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Reciprocating Bone Saw: Effect of Blade Speed on Cutting Rate

Authors:

Abstract

Power reciprocating saws are used in surgical procedures to cut bone. Improved cutting rates are desirable in order to reduce operative time and improve patient outcome. A fixture was developed to test the effect of blade speed on cutting rate of bovine cortical bone. It was hypothesized that the volumetric cutting rate would increase in a linear manner for a fixed stroke length and a constant thrust force. A 7.0 N thrust force was applied. The reciprocating stroke length was held constant at 3.0 mm. Using an 18 TPI blade, cutting rate was determined to increase in a slightly non-linear manner, with disproportionately higher cutting rate at higher blade speeds. The data implies that a higher reciprocating frequency may invoke more efficient cutting.
1 Copyright © 2011 by ASME
RECIPROCATING BONE SAW: EFFECT OF BLADE SPEED ON CUTTING RATE
Timothy B. Lannin, Matthew P. Kelly, and Thomas P. James
Laboratory for Biomechanical Studies
Department of Mechanical Engineering
Tufts University
200 College Avenue
Medford, Massachusetts 02155 USA
thomas.james@tufts.edu
ABSTRACT
Power reciprocating saws are used in surgical procedures
to cut bone. Improved cutting rates are desirable in order to
reduce operative time and improve patient outcome. A fixture
was developed to test the effect of blade speed on cutting rate
of bovine cortical bone. It was hypothesized that the volumetric
cutting rate would increase in a linear manner for a fixed stroke
length and a constant thrust force. A 7.0 N thrust force was
applied. The reciprocating stroke length was held constant at
3.0 mm. Using an 18 TPI blade, cutting rate was determined to
increase in a slightly non-linear manner, with
disproportionately higher cutting rate at higher blade speeds.
The data implies that a higher reciprocating frequency may
invoke more efficient cutting.
Keywords: Bone Sawing, Reciprocating
INTRODUCTION
Powered reciprocating saws are used to shape and transect
bone. While hand saws perform well, powered saws are
desirable due to their higher cutting rates and compact size.
Higher cutting rates reduce the time required for surgical
procedures. This is especially important in cases where a
patient extremity is under tourniquet. In addition, frictional
heating between the blade and the bony bed can occur, resulting
in elevated temperature at the bone surface. High temperatures
are known to cause thermal necrosis of bone cells [1]. The
death of bone cells due to high temperature can prolong patient
healing time and decrease the efficacy of implant fit.
In bone sawing processes, higher cutting rates can be
achieved by increasing blade speed [2]. For reciprocating saws,
blade speed is dependent on two parameters: stroke length and
reciprocating rate. It is preferable to increase blade speed
through an increase in reciprocating rate, rather than by
changing stroke length. In confined areas, a reciprocating saw
with a longer stroke length increases the likelihood of
damaging adjacent tissue.
Outside of orthopedic medicine, reciprocating saws for
construction and demolition employ orbital action, which is an
alteration to the straight-line reciprocating path of conventional
saws. For orbital action, the path of the blade is an ellipse in
which the major axis coincides with primary stroke direction
and the minor axis is normal to the surface of the workpiece.
The small amplitude chopping of orbital action is present in
construction/demolition reciprocating saws to improve cutting
rates in wood and metal. As of this writing, no data has been
published on the effectiveness of orbital action in sawing bone.
A novel bone saw with orbital action has been developed
for this study. The aim of this research was to investigate the
effect of blade speed on cutting rate of a reciprocating saw with
constant orbital action and constant stroke length.
BACKGROUND
Reciprocating saws are used to cut bone in a variety of
surgical procedures. The saws generally come in two sizes and
related configurations. For oral and maxillofacial surgery,
reciprocating saws and related blades are quite small, often
referred to by surgeons as micro reciprocating saws or simply
as “small bone” saws. These saws are being used, for example,
by oral surgeons for alveolar ridge reduction and removal of
mandibular tori, rather than the more time-consuming approach
of using burs [3,4]. In orthognathic surgery, reciprocating saws
are commonly used to perform maxillary osteotomies. Micro
reciprocating saws are also used in the retrieval of bone grafts,
such as in calvarial and iliac crest bone harvests [5]. The
configuration of a micro reciprocating saw is cylindrical as
shown in Figure 1.
