Keywords spinal fusion - pedicle screw - bone screws - biomechanical phenomena/physiology
Introduction
Pedicle screw-based construction is currently the most used method for fixation of
the thoracic and lumbar spine.[1 ]
[2 ] The clinical usefulness of pedicle screw-based construction is supported by the
high rate of fusion, deformity correction and clinical outcomes.[1 ]
[2 ]
[3 ] The average accuracy for pedicle screws inserted with free-hand or fluoroscopy is
of 85,1%, and of 95% for pedicle screws using navigation.[4 ] The main problems related to pedicle fixations include the mechanical properties
of pedicle screws, their accuracy and the use of intraoperative radiation for placement.
Failures such as screw loosening still occur despite technological advances, at rates
reported to as being between 0.6 and 11%.[5 ]
[6 ] The placement and radiation exposure to the surgeon, mainly in minimally invasive
procedures, are drawbacks of pedicle screw fixation.[3 ]
[4 ]
[5 ] The exposure of the surgeon to radiation during a fluoroscopic assisted thoracolumbar
pedicle screw surgery is 10 to 12 times greater when compared with other nonspinal
procedures assisted by the fluoroscopic technique.[2 ]
[7 ]
[8 ]
The Pediguard Threaded Device (PDT) was developed to prepare the pilot hole into the
vertebra and to overcome the problems related to pedicle screw insertion. The PTD
is a drilling instrument with a thread design and a sensor at the tip that can be
used to drill the pilot hole, directly followed by the screw insertion, reducing surgical
steps and radiation exposure, and with increased accuracy.[9 ]
[10 ]
[11 ]
The PTD is a drilling instrument with a threaded tip available in various sizes (4.0,
4,5, 5.5 mm) with different thread designs, that is used to streamline surgical steps
while maintaining the accuracy for pedicle preparation for screw placement. The use
of PDT allows to prepare the pilot hole of the pedicle to provide adequate mechanical
purchase of the screw, improving the pedicle screw accuracy using the impedance at
its tip.
Tapping the pilot hole is currently performed by spinal surgeons before pedicle screw
insertion.[10 ] Pilot hole tapping allows the inspection of the pedicle walls before screw insertion
and guides the insertion of the screw into the pedicle.[11 ] A thread on the inner surface of the pilot hole is produced by the tap, creating
a female surface for the pedicle screw. Although the use of a tap with the same diameter
of the pedicle screw results in a perfect match, it reduces the screw pullout strength,
mainly in osteoporotic bone, and it is not recommended.[3 ]
[4 ]
[12 ] Screw pullout strength is related to screw purchase and to the biomechanical stability
of the pedicle fixation system. The use of a tap 1 mm smaller than the diameter of
the screw (undertapping) increases pedicle screw pullout strength, and undertapping
is commonly used for pedicle screw placement.[10 ]
It was reported that undertapping with incongruent pitch (longitudinal distance between
thread crest) reduces the pedicle screw pullout strength.[10 ] Using a tap with a different pitch from that of the pedicle screw does not allow
the screw thread to fit the precut groove by the tap. A concerned raised with the
use of PDT for the introduction of pedicle screws with different pitch was the motivation
for the study.
The aim of the present study was to experimentally evaluate the influence of the pilot
hole tapping using PTD and a screw with a different pitch from that of the tap. We
tested the hypothesis that undertapping with congruent and incongruent threads will
have similar effect on screw pullout strength.
Methods
One hundred and five polyurethane blocks of 8 cm height, 5 cm width and 5 cm length,
with a density of 10 PCF (0.16g/cm3 ) (National Ltda., Santana de Parnaíba, São Paulo, Brasil) were used as test bodies
to introduce screws and to perform the mechanical pullout tests. A pre-hole of 40 mm
depth was made in the center of the polyurethane block using a 2.7mm drill. The screws
were inserted straight into the foam after tapping according to the experimental group.
Three types of tapping were performed: 1–Line-to-line tapping (tap with same pitch
and external diameter of the screw), 2–Undertapping with congruent pitch (tap with
the same pitch and 1 mm smaller external diameter than the screw) and 3 - Undertapping
with incongruent pitch (tap with different pitch and/or different number of lead and
1 mm smaller external diameter than the screw).
