Broken heart stents may be more common than thought

* Older, longer, or overlapped stents fracture prone

* Cypher more likely to break than Taxus

By Julie Steenhuysen

CHICAGO, Nov 9 (Reuters) - Heart devices known as drug-eluting stents may be far more prone to breaking than expected and may cause blood clots and scar tissue formation in some patients, U.S. researchers said on Monday.

They studied hearts obtained in autopsies and found 29 percent of drug-releasing heart stents -- tiny wire mesh coils that prop open clogged heart arteries -- had fractures in them.

Older stents, longer stents, and stents that overlapped one another were more likely to fracture, Dr. Renu Virmani of the Cardiovascular Pathology Institute in Maryland and colleagues reported in the Journal of the American College of Cardiology.

They also found the Cypher stent made by Johnson & Johnson (JNJ.N) was more likely to break than the Taxus stent made by Boston Scientific Corp (BSX.N).

Both are older, first-generation, drug-eluting stents -- devices that release drugs over time to prevent scar tissue from forming that could re-clog a treated artery.

They have largely been replaced by newer models, such as Abbott Laboratories (ABT.N) Xience, co-marketed as Promus by Boston Scientific.