Biomedical Engineering Reference
In-Depth Information
signifi cantly improve the attachment of the graft to the intima (van der Bas
et al. , 2004; Kusanagi et al. , 2007). As these studies were carried out in
animals, it remains to be seen whether improved healing response can be
achieved in atherosclerotic aortic necks.
The strength of haemodynamic force that causes migration depends upon
patient factors such as hypertension and aneurysm morphology, in addition
to stent-graft factors such as the diameter change from proximal to distal
attachment site (Mohan et al. , 2002). Curvature of the stent-graft within the
aneurysm also increases the haemodynamic distraction force (Liffman et
al. , 2001). Migration was reported in approximately one-tenth of the patients
by 3 years after endovascular repair (Vallabhaneni and Harris, 2001) with
the incidence increasing with the duration of follow-up.
8.8.3 Stent fracture
The function of the metallic stent is to provide structural integrity thereby
ensuring stability of the device fi xation and support for the graft compo-
nent. It is therefore important that the integrity of the stent is not impaired
after implantation. Stent fractures are commonly observed in vivo in most
devices whether they are manufactured from cobalt-chromium alloy,
stainless steel or nitinol (Riepe et al. , 2002; Jacobs et al. , 2003; Zarins et al. ,
2004).
Failure can be attributed to mechanical fatigue due to the cyclic loads the
device is subjected to in vivo . The presence of small cracks, either intro-
duced during the manufacture of the stent or inherent in the material, has
a signifi cant effect on fatigue failure. With cyclic load due to haemodynamic
forces, these cracks may grow, leading eventually to fracture of the metal
structure. In the presence of a corrosive environment such as blood, the
corrosion of the metallic stent may also contribute to the fatigue process,
i.e. corrosion fatigue. Although isolated stent fractures do not appear to
have immediate clinical consequences, cumulative damage could impair the
structure of the device, leading to loss of sealing and fi xation or even pro-
lapse into the aneurysm sac. Resistance to fatigue failure could be improved
by altering the dimensions and geometry of the stent struts but this will lead
to a bulky device requiring an unacceptably large delivery system. Metal-
lurgical processing and the stent manufacture may be further improved to
produce stents with much smaller and less numerous fl aws thereby enhanc-
ing fatigue properties.
Nitinol owes its resistance to corrosion from the spontaneous formation
of a thin titanium-based oxide layer on its surface that protects the base
material from further oxidation, a process known as passivation. On the
surface of stainless steel, chromium oxide forms as a tough fi lm which also
protects the underlying metal. Since the surface topography, the thickness
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