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2. Probing and Tapping: Are We Inserting Pedicle Screws Correctly?Q1
Q4 Vishal Prasada
, Addisu Mesfinb
, Robert Leec
, Julie Reigrut, MSd
, John Schmidt, PhDd,*Q2
a
Medway Maritime Hospital, NHS Foundation Trust, Gillingham, Kent, United Kingdom
b
Department of Orthopaedics and Rehabilitation, University of Rochester, Rochester, NY, USA
c
Royal National Orthopaedic Hospital NHS Trust, Brockley Hill, Stanmore, Middlesex HA7 4LP, United Kingdom
d
K2M Inc., 751 Miller Drive, Leesburg, VA 20175, USA
Received 10 November 2014; revised 25 May 2016; accepted 11 June 2016
Abstract
Purpose: Although there are a significant number of research publications on the topic of bone morphology and the strength of bone, the
clinical significance of a failed pedicle screw is often revision surgery and the potential for further postoperative complications; especially
in elderly patients with osteoporotic bone. The purpose of this report is to quantify the mechanical strength of the foam-screw interface by
assessing probe/pilot hole diameter and tap sizes using statistically relevant sample sizes under highly controlled test conditions.
Methods: The study consisted of two experiments and used up to three different densities of reference-grade polyurethane foam (ASTM
1839), including 0.16, 0.24, and 0.32 g/cm3
. All screws and rods were provided by K2M Inc. and screws were inserted to a depth of 25 mm.
A series of pilot holes, 1.5, 2.2, 2.7, 3.2, 3.7, 4.2, 5.0, and 6.0 mm in diameter were drilled through the entire depth of the material. A 6.5 Â
45-mm pedicle screw was inserted and axially pulled from the material (n 5 720). A 3.0-mm pilot hole was drilled and tapped with: no tap,
3.5-, 4.5-, 5.5-, and 6.5-mm taps. A 6.5 Â 45-mm pedicle screw was inserted and axially pulled from the material (n 5 300).
Results: The size of the probe/pilot hole had a nonlinear, parabolic effect on pullout strength. This shape suggests an optimum-sized probe
hole for a given size pedicle screw. Too large or too small of a probe hole causes a rapid falloff in pullout strength. The tap data
demonstrated that not tapping and undertapping by two or three sizes did not significantly alter the pullout strength of the screws. The data
showed an exponential falloff of pullout strength when as tap size increased to the diameter of the screw.
Conclusion: In the current study, the data show that an ideal pilot hole size half the diameter of the screw is a starting point. Also, that if
tapping was necessary, to use a tap two sizes smaller than the screw being implanted. A similar optimum pilot hole or tap size may be
expected in the clinical scenario, however, it may not be the same as seen with the polyurethane foam tested in the current study.
Ó 2016 Scoliosis Research Society.
Keywords: Probe hole; Pilot hole; Tapping; Screw pullout; Polyurethane foam
Introduction
A PubMed Central search of bone mechanical properties
revealed well over 10,000 articles including the effects of
genetics, biochemical signals, man-made chemical effects,
and a variety of disease states. In spite of all this research,
spine surgeons still do not know the quality of the patients’
bone until they open the surgical site and ‘‘poke around.’’
In many cases, the patient undergoing spine surgery has
osteoporosis. As has been stated previously, ‘‘Osteoporosis
is a major generalized bone disease characterized by a low
bone mass and the development of non-traumatic fractures,
especially of the vertebral bodies as a direct result of
osteopenia’’ [1]. Probing of the vertebral bodies is the best
indicator of whether the bone is normal, osteopenic, or
osteoporotic. Osteoporotic (weak) bone must be managed
more carefully than normal bone.
The clinical significance of a failed bone screw is often
revision surgery and the potential for further postoperative
complications. The holding power of a screw is also critical
in deformity corrections, and although this is less of a
problem in young healthy bone, it is an issue in elderly
Author disclosures: VP (none); AM (none); RL (none); JR (other from
K2M, Inc., during the conduct of the study; other from K2M, Inc., outside
the submitted work.); JS (other from K2M Inc., during the conduct of the
study).
*Corresponding author. K2M, Inc., 751 Miller Drive, Leesburg, VA
20175, USA. Tel.: (703) 554-1242; fax: (703) 779-7537.
E-mail address: jschmidt@k2m.com (J. Schmidt).
