Injury, Int. J. Care Injured 47S7 (2016) S20–S24
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Biomechanical testing to evaluate the cut-through resistance of intramedullary
nails for the proximal humerus
Maximilian Wanzla,*, Peter Foehrb, Ulrich Schreiberc, Rainer H. Burgkartb, Andreas Lenicha
a
Department of Orthopaedic Sports Surgery, Klinikum rechts der Isar, Technische Universität München, Munich, Germany
Department of Orthopaedics and Sportsorthopaedics, Klinikum rechts der Isar, Technische Universität München, Munich, Germany
University of Applied Science Munich, Munich, Germany
b
c
K E Y W O R D S
A B S T R A C T
proximal humerus fracture
Intramedullary nailing for stabilization of proximal humeral fractures is well-established. Complications as part of
a cut-through, such as backing out of locking screws, loss of reduction, and perforation of the screws into the
glenoid, are equally well-known. The test bench presented in this study enables testing of the cut-through
behavior of multiple intramedullary implants on a simulated osteoporotic three-part fracture configuration with
three different loading circumstances (A, B and C). In situation A, the glenohumeral dynamic force with
progressive loadings entered at an angle of 15° to the humeral shaft. In situation B the force entered at an angle of
35° and in situation C the angle measured 55°. Three different types of nails were tested: the Targon PH with the
optimal proximal screw length (T) and with all four proximal screws shortened (Tshort), the Synthes MultiLoc
PHN with (S5) and without (S4) the additional calcar screw and, lastly, the PolyAxNail PH, a polyaxial
intramedullary nail, in a neutral screw configuration (PAN) and a version with diametrically opposed crossed first
and fourth locking screws (PAN10).
Significant differences in the three cases were found with the evaluation of the failure load, which represents the
cut-through resistance. Case A: Tshort (245.4 ± 18.7 N) – S4 (346.8 ± 18.0 N) (adjusted p = 0.002); Tshort (245.4 ±
18.7 N) – S5 (368.5 ± 12.0 N) (adjusted p = <0.001); Tshort (245.4 ± 18.7 N) – T (323.5 ± 38.2 N) ( p = 0.004); Case B:
no significant differences between the study groups (adjusted significance). Case C: PAN (412.5 ± 16.0 N) – S5
(471.5 ± 21.5 N) (adjusted p = 0.007); T (414.0 ± 33.5 N) – S5 (471.5 ± 21.5 N) (adjusted p = 0.008).
The optimal screw length has a strong influence on the failure load. Choosing proximal screws that are too short,
produces a negative impact on the cut-through resistance. The additional calcar screw of the MultiLoc PHN and the
polyaxiality of the PolyAxNail showed a positive effect with regard to the failure load reached.
© 2016 Elsevier Ltd. All rights reserved.
intramedullary nailing
biomechanical testing
cut-through
Introduction
Fractures of the proximal humerus are the third most common
fractures found in the elderly population [1]. According to CourtBrown
et al. [2], the average age of patients is approximately 66 years, with a
higher percentage of afflicted women than men. The authors report an
increasing incidence with higher age. Additionally, an increase in
incidence over the last decades is reported [3,4]. Osteoporosis seems to
be a strong risk factor for fractures of the proximal humerus [5]. Stable
fractures with little or no dislocation can be treated conservatively.
Unstable and displaced fractures on the other hand, represent a clear
indication for operative intervention [6]. Tepass et al. [7] recommend
* Corresponding author at: Maximilian Wanzl. Tel: +49 17632542895.
E-mail address: m.wanzl@t-online.de (M. Wanzl).
0020-1383 / © 2016 Elsevier Ltd. All rights reserved.
plates and intramedullary nails, followed by fracture prosthesis for the
majority of surgical procedures. While the prosthetic restoration is
mainly used for luxation fractures, multi segment fractures, and
fractures with a head-split component, for two- to four-part fractures,
both plates and nailing systems are being used [8].
The use of intramedullary nailing for stabilization of proximal
humeral fractures has been expanded with good results through more
sophisticated locking methods [9–12]. Implant associated complications, attributed to the cut-through phenomenon, however, continue
to pose a challenge. These include complications like backing out or
cutting out of locking screws, loss of reduction, and perforation of the
screw into the glenoid [11–15]. Recent biomechanical studies have
broached these complications, however, there are only few studies
which cover the testing and evaluation of different intramedullary
implants and their cut-through behavior [16–18]. The selected load
cases usually correspond to in-vivo and in-vitro studies of the
biomechanics of the shoulder [19–22].
