Effects of resterilization on mechanical properties of polypropylene meshes.

Abstract

The re-use of sterile packaged polypropylene meshes in hernia surgery is not recommended by the manufacturers. However, especially in developing and underdeveloped countries, many surgeons are obliged to re-use the mesh pieces after resterilization because of economic problems. The purpose of this study was to determine the effects of ethylene oxide and autoclave resterilization on the mechanical properties of polypropylene meshes.
Repetitive ethylene oxide gas and autoclave sterilizations were applied to polypropylene meshes (Herniamesh S.r.l., San Mauro, Italy) up to 3 times and the effects on the mechanical properties were examined. Gas resterilizations were applied for 4.5 hours at 55 degrees C, whereas for autoclave resterilizations the specimens were kept at 134 degrees C and 3 atm pressure for 64 minutes. Ethylene oxide gas-sterilized samples were labeled as G(n) and autoclave-sterilized samples were labeled as A(n). Effects of the resterilizations on maximum load (Fmax), elongation at maximum load (deltaL), and energy required for complete failure of the specimen (E) were measured.
Fmax in the groups showed no significant differences. DeltaL values of groups A2, A3, and G3 were found to be significantly lower in comparison with the control group, whereas differences between the control group and other groups were not statistically significant. E values of A2 and A3 groups were significantly lower than that in the control group (P < .05), whereas the differences between the control group and other groups were not found to be statistically significant. No significant variations were determined between samples sterilized 1, 2, or 3 times in scanning electron microscopy micrographs, however, small irregularities were observed on autoclaved samples.
Single use of polypropylene meshes is always recommended because of biocompatibility and infection risks. However, if re-use of the open packages is needed, ethylene oxide sterilization is preferred over autoclave sterilization. If ethylene oxide sterilization is not available then 1 cycle of resterilization with an autoclave can be used.

Full text

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Scientific paper
Effects of resterilization on mechanical properties of
polypropylene meshes
Kemal Serbetci, M.D., Ph.D.a,*, Hakan Kulacoglu, M.D., F.A.C.S.b, Ali Onder Devay, M.D.b,
Nesrin Hasirci, B.S., Ms.C., Ph.D.c
aDepartment of Biomedical Engineering, Baskent University, Baglica Kampusu, Eskisehir Yolu 20. km, 06530, Ankara, Turkey
bDepartment of Surgery, Ataturk Teaching and Research Hospital, Ankara, Turkey
cDepartment of Chemistry, Middle East Technical University, Ankara, Turkey
Manuscript received July 20, 2005; revised manuscript November 28, 2005
Abstract
Background: The re-use of sterile packaged polypropylene meshes in hernia surgery is not recommended
by the manufacturers. However, especially in developing and underdeveloped countries, many surgeons
are obliged to re-use the mesh pieces after resterilization because of economic problems. The purpose of
this study was to determine the effects of ethylene oxide and autoclave resterilization on the mechanical
properties of polypropylene meshes.
Methods: Repetitive ethylene oxide gas and autoclave sterilizations were applied to polypropylene
meshes (Herniamesh S.r.l., San Mauro, Italy) up to 3 times and the effects on the mechanical properties
were examined. Gas resterilizations were applied for 4.5 hours at 55°C, whereas for autoclave resteril-
izations the specimens were kept at 134°C and 3 atm pressure for 64 minutes. Ethylene oxide gas–sterilized
samples were labeled as Gnand autoclave-sterilized samples were labeled as An. Effects of the resteril-
izations on maximum load (Fmax), elongation at maximum load (?L), and energy required for complete
failure of the specimen (E) were measured.
Results: Fmax in the groups showed no significant differences. ?L values of groups A2, A3, and G3 were
found to be significantly lower in comparison with the control group, whereas differences between the
control group and other groups were not statistically significant. E values of A2 and A3 groups were
significantly lower than that in the control group (P ? .05), whereas the differences between the control
group and other groups were not found to be statistically significant. No significant variations were
determined between samples sterilized 1, 2, or 3 times in scanning electron microscopy micrographs,
however, small irregularities were observed on autoclaved samples.
Conclusions: Single use of polypropylene meshes is always recommended because of biocompatibility
and infection risks. However, if re-use of the open packages is needed, ethylene oxide sterilization is
preferred over autoclave sterilization. If ethylene oxide sterilization is not available then 1 cycle of
resterilization with an autoclave can be used. © 2006 Excerpta Medica Inc. All rights reserved.