2212-134X/$ - see front matter Ó 2016 Scoliosis Research Society.
http://dx.doi.org/10.1016/j.jspd.2016.06.001
Spine Deformity xx (2016) 1e5
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3. patients with osteoporotic bone. The importance of correct
and positive fixation of bone screws is evident based on the
number of publications on the subject. Many of these
publications emphasize one particular aspect of fixation,
such as probe/pilot hole [2-8] or tapping [9-12]. Typical
shortcomings of these studies include the small sample
size, use of cadaveric or animal bone (with very large
scatter in the data), and an insufficient number of inde-
pendent variables tested. To combat the shortcomings of
cadaveric studies, polyurethane foam has often been used in
biomechanical studies to mitigate the variability of ca-
davers (cite).
The intent of this study is to quantify the mechanical
strength of the foam-screw interface by testing under highly
controlled conditions. Testing included:
Varying the probe/pilot hole diameter and then inserting
a standard-size screw.
Using one size pilot hole/screw and varying the tap size.
Materials and Methods
Two ASTM standards were used to guide the testing:
ASTM F1839 and ASTM F543 [19,20].
ASTM F1839-08e1 e Standard Specification for Rigid
Polyurethane Foam for Use as a Standard Material for
Testing Orthopedic Devices and Instruments. Based on a
literature review and previous testing, a general consensus
ranking of the foam can be made. The 0.16-g/cm3
foam is
similar to osteoporotic whereas the 0.32-g/cm3
foam is near
normal bone mineral density and the 0.24-g/cm3
foam is in
between [21-27].
All foam was purchased as sheet stock, 62 cm  244 cm
 5 cm in thickness (Last-A-Foam, General Plastics, Tacoma,
WA) and cut to final dimensions per ASTM F543. Pullout
testing was performed parallel to the foam rise in 0.16-, 0.24-,
and/or 0.32-g/cm3
foam per section A3 of the standard. There
was one deviation from the standard, which was that the
screws were inserted to a depth of 25 mm, not 20 mm.
A standard metal bushing housed the screws prior to
insertion. The bushing provided an interface with the
testing machine and allowed for full contact with the head
of the screw without slippage or tilting during pullout. All
screws were inserted to a depth of 25 mm, using a bushing
and spacer (see Fig. 1). Screws were never backed out.
Axial pullout strength tests were performed on an
Electropuls E3000 using Bluehill2 Software (Instron Cor-
poration, Norwood, MA). The bushing around each inserted
screw was placed in a custom axial pullout fixture whereas
the polyurethane foam block was positioned in a base
fixture clamped to the test frame base platen. Both fixtures
were designed to ensure that each pedicle screw was
centered directly under the load cell, Figure 1.
An axial preload of 20 Æ 5 N was established followed
by a tensile load at a rate of 5 mm/min until the screw
released from the test block. The ultimate axial pullout
strength was collected from the recorded load (N) versus
displacement (mm) curve.
Probe/Pilot hole size
To assess the effect of the initial pilot/probe hole
diameter, a series of pilot holes were drilled through the
entire depth of the foam blocks. Pilot hole diameters were
1.5, 2.2, 2.7, 3.2, 3.7, 4.2, 5.0, and 6.0 mm in diameter. A
6.5 Â 45-mm pedicle screw (K2M MESA) was inserted.
Axial pull testing was performed with n 5 30 for each
diameter pilot hole in each density of foam: 0.16, 0.24, and
0.32 g/cm3
(n 5 720).
Tap size
Based on the results of the Probe/Pilot hole study a
3.0-mm pilot hole was drilled and then tapped with the
following size taps: no tap, 3.5, 4.5, 5.5, and 6.5 mm.
print&web4C=FPO
Fig. 1. Insertion of the screw to the standardized depth. Screws were fitted as shown and inserted by hand to a depth of 25 mm. The spacers held the bushing Q5
level during screw insertion, ensuring screws were perpendicular to the foam surface. The bushing/test block were inserted into the axial pullout fixture
(right). The test block free floats in the base fixture, which is mounted to the load frame.
2 V. Prasad et al. / Spine Deformity xx (2016) 1e5
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4. A 6.5 Â 45-mm pedicle screw was inserted. Axial
pull testing was performed for each tap size in 0.16 and
0.32 g/cm3
foam (n 5 300).
All test data were analyzed using JMP 11.0 and SAS 9.4
(SAS Institute, Cary NC).
Results
For all plots the Maximum Load is defined as the peak
pullout force in Newtons.
Probe/Pilot hole size
Table 1 summarizes the mean pullout strength for each
probe/pilot hole tested in each of the three foams. Note that
eight different pilot holes (independent variables)
were assessed.
