AUTHOR AND EDITOR INFORMATION

Section 1 of 14  

• Authors and Editors
• INTRODUCTION
• INDICATIONS
• RELEVANT ANATOMY
• CONTRAINDICATIONS
• WORKUP
• TREATMENT
• COMPLICATIONS
• OUTCOME AND PROGNOSIS
• FUTURE AND CONTROVERSIES
• FURTHER READING
• ACKNOWLEDGMENTS
• Multimedia
• References

INTRODUCTION

Section 2 of 14  

• Authors and Editors
• INTRODUCTION
• INDICATIONS
• RELEVANT ANATOMY
• CONTRAINDICATIONS
• WORKUP
• TREATMENT
• COMPLICATIONS
• OUTCOME AND PROGNOSIS
• FUTURE AND CONTROVERSIES
• FURTHER READING
• ACKNOWLEDGMENTS
• Multimedia
• References

In the past 3 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.

History of the Procedure

Spinal fusion and instrumentation were developed and applied as independent techniques for treatment of spinal instability in the first half of the 20th century, before the biomechanical principles surrounding spinal instability were understood. 
 
Around the turn of the 20th century, the problem of progressive spinal deformity and disability caused by spinal tuberculosis (Pott disease) had become a focus of clinical inquiry. The problem did not yield itself to the decompressive procedures (eg, laminectomy) developed in the previous century. In 1911, Russell Hibbs and Fred Albee independently developed the concepts and methods for bony fusion of the spine to address the symptoms of Pott disease. These methods and their subsequent refinements consisted of applying autologous bone (harvested from laminae, iliac crest, or ribs) to the dorsal surface of spine. Although this constituted a major advance in spine surgery that was subsequently applied to a much wider range of pathological disorders and which remains in use today, the method of onlay posterior grafting, when performed in isolation, suffered from an unacceptably high rate of pseudarthrosis (failed fusion).
 
Around this time, spinal instrumentation, which mostly consisted of wiring of posterior elements, was employed sporadically for treatment of spine fractures.  This method was first employed by Berthold Hadra in 1891. In the 1950s, Paul Harrington pursued his historic work on correction of idiopathic and postpolio scoliosis by applying a combination of compression and distraction hooks and rods to the thoracolumbar spine.¹ The success of the Harrington rod system with deformity correction led to its subsequent use for treatment of overt spinal instability (eg, post-traumatic instability). However, it soon became apparent that the application of spinal instrumentation (without fusion) for treatment of spinal instability often ended in breakage or loosening of the hardware (hardware failure).
 
Harrington later expressed the idea that there is a “race between instrumentation failure and acquisition of spinal fusion.” This principle and the realization that the problems of pseudarthrosis and hardware failure could be resolved if bone grafting and instrumentation were used together laid the foundations of modern spine stabilization surgery. In current practice, bone grafting and instrumentation are often used concurrently based on the expectation that internal fixation of spine enhances the success of bone fusion while a successful bone fusion eliminates the possibility of hardware failure by reducing the chronic biomechanical stresses on the hardware construct.

Of note, the term "fusion" is used in this article and in spine literature to refer to the concept of internal stabilization of spine, generally accomplished by fusion with instrumentation (instrumented fusion), but also, albeit with decreasing frequency, accomplished by bone grafting alone.

Problem

Strictly defined, spinal fusion is an operation designed to treat spinal instability. In practice, however, this definition is not particularly useful as it fails to establish the indications for spinal fusion. The problem is threefold: (1) the current definitions of spinal instability are not uniformly accepted and applied; (2) it is difficult to measure instability in individual clinical circumstances; and (3) class I and II scientific evidence regarding spinal fusion is scarce. In this setting, clinical practice is guided by an understanding of the principles of spinal biomechanics and knowledge of the generally accepted indications, contraindications, and controversies regarding fusion surgery.

In their widely-quoted work, White and Panjabi defined spinal stability as the ability of the spine under physiological loads to limit patterns of displacement so as to not damage or irritate the spinal cord and nerve roots and, in addition, to prevent incapacitating deformity or pain due to structural changes.² Conversely, instability refers to excessive displacement of the spine that would result in neurological 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.³  
 
A simpler conceptual approach would be to think of instability as overt, anticipated, or covert. 
 
Overt instability refers to excessive motion that is readily documented by radiographic studies and results in pain, deformity, or neurological deficit. Those spine fractures, dislocations, tumors, and infectious processes that significantly disrupt one or more spinal motion segments produce acute overt instability.  Spondylolisthesis with abnormal dynamic displacement, documented on flexion/extension x-ray films, 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 internal means (fusion).

