Experimental study on stability of a high-porosity expanded polytetrafluoroethylene graft in dogs.

Abstract

The purpose of the present study was to evaluate the stability of a high-porosity expanded polytetrafluoroethylene (ePTFE) graft, which has been shown to possess excellent biocompatibility and tissue integration.
The graft used in the present study was a high-porosity ePTFE graft , which had an average internodal distance of approximately 60 microm and a random node architecture with tortuous path channels extending from the outer to the inner surface. Eleven beagle dogs (each group n = 3 or 4) weighing 10-12 kg were used. The graft, with a 6 mm inside diameter and a 30-40 mm length, was implanted into the canine abdominal aorta and retrieved after 2-80 weeks. The deformation of the graft was evaluated by conventional computed tomography (CT). The radial tensile strength, longitudinal tensile strength, and suture retention strength of the graft were measured after 2-80 weeks.
CT studies showed no anastomotic aneurysm or deformation of the graft. Physical tests demonstrated no significant deterioration in suture retention strength, radial tensile strength or longitudinal tensile strength for periods ranging from 2-80 weeks compared to pre-implantation grafts.
The graft possesses adequate stability that ensures safe and effective clinical use.

Full text

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37Ann Thorac Cardiovasc Surg Vol. 12, No. 1 (2006)
Original
Article
Introduction
Polytetrafluoroethylene (PTFE) is a fully fluorinated poly-
mer with a chemical formula (-CF2-CF2). Since a process
to expanded PTFE (ePTFE) was discovered by Oga of
Sumitomo Electric Industries while conducting experi-
ments on wire coating,1) the technique was ultimately re-
fined and applied to the development of a viable vascular
prosthesis. In 1975, the first ePTFE graft was introduced
to the market by two manufacturers, Impra and W.L. Gore
and Associates. Early observation of aneurysmal change
of ePTFE grafts led manufacturers to add a thin outer
reinforcing wrap of porous fibrous PTFE or sinter on the
outer surface.2)
The biological behavior of ePTFE grafts is largely in-
fluenced by their physical dimensions, which can be con-
trolled by their manufacturing process. The physical struc-
ture of ePTFE grafts comprises a microporous framework
of solid materials (nodes) connected by fine fibers (fibrils).
Several investigators were able to demonstrate improved
graft healing by making the graft more porous.3-7) How-
ever, in such experiments, high-porosity ePTFE grafts
lacked an outer wrap and were not clinically available
Experimental Study on Stability of a High-porosity
Expanded Polytetrafluoroethylene Graft in Dogs
Mitsuhiro Isaka, DVM, PhD,1 Toshiya Nishibe, MD, PhD,1 Yasuhiro Okuda, MS,3
Masaru Saito, DVM,2 Takahiro Seno, DVM, PhD,2 Kazuto Yamashita, DVM, PhD,2
Yasuharu Izumisawa, DVM, PhD,2 Tadao Kotani, DVM, PhD,2
and Keishu Yasuda, MD, PhD1
From 1Department of Cardiovascular Surgery, Hokkaido University
School of Medicine, Sapporo, 2Department of First Veterinary
Surgery, Rakuno-gakuen University, Ebetsu, and 3Biomedical
Research & Development Department, Sumitomo Electric Industries,
Co., Ltd., Osaka, Japan
Received August 9, 2005; accepted for publication September 1,
2005.
Address reprint requests to Mitsuhiro Isaka, DVM, PhD: Department
of Cardiovascular Surgery, Hokkaido University School of Medicine,
N14W5, Kita-ku, Sapporo, Hokkaido 060-8648, Japan.
Purpose: The purpose of the present study was to evaluate the stability of a high-porosity ex-
panded polytetrafluoroethylene (ePTFE) graft, which has been shown to possess excellent
biocompatibility and tissue integration.
Methods: The graft used in the present study was a high-porosity ePTFE graft , which had an
average internodal distance of approximately 60 μm and a random node architecture with tor-
tuous path channels extending from the outer to the inner surface. Eleven beagle dogs (each
group n=3 or 4) weighing 10-12 kg were used. The graft, with a 6 mm inside diameter and a 30-
40 mm length, was implanted into the canine abdominal aorta and retrieved after 2-80 weeks.
The deformation of the graft was evaluated by conventional computed tomography (CT). The
radial tensile strength, longitudinal tensile strength, and suture retention strength of the graft
were measured after 2-80 weeks.
Results: CT studies showed no anastomotic aneurysm or deformation of the graft. Physical
tests demonstrated no significant deterioration in suture retention strength, radial tensile strength
or longitudinal tensile strength for periods ranging from 2-80 weeks compared to pre-implantation
grafts.
Conclusion: The graft possesses adequate stability that ensures safe and effective clinical use.
(Ann Thorac Cardiovasc Surg 2006; 12: 37–41)
Key words: stability, high-porosity, expanded polytetrafluoroethylene

