Biomedical Engineering Reference
In-Depth Information
the stent-graft fabric against the internal surface of the aorta at the areas
of the neck or 'sealing-zones'. The fabric tube itself forms the blood fl ow
channel.
8.7.1 Stent-graft design and selection
Although most open AAA repair can be performed using a tube graft, it is
usually not feasible to use a tubular stent-graft in EVAR since infrarenal
aneurysms rarely contain a 'distal neck' required for the distal seal within
the aorta. This problem can be overcome by sealing within one or both iliac
arteries. The fi rst option, sealing within a single iliac artery is achieved by
using a tapered aortouniiliac stent-graft to bridge between the infrarenal
aortic neck and the iliac artery. This requires the addition of a 'plug'
(occluder) to seal the opposite common iliac artery (to prevent back bleed-
ing into the aneurysm sac). It also requires the addition of a prosthetic graft
femoro-femoral crossover bypass to restore circulation to the opposite
lower limb.
In the majority of cases, a bifurcated stent-graft is used so that the distal
seal is achieved in both iliac arteries. Bifurcated systems are available either
as one-piece (unibody) stent-grafts or as 'modular' stent-grafts that consist
of two or more components which must be constructed into a complete
bifurcated system within the patient. Modular systems are much more
common because of their versatility but a potential drawback is the possibil-
ity of component separation due to the action of haemodynamic forces. A
unibody stent-graft does not suffer from this problem but it is less adaptable
to variations in anatomy.
Achieving endovascular exclusion of an aneurysm depends on matching
a stent-graft to the patient's anatomy. Therefore some stent-grafts are man-
ufactured to fi t a particular patient's anatomy based on accurate measure-
ments obtained from imaging. Modular component designs reduce the
necessity for such custom manufacture. Each modular component of the
stent-graft is available in a range of sizes. A complete stent-graft system can
be built by combination of appropriately sized components. Such modular
designs are particularly useful when undertaking urgent and emergency
repairs, when there is no time to procure a custom made commercial device.
The introduction of EVAR represents a signifi cant step in the evolution
of aneurysm management. The benefi ts of EVAR for the high-risk patient
and the signifi cant reduction in early mortality rates compared with open
repair are well documented (EVAR Trial Participants, 2005). This tech-
nique, however, is associated with unique problems in the post-operative
period. There is a high incidence of complications in the late follow-up and
consequently a high incidence of secondary procedures to correct them. The
relative signifi cance of all complications is not completely understood and
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