Older Americans are increasingly reporting chronic knee pain and getting knee replacements, even though evidence of knee arthritis is not on the rise, according to a new report in the Annals of Internal Medicine.

Researchers from Boston University are puzzled by the disparity between the two trends, of people reporting knee pain to the number of people with apparent arthritis.

Researchers assumed the nation’s growing obesity epidemic and its aging population was probably the reason for the rise in chronic knee pain, however the study found that those were only part of the reason.

"We don't really know what the other reasons could be," said lead researcher Uyen-Sa D.T. Nguyen, of Boston University School of Medicine, according to Reuters.

One possibility, she told Reuters Health, is that older adults these days are more likely to admit to having pain.

"I suspect that what happened in the past is that people often wouldn't complain," agreed senior researcher Dr. David T. Felson, who is also with Boston University.

But it's hard to know for sure, the researchers say.

Regardless, it's possible that the rising rate of knee pain is one factor in the surge in knee replacement surgeries in recent decades, Felson said in a written statement.

Researchers compared data from 6 National Health and Nutrition Examination Survey (NHANES) surveys between 1971 and 2004 and from 3 examination periods in the Framingham Osteoarthritis (FOA) Study between 1983 through 2005, to determine the cause for the recent surge in knee replacements.

Researchers found that complaints of knee pain increased by about 65 percent in the NHANES from 1974 to 1994 among white and Hispanic men and women and among African American women, and the FOA study shows people with knee pain and symptomatic knee osteoarthritis doubled in women and tripled in men over two decades. 

Results from the X-ray evidence of arthritis show that over the same period of time there was little change, and that there was even a decrease among women, from 42 percent to 35 percent. 

Some suggest that Americans could be undergoing replacements at an earlier age or at lesser severity than in the past.