Evaluation of Dynamic Creep Properties of Surgical Mesh Prostheses—Uniaxial Fatigue

Evaluation of Dynamic Creep Properties of Surgical MeshProstheses—Uniaxial Fatigue

Shiny Velayudhan, Darren Martin, Justin Cooper-White

Australian Institute for Bioengineering and Nanotechnology, The University of Queensland, Brisbane, Australia 4072

Received 9 October 2008; revised 12 January 2009; accepted 2 March 2009Published online 29 May 2009 in Wiley InterScience (www.interscience.wiley.com). DOI: 10.1002/jbm.b.31401

Abstract: In this study, we examine the dynamic creep behavior of four commonly used

commercial hernia meshes (Prolene1, Ultrapro1, Vypro1, and Vypro1II). The meshes,

differing from each other with respect to composition and architecture, were tested under

uniaxial tension at simulated physiological loads and environmental conditions. The changes in

percentage strain elongation, secant modulus, and cyclic energy dissipation over 100,000 cycles

were compared. All of the meshes evaluated were found to be overengineered compared to

physiological-loading criteria and displayed good load-carrying performance. When all

meshes were tested at 378C in physiological saline, they survived 100,000 cycles of sinusoidal

loading without fracture, except Ultrapro1. Interestingly, irrespective of the differences in

structure and composition, all meshes underwent strain-hardening and permanent plastic

deformation. Scanning electron micrographs of the meshes showed evidence of yarn thinning,

decrimping, and fracture. The results of this study suggest that strain-hardening of the meshes

under dynamic loading could be a possible cause for complications related to abdominal

mobility during long-term implantations. ' 2009 Wiley Periodicals, Inc. J Biomed Mater Res Part B:

Appl Biomater 91B: 287–296, 2009

Keywords: dynamic creep; Hernia mesh; hysteresis; simulated physiological conditions

INTRODUCTION

The use of polymer meshes to assist in the repair of hernias

is an accepted procedure worldwide, with over one million

meshes being implanted per year in the United States

alone.1 Hernia meshes function by firmly augmenting the

debilitated area, providing a tension-free repair and facili-

tating the incorporation of fibro-collagenous tissue into the

mesh2,3 resulting in up to 50% lower recurrence rates than

tissue-to-tissue sutured hernia repairs.4–6 However, they are

not free of postoperative complications and the need for

recurrent surgery,7–9 although the reasons as to why one

mesh performs better than another remains contentious.

Conversely, there is abundant evidence indicating a rela-

tionship between postoperative complications and mesh

design,10–13 emphasizing the need for future insight, from

an engineering perspective, of the mesh design and their

functional and mechanical performance under representa-

tive service conditions.14

The functional and mechanical properties of commercial

meshes have been explored by many researchers1,15,16 and

various modifications suggested.17–19 Nonetheless, these

studies have tested the meshes to failure under static condi-

tions only, which is far from physiologically relevant. Fur-

thermore, the effect of strain rate has, in most cases, not

been evaluated and further, the elastic modulus of the

meshes is often only obtained at high stress levels. These

biomaterials are subjected to complex dynamic loading

conditions in a corrosive physiological environment and

hence prediction of their biomechanical performance, based

on the results from static data alone is often ambiguous.

In this study, we investigate the dynamic properties of

four commercial meshes under representative physiological

conditions using both static and dynamic (hysteresis) meth-

odologies. Hysteresis based testing methods have, interest-

ingly, only been sporadically exploited by previous

researchers in the area of biomaterials.20,21 The hysteresis

method is a powerful technique which enables the evalua-

tion of dynamic creep properties by measuring hysteresis

loops under stress-controlled or strain-controlled conditions.

The dynamic moduli, the energy dissipated and extent of

creep are then derived from the hysteresis loops. In particu-

lar, this study utilized single load testing (SLT) to elucidate

the dynamic creep of the meshes. The phase shift between

the stress and strain was minimized and the test was carried

out at low frequencies that coincided with the natural fre-

quencies of the physiologic loads acting on the abdominal

walls in the human body,22 which also avoided hysteretic

heat generation. It is well known that fatigue failure in syn-

thetic polymers may be accelerated in the simultaneous

Correspondence to: J. Cooper-White (e-mail: [email protected])

' 2009 Wiley Periodicals, Inc.

