagittal plane (flexion/extension), coronal plane (lateral bending), and axial plane (twisting)—in practice, most references to the IAR involve axial forces applied in the sagittal plane. The IAR commonly (but not necessarily) falls within Denis' middle column.

Force vectors are simple mathematical constructs that define not only the magnitude of a force but also its direction. A force vector applied at a distance to the IAR results in rotation of that vertebral segment about the IAR. The distance between the point of application of the force vector and the IAR is called the moment arm. The longer the moment arm is, the less force is required to produce rotation.

When unrestricted rotation or displacement of an object is not possible in response to a force vector, deformation of its material (in this case, bone) occurs. For solid objects, elastic deformation occurs if the material can resume its shape when the stress (force divided by cross-sectional area) is removed. With increasing stress, a threshold is reached (elastic yield point) beyond which irreversible but smooth deformation (plastic deformation) occurs.

With further increases in stress, another threshold is reached (ultimate tensile point or failure point), at which point a fracture occurs and the stress is relieved. In the case of vertebral bone, the elastic yield point and failure point are fairly close, so the very little plastic deformation takes place before a fracture occurs.

On the basis of these concepts, traumatic spinal instability can be categorized according to the underlying pathophysiologic mechanisms. When an axial force vector is applied anterior to the IAR, a compression fracture occurs as a result of isolated failure of the anterior column.

When the axial force vector is precisely directed over the IAR, no rotation occurs. In this situation, if the stress exceeds the ultimate tensile point of the vertebral bone, failure of both middle and anterior columns occurs, resulting in a burst fracture.

If the axial force vector is directed posterior to the IAR (hyperextension), fractures of laminae and facet joint may result. This is more common in the cervical spine because of its lordotic curvature.

Pure distraction forces are rarely applied to the spine. Distraction-flexion force vectors are composite vectors with components in the superior and anterior orientation in the sagittal plane, generally associated with seatbelt deceleration injuries of the thoracolumbar spine. The vertical (distractive) component of the vector is applied posterior to the IAR, whereas the flexion component is directed superior to the IAR, resulting in rupture of the posterior ligamentous complex and the middle column. The anterior column remains intact, acting as a hinge.

In this type of injury, if the orientation of the vector is such that the flexion component is stronger and is directly applied to the IAR, a true Chance fracture may occur, consisting of a horizontal shearing fracture across the pedicles, the vertebral endplates, or both. (See the image below.) With even larger flexion force vectors, a fracture-dislocation may occur, with failure of all three columns and bilateral jumped or fractured facets. If a rotational vector (twisting moment) is also present in the axial plane and the flexion vector is not overwhelming, a unilateral jumped facet may result.

(A) Compression fracture; (B) burst fracture; (C) (A) Compression fracture; (B) burst fracture; (C) hyperextension injury to lamina and facets; (D) flexion-distraction (seatbelt) ligamentous injury and Chance fracture; (E) shear fracture-dislocations.

Although these biomechanical concepts are often discussed in the context of traumatic instability, they can be extended to other forms of instability as well. Furthermore, they are commonly applied in devising fusion and instrumentation constructs to treat specific instances of spinal instability.

For instance, interbody bone grafts and cages are best applied as distraction constructs applied in the region of IAR. Pedicle screw constructs can act as cantilever beams, shifting the IAR to the rod-screw interface. (See the image below.) Consideration of IAR is of crucial importance in three-point-bending constructs (eg, universal hook, wire, screw, rod systems used for thoracolumbar posterior instrumentation), where application of compressive and distractive forces can have significant effects on spine contour.

Example of application of biomechanical principles Example of application of biomechanical principles to spine surgery. Insertion of special pedicle screws (Schanz screws) pivoting on rod transfers instantaneous axis of rotation (IAR) to screw-rod interface. Compression of proximal end of screws produces distraction-reduction of vertebral burst fracture. If posterior longitudinal ligament is intact, retropulsion is corrected by ligament taxis. Image courtesy of Synthes, Inc.

Biology of fusion

For fusion to succeed, osteoprogenitor cells must differentiate into osteoblasts, populate the fusion matrix, survive in the fusion environment, and deposit bone. Many local and systemic host factors and graft properties affect these processes. Graft material may have the following properties:

  • Osteoconduction
  • Osteoinduction
  • Osteogenesis

Osteoconduction refers to the capacity of the graft to serve as a matrix or scaffolding for infiltration of bone cells and supporting neovascular network. Allogeneic, autologous, and synthetic bone matrices made of hydroxyapatite or coral are osteoconductive.

Osteoinduction refers to the capacity of bone to direct differentiation, migration and attachment of osteoprogenitor cells. Many positive and negative osteoinductive influences exist. Bone morphogenic protein, a member of the transforming growth factor (TGF)-β family, induces differentiation of mesenchymal cells into osteoblasts.[6] It is found naturally in the bone fusion environment and is available in recombinant form for clinical use.

Compressive forces applied to the bone graft also promote increased bone deposition, accounting for the greater success of interbody bone grafts versus onlay bone grafts. Application of a direct electrical current to bone also has an osteoinductive influence,[7] a phenomenon that is put to use by implanting a bone stimulator in cases at high risk for pseudarthrosis.

Osteogenesis refers to the capacity of bone graft to initiate fusion by providing live osteoprogenitor cells. Only autologous bone graft has this property.

In addition to osteogenesis, autologous bone graft provides osteoinduction and osteoconduction and thus is the ideal graft material. A corticocancellous autograft (eg, a tricortical iliac crest autograft) is capable of providing structural support as an interbody implant in addition to the abovementioned favorable properties. The only drawback of using autograft material is the potential for donor-site complications associated with graft harvest.

Host factors that adversely affect fusion include the following:

  • Malnutrition
  • Corticosteroid use
  • Irradiation
  • Neoplastic disease
  • Local infection
  • Smoking

Of these, smoking is the most prevalent correctable risk factor. There is abundant experimental and clinical[8] evidence documenting the adverse effects of smoking on bone healing and fusion.

Finally, immobilization of the target motion segment has been shown to significantly enhance the success of fusion.[9] This is best accomplished by means of instrumentation. In absence of instrumentation, fusion should be supported by external bracing until it solidifies.

Etiology

Virtually every category of disease affecting the bones, disks, joints, or ligamentous support structures of the spine can produce spinal instability. These include the following:

  • Infections
  • Inflammatory diseases
  • Connective tissue disorders
  • Congenital disorders
  • Degenerative disorders
  • Iatrogenic (postoperative) etiologies

Epidemiology

Given that spinal instability is not a single disease but a pathologic consequence of a variety of different spine disorders (eg, traumatic fractures, metastatic tumors,[10] and degenerative conditions), each with its own epidemiology, it is not possible or meaningful to determine the incidence and prevalence of spinal instability in the population. Furthermore, because of the disagreements on indications for spine fusion (at least for degenerative disease), the incidence of spinal instability does not correlate with the observed frequency of spine fusion surgery.

More than 400,000 spinal fusions are performed in the United States annually. The vast majority of these operations are performed for degenerative disease of the spine. Between 1998 and 2008, the annual number of spinal fusion discharges increased 137%, from 174,223 to 413,171, and the mean age for spinal fusion increased from 48.8 to 54.2 years.[11]

Between 1996 and 2001, the number of spine fusions in the United States increased 76%.[12] Whereas in 1990 about 70% of cervical spine operations consisted of nonfused decompressions, by 2000 about 70% of cervical spine operations consisted of anterior cervical fusions.[13]  In New York, the number of subaxial spinal fusions increased by 114% between 1997 and 2012.[14]

An increase in the incidence of spinal instability certainly could not account for the increase in fusion surgery. Although the forces driving this trend may be debated, it is clear that the standard of care in the United States has been shifting toward greater use of fusion surgery.

Prognosis

Outcome after fusion surgery is measured in terms of the three cardinal clinical manifestation of spinal instability:

  • Neurologic function
  • Pain
  • Disabling deformity

With overt instability (eg, from trauma, tumor, or infection), neurologic function after surgery is directly related to preoperative neurologic status and cannot be used as a measure of success of fusion surgery. For instance, after a thoracolumbar fracture-dislocation with cord laceration and paraplegia, the success of fusion surgery should not be measured in terms of recovery of neurologic function, but in terms of addressing disabling deformity and pain.

In conditions with overt instability, deformity and pain outcome measures correlate closely with radiographic success of fusion. Modern fusion and instrumentation techniques ensure radiographic success in most of these cases; thus, the outcome of fusion surgery for treatment of overt instability is generally good, and the necessity of lumbar fusion for overt instability is not questioned.

The situation is entirely different in the case of covert instability as it applies to degenerative disease, where there is not a strong correlation between successful radiographic fusion and clinical improvement, and the former cannot be used as a surrogate marker for the latter. Consequently, there is now a great deal of interest in direct assessment of clinical outcome after fusion surgery for degenerative spine disease.

In a multicenter randomized controlled trial, the Swedish Lumbar Spine Study Group provided useful class I scientific evidence.[15]  Of the 294 patients with disabling back pain due to one- or two-level degenerative disease without spinal stenosis or spondylolisthesis (covert instability: dysfunctional motion segment), the lumbar fusion group did significantly better than the conservatively treated group in terms of pain relief, degree of disability, return to work status, and degree of satisfaction.

In contrast to this trial, a smaller Norwegian study of degenerative back pain[16]  failed to show a statistically significant difference between lumbar fusion and a very aggressive regimen of physical and cognitive treatment (25 hours of physical therapy weekly for 8 weeks, followed by a comprehensive home exercise program, individual counseling, lessons, group therapy sessions, and peer group discussions). Both groups experienced significant improvements over baseline, with a trend toward greater improvement in the surgical group.

This study was criticized not only for its small number of patients but also for the large confidence intervals in the data, which suggested that it lacked sufficient power to detect a statistical difference.[17]  Furthermore, it is unclear whether large-scale implementation of such a vigorous physical and cognitive program is realistic in the everyday clinical setting.

Such discrepancies in the spine literature have generally been the norm, not the exception, and they have often arisen from studies of far less scientific rigor than those mentioned above.

