J. Biomedical Science and Engineering, 2011, 4, 235-241 JBiSE
doi:10.4236/jbise.2011.44032 Published Online April 2011 (http://www.SciRP.org/journal/jbise/).
Published Online April 2011 in SciRes. http://www.scirp.org/journal/JBiSE
Biomechanical comparison of distal locking screws for
distal tibia fracture intramedullary nailing
Brennen L. Lucas1, Alexander C. M. Chong1,2, Bruce R. Buhr1,
Teresa L. Jones1, Paul H. Wooley1,2
1Department of Surgery, Section of Orthopaedics, The University of Kansas School of Medicine, Wichita, Kansas, USA;
2Orthopaedic Research Institute, Wichita, Kansas, USA.
Email: alexander.chong@viachristi.org
Received 23 February 2011; revised 1 March 2011; accepted 3 March 2011.
ABSTRACT
Background: Newer generation intramedullary (IM)
tibial nails provide several distal interlocking screw
options. The objectives were to determine: 1) if the
new oblique interlocking option provides superior
stability, 2) which screw orientation/ configuration
is the most biomechanically stable, and 3) if three
distal interlocking screws provide better stability.
Methods: A preliminary experiment was performed
in torsion, compression, and bending tests with four
different screw configurations: (I) one medial-to-
lateral and one oblique, (II) two medial-to-lateral,
(III) one medial-to-lateral and one anterior-to-po-
sterior, and (IV) one medial-to-lateral, one ante-
rior-to-posterior and one oblique in simulated distal
metaphyseal fracture tibiae. Twenty-four Synthes
EXPERT tibial IM nails were used for six speci-
mens of each screw configuration. Parts I and II,
tibial IM nails were locked with 5.0 mm interlock-
ing screws into simulated distal tibiae (PVC and
composite analogue tibia). Part III, the two most
stable configurations were tested using five pairs of
simulated cadaveric distal tibiae metaphyseal frac-
tures. Results: Significant differences were attrib-
utable to distal screw orientation for intrame-
dullary nailing of distal tibia fractures. Configura-
tions II and IV were found to be more stable than
the other two configurations. No significant differ-
ence was detected in construct stability in all modes
of testing between Configurations II and IV. Dis-
cussion: Configuration I did not provide superior
stability for the distal tibia fracture fixation. Con-
figurations II and IV provided equivalent stability.
When choosing IM fixation for treatment of distal
tibia metaphyseal fractures two medial-to-lateral
screws provide the necessary stability for satisfac-
tory fixation. Clinical Relevance: This study indi-
cated an option for operative treatment of distal
metaphyseal tibia fracture fixation where preserva-
tion of soft tissue and rigid stabilization are needed.
Keywords: Biomechanical; Locking Screws;
Intramedullary Nail; Distal Tibia
1. INTRODUCTION
The treatment of diaphyseal tibia fractures using in-
tramedullary nailing is widely accepted and has been
expanded to distal metaphyseal fractures [1-3]. Intrame-
dullary nailing o f diaphyseal tibia fractures offers a vari-
ety of advantages compared to other treatments includ-
ing fracture stabilization with early weight bearing and
good preservation of the soft tissue envelope [4-9]. One
reason for treating distal fractures with intramedullary
nailing is to protect the already damaged and violated
thin soft tissue envelope [1-3,10-12]. Newer generation
nail designs have been introduced that make prior nail
modification techniques unnecessary and provide several
distal interlocking screw options. For example, the Syn-
thes EXPERT tibial nail system (Synthes USA, West
Chester, PA) offers four different distal interlocking op-
tions. There are two medial to lateral screw o ptions with
the most proximal being 37 mm from the distal tip and
the other medial to lateral screw hole located 13 mm
from the tip of the nail. There is also an anterior to pos-
terior option located 22 mm from the nail tip and a 30˚
oblique (either direction) hole that is ju st 5 mm from the
tip. In the technique guide “three locking screws” are
advocated for distal fractures.
