A fake surgical procedure is just as good as real surgery at reducing pain and other symptoms in some patients suffering from torn knee cartilage, according to a new study that is likely to fuel debate over one of the most common orthopedic operations.

As many as 700,000 people in the U.S. undergo knee surgery each year to treat tears in a crescent-shaped piece of cartilage known as the meniscus, which acts as a shock absorber between the upper and lower portions of the knee joints. The tears create loose pieces of cartilage that doctors have long thought interfere with motion of the joints, causing pain and stiffness.

But researchers in Finland who studied two sets of patients—one that received the surgery, and another that was led to believe that it had—observed no significant differences in improvement between the groups after one year.

Surgery did provide a slight advantage in certain areas early on, including a decrease in pain felt after exercising and in some quality-of-life measures, but the differences disappeared by the end of the 12 months, the researchers said in a paper published Wednesday in the New England Journal of Medicine.

The patients agreed to participate in the study prior to the procedure and were informed they would either receive the surgery or not.