CHAPTER SYNOPSIS:
A large number of instrumentation systems are currently available for treating pathology of the lumbar spine. These include sublaminar wires, hooks, transpedicular fixation, translaminar facet screws, anterior screws with plates or rods, interbody fusion devices, or a combination of any of these. This chapter provides an overview of the role of instrumentation in lumbar spine degenerative conditions with emphasis on the technique of transpedicular fixation.
IMPORTANT POINTS:
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Pedicle screws have become the most widely used fixation method in modern spine surgery.
Advantages of Pedicle Screws
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Provide fixation and control in all three columns from the posterior approach
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Allow for enhanced, three-dimensional deformity correction
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Do not violate the spinal canal when inserted correctly
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May be used after laminectomy is performed
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Allow solid fixation at the lumbosacral junction
Disadvantages of Pedicle Screws
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Neurologic risk from improper insertion
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Risk for adjacent segment degeneration from rigid fixation
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Insertion requires extensive lateral dissection for proper exposure
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Difficulty in screw purchase in osteoporotic bone
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Increased cost
CLINICAL/SURGICAL PEARLS:
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In the absence of anterior column integrity, pedicle screw fixation will likely fail.
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Convergent placement of pedicle screws is generally recommended.
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Care should be used to ensure accurate pedicle screw placement, with mechanical probing of the pedicle tract to palpate five bony surfaces being the gold standard technique.
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The optimal length and diameter of the pedicle screws should be determined based on the axial cuts of the preoperative imaging.
CLINICAL/SURGICAL PITFALLS:
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Bicortical fixation is not recommended because the risk for injury to anterior structures outweighs the minimal increase in fixation strength.
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A caudally or medially misplaced screw risks injury to the nerve root, dura, or cauda equine.
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A laterally misplaced screw may not risk neurovascular injury but will have lower pullout strength if the screw is not positioned in the bone.
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Use care not to violate the superior facet joint during the procedure.
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Over the past few decades, significant advances have been made in our understanding of spinal anatomy and pathology, as well as in the design and techniques of spinal instrumentation. Currently, the spine surgeon has a large armamentarium of instrumentation techniques at his disposal, including sublaminar wires, hooks, transpedicular fixation, translaminar facet screws, anterior screws with plates or rods, interbody devices, or a combination of these. The goal of this chapter is to provide an overview of the role of instrumentation in degenerative conditions of the lumbar spine, with particular emphasis on transpedicular fixation and its complications.
DEGENERATIVE CASCADE
Three terms are commonly used to describe degenerative conditions of the lumbar spine: internal disc disruption, degenerative disc disease, and segmental instability . Internal disc disruption (“dark disc disease”) refers to an alteration in the internal structure of the intervertebral disc. Mild degenerative abnormalities are noted on plain radiographs, and the diagnosis is made by magnetic resonance imaging (MRI) and discography. Degenerative disc disease is characterized by osteoarthritic changes such as disc space narrowing, end plate sclerosis, osteophyte formation, and facet degeneration. In the absence of instability, decompressive procedures without fusion are recommended for the treatment of neurogenic claudication or radiculopathy. Fusion for low back pain without instability remains controversial because outcomes are not predictable. Segmental instability is the last stage of the degenerative cascade and represents abnormal motion between two or more vertebrae. Chronic disc and facet joint degeneration and ligamentous laxity with loss of the supporting capsuloligamentous structures of the spinal segment lead to instability. Segmental instability refers to either translational or rotational instability of the lumbar segment, and includes rotatory subluxation, degenerative scoliosis, and degenerative spondylolisthesis.
ROLE OF INSTRUMENTATION
The role of instrumentation in the treatment of degenerative conditions of the lumbar spine has been a controversial topic over the last two decades. Several prospective, randomized studies have been performed to assess the role of instrumentation in degenerative conditions of the lumbar spine. Although it generally accepted that posterior instrumentation enhances the rate of fusion, it is not clear whether it improves the functional outcome. In general, fusion should be reserved for patients with structural instability (i.e., segmental instability), such as spondylolisthesis or lateral listhesis caused by degenerative scoliosis. In the absence of structural instability, fusion remains controversial for the treatment of low back pain.
The goal of spinal instrumentation is to provide the stability needed to achieve fusion. However, the fundamental principles of fusion surgery must be adhered to, such as proper decortication, preparation of the fusion bed, and the use of appropriate bone graft. Instrumentation in and of itself will not compensate for these errors in technique. Instrumentation should be considered as a supplement to enhance the fusion rate.
