ANATOMIC TECHNIQUES
ACCURACY OF PEDICLE SCREW PLACEMENT FOR LUMBAR
FUSION USING ANATOMIC LANDMARKS VERSUS OPEN
LAMINECTOMY: A COMPARISON OF TWO SURGICAL
TECHNIQUES IN CADAVERIC SPECIMENS
Aftab Karim, M.D.
Department of Neurosurgery,
Louisiana State University
Health Sciences Center,
Shreveport, Louisiana
Debi Mukherjee, Sc.D.
Departments of Neurosurgery
and Orthopedics,
Louisiana State University
Health Sciences Center,
Shreveport, Louisiana
Jorge Gonzalez-Cruz, M.D.
Department of Neurosurgery,
Louisiana State University
Health Sciences Center,
Shreveport, Louisiana
Alan Ogden, B.S.
Departments of Neurosurgery
and Orthopedics,
Louisiana State University
Health Sciences Center,
Shreveport, Louisiana
Donald Smith, M.D.
Department of Neurosurgery,
Louisiana State University
Health Sciences Center,
Shreveport, Louisiana
Anil Nanda, M.D.
Department of Neurosurgery,
Louisiana State University
Health Sciences Center,
Shreveport, Louisiana
Reprint requests:
Anil Nanda, M.D.,
Department of Neurosurgery,
Louisiana State University
Health Sciences Center,
1501 Kings Highway,
P.O. Box 33932,
Shreveport, LA 71130-3932.
Email: ananda@lsuhsc.edu
Received, November 18, 2005.
Accepted, February 6, 2006.
NEUROSURGERY
OBJECTIVE: We determined whether the accuracy of lumbar pedicle screw placement is
optimized by performing a laminectomy before screw placement with screw entry point
and trajectory being guided by pedicle visualization and palpation (Technique 1). This
technique was compared with a technique using anatomic landmarks for pedicle screw
placement (Technique 2). The biomechanical stability of the instrumented constructs, in
the absence and presence of a laminectomy, was also compared.
METHODS: Twelve L1–L3 specimens were harvested from fresh cadavers. The intact
laminectomy and instrumented spines were biomechanically tested in flexion and
extension, lateral bending, and axial rotation. Laminectomies were performed in six of
the 12 specimens before pedicle screw placement using Technique 1. The remaining
six specimens underwent pedicle screw and rod fixation using Technique 2. Computed tomographic images were obtained for all instrumented specimens. Deviation of
the screws from the ideal entry point or trajectory was analyzed to quantitatively
compare the two techniques.
RESULTS: Computed tomographic analysis of the specimens showed that all screw
placements were within the pedicles. Scatter plot analysis demonstrated that screws
placed using Technique 2 were more likely to have the combination of entry points
and trajectories medial to the ideal entry point and trajectory. Laminectomy did not
weaken the final pedicle screw and rod-fixated constructs.
CONCLUSION: All screw placements were grossly within the confines of the pedicles,
regardless of technique, as evidenced by computed tomographic analysis. Furthermore, the anatomic landmark technique and the open laminectomy technique yielded
biomechanically equivalent pedicle screw and rod-fixated constructs.
KEY WORDS: Biomechanical stability, Lumbar laminectomy, Pedicle screw accuracy
Neurosurgery 59[ONS Suppl 1]:ONS-13–ONS-19, 2006
L
umbar fusion is frequently performed
for segmental instability and degenerative spondylolisthesis (18). Various techniques have evolved for the placement of
pedicle screws for lumbar fusion (2–5, 9, 13,
15, 19). Most surgeons use anatomic landmarks, often in conjunction with fluoroscopy,
to guide pedicle screw placement in the lumbar spine (1, 16, 17). Despite modern techniques, the incidence of pedicle screw misplacement in the lumbar spine remains
significant (6, 7, 11). Neuronavigation has
been shown to improve accuracy of screw
DOI: 10.1227/01.NEU.0000219942.12160.5C
placement (2, 10, 12). However, it adds to the
time and resources needed for surgery. In the
thoracic spine, however, an open lamina technique is often used to improve the accuracy of
pedicle screw placement for thoracic fusion (E.
