More than 4,000 people who came down with serious infections after joint surgery have sued 3M Co., blaming the Maplewood-based company for selling a warm-air blower that may have deposited infectious bacteria in their incisions during surgery.

Most of the patients developed deep-joint infections after orthopedic surgery. A dozen lawyers spent three days in U.S. District Court in Minneapolis this week debating which expert opinions and computer models should be allowed as evidence if the first cases go to trial early next year, as scheduled.

3M’s Bair Hugger device works by warming air and then blowing it into an inflatable “blanket” draped over a surgical patient, which 3M says is safe and effective. The company says it is in use in more than four of every five U.S. hospitals and has been used in more than 200 million procedures.

The plaintiffs want to present scientific studies and medical experts who will describe how the inner workings of the device harbor bacteria and disrupt the normal downward airflow inside an operating room that keeps bacteria-carrying particles on the floor and away from the surgical site.

None of the plaintiffs has definitive physical proof that the surgical warming device moved a specific bacterium into their wound.

Rather, pretrial arguments in a Minneapolis federal courtroom this week showed patients will rely on expert testimony and calculations by a supercomputer to try to prove that the Bair Hugger can cause infections by moving bacteria-laden particles into wounds during surgery.

3M lawyers said the plaintiffs’ simulation doesn’t agree with their scientific findings. And company lawyers urged U.S. District Judge Joan Ericksen to reject the unusual legal tactic of using a simulation to prove a point in a medical case: “There’s no case where a medical mass tort has been decided based on a computer simulation,” said Peter Goss, one of the attorneys for 3M, during oral arguments on pretrial motions.

Genevieve Zimmerman, one of the plaintiffs’ attorneys, argued that the use of a simulation was appropriate because the Bair Hugger isn’t like other medical mass torts. The simulation was produced from a computational fluid dynamics (CFD) model of how small particles in the operating room would be disturbed when a Bair Hugger is switched on.

“At its core, this case [is] dependent on engineering principles, as compared to pharmacology,” Zimmerman said via e-mail. “CFD is routinely used to test all manner of engineering issues, from airplane design and nuclear weapons, to weather patterns, cars, and skyscrapers. … Because it is so well-established and reliable, CFD is routinely admitted in court proceedings throughout the United States.”

It’s not clear when Ericksen will rule on the evidentiary matters or 3M’s motion for summary judgment.

Surgical infections are among the most common and costly types of adverse events that happen in hospitals, and organizations like the Centers for Disease Control and Prevention have long published guidelines for bringing the number of infections to as close to zero as possible.

Warming up patients before and during surgery has been hailed as a way of preventing infections. A surgical incision sheds heat quickly, which can inhibit healing and the body’s infection-prevention mechanisms.

On Aug. 30, the Food and Drug Administration reiterated its advice that using patient-warming systems including the Bair Hugger in surgery has been demonstrated to decrease the risk of infections. “After a thorough review of available data, the FDA has been unable to identify a consistently reported association between the use of forced air thermal regulating systems and surgical site infection,” the FDA wrote.

But the plaintiffs say 3M has ignored warnings about the risk that the Bair Hugger spreads germs.

The plaintiffs’ CFD simulations depict how 10-micron particles such as dead skin flakes can be blown up from the surgical floor in the turbulence created by heat rising after the device is switched on. The simulations, created through 2 million CPU-hours of calculations on a supercomputer, show that the device can move a statistically significant number of bacteria-harboring particles into a surgical site in what Zimmerman called “an extraordinarily short time.”

The simulation “reveals from an engineering standpoint the impact the Bair Hugger has on the movement of [small] particles in [an] operating room,” Zimmerman said. “The CFD is not offered alone to show causation but is part of the evidence and expert testimony plaintiffs have introduced to establish the Bair Hugger causes deep joint infections.”

But 3M lawyers ask, if the simulation is accurate, why aren’t the results borne out in dozens of scientific studies that show forced-air warming devices effectively warm patients before, during and after surgery and can reduce the risk of surgical site infections?

“All of the real science out in the real world rejects the theory promoted in these lawsuits,” Jerry Blackwell, one of the 3M attorneys, said via e-mail. “These [plaintiffs] lawyers have hired an expert to make an animation to assist them in making claims not supported by the real science. For this reason, we asked that it be excluded.”