ROLE OF INTERBODY FUSION
In recent years, the use of interbody fusion has become increasingly common. Interbody devices can be inserted from either an anterior (anterior lumbar interbody fusion) or posterior (posterior or transforaminal lumbar interbody fusion) approach. Interbody fusion procedures offer several potential advantages: the interbody grafts are placed in the weight-bearing position and are better able to resist axial loads; an increased surface area exists for fusion; good blood supply results after decortication of the vertebral end plates; the neural foramina are increased in height and diameter, thus relieving compression of nerve roots; and the disc, which is commonly believed to be a source of pain, is removed. Furthermore, proponents of interbody fusion argue that disc height and lumbar lordosis are restored, thereby improving overall sagittal balance. However, subsidence of the interbody graft into the vertebral end plates can occur with time, and little data exist to show that the improvement in lordosis observed in the immediate postoperative period is maintained at long-term follow-up.
It has been demonstrated biomechanically that the most effective means of eliminating motion between two vertebrae is through the disc space rather than the facet joints, transverse processes, or spinous processes. Furthermore, it has been shown that posterolateral fusion does not completely achieve immobilization of the motion segment despite a solid fusion.
Despite these theoretical advantages of anterior column fusion, it is not yet clear whether clinical outcomes are improved compared with an instrumented posterolateral fusion alone. The disadvantages of including an interbody device include the added cost, increased operative time, risk for neurologic injury because of nerve root or dural sac retraction, and long-term, potentially deleterious effects of complete immobilization of a motion segment on the adjacent lumbar levels.
TRANSPEDICULAR INSTRUMENTATION
Historical Perspective
King first described the transfacet screw in 1948 when he implanted metallic screws across facet joints for partial immobilization of the lumbar spine. King’s transfacet screw fixation was described even before hook-based systems were popularized by Harrington in the 1960s. Boucher introduced the concept of pedicle screws in the 1950s as he noted a high pseudarthrosis rate with the transfacet screw, and so he directed his screw more medially such that it entered the pedicle and vertebral body. introduced transpedicular instrumentation in the United States in 1983.
Advantages and Disadvantages of Pedicle Screw Constructs
Pedicle screws offer several advantages over other fixation methods. Unlike sublaminar wires or hooks, which provide fixation in the posterior column only, pedicle screws provide fixation and control in all three columns of the spine from the posterior approach. This rigid fixation enhances fusion rates and also diminishes the number of levels required for fusion to obtain adequate stability. The power of pedicle screws allows enhanced three-dimensional deformity correction, and the segmental control of the vertebrae allows distraction, compression, and derotation for deformity correction. They allow the surgeon to apply corrective forces in multiple planes to correct complex spinal deformities. Furthermore, unlike sublaminar wires or hooks, pedicle screws do not violate the spinal canal when inserted correctly. They can also be used after laminectomy is performed when other types of constructs (such as wires, sublaminar hooks, or translaminar facet screws) are contraindicated. Furthermore, pedicle screws allow solid fixation at the lumbosacral junction.
The use of pedicle screw systems has several disadvantages. Although with experience and proper technique, pedicle screws can be safely inserted, the risk for neurologic injury is always a concern with malpositioning of screws. Furthermore, the rigid fixation that pedicle screw constructs offer may accelerate adjacent segment degeneration. Insertion of pedicle screws also requires extensive lateral dissection to expose the transverse processes and to obtain the proper medial trajectory, which leads to greater blood loss and increased operative time. It is difficult to obtain good screw purchase in patients with significant osteoporosis. Finally, pedicle screws are expensive, and when multiple levels are included in the construct, costs can be exorbitantly high.
Some fundamental principles apply to the use of pedicle screws. Although pedicle screw constructs provide excellent rigidity, in the absence of anterior column integrity, pedicle screw fixation will likely fail, resulting in either loss of spinal alignment (kyphosis), screw breakage or pullout, or even breakage of the rod(s). Biomechanical principles must be adhered to, and inappropriate applications of pedicle screws will lead to poor outcomes.
