Biomedical Engineering Reference
In-Depth Information
Type I endoleaks occur when there is a failure of seal at one or more
stent-graft attachment sites. It is diffi cult to determine the true incidence of
late Type I endoleaks. Many reports combine all types of endoleak while
others refer only to early experience and fi rst generation stent-grafts
(Schurink
et al.
, 1999), Perhaps the best estimate can be found in a report
by Tonnessen
et al.
(2002) in which Type I endoleak had been documented
in nearly 10% of patients within 20 months of operation.
Reperfusion of the aneurysm sac through patent lumbar or mesenteric
side-branches has been classifi ed as Type II endoleak. Side-branch perfu-
sion ceases spontaneously in the majority of patients after endovascular
repair due to lack of a direct communication with the aortic lumen. The
high variation in the reported incidence of late type II endoleak (2-70%)
(Stelter
et al.
, 1997; Schurink
et al.
, 1999) is probably a refl ection of the
variety of techniques employed to detect endoleak. Ultrasound duplex
scan, magnetic resomance imaging (MRI) and computed tomography (CT)
have different sensitivity and specifi city (McWilliams
et al.
, 2002). Further-
more, detection of endoleak by CT scanning depends upon the imaging
protocols used. Biphasic scanning (scanning the patient fi rst when the bulk
of the contrast is in the arteries and again when it had passed into the
veins) improves detection rates compared with arterial phase scanning
alone (Golzarian
et al.
,
1998). It has been suggested that the majority of
isolated single vessel Type II endoleaks seal spontaneously (since there is
in effect no outlet for the blood-fl ow) and therefore a persistent Type II
endoleak may in fact be a marker for otherwise occult graft-related
endoleak.
Type III endoleak is due to dislocation of the components of a modular
device or to a breach in the fabric of the stent-graft. Its cumulative incidence
is approximately 10% at three years after EVAR (Vallabhaneni and Harris,
2001). The problem of modular disconnection appears to have been resolved
by newer designs with improved attachments between the components.
Steps are also being taken to address the issue of material fatigue and fric-
tional wear of the fabric.
Type IV endoleak is associated with porous thin-walled fabric grafts.
However, this is time limited and is usually resolved within a month of
aneurysm repair. Expanded PTFE graft material has much lower porosity
than woven PET fabric and is therefore less susceptible to this type of
endoleak.
8.8.2 Stent-graft migration
Migration, i.e. movement of a stent-graft in relation to its original site of
implantation, occurs if the haemodynamic distraction force, which tries
to displace the device in a caudal direction, is greater than the strength of
Search WWH ::
Custom Search