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(NBC) - There has been a big increase in the number of knee replacement surgeries in the past two decades, and the public is paying for much of it.

New research finds the number of total knee replacements has shot up in the past 20 years, mainly because of an aging but active population.

Dr. Brian Wolf and colleagues at the University of Iowa found the number of total knee replacements among Medicare patients went from 93,000 in 1991 to more than 240,000 in 2010.

That's an increase of 161 percent.

The study only looked at Medicare patients, and because knee replacement surgeries are also performed on those younger than age 65 -- when Medicare kicks in -- many experts say the numbers are actually quite higher.

"The numbers in the study, while impressive, really only represent a small portion of what we're actually going to be seeing down the road in terms of these procedures," says Dr. James Slover of New York University's Langone Medical Center.

With an increase in surgeries comes an increase in costs.

Medicare covers the majority of expenses related to total knee replacements.

"This is part of the overall baby boomer dilemma that medicine is facing not only with knee replacements but with a lot of medical issues that are going to be coming forward over the next 20 years," Dr. Wolf explains.

But 20 years is often the absolute maximum life span of a knee replacement, raising the likelihood many patients will need additional surgeries.

The study also found an increase in hospital readmission rates among patients.

A total knee replacement costs an average of $15,000.

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