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Figure 30–3
(A) S.M. is a 15-year, 9-month-old girl with a King II adolescent idiopathic scoliosis. The thoracic curve measures 53 degrees, and the lumbar curve 40 degrees. The
apex of the lumbar curve (L2-L3 disc) completely deviates from the midline (lumbar type C modifier). (B) The
thoracic sagittal plane shows thoracic kyphosis of +25
degrees and a lumbar lordosis of −59 degrees with a
fairly neutral thoracolumbar junction. (C) The right-side
bending radiograph demonstrates correction of the
thoracic curve to 33 degrees. (D) The left-side bending
radiograph demonstrates correction of the lumbar curve
to 15 degrees.
A
B
C
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150
SECTION II THE THORACIC SPINE
Eurostile
D
Figure 30–3 (continued)
(E) She underwent a posterior instrumentation and fusion
from T4-T12. T12 was the stable vertebra preoperatively.
T12 was kept tilted in the coronal plane to accommodate
the lumbar curve below. A standard instrumentation pattern as seen in the previous two cases was utilized with
mild apical translation performed without any rod rotation
maneuver. The postoperative coronal radiograph demonstrates equally matched thoracic and lumbar Cobb
measurements and a well-balanced spine. (F) The postoperative lateral radiograph demonstrates normalized
thoracic, thoracolumbar, and lumbar regional alignments
with overall good sagittal balance.
E F
Eurostile
30 POSTERIOR SCOLIOSIS CORRECTION OF KING II CURVES
151
■
Harvey CJ Jr, Betz RR, Clements DH, Huss G, Clancy M. Are there indica-
tions for partial rib resection in patients with adolescent idiopathic
scoliosis treated with Cotrel-Dubousset instrumentation? Spine
1993;18:1593–1598.
King HA, Moe JH, Bradford DS, Winter RB. The selection of fusion levels in
thoracic idiopathic scoliosis. J Bone Joint Surg Am 1983;65:1302–
1313.
Lenke LG, Betz RR, Harms J, et al. Adolescent idiopathic scoliosis: a new
classification to determine extent of spinal arthrodesis. J Bone Joint
Surg Am 2001;83:1169−1181.
Lenke LG, Bridwell KH, Baldus C, Blanke K. Preventing decompensation
in King type II curves treated with Cotrel-Dubousset instrumentation:
strict guidelines for selective thoracic fusion. Spine 1992;17:274–281.
Lenke LG, Bridwell KH, Baldus C, Blanke K, Schoenecker PL. Cotrel-
Dubousset instrumentation for adolescent idiopathic scoliosis. J Bone
Joint Surg Am 1992;74:1056–1067.
Lenke LG, Bridwell KH, Blanke K, Baldus C, Weston J. Radiographic re-
sults of arthrodesis with Cotrel-Dubousset instrumentation for the
treatment of adolescent idiopathic scoliosis: a five to ten-year followup study. J Bone Joint Surg Am 1998;80:807–814.
Richards BS. Lumbar curve response in type II idiopathic scoliosis after
posterior instrumentation of the thoracic curve. Spine 1992;17:S282−
S286.
Richards BS, Birch JG, Herring JA, Johnston CE, Roach JW. Frontal plane
and sagittal plane balance following Cotrel-Dubousset instrumentation for idiopathic scoliosis. Spine 1989;14:733–737.
Shufflebarger H. Theory and mechanisms of posterior derotation spinal
systems. In: Weinstein SL, ed. The Pediatric Spine: Principles and
Practice. Vol. 2. New York: Raven Press; 1994:1515.
Shufflebarger H, Crawford A. Is Cotrel-Dubousset instrumentation the
treatment of choice for idiopathic scoliosis in the adolescent who has
an operative thoracic curve? Orthopaedics 1988;11:1579–1588.
Thompson JP, Transfeldt EE, Bradford DS, Ogilvie JW, Boachie-Adjei O.
Decompensation after Cotrel-Dubousset instrumentation of idiopathic
scoliosis. Spine 1990;15:927–931.
Wood KB, Transfeldt EE, Ogilvie JW, Schendel MJ, Bradford DS. Rotational
changes of the vertebral-pelvic axis following Cotrel-Dubousset instrumentation. Spine 1991;16(suppl 8):S404−S408.
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31
Posterior Scoliosis Correction
Double Major Curves
Keith H. Bridwell
Goals of Surgical Treatment
To maintain balance, achieve acceptable correction, and minimize fusion
levels.
