The Journal of Bone and Joint Surgery (American) 82:1458 (2000)
© 2000 The Journal of Bone and Joint Surgery, Inc.
The Use of Pedicle-Screw Internal Fixation for the Operative Treatment of Spinal Disorders*
Robert W. Gaines, Jr., M.D.
Investigation performed at the Columbia Orthopaedic Group,
Columbia Spine Center, and the Department of Orthopaedic Surgery,
University of Missouri Health Sciences Center, Columbia, Missouri
*No benefits in any form have been received or will be received
from a commercial party related directly or indirectly to the subject
of this article. No funds were received in support of this study.
Department of Orthopaedic Surgery, University of Missouri Health
Sciences Center, One Hospital Drive, Columbia, Missouri 65212. E-mail
address: rgaines{at}cogmds.com
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Introduction
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Pedicle screws have dramatically improved the outcomes of
spinal reconstruction requiring spinal fusion.
Short-segment surgical treatments based on the use of pedicle
screws for the treatment of neoplastic, developmental, congenital,
traumatic, and degenerative conditions have been proved to be practical,
safe, and effective.
The Funnel Technique provides a straightforward, direct, and
inexpensive way to very safely apply pedicle screws in the cervical,
thoracic, or lumbar spine.
Carefully applied pedicle-screw fixation does not produce severe
or frequent complications.
Pedicle-screw fixation can be effectively and safely used wherever
a vertebral pedicle can accommodate a pedicle screw - that is, in
the cervical, thoracic, or lumbar spine.
Training in pedicle-screw application should be standard in orthopaedic
training programs since pedicle-screw fixation represents the so-called gold
standard of spinal internal fixation.
Pedicle screws have revolutionized the surgical treatment of
spinal disorders, although their introduction and widespread adoption
by spinal surgeons has created one of the most difficult regulatory
problems ever seen in orthopaedics.
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History of Spinal Internal Fixation with Pedicle Screws
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Hadra65 first used silver-wire
internal fixation for the treatment of a cervical fracture-dislocation
and tuberculous spondylitis in 1891. Later, King84 introduced
facet screws for the treatment of degenerative lumbar conditions.
Although Boucher17 has been widely
credited with the first use of pedicle screws in North America,
his report suggests that his innovation was a longer facet screw
that occasionally obtained oblique purchase across the pedicle.
His screws were not aimed down the long axis of the pedicle. Thus,
it seems that Harrington and Tullos68 deserve
credit for the first deliberate attempt to put pedicle screws through
the isthmus of the pedicle. Their report, published in 1969, described
the attempted reduction of two cases of high-grade spondylolisthesis.
Pioneering the use of pedicle-screw internal fixation proceeded
in France and Switzerland during the 1970s. Clinical success with
the screws was reported during the 1980s by investigators including Cotrel
and Dubousset31, Dick40, Roy-Camille et al.137,138, and Louis99.
A strong stimulus to North American use of the pedicle-screw
technique was the presentation by Roy-Camille at the 1979 American
Academy of Orthopaedic Surgeons meeting in San Francisco. Subsequently,
a number of American surgeons began to use pedicle screws in the
United States. Arthur Steffee used them most creatively.
Steffee et al.152 developed
the variable-screw-placement (VSP) plate, which permitted pedicle
screws to be placed according to individual patient anatomy. This
device provided much more clinical latitude than the Roy-Camille
plate137,138, which had fixed
screw-hole distances for application of the screws (Fig. 1).
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Fig. 1: The
Roy-Camille plate137,138 (left)
and the Steffee variable-screw-placement (VSP) plate152 (right). The former plate, with
its fixed screw-hole distances, was not able to accommodate the
individual patient anatomy as well as the latter plate.
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Recognizing the clinical utility of pedicle-screw fixation, the
North American Spine Society and the Scoliosis Research Society
collaborated on a series of introductory meetings to educate spinal surgeons
about this new technique. The first of these meetings was held in
1984. These two societies subsequently mutually sponsored the International
Meeting on Advanced Spine Techniques, and this annual meeting continues
to be held to the present day.
Since the introduction of pedicle screws, engineers, surgeons,
radiologists, neurophysiologists, anatomists, epidemiologists, and
statisticians have made fundamental efforts to improve what was recognized
universally as a clinical liability in surgical spine care: the
lack of truly high-quality spinal internal fixation similar to what
was available for internal fixation of long bones.
The basic and clinical science of pedicle screws developed from
their introduction and infancy in the early 1970s to widespread
acceptance, as indicated in "State-of-the-Art Treatment" statements by
the North American Spine Society in 1993 and 1996, which endorsed
the use of pedicle screws by experienced surgeons. Review articles
described the evolution of treatment methods as surgical experience
accumulated33,48,62,114,158.
