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Soft filamentous woven polyester arterial prosthesis from China
Soft filamentouswoven polyester arterial prosthesis from China* TianJian Rae and Chih Pan Research
Laboratory
of Cardiovascular
Diseases.
Shanghai
Chest
Hospital,
Shanghai.
China
Robert Guidoin,DanielMarceauand Paul-EmileRoy Laboratory
of ExperimentalSurgery,
Lava1 University
and8iomaterials
Institute.
St-Fraqois
d‘Assise
Hospital,
Quebec,
QC, Canada
Martin Kingand Sri@& Badour Clothing
and
Textiles,
University
of Manitoba,
Winnipeg,
Manitoba,
Canada
Jusheng Rui, HaiminChengand CilanLi Faculty
of Biology,
University,
Sample-Making
Factory
of Sozhoo,
10 September
Woven polyester
1989;
arterial
surface
which
external
capsules.
China
China 20
January
polyester
the new prototype
and flexibility. properties
a unique lightweight,
healing was achieved
ingrowth
was
Further
highly porous, satin weave
is needed
before implantation.
and 19 of the 20 grafts
with tightly
This study undertook in comparison
bound capsules,
to evaluate
with
a
four
in which
in 20 dogs for periods ranging from 4 h
was necessary
blood loss was observed,
Rapid full-width
and
Republic of China.
an animal trial was completed
aortic substitute
preclotting
work
softness
and smooth
of their internal
in the Peoples’
and physical
origin. In addition,
as an infrarenal
sacrifice.
observed.
in poor attachment
been developed
porosity,
its structure
permeability,
no significant
been known for their tight construction
and resulted
has recently
grafts of North American
was implanted
of its high water
and installed,
capacity
are its high physical to characterize
to 12 month. The in vitro tests confirmed Because
1990)
have traditionally
their healing
A new woven prototype
of in vitro tests
commercial
Jiangsu,
accepted
prostheses
has limited
Among its unique features series
Shanghai.
Qian and ChenhuaJAI-I
Xiaoping (Received
Fudan
its long-term
construction.
Once preclotted
remained
patent
and more extensive durability
at the tissue
as an arterial
substitute. Keywords:
Polyester.
blood,
vascular
prostheses
With more than 30 years of clinical use, vascular prostheses constructed from polyester fibres have proved to be the most reliable arterial substitutes for the replacement and bypass of large and medium vessels. Over the years, a number of different types have emerged which contain either different textile constructions, such as knitted and woven, or different types of yarn, such as flat or texturized’. The woven construction is preferred for the repair of abdominal aortic *Presented Surgery,
at the UMIST.
Correspondence St-Francols
0
1991
international Manchester,
to
d’Asslse
Dr
Conference UK
1 l-l
R. Guldotn,
Hospital,
Butterworth-Heinemann
on
2 July.
Biomaterials
10 de I’Espinay,
Textiles
in
Medicine
and
1989. Institute,
Quebec.
Room
QC G 1 L 3L5,
FI-304. Canada.
aneurysms2 and for the replacement of the thoracic aorta, because of its greater strength and superior ability to control blood 10~s~.4. On the other hand, the woven prosthesis is more difficult to suture, and suffers from inferior healing5,6 on account of the poor attachment of the internal and external capsules to its smooth fabric surface7. In an attempt to overcome these deficiencies, textile and surgical researchers in China have recently developed a new prototype woven prosthesis. This prosthesis differs from commercial products in that it is soft and more flexible and has a filamentous velour surface. The objective of the current study was to evaluate the healing capacity of this
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Biomaterials
199 1. Vol
12 April
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Polyester arterial prosthesis: T.J. Rae et al.
prototype using an animal model and to compare its in vitro properties with those of other virgin woven prostheses which are commercially available in North America and China.
MATERIALS Selection
AND
METHODS
of prostheses
In addition to the soft, filamentous, prototype prosthesis from China, which was used in both parts of the study, thein vitro evaluation included a regular woven Chinese graft and the following three woven grafts of North American origin: Soft Woven DeBakey (Bard Cardiosurgery Division, CR. Bard, Billerica, MA, USA), Cooley Verisoft and Woven Double Velour grafts (Meadox Medicals, Oakland, NJ, USA). These last three products have received widespread clinical acceptance in Western countries and therefore served as valid controls for comparison purposes.
Nominal linear density of yarns. The size or coarseness of a yarn is measured in units of mass per unit length. The higher the value in decitex (dg/km) the heavier or coarser the yarn. Lengths of yarn exceeding 1 m were removed from both directions of each virgin prosthesis and extended on a crimp tester to remove the residual crimp. Having determined the straightened length, the specimens were weighed on an analytical balance to the nearest 0.01 mg. The linear density of each was calculated and rounded to the nearest nominal value. Waterpermeability. The flow rate of water through a 1 cm* area of the dry prosthetic wall mounted flat with the crimps removed under an applied pressure head of 120 mm Hg is defined as the water permeability of the prosthesis. (This term is sometimes incorrectly referred to as ‘water porosity’ in the medical literature.) The volumes of water that flowed through five specimens of each virgin prosthesis were collected and measured during the first 5 min of the standard test method*. The mean values after the first minute were calculated and recorded.
In vitro evaluation Surface morphology. The virgin prostheses were observed by scanning electron microscopy. Small samples were exposed to osmium tetroxide vapours after being mounted on stubs, and coated with gold-palladium to improve their electron conduction before SEM examination in a Jeol JSM35CF scanning electron microscope (Soquelec, Montreal, QC, Canada) at an accelerating voltage of 15 kV. These observations provided information about the type of yarn and construction of the woven fabrics. Details of the weaves have been represented by point and cross-sectional diagrams to show the sequence of yarn interlacements. Woven fabric count. A measure of the tightness of a woven structure is given by its woven fabric count which indicates the frequency of yarns lying in the warp and weft directions. By viewing the prostheses flattened between glass slides through an optical microscope at 20 times magnification, the average number of warp yarns (ends) per cm width and the average number of weft yarns (picks) per cm length were counted. Porosity. The porosity of the virgin prostheses, which refers to the proportion of void space or pores within the wall of the graft compared with its total volumeg, was calculated using the following equation: M 1000 hd,
1
where P is the percentage porosity, M is the mass per unit area of the prosthetic wall (g/m’), h is the thickness of the wall (mm) and df is the density of the polyester fibres (assumed to be 1.38 g/cm3). The values for M and h were determined experimentally using standard test methodssZg. Fibre diameter. Individual fibres were removed from both directions of each virgin prosthesis and the diameter of at least 20 fibres selected at random was measured using an optical microscope at 400 times magnification with a micrometer eyepiece previously calibrated against a stage micrometer. The level of delusterant particles incorporated into the fibres during spinning was also observed using light field microscopy.
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Eiomaterials
199 1, Vol 12 April
Bursting strength. Measurements of bursting strength of the virgin prostheses were undertaken using a HoffmanTurner probe tester in a compression cell mounted on an lnstron Model 1 130 tensile tester*. Five specimens of each prosthesis were mounted one at a time with their crimps removed over the 8.1 mm diameter hole using a rubber O-ring of diameter 12.6 mm. A 6.4 mm diameter cylindrical probe with a hemispherical end was forced through the clamped specimen at a constant rate of 100 mm/min. The mean bursting strength (N) was calculated by averaging the five measured maximum values of force. Level of extractable material. The quantification of extractable finishes and contaminants present on the surface of the virgin prostheses was undertaken using established standard procedures fortextile products”. Approximately 2 g samples of each prosthesis were exposed to a series of multiple quantitative extractions in different solvents involving at least five syphoning cycles of each in a Soxhlet apparatus. The sequence of solvents was as follows: trichloroethylene, ethanol, acetone, water, 0.1 N hydrochloric acid. The mass of each extract was determined gravimetrically following evaporation of the respective solvent to dryness. The level of extractable material was expressed as a percentage of the mass of the original sample.
