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Keywords:

anterior cervical discectomy, bone grafting, cervical fusion, cervical plating, degenerative spine disease, indications for fusion, lumbar fusion, pedicle screws, spinal biomechanics, spinal fusion, spinal instability, spine fusion, spine instrumentation, spine hardware, spine stability

INTRODUCTION

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/instrumentation has increased. The question facing the modern spine surgeon is not 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's disease) had become a focus of clinical inquiry. The problem did not yield itself to the decompressive procedures (e.g., 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's 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’s glacial instability) falls in the category of chronic overt instability. Overt instability generally requires stabilization, either by external means (bracing) or internal means (fusion).

Click on the image to enlarge it!

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 two of Denis' 3 spinal columns.

Click on the image to enlarge it!

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 where 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/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 utilization 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 non-traumatic 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 three 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 three 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-pointbending 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.

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,8 a phenomenon that is put to use by implantation of a bone stimulator in cases at high risk for pseudarthrosis.

Osteogenic property 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,
thus providing the ideal graft material. A corticocancellous autograft, such as a tricortical iliac crest

autograft, is capable of providing structural support as an interbody implant in addition to the above-mentioned favorable properties. The only drawback of using autograft material is the potential for donor site complications associated with graft harvest.

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, or nerve root compression
  2. Pain
  3. Incapacitating deformity

INDICATIONS

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 were 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

Fracture/Dislocation

(mechanism)

Type/Issue

Treatment

C1 Jefferson’s Fracture

(axial loading)

1. isolated

2. with transverse lig. rupture

3. widely diasthetic

4. with odontoid fracture

1. hard collar

2. halo

3. consider occiput-C2 fusion

4. treat according to odontoid fx

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, <6mm displaced

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

4. Type 3

1. if no atlanto-occipital instability, collar x 3 months

2. Halo x 3-6 months

3. C1-2 fusion or odontoid screw

 

4. Halo x 6 months

C2 Hangman’s Fracture

(extension)

1. pars approximated

2. pars separated, reducible

 

3. pars separated, not reducible

1. hard collar x 3 months

2. Reduce in extension, then halo x 3 months

3. C2-3 fusion

Unilateral Jumped Facet

(flexion + rotation)

1. reducible

2. not reducible

 

3. with facet fracture

 

4. with disc herniation

1. reduce and halo x 3 months

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 disc herniation

2. not Reducible, without disc herniation

3. with disc herniation

1. closed reduction, then posterior fusion

2. open anterior or posterior reduction and fusion

3. anterior discectomy, reduction and fusion

Subaxial Spine Axial Loading Injuries

(axial +/- flexion)

1. simple compression fracture

2. burst fx +/- tear drop fx

3. burst + posterior column fx

1. hard collar

2. anterior corpectomy and fusion

3. anterior corpectomy and fusion

(+/- posterior fusion)

Clay Shoveler’s Fracture

(flexion)

Always stable

Soft collar and analgesics

Anterior Avulsion Fracture

(extension)

Always stable

Soft collar and analgesics

 

 

 

 

 

 

 

 

Table 2. Traumatic Instability of Thoracic and Lumbar Spine

 

Fracture

Denis Columns Involved

Treatment

Compression Fracture

Anterior column

Bracing (note that >50% vertebral body height loss or Cobb angle >30 degrees predicts worsening 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 w/ TLSO brace

 

If canal stenosis present, retropulsed fragment may be reduced by ligamentous taxis with posterior instrumented fusion in distraction.

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)

2 or 3 columns but with good bone contact

TLSO brace

Shear Fracture Dislocation

3 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 pathological 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 discs and vertebral bodies are highly destructive and destabilizing, requiring debridement/decompression and fusion, either simultaneously or in separate sessions.

Indications for fusion in chronic overt instability

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

Indications for fusion in 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 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 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 his 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, 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 below.

RELEVANT 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 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 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.

Because of these anatomical considerations, wires and hooks have been used more 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 that screws of appropriate diameter be selected based on preoperative CT studies and that breach of medial pedicle wall be avoided, 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 in the lumbar spine.

