(CNN) -- Patients who underwent simulated knee surgery fared just as well as those who got the real deal, according to a new study that's raising eyebrows about the most common orthopedic procedure performed in the United States.
The findings, published Thursday in the New England Journal of Medicine, add to a string of papers suggesting that arthroscopic partial meniscectomy fails to help many patients. The operation typically is performed to relieve knee pain, whether from wear or from an injury.
But other doctors say it's still too soon to draw sweeping conclusions.
The study, which was conducted in Finland, followed 146 patients between the ages of 35 and 65 with symptoms of degenerative wear and tear of the meniscus, a disk-shaped piece of cartilage that acts as a shock absorber between the shinbone and thighbone. They had no detectable arthritis, suggesting that any pain was due to a problem with the meniscus.
About half the patients underwent an arthroscopic meniscectomy, in which a surgeon inserts a blade through a tiny incision in the knee, and essentially shaves down the rough, frayed edges of the meniscus.
The other half underwent an elaborately staged "sham" surgery, in which the doctor made an incision and poked around without any actual manipulation, shaving or cutting.
A year later, there was no significant difference in the knee pain reported by patients in each group. Nearly two-thirds on each side said they were happy with the results, and most said they would do it all again.
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In patients without arthritis, the authors conclude, the procedure "provides no significant benefit over sham surgery."
As many as 700,000 arthroscopic partial meniscectomies are performed in the United States every year, at a direct cost of $4 billion, according to the study authors.
But the procedure has come under scrutiny with the publication of papers -- 2002, 2008 and earlier this year -- that found it provides little or no benefit in older patients, whose meniscus is frayed through simple wear and tear as opposed to a specific injury.
"It's pretty obvious to anyone who really has an interest in this that what we've called a meniscal tear isn't really a tear," says Dr. Teppo Jarvinen, who led the research team. "It has nothing to do with the tears we talk about in a 20-year-old athlete who twists or sprains their knee."
According to some, the new study draws a stronger conclusion because it includes patients with mechanical symptoms like popping, clicking or a sense of the knee locking up.
"When we hear a knee is locking, a bell goes off: 'this needs arthroscopy,'" says Dr. Dennis Cardone, an associate professor of orthopedic surgery at NYU Langone Medical Center in New York.
"If anything, this (study) swings it a little bit more. Even when a patient complains of locking, arthroscopy might not be necessary."
But Dr. Frederick Azar, vice-president of the American Academy of Orthopaedic Surgeons, says patients in the study are not typical.
"To have a degenerative medical meniscus tear and no evidence or arthritis is extremely unusual," says Azar. "It's well less than 1% of the patients we see."
Dr. Scott Rodeo, co-chief of the Sports Medicine and Shoulder Service at the Hospital for Special Surgery in New York, says it's likely that many of the patients in the study are in the early stages of developing arthritis, even if it's not detectable through X-rays.
He said the Finnish study reaffirms the sense that surgery is not likely to help these patients whose pain is due to arthritis.
Azar, a professor of medicine at the University of Tennessee and team physician for the Memphis Grizzlies basketball team, says he's worried about scaring away patients who might be helped.
"This is a very useful low-cost intervention, with a short recovery time and good results in most patients," he says.
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A worn meniscus can be a simple result of aging, but it's more common in people whose knees take a lot of pounding, including long-distance runners and people whose jobs have them standing for long periods on a hard floor. Obesity is an additional risk factor.
The Finnish study looks at meniscectomy, as opposed to meniscus repair, in which a surgeon actually sews together torn cartilage. According to Cardone, patients under 40 do especially well with meniscus repair, especially those whose injury stems from a single incident.
But orthopedists say many people with knee pain can be helped with physical therapy to strengthen muscles that support the knee.
Before recommending surgery, Azar also counsels patients to consider switching to low-impact activities -- for example, to mix biking or swimming into a workout routine, instead of just long-distance running. He says other patients may be helped by anti-inflammatories or injections of hyaluronic acid.
In advanced cases, when the meniscus is totally worn away, more drastic steps are an option.
"If it's bone-on-bone," says Azar, "their pain is coming from osteoarthritis and the only surgery to help is a knee replacement. But we try to exhaust all measures before doing that."
However, Jarvinen says the abundance of caution needs to start much earlier.
"All your fellow orthopedic surgeons will tell you, 'I already knew this.' But the facts are, this is still the most common orthopedic procedure," he says, and the vast majority of operations are unnecessary.