287

presence of stress and liquids or solvents.23–25 All tests

were thus carried out both in air as well as in Phosphate

Buffer Saline (PBS) solution at 378C.

MATERIALS AND METHODS

Four commercially available hernia meshes, namely Pro-

lene1, Ultrapro1, Vypro1, and Vypro1II, were compared

for their performance under quasi-static and dynamic sinu-

soidal loading. All the meshes for this study were procured

from Ethicon1, USA, and tested as received.

Before the long-term dynamic creep tests, quasi-static

tensile data were collected at 238C in air using an Instron

(Model 5584, USA) fitted with a 1 kN load cell (flat rub-

ber-faced jaws) operated at a crosshead speed of 10 mm/

min, up to a maximum load of 100 N, inline with ASTM

5034. The elongations of the meshes were measured in the

machine direction (referred to as warp hereafter) and per-

pendicular to the machine direction (referred to as weft

hereafter). Five specimens measuring 45 mm in length and

30 mm in width were cut in each direction using a sur-

geon’s scalpel to avoid distorting and unraveling the mate-

rial. Separate samples were used for testing warp and weft

directions. These samples were cut randomly from different

parts of the same mesh. The tensile modulus and % strain

of the prosthesis were calculated from the recovered data.

The tensile modulus was calculated from the linear portion

of the stress–strain curve. The % strain represents the strain

at the end of the experiment.

Dynamic creep tests were performed on an EnduraTEC

BioDMA (ELF 3200, Bose, USA) and fitted with a 250-N

load cell. Mesh specimens of dimensions of 30 mm in

length and 20 mm in width were cut along both warp and

weft directions and subjected to force controlled sinusoidal

oscillation at a frequency of 1 Hz for up to 100,000 cycles.

A mean load of 16 N/cm (physiological load)26 was main-

tained through out each experiment. For Vypro1 and Ultra-

pro1 in weft directions, the mean load was reduced to 4

N/cm as the elongation at 16 N/cm surpassed the displace-

ment limits of the BioDMA. The dynamic creep studies

were also carried out in Phosphate Buffer Saline (PBS, pH

5 7.3) at 378C. A force amplitude of 4 N was maintained

throughout the experiment. The secant modulus was calcu-

lated from the slope of the hysteresis loop. The area of the

hystereris loop gave the energy dissipation, and the %

strain was calculated from the initial and final gauge length

of the mesh sample.

Mesh architectures were examined by scanning electron

microscopy (Model JSM 6400, Joel, Japan), both before

and after the dynamic creep tests.

RESULTS

Architecture

Figure 1 shows the architecture of each of the meshes

before testing. The basic structure of the Prolene1 mesh

has been described as single-bar warp knit with tri-cot lap-

ping, wherein two filaments form a yarn.19 Prolene1

belongs to the class of heavy-weight small porous meshes

(pore size \1mm) and is constituted entirely of polypropyl-

ene (PP) fibers 150-lm diameter. Vypro1, Vypro II1, and

Figure 1. Architecture of (a) Prolene1, (b) Vypro1, (c) Vypro II1, and (d) Ultrapro1.

288 VELAYUDHAN, MARTIN, AND COOPER-WHITE

Journal of Biomedical Materials Research Part B: Applied Biomaterials

Ultrapro1 are large porous meshes, with their porosity

increased by up to 500–600% compared to Prolene1

(Vypro1 3–5 mm vs. Prolene1 \1 mm). Vypro1 and

Vypro II1 are made of multifilamentous PP combined with

an absorbable component made of Vicryl (which is primar-

ily polyglycolic acid). Ultrapro1 consists of PP monofila-

ments that are closely tangled with absorbable Monocryl1,

a copolymer of glycolide and epsilon-caprolactone.