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Anatomy of the Human Pedicle
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The anatomy of the human pedicle has been studied exhaustively
in different races6,25,26,73,83,
in children50,108,140, and in
adults123,146. Patterns of pedicle
anatomy unique to the cervical spine44,81,82,
the thoracic and the lumbar spine45,46,109,126,132,146,148,159,160,
and the sacrum47,94,120,172,173 have
been clearly identified. Measurements of the outer and inner diameters
of the pedicle have been performed extensively71,121.
The inner diameter of the pedicle - the critical surgical dimension
- has been shown to be more directly related to the height of the
patient than to the gender81.
However, wide individual variations within common patterns of anatomy
are the rule. This forces the surgeon to understand the individual
anatomy of the patient, in order to achieve clinical success, and
to appreciate the patterns of pedicle anatomy that are common to
the human race in general. There do not seem to be more than modest
differences in pedicle anatomy from race to race6,25,26,73,83.
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Biomechanical Studies of Pedicle Screws and Pedicle-Screw-Based Constructs
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In 1986, we52,53 clearly demonstrated
the improved quality of spinal internal fixation provided by a pedicle-screw implant
with a fixed link to a plate, in the first nonfailure stability
testing done for spinal fractures. We also showed the fundamental
importance of load transfer across the spinal column itself along with
the implant - a concept known as load-sharing53,
which was subsequently widely adopted by the spinal surgery community.
Biomechanical tests of pedicle-screw constructs have demonstrated
the fundamental importance of the bone-implant interface, bone density,
and screw pullout strength11,28,63,66,87,179.
The essential need for fit and fill of the screw in the isthmus
of the pedicle has been proved19,88,106.
The direct relationship between pullout strength and insertional
torque has been well demonstrated178,
and the fundamental improvement in pullout strength obtained by
cross-linking39,64,100,130 (that
is, attachment of the two screws in an individual vertebra with
use of a metal cross-link) has been documented. The stabilizing
influence of using converging screws (a high pedicle angle [Fig. 2]), if possible,
also was emphasized9,23,139.
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Fig. 2: Drawings
clearly illustrating the benefits of screw triangulation to improve
pullout strength, particularly when the screws are cross-connected
to one another9. (Reprinted, with permission, from: Barber, J. W.; Boden, S. D.; Ganey, T.; and Hutton,
W. C.: Biomechanical study of lumbar pedicle screws: does convergence
affect axial pullout strength? J. Spinal Disord., 11: 216, 1998.)
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The internal forces and moments of bilevel constructs have been
measured exhaustively with strain gauges on all components of rod
and plate-based constructs, and the fundamental importance of load-sharing
through the vertebral column has been reemphasized. Without load-sharing,
an unstable four-bar mechanism was clearly demonstrated when all
four pedicle screws were parallel (Fig. 3)23.
In addition to in vitro testing, finite-element
modeling has been used extensively59,96 to
analyze stress transfer through implants as well as through implant-based
constructs.
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Fig. 3: The 4R-4bar
linkage collapse associated with parallel pedicle screws (left)
can be resisted only by load-sharing through the fracture site (right)23.
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The bone-mineral density of vertebrae has been evaluated exhaustively
with densitometry. Japanese investigators demonstrated conclusively
that patients with multiple spontaneous compression fractures were
very poor candidates for pedicle-screw-based internal fixation because
their bone-mineral density was poor (one-fifth of normal bone density)
preoperatively (Fig. 4)151.
Other authors have strongly suggested the routine use of bone-mineral-density
testing before pedicle-screw-based surgery, although mild osteoporosis
(bone-mineral density of less than 100 milligrams per milliliter)
did not seem to affect screw purchase or clinical outcome adversely87.
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Fig. 4: The "JIKEI
Index" easily relates bone-mineral density and pedicle-screw pullout
strength with this simple x-ray-based scheme. Pullout strength is severely
limited when there are spontaneous compression fractures as in stages
2 and 3. Pedicle-screw fixation is contraindicated when there is
this much osseous deterioration151.
(Reprinted, with permission, from: Soshi, S.; Shiba, R.; Kondo,
H.; and Murota, K.: An experimental study on transpedicular screw
fixation in relation to osteoporosis of the lumbar spine. Spine, 16:
1336, 1991.)
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The success of hook-based constructs compared with that of screw-based
constructs was shown to be based on bone-mineral density28,112. In very osteoporotic specimens,
hook-based claw constructs performed as well as screw-based constructs28. A few authors have suggested use
of both a screw and a hook at the same level111,174.
