Osteoarthritis is More Than Wear and Tear

Knee osteoarthritis has more than doubled in prevalence since the mid-20th century after occurring at low frequencies since prehistoric times, a study has found.

Researchers from Harvard University and other centers concluded that rising levels of knee osteoarthritis-often considered an inevitable consequence of persons living longer-may be the result of modifiable risk factors, such as high body mass index (BMI), that have become more common in recent years.

Thus, they suggested, knee osteoarthritis may be more preventable than previously thought.

A Study of Skeletal Remains

The researchers studied cadaver-derived skeletal remains to investigate long-term trends in knee osteoarthritis prevalence in the United States and evaluate the effects of longevity and BMI on disease levels by comparing the prevalence among persons who lived in the 19th to early 20th centuries with persons in the late 20th to early 21st centuries. The samples included complete skeletons of persons aged 50 years and older who lived in major urban areas in the United States. They also analyzed knee osteoarthritis in a large sample of archeological skeletons of prehistoric Native American hunter-gatherers and early farmers.

Diagnosis of knee osteoarthritis was based on visual identification of the presence of eburnation on the articular surfaces of the right or left distal femur, proximal tibia, or patella.

Some key findings:

• The prevalence of knee osteoarthritis was 16% in the postindustrial sample but only 6% and 8% in the early industrial and prehistoric samples, respectively.

• The prevalence was 2.1-fold higher in the postindustrial sample than in the early industrial sample after the investigators controlled for age, BMI, and other variables.

• Females were more affected than males. After controlling for sex, knee osteoarthritis prevalence in the postindustrial sample was 2.6 times and 2 times higher than in the early industrial and prehistoric samples, respectively.

• Among postindustrial persons with knee osteoarthritis, 42% had the disease in both knees, a 2.5-fold and 1.4-fold higher proportion than in the prehistoric and early industrial samples, respectively.

The researchers tested whether the higher levels of knee osteoarthritis in the postindustrial era are attributable to greater longevity and higher BMIs. Age and BMI were positively associated with knee osteoarthritis prevalence, but at all ages, prevalence was at least twice as high in the postindustrial sample as in the early industrial sample, even after controlling for BMI.

A Role for Environmental Factors

The recent dramatic increase in knee osteoarthritis prevalence raises the question of what additional independent risk factors that are unique to or amplified in the postindustrial era might be, the authors suggested.

“Alleles of genes, such as GDF5, have been shown to influence knee OA susceptibility,” they said, “but the approximate doubling of knee OA prevalence in just the last few generations proves that recent environmental changes have played a principal role.”

A source of environmental change that warrants greater attention is whether and how joint loading has altered, they noted, but they pointed out that “while joint overloading from high BMI has become common only recently, our results indicate that the majority of knee OA today is not caused by high BMI per se.”

The authors also named physical inactivity as a feature of modern environments that might merit more study.

A “Mismatch Disease”

The researchers characterized knee osteoarthritis as a condition that from an evolutionary perspective fits the criteria of a “mismatch disease”-one that is more prevalent or severe because bodies are inadequately or imperfectly adapted to modern environments.

“Other well-studied mismatch diseases, such as hypertension, atherosclerotic heart disease, and type 2 diabetes, that also have become epidemic during the last few decades are strongly associated with knee OA, suggesting common causes and risk factors,” they stated, adding that prevention “will require a reappraisal of potential risk factors that have emerged or intensified only very recently.”

The researchers reported their findings online in Proceedings of the National Academy of Sciences.

References:

Wallace IJ, Worthington S, Felson DT, et al. “Knee osteoarthritis has doubled in prevalence since the mid-20th century.” Proc Natl Acad Sci U S A. Published online before print on August 14, 2017. doi: 10.1073/pnas.1703856114