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A man walks out of the hospital following knee surgery. - 

In recent years Americans have been replacing their knees at an remarkable rate.

A new study in the Journal of the American Medical Association finds total knee replacements have more than doubled in the past two decades. Around 600,000 of them are done each year.

Peter Cram at the University of Iowa Carver College of Medicine crunched these numbers and looked at the knee surgery surge could mean for health care costs.

Follow Jeff Horwich at @jeffhorwich