Cervical vertebrae have anatomical 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.

Click to enlarge the image! Comparison of vertebral anatomy in cervical, thoracic, and lumbar spine. Note the variation in anatomy and size of pedicles.

CONTRAINDICATIONS

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

  1. Diffuse multilevel neoplastic disease such that no adjacent normal segments exist for engagement of instrumentation
  2. Severe osteoporosis such that the bones would not support instrumentation and fusion would not be expected to solidify in absence of instrumentation
  3. 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:

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 are 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 is 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, x-ray evidence of loosening of screws or CT/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

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, 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/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 must 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 Percent Displacement of One Vertebral Body over Another
1 0-25%
2 25-50%
3 50-75%
4 75-100%
Spondyloptosis >100%

 

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

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 who fail conservative treatment (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 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 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 monofilaments wires used in the past are gradually being abandoned in favor of multi-stranded 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.

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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.

The newer Sonntag technique 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 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 (similar to Brooks technique) and the free ends of the cables are attached under C2 spinous process base. Since (unlike Brooks) there is no 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.

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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 months), creating a major drawback from the standpoint of patient comfort and rehabilitation.

C1-2 Transarticular screw fixation

Screw fixation of atlantoaxial segment provides immediate rigid fixation of the joint and eliminates the need for a halo. The main consideration here 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 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 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 C 1 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-rising 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

Odontoid Fracture Type Description
1 Fracture through odontoid tip (rare)
2 Fracture across the base of the odontoid process (most common)
2a Comminuted fracture across the base of odontoid (low chance of healing in halo or with odontoid screw)
3 Fracture extends from odontoid into C2 vertebral body (usually heals well in halo)

 

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

The technique is as follows: the procedure is performed from an anterior approach under biplanar fluoroscopy. 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 below). A plane is developed cephalad along the anterior aspect of the spine to the C2-3 disc 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 C2-3 disc 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 odontoid tip. The screw hole is tapped and screwed with a lag screw of appropriate length 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.

Subaxial cervical instability

Anterior cervical fusion

Anterior cervical fusion is one of the most commonly used fusion techniques in spine surgery. The anterior approach is used increasingly in preference to the posterior approach to the cervical spine,5 as it provides distinct advantages with regard to decompression, fusion, and instrumentation. Most pathological processes in the cervical spine, especially degenerative and neoplastic disorders, affect the structures anterior to the spinal cord. An anterior approach to the cervical spine permits thorough decompression of the spinal canal without manipulating the spinal cord. Furthermore, an anterior 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 compared to the posterior cervical elements render them ideal substrates for instrumentation.

Anterior cervical plate, applied in this case after 2-level anterior cervical discectomy and fusion. Image courtesy of Synthes, Inc.

The most commonly employed indications for anterior cervical fusion are in treatment of degenerative disorders. Large central disc herniations with cord compression and chronic disc-osteophyte complexes at one or more levels cannot be safely removed from a posterior approach. Cervical spinal stenosis associated with kyphosis is best treated through an anterior approach since a multilevel posterior decompression by laminectomy does not relieve the stretching of spinal cord over disc-osteophytes and may exacerbate the kyphosis in the long run. Anterior cervical discectomy is effective not only for treatment of neural compression (myelopathy and radiculopathy), but also for treatment chronic axial pain associated with disc degeneration and correction of spinal deformity (kyphosis).

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Large central disc herniations (A and B) and cervical spondylotic myelopathy with kyphosis

(C) are two common indications for anterior cervical discectomy and fusion.

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 posterior longitudinal ligament) or involves the vertebral body itself (eg, tumor or burst fracture). Traumatic cervical dislocations associated with disc herniation should be treated through an anterior approach.

Click on the image to enlarge it! C5-6 bilateral jumped facets associated with disc herniation (left) was treated with C6 anterior cervical decompression and fusion (right).

