The president of Medtronic’s spine business told Minnesota’s two U.S. senators this week that no one ever intended to hide a study of the company’s controversial Infuse product that went unreported for years.

Letters to Sens. Al Franken and Amy Klobuchar from Medtronic Spine President Douglas King revealed new details of the study, which the Star Tribune first reported April 10. After the article appeared, Franken and Klobuchar each asked for explanations of how the world’s largest medical device company misplaced a study of 3,647 Infuse patient records that revealed 1,045 “adverse events.”

The company responded that the reporting problem happened because Medtronic Spine’s policies for reporting patient injuries in such a study were unclear, the department did not have a master list of its clinical activities, and the study was closed down without following a formal process.

“While we regret that events from the retrospective chart review were not reported earlier, we have no information to suggest the failure to report was intentional,” King’s letter to Franken says.

Medtronic has said previously that employees misfiled data from the study, without reporting adverse events to the Food and Drug Administration as the law required. The company reported the adverse events to the FDA more than five years later, but neither the company nor the agency has made full details public.

Franken said in a statement to the Star Tribune that he welcomed the additional information Medtronic provided. But he said he still has questions.

“Ultimately, the fact that this could occur at one of the world’s leading medical device companies demonstrates that we need to improve the way we monitor the safety and efficacy of medical devices in the United States,” Franken said.

Infuse is a synthetic human protein that causes bone growth after lower-back surgery, among other approved uses. It has spawned thousands of product liability complaints and sparked multiple government investigations.

Medtronic says the product has also been used safely in more than a million patients. In a statement to the Star Tribune Wednesday, the company said it stands by Infuse.

In addition to his questions for Medtronic, Franken has also asked the FDA for an explanation of its handling of the Infuse study. Klobuchar met with FDA Commissioner Robert M. Califf to discuss what happened.

“Medtronic and the FDA have both admitted to making mistakes in this case,” Klobuchar said in a statement to the Star Tribune. “It is critical that we all work together to examine current practices and identify needed improvements to guarantee adverse events are reported as quickly and efficiently as possible.”

In the letters, Medtronic reveals that the study documented 82 different types of “conditions” found in the medical charts of the 3,647 patients who had received Infuse during surgery. Because the types of conditions were already documented in other studies, the company said the chart study was judged to have added nothing new to the risks facing Infuse patients.

“Only 122 events, or less than 12 percent of the total number of the subsequently reported adverse events, were identified by the surgeons as possibly related to Infuse, and the majority of those events were considered to be moderate or mild in severity,” King wrote to Franken.

The company also acknowledged for the first time, in the letter to Franken, that the “overwhelming majority” of the procedures it studied involved uses of Infuse never approved as safe or effective by the FDA.

These so-called “off-label” Infuse procedures are legal for doctors who think they are the best way to treat patients. In litigation, Medtronic has steadfastly denied that it has ever illegally promoted off-label uses. The FDA has issued two public health notices warning against off-label uses of Infuse.

Sarah Sorscher, an attorney with Public Citizen’s health research group, said the Infuse adverse events should have been reported as they were discovered. Even in its letters to Franken and Klobuchar, Medtronic still failed to reveal complete details, Sorscher told the Star Tribune. “I find that troubling,” she said.