Diagnosis
There is somewhat of a fine line between a type I curve and a double major
curve. I interpret a truly type I curve as one in which the lumbar curve is
structural and the thoracic curve is relatively nonstructural. A typical presentation is a right thoracic curve in conjunction with the left lumbar
curve. If the lumbar curve is substantially bigger, has more rotation, and
creates more deformity, one might consider a selective posterior or anterior
fusion of just the lumbar curve. On the other hand, if the thoracic curve is
more structural, and in particular if the right shoulder is substantially
higher than the left (assume a right thoracic, left lumbar pattern), then one
is more likely to fix both curves posteriorly. In most cases, I selectively fix
the lumbar or thoracolumbar curve anteriorly if the patient’s shoulders are
relatively level and if the lumbar curve has substantially more apical deviation and rotation than the thoracic curve. On the other hand, if the lum-
bar and thoracic curves are relatively similar in terms of apical deviation
and rotation, then I would be more likely to fix both curves posteriorly.
Also, if the right shoulder is substantially higher than the left, then I am
somewhat more apt to fix both curves posteriorly. This is determined in
part by physical examination and in part by the standing coronal and
lateral radiographs. If there is a substantial junctional kyphosis between
the two curves, then I am more apt to fix both of them. So in this chapter I
discuss the situation in which I would fix both curves as opposed to just
the thoracolumbar/lumbar curve.
Indications for Surgery
1. A double major curve pattern in which both the thoracic and the lum-
bar curves are substantial as demonstrated by their Cobb measurement,
rotation, and apical deviation.
2 Either the shoulders are level or the right shoulder is substantially
higher than the left, if it is a right thoracic/left lumbar curve.
3. The tidemark for fixing double curves is somewhat higher than the
tidemark for single curves.
Contraindications
The relative contraindications are a presentation in which either the
thoracic curve is substantially bigger than the lumbar curve or the lumbar
curve is substantially bigger than the thoracic curve. In those situations,
one is more apt to selectively fix one curve and not the other.
Advantages
The advantage of the posterior approach is a balanced correction of both
curves and therein less likelihood of throwing the patient off-balance in
the coronal plane by significantly correcting one curve more than the other.
Disadvantages
1. Long fusion.
2. Usually the lumbar curve is more flexible than the thoracic. Therefore,
even if one is fixing both curves posteriorly, there may be a tendency to
achieve more correction of the lumbar curve than the thoracic curve.
3. The disadvantages of a posterior approach are more trauma to the spi-
nal extensor muscles. The advantage of a posterior approach over an
anterior approach is less impact on pulmonary function and less pain
immediately postoperative. Pain with a posterior procedure is related
to damage to the posterior spinal extensor muscles. With an anterior
procedure, it is related to severance of the latissimus dorsi muscle, the
serratus anterior, the diaphragm, and the intercostal muscles as well as
the rib cage itself.
Procedure
Fusion levels: For the thoracic part of the deformity, the proximal fusion
level is usually a neutral level that is one or two vertebrae proximal to the
transitional level. This depends on sagittal balance. For the distal level, it
should always be to at least the neutral vertebra distally,which will also be
the transitional level (Fig. 31–1). At times, it is the stable vertebra distally.
One should attempt to horizontalize the last instrumented vertebra.
Whether or not one can stop at the neutral vertebra rather than the stable
vertebra is determined by the flexibility of the fractional curve below and
also the flexibility of the lumbar/thoracolumbar curve. It also depends
somewhat on whether hooks or pedicle screws are used. If pedicle screws
are used, in most cases it is possible to stop at the neutral rather than the
stable vertebra.
Incision Options
1. The incision is made in the midline. It is centered over the segments to
be fused and instrumented. The bone graft taken is usually cortical and
cancellous posterior iliac bone.
2. In most double major curves, it is less of a total skin incision to tunnel
over to the ilium rather than to make a separate incision over the ilium
as one might do if one were just fixing the thoracic curve.
Exposure Secrets
1. It is helpful to expose all the way out to the tips of the transverse
processes. This increases the surface area for fusion. It also facilitates
inserting in pedicle screws.
2. In placing pedicle screws, it is important that surgeons have direct
access to the angle of the pedicle, and that they don’t box themselves in
with the exposure. They should be sure the self-retaining retractors do
not keep them from angling proximally, distally, or medially enough.