In an effort to produce evidence-based treatment recommendations, the American Association of Neurological Surgeons (AANS)/Congress of Neurological Surgeons (CNS) Joint Section on Disorders of the Spine and Peripheral Nerves took up the monumental task of analyzing all of the available literature on lumbar fusion for degenerative disease.[17]  In 2005, they issued recommendations that were ranked according to the strength of the supporting evidence, as follows:

  • Standards - Highest level of clinical confidence; based on one or more well-designed comparative studies, or less well-designed randomized controlled trials (class I evidence)
  • Guidelines - Intermediate level of confidence; based on one or more well-designed comparative studies, or less well-designed randomized controlled studies (class II evidence)
  • Options - Lowest level of confidence; based on case series, comparative studies with historical controls, expert opinion, and flawed randomized controlled trials (class III evidence)

Most of the recommendations were considered to be based on class III evidence and thus were labeled as options. The following are some of the more salient 2005 recommendations from the AANS/CNS Joint Section[17] :

  • Lumbar fusion is recommended as a treatment for carefully selected patients with disabling low back pain due to one- or two-level degenerative disease without stenosis or spondylolisthesis (standards)
  • Lumbar fusion is not recommended as routine treatment following primary or recurrent disk excision, unless there is evidence of preoperative lumbar deformity, instability, or chronic low back pain (options)
  • Posterolateral lumbar fusion is recommended for patients with lumbar stenosis and associated spondylolisthesis who require decompression (guidelines)
  • Posterolateral lumbar fusion is not recommended as a treatment option in patients with lumbar stenosis without preexisting spinal instability (spondylolisthesis) or likely iatrogenic instability (options)
  • In the context of single-level standalone anterior lumbar interbody fusion (ALIF) or ALIF with posterior instrumentation, addition of posterolateral fusion is not recommended, as it increases operating time and blood loss without influencing the likelihood of fusion or functional outcome (guidelines)
  • In the workup of diskogenic pain, the following are recommended: magnetic resonance imaging (MRI) should be used as the initial diagnostic test instead of diskography; MRI-documented disk spaces that appear normal should not be fused; lumbar diskography should not be used as a stand-alone test for surgical decision-making; if diskography is performed, both a concordant pain response and morphologic abnormalities should be present before fusion is considered (guidelines)

In 2014, updates to the 2005 AANS/CNS recommendations were published.[18]  In an effort to enhance compatibility with other guidelines and recommendations, the standards/guidelines/options ranking system was replaced by a modified version of the North American Spine Society (NASS) strategy for evidence assessment and recommendation grading. Evidence was assigned to one of five levels (I-V), and recommendations were graded as follows:

  • Grade A (good evidence) - Two or more level I studies with consistent findings
  • Grade B (fair evidence) - Single level I study or multiple level II or III studies with consistent findings
  • Grade C (poor evidence) - Single level II study or multiple level IV or V studies
  • Grade I (insufficient evidence for recommendation) - Single level III, IV, or V study; studies of equivalent strength with conflicting findings or conclusions

For the most part, the evidence underlying the 2014 recommendations did not conflict with the recommendations provided in 2005. Additional detail is available in the 17 guideline articles published in the Journal of Neurosurgery: Spine.[18, 19, 20, 21, 22, 23, 24, 25, 26, 27, 28, 29, 30, 31, 32, 33, 34]

 

Presentation

History and Physical Examination

The clinical manifestations of spinal stability fall into three categories, as stated in the definition of instability (see Definition of Spinal Instability):

  • Neurologic deficit due to cord, cauda equina, or nerve root compression
  • Pain
  • Incapacitating deformity

Definition of Spinal Instability

In the widely quoted work by White and Panjabi,[1] spinal stability was defined as the ability of the spine under physiologic loads to limit patterns of displacement so as not to damage or irritate the spinal cord and nerve roots and, in addition, so as to prevent incapacitating deformity or pain due to structural changes. Conversely, instability was defined as referring to excessive displacement of the spine that would result in neurologic deficit, deformity, or pain.

Instability can be acute (eg, spine fractures and dislocations) or chronic (eg, spondylolisthesis). Acute instability has been further subcategorized as overt versus limited, whereas chronic instability has been subdivided to include glacial instability (progressive deformity) and instability associated with dysfunctional motion segment.[35]

A simpler conceptual approach would be to think of instability as overt, anticipated, or covert.

Overt instability

Overt instability refers to excessive motion that is readily documented by radiographic studies and results in pain, deformity, or neurologic deficit. Those spine fractures, dislocations, tumors, and infectious processes that significantly disrupt one or more spinal motion segments produce acute overt instability. (See the image below.)

Bilateral jumped facet syndrome is example of over Bilateral jumped facet syndrome is example of overt spinal instability due to trauma. Note grossly abnormal displacement of C5 relative to C6 with neck flexion.

Spondylolisthesis with abnormal dynamic displacement, documented on flexion-extension radiographs, is an example of chronic overt instability. In addition, any spinal deformity (kyphosis, hyperlordosis, scoliosis, or spondylolisthesis) that progresses with time as documented by serial radiographs (ie, Benzel glacial instability) falls in the category of chronic overt instability.

Overt instability generally requires stabilization, either by external means (bracing) or by internal means (fusion).[36]

Anticipated instability

Anticipated instability (see the image below) refers to instability that would be produced by a surgical procedure that is required for proper decompression of neural elements or resection of an offending lesion. 

Example of anticipated instability. (A) Large mass Example of anticipated instability. (A) Large mass affecting right C3-4 facet joint and lateral masses in patient with severe right-side neck and shoulder pain; (B, C) complete resection of tumor and simultaneous C3-4 anterior fusion to circumvent anticipated iatrogenic stability produced by radical resection of facet and lateral masses.

For instance, corpectomy or total facetectomy would constitute indications for fusion at the time of the original operation. A comprehensive anterior cervical diskectomy (with complete resection of the posterior longitudinal ligament and portions of both uncovertebral joints performed for adequate neural decompression) may also be considered in this category, in that it disrupts two of Denis' three spinal columns.

Covert instability

Covert instability is a more elusive concept, referring to circumstances in which excessive motion cannot be grossly demonstrated but is presumed to exist on the basis of the combination of clinical and radiographic findings. Fixed spondylolisthesis (without movement on flexion-extension radiographs) in the setting of progressively worsening back pain or radicular symptoms is a good example of covert instability (see the image below). Pseudarthrosis with intact instrumentation also falls in this category.

Spinal stenosis with fixed degenerative spondyloli Spinal stenosis with fixed degenerative spondylolisthesis in elderly patient is common example of covert instability. Acceptable surgical treatment options include decompression alone vs decompression with fusion.

Controversy arises when the concept of covert instability is applied to degenerative diseases of the spine. In this context, the concept of microinstability is sometimes evoked to justify fusion for a wider range of conditions, including recurrent disk herniation, disk degeneration with diskogenic pain, painful facet arthropathy, spinal stenosis, and failed back syndrome without overt instability.

 

Workup

Laboratory Studies

There are no laboratory studies that would assist in diagnosis of spinal instability. Laboratory studies can be helpful in diagnosing certain conditions that could result in spinal stability, such as spine infections (complete blood count [CBC], erythrocyte sedimentation rate [ESR], C-reactive protein [CRP], blood cultures), rheumatoid arthritis (rheumatoid factor [RF]), ankylosing spondylitis (HLA-B27), multiple myeloma (serum immunoelectrophoresis, urine Bence-Jones proteins), and others.

Laboratory studies are routinely performed as a part of preoperative preparation for spine surgery.

Imaging Studies

Magnetic resonance imaging (MRI) of the spine and plain radiographs with flexion and extension are the most useful imaging studies for evaluation of spinal instability. In addition to demonstrating vertebral displacement, vertebral deformation and neural compression, MRI provides invaluable information about spinal cord injury, neoplastic and infectious processes, and ligamentous disruption.

Computed tomography (CT) myelography is used when MRI cannot be obtained or has not provided the resolution necessary to assess the extent of neural compression. Plain CT is useful for assessing bone anatomy in the setting of vertebral fractures, spondylolysis, previous spine surgery, and congenital spine anomalies. CT may also be used to assess certain bony parameters (eg, pedicle size in the thoracolumbar spine, lateral mass anatomy in the cervical spine, and vertebral artery anatomy in the atlantoaxial [C1-2] region) in preparation for instrumentation of the spine.

To evaluate bone integrity before fusion when osteoporosis is suspected, a bone density scan is performed. Radionuclide bone scans have been supplanted by high-resolution CT for assessment of pseudarthrosis.

Other Tests

Electromyography (EMG) may be used to confirm nerve-root compression but does not play a direct role in establishing the diagnosis of spinal instability.

Diagnostic Procedures

Selective nerve-root injections can be used as a diagnostic tool to confirm that a particular nerve root is responsible for the pain syndrome. They are also used in a therapeutic capacity in nonsurgical management of spine disorders.

CT-guided biopsy/aspiration is used when tumor or infection is suspected and when the possibility of nonsurgical treatment is being entertained. When surgery has to be performed to decompress or stabilize the spine, the diagnosis can be obtained intraoperatively.

Substantial controversy exists regarding the value of diskography in diagnosis of diskogenic pain and in patient selection for fusion surgery. When performed, it should be accompanied by measurements of intradiskal pressure, documentation of severity and concordance of pain during injection, and postdiskography CT.

Histologic Findings

No histologic findings are relevant to the diagnosis of spinal instability, except when a neoplasm is the source of instability.

Staging

Because spinal instability is a heterogeneous disorder, no uniform staging or grading system exists that would be relevant to all forms of spinal instability.

Spondylolisthesis, defined as anterior translation of a vertebral body in relation to the adjacent caudal vertebral body, is graded according to the system in Table 1 below. (See the image below.)

Table 1. Grading of Spondylolisthesis (Open Table in a new window)

Slip Distance/Anteroposterior Diameter

of Vertebral Body

Grade

0-25%

1

25-50%

2

50-75%

3

75-100%

4

>100%

Spondyloptosis

Grade 1 spondylolisthesis in neutral position prog Grade 1 spondylolisthesis in neutral position progresses to grade 2 with flexion, indicating overt instability in this case.

In the lumbar spine, spondylolisthesis is either isthmic, degenerative, or traumatic. Isthmic spondylolisthesis occurs because of a congenital weakness and subsequent fracture of pars interarticularis (usually of L5; see the image below), resulting in uncoupling and glacial anterior translation of one vertebral body over another.

Grade I isthmic spondylolisthesis at L5-S1. Arrow Grade I isthmic spondylolisthesis at L5-S1. Arrow depicts L5 pars fracture.