Vypro1, Vypro II1, and Ultrapro1 are the members of

new generation lightweight and composite meshes in which

the content of PP has been drastically reduced.

Quasi-Static Tests in Air at 238C

The results from the quasi-static tests are shown in Figure 2.

Both the elastic modulus and the % elongation of all the

meshes were different in warp and weft directions, in agree-

ment with the earlier published studies.15,16,27–29 In all cases,

the modulus value was higher in the warp direction when

compared with the weft. The modulus values of the meshes

were also found to be highly dependent on the mesh archi-

tecture. Of the four meshes investigated, Prolene1 had the

highest modulus, followed by Vypro1, Vypro II1, and

Ultrapro1, respectively.

Dynamic Creep in Air at 238C

Typical hysteresis loops of Prolene1 in warp and weft

directions (in air at 238C) between 500 and 100,000 cycles

are shown in Figure 3. The hysteresis loops were found to

be regular and symmetrical throughout the 100,000 cycles.

Similar loops were generated for Vypro1, Vypro II1, and

Ultrapro1, and, for brevity, the hysteresis loops for Pro-

lene1 are presented here. It can also be seen from the fig-

ure that the hysteresis loops are displaced along the X-axis,

indicating creep in the material. The dynamic secant modu-

lus, cyclic energy dissipated (CED), and the percentage of

elongation of the meshes was thereafter calculated from

each of the series hysteresis loops for each mesh.

The secant moduli for the various meshes in the warp

and weft directions (in air at 238C) plotted against the

number of cycles are given in Figure 4. In general, the

dynamic modulus of all meshes increased with an increase

in the number of cycles, up to 100,000 cycles. The modu-

lus increase was quite rapid in the initial stages of the

experiment, gradually leveling off toward the end of

100,000 cycles. In line with the results from the quasi-static

test, Prolene1 displayed a higher secant modulus in the ini-

tial stages of the experiment. This was followed by Vypro

II1, Vypro1, and Ultrapro1, respectively. However, con-

trary to that observed in the quasi-static tests, the secant

modulii of Vypro II1 overlapped with that of Prolene1 to-

ward the end of 100,000 cycles.

The CED of all the meshes decreased with the increase

in number of cycles (Figure 5). The values of CED for Pro-

lene1, Vypro1, and Vypro II1 were found to lie very

close to each other. Ultrapro1, however, had comparatively

higher values of CED. The dynamic creep curves of all

meshes tested are shown in Figure 6. All the meshes

showed an asymptotic behavior. As noted with the secant

modulus, the creep curves are also dependent on the archi-Figure 2. The (a) modulus and (b) % Strain of the meshes (quasi-static test) tested in air at 238C.

Figure 3. Hysteresis loops of Prolene1 in (a) warp and (b) weft

directions tested in air at 238C.

289EVALUATION OF DYNAMIC CREEP PROPERTIES OF SURGICAL MESH PROSTHESES


tecture of the meshes. The creep curves also illustrate that

all the meshes showed a similar extent of permanent plastic

deformation.

The secant modulus of the meshes observed in warp

and weft directions tested in air at 238C are compared in

Figure 7. As expected, the modulus values of the meshes in

the warp direction were generally higher than that in the

weft direction; Prolene1 showed the highest secant modu-

lus value, which is in line with the results from quasi-static

studies. The values of secant modulus for Vypro1 and

Vypro II1 were found to be similar to that of Prolene1.

Dynamic Creep in Phosphate Buffer Saline at 378C

To study the dynamic creep behavior of the meshes under

physiologically relevant conditions, samples were immersed

in PBS at 378C for a maximum of 100,000 cycles. Hystere-

sis loops after a varying number of cycles, depending on

how long each mesh could withstand the test method, were

collected for all the meshes in warp and weft directions in

PBS at 378C (data not shown for brevity). The hysteresis

loops generated were apparently regular and symmetrical

throughout the course of each of the experiments and were

very similar to those observed at 238C in air. However,

surprisingly, for Prolene1 and Vypro II1 meshes, experi-

ments could not be performed beyond 76,000 and 64,000

cycles, respectively (in the warp direction), as the creep

observed in the meshes surpassed the displacement limits

of the machine, which is due to higher mesh elongation at

elevated temperature (378C). Furthermore, when tested in

Figure 6. Variation of creep curves for various meshes in (a) warp

and (b) weft directions tested in air at 238C.

