A type of knee surgery performed on more than 300,000 Americans each year to ease arthritis pain is worthless and perhaps even harmful, government researchers say.

The study looked at arthroscopic knee surgery for osteoarthritis, the painful, steadily worsening, wear and tear on joints that affects 12 percent of senior citizens. The operation is done to clear out debris or repair damaged cartilage.

In a type of study only rarely conducted, some patients got a real knee operation, while others underwent sham surgery. At every point over the next two years, those who had the fake surgery could climb stairs and walk slightly faster on average than those who had gotten real operations.

In arthroscopic surgery, a flexible viewing tube called an arthroscope lets the surgeon see inside a joint. Only tiny incisions are needed, and patients heal quickly.

Arthroscopic knee surgery for osteoarthritis costs roughly $5,000, or a total of $1.5 billion a year in the United States, said Dr. Nelda Wray, a researcher at the Houston VA Medical Center who led the study.

“We think that money could be better spent,” she said.

The findings could prompt insurance companies to refuse to pay for the procedure. Other treatments for arthritic knees include pain relievers, cortisone shots, hot packs, exercise and, as a last resort, knee-replacement surgery.

Dr. William Tipton Jr., executive vice president of the American Academy of Orthopedic Surgeons, said other researchers should duplicate the results before doctors, patients and insurance companies react. But he said the study confirms some doctors’ growing suspicions.

He said many orthopedic surgeons already feel that arthroscopic surgery should not be used except in certain circumstances, such as acute sports injuries to ligaments.

The study was published in today’s New England Journal of Medicine.