Benzel, personal communication, 2005) (20). In
this study, we determined whether the accuracy of lumbar pedicle screws could be similarly optimized by performing a laminectomy
before screw placement, with the screw entry
point and trajectory being guided by pedicle
visualization and palpation. The biomechanical stability of the instrumented constructs, in
VOLUME 59 | OPERATIVE NEUROSURGERY 1 | JULY 2006 | ONS-13
KARIM
ET AL.
the absence and presence of a laminectomy, was then compared to determine if performing the laminectomy introduced
a biomechanical disadvantage in the final pedicle screw and
rod-fixated construct.
MATERIALS AND METHODS
Twelve L1–L2 fresh-frozen human cadaver specimens (age
range, 60–93 yr) were randomly selected for laminectomy
(Technique 1) and nonlaminectomy (Technique 2) groups for
pedicle screw and rod fixation. The specimens were cleaned of
all soft tissue and were inspected for any visual abnormalities
and pathological features. No abnormalities were noted aside
from age-dependent osteoporosis. The average age of donors
for the six specimens undergoing Technique 1 was 80 years,
and the average age of the donors for the six specimens
undergoing Technique 2 was 72 years. There were two specimens from female donors in the Technique 1 group and three
specimens from female donors in the Technique 2 group. The
gross quality of specimens in the two groups was similar.
Because of custom testing fixtures designed specially for lumbar spine testing, no potting was used to assist in mounting
the specimens to the testing frame. Mechanical testing was
conducted on each group. The laminectomy group was tested
mechanically intact, after laminectomy, and after pedicle
screw and rod instrumentation, whereas the nonlaminectomy
group was tested intact and after pedicle screw and rod instrumentation.
In Technique 1, a wide laminectomy was performed (pedicle to pedicle) at L1. Approximately one-third of the facet was
resected to facilitate pedicle palpation. The L1 and L2 pedicles
could be palpated bilaterally after the laminectomy. The cadaveric specimen then underwent pedicle screw placement at
L1–L2 by one of the senior authors (AN) who routinely uses
this technique for screw placement. The pedicles were palpated with a three Penfield retractor and with the assistance of
a gearshift probe and lateral x-rays (fluoroscopy); screws were
placed at L1–L2 bilaterally. The remaining six specimens underwent screw placement using Technique 2 by one of the
senior authors (DS) who routinely uses this technique for
screw placement. In Technique 2, anatomic landmarks were
used to select the entry point and trajectory. The screw entry
point was identified by locating the intersection of the spine of
the transverse process with the corresponding facet. A highpowered drill was used to identify the cancellous bone of the
pedicle. The diameter of the pedicle screws was 5.5 mm. Screw
lengths per specimen were 35, 40, and 45 mm. Gearshift probe
and lateral x-rays (fluoroscopy) were used to determine screw
trajectory. After the placement of pedicle screws at L1–L2 in all
12 specimens (by either Technique 1 or Technique 2), appropriate length titanium rods were placed superiorly-toinferiorly at L1–L2 and were fixated using standard cap
screws (Medtronic Sofamor Danek, Inc., Memphis, TN). The
laminectomy group was tested mechanically intact, after laminectomy, and after pedicle screw and rod instrumentation,
ONS-14 | VOLUME 59 | OPERATIVE NEUROSURGERY 1 | JULY 2006
FIGURE 1. Photograph of the Instron 8874 biaxial testing frame with a
mounted lumbar specimen. This machine is used for biomechanical testing
of the specimens.
whereas the nonlaminectomy group was tested intact and
after pedicle screw and rod instrumentation.
All mechanical testing was conducted on an Instron 8874
biaxial testing frame (Instron, Corp., Canton, MA; Fig. 1).