In vivu evaluation Animal selection and preparation. Twenty healthy mongrel dogs weighing 14-30 kg were selected according to either the Chinese Medical Academy Regulations on Animal Care (Shanghai experiments) or the Regulations of the Canadian Council on Animal Care (Quebec experiments), i.e. only animals having normal haematology and blood biochemistry were selected. Before operation, each animal was starved for 24 h. Anaesthesia was induced with intravenous administration of sodium pentobarbital at a dose of 30 mg/kg. If required, supplemental anaesthesia was assumed by administering fluothane. The abdomen was shaved and the skin cleaned with antiseptics. Throughout the operation, the temperature of each dog was maintained at 37-38°C with a heating blanket.
Polyester
Implantation infrarenal
protocol.
A laparotomy
aorta was dissected
dog was administered
intravenous
heparin
3 min later, the abdominal
et al
graft
from
at a dose of 0.5-
aorta was clamped
aortic clamps and a short segment A sterile filamentous
was preclotted”
and anastomosed
arterial replacement
woven
The diameters
6, 7 or 8 mm depending was then closed
polypropylene
monofilament
was returned
as an
monofilament
of the prosthesis
selected
on the size of the host artery.
The abdomen
Antibiotics
of the
prosthesis
termino-terminally
using a 5-O polypropylene
(Prolene@) suture.
diet.
Rae
and the
aorta was resected.
The animal
TJ.
below the renal arteries to the
with two atraumatic
were
prosthes,s:
A few lumbar arteries were ligatured. The
aortic trifurcation. 1.5 mg/kg;
was performed
artenal
in layers using either
a 2-O
(Prolene@) or a 1-O silk suture.
to its cage and fed an unrestricted
were
administered
daily
for
5 d
post-
operatively. Graft harvesting.
The prescheduled
periods
of implantation
varied from 4 h to 12 month
with sacrifices
4 h, 1 wk, 1,2,3,4,5,6,7,8
and 12 month. At sacrifice,
animal
was
reanaesthetized
heparin
was
administered
grafts were explanted host arteries. with
They
heparinized
Examination healing
(4
intravenously
were
opened
physiologic
of explanted
were
taken
at three
and distal anastomotic solution
histopathology,
fibres
and
Fjgure
1
SEM
photomicrograph
(a, internal;
6, external
of the soft
filamentous
woven
surfaces).
standard
regions and
Following
and
fixation
embedding
using haematoxylin
staining
of
series of
China
or Epon resin, light microscopic
elastic
the extent
was performed.
region of the graft.
routine
The
and rinsed
the following
paraffin for
mg/kg).
To evaluate
buffered
trichrome
and
of the adjoining
neutral
were undertaken
the
room
longitudinally
grafts.
sections
sites: the proxtmal the central
(0.5
after
saline.
prostheses,
examinations
Histological
operating
with a few millimetres
of the explanted
microscopic
in the
occurring
in a
in either
examinations
and eosin staining
Masson’s
or
for collagen,
Verhoeff’s
Gordon-Sweet
stain
Mallory’s stain
for
for
reticular
fibres.
(b)
For scanning
electron
microscopy
were fixed in 2% glutaraldehyde
(SEM),
buffered
cut into small pieces, rinsed and post-fixed tetroxide.
Dehydration
ethanol,
followed
CO, as the transfer
was
observed
(cl
Those
point drying
medium.
The specimens
flow
that
surface
stereomicroscope
exhibited were
after
surface
microscope a
at a
smooth
examined
staining
nitrate and with haematoxylin observation
were then
and the luminal
5CF electron
graded
using liquid
voltage.
specimens
glistening
in
by critical
in a Jeol JSM-3
15 kV accelerating
in osmium
performed
coated with gold-palladium’*
samples
solution and
with
and
with
a light
0.3%
silver
and eosin. This facilitated
of the nuclear contour and cellular detail of
any endothelial-like
cells on the luminal
surface.
RESULTS In vitro Woven
structure.
Chinese seen
graft rather
woven
Chinese
The most unique
is its highly
in Figure
weave models
Figure
evaluation
7. The than
(Figure
graft 2).
structure
the
plain
and the
The
feature
filamentous
of the prototype
surface
incorporates
weave
found
traditional
differences
texture a 4/l
in the North
between
as
satin regular
American the
woven
2
SEM
photomicrograph
of the
6. external
surfaces).
designs
are seen in Figures
point diagrams Chinese three
woven
structure
supposing
Double
control
The point diagram
Cooley
Verisoft
3, 4 and 5, which
and cross-sectional
graft, the Woven
regular
Woven
illustrate
the
views of the filamentous Velour graft and the other
grafts
respectively.
represents
the luminal
it were
fa, internal;
a balanced
&omatenals
199
weave
1. Vol
view of the containing
12 Apnl
337
Polyester arterial prosthesis: T.J. Rao et al.
Figure 4 graft
Figure 3 Point diagram woven graft from China.
and cross-sectional
thickness and porosity were the highest. Indeed, this was the highest porosity value ever measured on a woven polyester prosthesis in our laboratoriesg. In this respect, it closely resembles that of velour knitted structures.
view of soft filamentous
black warp yarns and white weft yarns. The cross-sectional diagrams show how the first 10 warp yarns interlace starting with the extreme left-hand end. The long warp floats on the internal surface of the filamentous Chinese graft are clearly evident in Figure 3. This bears some similarity to the alternating 6/4 satin and l/l plain weave of the Meadox Woven Double Velour graft (Figure 4), except that the latter contains long warp floats on both internal and external surfaces. Tab/e 7 highlights other unique features of this graft, namely its light weight, loose, thick, open and porous structure. With only 45 ends per cm, the structure was not tightly packed. Consequently, the mass per unit area (or weight) was the lowest of all five grafts tested, while the Table 1
Type
Yarn structure. As well as having a satin weave, the Chinese filamentous graft was produced from texturized rather than flat yarns. Both warp and weft yarns were finer than those in the other four grafts, i.e. they had lower linear density (Tab/e 2). At the same time, the filaments that made up the yarn bundle were fewer in number (only 20) and thicker in diameter (20.9 pm) than in the four controls. This resulted in a more loosely packed yarn bundle and a more pronounced velour surface. Physical properties. The bursting strength of the Chinese filamentous prosthesis was considerably lower than that of any of the control grafts (Tab/e 3). Indeed, the value of 7 1 N
Characteristics of the woven fabric structure
of weave
Ends per cm satin
Filamentous
Regular
Soft
Cooley
Meadox
woven
woven
Woven
Verisoft
Woven
Chinese
Chinese
De&key
4/l
l/l
l/l
satin
total per
cm
Mass per unit area Thickness (mm)
plain
plain
Double
l/l
plain
45
plain
Picks
Point diagram and cross-sectional view of Woven Double Velour
(g/m’)
Porosity
(%)
338
Biomaterials
satln
l/l
plain
36 52
58
36
45
49
52
58
72
62
27
32
35
38
154
152
184
200 0.39
78.3
199 1, Vol 12 April
6/4
49
117
0.28 48.3
0.26 57.1
0.27 59.2
Velour
0.32 58.3
+
Polyester
Table 2
Type
artenal
prosthesis-
TJ.
