The studies were to be presented Saturday at the American College of Rheumatology annual meeting, in Chicago.
Nearly 6.5 million Americans between the ages of 35 and 84 will receive a diagnosis of knee osteoarthritis in the next decade, according to these new projections.
"The diagnosis of knee osteoarthritis is occurring much earlier," said study author Dr. Elena Losina, co-director of the Orthopedics and Arthritis Center for Outcomes Research at Brigham and Women's Hospital in Boston.
When she compared the age at diagnosis in the 1990s to ages in the 2010s, "the average age at diagnosis has moved from 69 to 56," she said.
It strikes some earlier than the average age, of course. Losina found that adults aged 45 to 54 will account for nearly 5 percent of all knee osteoarthritis (OA) cases in the 2010s, while they represented only 1.5 percent of the knee OA patients in the 1990s.
Losina suspects that obesity and knee injuries, both of which have become more common in the past decade, may be helping to drive the increase in knee OA among younger people.
Injuries to the knee have been linked with an increased risk of knee arthritis. And certain sports are riskier than others, said another researcher, Dr. Jeffrey Driban, an assistant professor of rheumatology at Tufts Medical Center in Boston. He reviewed studies that looked at a link between sports participation and knee OA. He focused on 16 studies, and then honed in on 10 that looked at athletes and nonathletes.
While there were not great differences later in the amount of knee OA for former sports players and nonathletes, he did find a risk linked with the type of sport and level of participation.
Soccer players, whether elite level or not, had a greater risk of knee OA, he found. So did elite long-distance runners, competitive weight lifters and wrestlers.
The increased risk of arthritis in these participants varied from about threefold to more than sixfold compared to nonathletes, he said.
For those who already have knee OA, the best medicine may be weight loss if they are overweight, and exercise.
In another study, Dr. Stephen Messier of Wake Forest University found that a program of diet and exercise reduced pain and improved mobility by as much as 50 percent in those with knee arthritis.
He assigned adults with knee OA and pain to one of three groups for the 18-month program. One group dieted only, one group exercised only and one group did both.
In all, 399 overweight or obese men and women, average age 66, completed the study. The diet and exercise group lost the most weight, averaging 11.4 percent of their body weight. The diet-only group lost 9.5 percent; the exercise-only group lost 2.2 percent.
When they compared pain and mobility, the diet and exercise group reported much less pain and had greater walking speed than the other groups.
Driban suggested that those who want to minimize the risk of knee OA later should consider sports with a lower knee injury risk, such as swimming and cycling.
However, a sports medicine specialist took issue with that suggestion. "There is no evidence that impact sports like running causes arthritis in a healthy knee," said Dr. Stephen Nicholas, director of the Nicholas Institute of Sports Medicine and Athletic Trauma at Lenox Hill Hospital, in New York City.
Like other experts, he does agree that once a knee injury occurs, a person is at higher risk for knee arthritis.
However, if someone has a healthy knee, Nicholas said he would tell them to pick the sport they enjoy.
Losina's research was funded by the U.S. National Institutes of Health's National Institute of Arthritis and Musculoskeletal and Skin Diseases. All three studies should be viewed as preliminary, as they were presented at a medical meeting and not yet peer-reviewed.
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