Two new studies presented recently at the annual meeting of the American Academy of Orthopaedic Surgeons (AAOS) are adding to the body of knowledge on total knee replacements.

One study examined who’s getting total knee replacements and revision surgeries – that is, a second surgery to make corrections after a first joint replacement surgery fails – and how fast those numbers are growing. The other study zeroed in to find out more about the causes of revision surgeries.

“We observed [in our practice] that joint replacement patients were becoming younger,” says lead author of the first study, Jacob Drew, MD, explaining why he and his colleagues at the University of Massachusetts Medical School (UMMS), in Worcester, undertook the study.

The study provided the proof. The researchers found the number of total knee replacements rose sharply over the past decade, with the greatest increase among people younger than 65. Using the Nationwide Inpatient Sample database, they reviewed hospital discharge data for more than 2 million total knee replacement patients to determine how many knee replacements and revisions were performed between 2000 and 2009. (A separate study, also presented at the AAOS conference found 4.7 million Americans are currently living with artificial knees.)

The researchers found the overall number of total knee replacements performed per year rose from more than 282,000 in 2000 to more than 621,000 in 2009 – an increase of 120 percent. The overall increase is steep, but even more so in younger age groups. While they found an 89 percent increase for those ages 65 to 84, they found a startling 188 percent increase for 45- to 64-year-olds.

Dr. Drew says the increase in total knee replacements isn’t due solely to population growth, although that is one factor. The driving force is a rise in the rate – that is, the percentage – of people in the under-65 age group opting for the procedure.

Although knee replacement surgeries are considered to have a good track record, “new knees” don’t last forever; earlier studies have found that 85 percent of knees last 20 years and the AAOS estimates 10 percent of patients at some point need a revision for one reason or another. The younger the patients are when they have the first surgery and the longer they live afterward, the more likely they will be to need revision surgery.

“We had some theoretical concerns about how young is too young,” says Dr. Drew.

The fast rate of growth of knee surgery among younger patients seems to support the concerns of Dr. Drew and his colleagues: The researchers found the number of revision surgeries increased 133 percent overall during the study period, with a higher rate of revisions among younger than older patients.