Materials Sciences and Applicatio ns, 2011, 2, 1367-1374
doi:10.4236/msa.2011.210185 Published Online October 2011 (http://www.SciRP.org/journal/msa)
1367
A Comparison between the Mechanical Behaviour
of Steel Wires and Ultra High Molecular Weight
Poly Ethylene Cables for Sternum Closure
Roelof Marissen1,2, Mischa Nelis2, Mildred Janssens2, Michelle D. M. E. Meeks3, Jos G. Maessen3
1Faculty of Aerospace Engineering, Delft University of Technology, Delft, The Netherlands; 2DSM Dyneema, Urmond, The Nether-
lands; 3University Hospital Maastricht, Maastricht, The Netherlands.
Received June 8th, 2011; revised June 30th, 2011; accepted August 8th, 2011.
ABSTRACT
Sternum closure after open heart surgery is typically done with steel wires. Final approximation of sternal parts and
connection is achieved by twisting the ends of the wire and bending the twisted assembly towards the sternum in order
to minimize outward protrusion. Though this routine procedure is highly effective, some failures do occur, e.g. due to
wire fracture. Fatigue fracture o f the wires, e.g. due to coughing implies a failure risk. An alternative development is to
make cables from gel spun Ultra High Molecular Weight Poly Ethylene (UHMWPE) fibres, such fibres are extremely
strong, yet flexible, and if made as a very pure grade, they are highly bio compatible. The optimal connection technique
will be different from that of steel. Connection will rather be with knotting than twisting. A new sternum closure and
fixation technique has been developed for the sternum. Additionally, a testing technique was developed, for a connec-
tion of simulated sternum parts, using different materials a ccording to their respective optima l connection method and
subsequently testing the mechanica l properties of the connectio n. Substantial differences were obs erved. The mechani-
cal behaviour of twisted steel wire conn ection showed more scatter than the knotted UHMWPE cables an d some initial
slack was sometimes present in the twisted cables. The maximum atta inable force in the steel wires was determined by
“untwisting” due to the external load. The maximum force in the UHMWPE cables was determined by the knot strength,
either slipping for small knots, or breaking of the cables at the knots for slip-improved knots. The maximum force on the
knotted UHMWPE cables was substantially larger than the maximum force on the twisted steel wires. Fatigue tests
were performed on both the steel solution and the UHMWPE cables solution. The performance was about similar, al-
though the simulated sternum opening was smaller for the UHMWPE cables at higher load levels. Summarizing, the
UHMWPE cables show two advantages namely higher maximum load and more reproducible mechanical behaviour
due to less scatter in the mechanical behaviour. On the other hand, the connection by knotting UHMWPE cables is
somewhat more elaborate than th e simple twisting connection of steel wires.
Keywords: Heart S urgery, Load, Displacement, Fracture, Deformation, Twisting, Knotting, Slip, Fatigue
1. Introduction
Open heart surgery may be inevitable if other treatments
of heart disease, like angioplasty, fail. Open heart surg ery
requires longitudinal splitting of the sternum by median
sternotomy, thus making the heart accessible for surgical
treatment. The sternum has to be closed after the treat-
ment. This is done by feeding steel wires, provided with
a needle between the ribs, around both sternum halves.
The needle and possible excess length is cut off and the
ends are connected by twisting th em together. This prac-
tice is adequate in most cases. However, fracture of the
wires does occur [1,2], sometimes in the surgery room,
sometimes later, e.g. due to fatigue loads if patients are
frequently coughing. Coughing may cause fatigue frac-
ture of the steel wires. Metals are especially sensitive to
fatigue after plastic deformation, see e.g. [3]. This is re-
lated to unfavorable residual stresses after plastic defor-
mation and to the large amounts of dislocations in the
metal crystals. The twisting of the ends implies consid-
erable plastic deformation. Figure 1 illustrates this prob-
lem. It shows two Scanning Electron Micrographs of an

A Comparison between the Mechanical Behaviour of Steel Wires and Ultra High Molecular Weight 1368 Poly Ethylene Cables for Sternum Closure
(a) (b)
Figure 1. SEM micrograph of a broken sternum closure wire, retrieved from a patient about five month after surgery. (a)
Overview; (b) detail of the non broken part showing micro cracks.
in-vivo broken wire that was removed from the patient,
about five month after surgery. The fracture surface oc-
curs at the first winding of the twisted area, because the
loads are highest at that location and full damage due to
the plastic deformation during twisting is present. The
adjacent counterpart shows surface cracks, indicating that
also this part of material has been loaded close to its limit.
