3M Co. declared legal victory Thursday after a federal judge dismissed lawsuits from more than 5,000 plaintiffs who said 3M’s Bair Hugger patient-warming device caused their post-surgical infections.

“There is no legitimate scientific support for the plaintiffs’ theory,” Dr. Todd Fruchterman, general manager at 3M’s medical solutions business, said in a statement. “We want physicians and patients to understand that the practice of patient warming is supported by leading health care institutions, professional societies and the U.S. Food and Drug Administration.”

The Bair Hugger, invented by Minnesota anesthesiologist and entrepreneur Dr. Scott Augustine, is used in more than 80% of U.S. hospitals. It includes a mechanical unit that blows warm air and an array of inflatable plastic “blankets” that are draped over a patient before, during and after surgery and filled with warm air. The goal is to keep the patient’s body from entering hypothermia during surgery, which causes vessels to constrict and can delay healing.

The plaintiffs alleged that the device could actually increase the risk of infection in several ways. The blower could be colonized with bacteria, they said; it could suck bacteria-laden particles from the operating-room floor and deposit them in the air over a surgical wound; and it could disrupt the normal ceiling-to-floor airflow in an operating room that is designed to remove the skin particles that all patients and doctors shed during surgeries.

However, U.S. District Judge Joan Ericksen in Minneapolis ruled late Monday that none of the plaintiffs’ experts could prove these theories. She placed her full ruling under seal until Aug. 6 to give the parties time to redact sensitive information.

“The federal court granted 3M’s request to exclude the plaintiffs’ general causation experts, leaving the plaintiffs with no scientific support for their claims,” 3M’s statement said. “The court previously had agreed to allow the plaintiffs’ experts to testify in court. After those experts testified in the first Bair Hugger system trial in 2018 and after the publication of a new study on surgical site infections, 3M asked the court to revisit its earlier decision.”

A spokeswoman for plaintiffs’ law firm Kennedy Hodges said no decision has been made on a potential appeal. She declined further comment.

A $2.6 billion market

The Bair Hugger is the dominant device in what Grand View Research estimates is a $2.6 billion market for patient temperature-management systems. It competes with patient-warming systems made by Medtronic PLC, Smiths Medical PLC, Zoll Medical Corp. and others.

Most of the plaintiffs who eventually sued 3M contracted deep-joint infections following hip or knee replacement procedures, often a year or more after the surgery. Unlike with soft-tissue incisions, which require hundreds of thousands of bacteria to trigger a catastrophic infection, the plaintiffs’ experts said a single bacterium on an artificial implant could lead to serious infection over time because the device has no immune system to prevent a bacterial biofilm from forming.

Prosthetic-joint infections happen in 1% to 2% of all hip- and knee-implant cases, and typically the root cause is never identified. They are serious medical events often requiring operations to replace the infected joints. The two-year mortality rate following a prosthetic-joint infection in a hip or knee is estimated between 4% and 8%, depending on patient characteristics, including age.

The multidistrict litigation against 3M featured more than 2,000 legal filings and hundreds of exhibits to support or discredit allegations contained in more than 5,000 individual lawsuits from plaintiffs across the country, whose claims were consolidated in 3M’s home district, the U.S. District Court for Minnesota. In the only federal case that went to trial, a jury digested two weeks of testimony and then took two hours to rule in favor of 3M on all counts.

At the time, the plaintiffs had vowed to press on, arguing that corporate executives had acted in bad faith regarding the Bair Hugger.

But the plaintiffs never presented evidence showing a specific microbial strain was spread by an individual device used in surgery and led to a patient infection. Instead, they relied on methods like advanced computer modeling and observational studies to prove their point.

FDA-approved device

Several medical societies and independent evidence-based reviews have considered the theoretical infection risks from devices like the Bair Hugger over the years, and none has said there is definitive evidence showing the devices pose more risks than benefits. 3M has said observational studies purporting to show infection risks are flawed and biased.

In 2017, the FDA said that after “a thorough review of available data,” the agency could not identify any consistently reported association between the use of forced air thermal regulating systems such as the Bair Hugger and surgical site infection. “Therefore, the FDA continues to recommend the use of thermoregulating devices [including forced air thermal regulating systems] for surgical procedures when clinically warranted,” the agency said.

3M noted that the federal judge’s decision to dismiss the lawsuits comes about six months after the Minnesota Court of Appeals upheld a state court decision to dismiss 61 similar cases from Minnesota plaintiffs. Two similar cases, filed in state courts in Missouri and Texas, are still pending.