n Feature Article
Biomechanical and Cost Comparisons of
Near-Far and Pin-Bar Constructs
Augusta Whitney Kluk, MD; Tina Zhang, BS; Joseph P. Russell; Hyunchul Kim, MS;
Adam H. Hsieh, PhD; Robert V. O’Toole, MD
abstract
Orthopedic dogma states that external fixator stiffness is improved by placing
1 pin close to the fracture and 1 as distant as possible (“near-far”). This fixator
construct is thought to be less expensive than placing pins a shorter distance
apart and using “pin-bar” clamps that attach pins to outriggers. The authors
therefore hypothesized that the near-far construct is stiffer and less expensive.
They compared mechanical stiffness and costs of near-far and pin-bar constructs commonly used for temporary external fixation of femoral shaft fractures. Their testing model simulated femoral shaft fractures in damage control
situations. Fourth-generation synthetic femora (n=18) were used. The near-far
construct had 2 pins that were 106 mm apart, placed 25 mm from the gap
on each side of the fracture. The pin-bar construct pins were 55 mm apart,
placed 40 mm from the gap. Mechanical testing was performed on a material test system machine. Stiffness was determined in the linear portion of the
load-displacement curve for both constructs in 4 modes: axial compression,
torsional loading, frontal plane 3-point bending, and sagittal plane 3-point
bending. Costs were determined from a 2012 price guide. Compared with
the near-far construct, the pin-bar construct had stiffness increased by 58% in
axial compression (P<.05) and by 52% in torsional loading (P<.05). The pinbar construct increased cost by 11%. In contrast to the authors’ hypothesis and
existing orthopedic dogma, the near-far construct was less stiff than the pin-bar
construct and was similarly priced. Use of the pin-bar construct is mechanically and economically reasonable. [Orthopedics. 2017; 40(2):e238-e241.]
E
xternal fixation is commonly used to
stabilize femoral shaft fractures in a
damage control scenario1 when a
patient is too clinically unstable to undergo
e238
intramedullary nail fixation. The surgeon’s
goal when using an external fixator in such
a situation is to provide adequate fracture
stability until the time of definitive fixation.
The classic studies of external fixation
systems, published from 1983 through
1993, used only tibial shaft fracture models.2-8 However, the studies bore out many
of the basic concepts that are still applied
today, such as the importance of pin diameter and clamp-to-bone distance. Orthopedic dogma states that external fixator stiffness is improved by a configuration with
1 pin close to the fracture and 1 as distant
The authors are from the R Adams Cowley
Shock Trauma Center (AWK, TZ, JPR, RVO),
Department of Orthopaedics, University of Maryland Medical Center, Baltimore, and the Orthopaedic Mechanobiology Laboratory (HK, AHH),
Department of Bioengineering, University of
Maryland, College Park, Maryland.
Dr Whitney Kluk, Ms Zhang, Mr Russell,
and Mr Kim have no relevant financial relationships to disclose. Dr Hsieh has received
grants from Synthes and Stryker. Dr O’Toole
is a paid consultant for Smith & Nephew and
CoorsTek.
The authors thank Senior Editor and Writer
Dori Kelly, MA, for manuscript editing and figure
formatting.
Correspondence should be addressed to:
Robert V. O’Toole, MD, R Adams Cowley Shock
Trauma Center, Department of Orthopaedics,
University of Maryland Medical Center, 22 S
Greene St, T3R62, Baltimore, MD 21201 (rvo3@
yahoo.com).
Received: July 14, 2016; Accepted: August 25,
2016.
doi: 10.3928/01477447-20161006-03
Copyright © SLACK Incorporated
n Feature Article
as possible (the “near-far” construct).9,10
Another commonly used configuration,
the “pin-bar” construct, includes pin-bar
clamps that attach the pins to a clamp
with outriggers for the bars such that the
pins are a shorter distance apart from each
other than with the near-far construct. To
the authors’ knowledge, no previous biomechanics work has assessed the relative
stiffness of these 2 constructs in a femoral
fracture model. The near-far construct is
used by many physicians who consider it
to be less expensive because it avoids the
use of the expensive pin-bar components
and because it is also the gold standard for
stiffness.
The aim of this study was to compare
the mechanical stiffness and the cost of
near-far and pin-bar fixation constructs
that are commonly used for temporary
spanning external fixation of femoral shaft
fractures. The authors’ hypothesis was
that the near-far construct has increased
mechanical stiffness at a lower cost.
