Maplewood manufacturer 3M Co. has recalled 33,000 cases of its Bair Hugger underbody warming blankets because of a design problem that may prevent the blankets from warming up patients during surgery as intended.

3M's recall notice, which was classified as a medium-severity Class 2 recall by the U.S. Food and Drug Administration, said health care providers should identify and discard any Bair Hugger underbody blankets affected by the recall. The affected lot numbers are R10359, R10360, R10361, R10362, R10363, R10364, R10365 and R10366, and they comprise about 33,108 cases of five units each distributed worldwide, including in the U.S.

Forced-air surgical blankets like the Bair Hugger system are intended to maintain normal body temperature in surgery to promote healing and prevent infection, according to the Food and Drug Administration. The Bair Hugger platform includes many types of inflatable blankets; the recall announced this week affects only the "underbody" blankets, which lie completely under the patient to allow unimpeded surgical access.

The FDA's alert said 3M's reason for the recall was a design change that makes these specific blankets more susceptible to having an airflow blockage, which can result in partial airflow. A field safety notice about the same problem, on file with Germany's medical device regulatory agency, said that if the blanket is only partly inflated during surgery, the patient could receive incomplete warming therapy.

The Bair Hugger is a Minnesota invention that 3M acquired in 2010. 3M said the system has been used on more than 200 million patients worldwide, including 50,000 new patients per day, and is installed in more than 80 percent of U.S. hospitals.

Yet thousands of plaintiffs blame the device for causing their deep-joint infections following orthopedic surgery, claiming that the system is responsible for blowing around bacteria-laden particles that may come to rest in a surgical wound. 3M strongly denies such allegations, saying the device has been proved safe in real-world experiments and computational fluid dynamics modeling.

Last August the FDA reported that although some health care providers and patients might avoid forced-air systems like the Bair Hugger, a thorough review of the available data did not show any consistently reported association between use of the systems and surgical site infections. The agency continues to recommend the devices be used during surgeries when clinically warranted.

A 3M spokeswoman said on Tuesday that the allegations in the lawsuits are in no way related to the problem described in the recall.