Relevant Anatomy and Techniques
From L1 to S1, the trend in pedicle morphology is an increasing diameter and a more medial orientation. The width of pedicles increases from L1 to S1. Most of the pedicles below T10 are greater than 7 mm in transverse diameter, and most below L1 are greater than 8 mm in diameter. The angle at which the pedicle emerges from the vertebral body changes in the transverse plane from L1 to the sacrum, progressively becoming more angulated posterolateral to anteromedial.
The pedicles have a close anatomic relation to the nerve roots. Approximately 2 mm of epidural fat separates the medial cortex of a lumbar pedicle from the dural sac. The nerve roots exit the neural canal immediately inferior to the pedicle, practically hugging the pedicle medially and inferiorly. The medial wall of the pedicle is thicker than the lateral wall, and this helps prevent perforation of the pedicle wall medially. There is very little room for error medially and inferiorly to the pedicle, and misplaced screws in these areas can cause nerve root injury, which may not be reversible with screw removal. The safest placement of the screw is within the cephalad portion of the pedicle.
The most common anterior structures at risk include the middle sacral artery and vein, the common iliac artery and vein (left greater than right), the L5 nerve root (especially with bicortical S1 screws directed straightforward in the coronal plane), the colon (particularly with S2 screws), and the sympathetic chain. The aorta and inferior vena cava lie anterior to the vertebral bodies in the lumbar spine. Bifurcation of these vessels occurs at the L5 vertebral body. In the lumbar spine, the aorta is the vascular structure most at risk when placing pedicle screws at or above L4.
When inserting sacral pedicle screws, several anatomic structures are at risk with bicortical fixation. These include the middle sacral artery and vein, the common iliac vessels or the external or internal iliac vessels, the L5 nerve root (especially with bicortical S1 screws directed anteriorly), the sympathetic chain, and the colon. Although bicortical screw purchase in the sacrum is desirable for fixation strength, the depth of penetration should not exceed 2 to 3 mm through the anterior sacral cortex to avoid injury to these structures. Furthermore, S1 pedicle screws should be angled medially because a straightforward approach risks damage to the L5 nerve root and the great vessels.
Biomechanics
Approximately 60% of the fixation strength of thoracic and lumbar pedicle screws is in the pedicle itself, and the pullout strength depends primarily on the structural characteristics of the pedicle rather than the vertebral body. The cancellous bone in the vertebral body adds another 15% to 20% of strength, whereas purchase in the anterior cortex offers another 20% to 25% increase. Bicortical purchase is not recommended in the lumbar spine because the risk for injury to the anterior vascular structures outweighs the benefits gained from the marginal increase in fixation strength.
Unilateral versus Bilateral Pedicle Screws
Some authors have advocated placing pedicle screws on only one side of the spine because of the risks associated with screw insertion and the added operative time. Advocates of unilateral instrumentation argue that adequate stability and immobilization of the spinal segment is provided by unilateral screws to obtain a solid fusion. In 1991, Goel et al. compared unilateral versus bilateral instrumentation in a biomechanical study of human cadavers, and found that unilateral pedicle screw fixation reduced motion by 40% and bilateral instrumentation by 70% when compared with the intact spine before decompression and instrumentation. Marked differences in rigidity were also noted with lateral bending (13% reduction vs. 65% reduction) and in axial modes (9% reduction vs. 65% reduction).
Despite the biomechanical support for bilateral instrumentation, clinical studies have not shown a difference between the two techniques. In 1992, Kabins et al. retrospectively evaluated fusion results with unilateral versus bilateral instrumentation in a one-level (L4-5) fusion, and found similar fusion rates and clinical outcomes between the two groups, although only plain radiographs were used to assess fusion. In a nonrandomized, prospective study of 87 patients, Suk et al. found no significant differences between unilateral and bilateral instrumentation groups, and concluded that unilateral pedicle screw fixation was as effective as bilateral pedicle screw fixation in lumbar spinal fusion independent of whether one or two segments were fused. Recently, a prospective, randomized study of 82 patients with degenerative lumbar spondylolisthesis who underwent posterolateral fusion with either bilateral or unilateral pedicle screw instrumentation showed no difference in clinical outcome or fusion rate at a mean of 4 to 5 years of follow-up.
Although most surgeons currently use bilateral instrumentation, evidence certainly exists in the literature to justify unilateral instrumentation in degenerative conditions of the lumbar spine. With the addition of an interbody fusion through a posterior approach, the justification for unilateral instrumentation may be even more compelling.
