Intrinsic Structural Failure of Polyester (Dacron) Vascular Grafts. A General Review

Transcription

Intrinsic Structural Failure of Polyester (Dacron) Vascular Grafts. A General Review
Acta chir belg, 2005, 105, 249-255
Intrinsic Structural Failure of Polyester (Dacron) Vascular Grafts. A General
Review
H. Van Damme*, M. Deprez**, E. Creemers*, R. Limet*
Service de Chirurgie Cardio-Vasculaire et Thoracique*, Département d’Anatomo-Pathologie (Prof Boniver)**, Hôpital
Universitaire de Liège, CHU du Sart-Tilman, Liège, Belgique.
Key words. Blood-vessel prosthesis ; prosthetic failure ; aneurysm, false ; polyethylene terephthalate, Polyester
(Dacron).
Abstract. Intrinsic structural failure of Dacron prostheses is a late exceptional complication, resulting from a loss of
structural integrity of the graft.
The authors report six cases of non-anastomotic false aneurysms in the mid-portion of a vascular Dacron graft, observed
at a mean of 12 years after insertion. It concerns four femoro-popliteal bypass grafts, one cross-over graft and a branch
of a bifurcated aorto-bifemoral graft, implanted between 1980 and 1990. This represents 0.2% of all vascular Dacron
grafts implanted in authors’ department since 1980.
The degenerated prosthesis was excised, and a new bypass graft was inserted. In three cases, histological analysis
revealed a foreign body giant cell reaction against fragmented Dacron fibers. In none of the cases there was evidence of
graft infection.
The authors discuss the evidence and etiopathogeny of this late, unusual complication, inherent to the Dacron graft
material. The most probable causative factor is material fatigue, leading to gradual breakdown and fragmentation of
individual fibers, and subsequent biodegradation of the basic material. Such an intrinsic weakness of prosthetic fabric
has only be observed in first and second generation Dacron grafts.
Introduction
Dacron (polyethylene terephthalate,PET) prostheses
are widely used as a durable and valuable vascular
conduit, since its introduction in 1957 (1). Dacron,
originally a trademark of DuPont(the first manufacturer
of the yarn), has become common use in surgical language, and will be used throughout the text, rather than
polyethylene terephthalate or polyester. Despite the
proven reliability and acknowledged good long term
performance of Dacron prostheses, structural defects,
resulting in graft rupture or false aneurysm formation,
have been sporadically reported (2-22). Such prosthetic
failures have been observed in a variety of knitted
Dacron grafts, manufactured by different companies.
Modifications in the manufacturing process made
Dacron grafts more resistant to cyclic pulsatile stretching and improved integration of the prosthetic material
into the host tissue. Opposingly, only few reports concern Teflon (polytetrafluoroethylene, PTFE) prostheses
(23-26). This graft inherent complication is observed
after seven years or more of implantation. The frequency of this potential complication is very low (0.5%
to 3%) (27).
The etiology of occasional structural textile failure is
supposed to be multifactorial (13, 27). Possible
causative factors, evoked in literature, are manufacturing
flaws (errors in the production process), erroneous storage conditions, inappropriate surgical handling, material fatiguing and biodegradation.
An extensive review of the literature is made,based on
a short series from author’s institution.The series concerns six defective prostheses in which holes and tears
were observed more than 12 years following implantation, leading to huge midgraft pseudoaneurysms and
perigraft haemorrhage.
The authors discuss in detail the supposed mechanisms of loss of integrity of Dacron grafts.