Tap with congruent pitch was a tap provided by companies as part of the instrumentation
set. A congruent pitch means that the thread pitch of the tap is the same as that
of the screw. Congruent taps of the same external diameter of the screw (5.5 and 6.5 mm)
(line-to-line tap) or 1 mm smaller (4.5 and 5.5 mm) (undertapping) were used according
to the experimental group.
Pediguard Threaded Devices of different diameters and pitches were used in the experimental
group of undertapping with incongruent pitch. Two PDTs of 4.5mm (Ped D1TA0011 and
Ped D1TA0013) were used as undertapping with incongruent pitch for 5.5 mm screws.
The D1TA0011 tap has a pitch of 2. 9mm with a double lead design, and the D1TA0013
has a pitch of 2.5 mm with a double lead design. One PDT of 5.5mm (D1TA0001) was used
as undertapping with incongruent pitch for a 6.5 mm screw. The D1TA0001 tap has a
pitch of 2.8mm with a single lead design. Compared with the respective manufacturer
taps, the Pediguard threaded devices also have a tip with a nonthreaded portion (∼
10 mm) for redirection ([Figure 1 ]).
Fig.
1 PediGuard Threaded Drill used in the study.
Three different types of pedicle screws of 5.5 and 6.5 mm in external diameter, and
with different types or design with different diameters and pitches were used. Screws
with diameter-tapered and homogeneous pitch (Legacy-Medtronic, Minneapolis, MN, USA),
with core and threads of two types (Solera-Medtronic, Minneapolis, MN, USA), and with
conical core homogeneous threads (Revere-Globus, Audubon, PA, USA) ([Figure 2 ]) were used.
Fig.
2 Screws of 5.5 mm and 6.5 mm in external diameter.
The experimental group was formed according to the type of the screw (external diameter
and manufacturer) and preparation of the pilot hole. Each experimental group was formed
by five polyurethane blocks.
After screw insertion, pullout strength was evaluated using a universal test machine
(EMIC-DL10000, São José dos Pinhais, PR, Brazil). A rod was attached to the head of
the screw and pullout force was applied vertically ([Figure 3 ]). The pullout force was applied at a speed of 2.0 mm/min until the screw was pulled
out of the polyurethane block ([Figure 1 ]).
Fig.
3 Universal test machine EMIC.
Statistical Methods
The nonparametric Kruskal-Wallis test was used to compare the pullout strength among
the different screws. The level of significance was set at 5% (p ≤ 0.05). To further define these differences, the Dunn multiple comparison post-test
was performed, and comparisons with a p-value below the adopted level of significance
(0.05) were indicative of a difference between groups.
Results
The results of screw pullout strength in the experimental groups according to the
tapping are illustrated in Tables and Figures.
There was no statistical difference between pullout strength of 5.5 and 6.5 mm Legacy
screws ([Table 1 ] and [Figure 4 ]) using undertapping with congruent pitch or undertapping with incongruent pitch.
Undertapping with incongruent screw showed a statistically lower pullout strength
compared with line-to-line tapping (p = 0.0089).
Fig.
4 Pullout strength for Legacy pedicle screw 6.5mm.
Table 1
Tap
Pullout Strength (N)
Tap Legacy 5.5
587.9 ± 18.19 (*) (**)
Tap Legacy 4.5
549.0 ± 21.92 (*)
Ped 4.5 D1TA 11
544.2 ± 3.176 (**)
Ped 4.5 D1TA 13
531.0 ± 34.03
The Legacy 5.5 mm screw presented lower pullout strength using line-to-line tap compared
with undertapping with congruent and incongruent pitch. A statistical difference was
observed between undertapping with congruent and incongruent pitch using the D1TA0011
tap (p < 0.05) compared with line-to-line tapping. No statistical difference was observed
between undertapping using incongruent pitch with the D1TA0013 tap. ([Table 2 ] and [Figure 5 ])
Fig.