Proceedings of the ASME 2011 International Mechanical Engineering Congress & Exposition
IMECE2011
November 11-17, 2011, Denver, Colorado, USA
IMECE2011-62457
2 Copyright © 2011 by ASME
Figure 1 TYPICAL CONFIGURATION OF A MICRO
RECIPROCATING SAW.
The second type of reciprocating saw is commonly referred
to as a “large bone” saw. This configuration resembles the
pistol grip configuration of a sagittal saw. Large bone
reciprocating saws and a representative blade are shown in
Figure 2. A typical application of this saw is in
unicompartmental knee arthroplasty or in a median sternotomy
[6]. The sternal version of the saw has an attachment to guard
the tip of the reciprocating blade, so as to avoid piercing tissue
and organs.
(A)
(B)
Figure 2 (A) LARGE BONE RECIPROCATING SAWS AND (B)
REPRESENTATIVE BLADE AS USED IN EXPERIMENT.
Reciprocating frequencies of large bone saws are
approximately 13,000 cycles per minute (217 Hz) and stroke
lengths are generally around 3.0 mm. When comparing micro
reciprocating saws to large bone saws, the reciprocating
frequency of the micro saws tends to be greater, but the stroke
length is less. Average blade speed is proportional to both the
reciprocating frequency and stroke length, so by this measure
both types of saws have comparable blade speeds.
Building on the knowledge base from metal machining
theory, bone sawing has been studied extensively as an
orthogonal cutting process [7, 8]. During orthogonal cutting, a
single cutting edge, which is wider than the workpiece, is
positioned so that the cutting edge is perpendicular to the
direction of cutting. For orthogonal studies, however, blade
speed has been limited by the speed at which the bed of a mill
or a shaper can traverse, which is an order of magnitude less
than blade speeds of a typical surgical reciprocating saw. In
order to study the effect of higher blade speeds on cutting rate,
a new reciprocating sawing fixture was developed.
EXPERIMENTAL FIXTURE
Reciprocating blade motion is created with a modified
slider-crank mechanism. However, instead of converting the
rotation of the crank into pure linear reciprocation, this
mechanism creates a blade path that resembles an ellipse. The
horizontal component (major axis of the ellipse) provides the
primary stroke action, and the vertical component (minor axis
of the ellipse) provides the orbital action. The magnitude of
orbit is controlled by adjusting the distance between the crank
center and the pivot point.
Referring to Figure 3, point A on the crank travels in a
circle of radius R around the origin, O. Link AB, of length L,
translates and rotates about point C. By moving the location of
the pivot point C (i.e. changing the offset distance W), the
degree of orbital path at point B is affected. The saw blade is
attached near point B and therefore reciprocates and plunges
into the bone on the return stroke of the saw.
Figure 3 SCHEMATIC REPRESENTATION OF MODIFIED
SLIDER-CRANK MOTION RESULTING IN ORBITAL CUTTING
ACTION.
The blade path is not exactly an ellipse, but for brevity, the
derivation of its exact path in terms of the variables in Figure 3
will be omitted. The magnitudes of the stoke length and orbit
are given by the following equations:
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
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
3 Copyright © 2011 by ASME
The physical implication of Eq. (1) and (2) is that by
adjusting the distance to the pivot point, W, and the magnitude
of the crank offset, R, the aspect ratio and scale of the elliptical
saw blade path are determined. For this experiment, an orbital
amplitude (magnitude of minor axis of ellipse) of 0.04 mm was
used.
Figure 4 DIAGRAM OF BLADE PATH FOR A SINGLE
TOOTH. (W=202.47 mm, L=200 mm, R=1.5 mm - Note that Y
is scaled at approximately 5x for visibility of the orbital
blade path).
Each tooth on the saw blade, however, is a different
distance from the crank, so L varies across the width of the
bone sample. The variation of L results in variation in orbital
amplitude for each saw blade tooth engaging the bone sample.
For example, for a bone sample width of 11 mm, the cutting
configuration in this study resulted in orbital amplitudes of 0.12
mm, 0.04 mm, and 0.05 mm at the base end, middle, and tip
end of the saw blade respectively.