The Implants and Correction
1. The size of rod depends on the philosophy of the surgeon. Many surgeons prefer a harder, stiffer rod. My preference is to use a fairly flexible rod that is 5 or 5.5 mm in diameter. My present preference for most
cases is the CD Horizon rod, which is 5.5 mm in diameter. It has an element of both elasticity and memory to it. It allows some in situ contouring and also protects the hooks and pedicle screws from pullout by
being somewhat flexible.
2. My preference is to use principally hooks in the thoracic spine, with
Wisconsin wire supplementation and then mostly screws in the lumbar spine. This depends somewhat on the patient’s anatomy. If the
patient has substantial-sized pedicles that are easy to hit, then I use
pedicle screws. If the patient has small, dysplastic pedicles, then I
settle on hooks (Fig. 31–2).
3. It is important to apply compression forces, rod rotation, or cantilever
forces to the convexity of the lumbar segments before any distraction is
applied to the concavity.
4. Correction of the thoracic curve is by translation performed by tightening the Wisconsin wires and in situ contouring of the left-sided rod
(concavity). No distraction is applied. The rod is engaged in the upper
hooks, but not tightened down, as the rod must slide through the hooks
during correction.
5. Correction of the lumbar curve is through applying compression forces
and in situ contouring of the convex rod (Fig. 31–3).
Pitfalls
1. One should be certain there is not a triple major curve, in other words,
a presentation with a high left thoracic curve as well.
2. In dealing with the lumbar curve, it is quite important that nothing reduces segmental lordosis.
3. Also, one does not want to put excessive compression force or excessive cantilever force on the fixation points. An excessive cantilever
force on the pedicle screws may predispose to pullout.
Complications of Instrumentation
1. It is important to be sure that the hooks at the thoracolumbar junction
are not imploded into the canal and they should have relatively small
blades.
2. It is important for the pedicle screws to be well seated.
3. A rod rotation maneuver using entirely hooks has a tendency to pull
off the distal hook.
4. One should also be careful not to cantilever the rod too much into the
proximal fixation points as this will tend to encourage the hooks to
pull off the lamina or the transverse processes.
Eurostile
31 POSTERIOR SCOLIOSIS CORRECTION OF DOUBLE MAJOR CURVES
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Figure 31–1
Preoperative standing coronal (A) and lateral (B)
radiographs of the spine.
isconsin
ires
A
R10
R11
R12
L1
L2
T12
T9
T11
T1
T7
T8
T10
B
Figure 31–2
Diagram of the placement of hooks and screws on the preoperative coronal radiograph.
T2
T3
T4
T5
T6
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154
L3
L4
L5
SECTION II THE THORACIC SPINE
Eurostile
A
Figure 31–3
Postoperative standing coronal (A) and lateral (B) radiographs of the
same patient.
B
Postoperative Care
1. This is surgeon-dependent, but my preference is to leave in the
hemovacs for the first 3 days postoperation. This decompresses the fascia and skin repair and allows healing of those layers. It may encourage
more blood loss, but I have not been impressed with that.
2. The patient can stand at the bedside and walk the day after surgery.
3. In most cases, it is not necessary to place the patient in a postoperative
brace.
Suggested Readings
Bridwell KH. Idiopathic scoliosis. In: Bridwell KH, DeWald RL, eds. The
Textbook of Spinal Surgery. 1st ed. Philadelphia: JB Lippincott;
1991:97–162.
Bridwell KH. Spinal instrumentation in the management of adolescent
scoliosis. Clin Orthop 1997;335:64–72.
Bridwell KH. Adolescent idiopathic scoliosis: surgical treatment. In: Wein-
stein SL, ed. The Pediatric Spine: Principles and Practice. 1st ed. New
York: Raven Press; 1994:511–555.
Bridwell KH. Adolescent idiopathic scoliosis: surgery. In: Weinstein SL,
ed. The Pediatric Spine: Principles and Practice. 2nd ed. Philadelphia:
Lippincott Williams & Wilkins; 2001:385−411.
Eurostile
31 POSTERIOR SCOLIOSIS CORRECTION OF DOUBLE MAJOR CURVES
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32
Posterior Scoliosis Correction
King Type V/Double Thoracic Curves
Keith H. Bridwell
Goals of Surgical Treatment
To balance the spine and level the shoulders.
Diagnosis
A double thoracic curve pattern is one in which there are two structural
thoracic curves. Most commonly, it is a high left thoracic, low right
thoracic curve. Usually the lower right thoracic curve is the bigger of the
two curves, but the upper curve determines the shoulder balance (Fig. 32–
1).