The enthusiasm for pedicle-screw fixation among spinal surgeons
created a group of engineering collaborators who quickly became
extremely knowledgeable regarding the engineering issues related
to structural and clinical success. A model for fatigue testing
that is now widely used was adopted by the American Society for
Testing and Materials (Fig. 5)34,60.
A corpectomy model is created with two polyethylene blocks to mimic
vertebrae, which are attached to a standard Instron tester (Instron, Canton,
Massachusetts). This model can be used for subcomponent or construct
testing.
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Fig. 5: Model
developed by the American Society for Testing and Materials for
construct or subconstruct testing. Polyethylene blocks are used
to mimic vertebral bodies. Here, under axial load, spinal deformity
secondary to distortion at the longitudinal member-connector interface
is seen34,60. (Reprinted, with
permission, from: Cunningham, B. W.; Sefter, J. C.; Shono, V.; and
McAfee, P. C.: Static and cyclical biomechanical analysis of pedicle
screw spinal constructs. Spine, 18: 1680, 1993.)
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The major diameter of a pedicle screw has been shown to control
pullout strength (Fig. 6)170,179.
Bicortical purchase both in individual vertebrae and in the sacrum
increased pullout strength fundamentally152,
although bicortical purchase, except at the sacrum, has not been
widely adopted by surgeons because of the risk of injury to the
aorta or vena cava. Bicortical sacral purchase, however, has been proved
to be extremely safe and has gained widespread acceptance. The first
sacral segment has been analyzed exhaustively, and several safe
and secure fixation sites have been demonstrated within it47,94,120,172,173. The anatomical liabilities
of the second sacral segment also have been clearly shown.
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Fig. 6: Bar graphs
showing the changes in pullout force (A) and bending
stiffness (B) when screws with five and six-millimeter
major diameters are compared. Improvements in both parameters are
related directly to the major diameter of the screw170,179. (Reprinted, with permission,
from: Wittenberg, R. H.; Lee, K.-S.; Shea, M.; White, A. A., III;
and Hayes, W. C.: Effect of screw diameter, insertion technique,
and bone cement augmentation of pedicular screw fixation strength.
Clin. Orthop., 296: 282, 1993.)
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The presence of one or more pedicle fractures has been shown
to reduce, although not to completely eliminate, the integrity of
pedicle-screw-based constructs85.
Many different styles of mechanisms have been developed by manufacturers
to attach a pedicle screw to a longitudinal member (a plate or a
rod); some are quite rigid and some, less rigid. All of these designs
have shown some clinical utility, although there were dramatic differences
in their biomechanical performance in vitro (Fig. 7)34. The patient's body weight has been
shown to be the main determinant affecting structural survival of
rods used for scoliosis correction75.
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Fig. 7: Bar graphs
showing differences in resistance to cyclic loading34. All of these implant systems have
been used extensively, although their biomechanical performance
differs dramatically. The healing of the fusion is much better in
patients with stiffer implants34.
VSP = variable screw placement (Steffee) plate, KPL = Kirschner
plate, DLO = Dyna-lok plate, ISO = Isola rod, TSR = Texas Scottish
Rite Hospital rod, CCD = Compact CD (Cotrel-Dubousset) rod, CDC
= CD cold rolled rod, FIX = AO fixateur interne, CDS = CD standard
rod, and KRO = Kirschner rod. (Reprinted, with permission, from: Cunningham,
B. W.; Sefter, J. C.; Shono, V.; and McAfee, P. C.: Static and cyclical
biomechanical analysis of pedicle screw spinal constructs. Spine, 18:
1681, 1993.)
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Nitrite has been added to the surface of the screw to improve
surface hardness97, and both titanium
and steel implants have been widely investigated, manufactured,
and used. Titanium implants are used for patients who are allergic
to nickel, chromium, or cadmium or for those who may require frequent
postoperative magnetic resonance imaging or computed tomographic scans43,128,163.
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Techniques of Safe Pedicle-Screw Application
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Roy-Camille et al.137,138 suggested
that a pedicle screw should be introduced by drilling the path and
then applying the screw. Most American surgeons realized the danger
of this approach; they adopted a blunt technique to identify the
pedicle and routinely used biplane image intensification during
the placement of pedicle screws152.
The use of taps of gradually increasing diameter to assess the quality
of cortical purchase through the isthmus of the pedicle, and the
use of image intensification to assess the length of the screw necessary
to obtain purchase in the vertebral body but not through the anterior
vertebral cortex, have become standard procedures for safe screw
application, resulting in strong fixation. This technique, called
the Funnel Technique, is now used widely (Fig. 8).