Bilateral jumped facet syndrome is an example of overt spinal instability due to trauma. Notice the grossly abnormal displacement of C5 relative to C6 with neck flexion.
 
Anticipated instability 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. For instance, corpectomy or total facetectomy would constitute indications for fusion at the time of the original operation. A comprehensive anterior cervical discectomy (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, as its disrupts 2 of Denis' 3 spinal columns.


Example of anticipated instability: Fig. A. Large mass affecting right C3-4 facet joint and lateral masses in a patient with severe right-sided neck and shoulder pain; Fig. B. and C. Complete resection of the tumor and simultaneous C3-4 anterior fusion to circumvent the anticipated iatrogenic stability produced by radical resection of facet and lateral masses.

Covert instability is a more elusive concept. It refers to circumstances in which excessive motion cannot be grossly demonstrated but is presumed to exist based on the combination of clinical and radiographic findings. Fixed spondylolisthesis (without movement on flexion and extension x-ray films) in the setting of progressively worsening back pain and/or radicular symptoms is a good example of covert instability. Pseudarthrosis with intact instrumentation also falls in this category. Controversy arises when the concept of covert instability is applied to degenerative diseases of the spine. In this context, the concept of micro-instability is sometimes evoked to justify fusion for a wider range of conditions, including recurrent disc herniation, disc degeneration with discogenic pain, painful facet arthropathy, spinal stenosis, and failed back syndrome without overt instability.

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

Frequency

Since spinal instability is not a single disease but a pathological consequence of a variety of different spine disorders such as traumatic fractures, metastatic tumors, 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.

It is estimated that more than 300,000 spine fusions are performed in the United States annually. The vast majority of these operations are performed for degenerative disease of the spine. Between 1996 and 2001, the number of spine fusions in the United States increased 76%.⁴ 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.⁵ An increase in the incidence of spinal instability could certainly not account for the increase in fusion surgery. While the forces driving this trend are debated, the standard of care in the United States is clearly shifting toward greater use of fusion surgery.

Etiology

Virtually every category of disease affecting the bones, discs, joints, or ligamentous support structures of the spine can produce spinal instability. These include trauma, tumors, infections, inflammatory diseases, connective tissue disorders, congenital disorders, degenerative disorders, and iatrogenic (postsurgical) etiologies.

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 3-column concept of the spine as defined by Denis is widely used as the conceptual framework for diagnosing acute overt spinal instability.⁶ Although originally devised based on 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. The anterior column consists of the anterior vertebral body (usually anterior two-thirds), the anterior annulus, and the anterior longitudinal ligament. The middle column refers to 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 due to failure of the anterior column with preservation of the middle column (stable). On the other hand, a burst fracture occurs due to compression failure of both anterior and middle columns (usually unstable), often resulting in bone retropulsion into spinal canal. A seat-belt 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 3 columns and are considered highly unstable.

Instantaneous axis of rotation (IAR) is the axis about which a vertebral segment would rotate when exposed to an asymmetric application of force. Although theoretically there are 3 axes of rotation corresponding to rotation in the sagittal plane (flexion/extension), coronal plane (lateral bending) and axial plane (twisting), most references to IAR correspond to axial forces applied in the sagittal plane. 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, 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.

Using these concepts, traumatic spinal instability can be categorized according to the underlying pathophysiological mechanisms. When an axial force vector is applied anterior to the IAR, a compression fracture occurs due to 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 due to 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 seat-belt deceleration injuries of the thoracolumbar spine. The vertical (distractive) component of the vector is applied posterior to the IAR, while 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 and/or vertebral endplates. 

With even larger flexion force vectors, a fracture-dislocation may occur with failure of all 3 column 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. 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, these principles are commonly applied when 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. Consideration of IAR is of crucial importance in 3-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 to spine surgery. Insertion of special pedicle screws (Schanz screws) pivoting on a rod transfers the IAR to the screw/rod interface. Compression of the proximal end of the screws, produces distraction-reduction of the vertebral burst fracture. If the 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 osteoconductive, osteoinductive, or osteogenic properties.
 
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-β (TGF-β) family, induces differentiation of mesenchymal cells into osteoblasts.⁷ It is found naturally in the bone fusion environment and is available in recombinant form for clinical use. 

The host factors that adversely affect fusion include malnutrition, corticosteroid use, irradiation, neoplastic disease, diabetes, local infection, osteoporosis, and smoking. Of these, smoking is the most prevalent correctable risk factor. There is abundant experimental and clinical⁹ 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.¹⁰ This is best accomplished by instrumentation. In absence of instrumentation, fusion should be supported by external bracing until it solidifies.