Skipping Levels in Multilevel Fusions
Some studies have suggested placing pedicle screws in only the most cephalad and caudad vertebrae of the fusion construct in a multilevel fusion. Krag suggests that screws are necessary only in the top and bottom vertebrae of a fusion segment regardless of its length. Other authors argue for placement of pedicle screws in each vertebral segment being fused. The biomechanical data support instrumenting every level of the construct. Dick et al. used calf lumbar spines to show that the addition of pedicle screws in the intermediate segment increased stiffness during axial, flexion, and torsional testing compared with nonsegmental pedicle screw constructs in a two-level construct. Brodke et al. found that the addition of intermediate pedicle screws significantly increased stiffness in both two- and three-level constructs. However, when interbody fusion is also performed, omission of screws from the intermediate segment of the posterior construct may not affect the rigidity of the construct. In a cadaveric study, Eskander et al. found that after a two-level ALIF procedure, posterior instrumentation significantly reduces motion and enhances stability, but omission of the middle segment pedicle screw in a two-level interbody fusion model did not affect construct stiffness or range of motion.
Effect of Using Cross-links
The cross-linking device connects the rods horizontally. The use of a cross-linked system was first described by Armstrong and Connock in 1970 with Harrington distraction-compression system in scoliosis surgery. They report that the cross-linked system demonstrated increased stability in axial loading compared with conventional systems. Since their introduction, several investigators have performed biomechanical studies to assess the value of cross-links in posterior fusion constructs. Horizontal transfixation of longitudinal rods has been shown by several investigators to provide additional stability in axial rotation and lateral bending but not in flexion and extension. It has also been shown that axial rotational stability is significantly improved with the use of two cross-links. Others have demonstrated that cross-linking increases torsional stiffness, although no significant differences could be measured in the flexion/extension and lateral bending moments. Brodke et al. performed a biomechanical analysis of two- and three-level pedicle screw constructs with and without segmental fixation and/or cross-links using calf lumbar spines. Cross-links had no effect on flexion/extension stiffness. The addition of two cross-links increased lateral bending stiffness in the longer three-level constructs, with little change in the two-level constructs. In axial torsion, the progressive addition of cross-links showed a tendency toward increased stiffness in both the two- and three-level constructs.
Complications of Pedicle Screws
Esses et al. report an overall complication rate of 9.6% in a survey of American Back Society members. Screw misplacement was the most common complication (5.2%), followed by pedicle fracture (2.3%), dural tear (1.9%), and significant vessel injury (0.16%). Yuan et al. report a 5% incidence rate of intraoperative events associated with the use of screws. Loss of purchase (1.7%), and pedicle fracture and screw breakout occurred in 1%. Neurologic injury, dural tear, and screw breakage were rare complications. In a series of 4790 pedicle screw insertions, Lonstein et al. note that 5.1% of screws were inserted outside the pedicle, and the incidence rate of permanent nerve root injury caused by pedicle screw insertion was 0.3%.
Misplaced Pedicle Screws
Misplaced pedicle screws represent the most common complication in pedicle screw instrumentation. The incidence of misplaced pedicle screws varies widely in the literature, with some studies reporting rates as low as 1.2% to as high as 28.8%. Studies that use a computed tomographic (CT) scan to assess pedicle screw position have a greater rate of pedicle perforation than those that use plain radiographs. In general, misplacement of screws occurs at a greater rate in scoliosis surgery and with more inexperienced surgeons. The actual incidence of misplaced screws is likely to be greater than is reported in the literature because many patients remain asymptomatic despite a malpositioned screw.
A screw that is misplaced laterally may not cause injury to any structures, but the pullout strength will be significantly lower if the screw is not positioned within bone ( Fig. 18-1 A ). A cephalad breach of the pedicle will likely lead to positioning of the screw within the intervertebral disc. This will reduce the pullout strength of the screw, and when it occurs at the superior end of a fusion construct, damage to the adjacent segment disc may precipitate degenerative changes at this level.
Related posts:
Rheumatoid Arthritis of the Cervical Spine
Indications for Fusion for Degenerative Disorders of the Lumbar Spine
Posterior Lumbar Interbody Fusion for Degenerative Disorders of the Spine
Failure of Cervical Spine Instrumentation
Failure of Lumbar Spine Instrumentation
Facet Joint Replacement for the Treatment of Degenerative Disorders of the Spine
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