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Fig. 8: Photographs
illustrating the Funnel Technique. A: The dorsal
projection of the pedicle (red circle) is localized. B: A
one-centimeter-diameter section of cortical bone is removed over
the top of the pedicle with a burr or Lexcel rongeur. C: The
cancellous bone within the pedicle is then visualized. D: The
cancellous bone is removed with a curet until the cortical wall
of the pedicle can be felt and visualized. This is followed by going
deeper into the pedicle toward the isthmus. E: The
Kerrison rongeur is used to remove the cortical bone peripherally
so that the isthmus of the pedicle can be seen. F: Once
the isthmus of the pedicle is directly palpated, a small two-millimeter
pedicle probe is passed through the isthmus into the vertebral body. G: A
larger (five-millimeter) probe then is used to enlarge the path
through the isthmus of the pedicle. H and I: Small
Steinmann pin segments (fifty-five millimeters in length) are placed
into the probed pedicles as radiographic markers. (The anteroposterior [H] and
lateral [I] c-arm images confirm the pedicle path.
The lateral c-arm image [I] also confirms the length
of the screw to be used; the depth of each Steinmann pin is measured
after it is removed.) J: Threads then are cut into
the pedicle with progressively larger taps until firm cortical purchase
is achieved. The feel achieved during the tapping process determines
the screw diameter that is used. K: A ball-tip
probe is used to feel the pedicle in all directions: the bottom
of the pedicle (in the vertebral body) and the superior, inferior,
medial, and lateral inner walls of the pedicle. L: The
screw then is inserted into the pedicle with the screwdriver. The
purchase (insertional torque) must progressively increase until
final seating. M: The anteroposterior and lateral
c-arm images confirm proper positioning after all of the screws, rods,
and connectors are inserted. (Grateful appreciation to Byron R.
Tarbox, M.D., Wicharn Yingsakmongkol, M.D., Michael R. Viau, M.D.,
and Eldin E. Karaikovic, M.D., Ph.D., for assistance with the "funnel
technique.")
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Irrigation with saline solution down the pedicle5, and visualization of the pedicle
by an endoscope51, have both been
used to assess proper screw placement. Routine monitoring of somatosensory135 and dermatomal somatosensory29 evoked potentials, as well as electromyography21,27,36,37,56,72,135,136,166, also
have been used for this purpose. Unpublished data have suggested
the need for routine laminectomy in patients with pedicle-screw
fixation, although this approach has not been adopted widely. Hertlein
et al.70 described the application
of pedicle screws from the anterior approach. The use of robotics1 and of computer-based guidance technology
also has been investigated14,22,58,79,115,116,124,125.
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Supplemental Techniques of Pedicle-Screw Fixation
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Methylmethacrylate has been used in vitro and in
vivo to improve the fixation of pedicle screws in vertebrae129,170,179 (Fig. 9). The improvement
in pullout strength is obvious. However, the occasional spread of
the cement into the spinal canal limits the successful application
of this technique. The use of calcium phosphate122 has
been reported, but there has been very limited acceptance of this
option. The use of carbonated apatite also has been reported98.
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Fig. 9: Graph
showing improved screw-bending stiffness due to the addition of
polymethylmethacrylate (PMMA) and polypropylene glycol-fumarate (PPF).
(Reprinted, with permission, from: Wittenberg, R. H.; Lee, K.-S.;
Shea, M.; White, A. A., III; and Hayes, W. C.: Effect of screw diameter,
insertion technique, and bone cement augmentation of pedicular screw
fixation strength. Clin. Orthop., 296: 284, 1993.)
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Early clinical reports of pedicle-screw fixation demonstrated
that most pedicle screws did not obtain cortical purchase (only
cancellous purchase was obtained), whereas recent reports have emphasized
the essential nature of cortical purchase and the direct relationship
between pullout strength, insertional torque, and bone-mineral density121,178. In our practice, the use of
the direct Funnel Technique to identify the pedicle isthmus permits
tapping the isthmus to obtain cortical purchase for each inserted
screw. Bicortical purchase is routinely used at the first sacral
level but never at any other level.
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Supplemental Screw Sites
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The improvements in fixation at individual vertebral levels stimulated
the need for improvements in spinopelvic fixation to manage spinopelvic
disorders with greater security. Iliosacral screws, iliac-wing screws,
intrasacral fixation, and lumbosacral end-plate fixation all have
been recommended, researched, and adopted in limited applications
for specific indications102,119.