5 Pullout strength for Legacy pedicle screw 5.5mm.
Table 2
Tap
Pullout Strength (N)
Tap Legacy 6.5
601.5 ± 27.40 (*)
Tap Legacy 5.5
633.6 ± 9.311
Ped 5.5 - D1TA 1
585.8 ± 12.43 (*)
There was no statistical difference between the pullout strength of 5.5 and 6.5 mm
Solera screws ([Tables 3 ], [4 ] and [Figure 6 ], [7 ]) using undertapping with congruent pitch or undertapping with incongruent pitch.
Fig.
6 Pullout strength for Solera pedicle screw 6.5mm.
Fig.
7 Pullout strength for Solera pedicle screw 5.5mm.
Table 3
Tap
Pullout Strength (N)
Tap Solera 5.5
541.3 ± 18.19
Tap Solera 4.5
546.6 ± 42.46
Ped 4.5 D1TA 11
501.1 ± 29.16
Ped 4.5 D1TA 13
513.8 ± 14.56
Table 4
Tap
Pullout Strength (N)
Tap solera 6.5
581.3 ± 42.10 (*)
Tap Solera 5.5
662.8 ± 48.32 (*)
Ped 5.5 - D1TA 1
607.8 ± 20.27
There was no statistical difference between the pullout strength of 5.5 and 6.5 mm
Revere (Globus) ([Tables 5 ], [6 ] and [Figure 8 ],[9 ]) using undertapping with congruent pitch or undertapping with incongruent pitch.
Fig.
8 -Pullout strength for Globus pedicle screw 6.5mm.
Fig.
9 Pullout strength for Globus pedicle screw 5.5mm.
Table 5
Tap
Pullout Strength (N)
Tap Globus 5.5
587.9 ± 18.19
Tap Globus 4.5
607.2 ± 21.88
Ped 4.5 D1TA 11
625.7 ± 24.54 (*)
Ped 4.5 D1TA 13
581.6 ± 9.776 (*)
Table 6
Tap
Pullout Strength (N)
Tap Globus 6.5
667.3 ± 14.36
Tap Globus 5.5
678.0 ± 21.03
Ped 5.5 - D1TA 1
652.9 ± 16.17
Discussion
Our findings support the hypothesis that undertapping of the pilot hole with incongruent
pitch does not reduce screw pullout strength compared with undertapping with congruent
pitch in every modality of pedicle screw tested. The effect of undertapping was not
uniform on the three pedicle screw types used in the study.
Although pedicle screws are largely used in spinal surgery, problems still occur due
to insufficient fixation of the interface between the screws and the bone.[13 ] The reported rate of pedicle screw loosening is of between 0.6 and 11%.[14 ]
[15 ] It is important to avoid screw loosening from the viewpoint of biomechanics and
to secure mechanical stability in the vertebrae. Several strategies have been made
on implant design and pilot hole preparation to improve fixation on the bone screw
interface to improve screw loosening.[10 ]
[16 ]
Pilot hole preparation is the only variable that can be controlled by the surgeons
during the operation. Parameters related to the preparation of the pilot hole, such
as diameter, mode of preparation, tapping and repetitive insertion of screw have been
reported,[10 ]
[17 ]
[18 ]
[19 ] and optimization of the pilot hole has been performed to enhance the mechanical
anchorage of the screws into the vertebrae.[20 ]
The use of PTD with a different pitch compared with the screw designs used in the
present study did not reduce the pullout strength of all types of screws used in the
study. The effect of undertapping with incongruent pitch observed in our study does
not correlate with the results reported by Bohl et al.,[10 ] who reported the decrease of screw pullout strength of undertapping with incongruent
pitch compared with undertapping with congruent pitch. Undertapping has a smaller
minor diameter and major diameter than the screw and, even though it is incongruent,
it allows the radial displacement and compaction of the cancellous bone by the core
of the screws. The design of the screw could interfere with the ultimate amount of
bone compacted and contained inside the screw thread that act on screw pullout strength.