The literature conveys neither a consensus on the ori-
entation of distal interlocking screws in distal metaphy-
seal tibia fractures, nor a consensus on the optimal
number of distal interlocking screws. Two medial to lat-
eral screws usually take less time to implant compared to
a medial to lateral screw and an anterior to posterior
B. L. Lucas et al. / J. Biomedical Science and Engineering 4 (2011) 235-241
Copyright © 2011 SciRes. JBiSE
236
screw, and it is much quicker to implant two screws
compared with three screws. However, the question re-
mains as to whether three distal interlocking screws
confer greater fixation stability than two distal inter-
locking screws for distal fracture patterns.
Controversy has emerged surrounding the optimal
number and configuration of interlocking screws for
intramedullary nailing [3,13-19]. To our knowledge, in
testing the detailed comparison of distal screw orienta-
tion and/or configuration for intramedullary nailing dis-
tal tibia fractures, little, if any, attempt has been made.
The objectives of this study were: 1) to determine if the
new distal oblique interlocking option provides superior
stability, 2) to determine which screw orientation or con-
figuration is the most biomechanically stable for the
intramedullary nailing of distal tibia fractures, and 3) to
determine if three distal interlocking screws provide an y
advantage in comparison to two distal interlocking sc re ws.
The null hypotheses for the present study were: 1) two
distal interlocking screws, one medial to lateral and one
oblique orientation , will provide equal constru ct stability
when compared to the other interlocking screw configu-
rations (two medial to lateral screws; a medial to lateral
and an anterior to posterior screw; and a three screw
configuration with a medial to lateral, an anterior to
posterior and an oblique screw), and 2) two distal inter-
locking screws will provide as much fixation stab ility as
three distal interlocking screws for distal metaphyseal
fractures fixation.
2. MATERIAL AND METHODS
The study was conducted in three parts: The first two
parts involved a preliminary experimental with four dif-
ferent screw configurations: (I) one medial to lateral and
one oblique, (II) two medial to lateral, (III) one medial to
lateral and one anterior to posterior, and (IV) one medial
to lateral, one anterior to posterior and one oblique in
simulated distal tibiae with a simulated distal metaphy-
seal fracture (Figure 1), while the third part compared
the two most biomechanically stable configurations us-
ing a simulated distal cadaveric tibiae metaphyseal frac-
ture. For all three parts, a total of twenty-four Synthes
EXPERT tibial intramedullary nails (10.0 mm diameter,
330 mm length, Synthes USA, West Chester, PA) (Fig-
ure 2) were used for this study.
For Part I of the biomechanical study, tibial intrame-
dullary nails were distally locked with either two or
three 5.0 mm interlocking screws into simulated distal
tibiae consisting of PVC pipe (outer diameter 25.0 mm
with inner diameter 17.0 mm) (Figure 3(a)). The PVC
pipe construct was chosen based on previous studies that
showed PVC pipe, which served as a synthetic homoge-
neous material, has a comparable pullout and torque
Figure 1. Screw configurations.
Figure 2. Synthes EXPERT tibial intramedullary nails.
(a) (b) (c) (d)
Figure 3. Samples of the experiment t est specimens. (a) Part I
test specimen (PVC), (b) Part II test specimen (composite ana-
logue bone), (c) Part III test specimen (Cadaver), and (d) Ra-
diograph of Part III test specimen.
strength for screws to those of bone [20,21]. To prepare
the PVC pipe for implantations, each PVC pipe and the
IM nail were placed in a custom designed holding jig to
standardize the implantation location for those inter-
B. L. Lucas et al. / J. Biomedical Science and Engineering 4 (2011) 235-241
Copyright © 2011 SciRes. JBiSE
237
locking screws were placed for each construct. The large
simulated canal was used to represent a worst-case sce-
nario with the distal tibia fracture with no endosteal to
nail contact at the fracture site, and was designed to not
allow the PVC pipe to limit deflection. Each specimen
was then locked with another custom designed holding
jig which centered with the nail, and fixed to the b ase of
a MTS materials-testing apparatus (Figure 4(a) and Fig-
ure 5(a)). A total of twenty-four specimens were u se d for
six specimens of each screw configuration.