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Use of Bone-Grafting Materials
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With improved spinal internal fixation, surgeons have attempted
to limit their use of extra autogenous bone graft to avoid morbidity
at the donor site. Bone stimulators134,
allograft materials18, synthetic
hydroxyapatite, calcium phosphate, and coral all have been used
with pedicle-screw-based constructs, with greater or lesser success
depending on the pathology and the host-related variables4. Smoking has been demonstrated to
be a risk factor for pseudarthrosis in some series but not others8,55.
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Alterations in Spinal Fusion Techniques Due to Improved Skeletal Fixation Provided by Pedicle-Screw-Based Implants
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Many surgeons have abandoned formerly routine techniques of spinal
fusion such as decortication of the posterior elements or facet
fusion when pedicle-screw-based implants are used. The time to healing
of a scoliosis fusion was reduced from six to twelve months in the
1960s to two to four months in the 1990s because of the secure internal fixation
provided by pedicle-screw-based implants67.
There has been a decrease in the need for postoperative bracing
of patients with constructs based on internal fixation with pedicle
screws. This has been particularly notable in patients with scoliosis10,67,95,110,154,155.
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Spinal Column Biology Around Pedicle-Screw-Based Implants
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Stress-shielding of bypassed vertebrae under implants that were
deliberately disproportionately large has been well documented in
dogs32. However, loosening of
these huge implants from the host spine occurred when the bypassed
spinal segment was osteoporotic. After screw-loosening, the stress-shielding
effect of the disproportionately large implants was alleviated and
the bone density of the bypassed segment of the spinal column returned
to normal.
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Imaging of Pedicle-Screw-Based Constructs to Assess Screw Placement and to Document Union of a Spinal Fusion
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The limitations of plain radiographs and computed tomography
in demonstrating proper screw position have been well documented54,117,165. Surgical exploration is
the reference standard for determining union. Only the radiographic
demonstration of trabeculation across the intertransverse (lateral)
or interbody area comes close to surgical exploration with regard
to accuracy in determining the presence or absence of solid union
of a spinal fusion15.
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Changes in Adjacent Segments
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Static kinematic testing has been used to examine and identify
the increased motion that occurs at motion segments adjacent to
spinal constructs immobilized by pedicle-screw-based implants175. Finite-element models also have
demonstrated this increased motion96.
Case reports30,93 of spinal
fractures occurring adjacent to spinal instrumentation constructs
have documented clinically the vulnerability of the spine after
a violent injury. Disc pressures are substantially changed adjacent
to a spinal instrumentation construct as well35,164.
Other case reports have demonstrated the necessity of protecting
the supraspinous and interspinous ligaments, the ligamentum flavum,
and the capsular ligaments at segments adjacent to a spinal instrumentation
construct, and they have shown the importance of restoring balance
in the sagittal plane to avoid degeneration of adjacent segments49.
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Indication for Pedicle-Screw-Based Fixation
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Although the initial clinical use of pedicle-screw-based implants
was based on their theoretical benefits, their effectiveness in
facilitating fusion now has been demonstrated conclusively55,144,145,149,177. Collaborative and
individual studies8,55,167,176,177 have
demonstrated statistically higher fusion rates associated with use
of these implants. The functional benefits for patients also have
been demonstrated conclusively with use of preoperative and postoperative
testing167.
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Addition of Interbody Fusion to Supplement Pedicle-Screw-Based Posterolateral Fusion
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Mainly on the basis of the results of treatment of scoliosis
and the desire to ensure load-sharing in the anterior column, many
low-back surgeons in the United States routinely began to use circumferential
fusion for all cases of degenerative spinal disease after anterior
and middle-column load-sharing was demonstrated to be beneficial
biomechanically55,152,156. Anterior
grafting was performed either with an anterior open procedure, a
laparoscopically based anterior procedure, or posterior lumbar interbody fusion
at the same time as the pedicle-screw-based surgery. Some authors
have suggested that the routine use of circumferential fusion improves
the healing rate in smokers55 and
that the union rate is better than that obtained with pedicle-screw-based
intertransverse fusion alone8.
The development of prosthetic devices that contain autograft
or allograft has added another variable to this clinical interface
that is still under investigation. An unpublished review of the
results in our patients demonstrated a 92 percent union rate with
pedicle-screw-based posterolateral fusion alone, without cages,
using only the autograft obtained from laminectomy for the majority
of patients. None of the patients had anterior surgery. The union
rate in smokers and nonsmokers was the same.
