INTRODUCTION
In the last few decades pedicle screw
placement has brought in a genuine scientific revolution in the
surgical care of spinal disorders, although their introduction
and widespread adoption by spinal surgeons all over the globe
has created one of the most difficult regulatory problems ever
witnessed in the history of orthopaedics. The technique has
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 straightforward, direct and inexpensive way
to very safely apply pedicle screws in the cervical (1),
thoracic (2,3) or lumbar spines (4).
Timeline of
Pedicle screws
In 1948 King introduced facet screws for
the treatment of degenerative lumbar conditions. Boucher has
been widely credited with the first use of pedicle screws in
North America. His innovation was a longer facet screw that
occasionally obtained oblique purchase across the pedicle. His
screws were not aimed at long axis of the pedicle. It seems that
Harrington & Tullos deserve credit for the first deliberate
attempt to put pedicle screw through the isthmus of the pedicle.
Pioneering the use of pedicle screws with clinical success
Roy-Camille (5), popularized the technique in European
countries. A strong stimulus to North American use of pedicle
screws techniques was the presentation by Roy-Camille (5) at the
1979 American Academy of Orthopaedic Surgeons meeting at San
Francisco. Subsequently a number of American surgeons began to
use it in US. Arthur Steffee used them most creatively. Steffee
et al developed 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 plate which had fixed screw hole
distances for application of the screws. Since the
introduction of pedicle screws, engineers, surgeons,
radiologists, neurophysiologist, anatomists, epidemiologist and
statisticians have made fundamental efforts to improve what was
universally recoganised as a clinical liability in surgical
spine care; the lack of a truly high quality spinal internal
fixation similar to what was available for internal; fixation of
long bones. What was introduced as an infant in 1970 for
clinical use, is now well accepted all over the world as
indicated in the “State of the Art treatment” statements by the
North American spine society in 1993 and 1996.
The long regulatory battle with the FDA
started in 1992, when pedicle screw was enjoying class three
clearance. Because of the litigation problems FDA was forced to
down grade pedicle screws to class 2 in 1995 for use in lumbar
spine. The legal battle still continues. It would seem that it
is because of their unproven clinical value rather than their
mechanical defects that the FDA has acted. In the treatment of
spinal fractures and tumors, and perhaps in some types of
spondylolisthesis, the usefulness of pedicle screws seem
evident, but they are most widely utilized in the treatment of
degenerative low back pain, in which their value has not been
established, despite what most spinal surgeons seem to believe.
Perhaps this confrontation will be of benefit by encouraging
manufactures to support, and clinicians to organize prospective
studies of the value of newly developed devices before they are
widely marketed. If they do not, they may find themselves from
1995 onwards, confronting not only the FDA but an array of
licensing authorities in the rest of the world.
Anatomy
The anatomy of the human pedicle has been
studied exhaustively in different races in children and in
adults. Measurements of the outer and inner diameters of the
pedicle have been performed extensively. 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 gender. However, wide individual variations with in common
pattern of anatomy are the rule. This forces the surgeons to
understand the individual anatomy of the patient, in order to
achieve clinical success.
Indications
The use of pedicle screws has resulted in
dramatic clinical improvements in the surgical care of spinal
diseases; Stabilization of spinal fractures, scoliosis,
spondylolisthesis, spinabifida, pars-interarticularis defects,
neoplasms , lesions of cervical spine and cervico-thoracic
junction. Spino pelvic trauma, traumatic spino pelvic disruption
and vertical fractures of sacrum have been successfully repaired
with either iliac or iliosacral screws.
In the correction of severe scoliosis, the
use of pedicle screws, because of their firm whole in 3 columns
of the spine, allows accurate correction at each segment in all
the 3 planes. There is however, a greater risk of serious neuro
injury when they are used in the thoracic spine and greater
caution is necessary. The rigid hold of the pedicle screws
also simplifies the reduction of spondylolisthesis. The most
contentious issue, however, is the use of pedicle screws in the
treatment of degenerative lumbar disc disease.
In 1986 Robert et al showed the fundamental
importance of load transfer across the spinal column itself
along with the implant in the treatment of spinal fractures – a
concept known as load sharing, which was subsequently widely
adopted by the spinal surgery community. The load sharing
classification provides a simple way to differentiate spinal
fractures according to comminution. It suggest 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. 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 patients.
Three parameters(A, B, C) are considered. A:1:- <30% comminution
2: 30 to 60% comminution. 3: >60% comminution on computed
tomographic sagittal plane sections. B: Apposition of fragments
1: minimal displacement. 2: at leat 2 mm of displacement of less
than 50% of the cross section of the body 3: at least 2 mm of
displacement of more than 50% of the cross section of the body
C: deformity 1: kyphosis of 3 degree of less 2: 4 to 9 degrees
3: 10 degrees or more on lateral plain radiograph.