Degenerative spondylolisthesis occurs because of severe degeneration of facet joints and incompetence of facet capsules, which lose the capacity to resist the flexion moment, resulting in translation. Traumatic spondylolisthesis represents a fracture-dislocation of the spine.

 

Treatment

Approach Considerations

Indications for spinal fusion

Acute overt instability

Conditions that result in acute overt instability require stabilization, either internally (by fusion) or externally (by reduction and bracing). In traumatic injuries, if instability is due to a fracture rather than ligamentous rupture, if the fracture fragments are (or can be reduced to be) in contact and in near-anatomic alignment, and if there is no significant neural compression, an external brace (eg, halo, collar, or thoracolumbosacral orthosis [TLSO] brace) is tried until the fracture heals. In all other circumstances and in cases where bracing has failed, fusion is indicated.

Tables 2 and 3 summarize treatment algorithms and indication for fusion in cervical, thoracic, and lumbar spine trauma.

Table 2. Treatment of Traumatic Instability of Cervical Spine (Open Table in a new window)

Fracture/Dislocation

(Mechanism)

Type/Issue

Treatment

C1 Jefferson fracture

(axial loading)

1. Isolated -->

2. With transverse ligament rupture -->

3. Widely diastatic -->

4. With odontoid fracture -->

1. Hard collar

2. Halo

3. Consider occiput-C2 fusion

4. Treat according to odontoid fracture

C1-2 Rotatory subluxation

(twisting moment)

1. Children, URI -->

2. Adults, tumor, trauma, infection -->

1. Bedrest, analgesics, halter traction, soft collar

2. Traction, hard collar, halo, or C1-2 fusion depending on cause and duration

Odontoid fracture

(flexion in young, extension in old)

1. Type 1 -->

2. Type 2, < 6 mm displaced -->

3. Type 2, >6 mm displaced or chronic or type 2A -->

4. Type 3 -->

1. If no atlanto-occipital instability, collar × 3 mo

2. Halo × 3-6 mo

3. C1-2 fusion or odontoid screw

4. Halo × 6 mo

C2 Hangman fracture

(extension)

1. Pars approximated->

2. Pars separated, reducible -->

3. Pars separated, not reducible -->

1. Hard collar × 3 mo

2. Reduce in extension, then halo × 3 mo

3. C2-3 fusion

Unilateral jumped facet

(flexion + rotation)

1. Reducible -->

2. Not reducible -->

3. With facet fracture -->

4. With disk herniation -->

1. Reduce and halo × 3 mo

2. Open reduction and posterior fusion

3. Open reduction and posterior fusion

4. Anterior decompression, open reduction, and anterior fusion

Bilateral jumped facet

(flexion)

1. Reducible, without disk herniation -->

2. Not reducible, without disk herniation -->

3. With disk herniation -->

1. Closed reduction, then posterior fusion

2. Open anterior or posterior reduction and fusion

3. Anterior diskectomy, reduction and fusion

Subaxial spine axial loading injuries

(axial ± flexion)

1. Simple compression fracture -->

2. Burst fracture ± teardrop fracture -->

3. Burst + posterior column fracture -->

1. Hard collar

2. Anterior corpectomy and fusion

3. Anterior corpectomy and fusion

(± posterior fusion)

Clay shoveler fracture

(flexion)

Always stable

Soft collar and analgesics

Anterior avulsion fracture

(extension)

Always stable

Soft collar and analgesics

Table 3. Treatment of Traumatic Instability of Thoracic and Lumbar Spine (Open Table in a new window)

Fracture

Denis Columns Involved

Treatment

Compression fracture

Anterior column

Bracing (note that >50% vertebral body height loss or Cobb angle >30º predicts worsening of kyphosis)

Compression fracture with splaying of spinous processes

Anterior and posterior columns

Posterior instrumented fusion

Stable burst fracture

(preserved posterior longitudinal ligament)

Anterior column and part of middle column

If no neural compromise, treat with TLSO brace

If canal stenosis present, retropulsed fragment may be reduced by ligamentous taxis in distraction

with posterior instrumented fusion

Unstable burst fracture

Anterior and middle columns with significant retropulsion,

or all three columns

Anterior decompression and instrumented fusion

Flexion-distraction seat belt injury (ligamentous)

Middle and posterior columns

Posterior reduction and instrumented fusion

Chance fracture (osseous)

Two or three columns but with good bone contact

TLSO brace

Shear fracture dislocation

Three columns

Instrumented fusion, anterior, posterior, or both

When overt instability is produced by a tumor, indications for surgery depend on the patient's life expectancy, physical condition, extent of cord compression, responsiveness to radiation and chemotherapy, number of motion segments involved by tumor, and severity of pain. The ideal candidate for decompression and fusion is a patient with limited systemic and spinal neoplastic disease who presents with an acute pathologic fracture with incomplete cord compromise.

Infections of the spine, if discovered early, may produce no neural compromise or instability and may be treated by antibiotics alone. However, advanced infections of the disks and vertebral bodies are highly destructive and destabilizing, requiring debridement/decompression and fusion, either simultaneously or in separate sessions.

Chronic overt instability

Chronic overt instability is initially managed conservatively (eg, with analgesics, anti-inflammatory drugs, physical therapy, or bracing). If and when the patient fails to respond to conservative management or if significant neurologic compromise exists, fusion is indicated.

Anticipated instability

Surgical removal of two columns of the spine (or removal of one column when another is known to be deficient), radical removal of one facet joint (see the image below), or partial substantial removal of both facet joints in one motion segment would be expected to produce instability. In these cases, it is prudent to consider fusion at the time of the original surgery.

Example of anticipated instability. (A) Large mass Example of anticipated instability. (A) Large mass affecting right C3-4 facet joint and lateral masses in patient with severe right-side neck and shoulder pain; (B, C) complete resection of tumor and simultaneous C3-4 anterior fusion to circumvent anticipated iatrogenic stability produced by radical resection of facet and lateral masses.

Covert instability

Like chronic overt instability, covert instability is initially managed conservatively, but with a much higher threshold for abandoning conservative treatment in favor of fusion.

Isolated spondylolysis without spondylolisthesis and spondylolisthesis without dynamic instability are typically treated conservatively with physical therapy and epidural steroid injections for at least 3-6 months. If back pain exists without radicular symptoms, greater effort is made to avoid surgery. The patient must quit smoking and demonstrate the ability to limit the intake of narcotics. With appropriate patient selection, good results can be achieved with fusion when conservative treatment has failed.

In symptomatic spinal stenosis without spondylolisthesis, decompression alone is the treatment of choice, but in spinal stenosis with degenerative spondylolisthesis of significance, fusion improves outcome.[17, 37]

Much more controversial is the treatment of that subcategory of covert instability that is known as microinstability or dysfunctional motion segment. Here, an abnormal disk or facet joint is presumed to be the pain generator. Provocative diskography and facet injections are often used in this setting to "locate" the pain generator. The idea is that fusion, by eliminating motion across the dysfunctional motion segment, may alleviate the pain.

This controversy and the relevant recommendations of the American Association of Neurological Surgeons (AANS)/Congress of Neurological Surgeons (CNS) Joint Section on Disorders of Spine and Peripheral Nerves are explored in greater detail elsewhere (see Prognosis).

Contraindications for spinal fusion

Absolute contraindications for fusion are relatively uncommon and include the following:

  • Diffuse multilevel neoplastic disease such that no adjacent normal segments exist for engagement of instrumentation
  • Severe osteoporosis such that the bones would not support instrumentation and fusion would not be expected to solidify in absence of instrumentation
  • Infection of soft tissues adjacent to spine or epidural infection that has not spread to the vertebral bones or disks, in which case the fusion construct would be at risk for infection (see below for established diskitis/osteomyelitis)

Relative contraindications for spinal fusion include the following:

  • Smoking
  • Malnutrition
  • Systemic infection
  • Chronic hypoxemia
  • Severe cardiopulmonary disease
  • Severe  depression, psychosocial issues, and secondary gain issues

As always, the contraindications for surgery have to be weighed against the risks of not performing the operation in each particular situation. For instance, smoking and severe depression may be contraindications for fusion in a patient with back pain and disk degeneration but should not deter the surgeon from fusing an unstable cervical spine fracture.

It is important to note that an active spine infection (diskitis/osteomyelitis) does not necessarily constitute a contraindication for fusion and instrumentation. To the contrary, advanced spine infections exert severe destabilizing effects on the spine, often necessitating stabilization at the time of debridement and decompression.

In this setting, careful clinical, laboratory, and radiographic follow-up are essential as the patient receives prolonged intravenous (IV) antibiotic treatment (for ≥6 weeks) to confirm eradication of the infection. Worsening pain or neurologic deficit, persistent fever, leukocytosis, or bacteremia and persistently elevated erythrocyte sedimentation rate (ESR) signal the possibility of persistent infection.

Similarly, evidence of loosening of screws on radiography (see the first image below) or evidence of increased bone destruction on computed tomography (CT) or magnetic resonance imaging (MRI) should be further investigated. However, persistent and stable vertebral enhancement on MRI (see the second image below) does not necessarily indicate persistent infection; this finding can lag behind microbiologic cure.

Loosening of this infected pedicle screw is eviden Loosening of this infected pedicle screw is evidenced by radiolucent halo (arrows) surrounding screw.
In this patient with T7-8 diskitis, vertebral enha In this patient with T7-8 diskitis, vertebral enhancement on MRI persisted 8 weeks after clinical and microbiologic cure.

Radionuclide bone scanning lacks specificity in this setting, but a tagged white blood cell (WBC) scan may be more useful. If there is doubt, CT-guided biopsy/aspiration of the region can help confirm the possibility of persistent infection, which would then be treated with reoperation.

Medical Therapy

In acute overt instability, stabilization of the spine is required in all cases. In this context, medical treatment refers to the use of external bracing for spine stabilization. If instability is due to an osseous fracture, if the fracture fragments can be reduced to near-anatomic alignment, and if there is no significant neural compression after reduction, the patient may be treated nonsurgically with a brace until the fracture heals.

In anticipated instability (eg, extensive diskitis and osteomyelitis treated with debridement, decompression and antibiotics), bracing may be used as a temporary means of stabilization, before fusion is undertaken or until spontaneous fusion occurs.

Many forms of external orthoses and braces are available. In the cervical spine, a halo offers the greatest amount of stabilization. Rigid cervical collars (eg, the Philadelphia collar and the Miami collar) and various cervicothoracic orthoses provide intermediate amounts of stabilization, whereas soft collars provide little stabilizing benefit. For the thoracic and lumbar spine, the only brace that provides significant stabilizing benefit is a rigid TLSO brace. Rigid lumbar braces that do not extend to the chest and soft braces/corsets provide little stabilizing benefit.