A 2 ¼ horsepower router motor (Porter Cable, Model 892,
Jackson, TN) was used to drive the crankshaft. The speed
control circuit was removed from the router so that a variable
transformer (Staco, Model SPN1510B, Staco Energy Products
Co., Dayton, OH ) could be used to manually control motor
speed from 0-20,000 rpm. The motor was oversized so that
reciprocating frequency could be held constant while under
load. The reciprocating saw mechanism is shown in Figure 5.
The reciprocating saw mechanism was fixed to one side of
a rigid rotating frame, as shown in Figure 6. A counterweight
was used to apply a fixed downward thrust force during sawing.
The variation in applied thrust force due to the angle of rotation
of the frame can be shown to be on the order of 0.1 N for the
angles of rotation used in this experiment. This variation is
smaller than the accuracy of the force gauge used, thus the
applied force was considered to be constant.
Figure 5 IMPORTANT FEATURES OF THE RECIPROCATING
SAWING MECHANISM. (1) DRIVE MOTOR, (2)
CRANKSHAFT, (3) CONNECTING ROD, (4) BLADE
Figure 6 CUTTING FIXTURE USED TO HOLD THE BONE
SAMPLE AND RECIPROCATING SAW.
EXPERIMENTAL PROCEDURE
Fresh adult bovine tibia were obtained from a local abattoir
and placed in a medical freezer at -20 degrees Celsius until time
of use. While still frozen, a meat cutting bandsaw (Grizzly,
Model G0560, Grizzly Industrial Inc., Muncy, PA) was used to
cut cortical bone samples from the mid-diaphysis region of the
bovine tibia as shown in Figure 7. The periosteum was removed
from the exterior surface of the bone and the medullary cavity
was scraped clean by hand to the bony surface. Each sample
was approximately 75 mm in length, with a rectangular cross
sectional area of approximately 8 mm by 11 mm. The length of
the sample corresponded with the primary osteon direction of
the long bone, shown by an arrow in Figure 7.
1
4
3
2
COUNTER-
WEIGHT
SAW
MECHANISM
ROTATION
AXLE
4 Copyright © 2011 by ASME
The counter weight was adjusted to provide a constant
force of 7.0 N throughout the cutting range. The thrust force
was measured at the tip of a saw tooth, in the middle of the
sawing portion of the blade. Force was measured with a
portable gauge (MG20, Mark 10 Co., Copiague, NY) to an
accuracy of ±0.4 N.
Figure 7 CORTICAL BONE SAMPLES TAKEN FROM THE
MID-DIAPHYSIS REGION OF AN ADULT BOVINE TIBIA.
Four blade speeds were used, 90.3 mm/s, 345 mm/s, 558
mm/s, and 760 mm/s, corresponding to reciprocating
frequencies of 14 Hz, 52 Hz, 84 Hz, and 114 Hz. Blade speeds
were calculated as a function of cutting frequency and reported
as root mean square values (RMS). Equation 3 was used to
calculate the RMS blade speed, where R (mm) is the crankshaft
offset, and is the cutting frequency (Hz).
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
The cutting frequency of the sawing mechanism was
measured using reflective tape on the motor drive shaft and a
handheld tachometer (Checkline A2108, Electromatic
Equipment Co., Cedarhurst, NY) with a precision of ±1 Hz
during sawing.
One 18 TPI (teeth per inch) reciprocating saw blade
(Brasseler USA, Model# KM-458, Savannah, GA) was used for
all cutting trials. The saw blade was inspected under a
microscope for wear, but it did not appear to have dulled during
the experiments. Four cuts were made for each reciprocating
frequency for a total of 16 cuts.
For each trial, the saw motor was allowed to warm up and
reach a steady-state speed. Once the desired speed was reached,
the saw was slowly lowered to approximately 0.5 mm above
the bone sample. The saw was then released and the timer
started. As the saw completed the cut, the total time was
recorded and the bone sample was collected. Bone sample cut-
offs were placed in sealed bags and returned to the refrigerator
for further analysis.
RESULTS
The volume of bone cut was determined from the cross
sectional area of the newly cut surface, multiplied by saw blade
kerf thickness, 1.0 mm. Given the irregular shape of the cut
cross sectional area, it was measured by taking a digital image
of the bone slice using an image processing technique with
ImageJ software (open source Java code from the US National
Institute of Health website http://rsbweb@nih.gov/ij/).