Images were acquired using MAX32 software (Instron, Corp.,
Canton, MA) and were analyzed in Microsoft Excel (Microsoft
Corp., Redmond, WA) as previously described (8, 14). Each
group was tested in axial rotation, flexion and extension, and
lateral bending using custom testing fixtures designed for
FIGURE 2. Illustrations of pedicle screw placement in the axial
plane were traced from axial CT
images. The solid line describes
the ideal screw trajectory in the
respective pedicle, and the dotted
line represents the off-target screw
trajectory. The proximal and distal
pedicle lines were identified on the
axial CT image. A, the ideal screw
trajectory was determined by
drawing a line through the midpoints of the proximal and distal pedicle line. Screw trajectories that deviated medially from ideal were designated negative angles, and those that
deviated laterally from ideal were designated as positive angles. The ideal
insertion point was identified as the midpoint of the proximal pedicle line.
B, the insertion points medial from ideal were designated as a negative
deviation distance. C, the insertion points lateral from ideal were designated as positive deviation distances.
www.neurosurgery-online.com
ACCURACY
TABLE 1. Deviation from ideal pedicle screw placement using
Technique 1a
OF
PEDICLE SCREW PLACEMENT
LUMBAR FUSION
TABLE 2. Deviation from ideal pedicle screw placement using
Technique 2a
Region
Specimen
no.
Insertion
(mm)
Angle degree
(radians)
Pedicle
width (mm)
Region
1-L1-right
2-L1-right
3-L1-right
4-L1-right
5-L1-right
6-L1-right
1-L1-left
2-L1-left
3-L1-left
4-L1-left
5-L1-left
6-L1-left
1-L2-right
2-L2-right
3-L2-right
4-L2-right
5-L2-right
6-L2-right
1-L2-left
2-L2-left
3-L2-left
4-L2-left
5-L2-left
6-L2-left
2641
2675
2637
2665
2657
2617
2641
2675
2637
2665
2657
2617
2641
2675
2637
2665
2657
2617
2641
2675
2637
2665
2657
2617
0.33
⫺0.23
0.33
⫺0.62
⫺1.39
1.39
⫺0.44
⫺1.35
⫺0.22
0
0.33
0.45
⫺0.95
⫺1.14
0.7
0.09
⫺0.07
⫺1.5
⫺0.69
0.41
⫺0.14
⫺0.96
⫺4.39
⫺0.84
9.39 (0.16)
7.96 (0.14)
7.76 (0.14)
4.21 (0.07)
5.72 (0.10)
⫺6.17 (⫺0.11)
4.59 (0.08)
0.79 (0.01)
2.63 (0.05)
8.79 (0.15)
4.98 (0.09)
⫺0.97 (⫺0.02)
5.16 (0.09)
7.44 (0.13)
2.82 (0.05)
12.89 (0.22)
12.29 (0.21)
8.64 (0.15)
6.18 (0.11)
⫺6.62 (⫺0.12)
4.78 (0.08)
15.56 (0.27)
31.53 (0.55)
10.21 (0.18)
7.98
10.57
7.72
13.32
9.63
8.67
8.89
10.87
7.21
9.51
11.46
9.52
10.02
10.20
8.67
9.60
11.38
9.65
9.46
9.77
8.41
10.07
12.86
9.90
1-L1-right
2-L1-right
3-L1-right
4-L1-right
5-L1-right
6-L1-right
1-L1-left
2-L1-left
3-L1-left
4-L1-left
5-L1-left
6-L1-left
1-L2-right
2-L2-right
3-L2-right
4-L2-right
5-L2-right
6-L2-right
1-L2-left
2-L2-left
3-L2-left
4-L2-left
5-L2-left
6-L2-left
a
FOR
Specimen Insertion
no.