In chronic overt instability and covert instability, medical treatment plays a more prominent role. If not at risk for imminent neurologic deterioration, patients with these forms of instability generally undergo conservative (nonsurgical) treatment first. Fusion is reserved for those in whom conservative treatment fails (see Approach Considerations).

Conservative treatment may include some or all of the items below:

  • Medications - Analgesics, anti-inflammatories, muscle relaxants, tricyclic antidepressants, antiepileptics
  • Physical therapy
  • Behavior modification (smoking cessation, weight loss)
  • Injection therapy (eg, epidural or facet steroid injections)
  • Transcutaneous electrical nerve stimulation (TENS)
  • Psychological treatment (especially for depression)
  • Alternative treatments (eg, acupuncture, biofeedback)

General Surgical Considerations

Once the decision has been made to fuse a particular spine segment, there may be several surgical methods by which this task can be accomplished. After a particular method is selected, the etiology of instability is no longer relevant, because the technical steps would be the same. The most commonly employed fusion techniques in various regions of the spine are described in subsequent sections.

The video below illustrates a spinal fusion procedure for relief of chronic lower back pain. 

Spinal fusion surgery for relief of chronic lower back pain. Procedure performed by John Ciccarelli, MD, Shawnee Mission Medical Center, Shawnee, KS. Video courtesy of BroadcastMed (https://www.broadcastmed.com/orthopedics/4033/videos/spinal-fusion-surgery-for-relief-of-chronic-lower-back-pain).

Preparation for surgery

Routine preoperative tests usually consist of complete blood count (CBC), electrolytes, blood urea nitrogen (BUN), creatinine, glucose, prothrombin time (PT), activated partial thromboplastin time (aPTT), international normalized ratio (INR), chest radiography, and electrocardiography (ECG). Blood is typed and screened. If extensive blood loss is anticipated, one or two units of packed red blood cells (RBCs) are cross-matched or a cell saver is used. Alternatively, if the procedure is scheduled electively, the patient may donate autologous blood several weeks beforehand.

Thigh-high compression stockings (TED hose) and sequential compression devices are applied preoperatively for prophylaxis of deep vein thrombosis (DVT) and are not removed until the patient is mobilized postoperatively.

In patients who are at particular risk for DVT and pulmonary embolism (PE; eg, those mwho are paraplegic, quadriplegic, or bedbound prior to surgery), subcutaneous injections of low-molecular-weight heparin (LMWH) may begin before the operation, with the individual patient's risk of postoperative epidural hematoma carefully weighed against the risk of PE. Meticulous attention to hemostasis, liberal use of closed wound drainage, and careful postoperative neurologic evaluation are indispensable when heparin is used.

An antibiotic with antistaphylococcal activity, usually a first-generation cephalosporin, is given within 1 hour prior to the skin incision and continued for three doses postoperatively.

Intraoperative details

The following are general concepts pertaining to intraoperative management of all fusion procedures. Intraoperative details specific to each fusion technique are provided in subsequent sections. 

Positioning

For posterior cervical procedures, a prone position is preferred. Although some surgeons use a sitting position to minimize bleeding from epidural veins, this position puts the patient at risk for intraoperative hypotension and venous air embolism. Meticulous surgical technique, judicious use of bipolar coagulation, and use of an operating microscope when necessary permit all posterior cervical procedures to be performed safely in prone position.

The patient's head may be immobilized by means of three-point skeletal fixation in a Mayfield head holder, which also permits precise control of cervical contour. If the head is positioned over a foam or horseshoe head holder instead, special attention should be given to avoiding compression of the eyes, which could result in raised intraocular pressure and retinal ischemia.

For posterior lumbar and thoracolumbar fusions, the patient is positioned prone over a frame or table that permits the abdomen to hang free. Otherwise, the increased intra-abdominal pressure would interfere with venous return and would increase intraoperative bleeding. The Wilson frame fulfills this requirement and provides the fastest and least cumbersome means for positioning the patient. Certain other spine frames and tables (eg, the Andrews table) allow the patient to be positioned in a knee-to-chest position.

The resultant lumbar flexion facilitates access to the spinal canal and disk spaces by increasing the interlaminar and posterior interbody distances. However, if the patient is fused in this position, the natural lumbar lordosis is lost, resulting in "flat back" syndrome. Other spine tables, such as the Jackson table, which allows the patient to be flipped from supine to prone position and vice versa, are useful for combined anterior-posterior procedures. Regardless of the position or frame used, all pressure points must be carefully padded to prevent compression neuropathy.

Fluoroscopy

Intraoperative fluoroscopy is essential for safe and accurate instrumentation of the spine. A radiolucent frame should be used when applicable to allow for lateral and anteroposterior (AP) fluoroscopy. The C-arm should be draped in sterile fashion and positioned so that it can be readily moved in and out of imaging position. (See the image below.)

Modern operating room setup for spine surgery with Modern operating room setup for spine surgery with fluoroscopy unit, neurophysiologic monitoring equipment, operating microscope, and digital radiology monitors.

Alternatively, fluoroscopy-based stereotactic navigation can be used, wherein a computer with sophisticated stereotactic software permits virtual fluoroscopic navigation throughout the surgery. In this case, AP and lateral fluoroscopic images are taken after the spine is exposed and a bone-mounted stereotactic frame is attached. The C-arm is then removed and the procedure is performed on the basis of computer-assisted navigation of the original fluoroscopic images.

Neurophysiologic monitoring

Intraoperative neurophysiologic monitoring for spine procedures consists of the use of somatosensory evoked potentials (SSEPs), motor evoked potentials (MEPs), or electromyography (EMG) to detect and correct factors that lead to neurologic compromise during the surgical procedure. During pedicle screw placement, stimulation of the pedicle probe with a nerve stimulator at subthreshold currents would result in EMG activity in the lower extremities if the probe is in contact with the nerve root, prompting its repositioning.

Microdissection under microscopy

Microdissection under an operating microscope allows safer decompression of neural elements, particularly in settings where visualization is limited (eg, anterior cervical diskectomy and osteophyte resection). The microscope can be moved out of position when the more delicate decompression phase of the procedure is completed before proceeding with fusion and instrumentation.

Anesthesia

Anesthetic considerations during fusion surgery include ensuring adequate blood pressure and fine-tuning muscle relaxation during the operation while maintaining the depth of anesthesia. Muscle relaxation facilitates the initial exposure of the spine but must be avoided or reversed if intraoperative MEP or EMG is to be used. When muscle relaxation is not present, it is important to maintain deep anesthesia to prevent patient movement during critical parts of the procedure, such as decompression of the spinal cord.

Adequate blood pressure must be maintained at all timed to avoid neural ischemia, which could exacerbate existing neural injury. In patients with labile blood pressure or cardiopulmonary risk factors, an arterial line may be inserted for continuous monitoring. If adequate peripheral venous access is not available, a central line is inserted. If the procedure is expected to last longer than 2 hours, a bladder catheter is inserted.

Surgical Therapy for Atlantoaxial (C1-2) Instability

Atlantoaxial instability may be caused by a variety of conditions, including the following:

C1-2 fusion with cable fixation

Before the advent of screw fixation techniques, wiring of C1-2 posterior elements with an interposed bone graft was the only method of fusion of the atlantoaxial segment. The monofilament wires once commonly used are being abandoned in favor of multistranded braided cables, which offer greater flexibility, strength, and fatigue resistance.

The following three techniques are available:

  • Gallie technique
  • Brooks technique
  • Sonntag technique

In the Gallie technique (see the image below), a cable loop is passed under C1 posterior arch from below, folded over C1, and hooked under the base of the C2 spinous process. The free ends of the cable are then brought together in the midline to secure a unicortical onlay bone graft against the decorticated surfaces of C1 and C2 laminae. Although relatively safe and easy to perform, this technique provides little rotational stability and has a higher pseudarthrosis rate as a consequence of the onlay nature of the graft.

C1-2 fusion with cable fixation (Gallie technique) C1-2 fusion with cable fixation (Gallie technique). In this case, fusion is supplemented with transarticular screws.

In the Brooks technique (see the image below), one (central) or two (lateral) bicortical bone grafts are wedged betweenthe  C1 posterior arch and the C2 lamina. A tension band is constructed by passing two separate cables under both C1 and C2 laminae and attaching their free ends posterior to the graft(s). The cancellous surfaces of the graft are in good contact with the decorticated undersurface of the C1 arch and the top rim of C2 lamina, placing the graft(s) under compression and thus enhancing fusion rates. In addition, this technique provides greater rotational stability by virtue of bilateral engagement of the C2 lamina. 

C1-2 fusion and cable fixation (Brooks technique). C1-2 fusion and cable fixation (Brooks technique). Image courtesy of Synthes, Inc.

The problem with the Brooks technique is that sublaminar passage of the cable under both C1 and C2 substantially increases the technical difficulty and risk of spinal cord injury during the procedure, particularly if the canal diameter is already compromised by the underlying pathology.

The Sonntag technique (see the image below) combines the ease and safety of the Gallie technique with the superior biomechanical features of the Brooks technique. Here, the cable loop is passed only under the C1 lamina and hooked under the C2 spinous process base, as in the Gallie technique. The difference is that the bicortical bone graft is wedged between C1 and C2 posterior elements, as in the Brooks technique, and the free ends of the cables are attached under the C2 spinous process base.

C1-2 fusion with cable fixation (Sonntag technique C1-2 fusion with cable fixation (Sonntag technique): coronal (left) and sagittal (right) CT reconstructions.

Because the Sonntag technique, unlike the Brooks technique, does not involve the use of a C2 sublaminar wire to protect against anteropulsion of the graft, a notch is made in the inferior portion of the graft, and it is wedged over the superior surface of the C2 spinous process.

Regardless of the technique used for C1-2 cable fixation, a halo is generally applied until fusion occurs (usually 3-6 months). This is a major drawback from the standpoint of patient comfort and rehabilitation.

C1-2 transarticular screw fixation

Screw fixation of the atlantoaxial segment provides immediate rigid fixation of the joint and eliminates the need for a halo. The main consideration is the risk of injury to the vertebral artery. In about 18% of cases, the vertebral artery rides high after emerging from the C2 transverse foramen, positioning itself in the path of the screw. Before surgery, it is imperative to perform high-resolution computed tomography (CT) with sagittal reconstructions to detect this variant anatomy and avoid screw insertion on that side.