Keywords: Hernia; Mesh; Polypropylene; Sterilization; Autoclave; Ethylene oxide; Re-use
Implantation of prosthetic materials in hernia surgery has
become popular worldwide. Polypropylene meshes, first pro-
posed by Usher et al [1] in the early 1960s, are the preferred
material for prosthetic repair today [2]. It also commonly is
used in many applications such as urinary incontinence slings,
vaginal prolapse suspension, and other soft-tissue surgical
mesh support. In general, the required strength for the meshes
is at least 50 N. Dora et al [3] investigated various materials
used for transvaginal anti-incontinence surgery on rabbits and
examined the time-dependent variations in tensile strength and
stiffness after implantation for up to 12 weeks. They reported
that polypropylene mesh did not differ in tensile strength from
baseline and even gained stiffness in time. Muller et al [4] used
different mesh materials to repair ovine infraspinatus tendons
andreportedthatpolypropylenemeshfailedatameanultimate
tensile strength of 75 ? 14 N.
* Corresponding author. Tel.: ?90-312-234-10-10, ext. 2153; fax:
?90-312-234-10-51.
E-mail address: kemals@baskent.edu.tr
The American Journal of Surgery 194 (2007) 375–379
0002-9610/07/$ – see front matter © 2006 Excerpta Medica Inc. All rights reserved.
doi:10.1016/j.amjsurg.2005.11.018

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These meshes are available commercially in sterile pack-
ages for single use, and re-use of the remaining pieces is not
recommended by the manufacturers: “The Herniamesh range
of hernioplasty products comprises several kinds of surgical
implants in monofilament polypropylene mesh. All the meshes
are ethylene oxide sterilized, disposable and cannot be re-
sterilized. The product is valid for 5 years from the packaging
date (www.herniamesh.com).”
In the product manual, warning 4 states that “If packag-
ing is open or damaged it is no longer considered sterile and
should not be used.” Warning 5 states that “Meshes are
intended for single use only, discard any unused product and
do not resterilize” (Monofilament Polypropylene Surgical
Mesh, Instructions for Use; HerniaMesh S.r.l., San Mauro
T.se (To), Italia).
The main problem when a mesh is unpackaged and then
resterilized is infectious complication risk, however, this
does not seem to be the only concern with the resterilization
procedure. The size of the prosthetic meshes may change
and they may shrink because of repetitive steam or auto-
clave sterilizations. In fact, this problem is known already
by the manufacturers and some of them warn surgeons to
avoid those sterilization techniques and instead to resort to
ethylene-oxide or gamma-ray sterilizations [5].
On the other hand, the resterilization procedure may alter
the mechanical properties of the prosthetic material as a
result of polypropylene macromolecular chain degradation.
Therefore, meshes may become less resistant to pressure
after resterilization. There is at least one known hernia
recurrence case caused by mesh disruption after a prosthetic
material repair with a resterilized mesh piece [6]. Unfortu-
nately, the re-use of mesh pieces after resterilization is a
reality for some surgeons today because of economic prob-
lems, especially in developing and underdeveloped coun-
tries. This study was designed to investigate the influences
of repetitive ethylene oxide and autoclave resterilizations on
the mechanical properties of polypropylene meshes.
Materials and Methods
Mesh
An undyed, knitted, pure polypropylene mesh piece, 30 ?
30cminsize,thatoriginallywassterilizedbythemanufacturer
(Herniamesh S.r.l.), was used.
Gas sterilization
Gas sterilizations were performed in the eto.krt 135 de-
vice (Ekol Medical, Ankara, Turkey). Ethylene oxide gas
was applied to the specimens for 4.5 hours at 55°C for each
sterilization process. After the sterilization phases, aeration
was applied to the samples for 12 hours. For repetitive
sterilizations, the same procedure was performed on the
samples at 1-day intervals. The packed samples were kept
on the shelf at room temperature between the sterilization
phases.
Autoclave sterilization
Autoclave sterilization was performed by using the Trans
T06 Autoclave device (Trans Medical Equipment Inc., An-
kara, Turkey). For each sterilization process, the specimens
were kept at 134°C temperature and 3 atm pressure for 64
minutes. Two or 3 sequential sterilizations were performed
at about 1-day intervals by using the same procedure. The
packed samples were kept on the shelf at room temperature
between the sterilization phases.
Specimen preparation
To avoid differences between mechanical properties of
meshes produced from different batches, only 1 square
polypropylene mesh piece that was 30 ? 30 cm was used. Test
samples were prepared by cutting the mesh into rectangular
specimens that were 100 ? 14 mm. Seven test groups were
arranged. Each group had 7 specimens chosen randomly.