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 usually made on the right side of the neck for a right-handed surgeon and the left side of the neck for a left-handed surgeon, as this significantly facilitates access. If C 7-T1 disc is the target, some surgeons prefer a left-sided approach to minimize the risk of recurrent laryngeal nerve (RLN) palsy, although 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 sternocleidomastoid muscle provides the best cosmetic result for one- and two-level fusions. With practice, a 3-level fusion can also be performed through a transverse incision. An oblique vertical incision along the anterior border of the sternocleidomastoid muscle 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 muscle 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 inferior-lateral to a superior-medial location. For C3-4, C4-5, and C5-6 discs, the plane of dissection is superior to the omohyoid muscle, while for C6-7 and C7-T1 disc, it is below the omohyoid muscle. Occasionally, for an extensive procedure that spans several segments above and below C6, the omohyoid muscle 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 attachment 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 monopolar electrocautery is avoided to prevent the risk of thermal damage to the nearby sympathetic chain with 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 are 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 medial-laterally but also superior-inferiorly, providing an 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 disc space(s). The anterior longitudinal ligament and the anterior annulus of the disc are resected. Under the operating microscope, the contents of the disc(s) are thoroughly evacuated to expose the converging posterior lips of the superior and inferior endplates and the intervening posterior annulus of the disc. 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 2 mm cutting bur under the operating microscope until they are reduced to thin shells of cortical bone. The posterior annulus of the disc is the completely resected. The herniated disc 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 hooks 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 preoperative CT or MRI axial images will provide useful information about the proximity of the vertebral artery to the neural foramina and any aberrant looping of that vessel.

If a corpectomy is to be performed, the discs 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 are removed. An appropriately sized anterior cervical plate is affixed to the vertebral bodies above and below the fused segment

(s) with cancellous screws under fluoroscopic guidance. The screws are locked utilizing the locking mechanism specific to the plate.

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.

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Anterior cervical discectomy and fusion: A. Disc removed and interspace prepared to receive graft; B. Iliac crest bone graft harvested; C. Bone graft; D. Graft inserted into disc space; E. Plate screwed to anterior surface of vertebral bodies.

Some surgeons do not perform a fusion after an anterior cervical discectomy. This is only appropriate if the operation consists of a limited central single level discectomy for a soft central disc herniation in 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 discectomy 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 discectomies and corpectomies.

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

Lateral mass screws

Excellent stabilization of the subaxial cervical spine can be achieved from 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. The technique can be employed whether or not a previous laminectomy has been performed.

Two techniques are available. In Magerl technique, the entry point is 2 mm medial and 2 mm superior to the center of lateral mass and the screw trajectory is –20-25 degrees lateral and parallel to facet surface in the sagittal plane. In Roy-Camille technique, the entry point is just above the center of the lateral mass and the trajectory is directed only 10 degrees laterally 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, it is not always easy to achieve the cranial angulation that the technique requires during surgery. Prior to placement of rods or plates, the facet joints and lateral mass surfaces are decorticated and packed/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, Bohlman triple-wire technique) allow firm attachment of bone graft strips to the sides of the spinous processes.

Sublaminar wiring technique involves the use of a loop of wire that is carefully passed under the lamina and the 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.

Lumbar spinal instability

Posterior and posterolateral noninstrumented lumbar fusion

Noninstrumented posterior or posterolateral fusion of the spine is fairly simple to perform and is an acceptable treatment for certain instances of degenerative instability, when the patient is not felt to be a candidate for pedicle screw insertion. Because of its greater susceptibility to pseudarthrosis, it is not recommended for situations where 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 standard fashion through a posterior midline incision. Bilateral exposure of the laminae is extended further laterally to completely expose of 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, 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 posterior iliac crest and placed in the "lateral gutters" over the lateral aspect of 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, 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 by 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 for a noninstrumented fusion. Pedicle screws are inserted into the pedicles above and below the motion segment to be fused. The main concern during pedicle 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 is 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. If the pedicle has not been exposed by laminectomy/laminotomy, AP and lateral fluoroscopy are usually used. The inferior-lateral aspect of the pedicle can also be exposed by subperiosteal dissection from a lateral approach along the base of the transverse process.