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Isaka et al.
Ann Thorac Cardiovasc Surg Vol. 12, No. 1 (2006)
because of their physical fragility.
Recently, we developed a new high-porosity ePTFE
graft with unique node-fibril morphology that possesses
excellent biocompatibility and tissue integration.8) The
purpose of the present study was to evaluate the stability
of the graft using a canine abdominal aorta replacement
model.
Material and Methods
Vascular grafts
The graft used in the present study was a high-porosity
ePTFE graft with unique node-fibril morphology. The
graft had an average internodal distance of approximately
60 μm and a random node architecture with tortuous path
channels extending from the outer to the inner surface
(Fig. 1).8) The graft had a 6 mm inside diameter and was
30-40 mm long. The graft was reinforced by a fluoro-
ethylenepropylene filament.
Animal models
Eleven beagle dogs of either sex weighing 10-12 kg were
used. The dog was chosen as a test animal because it has
been reported to approximate the human in terms of vas-
cular graft healing.9) All animals have received humane
care in compliance with the “Principles of Laboratory
Animal Care” formulated by the National Society for
Medical Research and the “Guide for the Care and Use
of Laboratory Animals” prepared by the Institute of Labo-
ratory Animal Resources and published by the National
Institutes of Health (NIH Publication No. 86-23, revised
1985).
The animals were anesthetized with an intravenous
injection of butruphanol (0.025 mg/kg), midazoram (0.3
mg/kg), and ketamine (5 mg/kg), and were endotracheally
intubated. Anesthesia was then maintained with 1% to
2% sevoflulene mixed with nitrous oxide and oxygen. A
midline abdominal incision was made, and the abdomi-
nal aorta below the renal artery was identified and iso-
lated from the surrounding structures. Heparin (150 IU/
kg) was given intravenously, proximal and distal control
of the abdominal aorta was obtained, and a 1-2-cm
section of the abdominal aorta was excised. The grafts
were interposed between the edges of the abdominal aorta,
using a continuous 5-0 monofilament polypropylene.
After complete hemostasis was secured, the abdomen was
closed layer to layer. Ampicillin (50 mg/kg) was given
intravenously before the operation. No anticoagulant or
antiplatelet drugs were given postoperatively. The grafts
were harvested at intervals of 2 weeks (n=4), 4 weeks
(n=4) and 80 weeks (n=3). The animals were anesthetised
and computed tomography (CT) was performed. The ab-
domen was then reopened. The entire graft with a portion
of the host artery was removed under systemic heparin-
ization (150 IU/kg). The specimen was fixed with 10%
buffered formalin for 2 weeks. Finally, the animals were
sacrificed by a sufficient dose of potassium chloride.
Computed tomography (CT)
The deformation of the graft was evaluated by conven-
AB
Fig. 1. Scanning electron micrographs of a high-porosity ePTFE graft with unique node-fibril morphology.
A: Longitudinal section.
B: Inner surface.