Indications for Surgery
1. The indications are the same as for other forms of idiopathic adolescent scoliosis. If the curves are substantial enough, then surgery is considered.
2. Because both curves are relatively structural, it is best to fix both of the
curves.
3. Fixing both the curves is indicated in the following situation:
a. Left shoulder higher than right.
b. Upper thoracic curve exceeding 40 degrees.
c. The upper thoracic curve not bending out beyond 25 degrees.
d. Substantial high, left thoracic hump on forward bending.
Contraindications
The only absolute contraindication is a skin problem that prevents poste-
rior fusion and instrumentation.
Advantages
The advantage of the posterior approach is that both curves are corrected,
and this creates a more balanced spine if, in fact, there is a double thoracic
pattern.
Disadvantages
The only disadvantage is that more of the upper thoracic segments are
fused. At present, there are no compelling data to suggest that this increases the likelihood of breakdown in either the cervical spine or the lum-
bar spine. Most commonly, fusion levels for a double thoracic scoliosis are
T2 to L1 or L2.
Procedure
Fusion levels: The most common levels are T2 to L1 or L2. This depends on
the segments that are within the curve.
Incision Options
A midline skin incision is made, and subperiosteal dissection is performed
out to the tips of the transverse processes from the segments being fused,
for example, T2 to L1. Next the facet joints are excised at each level and
fixation points are placed. Most commonly, hooks are used. The hooks are
placed in a compression mode on the left side of the upper curve and a distraction mode for the lower thoracic curve. The reverse is done on the other
side. Presently my preference is to use Wisconsin rods and in situ contouring of the rods to correct the lower right thoracic curve. To correct the
upper thoracic curve, my preference is to use compression forces.
My preference is to correct the upper thoracic curve from the convexity
rather than the concavity. This is because there tends to be somewhat of a
junctional kyphosis on the top, and therein correction by compression on
the left side seems to work better for the sagittal contour. It is also easier to
accomplish correction with one rod on one side first and then apply the
second rod. If the sagittal plane were otherwise, it would make sense to
apply potentially a distraction force or a translational force on the concave
right side of the upper curve.
The construct usually extends from T2 to L1 or L2. Cross-links at the
top and the bottom are advisable. I personally use autogenous iliac bone
grafting. The lower thoracic curve is corrected as described in the section
for double major curves.
Exposure Secrets
It is helpful to drape the patient’s shoulder and neck up very high so there
is ample working room. Otherwise, the exposure is straightforward.
Insertion of the Implants
My preference is to use 5.5-mm rods that are relatively flexible and also
have some memory to them to facilitate in situ contouring (Fig. 32–2).
Pitfalls
The principal pitfall is failing to recognize a double thoracic curve versus a
single curve. Tip-offs include the patient’s preoperative shoulder height,
apical translation of the upper curve, rotation of the upper curve, and
junctional kyphosis either radiographically or clinically.
Complications of Instrumentation
The posterior elements of T2 are somewhat smaller than lower in the
spine. Therefore, there is more risk of hook pullout at T2. On the left side, I
usually place a sublaminar hook. This can usually be placed with limited
dissection of the ligamentum flavum without disruption of the facet capsule or take down of the interspinous ligaments above. On the other side,
depending on the patient’s anatomy, one option is to place a hook on the
transverse process of T2 and a pedicle hook at T3 or to simply place a
pedicle hook at T2. Which of these constructs is preferable depends on
where the patient has the best bone stock.
Usually the lower thoracic curve is more flexible than the upper
thoracic curve, so there is the potential for making shoulder balance worse
even if both curves are instrumented and fused. This is because the tendency is to get more correction of the lower thoracic than upper thoracic
curve. As is the case for double major curves, the surgeon has to be careful
to correct the two curves equally and to obtain long cassette coronal radiographs before the patient leaves the operating room. If this is not feasible,
then at least be sure to study the patient standing and the long cassette
coronal standing radiograph before the patient is discharged from the
hospital.
Postoperative Care
For most idiopathic adolescents, it is possible to do the surgery without
any postoperative bracing or casting. It is usually possible to stand and ambulate the patient the day after the surgery.Whether the patient spends 3 or
5 days in the hospital varies (Fig. 32–3).
Suggested Readings
Bridwell KH. Idiopathic scoliosis. In: Bridwell KH, DeWald RL, eds. The
Textbook of Spinal Surgery. 1st ed. Philadelphia: JB Lippincott;
1991:97–162.