For Part II, the test setup is similar as Part I, except
the simulated distal tibiae consisting of composite distal
tibia which represents simulated distal tibial metaph yseal
fracture (Figure 3(b)). A total of twenty-four Four-Gen-
eration composite analogue distal tibiae (model #3402,
Pacific Research Laboratories, Inc., Vashon, WA) were
used for six specimens of each screw configuration. To
prepare the distal tibial metaphyseal fracture, each com-
posite analogue distal tibia was placed in a custom de-
signed cutting jig to generate a standardized transverse
section made 40 mm from the tibiotalar surface. This
fracture type was used to represent a very distal tibial
fracture (OTA 43A) which eliminates any stability ob-
tained from cortical contact between the proximal and
distal fracture fragments. The distal tibial canal was
reamed to 11 mm in diameter, then each composite ana-
logue distal tibia and the IM nail were then placed in
another custom designed holding jig to standardize the
implantation location for those interlocking screws were
placed for each construct. The results of Parts I and II
were analyzed, and the two most stable configurations
were determined and tested in Part III.
For Part III, the two most biomechanically stable
screw configurations were applied in a cadaver biome-
chanical study with the distal tibia as the only source of
bias (Figures 3(c) and 3(d)). Five pairs of cadaveric
distal tibiae were used for each screw configuration
tested. The bones were stripped of all soft tissue attach-
ments and a simulated metaphyseal fracture was created
by a transverse section made four centimeters from the
tibiotalar surface, and the distal tibial canal was reamed
to 11 mm in diameter, this procedure is similar to Part II.
The fibula was detached from the tibia and was not in-
cluded in the testing. The 10 mm Synthes EXPERT tibial
intramedullary nails were then inserted to 5 mm above
the distal tibial articular surface. Using fluoroscopy, the
5.0 mm distal interlocking screws were inserted in one
of the two predetermined configurations using freehand
technique. Each specimen was then potted in poly-
methylmethacrylate (PMMA) onto the mechanical test-
ing jig centered within the nail, and the other end of the
nail fixed to the base of a MTS materials-testing appara-
tus. The distal interlocking screws were protected with
(a) (b)
Figure 4. Compression and torsional experimental setup. (a)
Part I experimental setup, (b) Parts II and III experimental
setup.
(a) (b)
Figure 5. Bending experimental setup. (a) Part I experimental
setup, (b) Parts II and III experimental setup.
soft molding compound (Play-Doh® brand) to prevent
contact between the interlocking screw and the PMMA.
Each intramedullary nail/distal tibia model (Parts I, II
and III) was tested under compressive loading, ante-
rior-posterior bending, medial-lateral bending, and rota-
tion torque in a Bionix servohydraulic materials testing
system (MTS Model 858, Eden Prairie, MN). These tests
were carried out within the elastic range of the con-
structs. For the deflection measurements of compressive
loading, anterior-posterior bending, and medial-lateral
bending, a completed two-dimensional motion across the
fracture site in the loading (frontal) plane was measured
with two linear variable d ifferential transformers (LVDT)
which determined displacements produced by the ap-
plied loads. These two LVDTs provided real time dis-
placement measurements between the two points of at-
tachment during testing, and the angular deflections
were calculated using simple trigonometric principles.
Only for the torque-angular displacement measurements
B. L. Lucas et al. / J. Biomedical Science and Engineering 4 (2011) 235-241
Copyright © 2011 SciRes. JBiSE
238
were measured and collected from the MTS. For the load
measurements of all test parameters were measured and
collected by the MTS system for analysis. The average
and standard deviation of the series were calculated for
each type of construct in the corresponding test, and
each specimen was tested in random order for all test
parameters.