M. Wanzl et al. / Injury, Int. J. Care Injured 47S7 (2016) S20–S24
A dynamic fatigue test determines the cut-through behavior in
terms of the failure load of three different intramedullary implants
fixating a three-segment fracture. Unlike the aforementioned studies,
which simulated only a singular load case with a fixed glenohumeral
force application, the herein study considers an abduction movement divided into three separate load cases (Cases A, B and C). This
corresponds to a postoperative abduction, starting from the neutral
position (Case C) and reaching an abduction of 90° (Case A). By using an
artificial bone in connection with a standardized three-segment
fracture, this study can be easily reproduced. The three intramedullary
implants are used to investigate the following questions: The Targon
PH (Proximal Humerus Nail, Aesculap, Tuttlingen, Germany) is
used to determine the impact of short screw lengths while the
Synthes MultiLoc PHN (Proximal Humerus Nail, Synthes, Oberdorf,
Switzerland) is used to test an additional ascending calcar screw. The
impact of a polyaxial locking mechanism is tested using the PolyAxNail
PH (OT Medizintechnik, Munich, Germany).
Materials and methods
Three different standard proximal humeral nails were examined for
their cut-through behavior based on the failure load under a dynamic
sinusoidal force with progressive loadings. The artificial bone model
for the proximal part was a polyurethane foam with a density of ρ =
80 kg/m3 (Puren GmbH, Überlingen, Germany) and the model for the
distal part was a rigid polyamide shaft. The cut-through is defined in
the displacement of the humeral head in the direction of the force
vector and therefore describes the movement of the proximal screws
into the glenohumeral joint. The dimensions of a human humerus
were derived from CT-Data and transferred into the artificial bone
model. A three-segment fracture was simulated for the humeral head
and the greater tuberosity, whereas the fragments had no cortical bone
layer and a large fracture gap without medial support. The implantation position of each implant was detected based on virtual
implantations of all study groups on the segmented humerus [23].
The optimal screw length of each proximal screw was measured with
the tip of the screw 3 mm from the articular surface. This virtual
implantation was done in accordance with the manufacturers OR
manually. The position of the intramedullary nail, as well as the
position of the screws, was then transferred to the test specimen. The
intramedullary nails were subsequently placed into the test specimen
with a standardized implantation jig.
The following three intramedullary nails were tested: Targon PH
(Aesculap, Tuttlingen, Germany) in a version with an optimal screw
length for the four proximal locking screws and in a short version with
8 mm shortened screws; the MultiLoc PHN (Synthes, Oberdorf,
Switzerland) in two versions with four proximal locking screws and
an additional ascending calcar screw; and lastly, PolyAxNail PH
(Polyaxial Proximal Humerus Nail, OT Medizintechnik, Munich,
Germany) in a version with the four proximal screws in neutral
screw positions at an angle of 90° to the longitudinal axis of the nail
and in a version with the first and fourth proximal screw mirrorinverted 10° horizontally. This resulted in six study groups (Figure 1):
T = Targon PH, Tshort = Targon PH with 8 mm shortened screws, S4 =
MultiLoc PHN four proximal screws, S5 = MultiLoc PHN four proximal
screws and an additional calcar screw, PAN = PolyAxNail PH neutral
screw position, PAN10 = PolyAxNail PH with entanglement.
Three biomechanical situations of glenohumeral force entry with
A (15°), B (35°) and C (55°) were defined, according to studies by
Bergmann et al. [19,20]. Accordingly, situation A (15°) corresponds to
an arm abduction of approximately 90°, situation B (35°) corresponds
to an abduction of about 45° and situation C (55°) corresponds to a
neutral arm position. With the exception of Tshort tested only in 15°
(A), the implants were tested in the study groups in all three cases, with
a minimum number of n = 5 experiments. For each experiment, a new
biomechanical test specimen was used. The artificial bone specimens
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Fig. 1. Intramedullary nails and their tested screw configurations: T = Targon PH;
Tshort = Targon PH with 8 mm shortened screws; S4 = MultiLoc PHN with four proximal screws; S5 = MultiLoc PHN with four proximal screws and an additional calcar
screw; PAN = PolyAxNail PH neutral screw positon; PAN10 = PolyAxNail PH with
entanglement; *1 calcar screw (S5).
were exposed to a cyclic sinusoidal force with progressive loading
stages using the uniaxial dynamic test system Zwick HC10 (Zwick/Roell
GmbH, Ulm, Germany) (Figures 2 and 3). After a linear displacement
controlled ramp-up to a maximum load of 50 N, a sinusoidal force
controlled loading phase was applied, starting at mid-load value of
50 N with an amplitude of 10% and a frequency of 5 Hz. Every 250
cycles the step-by-step increase of the force’s mean value was
increased by 10 N. Once a mean force value of 200 N was reached,
the step-by-step increase was reduced to 5 N. The cut off criterion was
set at a transversal dislocation of the humeral head of 5 mm or the
achievement of a mean force value of 500 N. The axial force was
measured using a 2 kN load cell of the type 1010 AF 2KN (Huppert
GmbH, Herrenberg, Germany). Using the machine configuration
software testXpert R (Zwick/Roell GmbH, Ulm, Germany) the axial
load [N] and the dislocation [mm] were detected for every tenth cycle.