Intraoperative fluoroscopy for pedicle screw insertion

The entry point to the pedicle is located at the junction of lines bisecting the transverse process and 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-L5 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 screwed with the appropriate size screw. 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, although notched plates also exist for this purpose. 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. If reduction of spondylolisthesis is desired, the inferior screw is locked to the rod and the superior screw is pulled back toward the rod, 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.

Click on the image to enlarge it! Pedicle screw fixation of lumbar spine. Image courtesy of Synthes, Inc.

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

Lumbar interbody fusion

Lumbar interbody fusion refers to replacement of disc 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. It is ideally supplemented with pedicle screw instrumentation to provide internal fixation and may or may not be further supplemented with posterolateral fusion.

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

Interbody fusion can be performed through a posterior approach (posterior lumbar interbody fusion [PLIF]), a posterolateral approach (transforaminal lumbar interbody fusion [TLIF]), or an anterior approach (anterior lumbar interbody fusion [ALIF]). Recently, a far lateral approach (extreme lateral interbody fusion [XLIF]) has also been described.

The original PLIF technique was performed through a routine posterior exposure for lumbar discectomy. After the disc space was thoroughly evacuated, the endplates were decorticated with large angled curettes and bone rasps. The disc space was then packed with autologous cancellous bone. Nowadays, PLIF, TLIF, and ALIF are usually performed with the aid of interbody implants. The implants, which are made of a PEEK polymer machined cortical allograft bone, or metal cages are filled with cancellous bone before insertion into the disc space. The remaining disc space around the implant is also packed with cancellous bone. The 3 techniques differ only in the method of insertion of the interbody implant.

The PLIF technique is performed as follows: Laminectomy (or bilateral hemilaminectomy), bilateral medial facetectomy, and bilateral discectomy are carried out at the target segment. The traversing and exiting nerve roots are identified bilaterally and the intervening epidural veins are bipolar coagulated 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 disc space and insertion of the implant.

The TLIF technique is performed as follows: 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 discectomy is performed and the endplates are thoroughly decorticated using long angled curettes and bone rasps. A banana-shaped interbody implant packed with bone is inserted into the disc 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 disc space. The disc space posterior to the implant is packed with cancellous bone.

The ALIF technique is performed as follows: The appropriate lumbar or lumbosacral segment is reached through an anterior transperitoneal or extra-peritoneal approach (see below). This can be accomplished by an open or laparoscopic method. The L5-S1 disc is always approached between the iliac vessels, often requiring mobilization and lateral retraction of the left iliac vein. For the L4-5 disc 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 disc access. The anterior longitudinal ligament and the anterior annulus of the disc are incised and the disc contents are evacuated. The endplates are prepared and the interbody implants are inserted utilizing 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, a PLIF or TLIF can be performed in conjunction with pedicle screw instrumentation, thus performing a global fusion through a single approach without opening the abdomen.

Lumbar corpectomy

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

The technique is as follows: The L2-L5 segments are approached through a left retroperitoneal approach with the patient is 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 midportion of the vertebral bodies.

The vertebral body bone is removed from a left to right approach. The posterior margin the vertebral body is identified at the level of pedicle and followed inferiorly. Retropulsed bone fragments or ventral epidural tumor are removed. The discs 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 could 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, surrounded by methylmethacrylate can 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.

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

Thoracic and thoracolumbar instability

Posterior thoracic and thoracolumbar instrumentation

Since the advent of Harrington rods for posterior thoracolumbar stabilization and deformity correction, there has been a significant evolution in posterior instrumentation constructs. 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:

  1. They are segmental; that is, they are attached to the spine not only at proximal and distal ends of rods but at multiple intervening segments.
  2. They are modular and hybrid, allowing simultaneous use of hooks (laminar hooks,
    transverse process hooks, pedicle hooks), wires, and pedicle screws.
  3. They rely on 3-point-bending biomechanical principles more than 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 2 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 segment of the spine is instrumented (generally 3 segments above and 2 segments below) to prevent failure of the construct. The segments receiving bone graft would be shorter than the instrumented segments, known as the "rod long, fuse short" practice. Increasingly, pedicle screws are playing a prominent role in segmental modular constructs in the thoracic and thoracolumbar regions. The increased stability conferred by the screw-based systems allows construction of shorter constructs spanning the unstable motion segment.