If undertapped, incongruent tap pitch alone would not be able to change the screw
pullout strength, and other factors like screw design could play a role in it.[19 ]
[20 ]
[21 ]
[22 ]
Tapping the pilot hole is currently performed by spinal surgeons before pedicle screw
insertion.[10 ] However, same-size pretapped pedicle screws reduce the screw pullout strength, mainly
in osteoporotic bone, and they are not recommended.[3 ]
[12 ]
[19 ] The undertapped pilot hole promotes a radial displacement and compaction of cancellous
bone by the core of the screw during its insertion, resulting in greater bone-screw
contact and in a larger amount of bone inside the screw thread.[23 ]
Undertapping by 1 mm is considered safe and achieves the same screw pullout strength
compared with an untapped screw, which has the highest pullout strength.[19 ]
In the field of spinal surgery, tapping was also introduced to allow the inspection
of the pilot hole walls before screw insertion and to guide the insertion of the screw
into the pedicle.[19 ] The accuracy of the trajectory of the pedicle screw is improved by tapping the pilot
hole before screw insertion.[23 ] In clinical practice, it should also be considered that the use of PTD reduces the
number surgical steps for pedicle screw insertion, increases the accuracy and reduces
the radiation exposure to the surgeon.[20 ]
[24 ] The PTD combines the need to make a pilot hole and do a tap into one step without
a decrease in screw pullout strength.[25 ]
The limitation of the experimental model used in the present study should be considered.
Insertional torque, which has been used in many experimental studies related to pedicle
screws, was not performed. There is no correlation between insertional torque and
pullout strength,[20 ] and this was the reason for not including this type of evaluation in the present
study.
Another limitation of the experimental model used in the present study is the fact
that only pure axial force was applied, and no radial force. The test was performed
on a single screw setup and not on a complete construct with rods or plates. The screws
were not submitted to side load, which influences the mechanics of the bone-screw
interface.
Finally, the tests have been performed in soft foam blocks, representative of osteoporotic
bone. A harder model could give higher and different pullout values.
The pullout strength test may not be commonly seen in a clinical setting, but its
simplicity and reproducibility allow it to be considered as the most efficient method
to compare screw anchorage within the bone. It is accepted as a good predictor of
the mechanical performance of the screw. However, pedicle screws are subjected to
a complex mechanically demanding situation represented by an association of twisting,
bending and pullout force.[24 ]
[25 ]
The simplification of force studied here may not represent a realistic clinical condition,
but it may provide useful information when comparisons are made under controlled conditions.[24 ]
[25 ] Screw pullout strength does not represent the only mechanism of screw failure, but
it reflects the magnitude of screw purchase.[25 ]
The PTD is a drilling instrument with a threaded tip available in various sizes (4.0,
4,5, 5.5mm) with different thread designs that is used to streamline surgical steps
while maintaining the accuracy for pedicle preparation for screw placement. This device
has an impedance measurement capability at the tip. A change in the pitch and cadence
of the audio feedback indicates a change in the tissue around the tip of the PTD.
A mid-range-pitch and medium-cadence audio signal can be heard as the probe is being
advanced into the cancellous bone. A low-pitch and low-cadence audio signal can be
heard as the probe approaches the pedicle cortical wall, and is the first indication
of the potential breach.[26 ] The ability of the probe with impedance capability at the tip (Pediguard probe)
has been experimentally and clinically shown to improve accuracy and to reduce surgeon
radiation exposure.[6 ]
[9 ]
[10 ]
[11 ]
[12 ]
As the surgeon cannot change the bone quality, improvement in screw design and insertional
techniques are made to improve screw fixation, and undertapping of the pilot hole
should be included among them.[25 ]
[27 ] Although PDT has an incongruent pitch compared with the pedicle screws used in the
present study, undertapping the pilot hole with incongruent pitch showed no statistical
difference compared to undertapping with congruent pitch.
Conclusion
Undertapping of the pilot hole with congruent or incongruent pitch does not affect
the pedicle screw pullout strength. The pedicle screw pullout strength may be influenced
by many factors, such as screw thread design, and the use of an incongruent pitch
alone has no effect.