(mm)
2256
2581
2648
2652
2660
2674
2256
2581
2648
2652
2660
2674
2256
2581
2648
2652
2660
2674
2256
2581
2648
2652
2660
2674
⫺1.5
⫺1.09
1.17
0.31
0.66
⫺0.19
⫺4.37
⫺2.24
0.26
⫺0.59
0.52
⫺0.26
⫺2.68
⫺0.71
⫺2.58
⫺0.22
0.04
⫺1.2
⫺3.58
⫺0.29
⫺2.68
0.98
0.71
⫺4.75
Angle degree
(radians)
Pedicle
width (mm)
6.91 (0.12)
⫺9.2 (⫺0.16)
1.59 (0.03)
⫺4.8 (⫺0.08)
⫺1.42 (⫺0.02)
⫺1.84 (⫺0.03)
⫺3.85 (⫺0.07)
8.03 (0.14)
1.83 (0.03)
0.04 (0.00)
⫺2.45 (⫺0.04)
⫺1.01 (⫺0.02)
15.57 (0.27)
⫺2.52 (⫺0.04)
⫺2.69 (⫺0.05)
7.33 (0.13)
⫺6.02 (⫺0.11)
⫺11.78 (⫺0.21)
⫺9.75 (⫺0.17)
⫺6.35 (⫺0.11)
⫺9.03 (⫺0.16)
0.36 (0.01)
⫺2.3 (⫺0.04)
⫺1.11 (⫺0.02)
11.41
11.57
7.95
8.94
8.18
8.06
15.27
10.87
9.18
8.14
10.14
8.60
11.84
10.14
12.73
8.42
9.11
9.38
13.97
10.65
11.71
10.27
11.24
10.46
Screw trajectories that deviated medially from ideal were designated as
negative angles, and those that deviated laterally from ideal were designated as positive angles. The insertion points medial from ideal were
designated as negative distances, and insertion points lateral from ideal
were designated as positive distances.
a
Screw trajectories that deviated medially from ideal were designated as
negative angles, and those that deviated laterally from ideal were designated as positive angles. The insertion points medial from ideal were
designated as negative distances, and insertion points lateral from ideal
were designated as positive distances.
lumbar spines. For axial rotation, a 25 Newton axial preload
was applied to the segment and the specimen was rotated
⫾1.5 degrees (0.0216 radians) for five cycles at 0.25 Hz. The
torque and degree rotation data were graphed in Microsoft
Excel and a linear regression was plotted through the data to
generate a stiffness value. Flexion and extension were tested
by orienting the specimen 90 degrees from the axial testing
position and translating the specimen ⫾1.5 mm for five cycles
at 0.25 Hz in the anterior and posterior plane. Lateral bending
was tested by rotating the segments 90 degrees about the
spine’s vertical axis and translating the segments ⫾1.5 mm for
five cycles at 0.25 Hz laterally. The data for flexion and extension and for lateral bending were acquired and processed in
the same manner as those for axial rotation. Calculations of the
rotational stiffness (N–m/radian) and linear stiffness in both
the flexion and extension (N/mm) and lateral bending modes
(N–m/mm) were performed. This stiffness value is the slope
of the best linear fit. In rotational stiffness, torsion (N-meter) is
plotted on the y axis and the degree of rotation is plotted on
the x axis. The slope of the resulting best linear fit gives the
stiffness in N-m/radian. Similarly, in flexion and extension
and lateral bending modes, the linear load (Newton) is plotted
on the y axis and the displacement is plotted on the x axis. The
slope relates the stiffness as N/mm. After real-time collection
of the raw data via IEEE-488 using the MAX32 software, Excel
files were created for each testing mode. The stiffness values
were then calculated from the slope of the best fit line through
the data and were expressed in proper units such as rotational
stiffness in N-m/radian, flexion and extension in N/mm, and
lateral bending in N-m/mm. The stiffness values were charted
and normalized by calculating the fixed to respective intact
stiffness value. Statistical analysis of the data was performed
by paired t test using the SigmaStat software (Jandel Scientific
Software, San Rafael, CA) with a PC, and significance was
calculated at P ⫽ 0.05.
After mechanical testing of intact specimens and laminectomized specimens, pedicle screws were placed at L1–L2 using either Technique 1 or 2 in the 12 specimens. After pedicle
NEUROSURGERY
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KARIM
ET AL.
FIGURE 3. Representative axial CT slices at L1 for a specimen undergoing Technique 1 (A–C) and a specimen undergoing Technique 2 (D–F).
CT scans demonstrate that the screw trajectory with Technique 2 was
more medial than that of Technique 1. However, both techniques yielded
screw placement within the confines of the pedicle.