A modular posterior thoracolumbar instrumentation system, which is attached to the spine by a combination of screws and hooks, in turn attached to long rods. In this case, it is used for correction of scoliosis, using 3-point bending biomechanical principles. Image courtesy of Synthes, Inc.

Posterior systems also allow reduction of anterior (vertebral body) fractures using segmental distraction (see Image 5). 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 from a posterior versus an anterior approach.

Anterior thoracic and thoracolumbar instrumentation

There are instances where thoracic and thoracolumbar instability cannot be adequately addressed from a posterior approach. These include pathological fractures of the vertebral bodies due to tumor or infection, producing cord compression, and some traumatic burst fractures (associated with retropulsion and rupture of posterior longitudinal ligament) that cannot be reduced by posterior distraction. In these cases of anterior pathology, an anterior approach is preferred.

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 anterior cervical procedures.

The mid-thoracic (T4-T11) region is generally approached by 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-T11 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 by 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 has to be reconstructed in the same fashion as a lumbar corpectomy (see above). For this purpose, the most recent additions to the surgeon's armamentarium are expandable cages, which are placed in the corpectomy defect and then expanded to engage and distract the adjacent vertebral bodies. These 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 reconstruction with an expandable cage and a fixed plate/screw system. Image courtesy of Synthes, Inc.

In addition to permitting more thorough decompression of anterior pathology, anterior thoracic and thoracolumbar reconstructions allow one to limit the instrumented fusion to the pathological motion segment, sparing the adjacent segments.

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

Preoperative Details

Routine preoperative tests usually consist of complete blood count, electrolytes, BUN, creatinine, glucose, prothrombin time (PT), activated partial thromboplastin time (aPTT), international normalized ratio (INR), chest radiograph, and electrocardiogram. Blood is typed and screened. If extensive blood loss is anticipated, one or two units of packed red blood cells are cross-matched or a cell saver is used. Alternatively, if the procedure is scheduled electively, the patient may donate autologous blood several weeks before the surgery. Thigh-high compression stockings (TED hose) and sequential compression devices are applied for DVT prophylaxis preoperatively and are not removed until the patient is mobilized postoperatively.

In patients who are at particular risks for deep venous thrombosis and pulmonary embolism (eg, paraplegic, quadriplegic, or bed-bound prior to surgery), subcutaneous injections of low-molecularweight heparin may begin before the surgery, weighing the risk of postoperative epidural hematoma against the risk of pulmonary embolism. Meticulous attention to hemostasis, liberal use of closed wound drainage, and careful postoperative neurological evaluation are indispensable when heparin is used. An antibiotic with antistaphylococcal activity, usually a first generation cephalosporin, is given within one hour prior to the skin incision and continued for 3 doses postoperatively.

Intraoperative Details

Intraoperative details specific to each fusion technique have been summarized above. The following are general concepts pertaining to intraoperative management of all fusion procedures.

Positioning the patient for surgery is of utmost importance for safe and effective conduct of the procedure. 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 3-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 intra-ocular 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, Andrews table) allow the patient to be positioned in knee-to-chest position. The resultant lumbar flexion facilitates access to the spinal canal and disc 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 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 avoid compression neuropathy.

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 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. 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 based on computer-assisted navigation of the original fluoroscopic images.

Intraoperative neurophysiological monitoring for spine procedures consists of recording somatosensory evoked potentials (SSEP), motor evoked potentials (MEP), or electromyography (EMG) in order to detect and correct factors that lead to neurological compromise during the surgery. 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 an operating microscope allows safer decompression of neural elements, particularly where visualization is limited (eg, anterior cervical discectomy 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.

Anesthetic considerations during fusion surgery include maintenance of adequate blood pressure and fine-tuning of muscle relaxation during the surgery 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 more than two hours, a bladder catheter is inserted.

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

Postoperative Details

Pain is controlled aggressively after fusion surgery with parenteral opiates for the first 12-36 hours, after which 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, intravenous patient-controlled analgesia (PCA) with a continuous basal rate is employed.