The technique is as follows. The patient is placed in a prone position, and the head is carefully flexed under fluoroscopic guidance and fixed in a Mayfield head holder. An incision is made from the skull base to C7. In addition, small stab incisions may be necessary more inferiorly and laterally to permit placement of the drill in the correct trajectory. C1, C2, and C3 are exposed to the lateral margin of the lateral masses.

The ligamentum flavum above C2 is removed to expose the C2 nerve root (greater occipital nerve), which runs posterior to the facet joint, unlike any other location in the spine. The nerve and its surrounding venous plexus are retracted superiorly to expose the facet joint. A 2.5-mm drill is used to establish the crew trajectory, starting at the C2-3 facet edge about 2-3 mm lateral to the medial aspect of the lateral mass.

Drilling is performed in 2-mm increments pointing 10º medially in a cephalad trajectory aimed at the posterior cortex of the anterior arch of C1 under continuous lateral fluoroscopic imaging. As the drill crosses the C1-2 articulation, decreased mobility of C1 is often immediately palpable. To correct any subluxation, C1 may be pushed or pulled in anterior or posterior directions before the drill crosses the facet joint.

The drill hole is then filled with a self-tapping screw of the appropriate length; alternatively, the hole can be tapped before the screw is inserted (see the image below). The procedure is then repeated on the opposite side. The posterior surfaces of the C1 and C2 lateral masses and the posterior aspect of the facet joint are decorticated with a drill and packed with cancellous bone graft.

C1-2 transarticular screw. Note proximity of verte C1-2 transarticular screw. Note proximity of vertebral artery to typical screw trajectory.

If vertebral artery injury is encountered on one side, the screw is left in place, and screw placement on the opposite side is avoided in order to prevent bilateral injury. Postoperative vertebral angiography is performed to rule out pseudoaneurysm formation.

C1-2 transarticular screw fixation is best supplemented with C1-2 cable fixation in order to provide a better bone substrate for fusion than what can be packed in the facet joints.

Postoperatively, the patient is placed in a Philadelphia collar.

C1-2 lateral mass/isthmus fixation

When preoperative CT reveals that a high-riding vertebral artery would be in the trajectory of a C1-2 screw, an alternative technique can be employed. A screw is inserted in the C1 lateral mass. A second screw is inserted into the C2 isthmus. The two screws are then connected with a rod or a plate. The procedure is repeated on the opposite side. The technique for exposure is identical to C1-2 transarticular screw placement. After the ligamentum flavum is resected, the medial wall of the C2 isthmus is exposed and palpated with a Penfield 4 instrument.

Although the isthmus is sometimes called the C2 pedicle, this is not strictly correct. The isthmus is a tubular structure that courses medially and superiorly, connecting each C2 lateral mass to the body, and is more correctly identified as the pars equivalent.

The drill and screw are directed along the visualized trajectory of the isthmus. The entry point is at the center of the lateral mass, and the trajectory is angled 25º medially and 25º cephalad. The more medial trajectory of the screw in this technique helps avoid vertebral artery injury. Palpation of the medial wall of the isthmus during screw insertion helps avoid breach of the spinal canal.

Odontoid screw fixation

Odontoid fractures are categorized according to the scheme in Table 4 below and treated according to the algorithm in Table 2 above (see Approach Considerations).

Table 4. Odontoid Fracture Classification (Open Table in a new window)

Type

Fracture Anatomy

1

Fracture through odontoid tip (rare)

2

Fracture across base of odontoid process (most common)

2A

As in type 2, except with comminution of fracture line, reducing possibility of healing of fracture in halo or with odontoid screw

3

Fracture extension into C2 vertebral body; because of larger bone contact area, fracture usually heals well in halo

Odontoid screw fixation is reserved for certain type 2 odontoid fractures—specifically, those that are reducible to less than 3 mm of fracture fragment displacement and are not associated with rupture of transverse ligament or a tumor in C2. The main advantage of this technique is that it directly repairs the odontoid fracture, thus avoiding a C1-2 fusion and maintaining range of motion (ROM). Its shortcoming is the limited circumstances in which it can be employed.

In addition, type 2A fractures, fractures in patients older than 65 years, slanted fracture lines, and old nonhealing fractures may be better treated by means of C1-2 fusion; odontoid screw fixation is less likely to yield a satisfactory outcome in these circumstances.

The technique is as follows. An anterior approach under biplanar fluoroscopy is followed. The neck is slightly extended. A transverse incision is created on the right side of the neck, overlying the C5-6 interspace. Dissection is carried down to the spine, as in anterior cervical fusion (see Surgical Therapy for Subaxial Cervical Instability). A plane is developed cephalad along the anterior aspect of the spine to the C2-3 disk space.

An Apfelbaum retractor specifically designed for this technique is deployed. A Kirschner wire (K-wire) is placed at the anterior-inferior margin of C2, and a shallow pilot hole is drilled under fluoroscopy toward the odontoid tip. The entry point is along the midline if one screw is desired and lateral to the midline if two screws are intended.

A reamer is placed over the K-wire to create a tangential slot along the anterior margin of C3 and the C2-3 disk space. The reamer is removed and replaced with a drill guide. The spikes of the drill guide are tapped into the anterior body of C3 to stabilize it. The K-wire is removed and replaced with a drill through the drill guide.

Drilling is performed toward the odontoid tip. The screw hole is tapped, and a lag screw of appropriate length is screwed in to engage the cortex of the odontoid tip. The lag screw will help compress the fracture fragment against the C2 body. If a second screw is inserted, it does not have to be a lag screw. A Philadelphia collar is applied postoperatively for 3 months.

Surgical Therapy for Subaxial Cervical Instability

Anterior cervical fusion

Anterior cervical fusion (see the image below) is one of the most commonly used fusion techniques in spine surgery. The anterior approach is increasingly used in preference to the posterior approach to the cervical spine[13]  because it provides distinct advantages with regard to decompression, fusion, and instrumentation.

Anterior cervical plate, applied in this case afte Anterior cervical plate, applied in this case after two-level anterior cervical diskectomy and fusion. Image courtesy of Synthes, Inc.

Most pathologic processes in the cervical spine, especially degenerative and neoplastic disorders, affect structures anterior to the spinal cord. An anterior approach to the cervical spine permits thorough decompression of the spinal canal without manipulation of the spinal cord. Furthermore, such an approach permits placement of the bone graft in an interbody position under compression, which significantly enhances the success of fusion. Finally, the relatively large surface areas and volumes of the vertebral bodies as compared with the posterior cervical elements render them ideal substrates for instrumentation.

The most common indications for anterior cervical fusion are in treatment of degenerative disorders. Large central disk herniations with cord compression and chronic disk-osteophyte complexes at one or more levels cannot be safely removed via a posterior approach. (See the images below.) Cervical spinal stenosis associated with kyphosis is best treated via an anterior approach because a multilevel posterior decompression by laminectomy does not relieve the stretching of spinal cord over disk-osteophytes and may exacerbate the kyphosis in the long run. 

Large central disk herniations (A, B) and cervical Large central disk herniations (A, B) and cervical spondylotic myelopathy with kyphosis (C) are two common indications for anterior cervical diskectomy and fusion.
C5-6 bilateral jumped facets associated with disk C5-6 bilateral jumped facets associated with disk herniation (left) was treated with C6 anterior cervical decompression and fusion (right).

Anterior cervical diskectomy is effective not only for treatment of neural compression (myelopathy and radiculopathy), but also for treatment of chronic axial pain associated with disk degeneration and correction of spinal deformity (kyphosis).

Anterior cervical corpectomy (removal of vertebral body) is employed when the pathology extends behind the vertebral body anterior to the cord (eg, large osteophytes or ossification of the posterior longitudinal ligament) or involves the vertebral body itself (eg, tumor or burst fracture). Traumatic cervical dislocations associated with disk herniation should be treated via an anterior approach.

The technique is as follows. The patient is placed on the operating table in supine position. Rolls are placed under the shoulders and under the hip from which iliac crest graft is to be harvested. The arms are padded and tucked by the patient’s sides. The shoulders are taped to the foot of the bed to permit unencumbered visualization of the spine by fluoroscopy. The fluoroscopic C-arm is positioned in cross-table lateral orientation and draped.

The skin incision is typically made on the right side of the neck for a right-handed surgeon and on the left side for a left-handed surgeon; this significantly facilitates access. If the C7-T1 disk is the target, some surgeons prefer a left-side approach to minimize the risk of recurrent laryngeal nerve (RLN) palsy, though this puts the thoracic duct at risk. If unilateral RLN palsy is present preoperatively, surgery must be performed from the side of the palsy to avoid the risk of bilateral RLN palsy.

A transverse skin incision over a skin crease centered along the anterior border of the sternocleidomastoid muscle provides the best cosmetic result for one- and two-level fusions. With practice, a three-level fusion can also be performed through a transverse incision. An oblique vertical incision along the anterior border of the sternocleidomastoid is used if greater rostrocaudal exposure is required.

The platysma is divided in line with the skin incision. A subplatysmal dissection is carried out. A large external jugular vein is best mobilized and retracted to the side, whereas a smaller one can be ligated and divided.

An avascular plane is developed medial to the sternocleidomastoid and carried medial to the carotid sheath to reach the anterior border of the cervical spine. Gentle medial retraction of the midline structures significantly facilitates this task.

The omohyoid muscle courses obliquely in this region from an inferolateral to a superomedial location. For C3-4, C4-5, and C5-6 disks, the plane of dissection is superior to the omohyoid, whereas for C6-7 and C7-T1 disks, it is below the omohyoid. Occasionally, for an extensive procedure that spans several segments above and below C6, the omohyoid is divided and later reapproximated. 

Small tributaries to the internal jugular vein running transversely across the field of exposure can be coagulated and divided. If the common facial vein is hindering the exposure of the upper cervical spine, it can be ligated and divided. The carotid sheath is never entered. If the ansa cervicalis is encountered, it can be mobilized either medially or laterally.

In very high exposures, care is taken to prevent injury to the hypoglossal nerve deep to the digastric muscle. In very low exposures, care is taken to spare any neural structures that might correspond to a variant crossing of the recurrent laryngeal nerve.