The statistical analysis of the failure load was performed with SPSS
Statistics 22.0/23.0 (IBM, Armonk, United States). The normal distribution was calculated using the Kolmogorov-Smirnov or the ShapiroWilk test and the normal distribution charts. The Kruskal-Wallis test
with a subsequent pairwise 2-sided test was used to examine
differences within the three biomechanical cases and the study
Fig. 2. Test set up: (*1) load cell; (*2) X-Y-Mz-Compensator; (*3) piston with ball
joint; (*4) artificial osteoporotic humeral head segment; (*5) humeral shaft in fixation unit; (*6) digital photo camera; (α) 35° glenohumeral load (Case B).
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M. Wanzl et al. / Injury, Int. J. Care Injured 47S7 (2016) S20–S24
Fig. 5. Distribution of failure load Case B (35°).
The Kolmogorov-Smirnov test showed a normal distribution of the
failure load over all study groups. Statistical analysis of the failure load
shows significant differences between the study groups for the
biomechanical situations A and C. The highest average failure load
over all three cases was observed for the MulitLoc S5 with the
additional calcar screw (Case A: 368.5 ± 12 N; Case B: 421.3 ± 13.4 N;
Case C: 471.5 ± 21.5 N) (Figures 4–6; Table 1).
The distribution of the failure load shows a highly significant
difference between the study groups ( p < 0.001) for situation A (15°).
The pairwise comparisons deliver the following results: There is a
significant increase of the average failure load between Tshort - S4*
(adjusted p = 0.002) by 101.4 N and Tshort - S5** (adjusted p < 0.001) by
123.1 N. For all other combinations, no significant differences were
observed. Looking separately at the mating T (323.5 ± 38.2 N) – Tshort
(245.4 ± 18.7 N) reaching a difference of 78.1 N, extracted from the
multiple comparison with the other implants a significance ( p = 0.004)
was observed between T - Tshort***. The mating S4 (346.8 ± 18.0 N) –
S5 (368.5 ± 12.0 N) shows a difference of 21.7 N and PAN (289.0 ±
34.4 N) – PAN10 (307.5 ± 38.1 N) had a difference of 18.5 N. (Figure 4;
Table 1)
The pairwise comparisons for situation B (35°) do not show
significant differences between the study groups. The results for the
average failure load of the different mating are: S4 (403.7 ± 26.0 N) – S5
(421.3 ± 13.4 N) with a difference of 17.6 N and PAN (380.7 ± 44.9 N) –
PAN10 (378.7 ± 26.0 N) with a difference of 2 N. The Targon PH has
reached an average failure load of 376.1 ± 11.2 N (Figure 5; Table 1).
Fig. 4. Distribution of failure load Case A (15°).
Fig. 6. Distribution of failure load Case C (55°).
Fig. 3. Test set up (close-up) three-part fracture with fracture gap: (*1) piston with
ball joint; (*2) artificial osteoporotic humeral head segment; (*3) artificial osteoporotic tuberculum majus segment; (*4) intramedullary nail; (*5) humeral shaft in fixation unit
groups contained therein. The p-value was specified as an adjusted
p-value due to multiple comparisons. The level of significance used
across all tests had a value of 0.05.
Results
M. Wanzl et al. / Injury, Int. J. Care Injured 47S7 (2016) S20–S24
S23
Table 1.
Failure load of each study group (Case A, B and C)
Case A (15°)
T
Tshort
S4
S5
PAN
PAN 10
Case B (35°)
Case C (55°)
AV [N]
SD [N]
AV [N]
SD [N]
AV [N]
SD [N]
323.5
245.4
346.8
368.5
289.0
307.5
38.2
18.7
18.0
12.0
34.4
38.1
376.1
–
403.7
421.3
380.7
378.7
11.2
–
26.0
13.4
44.9
26.0
414.0
–
444.0
471.5
412.5
442.9
33.5
–
22.0
21.5
16.0
9.2
The distribution of the failure load of situation C (55°) shows a
significant difference for the study groups ( p = 0.004). Adjusted
significant differences arise between the groups in pairwise comparison. The mating PAN - S5* (adjusted p = 0.007) shows an increase of the
failure load by 59.0 N and the mating T - S5** (adjusted p = 0.008) an
increase by 57.5 N. For all other combinations of situation C (55°) no
statistically significant differences were observed. The results for the
average failure load of the mating are: S4 (444.0 ± 22.0 N) – S5 (471.5 ±
21.5 N) with a difference of 27.5 N and PAN (412.5 ± 16.0 N) – PAN10
(442.9 ± 9.2 N) with a difference of 30.4 N (Figure 6; Table 1).