FIGURE 4. Scatter plot analysis of data from Tables 1 and 2. The x axis
represents deviation from ideal entry point in millimeters, and the y axis
represents deviation in angle from ideal trajectory in radians. The
negative-negative quadrant represents a screw placed medial to the ideal
entry point along with a trajectory medial to ideal trajectory.
screw placement, the specimens were taken to our institution’s
radiology department for computed tomographic (CT) examination before biomechanical testing of the instrumented specimens. Axial CT scans were obtained in 1-mm slices over the
length of L1–L2. The resulting CT image data were archived
on a compact disc, translated to standard image format, and
analyzed using a three-dimensional graphics software package. The 1-mm CT images were archived on compact disc and
the images were converted to TIFF images using OSIRIS imaging software (Digital Imaging Unit, University Hospital of
Geneva, Geneva, Switzerland). The TIFF images were imported into Rhinoceros NURBS modeling software (Robert
McNeel & Associates, Seattle, WA). Screw position was analyzed qualitatively for placement within or outside the pedicle
on CT examination. The 12 instrumented specimens were then
tested biomechanically as described above.
A quantitative analysis was also developed to determine the
accuracy of screw entry and trajectory. Using the Rhinoceros
software, a horizontal line was drawn at the intersection of the
transverse process with the pedicle as the proximal pedicle
line. Another horizontal line was drawn in the distal pedicle tangent to the anterior canal to demarcate the anterior
pedicle line. The midpoint of the proximal pedicle line was
defined as the ideal entry point. The trajectory was determined by connecting the midpoints of the proximal pedicle
line and the distal pedicle line was identified as the ideal
screw trajectory. The screw entry point was determined at the
proximal pedicle line. Screw entry lateral from the ideal was
designated as a positive distance (in millimeters), and entry
points medial from ideal were designated as negative distances (in millimeters). Screw trajectories that deviated laterally from the ideal were designated positive angles, and those
that deviated medially from ideal were designated as negative
angles (Fig. 2). All 12 specimens with a total of 48 screws were
analyzed using this method. This technique was reproducible.
The data, including pedicle widths of L1 and L2 in the specimens, were calculated and recorded for Techniques 1 and 2 in
Tables 1 and 2, respectively.
ONS-16 | VOLUME 59 | OPERATIVE NEUROSURGERY 1 | JULY 2006
www.neurosurgery-online.com
ACCURACY
OF
PEDICLE SCREW PLACEMENT
FOR
LUMBAR FUSION
TABLE 3. Normalization of dataa
Laminectomy/intact
Pedicle screws and rods/intact
Specimen
Rotation
Lateral
bending
Flex and
extension
2641-LAM
2675-LAM
2637-LAM
2665-LAM
2657-LAM
2617-LAM
Average
Standard deviation
2256
2581
2648
2652
2660
2674
Average
Standard deviation
1.24
0.48
0.62
0.55
1.00
0.71
0.77
0.29
1.32
0.71
0.90
0.77
1.16
0.91
0.96
0.23
1.03
0.51
1.01
0.80
1.02
0.75
0.85
0.21
Rotation
Lateral
bending
Flexion and
extension
1.46
0.97
0.79
0.88
1.26
1.56
1.15
0.32
1.11
1.39
1.25
1.21
1.10
1.18
1.21
0.11
1.22
1.14
0.96
1.04
1.28
1.36
1.17
0.15
1.20
1.12
1.49
1.27
1.11
1.11
1.22
0.15
1.24
1.49
1.54
1.26
1.34
1.77
1.44
0.20
1.16
1.35
1.85
1.31
1.33
1.17
1.36
0.25
a
The normalized data were obtained by dividing the absolute value by the intact values for each specimen in each parameter (i.e., rotation, lateral bending, and
flexion and extension). Note that normalization of the data eliminates the unit.
FIGURE 5. Bar graph analysis of data in Table 3. Biomechanical comparison of Technique 1 and Technique 2 in rotation, lateral bending, and
flexion and extension. The biomechanical difference between instrumented
specimens undergoing Techniques 1 and 2 was not statistically significant.
shown in Figure 3. The scatter plot analysis of Tables 1 and 2 suggests
that Technique 1 is less likely to yield screw entry and trajectory
medial to the ideal value, as demonstrated by zero screw placements in the negative–negative quadrant using Technique 1 and 11
screw placements in the negative-negative quadrant using Technique 2 (Fig. 4).