Rao et al
Yarn character/sties
of multlfllament
Fllamentous
Regular
SOfl
Cooley
Meadox
woven
wove”
Woven
Verlsoft
Woven
Chinese
Chmese
DeBakey
texturized
flat
texturlzed
Double
Velour
yam
warp
flat
flat + texturtzed
weft
texturlzed
flat
texturlzed
flat
flat + texturlzed
Nommal
Itnear
density
(dtex)
warp
88
250
170
190
105
weft
88
250
170
100
105+
Filament
+ 120 120
count
warp
20
96
108
108
54
+ 54
weft
20
96
108
54
54
+ 54
Filament
diameter
(,um) 20.9
warp
+ 1.6
15.7
+ 0.5
11.6
+ 1.1
1 0.5
13.0
’ 0.7 + 0.6 1 0.8 1 0.4
13.5 14.4
20.9
weft
+ 2.3
1 1.5
15.9
f
12.5
1.2
13.3
0.5
13.5 14.4
Delusterant
level
warp
semldull
semldull
bright
semldull
weft
semldull
semldull
bright
semldull
semldull semtdull semldull semldull
Table 3
Water
Physical
properties
permeabtllty
Bursting
(ml/mln
cm’)
Regular
Soh
Cooley
Meadox
woven
Woven
Verisoh
Woven
Chmese
Chinese
DeBakey
4200
strength
is outslde knitted
the range normally
prostheses
developed
was considerably
94
-t2
expected
of the strengths
Table 3 also shows
407
for a woven
of the former
216
i
366
+ 8
graft and
the normal
range expected and high water
properties
can be explained
structure,
namely
the
in terms
combination
texturized
yarns
value
for a woven permeability
of the graft’s of a satin
in both
In vim
38
181 211
warp
unique
weave and
and weft
Grafts
prescheduled
periods
the
operation
mentous
were
without
prosthesis
difficulty
encountered
despite
a little bleeding
in the remaining
Chinese
As shown
grafts had higher levels of extractable
five solvents
than the two North
American
This suggests that the manufacturing employed
in China were
the North
American
Table 4
in Table 4, both
Levels
different
medical
of extractable
materials controls
device
prescribed
industry.
dogs for the
to
The
Chinese
install,
and
fila-
the
only
was when one prosthesis
to fray. Haemostasis
was easily achieved
in six cases and marginal
blood loss
14 cases (Tab/e 5). completed
their prescheduled
At the sacrifice,
had been implanted
for 2 month,
Implan-
all grafts but one, were
patent.
in all
tested.
practices and conditions from those
which
easy
in handling
a tendency
in 20
in Table 5. All dogs survived
complications. was
showed
All the animals
material.
276
implanted
indicated
tation period uneventfully. of extractable
I 18 44
395
evaluation
implantations.
directions. Level
+ 21 t 13
Velour
lightweight
permeability
These low strength
use of fine
+ 10
in the 1 960s13.
that the water
Double
+ 11
higher than those of the other prostheses
and in fact IS outside prosthesisg.
+ 300
71
is reminiscent
the
Filamentous woven
by
Pathology
of explanted
microscopic evidence Chinese
grafts.
examinations
of the
rate and
filamentous
The
histological
of the explanted sequence
of the
grafts
and other provided
healing
of the
prosthesis.
material
Fllamentous
Regular
Soft
Cooley
woven
woven
Woven
Verlsoh
Chinese
Chinese
DeBakey
trichloroethylene
0.76
0.70
0.42
ethanol
0.04
0.18
0.05
0.12
acetone
0.09
0.04
0.02
0.00
Loss
in mass
sequent0
(%,) following
extractlon
I”:
water 0.1 Total
N hydrochlortc
acid
0.59
0.09
0.03
0.00
0.00
0.13
0.18
0.00
0.00
1.1 1
1.13
0.49
0.71
Biomatenals
199 1, Vol
12 Apnl
339
Polyester
arterial prosthesis:
T.J. Rao et al.
Figure 6 Photomicrographs of histological sections through explanted graft wall after 2 month of implantation. (a) A compact thrombotic matrix is seen beneath the luminal lining (Massonb trichrome, original magnification x 180). (6) Extensive collagen penetration between polyester filaments (Masson’s tnchrome, original magnification x 280).
After one week, the thickness and cellular concentration of the thrombotic matrix had been reduced by the host’s fibrinolytic reaction. term. One month post-operatively the luminal surface of three of the four dogs appeared smooth and glistening in the region adjacent (6-l 3 mm) to the proximal and distal anastomoses. By 2 month, the smooth and glistening appearance extended almost the entire length of the graft. The original thrombotic matrix was much more compact and was now covered with an inner lining (Figure 6a). Staining with Masson’s trichrome showed extensive infiltration of collagen into the graft wall (Figure 66). Specimens stained with silver nitrate exhibited a Medium
Figure 5 Point diagram and cross-sectional with l/l plain weave.
view of regular woven grafts
term. After only 4 h of implantation, the luminal surface was covered by a thick adherent layer of unorganized thrombotic matrix, incorporating red blood cells and fibrin which penetrated the porous wall of the textile structure.
Short
Table 5
In viva evaluation:
observed haemostasis
and patency Evaluation at harvesting
Scheduled duration of implantation
Number of animals
None
Marginal
Little
Patent
Stenosed
Thrombosed
4h 1 wk 1 month 2 month 4 month 5 month 6 month 7 month 8 month 12 month
2 1 4 3 2 3 2 1 1 1
0 0 0 0 0 0 0 0 0 0
2 1 2 2 1 2 1 1 1 1
0 0 2 1 1 1 1 0 0 0
2 1 4 2 2 2 1 1 1 1
0 0 0 0 0 1 1 0 0 0
0 0 0 1 0 0 0 0 0 0
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Bleeding at implantation
Polyester artenal prosthesIs: TJ Rae et al
polygonal pattern of fusiform cells (Figure 7). SEM observations confirmed the presence of endothelial-like cells in the vicinity of the anastomoses, whereas no evidence of cellular development was found in the central regions of the explants (Figure 8a and b). By one month, the subendothelial layer incorporated a certain degree of vascularization as well as the ingrowth of reticularfibres (Figure 9). Observations of the explanted prostheses after 4- 12 month in vivo showed more extensive tissue infiltration and a smooth and glistening flow surface throughout the length of the graft. Mallory’s trichrome staining confirmed the absence of any remaining thrombotic matrix after 6 month or longer (Figure 70a). Indeed, the subendothelial layer of abundant collagenous fibres showed no evidence of
Long term.
Figure 9 Photomicrograph of histological section through explanted graft wall after 1 month of implantation showing infiltratIon of reticular fibres between polyester filaments (Gordon-Sweet stain, onginal magnification x 180).
tissue degeneration even after 12 month of implantation. Instead, elastic fibres and smooth muscle cells were confirmed with Verhoeff’s staining (Figure 106).
DISCUSSION The earliest synthetic materials used in the successful reconstruction and bypass of peripheral vessels were woven textile fabrics14. While not emanating from careful scientific studies, these early pioneering efforts were serendipitous in Figure 7 Photomicrograph of en facepreparation stainedwith silvernitrate showing the cellular detail of endothelial-like cells on the luminal surface after 2 month in viva (original magnification x280/.