Clinical

The clinical manifestations of spinal stability fall into 3 categories, as stated in the definition of instability:

1. Neurological deficit due to cord, cauda equina, or nerve root compression

2. Pain

3. Incapacitating deformity

INDICATIONS

Section 3 of 14  

• Authors and Editors
• INTRODUCTION
• INDICATIONS
• RELEVANT ANATOMY
• CONTRAINDICATIONS
• WORKUP
• TREATMENT
• COMPLICATIONS
• OUTCOME AND PROGNOSIS
• FUTURE AND CONTROVERSIES
• FURTHER READING
• ACKNOWLEDGMENTS
• Multimedia
• References

Indications for fusion in 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-anatomical alignment, and if there is no significant neural compression, an external brace (eg, halo, collar, thoracic lumbar sacral orthosis [TLSO] brace) is tried until the fracture heals. In all other circumstances and in cases where bracing has failed, fusion is indicated. Tables 1 and 2 summarize treatment algorithms and indication for fusion in cervical, thoracic, and lumbar spine trauma.

Table 1. Traumatic Instability of Cervical Spine

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 Image 2), 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.

Indications for fusion in 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.¹³

Much more controversial is the treatment of that subcategory of covert instability that is known as microinstability or dysfunctional motion segment. Here, an abnormal disc or facet joint is presumed to be the pain generator. Provocative discography 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/Congress of Neurological Surgeons Joint Section on Disorders of Spine and Peripheral Nerves are explored in greater detail in Outcome and Prognosis section.

RELEVANT ANATOMY

Section 4 of 14  

• Authors and Editors
• INTRODUCTION
• INDICATIONS
• RELEVANT ANATOMY
• CONTRAINDICATIONS
• WORKUP
• TREATMENT
• COMPLICATIONS
• OUTCOME AND PROGNOSIS
• FUTURE AND CONTROVERSIES
• FURTHER READING
• ACKNOWLEDGMENTS
• Multimedia
• References

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 axial load-bearing capacity of the vertebrae. The greater cancellous-to-cortical bone ratio in the vertebral body compared to the posterior vertebral elements makes it more susceptible to neoplastic and infectious diseases, while its relationship to the 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 requiring 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 provide for relatively free movements 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 provide for greater movement in the sagittal plane than in other directions. This facet orientation and the transitional location of the thoracolumbar spine between the rib cage-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 compared to 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 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), making them 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 has to be taken into account when inserting pedicle screws. In the thoracic spine, the pedicles have a narrow transverse diameter, a slight downward angle, and are located next to the narrow thoracic spinal canal. 

CONTRAINDICATIONS

Section 5 of 14  

• Authors and Editors
• INTRODUCTION
• INDICATIONS
• RELEVANT ANATOMY
• CONTRAINDICATIONS
• WORKUP
• TREATMENT
• COMPLICATIONS
• OUTCOME AND PROGNOSIS
• FUTURE AND CONTROVERSIES
• FURTHER READING
• ACKNOWLEDGMENTS
• Multimedia
• References

Absolute contraindications to 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 discs, in which case the fusion construct would be at risk for infection (see below for established discitis/osteomyelitis)

Relative contraindications to spinal fusion include the following:

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

As always, the contraindications to 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 to fusion in a patient with back pain and disc degeneration, but should not deter the surgeon from fusing an unstable cervical spine fracture.

Importantly, an active spine infection (discitis/osteomyelitis) does not necessarily constitute a contraindication to fusion and instrumentation. To the contrary, advanced spine infections exert severe destabilizing effects on the spine, often requiring 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 antibiotic treatment (for at least 6 wk) to confirm eradication of the infection. Worsening pain or neurological deficit, persistent fever, leukocytosis, or bacteremia and persistently elevated erythrocyte sedimentation rate signal the possibility of persistent infection. 

Similarly, radiographic evidence of loosening of screws or CT scan/MRI evidence of increased bone destruction should be further investigated.  However, persistent and stable vertebral enhancement on MRI does not necessarily indicate persistent infection, as this finding can lag behind microbiological cure. Radionucleotide bone scan lacks specificity in this setting, but a tagged-WBC scan may be more useful. When in doubt, a CT-guided biopsy/aspiration of the region can help confirm the possibility of persistent infection, which would then be treated with reoperation.

Loosening of this infected pedicle screw is evidenced by a radiolucent halo (arrows) surrounding the screw.

In this patient with T7-8 discitis, vertebral enhancement on MRI persisted 8 weeks after clinical and microbiological cure.