Clinical series
During the last decade, the authors observed six cases of
midgraft, non-anastomotic pseudoaneurysms in Dacron
vascular prostheses (four femoro-popliteal 8 mm grafts,
one 8 mm femoro-femoral cross-over graft and one
aorto-bifemoral bifurcation graft). The patients presented a sudden pulsating, painful, tender mass at mid-thigh
level (four degenerated femoro-popliteal grafts), at
250
Fig. 1
Angiographic image of a degenerated femoropopliteal Dacron
bypass graft. Two non-anastomotic pseudoaneurysms are evidenced at the distal segment of the prosthesis. There is also an
anastomotic pseudoaneurysm at popliteal level.
suprapubic level (one degenerated cross-over graft) and
at the right lower quadrant of the abdomen (a degenerated branch of a bifurcated aorto-bifemoral graft). There
were no signs of focal inflammation. The diagnosis was
readily apparent on clinical examination and on imaging
studies, which revealed a false aneurysm along the
H. Van Damme et al.
course of the graft. In two cases the patient presented
with hemorrage and rupture of the pseudaneurysm,
requiring urgent repair.
The mean delay after implantation was 149 months
(range : 13 to 180 months). The grafts were implanted
between 1980 and 1990. It concerned three Microvel®
Double Velour De Bakey (Meadox Medicals - Oakland,
NJ, USA) prostheses, one Weaveknit Dacron (Vascular,
USCI, United States Catheter and Instrument Company)
prosthesis, one Sauvage Filamentous Vascular (USCI)
prosthesis and one Cooley Double Velour Knitted
Dacron (Meadox Medicals - Oakland, NJ, USA) prosthesis. Two of the defective grafts had been thrombectomised with a fogarty balloon catheter 7 and 8 years
earlier. None of the grafts was externally supported by
rings. The six observations of intrinsic graft failure concern 0.2% of all Dacron grafts implanted at our institution since 1980.The defective graft was partially (n = 2)
or entirely (n = 4) excised, and a new bypass graft was
inserted. All patients had an uneventful postoperative
course. The new prosthetic bypass functioned well for a
mean of 41 months(range : 24 to 108 months).
Gross inspection of the defective graft revealed multiple, separate bleb-like saccular pseudoaneurysms
(mean size : 25 mm) along three of the four explanted
femoro-popliteal grafts, corresponding to distinct fenestration holes inside the graft texture (Figs. 1 and 2). In
the ruptured pseudoaneurysm of a limb of a bifurcation
graft, a vertical rent of 12 mm was evidenced, 4 cm
proximal to the femoral anastomosis, just underneath the
inguinal ligament. This led to subtotal disruption of the
graft and contained rupture. The remainder of the bypass
graft had a normal aspect. The degenerated cross-over
graft was surrounded by an incapsulated old
haematoma, probably due to interstitial blood seeping
throughout the dilated (35% increase in diameter) prosthesis. Two huge pseudoaneurysms of 45 mm and
50 mm diameter were close to both ends of the graft,
where a 4 mm hole was evidenced (Fig. 3). One of the
pseudaneurysms was rapidly expanding, secondary to
contained rupture. The anastomotic sites were intact. In
two of the defective femoro-popliteal grafts, there was a
concomitant,but independent anastomotic pseudoaneurysm, where the suture had pulled out of the fabric.
Release of the suture line was due to unravelling of the
prosthetic textile. Four grafts showed diffuse dilatation
(mean of 28% increase in diameter, compared to manufacturer’s stated size).
Bacteriologic analysis of the retrieved prosthesis was
negative in all cases. Histology was obtained on four
retrieved defective grafts. The wall of the saccular
pseudoaneurysm consisted of dense fibrous tissue, with
complete absence of Dacron fibers. The defect inside the
prosthetic wall contained broken filaments. In three
cases,these prosthetic debris elicited a foreign body
Polyester (Dacron) Vascular Grafts
251
Fig. 3
Two mid-graft pseudoaneurysms in a femoro-femoral crossover Dacron bypass-graft. The left sided pseudoaneurysm
shows a contained rupture.
A
giant cell reaction,characterized by the accumulation of
numerous multinucleated giant cells along the broken
filaments (Fig. 4). Histo-enzymatic analysis revealed a
pronounced activity of acid phosphatase inside the giant
cells, suggestive for active digestion of Dacron fibers.