Figure8 SEM photomicrographs of luminal surface of explants after 2 month I” vim (aJ Region nearproximal anastomosis showing endotheliallike cells (original magnification x 18OJ. (bJ Central region of graft showing an absence of cellular growth (original magnification x 18OJ.
Figure 10 Photomicrographs of histological sections through explanted graft wall after 6 month of implantation. faJ Complete resolution of the thrombotic matrix is observed (Mallory’s trichrome, orlgmal magnification xZZOJ. (bJ Elastic fibres laminated between smooth muscle cells were observed in the neomedia (Verhoeff‘s stain, original magnification x280).
&omatenals
199 1, Vol 12 April
341
Polyester
arterial
prosthesis:
TJ.
Rao et al.
developing a functional and durable conduit of polyester (Dacron@) yarns woven in a plain taffeta weave15. What is particularly impressive about this type of prosthesis is that today, 35 years later, it is still being used extensively by vascular surgeons all over the world. In spite of its evident clinical success, the plain woven Dacron graft has three limitations which are related to its particular type of fabric construction, and which have been responsible for certain complications and long-term failures16. These shortcomings are:
01
(ii)
(iii)
The tightness of the woven construction with the yarns jammed so closely together prevents the infiltration of fibrous tissue, hence slowing the rate and limiting the extent of the healing process. The smooth interior flow surface of the woven taffeta construction does not permit strong attachment of the thrombotic matrix or reorganized neointima which have a tendency to detach and form emboli. The tightly woven construction is also responsible for a rigid prosthesis with limited compliance and flexibility. This stiffness impairs the handling characteristics of the graft and makes it more difficult to suture.
The following discussion is an attempt to explain how the recent Chinese polyester arterial prosthesis with its novel filamentous woven design addresses these deficiencies.
Porosity
and the healing
process
It has long been recognized that the rate and extent of tissue incorporation during healing is directly related to the level of physical porosity within the graft wal116-1s. The presence of pores or void space within the wall ensures that the surrounding capsules of connective tissue maintain their viability through capillary regeneration and adequate blood supply. The porosity value of 78.3% for the filamentous Chinese graft is extremely high. In fact, it is the highest value for a woven prosthesis ever measured in our laboratoryg. It is therefore not surprising to find histological evidence for rapid healing with vascularization, proliferation of fibroblasts and infiltration of collagen within the first 2 month of implantation. The presence of elastic fibres and smooth muscle cells is no doubt due to the mechanical stimulation caused by the pulsatile blood flow. Other in vitro and in vivo studies have demonstrated that repeated stretching of arterial smooth muscle cells can initiate the synthesis of elastin’g-2’. While this novel Chinese design appears to enhance the rate of tissue infiltration, no evidence was found in this study to indicate improved endothelial coverage of the luminal surface. This was likely due to the limitation of endothelial cells to regenerate or to the inadequacy of the luminal surface to support the growth and migration of endothelium22,23. The observed cellular growth near the anastomoses was similar to that observed with other polyester (Dacron) vascular prostheses24, and because the origin of the cells and the mechanism which governs their shape and contour is still subject to controversy, we refer to these cells as ‘endothelial-like’ rather than ‘endothelium’25‘27.
Water
permeability
and haemostasis
The creation of a more loosely woven structure by using fewer yarns in the warp direction (ends), a finer yarn (lower
342
Biomaterials
199 1, Vol
12 April
linear density) and weaving a satin rather than a taffeta plain weave, will inevitably lead to a prosthesis with a high water permeability. Indeed, this is the case with the filamentous Chinese graft which has a particularly high water permeability value for a woven prosthesisg. This information in turn alerts the surgeon to the essential need for preclotting before installation. The results from this animal study have demonstrated that this particular prosthesis is easy to preclot, and that despite some blood oozing at the time of implantation, control over blood loss is readily achieved. No evidence of delayed haemorrhaging or transinterstitial bleeding during implantation was observed as has been the experience with certain very porous knitted grafts such as Microknit, which gave a 75% incidence of haemorrhage when implanted for periods of 7-35 d in dogs’*. This is an important observation since the canine model is known to have an active fibrinolytic system and insufficient reserves of fibrinogen to tolerate extended demands during the post-operative period**. Nevertheless, the filamentous Chinese graft appears to have an appropriate construction to ensure long-term haemostasis even though excessive fibrinolytic activity during the first few days after implantation may re-expose the thrombogenic polyester filaments to blood.
Adherence
of thrombotic
matrix
The unique satin weave of the filamentous Chinese graft with its long floats of texturized polyester yarn on the luminal surface provides an attractive rough or undulating profile to ensure the adherence of a thick, uniform and continuous layer of thrombotic matrix. This should be distinguished from the uneven, thick pile surfaces presented by certain knitted velour grafts, which have been observed to generate a very thick and uneven thrombotic matrix which subsequently forms a thick neointima and tends to cause graft stenosis24,2g. Certain knitted velour prostheses have also been known to leave exposed filament loops, or unincorporated ‘cilia’, projecting into the lumen even after several months of implantation3’, 31. Traditional smooth-walled woven polyester prostheses with a taffeta construction have been associated with problems of detachment of the inner and outer capsules in vivo 32-34. The Chinese filamentous graft does not appear prone to such problems. Our pathological results show that a strong and intimate bond is formed between the textile structure and the surrounding tissue capsules.
Handling
and suturability
An inevitable outcome of using a satin weave with fewer interlacements (crossover points between warp and weft yarns) is that the stiffness of the textile structure will be reduced and the bending, shearing and buckling characteristics increased35. This tendency is made more pronounced by using fewer warp yarns lying lengthwise around the circumference of thegraft. Such a loosely constructed weave provides the surgeon with excellent handling properties and a prosthesis that is soft, compliant, flexible and easy to suture.
long-term
durability
in vim
We have previously commented on the need for vascular prostheses to maintain their bursting strength over extended periods of implantation’3, 3o.This is particularly important for polyester prostheses which have been found to lose up to 25% of their initial strength after about 162 month in vivo36.
Polyester
Since
this animal
only
1 yr, additional
using
animal
study
involved
models
commenting
or
on the
filamentous
implantation
longer-term in
vitro
long-term
density
than that found
it produces
strength
Secondly,
with
a few
thick
we suspect
woven
structures,
filaments.
Since contains
the
A.D..
Surg
of this
strength
distance
from
Chinese
10
11
this
that surgeons
and hence
reduce
VA,
CONCLUSION
woven
represents
a new
high
permeability,
water
essential,
yet once
haemostasrs, neointima with
preclotting
installed,
and
external
woven
lightweight
durability
before
capsule
than
additional
On
III textlIes: Amencan
Research
Triangle
Yates,
animal
to assess
16
S.G
grafts’
17
18
long-term
P.. C&B,
vascular
prostheses
19
ACKNOWLEDGEMENTS
Research
Council
Recherche
of Canada
(Universite S.
D. Mattel
and L. Martin
their
gratitude
C. Gosselin, guidance. Ethicon, and
Bourassa,
K.
The
gifts
Hanburys,
The technical t-lorth,
D. Gagnon,
E. Huebner
D.
of surgical
are gratefully
and
21
Medical
Special
de la
assistance
of J.
King.
M.W..
Deterlmg.
and
i.e. sutures from
suitable
D.E
Szllagyl,
from
23
24
Allen
J.H.
Rouleau
C.. Garneau.
Ma/ 2
vast
Eastcott r
P.. Noel.
