In this procedure surgeons replace only the eroded side of the knee. Compared with total knee replacement, it offers a smaller incision, faster recovery, and superior function. Moreover, it might buy patients 10 to 15 years before they need total knee replacement.

But the operation isn't as durable or predictable as total knee replacement. In the same British study, which looked at more than 80,000 knee patients, early revision rates were twice as high for those who had partial knee replacement, especially among younger patients. Common reasons for failure include loosening and arthritis in other parts of the knee.

"Partial knees are great in the right person," Kraay says. "But only one in 20 people who need knee replacement are candidates." Patients should have arthritis in only one side of the knee, intact ligaments, and good range of motion. In addition, they can't be very heavy, nor too bowlegged or knock-kneed. Callaghan cautions that, like hip resurfacing, this operation might be overused.