Materials and Methods
The authors created a testing model
that simulated a typical femoral shaft
fracture with fourth-generation synthetic
femora (Pacific Research Laboratories,
Inc, Vashon, Washington). Each group
consisted of 9 femora. All fixators were
composed of Hoffmann 3 external fixator
components (Stryker, Mahwah, New Jersey). A 5-cm gap was created at the midpoint of each femur to replicate a comminuted mid-shaft femoral fracture.
The near-far construct had 2 pins that
were 106 mm apart, placed 25 mm from
the gap on each side of the fracture. Carbon fiber rods were connected to the pins,
and a third carbon fiber rod was used to
connect the 2 sides of the construct (Figure 1).
The pin-bar construct had pins placed
55 mm apart, 40 mm from the gap. A 10hole pin-bar clamp was used on either side
of the fracture, with pins placed in the 3
and 8 positions. Straight outriggers were
used on both sides of the pin-bar clamps
MARCH/APRIL 2017 | Volume 40 • Number 2
Figure 1: Near-far external fixator configuration.
(Note: Distances shown are not accurate.)
to connect carbon fiber rods between
clamps (Figure 2).
All pins were inserted in bicortical
fashion by a single orthopedic trauma
fellow (A.W.K.). The frames were tightened in a way that is typically done in the
operating room by the orthopedic trauma
fellow. Parameters such as pin and bar
diameters were identical between constructs. To simulate clinical conditions,
the distance between the bone model and
the fixator clamps was 100 mm to allow
for soft tissues in the thigh. The pin clusters were oriented at a 45° angle to simulate anterolateral femoral pin placement.
To fit custom testing fixtures to the bone
model, 6 cm and 8 cm were sawed off the
distal and proximal ends, respectively. All
testing was performed with a material test
system machine (MTS Model 858 Mini
Bionix II; MTS Systems, Minneapolis,
Minnesota).
Nine samples of each construct were
tested in random order to ascertain an
average stiffness value in 4 modes: axial
compression, torsional loading, frontal
plane 3-point bending, and sagittal plane
3-point bending. Constructs were loaded
in the linear portion of the load-displacement curve in all testing modes for 3 trials.
Stiffness was calculated from the slope of
the load-displacement curve.
Axially compressive loads were applied along the longitudinal axis of the
bone model. The applied load and resulting displacement were recorded for each
of the 9 constructs. At no point during
compression did the 2 bone ends meet.
Specimens were mounted vertically with
Figure 2: Pin-bar clamp external fixator configuration.
Figure 3: MTS apparatus (MTS Model 858 Mini
Bionix II; MTS Systems, Minneapolis, Minnesota)
for axial load testing.
the distal component constrained in the
bottom fixture. On the proximal end, an
aluminum hemisphere was brought into
unconstrained contact with an aluminum
surface under a tare load of 1 N (Figure
3). Compressive stiffness was calculated
as Newtons per millimeter. The frames
were axially compressed 110 mm at a rate
of 1 mm/s.
Torsion was applied along the bone
model axis through the potted bone model
ends to simulate twisting forces on the
limb. Both proximal and distal ends of the
bone model were instrumented with Cardan joints to eliminate parasite constraint
effects and enable testing in pure torsion
(Figure 4). The torsional stiffness was recorded as Newton-meters per degree. The
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n Feature Article
Results
Figure 5: MTS apparatus (MTS Model 858 Mini
Bionix II; MTS Systems, Minneapolis, Minnesota)
for medial-lateral and anterior-posterior 3-point
bending testing.
Figure 4: MTS apparatus (MTS Model 858 Mini
Bionix II; MTS Systems, Minneapolis, Minnesota)
for torsional testing.
models were loaded to a maximal torque
of 4.5 N-m at a rate of 0.5 N-m/s.
Frontal and sagittal plane 3-point
bending simulated forces occur when the
lower limb is elevated clinically or sustains varus or valgus stress. Proximal and
distal bone model ends were secured in
a custom 3-point bending jig (Figure 5).
Bone cement molds of the proximal and
distal bone model ends were made in both
the frontal and the sagittal planes and then
used to secure the ends from translating
laterally and to maintain orientation during a 3-point bending test. Specimens
were supported by aluminum rods 2 cm
from the proximal and distal ends. During
testing, a horizontally oriented aluminum
rod was brought down to make contact
with the distal bone model 10 mm away
from the osteotomy site under a tare load
of 1 N. Each bone model fragment was
displaced 5 mm at a rate of 1 mm/s. No
permanent deformity of the construct occurred during testing.