Discussion
B
Fig. 2
Multiple bleb like pseudoaneurysms along a femoropopliteal
Dacron graft, implanted 12 years ago. Angiographic (2A) and
CT-scan (2B) image.
Intrinsic structural failure of a fabric vascular graft is
defined as the inability of the graft to retain its structural integrity after implantation (13). By definition there is
no history of direct trauma in the area of the graft nor
evidence of prosthetic infection. It is clinically evident
as a pulsating mass along the course of the graft. The
focal ectasia or pseudoaneurysm is secondary to a hole
or tear in the body of the graft, outside the anastomotic
areas. The first published case concerned a woven
femoro-popliteal Dacron graft,and was reported by
Knox in 1962 (9).
A short historical survey of prosthetic vascular conduits is useful (28, 29). The first vascular conduit was
conceived by VOORHEES (30) in 1952, and its textile was
a Vinyon “N” polymer (used in parachute cloth) (Union
252
Fig. 4
Microscopic examination of a retrieved defective graft, H & E
50 (O.M.) and 200 (O.M., insert). The external wall of the
saccular pseudoaneurysm consists of a dence fibrous tissue
(F). Around the lumen (L), broken filaments of the defective
prothese elicit a marked foreign body giant cell reaction
(arrows). Numerous multinucleated giant cells are seen along
the dark-green prosthetic fibres (insert).
Carbide, Danbury, CT, USA). Other materials have been
tested such as Nylon (31) (duPont, Wilmington, DE,
USA) and Ivalon (Clay Adam Co, Bexton Dickinson,
Franklin Lakes, NJ, USA). These materials have fell in
disuse, since they did not withstand hemodynamic
strain, and because of their structural instability (32).
Dacron grafts became clinically available in 1957 (1).
Shortly thereafter, Teflon (originally used as electrical
wire insulator) has been introduced as basic material for
vascular prostheses. Teflon was considered too stiff and
difficult to handle, and prone to dilatation. Therefore, it
was rapidly abandoned until 1972, when a modified
Teflon material with an external wrap (expanded polytetrafluoroethylene, ePTFE) was elaborated. Dacron
became the most popular material for vascular prostheses. It is soft and pliable, which makes it easy to handle
and to suture. Dacron filaments are bundled into yarns
(20 to 54 filaments per yarn). The yarns are texturised as
woven (yarns densily interlaced at right angles to each
other in an “over and under” pattern - low porosity) or as
knitted (loosely interloped pattern, transversely (weft
knitting) or longitudinally (warp knitting) (13). The
knitted Dacron prostheses are more compliant and easier
to handle than the woven ones. Weft knitted and woven
grafts experience unravelling of the yarns and fraying at
the cut-edges or needle holes. Weft knitted grafts also
dilate more than warp knitted grafts. Therefore, weft
knitted grafts ware abandoned in favour of warp knitted
ones.
The large interstices of knitted grafts were considered
to favour tissue ingrowth and graft incapsulation.Knitted
H. Van Damme et al.
grafts have a porosity of 1500 ml/cm2/min, are more
compliant and manifest superior handling characteristics
compared to woven grafts. Later manufacturers fashioned ultra-light weight knitted Dacron fabrics
(USCI),spacing the yarns, with large interstices and high
porosity, in order to further promote graft incorporation
within and anchorage to the surrounding tissue.The
grafts showed excessive dilatation secondary to loss of
tensile strength and a decreased suture retention
strength (17, 33). Their use was discontinued after some
years.This lesson learned from the past should alert the
manufacturers of endovascular stent graft devices covered with ultrathin textile. Structural failure, such as
holes, tears and rents in the textile frabric near the bare
springs or metallic scaffold, have been observed some
years after implantation (34, 35).
