US health regulators are investigating a rare but potentially serious problem with a newer class of heart stents that causes the tiny tubular devices made of wire mesh to shrink, or lengthen, after implantation.

The problem appears to occur most frequently in the Promus Element and Ion devices made by Boston Scientific Corp , the US Food and Drug Administration said. Promus Element is sold outside the United States. The company submitted a pre-market application with FDA earlier this year.

The agency said it still views the Ion stent, which it approved for use in the spring, as safe when used for its authorized indications.

“FDA is actively working with (drug-eluting stent) manufacturers, including Boston Scientific, to better understand longitudinal stent deformation with respect to its causes, predisposing underlying anatomic conditions, operator techniques that can reduce the likelihood of its occurrence, and treatment strategies should it occur,” the agency said in an emailed response to a Reuters inquiry.

Boston Scientific, the biggest manufacturer of stents with more than a third of the $4 billion global market, declined to comment on discussions with the FDA. Boston Scientific had a 36 per cent share of the global market and a 49 per cent share of the US market at the end of the third quarter.

The company’s business has struggled on several fronts in recent years, including a series of stent recalls that date to 2004 and a heavy debt load from its ill-fated acquisition of stent and pacemaker manufacturer Guidant Corp in 2006.

Concerns about stent deformation were raised in two smaller cardiology journals published in the United States and Europe, and the issue was the subject of a discussion at the Transcatheter Cardiovascular Therapeutics meeting in San Francisco earlier this month.

Drug-eluting stents prop open diseased arteries and deliver medicine to keep the vessel from reclogging. In the case of longitudinal deformation, the stents become deformed inside the vessel, creating a clotting risk that could lead to heart attack.

Some factors that appear to predispose a patient to the problem include calcification of coronary arteries, implantation in a twisted blood vessel and improper positioning of the stent, FDA said.

The FDA said that only one instance of longitudinal stent deformation was reported from trials of more than 4,600 patients treated with a Promus Element stent. But it said it was also aware of recently published case reports on the problem and that an unspecified number of significant adverse events had been reported to the agency.

“At this time, additional data collection and analyses are ongoing, but the information available to date indicates that the Ion Paclitaxel-Eluting Platinum Chromium Stent remains safe and effective when used for its approved indications,” the agency said.

Heart stents have also come under scrutiny following research that shows taking conventional medication and exercise may help some patients more than the devices.

In June, Johnson & Johnson said it was getting out of the drug-eluting stent business. Other makers of drug-eluting stents include Medtronic Inc and Abbott Laboratories Inc.

Heart doctors see room for more data

Dr. Gregg Stone, a cardiologist at New York Presbyterian/Columbia Hospital who has led many clinical trials for all of the major stent makers, said the stent deformation seems to occur most often in complex cases of blocked blood vessels.

“It can happen with any stent if you’re aggressive enough with it,” he said. “The metal is very thin -- it’s 3/1,000 of an inch thick, so depending on the material and configuration ... some are more prone (to deformation) than others.” He would not characterize it as a design flaw or as user error. He cited instances where a balloon used to implant the device catches on the stent, pulls it and causes it to lose its proper form, or when the guide wire that carries the device pushes on the stent, causing it to shorten in length.

“It seems to be a rare phenomenon. Millions of these are implanted and there are just a handful of these cases,” he said.

Dr. Cindy Grines, a cardiologist at the Detroit Medical Center Cardiovascular Institute and editor of the Journal of Interventional Cardiology, said she reported the issues to FDA and to Boston Scientific when she first learned of it. She wrote an editorial on the problem in the latest issue of the Journal.

“The FDA was very interested and said they got other complaints and would do an extensive analysis,” Grines said in a telephone interview.