That was what almost happened with Mrs. Weinstein, an active, athletic 64-year-old who lives in New London, N.H. And it was her great fortune to finally visit a surgeon who told her so. He told her bluntly that her pain was caused by arthritis, not the torn cartilage.

No one had told her that before, Mrs. Weinstein said, and looking back on her quest to get a scan and get the cartilage fixed, she shook her head in dismay. There’s no surgical procedure short of a knee replacement that will help, and she’s not ready for a knee replacement.

“I feel that I have come full circle,” she said. “I will cope on my own with this knee.”

In fact, Mrs. Weinstein was also lucky because her problem was with her knee. It’s one of only two body parts  the other is the back  where there are good data on abnormalities that turn up in people who feel just fine, indicating that the abnormalities may not be so abnormal after all.

But even the data on knees comes from just one study, and researchers say the problem is far from fixed. It is difficult to conduct scans on people who feel fine  most do not want to spend time in an M.R.I. machine, and CT scans require that people be exposed to radiation. But that leaves patients and doctors in an untenable situation.

“It’s a concern, isn’t it?” said Dr. Jeffrey Jarvik, a professor of radiology and neurosurgery at the University of Washington. “We are trying to fix things that shouldn’t be fixed.”

As a rheumatologist, Dr. Felson saw patient after patient with knee pain, many of whom had already had scans. And he was becoming concerned about their findings.

Often, a scan would show that a person with arthritis had a torn meniscus, cartilage that stabilizes the knee. And often the result was surgery  orthopedic surgeons do more meniscus surgery than any other operation. But, Dr. Felson wondered, was the torn cartilage an injury causing pain or was the arthritis causing pain and the tear a consequence of arthritis?