The dark “netlike pattern” in Figure 1(a) is due to re-
maining organic material that was still resident, even
after ultrasonic cleaning. The remaining organic material
on the fracture surface is consistent to in vivo fracture.
Moreover, the flexural stiffness and hardness of the
steel wire will cause inhomogeneous load distribu tion on
the sternum, after closing and tightening. Locations with
high pressure on the sternum bone may cause bone cut-
ting and local bone degeneration. A possible solution for
these problems may be to close the sternum with polymer
cables. Such cables may be less sensitive to fatigue and
they are extremely flexible, thus a very homogeneous
load distribution on the sternum may be expected. Con-
sidering the occurring loads, it is clear that the tensile
strength of the polymer cable must about equalize the
strength of the steel wire. Cables made from gel-spun
UHMWPE do indeed offer that high tensile strength.
Moreover, gel-spun UHMWPE fibers are very resistant
to fatigue. Dyneema Purity® was chosen for the present
investigation. The material has already been implanted
successfully in humans [4] and animals [5]. Dyneema
Purity® has quickly become the “gold standard” for ro-
tator cuff repair in human shoulders. More information
about the gel-spun Dyneema® fibres is found in other
publications [6,7]. Dyneema Purity® is a specially pro-
duced grade with extreme purity, thus making it suitable
as an implant material. Section 2 describes the investi-
gated materials and the experiments and results. The re-
sults are discussed in Section 3, and some conclusions
are also presented in Section 3.
2. Materials and Experiments
Commercial steel wires are chosen as a representative of
a state of the art sternum closure technique. Ethicon sur-
gical steel M649C monofilaments were chosen. The
measured thickness was about 1 mm. Cables made of
Dyneema Purity® (registered trademark of Royal DSM
N.V.), further denoted as UHMWPE cables, were chosen
as a representative of the polymer solution. These
UHMWPE cables are by far the strongest biocompatible
polymer implant material that is available. The cables
were braided from 16 yarns of 165 dtex (linear density is
16.5 grams per km) Dyneema Purity® UG material. Fig-
ure 2 is a picture of both materials. The dimensions are
comparable. The diameters are about 1 mm.
Testing of both options is done by laying them around
a pair of aluminium blocks that can slide away from each
other. The blocks can be loaded in tension by a pin in a
hole that is present in each block. Figure 3 shows pairs
of blocks surrounded by a steel wire and an UHMWPE
cable. This pair of blocks simulates the split sternum in
such a way that it is possible to measure the response to
mechanical loading of the wires and cables and their con-
nection technique.
Quasi-static tensile tests are performed by mounting
the blocks in a Zwick/Ro ell 1474 tensile testing machine
and subjecting them to a separation displacement. The
displacement rate was 10 mm/minute. Forces and dis-
placements are measured during the tests.

A Comparison between the Mechanical Behaviour of Steel Wires and Ultra High Molecular Weight 1369
Poly Ethylene Cables for Sternum Closure
Figure 2. Photo of a steel wire and a cable made with Dy-
neema Purity® for sternum closure.
Figure 3. Pairs of blocks; left, a pair of blocks in open con-
dition; middle fixated with steel wire; right, fixated with a
cable made with Dyneema Purity®.
The connection of the steel wires was made by one
experienced surgeon, using the state of the art twisting
procedure. The connection of the UHMWPE cables was
done by knotting. The knots were tensioned with a pre-
determined force that was also provided by the tensile
testing machine. Two knotting procedures were investi-
gated.
1) A knot based on a racking hitch, allowing easy ten-
sioning. The knot is found in [8,9], The two cable ends
are passed through the racking hitch and closed with a
half-hitch and tightened with 360N, followed by two
square knot and tightening again with 400N. This will
further be denoted as “stabilized racking hitch”
2) Surgeons knot tightened with 360N, additionally
secured with two square knots tightening again with
400N. The surgical kno t was also made on a doub le wire ,
in order to obtain a comparable situatio n.