Figure 2 shows the results of the testing in 0.16 g/cm3
foam
(osteoporotic). The results for both the 0.24- and 0.32-g/cm3
foam show a similar shape, and all follow the general form of
Y5b0 þ b1X þ b2ðX À lÞ
2
ð1Þ
A look at the raw data shows that the probe/pilot hole
diameter has nonlinear effect and a parabolic shape. The
parabola indicates that there is an optimum probe/pilot hole
for any given screw size. For the 6.5-mm-outer-diameter
screw tested, that optimum appears at 2.7 mm. The
regression equation and correlation coefficient (r2
) for 0.16
g/cm3
foam were as follows:
Tap size
Table 2 and Figure 3 show the results of tapping a 3.0-
mm pilot hole.
Figure 3 shows an exponential curve with an ever-
increasing rate of decline from an asymptotic value. The
solid line represents the regression equation and the data
fits the form of:
Max loadðNÞ5a À b à expðlÃTap sizeÞ
ð3Þ
A nonlinear regression shows a plateau at 345.9 N
for the 0.16-g/cm3
foam and 1248.93 N for the
0.32-g/cm3
foam.
Max loadðNÞ5345:9 À 0:203 Ã expð1:011ÃTap SizeÞ
ð4Þ
Table 1
Mean pullout strengths.
Probe hole (mm) n 0.16 g/cm3
0.24 g/cm3
0.32 g/cm3
Mean SD Mean SD Mean SD
1.5 30 377.34 15.86 767.54 45.47 1,314.66 90.31
2.25 30 394.51 23.21 771.84 26.90 1,252.65 95.82
2.7 30 373.31 14.67 767.76 51.94 1,426.15 25.80
3.2 30 405.87 17.78 737.33 46.87 1,219.27 76.26
3.7 30 367.74 21.83 682.04 65.30 1,154.41 65.60
4.25 30 341.15 16.71 607.16 28.90 1,117.47 48.89
5 30 255.45 14.31 463.50 22.27 875.60 46.33
6 30 104.80 9.41 256.99 21.37 507.65 66.39
SD, standard deviation.
For each of the designated probe/pilot holes and foam densities. All
values are in Newtons (N), ntotal 5 720.
print&web4C=FPO
Fig. 2. Effects of pilot hole size on pullout strength in 0.16 g/cm3
foam.
Note the nonlinear, parabolic shape indicating an optimal pilot hole size.
The pedicle screw was a 6.5-mm-diameter screw. The solid line represents
the predicted values from the regression equation.
Table 2
Maximum Load pullout strengths after tapping.
Tap size (mm) N 0.16 g/cm3
n 0.32 g/cm3
Mean SD Mean SD
0.00 30 343.86 9.43 30 1,220.36 31.29
3.50 30 334.39 18.15 30 1,235.42 25.82
4.50 30 339.07 11.74 30 1,219.63 27.33
5.50 30 285.94 12.39 30 1,032.25 24.17
6.50 30 201.90 11.08 30 687.01 25.85
SD, standard deviation.
Student t-testing was performed with alpha 5 0.05 and a critical T
value of 1.97646. Note that the greatest decrease in strength for a 6.5-mm
screw occurs at tap sizes greater than 4.5 mm. All values are in newtons
(N); ntotal 5 300.
Max load ðNÞ5551:1 À 48:90 Ã Hole Diam À 25:3 Ã ðHole Diam À 3:58Þ
2
; r2
50:954; n5240 ð2Þ
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5. Discussion of Results
Probe/Pilot hole
The concept of an optimum probe/pilot hole has been
previously proposed [2-5]. However, in each of these
studies, only three or four pilot hole sizes were tested. In
the testing described by George [4], the method of hole
preparation involved either a drilled hole or probe hole.
Eight cadaveric spine segments from three separate ca-
davers provided vertebral bodies. They found no difference
in pullout strength based on how the probe/pilot hole was
made: drilled (mean 907.38 Æ 208.95N) or probed (mean
919.75 Æ 191.31N). This was confirmed by the work of
Zdeblick [5], who also reported no difference in pullout
strength based on how the probe/pilot hole was made.
These studies showed that it does not matter how the hole is
created; it is the size of the probe/pilot hole that determines
the final holding power of the screw.
Two nonlinear models were analyzed for the pilot hole
data: a parabolic model, Eq. (1), and an exponential one
very similar to Eq. (3). The exponential model took
the form
Max load5b0 À b1 Ã expðb2ÃHole DiameterÞ
ð5Þ
Both models were fit to the data, and the important metric
is the residual error. Residual error is the error that cannot be
accounted for by the fit of the model to the data and just like
in golf, the low score wins. In all cases, the parabolic data had
the lowest residual error. For the data of Figure 2, that error
was as follows: parabolic 5 101,809 and exponential 5
125,994. Therefore, the parabola is the best fit.