The cut surface of the bone slice was dyed black and
imaged against a white background with an 18.84 mm scale
marker. The blue plane of the image was extracted for the best
contrast, and the image was thresholded to highlight only the
dyed area. The software determined the real area of the bone by
scaling the number of pixels in the highlighted area by the scale
marker. The average volumetric cutting rate was determined by
dividing this volume by the time measured to cut through the
bone sample. Average results from the sawing experiments are
shown in Figure 8. The error bars represent one standard
deviation in the experimental data.
Figure 8 CUTTING RATES IN BOVINE CORTICAL BONE AS
A FUNCTION OF ROOT MEAN SQUARE BLADE SPEED.
DISCUSSION
For the lower three blade speeds (90.3 mm/s, 345 mm/s,
and 558 mm/s), the volumetric sawing rate increased in a linear
manner with blade speed. This is apparent from the data in
Figure 8. However, the cutting rate increased beyond a linear
extrapolation of the lower speeds when the saw was run at the
highest blade speed, 760 mm/s. This unexpectedly high cutting
rate could be an indication of a change in cutting mechanics at
higher blade speeds, which could have resulted from the
viscoelasticity of bone material. Alternatively, the higher
cutting rate could be related to an enhancement of the
effectiveness of orbital action at higher blade speeds. The
0.00
1.00
2.00
3.00
4.00
5.00
6.00
7.00
0200 400 600 800
Volumetric Cutting Rate (mm3/s)
RMS Blade Speed (mm/s)
Cutting Rate vs Speed
PRIMARY
OSTEON
DIRECTION
BONE
SAMPLE
5 Copyright © 2011 by ASME
orbital amplitude is quite small when compared to the cutting
stroke length, so its effect is analogous to vibratory cutting.
Krause [2] investigated the effect of imposing lateral
vibrations on a cutter during orthogonal machining of bone.
Forced vibrations improved cutting efficiency as apparent from
a reduction in cutting forces. Krause suggested that blade
vibrations reduced cutting forces by decreasing friction at the
interface between the bone and the cutter. He also postulated
that blade lateral vibrations could have improved cutting rate by
adding energy to aid in crack initiation.
In the experiments conducted here with an orbital action
reciprocating saw, the statically applied thrust force is
complimented by the dynamic thrust force due to orbital action.
On each return stroke of the saw, the mechanism is thrust
upward by the orbital amplitude. The mass of the saw resists
this upward motion, thereby creating a dynamic thrust force.
The dynamic thrust force increases with an increase in
reciprocating speed. It is possible that a combination of high
blade speed with orbital blade action could act to trigger a more
efficient cutting regime.
CONCLUSION
A novel mechanism was developed to create a dynamic
thrust force during the return stroke of a reciprocating bone
saw. The new mechanism caused the saw blade to follow an
elliptical blade path, commonly referred to as orbital action.
While holding the orbital amplitude constant at 0.04 mm,
experiments were conducted to test the effect of reciprocating
blade speed on volumetric cutting rate of bovine bone. It was
hypothesized that volumetric cutting rate would increase in a
linear manner with an increase in reciprocating frequency.
However, cutting rate increased in a non-linear manner at
higher blade speed. Volumetric cutting rate increased from 0.35
mm3/sec at a blade speed of 90 mm/s, to 6.1 mm3/sec at a blade
speed of 760 mm/s. Cutting rates corresponding to blade speeds
between 90 mm/s and 560 mm/s appeared linear, but diverged
from linearity to higher than expected cutting rates at blade
speeds of 760 mm/s. The data implies that a combination of
orbital blade action and higher reciprocating speed may invoke
more efficient chip formation.
ACKNOWLEDGMENTS
The authors would like to acknowledge the financial
support provided by Tufts University for this research.
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2. Krause, W.R., 1987, “Orthogonal Bone Cutting: Saw
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3. Goracy, E.S. and Rissolo A., 1993, “Use of a
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4. Turkyilmaz, I., 2010, “Use of Reciprocating Saw for
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A dividable titanium implant that, after insertion in the tibial metaphysis of an experimental animal, permits a numerical estimation of ingrowing bone was used to evaluate the effects of a defined temperature rise on bone regeneration. Heating the test implants to 47 degrees C or 50 degrees C for 1 minute caused significantly reduced bone formation in the implants, while no significant effects were observed after heating to 44 degrees C for 1 minute. The results reflect the importance of controlling the heat produced during surgery to avoid impaired bone regeneration.