The prevertebral fascia is opened and the esophagus and pharynx are retracted toward the contralateral side. A transverse cervical artery, often accompanied by a vein, is usually identified over the C7 vertebral body in the superior extension of the mediastinal fat pad. This artery and the fat pad can usually be swept inferiorly and preserved. If exposed, this artery should be carefully inspected at the end of the procedure to make sure that it is intact. If damaged by stretching, it must be ligated to avoid the risk of a catastrophic postoperative neck hematoma.

The attachments of longus coli muscles to the anterolateral aspects of the vertebral bodies above and below the target site are divided. Bleeding from muscle edges and bone is controlled with electrocautery and bone wax. Excessive use of the monopolar electrocautery is avoided so as to prevent the risk of thermal damage to the nearby sympathetic chain and resultant Horner syndrome.

If large anterior osteophytes are present, they are resected flush with the anterior surface of the vertebral bodies. An anterior cervical self-retaining retractor is inserted and its lips secured under the mobilized edges of the longus coli muscles. If sufficient dissection of the longus coli muscles is not carried out, the retractors will not remain in place, creating a significant nuisance during the remainder of the case. Many contemporary retractors have blades that distract not only medially and laterally but also superiorly and inferiorly, providing excellent exposure.

Caspar posts are inserted in the midportion of the vertebral bodies above and below the target site under fluoroscopic guidance. The Caspar distractor is then used to distract the disk space(s). The anterior longitudinal ligament and the anterior annulus of the disk are resected. Under the operating microscope, the contents of the disk(s) are thoroughly evacuated to expose the converging posterior lips of the superior and inferior endplates and the intervening posterior annulus of the disk.

The posterior annulus is temporarily left in place as a protective shield while the posterior lips of the endplates and the underlying osteophytes are meticulously drilled with a bur under the operating microscope until they are reduced to thin shells of cortical bone. The posterior annulus of the disk is then completely resected. The herniated disk material is removed.

The residual osteophyte shells are elevated away from the dura with a small hook or a small up-angled curette and removed. The posterior longitudinal ligament is completely resected from side to side to fully expose and decompress the dura. In rare instances, an ossified or thickened posterior longitudinal ligament is fused to the dura and cannot be completely resected without risking dural laceration.

The medial aspects of the uncinate processes are resected to ensure decompression of the symptomatic nerve roots. A nerve hook is passed laterally into the neural foramina to ensure their patency. Bleeding from the lateral epidural veins is readily controlled with Gelfoam.

Care is taken during lateral dissection to avoid injury to the vertebral arteries, which are located lateral and anterior to the uncovertebral joints. A careful review of the axial images from preoperative CT or MRI will provide useful information about the proximity of the vertebral artery to the neural foramina and any aberrant looping of that vessel into the normal course of the exposure.

If a corpectomy is to be performed, the disks above and below the vertebral body are first resected, as described above. The anterior aspect of the vertebral body is readily resected with a large rongeur. The posterior half of the vertebral body is carefully drilled until the posterior wall is thinned down to a shell of bone. Venous bleeding from the lateral walls of the resected vertebral body is controlled with bone wax. The posterior cortex and posterior longitudinal ligament are carefully elevated away from the dura and resected.

After satisfactory decompression is obtained, the endplates are decorticated with a drill, in preparation for fusion. The cartilaginous endplate is removed, but excessive removal of the bony endplate is avoided in order to minimize settling of the graft. Posterior ledges in the endplates are left behind to prevent retropulsion of the graft.

The height of the interspace is measured. An appropriately sized tricortical bone graft is harvested from the anterior iliac crest or fashioned out of cadaveric allograft bone. The graft is inserted into the interspace and tapped in place under fluoroscopic guidance. The distraction is then released and the distraction posts removed. An appropriately sized anterior cervical plate is affixed to the vertebral bodies above and below the fused segment(s) with cancellous screws under optional fluoroscopic guidance. The screws are locked with the locking mechanism specific to the plate. (See the image below.)

Anterior cervical diskectomy and fusion. (A) Disk Anterior cervical diskectomy and fusion. (A) Disk removed and interspace prepared to receive graft; (B) iliac crest bone graft harvested; (C) bone graft; (D) graft inserted into disk space; (E) plate screwed to anterior surface of vertebral bodies.

The retractors are removed. Hemostasis is secured. The wound is irrigated. The platysma is closed with absorbable sutures. The skin is closed in subcuticular fashion.

Some surgeons do not perform a fusion after an anterior cervical diskectomy. This is only appropriate if the operation consists of a limited central single-level diskectomy for a soft central disk herniation in the absence of spondylosis, wherein comprehensive removal of the posterior longitudinal ligament and uncovertebral joints has not been performed. Even if spontaneous fusion occurs as desired in these cases, the resultant reduction in foraminal height may predispose the patient to future nerve-root compression.

It is acceptable to perform a single-level diskectomy and fusion without plating, relying on the tension band provided by the middle and posterior columns to promote stability and fusion. A plate is employed for all multilevel diskectomies and corpectomies.

No collar may be needed, or a soft collar or rigid cervical collar may be applied for 1-6 weeks postoperatively, depending on the extent of the procedure. After multilevel corpectomies, a halo is considered.

Lateral mass screws

Excellent stabilization of the subaxial cervical spine can be achieved via a posterior approach by using lateral mass screws. Here, screws are inserted into the lateral masses at trajectories designed to avoid the vertebral artery and cervical nerve roots. The adjacent screws are linked by bilateral plates or rods. This procedure can be employed whether or not a previous laminectomy has been performed.

The following two techniques are available:

  • Magerl technique
  • Roy-Camille technique

In the Magerl technique, the entry point is 2 mm medial and 2 mm superior to the center of lateral mass, and the screw trajectory is 20-25º lateral and parallel to the facet surface in the sagittal plane. In the Roy-Camille technique, the entry point is just above the center of the lateral mass, and the trajectory is only 10º lateral and perpendicular to the lateral mass surface in the sagittal plane. Although the Magerl technique leaves a greater safety zone between the screw tip and the nerve root, achieving the cranial angulation that this technique requires during surgery is not always easy.

Before rods or plates are placed, the facet joints and lateral mass surfaces are decorticated and packed or covered with cancellous bone. If laminae are present, corticocancellous strips of bone graft can be placed over the decorticated laminae and wired in place.

Posterior cervical wiring

Monofilament wires, double-stranded twisted wires, Drummond wires (wire loops passing through a button), and braided multistranded cables can be used for wiring of posterior cervical elements.

Spinous process wiring is the simplest technique but provides no limitation of extension and little rotational stability. In this technique, a wire is passed through the center of one spinous process around the base of a lower spinous process. Multiple wire loops can be used to connect multiple spinous processes in an interlocking chain array. More complicated spinous process wiring patterns (eg, the Bohlman triple-wire technique) allow firm attachment of bone graft strips to the sides of the spinous processes.

Sublaminar wiring involves the use of a loop of wire that is carefully passed under the lamina and then cut in half. Each wire is then brought laterally to each side and tightened around a rod or bone graft. The technique is repeated for two or more laminae.

When a laminectomy has been performed, the only available structure for wiring is the facet joint. Wires passed through the midportion of the inferior articulating facet are tightened around a rod or bone graft strip.

In general, wiring techniques are inferior to anterior or posterior screw/plate/rod fixation methods.

Surgical Therapy for Thoracic and Thoracolumbar Instability

Posterior thoracic and thoracolumbar instrumentation

Since the advent of Harrington rods for posterior thoracolumbar stabilization and deformity correction,[38]  posterior instrumentation constructs have evolved substantially. Harrington rods were nonsegmental systems attached to the spine via hooks at proximal and distal ends of long rods and relied primarily on distraction for deformity correction. Modern posterior thoracolumbar instrumentation constructs differ from Harrington rods in several key aspects, as follows:

  • They are segmental; that is, they are attached to the spine not only at the proximal and distal ends of rods but also at multiple intervening segments
  • They are modular and hybrid, allowing simultaneous use of hooks (laminar hooks, transverse process hooks, pedicle hooks), wires, and pedicle screws
  • They rely more on three-point-bending biomechanical principles than on distraction for deformity correction and stabilization

These instrumentation systems can be used for treatment of acute overt instability due to trauma, neoplasms, or infections, in addition to deformity correction. Hooks can be applied above a lamina, facing down, or below a lamina, facing up. When two hooks are placed on the same lamina or adjacent laminae, facing each other, a "claw" construct is formed. Claw constructs help secure the ends of the rods against the spine.

When placement of laminar hooks is not possible (because of laminectomy) or desirable, transverse process hooks can be used. Hooks can also be placed under pedicles (inserted through the thoracic facet joint and screwed to the undersurface of the pedicle).

When the construct relies primarily on hooks, a long section of the spine is instrumented (generally three segments above and two below) to prevent construct failure. The segments receiving bone graft would be shorter than the instrumented segments (the "rod long, fuse short" principle). Pedicle screws are playing an increasingly prominent role in segmental modular constructs in the thoracic and thoracolumbar regions. The greater stability conferred by the screw-based systems allows construction of shorter constructs spanning the unstable motion segment. (See the image below.)

Modular posterior thoracolumbar instrumentation sy Modular posterior thoracolumbar instrumentation system, which is attached to spine by combination of screws and hooks, in turn attached to long rods. In this case, it is used for correction of scoliosis, using three-point bending biomechanical principles. Image courtesy of Synthes, Inc.

Posterior systems also allow reduction of anterior (vertebral body) fractures by using segmental distraction (see the image below). Above all, use of posterior instrumentation for thoracic and thoracolumbar spinal instability is clearly augmented by the fact that exposure of this portion of the spine is far easier via a posterior approach than via an anterior approach.

Example of application of biomechanical principles Example of application of biomechanical principles to spine surgery. Insertion of special pedicle screws (Schanz screws) pivoting on rod transfers instantaneous axis of rotation (IAR) to screw-rod interface. Compression of proximal end of screws produces distraction-reduction of vertebral burst fracture. If posterior longitudinal ligament is intact, retropulsion is corrected by ligament taxis. Image courtesy of Synthes, Inc.

Anterior thoracic and thoracolumbar instrumentation

There are instances in which thoracic and thoracolumbar instability cannot be adequately addressed through a posterior approach and for which an anterior approach is therefore preferred. These instances include the following:

  • Pathologic fractures of the vertebral bodies due to tumor or infection that produce cord compression
  • Some traumatic burst fractures (associated with retropulsion and rupture of posterior longitudinal ligament) that cannot be reduced by means of posterior distraction

The upper thoracic region (T1-3) is approached anteriorly by extending an anterior cervical approach inferiorly to the mediastinum via a partial or complete median sternotomy. The fusion and instrumentation methods here are identical to those employed in anterior cervical procedures.