For the compression load tests (Figure 4), each
specimen was loaded from 15 N in tension to a maxi-
mum load of 1000 N (weight of a 225lb man) in com-
pression at a rate of 10 N/sec. The 15 N in tension was
according to Osterkamp [22] where he concluded that
the proportions of the weight of a foot was 1.5% body-
weight, and the maximum load of 1000 N was chosen
based on the procedure previously used by Gorczca et al
[2]. Testing was initiated with two precondition ing load-
ing cycles, and then the load-deflection data from the
third to the fifth trials was recorded. Nail axial dis-
placement measurements were taken at two points 90˚
apart. This procedure was repeated three times for each
specimen, removing and repositioning the specimen every
time.
For the anterior-posterior and medial-lateral bend tests
(Figure 5), the load was sinusoidal cycled from ±10 N
to ±100 N for 5 cycles at a frequency of 0.05 Hz. The
corresponding bending moment varied from ±2.5 Nm to
±25.0 Nm. Testing was initiated with two precondition-
ing loading cycles and then followed by three data col-
lection loading cycles while force and displacement data
were collected. This procedure was repeated three times
for each specimen for each surface, removing and repo-
sitioning the specimen every time.
Rotation stability was tested at a frequency of 0.5 Hz
from 7.7 Nm to +7.7 Nm of torque with 10N of com-
pression (Figure 4). The maximum torque applied (7.7
Nm) in this investigation was approximately one-third of
physiologic torques observed during normal activities
[16]. Testing was initiated with two preconditioning
torque cycles and then torque and rotation angle were
recorded for the following three cycles. This procedure
was repeated three times for each specimen while re-
moving and repositioning the specimen each time.
3. STATISTICAL ANALYSIS
Data retrieved for Parts I and II from compression load-
ing, anterior-posterior bending, medial-lateral bending,
and rotation torque tests were analyzed with one-way
analysis of variance (ANOVA) of SPSS software (Ver-
sion 16.0; SPSS, Chicago, IL). Post hoc analysis of con-
struct was used the Least Significant Difference (LSD)
for multiple comparisons. The level of significant dif-
ference was defined as p < 0.05. These analyses were
used to compare the difference in each screw configura-
tion/orientation of distal locking screws in distal meta-
physeal tibia fractures and each testing mode between
constructs in term of stability. For Part III, paired t-tests
were used to statistically evaluate the difference between
the two most stable configurations, which determined
from Parts I and II, for constructs stability. The level of
significant difference was defined as p < 0.05.
4. RESULTS
For Part I, when compared in the compressive loading
(range: +15 N to 1000 N), anterior-posterior bending
with low (±10 N) and high (±100 N) applied load, me-
dial-lateral bending with low (±10 N) and high (±100 N)
applied load, and rotation torque with low (±0.1 Nm)
and high (±7.7 Nm) applied load, significant differences
were detected in stability between all four screw con-
figurations. Figure 6 shows the comparison of the con-
struct stability properties between the four different
screw configurations in Part I (PVC pipe) of the biome-
chanical study. For two screws configuration of all test
modes, Configuration II was found to be more stable
compared to the other two configurations. Configura-
tions II and IV were found to be more stable than the
other two configurations. Even though there were sig-
nificant differences were detected between all four con-
figurations, but from the post hoc analysis of construct
there were only two significant differences were detected
in stability in all modes of testing between two configu-
rations: 1) Configurations I and II, and 2) Configurations
II and III (Figure 6).
For Part II, significant differences were also detected
in stability between all four screw configurations, and
Figure 7 shows the construct stability properties for the
four different screw configurations using composite
analogue tibia. Configurations III and IV were found to
be more stable than the other two configurations. Even
though there were significant differences were detected
between all four configurations, but from the post hoc
analysis of construct there were only two significant
differences were detected in stability in all modes of
testing between two configurations: 1) Configurations I
and IV, and 2) Configurations II and IV (Figure 7).
