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A new study published in the British Journal of Sports Medicine suggests that surgeons are no better at determining which patients might benefit from operations to treat torn knee cartilage than if they just flipped a coin.

Researchers surveyed 194 surgeons to see whether they would recommend surgery or exercise therapy in 20 test cases with middle-aged patients who had tears in the meniscus, the cartilage that works as a cushion between the shin and thigh bones.

Overall, surgeons correctly predicted which patients would benefit from operations only half of the time. And experienced knee surgeons were no better at guessing correctly than other orthopedic surgeons.

These findings offer one reason these knee operations remain common despite a growing body of evidence suggesting many patients don’t benefit, said Dr. Victor van de Graaf, lead author of the study and a researcher at University Medical Centre, Utrecht, in the Netherlands.

“Orthopedic surgeons believe they are capable of identifying which patients may still benefit more from surgery,” van de Graff said by email. “Therefore, our findings may help to further decrease the number of unnecessary performed surgeries.”

During the operation, a surgeon makes a small incision in the knee and inserts a tiny camera called an arthroscope to view the inside of the joint, locate and diagnose the problem, and guide repairs. Sometimes surgeons remove all of the meniscus, and other times they only remove part of it.

While the procedure is minimally invasive, it’s not risk-free. Patients receive anesthesia, which in any surgery may lead to complications such as allergic reactions or breathing difficulties. In addition, this specific procedure might potentially damage the knee or trigger blood clots in the leg.

Among patients who didn’t respond to surgery, only 39% of the surgeons correctly foresaw this outcome. And among patients who didn’t respond to exercise therapy, only 29% of surgeons correctly predicted this outcome.

For patients who did benefit from surgery, surgeons correctly predicted this outcome 60% of the time. And for patients who benefited from physical therapy, surgeons correctly chose this outcome in 72% of cases.

In the same journal, a study published in June went further, concluding that it’s impossible to predict who might benefit from arthroscopic meniscus repair surgery.

Kenneth Pihl of the University of Southern Denmark in Odense and his colleagues built a model based on the real-world outcomes after one year for 641 patients who had undergone the surgery. Out of 18 preoperative factors that surgeons and the medical literature consider predictive of who will benefit, none predicted the actual results for these patients, Pihl’s team found.