Vascular implants for peripheral arterial bypass and aortic aneurysm repair - Biomaterials and Devices for the Circulatory System - page 227

condition), the AAA will eventually rupture. The risk of rupture is related
to diameter and it is estimated that 2% of aneurysms with diameter less
than 4 cm and 22% of aneurysms greater than 5 cm will rupture within 4
years (Hallin et al. , 2001).
Surgical repair is effective in preventing a rupture but is a complex and
major operation and a proportion of the patients are deemed unfi t for
surgery. In open surgery, the aneurysm is incised, the thrombus is removed
and a prosthetic tube graft is securely attached by a hand-sewn anastomosis.
The aneurysm sac is then placed around the graft to prevent the bowel from
adhering to the anastomotic areas. If a normal segment of aorta is unavail-
able, a bifurcated graft will be used to extend the bypass to common iliac
or femoral arteries. Surgical repair of intact aortic aneurysms is associated
with an average post-operative death rate of approximately 7% in the UK.

8.7 Endovascular aortic aneurysm repair

In endovascular aneurysm repair (EVAR) the aneurysm is excluded from
the circulation to prevent rupture while the aneurysm sac and its contents
remain in situ . This is achieved by means of a device consisting of a pros-
thetic tube or bifurcated graft, a metal stent support (stent-graft) and a
delivery system into which the stent-graft is folded and packed. The metal
skeleton consists of several wire structures (stent struts) and acts as a spring,
providing a radial force in an outward direction helping to fi x the device in
place. Vertical support (columnar strength) is provided by the relatively
rigid interconnection between adjacent stent segments. Fixation may be
augmented by the addition of hooks or barbs either to the metal skeleton
or the fabric component at the proximal sealing zone. The fabric and skel-
etal components of a stent-graft are bonded together by a variety of means
such as sutures, tapes or even by 'sandwiching' the metal within a 'double
skin' graft. The device is introduced through one of the femoral arteries and
guided to the aneurysm where it is anchored in place. The procedure is
performed under X-ray fl uoroscopy. The function of the delivery system is
to protect and constrain the stent-graft during deployment. Ideally it should
be fl exible enough to traverse tortuous blood vessels but rigid enough to
resist kinking and damage during its passage. It should add as little as pos-
sible to the total thickness of the device. The delivery system also contains
mechanisms that minimise blood loss during the procedure.
The ability of a stent-graft to effectively exclude an aneurysm from the
circulation depends predominantly on two factors, namely, seal and fi xation.
The term seal refers to the ability of a stent-graft to prevent blood from
entering the aneurysm sac and fi xation refers to the ability of the stent-graft
to stay anchored at the site of original implantation without slippage. Seal
immediately after stent-graft deployment is obtained by close apposition of

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