The midthoracic region (T4-11) is generally approached through a transthoracic approach from the left side. This usually involves a thoracotomy with opening of the pleura and deflation of the ipsilateral lung, which provides excellent exposure of the entire T4-11 region from an anterolateral perspective. Alternatively, an extracavitary (extrapleural) approach can be used, which provides a more limited rostrocaudal and anterior exposure.

The thoracolumbar region (T12-L2) is approached through an anterior thoracolumbar approach, which combines a thoracotomy with a retroperitoneal approach to the upper lumber spine and requires division and mobilization of the diaphragm.

When a thoracic or thoracolumbar corpectomy is performed, the corpectomy gap must be reconstructed in the same fashion as a lumbar corpectomy (see above). For this purpose, expandable cages are available, which are placed in the corpectomy defect and expanded to engage and distract the adjacent vertebral bodies (see the image below). The cages are then filled with bone and supplemented with a plate. The plate is applied to the lateral surface of the vertebral bodies and serves as a tension band construct.

Anterolateral thoracic corpectomy followed by reco Anterolateral thoracic corpectomy followed by reconstruction with expandable cage and fixed plate-screw system. Image courtesy of Synthes, Inc.

Besides permitting more thorough decompression of anterior pathology, anterior thoracic and thoracolumbar reconstructions enable the surgeon to limit the instrumented fusion to the pathologic motion segment, sparing the adjacent segments. (See the image below.)

Anteroposterior and lateral radiographs of anterio Anteroposterior and lateral radiographs of anterior thoracic corpectomy and reconstruction for pathologic fracture due to vertebral osteomyelitis.

Surgical Therapy for Lumbar Spinal Instability

Posterior and posterolateral noninstrumented lumbar fusion

Noninstrumented posterior or posterolateral fusion of the lumbar spine is fairly simple to perform and is an acceptable treatment for certain instances of degenerative instability, when the patient is not believed to be a candidate for pedicle screw insertion. Because of its greater susceptibility to pseudarthrosis, it is not recommended for situations in which overt instability is present. The patient is kept in a TLSO brace until the fusion solidifies.

The technique is as follows. The lumbar spine is exposed in a standard fashion through a posterior midline incision. Bilateral exposure of the laminae is extended further laterally to completely expose the facet joints and transverse processes of the vertebrae to be fused.

Usually, a decompressive laminectomy is carried out to treat neural compression. In this process, medial facetectomies may be carried out to fully decompress the lateral recesses, if necessary. The transverse processes, the lateral aspect of the facet joints, and the synovial facet surfaces are decorticated with a high-speed drill.

Corticocancellous strips of bone are harvested from the posterior iliac crest and placed in the "lateral gutters" over the lateral aspect of the facet joints and transverse processes. The space between the articulating surfaces of the facet joints is packed with cancellous bone graft. The fusion mass may be supplemented with cortical bone obtained from the laminectomy.

If the laminar surfaces have not been completely removed (eg, if only a laminotomy has been performed), bone graft can be applied to the decorticated laminae and spinous processes to produce a true posterior fusion. The posterolateral fusion may be supplemented with a posterior lumbar interbody fusion (PLIF).

Instrumented lumbar fusion with pedicle screws

Pedicle screw fixation is the most commonly used approach for internal stabilization of the lumbar spine. Screws are inserted into the pedicles of the vertebrae to be fused and connected to each other with bilateral rods or plates.

The technique is as follows. The spine is exposed (and decompressed if necessary) as it would be for a noninstrumented fusion. Pedicle screws are inserted into the pedicles above and below the motion segment to be fused. The main concern during screw insertion is to avoid breach of the pedicle wall and injury to the exiting nerve root.

If a laminectomy or upper laminotomy has been carried out, it will be possible to visualize or palpate the medial and inferior surfaces of the pedicle, which are in contact with the nerve root. In this case, only lateral fluoroscopy is necessary to guide the entry and trajectory of the screw in the sagittal plane. (See the image below.) If the pedicle has not been exposed via laminectomy or laminotomy, AP and lateral fluoroscopy are usually used. The inferolateral aspect of the pedicle can also be exposed by subperiosteal dissection via a lateral approach along the base of the transverse process.

Intraoperative fluoroscopy for pedicle screw inser Intraoperative fluoroscopy for pedicle screw insertion.

The entry point to the pedicle is located at the junction of lines bisecting the transverse process and the superior articulating facet. A starting hole is created at the entry point with an awl or a drill. A pedicle probe is then used to establish the path of the screw under fluoroscopic guidance. The probe may be stimulated with a nerve stimulator to elicit EMG activity from the lower extremities. If EMG is elicited at low stimulation currents, contact between the probe and the nerve root is suspected and the probe is removed and reinserted differently.

The trajectory in the sagittal plane is parallel to the superior endplate for L1-5 and toward the sacral promontory for S1. In the axial plane, the probe is directed slightly medially into the vertebral body. As one descends the lumbar spine, the medial angulation of the pedicles increases. After the probe is removed, the screw path is tapped and the appropriately sized screw inserted. In order to avoid penetration of the anterior cortex of the vertebral body, the screw should not extend beyond 80% of the diameter of the vertebral body on lateral fluoroscopy.

Once all of the screws have been inserted, they are usually linked with rods, though notched plates may also be used for this purpose. (See the image below.) Depending on the instrumentation system used, the screw-rod interface may be fixed, requiring extensive contouring of the rod to fit the screw heads, or variable, requiring minimal contouring.  Before the screw-rod interface is tightened and locked, the pedicle screws can be used to distract or compress the vertebral bodies. 

Pedicle screw fixation of lumbar spine. Image cour Pedicle screw fixation of lumbar spine. Image courtesy of Synthes, Inc.

If reduction of spondylolisthesis is desired, the inferior screw is locked to the rod and the superior screw pulled back toward the rod, with the latter acting as a cantilever beam. If adequate decompression of the spinal canal and nerve roots has been carried out by laminectomy and partial facetectomy, it may not be necessary to reduce the spondylolisthesis, in which case the fusion is said to have been performed in situ. Some surgeons prefer to attempt to reduce all spondylolisthesis in order to restore sagittal balance.

After pedicle screw insertion, a posterolateral bony fusion is performed as previously described. Instrumented posterolateral fusion can be further supplemented by interbody fusion (see below), thus producing a global fusion.

Lumbar interbody fusion

Lumbar interbody fusion refers to replacement of disk space with bone. Because of the substantial surface area of the vertebral endplates and the fact that the bone graft is under compression, interbody fusion enjoys a favorable fusion environment. Ideally, it is supplemented with pedicle screw instrumentation to provide internal fixation; it may or may not be further supplemented with posterolateral fusion. (See the image below.)

Combined interbody and posterolateral lumbar fusio Combined interbody and posterolateral lumbar fusion with pedicle screws: coronal (left) and sagittal (right) CT reconstructions.

Interbody fusion can be performed through the following approaches:

  • Posterior approach - PLIF
  • Posterolateral approach - Transforaminal lumbar interbody fusion (TLIF)
  • Anterior approach - Anterior lumbar interbody fusion (ALIF)
  • Far lateral approach - Extreme lateral interbody fusion (XLIF) 

The original PLIF procedure was performed through a routine posterior exposure for lumbar diskectomy. After the disk space was thoroughly evacuated, the endplates were decorticated with large angled curettes and bone rasps. The disk space was then packed with autologous cancellous bone.

In current practice, PLIF, TLIF, and ALIF are usually performed with the aid of interbody implants. The implants (made of a PEEK polymer, machined cortical allograft bone, or metal) are filled with cancellous bone before insertion into the disk space. The remaining disk space around the implant is also packed with cancellous bone. These three techniques differ only in the method of insertion of the interbody implant.

Posterior lumbar interbody fusion

Laminectomy (or bilateral hemilaminectomy), bilateral medial facetectomy, and bilateral diskectomy are carried out at the target segment. The traversing and exiting nerve roots are identified bilaterally, and the intervening epidural veins are coagulated with a bipolar device and divided. The endplates are thoroughly decorticated.

The traversing nerve roots and the dural sac are retracted medially as the interbody implants are inserted bilaterally under fluoroscopic guidance. Depending on the type of the implant, specific instruments are used for preparation of the disk space and insertion of the implant.

Transforaminal lumbar interbody fusion

This technique is usually performed unilaterally and does not require an extensive laminectomy. It lends itself to open or minimally invasive approaches.

A partial facetectomy is performed to unroof the neural foramen and identify the exiting and traversing nerve roots. A unilateral diskectomy is performed, and the endplates are thoroughly decorticated with long angled curettes and bone rasps.

A banana-shaped interbody implant packed with bone is inserted into the disk space via a transforaminal approach and tapped in place under fluoroscopic guidance. Because of its shape, as the implant is inserted and tapped, it gradually assumes a transverse orientation within the disk space. The disk space posterior to the implant is packed with cancellous bone.

Anterior lumbar interbody fusion

The appropriate lumbar or lumbosacral segment is reached through an anterior transperitoneal or extraperitoneal approach. This can be accomplished via either an open or a laparoscopic method.

The L5-S1 disk is always approached between the iliac vessels; this often requires mobilization and lateral retraction of the left iliac vein. For the L4-5 disk space, the level of aortic bifurcation and size of the left iliac vein determine the direction of vessel retraction. The iliolumbar vein is divided for L4-5 disk access. The anterior longitudinal ligament and the anterior annulus of the disk are incised and the disk contents evacuated. The endplates are prepared, and the interbody implants are inserted by using instruments and methods specific to the type of implant used.

ALIF is sometimes combined with pedicle screw instrumentation, necessitating an anterior-posterior approach, which avoids opening the spinal canal. In conditions that require decompression of the spinal canal, PLIF or TLIF can be performed in conjunction with pedicle screw instrumentation; in this way, a global fusion can be performed through a single approach without the need to open the abdomen.

Lumbar corpectomy

A lumbar corpectomy is generally performed for neoplastic disease affecting the vertebral body but may be performed for other indications as well, such as burst fractures with substantial retropulsion that cannot be reduced through a posterior approach or extensive vertebral osteomyelitis with pathologic fracture that cannot be adequately debrided and decompressed through a posterior approach.