Figure 4. Variation of secant modulus of various meshes in (a) warp

and (b) weft directions with the number of cycles tested in air at238C.

Figure 5. Variation of cyclic energy dissipation of various meshes in

(a) warp and (b) weft directions with the number of cycles tested inair at 238C.

Figure 7. Comparison of secant modulus of various meshes in

warp and weft directions tested in air at 238C. *The tests werecarried out at a mean load of 4N/cm.



the weft direction, the total creep in Prolene1 exceeded the

displacement limits of the machine at the end of only 32,000

cycles, whilst Vypro II1, on the other hand, fractured at the

end of only 16,000 cycles in the weft direction. Contrary to

all expectations, Ultrapro1 sustained fracture at the end of

20,000 cycles in both warp and weft directions.

As with the dry tests, the secant moduli, CED and %

strain of all meshes were calculated from these hysteresis

loops (see Figures 8–10). Except for the Ultrapro1, the se-

cant moduli of all the meshes, in both warp and weft direc-

tions increased steadily, leveling off toward the end of the

pertinent number of cyclic loads. In the case of Ultrapro1,

after an initial increase, the secant modulus was found to

decrease with the increasing number of cycles.

Each mesh was also tested at 238C in PBS to separate

the contribution of the liquid environment from that of the

increased temperature. The secant modulus calculated from

the hysteresis loops showed that all the meshes had slightly

higher values when compared with their dry counterparts

(Figure 11). The increase in modulus, however, is not very

significant and indicates that the presence of the PBS alone

does not affect the creep behavior of these meshes. This

observation is in line with that reported by El and Alt-

stadt.21 These investigators studied the fatigue behavior of

multiblock thermoplastic elastomers at various temperatures

and environmental conditions and reported that the hydro-

static forces imposed by the liquid surrounding the elasto-

mers did not affect their creeping behavior significantly.

Figure 11 also shows that the dynamic moduli values for

each mesh when tested in air at 238C were always higher

than when tested in PBS at 378C, as expected.

Fatigue Damage

Figures 12 and 13 show the photomicrographs of the

meshes following dynamic creep tests in air at 238C and in

PBS at 378C, respectively. Except for Ultrapro1 in PBS at

378C, all mesh samples withstood the experimental condi-

tions without any fiber fracture. However, all meshes suf-

fered significant changes with regard to their initial

structure following dynamic creep. The figures show that

Prolene1 was highly distorted postcreep in both wet and

Figure 8. Variation of secant modulus of various meshes in (a) warp

and (b) weft directions with the number of cycles tested in PBS at378C.

Figure 9. Variation of cyclic energy dissipation of various meshes in

(a) warp and (b) weft directions with the number of cycles tested in

PBS at 378C.

Figure 10. Dynamic creep curves of various meshes in (a) warpand (b) weft directions tested in PBS at 378C.



dry conditions. It is clear that after the initial loading, the

yarns underwent decrimping, followed by yarn elongation,

leading to a decrease in the yarn diameter. The PP mesh

obviously underwent significant plastic deformation.

The images of Vypro1 and Vypro II1 showed that

these meshes did not sustain significant changes in structure

postcreep. However, there had been a change in the aspect

ratio of the pores, with elongation in the direction of the

applied load.

Ultrapro1 did not undergo any deterioration with regard

to its structure postcreep under both dry or wet conditions

when tested at 238C. However, the mesh was completely

destroyed when tested at an elevated temperature of 378Cin an ionic environment. The yarns were untangled, and the

degradable moiety was missing in several locations of the

tested mesh. The remaining yarns (mostly PP) also seem to

have undergone ductile fracture.

