Screws-Medical, Surgical, Bone etc.

Tapered Pedicle Screw Pull Out Strengths

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2003 Summer Bioengineering Conference, June 25-29, Sonesta Beach Resort in Key Biscayne, Florida
INTRODUCTION
It is often necessary to back out the screw to increase its
height above the pedicle after initial insertion. This is done to achieve
alignment with other hardware elements. While the subsequent loss of
embedded screw length would be expected to reduce holding power,
there is a concern that tapered screws would be more affected than
cylindrical designs. This study investigated the impact on pull out
strength of backing fully inserted screws out one and three turns.
Three screw taper profiles were tested in an artificial cancellous bone
model. Each taper profile was tested with three screw thread shapes
and pitches.
METHODS
Screws were custom made with three taper profiles and three
thread shapes, for a total of nine designs. The major/minor (thread
tip/thread root) diameter combinations tested were
cylindrical/cylindrical (CC), cylindrical/tapered (CT), and
tapered/tapered (TT). For each taper profile, screws were made with
buttress (B), square (S), and “V” (V) thread shapes.
-
Figure 1. Screw designs tested. The panels show, from left
to right, the CC, CT and TT taper designs. Within each
panel the thread shapes are, left to right, Buttress, Square,
and “V”.
The B, S, and V threaded screws had pitches (the distance
between threads, or the distance the screw will advance in one
revolution) of 1.8, 2.375, and 3.0 mm respectively. All screws had a
base major diameter of 6.5mm and a threaded length of 40mm. The
screws are shown in Figure 1.
Figure 2. The test fixture. The screw was pinned to the
MTS ram and the foam block restrained by a clamp plate.
The plane of the clamp plate was adjustable to
accommodate any screw misalignment.
Polyurethane foam synthetic cancellous bone (Sawbones,
Pacific Research Laboratories, Inc., Vashon, WA) was used as a test
medium. Foam test blocks of 0.08, 0.16, and 0.24 g/cc densities were
used to simulate a range of cancellous bone densities. Holes were
prepared by pilot drilling to the minor diameter of each screw tip. The
holes were not tapped. The screws were fully inserted and pulled out
TAPERED PEDICLE SCREW PULL OUT STRENGTHS:
EFFECT OF INCREASING SCREW HEIGHT OUTSIDE THE PEDICLE
Vijay Goel, Ph.D. (1), David Dick (1), Setti Rengachary, M.D. (2),
Ishita Garg (1), Nabil Ebraheim, M.D. (1)
(1) Spine Research Center
Department of Bioengineering
The University of Toledo, Toledo, Ohio
Department of Orthopaedic Surgery
Medical College of Ohio, Toledo, Ohio
(2) Department of Neurological Surgery
Wayne State University School of Medicine
Detroit, Michigan
Starting page #: 0117
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2003 Summer Bioengineering Conference, June 25-29, Sonesta Beach Resort in Key Biscayne, Florida
axially using an MTS testing machine. Testing was repeated twice
more in new foam blocks with the screws backed off first one and then
three revolutions from fully inserted.
RESULTS
Three samples of each screw design were used for all
testing, with five trials made at each condition of screw design, foam
density, and back-out position. The standard deviation for each
condition averaged 6.4% of peak force across all testing. Failure was
by shearing of the foam at the thread tip diameter. Failure always
occurred within one thread pitch of displacement.
For the full insertion case, the CT taper profile and the
Buttress (and finest pitch) design produced the highest peak pull
forces, with the CTB producing the highest forces at each foam
weight. The S and V threads were generally very close, as were the
CC and TT profiles. Differences between screw designs became more
pronounced as the foam density increased, especially for the thread
shape. At the lowest foam weight, the thread shape/pitch made no
significant (two tailed t-test, p < 0.05) difference.
All screws showed significant increases in force with each
step in foam density, with six-fold increases typical over the tested
range. When the results were plotted vs. foam shear strength (as
determined from the measured density and the manufacturer’s
published density/strength data) peak pull force vs. density was found
to be highly linear for all screws.
At one turn back, most screws showed a significant drop in
strength, with an average loss of five to nine percent. At three turns
back, all screws recorded a significant drop, with average losses
ranging from 18 to 22 percent. Much of this drop was directly due to
the loss of embedded screw length as the screws were backed out.
Also, this loss of embedded length varied, with the screws backing out
different amounts for a given number of turns depending on pitch.
To correct for this variation, the pull forces were normalized
with respect to the length of screw remaining in the foam block. This
was calculated as the total length minus the number of back off turns
multiplied by the screw pitch. A graph of normalized forces for one
foam weight is given in Figure 3. After this normalization, most of the
CC designs showed no significant loss for either one or three turns.
The tapered designs were mostly unchanged at one turn. At three
turns, they generally lost strength, with the losses averaging less than
eight percent. An exception was the TT design, which lost strength in
the two lighter foams but actually gained some strength (two to four
percent) at one turn back in the 0.24 g/cc foam. No screw design lost
more than 11% per unit length.
The effect of thread shape and pitch did not appear to change
with back-out, as the screws generally maintained their relative
rankings as they were turned back.
DISSCUSION
While the CT designs did lose more strength with back-out
than the CC screws, they were still equivalent to the CC designs after
three turns out. The TT designs dropped relative to the CC in the
medium density foam, but the loss was small. And, in practice, the
performance of TT screws may be improved through the use of larger
base diameters than possible with the other designs. Pedicle screw
diameter is constrained by the inner diameter of the pedicle isthmus,
so a TT screw sized for the isthmus would be larger at the base than a
cylindrical screw.
Overall, these tests indicate that the CT taper profile
compares favorably to cylindrical designs even after three turns of
back-out, with the TT taper profile only slightly lower in performance.
A surprising observation was the small but statistically
significant increases that the TT designs all demonstrated at one turn
back in the highest density foam. This may have been due to the hole
preparation technique. Since the pilot holes were drilled to the minor
diameter at the screw tip, these tapered screws produced radial
compression of the foam as the screw widened towards the base. This
compressive stress may have contributed to early failure. When the
screws were backed off this stress was reduced, resulting in increased
apparent pull strength.
Finally, although screw pitch was not an independent
variable in this experiment, the best performing screws also had the
smallest pitch. A further advantage of finer screw pitch, at least with
uniaxial screws, is better control over screw height. To maintain
hardware alignment these screws must be rotated in half turn
increments, and finer pitch results in smaller possible height
adjustments.
ACKNOWLEDGEMENT
This work was funded in part with a grant from Stryker
Spine, Allendale NJ.
Normalized Pull Out Force vs Back-out
0.24 g/cc Foam
CTB
CTB
CTB
CTS
CTS
CTS
CTV
CTV
CTV
CCB
CCB
CCB
CCS
CCS
CCS
CCV
CCV
CCV
TTB
TTB
TTB
TTS
TTS
TTS
TTV
TTV
TTV
0
100
200
300
400
500
0
1
3
Back-Out Revolutions
N/
c
m
Figure 3. Peak pullout forces vs. revolutions of screw back-out in 0.24 g/cc Foam. Forces normalized to embedded screw
length. Error bars represent ± one standard deviation.

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