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Complications Associated with the Use of Pedicle Screws
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Complications have accompanied the use of pedicle screws. The
rate of screw misplacement has ranged from 0 percent10 to 2 percent20 to
25 percent95 in patients with
scoliosis and to nearly 4.2 percent8,16,49,101,160,168 in
those with degenerative diseases. A learning curve has been demonstrated
for surgeons in general54.
A variable prevalence of nerve-root and/or cauda equina injury
has been reported to be associated with pain and sensory deficit
in some patients20,55,113,167,168.
The prevalence has been reported to be as high as 11 percent114 and as low as 1 percent150 to 2 percent20,113.
However, most misplaced screws do not create nerve-root injury20,54,145,154. Safe removal of screws
that were drilled into the spinal canal has been reported41.
A high prevalence of screw misplacement associated with drilling160 rather than with the blunt technique
of screw application has been clearly demonstrated20. Computer-based navigation techniques
have been developed and have been suggested for clinical application14,22,58,79,115,116,124,125. Some authors
have suggested that the routine use of computer navigation improves
screw placement. Our own experience, however, has been that the
increased operating time does not improve the quality of screw placement
but increases tremendously the expense and potential danger.
A dural injury was reported in seven of 124 patients168 and in two of eighty-nine patients145 in two series of pedicle-screw
fixation for the treatment of degenerative spine disease. Deep-infection
rates have ranged from 1.1 percent (five of 470 patients)8 to 1.2 percent55 to
2.3 percent (two of eighty-five patients)57 to
4.2 percent (four of ninety-six patients)149.
Prompt wound débridement and administration of antibiotics, with
preservation of the implant and subsequent delayed primary closure,
have been adopted routinely, with acceptable intermediate-term clinical
outcomes57. The use of pedicle-screw-supplemented
fusion has been reported to enhance the results of primary operative
treatment of hematogenous discitis and osteomyelitis131.
The frequency of screw breakage has ranged from 2.6 percent8 to 4.9 percent55 to
9 percent6 to 36
percent114 to as high as 60 percent105. However, survivorship analysis
of a series of patients who had degenerative disease indicated that the
clinical survival of pedicle-screw-based constructs was similar
to that of total hip and total knee reconstructions101. The two alarmingly high screw-breakage
rates105,114 were reported when
highly comminuted spinal fractures were internally fixed with posterior short-segment
pedicle-screw-based instruments. The load-sharing classification103,127 provides a simple way to differentiate
fractures according to comminution. It suggests that anterior vertebrectomy,
strut-grafting, and instrumentation be performed in patients with
severely comminuted injury to avoid the high screw-breakage rates
that occur when short-segment posterior pedicle-screw-based instrumentation
is used for highly comminuted injuries (Fig. 10).
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Fig. 10: The load-sharing
classification permits quantification of comminution of spinal fracture
sites so that the load-sharing capability of the injured vertebral body
itself, along with the implant system, can be determined. This approach
has allowed surgeons to perform short-segment fixation for most
isolated spinal fractures in cooperative patients103,127. A: Comminution/involvement.
1 = little comminution (less than 30 percent), 2 = more comminution (30
to 60 percent), and 3 = gross comminution (more than 60 percent)
on computed tomographic sagittal plane sections. B: Apposition
of fragments. 1 = minimal displacement, 2 = spread of displacement
(at least two millimeters of displacement of less than 50 percent
of the cross section of the body), and 3 = wide displacement (at
least two millimeters of displacement of more than 50 percent of
the cross section of the body). C: Deformity correction.
1 = kyphotic correction of 3 degrees or less, 2 = kyphotic correction
of 4 to 9 degrees, and 3 = kyphotic correction of 10 degrees or
more on lateral plain radiographs. (Reprinted, with permission,
from: McCormack, T.; Karaikovic, E.; and Gaines, R. W.: The load
sharing classification of spine fractures. Spine, 19: 1742, 1994.)
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Screw pullout and screw-connector disengagement have been reported
both in in vitro testing and in patients8,16,55. Rod breakage has been reported
only rarely. Most authors have described problems with the screw
or the bone-screw interface but not with the longitudinal component8,16,55.
Screw-thread fracture has never been reported, to my knowledge.
There are established methods for removing broken screws42,104.
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Union Rate Associated with Pedicle-Screw-Based Constructs
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The union rate associated with pedicle-screw-based constructs
generally has been reported to be nearly 90 percent8,20,55,167,176,177. The benefits provided
by very stiff implants were demonstrated statistically in a prospective,
randomized trial comparing the rate of union associated with these
implants with the rates obtained without use of an implant and with
use of a semirigid implant177.
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Late Results Associated with the Presence of Implants
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Implant-related pain has led to the need to remove the implant
from some patients. The clinical benefits of implant removal have
not been uniformly demonstrated74.