DISCUSSION

Despite their widespread acceptance, hernia mesh implants

have been associated with many functional disturbances

(e.g., restricted abdominal wall mobility) during long-term

implantation. In this study, the functional performance of a

range of commercially available hernia meshes has been

evaluated under dynamic mechanical stresses in simulated

physiological conditions. In doing so, we aimed to provide

insight into how the dynamic loadings experienced by these

meshes in vivo may affect their long-term performance.

Effect of Architecture on Mechanical Performanceof the Meshes

Four strategies namely (a) quasi-static analysis at 238C in

air, (b) dynamic creep performance at 238C in air, (c)

dynamic creep performance at 238C in PBS, and (d)

dynamic creep performance at 378C in PBS were adopted

to evaluate the mechanical performance of the meshes. The

results from these four strategies confirmed that all the four

meshes displayed significant anisotropy—the properties

along the warp direction being always higher than the weft.

This can be explained by taking the mechanics of fabric

deformation into account. When a fabric or mesh is sub-

jected to tensile loading (static or cyclic), a change occurs

both with respect to its functional and morphological prop-

erties. Generally, the hierarchy of deformation modes of a

mesh under tension can be divided into two: macrolevel

fabric deformation modes and microlevel fabric deforma-

tion modes, each of which contains a number of different

Figure 11. Comparison of secant modulus of various meshes

tested in air at 238C, in PBS at 238C and in PBS at 378C.

Figure 12. Photomicrographs of (a) Prolene1, (b) Vypro1, (c) Vypro II1, and (d) Ultrapro1 post creep in air at 238C.



mechanisms.30 For knitted meshes, intrayarn or interfiber

friction is most influential at the initial stages of deforma-

tion. Intrayarn slip is where the continuous fibers within the

yarn slide past one another along the length of the fiber

because of changes in fiber curvature during bending and

unbending.31 This gives rise to jamming forces between

neighboring loops in the fabric. As the mesh is extended uni-

axially, the jamming forces between neighboring loops

increase in the direction normal to extension. In walewise

extension, (i.e., extension in the warp direction), the width

jamming yarns come closer together (Figure 14), thereby

increasing the force between the interlacing yarns.32,33 This,

in turn, reduces the tendency for the interlacing yarns to slip.

On the other hand, when uniaxial forces are applied in the

course direction (weft direction), length-jamming yarns

come closer together again, increasing the force between

interlacing yarns. This reduces the tendency for further yarn

slippage and increases the ease of yarn straightening in the

arc of the loop. For this reason, coursewise extensibility is

generally greater than walewise extensibility, which explains

the low modulus and higher elongation of all of the meshes

in the weft direction compared to the warp direction.

The architecture of the meshes also affected their modu-

lus and extent of elongation. It has been documented that

the mechanical properties of knitted fabrics vary with many

variables such as knit architecture, stitch density, pre-

stretching percentage on fabrics, inlay fiber bundles, tow

size of fibers, and so on.2,28,34–37 The four meshes investi-

gated differ from each other with respect to fiber composi-

tion, fiber diameter, and knit architecture; reflected in the

observed values of modulus and % strain. The small pore

size and larger fiber diameter of Prolene1 gives rise to its

higher values of modulus when compared with Vypro1,

Vypro II1, and Ultrapro1. Elastic modulus is a clinically

relevant property and measures the resistance of the mesh

to stretching. It is clear from the % strain results that most

of knitted fabrics are quite resistant to stretch, indicating

that they are able to provide good reinforcement.26 How-

Figure 13. Photomicrographs of (a) Prolene1, (b) Vypro1, (c) Vypro II1, and (d) Ultrapro1 postcreep in PBS at 378C.

