WASHINGTON - Three U.S. senators have asked Fridley-based Medtronic Inc. to supply detailed information about how the company keeps track of its medical devices after they go to market.

In a six-page letter that strikes an investigational tone, Sens. Charles Grassley, R-Iowa, Herb Kohl, D-Wis., and Richard Blumenthal, D-Conn., on Thursday called for specific explanations of how Medtronic monitors products once they are released to the public. The letter also seeks specific data about Medtronic's embattled spine product Infuse.

The letter to Medtronic was one of five individual letters sent to different medical device producers whose equipment has malfunctioned after being placed on the market. Grassley, Kohl and Blumenthal also have introduced legislation to require device makers to do clinical studies after their products are offered for sale to the public if those products are approved under a streamlined process that lets them avoid extensive pre-market testing.

Medtronic had no specific comments on the letter's content or contentions. "We have received the letter and are working to comply with their request," spokesman Steve Cragle said.

In the Medtronic letter, the senators lay out concerns about possible conflicts of interest in the existing screening regimen, in particular for Infuse.

"A researcher at Stanford University School of Medicine found a higher risk of cancer associated with Infuse, and there have been allegations that researchers who received funds from Medtronic, sometimes millinos of dollars, did not report negative findings from clinical trials," the letter states.

Doctors use Infuse to help bones fuse in spine surgery. In 13 premarket studies, researchers reported no adverse effects from the use of Infuse. But in June, a medical publication, Spine Journal, said another review revealed adverse affects in 10 to 50 percent of the cases studied, including cancer, inflammation and male sterility. The journal charged that Medtronic researchers had downplayed those problems.

In August, the company announced that it had hired researchers at Yale University to do independent studies of Infuse, which accounts for roughly $700 million a year in sales for Medtronic.

Jim Spencer • 202-408-2752