The knots that showed the best behaviour were ado-
pted for further testing under fatigue loads. Figures 4
and 5 illustrate both knots.
Fatigue testing is done at two load levels. The pair of
blocks is uploaded to 350N and subsequently unloaded to
35N and further cycled between 35N and 350N for low
load level cyclic fatigue tests. Additional high load level
cyclic fatigue tests were performed between 45N and
450N. Again the load and displacement was continuously
recorded. The fatigue tests are performed in a Zwick/Roell
Z010 tensile testing machine. The maximum displacement
rate in the fatigue tests was 25 mm/minute.
Figure 4. Schematic illustration of the “stabilized racking hitch” knot with security knots.
Figure 5. Schematic illustration of surgeons knot with two square knots.

A Comparison between the Mechanical Behaviour of Steel Wires and Ultra High Molecular Weight 1370 Poly Ethylene Cables for Sternum Closure
Figure 6 shows a load displacement diagram of both
knots in a quasi static tensile test. It indicates that the
stabilized racking hitch provides the best connection.
Moreover, this knot is more practical to make and it is
somewhat smaller. Therefore, this is the knot that is ch o-
sen for further in v e s ti g a t ions.
Figure 7 compares the load displacement diagram of
the metal wire and the stabilized racking hitch in a quasi
static tensile test. Some interesting differences can be
observed:
1) The maximum load level reached with the
UHMWPE solution is larger than that reached with steel
wire.
2) The maximum load of the steel wire specimens is
related to “plastic unwinding” of the wires. The unwind-
ing could be observed during the test. The upper load
region of the UHMWPE solution is less steep than the
Figure 6. Comparison of the simulated sternum force dis-
placement behavior for the two connections with cables
made with Dyneema Purity®.
Figure 7. Comparison of the simulated sternum force dis-
placement behavior for the steel wire connection and the con-
nection with the cable made with Dyneema Purity® with the
stabi liz ed ra ck ing h itch .
initial region. This is due to some knot slipping.
3) The nominal behavior at the onset of loading is
similar for steel and UHMWPE. However, the steel wire
solution shows a more variable load-displacement be-
havior. About half of the steel wire specimens exhibit a
kind of slack.
4) Some sternum opening under load occurs for both
solutions. The opening of the UHMWPE specimens
shows less scatter, especially at onset of loading, and is
on average less than the opening of the steel wire speci-
mens.
Figure 7 gives a good impression of the mechanical
behavior of both sternum closure methods. However, it is
not yet complete. Quasi-static loading in vivo is only
partly relevant. Breathing and coughing represent cyclic
loadings that may cause some sternum opening or frac-
ture. Therefore, some tests were performed under cyclic
loading. Two maximum load levels were chosen, 350N
and 450N. The minimum load level during cycling was
one tenth of the maximum load, so the so called load
ratio was 0.1. The results showed some scatter, so two
typical load displacement diagrams are presented per
material and load level. The results are presented in the
Figures 8-11. The Figures 8 and 9 show typical results
of loading at 350N. Every diagram shows the load dis-
placement excursions for the first 50 cycles, than a set of
cycles at cycle numbers (N) of about 1000 and finally a
set of cycles at N = 5000. The other cycles are not pre-
sented, in order to keep the figures readable. Figure 10
shows two tests on the steel wire connection at a maxi-
mum force of 450N. Figure 11 shows one (median) of
three tests on UHMWPE cables. The scale of Figures 10
and 11 is the same as in Figures 8 and 9. It can be ob-
served by comparing Figures 10 and 11 to 8 and 9, that
the displacements are considerably larger at 450N than at
350N. Moreover, the displacements at 450N tend to be
smaller for the UHMWPE cables than for the steel cables.
Figure 11 shows only one of three experiments in order
to keep the figure clear. Table 1 presents the data of all
cyclic tests.
It can be observed that the “Sternum opening” in-
creases during cycling. This occurs for both materials.