Data were recorded and plotted with
the use of Excel software (Microsoft, Redmond, Washington). Stiffness was determined by calculating the slope of the linear region of the load vs the displacement
curve for each of the 4 testing modes, as
described above. Three trials were performed for each construct in each testing
mode, and the average stiffness was calculated using the last 2 trials. The results were
analyzed using SPSS version 14.0 software
(SPSS Inc, Chicago, Illinois). An independent t test was used to compare the near-far
configuration with the pin-bar configuration in each of the 4 testing modes. P≤.05
was defined as statistically significant.
A cost comparison of the 2 constructs
was conducted based on publicly available 2012 company list prices.
Table
Comparison of Construct Stiffness
Mean±SD
Axial Compression,
N/mm
Torsional
Loading,
N-mm/degree
Medial-Lateral
Bending, N/mm
AnteriorPosterior
Bending, N/mm
Pin-bar
11±0.7
410±59
13±4.3
42±16
Near-far
7±1
270±33
11±2.7
33±10
Construct
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Compared with the near-far construct, the pin-bar construct had stiffness
that was increased by 58% in axial compression (11±0.7 N/mm vs 7±1 N/mm;
P<.05) and by 52% in torsional loading (410±59 N-mm/degree vs 270±33
N-mm/degree; P<.05). Compared with
the near-far construct, the pin-bar construct had stiffness increased by 26% in
medial-lateral bending (13±4.3 N/mm
vs 11±2.7 N/mm; P=.19) and by 25%
in anterior-posterior bending (42±16
N/mm vs 33±10 N/mm; P=.258) (Table
and Figures 6-9).
The near-far construct cost $7026 and
the pin-bar construct cost $7778. This
was an 11% increase in cost for the latter.
Discussion
External fixation is commonly applied
to stabilize femoral shaft fractures in a
damage control situation and in other clinical situations.9 The primary function of
the external fixator is to maintain fracture
length alignment until definitive fracture
fixation can be applied. It has previously
been shown that increasing pin spread
increases construct stiffness.4 The nearfar construct is based on that concept and
has been the classically taught technique.
Pin-bar clamps are appealing to some surgeons because of ease of placement and
perceived stability; however, cost is a concern. To the authors’ knowledge, no previous studies have specifically addressed
the mechanical merits of the 2 constructs
as they relate to femoral fractures.
The authors’ hypothesis was that the
near-far construct is stiffer than the pin-bar
construct at a decreased cost. They found
the opposite in terms of stiffness: the pinbar construct was equal to or stiffer than
the near-far construct in all modes tested.
Perhaps this is because the near-far construct has the mechanical advantage of a
larger pin spread, but the pin-bar construct
overcomes this with the mechanical advantage of having 2 bars cross the fracture at
Copyright © SLACK Incorporated
n Feature Article
a distance from each other instead of just
1 (Figures 1-2). Further, they were surprised to discover that the 2 constructs had
approximately equivalent relative costs.
Although the pin-bar components are the
most expensive used, the near-far constructs required more bar-to-bar and pin-tobar components, which are also expensive.
The prices might not have represented the
actual prices an institution pays because
they did not account for any discounts, but
they did likely accurately represent the relative cost for each component. Additional
components could likely be added to the
near-far fixator to match the stiffness of the
pin-bar at an increased cost.
These findings are clinically relevant
because they challenge existing dogma
and might allow surgeons to create a
stiffer external fixator construct with pinbar clamps for essentially equal cost. This
study did not address the amount of stiffness necessary to maintain fracture length
and relative alignment in a femoral shaft
fracture; however, with cost failing to be
an element distinguishing between constructs, little argument remains for use of
anything but the stiffer construct.
The authors used a saw bones model
as opposed to cadaveric femora because
their goal was to study the mechanical
performance of the fixator and not the
pin-bone interface, which would be identical with the 2 constructs. The saw bones
might have mechanical qualities that are
different from those of cadaveric bone.
However, the authors think the difference in fixator construct is independent of
bone quality in this situation. Their testing
model used only 1 company’s external fixator, assuming other companies’ external
fixators would show similar stiffness and
cost differences. Different specific parameters of the external fixators in terms of
spacing, components, and pricing strategy
might also have affected the results, but
the authors chose 2 commonly used constructs to try to maximize clinical applicability of the results.
MARCH/APRIL 2017 | Volume 40 • Number 2
Figure 6: Difference in stiffness for axial compression testing.
Figure 8: Difference in stiffness for medial-lateral
bending testing.
Conclusion
This study showed that in contrast to
existing orthopedic dogma, the near-far
construct was less stiff than the pin-bar
construct in a femoral shaft application
and was essentially equivalently priced.
Clinicians should be aware that use of the
pin-bar construct seems to be mechanically and economically reasonable.
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