Download What is a Stent?

Reviewed by:
AGNES Purwidyantri
Student ID No: D0228005
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Polyesters
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Lactide/Glycolide Copolymers
 Have been used for the delivery of steriods, anticancer
agent, antibiotics, etc.
 PLLA is found as an excellent biomaterials and safe for
in vivo (Lactic acid contains an asymmetric α-carbon
atom with three different isomers as D-, L- and DLlactic acid)
 PLGA is most widely investigated biodegradable
polymers for drug delivery.
 Lactide/glycolide copolymers have been subjected to
extensive animal and human trials without any
significant harmful side effects
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Poly(amides)
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Natural Polymers
 Remain attractive because they are natural products of
living organism, readily available, relatively
inexpensive, etc.
 Mostly focused on the use of proteins such as gelatin,
collagen, and albumin
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Polymer Processing
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Drug-incorporated matrices can be formulated either
compression or injection molding
Polymer & drug can be ground in a Micro Mill, sieve
into particle size of 90-120 µm, then press into
circular disc
Alternatively drug can be mixed into molten
polymer to form small chips, then it is fed into
injection molder to mold into desired shape
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A small tubular mesh usually made of either
stainless steel or Nitinol.
Inserted into stenotic arteries to keep the lumen
patent often used after PTCA.
Used at various sites including the coronary,
renal, carotid and femoral arteries.
Non-arterial uses e.g. in bronchus, trachea,
ureter, bile duct.
Current stent designs
Palmaz, the market leader
Palmaz “Corinthian” Iliac artery
stent
Gianturco-Roubin II Stent
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The concept of vascular stents is accredited to
Charles Dotter in 1969, who implanted
stainless steel coils in canine peripheral
arteries.
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Not followed up in humans because of
haemodynamically significant narrowing.
Not in clinical practice until 1980s.
Market leader is the Palmaz stent designed by
Julio Palmaz in 1985.
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Initially, 18 grafts placed in canine vessels, with
patency rates approaching 80% at 35 weeks.
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Smoking, high BP, toxins etc cause damage to the
vascular endothelium.
LDL and fibrin pass through and collect in the subendothelium.
Monocytes adhere to the damaged endothelium,
migrate to the sub-endothelial space and engulf LDL –
FOAM CELLS.
SMC migration and CT formation.
Two main types of plaque:
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Atheromatous (athere: gruel, oma: tumour)
Fibrous (like atheroma but with connective tissue cap)
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Heart and circulatory disease is the UK's
biggest killer.
In 2001, cardiovascular disease caused 40% of
deaths in the UK, and killed over 245,000
people.
Coronary heart disease causes over 120,000
deaths a year in the UK: approximately one in
four deaths in men and one in six deaths in
women.
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Coronary Artery Bypass Graft (CABG)
Percutaneous Transluminal Coronary
Angioplasty (PTCA)
Stents
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Major surgery
Complications
 Stroke
 Mediastinitis (1-4%)
 Renal dysfunction (8%)
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Minimally invasive procedure
 Percutaneous access either in
the brachial or femoral
arteries.
 A guide wire is advanced to
the stenotic region.
 A balloon is advanced along
the wire and
inflated/deflated several
times to fracture the plaque
and open the lumen.
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Plaque rupture, may lead to:
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Thrombus formation
Intimal flap
Arterial rupture
Acute closure
Sub-optimal result
Restenosis
Requires further intervention to
make vessel patent
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Prevents acute closure
Tacks back intimal flaps
Less restenosis:
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30–50 % restenosis with PTCA (coronary arteries).
Coronary stents are associated with fewer
repeat revascularisation procedures
Rates of death and MI are low and are not
significantly different between stents and PTA.
i shear stress
 Intimal Hyperplasia
 i lumen
 h shear stress
 If baseline shear stress not restored – continuing
intimal hyperplasia and RESTENOSIS
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Thrombus/platelet/fibrin adherence to stent struts.
Metabolic disorder/smoking/atherogenic diet.
Small lumen diameter.
Stress concentration at end of stent.
Flow disturbance within stented region.
Thrombus in Human Coronary Artery
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Thrombus
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Anticoagulants
 Heparin – systemically or coated on stent.
 Inhibition of the GP IIb-IIIa receptor:
 Prevents platelet aggregation.
 Available as Abciximab.
 Associated with h incidence of MI.
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PTFE coated stents.
Intimal hyperplasia in stented
Canine iliac artery.
After insertion of stent plus
PTFE graft material.
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Small diameter artery
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Combination of local and systemic medication and
covered stents.
Intimal hyperplasia
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Brachytherapy:
 Use of ionising radiation to stop cellular proliferation.
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Delivery: Radioactive stents, catheter radiation.
10% restenosis but may cause necrosis.
Anti-proliferative agents e.g. rapamycin (Sirolimus)
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Mechanical and flow disturbances:
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Compliance Matching Stent (CMS)
 This stent is rigid in the middle and becomes more
compliant near its ends.
 This compliance is achieved by parabolic and
cantilevered struts.
 The middle struts are straighter, providing some
resistance to recoil and support for the atherosclerotic
plaque.
Compliance Matching Stent
Parabolic and cantilevered struts cause
ends to be most
compliant.
Straighter struts in
middle provide stiff
support for plaque.
Transition in between.
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The gradual change from rigid to compliant
with the CMS reduces stress concentration at
the stent edges.
The geometry of this stent also fosters more
laminar flow through the stent.
Less flow disturbance means less intimal
hyperplasia.
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Durable polymer coatings on drug-eluting
stents have been associated with chronic
inflammation and impaired healing.
Potential advantages of bioabsorbable polymer stents:
• Reduced Polymer Load
• Short-term Polymer
Exposure
may • Reduce DAPT duration
• Reduce risk with DAPT
interruption
• Decrease stent thrombosis
Abluminal Bioabsorbable Polymer
Bioabsorbable polymer
(PLGA)
Applied only to the
abluminal surface (rollcoat)
Thin strut (0.0029”) PtCr
Stent
Current Durable
Polymer
Durable Permanent
Polymer
+
Drug
360° Around
Stent
Abluminal Bioabsorbable
Polymer
PLGA Bioabsorbable
Polymer
+
Everolimus
on Abluminal Side
of Stent