It's possible. Researchers followed 42 patients who had "normal" knee arthroscopy results. Sixty-four percent of the patients said they felt better three and a half years later. Only two percent felt worse. Sixty-eight percent said they had less pain, swelling, and knee locking and giving way.
The authors think that, once patients knew there were no abnormalities, they may have simply learned to live with their symptoms. Or there may be some benefit from the procedure itself. In any case, the authors conclude that "negative" knee arthroscopies--or those that come out "normal"--really aren't so negative after all.