Since there was no significant difference detected be-
tween Configurations II and III, and since Configuration
II is the standard of care fixation configuration used
currently, therefore, from Parts I and II results, Configu-
rations II and IV were chosen to perform the cadaver
biomec hanical stu d y.
For Part III, Figure 8 shows the construct stability
properties for Configurations II and IV using cadaveric
distal tibia. There was no significant difference detected
in construct stability in all modes of testing between
these two configurations. The one exception was medial-
lateral bending with low applied load.
B. L. Lucas et al. / J. Biomedical Science and Engineering 4 (2011) 235-241
Copyright © 2011 SciRes. JBiSE
239
Figure 6. Construct stability properties for each screw con-
figuration of Part I.
Figure 7. Construct stability properties for each screw con-
figuration of Part II.
Figure 8. Construct Stability Properties Using Cadaveric Dis-
tal T ibia.
5. DISCUSSION
This study compared biomechanical Synthes EXPERT
tibial nail stability using a va riety of distal tibial locking
screw configurations applied in intramedullary fixation.
This tibial nail system has two distal oblique holes to
enable the fixation of distal tibia fracture if necessary,
and offers four different distal interlocking options.
Our model assumed an unstable fracture pattern in the
worst-case scenario of cortical bone loss or extensive
comminution in which angular deflection would be lim-
ited only by distal interlocking screw configuration and
position. The authors recognize that in some fracture
patterns, cortical contact of the tibia or fibula would add
to the stability of the bone-implant construct, however,
this testing model was designed to examine the worst
case scenario.
Kneifel and Buckley [17] demonstrated an increased
rate of failure using one distal interlocking screw com-
pared to two distal interlocking screws with unreamed
nails in tibial shaft fractures. Chen et al. [15] showed
that there is no statistical difference in nail stability be-
tween two parallel (medial to lateral) and two perpen-
dicular (one medial to lateral, one anterior to posterior)
nails in anterior, posterior, medial or lateral directions, or
torsional loading. However, the results of this study in-
dicated significant differences in construct stability be-
tween all four screw configurations for distal tibia frac-
ture fixation using intramedullary nailing. This study
also investigated whether the new distal oblique inter-
locking option (Configuration I) add ed to the stability of
the fixation of the distal tibia fracture, but the results
showed that this obliqu e interlocking option did not pro-
vide superior stability for the fixation of the distal tibia
fracture.
The current biomechanical study (Part I and Part II)
using PVC pipe and composite analogue tibia as simu-
lated distal tibiae indicated that Configuration II and
Configuration IV were more biomechanically stable for
the intramedullary nailing of distal tibia fractures when
compared to the other two configurations. Our results
are in agreement with Chen et al. [15], and concur that
the instability of the screw fixation is probably due to
screw movement within the interlocking screw-hole, and
the mismatch between the screw threads and the inter-
locking screw-hole. Chen et al. [15] also determined that
there were possible two types of screw movements: Tilt
and shift. Our current study using the Synthes EXPERT
tibial nail system measured the worse possible tilt and
torsional movement using the dimension of the mis-
match between the screw threads and the interlocking
screw-hole, which was found to be approximately 2.9˚
(range: 2. 7˚ - 3.1˚) (Figure 9).
A relatively small number of cadaveric samples were
included in this study; however, the changes in the con-
struct stability properties between two distal in terlocking
screws (Configuration II) and three distal interlocking
B. L. Lucas et al. / J. Biomedical Science and Engineering 4 (2011) 235-241
Copyright © 2011 SciRes. JBiSE
240
Figure 9. Screw movement within the interlocking screw-hole.
screws (Configuration IV) were extremely small and
repeatable. Therefore, we are confident that two medial
to lateral distal interlocking screws (Configuration II)
and three distal interlocking screws (Configuration IV)
provide equivalent construct stability.
