Choosing the right stent

Covidien – Medtronic balloon stent

A stent is inserted by catheterization using two main methods:

The stent is dilated by the catheter balloon: the stent is placed during an angioplasty, under local anesthesia. The probe is brought to the blocked artery where a small balloon is inflated at the narrowed area to crush the atheromatous plaques against the arterial walls. The stent is then inserted in order to keep the artery open once the balloon has been removed.

A COOK Medical self-expanding stent

Self-expanding stent: the stent isn’t set on a balloon but is instead compressed in a thin sheath. Once it’s been placed inside the artery of the occlusion site, the sheath is removed; the stent then expands and moves the intima of the artery outwards. It keeps the artery open by attaching itself to the walls.

Stents are mainly made from the following five types of materials: metallic, coated, drug eluting, bioresorbable and radioactive.

• Metallic: Some stents are made of bare metal (i.e. only metal). We can distinguish between:
  • Stainless steel: this is the most common material used for stents. Its advantages include good X-ray visibility; a fairly elastic and resistant consistency; and good biocompatibility. These qualities make this type of stent very suitable for balloon insertion and preventing sudden vessel closure. Disadvantages include low MRI compatibility, a higher risk of injury from vessel wall damage than with nitinol stents, lower corrosion resistance and the release of metal ions.
  • Cobalt alloy: the most common cobalt alloy is cobalt chromium. These stents are more resistant than steel stents and have a higher radiopacity. They can therefore have a smaller mesh size, which reduces the risk of thrombosis. They also offer good biocompatibility, but with this type of stent, the risk of restenosis still needs to be monitored.
  • Nickel-titanium alloy: more commonly known as nitinol, this material is very common for carotid stents and is widely used for self-expanding stents. These stents are often larger in diameter than the vessel in which they are installed, which allows a constant pressure to be applied against the walls of the vessel. The main advantage of nitinol is its high elasticity since it is a shape memory alloy. A titanium oxide surface treatment makes it resistant to corrosion. This type of stent has a lower radial strength and X-ray opacity however. For the latter aspect, gold or tantalum markers can be used to remedy this.
• Coated: Some stents have a metal skeleton and are equipped with a passive or active coating. Passive coatings include gold, carbon, polymer and ceramic. Active coatings include drugs such as heparin or thrombomodulin.
  However, these types of stents do not offer proven clinical advantages over metal stents for carotid stents.

• Drug eluting: see question on the advantage of drug eluting stents.

• Bioresorbable: these stents are particularly biocompatible. Some have a magnesium alloy (with little magnesium elution), while others have an iron alloy. Some are made of polymers (PLLA) but they are less solid than metal stents and present risks of stenosis and inflammation which need to be monitored.

• Radioactive: these stents are very rare and are only intended for brachytherapy. They can be:
  • Metal stents bombarded by charged particles. This transforms stainless steel atoms into radioisotopes.
  • Stents that already have radioisotopes implanted in the structure.

A Rontis Medical drug eluting coronary stent

Drug eluting stents are primarily used to reduce the risk of restenosis. They are composed of three elements: the metallic stent, the active drug and its delivery system, which is most often made of polymer.

The drug slowly being released is generally mainly sirolimus or paclitaxel. Both drugs have obtained the European CE conformity marking on the basis of evidence acquired in randomized trials.

The purpose of these antimitotic and/or anti-inflammatory agents deposited on the stent is to block intimal proliferation in situ. Since they have been created, drug-eluting stents have significantly reduced restenosis, which is particularly common in angioplasty and bare stent implantation

The main risks associated with inserting stents are restenosis, clotting, kidney risks and vessel rupture.

• Restenosis: the risk of re-shrinking the artery is very frequent during an angioplasty. Drug eluting stents are the most effective in reducing this risk (see question on the advantage of drug eluting stents).

• Clotting: thrombosis can form inside the stent after the procedure. The blood clot can then block the artery. This is why it is important for the patient to take medication that helps reduce the risk of blood clots.

• Kidney risks: these are due to the use of contrast agents during stent placement, but they remain rare. They mainly affect people with a history of kidney failure.

• Risks of vessel rupture upon insertion: this may occur if the vessel is weak or too thin. In coronary stents, these complications may require emergency coronary bypass surgery.

Several parameters can be used to limit the risk of the restenosis associated with inserting a stent. The first, as we have seen, is the use of drug-eluting stents to contain intimal proliferation.

But that’s not the only parameter. The dimensions of the stent, i. e. its length and width, as well as the mesh size of the stent graft are also important. Technological developments over the past twenty years have led to many advances. In the past, the slotted tubes used were inflexible and caused major problems during implantation. The so-called “coil” stents had insufficient radial strength, which could lead to tissue prolapse.

Today, the physical parameters of stents have greatly improved, including better stent release, new mesh size for better support and better visibility during radiographic examinations. These improvements have had a significant impact on procedural success and restenosis rates.