We also determined whether a laminectomy performed to
guide screw placement affected the biomechanical stability of the
final pedicle screw and rod-fixated construct. The biomechanical
stiffness data for intact laminectomy and pedicle screw and
rod-instrumented specimens undergoing pedicle screw and rod
fixation using Technique 1 and Technique 2 are shown in Table 3
and Figure 5. The normalized data (Table 3) suggest that the
absence or presence of laminectomy did not alter the biomechanical stability in axial rotation, lateral bending, and flexion
and extension. A total of 12 cadaveric specimens were used for
this biomechanical study: six specimens for the laminectomy and
six specimens for the nonlaminectomy groups. The power of the
study is limited by the small number of samples and variability
in specimens (Table 4).
RESULTS
DISCUSSION
A total of 48 screws were placed by expert neurosurgeons using
the two techniques studied (24 screws per technique). The deviation
from ideal entry point and ideal trajectory were recorded for each
screw as described in Tables 1 and 2. CT scans of the specimens
demonstrated that all 48 screws were grossly within the confines of
the pedicles, regardless of technique. A CT scan of each technique is
Lumbar instability can occur in the presence or absence of
lumbar stenosis. Instrumented lumbar fusion for simple stenotic diseases of the spine is usually not indicated because
simple decompression without fusion results in resolution of
clinical symptoms in most patients. Likewise, patients undergoing lumbar fusions for degenerative segmental instability
NEUROSURGERY
VOLUME 59 | OPERATIVE NEUROSURGERY 1 | JULY 2006 | ONS-17
KARIM
ET AL.
In this article, we demonstrate that the two pedicle
screw placement techniques
Method
Normalcy/equal variance
Difference
P value Power
studied yielded comparable
Flexion and extension
Passed/passed
NS
0.57
0.05
results with respect to the acBending
Passed/passed
NS
0.57
0.05
curacy of screw placement
Rotation
Passed/failed
NS (Mann– Whitney rank test)
0.94
NA
and biomechanical properties.
a
NS, not significant; NA, not applicable, Paired t test analysis of biomechanical data in Table 3 in all three methods
Furthermore, we also describe
tested.
a two-dimensional technique
for quantitative analysis of
pedicle screw placement
based on axial CT images. The
undergo a concomitant decompressive laminectomy at the
quantitative analysis for pedicle screw placement described here
instrumented levels only if stenosis is present. As such, lumcan be enhanced by incorporating sagittal and coronal reconbar fusions do not always require a therapeutic lumbar lamistructions of axial CT images.
nectomy at the level of the fusion. However, accurate placement of pedicle screws may be guided by performing a
laminectomy, thereby permitting visualization and palpation
REFERENCES
of the pedicle to guide screw placement. In this study, we
compare two techniques for placement of lumbar pedicle
1. Assaker R, Reyns N, Vinchon M, Demondion X, Louis E: Transpedicular
screw placement: Image-guided versus lateral-view fluoroscopy—In vitro
screws in 12 cadaveric specimens. Six specimens underwent
simulation. Spine 26:2160–2164, 2001.
pedicle screw placement at L1–L2 using Technique 1, in which
2. Austin MS, Vaccaro AR, Brislin B, Nachwalter R, Hilibrand AS, Albert TJ: Imagea laminectomy was performed before pedicle screw placeguided spine surgery: A cadaver study comparing conventional open
ment. The remaining six specimens were subjected to Techlaminoforaminotomy and two image-guided techniques for pedicle screw placement in posterolateral fusion and nonfusion models. Spine 27:2503–2508, 2002.
nique 2, in which anatomic landmarks guided screw place3. Benzel EC, Rupp FW, McCormack BM, Baldwin NG, Anson JA, Adams MS:
ment. Fluoroscopy was used in both techniques. All screws in
A comparison of fluoroscopy and computed tomography-derived
this study were placed by two expert neurosurgeons (AN,
volumetric multiple exposure transmission holography for the guidance of
DS).
lumbar pedicle screw insertion. Neurosurgery 37:711–716, 1995.
