The number of first-time knee replacement surgeries among Medicare enrollees jumped 162% in the last 20 years, at a cost of roughly $9 billion annually to payers, a new study finds.

Investigators also noted increases in the number of hospital readmissions and infection rates. Investigators attribute this to shorter hospital stays after knee replacement surgery. Often, the next stop is a skilled nursing or rehab center.

An increase in obesity rates, and rapidly aging baby boomers are credited for the huge jump in knee replacements, according to University of Iowa researchers. They found that the number of replacement surgeries for artificial knees also soared. Between 1991 and 2010, revision surgeries jumped 106%. They analyzed Medicare Part A data files to reach their conclusions.

Knee replacement surgeries typically cost $15,000, and while that adds up to steep expenditures for Medicare, experts say the increase in mobility and productivity for those who undergo the procedure offsets initial outlays, according to Reuters.

The study was published Wednesday in the Journal of the American Medical Association.