The control group contained original samples that were
not treated with further sterilizations. Groups A1, A2, and
A3 samples all were autoclave resterilized for 1, 2, or 3
times, respectively, and packed separately. For A1 samples,
autoclave sterilization was applied only once and the sam-
ples were packed and kept at room temperature after the
sterilization process until the mechanical tests were com-
pleted. For repetitive sterilization to the A2 and A3 samples
the coverings of these samples were opened after 1 day and
autoclave sterilization was applied for the second time; the
samples were packed separately and kept at room temper-
ature. Coverings of A3 samples were opened after 1 day and
autoclave sterilization was applied for the third time; the
samples were packed and kept under the same conditions as
the other samples.
Groups G1, G2, and G3 were gas sterilized once, twice,
and 3 times, respectively, and the samples were packed
separately after each sterilization process. Gas sterilization
was applied only once to group G1 samples. For groups G2
and G3 samples, the coverings were opened after 1 day,
gas-sterilization was applied to the samples for the second
time, and then the samples were packed separately. For G3
samples the coverings were opened after 1 day and gas
sterilization was applied for the third time. All the packed
samples were stored on a shelf at room temperature until
they were used. Mechanical tests were completed the day
after the resterilization processes were applied. Therefore,
the samples can be considered freshly resterilized during the
application of tension tests.
Mechanical testing
The specimens were tested mechanically by using the
Lloyd LRX5K mechanical testing machine (Lloyd Instru-
ments Limited, Fareham, England). Gage lengths of the
specimens were adjusted to 40 mm. Tensile tests were
performed at a strain rate of 40 mm/min (100% strain). Each
tensile test ended when the specimen tore completely. For
the mesh-structured specimens a solid cross-sectional area
could not be obtained, therefore, tensile strength (force/
area) of the materials could not be calculated.
Maximum load before rupture (Fmax), elongation at
maximum load (?L), and quantity of energy required for
complete failure of the specimens (E) were measured and
calculated to investigate the mechanical characteristics of
specimens. The units are given as Newton, mm, and N.mm
for Fmax, ?L, and E values, respectively.
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Scanning electron microscopy
Topographic images of the meshes were obtained by
scanning electron microscopy (SEM) (JEOL, JSM-6400;
NORAN Instruments, Tokyo, Japan), after coating the sam-
ples with gold under vacuum.
Statistical analysis
Determination of the significance of the differences be-
tween the groups was performed by the Mann-Whitney U
test. All data are reported as means and SDs.
Because the meshes had a woven structure, the area of
the specimens changed considerably because of high elon-
gations resulting from the application of force. Therefore,
an exact cross-sectional area calculation was not possible.
The modulus of elasticity values therefore were unknown.
However, because the dimensions of all specimens were the
same, the slopes of the force/elongation graphs gave at least
a rough idea about the changes in elastic characteristics of
the specimens, and a meaningful comparison between the
samples was possible.
Results
The Fmax values of each group are shown in Table 1.
Although the values in the control group and the other
groups were somewhat different, these differences were not
found to be statistically significant (P ? .05).
The ?L values of each group are shown in Table 1.
Groups A2, A3, and G3 were found to have significantly
lower values in comparison with the control group (P ?
.05), whereas differences between the control group and
other groups were not statistically significant (P ? .05).
The E values of each group are shown in Table 1. The E
values of A2 and A3 groups were significantly lower than
that in the control group (P ? .05). Differences between the
control group and other groups were not found to be statis-
tically significant (P ? .05).
SEM micrographs
Although significant variations were not observed be-
tween samples sterilized 1, 2, or 3 times and the control
samples, small irregularities were observed on autoclaved
samples (Figs. 1 and 2).
Comments
Polypropylene meshes are the preferred material for ten-
sion-free inguinal hernioplasties and ventral hernia repairs
in many centers today [7–10]. These meshes are available in
sterile packages for single use. However, because it is cheaper,
some surgeons prefer cutting 1 large piece of mesh into seve-
ral smaller pieces and to re-use the pieces after resteriliza-
tion. Although it is not recommended by the manufacturers
mainly because of the risk for infection, recent studies have
shown that it also is inconvenient because of another reason:
mesh shrinkage after repetitive sterilizations [5].
Furthermore, at the 1999 Incisional Hernia Symposium
in Aachen, Germany, a central mesh recurrence case caused
by mesh disruption was reported after a prosthetic material
repair with a resterilized mesh piece [6].
Therefore, it should be kept in mind that resterilization
techniques may alter the mechanical properties and strength of
the prosthetic materials and render them less reliable in hernia
repair.
