According to recently released data from the Center for Disease Control, Bloomberg reports, the rate at which Americans over the age of 45 undergo knee replacements nearly doubled between 2000 to 2010. What’s more, the average age at which Americans got their knees replaced fell significantly, from 69 in 2000 to 66 in 2010. In total, there were over five million knee replacement surgeries performed during the decade.

As of 2008, the procedure, which costs somewhere around $16,600 per knee and entails replacing faulty bone and cartilage with an plastic or metal artificial joint, is the most frequent inpatient hospital procedure for Americans over 45.

So what gives? (Other than knees, of course.) According to Bloomberg, the growth in knee replacements can be explained by a longer life expectancy of Americans, which rose by a year-and-a-half for 65 year old Americans in the first decade of the new millennium, and the increased prevalence of obesity. Extra weight on the knees, after all, worsens osteoarthritis—the very condition a knee replacement is meant to alleviate.

David Teuscher, who heads the American Academy of Orthopaedic Surgeons, told Bloomberg that his knee replacement patients are often advised to lose weight. Often, though, they need a new knee or two before they can realistically get serious on the treadmill.

[Bloomberg]