Θεματική ενότητα: Stenting Μ. Ματσάγκας, MD, PhD, FEBVS Σάββατο 1...
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Transcript of Θεματική ενότητα: Stenting Μ. Ματσάγκας, MD, PhD, FEBVS Σάββατο 1...
Θεματική ενότητα: Stenting
Μ. Ματσάγκας, MD, PhD, FEBVSΣάββατο 1 Φεβρουαρίου
2014
Physics
Tensile strength of a material is the maximum amount of stress that the material can be subjected to before failure
Yield strength represents the stress at which material strain changes from elastic deformation to plastic deformation, causing it to deform permanently
Breaking strength is the stress coordinate on the stress–strain curve at the point of rupture
Physics
Below a certain stress known as the elastic limit, or the yield strength, the blood vessel demonstrates elastic recoil
As transmural pressure exceeds the elastic limit, the vessel demonstrates irreversible deformation
Elastic limit is a function of the lesion
Typically calcified lesions have low elastic limits as the brittle calcifications yield to moderate angioplasty pressures
Collagen-rich areas of myointimal hyperplasia have high elastic limits and require large transmural pressures to overcome the elastic limit.
Physics
Angioplasty involves the dilatation of a vascular
stenosis or occlusion with a balloon catheter
Angioplasty
Angioplasty
Biological response to balloon angioplasty
Initially thought to be the result of compression of atherosclerotic lesion followed by remodeling of the plaque
Predominant effect of balloon angioplasty is stretching the elastic components of the arterial wall
Inelastic portion of the plaque fracture or tear results in a definite but discrete arterial wall dissection
Histologically evident arterial dissection is nearly present in all diseased vessels following balloon angioplasty procedures
Biological response to balloon angioplasty
The injury to the endothelium exposes the subendothelial space and attracts platelets and fibrin that cover the damaged surfaces
All these events favor the local migration and proliferation of the SMC as a healing response, which may ultimately lead to restenosis, or intimal hyperplasia
Most angioplasty-induced dissections will ultimately heal within a month
Stenting
Within 15 min following stent implantation, there is an accumulation of red blood cells and platelets on the stent surface
At 24 h, this cellular layer is replaced by a layer of fibrin strands oriented in the direction of blood flow
In the third and fourth weeks after stent insertion, SMC proliferation and endothelialization resulted in a neointimal layer of approximately 1 mm in thickness
Biological response to stenting
Finally, several months after stent placement, the formation of the neointimal vessel begins
At 3–6 years, the fibromuscular tissue layer covering the stent surface is almost completely replaced by collagen
Biological response to stenting
The electrical charge of most metals and alloys used for intravascular devices is electropositive in electrolytic solutions, whereas all biologic intravascular substances are negatively charged
The positive electrical potential of the metallic struts attracts the negatively charged circulating proteins to form a thin layer of fibrinogen strands on the stent surface
The proteins neutralize the stent surface and decrease thrombogenicity
Biological response to stenting
Surface tension is another property that influences biological stent interaction
The initial layer of proteins that cover the metal within seconds of implantation helps reduce surface tension and thrombogenicity
Biological response to stenting
The technique of stent implantation itself may affect thrombogenicity and the rate of endothelialization
Stents should be deployed in such a way that the metal struts are embedded deep enough into the vessel wall to produce troughs where the struts are embedded surrounded by intima
If the struts are not properly embedded, the entire stented surface becomes covered with thrombus, preventing early endothelialization and thus predisposing to complete thrombosis and restenosis
Biological response to stenting
Cellular events analogous to a foreign body reaction are also seen, which include the thrombus formation organized around the stent
Following stent-graft implantation, the media of the underlying artery wall is partially replaced by collagen, perhaps due to the pressure from the stent-graft
Biological response to stenting
Mode of action
Fracture of the arterial plaque and a localized tear or dissection of the arterial wall
The tear may extend circumferentially or longitudinally in the vessel wall and may extend into the internal elastic lamina or into the media
The adventitial layer remains intact
Balloon dilatation also causes stretching of the medial layer if the balloon diameter is adequately oversized
Microscopic plaque material may become separated and embolize distally
This is usually asymptomatic in the peripheral circulation
In carotid artery angioplasty, this phenomenon has potentially more severe consequences
Mode of action
Concentric arterial lesions respond well to PTA, because the
arterial plaque and the arterial wall layers are dissected in a
uniform fashion, which improves the increase in the luminal
diameter
Mode of action
Mode of action
The balloon catheter is centered in a concentric arterial lesion
Mode of action
Balloon dilatation results in uniformly controlled wall dissection with adequate luminal gain
Eccentric arterial lesions may respond less well to balloon dilatation. This is because the wall opposite the plaque is stretched by the balloon rather than the plaque itself
Once the balloon is deflated, the normal elastic wall may recoil, resulting in an unsatisfactory result
Mode of action
Mode of action
The balloon The balloon catheter lies catheter lies within an within an eccentriceccentric arterial lesionarterial lesion
Mode of action
After balloon dilatation the wall opposite to the plaque is stretched
Mode of action
The stent provides an internal scaffold for the arterial lumen with excellent luminal gain
Advantages of stenting
Rapid, reliable and sustained increase in the luminal diameter
Entrapment of vulnerable plaque material that may cause embolization
Elimination of elastic recoil