In the present study we found that energy (E) values of
the A2 and A3 groups were significantly lower than values
in the control group. This means that a lesser amount of
energy is needed for mesh failure or disruption after 2 or
more autoclave resterilization procedures, compared with
the original sample. In addition, Fmax values, reflecting the
resistance of the mesh to maximum load before disruption,
Fig. 1. SEM micrographs of polypropylene mesh control group. (A) ?50
(general appearance). (B) ?1500 (fiber surface).
Table 1
Fmax, ?L, and E values of all groups
Sample Fmax, N
?L, mmE, Nmm
Control
A1
A2
A3
G1
G2
G3
62.78 ? 4.14
68.63 ? 5.64
56.82 ? 8.78
56.23 ? 11.73
61.96 ? 10.86
69.34 ? 4.97
59.95 ? 15.26
83.84 ? 6.44
78.10 ? 6.52
73.80 ? 7.15*
72.42 ? 7.13*
74.61 ? 1.36
76.50 ? 5.36
73.73 ? 11.14*
1991 ? 306
2081 ? 287
1563 ? 327*
1556 ? 451*
1888 ? 349
2028 ? 208
1741 ? 663
* P ? .05 in comparison with control group.
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showed a decrease after the second sterilization in the au-
toclaved groups, an increase after the first autoclave steril-
ization and a significant decrease after the second treatment.
This can be explained by the formation of some degree of
cross-linking between the polypropylene macrochains in the
mesh structure. A similar behavior was reported in the study
by Staggers and Margeson [11] even for orthodontic nickel
titanium wires. They found a significant increase in tensile
strength of the wires after 1 resterilization cycle with an
autoclave, but no decrease was recorded after even the fifth
resterilization cycle. On the other hand, it also has been
reported that no changes on surface physical topographies
were observed with SEM for different orthodontic wires
after autoclave sterilization [12].
Although the earlier-described results are given for me-
tallic wires, this might not be the case for plastic polypro-
pylene material. Both gas and autoclave sterilizations may
affect the molecular structure and/or fabric of polypro-
pylene meshes either by forming cross-links between the
polypropylene chains or by breaking them and creating
smaller chains and/or oligomers. SEM micrographs in the
present study showed small irregularities for the autoclaved
meshes, but no abnormalities were seen in the gas steriliza-
tion group. These structural changes may be responsible for
crack initiation before mesh failure.
The only study in the literature of resterilized polypro-
pylene meshes was reported recently by Broll et al [13]
from Lübeck University. In fact, they did not investigate the
mechanical properties of the meshes. However, their exper-
iment studied another important point: fibroblast growth on
the meshes. In their study, polypropylene mesh resterilized
by steam autoclave (only once) inhibited the growth of
fibroblasts significantly. The investigators believed that a
release of toxic substances from the resterilized mesh could
have a negative influence on cell proliferation. Because one
of the main advantages of polypropylene mesh in hernia
surgery is the stimulation of fibroblast growth, we believe
the results of Broll’s study should be considered as a rec-
ommendation against resterilization of the meshes.
SEM micrographs also showed almost no changes in the
physical and topographic shapes of the meshes. Some min-
imal alterations were observed for the samples that were
resterilized by autoclaving.
In light of these results, we could say that polypropylene
meshes become less resistant and less reliable after the
second autoclave sterilization. However, only 1 cycle of
resterilization with an autoclave seems to be acceptable in
respect to mechanical properties. During surgery, one can
cut a large piece of polypropylene mesh into small pieces,
package them separately, and resterilize the pieces once
Fig. 2. SEM micrographs of resterilized polypropylene meshes. (A) G1 ?50 (general appearance). (B) G1 ?1500 (fiber surface). (C) A1 ?50 (general
appearance). (D) A1 ?1500 (fiber surface). (E) G2 ?50 (general appearance). (F) G2 ?1500 (fiber surface). (G) A2 ?50 (general appearance). (H) A2
?1500 (fiber surface). (I) G3 ?50 (general appearance). (J) G3 ?1500 (fiber surface). (K) A3 ?50 (general appearance). (L) A3 ?1500 (fiber surface). G1 ?
gas sterilized once, A1 ? autoclaved once, G2 ? gas sterilized twice, A2 ? autoclaved twice, G3 ? gas sterilized three times, A3 ? autoclaved three times.
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more without any harm. On the other hand, repeated gas
sterilization had almost no adverse effects on the mechan-
ical properties of polypropylene meshes even after 3 con-
secutive cycles.
Conclusions
In summary, ethylene oxide sterilization results in min-
imal changes to the mechanical properties of polypropylene
meshes according to our testing methods. It is preferable to
use autoclave sterilization, which may be used for a single
cycle if gas sterilization is not available. However, our
findings apply only to the mechanical properties of polypro-
pylene meshes and do not apply to the potential issues of
infection or biocompatibility in patients.
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