When the results from the three simulated distal tibia
models with a simulated distal metaphyseal fracture
were compared (Figures 6-8), significant differences (p
< 0.05) were detected. This was especially true for ML
bending under high load, where the deflection with ca-
daveric model was more than twice than that with com-
posite analogue bone model. In this study, only one size
of the tibial intramedullary nail (10.0 mm dia meter) was
used, and different bone sizes occurring within the ca-
daveric specimens might be a limiting factor in this in-
vestigation. Therefore, the cadaveric findings might re-
flect a higher variable than the composite analogue bone
model within the comparison. In fact, when the com-
parison is performed within the same simulated test
model, the magnitude of the difference in all modes is
much smaller. Further clinical research is needed to con-
firm our results.
The information currently available in the literature
conveys neither a consensus on the orientation of distal
interlocking screws in distal metaphyseal tibia fractures,
nor agreement on the optimal number of distal inter-
locking screws. The reason could be because either ca-
daveric specimens or patient bones have significant
variations in size and bone quality and require the use of
different intramedullary nails sizes. Therefore a valid
conclusion cannot be construed. In this laboratory inves-
tigation using the composite analogue tibiae, low inter
sample variability, and construct stability for several
screw orientations allows direct comparison between
each configuration. However, the results of this labora-
tory studies cannot readily be extrapolated to clinical
situation. A larger study, with higher number of ca-
daveric bones and with varying bone density, may dem-
onstrate more clearly the screw configuration effect on
the stability for the intramedullary nailing of distal tibia
fractures.
There are some limitations to our study. Despite our
efforts to insert all screws in the same perfect central
screw placement, it is possible that drilling and inserting
the screw may result in a small degree of tilt that might
cause the nail and screw to interlock, in turn cause nail
stability. However, we believe that screw insertion was
performed as clinically intended (essentially by eye and
fluoroscope). In addition, it may be more difficult to
insert the oblique screw compared to the medial to lat-
eral or anterior to posterior, despite our best efforts to
standardize screw insertion. Drilling and redirection of
the screws was avoided in this study. Another inherent
limitation is that the in ter-hole distance varied depending
on the interlocking configuration used. Strict geometric
considerations suggest that this favors Configurations II
and IV, as the greater working length further restricts
angulations. However, this design parameter is of prac-
tical importance and should be considered when decid-
ing on distal interlocking screw orientation. George et al.
[18] have shown that the further the distal interlocking
screw is located from the fracture, the greater the rota-
tion stability of th e construct.
Overall, when choosing intramedullary fixation for
the treatment of distal tibia metaphyseal fractures, our
data suggest that two medial to lateral screws may pro-
vide the necessary biomechanical stability for satisfac-
tory fixation and clinical beneficial, because this not
only saves operative and fluoroscopy time, but also may
be economically advantageous compared to three distal
locking screws and/or an oblique screw.
6. CONFLICT OF INTEREST
STATEMENT
This study received a support grant from Synthes (West
Chester, PA, USA) with approved the study design, and
provide the Synthes EXPERT tibial nail system used in
this study. Pacific Research Laboratories, Inc. (Vashon
Island, WA) did provide the analogue tibia. Both Pacific
Research Laboratories, Inc. and Synthes USA had no
role in the collection , analysis and interpretation of data,
in the writing of the manuscript, or in the decision to
submit the manuscript for publication. This study also
did not receive any payments or other personal benefit or
a commitment or agreements that were related in any
way to the subject of the research that we conducted in
connection with the research .
7. ACKNOWLEDGEMENTS
The authors thank Synthes USA (West Chester, PA) for providing the
Synthes EXPERT tibial nail system used in this study. The authors also
wish to thank Mr. Joel D. White and Mr. Preston T. Saal for their as-
sistant on data entering on this study. No benefits of any form have
been received directly or indirectly to the subject of this article, and
B. L. Lucas et al. / J. Biomedical Science and Engineering 4 (2011) 235-241
Copyright © 2011 SciRes. JBiSE
241
there is no any potential of conflict of interest.
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