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Experimental Study on Stability of a High-porosity Expanded Polytetrafluoroethylene Graft in Dogs
Ann Thorac Cardiovasc Surg Vol. 12, No. 1 (2006)
tional CT. The CT was performed under the condition of
120 kV and 100 mA. The coronal sections were 1 mm
thick. The maximum and minimum diameters of the graft
were measured at the midpoint of the graft. The defor-
mity ratio (%) was calculated as a percentage determined
by dividing the maximum diameter into the minimum
diameter.
Physical properties
The radial tensile strength, longitudinal tensile
strength, and suture retention strength of the graft were
evaluated according to test methods similar to those
described by McClurken et al.10) The reinforced fila-
ment was stripped from the graft wall, and the graft
was cut into a test strip. To measure radial tensile
strength and longitudinal tensile strength for the graft
wall, a test strip, 1 mm in width and 10 mm in length,
was clamped between a pair of jaw grips and stretched
at a constant rate of 20 mm/minutes. The resulting
force required to maintain this constant crosshead
speed was indicated on a recorder that was synchro-
nized with an Instron tensile strength machine. The
radial and longitudinal tensile strengths were defined
as the force at break (unit: megapascal, Mpa).
To measure suture retention strength for the graft wall,
a test strip, 10 mm in width and 10 mm in length, was
mounted in an Instron tensile testing machine and a 5-0
monofilament suture with a tapered noncutting needle was
passed through one wall. The test was performed in the
longitudinal direction. A 3-mm bite was stressed at a con-
stant rate of 20 mm/minutes. The resulting force was re-
corded on a strip chart recorder with the peak force noted
in g. The suture retention strength was defined as the peak
force at the breaking point.
Statistics
Statistical analysis of distortion ratio, radial tensile
strength, and longitudinal tensile strength was performed
by analysis of variance (ANOVA), followed by the
Bonferroni-Dunn test. The suture retention strength was
analyzed by the Student’s t test. P values <0.05 were con-
sidered significant. All data are expressed as mean ± stan-
dard deviation.
Results
Deformity ratio
No grafts were found to be occluded when the CT was
performed. There were no anastomotic aneurysms or other
complications. The deformity ratio was 100±0% at 2
weeks, 106±7.3% at 4 weeks, and 107±4.1% at 80 weeks.
There was no significant difference in the deformity ratio
over the time course (Fig. 2).
Physical properties
The radial and longitudinal tensile strengths for the graft
wall were 4.3±0.4 Mpa and 17.2±2.6 Mpa at pre-implan-
tation, 4.4±0.4 Mpa and 15.0±1.5 Mpa at 2 weeks, 3.9±0.3
Mpa and 12.2±2.4 Mpa at 4 weeks, and 4.1±0.3 Mpa and
16.0±0.4 Mpa at 80 weeks, respectively. The strength did
Fig. 2. Deformity ratio measured by using CT. There was no
significant difference in the deformity ratio over the time course.
Fig. 3. Radial tensile strength. The strength did not change at all
points as compared to pre-implantation.