Discussion
Intramedullary nailing of the fractured proximal humerus has
reached a large indication spectrum and is being used successfully
[10,11,15,24–29]. Complications in terms of cut-through, however, still
remain a major problem. Our presented method, therefore, tests the
cut-through behavior of different intramedullary nails under postoperative and physiological load application. The defined situations A,
B and C cover a whole abduction cycle from a neutral arm position to a
maximum abduction of 90°. Whereas situation A (15°) corresponds to
an arm abduction of approximately 90°, situation B (35°) corresponds
to an abduction of about 45° and situation C (55°) corresponds to a
neutral arm position. A homogenous artificial bone model was used to
achieve the best possible quantitative comparison of the resulting load
stages between the test groups. This resulted in a maximum standard
deviation of 44.9 N.
No major differences could be seen in the failure load of the
individual implants while comparing the biomechanical situations A, B
and C and the simulated abduction. The highest failure load value was
reached by the Synthes MultiLoc S5 with the additional calcar screw,
followed by the S4. The influence of the selection of too short proximal
locking screws is examined on the basis of the Targon PH in case A
(15°). Optimal screw lengths are chosen for the variant T. In the case
of Tshort, all proximal screws are implanted with a distance shortened
by 8 mm. The average failure load is thereby reduced by approximately
78.1 N. In the multiple pairwise comparisons, however, no statistically
significant differences are found. If considering a separately examination of the influence of shortened screw length by the mating T –
Tshort extracted from the context of multiple comparison, a significant
difference is found ( p = 0.004). From a clinical point of view, stability
problems resulting from inadequate screw lengths of selected screws
that are too short seem to be obvious. As discovered by Lill et al. and
Liew et al. the length and subchondral positioning of the screws have
a direct and positive influence on the pull-out force [30,31]. In this
context and with these test results we can additionally mention a
higher cut-through resistance with the optimal subchondral screw
length.
Current literature reports increased medial stability by using calcar
screws in the treatment of proximal humeral fractures [32–34]. Also in
our test setup the study group S5 of MultiLoc PHN achieved the highest
failure load values across all three biomechanical loading cases. In
pairwise comparison with the variant S4 however, no significant
differences can be detected. A biomechanical study of Rothstock et al.
[17] provides similar results, with the S5 achieving the highest number
of cycles to failure load. The authors could not show any significant
differences in terms of number of cycles to failure load, using an
additional calcar screw. Therefore they compared the MultiLoc PHN
with two screw-in-screw in the first and second proximal screw with
and without an additional calcar screw. Both versions had no third
proximal screw set.
Polyaxial locking methods are already used in plate osteosynthesis
and provide both biomechanically and clinically comparable results
[35–37]. Freely positioned screws with angle stability are considered
beneficial. The tests conducted in this study also show values for the
failure load of a polyaxial implant compared to those of conventional
implants. Even though no statistically significant difference is found
between the PAN10 with entanglement and the PAN with neutral
screw position, the failure load did increase when using the crossed
version PAN10. In addition to these findings, the authors conclude that
the polyaxial locking could offer an additional positive influence
compared to conventional implants, if bone areas with higher density
and stability can be reached with a free orientation and positioning of
the locking screws [31,38,39].
Conclusion
Intramedullary nailing is currently successfully used for the
treatment of fractured proximal humeri. In this study, a testing
method was developed to investigate the cutting-through phenomena
in proximal humeri with osteosynthesis. The presented model matches
the typical implantation conditions: a three-segment fracture of
osteoporotic bone without cortical support with a large medial
non-contact fragment zone. The simulation of a physiological
glenohumeral force entry is ensured through three biomechanical
loading situations.
No major differences with regard to the failure load and the
glenohumeral force entry are noted across all implants: Targon PH,
MultiLoc PHN and PolyAxNail PH. However, the choice of the screw
length tends to have a strong influence on the stability of the implantbone interface. Extra screws, like the calcar screw of the Synthes
MultiLoc PHN or the use of a polyaxial locking mechanism of the
PolyAxNail PH, also have a positive impact on a higher failure load.
Conflict of interest
The authors declare no conflict of interest.
Acknowledgments
We thank all contributing students involved in this project for their
effort (especially to P. Kutschera and F. Franz). Special thanks to the
Institute for Medical Statistics and Epidemiology (IMSE), Klinikum
rechts der Isar, Technische Universität München, Munich Germany, for
their statistical analyses support.
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M. Wanzl et al. / Injury, Int. J. Care Injured 47S7 (2016) S20–S24
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