However, the increase becomes smaller at high cycle
numbers, so the behavior can be called stable. The in-
crease of the opening during cycling at maximum load of
350N is on average similar for the steel wires and for the
UHMWPE cables. Steel shows somewhat lower opening
at the maximum level. UHMWPE shows lower opening
at the minimum load level. The displacements at the
450N loading were lower for the UHMWPE cables in all
cases. Apparently, the UHMWPE cables have more sta-
ility at high cyclic load level. b

A Comparison between the Mechanical Behaviour of Steel Wires and Ultra High Molecular Weight 1371
Poly Ethylene Cables for Sternum Closure
Figure 8. Two measurements of the simulated sternum opening under cyclic loading of a steel wire connection, at a maximum
load of 350N.
Figure 9. Two measurements of the simulated sternum opening under cyclic loading of an UHMWPE stabilized racking hitch
connection, at a maximum load of 350N.
Figure 10. Two measurements of the simulated sternum
opening under cyclic loading of a steel wire connection, at a
maximum load of 450N.
Figure 11. A single (median) measurement of the simulated
sternum opening under cyclic loading of a stabilized rack-
ing hitch connection, at a maximum load of 450N.

A Comparison between the Mechanical Behaviour of Steel Wires and Ultra High Molecular Weight 1372 Poly Ethylene Cables for Sternum Closure
3. Discussion and Conclusions
The comparison between steel wires and UHMWPE ca-
bles in a simulated sternum closure test showed some
typical new results. Sternum closure techniques have
been investigated earlier on real human cadaveric ster-
num samples, e.g. by Losanoff et al. [10]. Of course such
tests on a real sternum yield the most relevant informa-
tion regarding the behavior of the total system (sternum
plus connection technique). However, the real sternum
may be subjected to local indentation due to wire or ca-
ble forces. Thus it reacts “soft” and this “softness” may
compensate for small displacement effects in the connec-
tion device. The present hard metal blocks do not show
such compensation effects and the isolated behavior of
the connection can be established more accurately. This
set up sacrifices the realistic simulation of a complete
in-vivo system on behalf of more precise observation of
structural behavior in detail. Indeed it is ob served that the
twisted steel wire connections sometimes show some
“slack”. This slack is different per connection, in spite of
the fact that it can hardly be observed on the specimen
and in spite of the fact that they are all made by one
trained surgeon in the same way at the same time. The
slack may not cause dramatic effects on a real sternum,
due to its “softness”. However, it will be related to a
somewhat decreased average stability. The loading on
the different wire connections of the sternum will be dif-
ferent due to different slack. On the other hand, the
UHMWPE cables did not show any slack. All UHMWPE
cable connections behaved very similar, especially in the
first part of the load-displacement diagram. The experi-
mental scatter at higher load and displacement levels are
about similar for both materials. However, the maximum
load that can be reached with the UHMWPE cable con-
nections is considerably higher. This implies a more fail
safe connection. Assume that a sternum is closed with
five steel wires, fracture of one wire due to whatever
cause will increase the average load on the other wires
with 20%. Of course this additional load will not be
equally distributed, wires adjacent to the broken ones
would even experience more than 20% load increase.
This might cause subsequent failure of other wires.
Moreover, possible initial slack of some steel wires
would decrease uniform loading and thus increase the
problem to some extent. The UHMWPE cable connec-
tions show about the double ultimate strength as com-
Table 1. Simulated sternum opening displacements in various cyclic tests.
Material-Load Min imu m Displ.
At N = 50 [mm] Maximum Displ. At N = 50 [mm] Min imu m Displ. At N = 1000 [mm]Maximum Displ. At N = 1000 [mm]Min imu m Displ. At N = 5000 [mm] Maximum Displ. At N = 5000 [mm]
Steel-350N 0.74 0.86 0.79 0.87 0.79 0.87
,, 0.89 1.05 1.01 1.10 1.01 1.10
Average ST-350 0.87 0.96 0.90 0.99 0.90 0.99
Steel-450N 1.50 1.62 1.54 1.65 1.55 1.65
,, 2.80 3.03 2.93 3.01 2.94 3.03
Average ST-450 2.15 2.32 2.24 2.33 2.25 2.38
UHMWPE-350N 0.29 0.85 0.53 1.00 0.64 1.08
,, 0.28 0.83 0.58 1.06 0.71 1.16
,, 0.70 1.19 0.97 1.40 1.08 1.48
Average PE-350 0.42 0.96 0.69 1.15 0.81 1.24
UHMWPE-450N 0.72 1.35 0.94 1.49 1.05 1.56
,, 1.00 1.64 1.28 1.84 1.41 1.93
,, 1.19 1.84 1.49 2.06 1.62 2.14
Average PE-450 0.97 1.61 1.24 1.80 1.36 1.88

A Comparison between the Mechanical Behaviour of Steel Wires and Ultra High Molecular Weight 1373
Poly Ethylene Cables for Sternum Closure
pared to the steel wires and thus will always resist addi-
tional load s.