Another innovative mile stone in the original concept
Dacron grafts was the velour coating, which gives a filamentous soft surface by an additional perpendicular pile
of microloops of yarns, added to the knitted texture of
ground yarns (36). Internal velour (1967) (36), external
velour (1973) (37) and double velour (1977) (38) prostheses exhibit an enhanced trapping and adhesion of
platelets and fibrin, favouring strong anchorage of
neointima and outer capsule. A further step was the substitution of cylindrical velour filaments by trilobar ones,
with a greater surface. It was an attempt to improve
“heal ability” and to produce a velour of superior quality. However, trilobar filaments appeared to be less
durable, and their use was discontinued in 1981. The
pursuit of a completely endothelialised flow surface was
elusive and a fault concept. The neointima on the flow
surface is mainly set up from fibrous tissue and is as so
far not antithrombotic, since almost devoided from
endothelial cells. The strong tissue integration of knitted
prostheses results in a reduced tendency to dilate and in
an increased resistance to infection.
Actually, all knitted prostheses are coated or impregnated by a sealing (collagen, albumin or gelatine
impregnation), making preclotting prior to implantation
no longer necessary. The sealants do not interfere with
the compliance of the knit trellis nor with the tissue
ingrowth. As mentioned before, crimped knitted Dacron
prostheses exhibit an inherent tendency to dilate after
implantation, once pressurised. The mean dilatation is
about 18% (39) and can vary enormously between grafts
of a given or different trademarks.
Intrinsic structural graft failure occurs after a mean
delay of seven years following implantation (2-22). In
literature, extremes of 12 months (26) and 19 years (5,
14-16, 22) have been reported. The defective graft segment typically shows multiple fenestrations (8, 10, 19,
21, 27). Total disruption, with pseudoaneurysm formation at midgraft level, has also been reported (3, 16, 23,
25). It mainly concerns first generation knitted Dacron
Polyester (Dacron) Vascular Grafts
grafts. Woven Dacron grafts are stronger and less prone
for structural defects.
The incidence has been estimated at 0.5% to 3% in
the earliest series (4, 19, 27). However, it is impossible
to determine the exact frequency of structural degeneration of vascular prostheses. Not all observations are
reported in the literature, possibly by fear for litigation.
Another cause of underestimation of the incidence is
that some cases of structural failure could lead to graft
thrombosis and consequently, the prosthetic defect is
ignored ; hence the patency rate of the bypass grafts is
variable at different levels, lower in femoro-popliteal
than in suprainguinal sites.Obviously only grafts patent
for several years are susceptible to manifest loss of
structural integrity. Finally, there has been a continuous
evolution in the manufacturing process of vascular grafts
and improvement of fabric design, making the comparison of sporadic case reports somewhat difficult. Even if
the true incidence is not known, it seems that this complication is uncommon and rare.
For the most recently marketed vascular Dacron prostheses, reports on structural failure are exceedingly
rare (3). Technological advances and improved quality
of textile fabrics and graft design,are claimed to yield
better resistance to cyclic continuous hydrodynamic
strain. But,a possible explanation could be that the
changes are almost observed five or more years following graft insertion.
Manufacturers of vascular prosthetic fabrics should
provide information on recent improvements in their
graft-design or in the production process. The modified
knitting pattern of Köper-interlacing of yarns (“sharkskin” stitches, Gelsoft, Sulzer - Vascutek - Inchinnan,
Scotland) is relevant. Köper-knitted grafts have a greater
burst strength and less tendency to dilate after implantation than warp-knitted Dacron grafts (18% increase in
diameter versus 27% compared to manufacturer’s stated
nominal diameter of the prosthesis) (40). For Albograft
(Edwards Lifesciences, Biomateriali - Brindisi, Italy),
the diameter of the yarn was increased and the number
of filaments per yarn was decreased(n = 24) at the same
time, in order to improve the burst ,radial and longitudinal resistance of the fabric.
Almost all Dacron grafts dilate to a variable degree
(from 5% to 84%, with average of 18%) shortly following insertion (39, 41). These dimensional changes early
after implantation are inherent to the graft material and
can be considered as a predictable phenomenon.
Moderate (10% to 20%) dilatation of a pressurised
Dacron graft is as so far not a matter of concern ,is clinically irrelevant and cannot be defined as an intrinsic
structural failure (28).
