The U.S. has seen a more than 100 percent increase in the number of people undergoing the knee replacement surgery, says a new study.

According to the study authors, there has been a rise of 161 percent in knee replacement surgeries over the past two decades. About 600,000 people undergo this surgery every year adding up to approximately $9 billion per year.

The study analysis included more than 3 million patients, aged 65 and older, enrolled with Medicare who underwent primary Total knee arthroplasty (TKA) and another 318,563 who underwent additional surgeries after the procedure or revision TKA

The researchers found that between 1991 and 2010, knee replacement surgeries in the U.S. grew by almost 161 percent; from 93,230 in 1991 to 243,802 in 2010. During the same period, per capita utilization of such operation through Medicare increased by more than 90 percent.

They also found that hospital stay duration decreased from 7.9 days between 1991 and 1994 to 3.5 days between 2007 and 2010.

However, the number of patients getting admitted again in the hospital within 30 days of the procedure increased from 4.2 percent during 1991-1994 to 5.0 percent during 2007-2010.

The authors attribute the growth of knee replacement procedures to the aging population, the prevalence of certain health complications like obesity that put people at a higher risk for osteoarthritis and an increase in criteria for being eligible for this procedure.

Lead author of the study, Peter Cram, MD, of the University of Iowa Carver College of Medicine attributes the increasing number of knee replacements to high levels of activities seen in the elderly.

"People are living longer and want to be active. They feel great after this surgery. They can hike in the mountains and ski. They can be active with their grandchildren," Cram said to USA Today.

According to the authors, guidelines for knee replacement suggest that these procedures are for "people with severe functional limitation unresponsive to conservative management (i.e., medications and physical therapy)." They added that the previous studies had shown that in Spain, about a quarter of such procedures were inappropriate. However, no such study has been done in the U.S.

"In the currently challenging and dynamic health care environment, critical evaluation and systematic data collection about total knee replacements will be needed to optimize outcomes and ensure access to these life-improving procedures," write James Slover, MD, and Joseph Zuckerman, MD, of the Hospital for Joint Diseases of New York University Langone Medical Center, New York, in an accompanying editorial.

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