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Robert Guidoin,DanielMarceauand Paul-EmileRoy Laboratory
of ExperimentalSurgery,
Lava1 University
and8iomaterials
Institute.
St-Fraqois
d‘Assise
Hospital,
Quebec,
QC, Canada
Martin Kingand Sri@& Badour Clothing
and
Textiles,
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of Manitoba,
Winnipeg,
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Jusheng Rui, HaiminChengand CilanLi Faculty
of Biology,
University,
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Factory
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Woven polyester
1989;
arterial
surface
which
external
capsules.
China
China 20
January
polyester
the new prototype
and flexibility. properties
a unique lightweight,
healing was achieved
ingrowth
was
Further
highly porous, satin weave
is needed
before implantation.
and 19 of the 20 grafts
with tightly
This study undertook in comparison
bound capsules,
to evaluate
with
a
four
in which
in 20 dogs for periods ranging from 4 h
was necessary
blood loss was observed,
Rapid full-width
and
Republic of China.
an animal trial was completed
aortic substitute
preclotting
work
softness
and smooth
of their internal
in the Peoples’
and physical
origin. In addition,
as an infrarenal
sacrifice.
observed.
in poor attachment
been developed
porosity,
its structure
permeability,
no significant
been known for their tight construction
and resulted
has recently
grafts of North American
was implanted
of its high water
and installed,
capacity
are its high physical to characterize
to 12 month. The in vitro tests confirmed Because
1990)
have traditionally
their healing
A new woven prototype
of in vitro tests
commercial
Jiangsu,
accepted
prostheses
has limited
Among its unique features series
Shanghai.
Qian and ChenhuaJAI-I
Xiaoping (Received
Fudan
its long-term
construction.
Once preclotted
remained
patent
and more extensive durability
at the tissue
as an arterial
substitute. Keywords:
Polyester.
blood,
vascular
prostheses
With more than 30 years of clinical use, vascular prostheses constructed from polyester fibres have proved to be the most reliable arterial substitutes for the replacement and bypass of large and medium vessels. Over the years, a number of different types have emerged which contain either different textile constructions, such as knitted and woven, or different types of yarn, such as flat or texturized’. The woven construction is preferred for the repair of abdominal aortic *Presented Surgery,
at the UMIST.
Correspondence St-Francols
0
1991
international Manchester,
to
d’Asslse
Dr
Conference UK
1 l-l
R. Guldotn,
Hospital,
Butterworth-Heinemann
on
2 July.
Biomaterials
10 de I’Espinay,
Textiles
in
Medicine
and
1989. Institute,
Quebec.
Room
QC G 1 L 3L5,
FI-304. Canada.
aneurysms2 and for the replacement of the thoracic aorta, because of its greater strength and superior ability to control blood 10~s~.4. On the other hand, the woven prosthesis is more difficult to suture, and suffers from inferior healing5,6 on account of the poor attachment of the internal and external capsules to its smooth fabric surface7. In an attempt to overcome these deficiencies, textile and surgical researchers in China have recently developed a new prototype woven prosthesis. This prosthesis differs from commercial products in that it is soft and more flexible and has a filamentous velour surface. The objective of the current study was to evaluate the healing capacity of this
Ltd. 0142-9612/91/030335-10
Biomaterials
199 1. Vol
12 April
335
Polyester arterial prosthesis: T.J. Rae et al.
prototype using an animal model and to compare its in vitro properties with those of other virgin woven prostheses which are commercially available in North America and China.
MATERIALS Selection
AND
METHODS
of prostheses
In addition to the soft, filamentous, prototype prosthesis from China, which was used in both parts of the study, thein vitro evaluation included a regular woven Chinese graft and the following three woven grafts of North American origin: Soft Woven DeBakey (Bard Cardiosurgery Division, CR. Bard, Billerica, MA, USA), Cooley Verisoft and Woven Double Velour grafts (Meadox Medicals, Oakland, NJ, USA). These last three products have received widespread clinical acceptance in Western countries and therefore served as valid controls for comparison purposes.
Nominal linear density of yarns. The size or coarseness of a yarn is measured in units of mass per unit length. The higher the value in decitex (dg/km) the heavier or coarser the yarn. Lengths of yarn exceeding 1 m were removed from both directions of each virgin prosthesis and extended on a crimp tester to remove the residual crimp. Having determined the straightened length, the specimens were weighed on an analytical balance to the nearest 0.01 mg. The linear density of each was calculated and rounded to the nearest nominal value. Waterpermeability. The flow rate of water through a 1 cm* area of the dry prosthetic wall mounted flat with the crimps removed under an applied pressure head of 120 mm Hg is defined as the water permeability of the prosthesis. (This term is sometimes incorrectly referred to as ‘water porosity’ in the medical literature.) The volumes of water that flowed through five specimens of each virgin prosthesis were collected and measured during the first 5 min of the standard test method*. The mean values after the first minute were calculated and recorded.
In vitro evaluation Surface morphology. The virgin prostheses were observed by scanning electron microscopy. Small samples were exposed to osmium tetroxide vapours after being mounted on stubs, and coated with gold-palladium to improve their electron conduction before SEM examination in a Jeol JSM35CF scanning electron microscope (Soquelec, Montreal, QC, Canada) at an accelerating voltage of 15 kV. These observations provided information about the type of yarn and construction of the woven fabrics. Details of the weaves have been represented by point and cross-sectional diagrams to show the sequence of yarn interlacements. Woven fabric count. A measure of the tightness of a woven structure is given by its woven fabric count which indicates the frequency of yarns lying in the warp and weft directions. By viewing the prostheses flattened between glass slides through an optical microscope at 20 times magnification, the average number of warp yarns (ends) per cm width and the average number of weft yarns (picks) per cm length were counted. Porosity. The porosity of the virgin prostheses, which refers to the proportion of void space or pores within the wall of the graft compared with its total volumeg, was calculated using the following equation: M 1000 hd,
1
where P is the percentage porosity, M is the mass per unit area of the prosthetic wall (g/m’), h is the thickness of the wall (mm) and df is the density of the polyester fibres (assumed to be 1.38 g/cm3). The values for M and h were determined experimentally using standard test methodssZg. Fibre diameter. Individual fibres were removed from both directions of each virgin prosthesis and the diameter of at least 20 fibres selected at random was measured using an optical microscope at 400 times magnification with a micrometer eyepiece previously calibrated against a stage micrometer. The level of delusterant particles incorporated into the fibres during spinning was also observed using light field microscopy.
336
Eiomaterials
199 1, Vol 12 April
Bursting strength. Measurements of bursting strength of the virgin prostheses were undertaken using a HoffmanTurner probe tester in a compression cell mounted on an lnstron Model 1 130 tensile tester*. Five specimens of each prosthesis were mounted one at a time with their crimps removed over the 8.1 mm diameter hole using a rubber O-ring of diameter 12.6 mm. A 6.4 mm diameter cylindrical probe with a hemispherical end was forced through the clamped specimen at a constant rate of 100 mm/min. The mean bursting strength (N) was calculated by averaging the five measured maximum values of force. Level of extractable material. The quantification of extractable finishes and contaminants present on the surface of the virgin prostheses was undertaken using established standard procedures fortextile products”. Approximately 2 g samples of each prosthesis were exposed to a series of multiple quantitative extractions in different solvents involving at least five syphoning cycles of each in a Soxhlet apparatus. The sequence of solvents was as follows: trichloroethylene, ethanol, acetone, water, 0.1 N hydrochloric acid. The mass of each extract was determined gravimetrically following evaporation of the respective solvent to dryness. The level of extractable material was expressed as a percentage of the mass of the original sample.