Methods of Pedicle
screw placement
Various methods have been described for
identifying the pedicle and placing the pedicle screws, but
basic steps include cleaning the soft issues, exposing the
cancellous bone of the pedicle canal by decortication at the
intersection of the base of the facet and the middle of the
transverse process, probing the pedicle, verifying the 4 walls
of the pedicle, tapping the pedicle and placing the screw (6).
In the lumbar spine, pedicle screws are
commonly inserted using anatomical land marks and conformatory
roentgenogrames are obtained. Because of the deformed pedicles
associated with scoliosis, many surgeons use fluoroscopic
guidance (7). Although Suk et al, used anatomical land marks,
and confirmed the position of the guide pins with plain
roentgenograms, most surgeons believe that because of the tight
confines of the pedicle in the thoracic spine, intra operative
fluoroscopic is indicated. Frameless stereo tactic technology is
available that allows 3 dimensional navigation, but requires
time consuming segmental registration and pre-operative CT data.
Investigations are currently underway using stereo tactic
technology combined with fluoroscopic. This technology has the
potential to greatly diminish radiation exposure to both the
surgeon and the patient and to allow spontaneous viewing of
instrument positions in as many projections as desired.
Great care is taken to advance the
instrument slowly and carefully. If resistance is encountered,
the probe is re-positioned. An intra operative roentgenogram or
C-arm image can be used to verify correct position. Instruments
should pass relatively easily and should not be forced in to the
pedicle. In addition to roentgenograms or image intensification,
laminotomy and medial pedicle wall exposure helps to confirm the
intra pedicular passage of the instrument. Once satisfactory
entry into the pedicle has been achieved and palpation from
within the pedicle demonstrates solid bone margins along the
pedicle wall throughout 360 degrees, the screw can be inserted.
If the screws are self tapping, the screw itself is inserted. If
the screws require tapping, the tap is inserted first and then
the screw.
In the original technique described by
Roy-Camille, the entrance point for screw insertion was situated
in the inter section between a vertical line passing through the
middle of the inferior facet and a transverse line passing
through the middle of the transverse process. The direction of
the screw was perpendicular to the posterior plane of the
vertebra and straight forward. No information on pedicle wall
violation was documented by Roy-Camille. Vaccaro et al were the
first to evaluate the feasibility of Roy-Camille technique. They
observe 41% penetration. A higher percentage of lateral (31.6%)
and inferior (11.6%) wall perforation was observed. Anatomically
the pedicle’s and anteromedial inclination in transverse plane
varies from cephalad to caudal (2). In general, this angle
decreases gradually from T1 to T12. In the lower thoracic
levels, screw placement with a straight forward direction may be
safe. However in the middle and upper thoracic levels, this
techniques will have a higher incidents of lateral wall
penetration (8) because of medial inclination of the pedicle.
The high incidence of inferior wall violation is due to too low
level of starting point. The incidence of pedicle wall violation
was significantly reduced when screws were placed using
open-lamina technique through a partial laminectomy. However,
this technique is not widely used.
Roy-Camille et al suggested that a pedicle
screw should be introduced by drilling the path and then
applying the screw. The danger of this approach is now realized
and a blunt technique to identify the pedicle along with a 2
plane image intensification during screw placement is adopted
now (7). The use of taps of gradually increasing diameter assess
the quality of cortical purchase through the isthmus of the
pedicle, and use of image intensification to assess the length
of the screw to obtain adequate purchase, result in strong
fixation. This technique named as the funnel technique is now
used widely.
Irrigation with Saline solution down the
pedicle and visualization of the pedicle by an endoscope, have
both been used to assess proper screw placement. Routine
monitoring of the somatosensory and dermatomal somatosensory
evoked potentials as well as electromyography, also have been
used for this purpose. Herthein et al described the application
of the pedicle screw from the anterior approach. The use of
robotics and computer assisted guidance technology also are
getting popular.
Complications
Routine use of pedicle screw has not been
free of complications (9,10). The rate of screw misplacement has
ranged from 10 – 25% and cortical violation upto 50% in some
reports. A learning curve has been demonstrated for surgeons in
general. A variable prevalence of nerve root and cauda equina
injury has been reported. Computer based navigation techniques
improve screw placement according to some authors. In one series
dural injury was reported in 7 of 124 patients (11). Deep
infections is reported to be 1.1%. The frequency of screw
breakage has ranged from 2.6 to 9%. Screw pull out and screw
connector disengagements has been reported both in vitro testing
and in patients.
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