Bridwell KH. Spinal instrumentation in the management of adolescent
scoliosis. Clin Orthop 1997;335:64–72.
Bridwell KH. Adolescent idiopathic scoliosis: surgical treatment. In: Wein-
stein SL, ed. The Pediatric Spine: Principles and Practice. 1st ed. New
York: Raven Press; 1994:511–555.
Bridwell KH. Adolescent idiopathic scoliosis: surgery. In: Weinstein SL,
ed. The Pediatric Spine: Principles and Practice. 2nd ed. Philadelphia:
Lippincott Williams & Wilkins; 2001:385−411.
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SECTION II THE THORACIC SPINE
Eurostile
A, B
Figure 32–1
Preoperative standing coronal (A) and lateral (B) radiographs of the
spine. (C) Right side-bending radiograph. (D) Left side-bending radio-
graph.
C
Eurostile
32 KING TYPE V/DOUBLE THORACIC CURVES
D
157
■
C7
T2
T3
T4
T1
T5
T6
T7
T8
T9
Wisconsin
wires
T10
T11
T12
L1
L2
L3
L4
L5
L5
Figure 32–2
Diagram of hook strategy.
A, B
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158
Figure 32–3
Postoperative long cassette coronal (A) and lateral (B) radiographs
showing satisfactory correction.
SECTION II THE THORACIC SPINE
Eurostile
33
Posterior Scoliosis Correction
Pedicle Screws
Se-Il Suk and Won-Joong Kim
Goals of Surgical Treatment
1. Halt the progression of the deformity.
2. Correct existing deformities.
3. Restore three-dimensional balance of the trunk and the spinal column,
with minimum sacrifice of motion segments.
Diagnosis
Scoliosis is defined as a three-dimensional deformity of the vertebral
column characterized by lateral deviation in the frontal plane with de-
rangement in the sagittal plane (hypokyphosis, hyperkyphosis, hyperlordosis, or hypolordosis) and torsion in the horizontal plane. Depending on
the etiology of the deformity, it is classified as idiopathic, congenital, neu-
romuscular, or neurofibromatosis, or of miscellaneous etiologies. Specific
diagnosis is made by appropriate physical examination and radiologic studies including magnetic resonance imaging (MRI) of the central nervous
system.
Indications for Surgery
1. Deformities with potential of adulthood progression
2. Idiopathic scoliosis
a Thoracic curve 쏜 40 degrees
b Lumbar or thoracolumbar curve 쏜 35 degrees
3. Congenital scoliosis
a Documented progression of more than 5 degrees on serial follow-up
b Deformities with known natural history of inevitable progression
(e.g., unilateral unsegmented bar with contralateral hemivertebra,
multiple unincarcerated unilateral hemiverterbae)
4. Progressive deformities from neuromuscular disorders, neurofibroma-
tosis, and other etiologies
5. Significant derangement of spinal and trunk balance in coronal and
sagittal planes
6. Significant thoracic hypokyphosis associated with coronal plane de-
formity
7. Poor trunk cosmesis
Contraindications
There is no absolute contraindication. Relative contraindications may include severe pedicle hypoplasia (e.g., neurofibromatosis); previous
lengthy laminectomies; and anticipated future posterior surgeries (e.g.,
partially resected spinal cord tumors).
Advantages of Posterior Pedicle Screw Fixation for Scoliosis
1. Improved three-dimensional deformity correction and maintenance
2. Improvement of thoracic hypokyphosis
3. Shorter fusion, preserving additional motion segments
4. Easier surgery design (do not need complex instrumentation patterns)
Disadvantages
1. Potential neurovascular complications related to screw misplacement
2. Overcorrection of the instrumented curve
Procedure
Determination of fusion levels: Fusion levels are determined using 14” ×
35” standing anteroposterior (AP) and lateral radiographs and appropriate
side bending studies (Fig. 33–1). Single thoracic curves that do not extend
into the lower lumbar spine (King types II, III, and IV): Selective thoracic
fusion from one level above the upper end vertebra to the distal neutral
vertebra on the standing radiograph.
Double thoracic curve (King type V): Fuse both the upper and the lower
thoracic curve. When the lower curve do not extend into the lower lumbar
spine, fuse to the distal neutral vertebra of the lower curve as in the single
thoracic curve.
Structural curves involving the thoracolumbar or lumbar region (King
type I, thoracolumbar, and lumbar curves): All structural curves are fused.
