The surgery is not recommended in the UK
An operation offered to ease the symptoms of osteoarthritis makes no difference, say Canadian doctors.
Patients given knee arthroscopy showed no improvement beyond that provided by physiotherapy and painkillers.
Arthritis experts in the UK said some surgeons were still carrying out the operation, against national guidance.
They said New England Journal of Medicine study showed doctors still relying on the technique to treat osteoarthritis were misguided.
Thousands of people in the UK suffer from osteoarthritis in the knee, which can be painful and limit movement.
The operation involves inserting instruments through small incisions to try to flush out loose fragments of cartilage, and to smooth the surfaces of the joints, in the hope that this will relieve symptoms.
A group of 178 men and women, with an average age of 60, were enrolled in the trial at the University of Western Ontario.
All of them were given physiotherapy and painkilling drugs such as ibuprofen, but half of the volunteers were also given the "lavage and debridement" procedure.
When their symptoms were compared at various points afterwards, the group who had the operation were faring no better than those who had not received it.
Dr Brian Feagan, one of the researchers, said: "This is definitive evidence that arthroscopic surgery provides no additional therapeutic value when added to physical therapy and medication for patients with moderate osteoarthritis of the knee."
This type of surgery is still recommended for some other knee conditions, including more severe osteoarthritis where the knee is "locked" in position, but is not recommended for moderate osteoarthritis by the National Institute for Health and Clinical Excellence, which formulates UK guidelines.
A spokesman for the Arthritis Research Campaign said there was no longer any excuse for performing it in patients with less severe arthritis.
"Arthroscopic lavage and debridement is still commonly performed in the US but more rarely in the UK over the past ten years, and is no longer accepted as an effective treatment for osteoarthritis of the knee in this country.
"Surgeons still performing this operation need to ask themselves why they are doing it."