A uniform dilatation of Dacron grafts is secondary to
flattening of the crimps once the graft is pressurised, to
rearrangement and separation of the loosely interlaced
253
knitted yarns (a phenomenon defined as gradual “yarn
slippage” leading to enlargement and expansion of the
interstices) and to progressive straightening of the yarns,
submitted to pulsatile flow (4, 13, 14, 17, 22, 28, 39, 42).
This results in loss of compactness and greater expansion of the knit, occasionally associated to interstitial
blood seeping (19, 43). Cases of excessive dilatation up
to 218% (10) and 367% (42) have been reported. Radial
distensibility is typical for knitted Dacron grafts and is
less observed with woven Dacron and with PTFE Teflon
grafts,which are dimensionally more stable (13, 44, 45).
Prior dilatation is even not an essential prerequisite to
late degenerative changes (13, 27, 29, 32, 33). There is
clinical evidence that graft dilatation is unrelated to late
structural graft failure, since midgraft pseudoaneurysms
have been observed in externally supported ringed prostheses (2, 16, 25, 26). Ringed prostheses do not manifest
generalised dilatation. Loss of structural integrity has
also been observed in non-reinforced PTFE grafts,
which exhibit less tendency to dilate (23). Many of the
sporadic case reports do not mention generalised dilatation of the graft and only describe focal midgraft
pseudoaneurysms, holes, tears or rents in the textile (8,
10-12, 16, 18, 21, 27).
Structural graft failure i.e. loss of integrity, occurs
preferentially at the remeshing line of knitted Dacron
prostheses (5). The remeshing line is the site where two
textile bands are sutured together to create a tubular conduit. At that site, the yarns (a bundle of 27 to 54 Dacron
filaments) are more stretched during the manufacturing
process and have absorbed more tension. These filaments are more prone to breakage, resulting in a longitudinal tear. Another predictable area of weakness is the
black guideline, allowing proper alignment during
implantation (5). Thermal dye fixation could damage
and weaken the yarns. Crimp ridges are more associated
with fiber breakage, pleading for structural fatigue as
underlying mechanism (27). On the peak of the ridges,
the fibers are more elongated and exhibit reduced resistance to hydrodynamic strain.Material wear of Dacron
filaments is the most probable cause of fiber breakdown
and late degeneration of Dacron grafts (5, 25,
27).Repetitive hydrodynamic microtrauma, due to pulsatile blood flow, results in progressive stretching and
thinning of the yarn filaments, with cracking or gradual
rupture of some filaments. The ends of these distorted
broken fibers appear as tapered and frayed (5, 27). All
the load is then transferred to the remaining neighbouring filaments of the yarn, which will finally snap. On
scanning electron microscopy the abrupt breaks appear
as square ended (27).
The mechanical stress during manufacturing and after
implantation is more important in small caliber straight
tubes. This explains why tears are mainly observed in
the branches of bifurcated grafts (6, 12, 15, 19, 22), in
H. Van Damme et al.
254
femoro-popliteal (2, 3, 8, 10, 16, 20, 23, 24) and in axillo-femoral (19, 21, 25, 26) grafts.
The loss of structural integrity observed in externally
supported ringed grafts, could be the consequence of a
compliance mismatch between the inextensible plastic
rings and the textile between the rings. Transversal rents
have been observed at the junction of the rings, so the
pseudoaneurysms protrude between the rings (2, 26).
Moreover, the thermal fixation of the rings to the textile
could fragilise the yarns and predispose them to breakage.
Additional external forces, such as compression and
stretching of a prosthetic branch of a bifurcated graft
underneath the inguinal ligament (7, 12, 15, 19), repeated bending at bone crossing (pelvic rim, ribs) or at the
knee joint, or clot extraction by balloon catheter during
graft revision (21, 24-27), could accelerate the degenerative process of graft wearing and fabric erosion. This
fatigue in regions of high stress was noted in three of the
six observations reported.