WORKUP

Section 6 of 14  

• Authors and Editors
• INTRODUCTION
• INDICATIONS
• RELEVANT ANATOMY
• CONTRAINDICATIONS
• WORKUP
• TREATMENT
• COMPLICATIONS
• OUTCOME AND PROGNOSIS
• FUTURE AND CONTROVERSIES
• FURTHER READING
• ACKNOWLEDGMENTS
• Multimedia
• References

Lab 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 (CBC, ESR, C-reactive protein, blood cultures), rheumatoid arthritis (rheumatoid factor), 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

Spine MRI and plain x-ray films 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. 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 in assessing bone anatomy in the setting of vertebral fractures, spondylolysis, history of previous spine surgery, and congenital spine anomalies. CT may also be used to assess certain bony parameters (such as pedicle size in thoracolumbar spine, lateral mass anatomy in cervical spine, and vertebral artery anatomy in C1-2 region) in preparation for instrumentation of the spine.

To evaluate bone integrity prior to fusion when osteoporosis is suspected, a bone density scan is performed. Radionucleotide 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 and/or stabilize the spine, the diagnosis can be obtained intraoperatively.

Substantial controversy exists regarding the value of discography in diagnosis of discogenic pain and in patient selection for fusion surgery. When performed, it should be accompanied by measurements of intradiscal pressure, documentation of severity and concordance of pain during injection, and postdiscography CT scan.

Histologic Findings

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

Staging

Since spinal instability is a heterogenous disorder, no uniform staging/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 3. 

Table 3. Grading of Spondylolisthesis

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), resulting in uncoupling and glacial anterior translation of one vertebral body over another. 

Grade I isthmic spondylolisthesis at L5-S1. Arrow depicts the 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

Section 7 of 14  

• Authors and Editors
• INTRODUCTION
• INDICATIONS
• RELEVANT ANATOMY
• CONTRAINDICATIONS
• WORKUP
• TREATMENT
• COMPLICATIONS
• OUTCOME AND PROGNOSIS
• FUTURE AND CONTROVERSIES
• FURTHER READING
• ACKNOWLEDGMENTS
• Multimedia
• References

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 discitis 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, Philadelphia collar, Miami collar) and various cervicothoracic orthoses provide intermediate amounts of stabilization, while soft collars provide little stabilizing benefit. In the case of thoracic and lumbar spine, the only brace that provides significant stabilizing benefit is a rigid TLSO (thoraco-lumbo-sacral orthosis) 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 neurological deterioration, the patients with these forms of instability generally undergo conservative (nonsurgical) treatment first. Fusion is reserved for those in whom conservative treatment fails (see Indications).

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

• Medications: Analgesics, anti-inflammatories, muscle relaxants, tricyclic antidepressants, anti-epileptics
• 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)

Surgical therapy

Once the decision has been made to fuse a particular spine segment, there may be several surgical methods to accomplish this task. After a particular method is selected, the etiology of instability is no longer relevant, as the technical steps would be the same. The following is a discussion of the most commonly employed fusion techniques in various regions of the spine.

Atlantoaxial (C1-2) instability 

Atlantoaxial instability may be caused by a variety of conditions, including odontoid fractures, rupture of the transverse ligament, ligamentous incompetence due to rheumatoid arthritis, and congenital instability associated with os odontoideum.
 
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 used in the past are gradually being abandoned in favor of multistranded braided cables, which offer greater flexibility, strength, and fatigue resistance. 
 
Three techniques are available. In the Gallie technique, a cable loop is passed under C1 posterior arch from below, folded over the C1, and hooked under the base of 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 due to the onlay nature of the graft.

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

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

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

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

Regardless of the technique used for C1-2 cable fixation, a halo is generally applied until fusion occurs (usually 3-6 mo), creating 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 the 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 a high-resolution CT scan 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 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 for 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 millimeters lateral to the medial aspect of the lateral mass. Drilling is performed in 2 mm increments pointing 10 degrees 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/posterior directions before the drill crosses the facet joint. The drill hole is then screwed with a self-tapping screw of the appropriate length (alternatively, it can be tapped and then screwed). 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. Notice the proximity of vertebral artery to the 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.  A postoperative vertebral angiogram is performed to rule out pseudoaneurysm formation.
 
C1-2 transarticular screw fixation is best supplemented with C1-2 cable fixation in order to provide better bone substrate for fusion than what can be packed in the facet joints. The patient is placed in a Philadelphia collar postoperatively.
 
C1-2 Lateral mass/isthmus fixation
 
When preoperative CT imaging 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 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 25 degrees medially and 25 degrees 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
 
This technique is reserved to certain type-2 odontoid fractures. Its main advantage is that it directly repairs the odontoid fracture, thus avoiding a C1-2 fusion and maintaining range of motion. Its shortcoming is the limited circumstances in which it can be employed. 
 
Odontoid fractures are categorized according to the following scheme (Table 4) and treated according to the algorithm in Table 1.
 
Table 4. Odontoid Fracture Classification
          

Type