Blending in: dissolvable stents promise to protect arteries

Science News,  June 9, 2007  by N. Seppa

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Biodegradable versions of the metal cylinders known as stents can keep blocked coronary arteries propped open long enough to free up blood flow, after which they disappear--a potential advantage. The new finding suggests that such dissolving stents might someday replace rigid, permanent stents, which carry health risks (SN: 10/28/06, p. 277).

Doctors insert mesh stents into jammed arteries to keep them open. In the new study, researchers implanted an experimental version of biodegradable magnesium stents in 63 people with dogged coronaries. The scientists chose magnesium because it's found in the body, poses no allergy risk, and in alloy form is slowly dissolved by body fluids, says study coauthor Raimund A. Erbel, an interventional cardiologist at the West German Heart Center in Essen.

At checkups 4 months after implantation, the researchers found that the stents had completely degraded and had been replaced by calcium and a phosphorus compound. Moreover, new vessel-lining cells had grown over the area, the team reports in the June 2 Lancet.

However, earlier checkups had revealed that the magnesium alloy began degrading within 14 days--too fast for practical use. Nearly half the vessels getting the magnesium stents narrowed again so much that the patients warranted an additional procedure, such as coronary bypass surgery.

As a result, the new stent differed little in effectiveness from angioplasty, in which a balloon-tipped catheter is inflated at a coronary blockage to simply push it aside. Over the past 15 years, doctors have increasingly used angioplasty to prepare a vessel for stent insertion. Without a stent, a vessel opened by angioplasty can soon collapse, a complication known as recoil.

Overall, the magnesium stent fared worse than did permanent stents, which stay rigid enough to prevent collapse. But unless they're coated with certain drugs, rigid stents can lead to renarrowing of the vessel from aberrant tissue healing. Coated metal stents, however, appear to carry the separate risk of triggering blood clots.

If biodegradable stents are to work, they will have to maintain rigidity just long enough to allow healthy layers of cells to reline a vessel and get past the recoil-risk stage, says interventional cardiologist Donaid E. Cutlip of Harvard Medical School in Boston. If so, they could provide better vessel flexibility than rigid stents do. "This is a novel concept. As with most early techniques, it raises a lot of questions, he says.

Erbel and his team are testing an alloy of magnesium that may take longer to dissolve and therefore reinforce vessels for several months.

Meanwhile, other groups are experimenting with biodegradable stents coated with compounds that help prevent the aberrant healing that can occur with rigid metal stents.

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