Biodegradation of the basic polymer is another probable contributing factor to fiber breakdown (46).
Historically, Dacron and PTFE material were considered
as ideal for vascular conduits, because of their inertness
and minimal tissue reactivity. It is possible that, years
after implantation, the macromolecular composition of
polyethylene is modified by osmotic splitting or by
hydrolytic fragmentation of weakened filaments. These
subtle changes could provoke a foreign body giant cell
reaction with recruitment of multinucleated giant cells
in the vicinity of the defective graft segment (46, 47).
This has been observed in three of the histologically
analysed degenerated grafts of this series, and also by
other authors (6, 7, 12, 16, 26, 27, 47).
Inappropriate rough handling of the graft by the surgeon and iatrogenic damage to the graft (i.e. application
of unpadded clamps, overstretching during tunnelisation) is a possible but unprovable factor implicated in
late structural failure of a graft. If structural graft failure
would be the consequence of damage by surgical manipulation, then its frequency would be much higher (28).
Manufacturing flaws (such as excessive heating during yarn texturisation and crimping by thermo-fixation,
excessive stretching of yarns during knitting, chemical
cleaning, gamma or beta ray sterilisation (48)) are also
questionable in relation to loss of structural integrity of
a graft. The fabrication process is standardised and computer controlled. A meticulous quality control is performed before the graft is marketed. A cold method for
compacting and crimping of the textile tubes has been
introduced recently for the Albograft (Edwards
Lifesciences, Biomateriali - Brindisi, Italy) to reduce
weakening of the textile structure.Since ionising radiation has been related to molecular chain scission, the
manufacturer of Albograft preferred gas sterilisation
with ethylene oxide over radiation sterilisation, in order
to preserve material integrity (48).
Chafke (5) and others (15, 17) make a plea for a cooperative clinical registry and retrieval program of defective grafts (an explant archive), in order to contribute to
a better knowledge and in an effort to provide documentation on this rare phenomenon of late graft deterioration.
Today, Dacron prosthetic fabrics remain one of the
most popular prosthetic conduits. Dacron prostheses can
be considered as reliable and durable arterial substitutes,
with a very low, unpredictable risk of structural failure
or degenerative changes.Biological substitutes for arterial bypasses (umbilical vein, bovine carotid arteries)
have not fulfilled the expectations and have been abandonned by many vascular surgeons, mainly because of
their structural instability and aneurysmal degeneration (28, 49).
The ideal vascular prosthetic conduit should be “on
shelf” available, durable and longstanding with a durability superior to the life-expectancy of the host, resist to
dilatation,resistant to infections, inert and biocompatible. The ideal graft should exhibit and maintain a thromboresistant flow-surface and elastic properties,comparable to those of the original undiseased artery to which it
is sutured,in order to approach the patency,compliance
and flexibility of a normal artery. Such an optimal graft
has not been produced. Some investigators work on the
incorporation of molecular substances, such as growth
factors, anticoagulants or antibiotics, and cellular layering (endothelial seeding or sodding), in order to resemble an endothelialised normal artery (47).
Finally, degeneration of Dacron textile fabrics is so
sporadic and its probability is extremely low, that there
is no evidence that regular periodic imaging by echoduplexscan of every graft is mandatory. This is opposite to
the recommendation of serial echoduplex of the
midgraft by other authors (7, 8, 11, 19-21, 39, 33, 42).
However,since structural failure is linked with the duration of patency,limbs of an aortic bifurcated graft should
be checked by ultra-sound every three year.Ultrasound
surveillance of aortic bifurcated grafts is all the more
justified since palpation of a pseudaneurysm at iliac
level is rather difficult,if not impossible.
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H. Van Damme
Service de Chirurgie Cardio-Vasculaire et Thoracique
Hôpital Universitaire de Liège
CHU du Sart-Tilman
B-4000 Liège, Belgique.
Tél.
: 32/4 366 71 63
Fax
: 32/4 366 71 64
E mail : hvandamme@chu.ulg.ac.be