In vivu evaluation Animal selection and preparation. Twenty healthy mongrel dogs weighing 14-30 kg were selected according to either the Chinese Medical Academy Regulations on Animal Care (Shanghai experiments) or the Regulations of the Canadian Council on Animal Care (Quebec experiments), i.e. only animals having normal haematology and blood biochemistry were selected. Before operation, each animal was starved for 24 h. Anaesthesia was induced with intravenous administration of sodium pentobarbital at a dose of 30 mg/kg. If required, supplemental anaesthesia was assumed by administering fluothane. The abdomen was shaved and the skin cleaned with antiseptics. Throughout the operation, the temperature of each dog was maintained at 37-38°C with a heating blanket.
Polyester
Implantation infrarenal
protocol.
A laparotomy
aorta was dissected
dog was administered
intravenous
heparin
3 min later, the abdominal
et al
graft
from
at a dose of 0.5-
aorta was clamped
aortic clamps and a short segment A sterile filamentous
was preclotted”
and anastomosed
arterial replacement
woven
The diameters
6, 7 or 8 mm depending was then closed
polypropylene
monofilament
was returned
as an
monofilament
of the prosthesis
selected
on the size of the host artery.
The abdomen
Antibiotics
of the
prosthesis
termino-terminally
using a 5-O polypropylene
(Prolene@) suture.
diet.
Rae
and the
aorta was resected.
The animal
TJ.
below the renal arteries to the
with two atraumatic
were
prosthes,s:
A few lumbar arteries were ligatured. The
aortic trifurcation. 1.5 mg/kg;
was performed
artenal
in layers using either
a 2-O
(Prolene@) or a 1-O silk suture.
to its cage and fed an unrestricted
were
administered
daily
for
5 d
post-
operatively. Graft harvesting.
The prescheduled
periods
of implantation
varied from 4 h to 12 month
with sacrifices
4 h, 1 wk, 1,2,3,4,5,6,7,8
and 12 month. At sacrifice,
animal
was
reanaesthetized
heparin
was
administered
grafts were explanted host arteries. with
They
heparinized
Examination healing
(4
intravenously
were
opened
physiologic
of explanted
were
taken
at three
and distal anastomotic solution
histopathology,
fibres
and
Fjgure
1
SEM
photomicrograph
(a, internal;
6, external
of the soft
filamentous
woven
surfaces).
standard
regions and
Following
and
fixation
embedding
using haematoxylin
staining
of
series of
China
or Epon resin, light microscopic
elastic
the extent
was performed.
region of the graft.
routine
The
and rinsed
the following
paraffin for
mg/kg).
To evaluate
buffered
trichrome
and
of the adjoining
neutral
were undertaken
the
room
longitudinally
grafts.
sections
sites: the proxtmal the central
(0.5
after
saline.
prostheses,
examinations
Histological
operating
with a few millimetres
of the explanted
microscopic
in the
occurring
in a
in either
examinations
and eosin staining
Masson’s
or
for collagen,
Verhoeff’s
Gordon-Sweet
stain
Mallory’s stain
for
for
reticular
fibres.
(b)
For scanning
electron
microscopy
were fixed in 2% glutaraldehyde
(SEM),
buffered
cut into small pieces, rinsed and post-fixed tetroxide.
Dehydration
ethanol,
followed
CO, as the transfer
was
observed
(cl
Those
point drying
medium.
The specimens
flow
that
surface
stereomicroscope
exhibited were
after
surface
microscope a
at a
smooth
examined
staining
nitrate and with haematoxylin observation
were then
and the luminal
5CF electron
graded
using liquid
voltage.
specimens
glistening
in
by critical
in a Jeol JSM-3
15 kV accelerating
in osmium
performed
coated with gold-palladium’*
samples
solution and
with
and
with
a light
0.3%
silver
and eosin. This facilitated
of the nuclear contour and cellular detail of
any endothelial-like
cells on the luminal
surface.
RESULTS In vitro Woven
structure.
Chinese seen
graft rather
woven
Chinese
The most unique
is its highly
in Figure
weave models
Figure
evaluation
7. The than
(Figure
graft 2).
structure
the
plain
and the
The
feature
filamentous
of the prototype
surface
incorporates
weave
found
traditional
differences
texture a 4/l
in the North
between
as
satin regular
American the
woven
2
SEM
photomicrograph
of the
6. external
surfaces).
designs
are seen in Figures
point diagrams Chinese three
woven
structure
supposing
Double
control
The point diagram
Cooley
Verisoft
3, 4 and 5, which
and cross-sectional
graft, the Woven
regular
Woven
illustrate
the
views of the filamentous Velour graft and the other
grafts
respectively.
represents
the luminal
it were
fa, internal;
a balanced
&omatenals
199
weave
1. Vol
view of the containing
12 Apnl
337
Polyester arterial prosthesis: T.J. Rao et al.
Figure 4 graft
Figure 3 Point diagram woven graft from China.
and cross-sectional
thickness and porosity were the highest. Indeed, this was the highest porosity value ever measured on a woven polyester prosthesis in our laboratoriesg. In this respect, it closely resembles that of velour knitted structures.
view of soft filamentous
black warp yarns and white weft yarns. The cross-sectional diagrams show how the first 10 warp yarns interlace starting with the extreme left-hand end. The long warp floats on the internal surface of the filamentous Chinese graft are clearly evident in Figure 3. This bears some similarity to the alternating 6/4 satin and l/l plain weave of the Meadox Woven Double Velour graft (Figure 4), except that the latter contains long warp floats on both internal and external surfaces. Tab/e 7 highlights other unique features of this graft, namely its light weight, loose, thick, open and porous structure. With only 45 ends per cm, the structure was not tightly packed. Consequently, the mass per unit area (or weight) was the lowest of all five grafts tested, while the Table 1
Type
Yarn structure. As well as having a satin weave, the Chinese filamentous graft was produced from texturized rather than flat yarns. Both warp and weft yarns were finer than those in the other four grafts, i.e. they had lower linear density (Tab/e 2). At the same time, the filaments that made up the yarn bundle were fewer in number (only 20) and thicker in diameter (20.9 pm) than in the four controls. This resulted in a more loosely packed yarn bundle and a more pronounced velour surface. Physical properties. The bursting strength of the Chinese filamentous prosthesis was considerably lower than that of any of the control grafts (Tab/e 3). Indeed, the value of 7 1 N
Characteristics of the woven fabric structure
of weave
Ends per cm satin
Filamentous
Regular
Soft
Cooley
Meadox
woven
woven
Woven
Verisoft
Woven
Chinese
Chinese
De&key
4/l
l/l
l/l
satin
total per
cm
Mass per unit area Thickness (mm)
plain
plain
Double
l/l
plain
45
plain
Picks
Point diagram and cross-sectional view of Woven Double Velour
(g/m’)
Porosity
(%)
338
Biomaterials
satln
l/l
plain
36 52
58
36
45
49
52
58
72
62
27
32
35
38
154
152
184
200 0.39
78.3
199 1, Vol 12 April
6/4
49
117
0.28 48.3
0.26 57.1
0.27 59.2
Velour
0.32 58.3
+
Polyester
Table 2
Type
artenal
prosthesis-
TJ.
