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Essential equipment: Stents
Purpose
Stents are scaffolds used to maintain luminal patency in many systems, e.g. vascular, biliary, gastrointestinal and tracheobronchial. They act by exerting an outward radial force to overcome stenoses/occlusions and elastic recoil after angioplasty. Stents may also reduce the likelihood of distal embolization during angioplasty by trapping plaque/thrombus against the vessel wall.
Description
Stents comprise a metal strut latticework, they have a high expansion ratio allowing them to be compressed to a small diameter for introduction into the patient and then expanded to many times that diameter. There are two principal stent types relating to how they are deployed.
Balloon-expandable stents
Most of these come premounted on an angioplasty balloon. As the balloon is inflated the stent expands to the same diameter.
Self-expanding stents
These come mounted, constrained on a delivery catheter. The stent is progressively unsheathed and opens due to intrinsic radial force. Note that self-expanding stents will often require additional balloon dilatation.
Stent construction
This is a complex subject and every manufacturer will try to tell you why their device is the best. Here is a simple guide, which is really all you need to know.
Material
Almost all stents are made from alloys of stainless steel or nickel and titanium (nitinol), some have finishes/coatings or properties such as drug elution or radioactivity intended to reduce thrombogenicity and restenosis. There are even novel stents designed to be resorbed once their work is done.
Nitinol
If you want to impress your boss you should know that nitinol is an acronym for Nickel Titanium Naval Ordnance Laboratory. Virtually all new self-expanding stents are made of nitinol for the simple reason that it has thermal ‘shape memory’ and ‘superelasticity’. This means that once the stent is made it can be deformed into one shape at a low temperature but will magically ‘spring’ back to its original shape when warmed. In practice, the stent is cooled and compressed for mounting on the delivery catheter but expands to its original shape at body temperature. Nitinol's properties make it particularly suitable for applications requiring flexibility and motion.
What about patients with nickel allergy?
No need to worry there, although nitinol is 50% nickel, the polished surface forms a stable titanium oxide coating. Do not forget that nickel it is also present in significant amounts in stainless steel and other alloys and in fact, these tend to release nickel faster!
Structure
There is generally a compromise between stent designs which favour radial force or flexibility, so there is no magic stent which will fulfil every therapeutic need. Self-expanding stents are either laser cut from nitinol or braided from overlapping wires.
As a general rule:
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Closed-cell structure stents (think trellis fence) in which every cell is linked to all adjacent cells tend to have greatest radial strength at the cost of reduced flexibility and trackability.
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Open-cell structure stents have fewer links between cells. This increases flexibility and trackability but reduces radial force. In addition, open cell stents cover less of the vessel surface.
This nomenclature applies to both balloon-mounted and self-expanding stents.
Measurements
Stent dimensions
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