Distal fusion to the bending stable vertebra that centralizes on sacrum and
bisected by the center sacral line. If the bending stable vertebra does not
derotate to less than Nash-Moe grade II on side bending, go down one level
caudally. In essence, the distal fusion level for the pedicle screw fixation is
identical to that of an anterior instrumentation.
Intraoperative neurologic monitoring: Neurologic complications related to pedicle screw instrumentation for scoliosis is extremely rare.
However, intraoperative neurologic monitoring [e.g., somatosensory
evoked potential (SSEP), multimodality evoked potential (MEP)] is a valuable assistance in making sure that everything is going fine.
Incision and exposure: A standard posterior midline incision is made
from the upper end of the spinous process two levels above the uppermost
pedicle instrumented to the lower end of the lamina of the lowest instrumented vertebra. The proximal incision should be long enough to allow
convergence of the pedicle screws in the uppermost vertebra. The spine is
exposed in the standard fashion, staying strictly subperiosteal to reduce
bleeding. In the thoracic and the lumbar spine, the vertebra instrumented
is exposed to the tip of the transverse processes bilaterally. In the course of
the exposure, care should be taken not to disturb the facets adjacent to the
uppermost pedicles instrumented, as damage to the facets may result in instability and precocious degenerative change.
Facetectomy: The facets included in the fusion are destroyed by inferior
facetectomy and removal of the articular cartilages to promote intraarticular arthrodesis.
Determination of pedicle entry sites: Presumed pedicle entry points
are decorticated with a rongeur to facilitate the insertion of the guide
pins. In the thoracic spine, the presumed pedicle entry point is at the
junction of the superior margin of the transverse process and the lamina.
In the lumbar spine, the point is at the junction of the line drawn through
the middle of the transverse process and the lateral margin of the facet
joint (Fig. 33–2). Screws are to be placed segmentally on the correction
sides of the curve (concave in the thoracic and convex in the lumbar) and
every other or third on the support sides. Then guide pins are inserted
shallowly through the exposed cancellous bone at the presumed pedicle
entry point. To facilitate radiograph interpretation, the guide pins are
directed along the axis of the pedicle in the frontal and the sagittal plane.
With the guide pins placed at planned pedicle screw sites, intraoperative
posteroanterior (PA) and lateral roentgenograms are taken to determine
the relationship between the presumed entry point and the ideal entry
point identifiable on the x-ray and to determine the direction of the
screws (Fig. 33–3). Taking the transverse angle of the pedicles into consideration, the ideal pedicle entry point (IPEP) in a neutrally rotated
vertebra is at the junction of the line parallel to the vertebral end plates
bisecting the pedicle and the lateral margin of the pedicle ring shadow on
a PA film. In rotated vertebrae, IPEP of the pedicles on the side of the rotation (convex side of scoliosis) moves more medially, whereas IPEP on
the opposite side (concave side) moves more laterally with increments of
vertebral rotation. On the lateral view, the IPEP is situated at the junction
of the line passing through the axis of the pedicle and the posterior
border of the facet joints (Fig. 33–4).
Pedicle entry: After determining the position of the ideal pedicle entry
points and the direction of the ideal pedicle paths relative to the guide pin,
the pedicle is entered through the point with a small diameter drill or a
small curet. It is very important to keep in mind the normal transverse
angle of the pedicles for the particular level to prevent inadvertent pedicle
perforation. Then the hole is checked with a blunt ended probe. A safe
entry into the pedicle is confirmed when the probe meets bony resistance
in all directions, meaning that the hole is globally surrounded by bone.
Hole preparation: Deep drilling is performed following the probe path
using a drill bit with a diameter same as the minor diameter of the screw
used. The pedicle screw offers best holding strength when the pilot hole is
about 60 % of the outer pedicle diameter.
Screw insertion: The pedicle screw is inserted after reconfirming the
bony containment of the pilot hole. When starting to insert the screw, turn
the screw with very gentle force so that the screw follows the predrilled
path. Undue force at the beginning may misdirect the screws into wrong
direction. The ideal screw diameter is about 80% of the pedicle diameter.
In pediatric patients, however, oversized screws up to 115% of the pedicle
diameter may be inserted without causing a fracture due to plasticity of the
pedicular cortex. The ideal screw length is about 80% of the length from
the posterior aspect of the facet joint to the anterior margin of the vertebral
body on a lateral radiograph to avoid complications of screw overpenetra-
Eurostile
33 POSTERIOR SCOLIOSIS CORRECTION: PEDICLE SCREWS
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