Rao et al
Yarn character/sties
of multlfllament
Fllamentous
Regular
SOfl
Cooley
Meadox
woven
wove”
Woven
Verlsoft
Woven
Chinese
Chmese
DeBakey
texturized
flat
texturlzed
Double
Velour
yam
warp
flat
flat + texturtzed
weft
texturlzed
flat
texturlzed
flat
flat + texturlzed
Nommal
Itnear
density
(dtex)
warp
88
250
170
190
105
weft
88
250
170
100
105+
Filament
+ 120 120
count
warp
20
96
108
108
54
+ 54
weft
20
96
108
54
54
+ 54
Filament
diameter
(,um) 20.9
warp
+ 1.6
15.7
+ 0.5
11.6
+ 1.1
1 0.5
13.0
’ 0.7 + 0.6 1 0.8 1 0.4
13.5 14.4
20.9
weft
+ 2.3
1 1.5
15.9
f
12.5
1.2
13.3
0.5
13.5 14.4
Delusterant
level
warp
semldull
semldull
bright
semldull
weft
semldull
semldull
bright
semldull
semldull semtdull semldull semldull
Table 3
Water
Physical
properties
permeabtllty
Bursting
(ml/mln
cm’)
Regular
Soh
Cooley
Meadox
woven
Woven
Verisoh
Woven
Chmese
Chinese
DeBakey
4200
strength
is outslde knitted
the range normally
prostheses
developed
was considerably
94
-t2
expected
of the strengths
Table 3 also shows
407
for a woven
of the former
216
i
366
+ 8
graft and
the normal
range expected and high water
properties
can be explained
structure,
namely
the
in terms
combination
texturized
yarns
value
for a woven permeability
of the graft’s of a satin
in both
In vim
38
181 211
warp
unique
weave and
and weft
Grafts
prescheduled
periods
the
operation
mentous
were
without
prosthesis
difficulty
encountered
despite
a little bleeding
in the remaining
Chinese
As shown
grafts had higher levels of extractable
five solvents
than the two North
American
This suggests that the manufacturing employed
in China were
the North
American
Table 4
in Table 4, both
Levels
different
medical
of extractable
materials controls
device
prescribed
industry.
dogs for the
to
The
Chinese
install,
and
fila-
the
only
was when one prosthesis
to fray. Haemostasis
was easily achieved
in six cases and marginal
blood loss
14 cases (Tab/e 5). completed
their prescheduled
At the sacrifice,
had been implanted
for 2 month,
Implan-
all grafts but one, were
patent.
in all
tested.
practices and conditions from those
which
easy
in handling
a tendency
in 20
in Table 5. All dogs survived
complications. was
showed
All the animals
material.
276
implanted
indicated
tation period uneventfully. of extractable
I 18 44
395
evaluation
implantations.
directions. Level
+ 21 t 13
Velour
lightweight
permeability
These low strength
use of fine
+ 10
in the 1 960s13.
that the water
Double
+ 11
higher than those of the other prostheses
and in fact IS outside prosthesisg.
+ 300
71
is reminiscent
the
Filamentous woven
by
Pathology
of explanted
microscopic evidence Chinese
grafts.
examinations
of the
rate and
filamentous
The
histological
of the explanted sequence
of the
grafts
and other provided
healing
of the
prosthesis.
material
Fllamentous
Regular
Soft
Cooley
woven
woven
Woven
Verlsoh
Chinese
Chinese
DeBakey
trichloroethylene
0.76
0.70
0.42
ethanol
0.04
0.18
0.05
0.12
acetone
0.09
0.04
0.02
0.00
Loss
in mass
sequent0
(%,) following
extractlon
I”:
water 0.1 Total
N hydrochlortc
acid
0.59
0.09
0.03
0.00
0.00
0.13
0.18
0.00
0.00
1.1 1
1.13
0.49
0.71
Biomatenals
199 1, Vol
12 Apnl
339
Polyester
arterial prosthesis:
T.J. Rao et al.
Figure 6 Photomicrographs of histological sections through explanted graft wall after 2 month of implantation. (a) A compact thrombotic matrix is seen beneath the luminal lining (Massonb trichrome, original magnification x 180). (6) Extensive collagen penetration between polyester filaments (Masson’s tnchrome, original magnification x 280).
After one week, the thickness and cellular concentration of the thrombotic matrix had been reduced by the host’s fibrinolytic reaction. term. One month post-operatively the luminal surface of three of the four dogs appeared smooth and glistening in the region adjacent (6-l 3 mm) to the proximal and distal anastomoses. By 2 month, the smooth and glistening appearance extended almost the entire length of the graft. The original thrombotic matrix was much more compact and was now covered with an inner lining (Figure 6a). Staining with Masson’s trichrome showed extensive infiltration of collagen into the graft wall (Figure 66). Specimens stained with silver nitrate exhibited a Medium
Figure 5 Point diagram and cross-sectional with l/l plain weave.
view of regular woven grafts
term. After only 4 h of implantation, the luminal surface was covered by a thick adherent layer of unorganized thrombotic matrix, incorporating red blood cells and fibrin which penetrated the porous wall of the textile structure.
Short
Table 5
In viva evaluation:
observed haemostasis
and patency Evaluation at harvesting
Scheduled duration of implantation
Number of animals
None
Marginal
Little
Patent
Stenosed
Thrombosed
4h 1 wk 1 month 2 month 4 month 5 month 6 month 7 month 8 month 12 month
2 1 4 3 2 3 2 1 1 1
0 0 0 0 0 0 0 0 0 0
2 1 2 2 1 2 1 1 1 1
0 0 2 1 1 1 1 0 0 0
2 1 4 2 2 2 1 1 1 1
0 0 0 0 0 1 1 0 0 0
0 0 0 1 0 0 0 0 0 0
340
Biomaterials
199 1. Vol 12 April
Bleeding at implantation
Polyester artenal prosthesIs: TJ Rae et al
polygonal pattern of fusiform cells (Figure 7). SEM observations confirmed the presence of endothelial-like cells in the vicinity of the anastomoses, whereas no evidence of cellular development was found in the central regions of the explants (Figure 8a and b). By one month, the subendothelial layer incorporated a certain degree of vascularization as well as the ingrowth of reticularfibres (Figure 9). Observations of the explanted prostheses after 4- 12 month in vivo showed more extensive tissue infiltration and a smooth and glistening flow surface throughout the length of the graft. Mallory’s trichrome staining confirmed the absence of any remaining thrombotic matrix after 6 month or longer (Figure 70a). Indeed, the subendothelial layer of abundant collagenous fibres showed no evidence of
Long term.
Figure 9 Photomicrograph of histological section through explanted graft wall after 1 month of implantation showing infiltratIon of reticular fibres between polyester filaments (Gordon-Sweet stain, onginal magnification x 180).
tissue degeneration even after 12 month of implantation. Instead, elastic fibres and smooth muscle cells were confirmed with Verhoeff’s staining (Figure 106).
DISCUSSION The earliest synthetic materials used in the successful reconstruction and bypass of peripheral vessels were woven textile fabrics14. While not emanating from careful scientific studies, these early pioneering efforts were serendipitous in Figure 7 Photomicrograph of en facepreparation stainedwith silvernitrate showing the cellular detail of endothelial-like cells on the luminal surface after 2 month in viva (original magnification x280/.
Figure8 SEM photomicrographs of luminal surface of explants after 2 month I” vim (aJ Region nearproximal anastomosis showing endotheliallike cells (original magnification x 18OJ. (bJ Central region of graft showing an absence of cellular growth (original magnification x 18OJ.