Prophylactic antibiotics, started before the surgery, are continued for 3 doses after surgery. Further antibiotic administration after this point in absence of infection has not been shown to be of benefit and may lead to emergence of antibiotic-resistant pathogens.

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

Intravenous fluids are used until the patient can tolerate food and drink by mouth. In anterior thoracolumbar procedures, nasogastric drainage may be required if paralytic ileus occurs. Routine orders for anti-emetics, 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 re-expanded and pneumothorax has resolved. If significant blood loss has occurred during or after the surgery (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 requires immediate surgical evacuation. Although rare, such hematomas develop within the first 24 hours after the surgery; hence, the general practice of keeping the patients in hospital overnight after such procedures. Neurological deterioration within the first 24-48 hours after surgery should raise suspicion of epidural hematoma, prompting immediate imaging studies or surgical re-exploration.

Early mobilization of the patient after fusion surgery not only expedites rehabilitation but also prevents certain complications such as 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 the first or second postoperative day is usually due to atelectasis and is treated with incentive spirometry and early patient mobilization. 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 which required significant muscle retraction and manipulation, in which case it 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 neurological or systemic injuries may require longer hospitalization.

Follow-up

Patient follow-up is geared toward assessment of functional recovery (pain and neurological function), radiographic assessment of fusion, and detection of delayed postoperative complications. The first follow-up visit is scheduled about 7-10 days after surgery to assess the condition of the wound, remove staples and sutures, and address the patient’s questions and concerns. The second and third followup visits are usually scheduled at 6 weeks and 3 months after surgery, although 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 neurological 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 by telephone, mailed questionnaires, or online, as dictated by specific practice patterns. Routine radiographic studies are performed at predefined intervals (eg, 6 wk, 6 mo, and 1 y postoperatively) until the fusion is deemed to be solid. Routine CT or MRI imaging is not required after fusion surgery, unless there is concern for a specific problem that requires such imaging.

COMPLICATIONS

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

General complications of fusion surgery include the following:


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:

- Deep venous thrombosis, pulmonary embolism
-
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. 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 disc herniation, accelerated disc or facet degeneration, or spinal stenosis at the adjacent segments above or below the level of 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.

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

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 as fusion of the symptomatic motion segment. 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).

OUTCOME AND PROGNOSIS

Outcome after fusion surgery is measured in terms of the 3 cardinal clinical manifestation of spinal instability: neurological function, pain, and disabling deformity. With overt instability (eg, trauma, tumor, infection), neurological function after surgery is directly related to preoperative neurological status and can not 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 neurological function, but in terms of

In conditions with overt instability, deformity and pain outcome measures correlate closely with radiographic success of fusion. Since modern fusion and instrumentation techniques assure radiographic success in most of these cases, 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 one of the rare instance of Class I scientific evidence in spine literature.11 Of the 294 patients with disabling back pain due to 1-or 2-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 study, a smaller Norwegian study of degenerative back pain12 failed to show a statistically significant difference between lumbar fusion and a very aggressive regimen of physical and cognitive treatment (25 h of physical therapy per wk for 8 wk, followed by a comprehensive home exercise program, individual counseling, lessons, group therapy sessions, and peer group discussions).

Importantly, both groups experienced significant improvements over baseline with a trend toward greater improvement in the surgical group. This study has been criticized for its small number of patients and large confidence intervals in the data, suggesting that it lacked sufficient power to detect a statistical difference.13 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 are the norm, not the exception, often dealing with 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/Congress of Neurological Surgeons (AANS/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.13 The ensuing recommendations were ranked according to the strength of the supporting evidence as follows:

1. 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)

2. Guidelines - Intermediate level of confidence; based on one or more well-designed
comparative studies, or less well-designed randomized controlled studies (class II
evidence)

3. 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 studies were felt to provide class III evidence; therefore, most recommendations were provided at the lowest options level. Some of the more salient recommendations are described below, along with their rankings:

- 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 disc 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 stand-alone 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 discogenic pain, the following is recommended:

- MR imaging be used as the initial diagnostic test instead of discography

- MR imaging-documented disc spaces that appear normal not be fused

- Lumbar discography not be used as a stand-alone test for surgical decision making

- If discography is performed, both a concordant pain response and morphological abnormalities be present prior to considering fusion (guidelines)

FUTURE AND CONTROVERSIES

It should be apparent from the foregoing discussion that there remains a great deal of controversy regarding application of fusion surgery to treatment of degenerative spine disease without overt instability. In the future, these controversies will be addressed by a 2-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.

