(CNN) - Hip, knee and other joint replacement procedures are among the most common elective surgeries in the United States.
The average age of patients undergoing hip replacement operations continues to decrease, but more men are undergoing these operations than in the past, according to a study presented Tuesday at the American Academy of Orthopaedic Surgeons' annual meeting.
The new study indicates that the average age of patients undergoing hip replacement surgery has declined from over 66 to just under 65, while the average age for knee replacement surgeries has declined from 68 to just under 66.
Though small, these declines are real, noted Dr. Matthew Sloan, co-author of the study and an orthopedic surgery resident in the Perelman School of Medicine at the University of Pennsylvania. The younger age of patients is "possibly due to increasing obesity or increased access to care in a generation that desires to maintain an active lifestyle into their retirement years," he said.
And although women continue to make up the majority of patients, "men are increasingly undergoing total joint replacement surgery," he added.
The history of total joint replacement dates to the late 1890s, Sloan explained, when surgeons first attempted to replace the ball in the hip joint's ball and socket. In the 1960s, Sir John Charnley popularized the contemporary style of total hip replacement in England. Total knee replacement surgeries -- though they began in the early 1900s -- did not become widespread until advances were made in bone cement in the 1970s.
Total joint replacement involves removing parts of an arthritic or damaged joint and replacing them with a metal, plastic or ceramic prosthesis that can replicate the movements of a healthy joint. These replacement surgeries are performed not only on hips and knees but on ankles, wrists, shoulders and elbows.
The most common total joint replacement procedures in the United States are total knee replacements (about 700,000 a year) and total hip replacements (about 400,000 a year), Sloan said.
Advances over time have been remarkable.
"Today, there are total joint replacements being performed on an outpatient basis, with patients returning home on the same day of surgery," Sloan said. Still, joint replacement surgeries "do not last forever."
Inevitably, a replacement will wear out and another operation will be needed to "revise" the implant, according to the American Academy of Orthopedic Surgeons. These are challenging, complicated operations in which part or the whole implant needs to be removed and replaced and bone needs to be rebuilt with metal pieces; therefore, they aren't as successful as the original surgery.
"Contemporary joint replacement procedures are expected to last 20 years or longer," Sloan said, a longer duration than in the past.
Greater knowledge of patients has helped surgeons advance their practice and "provide quality care," he said.
The need to better understand patients also inspired the study.
Arthritis pain and obesity
With co-author Dr. Neil P. Sheth, an assistant professor of orthopedic surgery in the Perelman School of Medicine, Sloan reviewed the National Inpatient Sample database, which contains over 116 million US patient records, from 2000 to 2014. They identified patients by code and looked at demographic data to understand whether there have been changes over time.
From 2000 to 2014, the average patient age decreased by 0.1 year annually for a first joint replacement surgery and by 0.2 year annually for a second total knee replacement surgery. During that 15-year study period, female patients accounted for up to 62% of all joint replacement procedures.
This is not unusual, Sloan explained. Prime candidates for joint replacement surgery are people with end-stage arthritis, and women have higher rates of arthritis than men (26% vs. 19%) possibly due to differences in anatomy, he said.
Still, the proportion of men undergoing a first knee replacement and a revision hip replacement is rising, Sloan said. He and Sheth also saw significant changes in the use of these elective surgeries by ethnicity.
"Over the past 15 years, non-Hispanic whites have declined in proportion, while blacks have increased," Sloan said. The proportion of blacks undergoing all joint replacement surgeries (except second hip surgeries) increased by 0.1 to 0.2% per year, the study found.
Explaining these results, Sloan noted that most people say the increasingly younger average age of joint replacement patients is caused by rising rates of obesity.
US data for the past decade indicate that 54% of patients undergoing total hip replacements and 79% of patients undergoing total knee replacements are either obese (with a body-mass index of greater than 30) or morbidly obese (with a BMI of greater than 40), he said.
Dr. Matthew Hepinstall, associate director of the Center for Joint Preservation and Reconstruction at Lenox Hill Hospital in New York, said obesity is not the only driver.
Injuries and changing expectations
A combination of factors is influencing the changing age of patients, said Hepinstall, who was not involved in the new study.
"A generation of people have been active and athletic through not just adolescence but through adulthood," he said. This sportiness in adulthood has led to injuries, which can cause arthritic damage.
There's also been "a change in expectations about the level of activity that should be possible when you're 50 and 60 and 70," Hepinstall said. More people are seeing friends and acquaintances who were limping but then had surgery and got back to their lives.
Additionally, doctors are more confident in this operation, which has improved over time and become "more reliable and more durable," he said.
Thirty years ago, a 50-something patient with pain may have been sent away with the words "Listen, you're not disabled. You're capable of working. You're capable of walking. Why would you have a hip replacement?" Hepinstall said. Today, a doctor might say to the same patient, "These are your years to still be active. If you wait till you're 75 and get your hip fixed, you will have heart disease, which will limit your mobility. Why not get it fixed now and enjoy these years?"
Overall, the study is true to what Hepinstall said he sees in his own practice. "We absolutely are seeing more and more people who need this. We're seeing them younger. Some are heavier, and some are more active and had injuries," he said.
Still, the percent increases or decreases might seem tiny to most.
"Even small changes in utilization can have relatively high impact on the country from an economic perspective," Hepinstall said. "The reality is, we're talking about one of the most common and most costly elective surgeries in our country." There are downstream effects of the choice to either fix a problem or live with it that may influence the economy in different directions, he said.
There are increased costs associated with more people having it done but also decreased costs associated with improvement in economic productivity when a surgery allows more people to stay active instead of collecting disability benefits or becoming a drain on a family's economic engine.
Wanting to keep health care costs under control, regulators are looking at hip and knee surgery to make sure use is "neither too high nor too low," Hepinstall said. "Getting it right is important."
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