Figure 10 Photomicrographs of histological sections through explanted graft wall after 6 month of implantation. faJ Complete resolution of the thrombotic matrix is observed (Mallory’s trichrome, orlgmal magnification xZZOJ. (bJ Elastic fibres laminated between smooth muscle cells were observed in the neomedia (Verhoeff‘s stain, original magnification x280).
&omatenals
199 1, Vol 12 April
341
Polyester
arterial
prosthesis:
TJ.
Rao et al.
developing a functional and durable conduit of polyester (Dacron@) yarns woven in a plain taffeta weave15. What is particularly impressive about this type of prosthesis is that today, 35 years later, it is still being used extensively by vascular surgeons all over the world. In spite of its evident clinical success, the plain woven Dacron graft has three limitations which are related to its particular type of fabric construction, and which have been responsible for certain complications and long-term failures16. These shortcomings are:
01
(ii)
(iii)
The tightness of the woven construction with the yarns jammed so closely together prevents the infiltration of fibrous tissue, hence slowing the rate and limiting the extent of the healing process. The smooth interior flow surface of the woven taffeta construction does not permit strong attachment of the thrombotic matrix or reorganized neointima which have a tendency to detach and form emboli. The tightly woven construction is also responsible for a rigid prosthesis with limited compliance and flexibility. This stiffness impairs the handling characteristics of the graft and makes it more difficult to suture.
The following discussion is an attempt to explain how the recent Chinese polyester arterial prosthesis with its novel filamentous woven design addresses these deficiencies.
Porosity
and the healing
process
It has long been recognized that the rate and extent of tissue incorporation during healing is directly related to the level of physical porosity within the graft wal116-1s. The presence of pores or void space within the wall ensures that the surrounding capsules of connective tissue maintain their viability through capillary regeneration and adequate blood supply. The porosity value of 78.3% for the filamentous Chinese graft is extremely high. In fact, it is the highest value for a woven prosthesis ever measured in our laboratoryg. It is therefore not surprising to find histological evidence for rapid healing with vascularization, proliferation of fibroblasts and infiltration of collagen within the first 2 month of implantation. The presence of elastic fibres and smooth muscle cells is no doubt due to the mechanical stimulation caused by the pulsatile blood flow. Other in vitro and in vivo studies have demonstrated that repeated stretching of arterial smooth muscle cells can initiate the synthesis of elastin’g-2’. While this novel Chinese design appears to enhance the rate of tissue infiltration, no evidence was found in this study to indicate improved endothelial coverage of the luminal surface. This was likely due to the limitation of endothelial cells to regenerate or to the inadequacy of the luminal surface to support the growth and migration of endothelium22,23. The observed cellular growth near the anastomoses was similar to that observed with other polyester (Dacron) vascular prostheses24, and because the origin of the cells and the mechanism which governs their shape and contour is still subject to controversy, we refer to these cells as ‘endothelial-like’ rather than ‘endothelium’25‘27.
Water
permeability
and haemostasis
The creation of a more loosely woven structure by using fewer yarns in the warp direction (ends), a finer yarn (lower
342
Biomaterials
199 1, Vol
12 April
linear density) and weaving a satin rather than a taffeta plain weave, will inevitably lead to a prosthesis with a high water permeability. Indeed, this is the case with the filamentous Chinese graft which has a particularly high water permeability value for a woven prosthesisg. This information in turn alerts the surgeon to the essential need for preclotting before installation. The results from this animal study have demonstrated that this particular prosthesis is easy to preclot, and that despite some blood oozing at the time of implantation, control over blood loss is readily achieved. No evidence of delayed haemorrhaging or transinterstitial bleeding during implantation was observed as has been the experience with certain very porous knitted grafts such as Microknit, which gave a 75% incidence of haemorrhage when implanted for periods of 7-35 d in dogs’*. This is an important observation since the canine model is known to have an active fibrinolytic system and insufficient reserves of fibrinogen to tolerate extended demands during the post-operative period**. Nevertheless, the filamentous Chinese graft appears to have an appropriate construction to ensure long-term haemostasis even though excessive fibrinolytic activity during the first few days after implantation may re-expose the thrombogenic polyester filaments to blood.
Adherence
of thrombotic
matrix
The unique satin weave of the filamentous Chinese graft with its long floats of texturized polyester yarn on the luminal surface provides an attractive rough or undulating profile to ensure the adherence of a thick, uniform and continuous layer of thrombotic matrix. This should be distinguished from the uneven, thick pile surfaces presented by certain knitted velour grafts, which have been observed to generate a very thick and uneven thrombotic matrix which subsequently forms a thick neointima and tends to cause graft stenosis24,2g. Certain knitted velour prostheses have also been known to leave exposed filament loops, or unincorporated ‘cilia’, projecting into the lumen even after several months of implantation3’, 31. Traditional smooth-walled woven polyester prostheses with a taffeta construction have been associated with problems of detachment of the inner and outer capsules in vivo 32-34. The Chinese filamentous graft does not appear prone to such problems. Our pathological results show that a strong and intimate bond is formed between the textile structure and the surrounding tissue capsules.
Handling
and suturability
An inevitable outcome of using a satin weave with fewer interlacements (crossover points between warp and weft yarns) is that the stiffness of the textile structure will be reduced and the bending, shearing and buckling characteristics increased35. This tendency is made more pronounced by using fewer warp yarns lying lengthwise around the circumference of thegraft. Such a loosely constructed weave provides the surgeon with excellent handling properties and a prosthesis that is soft, compliant, flexible and easy to suture.
long-term
durability
in vim
We have previously commented on the need for vascular prostheses to maintain their bursting strength over extended periods of implantation’3, 3o.This is particularly important for polyester prostheses which have been found to lose up to 25% of their initial strength after about 162 month in vivo36.
Polyester
Since
this animal
only
1 yr, additional
using
animal
study
involved
models
commenting
or
on the
filamentous
implantation
longer-term in
vitro
long-term
density
than that found
it produces
strength
Secondly,
with
a few
thick
we suspect
woven
structures,
filaments.
Since contains
the
A.D..
Surg
of this
strength
distance
from
Chinese
10
11
this
that surgeons
and hence
reduce
VA,
CONCLUSION
woven
represents
a new
high
permeability,
water
essential,
yet once
haemostasrs, neointima with
preclotting
installed,
and
external
woven
lightweight
durability
before
capsule
than
additional
On
III textlIes: Amencan
Research
Triangle
Yates,
animal
to assess
16
S.G
grafts’
17
18
long-term
P.. C&B,
vascular
prostheses
19
ACKNOWLEDGEMENTS
Research
Council
Recherche
of Canada
(Universite S.
D. Mattel
and L. Martin
their
gratitude
C. Gosselin, guidance. Ethicon, and
Bourassa,
K.
The
gifts
Hanburys,
The technical t-lorth,
D. Gagnon,
E. Huebner
D.
of surgical
are gratefully
and
21
Medical
Special
de la
assistance
of J.
King.
M.W..
Deterlmg.
and
i.e. sutures from
suitable
D.E
Szllagyl,
from
23
24
Allen
J.H.
Rouleau
C.. Garneau.
Ma/ 2
vast
Eastcott r
P.. Noel.
H.P
terephtalate 1980,
(Dacron”)
and
Gosseltn. Blats,
comme
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I”
J
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D
L.R., 1978,
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3
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en
H.H.G.,Anappra~saloftheuseandfunct~onofvasculargrahs
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Polyester
26 Maro~s.
la
1
prosthetic
Harrison,
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