Disc arthroplasty and posterior dynamic stabilization devices are 2 strategies that are currently being investigated. Some brands of artificial disc for treatment of symptomatic lumbar degenerative disc disease have received FDA approval. Short-term studies reveal equivalent results for disc arthroplasty and lumbar fusion. 14

Artificial lumbar disc. Image courtesy of Synthes, Inc.

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 few clinical trials that have been conducted have produced clinical outcomes comparable with fusion.15

The challenges facing artificial disks and posterior dynamic stabilization devices are 2fold. First, they have to improve upon lumbar fusion outcomes. Second, these mechanical devices have to 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, biological 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.

TEST QUESTIONS

Question 1:

Which of the following statements is correct regarding cervical bilateral jumped facet syndrome?

A. It represents an example of covert instability.

B. It rarely requires open surgery.

C. It is seldom associated with a neurological deficit.

D. If treated surgically, it is always treated via a posterior cervical approach.

E. It is produced by a flexion force vector.

The correct answer is E: Bilateral jumped facet syndrome constitutes an example of acute overt instability. Closed reduction may be difficult. Even if closed reduction is achieved, surgery (usually a posterior fusion) is required for stabilization. Because of the significant reduction in the diameter of the cervical spinal canal and neural foramina, it is frequently associated with neurological dysfunction. It can be treated by an anterior or posterior cervical approach. An anterior approach is preferred when there is an associated disk herniation. This injury is caused by a flexion force vector.

Question 2:

Which of the following statements regarding anterior cervical decompression and fusion is correct?

It is now performed more frequently than posterior approaches to the cervical spine in the

A.

United States.
It is contraindicated when multilevel cervical spondylosis and myelopathy with kyphosis is

B.

present.

C. It is reserved for covert instability caused by degenerative disease.

 

 

D. It is always accompanied by plating.

 

 

E. It should be performed on the side contralateral to an existing recurrent laryngeal nerve palsy.

 

 

 

The correct answer is A: Whereas most cervical procedures were performed from a posterior approach in the early 1990s, a major shift has occurred in practice patterns, so that now most of these procedures are performed anteriorly. Anterior cervical decompression and fusion is not contraindicated and is the method of choice when myelopathy due to multiple disk osteophyte complexes and kyphosis is present. A posterior decompressive laminectomy in this setting may exacerbate kyphosis over the long-term and result in recurrent myelopathy. Anterior cervical decompression and fusion is indicated for a wide range of pathology not limited to degenerative disease. It is appropriate for treatment of all forms of instability including acute and chronic overt instability and anticipated instability. Although it is frequently accompanied by plating when 2 or more motion segments are fused, plating is not strictly required for a single-level anterior diskectomy. It should always be performed on the side ipsilateral to an existing recurrent laryngeal nerve palsy to prevent the small but devastating possibility of bilateral laryngeal recurrent nerve (LRN) palsy.

Question 3 (T/F):

Type II odontoid fractures with greater than 6-mm displacement of the fracture fragment or accompanied by transverse ligament rupture are best treated by an odontoid screw.

The correct answer is False: These cases are best treated with posterior C1-2 fusion, ideally with transarticular screws. When the fracture fragment is displaced greater than 6 mm, it is difficult to engage it with an odontoid screw. If there is rupture of the transverse ligament, repair of the odontoid fracture does not address the persistent instability at C1-2.

Question 4 (T/F):

A 45-year-old man presents with an isolated T7 vertebral body metastasis from renal cell carcinoma, producing spinal cord compression. He has limited systemic disease. This is best treated by an anterior T7 corpectomy and instrumented reconstruction.

