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Advanced Technology for Vets and Pets |
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Technical Questions
What is a French and what is a gauge?
What are the two general categories of stents and how do they differ?
What is nitinol?
What is a French and what is a gauge?
A French (Fr) is a unit of measure of diameter. One Fr is equal to one third of a millimeter. Therefore a 6Fr catheter will be 2mm in diameter. A gauge is also a unit used to measure diameter. A gauge describes the number of objects that have to be lined up side by side to make an inch. Therefore, if 21 needles must be lined up to equal an each, the diameter of each needle is 21gauge. One could convert gauge to Fr, but this has no clinical applications and would be a tremendous waste of time.
What are the two general categories of stents and how do they differ?
Improvements in biomedical engineering and metallurgy have helped expand the clinical applications for stents. Stents can be classified as either balloon expandable or self expanding. The characteristics of each type vary, making one type more suitable for certain clinical applications than the other. There is significant overlap for uses of both types of stent and user preference plays a role in selection.
Balloon expandable stents are made from laser cut stainless steel. Such stents come packaged either individually or pre mounted on a balloon. When the balloon is inflated, the stent expands to the diameter of the balloon. As the stent expands, it changes very little in length. Their relatively constant size and method of delivery/deployment makes for precise and predictable placement. Balloon expandable stents may be permanently deformed by extrinsic compression and should therefore not be used in situations where they could be subject to these forces.
There are two broad categories of self expanding stents: those made from woven nitinol wires and those laser cut from nitinol tubes. Woven stents may be are reconstrainable, meaning that they can be almost entirely deployed, recaptured, moved and then deployed in a different location. The trade off however is that the length of the stent depends on its fully expanded diameter. These stents may shorten significantly as they expand over time, uncovering a region of pathology. Alternatively, if the stent does not expand to the degree suspected, the stent may remain too long.
Most laser-cut self expanding stents are not reconstrainable. Infiniti Medical does offer a variety of self expanding laser cut nitinol stent that is reconstrainable. This may be a good stent for operators with little experience deploying stents. Because laser cut stents are constructed of rings linked together, they are not subject to foreshortening and remain at a relatively stable length regardless of diameter. Self expanding stents exert a continuous outward force and resist deformation and are therefore preferable to balloon expandable stents in regions potentially subject to external compressive forces.
To ensure full expansion, self expanding stents may be dilated with a balloon of appropriate diameter after deployment.
What is nitinol?
Nitinol was developed by the US Navy and stands for nickel titanium alloy. The metal is particularly useful for medical applications because it has thermal memory. This property allows stents to be made at a certain diameter, cooled, and then compressed onto a delivery system. When the stent is deployed at body temperature, the stent attempts to regain its original configuration and diameter. Therefore a nitinol stent that is slightly oversized (by approximately 20%) with regard to the vessel will exert an outward force to keep the vessel open as the stent attempts to regain its original diameter.
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