By testing eight pilot hole sizes, the true, parabolic
shape of this relationship becomes apparent. The parabolic
shape indicates there is an optimal size hole that should be
made prior to screw insertion. For the 6.5-mm screws tested
in the PU foam, the peak of the parabola, which is the
optimum-sized hole, occurs at 2.6 mm. But why does the
pullout strength drop off at pilot hole sizes smaller than
this? The answer has to do with the materials involved. The
screw is made of metal, typically stainless steel, Ti-6Al-4V,
or cobalt/chrome. The modulus of the metal (Young’s
modulus, E) is a measure of the stiffness, and the metals
used in bone screws are roughly 10Â higher than bone.
Because of this mismatch, the screw will force its way,
fracturing the underlying material. With the substrate
fractured, the pullout strength will decrease.
The idea of making a pilot hole exactly at the optimum
hole size is tempting. However, because it is unusual to
know the true quality of the material, a probe/pilot hole
slightly larger than optimum should be considered. For the
case of the 6.5-mm screw tested in the current study tested
in PU foam, a pilot hole size of 3.2 mm would provide
approximately a 25% margin of safety (from fracturing the
underlying material) while sacrificing only 10 N in pullout
strength. (These values come from the predicted values of
the regression equation of Figure 2: peak 399.6 N at the
2.6-mm pilot hole, 390.9 N at the 3.2-mm pilot hole.)
Tap size
This aspect of the study suffers from a shortfall of the
number of dependent variables, tap sizes. The number of
tap sizes used in this study is a reflection of availability; all
taps that could be used were utilized in this experiment.
Nonetheless, the graph of Figure 3 clearly shows the effect
of undertapping.
Table 2 shows that the three smallest taps evaluated (0,
3.5, 4.5 mm) result in nearly identical pullout strengths.
The shape of the curve in Figure 3 is the important feature.
The influence of tap size on pullout strength is obvious for
the larger sizes tested; both the 5.5 and 6.5 mm taps. The
data illustrate that with a decreasing tap size (under-
tapping), a plateau is reached where tapping the pilot hole
allows for easier insertion of the screw without a decrease
in pullout strength. If the tap size is too large, pullout
strength literally ‘‘falls off a cliff.’’
Based on the data presented in the current study, a
similar optimum pilot hole or tap size may be expected in
the clinical scenario; however, it may not be the same as
seen with PU foam.
Limitations
One limitation of this study is that it was not performed
in cadavers and therefore is not representative of the
print&web4C=FPO
Fig. 3. The effect of tap size on probe/pilot hole tapping in 0.16-g/cm3
foam. There were small and statistically significant differences noted in
the pullout strengths for each tap size. But the primary feature of the curve
is the ‘‘cliff’’ that occurs just after the 4.5-mm tap size. The solid line rep-
resents the predicted values from the regression equation. The pedicle
screw was a 6.5-mm-diameter screw.
4 V. Prasad et al. / Spine Deformity xx (2016) 1e5
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6. clinical situation. This study is not intended to duplicate the
clinical situation.
Normal bone, found in the spine, consists of a high-
density cortical shell surrounding a cancellous, lower den-
sity, interior. The combination of these two types of bone
determines the insertion torque and pullout strength of a
bone screw. The material used in this study is uniform in
properties and as such cannot duplicate the combined
cortical/cancellous bone found in the human spine. How-
ever, the ASTM standard clearly states that polyurethane
foam is ideal for testing bone screws in cancellous bone.
Conclusions and Recommendations
In the current study, the ideal pilot hole size in PU foam
was half the diameter of the screw while data demonstrated
that if tapping was necessary, to use a tap two sizes smaller
than the screw being implanted.
The findings of this study further emphasize the impact
of pilot hole/tap sizes on screw pullout strength and suggest
there are optimum sizes for both. Although a similar ideal
pilot hole and tap size may be expected in a clinical sce-
nario, the optimum sizes will not be the same for bone.
Uncited References
[13-18].
Acknowledgments
We thank K2M for providing all of the instrumentation
used for this study. We acknowledge the contributions of all
of the summer interns who worked on this project over two
years. In alphabetical order: Morgan Brown, Karli Johnson,
Katherine Kamis, Daniel Schmidt, Kaci Schwarz, Griffin
Smith, and Peter Williams.
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FLA 5.4.0 DTD Š JSPD324_proof Š 25 July 2016 Š 11:32 pm Š ce