Saving Lives
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Nickel-titanium stents gain in popularity with patients, surgeons
Nickel magazine, Jun. 02 -- The market for nickel-titanium "stents" and related devices that shore
up blood vessels is growing as doctors and their patients seek less invasive medical techniques, while
engineers improve stent delivery systems.
Stents are tiny wire mesh tubes used to prop open damaged or clogged arteries. In most cases, the stent is pulled over a balloon catheter, which is then threaded up to the blockage site. When the balloon is inflated, the stent expands, forming a scaffold to hold the artery open and allow more blood flow. The stent remains in the artery permanently. After the balloon is deflated, the catheter is withdrawn.
But a new procedure to repair aneurysms in the abdominal portion of the aorta, the body's main artery, simplifies stent placement by using a self-expanding device. Abdominal aneurysms, like worn spots on a tire, sometimes rupture. Every year, ruptured aneurysms kill about 15,000 people in the United States alone. The stent graft procedure is emerging as a much less invasive alternative to abdominal surgery, the traditional way to repair the aorta.
In this case, a catheter is inserted into the femoral artery of the patient's leg, then threaded up to the aorta. Next, the stent is directed through the catheter on the tip of a wire to the section of the aorta with the aneurysm, where it expands in response to body heat.
Last year, surgeons at the Mayo Clinic in Rochester, Minnesota, U.S.A., performed their 100th aneurysm repair using this procedure. Two types of stents used by the Mayo surgeons were made from Nitinol (N01555), a nickel-titanium alloy that exhibits the shape memory effect. One of these, Medtronic's AneuRx stent graft system, received approval from the Food and Drug Administration in 1999.
"[The stent graft system] is truly a breakthrough in that it offers significant potential benefits to patients," said Christopher Zarins, M.D., chief of vascular surgery at Stanford University School of Medicine and clinical investigator in the AneuRx trial. "In addition to being as effective as open surgery, patients who received the AneuRx device also had fewer complications, spent less time in the hospital and experienced faster recoveries."
Although stents are made of various materials, including stainless steel, Nitinol is emerging as a favourite for some applications because it can change shape in response to thermal or mechanical changes and is highly flexible. A Nitinol stent that has been cooled into a compact shape and inserted into a blood vessel will gently return to its expanded shape when exposed to the body's warmth, reducing any trauma associated with stent placement.
Among the properties that render Nitinol useful for these medical applications are high loading plateau stress (450 megapascals at 3% strain), a low permanent set (0.2% at 6% strain), and transformation temperatures of 5 to 18[degrees symbol]C. The alloy consists of about 50-60% nickel by weight, with titanium making up the balance.
Guidewires, used to thread the catheter and deliver the stent, also benefit from Nitinol's super-elastic properties. Stringing a catheter is like trying to push a wet noodle through the vein, so a guidewire that can resist bending and kinking by springing back into shape has a significant advantage.
Besides the aorta, ailing body parts that may benefit from Nitinol stents include the esophagus, the
urinary tract and the liver. Cordis Endovascular, one of the world's largest manufacturers of shape memory
medical devices, recently introduced a Nitinol stent for the treatment of life-threatening obstructions in
bile ducts. Other companies marketing Nitinol stents include Boston Scientific Corp., Cook Inc., Endocare
Inc., Guidant Corp. and Memry Corp.
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