Cadaveric studies have significant limitations and do not
4. Calancie B, Lebwohl N, Madsen P, Klose KJ: Intraoperative evoked EMG
monitoring in an animal model. A new technique for evaluating pedicle
always simulate the clinical situation. In our comparative
screw placement. Spine 17:1229–1235, 1992.
study, the cadaveric specimens underwent removal of soft
5. Carl AL, Khanuja HS, Gatto CA, Matsumoto M, von Lehn J, Schenck J,
tissue. It is possible that a more clear view of the bone anatRohling K, Lorensen W, Vosburgh K: In vivo pedicle screw placement:
omy, particularly the lateral wall of the pedicle, facilitated
Image-guided virtual vision. J Spinal Disord 13:225–229, 2000.
6. Esses SI, Sachs BL, Dreyzin V: Complications associated with the technique
accurate screw placement in both techniques.
of pedicle screw fixation. A selected survey of ABS members. Spine 18:2231–
CT examination of the specimens demonstrated that all 48
2238, 1993.
screws were grossly within the confines of the pedicles, regard7. Jutte PC, Castelein RM: Complications of pedicle screws in lumbar and
less of technique. The scatter plot analysis of Tables 1 and 2 also
lumbosacral fusions in 105 consecutive primary operations. Eur Spine J
suggests that Technique 2 was more likely to yield screw entry
11:594–598, 2002.
8. Karim A, Mukherjee D, Ankem M, Gonzalez-Cruz J, Smith D, Nanda A:
and trajectory medial to the ideal value, as demonstrated by zero
Augmentation of anterior lumbar interbody fusion with anterior pedicle
screw placements in the negative-negative quadrant using Techscrew fixation: Demonstration of novel constructs and evaluation of
nique 1 and 11 screw placements in the negative-negative quadbiomechanical stability in cadaveric specimens. Neurosurgery 58:522–527,
rant using Technique 2 (Fig. 4). It is possible that Technique 1
2006.
9. Khoo LT, Palmer S, Laich DT, Fessler RG: Minimally invasive percutaneous
predisposes the surgeon to choose a more lateral trajectory away
posterior lumbar interbody fusion. Neurosurgery 51:S166–S181, 2002.
from the visualized or palpated medial wall of the pedicle.
10. Laine T, Lund T, Ylikoski M, Lohikoski J, Schlenzka D: Accuracy of pedicle
Placement of pedicle screw and rod fixation in the laminectomy
screw insertion with and without computer assistance: A randomised conspecimens consistently improved the biomechanical stability in axtrolled clinical study in 100 consecutive patients. Eur Spine J 9:235–240, 2000.
ial rotation, lateral bending, and flexion and extension when com11. Laine T, Makitalo K, Schlenzka D, Tallroth K, Poussa M, Alho A: Accuracy
of pedicle screw insertion: A prospective CT study in 30 low back patients.
pared with the corresponding specimens undergoing laminectomy
Eur Spine J 6:402–405, 1997.
alone (Table 3). Furthermore, the two groups (Techniques 1 and 2) of
12. Laine T, Schlenzka D, Makitalo K, Tallroth K, Nolte LP, Visarius H: Impedicle screw and rod-fixated constructs studied were of equal
proved accuracy of pedicle screw with computer-assisted surgery. Spine
stability in axial rotation, lateral bending, and flexion and extension,
22:1254–1258, 1997.
13. Merloz P, Tonetti J, Pittet L, Coulomb M, Lavallee S Sautot, P: Pedicle screw
regardless of whether a decompressive laminectomy was perplacement using image guided techniques. Clin Orthop Relat Res 354:39–
formed (Fig. 5). Because the biomechanical properties of the two
48, 1998.
techniques were identical and no screws violated the pedicles, the
14. Mitchell TC, Sadasivan KK, Ogden A, Mayeux RH, Mukherjee DP, Albright JA:
two techniques yielded equivalent results. Surgeon preference
Biomechanical study of atlantoaxial arthrodesis: Transarticular screw fixation versus modified Brooks posterior wiring. J Orthop Trauma 13:483–489, 1999.
should guide selection between these two techniques.