The mechanisms that limit further loading are different
for both systems. The twisted steel wires show plastic
un-twisting at maximum load. A significant further load
increase does not occur if the unwinding takes place. A
kind of plastic plateau is observed. Indeed, such a be-
havior is to be expected, because new twists are exposed
to the opening load if previous twists are untwisted. The
new twists show similar mechanical behavior as the pre-
vious twists, so similar loads will occur. Cash a et al. [11]
report that steel wires for sternum closure untwist at 20
kg-force. This is equivalent to 200N. The present results
on the blocks yield about 400N. This is consistent to the
value of 200N from [11], because the present load is car-
ried about equally by the two locations around the blocks,
so the double load is to be expected. It should however
be noted that in several cases some opening of the blocks
with steel wires occurred at this load. Moreover, due to
scatter some specimens never reached this load com-
pletely.
The UHMWPE cable connections are limited by knot
slipping. Of course the nose cannot slip, but, the half
hitch and square knots can slip. Indeed, knots in
UHMWPE cables do slip. This helps tensioning and ex-
plains the good initial behavior, but it also allows some
sternum opening at high loads. Typically knots in
UHMWPE cables tend to slip and tighten further under
increasing loads, thus the increasing opening under in-
creasing load can be understood. Of course, both plastic
untwisting and knot slipping are non reversible processes.
Indeed, the cyclic loading curves in the Figures 8-11 in-
dicate that some opening remains after (almost) unload-
ing.
Cyclic loading causes further opening of the simulated
sternum model. However, the amount of further opening
occurs merely at low cycle numbers. The difference in
opening between 1000 cycles and 5000 cycles is very
small as compared to the opening that is present at 1000
cycles. Moreover, most of the cyclic opening occurred
already at 50 cycles. Assume patients that cough force-
fully on average 3 times a minute during one days (quite
much). The total number of cycles is almost 5000. Con-
sequently, the number of 5000 load cycles of the present
investigation may be considered to be about a realistic
number for an upper design limit. Fracture due to cyclic
fatigue was not observed in the limited number of the
present tests, neither was this targeted for this small
number of tests. The cyclic opening of the simulated
sternum with both materials might not be expected on
forehand. However, cyclic loading of metals that are
(partially) loaded above the elastic limit, so where plas-
ticity occurs, may result in strain softening. That means
that the elastic yield stress decreases during cyclic de-
formation. In this case, this will be accompanied by in-
creasing displacement. The knots in the UHMWPE ca-
bles comprise a complex internal stress system with high
normal stresses due to knot tightening and friction shear
stresses that provide the holding power of the knot. Cy-
clic loading will influence the normal stresses in the knot
and consequently also influences the related friction
stresses. The cyclic variation of the friction stresses will
be related to some microscopic displacement under fric-
tion in the knot. The total knot slip displacement will of
course be in the direction of the average load. Fortu-
nately, also this complex mechanical system in the knot
tends to be stable. The further displacements decrease
strongly with increasing cycle number.
Summarizing, it can be stated that UHMWPE cables
are an attractive alternative for steel wires in sternum
closure. The present investigation demonstrated that they
provide a strong connection of the sternum parts. They
are flexible and highly biocompatible and it may be ex-
pected that they will hardly cause local pressure on the
sternum bone. Handling in the surgery room is simple
and they can be made from commercially provided
Dyneema Purity®. The present investigation shows that
the mechanical behaviour is attractive as compared to
state of the art steel wire methods. The initial load dis-
placement behaviour is similar to that of steel wires, but
the statistical scatter is lower. The behaviour is more
reproducible. The critical load level is much higher than
for steel wires, so the UHMWPE cables connections are
more redundant than steel and will provide more safety
against fracture. The simulated sternum opening behavior
under cyclic loading of the UHMWPE cables is similar
to that of steel. At high load levels it may even be better.
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