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Clinical Advances Secondary to the Use of Pedicle Screws and Pedicle-Screw-Based Constructs
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The use of pedicle screws has resulted in dramatic clinical improvements
in the surgical care of spinal diseases; these improvements can
be catalogued according to the disease process involved.
Scoliosis
Detailed assessment of patients who have been surgically treated
for scoliosis has demonstrated that the routine use of pedicle screws
produces better results, in terms of correction in the coronal and sagittal
planes and maintenance of the correction, than does the use of hook-based
constructs10,67. There is also
better tilt-angle correction and rotational correction (Fig. 11-A, Fig. 11-B, Fig. 11-C, Fig. 11-D, and Fig. 11-E)10,67. The routine use of pedicle-screw-based
constructs has induced many surgeons to abandon the use of braces
or casts postoperatively, with low rates of screw and rod breakage10,20,67, and it also has greatly facilitated
the care of adult patients with degenerative scoliosis38,110. However, when fusion of the
spine to the pelvis is necessary, it should include iliac fixation
for routine success38.
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Fig. 11-A: Figs.
11-A through 11-E: A patient with severe thoracic scoliosis who
had excellent correction of both the primary curve and the tilt
angle with use of pedicle screws for the caudal platform. The screws'
ability to control the caudal segments in the fusion in a very secure
manner allowed much better realignment of the tilt angle than would
have been possible with only hook systems10,67.
Fig. 11-A: Posteroanterior and forward-bending posteroanterior
photographs.
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Fig. 11-B: Posteroanterior
radiographs demonstrating an 88-degree curve with the patient standing
and a 53-degree right thoracic curve with the patient stretching.
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Fig. 11-C: Posteroanterior
radiograph made ten months postoperatively, demonstrating correction
and restored compensation.
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Fig. 11-D: Postoperative
posteroanterior and forward-bending posteroanterior photographs,
demonstrating good balance and mobility in the lumbar spine.
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Fig. 11-E: Preoperative
and postoperative standing lateral radiographs demonstrating correction
of the midthoracic kyphotic deformity.
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Spondylolisthesis
Routine use of pedicle screws for the treatment of spondylolisthesis
has increased the union rate to 94 percent and improved the ability
to reduce and maintain reduction of higher-grade slips7,80,141,149,152. The ability to spare
levels from fusion also has been demonstrated clearly in these patients152,156 (Fig. 12). Monosegmental fusion with use
of posterior lumbar interbody fusion and pedicle-screw-based reduction
has proved very successful in children.
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Fig. 12: Intermediate-grade
spondylolisthesis and/or degenerative spondylolisthesis can now
be well treated with reduction with the posterior approach only,
with use of interbody fusion with cages and pedicle screws. The
union rate is very high, and straightforward reduction of intermediate
slips is safe neurologically, provided that the roots are visualized
and protected carefully during the procedure.
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Vertebrectomy at the fifth lumbar level with reduction of the
fourth lumbar vertebra onto the first sacral vertebra and with use
of only single-level instrumentation and fusion has been widely
accepted as a routine treatment for spondyloptosis91.
Spinal Fracture
The use of pedicle screws for the treatment of spinal fracture
has allowed most surgeons to accomplish short-segment instrumentation (instrumentation
extending from only one vertebra cephalad to the damaged vertebra
and to one vertebra caudad to it)13,86,103,105,142.
Although initial screw-breakage rates were high105,
they can be reduced to zero when the influence of comminution of
the injured vertebral body is appreciated and the choice between
anterior and posterior short-segment instrumentation is based predominantly
on the load-sharing classification127 (Fig. 13). The limits
of in situ bending have been clearly defined63.
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Fig. 13: Reconstruction
of spinal fractures is much more secure with pedicle screws. In
this patient, realignment of adjacent-level injuries at the third
and fourth thoracic vertebrae was accomplished with short-segment
pedicle-screw instrumentation in the second and fifth thoracic vertebrae.
(Case contributed by Dr. Marc Asher.)
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Pedicle screws have dramatically facilitated the management of
burst fractures of the fifth lumbar vertebra118,147 as
well as the treatment of posttraumatic kyphosis in second and third-world
countries24,90,171.
Simple Lumbar Degenerative Disc Disease
The use of pedicle-screw-based implants has resulted in union
rates of approximately 95 percent after simple lumbar fusions for
two and three-level lesions8,55,107,167,176,177.
This union rate was possible previously only with a single-level
fusion.