Figure 14. Deformation mechanics in fabrics: (a) untensioned fabric(W and L indicate where width and length jamming will occur); (b)

walewise tension (the direction of slippage, labeled DS, puts more

yarn in the stem of the loop, bringing the width-jamming yarnscloser together); (c) coursewise tension (the direction of slippage

puts more yarn in the arc, bringing the length-jamming yarns closer

together) (adapted from Postle R, International Journal of Clothing

Science and Technology 2002;14:257).



ever, this is also a disadvantage because if the elastic mod-

ulus is too high, excessive stress concentrations along the

suture line can lead to complications.26

At this time point, it is worth highlighting the value of

performing dynamic testing. The difference in the modulus

of these meshes obtained from the quasi-static tests, even-

though measured at the physiological loads, varied from

each other by an order of magnitude, yet under dynamic

loading final moduli values were similar. Comparison of

results from the quasi-static and dynamic tests in terms of

absolute values makes little sense, owing to the diverse na-

ture of these experiments. Nevertheless, differences in the

two data sets can be qualitatively compared. First, we can

conclude that the effect of the different architectures sur-

veyed in this study is less evident when the meshes are

tested on a uniaxial tensile dynamic mode (at physiological

loads) than in a quasi-static test. Second, there is a clear in-

dication that the investigation of mechanical properties of

these meshes should include a variety of tests to allow for

the elucidation of their performance under real conditions;

for example, a complex range of dynamic and, to a lesser

extent, static stress situations.

Effect of Cyclic Loading on Mechanical Performanceof Meshes

The secant modulus, cyclic energy dissipation, and dynamic

creep of the meshes were calculated from the hysteresis

loops. Secant modulus (dynamic modulus) is the measure

of stiffness of the meshes subjected to cyclic dynamic load-

ing. Irrespective of the test environments used in all the

four meshes displayed an increase in secant modulus when

subjected to cyclic dynamic loading with exception of

Ultrapro1 (warp and weft) and Vypro1 (weft) when tested

in PBS at 378C. Repetitive mechanical loading on polymers

can lead to stress-induced crystallization (strain-hardening)

and realignment of the polymer chains, producing an

increase in the modulus.35,38,39 However, in the case of fab-

rics or meshes, the increase in modulus also depends on the

extent of fiber slippage that arises from interfiber friction in

the yarn structure.27,35,36,40 When yarn elongation increases,

its diameter decreases leading to an increase in fiber com-

pression. Consequently, fiber–fiber contacts or fiber entan-

glements are increased and the yarn becomes stiffer. After

this stage, the yarn elongation continues to increase by fiber

creep in a time-dependent manner.35 The noted significant

increase in modulus of each of the meshes with increasing

number of cycles represents a clinically relevant result.

Strain hardening is an undesirable property in the context

of hernia meshes and indicates that the meshes will increase

in stiffness over time. This decreases the flexibility of the

implanted mesh; possibly relating to observed complica-

tions such as restriction in the abdominal wall mobility and

long-termed pain and discomfort.32,41

Cyclic energy dissipation is often related to the loss

modulus of a polymer and is directly related to its resist-

ance to fatigue.42 Nasri and Lallam43 compared the loss

modulus and fatigue resistance of two industrial polyamide

fibers and expressed that, for a given level of loading, the

sample with a smaller loss modulus was more resistant to

failure by fatigue, when compared with one with a higher

value of loss modulus. It is also worthy noting that the var-

iation in CED of the meshes varied inversely to the

recorded values of secant moduli; in line with the observa-

tions reported by other researchers.44,45 An increase in

secant modulus and decrease in CED is indicative of

increasing polymer chain orientation, which normally

occurs during the initial stages of a fatigue experiment.

However, when the material undergoes structural deteriora-

tion or damage, the secant modulus drops with a corre-

sponding increase in the CED. Of the four meshes tested,

Ultrapro1 depicted the highest CED and lower secant mod-

uli compared to the other meshes, indicating that Ultrapro1

was more prone to failure by fatigue when compared with

other meshes (discussed in more detail in the section

below). Furthermore, the results from the dynamic creep

curves showed that Ultrapro1 displayed the highest value

of extension, most likely due to the presence of sufficient

space between junction points in the mesh for easier yarn

slippage and movement inside the latter group of fabric

structure.34

In case of Ultrapro1 (warp and weft) and Vypro1

(weft), a decline in the secant modulus was recorded when

tested in PBS at 378C. Deterioration in elastic modulus is a

strong indicator of the damage state of materials.46–48 As

mentioned previously, Ultrapro1 is a composite mesh and

consists of degradable Monocryl fibers interwoven with PP

fibers. Compared to the results obtained in both dry air and

PBS at 238C, the presence of an ionic environment coupled

with a higher temperature has significantly affected the

structural and compositional stability of the Ultrapro1

mesh, likely a result of the degradation or dissolution of

Monocryl fibers [Figure 13(d)] (detailed in the next section).

