Knee Pain More Common, but Age, BMI Not at Fault

Steven Fox

December 15, 2011

December 15, 2011 — The prevalence of knee pain and knee replacement surgeries has risen substantially during the last 20 years. However, the reasons for the increase remain obscure, according to a study published in the December issue of the Annals of Internal Medicine.

"A recent surge in knee replacements is assumed to be due to aging and increased obesity of the U.S. population," write Uyen-Sa D.T. Nguyen, DSc, Clinical Epidemiology Research & Training Unit, Boston University School of Medicine, and Brigham and Women’s Hospital, Boston, Massachusetts, and colleagues. However, the team's analysis suggest the rise in knee surgeries may be linked more to increasing knee pain or an increased awareness of knee pain.

"Using data from 2 community-based studies, we found that the age- and [body mass index (BMI)-]adjusted prevalence of knee pain over 20 years has increased for non-Hispanic white and Mexican American men and women and African American women," the authors write. "Adjustment for age did not substantially alter the prevalence estimate for knee pain or symptomatic knee osteoarthritis over time, whereas additional adjustment for BMI resulted in a 10% to 25% decrease in prevalence ratios."

To understand what is driving the increase in total knee replacements, Dr. Nguyen and colleagues evaluated results from 6 National Health and Nutrition Examination Surveys (NHANES) conducted between 1971 and 2004, and from 3 exam periods in the Framingham Osteoarthritis (FOA) Study carried out between 1983 and 2005.

"We examined whether a change in the prevalence of knee pain and symptomatic knee osteoarthritis could be attributed to age, [BMI], or radiographic knee osteoarthritis," the researchers write.

Participants in the NHANES surveys were white or black and were aged 60 to 74 years. More than 6900 people were included in that study. Participants in the FOA Study were predominately white and at least 70 years old. The number of people evaluated in the FOA Study were 902, 1132, and 671 from the 3 exam periods, respectively.

Participants in both studies were asked whether they experienced knee pain most days. In addition, participants in the FOA study underwent bilateral weight-bearing radiographs of their knees to assess the presence and extent of osteoarthritis. Radiographs were combined with self-reported knee pain to define symptomatic knee osteoarthritis.

The researchers found that from 1974 to 1994, several ethnic groups in the NHANES study, namely, non-Hispanic white and Mexican-American men and women and black women, experienced a 65% increase in age- and BMI-adjusted knee pain.

Among FOA participants, the prevalence of age- and BMI-adjusted knee pain and symptomatic osteoarthritis approximately doubled in 20 years among women, and tripled among men.

"These increases may explain the surge in knee replacement surgeries and suggest a bigger burden of knee pain in our society than previously thought," they write.

Remarkably, the researchers saw no such trend among FOA participants in terms of the prevalence of radiographic evidence of osteoarthritis. "[T]he age- and BMI-adjusted prevalence of radiographic knee osteoarthritis did not substantially change over this same period for men (P for trend = 0.82) and actually may have decreased for women (P for trend = 0.036)," the authors write.

They conclude that even though the prevalence of knee pain, independent of increasing age and rates of obesity, has risen during the last 20 years, obesity accounted for only part of the increase.

In an accompanying editorial, Allan Gelber, MD, MPH, PhD, from Johns Hopkins University School of Medicine in Baltimore, Maryland, comments, "These findings call both the health services researcher and the treating clinician alike to action."

He adds, "This reality compels the treating physician to carefully consider, from the signs and symptoms of the patient presenting with knee pain, a broad differential diagnosis. Not all knee pain in middle-aged and older adults is the result of osteoarthritis."

The study was supported by the National Institutes of Health and an American College of Rheumatology Research and Education Foundation Rheumatology Scientist Development Award. The Framingham Heart Study was funded by the National Heart, Lung, and Blood Institute, and the FOA Study was funded by the National Institute on Aging and National Institute of Arthritis and Musculoskeletal and Skin Diseases. The authors and Dr. Gelber have disclosed no relevant financial relationships.

Ann Intern Med. 2011;155:725-732. Article abstract, Editorial

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