Spinal Osteotomy
The fixation provided by pedicle screws has markedly altered
the clinical course in patients with fixed multiplanar spinal deformities
due to ankylosing spondylitis69,89,161 and
in those with severe flatback deformity92.
Patients now can walk with use of a removable brace after the osteotomy.
They spend no time in bed, whereas periods of one to two months
in bed were routine for patients who wore a plaster cast before
the advent of the secure fixation provided by pedicle screws. Although
it is difficult to prove, the postoperative neurological problems
encountered secondary to major osteotomy are also thought to be
greatly reduced by the secure short-segment fixation provided by
pedicle screws. The loss of correction also must be somewhat reduced
(Fig. 14-A, Fig. 14-B, Fig. 14-C, and Fig. 14-D).
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Fig. 14-A: Figs.
14-A through 14-D: Spinal osteotomy has become a much more predictable
surgical technique since the adoption of pedicle screws, which provide
more secure fixation than any previous technique.
Fig. 14-A: The site of the corrective osteotomy should be at
the apex of the deformity, with removal of the laminar arch, the
pedicles, and the part of the vertebral body and disc that represent the
apex of the deformity.
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Fig. 14-B: This
patient had a post-fracture flatback deformity after Harrington
instrumentation, with fixed deformity in the lumbar spine. The osteotomy
was performed through the third lumbar level.
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Spina Bifida
The unique benefits of pedicle screws in the correction, stabilization,
and maintenance of correction of spina bifida and postlaminectomy deformities
have been well documented133.
The reduction in the period of immobilization and the quality of
the final outcome are difficult to demonstrate statistically because
these deformities are unusual and the treatment is rarely standardized.
However, the acceptance of pedicle screws for the treatment of these
particularly difficult problems has been notable.
Pars Interarticularis Defects
Repair of pars interarticularis defects has been facilitated
considerably by the use of a pedicle screw at the cephalad end of
the defect; a hook, wire, or cable then is used to provide internal
fixation to the lamina78,141,150.
The infrequent need for surgical repair of these defects makes statistical
documentation of the effectiveness of screws difficult to prove.
However, their use has made a number of new and different techniques
available to surgeons and patients.
Neoplasms
Pedicle screws have facilitated the short-segment treatment,
including total vertebrectomy, of primary and metastatic neoplasms61,77. Short-segment instrumentation
is more appropriate in some of these patients than it is in others. The
use of short-segment treatment facilitated by pedicle screws has
provided the opportunity to perform safe radical resection of primary
spinal tumors for the first time153,157 (Fig. 15). Sacral and
lumbosacral resection of chordomas is now practical, and the outcomes
are predictable61.
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Fig. 15: In tumor
reconstructions, pedicle screws permit segmental resection, realignment,
and reconstruction over shorter segments than was possible before
their introduction. In this patient, a chordoma of the sacrum was
resected, and reconstruction was performed with use of short-segment
anterior and posterior approaches along with pedicle screws and
sacral and iliac screws155. (Case
contributed by Dr. Stefano Boriani.)
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Lesions of the Cervical Spine and the Cervicothoracic Junction
The use of pedicle screws has been very helpful in the treatment
of traumatic and developmental lesions at the cervicothoracic junction.
Pedicle screws were used frequently from the second to the seventh
cervical vertebra by Abumi and Kaneda2 and
at the cervicothoracic junction by Albert et al.3.
Pedicle-screw constructs have been compared favorably with lateral-mass
plate constructs76.
Spinopelvic Trauma
Severe, traumatic spinopelvic disruption and vertical fractures
of the sacrum have been repaired successfully with surgical use
of a combination of either iliac or iliosacral screws and caudad
lumbar pedicle screws12,143.
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Overview
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Although the routine use of pedicle screws has not been free
of complications, it has dramatically improved the prevalence of
union after spinal fusion. It also has facilitated shorter-segment
instrumentation for fractures and spondylolisthesis and has made
total vertebrectomy for neoplasms practical, with predictable outcomes.
In addition, the chance of achieving proper realignment and relief
of pain has been improved greatly, with fewer complications after
the surgical procedure.
The proper use of pedicle screws requires a detailed knowledge
of the patient's pedicle anatomy, the routine use of image intensification
during application of the screws, the use of blunt identification
of the isthmus of the pedicle with employment of the Funnel Technique,
meticulous attention to wound care, the use of primarily autograft
material to achieve fusion, and the availability of a full inventory
of implants. Slow, supervised rehabilitation, with acknowledgment
of the fundamental importance of load-sharing by the spinal column itself
along with the implant, is also essential until healing of the fusion
occurs.
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Introduction
History of Spinal Internal...