TABLE 4. Statistical analysis of the biomechanical dataa
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www.neurosurgery-online.com
ACCURACY
15. Nolte LP, Zamorano LJ, Jiang Z, Wang Q, Langlotz F, Berlemann U: Imageguided insertion of transpedicular screws. A laboratory set-up. Spine 20:
497–500, 1995.
16. Odgers CJ, Vaccaro AR, Pollack ME, Cotler JM: Accuracy of pedicle screw
placement with the assistance of lateral plain radiography. J Spinal Disord
9:334–338, 1996.
17. Robertson PA, Novotny JE, Grobler LJ, Agbai JU: Reliability of axial landmarks for
pedicle screw placement in the lower lumbar spine. Spine 23:60–66, 1998.
18. Tay BB, Berven S: Indications, techniques, and complications of lumbar
interbody fusion. Semin Neurol 230:221–230, 2002.
19. Wiesner L, Kothe R, Ruther W: Anatomic evaluation of two different techniques for the percutaneous insertion of pedicle screws in the lumbar spine.
Spine 24:1599–1603, 1999.
20. Xu R, Ebraheim NA, Ou Y, Yeasting RA: Anatomic considerations of pedicle
screw placement in the thoracic spine. Roy-Camille technique versus openlamina technique. Spine 23:1065–1068, 1998.
OF
PEDICLE SCREW PLACEMENT
FOR
LUMBAR FUSION
K
arim et al. examined two techniques for placing pedicle screws in
L1 and L2. One technique involved resecting one-third of the
medial facet and palpating the pedicle. The other technique used
standard landmarks without a facetectomy to determine the entry site.
Fluoroscopy was used to guide screw trajectories in both situations.
Biomechanical stability of the constructs was assessed using standard
techniques, and the placement of the screws was determined with
computed tomographic scans. The two techniques were found to be
equivalent. All screw placements were within the confines of the
pedicles regardless of the technique utilized. Biomechanical testing
did not show any significant difference between the two groups.
Based on the results of this study, surgeons comfortable with one of
these two techniques should not change their method of implanting
lumbar pedicle screws.
Vincent C. Traynelis
Iowa City, Iowa
COMMENTS
T
he authors compared two techniques of pedicle screw fixation in
cadavers. Both techniques were acceptable. Biomechanically, a
laminectomy did not decrease the strength of the constructs. This is a
well-founded study, although the number of specimens in each group
was small. The authors noted that when the pedicle screws were
placed under direct visualization, they tended to be placed less ideally
than when landmarks were used to guide placement. Nevertheless, all
pedicle screws were in an acceptable position. The authors’ conclusion
is something that most neurosurgeons already think. Nevertheless, it
is sensible to have it documented in an appropriate fashion.
K
n this study, the authors compare the radiographic and biomechanical results obtained by using two common methods of pedicle screw
placement. The two methods, not surprisingly, were essentially equivalent and the differences were minor. The method used for evaluating
ideal screw position is interesting, and it may be of use to other spinal
instrumentation researchers.
arim et al. have reviewed the accuracy of placing pedicle screws
using anatomic landmarks versus using an open laminectomy
technique. The accuracy of these two techniques was found to be
identical; however, the anatomic landmark technique is somewhat
flawed methodologically. Due to the spine being dissected free of soft
tissues, the lateral pedicle was visible in this experiment, but would
not have been observable were this test performed in vivo. Interestingly, with the lateral pedicle exposed, screw placement using the
second technique tended to have more medial entry sites and trajectories. Furthermore, the authors demonstrated that the ultimate
strength of the two constructs was equivalent whether or not laminectomy was performed. The “take-home” message for readers of this
in vitro experiment is that performing a laminectomy is an acceptable
maneuver to aid in placement of a pedicle screw from the point of
view of ultimate spinal stability. It is not illogical to assume that the
same results would apply to the clinical in vivo situation. Although
the message of this article initially might appear self-evident, the
formal testing to prove this hypothesis provides valuable data to the
readers.
Nevan G. Baldwin
Lubbock, Texas
Robert F. Heary
Newark, New Jersey
Volker K.H. Sonntag
Phoenix, Arizona
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NEUROSURGERY
VOLUME 59 | OPERATIVE NEUROSURGERY 1 | JULY 2006 | ONS-19