The operation -- arthroscopic surgery for the pain and stiffness caused by osteoarthritis -- is done on at least 225,000 middle-age and older Americans each year at a cost of more than a billion dollars to Medicare, the Department of Veterans Affairs and private insurers.

It involves making three small incisions in the knee; inserting an arthroscope, a thin instrument that allows surgeons to see the joint; and then flushing debris from the knee or shaving rough areas of cartilage from the joint and then flushing it.

In the study, to be published today in The New England Journal of Medicine, investigators at the Houston Veterans Affairs Medical Center and Baylor College of Medicine report that while patients often said they felt better after the surgery, their improvement was just wishful thinking. Tests of knee functions revealed that the operation had not helped, and those who got the placebo surgery reported feeling just as good as those who had had the real operation.

''Here we are doing all this surgery on people and it's all a sham,'' said Dr. Baruch Brody, an ethicist at Baylor who helped design the study. ...

The 180 participants in the study were randomly assigned to have the operation or to have placebo surgery in which surgeons simply made cuts in their knees so the patients would not know if they had the surgery.

After they recovered from the procedures, most patients said their knee pain had improved, and they continued to say they were better for the two years that the researchers followed their progress. But Dr. Nelda P. Wray, who is chief of the section of health services research at Baylor, said, ''On the objective scale, no one was better at any time point.''

Some orthopedists interviewed about the study said they had wondered for some time about the operation's effectiveness. Dr. Kenneth Fine, an orthopedic surgeon at the George Washington University School of Medicine, said the procedure had long seemed to do nothing for patients' underlying arthritis.

''There are pretty good success rates in terms of patient satisfaction,'' Dr. Fine said, ''but I have always been skeptical.''

Dr. William J. Tipton Jr., executive vice president and chief executive of the American Academy of Orthopedic Surgeons, also said he had questioned the operation.

''I'm both a patient and a physician,'' Dr. Tipton said, explaining that he has osteoarthritis. ''My knee is buckling now, but I'm not going to have arthroscopy done. I recognize that it's not going to help.'' ...

The research began when an orthopedic surgeon at the Houston veterans' hospital, Dr. J. Bruce Moseley, who is now the team physician for Houston's two professional basketball teams, approached Dr. Wray suggesting a study that would compare washing the knee joint with washing and scraping in patients with arthritis.

Dr. Wray had a bolder idea.

''She said, 'How do you know that what you are seeing is not a placebo effect?' '' Dr. Moseley recalled. ''My response was, 'This is surgery.' She said, 'I hate to tell you this, but surgery may have the biggest placebo effect of all.' ''

Placebo studies of surgery are almost never done. Many doctors consider them unethical because patients could undergo risks with no benefits. Working with Dr. Brody, the ethicist, the group tried to make the placebo treatment no more dangerous than daily life. Still, of 324 consecutive patients who were asked to participate, 144 declined.

For those who agreed, the day of surgery meant being wheeled into an operating room while neither they nor any of the medical staff knew what their treatment would be. When they were on the operating table, Dr. Moseley, who did all the operations, opened a sealed envelope telling him whether the patient was to have the surgery or not.

Those in the placebo group received a drug that put them to sleep. Unlike those getting the real operation, they did not have general anesthesia.

Dr. Moseley made small cuts in their knees to simulate an operation. He bent and straightened the knee and asked for surgical instruments, just in case the patient was partly conscious. An assistant sloshed water in a bucket to make the sound of a knee being flushed clean.

...Dr. David T. Felson of Boston University and Dr. Joseph Buckwalter of the University of Iowa note that if there were large beneficial effects from the surgery, the study should have found them.

''Although the study may not have been large enough to permit the detection of any small effects,'' they wrote, ''the data presented do not suggest that there were any.,''

In a telephone interview this week, Dr. Felson, a professor of medicine and a rheumatologist by training, praised the research but said it remained to be seen whether doctors and patients would abandon the procedure.

''There's a pretty good-sized industry out there that is performing this surgery,'' Dr. Felton said. ''It constitutes a good part of the livelihood of some orthopedic surgeons. That is a reality.''

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