The technique is as follows. The L2-5 segments are approached through a left retroperitoneal approach with the patient in a lateral decubitus position. The retroperitoneal approach is usually provided by a general surgeon. The peritoneum is mobilized forward until the psoas muscle is visualized.

The kidney is mobilized forward. The ureter is found over the psoas muscle and mobilized forward with its surrounding fat. The sympathetic chain and the genitofemoral nerve are identified over the psoas muscle and preserved. The psoas muscle attachments to the lateral aspects of the vertebral bodies are mobilized posteriorly with a Cobb periosteal elevator to expose the lateral aspect of the pedicles. The segmental vessels are ligated and divided over the midportion of the vertebral bodies.

The vertebral body bone is removed in a left-to-right approach. The posterior margin of the vertebral body is identified at the level of the pedicle and followed inferiorly. Retropulsed bone fragments or ventral epidural tumor is removed. The disks above and below the level of corpectomy are removed and the endplates decorticated.

Reconstruction across the gap produced by corpectomy requires a large interbody implant. This may be a piece of tibial or femoral allograft, cored out and filled with autologous bone. Alternatively, a Steinman pin wedged between the adjacent vertebral bodies and surrounded by methylmethacrylate may be used in the case of malignant disease, when bony fusion is not realistically expected.

Most commonly, metal cages of fixed height (Harms cage) filled with bone or expandable metal cages are used to reconstruct the vertebral body defect. A plate or plate-rod system is screwed to the lateral aspect of the vertebral bodies above and below the level of corpectomy, the latter providing the advantage of compression across the cage. (See the image below.)

Anterolateral lumbar corpectomy followed by recons Anterolateral lumbar corpectomy followed by reconstruction with fixed-height cage and dynamic rod system that allows compression across cage. Image courtesy of Synthes, Inc.

Postoperative Care

Postoperative pain is controlled aggressively with parenteral opiates for the first 12-36 hours, after which period the patient is converted to oral analgesics. Muscle relaxants and benzodiazepine anxiolytics can help reduce the requirement for opiates. If significant postoperative pain is anticipated, IV patient-controlled analgesia (PCA) with a continuous basal rate is employed.

Prophylactic antibiotics, started preoperatively, are continued for three doses (24 hours) after the procedure. In the absence of infection, further antibiotic administration after this 24-hour period has not been shown to be beneficial and may lead to the emergence of antibiotic-resistant pathogens.

If dexamethasone is used preoperatively and intraoperatively for neuroprotection, it is discontinued after the procedure as quickly as the patient’s neurologic condition permits. Prolonged postoperative use of corticosteroids may increase the risks of wound infection and dehiscence.

IV fluids are administered until the patient can tolerate oral feeding and drinking. In anterior thoracolumbar procedures, nasogastric drainage may be required if paralytic ileus occurs. Routine orders for antiemetics, antacids, and stool softeners are written.

If closed wound drainage is employed, the drain is removed when its output diminishes (usually on postoperative day 1). In transthoracic procedures, the chest tube is removed when the lungs have fully reexpanded and pneumothorax has resolved. If significant blood loss has occurred during or after the operation (through the drain or chest tube), hemoglobin levels are checked. If symptomatic anemia exists, blood transfusion is considered.

Wound hematoma is of particular significance after anterior cervical surgery. Small neck hematomas may cause dysphagia, odynophagia, hoarseness, or sore throat, and are treated conservatively. However, a large neck hematoma can result in upper airway compromise, which constitutes a life-threatening emergency and necessitates immediate surgical evacuation. Although rare, such hematomas develop within the first 24 hours after the operation—hence the general practice of keeping patients in hospital overnight after anterior cervical fusion procedures.

Neurologic deterioration within the first 24-48 hours after surgery should raise clinical suspicions of epidural hematoma, prompting immediate imaging studies or surgical reexploration.

Early mobilization of the patient after fusion surgery not only expedites rehabilitation but also prevents certain complications (eg, DVT, atelectasis, and pneumonia). If fusion is performed without instrumentation, an external orthosis is employed until the fusion has matured. After instrumented fusion, an external orthosis may still be applied to supplement the internal instrumentation, depending on the type and extensiveness of the procedure and the risk of instrumentation failure.

Fever is not uncommon after fusion surgery. A low-grade fever on postoperative day 1 or 2 is usually due to atelectasis and is treated with incentive spirometry and early patient mobilization. A high-grade or protracted fever should be worked up to exclude pneumonia, urinary tract infection, wound infection, and bacteremia. The incision should be examined daily (by the healthcare staff during the hospitalization and by the patient and family after discharge).

Occasionally, fever occurs in absence of any evidence of infection after an operation that required significant muscle retraction and manipulation. In such cases, the fever may be due to pyrogenic substances released as a result of muscle necrosis.

The patient is usually discharged from the hospital within 24-48 hours after an elective fusion operation. Debilitated and elderly patients and those with neurologic or systemic injuries may require longer hospitalization.

Although initiation of exercise therapy is often delayed until more than 3 months after lumbar fusion, Abbott et al showed that rehabilitation can be conducted during the first 3 postoperative months.[39] All patients in this study received a home program focusing on the following:

  • Pain-contingent training of functional strength and endurance of back, abdominal, and leg muscles
  • Stretching
  • Cardiovascular fitness

One group also received three outpatient sessions focusing on modifying maladaptive pain cognitions, behaviors, and motor control.[39] Patients who also received psychomotor therapy demonstrated significant improvement with respect to functional disability, self-efficacy, outcome expectancy, and fear of movement/(re)injury.

Complications

Specific complications of fusion surgery include injury to nearby structures specific to the particular operation or approach (eg, RLN palsy after anterior cervical surgery).

General complications of fusion surgery include the following:

  • Infection of soft tissues, epidural space, bone, disk space, fusion mass, or hardware, which may necessitate removal of instrumentation, prolonged IV antibiotic treatment, or both
  • Bleeding, which may necessitate transfusion - Studies from Asia have suggested that tranexamic acid and Batroxobin may be helpful for reducing blood loss in spinal surgery [40, 41]
  • Wound hematoma, which may necessitate surgical evacuation, particularly if it causes airway compromise after anterior cervical surgery
  • Epidural hematoma, necessitating surgical evacuation
  • Nerve-root or spinal cord injury
  • Cerebrospinal fluid leakage or pseudomeningocele formation, necessitating repair
  • Vascular injury (eg, vertebral or carotid artery injury in the cervical spine or iliac vessel injury in the lumbar spine)
  • Organ injury (eg, to the esophagus, pharynx, or rectum)
  • Nerve injury (eg, to the RLN or the hypoglossal nerve)
  • Pseudarthrosis, necessitating redo fusion
  • Excessive subsidence, displacement, or breakage of a structural graft
  • Hardware failure (eg, loosening, pullout, or breakage), necessitating reoperation
  • Discomfort associated with the hardware
  • Wound dehiscence

Complications associated with iliac crest bone graft harvest include the following:

  • Infection
  • Bleeding, hematoma formation, bruising
  • Pelvic fracture
  • Pain of musculoskeletal or neuralgic origin
  • Numbness around or related to the incision
  • Sacroiliac joint dysfunction and pain

Systemic complications of fusion surgery include, but are not limited to, the following:

  • DVT, PE
  • Myocardial infarction, congestive heart failure
  • Atelectasis, pneumonia
  • Urinary tract infection
  • Peripheral nerve injury related to patient positioning
  • Blindness related to intraoperative retinal ischemia

A potential long-term complication of fusion in the cervical or lumbar spine is adjacent segment degeneration, also known as transition level syndrome (see the image below). The intended loss of motion across the fused motion segment or segments increases the biomechanical stress on the adjacent motion segments. Over time, this may result in disk herniation, accelerated disk or facet degeneration, or spinal stenosis at the adjacent segments above or below the level of fusion.

Transition level syndrome: C6-7 disc herniation de Transition level syndrome: C6-7 disc herniation developed 6 years after C4-5 and C5-6 anterior cervical diskectomy and fusion.

Although some instances of adjacent segment disease are undoubtedly due to the natural history of the underlying degenerative disease, and others are due to unintended injury to the adjacent segment elements (eg, facet joints) during the original fusion operation, the rest are thought to be caused by the biomechanical mechanism described above.

Symptomatic adjacent segment disease is more likely to develop if the adjacent segment is already diseased, albeit asymptomatically, at the time of the original fusion operation. In order to avoid reoperation in this situation, it is common practice to fuse the adjacent degenerated motion segment at the same time that the symptomatic motion segment is fused. If the degenerated adjacent segment is felt to be contributing to the patient’s pain syndrome, its fusion is further justified.

By the same token, if a patient presents with significant multilevel degenerative disease, fusion should be avoided if at all possible, unless sufficient indication exists for fusion of all of the affected motion segments (eg, multilevel cervical spondylosis with myelopathy).

A very rare complication of anterior or posterior decompression and fusion is the "white cord syndrome" of acute hemiparesis, which is believed to be due to acute reperfusion of chronically ischemic areas of the spinal cord.[42] The hallmark finding is an intramedullary hyperintense signal on T2-weighted MRI in a patient with unexplained neurologic deficits after spinal cord decompression. Patients with this syndrome have improved with steroid therapy and acute rehabilitation.

Long-Term Monitoring

Patient follow-up is geared toward assessment of functional recovery (reduced pain and improved neurologic function), radiographic assessment of fusion, and detection of delayed postoperative complications.

The first follow-up visit is scheduled about 7-10 days after the procedure to assess the condition of the wound, remove staples and sutures, and address the patient’s questions and concerns.

Usually, the second and third follow-up visits are scheduled at 6 weeks and 3 months after the operation, though considerable variation exists in practice patterns. The focus of these visits is to ensure that the wound has healed properly, the fusion has progressed well, the patient’s neurologic function has improved as expected, the patient’s preoperative pain syndrome has resolved or diminished, the pain medications are tapered off, the brace is discontinued, and rehabilitation measures are instituted when appropriate.

If all has progressed well, additional follow-up can be performed through telephone calls, mailed questionnaires, or online communications, as dictated by specific practice patterns. Routine radiographic studies are performed at predefined intervals (eg, 6 weeks, 6 months, and 1 year postoperatively) until the fusion is deemed to be solid. Routine CT or MRI is not required after fusion surgery; these imaging methods are indicated only when there is concern regarding a specific problem that requires them for diagnosis.