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Isaka et al.
Ann Thorac Cardiovasc Surg Vol. 12, No. 1 (2006)
not change at all points as compared to pre-implantation
(Figs. 3 and 4). The suture retention strength for the graft
wall was measured at pre-implantation and 80 weeks. It
was 479.3±51.0 g at pre-implantation and 550.7±81.6 g
at 80 weeks. There was no significant difference in the
suture retention strength between pre-implantation and
80 weeks (Fig. 5).
Comment
There is general agreement about the desirable charac-
teristics of a vessel substitute that contribute to its func-
tion as an optimal vascular graft; it should be biocom-
patible with the host, resistant to infection, easy to steril-
ize and store, available in a variety of sizes, easy to im-
plant, impervious to blood leakage, nonthrombogenic,
compliant, low cost, easy to manufacture and physically
durable.11) The graft should be free from deleterious di-
mensional instability over time, which would result in
significant dilatation, aneurysm formation, rupture, or
excessive elongation that could promote tortuosity, kink-
ing, and eventual thrombosis. The graft should also be
capable of being adequately sterilized without deteriora-
tion and conveniently stored in a sterilized condition for
prolonged periods. Although a variety of synthetic mate-
rials were investigated as possible vascular grafts, includ-
ing Vinyon-N, Nylon, PTFE, Ivalon, Orlon, and polyeth-
ylene terephthalate (Dacron), the fabric materials, except
PTFE and Dacron, lost significant tensile strength fol-
lowing implantation or exhibited other problems for
manufacturing or sterilization. Therefore, in Japan, only
PTFE and Dacron grafts are commercially available as
prosthetic grafts for peripheral vascular surgery.
It has been generally known that ePTFE grafts func-
tion well as vascular grafts. The internodal distance, which
is defined as the average of the spacing between the nodes,
has been widely used to describe the biological behavior
of ePTFE grafts. In 1975, Campbell et al. performed ex-
tensive testing of ePTFE and concluded that an intern-
odal distance of less than 22 μm was optimal.12) They
actually correlated increasing internodal distance with
inferior patency in dogs, which appeared to be due to the
development of a thicker pseudointima. Since then, the
internodal distance in most commercially available grafts
was set in the 17-20-μm range. However, in the 1980-
1990s, several laboratories including ours revisited the
relationship of internodal distance to graft healing.3-7)
ePTFE grafts with greater internodal distance, i.e., high-
porosity ePTFE grafts, reportedly resulted in better graft
healing as compared to standard ePTFE grafts. The large
pore size allows rapid and unencumbered ingrowth of
areolar tissue from the surrounding tissue through the
interstices of the graft, thereby achieving a “healed” graft
with an organized cellular, antithrombotic flow surface.
However, high-porosity ePTFE grafts without an outer
wrap were not clinically available because of their physi-
cal fragility.
Recently, we developed a new high-porosity ePTFE
graft with unique node-fibril morphology.8) The graft had
a random node architecture with tortuous path channels
extending from the outer to the inner surface. Our previ-
Fig. 4. Longitudinal tensile strength. The strength did not change
at all points as compared to pre-implantation.
Fig. 5. Suture retention strength. There was no significant differ-
ence in the suture retention strength between pre-implantation
and 80 weeks.

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Experimental Study on Stability of a High-porosity Expanded Polytetrafluoroethylene Graft in Dogs
Ann Thorac Cardiovasc Surg Vol. 12, No. 1 (2006)
ous study demonstrated that, in the dog carotid interposi-
tion model, the graft exhibited satisfactory biocom-
patibility and tissue integration. The graft allowed ad-
equate biologic communication to occur where cells and
tissues could penetrate and vaginate, resulting in a faster
rate of healing.
The graft exhibited a degree of physical properties simi-
lar to other ePTFE grafts on the market. Some data has
been published indicating that the suture retention
strength, radial tensile strength and longitudinal tensile
strength were 700 g, 5.5 Mpa and 12.0 Mpa for an Impra
graft, 600 g, 9.0 Mpa and 11.8 Mpa for a Gore-Tex graft,
and 520 g, 3.6 Mpa and 12.4 Mpa for a Vitagraft,9) while
479.3 g, 4.3 Mpa and 17.2 Mpa for the graft, respectively.
This result led us to test the stability of the graft, which is
one of the desirable characteristics of an optimal vascu-
lar graft.
There have been several reports indicating that degen-
eration can occur in currently commercially available
ePTFE grafts. Geiger suggested that, 12 months after im-
plantation, the structure of ePTFE grafts was inhomoge-
neous with a widening of micropores, and both com-
pressed and ruptured fibrils.13) Vanmaele et al. also re-
ported loss of mechanical strength in ePTFE grafts within
a week.14) However, the present study demonstrated that,
in a canine abdominal aorta replacement model, the graft
showed no significant deterioration in suture retention
strength, radial tensile strength, or longitudinal tensile
strength for periods ranging from 2-80 weeks. CT stud-
ies as well as macroscopic observation at sacrifice also
demonstrated no anastomotic aneurysm or deformation
of the graft.
In conclusion, the present study demonstrates that the
graft possesses excellent stability. Taken together with
our previous study, it is suggested that the graft be con-
sidered for evaluation in clinical trials.
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