This significant decrease in the secant moduli for Ultrapro1

was accompanied by an increase in the CED (Figure 10),

whilst the variation in the % strain percentage (Figure 11)

was observed to be similar to that noted in dry air at 238C.

Effect of Testing Environment of MechanicalPerformance of Meshes

The secant modulus of the meshes at 378C in PBS was

lower than that measured at 238C in air. This is expected

as elevated temperatures (378C) aids in increasing polymer

chain mobility, allowing them to slide past each other with

greater ease resulting in a higher elongation of each fiber

for the same load applied, with a subsequent decrease in

the overall stiffness or modulus of the mesh.

Another notable environmental effect was with regard to

the mesh architecture. All meshes suffered significant

changes with regard to their initial structure following

dynamic creep. The micrographs of all meshes postcreep



showed that they underwent diverse degrees of structural

damage depending on their architecture. Prolene1 was

highly distorted postcreep in both wet and dry conditions.

It is clear that after the initial loading, the yarns underwent

decrimping, followed by yarn elongation, leading to a

decrease in the yarn diameter [Figure 12(a)]. The PP mesh

obviously underwent significant plastic deformation.

The images of Vypro1 and Vypro II1 showed that

these meshes did not sustain significant changes in structure

postcreep. However, there had been a change in the aspect

ratio of the pores, with elongation in the direction of the

applied load [see arrows in Figures 12(b,c) and 13(b,c)].

Ultrapro1 did not undergo any deterioration with regard

to its structure postcreep in the dry condition. However, the

mesh was completely destroyed when tested at an elevated

temperature in an ionic environment. The yarns were

untangled [Figure 13(d), arrow 1] and the degradable moi-

ety was missing in several locations of the tested mesh

[Figure 13(d), arrow 2]. The remaining yarns (mostly PP)

also seem to have undergone ductile fracture [Figure 13(d),

arrow 3]. It seems that the presence of an ionic environ-

ment, coupled with a temperature of 378C, equivalent to a

physiological environment, adversely affected the structural

and compositional stability of the component of the yarn,

resulting in them suffering significant degradation. As a

result, the fully applied load was transferred to the remain-

ing nondegradable PP component, leading to their fracture

due to the excessive loading.

CONCLUSIONS

The quasi-static and dynamic creep behavior of four popu-

lar hernia meshes were evaluated in air at room tempera-

ture and at 378C under physiologically relevant conditions.

Despite their different compositions and architecture, all

the four meshes strain-hardened and underwent permanent

plastic deformation; both of which will produce decreases

in mesh flexibility over time. This behavior was exagger-

ated when the meshes were tested at 378C in PBS.

Although a direct relationship between creep in mesh mate-

rials and postoperative complications has not yet been

established, the observed high percentages of irreversible

elongation at physiological loading at physiological envi-

ronmental conditions in three of the four meshes, and fur-

ther the failure of Ultrapro1 after only 16,000 cycles is

certainly worrisome. The strain hardening of these meshes

could be reduced if the design of the mesh was modified to

accommodate a yield strength equivalent to the maximum

abdominal wall force. A mesh with this property should

eradicate long-term complications of abdominal wall stiff-

ness and may thus in some circumstances, subsequently

eliminate the need for a recurrent surgery. Further work is

underway investigating the performance of each of these

meshes within an in vivo rabbit model in an attempt to es-

tablish the relationship between these new in vitro insights

and their in vivo performance.

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Evaluation of Dynamic Creep Properties of Surgical Mesh Prostheses—Uniaxial Fatigue

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