The correct answer is True: Generally, anterior compression of the cord by tumor should be addressed from an anterior approach, if the patient's systemic condition permits such an operation. An anterior approach in this case would permit a more comprehensive resection of tumor and stabilization.

Question 5 (T/F):

A seatbelt deceleration injury resulting in a true (osseous) Chance fracture can be treated in a TLSO brace without surgery.

The correct answer is True: Two types of flexion distraction seatbelt injuries are recognized: (1) Ligamentous and osseous disruption of posterior and middle columns associated with hinging on a preserved anterior column is an unstable fracture that requires fusion. (2) A true Chance fracture consists of a transverse fracture through the osseous elements of the spine, specifically the pedicles and endplates. Because of the good contact across the fracture surfaces, this injury can heal spontaneously in a brace.

Question 6 (T/F):

Lumbar fusion is contraindicated in patients with chronic low back pain with degenerative disease without spinal stenosis or spondylolisthesis.

The correct answer is False: The American Association of Neurological Surgeons/Congress of Neurological Surgeons (AANS/CNS) evidence-based guidelines support lumbar fusion in "carefully selected" patients in this category. It is generally recognized, however, that fusion is far from an ideal treatment for this group of patients and that novel treatment strategies are needed.

FURTHER READING

  1. Benzel EC. Biomechanics of Spine Stabilization, Principles and Clinical Practice. McGraw-Hill; 1995.
  2. Benzel EC (ed). Spine Surgery: Techniques, Complication Avoidance, and Management. 2nd Edition. Elsevier; 2005.
  3. Fourney DR, Gokaslan ZL. Spinal instability and deformity due to neoplastic conditions. Neurosurgery Focus. 2003;Jan 15;14(1):e8.
  4. Resnick DK, Groff MC. Evidence-based guidelines in lumbar spine surgery. Prog Neurol Surg. 2006;19:123-34.
  5. Sonntag VK, Marciano FF. Is fusion indicated for lumbar spine disorders? Spine. 1995;20(24 suppl):138S-142S.

REFERENCES

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  9. Brown CW, Orme TJ, Richardson HD. The rate of pseudarthrosis (surgical nonunion) in patients who are smokers and patients who are nonsmokers: a comparison study. Spine. Nov 1986;11 (9):942-3.
  10. Bridwell KH, Sedgewick TA, O'Brien MF, Lenke LG, Baldus C. The role of fusion and instrumentation in the treatment of degenerative spondylolisthesis with spinal stenosis. J Spinal Disord. Dec 1993;6(6):461-72.
  11. Fritzell P, Hagg O, Wessberg P, Nordwall A,. 2001 Volvo Award Winner in Clinical Studies: Lumbar fusion versus nonsurgical treatment for chronic low back pain: a multicenter randomized controlled trial from the Swedish Lumbar Spine Study Group. Spine. Dec 1 2001;26(23):2521-32; discussion 2532-4.
  12. Brox JI, Sørensen R, Friis A, Nygaard Ø, Indahl A, Keller A, et al. Randomized clinical trial of lumbar instrumented fusion and cognitive intervention and exercises in patients with chronic low back pain and disc degeneration. Spine. Sep 1 2003;28(17):1913-21.
  13. Resnick DK, Choudhri TF, Dailey AT, Groff MW, Khoo L, Matz PG, et al. Guidelines for the performance of fusion procedures for degenerative disease of the lumbar spine. Part 1: introduction and methodology. J Neurosurg Spine. Jun 2005;2(6):637-8.
  14. Lin EL, Wang JC. Total disk arthroplasty. J Am Acad Orthop Surg. Dec 2006;14(13):705
  15. Schwarzenback O, Berlemann U, Stoll TM, Dubois G. Posterior dynamic stabilization systems: DYNESYS. Orthop Clin North Am. 2005;36(3):363-72.

ACKNOWLEDGMENTS

Images 5, 12, 15, 20, 22, 23, 24, and 28 are protected by copyright under Synthes, Inc. or its affiliates; West Chester, Pennsylvania. The efforts of Mr. Jamison Wolocko and Ms. Cynthia Ryen in securing and providing these illustrations for this article are gratefully acknowledged.